Structural Design of Tris(bipyridyl)ruthenium(II)Derivatives for High-Performance Solid-State LightEmitting Devices
by
Erik S. Handy
B.A., Chemistry, 1994
Hampton University, Hampton, VA
SUBMITTED TO
THE DEPARTMENT OF CHEMISTRY
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF
DOCTOR OF PHILOSOPHY IN CHEMISTRY
AT THE
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 1999
C Massachusetts Institute of Technology, 1999. All Rights Reserved.
Author :
Certified by:
.............................. . . . . .
. .. ....
. ..
......... .. ..
Department Chemistry
May 11, 1999
...........................................
Michael F. Rubner
TDK Professor of Materials Science and Engineering
-
Thesis Supervisor
Accepted by: .............................................
Dietmar Seyferth
Chairman, Departmental Committee on Graduate Students
MASSACHUSETTS INSTITUTE
OF TECHNW
LBRE1999
LIBRARIES
Thesis Committee:
I
Michael F. Rubner (Thesis Advisor)
Richard R. Schrock
Timothy M.
NWaler
2
Structural Design of Tris(bipyridyl)ruthenium(II)Derivatives for High-Performance Solid-State LightEmitting Devices
by
Erik S. Handy
Submitted to the Department of Chemistry
on May 11, 1999 in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy in
Chemistry
Abstract
A systematic study of the molecular and supramolecular parameters important for
realizing high-performance Ru(II)-based solid-state light-emitting devices is presented.
In particular, the device properties of RuL 32+ derivatives are investigated, where L =
bipyridine (bpy) or phenanthroline (phen). Thin films of both small-molecule and
polymeric systems are explored, as well as films embodying small-molecule/
macromolecule combinations. When sandwiched between two electrodes, thin films of
RuL 32+ complexes produce light with intensities comparable to those of conventional
cathode ray tube (CRT) screens. Such devices operate at efficiencies (light/ current
ratios) on par with the very best light-emitting diodes (LED's) based on conjugated
polymers. As such, these metal complexes could be used in practical flat-panel display
applications as backlighting sources for liquid crystal displays (LCD's), or as the
emissive elements in ultrathin television screens.
The well-studied Ru(bpy) 32+ complex was first employed in liquid light-emitting
electrochemical cells almost three decades ago, but the solid-state light-emitting
properties of this material have only recently received attention. Much of this solid-state
research has centered around polymeric Ru(bpy) 32+-based devices. While such research
has helped to establish a framework for understanding how these devices operate, the
performance of the polymeric systems used, for the most part, has been unremarkable.
A series of Ru(bpy) 32+-based polyurethanes was developed in the present study. These
polyurethanes represent, for the first time, a Ru(bpy) 32+-based system which operate at
both high brightness and high efficiency. Significant improvements in performance were
made with Ru(bpy) 32+-based small molecules. The small-molecule systems produce light
of even greater intensity than the polyurethanes, and offer the advantages of low-voltage
operation and long operating lifetimes. The mechanism underlying the behavior of both
the polymeric and small-molecule Ru(II) devices parallels that of the liquid
3
electrochemical cells very closely. The nature of this parallel is discussed, in conjunction
with the current theories concerning operation of similar solid-state light-emitting
electrochemical cells based on conjugated polymers.
Thesis Supervisor: Michael F. Rubner
Title: TDK Professor of Materials Science and Engineering
4
Table of Contents
............. . 1
... ...
T itle.................................................................
T hesis C om m ittee..................................................................................2
........ .3
....
A bstract.......................................................................
List of Figures and Schemes...................................................................8
List of Tables......................................................................................15
16
.......................................
.
Acknowledgments....................
1.
17
Introduction .................................................................................
17
1. B ackground................................................................................
2. Light-Emitting Device Fabrication......................................................18
3.
Liquid Electrogenerated Chemiluminescence (ECL).................................21
4. Solid-State Light-Emitting Devices....................................................27
4.1. Mixed-Valent State Formation....................................................27
4.2. "Ion-Budgeted" Mixed-Valent Films.............................................29
4.3. Light-Emitting Electrochemical Cells (LEC's)...................................31
2.
5. The State of the Art-Recent Developments in Ru(II)-Based
........
Light-Emitting Device Research...............................................
6. Sum m ary..................................................................................
7. References.................................................................................
40
42
44
........
Experimental......................................................................
1. Syntheses.................................................................................
46
46
1.1. Sulfonated Ru(phen') 32+(I)..........................................................46
47
1.2. Bipyridine-based Ligands ........................................................
1.3. Ru(bpy) 32+-based Lumophores.................................................50
1.3.1.
1.3.2.
1.3.3.
1.3.4.
Lumophore
Lumophore
Lumophore
Lumophore
II...............................................................50
III.............................................................
IV..............................................................54
V..................................................................57
54
60
1.4 . Polyurethanes...................................................................
2. Device Fabrication........................................................................74
2.1 Single-Layer Devices Based Solely on Lumophore I.........................74
2.2 Multi-Layer Ru(II)/ PPV "Heterostructures"..................................74
2.3 Lumophores II-V and Polyurethanes..........................................76
3. Device Testing...........................................................................77
4. Ionic Conductivity Measurements...................................................78
5. Additional Characterization..........................................................79
5.1. UV-Vis Absorption Spectra...................................................79
5.2. Photoluminescence Measurements.............................................89
5.3. Electroluminescence Spectra....................................................97
6. R eferences...................................................................................100
5
3.
Emitter Design Considerations.........................................................101
1. Anatomy of a Ru(II)-based Light-Emitter.............................................101
2. Implications of Emitter Structural Modifications....................................103
3. References..................................................................................106
4.
Ru(phen') 2 -only and "Heterostructured" Ru(II)/ PPV Devices.................107
1. B ackground..................................................................................107
Previous Work....................................................................107
1.1.
Ru(II)/ Poly(p-phenylene vinylene) [PPV] "Heterostructures"............109
1.2.
2. Results
Spin-cast Ru(phen') 32+-based Devices.........................................110
2.1.
2.2.
Ru(II)/ PPV "Heterostructured" Devices......................................112
113
3. D iscussion ....................................................................................
117
4 . Sum mary ....................................................................................
5. References.................................................................................119
5.
Polymer-Based Devices....................................................................120
120
1. Introduction. ...............................................................................
Motivation for Polymeric Ru(II) Complexes.................................120
1.1.
1.2.
Polyurethanes.....................................................................121
1.3.
Ionic Conductivity in Polymer Films..........................................124
2 . R esults.....................................................................................127
2.1.
E arly W ork.......................................................................127
Multiple 0-10-OV Iterations...................................................134
2.2.
2.3.
Constant-Voltage Experiments.................................................137
142
3. D iscussion ..................................................................................
3.1.
D evice Fabrication...............................................................143
3.2.
General Device Features.........................................................144
3.2.1. The "Charging" Phenomenon..............................................144
3.2.2. Device Efficiency............................................................149
3.2.3. Operating Stability..........................................................154
4. Sum m ary ...................................................................................
159
5. References..................................................................................16
1
6.
Small-Molecule Devices...................................................................163
1. B ackground..................................................................................163
2 . Results...........................................................................
............ 165
2.1.
Lumophore Il-Small-Molecule Diol.........................................165
2.1 .1. General Performance Features..............................................167
2.1.2. The Voltage-Pulsing Scheme and Structural Modifications........ 170
2.1.2.1. Instantaneous High Brightness.......................................171
2.1.2.2. The Use of Different Counterions...................................172
6
2.2.
2.3.
. .. .. .. . .. .. . .. ...
Devices Based on Underivatized Ru(bpy) 32
2
Devices Based on Ester-Functionalized Ru(bpy) 3 ..........
174
. . . . . . . . . . . . . . 175
Devices Based on a Ru(bpy) 32 +-Derivative Tris-chelated with
Esterified Ligands..................................................................178
179
3. D iscussion ...................................................................................
Device Fabrication...............................................................179
3.1.
General Small-Molecule Device Performance..................................180
3.2.
Operant Mechanisms in Small-Molecule Devices...........................184
3.3.
Temporal Response..............................................................188
3.4.
192
4. Sum m ary ....................................................................................
5. References..................................................................................195
2.4.
196
7. Con clusion .....................................................................................
1. General Features of Solid-State Ru(II)-based Light-Emitting Devices..............196
2. U nifying Them es...........................................................................201
3. Design of Future High-Performance RuL32,- based Light-Emitting
D ev ices.........................................................................................208
15
4. R eferences...................................................................................2
7
List of Figures and Schemes
Figure 1-1. Schematic of thin-film fabrication schemes: (a) spin-casting, and (b) layer19
by-layer sequential adsorption...............................................
Figure 1-2. For the light-emitting devices in this study, an ITO-patterned glass slide is
used as the substrate. A thin film of the emitter is deposited onto the substrate, and a
thermally-evaporated aluminum electrode completes the "sandwich". Light is emitted
through the ITO....................................................................20
Figure 1-3. Schematic of the architecture of a completed device (as viewed from the
underside, through the glass substrate). The intersection of the ITO and aluminum
electrodes is shown, with a thin film of the emitter in between them.
....... . . ..2 1
....
................................................................................
Scheme 1-1. General reaction for light emission for liquid electrogenerated
chemiluminescence (ECL). The nomenclature, "Ru+" indicates that "X+" is the overall
charge on the whole lumophore (without consideration of the charge on the
accompanying anions), not just the metal center............................................22
Scheme 1-2. ECL based on oxidation of Ru(bpy) 3 2 +and oxalate anion. A reducing agent
is generated in situ, and reacts with the Ru(III) species with light emission. ............ 24
Figure 1-4. Electron- transfer via redox site-to-site hopping. Oxidized Ru(III) and
reduced Ru(I) species are generated at the electrode-film interfaces and transferred into
the bulk. When the two redox species meet, light is emitted from the cell. (Adapted from
Maness et al, J. Am. Chem. Soc. 1996, 118, 10609-10616.)................................26
Figure 1-5. (a.) Before a voltage is applied to the electrochemical cell, all of the
lumophores are in the Ru(II) state. (b.) Concentration gradients of Ru(III/II) and Ru(II/I)
species form with time after a voltage is applied. The recombination of Ru(III) and Ru(I)
species in the bulk of the film generates excited state Ru(II)* species......................28
Figure 1-6. Idealized steady-state concentration profiles of the oxidized Ru(III), Ru(II),
and reduced Ru(I) states as a function of distance from the anode, where x is the
thickness of the film. The concentrations are depicted as the ratio (C/CT) between the
local concentrations of the various species (C) to the total concentration of Ru species in
the film (CT). The Ru(III) species meet Ru(I) species in the bulk of the film, as indicated
by the vertical dotted line. (Adapted from Pichot et al in J. Phys. Chem. B 1998, 102,
28
3523-3530.)........................................................................................
Figure 1-7. Redistribution of the small counterions under the externally-applied electric
field is required for charge transport and light-emission...................................30
Figure 1-8. Parker model for charge injection into the light-emitting material,
sandwiched between two electrodes, under applied bias. Electrons, for example, must
8
tunnel through or hop over the triangular energy barrier formed by the offset in energy
levels between the cathode is use (e.g., aluminum, as shown above) and the emitter.
(Adapted from Neher et al. in Polym. Adv. Technol. 1998, 9, 461-475.)................33
Figure 1-9. The electric field distribution in a typical LEC. When a voltage is applied to
the two electrodes, the electric field (E) is initially uniform across the thickness of the
film (at time, t = to). The field strength does not change with increasing distance from the
electrodes. As time passes, with the device held at constant voltage, the mobile ions move
toward the electrode-film interfaces (cf. Figure 1-6), forming space-charge fields. These
space-charge fields constrain the electric field to the interfaces, easing charge injection
... 35
into the film...........................................................
Scheme 2-1. Synthesis of lumophore I..........................................................46
Scheme 2-2. Synthesis of diol ligand, iii. ...................................................
47
Scheme 2-3. Synthesis of lumophore II. .....................................................
50
Scheme 2-4. Synthesis of lumophore II-TPB. ..............................................
51
Figure 2-1. 1H NMR spectrum of lumophore II. ..........................................
52
Figure 2-2. FT-IR spectrum of lumophore II. ..............................................
53
Scheme 2-5. Synthesis of lumophore III. ...................................................
54
Figure 2-3. 'H NMR spectrum of II-TPB. .................................................
55
Figure 2-4. 1H NMR spectrum of lumophore III. ..........................................
56
Scheme 2-6. Synthesis of lumophore IV. ...................................................
57
Figure 2-5. 'H NMR spectrum of lumophore IV. .............................................
58
Figure 2-6. FT-IR spectrum of lumophore IV. ................................................
59
Scheme 2-7. Synthesis of lumophore V. .....................................................
60
Figure 2-7. 'H NMR spectrum of lumophore V. ..........................................
61
Figure 2-8. FT-IR spectrum of lumophore V. ...............................................
62
Scheme 2-8. General structure of the Ru(I)- based polyurethanes used in this study....63
Scheme 1-9. General scheme for polyurethane syntheses. ................................
9
64
Figure 2-9. 'H NMR spectrum of poly(II-6). ...................................................
65
Figure 2-10. 'H NMR spectrum of poly(II-1 2 ). ................................................
66
Figure 2-11. 'H NMR spectrum of poly(II-4). ................................................
67
Figure 2-12. FT-IR spectrum of poly(II-6). ....................................................
68
Figure 2-13. FT-IR spectrum of poly(II-12). ...................................................
69
Figure 2-14. FT-IR spectrum of poly(II-4). .....................................................
70
Figure 2-16. UV-Vis spectrum of lumophore I. ..............................................
81
Figure 2-17. UV-Vis spectrum of lumophore II. ..............................................
82
Figure 2-18. UV-Vis spectrum of lumophore III. ............................................
83
Figure 2-19. UV-Vis spectrum of lumophore IV. ............................................
84
Figure 2-20. UV-Vis spectrum of lumophore V. ............................................
85
Figure 2-21. UV-Vis spectrum of poly(II-4). ..................................................
86
Figure 2-22. UV-Vis spectrum of poly(II-6). ..................................................
87
Figure 2-23. Photoluminescence spectrum of lumophore II. ...............................
90
Figure 2-24. Photoluminescence spectrum of lumophore III. ...............................
91
Figure 2-25. Photoluminescence spectrum of lumophore IV. ...............................
92
Figure 2-26. Photoluminescence spectrum of lumophore V. .............................
93
Figure 2-27. Photoluminescence spectrum of poly(II-4). ....................................
94
Figure 2-28. Photoluminescence spectrum of poly(II-6). ..................................
95
Figure 2-29. Electroluminescence spectrum of lumophore II. ............................
98
Figure 2-30. Electroluminescence spectrum of lumophore IV. ..............................
99
Figure 3-1. Component parts of a generic Ru(II)- based emitter............................102
General structure of the Ru(II)-based polymers used in this
Figure 3-2.
10 3
stu dy .................................................................................................
10
Figure 3-3. Structure of a Ru(II)-based polyester used in high-efficiency thin-film lightemitting devices. The films were deposited using the layer-by-layer sequential
adsorption process.................................................................................105
Scheme 4-1. Chemical structure of lumophore I, the first Ru(II)- based emitter
107
investigated in this study. ........................................................................
Scheme 4-2. Chemical structures of poly(ethylene imine) (PEI) and poly(allylamine
hydrochloride) (PAH), polyelectrolytes used in the layer-by-layer sequential adsorption
process for surface m odification.................................................................108
Figure 4-1. Schematic of a "heterostructured" device. A "platform" of PPV layers
(alternating with electrically-insulating polyanions) was deposited onto the ITO first.
Next, a layer of lumophore I was spin-cast on top of the "platform". A thermallyevaporated aluminum cathode completed the device.........................................109
Figure 4-2. Luminance and current density vs. voltage plot for a device based solely on
lum ophore I.........................................................................................111
Figure 4-3. Luminance vs. voltage profile for a Ru(II)/ PPV "heterostructured" device.
For comparison, the luminance- voltage profile for a device based solely on lumophore I
112
is also show n. .......................................................................................
Figure 4.4. The original proposed structure of lumophore I is shown on the left.
Elemental analysis results, which show negligible levels of chlorine in I, support the
structure on the right...............................................................................114
Figure 5-1. General structure of the polyurethanes used in this study. The polyurethanes
are differentiated on the basis of the length, n, of the aliphatic interlumophore spacer
used, e.g. poly(II-6) has a 6-methylene spacer.................................................128
Figure 5-2. Luminance and efficiency from a poly(II-6) device during a 0-12-OV scan.
This device had been scanned several times to voltages less than or equal to 12V
130
immediately prior to the scan shown here. ....................................................
Figure 5-3. Shift in the turn-on voltage for poly(II-6) with multiple 0 - 8V scans. This
device was ramped once to 4V, then once to 6V prior to the first 8V scan. The horizontal
dotted line in the top plot designates roughly the emitted light intensity required for
device turn-on (0.003 cd/m 2 ). For clarity, most of the luminance values greater that
required for turn-on have not been plotted. In the bottom plot, the turn-on voltage for
each run to 8V is plotted explicitly..............................................................131
Figure 5-4. Hysteresis in emitted light intensity from a poly(II-6) device during a 0 - 8V
132
scan . .................................................................................................
11
Figure 5-5. The maximum efficiency for a poly(II-1 2 ) device approaches 1% during a
12-V scan. The spin-cast film thickness was roughly 1600A. Prior to the scan shown
here, the device was ramped several times to voltages in the 4-12V range...............134
Figure 5-6. Luminance, current density, and efficiency profiles with increasing 0-10-OV
.... ........ 135
runs for a poly(II-12) device. .............................................
Figure 5-7. Efficiency and turn-on voltage profiles with increasing 0-10-OV runs for a
136
poly(II-12) device. ..............................................
Figure 5-8. Luminance and current density profiles with increasing 0-10-OV runs for a
136
poly(II-4) device. ...............................................
Figure 5-9. Efficiency and turn-on voltage profiles with increasing 0-10-OV runs for a
....... 137
poly(II-4) device..............................................................
Figure 5-10. Luminance and efficiency vs. time profiles at 1OV for a device based on
138
poly(II-12). ...............................................................
Figure 5-11. Luminance and efficiency vs. time profiles at 1 OV for a device based on
poly(II9
4 ).......................................................................................................13
Figure 5-12. Poly(II-12) device held at IOV until failure (top plot), then allowed to
discharge overnight, and held again at 10V the next day (bottom plot)...................140
Figure 5-13. A device based on poly(II-12) was ramped to and held at IOV only until the
efficiency began to decrease. This was repeated several times for the same device. The
maximum efficiency can be seen to drop as the number of these 1 OV-hold experiments
141
increased. ............................................................................
Figure 5-14. Inter-chain and urethane-counterion hydrogen-bonding lead to diminished
ionic conductivity. (Adapted from Coleman et al., Macromolecules 1988, 21, 59-65)..146
Figure 5-15. Efficiency profile for a poly(II-12) device during a 10V multiple-scan
experiment. The efficiency reaches a maximum at roughly the same time that the turn-on
voltage drops to 3V, as indicated by the vertical dotted line. The efficiency begins to
156
decrease soon thereafter. .........................................................................
Scheme 6-1. Structures of the four Ru(bpy)3 2 -based small molecules employed in
light-em itting devices. .............................................................................
165
Figure 6-1. A device based solely on lumophore II produces high brightness.
during a 0-1OV scan. ...............................................................................
166
12
Figure 6-2. Luminance vs. time profiles for lumophore II at 2.5-5V. (Reprinted with
permission from J. Am. Chem. Soc., Volume 121, Number 14, Pages 3525-3528,
168
Copyright 1999 American Chemical Society.) ................................................
Figure 6-3. Efficiency vs. time profiles for lumophore II at 2.5-5V. (Reprinted with
permission from J. Am.. Chem. Soc., Volume 121, Number 14, Pages 3525-3528,
169
Copyright 1999 American Chemical Society.). ...............................................
Figure 6-4. Low- temperature (180 K) efficiency and luminance vs. time profiles for a
170
device based on lum ophore II. ...................................................................
Figure 6-5. With an initial higher-voltage pulse, high brightness could be realized at low
voltage (2.8V) almost instantaneously with devices based on lumophore II. (Reprinted
with permission from J. Am. Chem. Soc., Volume 121, Number 14, Pages 3525-3528,
171
Copyright 1999 American Chemical Society.). ...............................................
Figure 6-6. Current density (open circles) and luminance (closed circles) vs. time
profiles for a device based on II-TPB, the tetraphenylborate salt of lumophore II. The
device was initially ramped to and held at 3V. When no light was emitted after almost 15
174
minutes, higher voltages were applied. .........................................................
Figure 6-7. Luminance vs. time profiles for underivatized Ru(bpy) 3(PF 6)2 (lumophore
III) at 2.5-5V. (Reprinted with permission from J. Am. Chem. Soc., Volume 121,
Number 14, Pages 3525-3528, Copyright 1999 American Chemical Society.)...........175
Figure 6-8. Efficiency vs. time profiles for underivatized Ru(bpy) 3(PF 6)2 (lumophore III)
at 2.5-5V. (Reprinted with permission from J. Am. Chem. Soc., Volume 121, Number 14,
Pages 3525-3528, Copyright 1999 American Chemical Society.)..........................176
Figure 6-9. Luminance and efficiency vs. time profiles for lumophore IV at 2.5-5V.
(Reprinted with permission from J. Am. Chem. Soc., Volume 121, Number 14, Pages
3525-3528, Copyright 1999 American Chemical Society.)..................................177
Figure 6-10. Luminance and efficiency vs. time profiles for devices based on lumophore
17 9
V .......................................................................................................
Figure 6-11. Luminance and current density as a function of time for a device based on
184
lum ophore II. .......................................................................................
Figure 6-12. Luminance vs. time profiles at 3V for devices based on spin-cast films of
II and IV of comparable thickness (1700 and 1800A, respectively)........................190
Figure 7-1. A comparison of the luminance-time profiles at 3V for four of the
hexafluorophosphate counterion-based small-molecule lumophores studied..............199
13
Figure 7-2. Demarcation of three distinct regions in the luminance (closed circles) and
efficiency (open cirlces) vs. time profile for poly(II-12). This device was ramped to and
held at 1 OV. The times to maximum luminance and efficiency shown here are slightly
less than average for this material (perhaps due to film thickness variations), but the
profile is qualitatively the same as that shown in Chapter 5, Figure 5-10, for
201
example...........................................................................
Figure 7-3. Luminance and efficiency vs. time profiles for lumophore II at 5V. The two
profiles track each other almost perfectly, unlike those of the polyurethanes.............204
Figure 7-4. The difference between the time to maximum efficiency (depicted in black)
and time to maximum luminance (in grey) increases with decreasing ionic conductivity,
as shown here for some the hexafluorophosphate-based small-molecule systems. For the
least conductive system, (lumophore V), for example, maximum efficiency is reached
after ~350 minutes, but maximum luminance is not reached for some 1140
m inutes..............................................................................................206
14
List of Tables
Table 2-1. Ionic conductivities (at OV dc bias) were determined for most of the materials
investigated in this study, and are listed here. Some difficulty was encountered in making
and interpreting the results of the ac impedance measurements due to the thin spin-cast
films in use, as electrical short-circuiting would skew the impedance data, and hence the
calculated sample resistance and conductivity values. As such, these should be taken as
relative, rather than absolute values.............................................................79
Table 5-1. The performance of the polyurethanes is compared on the basis of the length
of the interlumophore spacer (i.e., the number, n, of methylenes in the spacer) used.
Values are averages from several devices based on each material. aTaken from the results
of the 10V multiple-iteration experiments. bTaken from the results of the 1 OV constantvoltage experiments. cThe ionic conductivity of each material, (, was calculated based
on the bulk resistance of a spin-cast thin film, as determined by ac impedance
spectroscopic (cf. Chapter 2, Sect. 4) measurements. Multiple measurements were not
148
averaged for the values in this column. ........................................................
15
"Now to him who is able to do immeasurably more than all we ask or imagine, according
to his power that is at work within us, to him be glory in the church and in Christ Jesus
throughout all generations." (Ephesians 3:20, New International Version) God is great.
Many thanks to my advisor, Prof. Michael Rubner, for an exciting graduate research
experience, and for his support and encouragement, especially during the tough times
when good experimental results were not forthcoming. Thanks to the other members of
my thesis committee, Profs. Timothy Swager and Richard Schrock, for their assistance in
the preparation of this thesis. Many thanks are also due to the David and Lucile Packard
Foundation, which helped to fund my graduate education.
Special thanks to Mom, Dad, Darryl, Gramma, Zia, and my late Uncle Ray for supporting
me in so many ways throughout my academic career. Special thanks also to Alison
Daniel, my one and only, for her love and endless support, company during many late
nights at the office before and throughout the writing of this thesis, and countless
tupperware dinners. Thanks to the Daniel/ Gibson/ Lancaster/ Small family as a whole
for their friendship and support. Thanks to my roommate, Eric Brittain, for the many
much-needed laughs. Thanks also to the many Rubner group members and others I've
had the opportunity to work with, especially Jin-Kyu Lee, Augustine Fou, Jeff Baur,
Dongsik Yoo, Bill Stockton, Ken Zemach, Marysilvia Ferreira, Osamu Onitsuka, Doug
Howie, Aiping Wu, Cormac Lyons, Hedi Mattoussi, Jason Pinto, Mike Durstock, Erika
Abbas, Hartmut Rudmann, Amlan Pal, Jonas Mendelsohn, Peter Wan, Tom Boyd, Phil
Soo, Bindu Nair, Kathy Vaeth, and Nick Benfaremo. Special thanks to Mike, Erika,
Amlan, and Hartmut for the many impromptu discussions at the blackboard stemming
from "five-minute questions", which we always addressed in no more than three hours.
In all seriousness, I will remember these discussions fondly as some of the best times of
my graduate career. Thanks to Beni Dair, Vanessa Chan, and Mike Fasolka for an
enjoyable first year at MIT as part of the Program in Polymer Science and Technology.
Thanks also to the many members of "Polymertalk", our polymer synthesis discussion
group. I appreciate your support and experimental insights.
To my extended family at Pentecostal Tabernacle Church-- special thanks to Pastor
Brian Greene for his leadership and counsel, to Elder Kenrith Plummer for helping me
keep my focus on graduate school while mentoring me in Christian ministry, and to
Anthony Gomes and Kevin Greene for their friendship and encouragement over the
years.
16
Chapter 1. Introduction
1. Background
One of the latest advances in flat-panel display research involves the use of organic
materials as light-emitters. In 1987, a seminal report by Tang and van Slyke described
solid-state, thin films of organic small molecules which would emit light at high
efficiency when an electrical potential was applied to them.' This phenomenon is known
as
"electroluminescence".
In the following years, many similar reports regarding
organic light-emitting diodes (LED's) emerged, employing different electrode materials,
different electroluminescent materials, and different device architectures. For example,
emitters were layered with other materials used to assist in the transport of electrical
charge carriers through the device.
Such devices may soon find use in practical
applications as light sources for back-lit liquid crystal displays (LCD's), or as the active
material in flat-screen televisions and computer monitors. The ease with which organic
materials can be manipulated opens the door to their use in even larger, ultrathin displays,
and perhaps displays which are mechanically flexible, as well.
A large body of literature covering the mechanism of LED operation has emerged in
recent years. In general, electrons are injected into the thin film at one electrode, and
"holes" (positively-charged carriers) are injected (i.e., electrons are removed) at the other.
When electrons and "holes" meet in the bulk of the thin film, excited state species are
generated, which relax to the ground state with emission of photons. These devices
usually produce light with wavelengths in the visible range. To date, devices which emit
light of all colors have been produced, as have white- light emitting devices. Emitted
light intensities from the very brightest devices rival those of conventional phosphor-
17
based cathode-ray tube (CRT) screens, and are usually reported as luminance values in
units of candelas/ meter 2 (cd/M 2 ). For reference, a CRT screen operates at roughly 100
cd/m 2 . The efficiency with which a device produces light is defined as the number of
photons emitted from the device per electron which passes through the device.
Many different polymeric materials are employed in these devices, including the
poly(thiophene)'s and the poly(para-phenylene)'s. In particular, poly(para-phenylene
vinylene) (PPV) and its derivatives have received a good deal of attention. Beginning
with the report by Tang and VanSlyke, metal complexes, such as Alq 3 [tris-(8hydroxyquinoline)aluminum], have also come to the fore as emitter materials. This study
focuses on the light-emitting properties of ruthenium polypyridyl complexes. Unlike
PPV(which, in general, is very prone to photooxidation), these red-orange metal
complexes are quite stable.
Their photophysical properties have been studied
extensively, and Ru(II) complexes have recently found application in photovoltaic cells.2
In 1972, Tokel and Bard issued the first report of luminescence from liquid
electrochemical cells based on Ru(bpy) 3C 2 (where bpy = 2,2'-bipyridine).3 This report
set off a flurry of research along similar lines, and research of this kind continues to this
day in many laboratories. The application of Ru(bpy) 32 in solid-state light-emitting
devices is new, however. What follows is an investigation of the molecular and
supramolecular parameters important for high performance in solid-state devices based
on this well-studied lumophore.
2. Light-Emitting Device Fabrication
In this work, devices based on Ru(II) complexes are constructed in a thin-film
"sandwich" configuration, with the light-emitting material cast between two electrodes.
18
Typically, the thin film has a thickness on the order of 1000-2000A, and is coated onto a
glass substrate in one or a combination of two ways. The most straightforward is spincasting. Several drops of an organic solution of the emitter are added to the surface of the
substrate, which is fitted onto a rotating chuck. The solution is subsequently spread over
the surface of the substrate by spinning at high speeds, and a uniform film formed within
a few seconds. This is shown below in Figure 1-1. Alternatively, polymeric emitters may
repeat
rnns
a.
b.
Figure 1-1. Schematic of thin-film fabrication schemes: (a) spin-casting, and (b)
adsorption.
layer-by-layer sequential
be assembled into thin films from aqueous solution via a layer-by-layer sequential
adsorption process. Given that the Ru(II) lumophore has a positive charge, polymeric
Ru(II) complexes are polycations. When rendered water-soluble by proper counteranion [hereafter referred to as the "counterion"] choice, these polycations can be
assembled into thin films with a variety of polyanions on a variety of substrates.
Electrostatic attraction between polycations and polyanions drives the film build-up
process, affording highly-interpenetrated layers and uniformly-coated substrates. For
making light-emitting devices, the substrate of choice for spin-casting and layer-by-layer
19
assembly is indium- tin oxide (ITO)- treated glass. ITO is a commercially-available,
transparent electrode material that is sputter-deposited onto glass slides. When the device
is complete, the luminescence, while emitted in all directions, is best observed through
the ITO.
ITO serves as the anode, the electrode which will be positively-biased during device
operation (so-called, "forward bias"). Thermally-evaporated aluminum serves as the
cathode, the negatively-biased electrode. Small pellets of aluminum are resistively
heated under high vacuum, and the aluminum vapor deposits onto the film-coated ITO
substrate in mask-defined regions. The ITO is typically patterned into 2 - 3-mm lines
roughly 2000A thick (Figure 1-2). Aluminum lines 2 mm in width, and roughly the same
Patterned indium tin
oxide (ITO) on elass
Completed device
Ru(II Film
ITO
Alu inum
Glass
hv
Figure 1-2. For the light-emitting devices in this study, an ITO-patterned glass slide Is used as the
substrate. A thin film of the emitter is deposited onto the substrate, and a thermally-evaporated aluminum
electrode completes the "sandwich". Light is emitted through the ITO.
20
2mm
Aluminum
--
ITO
2-3mm
Figure 1-3. Schematic of the architecture of a completed device (as viewed from the underside, through the
glass substrate). The intersection of the ITO and aluminum electrodes is shown, with a thin film of the
emitter in between them.
thickness as the ITO, are defined in bands running perpendicular to those of the ITO.
Individual devices, or pixels, are thus defined where the Al and ITO lines intersect,
with the light-emitting film in between them. The emitted light intensity can then be
measured when an individual device is positioned atop a photodiode, ITO-side down.
The final device architecture, as viewed from the underside, is shown in Figure 1-3.
3. Liquid Electrogenerated Chemiluminescence (ECL)
Several different Ru(II)- based systems were used in this study, including
commercially-available tris(bipyridyl)ruthenium(II) [Ru(bpy)32 ].
While chemically
distinct from one another, all systems share the same fundamental operating mechanism
in light-emitting devices. In their work with solution- based electrochemical cells, Tokel
21
and Bard first identified this mechanism as one which involved Ru(bpy)32+ in three
different electronic states: Ru(bpy)33+, Ru(bpy)32 +, and Ru(bpy) 31+ (ref. 3). Their work
employed a dilute acetonitrile solution of Ru(bpy)32+, in the presence of tetra-nbutylammonium tetrafluoroborate as supporting electrolyte. The reaction scheme later
outlined was as follows:
2 Ru2+ --+
Ru*+ Ru3 +
Ru1+ + Ru3 + -> Ru2+ RU2+*
RU2+* -> RU2+ + hv
Scheme 1-1. General reaction for light emission for liquid electrogenerated chemiluminescence (ECL).
The nomenclature, "Ru" indicates that "X+" is the overall charge on the whole lumophore (without
consideration of the charge on the accompanying anions), not just the metal center.
Orange light was emitted from the electrochemical cell when an alternating -current (ac)
voltage was applied to the platinum working electrode.
It is instructive to walk through the mechanism by which luminescence is produced.
En route to light emission, the Ru(bpy)32+ species undergoes a disproportionation
reaction, being electrochemically oxidized to the Ru(bpy) 33+ state and reduced to the
Ru(bpy)31' state. Oxidation of the complex is metal-centered, i.e., an electron is removed
from the metal. Reduction of the complex is ligand-centered, i.e., an electron is added to
one of the ligands. For the rest of this study, the oxidized Ru(bpy) 33species will be
abbreviated, "Ru(III)", and the reduced Ru(bpy) 3 1+ species will be abbreviated, "Ru(I)",
following the convention used in the liquid and solid-state literature.
