Reducing Recombination in Organic Photovoltaics
by
Jason M. Sussman
B.S., B.A., The University of Texas at Austin (2007)
M.Phil., Cambridge University (2010)
Submitted to the Department of Materials Science and Engineering
in partial fulfillment of the requirements for the degree of
Master of Science in Materials Science and Engineering
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
September 2011
c Massachusetts Institute of Technology 2011. All rights reserved.
Author . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Department of Materials Science and Engineering
August 17, 2011
Certified by . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Marc A. Baldo
Associate Professor of Electrical Engineering and Computer Science
Thesis Supervisor
Certified by . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Harry L. Tuller
Professor of Ceramics and Electronic Materials
Accepted by . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Christopher A. Schuh
Chairman, Department Committee on Graduate Students
2
Reducing Recombination in Organic Photovoltaics
by
Jason M. Sussman
Submitted to the Department of Materials Science and Engineering
on August 17, 2011, in partial fulfillment of the
requirements for the degree of
Master of Science in Materials Science and Engineering
Abstract
In this thesis, I consider two methods to improve organic photovoltaic efficiency:
energy level cascades and promotion of triplet state excitons. The former relies on
a thin layer of material placed between the active layers of a photovoltaic device to
destabilize excitons. If the interfacial material is chosen properly, it can significantly
improve device performance. The second method proposes to use quantum mechanical
rules to reduce the rate of loss in organic photovoltaic devices. An electron in a
triplet state cannot directly drop to the ground state by emitting a photon, so triplet
excitons have longer lifetimes, and are thus more likely to diffuse to an interface to
be dissociated. But this work suggests that, once they are at the interface, they are
less likely to be dissociated than a singlet.
Thesis Supervisor: Marc A. Baldo
Title: Associate Professor of Electrical Engineering and Computer Science
Acknowledgments
I’ve been lucky to work with a great group of people. Confining myself to examples
from this project: Priya Jadhav taught me how to make and measure devices; Tim
Heidel drove a large part of this work; Carlijn Mulder helped me resolve a pressing
yield issue; Phil Reusswig helped me machine a vital piece of equipment; Varunesh
Singh streamlined the sample-making process; David Ciudad streamlined cryostat use;
Nicholas Thompson improved my optical measurement technique; Matthias Bahlke
offered valuable editorial suggestions; Jiye Lee helped me better understand the dissociation process; and my advisor, Marc Baldo, provided valuable guidance and mentorship from start to finish.
Outside of the lab, I cannot forget to acknowledge BP and the MIT Energy Initiative, who helped fund my tenure here; and my family, who never fail to encourage
me.
Contents
1 Introduction
17
1.1
The case for solar power . . . . . . . . . . . . . . . . . . . . . . . . .
17
1.2
The varieties of solar power generators . . . . . . . . . . . . . . . . .
19
1.3
Outline of the remainder of the thesis . . . . . . . . . . . . . . . . . .
21
2 Theory
23
2.1
Photovoltaic fundamentals: the pn junction . . . . . . . . . . . . . .
23
2.2
Organic photovoltaics . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
2.2.1
CT state destabilization by charge separation . . . . . . . . .
28
2.2.2
Spin dependence of recombination . . . . . . . . . . . . . . . .
30
3 Experimental Methods
33
3.1
Device fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
3.2
Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
37
3.3
Materials selection . . . . . . . . . . . . . . . . . . . . . . . . . . . .
38
3.3.1
Cascaded energy level alignment devices . . . . . . . . . . . .
38
3.3.2
Devices to probe spin dependence of recombination . . . . . .
39
4 Results and Analysis
4.1
4.2
43
Interfacial layer devices . . . . . . . . . . . . . . . . . . . . . . . . . .
43
4.1.1
m-MTDATA/rubrene/PTCBI devices . . . . . . . . . . . . . .
43
4.1.2
CuPC/ClAlPC/C60 Devices . . . . . . . . . . . . . . . . . . .
45
Spin-dependent recombination . . . . . . . . . . . . . . . . . . . . . .
48
7
5 Conclusions
53
References
54
8
List of Figures
1-1 (a) Energy demand is expected to grow significantly over the next
quarter-century: the increase in energy demand from 2007 to 2035 is
greater than the total coal consumption in 2007 [2]. Figure from [2].
(b) Yearly energy from solar irradiation to land versus energy from
other sources. Fossil fuels and uranium expressed as total reserves,
renewables by yearly potential. Figure from [11]. . . . . . . . . . . . .
18
1-2 Chart of the best attained efficiencies for various PV technologies, by
Wolden et al. [16]. They refer to DSSCs as DSCs, which have an
efficiency above OPVs but below inorganic PVs. . . . . . . . . . . . .
20
2-1 (a) When a pn junction is created, mobile charges (circles) diffuse to
areas of lower concentration, leaving behind immobile ions (squares).
This develops an internal electric field that ultimately balances the diffusion potential. Figure from [24]. (b) A pn junction exposed to light.
Excited electrons far from the junction will recombine before they can
diffuse to the junction; charges close to the junction will be accelerated by the change in potential energy. Electrons move “downhill,”
and holes move “uphill,” creating a conventional current to the left.
Figure from [24]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
24
2-2 The response of an ideal pn junction to changes in incident light intensity. As the rate of electron photoexcitation is increased in a pn
junction, the reverse current increases proportionally. The higher the
proportionality, the better the solar cell. Figure from [24]. . . . . . .
9
25
2-3 Diagram of a heterojunction. Black dots represent electrons, white
dots holes. When excited, electrons will move from their position in the
highest occupied molecular orbital (HOMO) to the lowest unoccupied
molecular orbital (LUMO), leaving a hole behind. The two are bound
by the Coulomb potential, and are unlikely to dissociate unless they
diffuse to a heterojunction with a HOMO–LUMO split smaller than
that of the material on either side. If they reach such a heterojunction,
it is energetically favorable for the exciton (regardless of whether it was
generated in the donor or the acceptor) to dissociate at the interface,
leaving the electron on the acceptor side and the hole on the donor
side and increasing the likelihood of the two dissociating entirely [26].
But the two are still be bound and may still recombine [36]. . . . . .
27
2-4 Diagram of a three-layer CELA device. The material at the left serves
strictly as a donor, and the material at the right as an acceptor; with
a HOMO and LUMO intermediate between those of the materials to
either side, the middle material serves as an acceptor for the material
at the left and a donor for the material at the right. There are thus two
heterojunctions serving to dissociate excitons. Note that the HOMO–
LUMO offset is greater at each heterojunction than it would be in a
bilayer device made with the two outer materials. . . . . . . . . . . .
10
28
3-1 (a) Pattern of ITO on the glass substrate. Black represents the ITO,
white the glass. Testing contacts will be placed on the five squares
around the edge of the substrate. The lollipop shape allows electrical
contact to be made at the edge while the devices sit in the center;
the four corners will be coated with silver, but if any flakes off in the
process of making contact, the ITO underneath maintains electrical
conductivity. (b) Substrate after deposition of active materials and
BCP. Green represents the deposited organic materials; the mask is a
photo negative of the square, allowing the materials to be deposited
as pictured and nowhere else. (c) Substrate after deposition of silver
(the mask is, again, a photo negative of the pattern depicted). The
silver deposition defines the active area of four devices (the circles in
the middle), and creates silver contacts that can easily be connected
to the testing apparatus and silver wires connecting the contacts with
the device. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
34
3-2 Diagram of vacuum sublimation purification at the beginning of the
process. An outer quartz tube serves to hold a vacuum, while segmented inner tubes allow the material to be readily extracted postpurification. The material to be purified is placed in one end of the
inner tubes as shown, evacuated to a pressure lower than 10−5 Torr,
and subjected to a thermal gradient. The material end of the tube is
kept above the material’s sublimation temperature, while the far end
of the tube is set to as low a temperature as possible, so impurities with
higher or lower sublimation temperatures are deposited away from the
deposits of the desired material. . . . . . . . . . . . . . . . . . . . . .
