Poly(m-phenylene): Conjugated polymer host with high triplet energy for efficient blue electrophosphorescence

Article abstract of DOI:10.1021/ma102091g

A poly(m-phenylene) derivative tethering carbazole unit (PmPCz) is reported to have a triplet energy level (E_T) as high as 2.64 eV, sufficient to host blue electrophosphorescence. To our best knowledge, this is the first conjugated polymer with

Full text of DOI:10.1021/ma102091g

9608 Macromolecules 2010, 43, 9608–9612
DOI: 10.1021/ma102091g
Poly(m-phenylene): Conjugated Polymer Host with High Triplet Energy
for Efficient Blue Electrophosphorescence
Jun Liu and Qibing Pei*
Department of Materials Science and Engineering, University of California, Los Angeles,
California 90095, United States
Received September 9, 2010; Revised Manuscript Received October 20, 2010
ABSTRACT: A poly(m-phenylene) derivative tethering carbazole unit (PmPCz) is reported to have a triplet
energy level (ET) as high as 2.64 eV, sufficient to host blue electrophosphorescence. To our best knowledge,
this is the first conjugated polymer with ET higher than that of state-of-the-art blue phosphorescent dopant,
iridium(III) bis(4,6-(difluorophenyl)pyridinato-N,C2)picolinate (FIrpic). PmPCz exhibits good thermal
stability and excellent miscibility with the blue phosphorescent dopant, FIrpic. Owing to the high ET and
good miscibility, blends of PmPCz with FIrpic show no triplet energy back-transfer and exhibit emission
exclusively from FIrpic, even at FIrpic concentration as low as 1 wt %. Single-layer blue phosphorescent
polymer light-emitting diode based on the blend exhibits a luminance efficiency of 4.69 cd/A.
Introduction
identified in the molecular structure.17,18 In poly(3,6-fluorene),
the longest poly(p-phenylene) chain is biphenyl structure
Phosphorescent polymer light-emitting diodes (PPLEDs)
based on a phosphorescent dopant dispersed in a polymer host
have received great attention thanks to solution processability
and internal quantum efficiency potentially as high as 100%.1-13
For both display and lighting applications, all three primary
colors (red, green, and blue) are necessary. At present, the
performance of blue PPLEDs lags far behind that of green and
red PPLEDs, largely due to lack of a suitable polymer host.6-10
Blue PPLEDs typically use nonconjugated polymers as the host,
such as poly(vinylcarbazole) (PVK).6-9 However, PVK trans-
ports only holes and requires the use of additional electron
transporting materials. Their high resistivity leads to rather high
operating voltages of the resulting devices. Recently, an alter-
native approach for blue PPLEDs is to use solution-processable
small molecules as the host.14,15 However, this approach suffers
from tedious multistep organic synthesis of the materials and
complicated device fabrication (extra vacuum-deposited elec-
tron-transporting layer is necessary). Conjugated polymers have
been demonstrated tobe excellent host for red and green PPLEDs
with low driving voltage and high electroluminescence (EL)
efficiency.3-5 However, they are scarcely used in blue PPLEDs
due to their low triplet energy level (ET). A host should have an
ET higher than that of the phosphorescent dopant. Otherwise,
triplet energy back-transfer from the dopant to the host will
occur and lead to low EL efficiency.16 The state-of-the-art blue
phosphorescent dopant, iridium(III) [bis(4,6-difluorophenyl)-
pyridinato-N,C2]-picolinate (FIrpic), has an ET of 2.62 eV.
Unfortunately, no conjugated polymer is known to exhibit an
ET higher than 2.6 eV. The only conjugated polymer reported as
the host for FIrpic is poly(3,6-fluorene) with an ET of 2.58 eV,
which is slightly lower than that of FIrpic.13 The resulting
PPLEDs show a maximum luminance efficiency of only 0.4 cd/A.
Therefore, it is challenging but critically important to develop
conjugated polymer hosts with high ET.
(marked as blue in Figure 1). We observe that the moieties
adjacent to the biphenyl structure also affect the ET. For instance,
1,3-diphenylbenzene has a lower ET (2.82 eV) than that of
biphenyl (2.85 eV).19 In poly(3,6-fluorene), owing to the nature
of fluorene, the phenyl rings (marked as red) adjacent to the
biphenyl structure (marked as blue) are coplanar with the
biphenyl structure. Such a coplanarity increases conjugation
and lowers the ET of the biphenyl structure and of the conjugated
polymer backbone. This effect may be circumvented in poly-
(m-phenylene), in which the phenyl rings (marked as red) adjacent
to the biphenyl structure (marked as blue) are twisted with the
biphenyl structure (see Figure 1). Therefore, poly(m-phenylene) is
expected to have a high ET that can be sufficient to host FIrpic.
