Conjugated polymer as host for high efficiency blue and white electrophosphorescence

Article abstract of DOI:10.1021/ma200282x

We demonstrate that conjugated polymers are able to efficiently host blue and white electrophosphorescence if the conjugated polymer has both high triplet energy level (E_T) and high-lying HOMO energy level. A novel conjugated polymer host (PmPTPA) is developed by attaching triphenylamine unit to poly(m-phenylene) backbone. The poly(m-phenylene) backbone endows PmPTPA an E_T as high as 2.65 eV, which is sufficiently high to prevent triplet energy back transfer. The tethering triphenylamine unit leads to the HOMO energy level of -5.35 eV for PmPTPA and facilitates hole injection. As the result, blue phosphorescent polymer light-emitting diodes (PPLEDs) based on PmPTPA exhibit the luminance efficiency of 17.9 cd/A and external quantum efficiency of 9.3%. White PPLEDs with blue, green and red phosphorescent dopants dispersed in PmPTPA show the luminance efficiency of 22.1 cd/A and external quantum efficiency of 10.6%. For both the blue and white PPLEDs based on the conjugated polymer host PmPTPA, the EL performance are fairly comparable to those of the state-of-the-art nonconjugated polymer host, poly(vinyl-carbazole) (PVK). These results indicate that conjugated polymers are suitable host materials for PPLEDs with all emission colors.

Full text of DOI:10.1021/ma200282x

ARTICLE
pubs.acs.org/Macromolecules
Conjugated Polymer as Host for High Efficiency Blue and
White Electrophosphorescence
Jun Liu,† Lu Li,†,‡ and Qibing Pei*,†
†Department of Materials Sciences and Engineering, University of California, Los Angeles, California 90095, United States
‡State Key Laboratory of Electronic Thin Films and Integrated Devices, Department of Optoelectronic, Information,
University of Electronic Science and Technology of China, Chengdu 610054, P. R. China
ABSTRACT: We demonstrate that conjugated polymers are able to
efficiently host blue and white electrophosphorescence if the con-
jugated polymer has both high triplet energy level (ET) and high-lying
HOMO energy level. A novel conjugated polymer host (PmPTPA) is
developed by attaching triphenylamine unit to poly(m-phenylene)
backbone. The poly(m-phenylene) backbone endows PmPTPA an ET
as high as 2.65 eV, which is sufficiently high to prevent triplet energy
back transfer. The tethering triphenylamine unit leads to the HOMO
energy level of ꢀ5.35 eV for PmPTPA and facilitates hole injection. As
the result, blue phosphorescent polymer light-emitting diodes
(PPLEDs) based on PmPTPA exhibit the luminance efficiency of
17.9 cd/A and external quantum efficiency of 9.3%. White PPLEDs
with blue, green and red phosphorescent dopants dispersed in
PmPTPA show the luminance efficiency of 22.1 cd/A and external
quantum efficiency of 10.6%. For both the blue and white PPLEDs
based on the conjugated polymer host PmPTPA, the EL performance
are fairly comparable to those of the state-of-the-art nonconjugated
polymer host, poly(vinylꢀcarbazole) (PVK). These results indicate that conjugated polymers are suitable host materials for
PPLEDs with all emission colors.
’ INTRODUCTION
all exhibit short lifetime. Therefore, it is strongly required to
develop polymer host with both high ET and proper LUMO/
HOMO energy levels.
Polymer light-emitting diodes (PLEDs) have been intensively
investigated for important applications such as flat-panel displays
and solid-state lighting with low-cost solution processing.1
Phosphorescent PLEDs (PPLEDs) employing phosphorescent
dopants dispersed in a polymer host have the potential to harvest
both singlet and triplet excitons and obtain 100% internal
electron-to-photon quantum efficiency.2ꢀ22 Among variable
emission colors, blue and white are the most difficult to achieve
in PPLEDs:3ꢀ7,17ꢀ22 the efficiencies of blue and white PPLEDs
are rather low compared to red and green devices, largely due to
lack of a suitable polymer host for blue phosphorescent dopants.
An ideal polymer host should have a triplet energy level (ET)
higher than that of the phosphorescent dopants. Otherwise,
triplet energy back transfer from the phosphorescent dopants
to the host will quench triplet excitons and leads to low
electroluminescence (EL) efficiency.23 Meanwhile, the polymer
host should have proper LUMO/HOMO energy levels to
facilitate the injection and transport of both electrons and holes.
