molecules
Article
SimCP3—An Advanced Homologue of SimCP2
as a Solution-Processed Small Molecular Host
Material for Blue Phosphorescence Organic
Light-Emitting Diodes
Yi-Ting Lee 1 , Yung-Ting Chang 2 , Cheng-Lung Wu 2 , Jan Golder 3 , Chin-Ti Chen 2,3, *
and Chao-Tsen Chen 1, *
1
2
3
*
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan; likewithyou@gmail.com
Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan; eveningtin@gmail.com (Y.-T.C.);
cdstant@yahoo.com.tw (C.-L.W.)
Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30050, Taiwan;
jangolder@gmx.net
Correspondences: chintchen@gate.sinica.edu.tw (C.-Ti Chen); chenct@ntu.edu.tw (C.-Tsen Chen);
Tel.: +886-2-2789-8542 (C.-Ti Chen); +886-2-3366-4200 (C.-Tsen Chen)
Academic Editor: Jwo-Huei Jou
Received: 5 September 2016; Accepted: 27 September 2016; Published: 30 September 2016
Abstract: We have overcome the synthetic difficulty of 9,90 ,900 ,9000 ,90000 ,900000 -((phenylsilanetriyl)
tris(benzene-5,3,1-triyl))hexakis(9H-carbazole) (SimCP3) an advanced homologue of previously
known SimCP2 as a solution-processed, high triplet gap energy host material for a blue
phosphorescence dopant. A series of organic light-emitting diodes based on blue phosphorescence
dopant iridium (III) bis(4,6-difluorophenylpyridinato)picolate, FIrpic, were fabricated and tested to
demonstrate the validity of solution-processed SimCP3 in the device fabrication.
Keywords: solution process; molecular host material; high triplet gap energy; SimCP2; blue
phosphorescence OLED
1. Introduction
An increasing number of high-performance blue phosphorescence organic light-emitting
diodes (OLEDs) can be found in literature reports. Although remarkable electroluminescence
(EL) efficiency as high as ~27%, ~51 lm/W, or ~54 cd/A based on sky-blue phosphorescence
dopant iridium (III) bis(4,6-difluorophenylpyridinato)picolate (FIrpic) has been achieved [1–7],
vacuum-thermal-evaporation is a common device fabrication process, but it is not favorable
for mass production or large panel fabrication. Accordingly, for more desirable production or
fabrication process, there have been a number of solution-processable small molecular host materials
for blue phosphorescence organic light-emitting diodes in recent years [8–14]. We and others
have developed and demonstrated bis(3,5-di(9H-carbazol-9-yl)phenyl)diphenylsilane (SimCP2
in Scheme 1) as one of high-performance solution-processable small molecular host materials
for yellow, green, and blue phosphorescence OLEDs [15–22]. More recently, SimCP2 hosted
FIrpic OLEDs were reported as having EL efficiency reaching ~11%, ~11 lm/W, or 23 cd/A [23].
Nevertheless, the EL of these FIrpic OLEDs with solution-processed light-emitting layer is far less
efficient than those OLEDs fabricated by vacuum-thermal-evaporation. For blue phosphorescence
OLEDs, solution-processable small molecular hosts with a high triplet energy gap (ET ) greater than
2.6 eV of the benchmark sky-blue FIrpic are still rare and highly demanded in the field. As our
ongoing effort to develop high-performance solution-processable small molecular host materials
Molecules 2016, 21, 1315; doi:10.3390/molecules21101315
www.mdpi.com/journal/molecules
Molecules 2016, 21, 1315
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Molecules
2016, 21, 1315
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for blue
phosphorescence
OLEDs, we have recently overcome synthetic difficulties and successfully
0
00
000
0000
00000
prepared
9,9 ,9 ,9 ,9 ,9 -((phenylsilanetriyl)tris(benzene-5,3,1-triyl))hexakis(9H-carbazole)
(benzene-5,3,1-triyl))hexakis(9H-carbazole)
(SimCP3) (Scheme 1) as an advanced homologue
Molecules 2016, 21, 1315
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11
(SimCP3)
(Scheme 1) as an advanced homologue of SimCP2. Herein, we report the synthesis
and
SimCP2. Herein, we report the synthesis and characterization of SimCP3 in full detail. A series of
characterization
of
SimCP3
in
full
detail.
A
series
of
SimCP3-hosted
OLEDs
are
fabricated
and
tested
SimCP3-hosted
OLEDs are fabricated and tested
to demonstrate
its as
viability
as a solution-processed
(benzene-5,3,1-triyl))hexakis(9H-carbazole)
(SimCP3)
(Scheme 1)
an advanced
homologue of
to demonstrate
itsfor
viability
asphosphorescence
a the
solution-processed
host materialoffor
sky-blue
phosphorescence
host
material
sky-blue
OLEDs.
SimCP2.
Herein,
we report
synthesis and
characterization
SimCP3
in full
detail. A series OLEDs.
of
SimCP3-hosted OLEDs are fabricated and tested to demonstrate its viability as a solution-processed
host material for sky-blue phosphorescence OLEDs.
Scheme 1. Chemical structure of SimCP2 and SimCP3. MM2 energy minimized structures (by Chem 3D)
Scheme 1. Chemical structure of SimCP2 and SimCP3. MM2 energy minimized structures (by Chem
are shown on the right, respectively.
3D) are shown on the right, respectively.
Scheme 1. Chemical structure of SimCP2 and SimCP3. MM2 energy minimized structures (by Chem 3D)
2. Results
and on
Discussion
are shown
the right, respectively.
2. Results and Discussion
In our earlier reports [16,24,25], both 9,9′-(5-(triphenylsilyl)-1,3-phenylene)bis(9H-carbazole)
2. Results and Discussion
0 -(5-(triphenylsilyl)-1,3-phenylene)bis(9H-carbazole)
(SimCP)
and SimCP2
were
synthesized
in a9,9
similar
method (Scheme 2). Either chlorotripheylsilane
In
our earlier
reports
[16,24,25],
both
or dichlorodiphenylsilane
was
reactedin
with
monolithiated
to furnish 1,3In our
earlier were
reports
[16,24,25],
both
9,9′-(5-(triphenylsilyl)-1,3-phenylene)bis(9H-carbazole)
(SimCP)
and
SimCP2
synthesized
a similar
method 1,3,5-tribromobenzene
(Scheme 2). Either chlorotripheylsilane
dibromophenyl
substituted
thriphenylsilane
and
bis-1,3-dibromophenyl
substituted
diphenylsilane,
(SimCP)
and
SimCP2
were
synthesized
in
a
similar
method
(Scheme
2).
Either
chlorotripheylsilane
or dichlorodiphenylsilane was reacted with monolithiated 1,3,5-tribromobenzene to furnish
respectively.
The silane compounds
werewith
then monolithiated
brought to a palladium-catalyzed
amination
reaction
or dichlorodiphenylsilane
reacted
1,3,5-tribromobenzene
to furnish
1,31,3-dibromophenyl
substitutedwas
thriphenylsilane
and
bis-1,3-dibromophenyl
substituted
diphenylsilane,
with
excess amount
of carbazole.
Both SimCPand
andbis-1,3-dibromophenyl
SimCP2 were obtainedsubstituted
with a reasonable
isolation
dibromophenyl
substituted
thriphenylsilane
diphenylsilane,
respectively. The silane compounds were then brought to a palladium-catalyzed amination reaction
yield
of 54% and
thethen
finalbrought
amination
It is conceivable
that the reaction
respectively.
The~40%,
silanerespectively,
compounds in
were
to reaction.
a palladium-catalyzed
amination
with yields
excesswill
amount
of carbazole.