Here, we report a poly(meta-phenylene) derivative (PmPCz)
with an ET of 2.64 eV as a conjugated polymer host for high-
efficiency blue PPLEDs. The chemical structure of PmPCz shown
in Figure 1 has a poly(m-phenylene) backbone and carbazole unit
tethered as a side group to facilitate hole injection and to improve
compatibility with phosphorescent dopants.20 To our best knowl-
edge, this is the first time for poly(m-phenylene) derivative to be
used in PPLEDs. Moreover, this is the first conjugated polymer
with ET higher than that of FIrpic. The higher ET of PmPCz than
that of FIrpic prevents triplet energy back-transfer from FIrpic to
PmPCz and leads to high EL efficiency. The resulting PPLEDs
emits blue light with a luminance efficiency that is more than
10 times higher than that of the device based on poly(3,6-fluorene).13
Moreover, the high ET of PmPCz and high compatibility between
PmPCz and FIrpic allow the use of the expensive iridium(III)
dopant at a very low concentration (1 wt %), which is very
desirable for low-cost PPLEDs.
Experimental Section
General. 9-Hexyl-9H-carbazole (Cz),20 1,3-dibromo-5-butoxy-
benzene (4),21 and poly(5-butyloxyphenylen-1,3-yl)21 were synthe-
sized following the procedures in the literature. All the reactions
were carried out under an argon atmosphere.
Brunner et al. reported that ET of conjugated polymers were
determined by the longest poly(p-phenylene) chains that can be
9-(6-Bromohexyl)-9H-carbazole (1). To a solution of carba-
zole (8.35 g, 50 mmol) in dry THF (40 mL) was added NaH
*Corresponding author. E-mail: qpei@seas.ucla.edu.
pubs.acs.org/Macromolecules
Published on Web 11/15/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 23, 2010 9609
and stirring overnight. After work-up, the mixture was poured
into water and extracted with CH2Cl2. The organic layer was
washed with water for three times and dried over anhydrous
Na2SO4. After being filtered and concentrated, the residual was
purified by column chromatography with CH2Cl2/hexane=1/1
as eluent to give the title compound as a white solid. Yield: 0.68 g
(31%). 1H NMR (400 MHz, CDCl3) δ (ppm): 8.10 (d, 2H), 7.47
(m, 5H), 7.23 (m, 3H), 7.10 (dd, 1H), 4.43 (t, 2H), 3.91 (t, 2H),
1.92 (m, 2H), 1.73 (m, 2H), 1.46 (m, 4H), 1.34 (s, 12H). 13C
NMR (100 MHz, CDCl3) δ (ppm): 159.41, 140.43, 129.63,
125.63, 122.85, 122.9, 120.37, 118.76, 108.64, 84.19, 67.97,
42.94, 29.07, 28.92, 27.02, 25.89, 24.86. HR-MS (m/z (%)):
550.18 (100) [M þ H]þ. Anal. Calcd for C30H35BBrNO3: C,
65.71; H, 6.43; N, 2.55. Found: C, 65.96; H, 6.49; N, 2.52.
Poly(5-(6-(9H-carbazole)hexyloxy)-1,3-phenylene) (PmPCz).
A mixture of 9-(6-(3-bromo-5-(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)phenoxy)hexyl)-9H-carbazole (3) (0.45 g, 0.90 mmol),
Pd(PPh3)4 (0.010 g, 0.01 mmol), K2CO3 (2.00 mL 2.0 M aqueous
solution, 4.00 mmol), toluene (6.0 mL), and Aliquat 336 (1 drop)
was stirred at 90 °C for 48 h. After being cooled down, the
mixture was poured into CH2Cl2 and washed with water three
times, followed by drying over anhydrous Na2SO4. After being
filtered and concentrated, the solution was precipitated in
methanol. The white powder was collected, reprecipitated in
CH2Cl2/CH3OH two times, and dried in vacuum overnight.