Currently, the most widely used polymer host for blue PPLEDs is
a nonconjugated polymer, poly(vinylꢀcarbazole) (PVK).3ꢀ7
However, PVK has a low lying HOMO energy level of ꢀ5.9
eV, which mismatches work function of common anode and
results in difficult hole injection. Moreover, PPLEDs using PVK
Although conjugated polymers have been demonstrated to be
suitable host materials for green and red PPLEDs,8ꢀ16 the
reports of conjugated polymers in blue and white PPLEDs have
been scare due to the generally low ET of conjugated polymers.
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. However, only until very recently, we
reported the first conjugated polymer, poly(m-phenylene) con-
taining carbazole unit as a side group (PmPCz), that has an ET >
2.62 eV.24 Owing to the absence of triplet energy back transfer,
PPLEDs employing PmPCz doped with FIrpic exhibit a lumi-
nance efficiency of 4.7 cd/A, which is a 10 times improvement
over previously reported blue PPLEDs based on a conjugated
polymer host.25 However, this efficiency is still lower than that of
the blue PPLEDs based on the blend of nonconjugated polymer
PVK and an electron-transport material (15ꢀ20 cd/A).3ꢀ7
The low efficiency is caused by the mismatch of the
Received: February 8, 2011
Revised:
March 19, 2011
Published: March 30, 2011
r
2011 American Chemical Society
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HOMO energy level of PmPCz with the work function of the
anode.24
at 70 °C for 20 min. The poly(3,4-ethylenedioxythiophene) doped with
poly(styrenesulfonate) (PEDOT:PSS, Clevios VP Al 4083 from H. C.
Starck Inc.) layer was spin-coated on 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 (approximate 70 nm) was then spin-coated at 1500 rpm for
60 s from the solution of PmPTPA (20 mg/mL) and the phosphorescent
dopants in chlorobenzene. After the spin-coating, the devices were trans-
ferred to a vacuum chamber. At a pressure of 10ꢀ6 Torr, 1,3-bis[(4-tert-
butylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD-7, 30 nm), CsF
(1.0 nm) and aluminum (100 nm) were sequentially deposited by thermal
evaporation through a shadow mask. The active area of the devices was
0.12 cm2.
White PPLEDs are of particular importance because of their
great potential for energy saving solid-state lighting and low cost
flat-panel display.17ꢀ22 The development of white PPLEDs is
also hampered by the lack of a polymer host for blue phosphor-
escence. Until now, all efficient (>20 cd/A) white PPLEDs use
PVK as the host, which suffers from difficult hole injection and
short lifetime as mentioned above.17ꢀ19 There is no report of
high efficiency (>20 cd/A) white PPLEDs based on a conjugated
polymer host.
Here, we report a novel conjugated polymer host (PmPTPA)
based on poly(m-phenylene) backbone containing triphenylamine
tethered side groups. The poly(m-phenylene) backbone and tri-
phenylamine moiety impart PmPTPA ahighET (2.65 eV) as well as
a high-lying HOMO energy level (ꢀ5.35 eV), which prevents triplet
energy back transfer and facilitates hole injection, respectively. Blue
PPLEDs based on PmPTPA doped with FIrpic exhibit a luminance
efficiency of 17.9 cd/A and an external quantum efficiency of 9.3%.
When doped with blue, green and red emissive Ir complexes with
appropriate ratio, white PPLEDs are obtained with the CIE
coordinates of (0.37, 0.47), luminance efficiency of 22.1 cd/A and
external quantum efficiency of 10.6%. For both the blue and the
white PPLEDs, the EL performance of this conjugated polymer host
PmPTPA are fairly comparable to those of the state-of-the art
nonconjugated polymer host PVK.3ꢀ7,17ꢀ22 These results indicate
that conjugated polymers are suitable host materials for PPLEDs
with all emission colors.