11
34
3-3 (a) Diagram of mask assembly. Substrates sit in the depressions on the
piece depicted on the left; a mask that is the negative of the pattern in
Fig. 3-1(b) or (c) sits over the holes on the right-hand piece. The righthand piece fits over the left-hand piece, holding the substrates in place
and masking them as desired. (b) Rotating substrate holder schematic.
The apparatus is held above the materials sources by bolting the top
plate to a flange in the evaporation chamber. An axle is secured to the
top plate by a nut, but is allowed to rotate freely. A motor attached
to the top plate turns a gear pinned to the axle, while the substrate
holder from (a) is secured to the rotating axle by nuts above and below. 35
3-4 Exaggerated depiction of the shadow effect. When material is deposited on a substrate at a constant angle, it will pile up according
to the angle of incidence. Another material deposited in the same
fashion will pile up in different places: the contact deposited on top
may then be separated from the contact on the bottom by a resistor,
not a diode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36
3-5 Energy levels of m-MTDATA/rubrene/PTCBI interfacial device [80,
81, 82]. Dashed lines represent uncertainty in energy levels; x was
varied from 0 to 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
39
3-6 Energy levels of CuPC/ClAlPC/C60 interfacial device. CuPC’s HOMO
is 5.2 ± 0.15 eV and its LUMO is 3.4 ± 0.5 eV [78]; ClAlPC’s HOMO
is 5.4 ± 0.15 eV and its LUMO is 3.7 ± 0.5 eV [88]; C60 ’s HOMO is
6.4 ± 0.15 eV and its LUMO is 4 ± 0.5 eV [78]. Dashed lines represent
uncertainty in energy levels; x was varied from 0 to 5. . . . . . . . . .
12
40
3-7 (a) Energy levels of pentacene/PTCBI device [78]. When excited, pentacene produces triplets through singlet fission [87]; PTCBI produces
singlets [86]. The PTCBI layer was made thick to improve production
yield. (b) EQE of pentacene/PTCBI device and modeled contributions
of pentacene and PTCBI to the EQE. At 532 nm, roughly half of the
excitons contributing to the current are from PTCBI; at 635 nm, most
of the contributing excitons are from pentacene. . . . . . . . . . . . .
41
4-1 (a) J–V curves for m-MTDATA/PTCBI devices with varying rubrene
interfacial layer thicknesses. 0.5 nm and 1.5 nm of rubrene increase
the short circuit current and the open circuit voltage; greater thicknesses (3.5 nm and 5 nm) further increase the open circuit voltage, but
decrease the short circuit current. Figure from [89]. (b) External quantum efficiency versus wavelength in m-MTDATA/PTCBI devices with
and without rubrene interfacial layers. The interfacial layer improves
performance across the spectrum, with the peak enhancement from a
1.5 nm layer. The absorption peaks in the 450–550 nm range correspond to rubrene’s absorption spectrum [90]; enhanced EQE outside
of this range is caused by enhanced charge separation. Figure from [89]. 44
4-2 Peak-wavelength EQE contributions of m-MTDATA and PTCBI as a
function of rubrene interfacial layer thickness, calculated from overall
EQE and absorptivities following the method of Peumans et al. [28].
Dashed lines serve as guides to the eye. Figure from [89]. . . . . . . .
13
46
4-3 (a) J–V curves for CuPC/C60 devices with varying ClAlPC interfacial layer thicknesses. All interfacial layer thicknesses considered increase both VOC and JSC , but the peak JSC enhancement comes from
a 1.5-nm-thick interfacial layer. As the ClAlPC interlayer thickness
increases, the J–V curves move closer to a ClAlPC/C60 device, shown
as a solid pink line. Figure from [89]. (b) External quantum efficiency
versus wavelength in CuPC/C60 devices with varying ClAlPC interfacial layer thicknesses. A thin layer of ClAlPC increases EQE below 500
nm, where C60 absorbs and ClAlPC largely doesn’t, so it must improve
C60 exciton dissociation. It also hurts CuPC exciton dissociation, as
can be seen from the decrease in EQE near 600 nm with increasing interlayer thickness. Increased EQE in the 700–800 nm range comes from
absorption by ClAlPC, as can be seen from the EQE of a ClAlPC/C60
device. Figure from [89]. . . . . . . . . . . . . . . . . . . . . . . . . .
47
4-4 Short-circuit current versus intensity at several temperatures. Both
JSC ’s linear relationship with intensity and its fall with temperature
comports with Giebink et al.’s model of OPV behavior [95]. . . . . .
50
4-5 Calculated dissociation efficiency as a function of temperature. Lines
are drawn as guides to the eye. The apparent decline in dissociation
efficiency with falling temperatures may stem from the calculation’s
failing to consider a temperature dependence of diffusion. . . . . . . .
51
4-6 Open-circuit voltage as a function of the logarithm of the short-circuit
current, taken at temperatures ranging from 10K to 293K in steps of
20K. Giebink et al. predict a linear relationship between VOC and the
logarithm of JSC [95]; the discontinuity may represent a reduction in
the number of available states at low temperatures. . . . . . . . . . .
14
52
List of Tables
4.1
Percentage of incoming photons that are converted to excitons that arrive at the heterojunction. Calculated following the numerical method
of Peumans et al. [28], using measured materials absorptivities, a
PTCBI exciton diffusion length of 3 nm [28], and a pentacene exciton diffusion length of 65 nm [96]. . . . . . . . . . . . . . . . . . . . .
15
49
16
Chapter 1
Introduction
1.1
The case for solar power
It is easy to forget the depth of our reliance on energy. Information and transportation
rely on energy, of course, but so does agriculture: at least a third of the world is fed
with man-made (i.e., fossil fueled) fertilizers [1]. And energy use will only become
more important: the U.S. Energy Information Administration estimates that world
energy demand will increase by nearly 50% over the next 25 years [2]. A more
ambitious goal, raising worldwide standards of living to match the West’s in 1990,
would quadruple energy production [2, 3, 4]. We need more energy. As twenty days’
worth of sunshine is equal to the planet’s fossil fuel reserves [5], solar power can help
meet those needs.
Solar power will also make our energy supply safer and more reliable. While
the sun does not always shine on a given solar panel, we seem to be learning how
to better store energy [6] more quickly than we are learning how to stabilize our
energy sources [7] and prevent energy blackmail [8]. And of course our current energy
supplies have more than just geopolitical problems. Particulate and ozone pollution
from fossil fuel use cause on the order of four million deaths annually [9]; nuclear
waste disposal is stymied by a lack of communities willing to be the dumping ground
[10]; nuclear power itself now raises thoughts of Fukushima as well as Chernobyl and
Three Mile Island; hydroelectric dam construction in China may have triggered an
17
(a)
(b)
Annual Solar Irradiation
Wind
Biomass
Geothermal
Ocean and Wave
Hydro
Coal
Oil
Natural Gas
Uranium
Global Annual Energy Consumption
Figure 1-1: (a) Energy demand is expected to grow significantly over the next quartercentury: the increase in energy demand from 2007 to 2035 is greater than the total
coal consumption in 2007 [2]. Figure from [2]. (b) Yearly energy from solar irradiation
to land versus energy from other sources. Fossil fuels and uranium expressed as total
reserves, renewables by yearly potential. Figure from [11].
18
earthquake that killed 80,000 [12]; and everyone has heard how fossil fuels produce
the greenhouse gases threatening us with unpredictable and potentially disastrous
climate changes. Photovoltaic solar power is largely free of these problems, and, as a
scalable technology, can be adapted to existing electrical grids relatively easily.
Solar power offers a wealth of benefits, but it must become cheaper if it is to be
adopted. The better we understand the mechanisms to harvest it, the more cheaply
it can be tapped.
1.2
The varieties of solar power generators
Solar power generators come in several different flavors, each falling into one of two
categories. The first, solar thermal power, uses sunlight to produce heat, whether for
the sake of the heat itself (e.g., residential water heating) or to drive a heat engine. As
of 2009, 431 of the 2,108 megawatts of solar power generation in the U.S. came from
solar thermal generators [13]. Photovoltaic (PV) generation makes up the balance,
and offers a broader scope for research to boot. PVs are divided into three groups
according to the material used: inorganic semiconductors, dye-sensitized solar cells
(DSSCs), and organic photovoltaics (OPVs).