When Ru(III) species meet Ru(I) species, ground state Ru(II) and excited-state
Ru(II)* species are produced. As in photoluminescence (PL) studies, where the excitedstate Ru(II)* state is generated by photons, emission from these electrogenerated excitedstate species is dominated by phosphorescence, resulting from triplet-state relaxation.4
22
(Singlet excited state emission also contributes to the luminescence.) As relaxation of
this kind is formally a Pauli-forbidden process, the lifetime of the excited state is fairly
long , i.e., on the order of microseconds. In the experimental set-up employed by Tokel
and Bard, 3 neither redox reaction would occur until a potential of sufficient magnitude
was applied to the working electrode. Oxidation of Ru(II) occurred at +1.6V, and
reduction occurred at -1. 1V (vs. Ag/Age reference). When an ac bias is applied to the
working electrode, the sign of the potential is changed over the course of a given cycle,
such that oxidation occurs during one phase of the cycle, and reduction during the other.
Ru(II) species diffuse toward the electrode in the unstirred solution, and diffuse
away following electron gain or loss. After one complete cycle, both redox species have
been generated, and they recombine in solution, generating two Ru(II) species, one of
which is in the excited state. As mentioned earlier, relaxation of the excited state to the
ground state is often radiative, producing a photon with a wavelength of roughly 620-630
nm, or red-orange light.
This phenomenon is known as electrogenerated
chemiluminescence, or ECL. McCord et al. (and other workers) have found that the
efficiency of radiative excited-state decay improves with increasing ligand aromaticity.
Room temperature ECL efficiency increases from a low of 5% for bipyridine to a high of
24% for diphenylphenanthroline ligands.5
(Efficiencies as high as 30% have been
reported with diphenylphenanthroline ligands at subambient temperatures. 6)
Liquid ECL experiments based on Ru(bpy) 32+ and its derivatives have been
performed in various incarnations. One of the marvels of the ECL phenomenon is that it
persists down to very low solution concentrations of the emitter, i.e., the nanomolar
range. As such, this family of emitters has been used in sensor applications as tagging
23
9
agents for various analytes, including DNA, viruses, antibodies, etc. There have been
many take-offs from the early work of Tokel and Bard, where the Ru(II) complex,
dissolved in an organic solvent, undergoes disproportionation. An alternate scheme
involves solely oxidation of Ru(II) to Ru(III), and the use of a sacrificial species, such as
tripropylamine or the oxalate anion, to generate a reducing agent. This reaction is
outlined below in Scheme 1-2 and, as in Scheme 1-1, produces light. The oxidation- only
scheme is particularly useful for aqueous, rather than organic solutions, as the
electrochemical
window defined by the oxidation and
reduction potentials for
Ru(II) -+ Ru(III)
C20 4- -+CO
2+
CO 2 ~
Ru(III)+ C02- -+Ru(II)* + CO 2
Ru(II)* -+Ru(II) + hv
Scheme 1-2. ECL based on oxidation of Ru(bpy)3 2 *and oxalate anion. A reducing agent is generated in
situ, and reacts with the Ru(III) species, resulting in light emission.
Ru(bpy)
2
is larger than that of water. That is, water itself undergoes electrolysis at
potentials required for reduction of Ru(II) to Ru(I).
In addition to solution-dissolved emitters, ECL experiments have also been
performed with redox sites embedded in polymer membranes (e.g., Nafion*, a
perfluorinated sulfonic acid-bearing polymer 7), or otherwise immobilized on the surface
of the working electrode. 8 In direct-current (dc) experiments, a porous gold electrode
may be thermally-evaporated on top of the film to serve as the cathode. Many workers
have directly electropolymerized derivatives of Ru(bpy)3 2 (refs. 9a,f-h) and Os(bpy) 32 (a
closely-related analog)
8b~d,
functionalized with vinyl groups, onto platinum electrodes.
Of these systems, most ECL studies have focused on Ru(bpy)32 and its ruthenium
24
derivatives. [Ouyang and Bard described Os(bpy)32 - based ECL.10 A recent report by Ma
et al. also reported light-emitting Os(II)-based devices, but the luminescence was not
ascribed to electrochemical processes.'1 ]
Like the solution studies, in which the Ru(II) emitters freely diffuse, these metal
complex-containing films remain in contact with the organic solvent and supporting
electrolyte.
In this scheme, however, the redox sites are immobilized, and cannot
physically diffuse to a large extent. The electrons must be transported to the electrode
surface by "hopping" from discrete redox site to redox site through the film. 9c That is, the
lumophores engage in a process known as electron self-exchange as they "exchange"
charge states to transport electrons through the film. When operated in dc mode, only the
oxidized Ru(III) state is generated. In addition to supporting electrolyte, the solution
bathing the film also contains some sacrificial moiety used to generate a reducing species
in situ, as discussed earlier. The electrogenerated reducing species reacts with oxidized
Ru(III) species, and light is emitted from the cell.
A closer look at what occurs during oxidation, for example, is illustrative. At the
electrode-film interface, a Ru(II) site is oxidized to Ru(III), and gives up an electron to
the electrode. Adjacent to this now-oxidized species (at a position farther away from the
anode and deeper into the bulk of the film) is another unreacted Ru(II) species. An
electron is transferred from the unreacted Ru(II) to the new Ru(III) species, and the two
lumophores exchange valence states, as shown in Figure 1-4. This electron transfer
process continues on into the bulk of the film, farther and farther from the electrode-film
interface. Electrons are continually transported to this interface via site-to-site hopping,
and oxidation continues.
12
25
4.
+
Ru
Ru 3
R +
'
RRu
Ru
ho
Ru2+
-
<-= Ru*
-
R
Ru:
Figure 1-4. Electron- transfer via redox site-to-site hopping. Oxidized Ru(III) and reduced Ru(I) species
are generated at the electrode-film interfaces and transferred into the bulk. When the two redox species
meet, light is emitted from the cell. (Adapted from Maness et al., J. Am. Chem. Soc. 1996, 118, 10609-
10616.)
Whereas all of the lumophores were +2-charged prior to voltage application, a mixedvalent environment composed of both Ru(II) and Ru(III) [formally, a Ru(III/II) couple] is
established once oxidation begins. There will be a gradient in the local concentration of
these two electronic states, with the concentration of Ru(III) decreasing and that of Ru(II)
increasing farther and farther into the bulk of the film (Figure 1-5).
As mentioned
earlier, charge-compensating counterions (which accompany every positively-charged Ru
complex) and supporting electrolyte also participate in the charge transport process.
While electrochemically inert, the presence of the mobile anions serves to maintain local
electroneutrality, and, in so doing, maintains the electroactivity of the film as the charge
of the Ru complex changes from +2 to +3.9c Were the counterions not present, the buildup of charge due to injection and removal of electrons from the film at the electrode-film
interfaces would eventually render the device inoperable. At the anode, for example, a
Ru(II)lumophore oxidized to Ru(III) would only be partially-compensated by its original
two counterions.
The complex as a whole would have an overall charge of +1.
A large
number of +1 - charged moieties would accumulate at the anode-film interface, which
would preclude removal of any more electrons from the film. If an additional counterion
can enter the outer coordination sphere of the complex, charge compensation will be
26
complete, and the overall charge on the complex will be zero, i.e., the complex will be
neutral. Oxidation of the film by electron self-exchange can continue.
4. Solid-State Light-Emitting Devices
4.1 Mixed-Valent State Formation
Mixed-valent film (hereafter, "MVF") devices can also be operated in the complete
absence of solvent, and, often, in the absence of supporting electrolyte. In solid-state
devices, a dry, redox-active thin film (1000-2000A) is "sandwiched" between two
electrodes, one serving as the negatively- biased cathode, and the other as the positivelybiased anode, as in some of the liquid ECL work discussed earlier. When a potential
difference is applied to the two electrodes, Ru(II) undergoes disproportionation, where
both oxidation of Ru(II) to Ru(III) and reduction of Ru(II) to Ru(I) occur. The Ru(II)
species is oxidized at the anode and reduced at the cathode, and the disproportionation
products are transferred away from the electrodes and toward each other via electron
hopping.
Concentration gradients of Ru(III/I) and Ru(II/I) redox couples are formed in
series (so-called "serial" concentration gradients) across the thickness of the film (Figure
1-5). As such, Ru(III) and Ru(I) species are delivered to the bulk of the film, where they
recombine, as described earlier, and produce photons. This is shown schematically in
Figures 1-5 and 1-6. Paralleling the original dilute solution ECL work by Tokel and
Bard, ,Ru(I) serves as the reducing agent required for light emission, rather than some
sacrificial species (e.g., oxalate ion) which would be consumed during device operation.
The operation of solid-state MVF devices is therefore regenerative and, barring parasitic
reactions (e.g., with ambient moisture, oxygen) which consume highly-reactive Ru(III)
and Ru(I) species, should continue indefinitely.
27
---___
-
-----
-
=.VRO-
& __
a.
Ru(III)
Ru(I)
+
0 Ru(II)
b.+
+
Figure 1-5. (a.) Before a voltage is applied to the electrochemical cell, all of the lumophores are in the
Ru(I) state. (b.) Concentration gradients of Ru(III/II) and Ru(II/I) species form with time after a voltage is
applied. The recombination of Ru(III) and Ru(I) species in the bulk of the film generates excited state
Ru(II)* species.
1
C/CT
0
.4
Ru 2+
Ru 2+ ...---
U310
Ruu
Distance
X
Figure 1-6. Idealized steady-state concentration profiles of the oxidized Ru(III), Ru(II), and reduced Ru(I)
states as a function of distance from the anode, where x is the thickness of the film. The concentrations are
depicted as the ratio (C/CT) of the local concentrations of the various species (C) to the total concentration
of Ru species in the film (CT). The Ru(III) species meet Ru(I) species in the bulk of the film, as indicated
by the vertical dotted line. (Adapted from Pichot et al in J. Phys. Chem. B 1998, 102, 3523-3530.)
28
A large body of work 9b, 9 c,9i,12,1 3 has been done with solid-state Os(II)-based
electrochemical cells, which exhibit behavior similar to that of Ru(II)-based devices.
[The solid-state electrochemical properties of non-emissive metals, such as Fe (ref. 14a)
and Co (ref. 14b), have also been investigated in metal- tris(bipyridine) MVF's.] The
Os(II)-containing film generally used is based on poly[Os(bpy)2(vpy)2](C04)2, [vpy
=
4-
methyl-4'-vinyl-2,2'-bipyridine], a rigid, cross-linked polymer in which the Os(II) sites
are largely immobile. After this material is electropolymerized onto the anode, the film is
dried, and a porous gold cathode is thermally-evaporated on top. Alternatively, the film
may be deposited onto interdigitated-array (IDA) electrodes.
In either case, the
completed device is then immersed in a solution of supporting electrolyte (e.g.,
Et4NClO 4 ), and the bias on each electrode is independently-controlled. The anode and
cathode are set to the potentials required for oxidation and reduction, respectively, of the
Os(II) film between them, as determined by cyclic voltammetry measurements. Whether
a "sandwich" or IDA scheme is used, charge-compensating ions from the solution are
free to diffuse into the film as oxidation and reduction proceed. When a steady-state
current is measured, indicating that the Os(III/II) and Os(II/I) concentration gradients are
maximal, the film is removed from the electrolyte solution with the potentials still
applied, and dried. The potentials are then removed, and electrochemical measurements
made.
4.2 "Ion-Budgeted" Mixed-Valent Films
Alternatively, a mixed-valent state may be induced in an ML 32 film [M= Ru, Os,
etc.; L = bpy, phenanthroline (phen), etc.] sandwiched between two electrodes, without
initial exposure to solvent and without benefit of supporting electrolyte. Such MVF's are
29
said to operate on a so-called "ion budget", where no ions can ever enter or leave the film
during operation.13 The only ionic species present in the film are true components of the
neat material, i.e., the divalent ML32 + cation and its two accompanying counterions
e.-
e,
4mm@
anode
(+)
cathode
(-
Figure 1-7. Redistribution of the small counterions under the externally-applied electric field is required
for charge transport and light-emission.
(the anions).
The pivotal role of the counterions in solid-state devices must now be addressed.
Their overall function is the same: to support oxidation and reduction by maintaining
local electroneutrality. As in the dilute solution and solvent-swollen film cases, these
counterions will migrate toward the positively-charged anode to neutralize its charge, as
shown in Figure 1-7. In the solid-state, however, the solution is a solvent-free thin film
of the lumophore. Transport of the counterions away from the cathode and toward the
anode is now greatly retarded due to the rigidity of the unswollen matrix. In dilute liquid
solution, counterion redistribution is largely unhindered by steric and attractive
interactions with the lumophore population. Most counterion interactions are with high30
fluidity solvent molecules. As a result, in liquid-state cyclic voltammetry measurements,
there is little hysteresis in current- voltage sweeps up to very high sweep rates. It is true
that charge transport via electron hopping in the solid-state is somewhat slower than that
due to physical diffusion of the redox sites in solution. A 10-fold decrease in the electron
self-exchange rate constant in films of immobilized Os(II) sites relative to solutions of
freely-diffusing Os(bpy)32+ has been estimated.
5
However, several workers have
demonstrated that the long response times observed in solid-state devices are attributable
to mass transport of the counterions as they redistribute themselves. That counterion
redistribution is hindered in the solid-state is significant in that, with regard to the redox
reactions which must occur, this redistribution is also rate-determining. In general, the
electrochemistry cannot proceed any faster than the counterions redistribute.
4.3. Light-Emitting Electrochemical Cells (LEC's)
There is a growing body of scientific literature which supports these assertions.
Several workers have described studies with fixed-site MVF's, as well as light-emitting
devices based on conjugated polymers, e.g., PPV. While the electroactive components in
these two types of systems are quite different from one another, both groups share several
key features. Neither class of materials possesses high intrinsic electronic conductivity.
The Os(II) and Ru(II) states are only ionically-conductive; it is the corresponding
oxidized and reduced species which are both ionically- and electronically- conductive.
Typical LED's based on PPV operate at high voltages, much higher than the magnitude
of the HOMO-LUMO band gap would suggest. Unlike Os(bpy) 32+and Ru(bpy) 32+-based
films, however, neat, underivatized PPV films contain no mobile ionic species.
Beginning with a seminal report by Pei et al., many reports have emerged concerning
31
PPV-based devices into which high concentrations of salt had been incorporated, socalled light-emitting electrochemical cells (LEC's).16 Accordingly, many features of
LEC's parallel those of devices based on Ru(II) dyes. Notable among these features are:
(1)
a low turn-on voltage at or near the redox potential of the emitter;
(2)
a turn-on voltage which does not change with film thickness;
(3)
a turn-on voltage which does not change with the work function of the cathode
employed; and,
(4)
a long induction period, or "charging" time between voltage application and
device response.
Current flow and light emission which are symmetrical about zero bias (OV) have
also been observed in Ru(bpy) 32 - based devices and in LEC's under the proper
conditions. That is, the device responds in the same way regardless of which electrode is
biased as the cathode, for example (i.e., forward or reverse bias).
This kind of
symmetrical response to potential biases of opposite signs is yet another signature of an
electrochemical mechanism.
Conjugated polymer LED's typically exhibit high
rectification ratios, i.e., emit light only when biased in a certain manner. In addition,
LED's, which ordinarily contain negligible levels of mobile ions, have much shorter
response times, but generally require the use of reactive cathode materials (e.g., Ca, Mg)
for high performance.
An explanation of basic LED operation follows, as outlined by Neher et al.
7
In
LED's, charge injection from a metal electrode to a neutral emissive species is governed
both by the externally-applied electrical potential and the energetic differences between
32
the two materials.
Parker has described the triangular energy barriers which result from
this energy mismatch, which are shown schematically below (Figure 1-8). The energetic
LUMO
Al
ITO
-- -000
HOMO
Figure 1-8. Parker model for charge injection into the light-emitting material, sandwiched between two
electrodes, under applied bias. Electrons, for example, must tunnel through or hop over the triangular
energy barrier formed by the offset in energy levels between the cathode is use (e.g., aluminum, as shown
above) and the emitter. (Adapted from Neher et al. in Polym. Adv. Technol. 1998, 9, 461-475.)
barrier to electron injection is the difference between the work function of the cathode
(typically aluminum) and the LUMO of the light-emitting material. Similarly, for hole
injection, the anode (e.g., ITO) work function/ emitter HOMO difference must be
considered. Charge injection in LED's may be accomplished in one of, or a combination
of two ways. At each electrode/ film interface, the electron may "hop" over these energy
barriers, or travel through the width of the barrier by what is known as a "tunneling"
process. When a potential difference, AE, is first applied to the two electrodes, the slope
of the potential drop between the electrodes is linear and the electric field constant across
the thickness of the film. At the cathode, for example, an electron will be injected into
the film at a potential greater than or equal to that corresponding to the work function/
LUMO energy difference referred to above. At low voltages, the distance over which the
33
potential drops across the film sufficiently is too large for electron tunneling. As such,
for LED's, the potential which must be applied for carrier injection is generally very
much in excess of the calculated energy gap. At higher voltages, the potential drop is
steeper, and the hopping/ tunneling distance is shorter. The high-voltage requirement is
generally circumvented by the use of low work-function metals, such as calcium or
magnesium, for the cathode.
At the heart of a typical ion-containing LEC, however, is a 1000-2000A blend of
PPV, for example, and an ion-conducting polymer, such as polyethylene oxide (PEO). A
highly-dissociable salt, such as lithium triflate, is added to the blend prior to spin-casting,
and the film is sandwiched between two electrodes (e.g., ITO and Al). LED's contain no
significant levels of mobile ions. With proper tuning of the spin-casting solvent mixture
(e.g., by addition of a small amount of water) 18 to the acetonitrile solvent, or use of a
surfactant (such as octylcyanoacetate), 1 9 the PPV and PEO form a two-phase
interpenetrating network. When an electric field is applied, the lithium cations and
triflate anions migrate preferentially through the PEO toward the cathode and anode,
respectively.
As this ionic redistribution proceeds, an electric double layer forms at the electrodefilm interfaces. This double layer is similar to that described in liquid electrochemical
cells at the electrode-solution interface.20 Static charges on the electrodes comprise one
side of the layer, and redistributed ions of opposite charge comprise the other. The effect
of forming these double layers is to constrain the externally- applied electric field to the
regions most immediately near the electrodes, forming so-called "space-charge" fields
(Figure 1- 9). At a sufficiently- high ionic concentration, there will be no electric field
34
t > to
Electric field concentrated
time, t = to
Constant electric field
interfaces
E
E
0
Distance
0
X
Distance
X
Figure 1-9. The electric field distribution in a typical LEC. When a voltage is applied to the two electrodes,
the electric field (E) is initially uniform across the thickness of the film (at time, t = to). The field strength
does not change with increasing distance from the electrodes. As time passes, with the device held at
constant voltage, the mobile ions move toward the electrode-film interfaces (cf. Figure 1-7), forming spacecharge fields. These space-charge fields constrain the electric field to the interfaces, easing charge
injection into the film.
outside of the double layers. That is, the electric field will exist only between the cathode
and cations,and between the anode and anions at the electrode-film interfaces, and no
electric field will be exerted on the bulk of the film.
In order for the device to operate, one electrode must be biased to a higher potential
than the other, such that electrical current will flow between them.
The most crucial
parameter in LEC operation is not the absolute potential applied to each electrode,
however, but rather the manner in which the potential drops across the intervening film.
Formally, the potential drop is the difference in potential between an electrode and some
point in the film some distance from the electrode. The potential drop at the interfaces in
LEC's is higher at a given voltage than that in LED's, as LED's are devoid of mobile,
charge-compensating ions.
The height of the energy barriers at the interfaces is
unchanged by the incorporation of salt into the polymer film. At sufficiently high ionic
concentrations, however, the width of the energy barrier is reduced to 10A or less, which
is narrow enough for facile electron tunneling into the film. The effect of reducing
35
barrier widths is to mitigate differences in barrier heights brought on by the use of
different electrode materials for anode and cathode. As such, the levels of electron and
hole injection are better matched. As mentioned earlier, reactive metals are often used to
reduce the energy barrier to electron injection in LED's.
With the incorporation of
mobile ions, the specific cathode material in use is less important, and stable, low-work
function metals such as aluminum can be used to fabricate high-brightness, highefficiency devices. Device efficiency is improved as the likelihood of carrier
recombination is increased.
Only charge injection at the interfaces has been considered up until now. As
mentioned earlier, the presence of mobile ions ensures that the electric field across the
film is localized to the region most immediately near the electrodes. In the limit of high
ion concentration, charge carriers (electrons and "holes") move through the bulk of the
film by diffusion only, and not under the influence of an electric field.
There will be a
higher concentration of injected electrons and holes in the film just outside the double
layers, and a lower concentration further into the bulk of the film. Charge carriers will
be transported along these gradients and will recombine in the bulk with emission of
photons. Neher et al. have suggested that a salt concentration of 1018 cm- 3 is required for
the total potential drop across 10A-barriers to equal the applied voltage in a 1000A
film.' 7 If the ion content is too low to compensate for the applied voltage, electron
injection, for example, will be limited due to the width of the energy barrier. That is, for
a salt concentration of 1017 cm 3 , this energy barrier will be greater than 100A, and the
device will behave somewhat more like a traditional LED containing no mobile ions.
Specifically, rectification is expected, and there will be a finite electric field within the
36
bulk of the film. As a result, carrier transport in the bulk will occur due to both diffusion
and the electric field.
Diffusion-controlled electron transport persists into the dry, "ion-budgeted" MVF
device scenario, with which this study is principally concerned. As discussed for the
conjugated polymer LEC's, there is no electric field in the bulk of the film at sufficientlyhigh mobile ion concentrations. The electric field is constrained to the space-charge
layers at the electrode-film interfaces. For immobilized redox sites, then, localization of
the electric field leaves electron diffusion along the Ru (III/II) and Ru(II/I) gradients as
the only driving force for transport of the Ru(II) state in the bulk. Electrons hop from
redox site to site along the concentration gradients, and not in response to any voltage
gradient. (If the electric field is sufficiently high, the counterion concentration too low,
or the ionic mobility too low, an electric field will develop in the bulk of the film,
however. As such, transport of electronic charge carriers will be driven by both
concentration gradients and voltage gradients,8e as just mentioned for LEC's.)
This
represents a striking parallel to the mechanism of liquid ECL cell operation, where the
whole Ru(II) lumophore diffuses to the working electrode surface in an unstirred
solution. The effect is the same in both the solid- and liquid-states: Ru(II) species are
continually delivered to the electrode(s) via diffusion, where they engage in electron
transfer reactions, and the redox species diffuse away. In dilute solution, the whole
lumophore diffuses. When the lumophore is immobilized on the electrode, it is the
electrons (and, effectively, the valence states) that diffuse.
In contrast to the liquid solution case, the potentials applied to each electrode in the
solid-state "sandwich" configuration are not individually controlled, relative to some
37
reference electrode. Rather, it is the potential difference between the electrodes that is
controlled. The difference in formal potentials (the "redox potential"), AE 0 [where AE0 =
E 0(III/II) - E"(II/I)], between the oxidation and reduction potentials Tokel and Bard
originally reported for Ru(bpy) 3 is roughly 2.7V. 3 It is necessary, then, that a potential
difference, AE, roughly equal to or greater than AE 0 be applied to the two electrodes for
electrolytic current to flow and light to be emitted, i.e., for Ru(III) and Ru(I) states to be
generated. Given that the electrode potentials are not individually controlled, however, it
is unclear, then, what the exact potential will be on a given electrode at any given time.
What is more crucial, however, is the potential drop across each electrode-film interface.
It is the magnitude of the potential drop that will determine whether redox reactions occur
or not.
The magnitude of the drop at any given time during device operation is
determined primarily by the potential difference applied externally to the electrodes, but
also by the extent of counterion redistribution which has occurred. When the potential
drop across the two electrode-film interfaces is sufficiently high that both oxidation and
reduction can occur, electrolytic current flow and light emission are expected.
When a potential difference, AE, is first applied, the potential drop is linear and the
electric field constant across the thickness of the Ru(II)-containing film, prior to any
response on the part of the device (Figure 1-9).
The situation is similar to that of a
typical conjugated polymer LED containing no mobile ions. One can model the device at
this point as a parallel-plate capacitor with a thin-film dielectric material in between the
plates.
Immediately, however, the counterions begin moving under the applied field,
away from the cathode and toward the anode, to neutralize the charges on the plates.
The result is a high-concentration of negatively-charged anions (the counterions) at the
38
anode, and a high concentration of positively-charged cations (the +2-charged
lumophores) left behind at the cathode.
If AE is sufficiently large, oxidation and
reduction will begin taking place to generate the mixed-valent environment required for
electron hopping and light emission. [As mentioned earlier, the Ru(II) state is only
ionically conductive; the redox products Ru(III) and Ru(I) are both ionically- and
electronically- conductive.]
Continued counterion redistribution is required to offset the buildup of positive and
negative charges at the interfaces as the concentration gradients are formed.
As the
counterions move, an internal electric field is established, which neutralizes the
externally-applied electric field in the bulk of the film and concentrates it at the
interfaces. The potential drop across the interfaces rises as redistribution progresses. At
the anode, for example, oxidation will occur if the drop across it is approximately equal
to the formal oxidation potential, Eo, of the emitter.
Note: in this "sandwich"
configuration, however, charge balance requires that oxidation and reduction occur at the
same time, or not at all. When a thin film device is immersed in a solution of supporting
electrolyte or when extra salt is blended into the film, it is possible to generate a
concentration gradient across the film based on only one redox couple, e.g., a gradient of
solely Ru(III) and Ru(II) states.
In the absence of extra electrolyte, i.e., on an "ion
budget",13 an equal number of oxidized and reduced species must be generated in order
for current to flow. That is, it is theoretically impossible to generate any Ru(III) species
without simultaneously generating an equivalent number of Ru(I) species, regardless of
the magnitude of the drop across the anode-film interface. Oxidation cannot occur if the
39
potential drop across the cathode-film interface is not sufficiently high to support
reduction, as well.
Assuming that both redox species can be formed, the electric field will drive electron
transfer across the electrode-film interfaces. In the bulk, where, for AE > AE', there is no
electric field, electron transport through the film will occur by diffusion along the
Ru(III/II) and Ru(II/I) concentration gradients.
The reaction of Ru(III) with Ru(I)
generates excited-state Ru(II)*, which decays radiatively, and light is emitted. Both
electric field- and concentration gradient-driven electron conductivity are orders of
magnitude higher than that of the counterions. Nevertheless, no Ru(II) electrolyses can
occur without the requisite counterion redistribution.
5. The State of the Art-Recent Developments in Ru(II)-Based Light-Emitting
Device Research
As noted earlier, Tokel and Bard made the original report of light emission from
solution-dissolved Ru(bpy) 32+ in electrochemical cells almost 30 years ago. 3 Only
within the last five years or so have attempts been made to fabricate solid-state lightemitting devices based on Ru(bpy) 32+ derivatives, however. In 1996, Maness et al.
described diode-like chemiluminescence from solid-state films of poly[Ru(vbpy) 3](PF 6)2
(where vbpy = 4-vinyl-4'-methyl-2,2'-bipyridine).*
The Ru(II) monomer was
electropolymerized onto IDA electrodes to a thickness of roughly 2000A. The spacing
between the fingers of the electrode array was 5 gm. Concentration gradients of
Ru(III/II) and Ru(II/I) were formed while the film was immersed in a solution of
supporting electrolyte. The applied potentials (equivalent to the oxidation and reduction
potentials of the lumophore) were removed after the film was removed from the solution,
40
dried, and cooled to subambient temperatures.
Devices processed in this manner
operated at 0.03% quantum efficiency, and produced luminance which was undetectable
by eye.
In 1997, Maness et al. reported improved efficiencies (0.1- 0.2%) from films of a
different Ru(bpy) 32+ moiety (fitted with PEO arms) drop-cast onto 2-gm-spaced IDA
electrodes. No luminance values were reported.
Elliott et al. reported luminance of 25
cd/m 2 and 0.06-1% efficiency from a highly-crosslinked, acrylate-functionalized
Ru(bpy) 32 + derivative.
This material was "sandwiched" between an ITO anode and a
gold mesh cathode as a 3000A-thick film. Considerable variability was observed in the
performance of these devices. High-efficiency devices, operating in the 1-3% external
efficiency range, were realized by Lee et al. and Wu et al. at MIT with Ru(bpy) 32+-based
polyesters. These polyesters had been assembled into thin films by an aqueous layer-bylayer sequential adsorption process, and sandwiched between ITO and thermallyevaporated aluminum electrodes.
24,25,26
These polyester devices exhibited low
luminance, however.
The numerous liquid ECL studies suggested that Ru(bpy) 32+ could produce light
rapidly at high brightess and high efficiency at near-redox potentials (2.5-3V). Many
different schemes have been devised to realize high performance with this emitter in the
solid-state, but none has been very successful. Each system had its own merits (e.g, high
efficiency, respectable luminance, instantaneous response, etc.), but no single system
embodied all of the features needed for practical display applications. Hence, many
opportunities remained for the introduction of superior RuL 32+-based materials and
device fabrication schemes, both prior to and throughout the course of this study.
41
6. Summary
This introductory section has presented an overview of Ru(II)- based light-emitting
device fabrication, history, and operating mechanisms. In addition, an assessment of the
progress other workers have made in optimizing these devices has been provided.
Briefly, Ru(II)- based films are generally sandwiched between two electrodes, a
positively-charged anode and a negatively-charged cathode. In the past, light emission
has been realized in liquid ECL cells by the alternate generation of Ru(III) and Ru(I) at
the working electrode.
These redox species react with one another and produce an
excited state Ru(II)* moiety, which relaxes to the ground state with emission of redorange light. This mechanism is mimicked in the solid-state in the absence of solvent and
supporting electrolyte. Devices operated in this way are said to do so on an "ion budget".
As such, an equivalent number of oxidized and reduced Ru complexes are created during
device operation.
As in LEC's, based on conjugated polymer films seeded with mobile ions, the
counterions (anions) associated with Os(bpy)32+ and Ru(bpy) 32+ in MVF's redistribute
themselves when a potential bias is imposed. As expected, these ions shift away from the
cathode and toward the anode. The rate at which the oxidized and reduced Ru species are
electrogenerated is governed by the rate at which the counterions can redistribute.
Counterion redistribution establishes electric double layers at the electrode-film
interfaces, localizing the electric field to these interfaces and nullifying the field in the
bulk.
When the potential drop across the interfaces is sufficiently large, the
electrochemistry required for light emission can occur.
In dilute solution, the whole
Ru(II) lumophore diffuses to the surface of the working electrode and undergoes
42
electrochemical reactions. Similarly, in the solid-state, the Ru(II) state is transported by
electron diffusion along Ru(III/II) and Ru(II/I) concentration gradients. At sufficiently
high voltages, electron transport is driven by a combination of concentration and voltage
gradients.
It is not surprising that LEC's and MVF devices, though compositionally-different,
operate in much the same way. Unlike many LEC's, however, the bulky Ru(II) cation in
MVF's is largely immobile, and will not undergo gross translation when the field is
applied. This effects a subtle mechanistic difference between typical LEC's and MVF
devices with regard to the formation of the interfacial ionic space-charge layers.
Nevertheless, the detailed explanations offered for LEC behavior complement the large
body of ECL and MVF literature well, and both are brought to bear in understanding the
Ru(II)-based devices examined in this work. An exploration of some of the design
parameters important for fabricating high-performance Ru(II) devices is outlined in the
chapters which follow.
43
References
1. Tang, C.W.; VanSlyke, S.A. Appl. Phys. Lett. 1987, 51, 913-915.
2. Nazeerudin, M.K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Muller, E.; Liska, P.;
Vlachopoulos, N.; Gratzel, M. J. Am. Chem. Soc. 1993, 115, 63 82-6390.
3. Tokel, N. E.; Bard, A. J. J. Am. Chem. Soc. 1972, 94, 2862.
4. Roundhill, D.M. Photochemistry and Photophysics of Metal Complexes. Plenum
Press, New York: 1994.
5. McCord, P.; Bard, A.J. J. Electroanal. Chem. 1991, 318, 91-99.
6. Kapturkiewicz, A. Chem. Phys. Lett. 1995, 236, 389-394.
7. (a.) Rubenstein, I.; Bard, A.J.; J. Am. Chem. Soc. 1980, 102, 6641. (b) Rubenstein, I.;
Bard, A.J.; J. Am. Chem. Soc 1981, 103, 5007
8. (a.) Abruna, H.D.; Meyer, T.J.; Murray, R.W. Inorg. Chem. 1979, 18(11), 3233-3240.
(b.) Xu, X.-H..; Bard, A.J. Langmuir 1994,10, 2409-2414. (c.) Obeng, Y.S.; Bard,
A.J. Langmuir 1991, 7, 195-201. (d.) Miller, C.J.; McCord, P.; Bard, A.J. Langmuir
1991, 7, 2781-2787. (e.) Maness, K.M.; Terrill, R.H.; Meyer,T.J.; Murray, R.W.;
Wightman, R.M. J. Am. Chem. Soc. 1996, 118, 10609-10616.
9. (a.)Abruna, H.D.; Bard,A.J. J. Am. Chem. Soc. 1982, 104, 2641-2642. (b.) Jernigan,
J.C.; Surridge, N.A.; Zvanut, M.E.; Silver, M.; Murray, R.W. J. Phys. Chem. 1989,
93, 4620-4627. (c.) Pickup, P.G.; Kutner, W.; Leidner, C.R.; Murray, R.W. J. Am.
Chem. Soc. 1984, 106, 1991-1998. (d.) Facci, J.S.; Schmehl, R.H.; Murray, R.W. J.