Figure 1. Chemical structure, schematic illustration, and triplet energy
of poly(3,6-fluorene) and PmPCz. The ET of the polymer backbones are
determined by the biphenyl structure marked as blue. Moreover, the
moieties adjacent to the biphenyl structure also influence the ET. In
poly(3,6-fluorene), the adjacent two phenyl units (marked as red) are
coplanar with the corresponding phenyl ring in the biphenyl structure
(marked as blue) due to the nature of fluorene. Such coplanarity
decreases the ET. In contrast, the coplanarity effect is avoided in the
case of poly(m-phenylene) backbone. Therefore, PmPCz is expected to
exhibit higher ET than that of poly(3,6-fluorene).
1
Yield: 0.16 g (51.2%). H NMR (400 MHz, CDCl3) δ (ppm):
8.04 (br, 2H), 7.45-7.33 (br, 5H), 7.19-7.11 (br, 4H), 4.31
(br, 2H), 3.93 (br, 2H), 1.84 (br, 4H), 1.30 (br, 4H). 13C NMR
(100 MHz, CDCl3) δ (ppm): 159.78, 143.14, 142.88, 140.33,
125.58, 122.76, 120.30, 118.72, 112.36, 108.61, 67.96, 42.80,
29.15, 18.90, 26.98, 25.85. Anal. Calcd for C24H23NO: C,
84.42; H, 6.79; N, 4.10. Found: C, 84.37; H, 6.95; N, 3.88.
Characterization. 1H and 13C NMR spectra were recorded on
a Bruker arx-400 spectrometer. Elemental analysis was carried
out with a Perkin-Elmer 2400 elemental analyzer. Molecular
weight of the polymers was measured by the gel permeation
chromatography (GPC) method using polystyrene as the stan-
dard and tetrahydrofuran (THF) as the eluent. Cyclic voltam-
metry (CV) was conducted in a solution of Bu4NBF4 (0.1 M) in
acetonitrile with Pt wire, Pt plate, and saturated calomel elec-
trode (SCE) as the working electrode, counter electrode, and
reference electrode, respectively. The polymer film was dip-coated
on the working electrode from its solution in methylene chlo-
ride. Absorption spectra were obtained from a Shimadzu UV-
1700 UV/vis spectrophotometer. Fluorescence spectra at room
temperature and phosphorescence spectra at 77 K were mea-
sured with a PTI QuantaMaster 30 spectrofluorometer. Current-
voltage and brightness-voltage curves of electroluminescent
devices were recorded by a computer-controlled Keithley 2400/
2002 source unit calibrated with a Photoresearch PR-655
spectrophotometer.
Device Fabrication. Indium tin oxide (ITO) glass substrates
were ultrasonically cleaned for 30 min each sequentially with
detergent, deionized water, acetone, and isopropanol. Then they
were dried in a heating chamber at 70 °C. The PEDOT:PSS
(Clevios VP Al 4083 from H. C. Starck Inc.) layer was spin-
coated on the ITO at 3000 rpm for 60 s and then baked at 120 °C
for 15 min to give an approximate thickness of 40 nm. The
emissive layer (∼90 nm) was then spin-coated from the solution
of PmPCz (20 mg/mL) and FIrpic (from American Dye Source)
in chlorobenzene. Finally, a thin layer of CsF (1.0 nm) followed
by a layer of aluminum (100 nm) was deposited in a vacuum
thermal evaporator through a shadow mask at a pressure of
10-6 Torr.
(2.52 g, 63 mmol) in several portions. The resulting solution was
stirred for 10 min at room temperature, followed by added dropwise
to a refluxing solution of 1,6-dibromohexane (45.00 mL, 292 mmol)
in THF (100 mL). Then the mixture was refluxed for 24 h. After
being cooled to room temperature, the mixture was added several
drops of water to destroy the excessive NaH, followed by remove of
THF with rotatory evaporation and remove of excessive 1,6-dibromo-
hexane with vacuum distillation. The residual was dissolved in
CH2Cl2, washed with water three times, dried over anhydrous
Na2SO4, filtered, and concentrated. Further purification by flash
column chromatography with CH2Cl2/hexane=1/4 as eluent
gave the title compound as a white solid. Yield: 13.81 g (82.9%).
1H NMR (400 MHz, CDCl3) δ (ppm): 8.10 (d, 2H), 7.48 (t, 2H),
7.40 (d, 2H), 7.22 (t, 2H), 4.32 (t, 2H), 3.36 (t, 2H), 1.90 (m, 2H),
1.81 (m, 2H), 1.47 (m, 4H). 13C NMR (100 MHz, CDCl3) δ(ppm):
140.41, 125.63, 122.86, 120.39, 118.79, 108.60, 42.87, 33.72, 32.58,
28.84, 27.94, 26.49. HR-MS (m/z (%)): 330.08 (100) [M þ H]þ.