Inorganic semiconductors are the oldest (developed as early as 1883) and thus
most mature branch of photovoltaics [14] (see Fig. 1-2). They boast high sunlight-toenergy conversion efficiency—25% for crystalline silicon, 27.6% for thin-film gallium
arsenide [15]—and dominate the market, with 86% of the PV market taken up just
by silicon-based devices [16]. But such materials require more energy-intensive processing: silicon PVs are less sensitive to impurities than are integrated circuits, but
that does not mean they are insensitive to impurities [17], and solar cells must be
processed at temperatures exceeding 800◦ C [18].
DSSCs, as their name suggests, have a dye to absorb light and a semiconductor to
provide charge transport [19]. The cell contains a redox species, so under illumination
the light injects electrons into the semiconductor and is regenerated by the solution
[19]. DSSCs can be produced more cheaply than some inorganic PVs [20] and their
19
030801-10 Wolden et al.: Photovoltaic manufacturing: Present status
former offers advantages with
the latter may be more amena
strates include glass, metal, and
formance being obtained on gla
bility and the production of la
minimodules with areas "100
losses being one of the major
DSC module is strongly relate
Standard practices for lifetime
IEC61646 for thin-film PV, do
long-term light soaking at 55– 6
shown that efficiencies remai
value for over 25 000 h.113 The
cell indicate that carefully enca
last for over 20 years in a typi
The longest outdoor test of D
Toyota and Asin was 2.5 years
FIG. 5. "Color online# Evolution of champion cell efficiencies since 1995 for
6% "relative# per year.114 Faste
various PV technologies.
uted to differences
related to
Figure 1-2: Chart of the best attained efficiencies for various PV technologies,
by
area, above
or environment. Hermeti
Wolden et al. [16]. They refer to DSSCs as DSCs, which have an efficiency
more
challenging, and current p
OPVs but below inorganic PVs.
chargeable batteries for portable electronics. A beneficial feasubstrates have lifetimes of on
ture of DSC is that their performance improves under diffuse
applications, the sealing mater
efficiency is comparable
to some
silicon111PVs
[15], but
presents
enabling
theirtheir
use liquid
indoorselectrolyte
and
and low light
conditions,
chanically and thermally stabl
without[20].
direct solar exposure. Devices can be fabricated in a
and chemically inert to the ele
packaging difficulties
number of colors and levels of transparency, which is an
prevent mass transport betwee
OPVs are named
for feature
their carbon-backboned
active
layers,
whether theso
layer
con- that Hagfeldt et al
attractive
for architectural and
BIPV
applications.
important
Manufacturing can also be done at low temperature using
approach for D
sists of a polymer (e.g., poly[2-methoxy-5-(20 -ethyl-hexyloxy)-1,4-phenylenemanufacturing
vinylene]
flexible substrates.
vides the most functional enca
Unfortunately,
cell efficiency more
has been
stagnant known
[21]) or a small molecule
(e.g., champion
buckminsterfullerene,
commonly
as C60electrolyte with a gel
the liquid
at !11% for the past 15 years "Fig. 5#. The three main comencapsulation
[21]). OPVs can
be made to be flexible [20], allowing them to be easily arranged
in requirements, bu
ponents in a DSC, the Ru-based dye, the photoanode, and the
in decreased efficiency. Elimin
iodine-based
redox couple,
have also remained
un- theofcheapest
the most advantageous
geometry;
more importantly,
they’relargely
potentially
R2R manufacturing method
changed. Further optimization of any one of these compobe critical to economics partic
solar cell to produce
[20]. The materials
they
from and
the manufacturing
nents individually
is not likely
to are
yieldmade
significant
improvemains below 12%.
112
ments
in
efficiency.
The
recent
review
by
Hamann
et
al.
processes that can be used to make them are cheap [20, 22], and often show good
provides an excellent overview of the complexity of the isB. chapter,
OPV
absorption to boot
[23]. But,First,
for reasons
thatdye
will
be not
explained
the next
sues involved.
the leading
does
capture in
much
Carbon
light past
750the
nm,
and varieties
harvestingofthe
red and near-infrared
they are less efficient
than
other
photovoltaics,
and have therefore
notis abundant, and th
manufacturing of flexible pho
portions of the spectrum is needed to increase current densicome into widespread
use [16].replacing Ru is an important long-term consive efforts to develop solar ce
ties. In addition,
tors. Brabec et al.115 recently p
cern with respect to material availability. Second, the I3− / I−
view of the developments in O
redox couple is positioned with a 550 mV overpotential relathe challenges that lie ahead. F
tive to dye regeneration. An alternative redox couple could
champion cell efficiencies for
potentially allow the Voc to be improved by up to 300 mV,
technologies have been relative
but recombination rates are typically much faster with nonefficiency, organic PV has mad
iodine redox couples. A combination of these two changes
cade, with Heliatek and Konar
could elevate device performance to !16%. However, as
112
ons, each with devices certified
cautioned by Hamann et al., 20 this will most likely require
developments have occurred in
simultaneous optimization of both dye and electrolyte and
companies such as Solamer an
perhaps the development of new photoanodes with faster
efficiency record in recent year
charge transport as well. Photoanode designs based on wide
1.3
Outline of the remainder of the thesis
Chapter 2 explains the photovoltaic process in more detail to give a better understanding of the challenges facing OPVs. An inorganic PV models basic solar cell
concepts before we detail how OPVs work and why they are less efficient. From there
we can understand how we may improve OPV efficiency by adding a thin layer of
material or adjusting electron spins.
Chapter 3 relates how devices were produced and tested, and from which materials.
Chapter 4 presents and discusses experimental results. Interfacial layers prove capable of improving OPV efficiency beyond their ability to absorb light, while electron
spin engineering presents mixed results, potentially improving some aspects of the
photovoltaic process while harming others.
Chapter 5 summarizes findings and discusses their implications for future research.
21
22
Chapter 2
Theory
Solar cells are built on the photoelectric effect: sufficiently energetic light can free a
solid’s normally bound electrons to power a circuit. But an excited electron can simply
relax again (an action known as recombining), so that isn’t enough. To understand
how a solar cell works and judge its performance, let us first consider a model system:
the pn junction.
2.1
Photovoltaic fundamentals: the pn junction
As its name implies, the pn junction is an interface between a p- and an n-type
semiconductor. The latter is a semiconductor leavened with atoms lying to its right
on the periodic table: the dopants have more electrons than the semiconductor atoms
they replace, and these extra electrons are available as charge carriers. A p-type
semiconductor, of course, is the opposite, in which dopants have too few electrons
to make all of the bonds the lattice structure require and are therefore a source
of positively charged “holes” (a pseudoparticle consisting of the state in which an
electron should be). More detail is available in Pierret [24], but for this work it
suffices to say that when a p- and an n-type semiconductor are joined, the dopant
atoms remain in place while their loosely bound charge carriers diffuse across the
boundary. This creates an internal electric field across the junction (see Fig. 2-1(a)),
which serves to accelerate any charge carriers generated near the junction in opposite
23
(a)
(b)
Figure 2-1: (a) When a pn junction is created, mobile charges (circles) diffuse to
areas
(c)of lower concentration, leaving behind immobile ions (squares). This develops
an internal electric field that ultimately balances the diffusion potential. Figure from
[24]. (b) A pn junction exposed to light. Excited electrons far from the junction will
recombine before they can diffuse to the junction; charges close to the junction will
be accelerated by the change in potential energy. Electrons move “downhill,” and
holes move “uphill,” creating a conventional current to the left. Figure from [24].
directions, creating a (reverse) current (see Fig. 2-1(b)). Thus the pn junction is the
classic photovoltaic device.