Am. Chem. Soc. 1982, 104, 4960-4962. (e.) Abruna et al., J. Am. Chem. Soc. 1981,
103, 1. (f) Ellis, C.D. (g.) Margerum, L.D.; Murray, R.W.; Meyer, T.J. Inorg. Chem.
1983, 22, 1283-1291. (h.) Gould, S.; Strouse, G.F.; Meyer, T.J.; Sullivan, B.P. Inorg.
Chem. 1991, 30, 2942-2949. (i.) Sosnoff, C.S.; Sullivan, M.; Murray, R.W. J. Phys.
Chem. 1994, 98, 13643-13650.
10. Ouyang, J.; Bard, A.J. Bull. Chem. Soc. Japan. 1988, 61, 17-24.
11. Ma, Y.; Zhang, H.; Shen, J.; Che, C. Synth. Met. 1998, 94, 245
12. Jernigan, J.C.; Murray, R.W. J. Am. Chem. Soc. 1987, 109, 1738-1745.
13. Jernigan, J.C.; Chidsey, C.E.D.; Murray, R.W. J. Am. Chem. Soc., 1985, 107, 28242826.
14. (a.) Long, J.W.; Velazquez, C.S.; Murray, R.W, J. Phys. Chem. 1996, 100, 54925499. (b) Williams, M.E.; Masui, H.; Long, J.W.; Malik, J.; Murray, R.W. J. Am.
Chem. Soc. 1997, 119, 1997-2005.
15. Chan. M.-S.; Wahl, A.C. J. Phys. Chem. 1982, 86, 126.
16. Pei, Q.; Yu, G.; Zhang; Yang, Y.; Heeger, A.J. Science, 1995, 269, 1086.
17.Neher, D.; Gruner, J.; Cimrova, V.; Schmidt, W.; Rulkens, R.; Lauter, U. Polym.
Adv. Technol. 1998, 9, 461-475.
18.Pei, Q.; Yang, Y; Yu, G.; Zhang, C.; Heeger, A.J. J. Am. Chem. Soc. 1996, 118,
3922.
19. Cao, Y.; Yu, G.; Heeger, A.J.; Yang, C.Y. Appl. Phys. Lett., 1996, 68, 3218.
20. Atkins, P.W. Physical Chemistry, 4 th ed. W.H. Freeman and Company, New York:
1990.
21.deMello, J.C.; Tessler, N.; Graham, S.C.; Friend, R.H. Phys. Rev. B, 1998, 57,
12951-12963.
44
22. Maness, K.M.; Masui, H.; Wightman, R.M.; Murray, R.W. J. Am. Chem. Soc. 1997,
119,3987.
23. Elliott, C.M.; Pichot, F.; Bloom, C.J.; Rider, L.S. J. Am. Chem. Soc. 1998, 120, 6781.
24. Lee, J.-K.; Yoo, D.; Rubner, M.F. Chem. Mater. 1997, 9, 1710.
25. Wu, A.; Lee, J.-K.; Rubner, M.F. Thin Solid Films 1998, 329, 663.
26. Wu, A.; Yoo, D.; Lee, J.-K.; Rubner, M.F. J. Am. Chem. Soc., in press.
45
Chapter 2. Experimental Section
1. Syntheses
1.1 Sulfonated Ru(phen') 2 (I)
Lumophore I was synthesized following a modification of the procedure outlined in
Inorg. Synth. 1990, 28, 338. The ligand, the disodium salt of bathophenanthroline
disulfonic acid (" phen' "), was used as received from Aldrich Chemical Co. Prior to use,
ruthenium(III) chloride was ground to a fine powder with a mortar and pestle, then dried
for several hours under dynamic vacuum at 180 0C. Sodium hypophosphite, the reducing
agent, was used as received. A slight excess of the ligand (2.52 g, 4.69 mmol) was
refluxed in 50 mL water with ruthenium(III) chloride (0.32 g, 1.54 mmol) and sodium
hypophosphite (0.17 g, 1.94 mmol) for 40 minutes. As the reaction proceeded, the
SO 3 -Na+
Na+-0
Na+-0
3S
SO 3 -Na+j 2 CI -
3
SO3-Na+
N
N
RuC1 3, NaHPO2
Ni1
H 2 0, reflux
N
''N
RU 2+
IN
N
I
NN
Na+-03S
SO3-Na+
SO3-Na+
Scheme 2-1.
reaction mixture changed from a dark brown to a red-orange color. After filtration on a
Bchner funnel, the reaction mixture was concentrated to dryness on a rotary evaporator.
Purification was accomplished either by precipitation of the complex from aqueous
solution with acetone as a non-solvent, or recrystallization from water/ acetone. Yield:
46
>54%. Elemental analyses were performed by Galbraith Laboratories (Knoxville, TN).
Anal. Calculated for I: C 49.03%, H 2.75%, N 4.77%, Na 7.82%, Cl 4.02%. Found: C
48.76%, H 3.12%, N 4.81%, Na 5.36, Cl < 0.9%.
1.2 Bipyridine-based Ligands
Lumophore II was synthesized from the reaction of commercially-available cisbis(2,2'-bipyridine)dichlororuthenium(II) hydrate with 4,4-dihydroxymethyl-2,2'bipyridine (structure iii, Scheme 2-2). Two synthetic schemes were followed in the
generation of the bipyridine diol ligand, iii. The first was a procedure described by Lee et
al.' Briefly, 4,4'-dimethyl-2,2'-bipyridine was treated with hydrogen peroxide, affording
HOOC
COOH
Cr03
N
N
MeOH
conc. H2SO 4
N
N
conc. HS0 4,
reflux
-100%
i
COOMe
MeOOC
1. NaBH 4
OH
HO
EtOH, reflux/
2. NII 4C1 (aq.)
> 64%
88%
Hi
iii
Scheme 2-2.
the corresponding N-oxide. This N-oxide was then refluxed with acetic anhydride, and
the resulting diester hydrolyzed to the corresponding diol in ethanol. A more efficient
synthesis was described by Kocian et al.,2 and mostly this scheme was followed for the
materials described in this work. The starting material, 4,4'-dimethyl-2,2'-bipyridine was
used as received (Aldrich Chemical Co.). As shown in Scheme 1-2, 4,4'-dimethyl-2,2'bipyridine (10.0g, 0.0543 mol) was added to 200 mL of concentrated sulfuric acid, and,
47
after complete addition, dissolved over a period of 20 minutes, affording a yellow
solution. The three-necked round-bottom flask containing the solution was immersed in
a water bath to keep the temperature of its contents below 40'C. A three-fold excess of
chromium(III) oxide (32.076g, 0.32 mol) was then added slowly, and a large temperature
increase was observed in the reaction mixture. The water bath was used to keep the
temperature below 70*C.
After 2 hours, the reaction mixture was poured over roughly
1500 mL of crushed ice in a large beaker, and a pale yellow precipitate formed. This
solid material was filtered on a Buchner funnel and washed with water until the filtrate
was mostly clear.
Filtration of the precipitate was most effective when the suspension was split into two
parts and heavily diluted with water; the precipitate then settled cleanly to the bottom of
the beaker.
Otherwise, the solid material remained suspended in the solution. More
importantly, when there was no such dilution, the filter paper used to capture the solid
material would disintegrate in the highly-acidic supernatent, resulting in poor separation.
The conversion of the starting material to the corresponding diacid was quantitative, as
confirmed by 'H NMR (DMSO-d 6 ) 1H NMR spectra were obtained using a Varian XL
300 MHz spectrometer. The diacid, i, was dried overnight at 60'C under dynamic
vacuum.
Diacid i was not soluble in many room-temperature organic solvents. This material
would dissolve, however, in hot DMSO, and at high concentrations would reprecipitate
as a greyish-white solid as the DMSO cooled to room temperature.
Alternatively, the
diacid could be dissolved in 150 mL IN NaOH, and reprecipitated as a bright white solid
with the addition of 70 mL of 37% HCl. Reprecipitation in this manner was quantitative.
48
The as-synthesized diacid could be used directly, however, without further purification.
After its initial generation and overnight drying, i (14g) was then added to 300 mL
methanol, forming a dull green-brown dispersion. Concentrated H2 SO 4 was then added
dropwise, effecting an increase in solution temperature. The reaction mixture was
refluxed under nitrogen for 24 hours, during which time the mixture turned a clear, dark
red color. When the red solution was cooled to room temperature, it was poured into 700
mL of water, and a greyish-pink precipitate formed instantaneously. The suspension was
neutralized slowly with 25% NaOH, which turned the suspension to a dull yellow- green
color. The solid product was filtered out on a Blchner funnel, and dried overnight at
60*C under dynamic vacuum. After drying, 8.8g of product were collected, representing
a yield of 56%.
(A yield greater than 72% was later realized.)
Synthesis of the
bipyridine diester, ii, was confirmed by 'H NMR (CDCl 3).
Diester ii was reduced to the corresponding diol, iii, with sodium borohydride. The
diester (2.4945, 9.17 mmol) was refluxed in 200 mL of absolute ethanol for 1-1.5 hours
until the solid dissolved. The sodium borohydride (6.9421g, 0.1835 mol) was added to
the reaction mixture all at once, and the mixture was returned to reflux for another 4.5
hours. Much gaseous evolution was observed. The reaction mixture was allowed to cool
overnight, and 30 mL of a saturated aqueous solution of ammonium chloride was added.
A large amount of a white solid formed as a result. After rotary evaporation, a moist
white solid remained, which was dried on a vacuum line for 2 hours. After the solid was
dried, 200 mL of ethyl acetate were added to the solid, and the mixture was refluxed for
1-1.5 hours to extract the product. After the mixture had cooled to room temperature, the
ethyl acetate was decanted and filtered on a Buichner funnel through a 0.7 jim borosilicate
49
filter to remove insolubles. (Incidentally, the ethyl acetate- insoluble material was
soluble in water.) Multiple ethyl acetate-extractions were performed to remove diol iii
from the white ethyl acetate-insoluble product. When the ethyl acetate was removed
from the filtrate by rotary evaporation, a very fine white powder remained, which was
dried on a vacuum line. This solid could be recrystallized from hot water with
refrigeration. 'H NMR confirmed reduction of the diester to the corresponding diol.
1.3 Ru(bpy) 3
-
Based Lumophores
Ligands ii and iii were used in the synthesis of lumophores II, IV, and V (Schemes
2-3, 2-4, and 2-5). The commercially-available precursors, cis-bis(2,2'-bipyridyl)dichlororuthenium(II) hydrate and ruthenium(III) chloride were used as the ruthenium
sources. Cis-bis(2,2'-bipyridyl)dichloro-ruthenium(II) hydrate was used as received
(Aldrich Chem. Co.).
1.3.1. Lumophore II
OH
HO
N
N
Ru(bpy) 2 C12 H2 0
1:1 MeOH/H
reflux
2
0
NI
N
2 Cl-
N
4
H2 0
-
N
N
OH
HO
OH
HO
F
N\
N
N
N
78%
75%
iv
I
2 PF,
Scheme,2-3.
Ligand i (1.2071g, 5.58 mmol) was added to a 1:1 methanol/ water (roughly 90 mL
each) mixture with cis-bis(2,2'-bipyridyl)dichlororuthenium(II) (2.7015g, 5.58 mmol), as
shown above in Scheme 2-3. The reaction mixture was refluxed for 20 hours, after which
it was allowed to cool, and. the resulting red solution was filtered through a BUchner
50
funnel to remove any insolubles. The solvents were removed by rotary evaporation. The
red solid which remained was re-dissolved in a minimum of water, and recrystallized
with THF as a non-solvent with refrigeration. A 78% yield of the Cl- salt of lumophore
II, iv, was typical. An aqueous solution of this salt was then combined with an aqueous
solution of ammonium hexafluorophosphate (used as received from Aldrich Chem. Co.),
and an orange precipitate was obtained. (A five-molar excess of NH 4PF6 was used.) This
precipitate was filtered off on a Bnchner funnel, washed with 100 mL deionized water,
and air dried. Lumophore II, as the PF6 ~salt, was obtained in 75% yield from the Clsalt. Anal. Calculated for II: C 41.7%, H 3.08%, N 9.14%. Found: C 40.04%, H 3.92%,
N 8.96%. The 'H NMR spectrum of II (DMSO-d 6) is shown in Figure 2-1, and is similar
to that described by Lee et al. for this diol 1 (synthesized by the route outlined earlier).
The infrared spectrum of II is shown in Figure 2-2. A Nicolet 51 OP FT-IR spectrometer
was used to generate this spectrum and the other FT-IR spectra provided later.
The Cl~ salt of II (0.3039g, 0.433 mmol), iv, was dissolved in water and combined
with an aqueous solution of sodium tetraphenylborate (used as received from Aldrich
Chem. Co.), affording a red, flaky precipitate. BtIchner funnel filtration of this mixture
OH
HO
I
N
N N2
\H20
HO
2ci-
NaBPh
OH
.
N
24
IV
II-TPB
Scheme 2-4.
51
fl2BPh-
H
HO
IN
/ N
N-
s
N
,\
NP
e'
NN
(PF6-)2
,
II
II~
(NI
jill-i
-
9
11
- -
r
r
7
-
6
I
I FIPM
Lumophore II
1.00-
yfV~
0.95
0.90-
0.85
A
0.80
0.75D
E
0.70
0.650.600.550.50
0.45
0.40
0.35
0.30
4000
3500
3000
2500
2000
Wavenumbers (cm-1)
1500
1500
1000
1000
500
500
was very slow. The solid which remained, lumophore II-TPB, was soluble in acetone,
and exhibited orange luminescence under a UV lamp. The 1H NMR spectrum of II-TPB
(DMSO-d 6 ) is shown in Figure 2-3, and is similar to the spectrum of lumophore II, with
the addition of the resonances due to
the protons on the phenyl groups of the
tetraphenylborate counterion (7.18, 6.89, and 6.67 ppm).
1.3.2. Lumophore III
Lumophore III was derived from commercially-available Ru(bpy) 32 via anion
metathesis (Scheme 1-5).
Ru(bpy) 3Cl 2 was used as received (Alfa Chemicals). An
aqueous solution of Ru(bpy) 3Cl 2 was combined with an aqueous solution of ammonium
hexafluorophosphate resulting in a bright orange precipitate. This precipitate was filtered
off on a BUchner funnel, washed with 100 mL of deionized water, and air-dried. A 90%
yield of lumophore III was realized. Anal. Calculated for III: C 41.9%, H 2.82%, N
9.78%. Found: C 41.67%, H 3.47%, N 9.92%. The H NMR spectrum of III (DMSOd6 ) is shown in Figure 2-4.
N
N
N
\.
N
2 Cl- NH 4PF
H2 0
N
~
N
I
NJ
2 PF6 ^
\
N
90%
III
Scheme 2-5.
1.3.2.
Lumophore IV
Diester ii (0.3 007g, 1.11 mmol) was combined with cis-bis(2,2'-bipyridyl)dichlororuthenium(II) (0.5g, 1.03 mmol) in a 1:1 ethanol/ water mixture (-20 mL each) and
54
Hdd~ I
1 ,
L
1
1I
L
11 i
.ij..Lli A~. .A I
J A1iIL
L IL AIL-L
tI
I I A..IJ
.
(,J~
N
4,3L
t L
O
Ho-
-N (PF6~)
Na
N2
NN
I11
)V
I F T
\~
-I
T 1 1
9
- -
- -
OMe
N
N
f
Ru(bPY) 2 C12 H2 0
iN
\
//\
0/\
0
MeO
OMe
MeO
OMe
MeO
1:1 Et0H/H 2 0
reflux
(C-)
\
I"
2
/
NI
N
N
(PF~
(PF&XN
\/
N?
N/
IV
Scheme 2-6.
refluxed for 68 hours (Scheme 1-6).
The solvents were removed from the reaction
mixture by rotary evaporation. The shiny black solid which remained was dissolved in a
minimum of water, then recrystallized with THF as a non-solvent with refrigeration. The
yield was quantitative. To convert the lumophore to the PF6~ salt, an aqueous solution of
the Cl- salt was combined with an aqueous solution of ammonium hexafluorophosphate,
and a brown solid precipitated out of solution. This solid, lumophore IV, was filtered off
on a BUchner funnel, washed with water, and air-dried. The lumophore was used in this
form for devices. Prior to NMR characterization, lumophore IV was then dissolved in
acetone, and precipitated into ethyl ether, filtered off on a BUchner funnel, and dried at
room temperature under vacuum for several days. The 1H NMR spectrum of IV (DMSOd 6)
is shown in Figure 2-5, and its FT-IR spectrum shown in Figure 2-6.
1.3.3.
Lumophore V
Diester ii (0.9996g, 3.67 mmol) was combined with ruthenium(III) chloride
(0.2487g, 1.20 mmol) and sodium hypophosphite hydrate (0.1496g, 1.70 mmol) in a 1:1
mixture of absolute ethanol and water (30 mL each). Ruthenium(III) chloride and
sodium hypophosphite hydrate were used as received (Aldrich Chem. Co.). The reaction
mixture was refluxed for 24 hours, after which it was allowed to cool and passed through
a BUchner funnel. A large quantity of a dark, insoluble material was caught on the filter,
57
0
0
N \
U
N
(PF 6 jI2
II
N
IV
00
F-1~ F1ri~
-111
111
1
1
1
1 1
1
1
-11
[
I
I~
I II I
1
I
T1 I
I
i
I
I
III
I~ -
1F
I
ur
-[.
-I
FI
r
vTrT-
Lumophore IV
I.05
1.00A(v
0.95
I
Ph
0.90-
0.85
V
0 800.75
0~~
I'-)
0.70
0.65
0.60
0.550.50
0.45
0.40
4000
3500
3000
2500
2000
Wavenumbers (cm-1)
1500
1000
500
and a dark red-brown filtrate was collected. Under a UV lamp, the filtrate exhibited
weak orange luminescence. The insoluble material exhibited no apparent luminescence.
The filtrate was added to an aqueous solution of ammonium hexafluorophosphate, which
effected a rust-colored precipitate. This mixture was refrigerated, then filtered on a
BUchner funnel, affording collection of a small amount of solid, as the particle size was
too small for the filter in use. When 100-150 mL of water were added to the filtrate, the
particle size of the precipitate increased, and the mixture was filtered more successfully.
The yield of lumophore V was greater than 50%. This material was soluble in methanol
and acetone.
Prior to NMR
The lumophore was used in this form for devices.
characterization, lumophore V was dissolved in acetone, then precipitated into ethyl ether
and filterd off on a B chner funnel. The 1H NMR spectrum of V (DMSO-d 6 ) is shown in
Figure 2-7, and its FT-IR spectrum is shown in Figure 2-8.
O M e RuC
H20M
0
e
N
O Me
N
N
M eO
1
N
/A N 1 7+') N
N
N
1:IEtOH/H20
0
0
0
0
OMe
H20
0
MeO
_N
1'R
+'
OMe
N
-0
N
N
N
reflux
(PF6~)2
M(oC-)
OMe
MeO
M eO
0
0
0
0
0
0
MeO
OMe
> 50%
V
Scheme 2-7.
1.4. Polyurethanes
A series of polyurethanes was synthesized based on lumophore II (a diol) and
commercially-available diisocyanates. The diisocyanates differed in the number of
methylenes between the terminal functional groups. Diisocyanates with 4, 6, and 12
methylenes were employed. The general structure of these polyurethanes is shown in
60
Lurnophore V
98765
43
2
1 PPM
Lumophore V
1.00
0.95-0.90
0.85 -
1
0.75-
mU 0.65
0.70-
*-'
f
0.65
0.60
0.55
0.50
400O
3500
3000
2500
2000
Wavenumbers (cm-1)
1500
1000
500
Scheme 2-8. The synthetic procedure was mostly the same for each of the three
polyurethanes, regardless of the length of the diisocyanate in use. Each polymer is
designated as "poly(II-n)", where "II" indicates the lumophore on which the polymers
were based, and "n" is the number of methylenes in the diisocyanate used for a given
polymer. An outline of the synthesis of poly(II-6) is given below, and is representative
of the procedure followed for all three polyurethanes.
O
HO
C N-(C H-2
N
x
n =4,6,12
N
/
.N
/~ \
NI\ I..Ri.
N
/ \
(PF 6 ~)2
N
, \
Scheme 2-8. General structure of the Ru(II)- based polyurethanes used in this study.
All solids were dried on a vacuum line overnight (or longer) prior to polymerization.
Dibutyltin bis(2-ethylhexanoate) (used as received from Aldrich Chem. Co.) was used as
a catalyst for the polymerization. 3 HDI (1,6-hexanediisocyanate, Aldrich Chem. Co.) was
vacuum distilled prior to use, and the distillation fractions checked for purity by infrared
analysis. A rubber septum-sealed, three-necked round-bottom flask/ condenser reactor
containing a magnetic stir bar was assembled while hot, and dried under vacuum with a
heat gun. The reactor was then evacuated and back-filled with dry nitrogen three times.
Lumophore II (0.4989g, 0.542 mmol) and dibutyltin bis(2-ethylhexanoate) (0.0038g,
0.012 mmol) were added as solids to the reactor under flowing N2 , after which the reactor
was evacuated and N2 - backfilled an additional three times. Anhydrous DMF (2.8115g)
was injected into the reactor via syringe to dissolve the solids, and the reactor was heated
63
to 60-65"C in an oil bath with stirring. HDI (88 gL, 0.544 mmol) was injected into the
reactor via syringe, and the residual contents of the syringe were rinsed with 0.2087g of
anhydrous DMF into the reactor. The polymerization (Scheme 1-9) was allowed to run
for two days.
OlH
0 +x
-(- N---(CH 2)--4N
OH
H
HO
n =4,6,12
1 eq. BDI, HDI, or DDI
N
N
-N
/ "
-.
(Pr6 )
(PF)6)2
N"Ru-
N
/
2NDMF, 60-70'C
Sn cat.N
.
u--N
N
~~~
N/
BDI = 1,4- butanediisocyanate
HDI = 1,6- hexamethylenediisocyanate
DDI = 1,12- dodecanediisocyanate
Scheme 1-9. General scheme for polyurethane syntheses.
The reaction was deemed complete when an aliquot removed from the reactor
showed no FT-IR absorption due to the -NCO (isocyanate) group (2272 cm- 1). The
reaction mixture was then precipitated into 50 mL of deionized water, forming a bright
orange solid. This solid was filtered off on a Buchner funnel and washed with 60 mL
each of THF and hexane to remove residual catalyst, etc. A yield of 0.3755 g of poly(II6) was realized.
As mentioned earlier, similar procedures were followed for the
syntheses of poly(II-12) and poly(II-4), the 12- and 4- methylene analogs, respectively,
of poly(II-6), and were used in this form for devices. Prior to NMR characterization,
poly(II-12) and poly(II-4) were dissolved in acetone and precipitated into ethyl ether.
The fine orange precipitated was isolated by centrifugation. The 1H NMR spectra of
poly(II-6), poly(II-12), and poly(II-4) (all in DMSO-d 6) are shown in Figures 2-9, 2-10,
and 2-11, respectively. Their FT-IR spectra are shown in Figures 2-12, 2-13, and 2-14.
64
OH
H0.
_N
\I
g
S7
64
32
IPPH
qj 2N
]
(PFS6 )2
Poly(1I- 12), acetone-soluble
T
1
IV1~
F1
tT-1
-r1
1--
1~
1r
--
i
i
F
I
VjT
1
1
f-
T
-
FI
-T
4)
1-
1T-
1
F
-r
3I6
I
I-
~
T-T
--
r
t1-
I
-1
1-
Ill
T
IT
I
1
.1
1-F
2
_T
I
1 PPM
Poly(1l- 4 )
-rlrrnT~rmrI~rnFVT
I
1 T
T---- F TIT-IVT
9
97
T T
Tt1T
6
TT v rr7
V1rT7
~
rTh1-FTr-p
64
v--r-n---TrI1
T[rw
3
vT
PPM
Poly(II-6), acetone-soluble
1.00-
{'
0.950.90
/
0.85
I
0.800.75
fi I~
0.70
(D
0.65-
00
0.60
E
U,
0.55
2U
0.50
0.45
0.40
763.58-
"amide V"
0.35
3344.52-- H- bonded N-H
0.30-
1239.83-- "amhi
025
0.20
3431.58-- "free" N-H
1526.60--"amide
11"
3580.09-- unreacted Ru diol
1726.3 2-- urethane C=O
0.15
4000
Ill"
3500
3000
2500
2000
Wavenumbers (cm-1)
1500
1000
500
Poly(II-12), acetone-soluble
0.90I.'
P .
0.85
/i~
0.80
E-
p
I,
V
0.75
0.70
0.65
a,
[
0.60
0.55
0.50
0.45
0.40
0.35
0.304000
3500
3000
2500
2000
Wavenumbers (cm-1)
1500
1000
500
Poly(II-4)
0.86
-
--
0.84 0.82
0.80
t
I
0.78
0.76--
0.74-
0.70
0.68
0.66
0.640.62
0.60
0.58
4000
3500
3000
2500
2000
Wavenumbers (cm-1)
1500
1000
500
In these NMR spectra, several points are noteworthy. First, in moving from the 4- to
the 6- to the 12-methylene-spacer polyurethane, the presence of the protons on the
aliphatic spacer (1.2- 1.6 ppm) is seen to increase. The presence of the hydroxymethyl
groups (at the ends of the polymers) from the Ru(II) monomer (lumophore II) is
indicated by the resonances at 4.7 ppm and 5.7 ppm, suggesting that the polyurethanes
are not of very high molecular weight. [Indeed, differential scanning calorimetry (DSC)
thermograms made based on samples of these polymers were featureless, i.e., no glass
transition temperature could be determined. DSC measurements were made using a
Perkin-Elmer DSC 7 calorimeter.] These signals correspond to the four pseudo-benzylic
protons (giving rise to a doublet in the spectrum of II) and two hydroxylic protons
(giving rise to a triplet) of the monomer. Nevertheless, the formation of a new linkage is
indicated by the new resonance at 5.2 ppm, corresponding to the protons at the pseudobenzylic positions of the lumophore adjacent to the urethane linkage. The aromatic
regions of the polyurethane spectra are slightly perturbed from that of lumophore II.
The formation of the urethane linkages is more apparent on FT-IR analysis of these
materials, and is evidenced by the presence of the so-called, "amide I" urethane carbonyl
absorption band at 1725-1730 cm 4 . Secondary bands due to other absorptions of the
urethane are found at roughly 1527 cm' ("amide II"), 1240 cm 1 ("amide III"), and 763
cm
1
("amide V").
The strong absorption at roughly 840 cm' is attributable to P-F
stretching of the hexafluorophosphate counterions. As in the NMR spectra, the presence
of the interlumophore spacer is clear in the 2800- 3000 cm 1 range, with absorptions
arising from symmetric and antisymmetric aliphatic C-H stretching. At slightly higher
wavenumbers, one can detect aromatic C-H absorptions.
71
In the 3200-3500 range,
absorptions due to so-called, "free" and hydrogen-bonded urethane N-H stretching is
observed.
Residual -OH polymer chain end groups are also seen at 3580 cm-1,
supporting the likelihood of the polymers being of low molecular weight (i.e., 10,00015,000 g/ mol or less). (More direct techniques of molecular weight determination, such
as gel permeation chromotography, were not made due to the anticipated difficulty of
eluting these hydrogen-bonding, electrostatically-charged macromolecules from the
separation columns.)
All three polymers were soluble in acetone and pyridine. Poly(II-6) and poly(II-12)
could be rendered water-soluble by combining acetone solutions of the polymers with an
acetone solution of tetra-n-butylammonium chloride. FT-IR analysis of a solid based on
water-soluble poly(II-6) revealed the presence of some hexafluorophosphate ions,
suggesting that the metathesis to chloride anions was not quantitative. A water-soluble
derivative of poly(II-12), poly(iv-12) (based on lumophore iv), was synthesized in the
manner described for the other polyurethanes, but with difficulty, as lumophore iv was
only sparingly soluble in anhydrous DMF. An aqueous solution of poly(iv-12) was
combined with an aqueous solution of sodium acrylate, affording an orange precipitate.
This solid material was, presumably, the acrylate salt of poly(II-12), the chloride
counterions of poly(iv-12) having been replaced. The poly(iv-12)-acrylate was sparingly
soluble only in 2-methoxyethanol and DMF.
Devices based on this material were
fabricated and tested, but this material was not characterized further.
72
73
2. Device Fabrication
2.1 Single-Layer Devices Based Solely on Lumophore I
Using a photoresist spinner (Headway Research Inc.), films for devices based on I
alone were spin-cast from 4% (w/w) solutions of the lumophore in 2-methoxyethanol
solvent. The 1-in. X 1-in. glass substrates were sputtered with ITO (indium tin oxide, the
anode) by Donnelley Applied Films, and patterned in 2-mm-wide strips by DCI, Inc.
Prior to spin-casting the film, the ITO- patterned substrate was subjected to ultrasonic
cleaning in water, then rinsed with acetone. Five drops of solution were added to the
substrate, and the substrate was spun at 3000 rpm for 30 seconds. This yielded a thin
film roughly 700A thick. Film thickness measurements were made by profilometry
(using a Tencor® P10 Surface Profiler). An aluminum cathode roughly 2000A thick, 2mm wide, and 1" long was thermally- evaporated on top of the film in bands running
perpendicular to those of the underlying ITO.
In this way, a light-emitting active area
(hereafter referred to as a "device") of 4mm2 was defined by the intersection of the two
electrodes, with the film in between.
2.2 Multi-layer Ru(II)/ PPV "Heterostructures"
For Ru(II)/ PPV multi-layer films, the lumophore I film was spin-cast onto a
"platform" of PPV built up on the ITO- patterned substrate. Prior to deposition of the
Ru(II) film, the ITO substrates were alternately immersed in aqueous solutions of
commercially-available polyanions and a solution of a cationic PPV-precursor. The
polyanions used were SPS [the sodium salt of poly (styrene sulfonate)], and PMA [the
sodium salt of poly(methacrylic acid)] (Scheme 1-10). In its fully-conjugated form, PPV
74
CH 3
-F-CH 2 CH-+
x
-[-CH
2 C+ x
N+ -0
NaGO
0
SO 3 *Na
SPS
PMA
Scheme 1-10.
is insoluble in most solvents, including water, so a water-soluble thiophenium chloride
precursor 4 polymer (Scheme 1-11) was used. This precursor polymer could be thermallyconverted to PPV when the film build-up process was complete. A description of this
process follows. When a layer of a polyanion, for example, was deposited on the ITO, the
surface of the substrate would become negatively- charged.
The PPV-precursor
polycation (p-PPV) would adsorb to the surface and remain there due to electrostatic
forces of attraction, imparting a net positive charge to the surface. The substrate could
then be immersed in a solution of a polyanion, and the surface charge reversed once
again. Each pair of oppositely-charged polymers deposited on the surface constituted a
"bilayer".5
As received, the p-PPV was a clear, viscous liquid, and was diluted 1:50 with
deionized water for film deposition. A 102 M SPS solution was used. This solution was
adjusted to pH 4.0, and had a NaCl concentration of 0.1 M. Similarly, a 102 M PMA
solution was adjusted to pH 3.5 for use in making multi-layer films, and was 0.1 M in
NaCl. A computer-controlled HMS Series slide stainer (Carl Zeiss, Inc.) was used to
automate immersion of the substrates in the various solutions.
75
Five bilayers of p-PPV/
SPS were deposited onto the ITO first, followed by 20 bilayers of p-PPV/ PMA. The
multilayer films were dried overnight in a vacuum oven at room temperature. Next, the
p-PPV was converted to the fully-conjugated PPV form when the films were heated at
210"C for 11 hours under vacuum (Scheme 1-1 1). In this way, the byproducts of the
/C1 -
10aC
+
p-PPVin vacuc
-
+
+ HC
~
t
+
Ct
PPV
p-PPV
Scheme 1--11.
thermal conversion process were removed from the 340A-thick film. Next, a 4% (w/w)
solution of lumophore I in 2-methoxyethanol was spin-cast onto the PPV "platform" at
4000 rpm. On a bare substrate, (i.e., non-PPV/ polyanion coated), this would have
afforded a Ru(II) film 600-700A thick. An aluminum cathode roughly 2000A thick, 2mm wide, and 1" long was deposited on top of the film (by thermal evaporation) in bands
running perpendicular to those of the underlying ITO. In this way, a light-emitting active
area of 4mm 2 was defined by the intersection of the two electrodes, with the film in
between.
2.3 Lumophores 1I-V and Polyurethanes
Devices were fabricated in the following way. ITO was used as the anode. The ITO
had been sputtered onto 1-in. X 1-in. glass substrates by Donnelley Applied Films and
patterned into 3-mm-wide strips by DCI, Inc. The ITO substrates were cleaned by one
76
10-min. ultrasonication step in a 2:1 H20/ LYSOLO mixture, followed by three 10-min.
ultrasonication steps in H2 0 only.
The substrates were rinsed with H2 0, then air-dried
using compressed air. Using a photoresist spinner, five drops of filtered pyridine were
spun off of the substrates three times at 7000 rpm for final rinsing. Next, thin films of the
various emitters were spin-cast onto the substrates from five drops of filtered 4% (w/w)
pyridine solutions. For lumophores II, III, and IV, the solution was spun on at 1500 rpm
for 30 seconds. This afforded films of II roughly 800-900A thick, films of III 900-
ioooA
thick, and films of IV 500-700A thick. Prior to the pyridine rinsing step, the ITO
substrate used for lumophore V was immersed in 1M hydrochloric acid for 1 minute, then
rinsed with H20. The pyridine solution of V was spun onto the ITO substrate at 1000
rpm, affording a film of roughly 1800A. For poly(II-4), poly(II-6), and poly(II-12),
films in the 1000-2000A thickness regime were generally employed, and the ITO
substrates were not immersed in hydrochloric acid. In all cases, however, the spin-cast
films were annealed at 100 0 C under dynamic vacuum to remove residual pyridine. A
2000A-thick aluminum cathode was then thermally-evaporated on top of the film in 1in.-long, 2-mm-wide strips, defining a 6 mm2 light-emitting active area.