Both
SimCP
andhigher
SimCP2
were obtained
with a reasonable
isolation
lowerof
than
that of Both
SimCP2
if the
homologue
SimCP3
byisolation
a same
with excessbe
amount
carbazole.
SimCP
and SimCP2
were obtained
withisaprepared
reasonable
yieldreaction
of
54%
and
~40%,
respectively,
in
the
final
amination
reaction.
It
is
conceivable
that
the
reaction
sequence
shown
in Scheme 2.
yield of 54%
and ~40%,
respectively,
in the final amination reaction. It is conceivable that the reaction
yieldsyields
will will
be lower
than
that
ofofSimCP2
homologue
SimCP3
is prepared
a same
be lower
than
that
SimCP2ififthe
the higher
higher homologue
SimCP3
is prepared
by aby
same
reaction
sequence
shown
in
Scheme
2.
reaction sequence shown in Scheme 2.
Scheme 2. Synthetic preparation of SimCP and SimCP2.
Due to the much more
partial
amination
products
formed SimCP2.
in the reaction of synthesizing
Scheme
Synthetic
preparation
of SimCP
Scheme
2. 2.
Synthetic
preparation
of SimCPand
and SimCP2.
SimCP3, we obtained a series of product mixtures, which were hard to separate by column
chromatography.
In addition,
the purification
SimCP3 formed
requiredina the
tedious
process
in order to
Due to the much
more partial
aminationofproducts
reaction
of synthesizing
Due
to the
the
much
more
partial
formed
thetoreaction
of
remove
amount
of
carbazole
usedmixtures,
in products
the reaction.
Alternatively,
we
have been
able to
SimCP3,
weexcess
obtained
a series
of amination
product
which
were in
hard
separate
bysynthesizing
column
synthesize
SimCPIn
byaddition,
mono-lithiated
9,9′-(5-bromo-1,3-phenylene)bis(9H-carbazole)
SimCP3,
we obtained
ausing
series
of product
mixtures,
which
werea tedious
hard toprocess
separate
by column
chromatography.
the
purification
of SimCP3
required
in (BrmCP)
order
to
to
react
with
chlorotripheylsilane.
However,
the
same
synthetic
approach
has
been
problematic
in
remove
the
excess
amount
of
carbazole
used
in
the
reaction.
Alternatively,
we
have
been
able
to to
chromatography. In addition, the purification of SimCP3 required a tedious process in order
the
preparation
of
SimCP2
and
SimCP3,
even
though
a
very
recent
literature
has
reported
the
same
synthesize
SimCP
by using
9,9′-(5-bromo-1,3-phenylene)bis(9H-carbazole)
remove
the excess
amount
of mono-lithiated
carbazole used
in the reaction. Alternatively, we have (BrmCP)
been able to
synthetic
approach
in the preparation
of SimCP2
and SimCP3
We also
approach of
0 -(5-bromo-1,3-phenylene)bis(9H-carbazole)
to reactSimCP
with
chlorotripheylsilane.
However,
same
synthetic[23].
approach
hastried
beenthe
problematic
in
synthesize
by using
mono-lithiated
9,9the
(BrmCP)
Grignard
reaction
instead
n-butyllithium),
but it
did not
work
out wellhas
either.
It seems
to us
the preparation
of (Mg
SimCP2
andofSimCP3,
even though
a very
recent
literature
reported
the same
to react with chlorotripheylsilane. However, the same synthetic approach has been problematic in
that
eitherapproach
lithiated or
BrmCP
is relatively
thealso
reaction
In the
synthetic
in magnesiated
the preparation
of SimCP2
and unstable
SimCP3 under
[23]. We
tried condition.
the approach
of
the preparation
of SimCP2
andthe
SimCP3,
even
though
a very
recent literature
has
reported
the same
case
of
SimCP2
and
SimCP3,
chance
of
either
lithiated
or
magnesiated
BrmCP
reacting
more
than
Grignard reaction (Mg instead of n-butyllithium), but it did not work out well either. It seems to
us
synthetic
approach in the preparation of SimCP2 and SimCP3 [23]. We also tried the approach of
that either lithiated or magnesiated BrmCP is relatively unstable under the reaction condition. In the
Grignard
reaction
instead of
but it did
not work out
wellreacting
either. more
It seems
case of
SimCP2(Mg
and SimCP3,
then-butyllithium),
chance of either lithiated
or magnesiated
BrmCP
thanto us
Molecules 2016, 21, 1315
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that either
lithiated
or magnesiated BrmCP is relatively unstable under the reaction condition.
In the
Molecules
2016, 21, 1315
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case of SimCP2 and SimCP3, the chance of either lithiated or magnesiated BrmCP reacting more than
dichlorophenylsilane
or trichlorophenylsilane
ratherlow.
low.InInaddition,
addition,the
thereaction
reactionisisquite
once once
with with
dichlorophenylsilane
or trichlorophenylsilane
is is
rather
quite
sensitive
to
the
purity
of
dichlorophenylsilane
or
trichlorophenylsilane,
which
is
prone
to
sensitive to the purity of dichlorophenylsilane or trichlorophenylsilane, which is prone to
hydrolysis
hydrolysis and generates hydrochloric acid that is detrimental to the reaction. Finally, we were able
and generates hydrochloric acid that is detrimental to the reaction. Finally, we were able to synthesize
to synthesize and isolate pure SimCP3 in multi-gram scale via reaction with sodium metal, i.e., a
and isolate
pure
SimCP3
Molecules
2016,
21, 1315 in multi-gram scale via reaction with sodium metal, i.e., a Würtz–Fittig-type
3 of 11
Würtz–Fittig-type coupling reaction between chlorosilane and aryl halide (Scheme 3) [26,27]. Instead
coupling
reaction
between
chlorosilane
and
aryl
halide
(Scheme
3)
[26,27].
Instead
of
a
carbon
anion of
of aonce
carbon
of BrmCP, a more
feasible silyl anion is
of rather
trichlorophenylsilne
involved
withanion
dichlorophenylsilane
or trichlorophenylsilane
low. In addition,is the
reactioninisthe
BrmCP,
a more
feasible silyl anion of trichlorophenylsilne is involved in the coupling reaction.
coupling
reaction.
quite sensitive to the purity of dichlorophenylsilane or trichlorophenylsilane, which is prone to
hydrolysis and generates hydrochloric acid that is detrimental to the reaction. Finally, we were able
to synthesize and isolate pure SimCP3 in multi-gram scale via reaction with sodium metal, i.e., a
Würtz–Fittig-type coupling reaction between chlorosilane and aryl halide (Scheme 3) [26,27]. Instead
of a carbon anion of BrmCP, a more feasible silyl anion of trichlorophenylsilne is involved in the
coupling reaction.
Scheme
3. SynthesisofofSimCP3
SimCP3 from
from BrmCP
reaction.
Scheme
3. Synthesis
BrmCPvia
viaWürtz–Fittig
Würtz–Fittig
reaction.
To facilitate the SimCP3 preparation, we improved the synthesis of BrmCP with a significantly
Td = 620 oC
60
100
40
Weight (%)
Weight (%)
80
20
0
80
Td = 620 oC
60
40 100 200 300 400 500 600 700 800 900
20
0
Temperature (oC)
(a)
Heat Flow Heat
(Endothermic
-->)
Flow (Endothermic
-->)
To
facilitate
theyield
SimCP3
preparation,
we improved
the synthesis of
BrmCPreaction,
with a SimCP3
significantly
better
isolation
of >90%
[12,28]. Regarding
the Würtz–Fittig-type
coupling
betterwas
isolation
yield
of >90%
[12,28].