Of course, we want to know not only whether a photovoltaic device is working,
but how well it is working. The first criteria are found in the relationship between
voltage and current. As seen in Fig. 2-2, the greater the rate of photoexcitation, the
further the J-V curve will be shifted down. In numbers, a given device illuminated
by a given spectrum and intensity of light may be characterized by:
• An open-circuit voltage, VOC , the voltage at which no current flows through the
device. The highest voltage that can be supplied by a device for a given input
[24];
• A short-circuit current, JSC , the current through the device when no voltage is
placed across it [24];
24
Figure 2-2: The response of an ideal pn junction to changes in incident light intensity.
As the rate of electron photoexcitation is increased in a pn junction, the reverse
current increases proportionally. The higher the proportionality, the better the solar
cell. Figure from [24].
• A maximum power output, Pm = Jm × Vm , which, from above, will be less than
JSC × VOC [24];
Pm
, a measurement of how close the device comes
JSC × VOC
to ideal operation [24];
• A fill factor, F F =
• A power conversion efficiency, η, the maximum power output divided by the
input power [24].
These numbers quantify overall device performance under set circumstances, but
are unwieldy for understanding how a device responds to different parts of the spectrum. We therefore test devices’ external quantum efficiency (EQE), the JSC divided
by the number of incoming photons per second, as a function of wavelength. EQE is,
by definition, a function of a device’s absorption and recombination rate: the higher
the percentage of light absorbed, the higher the percentage of photoexcited electrons
extracted, the greater the EQE. If an active material has a high absorptivity but the
device’s EQE is low at the same wavelength, the device has poor optics or suffers
from a high recombination rate.
2.2
Organic photovoltaics
Permittivity is the most important difference between organic and inorganic semiconductors. Coulomb binding potential is inversely proportional to relative permittivity,
25
and inorganic semiconductors have higher permittivities than organic semiconductors. Silicon’s permittivity is three to four times larger than organic semiconductors’
[24, 25]. An electron in silicon, when photoexcited, is immediately a free (or effectively
free) charge carrier [26]; an electron in an organic semiconductor, when photoexcited
to the lowest unoccupied molecular orbital (LUMO), remains bound to the hole it
left behind in the highest occupied molecular orbital (HOMO) in a tightly (∼0.3–1.2
eV) bound exciton [26]. This tight binding causes significant loss in OPVs: while a
single-junction inorganic PV is thermodynamically limited to a maximum efficiency
of 31% [27], exciton binding strength reduces the theoretical maximum efficiency of
standard OPVs to 22–27% in current state-of-the-art OPVs [26].
Excitons are broken up by putting them in a situation in which remaining bound
is energetically unfavorable. As we saw, this can be accomplished in an inorganic
semiconductor by a pn junction; OPVs usually do it by means of a donor-acceptor
heterojunction [26, 28, 29, 30]. Like a pn junction, a heterojunction is an interface
between two materials presenting different energy levels to charge carriers. When an
exciton reaches the heterojunction, if the difference between the donor’s HOMO and
the acceptor’s LUMO is smaller than the HOMO–LUMO gap in either the donor or
acceptor material, it will be thermodynamically favorable for the electron will go to
the acceptor and the hole to the donor (see Fig. 2-3). But excitons must survive long
enough to diffuse to this heterojunction: the thicker an active layer, the more light it
will absorb but the smaller the chances of its excitons reaching the heterojunction to
be dissociated [31].
There is another design tension associated with heterojunctions: while they break
up excitons, the electron and hole may still be bound in a charge transfer (CT) state
[32, 33]. The physics of the final dissociation (or lack thereof) at the interface remain
a topic of active research [32, 34, 35], but it is clear that the CT binding state energy
cuts an OPV’s power conversion efficiency by decreasing its VOC [26]. But the VOC is
proportional to the CT state binding energy [36, 30]!
So, generating current requires an OPV to first absorb light, then the resulting
exciton must diffuse to a heterojunction, then the heterojunction must break up the
26
Figure 2-3: Diagram of a heterojunction. Black dots represent electrons, white dots
holes. When excited, electrons will move from their position in the highest occupied
molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), leaving a hole behind. The two are bound by the Coulomb potential, and are unlikely to
dissociate unless they diffuse to a heterojunction with a HOMO–LUMO split smaller
than that of the material on either side. If they reach such a heterojunction, it is
energetically favorable for the exciton (regardless of whether it was generated in the
donor or the acceptor) to dissociate at the interface, leaving the electron on the acceptor side and the hole on the donor side and increasing the likelihood of the two
dissociating entirely [26]. But the two are still be bound and may still recombine [36].
exciton, and then the resulting CT state must break into the bare charges that run
the circuit. Thicker active layers mean a greater chance of absorption and a smaller
chance that excitons will diffuse to the interface before recombining; increasing the
CT binding energy increases both the potential VOC and the recombination rate. Several device architectures have been proposed to cut through the first knot: optical
techniques can increase the optical path through a thin film without affecting exciton
diffusion lengths [28]; antenna structures decouple light absorption from exciton diffusion [37]; bulk heterojunctions (interpenetrating donor–acceptor networks) reduce the
distance an exciton must travel to reach a donor–acceptor interface [38, 39]; tandem
cells, stacks of heterojunction devices, serve to increase the heterojunction surface
area [30] and can also be designed to broaden the absorption spectrum [40]; some
devices are designed to produce triplet excitons to take advantage of their potentially
longer diffusion lengths, as explained below. We here consider the second knot: how
to cut through it by means of cascade energy level alignment (CELA), and how using
triplet states to resolve the first tension of OPVs may exacerbate the second.
27
LUMO
Donor
Acceptor*
Donor*
Acceptor
HOMO
Figure 2-4: Diagram of a three-layer CELA device. The material at the left serves
strictly as a donor, and the material at the right as an acceptor; with a HOMO and
LUMO intermediate between those of the materials to either side, the middle material
serves as an acceptor for the material at the left and a donor for the material at the
right. There are thus two heterojunctions serving to dissociate excitons. Note that
the HOMO–LUMO offset is greater at each heterojunction than it would be in a
bilayer device made with the two outer materials.
2.2.1
CT state destabilization by charge separation
We reduce CT state recombination with a photosynthesis-inspired method. Chloroplasts must also minimize recombination, and do so by means of an electron “staircase”: the molecule absorbing the photon is the first in a chain of molecules, each
with a slightly lower LUMO than the previous [41]. The electron tends to fall down
the staircase, at the bottom of which it is spatially separated from the hole [41] and
therefore unlikely to recombine. OPVs that exploit this concept are said to have a
CELA structure [42]. A CELA device has multiple heterojunctions, each with energy offsets between the materials on either side (see Fig. 2-4). Previous work on
the device architecture is detailed below; unlike the others, we demonstrate that a
thin (nanometer-scale) interfacial layer can serve to increase both VOC and JSC by
improving the efficiency of CT state dissociation.
The eponymous cascade of a CELA device tends to increase VOC over a bilayer
device with only the opposite ends of the cascade [43, 44, 45, 46, 47, 48, 49]. By
definition, adding the CELA device’s interfacial layer creates two heterojunctions
with larger CT state binding energies than were in the bilayer device (see Fig. 2-4),
and, as mentioned, VOC increases with CT state binding energy. While there have
28
been some studies that found no increase in VOC [42, 50, 51, 52], those used interfacial
layers at least 10 nm thick, broadening their device’s absorption spectrum at the cost
of increasing the likelihood of recombination in the interfacial layer .
Even among those CELA studies showing a VOC increase, many show no improvement in the dissociation of CT states. Sista et al. [44], Lai et al. [45], Hong et al. [46],
and Huang et al. [48] show a fall in JSC with the addition of an interfacial layer. Kinoshita et al. [43], studying a copper phthalocyanine (CuPC) interfacial layer between
pentacene and C60 , include one data point replacing CuPC with zinc phthalocyanine
(ZnPC), and ascribe the resulting increase in JSC over pentacene/C60 control devices to ZnPC’s longer excited state lifetime; they do not, however, offer any data
beyond the device’s J–V curve to help isolate the ZnPC interfacial layer’s specific
effect. Kumar et al. [47] show no appreciable change in VOC with an interfacial layer
thinner than 3 nm, but report that the thinner interfacial layers increase JSC —which
they ascribe to a broader absorption spectrum without reporting the EQE. A subsequent paper by Huang et al. [49] also shows CELA architecture improving both VOC
and a JSC , but their EQE measurements also indicate that enhanced absorption is
improving JSC .