3.
Device Testing
Unless otherwise indicated, device testing was performed in a glovebox in a nitrogen
environment.
Device testing was computer-automated using a Labview* (National
Instruments) program. Unless otherwise indicated, the voltage sweep rate was 0.5V/ 5
seconds. In some experiments, the voltage was ramped to and held at a given voltage,
rather than immediately ramped back down to OV. In such cases, the light output and
current flow were monitored as a function of time, as well as voltage. A Newport 1830C
77
optical power meter, coupled to a silicon photodiode, was used to measure light output.
The optical power output, measured in nanowatts, was converted to luminance (candelas/
meter 2 , cd/M 2 ) by dividing by a factor of 30-36. This conversion factor was determined
for the experimental set-up in use by comparing power output measurements to direct
luminance measurements.
Luminance values were determined using a Graseby
Optronics S370 optometer. A Hewlett- Packard 34401A multimeter was used to measure
current flow.
Device efficiency was defined as the ratio of light emitted from the front face of the
device (i.e., emission is taken to be Lambertian) to current flow. Calculation of external
quantum efficiency values for solid-state Ru(II)-based devices such as these has been
described by Lyons et al.6 Briefly, light was collected over emission angles 0 to 21 by
the photodiode. The measured power output was corrected for losses due to waveguiding
(whereby light is directed out towards the sides of the glass substrate, for example), and
the external efficiency,
Tjext(%),
was calculated as follows:
7qext(%) =
(P/hv) / (I/e)
where P = the corrected power output;
h = Planck's constant;
v = the center frequency of the emitted light;
I = the current; and,
e = the elementary charge.
4. Ionic Conductivity Measurements
Ac impedance measurements were made with spin-cast thin films using a HewlettPackard 4284A Precision LCR Meter, with a 10 mV ac signal (OV dc bias) over the
frequency range of 20 Hz to 1 MHz. 7 Ionic conductivity calculations were, made based
on measurements of the bulk resistances of the films, as derived from Cole-Cole plots
78
(plots of the complex impedance, z", vs. the real impedance, z', of the films). 8
Specifically, the width of the semicircular plots was taken to be the bulk resistance of the
samples. Semicircles were fitted to the plots using the Zview program (Scribner and
Associates, Inc.).
The ionic conductivity, a [in units of (ohm cm)', or siemens/
centimeter (S/cm)], was calculated as follows:
c=I/RA
where 1 = film thickness [cm];
R = resistance of the sample [ohm]; and,
A = area of the device [cm 2
Table 2-1. Ionic conductivities (at OV dc bias) were determined for most of the materials investigated in
this study, and are listed here. Some difficulty was encountered in making and interpreting the results of
the ac impedance measurements due to the thin spin-cast films in use, as electrical short-circuiting would
skew the impedance data, and hence the calculated sample resistance and conductivity values. As such,
these should be taken as relative, rather than absolute values.
Lumophore
II
Ionic Conductivity, a (S/cm)
1.7 x 10~
IV
V
Poly(II- 12)
Poly(II-4)
6.8 x 10-9
3.9 x 109
2.0 x 10 9
1.5 x 10-0
5. Additional Characterization9
5.1 UV-Vis Absorption Spectra
The absorption spectra of lumophores I, II, III, IV, and V, as well as those of
poly(II-4) and poly(II-6) are provided in the Figures which follow (Figures 2-16 - 222). Generally, the large absorption feature (in the 460 nm range for the non-esterified
lumophores) is due to a metal-to-ligand charge transfer (MLCT) transition in which
electrons are photoexcited from a metal d-orbital to a ligand 71* orbital. Metal d-d and
ligand n-n* absorptions occur at shorter wavelengths.
79
0
80
Lumophore I -- UV Vis Absorption
200
250
300
350
400
wavelength (nm)
81
450
500
Lumophore II
200
250
300
--
UV Vis Absorption
350
400
wavelength (nm)
82
450
500
Lumophore III -- UV Vis Absorption
200
250
300
350
400
wavelength (nm)
83
450
500
Lumophore IV
--
UV Vis Absorption
0
200
250
300
350
400
wavelength (nm)
84
450
500
Lumophore V
200
250
300
--
UV Vis Absorption
350
400
wavelength (nm)
85
450
500
Poly(II- 4 )-- UV Vis Absorption
0
200
250
300
350
400
wavelength (nm)
86
450
500
Poly(II-6)-- UV Vis Absorption
0
200
250
300
350
400
wavelength (nm)
87
450
500
88
5.2. Photoluminescence Measurements
When photoexcited at a suitable wavelength, these Ru(bpy)3 2 -based lumophores
emit light in the 630-700 nm range. As such, the emitted light is orange to red in color.
As mentioned in Chapter 1, emission occurs largely due to relaxation of electrons from
triplet excited states to the ground state, although some excited state singlet emission also
contributes to the luminescence. The photoluminescence (PL) spectra of lumophores II,
III, IV, and V, as well as those of poly(II-4) and poly(II-6) are shown in Figures 2-23 2-28. Photoluminescence measurements were made using a Spex Fluorolog
spectrophotometer fitted with a Xe light source. An excitation wavelength of 451.7 nm
was used for all lumophores.
89
Photoluminescence
Lumiophore II
CC
530
570
610
650
690
Wavelength (nm)
730
770
810
850
Photoluminescence
Lumophore III
CdN
530
4
570
610
650
690
-
730
Wavelength (nm)
-
770
810
850
Lumophore IV
St
530
570
610
650
690
wavelength (nm)
730
770
810
850
Photoluminescence
Lumophore V
7
-Sj
530
570
610
650
690
Wavelength (nm)
730
770
810
850
Poly(II-4)
C7
530
570
610
650
690
Wavelength (nm)
730
770
810
850
Photoluminescence
Poly(II-6)
C
530
570
610
650
690
Wavelength (nm)
730
770
810
850
96
5.3. Electroluminescence Spectra
The overall focus of this study is the emission of light from these lumophores due to
application of an electric field, also known as electroluminescence (EL). Typically, for
light-emitting devices, the EL spectra track the PL spectra very closely, indicating that
the nature of the excited states are the same in both phenomena.
This is true of
Ru(bpy) 3 2 emitters, as well. The EL spectra of lumophores II and IV are shown in
Figures 2-29 and 2-30.
97
Lumophore
I-- Electroluminescence
a)
00
U)
S
C
.iIIii
A6
530
i
580
I
630
I
I
1
0
680
730
780
830
Wavelength (nm)
it
Lumophore IV-- Electroluminescence
1200
--
1000
800
0*
600
400
200
0 1
530
570
610
650
690
Wavelength (nm)
730
770
810
850
References
1. Lee, J.-K.; Yoo, D.; Rubner, M.F. Chem Mater. 1997, 9, 1710.
2. Kocian, 0.; Mortimer, R.J.; Beer, P.D. Tetrahedron Lett. 1990, 31, 5069. This
scheme was adapted for the synthesis of lumophore 11 by Dr. Jin-Kyu Lee, MIT.
3. (a.) Sun, S.-J.; Chang, T.-C. J. Polym. Sci. A: Polym. Chem. 1996, 34, 771-779. (b.)
Houghton, R.; Mulvaney, A. J. Organometallic Chem. 1996, 518, 21-27.
4. Provided by Dr. Bing Hsieh, Xerox Corporation.
5. The PPV "platforms" were deposited by Mr. Michael Durstock, Department of
Materials Science and Engineering, MIT.
6. Lyons, C.H.; Abbas, E.D.; Lee, J.-K.; Rubner, M.F. J. Am. Chem. Soc. 1998, 120,
12100-12107.
7. The assistance of Mr. Michael Durstock, Ms. Erika Abbas, and Mr. Hartmut
Rudmann (Department of Materials Science and Engineering, MIT), and Dr. Amlan
J. Pal (a visiting scientist from the Indian Association for the Cultivation of Science,
Department of Solid- State Physics) is gratefully acknowledged in making the ac
impedance measurements and ionic conductivity calculations.
8. Bruce, P. "Electrical Measurements on Polymer Electrolytes", in Polymer Electrolyte
Reviews-1. MacCallum, J.R., and Vincent, C.A. (eds.). Elsevier Applied Science,
London: 1987.
9. The assistance of Ms. Erika Abbas (MIT) is gratefully acknowledged for the UV-VIS,
photoluminescence, and electroluminescence measurements.
10. Roundhill, D.M. Photochemistry and Photophysics of Metal Complexes. Plenum
Press, New York: 1994.
100
Chapter 3. Emitter Design Considerations
1. Anatomy of a Ru(II)-based Light-Emitter
The objective of this study was to design a Ru(bpy)32 +- based material with optimal
light-emitting device characteristics.
The model device would exhibit several key
features:
1.
high emitted light intensity at low current densities, affording high device efficiency
(high light/ current ratio);
2. operation at low driving voltages (i.e., with a low power requirement);
3. a short "charging" time, or time required to reach maximum light output; and,
4. a long operating lifetime.
The "charging" time is a hurdle unique to solid-state electrochemically-based lightemitting devices. As discussed earlier, in an electrochemical mechanism of operation,
the rate at which the redox processes mature in mixed-valent films is mostly determined
by the rate of counterion redistribution. Hence, a distinct "charging" time (on the order
of seconds to hours) is often observed in devices based on these films. Typical mobile
ion-free conjugated polymer LED's emit light instantaneously at all voltages above some
threshold voltage (defined by the emitter, film thickness, and electrodes in use).
Similarly, the ideal Ru(II)-based emitter would reach high brightness with very little
delay, which, in combination with the other criteria listed above, would make the emitter
attractive for use in practical flat panel displays.
In the present study, several different redox-active Ru(II) species were employed en
route to this target, including both small-molecule and polymeric derivatives. All the
emitters were composed of a ruthenium(II) metal center tris-chelated with three aromatic,
101
bidentate ligands, and charge-balanced with two anions (the "counterions"). The various
components of such an emitter are shown schematically in Figure 3-1, below:
N
2X
I' a N~N N
counterions
N
electroactive lumophore
Figure 3-1. Component parts of a generic Ru(II)- based emitter.
Spin-cast films of organic small molecules are typically fraught with defects, e.g.
microcrystallites/ aggregates, or pinholes. "Pinholes" are formed when the material to be
cast fails to uniformly coat the substrate, leaving regions of exceptionally-thin film, or
actual "holes" where no material deposits. In light-emitting devices, electrons flowing
from the aluminum to the ITO "leak" through the film at these defects without producing
photons. As such, defects give rise to what is known as "leakage current", and cause a
lowering of the device efficiency.
Polymers typically provide surface coverage superior to that of small molecules.
Suitably-functionalized macromolecules lend themselves well to both spin-casting and
the layer-by-layer sequential adsorption process described earlier. The Rubner group at
MIT, among others, has shown that the layer-by-layer technique can be used to fabricate
highly-uniform films on a variety of surfaces. With a single polyanion/ polycation
102
bilayer, for example, it is possible to cause a bead of water to completely wet an
otherwise-hydrophobic surface (reducing the water contact angle from 700 to less than
50), clearly inducing hydrophilicity. 1 In general, the layer-by-layer process is an
excellent tool for surface modification, and, with respect to light-emitting devices, is
thought to be beneficial for smoothing out any problematic features on the ITO surface
(e.g., ITO "spikes" and residual photoresist left over from the patterning process), thereby
improving device efficiency.
In the polymeric Ru(bpy) 32+-derivatives described later (Chapter 5), the lumophore
is incorporated into a polymer backbone via urethane linkages. An aliphatic hydrocarbon
spacer, with a variable number of methylene units, is used to separate the lumophores
from one another along the polymer backbone (Figure 3-2).
hydrocarbon spacer
n
N
N
/'Nii.~Zun
N
2X-
N
Figure 3-2. General structure of the Ru(II)-based polymers used in this study.
2. Implications of Emitter Structural Modifications
The performance of the RuL 3 2+ emitter in thin films can be tuned at both the
molecular and the supramolecular levels. At the molecular level, exchan1ging one or
more of the ligands for different closely-related bidentate analogs affords changes in the
103
oxidation and reduction potentials of the emitter, as well as the quantum efficiency and
color of its photoluminescence. By fitting the cationic lumophore with different anions,
the impact of the counterions on device response-time can be investigated. Changing the
length of the aliphatic interlumophore spacer presumably allows modulation of the siteto-site distance between lumophores in polymeric films, which may affect the extent of
various lumophore- lumophore interactions.
Using the layer-by-layer approach, it is possible to influence device performance at
the supramolecular level, as well. Previous work suggested that use of the layer-by-layer
film fabrication technique is facilitated with polymeric Ru(II) emitters. As discussed
earlier, the layer-by-layer technique affords highly-uniform thin films of molecularlyblended polyelectrolytes.
This technique has been successfully employed in the
fabrication of light-emitting devices based on poly(p-phenylene vinylene), or PPV.2
With small changes in polyelectrolyte solution pH or salt content (ionic strength), the
architecture and composition of the resulting films could be precisely tuned. Lee et al.
and Wu et al. described high-efficiency light-emitting devices based on layer-by-layerfabricated thin films of a Ru(bpy) 32+-based polyester (Figure 3-3).3
Devices with
comparable performance were realized with layer-by-layer-fabricated films of a watersoluble Ru(bpy) 32 +-based polyurethane (Chapter 2, Section 1.4),4 whose structure was
analogous to the organic solvent-soluble polyurethanes described later in Chapter 5. In
the present study, however, thin films of RuL32+-based lumophores were simply spin-cast
onto ITO-patterned substrates. Films with thicknesses in the range of 500-2000A were
employed.
As described later, the layer-by-layer technique was only used in two
104
instances (Chapters 4 and 6) to modify the substrate surface. Thin films of Ru(II)
complexes were then spin-cast onto the modified substrates.
0
0
---
(C
H2 -)12
0+
CO
/ \
/
/ \
N
\
.N
\.-+2
N'' 'Ru-
N/ \
~7
(0I ~)2
N
Figure 3-3. Structure of a Ru(II)-based polyester used in high-efficiency thin-film light-emitting devices.
The films were deposited using the layer-by-layer sequential adsorption process.
105
References
1. Yoo, D.; Shiratori, S. S.; Rubner, M.F. Macromolecules, 1998, 31, 4309-4318.
2. Fou, A.C.; Onitsuka, 0.; Ferreira, M.; Rubner, M.F.; Hsieh, B.R. J. Appl. Phys.
1996, 79, 7501-7509.
3. (a.) Lee, J.-K.; Yoo, D.; Rubner, M.F. Chem. Mater. 1997, 9, 1710. (b.) Wu, A.; Lee,
J.-K.; Rubner, M.F. Thin Solid Films 1998, 329, 663. (c.) Wu, A.; Yoo, D.; Lee, J.K.; Rubner, M.F. J. Am. Chem. Soc., in press.
4. These devices were fabricated by Ms. Erika Abbas, Department of Materials Science
and Engineering, MIT.
106
Chapter 4. Ru(phen') 2 -only and "Heterostructured" Ru(II)/PPV
Devices
1. Background
1.1. Previous Work
The great flexibility afforded by the layer-by-layer technique led to the design of a
water-soluble RuL 32+- based complex with bathophenanthroline ligands, I [Ru(phen') 2+]
3
to which this technique could be applied.. The structure of I is shown in Scheme 4-1,
below. Bard et al. demonstrated that tris(diphenylphenanthroline)ruthenium(II)
perchlorate (the non-sulfonated analog) is largely water-insoluble, and only partly
solubilized with the use of surfactants.' Hence, each ligand on lumophore I is
functionalized with two sodium sulfonate groups to impart water-solubility. For use of I
in spin-cast films, it was hoped that its bulky ligands would discourage crystallization, a
common impediment to the fabrication of functional devices. As mentioned earlier,
microcrystallites act as point defects, serving to concentrate the electric field in small
areas and cause electrical shorting-out of the device.
SO 3-Na+
S03-Na
Na+-0 3 S
N
SNi1,, NINj,,
u 2+
'
N
N,
Na+-0 3S
SO 3-Na+
SO3 Na+
Scheme 4-1.
107
2 CI -
Attempts to fabricate high-performance thin-film devices based on I with the
layer-by-layer technique had been unsuccessful. 2
Yoo described the use of both
protonated poly(ethyleneimine), PEI, and poly(allylamine hydrochloride), PAH, as
polycations for assembly with lumophore I. The structures of PEI and PAH are shown in
CH 2 CH 2NH
2
_ 21+
CH 2 CH-]
X
Cl
X
CH 2 NH 2 + -Cl
PEI
PAH
Scheme 4-2.
Scheme 4-2. [Without its sodium cations and chloride anions, the Ru(II) complex was
taken to be anionic overall.] In PEI-based films roughly 500 A thick [i.e., 30 bilayers of
Ru(II)/ PEI], the maximum emitted light intensity and device efficiency were 10 cd/M2
and 0.004%, respectively, at 1 OV. The maximum luminance dropped to 1 cd/m 2 at 1OV
when PEI was replaced with PAH in films with the same number of bilayers and of
comparable thickness. By UV-VIS spectroscopy, the amount of I incorporated into the
device was the same whether PEI or PAH was used. Coulombic repulsion between the
+2 charge on the metal center of I and positive charges on the polycations was blamed for
the poor device performance, owing to poor film quality.
No improvement in device quality was realized when the Ru(bpy) 32 -based
polyester 3 4 described in Chapter 3 (Figure 3-3) was used as the polycation with I
instead of PEI and PAH.
These polyester-based devices typically shorted out (as
indicated by high current flow without significant light emission), unless a large number
108
of bilayers was employed (i.e, 50 bilayers for a 300
A film).
Similar attempts were made
to fabricate high-performance layer-by-layer devices based on I and the cationic
precursor to PPV 5 (Chapter 2, Scheme 1-11), but these were also unsuccessful. 6 While
the layer-by-layer technique proved incompatible with I, spin-casting was more effective
for making thin films of this material, and was used in the present study.
1.2. Ru(II)/ PPV "Heterostructures"
In addition to spin-cast films based solely on I, the use of multi-component,
"heterostructured" films was also explored in this study. While not effective for making
high-performance devices based on I, the layer-by-layer approach had been successfully
employed in PPV devices. The structure of PPV is shown in Figure 4-1, below. It was
Ru(II) complex
PPV platform
hv
PPV =
Figure 4-1. Schematic of a "heterostructured" device. A "platform" of PPV layers (alternating with
electrically-insulating polyanions) was deposited onto the ITO first. Next, a layer of lumophore I was spincast on top of the "platform". A thermally-evaporated aluminum cathode completed the device.
109
thought that if PPV were used as a "platform", so to speak, onto which I could be spincast, such a platform might compensate for defects in the spin-cast film quality and
decrease the extent of leakage current. If electrically- insulating polymers (e.g., PEI,
PAH, etc.) were used as a platform, however, the injection of electronic charge carriers
into the film from the ITO would be more difficult, and significantly higher operating
voltages would be required.
PPV, like most conjugated polymers, is primarily a hole- conducting polymer. The
performance of conjugated polymer LED's is generally improved with the use of
electron-transporting additives (e.g., oxadiazoles).
It was envisioned that the
Ru(phen') 32+top layer might play a similar role in improving transport of electrons from
the cathode to the underlying PPV layer. Emissive excited states generated in the PPV
during device operation
than Ru(phen')
(Xmax, emission
32+ excited
-
530 nm, i.e., green light), being higher in energy
states, would likely undergo energy transfer to the metal
complex. As such, Ru(phen') 32 +* would be formed, and the emitted light would be redorange anyway.
2. Results
2.1. Spin-Cast Ru(phen') 32+-based Devices
In addition to water, lumophore I was soluble in methanol and 2-methoxyethanol,
and 2-methoxyethanol was used as the solvent from which I was spin-cast. In this study,
devices based on spin-cast films of I produced a maximum of 20-25 cd/m 2 at 5V, as
shown in Figure 4-2, and operated at a maximum efficiency of 0.05%.
Scanning to
voltages greater than 4.5 -5V caused a marked decrease in efficiency, however, as did
110
120
25
-- -0-
100
80
Current
020
-Luminance
-L
15
E
*7 60
5
:-I10
rM
40
a
5
20
0
0
0
1
2
3
4
5
Voltage (V)
Figure 4-2. Luminance and current density vs. voltage plot for a device based solely on lumophore I.
multiple scans to 4.5-5V. In later work by others in this research group, the maximum
luminance was increased to the 50-200 cd/M2 range, but without a concomitant
improvement in efficiency.
In devices based on the non-sulfonated analog of I, the emitted light intensity only
reached a maximum of 0.3 cd/M 2 when they were ramped to and held at 2.5-5V. The
maximum efficiency was on the order of 10-5- 10-4 % . When the chloride counterions
were later replaced with hexafluorophosphate (PF6-) counterions, the voltage could be
ramped from OV to 1 OV to OV without device failure. A device operated in this manner
reached an efficiency of 0.04%. When the voltage was ramped to and held at 1 OV (rather
than ramped back down to OV), the maximum luminance from this device reached 200300 cd/M 2 , and the efficiency reached 0.3%.
111
2.2 Ru(II)/ Poly(p-phenylene vinylene) [PPVI "Heterostructured" Devices
At the time, the best layer-by-layer sequential absorption architecture for PPV-based
devices consisted of five bilayers of PPV/ SPS [SPS = the sodium salt of poly(styrene
sulfonate)], followed by twenty bilayers of PPV/ PMA [PMA = the sodium salt of
poly(methacrylic acid)]. The layer-by-layer process itself required the use of a cationic
PPV tetrahydrothiophenium chloride precursor,5 as discussed in Chapter 2. Once
completed, the layer- by -layer film was annealed under vacuum at high temperature to
10 0
11,
-
80
-0---
, ,
,
I, I,
, ,
, I I, I I, I,,!I
I
Ru(II) only
Ru(II)/ PPV "heterostructure"
~-60
$
40
20
0
5
10
15
Voltage (V)
Figure 4-3. Luminance vs. voltage profile for a Ru(lI)/ PPV "heterostructured" device. For comparison,
the luminance- voltage profile for a device based solely on lumophore I is also shown.
thermally-convert the precursor to the conjugated, hole-transporting PPV form. After the
conversion process, an organic solution of I was spin-cast onto the PPV platform, and
aluminum was thermally-evaporated on top of this "heterostructured" film (Figure 4-1).
These devices produced luminance levels in the range of 25-50 cd/M2 at 15V, comparable
112
to that of the best spin-cast Ru(II) complex-only devices based solely on I. Device
efficiencies were in the range of 0.1%, however. Higher luminance levels were achieved
by drying the completed "heterostructured" films under vacuum overnight at room
temperature. In this way, residual spin-casting solvent was presumably removed prior to
aluminum evaporation. With dried films, the maximum luminance increased to the 70-90
cd/m2 range 28
at 13-15V (Figure 4-3), but the device efficiency was not affected.
3. Discussion
The spin-cast films of I-only (with no platform) produce luminance in the 50-200
cd/m2 range. Luminance levels of this magnitude would make I attractive for use in
some practical, low-brightness flat-panel displays, e.g., as backlighting sources for
illuminated wristwatch faces, cellular telephones, etc. The poor device efficiency,
however, remains a significant hurdle. That is, the current densities required for light
emission are unacceptably high for practical applications. That the solid-state device
efficiency is so low, however, is remarkable given that, barring any deleterious effects of
ligand sulfonation, this material would be expected to have a liquid cell ECL efficiency
(analogous to a solid-state internal efficiency) on the order of 24%.1 In the solid state,
barring losses due to waveguiding, etc., 9 a 24% ECL efficiency should translate to
roughly a 10-15% external efficiency.
There are several possible explanations for the low luminance and efficiency. At the
supramolecular level, photoluminescence (PL) studies by Damrauer et al. demonstrated
that formation of the emissive excited states in phenyl-substituted Ru(bpy)3 2 + complexes
requires rotation of the phenyl rings about the carbon-carbon bonds attaching them to the
parent ring system.1
The optimal conformation is one in which the phenyl rings are
113
coplanar with the rest of the bipyridine ligand. When this rotation is chemicallyhindered by the addition of bulky substituents to the phenyl rings, the PL quantum yield
decreases and the rate constant for nonradiative (non-light emitting) decay of the excited
state increases. It is possible that, at the supramolecular level, such rotation is similarly
hindered in the solid-state due to close packing of the lumophores. Formation of the
excited state would thereby be less facile.
At the molecular level, a closer inspection of the true structure of I provides the basis
for another explanation. In the proposed structure (Figure 4-4), the Ru metal center is
chelated by three bathophenanthroline ligands, each ligand functionalized with two
0-Na+
03-Na+
SO 3 NI
N N
Na+-03S
1
2 Cl-
So -4Na+
_03S
N
N.
2+
N
NI
N
N
7
N
Iu 2.
I
N
Na+ -03
N
I
S03-Na+
-0 3S
SO 3 -
03-Na+
03-
PROPOSED
ACTUAL
Figure 4.4. The original proposed structure of lumophore I is shown on the left. Elemental analysis
results, which show negligible levels of chlorine in I, support the structure on the right.
sodium sulfonate groups on the phenyl rings. The +2 charge on the metal center is
compensated by two chloride counterions.
Elemental analysis shows that chlorine is
present in this material only in negligible quantities, however (Chapter 2, Section 1.1)- It
is believed that during the purification process (precipitation or recrystallization), the
chloride counterions of one lumophore are replaced by two sulfonate groups of another.
114
In this way, two chloride anions and two sodium cations are ejected from the structure of
this zwitterion, affording the structure on the right-hand side of Figure 4-4.11 Only two of
the original six sodium cations remain. The elemental analysis determination of the
sodium content in I is in agreement with this structure.
If local mobility of the
lumophore is important for electron transport (to establish a mixed-valent environment),
2
rotation or small-scale translation would be more difficult in this ionically-crosslinked
matrix. On these grounds, a reduction in luminance and efficiency might be expected.
The electrochemical instability of any residual chloride counterions might also be
problematic. Indeed, Tokel and Bard reported irreversible oxidation of chloride anions in
their original dilute solution ECL work with Ru(bpy) 3C 2 .13 In the present study, the
highly-unstable products of chloride ion oxidation may react with I or some other species
to form an electrically-insulating barrier at the ITO anode, hindering charge-injection. In
the chloride counterion-based, non-sulfonated analog of I, the chloride counterions are
presumably preserved on purification of the complex, and the device efficiency decreases
by several orders of magnitude, relative to that of I. When electrochemically-inert
hexafluorophosphate counterions are employed, however, there is a clear increase in
device efficiency at high voltage.
With only negligible levels of mobile chloride anions available, space-charge layer
formation at the anode is minimal (cf. Chapter 1, Sect. 4.3). Oxidation of the lumophore
is likely more difficult; hence the low luminance and efficiency in Ru(phen')3 2 - only
films. The only other negatively-charged species present in the film, the sulfonate
anions, are involved in the extensive intermolecular "crosslinking" described earlier, as
they are bound up with the dicationic metal centers. Hence, their mobility is greatly
115
restricted, and they have little freedom to redistribute themselves under the applied field.
Even if there are some sulfonate groups which are not ionically-bound, that that they are
covalently attached to the largely-immobile lumophore suggests that they have low
mobility.
The role of the PPV platform is considered next. In addition to minimizing leakage
current by improving surface coverage, it is possible the platform serves to compensate
electronically for the dearth of mobile anions. That is, it may be that PPV is oxidized in
the normal LED manner (i.e., via withdrawal of electrons from, or injection of "holes"
into the HOMO), and that lumophore I is somehow more easily oxidized at the
Ru(phen') 3 2 / PPV interface in the "heterostructures" than at the Ru(phen')
32+/
ITO
interface in the neat spin-cast films. The nature of the electron-transfer reactions at the
Ru(II)/ PPV interface is not well-understood, and perhaps may be non-electrochemical in
origin (i.e., not dependent on ion redistribution for maturation).
Nevertheless, the
increase in efficiency (and luminance, to some degree) in the "heterostructures" is clear.
Additional evidence for an alternative operating mechanism stems from the lack of a
"charging" phenomenon in the Ru(phen')
2
/ PPV devices. In the initial work with
devices based solely on lumophore I, the voltage was generally ramped back down
immediately from its highest value. Hence, the "charging" effect was not observed in
these devices, either. Early on, it was not apparent that an electrochemical mechanism of
operation might be at work, and these devices were modeled like conjugated polymer
LED's, in which the time to maximum luminance is negligible. Positive hysteresis in the
luminance- voltage curves (which might have been indicative of small-scale charging)
would not have been a complete surprise. Even conjugated polymer-based LED's exhibit
116
such behavior as the devices heat up (due to the flow of current through the somewhatresistive film) during operation.
This heating can increase charge carrier injection
slightly, giving rise to higher luminance levels.
In later work, however, when the
voltage was held constant across lumophore I films, the luminance was observed to
increase significantly with time as the devices charged up.7
review of the Ru(bpy)
2
This observation, and a
liquid ECL literature, suggested that migration of mobile ions
(sodium cations for lumophore I) might be important.
As discussed in Chapter 1, in non-electrochemical LED's, electron injection into
the emitter LUMO is significantly enhanced when low work function metals (e.g., Ca,
Mg) are used as the cathode. This effect is attributable to a lowering of the energetic
barrier to electron injection. It is possible that lumophore I behaves, in part, like PPV,
and not solely by electrochemical processes in which the specific cathode material in use
is less important.
As such, it may be that, even with mobile cations (Na') to help
establish a space charge field at the cathode, the work function of aluminum is simply too
high for facile charge injection, and that a more reactive metal is needed.
4. Summary
In general, using the PPV-based polymer "platform" serves to increase both the
emitted light intensity and efficiency of Ru(phen') 3 2 +-based devices.
Like the
Ru(phgn') 3 2 +-only devices, however, reproducibility is generally poor from
"heterostructured" device to device. The efficiencies are still an order of magnitude
lower than those of the best conjugated polymer devices, which operate at upwards of 4%
external efficiency. Later work with other conjugated polymer "platforms'" (e.g., other
PPV/ polyanion combinations, poly(p-phenylene)- based platforms, etc.) failed to yield
117
further improvements in device efficiency.' 4 (Howie has suggested that devices based on
blends of lumophore I with PEO and FeC12 operate at 1% efficiency, but no data were
presented.15) The most obvious drawback to the "heterostructure" approach is the need
for high voltage to realize high luminance. The insertion of a marginally-resistive
polymer film between the Ru(II) complex and the anode necessitates the use of higher
electric fields for charge injection. Nevertheless, the luminance levels achieved using
this architecture approach 100 cd/m 2 , roughly the brightness of a CRT computer screen.
Prior to the 200 cd/n
2
luminance later realized with improved substrate processing of
spin-cast I-only devices, this was the highest luminance reported for solid-state RuL 32+
devices, and demonstrated that the emissive properties of these metal complexes
warranted further study.
118
References
1.
2.
3.
4.
5.
McCord, P.; Bard, A.J. J. Electroanal. Chem. 1991, 318, 91-99.
Yoo, D. PhD Thesis, Massachusetts Institute of Technology, 1997.
Lee, J.-K.; Yoo, D.; Rubner, M.F. Chem. Mater. 1997, 9, 1710-1712.
Yoo, D.; Wu, A.; Lee, J.-K.; Rubner, M.F. Synth. Met. 1997, 85, 1425-1426.
Briefly, a tetrahydrothiophenium chloride precursor to PPV was used in the layer-bylayer process. High-temperature annealing is required to convert this precursor to the
fully-conjugated, light-emitting form of PPV. Tetrahydrothiophene and hydrochloric
acid are eliminated from the precursor and removed from the film under vacuum. The
precursor is water-soluble; fully-conjugated PPV is insoluble in water and most
organic solvents.
6. Howie, D., Massachusetts Institute of Technology. Unpublished results.
7. (a.) Lyons, C.H.; Abbas, E.D.; Lee, J.-K.; Rubner, M.F. J. Am. Chem. Soc. 1998,
120, 12100. (b.) Yoo, D. PhD Thesis, Massachusetts Institute of Technology, 1997.
8. Lee, J.-K.; Yoo, D.; Handy, E. S.; Rubner, M.F. Appl. Phys. Lett. 1996, 69 (12),
1686.
9. As outlined in the Chapter 2, only the light emitted from the front face of these
devices is collected. However, light can also travel through the glass substrate in a
process known as "waveguiding", and be emitted from the sides of the substrate. As
in fiber optics, the light propagates via multiple internal reflections from the upper
and lower surfaces of the glass, and can be emitted in angles as much as 900 or more
from the direction normal to the front face.
10. Damrauer, N.H.; Boussie, T.R.; Devenney, M.; McCusker, J.K. J. Am. Chem. Soc.
1997, 119, 8253-8268.
11. Williams, M.B., University of North Carolina, Chapel Hill. Private conversation.
12. (a.) Williams, M.E.; Masui, H.; Long, J.W.; Malik, J.; Murray, R.W. J. Am. Chem.
Soc. 1997, 119, 1997-2005. (b.) Long, J.W.; Velazquez, C.S.; Murray, R.W, J. Phys.
Chem. 1996, 100, 5492-5499.
13. Tokel, N. E.; Bard, A. J. J. Am. Chem. Soc. 1972, 94, 2862.
14. Abbas, E.D. PhD Thesis, Massachusetts Institute of Technology, 1999.
15. Howie, D.W. Bachelor's Thesis, Massachusetts Institute of Technology, 1997.
119
Chapter 5. Polymer-Based Devices
1. Introduction
1.1. Motivation for Polymeric Ru(II) Complexes
Respectable luminance levels were realized with light-emitting devices based on
lumophore I, the sulfonated small-molecule Ru(II) complex. Improvements in the initial
performance of I were made by using the layer-by-layer, sequential adsorption technique
to first build up a thin "platform" of PPV, onto which I could then be spin-cast. One of
the selling points of organic light-emitting mfaterials, however, is the ease of with which
they can be fabricated into devices. With the "heterostructure" approach, this element of
simplicity had been somewhat lost. Multiple, iterative steps were required to build up the
"platform", followed by thermal conversion of the PPV, followed by spin-casting. It was
clear that a polymeric matrix, in some form, was important for enhancement of device
performance. The fabrication process necessary to realize this improvement with the
"heterostructures", however, was overly complex.