SimCP3
furnished
in only
modest
yieldsRegarding
around 30%the
dueWürtz–Fittig-type
to the competition coupling
reaction ofreaction,
the potential
Scheme 3. Synthesis of SimCP3 from BrmCP via Würtz–Fittig reaction.
silicon–silicon
bond modest
formationyields
of oligosilane
polysilane
However, thereaction
desired product
was
was furnished
in only
aroundor30%
due to[29,30].
the competition
of the potential
much easier
to isolate
from the
product mixture.
In this
we
also observed
silanol side
silicon–silicon
bond
formation
ofpreparation,
oligosilane
polysilane
[29,30].of
However,
desired
product
To facilitate
the SimCP3
weor
improved
the reaction,
synthesis
BrmCP
withthe
a significantly
products,
although
they
were
readily
removed
by
column
chromatography.
In
addition,
there was
better
isolation
yield of from
>90% [12,28].
Regarding
the Würtz–Fittig-type
coupling
SimCP3
was much
easier
to isolate
the product
mixture.
In this reaction,
we reaction,
also observed
silanol
no was
excess
amountinof
carbazole
involved
in the
reaction;
hence,
no extra reaction
purification
process
was
furnished
modest
around
30%
due
the
competition
of the
potential
side products,
althoughonly
they
wereyields
readily
removed
bytocolumn
chromatography.
In addition,
there
required
for SimCP3.
are the
main advantages
of the[29,30].
Würtz–Fittig
reaction
over other
synthetic
silicon–silicon
bond These
formation
of oligosilane
or polysilane
However,
the desired
product
was
was no excess amount of carbazole involved in the reaction; hence, no extra purification process
methods
of preparing
Several mixture.
spectroscopic
and physical
much easier
to isolate SimCP3.
from the product
In this reaction,
we alsomethods
observedwere
silanolused
side to
was required
forthe
SimCP3.
These are
main
advantages
of the Würtz–Fittig
reaction
over other
characterize
homogeneity
thinthe
film
and the
optoelectronic
properties
SimCP3
products, although
they wereofreadily
removed
bynecessary
column chromatography.
In addition,ofthere
was as
synthetic
of preparing
SimCP3.
Several
spectroscopic
and
physical
methods
were
nomethods
excess amount
ofhost
carbazole
involved
in the
reaction;
hence, no
extra
purification
process
wasused to
a solution-processable
material
for FIrpic
OLEDs.
characterize
theas
homogeneity
ofare
thin
film
necessary
optoelectronic
properties
of
SimCP3 as a
required
for
SimCP3.
mainand
advantages
of the Würtz–Fittig
reaction
over other
synthetic
First,
shown
in These
Figure
1,the
SimCP3
is the
very
thermally
stable with
a thermal
decomposition
methods of
SimCP3.
Several
spectroscopic
methods
were used to
solution-processable
host
material
temperature
(Td)preparing
as
high
as
620 for
°C, FIrpic
where OLEDs.
5%
weight lossand
wasphysical
determined
by thermogravimetric
characterize
the
homogeneity
of
thin
film
and
the
necessary
optoelectronic
properties
of SimCP3
as
analysis
(TGA)
under
a
nitrogen
atmosphere.
Second,
SimCP3
exhibits
amorphous
or
semi-amorphous
First, as shown in Figure 1, SimCP3 is very thermally stable with a thermal
decomposition
a solution-processable
hostthermograms
material
for FIrpic
OLEDs. scanning calorimetry (DSC). As shown in
◦
characteristic
based
on
the
of
differential
temperature (Td ) as high as 620 C, where 5% weight loss was determined by thermogravimetric
First,
as shown
Figure 1,temperatures
SimCP3 is very
stable with
a thermal decomposition
Figure
1, SimCP3
hasinmelting
(Tmthermally
),SimCP3
crystallization
temperatures
(Tcsemi-amorphous
), and glass
analysis
(TGA)
under
nitrogen
exhibits
amorphous
or
temperature
(Td)a as
high asatmosphere.
620 °C, whereSecond,
5% weight
loss was
determined
by thermogravimetric
transition temperatures (Tg) of 330, 223, and 174 °C, respectively. Two very weak exothermic signals
characteristic
on the
thermograms
of differential
scanning
calorimetry
(DSC). As shown in
analysisbased
(TGA) under
a nitrogen
atmosphere.
Second, SimCP3
exhibits amorphous
or semi-amorphous
around 290 and 330 °C may be the inhibited crystallization process of thermally annealed SimCP3.
based
on the
thermograms(T
ofmdifferential
scanning
calorimetry (DSC).
As shown
in
Figure 1,characteristic
SimCP3 has
melting
temperatures
), crystallization
temperatures
(Tc ), and
glass transition
The Tm and Tc of SimCP3 disappeared,
and Tg was observed after the first heating scan, indicative of
◦
Figure
1,
SimCP3
has
melting
temperatures
(T
m
),
crystallization
temperatures
(T
c
),
and
glass
temperatures
(Tg ) oftendency
330, 223,ofand
174 C, respectively. Two very weak exothermic signals around 290
the amorphous
SimCP3.
temperatures (Tg) of 330, 223, and 174 °C, respectively. Two very weak exothermic signals
◦ C may be
and 330 transition
the inhibited crystallization process of thermally annealed SimCP3. The Tm and Tc
around 290 and 330 °C may be the inhibited crystallization process of thermally annealed SimCP3.
of SimCP3 disappeared,
and Tg was observed after the first heating
scan,
T 330
C indicative of the amorphous
The Tm and T100
c of SimCP3 disappeared, and Tg was observed after the first heating scan, indicative of
tendency
SimCP3.tendency of SimCP3.
theofamorphous
Heating
100 200 300 400 500 600 700 800 900
Figure
1. TGA (a) and DSC (b)
Temperature (oC)
(a)
o
m
st
1
nd
2
rd
3
o
Tc 223 C
o
Tm 330 C
st
Cooling 1nd
2
rd
Heating 1st 3
o
Tg 174 Co
nd
2
rd
3
Tc 223 C
st
Cooling 1nd
100
150
200
250
o
T 174 C
300
350
o
2
rd
3400
g
Temperature
( C)
(b)
100
150
200
250
300
o
thermograms
of SimCP3.
Temperature ( C)
350
(b)
Figure 1. TGA (a) and DSC (b) thermograms of SimCP3.
Figure 1. TGA (a) and DSC (b) thermograms of SimCP3.
400
Molecules 2016, 21, 1315
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Molecules 2016, 21, 1315
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Similarly,
or
Similarly,
amorphous
or semi-amorphous
semi-amorphous characteristics
characteristics are
are evident
evident from
from the
the X-ray
X-ray diffraction
diffraction
Molecules
2016, 21,amorphous
1315
4 of 11
(XRD)
spectrum
(Figure
2a)
of
the
solution-casting
thin
film
of
SimCP3.
The
XRD
spectrum
(XRD) spectrum (Figure 2a) of the solution-casting thin film of SimCP3. The XRD spectrum is
is basically
basically
aa featureless
halo,
indicative
of semi-amorphous
an
nature.
Macroscopically,
thefrom
homogeneity
the solution
Similarly,
amorphous
or
characteristics
are evident
thehomogeneity
X-rayof
diffraction
featureless
halo,
indicative
of amorphous
an amorphous
nature.
Macroscopically,
the
of the
(XRD)
spectrum
(Figure
2a)
of
the
solution-casting
thin
film
of
SimCP3.