Synthesis of 1-(6-Bromohexyloxy)-3,5-dibromobenzene
(1). A mixture of 3,5-dibromophenol (2.58, 10.24 mmol), 1,6-dibro-
mohexane (3.2 mL, 20.85 mmol), NaOH (aq., 50 wt %, 7 mL), toluene
(30 mL) and tetrabutylammonium bromide (0.32 g, 1.00 mmol) was
stirred at 80 °C for 24 h. After work-up, the organic layer was separated
and washed with water for three times. The organic layer was dried over
anhydrous Na2SO4, filtered and concentrated. Column chromatography
on silica gel with CH2Cl2/hexane = 1/4 as eluent afforded the title
compound as a white solid. Yield: 3.21 g (75.5%). 1H NMR (400 MHz,
CDCl3), δ (ppm): 7.23 (s, 1H), 6.98 (s, 2H), 3.92 (t, 2H), 3.43 (t, 2H),
1.89 (m, 2H), 1.80 (m, 2H), 1.50 (m, 4H). Anal. Calcd for C12H15Br3O:
C, 34.73; H, 3.64; Found: C, 35.52; H, 3.38.
Synthesis of (4-(Diphenylamino)phenyl)methanol (2).
To a dispersion of LiAlH4 (0.76 g, 20.00 mmol) in THF (20 mL) at
room temperature was added dropwise a solution of 4-(diphenyl
amino)benzaldehyde (2.50 g, 9.16 mmol) in THF (20 mL). After
finished, the mixture was stirred at room temperature overnight. Several
drops of water were carefully added to the mixture to destroy the
excessive LiAlH4, followed by removal of the solvent. The residual was
dissolved in CH2Cl2, washed with water for three times, dried with
anhydrous Na2SO4, and then filtered. Removal of the solvent gave the
title compound as a white solid. Yield: 1.90 g (75.4%). 1H NMR (400
MHz, CDCl3) δ (ppm): 7.24 (m, 6H), 7.07 (m, 6H), 7.01 (m, 2H), 4.64
(s, 2H). Anal. Calcd for C19H17NO: C, 82.88; H, 6.22; N, 5.09; Found:
C, 82.50; H, 6.22; N, 4.94.
Synthesis of N-(4-((6-(3,5-Dibromophenoxy)hexyloxy)
methyl)phenyl)-N-phenylbenzenamine (3). A mixture of 1-(6-
bromohexyloxy)-3,5-dibromobenzene (1) (0.89 g, 2.15 mmol),
(4-(diphenylamino)phenyl)methanol (2) (0.60 g, 2.18 mmol), NaOH
(aqueous, 20 wt %, 4 mL), toluene (2 mL), and tetrabutylammonium
bromide (0.06 g, 0.20 mmol) was stirred at 80 °C for 24 h. After work-
up, the organic layer was separated and washed with water for three
times. The organic layer was dried with anhydrous Na2SO4, filtered, and
concentrated. Column chromatography on silica gel with CH2Cl2/
hexane = 1/4 as eluent afforded the title compound as a viscous liquid.
Yield: 0.90 g, (68.7%). 1H NMR (400 MHz, CDCl3), δ (ppm): 7.24 (m,
6H), 7.06 (m, 11H), 4.44 (s, 2H), 3.92 (t, 2H), 3.51 (t, 2H), 1.78 (m,
2H), 1.66 (m, 2H), 1.50 (m, 4H). Anal. Calcd for C31H31Br2NO2: C,
61.10; H, 5.13; N, 2.30; Found: C, 60.78; H, 5.21; N, 2.15.
Synthesis of PmPTPA. To a mixture of bis(1,5-cyclooctadiene)
nickel(0) (0.33 g, 1.20 mmol), 1,5-cyclooctadiene (0.13 g, 1.20 mmol),
bipyridine (0.19 g, 1.20 mmol), N,N-dimethylformamide (5 mL) and
toluene (3 mL) in argon at 80 °C was added a solution of N-(4-((6-(3,5-
dibromophenoxy)hexyloxy)methyl)phenyl)-N-phenylbenzenamine (3)
(0.60 g, 1.00 mmol) in toluene (2 mL). The mixture was stirred in dark
at 80 °C for 2 days. After work-up, the mixture was poured to methanol.
The solid was collected and dissolved in CH2Cl2. The resulting solution
was washed with water for three times, dried over Na2SO4, filtered and
concentrated. The residual was poured into methanol to give white solid.
Reprecipitation in CH2Cl2/CH3OH twice afforded the title polymer as
white powder. Yield: 0.13 g. (35.2%) 1H NMR (400 MHz, CDCl3), δ
(ppm): 7.28ꢀ7.13 (br, 6H), 7.05ꢀ6.90 (br, 11H), 4.34 (br, 2H), 3.94
’ EXPERIMENTAL SECTION
Materials. All chemicals and reagents for synthesis were purchased
from Aldrich Chemical Co. and used as received without further
purification. 4-(Diphenylamino)benzaldehyde was synthesized accord-
ing to the literature procedures.26 All the reactions were carried out
under argon atmosphere. The three phosphorescent dopants, iridium
(III) [bis(4,6-difluorophenyl)pyridinato-N,C2]picolinate (FIrpic), iridium
(III) tris[2-(p-tolyl)pyridine] (Ir(mppy)3) and iridium(III) tris(1-phenyl
isoquinoline) (Ir(piq)3), were purchased from American Dye Source.