In contrast with previous studies, we find that a thin, cascade-creating interfacial
layer can improve all of the overall device performance metrics by reducing the recombination rate. As a heterojunction destabilizes an exciton by offering a lower-energy
state, so too can the resulting CT state be destabilized. For a second heterojunction
to do so, it must fulfill two requirements. First, the decrease in the charge carrier’s
energy (i.e., the decrease in LUMO for an electron, or in HOMO for a hole) must
outweigh the CT binding energy to provide a driving force for dissociation. Second,
the interfacial layer must be thin: CT states have a spatial extent on the order of
1 nm [53] and cannot diffuse like excitons can, so if the second heterojunction isn’t
close at hand, it can’t influence the CT state. The materials we use to demonstrate
this are detailed in the next chapter.
29
2.2.2
Spin dependence of recombination
To discuss the spin dependence of recombination, we should first review spin, the
intrinsic angular momentum of fundamental particles; a more complete account is
available in a text like Griffiths [54], on which this is based. Electrons have spin- 12 ,
and in an unexcited molecule these electrons are paired up antiparallel to each other:
the total spin angular momentum of the molecule, s, is zero. When the least-bound
electron is excited, there will be two unpaired electrons in the molecule: the excited
electron and the electron that remains in what we still call the HOMO. The other
electrons are paired up and sum to zero, but the two unpaired electrons may be
parallel or antiparallel, and may thus be considered a two-electron system for our
purposes. A two-electron system will have a total spin of either zero ( 12 − 12 ) or one
( 12 + 12 ), and there are four ways for the system to be in one of those two states:
n
− ↓↑)
o



|1 1i = ↑↑


|1 0i = √12 (↑↓ + ↓↑)



 |1 -1i = ↓↓





|00i
=
√1 (↑↓
2
s = 0 (singlet)
s = 1 (triplet)




There are two key consequences a system’s being in a singlet or triplet state: lifetime
and energy.
Triplets are longer-lived than singlets as a direct consequence of spin. Photons are spin-1 particles: they cannot change spin by
1
,
2
so absorption or emis-
sion of a photon is very unlikely to take the system from a singlet to a triplet
state or vice-versa [54]. As the ground state is itself a singlet state, the first excited triplet state is long-lived, on the order of micro- or milliseconds [55, 56] when
singlet states last for mere nano- or picoseconds [55, 57]. Longer exciton lifetimes
mean more time to travel to a heterojunction, allowing thicker, more light-absorbant
layers. But diffusion length is not merely a function of lifetime. Singlets in amorphous 40 -bis(9-carbazolyl)-2,20 -biphenyl (CBP) have been reported to have a somewhat longer diffusion length (16.8 nm [58]) than triplets in N, N 0 -di-1-naphthalenyl30
N, N 0 -diphenyl-[1,10 :40 ,100 :400 ,1000 -quaterphenyl]-4,4000 -diamine (4P-NPD, 11 nm [59]).
Still, longer triplet exciton diffusion lengths have been reported: 40 nm in C60 [28],
60 nm in Alq3 [60], 3.9 µm in a polymer [61]. And while this may seem like a
moot point—if a photon can’t take the system from the ground to a triplet state,
what good are triplets?—but there are several ways to optically generate triplets [22].
Some materials, including C60 , mainly produce triplets when excited [62], and the
rate of energy transfer from singlets to triplets in other materials can be adjusted
[55, 56, 63]. Generating triplets in OPVs is worth considering.
Triplet and singlet states also differ in energy, a consequence of the Pauli exclusion
principle [64]. A two-electron wavefunction must be antisymmetric under particle
exchange, i.e., ψ(r1 , r2 ) = −ψ(r2 , r1 ). The electrons’ states are described by their
position and spin, so if the electrons are in the (antisymmetric) singlet spin state,
they will have a symmetric spatial wavefunction; they will have an antisymmetric
spatial wavefunction if they are in a (symmetric) triplet state. Naturally, the spatial
wavefunctions affect the system’s potential energy:
e2
1
hψ(r1 , r2 )| |ψ(r1 , r2 )i
4π0
r12
2
e
1
K=
hψ(r1 , r2 )| |ψ(r2 , r1 )i
4π0
r12
J=
J the Coulomb integral, K the exchange integral, e an electron charge, 0 the permittivity of free space, r12 the distance between the two electrons [65]. The Coulomb
integral corresponds to the Coulomb potential that we are familiar with; the exchange
integral stems from the Pauli exclusion principle [64]. The symmetry of antisymmetry
of the spatial wavefunctions have no effect on J, but change the sign of K: all else
equal, a triplet state’s energy is 2K smaller. So triplets are more tightly bound than
singlets, and are thus harder to break up—but maybe not much harder. Pentacene
may have a 0.97 eV singlet-triplet gap [66], but C60 ’s singlet-triplet splitting is a mere
0.2 ± 0.1 eV [67].
It is not obvious whether an OPV generating triplet excitons will differ from an
OPV generating singlets. Triplets’ longer lifetimes sometimes improve their odds
31
of diffusing to the heterojunction, but their tighter binding reduces their odds of
being successfully dissociated. Several groups have reported simultaneously increasing
triplet yields and device efficiencies by, variously, doping polymers with heavy metals
[56]; doping polymers with phosphorescent dyes [21, 68, 69, 70, 71, 72, 73]; tethering
an iridium complex to a conjugated polymer [74]; and doping small molecule films with
a platinum complex [75]. But none of these experiments directly compared singlet
behavior with triplet behavior. We compare singlet and triplet exciton dynamics
in an OPV by making a bilayer device with two materials of minimally overlapping
absorption spectra, one producing singlets, the other triplets.
32
Chapter 3
Experimental Methods
3.1
Device fabrication
Our devices consist of a glass substrate with patterned indium tin oxide (ITO; see
Fig. 3-1(a) for the pattern), produced by Luminescence Technology Corporation of
Taiwan; thin films of materials, detailed below, that comprised the active layers of
the devices; 9 nm of bathocuproine (BCP) as an exciton blocking layer [76], standard
for OPVs; and 50 nm of silver. Making them involved three basic steps: purifying
the organic materials to be deposited, cleaning the device substrates, and depositing
the films.
The first of those three steps was usually accomplished by purchasing 99.99% pure
material from a vendor. When high purities were unavailable, we purified material
ourselves using vacuum thermal sublimation (see Fig. 3-2). Once purified, materials
are kept under nitrogen to minimize contamination by water or oxygen.
We also make sure the substrates are clean before we make devices on top of
them. We first sonicate the substrates in a 2% solution of Cole-Palmer’s Micro-90
for five minutes to clean off any organic detritus. This is followed by two steps each
of five minutes’ sonication in distilled water (removing the Micro-90), two minutes’
sonication in acetone (removing the water), and two minutes of immersion in boiling
isopropanol (removing the acetone). After this solution processing, we finish by exposing the substrates to an oxygen plasma for five minutes, also standard for OPVs
33
!"#$%&'$()*(+,(+%-#$'./('$%&+-0#,+1(!"#$%&-
Figure 3-1: (a) Pattern of ITO on the glass substrate. Black represents the ITO,
white the glass. Testing contacts will be placed on the five squares around the edge
of the substrate. The lollipop shape allows electrical contact to be made at the edge
while the devices sit in the center; the four corners will be coated with silver, but
if any flakes off in the process of making contact, the ITO underneath maintains
electrical conductivity. (b) Substrate after deposition of active materials and BCP.
Green represents the deposited organic materials; the mask is a photo negative of
the square, allowing the materials to be deposited as pictured and nowhere else. (c)
Substrate after deposition of silver (the mask is, again, a photo negative of the pattern
depicted). The silver deposition defines the active area of four devices (the circles in
the middle), and creates silver contacts that can easily be connected to the testing
apparatus and silver wires connecting the contacts with the device.