The
XRD
spectrum
is
basically
spin-coated
SimCP3
thin
film
is
evident
from
the
photoluminescence
(PL)
images
of
the
neat
film
and
solution spin-coated SimCP3 thin film is evident from the photoluminescence (PL) images of the neat
a
featureless
halo,
indicative
of
an
amorphous
nature.
Macroscopically,
the
homogeneity
of
the
FIrpic-doped
SimiCP3
thin
films
(Figure
2b),
which
show
uniform
color
and
no
sign
of
agglomeration
film and FIrpic-doped SimiCP3 thin films (Figure 2b), which show uniform color and no sign of
solution
spin-coated
SimCP3
thin film
is evident
from
the
photoluminescence
(PL)(AFM)
images
of
the neat
or
phase separation.
Microscopically,
atomic
force
microscopy
(AFM)
demonstrated
thedemonstrated
homogenous
agglomeration
or phase
separation.
Microscopically,
atomic
force
microscopy
film
and
FIrpic-doped
SimiCP3
thin films
2b),orwhich
uniform
color(Figure
and no3).sign
of
and
flat
surface
morphology
of SimCP3
thin(Figure
films
with
without
a FIrpic
the
homogenous
and flat
surface
morphology
of SimCP3
thin show
films
with dopant
or without
a FIrpicWhereas
dopant
agglomeration
or
phase
separation.
Microscopically,
atomic
force
microscopy
(AFM)
demonstrated
thin
film3).
ofWhereas
poly(3,4-ethylenedioxythiophene)
doped with poly(styrenesulfonate)
(PEDOT:PSS[CC1])
(Figure
thin film of poly(3,4-ethylenedioxythiophene)
doped with poly(styrenesulfonate)
theahomogenous
and
flat surface
morphology
of SimCP3
thin films
with or without a (RMS)
FIrpic dopant
had
higher
surface
roughness
of
0.68
nm,
a
rather
small
values
of
root-mean-square
roughness
(PEDOT:PSS[CC1]) had a higher surface roughness of 0.68 nm, a rather small values of root-mean(Figure
3).
Whereas
thin
film
of
poly(3,4-ethylenedioxythiophene)
doped
with
poly(styrenesulfonate)
of
0.25
and
0.33
nm
was
observed
for
the
thin
film
of
SimCP3
and
SimCP3:FIrpic
(10
wt
%),
respectively.
square (RMS) roughness of 0.25 and 0.33 nm was observed for the thin film of SimCP3 and
(PEDOT:PSS[CC1]) had a higher surface roughness of 0.68 nm, a rather small values of root-meanRegarding
the (10
flatness
the thin films,
none ofthe
theflatness
discernable
features
AFMofsurface
images
SimCP3:FIrpic
wt %),ofrespectively.
Regarding
of the thin
films,innone
the discernable
square (RMS) roughness of 0.25 and 0.33 nm was observed for the thin film of SimCP3 and
had
a
height
over
5.5
nm,
2.1
nm,
and
2.7
nm,
for
PEDOT:PSS,
SimCP3,
and
SimCP3:Firpic
thin
features
in AFM(10
surface
images
had a height
over the
5.5 flatness
nm, 2.1 nm,
2.7
nm, none
for PEDOT:PSS,
SimCP3,
SimCP3:FIrpic
wt %),
respectively.
Regarding
of theand
thin
films,
of the discernable
films,
respectively.
and
SimCP3:Firpic
thin images
films, respectively.
features
in AFM surface
had a height over 5.5 nm, 2.1 nm, and 2.7 nm, for PEDOT:PSS, SimCP3,
and SimCP3:Firpic thin films, respectively.
(a)
2000
1500
(a)
Intensity
Intensity
(a.u.)(a.u.)
2000
1500
1000
1000
500
500
5 10 15 20 25 30 35 40 45 50 55 60
2 Theta
5 10 15 20
25 30 (degree)
35 40 45 50 55 60
2 Theta (degree)
Figure 2.
2. (a)
(a) XRD
XRD spectra
spectra of
of the
the solution
solution drop-casting
drop-casting thin
SimCP3 on
on silicon
silicon wafer;
wafer; (b)
(b) PL
PL
Figure
thin film
film of
of SimCP3
Figureof
2. solution
(a) XRD spin-coated
spectra of thethin
solution
drop-casting
thin film
ofwt
SimCP3
on
silicon wafer; (b) PL
images
films
of
SimCP3:
FIrpic
(~10
%)
and
SimCP3.
images of solution spin-coated thin films of SimCP3: FIrpic (~10 wt %) and SimCP3.
images of solution spin-coated thin films of SimCP3: FIrpic (~10 wt %) and SimCP3.
(a)
(a)
(b)
(b)
(c) (c)
Figure
images of
of ITO/PEDOT:PSS
ITO/PEDOT:PSS(a);
(a);ITO/PEDOT:PSS/SimCP3
ITO/PEDOT:PSS/SimCP3
Figure 3.3. 55 ×× 55 μm
μm AFM
AFM images
(b);(b);
andand
Figure 3. 5 × 5 µm AFM images of ITO/PEDOT:PSS (a); ITO/PEDOT:PSS/SimCP3 (b); and
ITO/PEDOT:PSS/SimCP3:FIrpic
(10
wt
%)
(c).
ITO/PEDOT:PSS/SimCP3:FIrpic (10 wt %) (c).
ITO/PEDOT:PSS/SimCP3:FIrpic (10 wt %) (c).
The
of the
the host
hostmaterial
materialfor
fora ablue
blue
phosphorescence
dopant
such
Themost
mostimportant
important property
property of
phosphorescence
dopant
such
as as
The
of the host
for
awith
blue
phosphorescence
dopant
such
as
FIrpic
ET.T.We
Weimportant
estimated property
as
neat
time-gated
acquisition
PL PL
spectra
FIrpic
isisEmost
estimated
the ETT of
of SimCP3
SimCP3
asaamaterial
neatthin
thinfilm
film
with
time-gated
acquisition
spectra
FIrpic
istemperatures
ET . We estimated
the 4a,b).
ET of SimCP3
as a neat
thin
film
with time-gated
acquisition
PL spectra
low
temperatures
(Figure
From
with
either
variable
delayed
time
or variable
lowlow
atatlow
(Figure
4a,b).
From spectra
spectra
with
either
variable
delayed
time
or variable
at
low
temperatures
(Figure
4a,b).
From
spectra
with
either
variable
delayed
time
or
variable
low
max
around
400
nm
is
the
delayed
temperature,
it
can
be
confirmed
that
the
emission
band
with
a
λ
temperature, it can be confirmed that the emission band with a λmax around 400 nm is the delayed
fluorescence
and
a
λ
max
around
525
nm
is
the
phosphorescence.
By
taking
the
onset
wavelength
of
the
temperature,
it can
emission
band with By
a λmax
around
400 nm
is the delayed
fluorescence and
a λbe
maxconfirmed
around 525that
nm the
is the
phosphorescence.
taking
the onset
wavelength
of the
high
energy
edge
of
phosphorescence
bands,
we
obtain
~461–468
nm
(2.69–2.65
eV)
as
the
optical
fluorescence
and
a
λ
around
525
nm
is
the
phosphorescence.
By
taking
the
onset
wavelength
max
high energy edge of phosphorescence bands, we obtain ~461–468 nm (2.69–2.65 eV) as the optical
energy
gapfor
forEETT..From
From
our
measurements, the
EET TofofSimCP3
is is
sufficiently
larger
than
thatthat
ofeV)
FIrpic,
of
the high
energy
edgeour
of phosphorescence
we obtain
~461–468
nm
(2.69–2.65
as the
energy
gap
measurements,
thebands,
SimCP3
sufficiently
larger
than
of FIrpic,
ensuring
triplet
state
exciton
on
the
phosphorescence
dopant
rather
than
its
host
SimCP3.
