Hindrance-functionalized π-stacked polymer host materials of the cardo-type carbazole-fluorene hybrid for solution-processable blue electrophosphorescent devices

Full text of DOI:10.1021/ma200624u

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pubs.acs.org/Macromolecules
Hindrance-Functionalized π-Stacked Polymer Host Materials of the
Cardo-Type CarbazoleꢀFluorene Hybrid for Solution-Processable
Blue Electrophosphorescent Devices
Cheng-Rong Yin,† Shang-Hui Ye,† Jie Zhao,‡ Min-Dong Yi,† Ling-Hai Xie,*,† Zong-Qiong Lin,†
Yong-Zheng Chang,† Feng Liu,† Hui Xu,‡ Nai-En Shi,† Yan Qian,† and Wei Huang*,†
†Key Laboratory for Organic Electronics & Information Displays (KLOEID) and Institute of Advanced Materials (IAM),
Nanjing University of Posts & Telecommunications, Nanjing 210046, P. R. China
‡Key Laboratory of Functional Inorganic Material Chemistry & Ministry of Education, School of Chemistry and Materials,
Heilongjiang University, Harbin 150080, P. R. China
S Supporting Information
b
’ INTRODUCTION
π-conjugation-interrupted technique also provides a useful strat-
egy to improve the ET levels of polymer host materials through
the introduction of sp3-hybrized carbon, silicon, or oxygen.22
However, there are still rare polymer host materials for blue
electrophosphorescent emitters up to date.
Phosphorescent organic light-emitting devices (PHOLEDs)
have attracted tremendous research interests owing to the spinꢀ
orbit coupling interactions of heavy metals, resulting in the harvest
of both singlet and triplet excitons to give the approaching 100%
internal quantum efficiency theoretically.1ꢀ4 However, it is a pre-
requisite for high-performance PHOLEDs to blend the triplet
emitters of heavy-metal complexes into host matrixes to reduce
their concentration-quenching effect. Recently, considerable
progresses have been achieved in small molecular host materials
based blue PHOLEDs in terms of brightness, power efficiency,
and durability.5ꢀ8 However, small-molecule based devices require
complex coevaporation techniques, high vacuum, and tedious
and precise control process during device fabrication, thus
greatly hindering the success of product development and
commercialization. In contrast, solution process methods, such
as spin-coating, inkjet printing, or screen printing, show more
advantages of easier fabrication process and lower cost with
phosphorescent polymer light-emitting diodes (PPLEDs).9 In
this context, the key issue is to develop the excellent polymer host
materials for PPLEDs, especially for blue PPLEDs.
Similar to small molecular host materials, high triplet energy
(ET) levels are crucial for suitable polymer host materials to
effectively prevent reverse energy transfer from emitters to hosts
and confine triplet excitons on the triplet emitters in PPLEDs.10ꢀ12
In general, π-conjugated polymers usually have low ET levels,
which are not suitable as host materials for blue and green
phosphorescent heavy-metal complexes,13,14 such as poly(2,7-
fluorene)s and its derivatives which are the widely used π-con-
jugated polymer host materials possess low ET of 2.15ꢀ2.3 eV.15,16
One effective method to increase the ET levels is to design the
nonlinear π-conjugated polymers via the method, limiting the
delocalization length of carriers and excitons. However, the
obtained π-conjugated polymers, such as poly(3,6-carbazole)s
(ET = 2.53ꢀ2.6 eV),17,18 poly(3,6-fluorene)s (ET = 2.58 eV),19
poly(3,6-silafluorene)s (ET = 2.55 eV),20 and poly(m-pheny-
lene) derivatives (ET = 2.64 eV)21 still have too low ET levels to
act as efficient host materials for typical blue electrophosphores-
cent emitters, such as bis[(4,6-difluorophenyl)pyridinato-N,
C2(picolinato)iridium(III) (FIrpic) (ET = 2.65 eV). Other
With respect to π-conjugated polymers, π-stacked polymers
as supramolecular semiconductors exhibit the unique carrier and
exciton features in the applications of various polymer semicon-
ducting devices.23,24 The design of π-stacked polymers opens
another way to develop the polymer hosts for red-, green-, and
blue-light-emitting triplet emitters. One typical and successful
π-stacked polymer host material for triplet emitters is poly(N-
vinylcarbazole) (PVK) with the relative high triplet energy state
along with excellent hole-transporting ability,9,25ꢀ27 although the
ET value of PVK is still ambiguous in the reported litera-
tures.14,28,29 Bilayer blue PPLEDs using PVK as polymer host
exhibited power efficiency of 14 lm/W and luminous efficiency of
22 cd/A according to the reports by So and co-workers.30
Nevertheless, the design of π-stacked polymer host materials
with high triplet energy and bipolar carrier transporting ability
affords one promising alternatives for the high-performance blue
PPLEDs.31ꢀ36 In this paper, we proposed hindrance-functiona-
lized π-stacked polymers to design high-triplet-energy polymer
host materials for solution-processable blue electrophosphores-
cent devices. A model poly(N-vinyl-3-(9-phenylfluoren-9-yl)car-
bazole) (PVPFK) has been designed and synthesized. Bulky
9-phenylfluorenyl moieties (PFMs) with steric hindrance effect
exhibits high ET levels8 and good electron-transporting ability.37
Futhermore, the introduction of PFMs into the carbazole units of
PVK through the 3D cardo-type linkers of the sp3-hybridized
carbon can not only block the intrachain face-to-face overlaps of
carbazole groups but also improve the chain rigidity of PVK,
resulting in high thermal and morphological stabilities. In addi-
tion, PVPFK has the enhanced electron-transporting abi-
lity with respect to PVK. PPLEDs using PVPFK as the
host and FIrpic as the guest exhibited lower turn-on voltage,
Received: March 18, 2011
Revised:
May 10, 2011
Published: May 31, 2011
r
2011 American Chemical Society
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Macromolecules
COMMUNICATION TO THE EDITOR
higher efficiencies, and brightness than that of PVK-based
counterpart.