!"#$%&'$()*(+,(+%-#$'./('$%&+-0#,+1(!"#$%&!"#$%&'$()*(+,(+%-#$'./('$%&+-0#,+1(!"#$%&!"#$%&'$()*(+,(+%-#$'./('$%&+-0#,+1(!"#$%&-
!"#$%&'$()*(+,(+%-#$'./('$%&+-0#,+1(!"#$%&-
(c)
Outer Tube
Inner Tubes
"T #
Vacuum
Pump
Impure Material
Figure 3-2: Diagram of vacuum sublimation purification at the beginning of the process. An outer
! quartz tube serves to hold a vacuum, while segmented inner tubes
allow the material to be readily extracted post-purification. The material to be purified is placed in one end of the inner tubes as shown, evacuated to a pressure lower
than 10−5 Torr, and subjected to a thermal gradient. The material end of the tube is
kept above the material’s sublimation temperature, while the far end of the tube is
set to as low a temperature as possible, so impurities with higher or lower sublimation
temperatures are deposited away from the deposits of the desired material.
34
!"#$%&'$()*(+,(+%-#$'./('$%&+-0#,+1(!"#$%&-
(b)
!"#$%&'$()*(+,(+%-#$'./('$%&+-0#,+1(!"#$%&-
(a)
(a)
(b)
Top plate (attaches to flange)
Motor with gear
Substrate holder
Nut
Ball bearing
Gear (pinned to axle)
Axle
Nut
Nut
Figure 3-3: (a) Diagram of mask assembly. Substrates sit in the depressions on the
piece depicted on the left; a mask that is the negative of the pattern in Fig. 3-1(b)
or (c) sits over the holes on the right-hand piece. The right-hand piece fits over
the left-hand piece, holding the substrates in place and masking them as desired.
(b) Rotating substrate holder schematic. The apparatus is held above the materials
sources by bolting the top plate to a flange in the evaporation chamber. An axle is
secured to the top plate by a nut, but is allowed to rotate freely. A motor attached
to the top plate turns a gear pinned to the axle, while the substrate holder from (a)
is secured to the rotating axle by nuts above and below.
[92].
Once the substrates have been cleaned, they are held above the material sources
with a mask assembly (see Fig. 3-3(a)). The mask ensures that materials are only
deposited where they are needed (see Fig. 3-1): active-layer materials and BCP cover
a large patch of ITO to ensure electrical contact and reduce the risk of shorts, but are
kept from blocking the electrical contacts on the side; we deposit silver on a smaller
region to create four distinct devices of a known, manageable size. While the organic
materials are being deposited, a custom-made rotating substrate holder keeps them
turning (see Fig. 3-3(b)). This ensures an even deposition of the thin films, preventing
35
Figure 3-4: Exaggerated depiction of the shadow effect. When material is deposited
on a substrate at a constant angle, it will pile up according to the angle of incidence.
Another material deposited in the same fashion will pile up in different places: the
contact deposited on top may then be separated from the contact on the bottom by
a resistor, not a diode.
a “shadow” from forming that would cause devices to short out (see Fig. 3-4).
The deposition itself occurs in a custom Angstrom deposition chamber, which we
evacuate to a pressure below 2 × 10−6 Torr before depositing materials. Deposition
of a layer is computer-controlled using the bundled organic deposition software: the
computer heats a material until the crystal monitor inside the chamber indicates that
the material is evaporating at the desired rate (1 Å/s for organics, 3 Å/s for silver), at
which point the computer opens up a shutter inside the chamber, allowing material
to be deposited. When the desired amount of material has been deposited (again
measured by the crystal monitor), the computer closes the shutter and stops heating
the material. To the greatest extent possible, this is done without breaking vacuum,
which could expose the devices to contamination, but we have to break vacuum to
change to the silver mask.
Once the devices are made, there is an optional extra step of packaging them.
Organic semiconductors are often sensitive to oxygen and water [31], but not all of
our measuring equipment fits in a glovebox. Devices that had to be measured in
atmosphere were packaged by using Epoxy Technologies PB057888 to stick a square
quarter-inch of glass over the active area.
36
3.2
Characterization
We measure absorptivity with an Aquila nkd-8000 optical metrology instrument. To
do so, we measured the reflection from and transmission through a 1000 Å-thick film of
the material of interest for both p- and s-polarized light in the relevant spectrum. We
extract the wavelength dependence of the material’s refractive index and absorptivity
from that data by using ProOptix software to fit a model of the material response to
the data.
J–V curves are measured with an Agilent 4156C Precision Semiconductor Paramter
Analyzer. We connect the ITO and a silver contact to the Agilent, which varies voltage and measures the resulting current. Device performance under AM1.5 one-sun
illumination (1000W/m2 ) was simulated by a Newport Oriel Model 91191 solar simulator; singlet performance was compared to triplet performance by comparing J–V
curves under varying intensities of monochromatic illumination, with one wavelength
exciting more singlets than triplets and another with the opposite effect.
EQEs were measured using a xenon lamp with an Oriel Cornerstone monochrometer, a Stanford Research Systems SR830 DSP lock-in amplifier, a chopper, and a
calibrated Newport 818 silicon photodiode. The chopper and the lock-in eliminate
background noise from the device’s optical response by blocking and unblocking the
light source at 77 Hz and subtracting the “off” from the “on” signal. The monochrometer adjustably selects one wavelength of light from the lamp’s spectrum, and we
calculate the device’s EQE by comparing the device’s output at that (and all other)
wavelength(s) to the photodiodes.
Measurements at low temperatures are taken with a Janis STVP-100-2 optical
cryostat. This makes little difference to the actual data-taking—once samples are
loaded into the cryostat, the Agilent is hooked up as usual and light sources are
focused onto the devices through the cryostat’s optical window. Otherwise, by modulating the flow of liquid helium and by using a Cryo-Con 32B temperature controller
to control a resistive heater in the cryostat, the sample can be held at a given (subroom) temperature.
37
3.3
3.3.1
Materials selection
Cascaded energy level alignment devices
Energy level uncertainty makes it hard to design a CELA structure. HOMO levels
present little difficulty: they can be measured fairly precisely by ultraviolet photoelectron spectroscopy (UPS), in which a sample is excited with UV light and the kinetic
energy of the emitted electrons is compared with the energy of the photons [77]. But
LUMO levels are measured through inverse photoelectron spectroscopy (IPES), in
which electrons are fired into a film and a photodetector measures the photons emitted by their decay into empty states [78]. IPES has a low yield and low signal-to-noise
ratios, so LUMO levels generally have measurement uncertainties of at least 0.5 eV
[79]. The uncertainty in LUMO levels is thus on par with the HOMO–LUMO gap
in current state-of-the-art devices [26]. The HOMO (i.e., the hole) cascade is therefore reasonably easy to design, but the LUMO (electron) cascade is far less certain.
Under the circumstances, we made two sets of test devices: the first with a broad
gap between the donor and acceptor LUMO levels to demonstrate the principle in
an unambiguously cascaded device, and the second with standard materials to show
that the concept is relevant to real-world devices.
Our earliest devices, made and characterized by Dr. Tim Heidel, therefore used
materials that were ideal only to prove the concept, not as actual solar cells. 4,40 ,400 tris-(3-methyl-phenyl phenylamino) triphenylamine (m-MTDATA) has a wide bandgap
(HOMO = 5.1 ± 0.15 eV, LUMO = 2.0 ± 0.5 eV [80]) that keeps it from absorbing
much solar radiation, but there is little question that 5,6,11,12-tetraphenylnapthacene
(rubrene; HOMO = 5.36 ± 0.15 eV, LUMO = 3.15 ± 0.5 eV [81]) has LUMO and
HOMO levels intermediate between those of m-MTDATA and 3,4,9,10-perylenetetracarboxylic bis-benzimidazole (PTCBI; HOMO = 6.2 ± 0.15 eV, LUMO = 3.6 ± 0.5 eV
[82]): see Fig. 3-5. What’s more, the absorption spectra of the three are sufficiently
different to identify their effect with a glance at a graph of EQE against spectrum:
m-MTDATA’s absorption peak is between 350–400 nm, rubrene’s is between 450–550
nm, and PTCBI absorbs broadly from 425–800 nm, with a peak at 550 nm.