The
UVoptical
energy
gap
for exciton
ET . From
the dopant
ET of SimCP3
sufficiently
larger than
that
of
ensuring
triplet
state
onour
themeasurements,
phosphorescence
rather is
than
its host SimCP3.
The
UVvisibleensuring
absorption
and state
fluorescence
spectra
of SimCP3 in solution
are rather
illustrated
initsFigure
4c.
FIrpic,
triplet
exciton
on
the
phosphorescence
dopant
than
host
SimCP3.
The
visible absorption and fluorescence spectra of SimCP3 in solution are illustrated in Figure 4c.
UV-visible absorption and fluorescence spectra of SimCP3 in solution are illustrated in Figure 4c.
5 of 11
5 of 11
60000
50000
Neat film @14 K
Delayed time
1 ms
7 ms
13 ms
22 ms
28film
ms@14 K
Neat
(a)
40000
Molecules 2016, 21, 1315
60000
20000
Delayed time
1 ms
7 ms
40000
13 ms
0
30000
22 ms
350 400 450 500 550 600 650 700 750
28 ms
50000
(a)
461 nm
10000
200000
180000
160000
140000
120000
100000
80000
200000
60000
180000
40000
160000
140000
20000
1200000
Neat film @ 121 ms delayed time
Temperature
250K
200K
150K
100K
50K
Neat film
@ 121 ms delayed time
14K
Temperature
(b)
5 of 11
(b)
468 nm
250K
200K
150K
100K
350 50K
400 450 500 550 600 650 700 750
14K
Wavelength (nm)
2
120000
40000
100000
ε (M-1cm-1)
ε (M-1cm-1)
100000
Wavelength
(nm)
80000
140000 (c)
60000
0.8
368 nm
0.80.2
0
60000
20000
1.0
Wavelength
(nm)
UV-Visible absorption 1.4
0.6
Fluorescence
347 nm
1.2
in CH2Cl2
1.00.4
20000
80000
250
40000
2
300
350
347 nm
400
450
500
Wavelength (nm)
550
Fluorescence Intensity (a.u.)
100000
80000
Wavelength (nm)
60000
468 nm
461 nm
40000
10000
140000 (c)
UV-Visible
20000 absorption 1.4
0
Fluorescence
0
120000
1.2
368 nm
350 400 450 500 550 600 650 700 750
350
400Cl450 500 550 600 650 700 750
in CH
20000
Fluorescence Intensity (a.u.)
Emission Intensity (Counts)
30000
Emission Intensity
(Counts)
Emission
Intensity (Counts)
Emission Intensity (Counts)
Molecules 2016, 21, 1315
Molecules 2016, 21, 1315
0.60.0
600
0.4
0.2
0
0.0
(a)Variable
spectra
of
(b)
(a) Variabletime-gated
time-gatedPL
PL
spectra
ofSimCP3
SimCP3
thin
film;
(b) Variable
Variable temperature,
temperature, 121
121 msec
msec
250
300 350
400 450 thin
500 film.
550 600
Figure
Figure 4.
4.
Wavelength
(nm)temperature
time-delayed
(c)
Room
time-delayed PL
PL spectra
spectra of
of SimCP3
SimCP3 thin
thin film.
film;
(c) Room
temperature absorption
absorption and
and PL
PL spectra
spectra of
of
SimCP3
in
dichloromethane
solution.
SimCP3 in dichloromethane solution.
Figure 4. (a)Variable time-gated PL spectra of SimCP3 thin film. (b) Variable temperature, 121 msec
time-delayed PL spectra of SimCP3 thin film. (c) Room temperature absorption and PL spectra of
Knowing
an
of of
3.573.57
eV (347
nm shown
in Figure
4c) and
theand
HOMO
energy
SimCP3
inoptical
dichloromethane
solution.
Knowing
an
opticalenergy
energygap
gap
eV (347
nm shown
in Figure
4c)
the HOMO
level
of level
5.95 eV
(Figure
5a), which
acquired
by a low-energy
photoelectron
spectrometer
Rikenenergy
of 5.95
eV (Figure
5a),was
which
was acquired
by a low-energy
photoelectron
spectrometer
Knowing
an optical
energy
gap
ofSimCP3
3.57 eV (347
nm
shown
in Figure as
4c)~2.4
and the
HOMO
energy
Keiki
AC-2,
the
LUMO
energy
level
of
can
thus
be
calculated
eV.
Together
with
the
Riken-Keiki AC-2, the LUMO energy level of SimCP3 can thus be calculated as ~2.4 eV. Together
level
of
5.95
eV
(Figure
5a),
which
was
acquired
by
a
low-energy
photoelectron
spectrometer
RikenHOMO
LUMO
andlevels
ET ofand
TAPC
transporting
or electronorblocking
with theand
HOMO
andenergy
LUMOlevels
energy
ET (hole
of TAPC
(hole transporting
electron material)
blocking
Keiki
AC-2,[32],
the LUMO
energy level
of SimCP3
can thus be
calculated
as ~2.4material)
eV. Together
the
[31,32],
FIrpic
and
TmPyPB
(electron
transporting
or
hole
blocking
[33],with
the[33],
energy
material)
[31,32],
FIrpic
[32],
and
TmPyPB
(electron
transporting
or
hole
blocking
material)
HOMO and LUMO energy levels and ET of TAPC (hole transporting or electron blocking material) the
level
alignment
of
materials
involved
in
OLEDs
is
depicted
in
Figure
5b.
Therefore,
TAPC,
SimCP3,
energy
levelFIrpic
alignment
of materials
involved
in OLEDsorishole
depicted
in Figure
5b.[33],
Therefore,
TAPC,
[31,32],
[32], and
TmPyPB (electron
transporting
blocking
material)
the energy
and
TmPyPB
all
have HOMO
and
LUMO
energy
levels
that levels
encompass
those of FIrpic,
and
FIrpic
SimCP3,
TmPyPB
all have
HOMO
LUMO
energy
thatTherefore,
encompass
those
of FIrpic,
level and
alignment
of materials
involved
in and
OLEDs
is depicted
in Figure 5b.
TAPC,
SimCP3,
has
the
smallest
E
T among all materials involved in the OLED fabrication. This is a good set of materials
and TmPyPB
all smallest
have HOMO
and LUMO
energy levels
that encompass
those offabrication.
FIrpic, and FIrpic
and FIrpic
has the
ET among
all materials
involved
in the OLED
This is a
enabling
the
confinement
of
triplet
exciton
on
the
FIrpic
phosphorescence
dopant.
Accordingly,
we
has
the
smallest
E
T
among
all
materials
involved
in
the
OLED
fabrication.
This
is
a
good
set
of
materials
good set of materials enabling the confinement of triplet exciton on the FIrpic phosphorescence
fabricated
three
types
of
FIrpic
OLED
having
SimCP3
as
a
solution-processed
host
material:
a
singleenabling
the
confinement
of
triplet
exciton
on
the
FIrpic
phosphorescence
dopant.