Anal. Calcd for C18H20BrN: C, 65.46; H, 6.10; N, 4.24. Found:
C, 65.48; H, 6.14; N, 4.11.
9-(6-(3,5-Dibromophenoxy)hexyl)-9H-carbazole (2). A mixture
of 3,5-dibromophenol (2.32 g, 9.2 mmol), 9-(6-bromohexyl)-9H-
carbazole (1) (3.63 g, 11.0 mmol), K2CO3 (3.94 g, 28.6 mmol), and
DMF (30 mL) was stirred at 110 °C overnight. After being cooled
to room temperature, the mixture was poured into brine and
extracted with CH2Cl2. The organic layer was washed with brine
four times, dried over anhydrous Na2SO4, filtered, and concen-
trated. The residual was purified by flash column chromatography
with CH2Cl2/hexane=1/4 as eluent to afford the title compound
1
as a white crystal. Yield: 4.10 g (88.9%). H NMR (400 MHz,
CDCl3) δ (ppm): 8.11 (d, 2H), 7.47 (td, 2H), 7.40 (d, 2H), 7.24
(m, 3H), 6.95 (d, 2H), 4.33 (t, 2H), 3.84 (t, 2H), 1.92 (m, 2H), 1.72
(m, 2H), 1.45 (m, 4H). 13C NMR (100 MHz, CDCl3) δ (ppm):
160.25, 140.43, 126.23, 125.63, 123.10, 122.87, 120.42, 118.81,
116.89, 108.62, 68.33, 42.90, 28.92, 28.85, 26.98, 25.83. HR-MS
(m/z (%)): 502.01 (100) [M þ H]þ. Anal. Calcd for C24H23Br2NO:
C, 57.51; H, 4.62; N, 2.79. Found: C, 57.77; H, 4.53; N, 2.76.
9-(6-(3-Bromo-5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
phenoxy)hexyl)-9H-carbazole (3). To a solution of 9-(6-(3,5-
dibromophenoxy)hexyl)-9H-carbazole (2) (2.04 g, 4.0 mmol)
in dry THF (60 mL) at -78 °C was added n-BuLi (1.60 mL 2.5 M
solution in hexane, 4.0 mmol). The resulting solution was stirred
at -78 °C for 1 h, followed by addition of 2-isopropyloxy-
4,4,5,5-tetramethyl-1,3,2-dioxaboronate (1.10 mL, 5.5 mmol)
Results and Discussion
As shown in Scheme 1, our synthesis began with attaching
carbazole to m-dibromobenzene with an alkyl spacer to obtain
the intermediate 2. Subsequent treatment withn-butyllithium and

9610 Macromolecules, Vol. 43, No. 23, 2010
Scheme 1. Synthetic Routes for PmPCz, PmP, and Cza
Liu and Pei
a Reagents and conditions: (i) NaH, THF, 1,6-dibromohexane, reflux; (ii) 3,5-dibromo-1-phenol, DMF, K2CO3, 110 °C; (iii) (a) n-BuLi, THF, -78 °C,
(b) 2-isopropyloxy-4,4,5,5-tetramethyl-1,3,2-dioxaboronate; (iv) Pd(PPh3)4, K2CO3 (aqueous, 2 M), Aliquat 336, toluene, 90 °C.
Figure 2. AFM phase image of the film spin-coated from the blend of
PmPCz and FIrpic (10 wt %).
Figure 3. Cyclic voltammogram of PmPCz.
2-isopropyloxy-4,4,5,5-tetramethyl-1,3,2-dioxaboronate afforded
the monomer 3. The polymer host PmPCz was synthesized by
Suzuki polycondensation using Pd(PPh3)4 catalyst. For compar-
ison, we also synthesized poly(5-butyloxy-1,3-phenylene) (PmP)
and N-hexylcarbazole (Cz), which can be regarded as the model
compounds for poly(m-phenylene) backbone and carbazole unit,
respectively.
PmPCz is readily soluble in common organic solvents, such as
toluene, chloroform, and tetrahydrofuran. The weight-average
molecular weight (Mw) is 9900 with a polydispersity index (PDI)
of 1.26, as determined by gel permeation chromatography using a
poly(styrene) standard. Differential scanning calorimetry (DSC)
analysis of PmPCz shows neither crystallization nor melting
behavior. The glass transition temperature (Tg) is 80 °C, suggest-
ing a stable glassy morphology at normal PPLEDs operation
conditions. The compatibility of PmPCz and the dopant FIrpic is
Figure 4. Absorption (blue dot), fluorescence (red dash), and phos-
phorescence (solid line) spectra of PmPCz, PmP, Cz, and FIrpic in
solution.
investigated by atom force microscopy (AFM). As shown in
Figure 2, the phase image of the film spincoated from the blend
of PmPCz and FIrpic (10 wt %) implies smooth and uniform
surface without pinholes, particles, and phase separation, indi-
cating good compatibility of the host and the dopant.