38
LUMO
(2.0 ± 0.5) eV
m-MTDATA
(25 nm)
(3.6 ± 0.5) eV
Rubrene
(x nm)
ITO
HOMO
(3.2 ± 0.5) eV
BCP
(9 nm)
PTCBI
(25 nm)
Ag
(5.1 ± 0.15) eV
(5.4 ± 0.15) eV
(6.2 ± 0.15) eV
Figure 3-5: Energy levels of m-MTDATA/rubrene/PTCBI interfacial device [80, 81,
82]. Dashed lines represent uncertainty in energy levels; x was varied from 0 to 5.
Copper phthalocyanine (CuPC) and C60 are more effective, and thus more common, OPV materials [83, 84], so devices made with these serve to test the breadth
of the CELA technique’s validity. But their LUMO level uncertainties make finding
an appropriate interfacial layer a matter of ill-informed guessing: CuPC’s LUMO is
3.4 ± 0.5 eV [78], C60 ’s is 4 ± 0.5 eV [78], and any possible interfacial material has
a similar uncertainty in its LUMO. Still, while a LUMO cascade can’t really be rationally designed, the HOMO cascade can be, and chloroaluminum phthalocyanine
(ClAlPC) fits the bill as well as possible (see Fig. 3-6): it creates a hole transport
cascade and may create an electron transport cascade. It also has a convenient absorption spectrum: C60 absorbs most strongly below 550 nm [52], CuPC between
550–700 nm [85], and ClAlPC near 750 nm [85], so the interlayer’s effects are obvious
on an EQE–spectrum graph.
3.3.2
Devices to probe spin dependence of recombination
When excited with visual light, PTCBI produces singlet excitons [86]; pentacene
produces mainly triplet excitons [87]. Fig. 3-7(a) shows that the former can serve as
an acceptor and the latter as a donor in a heterojunction solar cell, and Fig. 3-7(b)
that we can change the ratio of singlet to triplet excitons arriving at the heterojunction
39
LUMO
(3.4 ± 0.5) eV
(3.7 ± 0.5) eV
(4.0 ± 0.5) eV
ITO
CuPC
(20 nm)
HOMO
BCP
(9 nm)
ClAlPC
(x nm)
(5.2 ± 0.15) eV
(5.4 ± 0.15) eV
Ag
C60
(40 nm)
(6.4 ± 0.15) eV
Figure 3-6: Energy levels of CuPC/ClAlPC/C60 interfacial device. CuPC’s HOMO
is 5.2 ± 0.15 eV and its LUMO is 3.4 ± 0.5 eV [78]; ClAlPC’s HOMO is 5.4 ± 0.15
eV and its LUMO is 3.7 ± 0.5 eV [88]; C60 ’s HOMO is 6.4 ± 0.15 eV and its LUMO
is 4 ± 0.5 eV [78]. Dashed lines represent uncertainty in energy levels; x was varied
from 0 to 5.
by changing the wavelength of light used to excite the device.
40
(a)
(b)
Figure 3-7: (a) Energy levels of pentacene/PTCBI device [78]. When excited, pentacene produces triplets through singlet fission [87]; PTCBI produces singlets [86].
The PTCBI layer was made thick to improve production yield. (b) EQE of pentacene/PTCBI device and modeled contributions of pentacene and PTCBI to the
EQE. At 532 nm, roughly half of the excitons contributing to the current are from
PTCBI; at 635 nm, most of the contributing excitons are from pentacene.
41
42
Chapter 4
Results and Analysis
4.1
4.1.1
Interfacial layer devices
m-MTDATA/rubrene/PTCBI devices
As expected, only very thin rubrene interfacial layers improved device performance.
Devices without an interfacial layer had a short-circuit current density JSC = 0.19
mA/cm2 , VOC = 0.51 V, fill factor F F = 0.34, and power conversion efficiency
ηp = 0.066%; devices with a 1.5 nm rubrene layer showed JSC = 0.41 mA/cm2 ,
VOC = 0.61 V, F F = 0.45, and ηp = 0.23%. Beyond this thickness—approximately
one monolayer of interfacial material—the VOC continued to increase, but the JSC ,
F F , and ηp all fell (see Fig. 4-1(a)). Naturally, this is reflected in the external
quantum efficiency: the EQE increased as the interfacial layer was thickened, peaked
when the interfacial layer was 1.5 nm thick, and fell thereafter (see Fig. 4-1(b)).
The rise and fall in JSC with small increases in interfacial layer thickness suggest
that the layer is acting as more than an absorber. The EQE–wavelength graph of
Fig. 4-1(b) confirms it: although rubrene’s absorption peaks in the 450–550 nm range,
adding the interfacial layer increases the EQE for other parts of the spectrum, and
making it too thick decreases the EQE for other parts of the spectrum once more.
Dr. Heidel quantified this observation by modeling the direct contributions of
each material to the EQE. He supposed that a material’s contribution to EQE is
43
(a)
(b)
Figure 4-1: (a) J–V curves for m-MTDATA/PTCBI devices with varying rubrene
interfacial layer thicknesses. 0.5 nm and 1.5 nm of rubrene increase the short circuit
current and the open circuit voltage; greater thicknesses (3.5 nm and 5 nm) further increase the open circuit voltage, but decrease the short circuit current. Figure
from [89]. (b) External quantum efficiency versus wavelength in m-MTDATA/PTCBI
devices with and without rubrene interfacial layers. The interfacial layer improves
performance across the spectrum, with the peak enhancement from a 1.5 nm layer.
The absorption peaks in the 450–550 nm range correspond to rubrene’s absorption
spectrum [90]; enhanced EQE outside of this range is caused by enhanced charge
separation. Figure from [89].
44
proportional to its absorptivity and empirically fitted each material’s proportionality
constant to match the overall observed EQE. The result, shown in Fig. 4-2, indicates that an incomplete interlayer (5 Å corresponds to approximately
1
20
coverage by
rubrene [91]) gives incomplete benefits, while a 1.5-nm-thick rubrene layer increased
m-MTDATA’s peak EQE by nearly 40% and more than doubled that of PTCBI. At
greater thicknesses, the rubrene–PTCBI heterojunction is too far away to influence
the m-MTDATA–rubrene CT state (or vice-versa), and each heterojunction must not
only dissociate excitons unaided, but have the dissociated charge carriers travel further to reach an electrode. Thus, a device with a 10-nm-thick interfacial layer has
a PTCBI-associated EQE on par with the control device’s, while its m-MTDATAassociated EQE is lower than the control device’s, possibly because rubrene’s electron
mobility is 10% smaller than PTCBI’s [93, 94].
4.1.2
CuPC/ClAlPC/C60 Devices
Like the rubrene layer in the devices above, a sufficiently thin ClAlPC interfacial layer
improves CuPC/C60 device performance. Plain CuPC/C60 devices have an overall
conversion efficiency of 2.2%; a 1.5-nm-thick ClAlPC interfacial layer improves that
by 50%, but 4.5 nm of ClAlPC makes efficiency fall 10% from its peak (see Fig.
4-3(a)).
Although an interfacial layer once again improves overall power conversion efficiency, Fig. 4-3(b) muddies our story. Adding an interfacial layer increases the EQE
of the device below 520 nm and above 700 nm, but decreases it near 600 nm. The
increase above 700 nm may be ascribed to ClAlPC absorption, but the increase below
520 nm comes from more efficient C60 exciton dissociation. At the same time, the fall
in EQE near 600 nm must come from weaker dissociation of CuPC excitons.
This implies that, as feared, excitons are only dissociated at the ClAlPC/C60
interface, not on both sides of the interfacial layer as in the previous devices. Excitons
generated in C60 face a cascade as intended and therefore dissociate more efficiently,
but excitons generated in CuPC don’t. As the control devices work whether C60 or
CuPC is being excited, there are two possibilities: either ClAlPC has too low a LUMO
45
Figure 4-2: Peak-wavelength EQE contributions of m-MTDATA and PTCBI as a
function of rubrene interfacial layer thickness, calculated from overall EQE and absorptivities following the method of Peumans et al. [28]. Dashed lines serve as guides
to the eye. Figure from [89].