Accordingly,
we
dopant. Accordingly, we fabricated three types of FIrpic OLED having SimCP3 as a solution-processed
layer
device (SL)—ITO/PEDOT:PSS/SimCP3:FIrpic/CsF/Al—a
bilayer device
(BL)—ITO/PEDOT:PSS/
three
types of FIrpicdevice
OLED having
SimCP3 as a solution-processed
host
material: a singlehost fabricated
material:
a single-layer
(SL)—ITO/PEDOT:PSS/SimCP3:FIrpic/CsF/Al—a
bilayer
layer
device
(SL)—ITO/PEDOT:PSS/SimCP3:FIrpic/CsF/Al—a
bilayer
device
(BL)—ITO/PEDOT:PSS/
SimCP3:FIrpic/TmPyBP/CsF/Al—and
a
trilayer
device
(TL)—ITO/PEDOT:PSS/TAPC/SimCP3:FIrpic/
device (BL)—ITO/PEDOT:PSS/SimCP3:FIrpic/TmPyBP/CsF/Al—and a trilayer device (TL)—ITO/
SimCP3:FIrpic/TmPyBP/CsF/Al—and
trilayerconcentration
device (TL)—ITO/PEDOT:PSS/TAPC/SimCP3:FIrpic/
TmPyBP/CsF/Al.
After optimization, theadopant
FIrpic was 4the
wt dopant
% in all concentration
three devices.
PEDOT:PSS/TAPC/SimCP3:FIrpic/TmPyBP/CsF/Al.
After of
optimization,
TmPyBP/CsF/Al.
AfterCsF/Al
optimization,
the each
dopant
concentration
of
FIrpic
was
4 wt
% in allwas
threefabricated
devices. by
Except
for
TmPyBP
and
cathode,
thin
film
layer
of
such
FIrpic
OLEDs
of FIrpic
was
wt % in and
all three
devices.
for film
TmPyBP
and
CsF/Al
cathode,
each
thin film
Except
for4 TmPyBP
CsF/Al
cathode,Except
each thin
layer of
such
FIrpic OLEDs
was
fabricated
by layer
solution
process,
i.e., the
method.
of such FIrpic
OLEDs
wasspin-coating
fabricated by
solution process, i.e., the spin-coating method.
solution process, i.e., the spin-coating method.
CPS^0.5
CPS^0.5
12
(a)
11 12 (a)
11
10
10
9
9
8
8
7
7
6
6
5
5
5.955.95
eVeV
4
4
3
3
2
2
1
1
0
0
5.3 5.4 5.5 5.6 5.7 5.8 5.9 6.0 6.1 6.2 6.3 6.4 6.5
5.3 5.4 5.5 5.6 5.7 5.8 5.9 6.0 6.1 6.2 6.3 6.4 6.5
Energy (eV)
Energy (eV)
Figure 5. (a) Low-energy photoelectron spectra of SimCP3; (b) Energy level alignment of materials
Figure 5.
5. (a)
(a) Low-energy
Low-energy photoelectron
photoelectron spectra
spectra of
of SimCP3;
SimCP3; (b)
(b) Energy
Energy level
level alignment
of materials
materials
Figure
alignment of
involved
in OLEDs.
involved in
in OLEDs.
OLEDs.
involved
Molecules 2016, 21, 1315
6 of 11
EL characteristics of SL, BL, and TL devices are summarized in Figure 6 and Table 1. All three
devices exhibit typical sky-blue emissions of FIrpic with a λmax of ~468 nm, although a variation of
emission shoulder band around 500 nm (Figure 6a) was observed. Since both BL and TL devices
Molecules 2016, 21, 1315
6 of 11
have a similar intensity of the shoulder band and are both more intense than that of the SL device,
it can be logically
inferredofthat
TmPyPB
effectively
blocks the
triplet6 and
exciton
FIrpic,
confining the
EL characteristics
SL, BL,
and TL devices
are summarized
in Figure
Table of
1. All
three devices
charge exhibit
recombination
mostly
in the of
SimCP3:FIrpic
light-emitting
layer (i.e.,
farther
away from the
typical sky-blue
emissions
FIrpic with a λmax
of ~468 nm, although
a variation
of emission
band
around
500 nm (Figure
6a) was
observed.
Since
both BLto
and
TLlow
devices
a similar
cathodeshoulder
surface)
[19].
Therefore,
the same
reason
can be
ascribed
the
EL have
efficiency
of the SL
intensity
of
the
shoulder
band
and
are
both
more
intense
than
that
of
the
SL
device,
it
can
be
logically
device (Figure 6b,c), where the triplet exciton of FIrpic generates in an area too close to the cathode
inferred that TmPyPB effectively blocks the triplet exciton of FIrpic, confining the charge recombination
surface and hence is prone to being quenched. Similarly, the TAPC of the TL device plays a role
mostly in the SimCP3:FIrpic light-emitting layer (i.e., farther away from the cathode surface) [19].
of inhibiting
triplet
exciton
from
the
the device.
the EL
efficiency
Therefore,
the same
reason
can contacting
be ascribed to
theanode
low ELofefficiency
of the Therefore,
SL device (Figure
6b,c),
of the TL
device
is promoted
when compared
with
the
device.surface
The maximum
where
the triplet
exciton offurther
FIrpic generates
in an area too
close
to BL
the cathode
and hence isexternal
prone
to being(EQE),
quenched.
Similarly,
the TAPC
of the
TL power
device plays
a role of(PE)
inhibiting
excitonare ~9%,
quantum
efficiency
current
efficiency
(CE),
and
efficiency
of thetriplet
TL device
fromand
contacting
the anode
of the device.
Therefore,
the device
EL efficiency
of the
TL device
is promoted
~20 cd/A,
12 lm/W,
respectively.
However,
the TL
has the
lowest
current
density and the
when compared with the BL device. 2The maximum external quantum efficiency (EQE),
highestfurther
turn-on
voltage—around 5 V at 1 cd/m (see Figure 6d)—among the three devices. This can
current efficiency (CE), and power efficiency (PE) of the TL device are ~9%, ~20 cd/A, and 12 lm/W,
be attributed
to
the two exciton blocking layers in the device, which limits the current density and
respectively. However, the TL device has the lowest current density and the highest turn-on
elevatesvoltage—around
the driving voltage
the2 (see
device.
three
devices
relatively
mild
5 V at 1 of
cd/m
FigureNevertheless,
6d)—among theall
three
devices.
Thisexhibited
can be attributed
to
2 (about 38, 30, and 19 mA/cm2 of the SL,
efficiency
Even
at thelayers
highinbrightness
of 3000
cd/m
theroll-off.
two exciton
blocking
the device, which
limits
the current
density and elevates the driving
of the device.
Nevertheless,
three devices
exhibited
mild
Even
BL, andvoltage
TL devices),
an EQE
of 3.6%,all4.5%,
and 7.4%
wererelatively
achieved
byefficiency
the SL,roll-off.
BL, and
TLatdevices,
2 (about 38, 30, and 19 mA/cm2 of the SL, BL, and TL devices), an EQE of
the
high
brightness
of
3000
cd/m
which are 95%, 62%, and 81% of the maximum EL efficiency, respectively (Figure 6c). Compared with
3.6%, 4.5%, and 7.4% were achieved by the SL, BL, and TL devices, which are 95%, 62%, and 81% of
those of vacuum-thermal-evaporation OLEDs [2,4], the efficiency roll-off observed for the three devices
the maximum EL efficiency, respectively (Figure 6c). Compared with those of vacuum-thermalis relatively
modest.
evaporation
OLEDs [2,4], the efficiency roll-off observed for the three devices is relatively modest.
0.5
450
500
550
600
650
10
10
1
0
1
10 20 30 40 50 60 70 80 90 100
2
Wavelength (nm)
Current Density (mA/cm )
EQE (%)
10
0
10 20 30 40 50 60 70 80 90 100
2
Current Density (mA/cm )
10000
2
SL
BL
TL
Current Density (mA/cm )
(c)
1
100
1000
100
10000
(d)
SL
BL
TL
2
0.0
400
SL
BL
TL
(b)
Brightness (cd/m )
EL Intensity (a.u.)