To this end, an exploration of
polymeric Ru(II) complexes was begun, in search of a one-component system that would
satisfy the need for high brightness and efficiency. Ideally, this system could be simply
spin-cast and metallized to form a device.
Many other workers have developed Ru(bpy) 32+- and Os(bpy) 32+-based polymer
systems. Most often, these complexes, functionalized with vinyl groups, have been
electropolymerized onto conducting substrates.'
Alternatively, Elliott et al. have made
drop-cast films of acrylate-functionalized Ru(bpy) 32+ complexes, then thermally
polymerized the films by annealing them at 150 0 C for several hours. 2 A different scheme
involved binding Ru(bpy) 32+ complexes to the backbone of various polyanions (e.g.,
120
Nafion*) via electrostatic attractiona b or binding bis(bipyridyl)ruthenium(II) complexes
to poly(pyridine)s via dative bond formation. Jones et al. performed electron and energy
transfer studies on polystyrenes with covalently-attached pendant Ru(bpy) 32+ and
Os(bpy)32+ complexes.R
There have been few reports of linear, main-chain Ru(bpy) 32+-based
macromolecules, however. Several workers have reported Co(bpy)32+ (ref. 4), Fe(bpy) 32+
(ref. 4,5) and Ru(bpy) 32+ (ref. 6) complexes fitted with low-molecular-weight PEO
"arms"; such complexes are high-viscosity melts at room temperature. 4 Zhu and Swager
described conjugated polythiophene/ bipyridines, to which ruthenium(II) centers were
later coupled.7 Similarly, fully-conjugated PPV- Ru(bpy)32 + polymers have been
synthesized for photorefractive devices. In the present work, it was thought that linear
polymers would lend themselves well to the layer-by-layer sequential adsorption and
spin-casting film deposition techniques.
The morphology of polymers in
electropolymerized films may be inhomogeneous, potentially complicating the
interpretation of electrochemical measurements. 1 b Electropolymerized films also tend to
be intractable in organic solvents, which makes them excellent surface coatings, but they
are difficult to characterize by many spectroscopic techniques.
1.2. Polyurethanes
A series of polyurethanes was generated in this study, with the Ru(II) lumophore as
part of the main chain. Polyurethanes are well-characterized in the scientific literature,
and are known to be excellent coating materials due to their hydrogen bonding
capabilities.
The quality of spin-cast films based on these materials could then be
expected to be very good.
Much industrial use is made of so-called, "segmented"
121
polyurethane copolymers. At the molecular level, these copolymers are made up of a
"rigid", inflexible segment, and a "soft", flexible segment. At the supramolecular level,
the "rigid" segments associate via hydrogen bonding between the urethane groups, often
forming so-called "hard" domains in a sea of the "soft", flexible material (e.g., PEO).
These "hard" domains act as virtual intermolecular crosslinks, in analogy to the sulfursulfur bonds of vulcanized rubber.8
Similarly, segmented polyurethanes show
elastomeric properties; the "crosslinked" rigid domains keep the material from deforming
irreversibly when subjected to large strains.
Unlike vulcanized rubber, however, the
"crosslinking" is not permanent. By heating the material up, the hydrogen bonds holding
the "hard" domains together can be overcome, and the material will lose its elastomeric
properties. As such, these materials are known as "thermoplastic elastomers", and find
application in ski boots, tractor tires, automobile bumpers, etc. 9
Just prior to the observation of the "charging" phenomenon in devices based on I,
Pei et al. reported impressive results from devices based on PPV and PEG,' 0 as described
in Chapter 1.
The operation of these devices was electrochemical in origin, with the
PPV being oxidized and reduced, and the PEO supporting ion transport. (PEO is known
in polymer battery research community as an excellent ion conductor.) With the addition
of lithium triflate to the film, light emission occurred immediately at low voltage.
The
observation of the "charging" effect and a review of the literature suggested that an
electrochemical mechanism was also operant in solid-state Ru(bpy) 32 + devices. As such,
polyurethanes appeared a good material choice. Polyurethanes would be amenable to
incorporation of both a rigid light-emitting segment based on a metal-organic complex,
and a "soft", flexible segment for the required counterion transport. Similar metal
122
complex-containing polyurethanes have been reported by Chen et al., who described
described the thermal properties of polyurethanes based on PTMO [poly(tetramethlene
oxide)] and cobalt- acetaldimine complexes. "
In the present study, by changing the length and identity of the flexible segment
between the lumophores, several issues of device performance could be addressed. For
example, with spacers of varying length, the average electron hopping distance from
redox site to site could be tuned. It was envisioned that there would be an ideal Ru-Ru
separation distance. The lumophores would have to be close enough for facile electron
transfer between them, but far enough apart to frustrate any self-quenching of
electrogenerated excited-states which might occur.' 2 In addition, it was thought that the
polyurethanes might afford a way that the otherwise transient "charging" effect could be
made more permanent. That is, it was observed that devices based on I would "charge"
up under voltage, as manifest by an increase in luminance with time, but would discharge
when the voltage was removed. Presumably, the counterions would distribute themselves
more uniformly throughout the film when the field was turned off.
In the devices of Pei et al., the PPV and PEO formed a two-phase interpenetrating
network, as described earlier. The electrons and "holes" would travel through the PPV
phase, and the mobile anions and cations would travel through the PEO.
It was
envisioned that, under the proper conditions, a similar environment could be established
in thin films of Ru(bpy) 3 2 -based segmented polyurethanes. By proper tuning of the
polyurethane structure, the emitters could be sequestered at a strategic time from the ionconducting pathways (e.g., PEO, or just "soft", flexible, aliphatic spacers) by formation
of "hard" and "soft" domains in a phase-segregated film. That is, it would be possible to
123
charge up a device at elevated temperature, when the film was homogeneous, then cool
the film down to room temperature under voltage to allow phase segregation to occur. In
this way, the mobile counterions might be prevented from moving in and out of the
"hard" Ru domains. The counterions would effectively be "locked" in their optimized,
redistributed state when the applied voltage was removed. Consequently, the device
would not have to be charged up every time it was operated. Counterion mobility
through the "soft" domains would be expected to be high, such that any "charging" which
did have to occur the next time the device was operated would be rapid.
1.3. Ionic Conductivity in Polymer Films
A quantitative assessment of counterion mobility in a polymer sample can made by
measurements of the sample's ionic conductivity. The number (concentration) of and
elementary charge on ionic species present are also considered in such measurements.1 3
Druger, Nitzan, and Ratner have described ion mobility in dry polymer films in terms of
a "dynamic percolation" model,14 "5 several components of which are described below.
In this model and in others, ionic mobility is based on the availability of vacant sites in
the polymer matrix into which an ion can "hop". The frequency with which such sites
become available is a function of the molecular and supramolecular properties (e.g.,
morphology) of the polymer matrix.
At the molecular level, interaction (e.g.,
complexation of some kind) between the polymer and the ionic species plays a role in
ionic conductivity.
Ionic conductivity measurements are typically made using ac
impedance spectroscopy. Dc ionic conductivity measurements are difficult because,
unlike electrons, the ions cannot enter or leave the film, so oscillatory techniques are
usually employed.16 Ac impedance measurements allow determination of the bulk
124
resistance of a polymer film, from which the resistivity, and hence the conductivity can
be determined. Wright first reported ionic conductivities on the order of 10-8 - 10- Scm-1
using PEO/ salt combinations; this report sparked extensive research in the area of solid
polymer electrolytes.' 7 To date, the best polymeric ionic conductors boast conductivities
on the order of 10-4 Scm-1.13
A polymer will exhibit high ionic conductivity at temperatures above its glasstransition temperature (Tg), when large-scale polymer segmental motions make new sites
within the bulk readily available for an ion to hop into.15
These length scales of these
motions are generally on the order of 1-10A, and occur on the 10-1 second time scale. 18
The kinetic considerations of the dynamic percolation model are centered around a given
polymer's "characteristic rate of renewal", referring to the time required for a "jump" site
to become accessible for complexation to a diffusing ion.' 5 Multiple renewal times are
necessary to account for the different conductivities of amorphous and crystalline regions
in semicrystalline polymers. Several classes of polymers have been studied, including
PEO (the most popular system), poly(ethylene imine)'s, as well as low-Tg
poly(phosphazene)- and poly(siloxane)-based comb polymers.9
In general, the polymer matrix should be polar enough to solvate the constituent ions
of the salt with which it has been blended.
That is, if the free energy of complex
formation between the polymer and the salt is not greater than the lattice energy of the
salt itself, the salt will not dissociate considerably, and the ionic conductivity will be
quite low. (The exact chemical make-up of the charge carriers will be complicated by
the formation of so-called ion "triplets" and higher-order ionic clusters. These are more
likely to form in polymeric matrices of low polarity, where dissociation of the salt is less
125
favorable.) Assuming that dissociation of the salt is fairly extensive, ions "jump" from
polar site to polar site within the film in the dynamic percolation model. Typically,
cation transport is more laborious than anion transport, as cations are usually much better
solvated by Lewis basic sites (e.g., ethereal oxygen, nitrogen, and sulfur atoms) within
the polymer matrix. Spectroscopically, complexation of PEO with cations has been
confirmed using FT-IR, where vibrations due to ether oxygen- cation interactions have
been assigned. More specifically, it is possible to probe the extent of such interactions by
spectroscopic examination of polyatomic ion vibrations. The vibrational spectra of more
tightly-bound ions will be perturbed to a greater extent than those of loosely-bound ions,
which may, accordingly, exhibit higher mobilities. For example, Ratner and Shriver
discuss the borohydride and borofluoride anions in a matrix of PEO, in which the
conductivity of the latter was considerably higher, being the more loosely-bound.1 5
Ionic conductivity is affected both by salt concentration and temperature.
Conductivity rises with increasing salt content in the polymer until the effect of
increasing the charge carrier concentration is outweighed by an decrease in ion mobility.
Such a mobility decrease can occur when "free", mobile ions begin to be trapped in ion
pairs again (as cation- anion re-association begins). The number of mobile ions would
also decrease as a result, further decreasing the measured conductivity. In addition, as
the salt concentration increases, vitrification of the polymer matrix begins. The addition
of more salt begins to rigidify the matrix via the formation of pseudo-crosslinks. This
occurs when ions interact with the polar groups on multiple chains at a time, reducing the
ability of the chain segments to rearrange themselves in space. With increasing matrix
rigidification, either due to added salt or reduction in temperature, the amount of free
126
volume (voids, unoccupied space) within the polymer is reduced. In addition, the ability
of polymer segments to jump into these spaces is diminished. As such, the ionic mobility
(and hence the ionic conductivity) drops. Accordingly, ionic conductivity drops with
increasing degrees of crystallinity, although crystalline phases of PEO, for example, do
exhibit limited conductivity.
In this study, the device properties of what might later have become the "hard"
segments of true "segmented" polyurethanes were investigated.
That is, only
polyurethanes rich in the light-emitting Ru(II) complex were synthesized and evaluated,
without incorporation of the long, flexible, "soft" segment. "Segmented" copolymers
were not developed. The light-emitting characteristics of these "hard segment" materials
were interesting in and of themselves, however, and are described next.
2. Results
2.1 Early Work
The polyurethanes in this study were based on lumophore II, a diol (Chapter 2,
Section 1.3.1), and commercial diisocyanates. The general structure of the polyurethanes
is shown in Figure 4-1. While unmodified Ru(bpy) 32 + is commercially-available,
derivatives suitable for condensation polymerization were not. Aliphatic diisocyanates
were available with varying numbers of methylene groups, n (Figure 5-1), between the
functional termini, with n ranging from 4 to 12. A very commonly-used diisocyanate in
polyurethane synthesis is the six-methylene derivative, hexamethylene diisocyanate
(HDI). HDI was used to stitch lumophore II into the backbone of the first polyurethane
in this study, dubbed "poly(II-6)" ("II" indicating the lumophore used, "6" indicating the
127
OH
HO
O-NC+
+(CN-( CH2
n=4, 6, 12
/
/
N
N
/ NII.R+2~
/i
N,
N
/
\
\-1
(PF 6 -)2
-
poly(II-n)
Figure 5-1. General structure of the polyurethanes used in this study. The polyurethanes are differentiated
on the basis of the length, n, of the aliphatic interlumophore spacer used, e.g. poly(II-6) has a 6-methylene
spacer.
number of methylenes in the aliphatic spacer). Poly(II-6) was spin-cast from 3% (w/w)
pyridine solutions at three different spinning rates, affording films of different
thicknesses. As with many new light-emitting materials, it was unclear what voltages
would be required for light emission from this polymer, so the applied bias was initially
scanned from OV to 4V to OV, then to higher voltages in the same way. The scan rate
was 0.5V/ 5 sec.
The devices based on the thinnest film (-430A), where the spin rate was 3000 rpm,
produced average luminance levels of 50 cd/m 2 in the range of 7- 7.5V. At higher
voltages, the devices failed, as evidenced by a dramatic decrease in light intensity. The
device efficiency was improved from 0.01% to 0.1% by increasing the film thickness to
800A (spinning at 2000 rpm). In addition, the maximum luminance increased to the 90100 cd/M2 range at 7-8V. Again, failure occurred at voltages in excess of 7V, or after
multiple scans to 7V. Typically, films were deposited from pyridine solutions in a single
spin-casting cycle. In one set of devices, a film was cast onto ITO at 2000 rpm, and then
the process of dropwise addition of solution and spin-casting at 2000 rpm was repeated
128
on the same film-coated substrate. Devices fabricated in this way operated with an
average efficiency of 0.05%, produced luminance in the 130-200 cd/M2 range at 7-8V,
and failed in the same voltage range. By profilometry, the thickness of this film was
found to be roughly 560A, but very non-uniform across the surface of the substrate.
When poly(II-6) was spin-cast at 1000 rpm (yielding an 800-900A film), the emitted
light intensity increased to the 200-300 cd/m 2 range at 8-1bV.
The best devices
processed in this way could be operated at an efficiency of 0.4-0.5%. The concentration
of the spin-casting solution was then increased to 4%, and 1 000A films deposited. Under
these conditions, the maximum luminance increased to the 300-500 cd/m 2 range at 912V, and the maximum efficiency to the 0.6-0.8% range (Figure 5-2). For reference,
luminance of this intensity is comparable to that of a PPV-only LED, but only when a
calcium cathode is used. When aluminum is used instead as the cathode, the luminance
is in the range of 10-50 cd/m 2 for PPV.
Several trends emerge on closer inspection of the data from these poly(II-6) devices.
In general, the device efficiency increased as the film thickness increased, as did the
maximum driving voltage that could be applied. Whereas the 400-600A devices would
fail at 7.5-8V, the thicker 1000A-devices could often be driven at up to 12V before
breakdown occurred.
As a consequence, the maximum emitted light intensity also
increased 10-fold with thickness from 50 to 500 cd/m 2 . The highest device efficiencies
were realized when, after the voltage was slowly ramped up to its highest value, it was
immediately dropped to OV, and another ramp-up begun without slowly ramping the
voltage back down first. As such, this practice was generally employed in voltage- sweep
experiments. When these devices were biased negatively (i.e., negative voltages were
129
0.8
500
0.7
--- O-- E fic iency
0.6
Lu m inance
400 t
-
0.5
300
0.4
. I
0.3
0.2
200
100
0.1
0
0
0
2
6
4
10
8
12
14
Voltage (V)
Figure 5-2. Luminance and efficiency from a poly(II-6) device during a 0-12-OV scan. This device had
been scanned several times to voltages less than or equal to 12V immediately prior to the scan shown here.
applied), only a small amount of light was emitted (0.01- 0.3 cd/M 2 in the -10 to -16V
range), regardless of the film thickness.
Another phenomenon observed in all of these devices was a shift in turn-on voltage
[the voltage at which the poly(II-6) devices first produced luminance in excess of 0.003
cd/m 2 , the noise level of the photodetector].
In forward (positive) bias the turn-on
voltage steadily decreasedas the bias was repeatedly scanned upwards from 0V. For the
thickest films, for example, the turn-on voltage on the first scan was typically on the
order of 8-10V. The turn-on voltage then dropped 0.5-2V with each scan, finally settling
at 3.0.' This is shown in Figure 5-3. Regardless of the thickness of the device, the turn-on
voltage always dropped to 3V. Even in a device with a film thickness on the order of
3300A, where the initial turn-on voltage was 15V, the turn-on voltage was observed to
decrease with each run to high voltage. (Due to the high voltages applied, i.e., 20-30V,
the device failed before the measured turn-on voltage reached 3V.) This behavior is not
130
0.025
#1
--0--run
0.02
--
0.
-
run#2
s-- run #3
run #4
0.015
m
run #5
run #6
- 8-U
0.01
0.005
0
2
1
0
7
6
5
4
3
8
Voltage (V)
8
7
65
S 4
3
2
1
mm e
2
I
mu
mm
I
4
3
m
I
5
*
m m
I
6
I
i
ai
7
#runs to 8V
Figure 5-3. Shift in the turn-on voltage for poly(II-6) with multiple 0 - 8V scans. This device was ramped
once to 4V, then once to 6V prior to the first 8V scan. The horizontal dotted line in the top plot designates
roughly the emitted light intensity required for device turn-on (0.003 cd/M 2 ). For clarity, most of the
luminance values greater that required for turn-on have not been plotted. In the bottom plot, the turn-on
voltage for each run to 8V is plotted explicitly.
observed in conjugated polymer or small-molecule- based LED's, in which the
mechanism is not electrochemically-based. As described in Chapter 1 for lumophore I
[the sulfonated Ru(II)- based emitter], the turn-on voltage is often observed to increase
131
with increasing scans to high voltage. Such increases are usually attributed to some
degradative mechanism which renders the light-emitting material less electro- and/or
photoactive.
When complete 0-10-OV scans were run, hysteresis in the luminance vs. time plots
(shown below in Figure 5-4) was often observed, with the luminance levels higher during
the ramp down than they were during the ramp up over a large voltage range. As such, it
appeared that the devices were being "conditioned" in some way with each scan. The
conditioning imposed in forward bias (e.g., +6V) could be "erased" with repeated scans
to negative bias (e.g., -6V), but not permanently so. High luminance could be restored
when the device was positively -biased once again. These devices would also produce
3.5
3
2.5 2
1.5
0
0
2
4
6
8
10
Voltage (V)
Figure 5-4. Hysteresis in emitted light intensity from a poly(II-6) device during a 0 - 8V scan.
132
high luminance levels if repeatedly stepped (pulsed) directly to high voltages (e.g., 1 OV)
for a few seconds, rather than slowly ramped up. As in the scanning experiments,
however, the devices would "charge up" with the pulsing technique, as well, as manifest
by a steady increase in the emitted light intensity with more and more pulses.
In an effort to improve the spin-cast film quality, the length of the aliphatic spacer
between the lumophores was increased from six to twelve methylenes.
It was thought
that this would make the polymer less rigid and possibly a better film-former, minimizing
leakage current. If less current could leak through the devices, they would be more
efficient. The resulting polyurethane, poly(II-12), was fabricated into thin film devices
in the same way as was poly(II-6). Using commercially-available butanediisocyanate, a
4-methylene analog, poly(II-4), was also synthesized, having a shorter interlumophore
spacer than the reference poly(II-6) polymer. While the poly(II-4) film quality was not
necessarily expected to be superior to that of poly(II-6), the lumophore density would be
higher in this polymer, and the redox site-to-site distance smaller as a result. The impact
these differences would have on device performance was unclear.
Films of poly(II-12) and poly(II-4) were spin-cast to thicknesses in the 1400-1600A
range. As usual, the voltage was ramped to progressively higher voltages from OV, and
not allowed to ramp back down from the highest voltage before the next ramp-up was
begun. At 1 OV, a device based on poly(II-12) produced luminance of roughly 140 cd/m 2
at an efficiency of 0.6-0.7%, comparable to the best poly(II-6) devices. When the
thickness was increased to roughly 1600A, the maximum efficiency increased to the 0.81% range at 12V (Figure 5-5).
As usual, large hysteresis loops were observed in
luminance vs. voltage plots for poly(II-12), and the turn-on voltage dropped with
133
increasing runs to high voltage, finally settling at 3V. The number of runs required to
minimize the turn-on voltage was a function of the film thickness and maximum voltage
applied. In both materials, however, with subsequent runs beyond the 3V-turn-on point,
the device efficiency soon began to decrease. The light output generally continued to
increase beyond this point.
1
80
Efiency
0.8
p-
70
60
Lumiance
50
0.6
40
0.4
30
20
0.2
10
-
0
0
2
4
6
8
10
12
0
14
Voltage (V)
Figure 5-5. The maximum efficiency for a poly(II-12) device approaches 1% during a 12V scan. The
spin-cast film thickness was roughly 1600A. Prior to the scan shown here, the device was ramped several
times to voltages in the 4-12V range.
2.2. Multiple 0-10-OV Iterations
In, order to better understand the turn-on voltage drop and elucidate structureperformance relationships for these materials, the applied voltage was repeatedly scanned
from 0 to 10 to OV. That is, once the applied voltage reached 1 OV, it was allowed to
ramp back down to OV, then begin the ramp-up process again. Unlike the earlier
experiments, the time between scans was now also controlled, i.e., the program was no
134
longer manually re-started at the end of each run. The multiple 1 OV-iterations were
completely computer-controlled, such that, for a given number of runs, each device spent
the same amount of time under bias. Devices based on poly(II-12) and poly(II-4) were
spin-cast from 4% solutions at 1000 rpm, yielding 1600 and 1470A films, respectively.
The change in poly(II-12) current density and luminance with number of 10V runs is
shown in Figure 5-6. Figure 5-7 shows the change in device efficiency over the course of
this experiment, and the reduction in turn-on voltage (again, the voltage at which the
luminance exceeds roughly 0.003 cd/m 2) with increasing runs. The turn-on voltage starts
20
25
15
20
10
15
5
10
0
0
5
10
-
15
20
25
30
- I- o-- 5
35
40
# runs to 10V
Figure 5-6. Luminance and current density profiles with increasing
0-10-OV runs for a poly(II-12) device.
out at 1 OV in the first run, then drops to 7V in the second as the device is "conditioned".
The decrease in turn-on voltage is more gradual thereafter, generally dropping 0.5V with
each run, and finally settling to 3V, as described earlier. In subsequent runs, the turn-on
voltage remains largely unchanged, out to 40 runs. (Were the device allowed to sit at OV
135
11.g.. Il.IllI NI.
8
7
-
P"
0.25
g g
....
I Ulm 1 i..1 .. I
0.2
.
S6
0.15
5
-.
4
-
- 0.1
-
0.05
3
3
5
0
10
15
20
25
30
35
40
# runs to 10V
Figure 5-7. Efficiency and turn-on voltage profiles with increasing
0-10-OV runs for a poly(II-12) device.
20
55
50
15
-45
10
40
--
-
.
- 35
30
0
25
0
5
10
15
20
25
30
35
40
# runs to 10V
Figure 5-8. Luminance and current density profiles with increasing
136
0-10-OV runs for a poly(II-4) device.
for several hours, the turn-on voltage would begin to increase. Repeated scans to 1 OV
could be used again to bring the turn-on voltage back down to 3Vonce more.) The
luminance eventually reaches a maximum of 20-25 cd/m 2 , then begins to decrease
slowly. As shown in Figures 5-8 and 5-9, some features of the poly(II-12) devices are
mimicked by those of poly(II-4), the polyurethane with the shorter interlumophore
spacer. Namely, both the efficiency and luminance increase with more and more 1OVruns. By 40 runs, neither the efficiency nor the luminance has reached a maximum. The
turn-on voltage drops, and eventually settles to 3V.
1.14
10
0.12
8PEN 660.08
0.06
4
-e
0.04
m
0.02
0
0
1-oi-1
5
10
0
15
25
20
30
35
40
# runs to 10V
Figure 5-9. Efficiency and turn-on voltage profiles with increasing
0-10-OV runs for a poly(II-4) device.
2.3. Constant-Voltage Experiments
In an analogous set of experiments, the applied voltage was ramped to and held at
1OV, not repeatedly ramped between 0 and 1OV. The same general behavior was
observed for devices based on both polyurethanes.
137
As shown in Figure 5-10 for a
poly(II-12) device (below), both the emitted light intensity and device efficiency
increased with time at 1OV, as had been observed with repeated scans to 1OV. The time to
maximum efficiency at 1OV was on the order of 10 minutes for poly(II-12) and for
poly(II-4). A maximum efficiency in the range of 1% was realized with poly(II-12),
comparable in magnitude to the best LED or LEC efficiencies reported. Poly(II-4)
devices exhibited efficiencies on the order of 0.6%. Poly(II-12) generally charged up
.
1.2
200
-C--
Efficiency
-
1
. -e--Luminance
- 150
0.8
0.6
100
0.4
50
0.2
-
0
0
0
5
10
15
20
25
30
35
time (min.)
Figure 5-10. Luminance and efficiency vs. time profiles at 1OV for a device based on poly(II-12).
to 50 cd/m 2 or so within 15- 20 minutes. A few minutes after the efficiency reached a
maximum, the luminance reached a plateau, holding fairly constant for the next 10 to 15
minutes. While both polyurethanes exhibited the same time to maximum efficiency, their
respective luminance- time profiles were a point of divergence. No such plateau was
observed in devices based on poly(II-4). Rather, as shown in Figure 5-11, the luminance
increased smoothly through the 10-20-minute time period for poly(II-4). What both
138
polymer systems had in common, however, was a rapid increase in light output after 2025 minutes. The average time to maximum luminance was roughly 6 minutes longer for
poly(II-12) than for poly(II-4). In general, both systems produced luminance of 200
0.8
-
I
I
I
I
I
*
*
*
*
~
*
~
I
*
I
I
I
I
I
*
*
200
*
0.7
0.6
0-N
150
0.5
0.4
100
0.3
0.2
50
0.1
0
I
0
I
5
I
I
I
I
I
10
15
.
I
20
0
25
time (min.)
Figure 5-11. Luminance and efficiency vs. time profiles at 1 OV for a device based on poly(II-4).
cd/m2 or greater during this event. The emitted light intensity then decreased as rapidly as
it had increased, and continued to drop without limit thereafter. This high- brightness
event appeared to be catastrophic. Figure 5-12 shows the luminance and efficiency vs.
time profiles for a poly(II- 12) device which was ramped to and held at 1 OV, then held at
1 OV again the next day. The device efficiency dropped an order of magnitude from
1.5% to roughly 0.1% between the two runs. In order to assess the permanence of this
decrease, poly(II-12) devices were ramped to and held at 1 OV just until the efficiency
began to drop. The voltage was then ramped back down to OV, and the devices were
139
Run #1
250
, , , i , , ,
, , ,
, , ,
- ,
, , , , , , ,
1.5
200
2
1
-.
:g
e
150
100
?
= 0.5
50
0
0
0
2
6
4
12
10
8
14
time (min.)
Run #2
7
5
0.1
Q-
4
-40-
3
S0.05-
0
1
0
0
10
20
30
40
50
60
70
80
time (min.)
Figure 5-12. Poly(II-12) device held at IOV until failure (top plot), then allowed to discharge overnight,
and held again at 1 OV the next day (bottom plot).
140
1
.
0.8
increasing #10-V
experiments
-hold
~0.4
~0.2
0
1
2
3
4
5
6
7
8
9
time (min.)
Figure 5-13. A device based on poly(II-12) was ramped to and held at 10V only until the efficiency began
to decrease. This was repeated several times for the same device. The maximum efficiency can be seen to
drop as the number of these 1 OV-hold experiments increased.
allowed to discharge overnight (12-24 hours). A turn-on voltage in the range of 9-10V
each day confirmed that the allotted time was sufficient for complete discharge. The
experiment was repeated for five days on the same devices. As Figure 5-13 shows, the
device efficiency dropped fairly steadily with each 1 OV run.
The magnitudes of the efficiency and luminance, as well as the device response time,
were a strong function of the applied voltage. For example, when poly(II-12) devices
were ramped to and held at voltages less than 1OV, the charging time increased
significantly. For reference, this system reached maximum efficiency and luminance
after 10 and 26 minutes, respectively at IV. At 8V, the charging time rose to 4.5 hours
for a maximum of 50 cd/m 2 . The maximum efficiency had dropped 0.7% (30 minutes)
from the 10V value of 1%. (Interestingly, the devices which were operated at sub-10V
potentials employed films spin-cast onto ITO which had been first immersed in IM HCL.
141
Such processing usually serves to increase the device efficiency of RuL3 2 devices.) At
12V, however, the time to maximum efficiency and luminance decreased to 3 and 5.5
minutes, respectively. While the 12-V maximum luminance was comparable to that at
1 OV (vide supra), the efficiency increased to 1.5%, a 50% improvement over the average
10-V efficiency. In addition, the luminance-time curve for poly(II-12) at 12V began to
look more like that of poly(II-4) at 10V (i.e, the plateau in the luminance-time curve
disappeared).
In an attempt to make the conditioning imposed on devices more permanent, the
hexafluorophosphate counterion was exhanged for an acrylate anion in poly(II-12). It
was thought that the acrylate counterion would redistribute under the applied field, and
then could be thermally-polymerized once redistribution was complete.
Once
polymerized, the acrylate counterions would not be able to return to their OV positions
when the field was removed. Hence, the device would not have to be charged up again
each time it was operated. The acrylate salt of poly(II-12), however, was only sparingly
soluble in organic solvents. Drop-cast films of this material only produced a maximum
luminance of 1-2 cd/m 2 , and were not studied further. When chloride counterions were
employed instead, the maximum luminance reached into the range of 20 cd/m 2 at 7-8V.
3.
Discussion
The following general features of these Ru(II)-based polyurethanes as light-emitters
were observed:
1. the need for high voltage (> 7V) for device response on a reasonable timescale;
2. a dynamic turn-on voltage, which drops to 3V with successive scans to 1OVor
while holding at 1 OV, and rises with time at OV;
142
3. luminance and efficiency which increase with time at 1 OV as the devices "charge"
up (over minutes to hours);
4. an increase in the 1OV efficiency with the length of the aliphatic interlumophore
spacer; and,
5. catastrophic device failure after 20-30 minutes at 10V for 1000-1600A films.
An explanation of the foregoing experimental results and a look at the impact of
emitter structure on device fabrication, charging time, efficiency, and operating stability
follows.
3.1. Device Fabrication
The polyurethanes represented a one-component system on which high-performance
devices could be based. There was no need for multi-step procedures for depositing
uniform films of these materials, as they lent themselves well to simple "one-shot" spincasting. Devices were also fabricated based on a PF6~ -bearing polyester (Chapter 3,
Figure 3-3) whose structure was analogous to that of poly(II-12); the two polymers
differed primarily in their backbone linkages. Solutions of this polyester had to be spun
onto the same substrate more than once to realize device- quality films.
It was
anticipated that this procedure would afford better overall substrate coverage (i.e., by
minimizing pinhole formation). When this procedure was applied to the polyurethanes,
however, the device efficiency actually worsened. It was thought that some of the
polymer deposited in the first spin-casting cycle eroded away during the second cycle,
giving rise to considerable thickness variations. The electric field would have been
concentrated in regions where the film was exceptionally thin, and the device would have
failed in these regions first. Consequently, the overall efficiency would have been
143
decreased. This suggests that the urethane linkages readily afforded uniform films of
these polymers, and that their macromolecular character was not sufficient to ensure high
performance.
3.2. General Device Features
3.2.1
The "Charging" Phenomenon
A physical description of what occurs during device operation follows. The long
delay between voltage application and maximum light emission in these polyurethane
devices is consistent with a mass transport-limited mechanism of operation.
This
mechanism is likely the same as that described by Bard and Murray and their many
respective co-workers, as outlined in Chapter 1. At the heart of this mechanism is an
electrochemical process in which Ru(II) lumophores are oxidized at one electrode and
reduced at the other. The resulting redox products are transported into the bulk of the
film via electron self-exchange (site-to-site hopping). The rate at which this process
matures is determined by the rate at which the counterions associated with each
lumophore redistribute themselves in the film under the applied field.
The sluggish response of these polyurethanes can be explained on several fronts. The
first is the polymeric nature of the materials. Even before a consideration of the specific
chemical character of the polymers in question is taken up, the very fact that these are
macro~molecular systems suggests that ion migration through them might be slow. Ratner
and Shriver,
5
among others, have treated this subject thoroughly, as was outlined in
Section 1.3. Briefly, these workers have demonstrated that ions, like electrons, migrate
through polymer films via a site-to-site hopping mechanism. In general, an ion at one
144
site must wait for polymer chain segments to rotate out of the way and make free volume
available at another before it can hop.
This site-to-site hopping process is complicated by formation of the inter- and
intramolecular hydrogen bonds which are characteristic of polyurethanes. These bonds
form pseudo- crosslinks between the chains. As discussed in Section 1, much industrial
use is made of hydrogen bonding in segmented polyurethanes as thermoplastic
elastomers. So strong are these bonds that some workers believe them to persist even
after macroscopic dissolution in aggressive dipolar aprotic solvents such as DMF, a
solvent typically used to dissolve hydrogen-bonding solids.19 DSC did not reveal the
presence of these bonds probably due to the low molecular weight of the polymers
analyzed (10-15k).
Nevertheless, it is reasonable to assume that hydrogen bonds are a
force to be reckoned with in the polyurethane devices under investigation here, as
supported by FT-IR. It is through this dense network of hydrogen-bonded chains that the
PF 6 ions must travel as they redistribute themselves throughout the polymer film.