46
(a)
(b)
Figure 4-3: (a) J–V curves for CuPC/C60 devices with varying ClAlPC interfacial
layer thicknesses. All interfacial layer thicknesses considered increase both VOC and
JSC , but the peak JSC enhancement comes from a 1.5-nm-thick interfacial layer.
As the ClAlPC interlayer thickness increases, the J–V curves move closer to a
ClAlPC/C60 device, shown as a solid pink line. Figure from [89]. (b) External
quantum efficiency versus wavelength in CuPC/C60 devices with varying ClAlPC interfacial layer thicknesses. A thin layer of ClAlPC increases EQE below 500 nm,
where C60 absorbs and ClAlPC largely doesn’t, so it must improve C60 exciton dissociation. It also hurts CuPC exciton dissociation, as can be seen from the decrease
in EQE near 600 nm with increasing interlayer thickness. Increased EQE in the 700–
800 nm range comes from absorption by ClAlPC, as can be seen from the EQE of a
ClAlPC/C60 device. Figure from [89].
47
and thus traps electrons between C60 and CuPC, or ClAlPC has too high a LUMO
and CuPC excitons must diffuse through it to dissociate at the ClAlPC/C60 interface.
Our results suggest the latter. If the LUMO level of ClAlPC were lower than that of
C60 , excitons would not dissociate at the ClAlPC–C60 interface, but ClAlPC excitons
clearly do (see, again, Fig. 4-3(b)). So ClAlPC must have too high a LUMO to
break up CuPC excitons, which must instead diffuse through the interfacial layer to
be dissociated. According to Bailey-Salzman et al. [85], ClAlPC is a worse exciton
transporter than CuPC, which is confirmed by both the ClAlPC/C60 device EQE and
the fall in the CuPC-associated EQE with the introduction of the interfacial layer.
4.2
Spin-dependent recombination
We found that triplets have a lower dissociation efficiency than singlets, but that the
CT states produced from their dissociation have same probability of recombination.
To find the former, we examine JSC under illumination. Giebink et al. find that, in
neat heterojunction devices,
JSC = −q ηCT d JX ,
(4.1)
where q is the charge on an electron, ηCT d is the efficiency of CT state dissociation, and
JX is the exciton current density reaching the heterojunction [95]. JX is a function of
the exciton diffusion length, the layer thickness, the generation rate, and the incident
intensity. This is reflected in the linear relationship between intensity and JSC seen
in Fig. 4-4. Using the materials’ absorptivities (which we measured) and exciton
diffusion lengths (available in the literature [28, 96]) and the numerical method of
Peumans et al. [28], we can calculate JX for each material and wavelength (see Table
4.1); combining these with Equation 4.1, we may compute the exciton dissociation
efficiency. As both materials are excited to a greater or lesser extent by the two lasers,
we must simultaneously solve the equations
pent
P T CBI
P T CBI
JSC (635) = −q (JXpent (635) ηCT
(635) ηCT
)
d
d + JX
pent
P T CBI
P T CBI
JSC (532) = −q (JXpent (532) ηCT
(532) ηCT
)
d
d + JX
48
Material
Pentacene
PTCBI
JX /(Incident Intensity) [%]
532 nm
635 nm
10.44
18.0175
6.8075
5.3265
Table 4.1: Percentage of incoming photons that are converted to excitons that arrive
at the heterojunction. Calculated following the numerical method of Peumans et al.
[28], using measured materials absorptivities, a PTCBI exciton diffusion length of 3
nm [28], and a pentacene exciton diffusion length of 65 nm [96].
We did so at a series of temperatures, as seen in Fig. 4-5. At room temperature, our
data suggests that pentacene’s triplet excitons are, at most, 80% as likely to dissociate at the heterojunction as PTCBI’s singlet excitons. The CT state dissociation
efficiency appears to fall with temperature, although it is unclear how much of that
effect comes from falling JX : the JX model takes diffusion length as a given and does
not account for temperature effects.
To find a difference in CT state recombination rates, we considered the relationship
between JSC and VOC . JSC is proportional to the number of CT excitons at the
heterojunction; at VOC , there is no current through the device, so we may consider
the CT state recombination rate to be unity. If the recombination rate is intrinsically
high, then a smaller voltage is required to prevent excitons from dissociating. So if
triplet-derived CT states are less likely to recombine than singlets, we would expect
them to have a different JSC –VOC slope than singlet excitons; we see no such thing
in Fig. 4-6.
Both the generally higher singlet dissociation probability and the apparently different relationships between dissociation probability and temperature are in keeping
with our understanding of OPVs. As mentioned in Chapter 2, triplets are more
strongly bound than singlets, so there is a weaker driving force for their dissociation
at the heterojunction. The CT states that result, though, appear to have the same
recombination rate.
49
Pentacene (40 nm) / PTCBI (45 nm)
0
Short-Circuit Current (mA/cm2)
-2.00E+20
8.00E+20
1.80E+21
-0.1
532nm 250K
635nm 250K
-0.2
532nm 200K
635nm 200K
532nm 150K
-0.3
635nm 150K
Linear(532nm 250K)
Linear(635nm 250K)
Linear(532nm 200K)
-0.4
Linear(635nm 200K)
Linear(532nm 150K)
Linear(635nm 150K)
-0.5
-0.6
Light Intensity (photons/s)
Figure 4-4: Short-circuit current versus intensity at several temperatures. Both JSC ’s
linear relationship with intensity and its fall with temperature comports with Giebink
et al.’s model of OPV behavior [95].
50
Calculated Dissociation Efficiency At
Heterojunction
Dissociation Efficiency (%)
5
0.5
0
50
100
150
200
250
300
0.05
Pentacene
PTCBI
Expon.(Pentacene)
Expon.(PTCBI)
0.005
!"#$"%&'(%")*+,)
Figure 4-5: Calculated dissociation efficiency as a function of temperature. Lines
are drawn as guides to the eye. The apparent decline in dissociation efficiency with
falling temperatures may stem from the calculation’s failing to consider a temperature
dependence of diffusion.
51
Pentacene (40 nm) / PTCBI (45 nm), 10–293K
0.25
0.2
VOC (V)
0.15
0.1
532nm
635nm
0.05
0
5E-05
0.0005
0.005
Absolute Value of JSC (mA/cm2)
Figure 4-6: Open-circuit voltage as a function of the logarithm of the short-circuit
current, taken at temperatures ranging from 10K to 293K in steps of 20K. Giebink
et al. predict a linear relationship between VOC and the logarithm of JSC [95]; the
discontinuity may represent a reduction in the number of available states at low
temperatures.
52
Chapter 5
Conclusions
We have considered two methods to improve organic solar cell performance. We inserted a thin interfacial layer to create a double heterojunction, destabilizing not only
excitons but the charge-transfer states that are formed at donor-acceptor heterojunctions; and we examined how long-lived triplet excitons behave at heterojunctions.
Creating a cascade of energy levels can simultaneously improve all key metrics
of solar cell performance, but the method will benefit from more precise knowledge
of organic semiconductors’ electronic properties. Even archetypal CuPC/C60 devices
showed VOC , JSC , and conversion efficiency improvements upon adding a thin interfacial layer with an intermediate LUMO level, suggesting that still better OPVs are
possible if HOMO levels were only known more precisely. As the CELA method is
sensitive to those levels, it may itself serve as a probe in those efforts. And as other
groups did not see similarly improved exciton destabilization in similar devices, this
work calls for a better understanding of the character of bound CT states.
The long lifetime of triplet excitons makes them attractive for OPVs, but not
overwhelmingly so. Singlet excitons dissociate more efficiently than triplet excitons
even when facing the same heterojunction and device architecture. All else equal, if
a given triplet state does not diffuse appreciably further than a given singlet state, a
device based on the singlet state will offer better performance.
53
54
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