1.0
Current Efficiency (cd/A)
SL
BL
TL
Power Efficiency (lm/w)
(a)
1000
10
100
1
10
0.1
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Voltage (V)
Figure 6. Electroluminescence characteristics of SimCP3-hosted FIrpic OLEDs. (a) EL spectra at ~10
Figure 6. Electroluminescence characteristics of SimCP3-hosted FIrpic OLEDs. (a) EL spectra
volt; (b) Current density dependent current efficiency and power efficiency; (c) Current density
at ~10 volt;
(b) Current density dependent current efficiency and power efficiency; (c) Current density
dependent external quantum efficiency; (d) Voltage dependent current density and brightness.
dependent external quantum efficiency; (d) Voltage dependent current density and brightness.
Table 1. Electroluminescence characteristics of SimCP3-hosted FIrpic OLEDs
Table 1.
Electroluminescence
characteristics
of SimCP3-hosted
FIrpic
2)
e (%) OLEDs.
Device
λmax
(nm) Von a (V)
Lmax b (cd/m
CE c (cd/A)
PE d (lm/W)
EQE
CIEx,y f
Device
SL
BL
TL
a
SL
472, 492
5.0
8078
7.7, 7.0, 7.7
3.4, 3.4, 3.0
3.8, 3.4, 3.8 0.15, 0.32
c
e
d
BL (nm)476, 500
94402 )
16.0,
12.7,
10.8 11.1, 7.2,
7.3, 5.8, 4.9
λmax
Von a (V)4.5 Lmax b (cd/m
CE
(cd/A)
EQE 0.16,
(%)0.34
PE 5.2
(lm/W)
CIEx,y f
TL
476, 500
5.0
8880
19.6, 18.0, 17.4 12.3, 9.5, 7.3 9.1, 8.4, 8.1 0.16, 0.34
472, 492
5.0
8078
7.7, 7.0, 7.7
3.4, 3.4, 3.0
3.8, 3.4, 3.8
0.15, 0.32
a Turn-on
b Maximum luminance.
c Current
voltage of4.5
10 cd/m2 (Von).
efficiency:
476, 500
9440
16.0, 12.7, 10.8
11.1,
7.2, 5.2 maximum,
7.3, 5.8, 4.9at 100,0.16, 0.34
476, 500
5.0
8880 efficiency:19.6,
18.0, 17.4at 100,12.3,
7.3 cd/m
9.1,
8.4, 8.1
0.16, 0.34
d Power
2, respectively.
maximum,
and9.5,
1000
and 1000
cd/m2, respectively.
b Maximum luminance. 2 c Current efficiency:
e External
Turn-on
voltage
of 10 efficiency:
cd/m2 (Vmaximum,
maximum,
quantum
at 100, and 1000 cd/m , respectively. f Measured
at ~10 volt. at 100,
on ).
2
d
and 1000 cd/m , respectively. Power efficiency: maximum, at 100, and 1000 cd/m2 , respectively. e External
quantum efficiency: maximum, at 100, and 1000 cd/m2 , respectively. f Measured at ~10 volt.
Molecules 2016, 21, 1315
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3. Materials and Methods
3.1. General Instrumentation
1 H-
and 13 C-NMR spectra including distortionless enhancement by polarization (DEPT) were
measured on a Bruker AV-400 MHz or AV-500 MHz NMR Fourier transform spectrometer (Bruker,
Billerica, MA, USA) at room temperature. Elemental analyses (on a FLASHEA 1112 Series, Thermo
Electron Corporation, Waltham, MA, USA), matrix-assisted laser desorption/ionization time-of-flight
mass spectroscopy (MALDI-TOF MS, The New UltraflextremeTM , Bremen, Germany), and fast
atom bombardment (FAB) mass spectroscopy (FAB-MS, JMS-700 double focusing mass spectrometer,
JEOL, Tokyo, Japan) were performed by the Elemental Analyses Laboratory and Mass Spectroscopic
Laboratory, respectively, in-house service of the Institute of Chemistry, Academic Sinica. Infrared
absorption spectra were recorded on a Varian 640-IR FT-IR spectrometer (Palo Alto, CA, USA). The
sample of SimCP3 was ground together with KBr powder and compressed into a thin disc. Thermal
decomposition temperatures (Td 0 s) of the host materials were measured by thermogravimetric analysis
(TGA) using Perkin-Elmer TGA-7 analyzer systems (Waltham, MA, USA). Melting temperatures (Tm 0 s),
crystallization temperatures (Tc 0 s), and glass transition temperatures (Tg 0 s) of the host materials were
measured by differential scanning calorimetry (DSC) using Perkin-Elmer DSC-6 analyzer systems
(Waltham, MA, USA), using a scan rate of 10 ◦ C/min. The X-ray diffraction measurement of the
solution-casting thin film was carried out by using a Bruker D8 Advance diffractometer (Billerica,
MA, USA) equipped with a Lynxeye detector. The radiation used was a monochromatic Cu Kα
bea‘m of wavelength λ = 0.154 nm. AFM measurements were carried out with XE-100 from Park
Systems (Suwon, South Korea) using a non-contact mode to obtain topography images of the thin
films, which were prepared by a spin-coating chlorobenzene solution of SimCP3 or SimCP3:FIrpic
on a PEDOT:PSS-coated ITO-glass substrate. UV-visible absorption spectra were recorded on a
Hewlett-Packard 8453 diode array spectrophotometer (Palo Alto, CA, USA). Room temperature
fluorescence spectra were recorded on a Hitachi fluorescence spectrophotometer F-4500 (Tokyo, Japan).
The ionization potentials (or HOMO energy levels) of SimCP3 were determined with a low energy
photo-electron spectrometer (Riken-Keiki AC-2, Riken Keiki Co., Ltd., Tokyo, Japan). LUMO energy
levels were estimated by subtracting the optical energy gap (Eg ) from HOMO energy levels. Eg was
determined by the on-set absorption energy from the absorption spectra of the materials as thin
film. To measure the triplet energy gap (ET ) of a thin film sample, we established a system with a
temperature control of Model 350 (LakeShore Company, Westerville, OH, USA) as low as 10 K by
Model 22C/350C Cryodyne Refrigerators (Janis Research Company, Woburn, MA, USA) and a tunable
laser with excitation wavelengths of 213, 266, 355, 532, and 1064 nm (Brilliant B laser, Quantel Company,
Newbury, Berkshire, UK), which was coupled with a time-delay controller of LP920 flash photolysis
spectrometer (Edinburgh Instrument, Kirkton Campus, Livingston, England). We also acquired
the triplet energy gap of SimCP3 in frozen 2-Me-THF by using Fluorolog III photoluminescence
spectrometer (Kirkton Campus, Livingston, UK), which was equipped a xenon lamp as excitation
light source.
3.2. Device Fabrication and Performance Measurements
The ITO substrate was purchased from Buwon Precision Sciences (Taoyuan, Taiwan with a sheet
resistance around 25 Ω/sq and a thickness of 100 nm. The hole transport layer—PEDOT:PSS[CC2]
(CH8000)—was purchased from Sigma-Aldrich (St. Louis, MO, USA). The blue phosphorescence
dopant FIrpic, the electron transport layer of TmPyPB, and the hole transport layer of TAPC were
obtained commercially from Lumtec (Hsinchu, Taiwan). The indium tin oxide (ITO) substrates were
pre-coated with the PEDOT:PSS hole transporting layer (HTL) and baked in the air at 150 ◦ C for 10 min.