Characterization. 1H NMR spectra were conducted on a Bruker
arx-400 spectrometer. Elemental analysis was performed with a Perkin-
Elmer 2400 elemental analyzer. Molecular weights of the polymer were
measured by gel permeation chromatography (GPC) method using
tetrahydronfuran (THF) as the eluent and polystyrene as the standard.
Cyclic voltammetry (CV) was carried out in a solution of Bu4NBF4 (0.1
M) in acetonitrile with Pt wire, Pt plate and saturated calomel electrode
(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 chloride. Absorption spectra
were obtained from a Shimadzu UV-1700 UV/vis spectrophotometer.
Fluorescence spectra at room temperature and phosphorescence spectra
at 77 K were measured with a PTI QuantaMaster 30 spectrofluorometer.
Currentꢀvoltage 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.
Electroluminescence spectra were measured by the Photoresearch PR-
655 spectrophotometer. Luminance efficiencies, power efficiencies and
external quantum efficiencies of the devices were calculated according to
the voltageꢀcurrent-brightness curves and the electroluminescence
spectra.
Device Fabrication. Indium tin oxide (ITO) glass substrates were
ultrasonically cleaned for 30 min each sequentially with detergent, deionized
water, acetone and 2-propanol. Then they were dried in a heating chamber
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Scheme 1. Chemical Structure and Synthetic Route of
PmPTPAa
Figure 2. CV of PmPTPA in solid film.
PmPTPA is readily soluble in common organic solvents, such
as toluene, chlorobenzene, chloroform and tetrahydrofuran.
1
Its chemical structure is verified by H NMR and elemental
a Reagents and conditions: (i) 1,6-dibromohexane, NaOH(aq), toluene,
(n-C4H9)4NBr, 80 °C; (ii) LiAlH4, THF, room temperature; (iii) NaOH-
(aq), toluene, (n-C4H9)4NBr, 80 °C; (iv) bis(1,5-cyclooctadiene)nickel-
(0), 1,5-cyclooctadiene, bipyridine, DMF, toluene, 80 °C.
analysis. The number-average molecular weight (Mn) is 5400
with a polydispersity of 1.42, as determined by GPC against
poly(styrene) standards. The reason for the moderate molecular
weight of PmPTPA is not clear yet. Thermogravimetric
analysis (TGA) of PmPTPA shows a decomposition tempera-
ture (Td, corresponding to 5% weight loss) of 380 °C, indicative
of good thermal stability.
Cyclic voltammetry was employed to estimate the LUMO/
HOMO energy levels of the conjugated polymer host. As shown
in Figure 2, cyclic voltammogram of PmPTPA shows two
oxidation peaks with the onset potentials at 0.95 and 1.45 V
(vs SCE), respectively. The first peak is attributed to the
triphenylamine unit and the second to the poly(m-phenylene)
backbone. No reduction wave is detected. According to the
first onset oxidation potential and the empirical formula, EHOMO
= ꢀe(Eox þ 4.4) [eV], the HOMO energy level of PmPTPA is
estimated to be ꢀ5.35 eV. On the basis of the absorption onset of
the polymer in film, the optical bandgap of PmPTPA is 3.49 eV.
Therefore, the LUMO energy level is calculated to be ꢀ1.86 eV.
The HOMO of PmPTPA is much higher than that of our
previous polymer host PmPCz (HOMO: ꢀ5.56 eV)24 and
PVK (HOMO: ꢀ5.90 eV) due to the strong electron donating
ability of triphenylamine unit in PmPTPA. The HOMO energy
level of PmPTPA is close to the Fermi level of PEDOT:PSS
(ꢀ5.2 eV), which can lead to a small hole-injection barrier and
improved hole injection for PmPTPA. Thus, low driving voltage
and high device efficiency can be expected.