Redistribution is further retarded by attractive interactions between the ions themselves
and the urethane linkages. These urethane-urethane and urethane-ion interactions are
shown schematically in Figure 5-14.
Under this model, in the multiple-scan experiments (Section 2.2), as the number of
10V runs increases, the PF6~ counterions are swept farther and farther from their initial
positions, and closer to the positively-biased ITO anode. The effect of counterion
redistribution is to establish a high concentration of negatively-charged species at the
ITO-film interface, leaving behind a high concentration of positively-charged Ru(II) and
Ru(III) species at the aluminum-film interface. Ionic space charge fields are established
145
0
1
N-1+ -=c
N-It
N-I+*--F=C
polyurethane
1
"00
backbone
Figure 5-14. Inter-chain and urethane-counterion hydrogen-bonding lead to diminished ionic conductivity.
(Adapted from Coleman et al., Macromolecules 1988, 21, 59-65.)
at the interfaces, as described in Chapter 1. As such, the potential drop at each interface
increases, such that oxidation and reduction of Ru(II) become progressively more facile.
Hence, a decrease in the applied voltage required for light emission (i.e, a decrease in the
turn-on voltage) is observed with continued 1 OV- ramping.
Clearly, the device is being conditioned in some way with increasing scans, as
evidenced by the dynamic turn-on voltage.
That this conditioning persists into
subsequent scans supports a mechanism which is not purely electronic, as electronic
processes (e.g., charge transport) occur on considerably shorter timescales. Relaxation of
the vcltage-imposed conditioning would have been much quicker had the conditioning
been solely electronic in nature.
Barring degradation, if the mechanism is purely
electronic, the devices should behave identically each time they are biased. Instead, the
turn-on voltage decreases steadily with each 1 OV run. In addition, the -light output
increases with increasing runs, as more and more electrochemistry is induced. Similarly,
146
in the constant-voltage experiments (Section 2.3), more extensive counterion
redistribution occurs the longer the 10V bias is maintained. The more the counterions
redistribute, the more oxidation and reduction occur throughout the film, as evidenced by
the increase in luminance with time.
The macromolecular nature of the polyurethanes, polymer-polymer interactions, and
polymer-ion interactions all conspire to make counterion redistribution more difficult.
(This effect was even more pronounced when the bulky, perhaps tightly-binding acrylate
counterions were employed, and diminished with the use of the small, yet
electrochemically-unstable chloride ions.) Their interplay provides some basis for the
experimentally-observed charging profiles for the different polyurethanes in this study.
In the multiple-scan experiments, both poly(II-12) and poly(II-4) required roughly the
same number of 1OV runs (8-11) for a 3V turn-on voltage to be realized.
Whereas
poly(II-12) reached maximum luminance (15-30 cd/m 2 ) within 22-24 runs, however, a
poly(II-4) device operated in this manner still had not done so after 40 runs. It is likely
that the maximum observed in the poly(II-12) luminance-time profiles in these
experiments was only what appeared to be a plateau in the constant-voltage experiments.
Indeed, in experiments with multiple scans to 14V (instead of IV), a high-brightness
(200 cd/m 2 ) failure event was observed within 8 runs.
In the constant-voltage experiments, the time to maximum efficiency was roughly the
same for both polymers. However, poly(II-12) required an average of 6 minutes more to
reach maximum luminance than did poly(II-4). The exact reason for the more sluggish
response of poly(II- 12) relative to poly(II-4) at 1 OV is not clear. At the molecular level,
poly(II-12) had a longer aliphatic spacer between the lumophores. It is likely, then, that
147
the site-to-site distance between the lumophores was larger as well, possibly slowing the
electron hopping process. Counterion mobility might also be affected. According to the
dynamic percolation model, more aliphatic material would have to reorient itself in
poly(II-12) in order to make a new hopping site available for ion transport. As such, it
would seem that the ionic conductivity (a) of poly(II-12) would be lower than that of
poly(II-4). Preliminary conductivity measurements, outlined below in Table 5-1, do not
bear this out, however. In light of the overwhelming evidence for a counterion
redistribution rate-limited mechanism, more exhaustive measurements should be made to
confirm these values. That the poly(II-12) film was somewhat thicker than that of
poly(II-4) may explain the longer IOV charging time. In addition, if poly(II-4) is truly
less conductive than poly(II-12), the enhanced urethane linkage density in the former
(which might cause greater diminution of counterion mobility) may be the cause.
The polyurethanes cannot be compared on the basis of the magnitudes of their
maximum luminance, however. As mentioned above, in the multiple-scan experiments,
maximum luminance was not ever reached with poly(II-4). In the constant-voltage
experiments, during the final catastrophic failure event, the light output would rapidly
Table 5-1. The performance of the polyurethanes is compared on the basis of the length of the
interlumophore spacer (i.e., the number, n, of methylenes in the spacer) used. Values are averages from
several devices based on each material. aTaken from the results of the 1OV multiple-iteration experiments.
bTaken from the results of the 1OV constant-voltage experiments.
c The ionic conductivity of each
material, a, was calculated based on the bulk resistance of a spin-cast thin film, as determined by ac
impedance spectroscopic (cf. Chapter 2, Sect. 4) measurements. Multiple measurements were not
averaged for the values in this column.
Spacer
length (n)
# 1OV runs to
3V turn-ona
4
12
Max.
efficiencyb
10.7
Time to max.
efficiencyb
(min.)
11.2
0.6
Time to max.
luminanceb
(min.)
21
8.5
9.75
1
27
148
(%)
a
(S/cm)'
1.5 x10'
2 x 101
increase, reach a maximum, and fall again within the 5-second interval between
computer-automated measurements. (One could observe this occurrence by visually
tracking the real-time emitted light intensity values on the optical power meter read-out.)
As such, the true maximum light emission produced during this event was usually not
registered by the software, and, therefore, was not plotted on the luminance-time profiles.
On these grounds, the recorded maximum luminance is not a valid parameter on which to
compare the two systems.
3.2.2. Device Efficiency
A particularly interesting observation in both sets of experiments was the increase in
device efficiency with continued application of 1 OV. One would expect that the ratio of
light output to current flow would remain constant, even though the magnitudes of each
parameter would increase with more counterion redistribution. A closer look at the
physical processes involved in light emission is warranted. On an "ion budget", charge
balance requires that the number of oxidized lumophores be equivalent to the number of
reduced lumophores. That is, it is theoretically impossible to remove an electron from
the film during oxidation without injecting an electron back into the film in a reduction
event. Charge balance makes no demands on the magnitude of the light that is emitted as
a result, however. It is entirely possible that an electron injected into the film at the
cathode could pass completely through the film to the anode without encountering the
hole that was produced at the same time. If the charge carriers do not recombine, no light
will be emitted, even though current is passing through the device. Hence, the efficiency
will be zero.
149
This picture becomes clearer when the relative dimensions of the Ru(II) lumophores
and the electrodes are considered. If the lumophores are modeled as hard spheres having
a diameter of roughly 1 OA, the electrodes in a 6 mm 2 device would be in direct contact
with slightly fewer than 6 x 1010 lumophores. (The number of lumophores passivating
the electrodes in a polyurethane device would be slightly lower due to the presence of the
aliphatic spacer, which would occupy some finite area.) The chances of an electron
injected at one electrode finding a "hole" injected at the other are slim until the number
of carriers rises to some threshold value. In short, the higher the number of injected
carriers, the higher the light output. The so-called, "capture cross-section", or region in
which the charge carriers can recombine, increases in size with the number of injected
carriers. As the likelihood of carrier recombination rises, the device efficiency rises.
Pichot et al. have argued that, under the constraints of the "ion budget", the flux of
oxidized species to the recombination zone has to be the same as that of the reduced
species.20 As such, the rates at which the two are transported into the bulk of the film
should be roughly the same, theoretically speaking. This argument would place the
recombination zone very near to the geometric center of the film, i.e., at a location
roughly equidistant from both electrodes. On an "ion budget", when no ionic species can
enter or leave the film during operation, the number of counterions present in the film is
insufficient to support formation of a Ru(II/I) gradient, for example, through the
thickness of the film. However, this theory was formulated on the basis of experimental
work with films some 1-5gm thick. It is unclear whether the constraints of the "ionbudget" argument hold at the higher electric fields reached in these 1000 - 2000A films.
There is evidence
21
to suggest that one charge carrier may be injected prior to the other.
150
2
Abbas reported low-voltage current flow in thin-film Ru(bpy) 3 -based devices prior to
light emission. The magnitude of the current density is too high, however, to be simply
due to capacitive charging of the electrodes (i.e, due to low-level counterion
redistribution). In fact, the emission of light is roughly coincident in time with a drop in
current density.
While all features of this kind of device response are not well-
understood, this phenomenon suggests that injection of the two electronic charge carriers
may not be simultaneous.
In the event that the reduced species are considerably more populous (or are
transported faster) than the oxidized species, or vice versa, the recombination zone would
be very close to one or other of the electrodes. (Recombination would occur in an area
closer to the ITO, for example, in the first case.) Energy-transfer quenching of excited
states by electrodes is a well-known phenomenon in the LED community.
Metal
electrodes (e.g., platinum, gold, aluminum) have been cited many times as quenchers for
Ru(II)*, and metal oxides (e.g., ITO) to a lesser extent. In conjugated polymer LED's,
excitons (the products of electron-hole recombination) have been reported to diffuse up to
1 ooA from the point of their generation to a quenching site. Therefore, the farther the
recombination zone can be removed from the electrodes, the more efficient the device.
Electrode quenching would reduce the number of photons emitted from the device
without effecting the amount of current flowing through it, thereby lowering the device
efficiency.
As the injection and transport of the two charge carriers becomes more
balanced (e.g., with increased 1 OV scans, or at longer times under a constant 10V bias),
the recombination zone may move away from one of the electrodes, and closer to the
geometric center of the film, reducing electrode quenching.
151
This shifting of the
recombination zone would also give rise to a progressive increase in device efficiency.
The extent of excited state quenching by the electrodes would be lessened outright with
thicker films (deposited by the "one-shot" spin-casting method).
While substrate
coverage was presumably improved in these thicker films (vide supra), the region in
which oxidized Ru(III) species would encounter reduced Ru(I) species during operation
would also be further removed from the electrodes.
Device efficiency appeared to be a function of the voltage protocol used, as well. In
some of the early multiple-scan experiments, the scan was stopped when the maximum
voltage was reached, and the voltage was not allowed to slowly ramp back down to OV.
Instead, the program was manually restarted at OV, and the voltage was ramped up again.
This was a practice adopted from work with PPV-based LED's, the rationale being that
the less time the devices spent at high voltage (i.e., during voltage scans), the more stable
they would be. To some extent, this was true for the Ru(II)-based devices. Initially,
however, this testing protocol served to better preserve the conditioning imposed on the
poly(II-6) devices (during the voltage ramp-up) for the next scan. That is, at the scan rate
used (0.5V/ 5 sec.), a complete 0-10-OV scan of forty 0.5V steps would take roughly
210 seconds. The Labview software (used to automate device testing) always took two
current flow and light output readings at the highest applied voltage (cf. Figure 5-4),
which added the extra ten seconds to the length of each scan. Were the program stopped
after the second reading at 1 OV, for example, it was possible to save roughly 100 seconds
of ramp-down time. The program was immediately restarted at OVand ramped up again
to 1OV, usually within 10 seconds or so, rather than after 100. In between runs, the
counterions would not have sufficient time to return to their initial OV-positions, so more
152
of the "conditioning" imposed on the device during one run would be preserved into the
next run. Hence, the counterion gradient developed during a given scan would have a
greater contribution from the previous scan, so the number of electrogenerated excited
states and the efficiency would be higher.
Evidence for this phenomenon was also garnered by visually tracking the real-time
light output values on the optical power meter during multiple 1OVscans. When the
voltage was allowed to slowly ramp back down from 1OV, the emitted light intensity
could be seen to decrease in between steps to lower and lower voltages. (Normally, when
ramping up to high voltage, the light intensity would increase at each voltage in between
steps.) This suggested that the counterions were relaxing somewhat from their highvoltage positions, no longer facilitating light-producing redox chemistry to the same
extent. High voltage was clearly required to keep the counterions in place. Without it,
they would quickly relax under the influence of the internal electric field established by
their initial redistribution. The efficiency improvement realized by increasing the time at
high voltage in the multiple-scan experiments was even greater in the constant-voltage
experiments. Whereas, for poly(II-12), maximum efficiencies in the range of 0.2-0.9%
were realized in the former, the latter routinely produced efficiencies in the 1-1.5% range.
The same was true of the luminance levels produced by the two types of experiments.
The multiple-scan experiments typically produced luminance levels in the range of 15-70
cd/M 2
Luminance in the range of 50-100 cd/m 2 were the norm for the constant-voltage
experiments prior to the higher-brightness catastrophic failure event.
Finally, device efficiency could be tuned at the molecular level, as work with
poly(II-12) and poly(II-4) demonstrated.
153
It is unclear from the multiple-scan
experiments which of the two polymers is the more efficient, but the constant-voltage
work makes the impact of interlumophore spacer length clear. At 1OV, the average
maximum efficiency for the poly(II-4) devices was 0.6%, while that of the poly(II-12)
devices was 1%. One possible explanation is that, when a longer aliphatic spacer is used,
the lumophores are separated more from one another in the spin-cast film, and excitedstate self-quenching is minimized. The poly(II-12) devices were also somewhat thicker
than those based on poly(II-4), which may have slightly reduced electrode quenching, as
described earlier. When devices based on mixed PF 6~/ C~ poly(II-6) were ramped to
1oV, the device efficiency reached a maximum of 0.01% and the luminance reached a
maximum of 20 cd/m 2 at 6.5- 8V. The devices failed at higher voltages. While these
devices charged up quickly, their poor performance is almost certainly due to the
electrochemical instability of the chloride anions, as described in Chapter 4. In their
original ECL work, Tokel and Bard cited the oxidation of Cl~ at 1.4V. The production of
highly-reactive chlorine atoms, and the ensuing in situ generation of chlorine gas or nonelectroactive Ru(bpy)xCly byproducts can be envisioned. In any event, it is clear that the
use of electrochemically-stable counterions (e.g., PF6~, C10
4
, etc.) is important in high-
performance solid-state Ru(II)-based devices.
3.2.3.
Operating Stability
In the constant-voltage experiments, the time to maximum luminance seemed to
increase slightly with increasing spacer length. As mentioned above, it is possible that
this effect is somewhat due to film thickness differences, as the thickness of the poly(II12) films was some 100-200A greater than that of the poly(II-4) devices. Indeed, by
154
spin-casting a thinner film of poly(II-12) or operating a poly(II-12) device at higher
voltage, one can effect shorter charging times, as well as luminance and efficiency vs.
time profiles which are qualitatively similar to those of poly(II-4). However, as shown in
Figures 5-10 and 5-11, the time to maximum efficiency in these experiments was roughly
the same for both materials. This suggests that any film thickness differences affect the
time to maximum luminance more than the time to maximum efficiency.
The point at which the efficiency reaches a maximum and then begins to drop may
shed some light on why these devices failed at all. Unlike most LED's, Ru(bpy) 32+-based
light-emitting devices employ stable, largely unreactive electrodes. Unlike liquid ECL
cells, in which some sacrificial species (e.g., oxalate ion) is generally consumed over the
lifetime of the cell, solid-state Ru(bpy) 32+-based devices are, in theory, regenerative. As
long as oxidized Ru(III) species and reduced Ru(I) species are produced and allowed to
recombine, light will be emitted.
The experiments with poly(II- 12), in particular, are illustrative as to why this process
does not continue indefinitely in practice. Figure 5-13 showed that the downturn in
efficiency in the poly(II- 12) constant-voltage experiments was permanent, indicating that
some irreversible process was occurring. Figure 5-14 (below) shows that the point at
which the devices operate at maximum efficiency is roughly coincident with that at
which the turn-on voltage settles to 3V. (When multiple 12V scans were employed, the
efficiency began to drop immediately after the turn-on voltage reached 3V.) The final
turn-on voltage is very close to the Ru(bpy) 32+ redox potential of 2.7V, as measured
initially by Tokel and Bard.22 This suggests that when the minimum turn-on voltage is
155
.
0.15
0.05
3
0
5
10
20 25
15
# runs to 1OV
30
35
40
Figure 5-14. Efficiency profile for a poly(II-l2) device during a 1OV multiple-scan experiment. The
efficiency reaches a maximum at roughly the same time that the turn-on voltage drops to 3V, as indicated
by the vertical dotted line. The efficiency begins to decrease soon thereafter.
reached, the solid- state device begins operating much like a liquid cell. This parallel
between liquid- and solid-state devices is significant in that it correlates the downturn in
efficiency with a materials parameter. That is, it implies that device failure is due to
some intrinsic property in the Ru(II) emitter, and not to a more general, albeit elusive
global device issue (e.g., cathode instability, delamination of the cathode from the film,
poor film quality, etc.).
The actual mechanism of degradation is not clear. Given the instability of the
reducbd Ru(I) state, however, degradation most likely occurs at the cathode-film
interface.
It is possible that the Ru(I) species engage in some parasitic side reaction
which renders them non-electroactive and effectively lowers the overall device
efficiency. From an electrochemical perspective, injection of electrons into the film is
most efficient when the concentration of Ru(I) species at the cathode is equivalent to the
156
concentration of Ru(II) species.
[Correspondingly, development of the Ru(III/II)
concentration gradient is most facile when the Ru(III)/Ru(II) ratio at the anode is unity.]
Under "ion-budget" constraints, when the 3V turn-on voltage was reached, this condition
was satisfied in some areas of both electrode-film interfaces. This condition would be met
first in regions where the film was slightly thinner, as the electric field would be higher
and counterion redistribution fastest here. The electrochemistry in the thicker-film
regions would develop more slowly.
With continued counterion redistribution, however, the potential drop across the
interfaces would continue to rise, and the population of Ru(I) states at the cathode would
rise, accordingly. In the thinner-film regions of the cathode-film interface, the cathode
would become more and more concentration-polarized, as Ru(II) species are converted to
Ru(I) faster than new Ru(II) species are delivered from the bulk. At a potential drop of
sufficient magnitude, it is possible that Ru(O), the two-electron reduction product, could
have been formed. Ru(O) is even more highly-reactive than Ru(I),6 and previous attempts
to study the electrochemical properties of this species have been largely unsuccessful due
to its instability. Reactions of Ru(O) with the aluminum cathode, adsorbed moisture, etc.,
can be envisioned, the products of which might not be electroactive, and perhaps may act
as quenchers for electrogenerated excited Ru(II)* states. The thinner-film regions in
which this material was formed would begin to degrade, but concentration gradient
development would still be in its early stages in thicker-film regions.
Hence, the
simultaneous decrease in efficiency and increase in luminance with time.
The final catastrophic, high-brightness event (e.g., Figure 5-10) is even more difficult
to explain. Generally, no damage to the aluminum cathode could be observed following
157
this event. Aluminum damage might have indicated that the device had simply shorted
out in some areas. Such short-circuiting is usually precipitated by concentration of the
electric field at some defect in the film (e.g., a dust particle, pinhole, region of
exceptionally-thin film), and attended by anomalously high current. Such high current
might also have been accompanied by high luminance. The current flow during this
failure event was relatively high, always reaching into the 300-500 mA/cm 2 range, but
did not approach the current limit of the ammeter (1300-2000 mA/cm 2 ), where aluminum
damage would have been expected. Device failure likely began much earlier with the
downturn in efficiency, as described earlier, which may be attributable to instability of
the Ru(I) or Ru(0) species.
Similarly, the moderately-high current density and
undamaged aluminum suggest that the final failure event, while catastrophic, is fairly
well-controlled, and is also related to the emitting materials in use, not to some extrinsic
factor.
It is possible that, after 20-25 minutes at 1 OV, the requirement for equality in the
Ru(II) and Ru(I) concentrations has been met in all regions across the entire cathode-film
interface. Hence, charge injection is facile, and both the current density and light output
increase dramatically, and seemingly without limit until gross device failure occurs.
Maness et al. described a similar observation in devices based on an electropolymerized
Ru(bpy) 32 + derivative."
After it was charged up, the thin film was frozen at 00 C,
reportedly locking both the PF~ counterions and Ru(III), Ru(II), and Ru(I) species in
place. No limiting current was observed in these frozen devices, either, during voltage
sweeps. As the counterions, were no longer mobile, electron hopping was said to occur
under the influence of a voltage gradient, and not by a gradient in the concentration of
158
Ru(III), Ru(II), and Ru(I) species. Perhaps, in the present study, once the counterions
redistribute as much as possible, carrier transport is driven mostly by the voltage
gradient, and the redox site concentration gradient (which along which carriers would
normally be driven by diffusion in the bulk) becomes less important.
In any event, as mentioned earlier, potentials of 7V or greater are needed in the first
place to realize high luminance on a reasonable timescale in these devices. When ramped
to and held at only 3V, a device based on poly(II-12) did not produce more than 0.02
cd/M 2 after 72 hours. Another based on poly(II-4) produced only 0.1 cd/m 2 after 15
hours at 3V. However, the high voltage required to accelerate counterion redistribution
is no longer needed after 20-25 minutes. While high voltage is needed to keep the
redistributed counterions in place, a potential of 1 OV, for example, is excessively high for
simply driving charge injection. To a first approximation, the potential drop across each
electrode-film interface at this point should be in the range of one-half of the applied
voltage. With a 5V interfacial potential drop, many different degradative scenarios can be
envisioned for a material whose one-electron oxidation and reduction potentials are in the
range of only ± 1-2V. For example, Tokel and Bard reported that bipyridine ligands
separate from the ruthenium metal center and are themselves reduced at sufficiently high
voltage.
In the present study, preliminary attempts to identify the chemical species
formed at the cathode-film interface during this event using X-ray photoelectron
spectroscopy (XPS) have been unfruitful due, in part, to the small 6 mm2 pixels in use.
4. Summary
In closing, it is significant that the polyurethanes can be simply spin-cast and
metallized in the fabrication of high-performance devices. Complicated, multi-step
159
schemes are not required for deposition of high-quality films. The polyurethane devices
reproducibly exhibit both high brightness (with luminance as high as 500 cd/m2 ) and high
efficiency (1%
or better), unlike previously-reported solid-state Ru(bpy) 32+-based
devices. At the time of these findings, this combination of high efficiency and high
luminance was a first.24
The long charging time and high-voltage requirement were not anticipated, however.
As such, these polyurethanes may not be technologically- useful materials. It is possible
that the performance of the poly(II-n)'s could have been improved by changing the
identity of the interlumophore spacer rather than just its length, as described earlier. True
"segmented" polyurethanes might have afforded a means to make the charging-up
process more permanent by if phase segregation were induced when counterion
redistribution was optimal. Given the sluggish device response of these "hard segment"only polymers, however, full-blown segmented copolymers were not pursued.
In
addition, new developments with small-molecule Ru(bpy)32+- based devices (described in
the next chapter) suggested that other approaches would be more fruitful. Nevertheless,
this polyurethane study demonstrated that, contrary to the suggestions of others, i
Ru(bpy) 32+is a viable lumophore for eventual use in practical applications, and should be
numbered among promising new solid-state light-emitting materials.
160
5. References
1. Abruna, H.D.; Bard,A.J. J. Am. Chem. Soc. 1982, 104, 2641-2642. (b.) Jernigan,
J.C.; Surridge, N.A.; Zvanut, M.E.; Silver, M.; Murray, R.W. J. Phys. Chem. 1989,
93, 4620-4627 (c.) Pickup, P.G.; Kutner, W.; Leidner, C.R.; Murray, R.W. J. Am.
Chem. Soc. 1984, 106, 1991-1998. (d.) Facci, J.S.; Schmehl, R.H.; Murray, R.W. J.
Am. Chem. Soc. 1982, 104, 4960-4962. (e.) Abruna et al., J. Am. Chem. Soc. 1981,
103, 1. (f.) Ellis, C.D.; Margerum, L.D.; Murray, R.W.; Meyer, T.J. Inorg. Chem.
1983, 22, 1283-1291. (g.) Gould, S.; Strouse, G.F.; Meyer, T.J.; Sullivan, B.P. Inorg.
Chem. 1991, 30, 2942-2949. (h.) Sosnoff, C.S.; Sullivan, M.; Murray, R.W. J. Phys.
Chem. 1994, 98, 13643-13650. (i.) Maness, K.M.; Terrill, R.H.; Meyer, T.J.; Murray,
R.W.; Wightman, R.M. J. Am. Chem. Soc. 1996, 118, 10609.
2. Elliott, C.M.; Pichot, F.; Bloom, C.J.; Rider, L.S. J. Am. Chem. Soc. 1998, 120, 6781.
3. (a.) Rubenstein, I.; Bard, A.J.; J. Am. Chem. Soc. 1980, 102, 6641. (b) Rubenstein, I.;
Bard, A.J.; J. Am. Chem. Soc 1981, 103, 5007 (c.) Jones Jr., W.E.; Baxter, S.M.;
Strouse, G.F.; Meyer, T.J. J. Am. Chem. Soc. 1993, 115, 7363-7373.
4. Williams, M.E.; Masui, H.; Long, J.W.; Malik, J.; Murray, R.W. J. Am. Chem. Soc.
1997, 119, 1997-2005
5. Long, J.W.; Velazquez, C.S.; Murray, R.W, J. Phys. Chem. 1996, 100, 5492-5499.
6. Maness, K.M.; Masui, H.; Wightman, R.M.; Murray, R.W. J. Am. Chem. Soc. 1997,
119, 3987.
7. Zhu, S.S.; Swager, T.M. Adv. Mat. 1996, 8, 497-500
8. Sperling, L.H. Introduction to Physical Polymer Science. John Wiley & Sons, Inc.,
New York: 1986, pp. 302-304.
9. Odian, G.W. Principles of Polymerization., 3rd ed. John Wiley & Sons, Inc. New
York: 1991.
10. Pei, Q.; Yu, G.; Zhang; Yang, Y.; Heeger, A.J. Science, 1995, 269, 1086.
11. Chen, L.; Xu, H.; Zhu, Y.; Yang, C.; Shen, G. Polym. J. 1996, 28, 481-488.
12. Maness, K.M.; Masui, H.; Wightman, R.M.; Murray, R.W. J. Am. Chem. Soc. 1997,
119, 3987.
1 3.Tsuchida, E.; Takeoka, S. "Electronic Processes in Macromolecular Metal
Complexes", in Macromolecular-Metal Complexes. Ciardelli, F., Tsuchida, E., and
Wohrle, D. (eds.) Springer-Verlag, Berlin: 1996.
14. Druger, S.D.; Nitzan, A.; Ratner, M.A. Solid State Ionics 1983, 9, 1115.
15. Ratner, M.A.; Shriver, D.F. Chem. Rev. 1988, 86, 109-124.
16. Bruce, P. "Electrical Measurements on Polymer Electrolytes", in Polymer Electrolyte
Reviews-1. MacCallum, J.R., and Vincent, C.A. (eds.). Elsevier Applied Science,
London: 1987.
17. Wright, P.V. Br. Polym. J. 1975, 7, 319.
18. Poinsignon, C.; Berthier, C. Abstracts. First International Symposium on Polymer
Electrolytes, St. Andrews, Scotland, 1987.
19. (a.) Lu, X.; Hou, M.; Gao, X.; Chen, S. Polymer 1994, 35, 2510-2515.. (b.) Lu, X.;
Wang, Y.; Wu, X. Polymer 1994, 35, 2315-2320.
161
20. Pichot, F.; Bloom, C.J.; Rider, L.S.; Elliott, C.M. J. Phys. Chem. B. 1998, 102, 35233530.
21. Abbas, E.D., PhD Thesis, Massachusetts Institute of Technology, 1999.
22. Tokel, N. E.; Bard, A. J. J. Am. Chem. Soc. 1972, 94, 2862.
23. The XPS work was performed by Prof. James Whitten, University of Massachusetts,
Lowell.
24. Handy, E.S.; Abbas, E.D.; Pal, A.J.; Rubner, M.F. Proc. SPIE- Int. Opt. Soc. Opt.
Eng. 1998, 3476, 62.
162
Chapter 6. Small-Molecule Devices
1. Background
The spin-cast polyurethanes, based on lumophore II (Chapter 2, Scheme 1.3.1),
produced the highest luminance levels reported at that time for RuL 32+-based devices,
operating at 50-500 cd/m 2 and 1% efficiency.
Unlike the high-brightness lumophore I/
PPV "heterostructures" and the high-efficiency, layer-by-layer-fabricated Ru(bpy) 32+_
polyester devices, the polyurethanes did not require multi-step build-up schemes for high
performance.
In short, they represented a flexible, well-controlled system for the study
of Ru(bpy) 32+-based device optimization.
Other workers have described similar solid-state light-emitting oligomer- and
polymer-based Ru(bpy)3 2+ systems. Like the polyurethanes, these systems charged up
very slowly, and elaborate schemes were often devised to circumvent the otherwiselengthy initial charging period. As discussed in Chapter 1, Maness et al. reported
electropolymerized Ru(bpy)32+
systems
and oligomeric Ru(bpy) 32+ moieties
functionalized with short PEO arms.2' 3 When the electropolymerized system was
employed, devices were initially charged up while the crosslinked film was swollen with
solvent. The film was then dried to lock the counterions in place, after which the applied
bias was removed, and the device was operated in the normal way. Alternatively, the
oligomeric system could be charged up at room or elevated temperature, then cooled to
subambient temperatures to keep the counterions from returning to their OV positions
when the bias was removed.
In both cases, diode-like behavior was observed, as
evidenced by the highly- rectified current flow and light emission. More recently, Elliott
et al. prepared crosslinked films from thermally-polymerized acrylate-functionalized
163
Ru(bpy)3 2 + moieties. 4 Devices based on these films were similarly "pre-charged" (while
solvent-swollen) to minimize the time to maximum luminance, like those of Maness et al.
While employing low voltages (2.5-5V), none of these systems or pre-charging schemes,
however, afforded devices with both high brightness (> 25 cd/M 2 ) and efficiency.
The principal drawback to the use of polyurethanes in practical flat-panel displays
was their long charging time. Their sluggish response of these materials to applied bias
necessitated the use of high voltages (>7V), which destabilized them and eventually led
to their catastrophic failure within minutes to a few hours. At the molecular level, device
properties could be tuned by simply changing the length of the aliphatic spacer between
the lumophores.
As discussed in Chapter 5, the interlumophore spacer used in the
polyurethanes was initially six methylenes long (not including the reactive terminal
isocyanate groups). From there, in an attempt to understand the impact of this spacer on
spin-cast film quality, device efficiency, redox site-to-site distance, etc., the spacer was
lengthened to twelve, then shortened to four methylenes.
As a control experiment, the spacer was next removed altogether to assess the device
performance of lumophore II alone in spin-cast films. No additional aliphatic material
was employed to serve as a polymeric backbone, or as a "platform" to modify the ITO
substrate. This approach had also been attempted with lumophore I, as described in
Chapter 4, but with little success.
Among others, poor film quality and meager
concentration gradient development were fingered as the culprits for the poor
performance, invoking the use of the PPV "platform" onto which the small molecule
could be deposited.5
Conventional wisdom in the LED community held that small
molecules, in general, could not be successfully spin-cast into uniform films.
164
On the
contrary, it was discovered that lumophore II could be spin-cast quite easily into
optically-clear, device-quality films. By eye, these 1000-2000A small-molecule films
were as macroscopically-uniform as the best polyurethane films. High-quality spin-cast
films were also realized with other related Ru(bpy) 32+-based small molecules. The device
fabrication and performance of four such small molecules (Scheme 6-1) is described
below.
H
HO
/
eR*
Ni
N\
(PF6)2
NO
N
/
6~) 2
NesN(PF
N;
,\
N
III
II
o
0
OMe
MeO
OMe
MeO
S(PF 6-)2
NI
N
(PF 6 )
o
/
R
N1
N
OMe
OMe
N
N
MeO
N
0
N
MeO
V
IV
Scheme 6-1. Structures of the four Ru(bpy) 32 ,-based small molecules employed in light-emitting devices.
2. Results
2.1 Lumophore II-- Small-Molecule Diol
The liquid ECL work of Tokel and Bard6 (among others) demonstrated that RuL 32+
emitters could produce high luminance and do so efficiently, rapidly, and at low driving
165
800
1500
-700
S--Current
-
1000
600
: 0
-I500
- Luminance
.
m
400
300
500
200
0
0
2
MW -W
4
-100
.
.-
0 0:
0
1
6
8
10
12
Voltage (V)
Figure 6-1. A device based solely on lumophore II produces high brightness during a
0-10V scan.
voltages. Until lumophore II was employed, however, every attempt to reproduce these
four performance features simultaneously in solid-state devices had failed. As control
experiments, the first devices based on neat, spin-cast films of II were operated in the
same way as the polyurethanes into which II had been incorporated. That is, the applied
voltage was ramped from OV up to 1 OV and held there. Surprisingly, the emitted light
intensity rose immediately into the 700-800 cd/m 2 range at 9-10V (Figure 6-1). The
devices failed before the maximum voltage was reached, i.e., before the devices reached
the point at which a constant 10V bias would have been maintained.
The device
efficiency was on the order of 0.1%. Comparable results have been reported for
suitably-derivatized PPV devices (e.g., MEH-PPV, a red light-emitter), and more recently
for a trivalent europium complex 7 sandwiched between thermally-evaporated
triphenylenediamine and Alq 3 layers. However, there are no other examples in the
166
literature of neat, spin-cast, non-polymeric organic or metal complex thin-film devices
which produce red or red-orange luminance of this intensity.
In these early experiments, lumophore II presented itself as a system that had the
potential to operate at substantially lower voltages than the parent polyurethanes with
minimal charging time. This lumophore could be driven to unprecedented luminance
levels and operate at impressive efficiencies for a spin-cast small molecule.