Blends of SimCP3 and FIrpic in chlorobenzene were spin-coated on ITO/PEDOT:PSS. The active layer
was then annealed at 90 ◦ C for 30 min. After spin-coating the active single-layer, the TmPyPB
(50 nm), an ultrathin CsF (2 nm) interfacial layer, and then an aluminum cathode (100 nm) were
Molecules 2016, 21, 1315
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vacuum-thermal deposited. In the triple-layer device, a thin layer (~20 nm) of TAPC was spin-coated
first on PEDOT:PSS. In this case, TAPC layer was thermally annealed at 90 ◦ C for 10 min before
spin-coating SimCP3:FIrpic. A surface profiler (Dektak 150, Veeco Instruments, Plainview, NY, USA)
was used for calibrating the thickness of HTL and the active layer. After the deposition of the cathode,
the devices were hermetically sealed with glass and UV-cured resins in a glove box (O2 and H2 O
concentration below 0.1 ppm). The device’s active area was 0.04 cm2 and was defined by the self-made
shadow mask applied in the cathode deposition. Current density and voltage characteristics were
measured by a dc current–voltage source meter (Keithley 2400, Tektronix, Beaverton, OR, USA), and
the device brightness (or electroluminance, cd/m2 ) and EL spectra were monitored and recorded with
a spectrophotometer (PR650; Photo Research, Syracuse, NY, USA). The EQE of OLEDs was calculated
from the luminance, current density, and the EL spectra, assuming the EL of the OLED is isotropic, i.e.,
a Lambertian emission [34].
3.3. Materials Preparation
All chemical and reagents were obtained from commercial suppliers and used without further
purification. Solvents were purified according to the standard procedures. Unless specified condition,
all reaction were performed under a nitrogen atmosphere using standard Schlenk techniques. Materials
involved in OLED fabrication, 4,40 -(cyclohexane-1,1-diyl)bis(N,N-di-p-tolylaniline) (TAPC), and
3,30 -(50 -(3-(pyridin-3-yl)phenyl)-[1,10 :30 ,100 -terphenyl]-3,300 -diyl)dipyridine (TmPyPB) were purchased
from Lumtec (Hsinchu, Taiwan) and Ultra Fine Chemical Technology Corp. (Degussa, Dusseldorf,
Germany), respectively. They were used as received without further purification.
Synthesis and characterization of 9,90 -(5-Bromo-1,3-phenylene)bis(9H-carbazole), BrmCP. The synthesis
and isolation of BrmCP are modified from a known procedure of a patent report [28]. To an anhydrous
DMF solution (200 mL) containing sodium hydride (60% suspension in oil, 13 g, 0.31 mole), carbazole
(52 g, 0.31 mol) was slowly added. The mixture was stirred for 1 h at room temperature. After cooling
in an ice bath, 3,5-difluorobromobenzene (12 mL, 0.1 mol) was added dropwise. The solution mixture
was then heated at 130 ◦ C for 12 h. After cooling, an excess amount of ethanol–water mixture (10:1)
was added and stirred, resulting in white precipitates. The product was isolated by suction filtration,
and further purification was achieved by zone-temperature sublimation to afford white solid (44.7 g,
92%). 1 H-NMR (400 MHz, CDCl3 ): δ (ppm) 8.13 (d, 4H, J = 7.6 Hz), 7.85 (s, 2H), 7.78 (s, 1H), 7.53 (d,
4H, J = 7.6 Hz), 7.45 (t, 4H, J = 7.6 Hz), 7.32 (t, 4H, J = 7.6 Hz). 13 C-NMR (100 MHz, CDCl3 ): δ (ppm)
140.72, 140.55, 128.89, 126.58, 124.32, 124.18, 124.04, 120.98, 120.77, 109.81. MALDI-HRMS: calcd MW
486.0726, m/z = 486.0705 (M+ ).
Synthesis and characterization of 9,90 ,900 ,9000 ,90000 ,900000 -((phenylsilanetriyl)tris(benzene-5,1,3-triyl))
hexakis(9H-carbazole) (SimCP3). To a refluxing toluene (4 mL), sodium metal (0.2 g, 8.3 mmol) was
added. The mixture was stirred for 10 min. Then, BrmCP (2.0 g, 4.0 mmol) and trichloro(phenyl)silane
(0.2 mL, 1.3 mmol) dissolved in toluene (4 mL) were added slowly to the refluxing toluene
containing sodium metal through an additional funnel. After 3 h of reaction, the reaction solution
was cooled to room temperature and an excess amount of methanol was added. The resulting
precipitations were isolated by suction filtration and subjected to column chromatography (silica
gel, dichloromethane/hexanes: 2/3). A white solid was obtained with a yield of 34% (0.6 g). FT-IR
(λ, cm−1 ): 3044 (w), 3013 (w), 1623 (w), 1580 (s), 1490 (m), 1480 (m), 1447(s), 1416 (m), 1374 (m), 1330
(s), 1309 (s), 1227 (s), 1191 (m), 1155 (m), 1120 (m), 1098 (w), 1027 (w), 1002 (w), 959 (w), 923 (w), 904
(w), 882 (w), 840 (w), 798 (w), 778 (w), 746 (s), 706 (s), 722 (s). UV-Vis (CH2 Cl2 ): λmax 293, 311, 325,
339 (3.61 × 104 cm−1 ·M−1 ). 1 H-NMR (400 MHz, CDCl3 ): δ (ppm) 8.07–8.03 (m, 18H), 7.87 (m, 5H),
7.48–7.47 (m, 3H), 7.27 (d, 12H, J = 8.4 Hz), 7.15 (t, 12H, J = 7.6 Hz), 7.00 (t, 12H, J = 7.6 Hz). 13 C- and
DEPT NMR (125 MHz, CDCl3 ): δ (ppm) 140.54, 140.04, 137.07, 136.27 (DEPT-90), 133.69, 132.92
(DEPT-90), 131.40 (DEPT-90), 129.19 (DEPT-90), 127.27 (DEPT-90), 126.47 (DEPT-90), 123.89, 120.74
Molecules 2016, 21, 1315
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(DEPT-90), 120.62 (DEPT-90), 109.62 (DEPT-90). MALDI-HRMS: calcd MW 1326.4800, m/z = 1326.4757
(M+ ). Anal. Found (calcd) for C96 H62 N6 Si: C 86.66 (86.85), H 4.92 (4.71), N 6.37 (6.33).
4. Conclusions
In summary, we have overcome the synthetic difficulty of SimCP3, an advanced homologue of
previously known SimCP2. We have examined the amorphous nature and the spectroscopic properties
(HOMO and LUMO energy levels as well as the triplet energy gap) of SimCP3. Three sky-blue
phosphorescence OLEDs have been fabricated with a SimCP3:FIrpic light-emitting layer, which
was spin-coated from solution. A trilayer device exhibits the highest EL efficiency of ~9%, ~20 cd/A,
or ~12 lm/W, demonstrating the viability of SimCP3 as a solution-processed host material for sky-blue
phosphorescence OLEDs.
Acknowledgments: We thank the Ministry of Science and Technology (MOST 103-2113-M-001-021-MY3),
Academia Sinica, and National Taiwan University for financial support. We acknowledge Jiun-Haw Lee of
National Taiwan University in the calculation of EQE of the OLEDs in this work.
Author Contributions: Chin-Ti Chen and Chao-Tsen Chen conceived and designed the experiments;
Cheng-Lung Wu, Yung-Ting Chang, Jan Golder, and Yi-Ting Lee performed the experiments; Yung-Ting Chang
and Yi-Ting Lee analyzed the data; Chin-Ti Chen and Chao-Tsen Chen wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of SimcP3 are available from the authors.
© 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).