Figure 3 shows the absorption spectrum and fluorescence
spectrum of PmPTPA in dilute solution and in thin film. The
absorption band at 309 nm in film is attributed to the triphenyla-
mine unit.24 The fluorescence spectrum of PmPTPA in solution
exhibits two bands at 365 and 428 nm, which are assigned to the
emission from triphenylamine unit itself and excimer, respectively.
In thin film, the fluorescence spectrum shows emission predomi-
nantly from the excimer due to the close packing of triphenylamine
unit in solid state. The phosphorescence spectrum of PmPTPA at
77 K (see Figure 4) is characteristic of poly(m-phenylene)
backbone.24 Deconvolution of the broad phosphorescence spec-
trum gives the higher energy band at 469 nm. Therefore, the ET of
PmPTPA is determined to be 2.65 eV. This value is higher than that
of the state-of-art blue phosphorescent dopant, FIrpic (2.62 eV),
indicating that PmPTPA is able to host FIrpic without detrimental
triplet energy back transfer.
Figure 1. AFM phase image of the film spincoated from the blend of
PmPTPA and FIrpic (15 wt %).
(br. 2H), 3.50 (br, 2H), 1.84 (br, 4H), 1.30 (br, 4H). Anal. Calcd for
C31H31NO2: C, 82.82; H, 6.95; N, 3.12; Found: C, 82.75; H, 7.23;
N, 2.89.
’ RESULTS AND DISCUSSION
Scheme 1 outlines the synthetic route for the polymer host
PmPTPA. Reduction of 4-(diphenylamino)benzaldehyde with
LiAlH4 readily afforded 2. The key monomer 3 was readily
synthesized via two etherification reactions of 1,6-dibromohex-
ane with 3,5-dibromophenol leading to ether 1 and 1 with 2. The
overall yield was 54%. The monomer 3 was polymerized to afford
PmPTPA using the Yamamoto condition with bis(1,5-
cyclooctadiene)nickel(0) as the catalyst. The yield of the polym-
erization was 35% because of the loss in the purification process.
For a host material, the compatibility of the host with the
dopants is very important for the dispersion of the dopants in the
host and for the energy transfer from the host to the dopants. The
compatibility of PmPTPA and FIrpic is investigated by atom
force microscopy (AFM). As shown in Figure 1, the phase image
of the film of the blend of PmPTPA and FIrpic (15 wt %) shows
smooth and uniform surface without phase separation, indicating
the good compatibility of the host and the dopant.
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Figure 3. Absorption and fluorescence spectra of PmPTPA in solution
and in solid film.
Figure 4. Deconvolution of the phosphorescence spectrum of
PmPTPA in dilute solution.
Figure 6. EL spectra (a), current densityꢀluminance efficiencyꢀpower
efficiency curve (b), and voltageꢀcurrent densityꢀbrightness curve (c)
of the blue PPLEDs made from PmPTPA and 15 wt % FIrpic.
To evaluate PmPTPA as a host material for blue electropho-
sphorescence, blue PPLEDs were fabricated with the configuration of
ITO/PEDOT:PSS (40 nm)/PmPTPA: FIrpic (15 wt %, 70 nm)/
OXD-7 (30 nm)/CsF (1 nm) /Al (100 nm). OXD-7 was used as an
electron transporting layer to boost electron injection. As shown in
Figure 6a, the EL spectra of the device exhibit sky-blue emission
exclusively from FIrpic and no emission from the polymer host. The
absence of the polymer host emission is owing to the energy transfer
and charge trapping from PmPTPA to FIrpic in the EL process.
Figure 6b presents the current density-luminance efficiencyꢀpower
efficiency characteristic and Figure 6c shows the voltageꢀcurrent
density-brightness curve of the blue PPLEDs. The devices exhibit a
turn-on voltage of 6.0 V. At a brightness of 1000 cd/m2, the luminance
efficiency, power efficiency and external quantum efficiency of the
device are 17.9 cd/A, 5.2 lm/W, and 9.3%, respectively, with the CIE
Figure 5. PL spectra of the blend of PmPTPA and FIrpic in thin film
with specified weight ratios of FIrpic.
Figure 5 shows the photoluminescence (PL) spectra of the
blends of PmPTPA and FIrpic at different FIrpic weight ratios in
solid film. The PL spectra are dominated by FIrpic’s emission even
at a FIrpic concentration as low as 1 wt %, indicative of efficient
energy transfer from PmPTPA to FIrpic. The relative intensity of
the host emission decreases with increasing FIrpic concentration.