Its
incorporation into working devices was straightforward, as II could be simply spin-cast
and metallized.
Like the polyurethanes, deposition of films of II required no multi-step
operations, using other polymers to tailor the ITO substrate prior to deposition.
Lumophore II was much simpler to synthesize than the polyurethanes, however, making
it considerably more attractive for future study and practical application in real-world
devices. The impact of different applied voltages, subambient temperature, an alternative
biasing schemes, substrate processing schemes, and different counterions on devices
based on II is described below.
2.1.1. General Performance Features
Lumophore II continued to operate at high brightness and high efficiency with
minimal charging time when the voltage was lowered to 2.5-5V. As shown in Figure 6-2,
luminance levels in the range of 600-1000 cd/m 2 were realized at 4 -5V, and luminance in
excess of 200 cd/m 2 was attainable at 3V.8 Even at 2.5V, the emitted light intensity was
still in the 50 cd/m 2 range. The efficiency, however, varied inversely with the applied
voltage, reaching a maximum of 1% at 2.5V from a low of 0.2-0.3% at 5V (Figure 6-3).
In addition, the charging time increased with decreasing voltage. Whereas the light
output reached into the 1000 cd/M 2 range within 2 minutes at 5V, maximum luminance
167
was not realized for roughly 1 hour at 2.5V. At lower voltages, however, the long-term
operating stability was significantly better than at higher voltages. Device stability is
defined in terms of the time required for the emitted light intensity to drop to one- half of
1000
0..................................
o
800
-
5V hold
4V hold
3V hold
V
2.5V hold
-
j600-0
600
200
--
0
0
10
30
20
40
50
60
time (min.)
Figure 6-2. Luminance vs. time profiles for lumophore II at 2.5-5V. (Reprinted with permission from J.
Am. Chem. Soc., Volume 121, Number 14, Pages 3525-3528, Copyright 1999 American Chemical
Society.)
its maximum value (tj12), relative to the time that the bias was first applied. The average
t1/2 at 5Vwas 5 minutes, whereas the 3V-tl/ 2 was on the order of 2 hours.
Clearly, additional modification of the ITO substrate was not required for high
performance with lumophore II, as it was with I. However, when the substrate was
immersed in 1M HCl prior to film deposition, the emitted light intensity from II at 33.5V increased to the 300-700 cd/M2 range. 9 A concomitant increase in device efficiency
was also noted, with efficiencies in the 1% range attainable at these voltages.
168
' '
a
-
1.2
0
5V hold
.
V
4V hold
3V hold
2.5V hold
0.8
0.6
--
0.4
0=
--
r
0.2
0
0
10
30
20
40
50
60
time (min.)
Figure 6-3. Efficiency vs. time profiles for lumophore II at 2.5-5V. (Reprinted with permission from J.
Am. Chem. Soc., Volume 121, Number 14, Pages 3525-3528, Copyright 1999 American Chemical
Society.)
Alternatively, when the ITO was pre-coated with roughly 30A of poly(ethylene
imine) (PEI) prior to spin-casting, the performance of lumophore I actually worsened
somewhat. Preliminary work showed that these devices produce luminance in the 100
cd/M2
range at 3V and 300-400 cd/M2 range at 4V. At neither voltage did the device
efficiency ever exceed 0.3-0.5%.
When operated at subambient temperatures, devices based solely on II exhibited
much longer response times. For example, at 180 K, a device ramped to and held at 5.5V
required 14-15 minutes for turn-on. This is shown in Figure 6-4. The luminance then
rose quickly over the next 8 minutes to a maximum of ca. 900 cd/m 2 at 22 minutes, then
decreased just as quickly, dropping to one-half that intensity 3 minutes later. (This
169
0.4
1000
0.35
0.3
- --- C-
Efficiency
-
Luminanc
I-
800
0.25
600
0.2
400
0.15
0.1
200
0.05
0
0
0
5
10
15
20
25
30
35
time (min.)
Figure 6-4. Low- temperature (180 K) efficiency and luminance vs. time profiles for a device based on
lumophore II.
experiment was performed in a cryostat at near- liquid nitrogen temperatures. In this setup, the device was positioned farther from the photodiode than in the normal
arrangement, such that the photodiode collected less of the emitted light. The luminance
shown in Figure 6-4 is corrected accordingly.) The efficiency also increased over time
(tracking the increasing light output very closely), and reached a maximum of 0.4% at 22
minutes, as well. 10
2.1.2. The Voltage-Pulsing Scheme and Structural Modifications
2.1.2.i. Instantaneous High Brightness
Assuming an electrochemical mechanism of operation, the long charging period
associated with II is attributable to the time required for redistribution of the PF 6 ~
counterions. Even at ambient temperatures, this redistribution is slow at low voltages,
but both faster and more extensive at higher voltages. As such, the maximum luminance
170
is higher and is reached more quickly at 5V than at 2.5V. Application of higher voltages
also accelerates degradative processes (discussed in Chapter 5), however, accounting for
the more rapid device failure. In between these two voltage extremes, at a potential of
2
3V, devices based on II exhibited high brightness (>100 cd/M , the brightness of a
conventional CRT screen) and respectable efficiencies (0.6 - 0.7%). Nevertheless, the
time to maximum luminance at 3V was still on the order of 20 minutes, an unacceptably
long waiting period for most practical display applications.
If high voltage was applied only briefly, however, to accelerate the charging-up
process, and the voltage then reduced, high brightness could be realized at low voltage
within only a few seconds (Figure 6-5).9 With such a protocol, the need for a long
600
5
-r-Vo-- Voltage
500
4
-
Light
400
300
e
2200
100
0
0
0
20
40
60
80
100
time (sec.)
Figure 6-5. With an initial higher-voltage pulse, high brightness could be realized at low voltage (2.8V)
almost instantaneously with devices based on lumophore II. (Reprinted with permission from J. Am.
Chem. Soc., Volume 121, Number 14, Pages 3525-3528, Copyright 1999 American Chemical Society.)
171
charging time could be circumvented. In Figure 6-5, the device was pulsed from OV to
4.4V for 6 seconds, during which time the emitted light intensity can be seen to rise
quickly to the 500-600 cd/m 2 range. The applied voltage was then lowered to 2.8V, and
the light output dropped to 100 cd/m 2. As such, high brightness was attainable at low
voltage in less than 10 seconds.. Without the initial high-voltage pulse, luminance of this
intensity would likely not have been realized for several tens of minutes at 2.8V.
2.1.2.2. The Use of Different Counterions
All of the experimental results presented thus far have been based on devices of II
as the PF 6 ~salt. When chloride counterions were employed instead [i.e., when lumophore
iv (Chapter 2, Section 1.3.1) was used, prior to the anion metathesis which afforded II],
it was not possible to spin-cast homogeneous films of II. The spin-casting solvent used
was 2-methoxyethanol, in which the Clf salt appeared to be quite soluble. The resulting
solutions afforded cloudy, unusable films, which, in the past, have yielded high-current,
non-emissive devices.
In the continuing attempt to circumvent the long charging time exhibited by the PF 6 ~
salt of II, the counterions were changed to species which might make any pre-imposed
"conditioning" more permanent, as was attempted with the polyurethanes. That is, an
alternative to the voltage-pulsing technique would be to charge the device up at a suitable
voltage, then "lock" the counterions in place. With such a scheme, the counterions would
be unable to return to their OV positions when the bias was removed, and the device
would not have to be re-charged the next time it was operated. In this way, the device
would produce light instantaneously whenever a voltage was applied, irrespective how
much time had passed since it was last operated.
172
(The reader should note that the
voltage-pulsing technique described in Section 2.1.2.1 was most effective with devices
on which a bias had been recently imposed.) This was the theme underlying the work of
Maness et al.2'3 and that of Elliott et al. 4 In their work, diode-like behavior and
instantaneous responses were observed after films which had been charged up at room or
elevated temperatures (or while solvent-swollen) were cooled down (or dried).
To this end, an attempt was made to substitute the acrylate anion for the chloride
anion in the C1 salt of II (lumophore iv) via metathesis. Ideally, the acrylate counterions
would have redistributed as the device was charged up, after which the counterions could
have been thermally-polymerized to form poly(acrylic acid) in situ. As discussed in
Chapter 5, a polymeric counterion would have had great difficulty assuming an
equilibrium OV distribution throughout the film when the bias was removed, thereby
making the initial conditioning relatively permanent. The metathesis was unsuccessful,
however, as the acrylate salt of II appeared to be water-soluble, even on refrigeration of
the aqueous solution.
A somewhat more fruitful attempt involved the use of tetraphenylborate (BPhW)
counterions. It was thought that these anions, due to their bulk, would have similar
difficulty returning to a OV distribution. The metathesis reaction to replace the Cl- with
the BPh- counterions was successful, and the tetraphenylborate salt of II precipitated
cleanly from aqueous solution. When DMF was used as the spinning solvent, spin-cast
films of this complex, II-TPB, appeared to be macroscopically highly-uniform. As
shown in Figure 6-6, luminance levels of 1 cd/M2 or less are typical of devices based on
II-TPB. In Figure 6-6, the voltage was initially ramped to and held at 3V. As the device
did not turn on at 3V, the voltage was manually stepped to and held at higher voltages
173
0.3
2000
8V
0.25
1500 -V
E
.
6V
)0.
-0
0.15.n
1000
0.1 2'
0.05
0
0
10
12
16
14
18
20
time (min.)
Figure 6-6. Current density (open circles) and luminance (closed circles) vs. time profiles for a device
based on II-TPB, the tetraphenylborate salt of lumophore II. The device was initially ramped to and held
at 3V. When no light was emitted after almost 15 minutes, higher voltages were applied.
after 14-15 minutes. At 6V and 8V, the luminance can then be seen to initially rise, then
drop off.
The device efficiency rose to a maximum of roughly 0.1% at these higher
voltages.
Even when a II-TPB device was held at 7V for 3 hours and at elevated
temperature (11 00 C), there was no light emission. Given the low emitted light intensity
and difficulty involved in charging up these devices, the possibility of permanent
conditioning using II-TPB was not pursued further.
2.2. Devices Based on Underivatized Ru(bpy)32
It, was found that the benefits of using small-molecule II in light-emitting devices,
rather than polymers, could be extended to the even simpler lumophore III, underivatized
Ru(bpy) 32 . As outlined in the Experimental section, the hexafluorophosphate salt of
this well-studied emitter is accessible from commercially-available material, via a simple
anion metathesis reaction. When sandwiched between ITO and aluminum electrodes, III
174
In
.
.
I
I
I
I
I
.
- I
I
I
I
I
I
I
I
-
600
I
I
I
I
5V hold
-
4V hold
3V hold.
I
.
]hold
___2.5V
500
I
I
0
*
2
I
400
300
Q
200
100~
0
0
10
30
20
40
50
60
time (min.)
Figure 6-7. Luminance vs. time profiles for underivatized Ru(bpy) 3(PF 6)2 (lumophore III) at 2.5-5V.
(Reprinted with permission from J. Am. Chem. Soc., Volume 121, Number 14, Pages 3525-3528,
Copyright 1999 American Chemical Society.)
produces luminance levels in the range of 600-700 cd/m 2 at 4-5V(like lumophore II), and
400 cd/m 2 at 3V. As shown in Figure 6-7, at 2.5V, the emitted light intensity had not
reached a maximum after roughly 1 hour, at which point the luminance was only 20
cd/rn or so. The device efficiency was somewhat lower than that of II in the 2.5 - 5V
range, and reached a maximum of 0.3% at 4V (Figure 6-8). At 2.5V, the voltage at
which II was most efficient, III exhibited an efficiency of 0.05% after roughly 1 hour.
2.3. Devices Based on Ester-Functionalized Ru(bpy)32 +
A methyl ester derivative of 2,2'-bipyridine was generated en route to the target
dihydroxymethylated ligand used in the synthesis of lumophore II. When this esterified
ligand was refluxed with commercially- available Ru(bpy) 2 C 2, lumophore IV resulted
175
0.35
0
0.3
4V hold
--
--
-
*Q
0.2
0.25
e
V
--
3V hold
2.5V hold
--
"g 0.15
o
5V hold
*-
0.1
0.05
0
0
10
30
20
40
50
60
time (min.)
Figure 6-8. Efficiency vs. time profiles for underivatized Ru(bpy) 3(PF6 )2 (lumophore III) at 2.5-5V.
(Reprinted with permission from J. Am. Chem. Soc., Volume 121, Number 14, Pages 3525-3528,
Copyright 1999 American Chemical Society.)
(Scheme 5-1). The PF6~ salt of this material was also soluble in pyridine, and afforded
highly-uniform films when spin-cast onto ITO substrates. As shown in Figure 6-9,
devices based on IV produced luminance levels of the roughly same magnitude as did
those based on II and III. Luminance levels in the range of 900 cd/m 2 were realized at
5V, and 50 cd/M 2 at 2.5V, with light emission of intermediate intensity at voltages in
between. As with lumophores II and III, the devices biased at lowest voltage (2.5V)
operated at the highest efficiency (0.5%) The 5V efficiency for IV was comparable to
the efficiencies of its non-esterified analogs .
Two differences in the device behavior of this esterified emitter are. noteworthy,
however. On inspection of Figure 6-9, the most obvious difference is the significantly
176
1 1
a1
1000
V
800
6
-
*
0
1.
2.5V hold
3V hold
4V hold
5V hold
600
5
400
0
-
200
0
-
100
10
1
1000
time (min.)
? 0.5
V
.
0.4
a
0
2.5V hold
3V hold
4V hold
5V hold
0.3
M
~0.2
-
0.1
1
100
10
1000
time (min.)
Figure 6-9. Luminance and efficiency vs. time profiles for lumophore IV at 2.5-5V. (Reprinted with
permission from J. Am. Chem. Soc., Volume 121, Number 14, Pages 3525-3528, Copyright 1999
American Chemical Society.)
longer charging time. Whereas, at 3V, devices based on II reached maximum luminance
in 20 minutes,
lumophore IV required more than 250 minutes to charge up. The
increased charging time is attended by a considerable enhancement in device lifetime,
177
however. Whereas II and III exhibited 3V-tU2 values of 1.5-3 hours, the 3V-t/ 2 for IV
was on the order of 30-70 hours. In addition, unlike II and III, the color of the emitted
light from IV was red
(Xmax~
700 nm), rather than red-orange
(Xmax~
630 nm) (cf.
Chapter 2).
2.4. Devices Based on a Ru(bpy)32+- Derivative Tris-chelated with Esterified
Ligands
Given the stability enhancements and technologically- interesting bright red
emission afforded by lumophore IV, increasing the number of ester-functionalized
ligands chelating the central metal ion was the next logical step. In addition, Wacholtz
and Auerbach have reported that the ethyl ester analog of lumophore V (Scheme 5-1)
exhibits a solution PL efficiency of roughly 30%." (For reference, the PL efficiency of
underivatized Ru(bpy) 32+ is in the range of 5%.12) Needless to say, in light of their
report, lumophore V was a very attractive synthetic target for use in solid-state devices.
At spin-cast film thicknesses comparable to those of the previous three small molecules ,
however, devices based on V were characterized by high current, low luminance levels,
and poor efficiency.
When the film thickness was increased to 1800A, luminance as high as 400 cd/M2
was realized with lumophore V at 5V, and 50 cd/m 2 at 3V (Figure 6-10).
As with
lumophore IV, the light emitted from V was red in color. The device efficiency was not
quite as high as expected, reaching a maximum of 0.3% at 5Vand 0.4% at 3V. An
unexpected benefit to fabricating devices based on V, however, was the greatlyimproved device stability. This material exhibited a 3V-t1/ 2 of 240 hours. As shown in
Figure 6-10, the 3V-charging time was roughly 20 hours (1200 minutes).
178
100
_----
80
3V hold
5V hold-
60
S40
20
0
0
1 104
5000
time (min.)
1.5 104
0.5
--- o--
0.4
3V hold
5V hold-
0.3
0.2
c
0.1
S0
0
1 104
5000
1.5 104
time (min.)
Figure 6-10. Luminance and efficiency vs. time profiles for devices based on lumophore V.
3.
Discussion
3.1. Device Fabrication
As noted earlier, it is remarkable that device- quality films of these small molecules
can be fabricated by spin-casting. Spin-casting is a technique that has traditionally been
179
reserved for macromolecules. When small molecules are incorporated into light-emitting
devices, vacuum deposition techniques are usually employed. (When an attempt was
made to thermally-evaporate lumophore II, however, the material appeared to
decompose, and turned completely black on heating without film deposition.") This
work demonstrates that small-molecule Ru(bpy)32+ derivatives are amenable to
contemporary high-throughput, industrial fabrication schemes, making them
technologically-interesting, as well as scientifically important.
Films of more planar lumophores, such as anthracenes, are generally plagued by
defects due to crystalline packing. It is likely that the steric bulk of Ru(bpy)32+-type
lumophores like II - V (in which the three ligands are not coplanar) is instrumental in
frustrating crystallization. As such, there is no need for any auxiliary functional groups,
such as the sulfonate groups employed in lumophore I (Chapter 4). Given the problems
encountered with the films of the chloride salt of II (lumophore iv), however, it appears
that the bulkiness of the ligands is a necessary, but not sufficient condition for highquality films. Assuming complete solubility of the lumophore in the spin-casting solvent,
it may be that the identity of the counterions is also important in preventing extensive
aggregation in spin-cast films. Possibly, if the counterions (e.g., C-, other halides), are
small enough to pack neatly in between the ligands and are not sufficiently large so as to
keep the metal complexes apart, more extensive aggregation can occur.
3.2 General Small-Molecule Device Performance
In addition to their ease of fabrication, the small-molecule Ru(bpy) 32 -based devices
embody many desirable properties. Between the four of them, lumophores II - V
demonstrated that small-molecule Ru(II) devices can operate at low voltage (2.5-5V)
180
with high brightness (~1000 cd/m 2) and high efficiency (1%), with minimal charging
time (2 minutes at 5V). In addition, they exhibit high operating stability (t1 /2 of 240
hours).
At low voltages, Jernigan et al. have suggested that current flow through
considerably thicker poly [Os(III/II)(bpy)2(vpy)2(C04)2.5 films is particularly low due to
back-diffusion of charge carriers." When the driving force for electron tranport is lower
(i.e., at low voltage), it was noted that an electron is more likely to hop back from a new
Os(II) site to the Os(III) site is just left due to electrostatic attractive forces. The
likelihood of such a back-reaction is lower at higher voltage, when the concentration
gradients are steeper. This might also be true of the small-molecule light-emitting
devices investigated here.
As noted earlier, the polymeric systems described by others produced luminance
which was not visible by eye, or they were difficult to work with and their performance
in light-emitting devices was not reproducible. The polyurethanes required voltages
outside of the 2.5-5V range for light emission on a reasonable timescale. Although they
employed lumophore II as their active component, the polyurethanes required high
voltage (>7V) to impart sufficient mobility to the PF6~ counterions for light emission.
(Presumably, the electrochemical properties of II remained largely unperturbed by
polymerization. The 10V multiple-iteration experiments, in which the turn-on voltage
always settled to 3V, supported this contention.) The small-molecule systems, on the
other hand, can be operated at near-redox potentials due to their considerably higher ionic
conductivity. Applied voltages in the 2.5-5V range are sufficient to both establish the
mixed-valent Ru(III/II) and Ru(II/I) gradients and induce extensive counterion
redistribution.
181
The performance of II can be improved significantly by immersing the ITO substrate
in acid for a short period of time, prior to spin-casting. Other workers have described
similar increases in luminance and efficiency in LED's with such processing. Exposure
to acid may serve to etch away residual organic photoresist leftover from the ITO
patterning process. If not removed, this photoresist would serve as a barrier to the
injection of charge into the film. In addition, residual photoresist could give rise to large
variations in the thickness of the film cast onto the ITO, affording high electric field
concentration in thinner areas. The likelihood of current leaking through these thinner
areas is high, which would lower the overall device efficiency. [One might have thought
that treating the un-etched ITO with PEI would serve to overcoat any non-uniformities on
the ITO surface (e.g., due to residual photoresist). No performance improvements were
noted, however.]
The luminance and efficiency enhancements have also been ascribed
to etching and roughening of the ITO by the acid, which etching increases its surface
area, and, in turn, increases the area over which it contacts the spin-cast film. As such,
the number of carriers injected into the film per unit time is higher, as is the likelihood of
their recombination, resulting in higher efficiency and higher brightness.
It is quite surprising that, in the nearly thirty years of literature covering Ru(bpy) 32+
photo- and electrochemistry, its superior solid-state light-emitting properties have never
been reported. Many have undertaken to synthesize emissive polymers and multilayer
films based on this lumophore, but discussions of its solid-state properties in neat films
are conspicuous by their absence. It is technologically-significant that this well-known,
virtually off-the-shelf lumophore can also be easily fabricated into high-brightness
devices. As such, Ru(bpy) 32+would be a very promising candidate for incorporation into
182
the new generation of commercial electrochemically-based light emitting devices.
Unlike conjugated polymer LEC's, however, devices based on lumophore III require no
additives (e.g., PEO, lithium triflate salts, etc.) for high performance. The efficiency of
devices based on this lumophore can presumably be improved by acid-etching the ITO,
as observed with lumophore II. To this end, increasing the thickness of the thin film
might also be successful, providing better surface coverage, and minimizing leakage
current. Acid-etching the ITO also served to increase the intensity of light emitted from
II at all voltages employed, and similar enhancements are to be expected from III.
That the emission from lumophores IV and V was red, rather than red-orange, was
unexpected. This observation is supported, however, by the solution PL spectrum of IV,
in comparison to that of II. The wavelength of peak intensity for the former (kmax,IV =
705 nm) is red-shifted from that of the latter by roughly 60 nm. Other workers have also
noted this shift in emission wavelength for the ethyl ester analog of V (discussed below),
relative to that of II.11,15 Assuming that the excited states which give rise to ECL are the
same as those responsible for PL, the red color emitted from the solid-state devices is
understandable. In actuality, a red-shift in solution PL is observed for many Ru(bpy)
32+
derivatives. That is, regardless of whether Ru(bpy) 32+ is functionalized with electron
donor groups or acceptor groups, a red-shift is the norm. Red light emission is also
technologically-significant. If Ru(bpy) 32+derivatives were incorporated into full-color
flat-panel displays (e.g., television screens), emitted light of a more pure red color would
be of greater value than red-orange in mixing with green and blue light emitters, as more
new colors could be produced.
183
3.3. Operant Mechanisms in Small-Molecule Devices
A discussion of the physical processes at work in these small- molecule devices is
illustrative here. The behavior of these devices at near-redox potentials is significant in
that it can be correlated more closely with liquid ECL work than the high-voltage
polyurethane results. The device response of lumophore II will be taken as representative
of the small-molecule systems studied. At 3V, for example, the current flowing through
the device increases with time, as does the emitted light intensity. This is shown in
Figure 6-11. Typical of electrochemical cells, the current reaches a maximum, levels off,
then begins to decrease. In most electrochemical cells, however, the current increases
with increasing voltage, and the voltage sweep rate is low enough that the cell can
250
, , ,
, ,*
,
*
,
,
, ,
100
80
e~200
150
-
..
-60
5 100
40 4
50
20
0
0
0
50
100
150
200
time (min.)
Figure 6-11. Luminance and current density as a function of time for a device based on lumophore II.
respond with minimal delay. In these "ion-budgeted" thin films, however, the potential is
often held fixed. In such experiments, the current increases with time due to an increase
184
in the magnitude of the potential drop across the electrode-film interfaces. As discussed
earlier, the potential drops increase as the counterions redistribute to form ionic space
charge layers at the interfaces.
As these space charge layers form, the electric field
becomes localized more and more at the interfaces, and less and less in the bulk of the
film. Overall, this process makes charge injection easier and easier with time; hence, the
increase in current.
It is reasonable to believe that the current reaches a maximum and begins to drop as
one of the electrodes starts to become concentration- polarized. This is typical of purely
electrochemical cells. When this maximum current is reached in such cells, there are no
more redox-active moieties in contact with the electrode. Charge injection at the
interface, which occurs under influence of the electric field, is faster than charge transport
in the bulk, which occurs mostly by diffusion. Overpotentials (potentials greater than the
putative difference between the oxidation and reduction potentials of the emitter) are
applied in this study. As such, it may be that the applied electric field is never
completely localized to the electrode film interfaces. However, it is probable that the
electric field strength is much higher at these interfaces than in the bulk, such that the rate
of carrier transport is higher in the former. Consequently, the reduced Ru(I) species at
the cathode-film interface must wait for more Ru(II) species to be delivered from the
bulk, and the number of charge carriers which pass through the device with time
decreases.
During voltage sweeps in typical electrochemical cells, however, the current rises to
a maximum value, the so-called "limiting current", then drops and reaches a steady-state
value, indicating that concentration-polarization of one of the electrodes is complete. No
185
additional current flows at higher voltages. A true limiting current is not observed in the
solid-state Ru(II)-based devices studied here, however. At 3V, a maximum in the current
is reached with time. However, when voltages in excess of 3V are applied, there is a
concomitant increase in maximum current flow. In addition, after reaching its maximum
value over time at 3V, for example, the current drops steadily thereafter without a lower
limit. The absence of an upper limit to the current can be explained, in part, on the basis
of very thin films in use. The electric field across the device is considerable, and may
never be completely confined to the interfaces by the ionic space charge layers. At 3V,
the electric field strength would be 3 x 105 V/cm across a 1 oooA film (compared to fields
as small as 6 x 103 V/cm in the IDA devices of other workers with 5 tm- spaced fingers).
As such, there may be some residual electric field within the bulk of the film, such that
carriers in that region are transported along both concentration and voltage gradients
simultaneously.
Since carrier transport in the bulk would no longer be completely
diffusion- controlled, an upper limit to the current would not be expected at higher
voltages. As described for the polyurethanes, which were generally driven at IV, there
may be a gradual conversion with time from a pure concentration gradient-driven mode
of operation to one dominated by the voltage gradient in these small-molecule devices,
particularly at large overpotentials. Hence, no limiting current would be expected.
The current also continued to drop without limit, however, after reaching its
maximum value at 3V. This is uncharacteristic of electrochemical processes in which
stable one-electron redox products are involved. As mentioned in Chapter 5, several
workers have described the chemical instability of the Ru(I) state. Were this species to
engage in parasitic side reactions (e.g., with the aluminum cathode, adsorbed water
186
molecules, etc.) it might be rendered permanently non-electroactive.
This non-
conductive material would serve as a barrier to electron injection. Some of the applied
potential would then drop across this thin electrically- insulating barrier, and the effective
bias imposed on the rest of the film would be somewhat lower. As a result, a progressive
decrease in current flow would be expected as the amount of this insulating material at
the interface increased with time.
Even before the decrease in current flow, the operating efficiency (ratio of light
output to current) of the device began to decrease. While direct evidence for the
formation of an insulating barrier was not obtained, the polyurethane work demonstrated
that when the efficiency decreased, the effect was permanent.
Briefly, when a
polyurethane device was held at high voltage beyond the time at which the efficiency
began to drop, it never recovered its original efficiency, even in subsequent runs to the
same voltage (Chapter 5, Figure 5-13). This suggested that, even before the highluminance, catastrophic failure event occurred (Figures 5-10 - 5-12), the redox-active
material was beginning to degrade.
In addition, the multiple-scan experiments
demonstrated that the point at which the device operates at maximum efficiency is
roughly coincident with that at which the device turn-on voltage settles to 3V. At this
point, the thin film device begins operating like a liquid cell. The difference between the
solution oxidation and reduction potentials (the so-called, "redox potential") for
Ru(bpy) 3 , as measured initially by Tokel and Bard,6 is 2.7V, which is very close to the
3V observed as the final turn-on voltage. This parallel between liquid- and solid-state
devices correlated the downturn in efficiency with a materials parameter, suggesting that
degradation is not due to some external factor.
187
It is reasonable to assume that this correlation also holds for the small molecules,
i.e., that some irreversible process has begun to compete successfully with normal
electrochemical processes when the device efficiency decreases. A side reaction of Ru(I)
with some atmospheric contaminant, for example, would certainly qualify as such a
process. In addition, without violating the constraints of the "ion budget", a Ru(I) species
might have time to take on another electron and be reduced to Ru(0) under these high
electric fields before it was carried away into the bulk. The over-reduction product,
Ru(0), is an even more unstable moiety than Ru(I), and could also participate in
irreversible processes. These processes would be accelerated at voltages higher than 3V,
and decelerated at voltages lower than 3V. Hence, the inverse relationship between
applied bias and device efficiency.
3.1.
Temporal Response
Lumophore II alone embodies many of the desirable attributes outlined earlier, and
is the most promising derivative in this study.
Without acid- etching of the ITO
substrate, at 3V, II produces luminance of over 200 cd/m 2 at 0.6% efficiency, but with a
20-minute charging time.
Several attempts to circumvent this long induction period
were attempted. With regard to the voltage-pulsing protocol, it should be noted that
different high and low voltages were required for different devices than for that profiled
in Figure 6-5. That is, devices which had been previously operated were more amenable
to the technique than "fresh" devices on which a potential bias had never been imposed.
The nature of these variations in efficacy of the protocol were not fully investigated.
Device-to-device film thickness variations (±100A), for example, could be the basis for
one explanation. That is, a thicker film would require higher voltages and/or longer
188
pulses for light output comparable to that of a thinner film. The electric field across a
thicker film would be smaller at a given voltage than that across a thinner film.
Consequently, the required counterion redistribution would be slower, the redox
chemistry induced in a given time less extensive, and the light output lower.
Nevertheless, it is clear that the long charging time observed by the normal, slower
charging-up route is not an insurmountable obstacle.
Virtually-instantaneous high
brightness has been demonstrated with this voltage-pulsing protocol, and this protocol
could easily be utilized in practical Ru(bpy)
2
+- based
flat-panel displays
An alternative scheme was to charge the device up once, then preserve this
conditioning chemically using the large tetraphenylborate counterion in II-T PB. It
appeared, however, that this counterion was perhaps too bulky even for initial
redistribution to occur under applied bias. Mechanistically, this demonstrates the
significant role of the counterions in the operation of solid-state RuL32 devices. The low
luminance and sluggish response of the II-TPB devices supports an electrochemical
mode of operation in which the response time of the device is determined by the rate of
counterion redistribution. Given the possible short response-time benefits (in postconditioning runs) of using a large, bulky counterion, however, further work to
chemically-modify lumophore II along similar lines is warranted.
The four lumophores discussed in this chapter can be divided into two groups with
regard to their temporal device response at 3V, for example: (1) II and III, the fastresponding systems with modest operating lifetimes, and (2) IV and V, the long-lived,
albeit more initially-sluggish systems. The difference in charging behavior for the two
groups is most starkly profiled in Figure 6-12, which compares the 3V luminance-time
189
120
100
-I
S80
60
40
20
20
0.001
0.01
0.1
1
10
100
1000
time (hrs.)
Figure 6-12. Luminance vs. time profiles at 3V for devices based on spin-cast films of II and IV of
comparable thickness (1700 and 1800A, respectively).
profiles of II and V. Whereas II normally reached maximum luminance in roughly 20
minutes, V required some 20 hours to charge up. One might be tempted to attribute this
effect to film thickness differences, given that the thickness of the film of V used in
Figures 6-10 and 6-11 was roughly twice that of the films based on II normally used
(1800 vs. 900A).
Figure 6-11 shows that even when a film of II with a thickness
comparable to that of V was used (-1700A), however, the 3V charging time only
increased by 5-10 minutes. Further, the devices profiled in Figure 6-9 were based on
films of lumophore IV that were actually 100-200A thinner than those of II and III.
Nevertheless, the pronounced increase in time to maximum luminance for the former is
unmistakable. Hence, the significantly longer charging time observed for the group of
esterified lumophores cannot be explained on the grounds of diminished electric field
strength alone.
With film thickness eliminated as a significant consideration, this increased charging
time was most unusual. One possible supramolecular explanation is that esterifying the
190
bipyridine ligands in IV and IV effectively made the thin-film matrix less ionicallyconductive, almost as if it were truly polymeric. Indeed, the 3V charging time for V is
more like that of the polyurethane poly(II-12) (Chapter 5, Sect. 3.2.3). It is possible that
the increased polarity of the lumophore, relative to unmodified Ru(bpy) 3 (PF 6)2 , for
example, gave rise to more intermolecular interaction, thereby rigidifying the matrix
somewhat. Invoking the "dynamic percolation" model (Chapter 5, Sect. 1.3), a lessflexible matrix would have longer characteristic renewal times. That is, new free volume
sites into which a mobile ion could hop would become available less frequently. Hence, a
longer charging time would be observed. The 3V current density, however, in devices
based on V was higher than in those based on the other hexafluorophosphate- based small
molecules with equivalent film thicknesses. It is possible that this was due to poor film
quality, and that much of the current flow is leakage current. If this is the case, then it
suggests that the extent of intermolecular interaction is minimal. Alternatively, the polar
ester groups may interact with the redistributing counterions via dipole-dipole attraction,
making ionic migration under the applied field more difficult.
While the supramolecular properties of IV and V are nebulous, their properties at the
molecular level are easier to reckon with. The most relevant molecular-level explanation
for the lengthy charging time relates to the change in the oxidation potential of
Ru(bpy) 32+ on esterification. Elliott and Hershenhart reported that the redox potentials of
the ethyl ester analog of V were significantly different from those of underivatized
Ru(bpy) 32 . 16 Specifically, the reduction potential for the analog was shifted positively
some 0.38V, and the oxidation potential some 0.3V in the same direction. That is, the
electron-withdrawing esters made Ru(bpy) 32+easier to reduce and harder to oxidize. It is
191
likely that, in making the metal center more electron-poor (i.e., more difficult to oxidize),
the presence of the esters also increased the binding energy between the metal and the
counterions.
As such, it would be more difficult for the counterions to liberate
themselves from the lumophore when an electric field is applied.