On the basis of the absorption onset of the polymer in film, the
optical bandgap of PmPCz is 3.44 eV. Thus, the LUMO energy
level is calculated to be -2.12 eV.
Figure 4 shows the absorption spectra, fluorescence spectra
at room temperature, and phosphorescence spectra at 77 K of
PmPCz, PmP, Cz, and FIrpic. Poly(m-phenylene) has a larger
singlet energy than that of carbazole, yet its ET is smaller.
Therefore, the fluorescence of PmPCz is dominated by the
carbazole moiety, while its phosphorescence is characteristic of
the poly(m-phenylene) backbone. The phosphorescence spectrum
Cyclic voltammetry was employed to estimate the LUMO and
HOMO energy levels of the polymer host. As shown in Figure 3,
the cyclic voltammogram of PmPCz shows two oxidation waves
with onset potential at 1.16 and 1.50 eV, which are attributed to
the carbazole unit and the poly(m-phenylene) backbone, respec-
tively. No reduction behavior is observed despite many attempts.
According to the empirical formula EHOMO = -e(Eox þ 4.40) [eV],
the HOMO energy level of PmPCz is estimated to be -5.56 eV.

Article
Macromolecules, Vol. 43, No. 23, 2010 9611
of PmPCz has two peaks. Deconvolution gives the higher energy
peak at 470 nm, which is used to calculate the ET of PmPCz to be
2.64 eV. This value is higher than those of conjugated polymers
reported so far with high ET, including poly(3,6-carbazole) and
its derivatives (ET = 2.53-2.60 eV),18 poly(3,6-fluorene) (ET =
2.58 eV),13 and poly(3,6-silafluorene) (ET = 2.55 eV),22-24 due to
the twisted conjugated polymer backbone in PmPCz. Most
PmPCz and is consistent with the higher ET of PmPCz than that
of FIrpic. The energy transfer from PmPCz to FIrpic is supported
by the overlap of the absorption spectrum of FIrpic and fluor-
escence spectrum of PmPCz (see Figure 4). Moreover, the effi-
cient energy transfer from PmPCz to FIrpic also results from their
good compatibility, which makes FIrpic well dispersed in PmPCz.
To test the capability of PmPCz as a polymer host, preliminary
single-layer PPLEDs were fabricated with the configuration of
ITO/PEDOT:PSS (40 nm)/PmPCz þ FIrpic (90nm)/CsF (1 nm)/
Al (100 nm). The EL spectra of the devices are displayed in
Figure 5b. In the concentration range of 1-10 wt % of FIrpic in
the blends, all PLEDs exhibit sky-blue emission from FIrpic. No
detectable emission from PmPCz is observed, even at FIrpic
content as low as 1 wt %. The emission from the polymer host is
more suppressed in EL than in PL due to the charge trapping
effect of FIrpic.25-27 The EL performance of the PPLEDs is listed
in Table 1. The devices show increased turn-on voltage and
decreased luminance efficiency with increasing FIrpic content
from 1 to 10 wt % (see Figure 6a). The increased turn-on voltage
results from the charge trapping of FIrpic, whereas the decreased
EL efficiency is possibly caused by concentration quenching of
importantly, ET of PmPCz is higher than that of FIrpic (ET
=
2.62 eV), the state-of-the-art blue phosphorescent dopant. This
will prevent the triplet back energy transfer from FIrpic to
PmPCz and make high EL efficiency possible. For other deep
blue phosphorescent dopants with high ET, such as iridium(III)
bis(40,60-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate
(FIr6, ET = 2.7 eV), the ET of PmPCz is not high enough to
prevent triplet energy back-transfer.
Figure 5a shows the photoluminescence (PL) spectra of the
blends of PmPCz and FIrpic at different weight ratios in solid
films. The PL spectra of the blends are dominated by FIrpic’s
emission peaked at 472 nm even at a FIrpic concentration as low
as 1 wt %, indicative of an efficient energy transfer from PmPCz
to FIrpic. When the content of FIrpic is >2 wt %, the emission
from PmPCz is almost completely quenched. This phenomenon
obviously indicates no triplet energy back-transfer from FIrpic to
Figure 6. (a) Luminance efficiency-current density curves of PPLEDs
with the single-layer configuration of ITO/PEDOT:PSS/PmPCz þ
FIrpic (blend with specified FIrpic content)/CsF/Al. (b) Voltage-
current density-brightness curves of the device containing 1 wt %
FIrpic in the blend.