When the content of FIrpic is more than 5 wt %, the emission from
PmPTPA is almost completely quenched, which indicates absence
of triplet energy back transfer from FIrpic to PmPTPA and is
consistent with the higher ET of PmPTPA than that of FIrpic.23
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red dopants, respectively. The concentrations of the three
phosphorescent dopants were optimized for balanced blue, green
and red emission to give white light. As the result, high quality
warm white emission was obtained at FIrpic, Ir(mppy)3, and
Ir(piq)3 contents of 15, 0.2, and 0.3 wt %, respectively. The EL
spectra of the white PPLEDs (see Figure 7a) exhibit three bands
at 475, 506, and 619 nm, which are attributed to the emissions
from FIrpic, Ir(mppy)3, and Ir(piq)3, respectively. No emission
from the host is observed. It is worthy to mention the excellent
bias independence of the EL spectra. When the bias is increased
from 9 to 15 V, the EL spectra remain almost unchanged (see
Figure 7a) with the CIE coordinates vary slightly from (0.398,
0.480) to (0.378, 0.470). The bias independent EL spectra
are essential for white PPLEDs in lighting and display
applications.27ꢀ29 The white PPLEDs have a turn-on voltage
of 6.5 V. At a brightness of 1000 cd/m2, the device gives a
luminance efficiency of 22.1 cd/A, power efficiency of 4.7 lm/W
and external quantum efficiency of 10.6%. Figure 7b displays the
dependence of luminance efficiency and power efficiency on
current density of the white light emitting device and Figure 7c
shows the voltageꢀcurrent density-brightness curve of the
device. Similar to the blue PPLEDs, the white PPLEDs based
on PmPTPA also exhibit efficiencies comparable to those of
three-color white PPLEDs using PVK as the host.19This result
further confirms that PmPTPA is an excellent host polymer for
electrophosphorescence.
’ CONCLUSION
In summary, we develop a conjugated polymer host by
covalently attaching triphenylamine unit to the side chain of
poly(m-phenylene) backbone. The polymer can efficiently host
blue and white electrophosphorescence thanks to its ET as high
as 2.65 eV and high-lying HOMO energy level of ꢀ5.35 eV. The
blue and white PPLEDs show external quantum efficiency of
9.3% and 10.6%, respectively, which are both fairly comparable to
those of nonconjugated polymer hosts. Work is underway to
hopefully improve the lifetime of blue and white PPLEDs. The
prior work in achieving extremely long lifetime PLEDs using
conjugated polymers30 provides insights and encouragement in
developing PPLEDs based on poly(m-phenylene) derivative host
with variable emission colors, high efficiency, and long lifetime.
Figure 7. EL spectra (a), current densityꢀluminance efficiencyꢀpower
efficiency curve (b) and voltageꢀcurrent densityꢀbrightness curve (c)
of the white PPLEDs made from PmPTPA, 15 wt % FIrpic, 0.2 wt %
Ir(mppy)3 and 0.5 wt % Ir(piq)3.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: qpei@seas.ucla.edu.
coordinates of (0.16, 0.33). The EL efficiencies are much higher than
those of previous blue PPLEDs based on conjugated polymer hosts,
such as carbazole-tethering poly(m-phenylene) (4.7 cd/A)24 and
poly(3,6-fluorene) (0.4 cd/A).25 Moreover, the device efficiencies of
PmPTPA are fairly comparable to those of nonconjugated polymer
host PVK (about 15ꢀ20 cd/A at practical brightness).3ꢀ7 The high
device efficiencies of PmPTPA are attributed to two factors. First, the
high ET of PmPTPA prevents the detrimental triplet energy back
transfer from the dopant to the host. Second, owing to the high-lying
HOMO energy level of PmPTPA, the hole injection barrier from
PEDOT:PSS to PmPTPA is small, enhancing hole injection and
improving balance of electrons and holes.
On the basis of the successful blue PPLEDs, we fabricated
white PPLEDs with PmPTPA host. Emissive layer of ideal white
PPLEDs involved blue, green, and red phosphorescent dopants
dispersed in a polymer host. Here, we selected commercially
available FIrpic, Ir(mppy)3, and Ir(piq)3 as the blue, green, and
’ ACKNOWLEDGMENT
The work reported here was supported by the United States
Department of Energy Solid State Lighting Program (Grant No.
DE-FC26-08NT01575). The authors thank Dr. Zhibin Yu for
fruitful discussion.
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