Counterion
redistribution would be hindered, and the charging time increased. This effect would be
enhanced in V, which bears six ester groups, relative to IV, which bears only two. Using
FT-IR spectroscopy, other workers have shown that anions were more tightly-bound to
their cations in polymer films which exhibited lower ionic conductivity."
Specifically,
the vibrational modes of polyatomic anions (e.g., BH 4 , NCS~) were more disturbed when
there was strong ion-pairing.
Infrared analysis of the PF6 ~ion absorption in II vs. V
films, for example, has not borne this out, however.
It may be that the
hexafluorophosphate anion is too weakly bound to the cationic metal center in either of
these emitters for significant spectral differences to be observed.
4.
Summary
This small-molecule study demonstrates several key points. First, it is clear that
elaborate film fabrication techniques (e.g., multi-step layer-by-layer deposition) are not
required for high performance with Ru(bpy) 32+ emitters, as the straightforward spincasting approach is sufficient.
One might have concluded this already from the
discussion of the polymer-based devices (Chapter 5).
However, the results of this
chapter show that it is not even necessary to polymerize these metal complexes, as
commercially-available Ru(bpy) 32+ outperforms the poly(II-n)'s with respect to
maximum luminance and required driving voltages. Device efficiencies comparable to
those of the best polyurethane devices (and approaching those of the best conjugated
192
polymer LED's) can be realized with small molecules. Further, this study shows that,
contrary to popular belief, spin-casting need not be reserved for macromolecular
materials.
Small-molecules of sufficient steric bulk afford high-quality films by
themselves, without benefit of a polymeric backbone or pre-deposited polymer
"platform". Like the polyurethanes, reactive cathode materials (such as calcium) are not
required for high brightness and efficiency.
Since they can be driven at low voltage,
however, the small-molecule devices show significantly-enhanced operating stability,
relative to their polymeric counterparts.
At the molecular level, with proper ligand functionalization, the color of the light
emission can be tuned from red-orange to a more technologically-useful red color at
comparable brightness and efficiency. The counterion in use is clearly very important, as
well, as the device response-time is dependent on the ability of the counterions to
redistribute in the thin-film matrix. Large or tightly-binding counterions exhibit low
mobility, and in some cases, may not redistribute even under high electric fields and with
considerable thermal assistance (i.e., when the device is operated at high temperature).
When smaller counterions are employed, redistribution is more facile, and can be
accomplished at near-redox potentials at ambient temperature. As such, it is clear that the
operation of small-molecule solid-state devices closely parallels that of liquid, solutionbased ECL cells.
Even with more-mobile counterions, the solid-state device response- time is still too
long for many practical applications. The response-time can be decreased with an initial
short-duration, high-voltage pulse, however, after which these devices can be operated at
low voltage. In this manner, brightnesses comparable to those of CRT screens can be
193
realized in small-molecule devices within only a few seconds. With overall performance
of this caliber, ultrathin small-molecule Ru(bpy)3 2+-based devices may one day replace
their larger, bulkier CRT counterparts in many real-world applications.
194
4. References
1.
Handy, E.S.; Abbas, E.D.; Pal, A.J.; Rubner, M.F. Proc. SPIE- Int. Soc. Opt. Eng.
1998, 3476, 62..
2. Maness, K.M.; Terrill, R.H.; Meyer,T.J.; Murray, R.W.; Wightman, R.M. J.Am.
Chem. Soc. 1996, 118, 10609-10616.
3. Maness, K.M.; Masui, H.; Wightman, R.M.; Murray, R.W. J.Am. Chem. Soc. 1997,
119, 3987.
4. Elliott, C.M.; Pichot, F.; Bloom, C.J.; Rider, L.S. J.Am. Chem. Soc. 1998, 120, 6781.
5. Lee, J.-K.; Yoo, D.; Handy, E. S.; Rubner, M.F. Appl. Phys. Lett. 1996, 69 (12),
1686.
6. Tokel, N. E.; Bard, A. J. J Am. Chem. Soc. 1972, 94, 2862.
7. Kido, J; Hayase, H; Hongawa, K.; Nagai, K; Okuyama, K. Appl. Phys. Lett. 1994,
65, 2124-2126.
8. Handy, E.S.; Pal, A.J.; Rubner, M.F. J Am. Chem. Soc. 1999, 121, 3525-3528.
9. This work was done in collaboration with Dr. Amlan J. Pal, Indian Association for
the Cultivation of Science, Department of Solid-State Physics, during his visit to
MIT.
10. This work was done in collaboration with Ms. Erika Abbas, Department of Materials
Science and Engineering, MIT.
11. Wacholtz, W.F.; Auerbach, R.A.; Schmehl, R.H. Inorg. Chem. 1986, 25, 227.
12. Roundhill, D.M. Photochemistry and Photophysics of Metal Complexes. Plenum
Press, New York: 1994.
13. This work was done in collaboration with Ms. Kathy Vaeth, Department of Chemical
Engineering, MIT.
14. Jernigan, J.C.; Surridge, N.A.; Zvanut, M.E.; Silver, M.; Murray, R.W. J. Phys.
Chem. 1989, 93, 4620-4627.
15. Cook, M.J.; Lewis, A.P; McAuliffe, G.S.G.; Skarda, V.; Thomson, A.J. J.Chem.
Soc. Perkin Trans. II 1984, 8, 1293.
16. Elliott, C.M.; Hershenhart, E.J. J Am. Chem. Soc. 1982, 104, 7519-7526
17. Dupon, R.; Papke, B.L.; Ratner, M.A.; Whitmore, D.H.; Shriver, D.F. J. Am. Chem.
Soc. 1982, 104, 6247.
195
Chapter 7. Conclusions
1. General Features of Solid-State Ru(II)-based Light-Emitting Devices
It has been known for quite some time that the room-temperature solution
photoluminescence (PL) wavelength for Ru(bpy) 32+and its derivatives generally falls in
the range of 605-700 nm, i.e., red-orange light is emitted. Any functionalization of the
bipyridine ligands generally results in a red-shift of the PL from that of unmodified
Ru(bpy) 3 2+. As is usually observed for conjugated polymer LED's, the spectrum of solidstate RuL 32+electrogenerated light emission tracks that of its PL very closely.
Light is
generated in these devices by disproportionation of Ru(II) and mutual annihilation of the
resulting one-electron redox species, as shown below:
Ru(I) + Ru(III)
--
Ru(II)+ Ru(II)*
-*
2 Ru(II)+ hv.
Emission is dominated by triplet excited-state relaxation of electrons from ligand n* to
metal d-orbitals.
At the outset of this work, the sluggish response of these materials to applied bias was
a surprise, and represented a significant departure from LED operation. Several reasons
for the long charging time were considered. The reports of other workers suggested that
small-scale rotation and/or translation ("physical diffusion") of the lumophores to some
optimal orientation in the thin film occurs during device operation. Most of the evidence
suggests that the redistribution of the counterions associated with these lumophores is
more important for device operation. The work of Pei et al (and others) indicated that
ionic redistribution and the ensuing space-charge layer formation were central to the
performance enhancements in conjugated-polymer LEC's over LED's.'
196
A consideration of the small-molecule devices is undertaken first.
That these
complexes can be spin-cast into high-performance devices is remarkable, to begin with.
As noted earlier, spin-casting is a technique traditionally reserved for polymeric
materials. It is presumed that the steric bulk of these lumophores (with ligands projecting
out from the metal center in three dimensions) and their counterions is sufficient to
frustrate crystalline packing. Spin-casting is a simple, rapid technique currently in use in
semiconductor device fabrication lines. That these Ru(II)-based materials are amenable
to this technique makes them even more attractive for application in practical flat-panel
displays.
The first system investigated was the sulfonated Ru(phen')
2
+ complex,
I. Maximum
luminance in the range of 20 cd/m 2 (and later as high as 200 cd/M2 ) was realized at an
efficiency of 0.05% from spin-cast films. At the molecular level, it is surprising that this
material did not perform better. In solution, the ECL efficiency of the non-sulfonated
analog is roughly 24%. The high efficiency enhancement has been attributed to the
presence of the phenyl rings, which increase the extent of conjugation. In addition, the
bridging carbons of the parent phenanthroline ring system prevent rotation of the pyridyl
moieties. However, it may be that formation of the emissive excited states requires
rotation of these phenyl rings about the carbon-carbon bonds attaching them to the
phenanthroline ring, as demonstrated by Damrauer et al. 2 It is possible that the close
proximity of the lumophores in the solid-state precludes such rotation, effecting a
decrease in the observed efficiency. Further, elemental analysis showed that, in what is
probably the actual structure of lumophore I, the chloride counterions of one molecule
are largely replaced by the sulfonate groups of another (Chapter 4, Figure 4-4). Hence,
197
it might be expected that rotation of the phenyl rings to which the sulfonate groups are
attached would, indeed, be difficult. If local mobility of the lumophore is generally
important for electron transport, this mobility would be minimized in such an ionically"crosslinked" matrix. Any chloride counterions which remain might themselves undergo
oxidation, and the byproducts of this reaction might be detrimental to device
performance.
The best "heterostructured" Ru(II)/ PPV devices operated at 0.1-0.2% efficiency and
produced luminance levels of 70-90 cd/m 2 . That is, the efficiency improved by an order
of magnitude from that of the devices based on I without the PPV-based platform. The
reason for this improvement is not well-understood. Use of the platform may have
improved the overall quality of the thin film by minimizing non-productive "leakage"
current, although later work with thicker I-only films (which would have suffered less
from defects) did not exhibit higher device efficiencies. With very few (if any) chloride
counterions remaining, formation of an ionic space charge layer at the ITO- film interface
would be minimal in I-only films, and oxidation of the lumophore would be difficult. It
may be that PPV compensates for this effect, facilitating oxidation of the lumophore by
some unknown, perhaps non-electrochemical mechanism. Indeed, the "charging"
phenomenon was not observed in "heterostructured" devices, suggesting a mechanism
more akin to that of mobile ion-free LED's.
Later work with different platform
architectures failed to yield any further improvements.
-The
work with the bipyridine ligand-based small molecules demonstrated that no
auxiliary functionalization of the Ru(II) emitter is needed for high-performance devices,
as even unmodified Ru(bpy)3
+
(lumophore III) outperforms lumophore I, despite the
198
250
.
, .
II
200
111...
1
. * .111.
.
1
a
III
+ -IV
V
'-150
100
-
50
0
10
1
100
1000
104
time (min.)
Figure 7-1. A comparison of the luminance-time profiles at 3V for four of the hexafluorophosphate
counterion-based small-molecule lumophores studied.
differences in ECL efficiencies. Lumopore III and the polyurethane diol monomer,
lumophore II, have very similar structures, and, accordingly, devices based on these
materials perform in much the same way. For example, when held at 5V, both 11 and III
produce luminance levels in the range of 800-1000 cd/M2 within 2 minutes, then rapidly
become non-emissive thereafter. At lower voltages, the red-orange luminescence
(Xmax
~
630-640 nm) is not as intense, but the efficiency and long-term operating stability are
enhanced. As shown in Figure 7-1, lumophore II produces luminance in the range of 200
cd/m 2 at 3V. At 2.5V, this emitter can be operated at a maximum of 1% efficiency.
Esterification of Ru(bpy)
2
affords devices which produce more technologically-
useful red light (kmax ~ 700 nm), as work with lumophores IV and V demonstrated. The
former is chelated by one esterified bipyridine ligand, and the latter is chelated by three.
Lumophore IV produces luminance in the 90-900 cd/M 2 range and operates at 0.2-0.5%
efficiency at 3-5V. Luminance as high as 400 cd/M2 can been realized with lumophore V
at 5V (0.3% efficiency), and 50 cd/m 2 at 3V (0.4% efficiency). While the luminance and
199
efficiency values for these emitters are comparable to those of non-esterified II and III,
the response time is considerably longer for IV and V at all voltages tested. Whereas II,
for example, reached maximum luminance after 20-30 minutes at 3V, the 3V-charging
time for IV is 1.5 -5 hours, and 20 hours for V. The slower response in the esterified
emitters is attributable to sluggish counterion redistribution, but the exact reasoning is
unclear. Electrochemical measurements 3 made with a closely-related ethyl ester analog
suggest that ester groups, which are directly attached to the conjugated pyridine rings,
make the metal center more electron-poor in IV and V.
As discussed earlier, this
probably results in tight ion-pairing between the dicationic metal center and the
counterions, making separation of the two more difficult. Hence, the ionic mobility is
lessened, which would serve to lower the measured ionic conductivity.
Like the monomer on which they are based, the polyurethanes produce high
luminance, but require high voltage to do so. Luminance levels in the range of 50-500
cd/M2
were realized with these materials in 30 minutes or less at 1OV. Also like
lumophore II and its small-molecule analogs, the long charging time required by the
polyurethanes is attributable, in large part, to the need for counterion redistribution. The
1OV luminance and efficiency vs. time profile is shown in Figure 7-2 for poly(II- 12), the
polyurethane with the 12-methylene interlumophore spacer. This profile can be divided
into three distinct regions over the useful life of the device. In Region 1, the device is
charging up, and, at the beginning, has not engaged in any degradative processes. In
Region 2, the efficiency reaches a maximum and begins to decrease. A few minutes
later, the slope of the luminance-time curve begins to decrease. For poly(II-4), the
200
magnitude of the light output continues
to increase, and that of poly(II-12) actually
begins to decrease slightly, reminiscent of the
small- molecule luminance-time curves at
1.4
M
1.2
250
2
-
200
~,0.8
150
02
S0.6
T
100
V
0.4
0.2
I
0
0
'' '
0
5
10
"I
Qs
i
15
20
50
0
time (min.)
Figure 7-2. Demarcation of three distinct
regions in the luminance (closed circles)
and efficiency (open
cirlces) vs. time profile for poly(II-12).
This device was ramped to and held at
1OV. The times to
maximum luminance and efficiency shown
here are slightly less than average for this
material (perhaps due
to film thickness variations), but the profile
is qualitatively the same as that shown in Chapter
10, for example.
5, Figure 5-
low voltage (Figure 7-1). The mechanism
for these changes is taken to be the same
in all
materials, however. While the
counterions are still continuing
the redistribute,
degradative mechanisms which shut down
the luminescence begin to compete more
successfully with the induction of more electrochemistry.
2. Unifying Themes
The high-brightness, catastrophic failure
event in Region 3 of Figure 7-2 is not
unique to the polyurethanes. As shown
in Chatper 6, Figure 6-1, devices based
on
201
lumophore II exhibited similar luminance behavior when biased to 1 OV. Although it was
actually the voltage which was varied in Figure 6-1, the ionic conductivity of the thin
film was still too low for the emitted light intensity to keep up with the scan rate
employed. The potential drop across the electrode-film interfaces never maximized at a
given voltage before the voltage was increased a few seconds later. Hence, Figure 6-1 is
actually a plot of light output vs. a superposition of voltage and time. The ionic mobility
increased with voltage, and the rate at which it increased was finally commensurate with
the scan rate at 8-9V, when the luminance increased from less than 50 cd/M2 to over 700
cd/M 2.
This phenomenon is demonstrated more explicitly in Chapter 6, Figure 6-4, in
which the mobility of the counterions is reduced by operating the device at subambient
temperature (180 K). When the device was ramped to and held at 5.5V, it did not even
turn on for 10 minutes.
The device eventually reached maximum luminance and
maximum efficiency after 20 minutes. Operating the device at low temperature did not
affect its overall performance, as its maximum luminance and efficiency are comparable
to what would be observed at room temperature at 5V (cf. Figures 6-2 and 6-3). It
appears that only the response time is affected.
For comparison, a similar room-
temperature device based on II reached maximum luminance and efficiency in less than 2
minutes at 5V. The parallel with the polyurethane devices is striking. Whether ionic
mobility is reduced chemically (affording a hydrogen-bonded, polymeric matrix) or
thermally (by operating at low temperature), a sudden increase in luminance is observed
when large overpotentials are applied. The occurrence of the high-brightness failure
event shown in Region 3 of Figure 7-2 is clearly not restricted to polyurethanes.
202
The parallel is not perfect, however. Although both the low-temperature smallmolecule device and the polyurethane devices exhibit a precipitous increase in luminance
up to a maximum, the point at which the device efficiency reaches its highest value,
relative to the maximum luminance point, appears to be more system-dependent. That is,
the time between the maximum efficiency and maximum luminance is a function of the
emitter in use. In a device based on II, the luminance reached a maximum (and the
device failed) only 0.5V after the efficiency did so during a single 0-10V scan. The
polyurethanes behave quite differently.
In the 0-10-OV multiple-scan poly(II-12)
experiments, the luminance did not reach a maximum until some 10-15 runs after the
efficiency. The constant-voltage experiments are even more demonstrative. The 5V
room-temperature luminance and efficiency values for II (taken from Figures 6-2 and 63) are overlaid in Figure 7-3, below. In the ambient and subambient work with II, the
efficiency maximized just 6 seconds before the luminance did so at 5.5V.
In the
polyurethane devices, however, the efficiency was maximal some 10-20 minutes prior to
the luminance, as evidenced by Figure 5-10. Granted, the applied voltage was much
higher in the polyurethanes, but this effect was even more pronounced when lower
voltages (7-9V) were applied.
From an electronic perspective, electron self-exchange becomes more difficult at low
temperature because the electrons are less able to surmount the activation barrier to
hopping. This barrier is higher in polymers due to the presence of the intervening
electrically- insulating aliphatic spacer between the redox sites. With regard to the
counterions, these observations agree well with the "dynamic percolation" model, in
203
0.3
1000
0.25
-
-
-2800
t
0.2
600
0
400
e
0.15
0.1200
- 0.05
0
0
0
1
2
3
4
5
time (min.)
Figure 7-3. Luminance and efficiency vs. time profiles for lumophore II at 5V. The two profiles track
each other almost perfectly, unlike those of the polyurethanes.
which ions hop from vacancy to vacancy in their migration through the thin film. When
these vacancies become available less frequently, redistribution of the counterions is
hindered. It has been established that the times to maximum efficiency and luminance
are coupled most directly to the rate of counterion redistribution. However, the extent of
temporal resolution between these two events varies from material to material. This
suggests that the counterion activity underlying device behavior is coupled to some other
mechanism, such as degradation. An understanding of exactly how the two are coupled
would be instrumental in designing future RuL3 -based light-emitters with even better
performance.
As a first step toward gaining such an understanding, a comparison of the resolution
between maximum luminance
and maximum
204
efficiency
for four of the
hexafluorophosphate-based small molecules is made next. The time between the two
maxima (hereafter, the "temporal resolution") for these lumophores increases in the order
III < II < IV < V. For example, while the 3V efficiency and luminance vs. time profiles
are nearly superposable for lumophore II, the two maxima are separated in time by some
45 minutes in a thinner film of IV. Interestingly, the measured ionic conductivity for II,
IV, and V decreases in the same order. That is, the temporal resolution varies inversely
with the conductivity. The conductivity decrease in IV and V was explained on the
grounds of tight ion-pairing between the counterions and the metal center. Aside from its
two hydroxymethyl groups, lumophore II is very structurally-similar to III. While the
ionic conductivity of III was not determined in this study, the resolution is some 4
minutes greater in II than in III at 3V. Given the trend observed with lumophores II, IV,
and V, it is reasonable to assume that III is slightly more conductive than II. It is not
expected that the hydroxymethyl groups on lumophore II greatly perturb its oxidation
and reduction potentials from that of underivatized Ru(bpy) 3 2 +, as may be the case with
IV and V.
Perhaps it is hydrogen bonding between the hydroxyl groups on different
molecules of II (or between the lumophores and the counterions) that makes
redistribution more difficult. Nevertheless, the relationship between ionic conductivity
and temporal resolution of the two maxima is clear.
With regard to the polyurethanes, maximum efficiency is reached when the turn-on
voltage settles to 3V, as discussed in Chapter 5. At this point, the operation of the solidstate devices closely resembles that of liquid ECL cells. That is, the total potential drop
across the two electrode-film interfaces is sufficiently large to allow for oxidation,
reduction, and light emission at near-redox potentials in some areas of the film. (It was
205
postulated that the thinner-film regions would emit light first.) This is taken to be true for
all of the hexafluorophosphate-based small molecules, too. As counterion redistribution
continues, however, the potential drops continue to rise, and it is likely that the instability
of reduced Ru(I) [and perhaps that of over-reduced Ru(O)] begin to play a role in
decreasing device efficiency. The luminance continues to increase however, suggesting
that the Ru(III/II) and Ru(III) concentration gradients are still developing in other areas
of the film, producing more and more light.
The time required for development of these gradients increases significantly with
decreasing ionic conductivity. This was manifest both in the absolute time to maximum
efficiency and luminance values for these materials. Figure 7-4 shows that the times to
1500
Efficiency
Luminance
....................... .... ........................
1000
V
500
IV
II~
0
3.9 10 -9
6.8 10
-9
1.7 10
-8
Ionic conductivity (S/cm)
Figure 7-4. The difference between the time to maximum efficiency (depicted in black) and
time to
maximum luminance (in grey) increases with decreasing ionic conductivity, as shown here
for some the
hexafluorophosphate-based small-molecule systems. For the least-conductive system (lumophore
V), for
example, maximum efficiency is reached after -350 minutes, but maximum luminance is not
reached for
some 1140 minutes.
206
efficiency and luminance maxima have different dependencies on the ionic conductivity
of the matrix, however. Mechanistically, some regions of the film at the cathode-film
interface begin to degrade (and perhaps acts as Ru(II)* quenching sites) as other regions
are just beginning to emit light. It may be that, the longer the device takes to charge up
completely (i.e, the longer it takes for the electrochemistry to reach full maturity across
the entire electrode-film interfaces), the more time these degraded sites have to develop,
as well, and act to lower the overall device efficiency.
On these grounds, one might be tempted to suggest that the as-cast films with lower
ionic conductivity simply have more thickness variations than those with higher
conductivity.
This might afford a decrease in efficiency prior to a maximum in
luminance due to degradation in thinner- film regions.
microscopy
Preliminary atomic force
(AFM) measurements, however, indicate that spin-cast films of II and IV,
which exhibit very different charging times (20 and 250 minutes, respectively, at 3V),
have comparable surface roughness.4 This suggests that the temporal resolution between
efficiency and luminance maxima for the various emitters is materials-dependent, not a
device-processing artifact.
An alternative explanation for the resolution differences stems from measurements of
the capacitance of poly(II-n) devices, as described by Handy et al.5 These measurements
were performed using an ac bias imposed on a dc voltage.
When a de voltage of
significant magnitude was applied, the capacitance immediately dropped several tens of
picofarads, and returned to its initial value when the dc bias was removed minutes later.
No device turn-on occurred for several seconds when the dc bias was first applied.
Nevertheless, the initial drop in capacitance was attributed to injection of charge carriers.
207
If charge injection truly occurred, then some Ru(II) lumophores must have been oxidized
and/ or reduced, even if only at the electrode- film interfaces. This suggests that gross
counterion redistribution is not absolutely necessary for charge injection. Indeed,
Jernigan and Murray have proposed that the formation of Os(II/I) gradients in
poly[Os(bpy)2(vpy)2](C104) 2 films is less dependent on counterion mobility than that of
the Os(III/II) gradients. 6 Their results suggest that the requirement for global charge
balance prior to the occurrence of redox chemistry, while largely met, is not absolute in
these mixed-valent film devices.
If redistribution is absolutely required, it may be that the mobility of the counterions
is significantly higher at the electrode-film interfaces than it is in the bulk. Indeed, other
workers have noted that the Tg of polymer films is often lower at the surface than it is in
the bulk. As such, reduction of the lumophores directly in contact with the cathode could
occur very quickly. In either case, it appears that unstable Ru(I) species [and perhaps
even more reactive Ru(O) species] can be formed quite readily at the cathode-film
interface, regardless of the measured ionic conductivity of the thin-film matrix. The
lower the bulk ionic conductivity, the more time will be required for the redox gradients
to mature to the point where maximum light emission is realized.
Hence, as the ionic
conductivity is lowered, more time becomes available for degradation to progress, and
the device efficiency will reach a maximum sooner than the luminance and begin to drop.
This is precisely what is observed, as shown in Figure 7-4.
3. Design of Future High-Performance RuL3 2 - based Light-Emitting Devices
In light of the foregoing, several conclusions can be drawn about RuL3 2 +devices with
an eye toward their optimization.
With regard to fabrication, there is no need to
208
synthesize polymeric RuL32+emitters if thin films are to be spin-cast, given that small
molecules afford high-quality films. In fact, viable small-molecule lumophores can even
be obtained from commercially-available Ru(bpy) 32+ after a simple anion-metathesis
reaction.
High performance was not realized with previously-reported oligomeric or
polymeric Ru(bpy) 32+- based films because the films were generally too thick for the low
voltages employed. The thin films of the small molecules in this study, in particular,
were sufficiently ionically-conductive that near-redox potentials could be applied,
yielding high brightness and efficiency.
Moreover, small-molecule demonstration
devices could be built from simple hobby shop parts, and driven with a 3V watch battery.
Such devices could even be operated for several hours in air, without encapsulation, at
high brightness.
Other devices were stored in air for several months, and still exhibited
high brightness and efficiency when operated in air.
The AFM measurements mentioned earlier did reveal what appeared to be "craters"
in the topology of a film based on lumophore II.4 These defects were roughly 2000A in
diameter and 60-70A deep. It is possible that AFM images of all of the spin-cast films
used in this study would reveal similar pinholes.
While high efficiencies were still
realized with such films, modification to the device processing scheme may afford even
better performance. These pinholes may have formed when residual spin-casting solvent
in the freshly-cast film was quickly removed at high temperature under vacuum. Perhaps
a gentler film drying/ annealing process would yield devices with efficiencies
approaching the 5% realized in liquid ECL cells.
In addition, preliminary work showed
that etching the ITO afforded considerably higher luminance and efficiency values. This
should be adopted as a general practice.
209
In addition to exhibiting high brightness and efficiency, the ideal emitter would also
respond quickly with short charging times, all at low voltages. An added advantage
would be long-term operating stability. With regard to the molecular design of new
RuL 32+- based emitters, two general observations can be made:
1 . The
most luminescent materials
are those in which redistribution
of
electrochemically-inert counterions is the fastest and most extensive.
2. The most stable materials are those in which redistribution is slower and least
extensive.
Given that lumophore I had few mobile anions (and that any anions which were
present would have been unstable chloride ions), it is difficult to relate its properties to
those of the hexafluorophosphate group. At 1OV, the polyurethanes produced higher
luminance levels than another other polymeric Ru(bpy) 32+materials reported at that time.
To some extent, the polyurethane devices were driven at 1 OV as a matter of convenience,
not necessity. At a near-redox potential of 3V, the maximum luminance from a poly(II12) device was very low (ca. 0.05 cd/m 2 ), and required a long charge-up time (roughly
13.5 hours).
the
t1/ 2
Device lifetime at 3V was longer, as well, for this device, as indicated by
value of > 65 hours. This was a general trend for the hexafluorophosphate
group-the longer the 3V-charging time, the better the operating stability. At 3V,
lumophore II produced luminance levels as high as twice that of a CRT screen within 2030 minutes. The esterified lumophore V produced 50 cd/M2 or so at 3V, and exhibited a
charging time on the order of 20 hours. The device lifetime, however, varied from 1-2
hours in the former to 10 days in the latter.
210
The electrochemical measurements of Elliott and Hershenhart 3 suggested that
esterification made Ru(bpy) 32+easier to reduce. If the formation of unstable Ru(I) and
Ru(O) species is to be cited as the primary mechanism for device failure in these systems,
lowering the reduction potential should have made for a less stable material, not a more
stable one.
Practically speaking, it is true that one would have to endure a lengthy wait
for polyurethane- or esterified lumophore-based flat-panel displays to reach useful
brightness. The mobility of the hexafluorophosphate counterions is simply too low in
these materials. With regard to operating stability, however, retardation of counterion
redistribution is beneficial. Even though the putative reduction and over-reduction
potentials are smaller in the esterified lumophores than for unmodified Ru(bpy) 32+, more
time is required for the potential drop to reach these potentials at the cathode-film
interface. Hence, degradation occurs more slowly.
In this light, lumophore IV, for example, represents a good system for further study,
as its stability is enhanced relative to unmodified Ru(bpy) 32+, and its luminance appears
to be uncompromised by its long response time. If lumophore IV, however, were to be
implemented in practical display applications, one should bear the temporal resolution
between its luminance and efficiency maxima in mind. That is, although its 3V-t 1 2 is in
the range of 30-70 hours, its useful life in battery-operated applications (e.g., cellular
phones, personal pagers, etc.) might be much shorter. After 80 minutes or so, its
efficiency begins to decrease (Chapter 6, Figure 6-9), so the current density required to
maintain a given luminance level would increase thereafter.
Aside from the drop-off in efficiency, the long charging time remains an obstacle to
use of some these Ru(bpy) 32+- based emitters in practical displays. Even short-duration,
211
low-brightness (-10 cd/m 2 ) applications like watch-face backlighting require
instantaneous luminance. As described in Chapter 6, lumophores II and III produce
high-brightness within a few seconds after an initial high-voltage pulse. In general, this
technique could not be applied to the slower-responding systems, although a device based
on IV produced >10 cd/M2 at 2.8V after a short 6V pulse. As mentioned above for the
polyurethanes, it appeared that the high voltage required to accelerate counterion
redistribution was also required to keep them in their redistributed positions. While
lumophores II and III are more amenable to high-voltage pulsing, the technique was
generally more successful when the devices had been recently operated, and less so with
"fresh" devices. In addition, the pulsing would have to be repeated every time the device
was operated.
Of those studied here, it seems that no single emitter embodies all of the properties
needed for commercialization of RuL 32 -based devices. The quintessential device would
produce red light and exhibit long-term stability, like IV and V, and respond quickly to
applied bias without loss of efficiency prior to maximum luminance, like II and III. In
addition, it would not have to be "re-charged" after sitting at OV for an extended period
of time. To this end, there are several possible approaches. A film, in which lumophore
IV was dispersed in a matrix of lumophore II, could be spin-cast onto acid-etched ITO.
Given that the excited state for lumophore II is higher in energy than that of IV, energy
transfer might occur from the former to the latter, affording red, rather red-orange
emission. Other electrochemically-stable counterions could also be employed, such as
the tetrafluoroborate (BF 4 -) or triflate (SO 3CF3~) anions. As discussed briefly in Chapters
212
5 and 6, these anions would bind more weakly to the cationic metal center, and might be
faster-charging as a result.
While most of the hexafluorophosphate-based small molecules employed in this study
performed better than the lumophore IT-based polymers, there may yet be some
advantage to adding a macromolecular component to these devices. One of the original
ideas driving the development of polyurethanes was that "segmented" copolymers could
be synthesized. These copolymers were not pursued because of the sluggish response
and high-voltage requirement of the "hard segment-only" poly(II-n)'s.
However, if
RuL 32+-based copolymers were developed, spin-cast films of these materials could be
initially charged-up under voltage at high temperature when the thin film was
homogeneous, as discussed earlier. Once counterion redistribution was optimal, the
devices could be cooled to room temperature under bias, such that the material would
phase-separate into "hard", light-emitting domains and "soft", ionically-conducting
domains, and the bias then removed. The resulting microstructure would resemble that of
the PPV- PEO blends used in ion-containing, conjugated polymer LEC's. A RuL 32+_
based device processed in this way might then exhibit rapid response times, as the ions
would only have to redistribute through the conductive domains, and would not have to
interact with the rigid, cationic lumophores.
Alternatively, a neat film of a polymerizable derivative of IV (similar to that
employed by Elliott et al. 3) could be spin-cast onto acid-etched ITO, then polymerized at
the proper time to reduce counterion mobility. Continuing the preliminary work with
acrylate (Chapter 5) and tetraphenylborate (Chapter 6) counterions, this polymerizable
derivative could also be functionalized with crosslinkable counterions (e.g.,
213
vinylsulfonate ions) to more securely lock the counterions in place once they had
rearranged themselves in the film. The redistribution of such bulky anions to their
original positions would be difficult at OV (i.e., when the device was turned off). The
OV-redistribution of a thermally- or photochemically-polymerized polvanion would be
even more difficult. Further, by polymerizing the counterions at the proper time, their
redistribution could be strategically halted when the device efficiency was maximal. This
would ensure that the magnitude of the potential drop across the cathode-film interface
did not exceed the putative reduction potential for the lumophore in use, and that
degradation would not ensue.
In whatever form it next appears, it is clear that the well-studied Ru(bpy) 32+complex
is, in fact, a viable solid-state lumophore, and should be numbered among recognized
light-emitters. Ru(bpy) 32+ derivatives show great promise as the active materials in
future flat-panel display applications. Outside of this and other studies performed by the
Rubner research group, little success has been realized in making practical RuL 32+-based
devices, however. As the mechanisms underlying their operation become more clear and
relevant processing parameters better understood, great improvements in devices based
on this exciting class of materials are expected.
214
References
1. Pei, Q.; Yu, G.; Zhang; Yang, Y.; Heeger, A.J. Science, 1995, 269, 1086.
2. Damrauer, N.H.; Boussie, T.R.; Devenney, M.; McCusker, J.K. J. Am. Chem. Soc.
1997, 119, 8253-8268.
3. Elliott, C.M.; Hershenhart, E.J. J. Am. Chem. Soc. 1982, 104, 7519-7526.
4. The AFM measurements were made by Mr. Jonas Mendelsohn, Department of
Materials Science and Engineering, MIT.
5. Handy, E.S.; Abbas, E.D.; Pal. A.J.; Rubner, M.F. Proc. SPIE - Int. Soc. Opt. Eng.
1998, 3476, 62. The capacitance measurements were made by Dr. Amlan J. Pal,
Indian Association for the Cultivation of Science, Department of Solid-State Physics,
during his stay at MIT.
6. Jernigan, J.C.; Murray, R.W. J. Am. Chem. Soc. 1987, 109, 1738-1745.
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