Figure 5. (a) PL spectra of spin-cast films of PmPCz and the blend of
PmPCz and FIrpic (weight ratios specified). (b) EL spectra of the
PPLEDs based on the blends of PmPCz and FIrpic (weight ratios
specified).
Table 1. EL Performance of the Devices Based on the Blend of PmPCz and FIrpic with Different Ratios
turn-on luminance power maximum
content of
FIrpic (wt %)
CIE coordinates
(x, y)
voltagea (V) brightness (cd/m2)
efficiency (cd/A) efficiency (lm/W)
1
2
5
10
10.0
12.0
13.8
15.0
4.69
3.53
2.74
1.60
1.04
0.68
0.45
0.26
2742
1991
1530
(0.16, 0.34)
(0.17, 0.35)
(0.18, 0.35)
(0.18, 0.37)
b
-
a Voltage at the brightness of about 1 cd/m2. b The device did not reach maximum brightness at the bias limit of our voltage source.

9612 Macromolecules, Vol. 43, No. 23, 2010
Liu and Pei
FIrpic, reduced charge carrier balance, or both. The best EL perfor-
mance is achieved at a FIrpic contentof1wt%. Incomparison, blue
PPLEDs reported so far generally use 5-10 wt % of FIrpic for
optimal EL performance.25 The low optimal FIrpic content in this
work is very attractive because low content of expensive phosphor-
escent complex means low cost of PPLEDs. The low optimal
content here is probably related to the high ET of PmPCz, which
prevents triplet energy back-transfer and makes triplet excitons
confined in FIrpic to generate light emission.
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Figure 6b shows the voltage-current density-brightness curves
of the device with 1 wt % FIrpic. The turn-on voltage is 10.0 V, and
the brightness reaches 2742 cd/m2 at 17.5 V. At a current density of
19.4 mA/cm2, the device exhibits the maximum luminance effi-
ciency of 4.69 cd/A and power efficiency of 1.04 lm/W. Although
this performance is moderate compared to the values of blue
PPLEDs in the literature,6-9 the luminance efficiency is 10 times
higher than the 0.4 cd/A reported for a single-layer device based on
a conjugated polymer host, poly(3,6-fluorene).13 A control device
with the typical nonconjugated polymer, PVK, as the host shows a
turn-on voltage of 12.6 V and maximum luminance efficiency of
1.7 cd/A. It is well-known that PVK only transport holes. The
device of PmPCz exhibits lower turn-on voltage and higher EL
efficiency than those of PVK, probably because the conjugated
poly(m-phenylene) backbone in PmPCz facilitates electron injection/
transporting and improves charge carrier balance.
PPLEDs generally exhibit higher luminance efficiency with
decreasing current density because of long diffusion distance of
triplet excitons and triplet-triplet annihilation.9 However, the
devices based on PmPCz have abnormally low luminance effi-
ciency at low current densities. This may be explained by unbal-
anced charge carriers: The high-lying LUMO energy level of
PmPCz induces a large barrier for electron injection from cathode
to the emissive layer. Device optimization with improved charge
carrier balance24 is expected to lead to improved EL performance.
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In conclusion, a new conjugated polymer with a poly-
(m-phenylene) backbone and tethered carbazole side groups
has been developed. PmPCz exhibits good thermal stability and
excellent miscibility with FIrpic. The ET of PmPCz at 2.64 eV is
higher than that of FIrpic, making PmPCz a suitable polymer
host for blue PPLEDs. The high ET prevents triplet energy back-
transfer, confines triplet excitons in FIrpic to emit light, and
allows the use of low content of FIrpic for high EL efficiency.
Single-layer PPLEDs using the blend containing 1 wt % of FIrpic
emits blue light exclusively from FIrpic with a maximum lumi-
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for the design of polymer host materials for phosphorescent
dopants. Study is ongoing to use PmPCz in blue and white
polymer light-emitting electrochemical cell (PLEC) with low
driving voltage and balanced charge carriers.
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Acknowledgment. The work reported here was supported by
the United States Department of Energy Solid State Lighting
Program (Grant DE-FC26-08NT01575).