electron-transporting/hole-blocking layer as well as excition-
confining layer. The cathode Ca:Ag alloy was subsequently
deposited onto the TPBI layer. EL spectra and chromaticity
coordinates were measured with a SpectraScan PR650 photo-
meter. Current densityꢀvoltageꢀluminance (JꢀVꢀL) mea-
surements were conducted simultaneously using a Keithley
4200 semiconductor parameter analyzer combined with a New-
port multifunction 2835-C optical meter, with luminance being
measured in the forward direction. All device characterizations
were carried out under ambient laboratory air at room temperature.
Synthesis of 3-(9-Phenylfluoren-9-yl)carbazole (2). A solu-
’ EXPERIMENTAL SECTION
Chemicals and Materials. Carbazole, 1,2-dichloroethylene,
2,20-azobis(isobutyronitrile) (AIBN), and other reagents were
used as received from commercial sources. All the solvents were
treated according to the standard procedures. 9-Phenylfluoren-9-
ol (1) was prepared according to our previous report.38
Characterization. 1H and 13C NMR in DMSO-d6 or CDCl3
were recorded at 400 MHz using a Varian Mercury 400 plus
spectrometer. Absorption and photoluminescence (PL) emis-
sion spectra of the materials were recorded on UV-3600 Shi-
madzu UVꢀvisꢀNIR spectrophotometer and a Shimadzu RF-
5301PC spectrophotometer, respectively. The solution state
spectra were measured in chlorobenzene solution. The film
was prepared by spin-coating from chlorobenzene solution.
Thermogravimetric analyses (TGA) were conducted on a Shi-
madzu DTG-60H thermogravimetric analyses with a heating rate
10 °C/min under a nitrogen atmosphere. The differential scan-
ning calorimetry (DSC) analyses were performed on a Shimadzu
DSC-60A Instrument 0 at a heating rate of 10 °C/min. Gel
permeation chromatography (GPC) was used to obtain the
molecular weight of the polymers with reference to polystyrene
standards using THF as eluant. The cyclic voltammetric (CV)
studies were conducted at room temperature on the CHI660E
system in a typical three-electrode cell with a platinum sheet
working electrode, a platinum wire counter electrode, and a
silver/silver nitrate (Ag/Agþ) reference electrode. All electro-
chemical experiments were carried out under a nitrogen atmo-
sphere at room temperature in an electrolyte solution of 0.1 M
tetrabutylammonium hexafluorophosphate (Bu4NPF6) in dichloro-
methane (CH2Cl2) at a sweeping rate of 0.1 V/s. According to the
redox onset potentials of the CV measurements, the highest
occupied molecular orbital (HOMO)/lowest unoccupied mo-
lecular orbital (LUMO) of the materials were estimated based
on the reference energy level of ferrocene (4.8 eV below the
vacuum): HOMO/LUMO = ꢀ(Eonset ꢀ 0.02 V) ꢀ 4.8 eV,
where the value 0.02 V is for FOC vs Ag/Agþ. The phosphor-
escence spectrum was tested with combined steady-state fluor-
escence and phosphorescence lifetime spectrometer (Edinburgh
Instrumennts, Plsp 920). The time-resolved measurements
(time-resolved emission spectra, TRES) were performed in
CH2Cl2 glass at 77 K using time-correlated single photon
counting (TCSPC) method with a microsecond flash lamp and
the synchronization photomultiplier for signal collection and
TCC900 plug-in PC card incorporating START and STOP
constant fraction discriminators for data processing. The values
of ET levels of these compounds have been estimated from the
onset peak of their respective phosphorescence spectrum.
Fabrication of Phosphorescent Polymer Light-Emitting
Devices. PPLEDs were fabricated in the following configuration:
indium tin oxide (ITO)/poly(ethylenedioxythiophene)ꢀpoly-
(styrenesulfonic acid) (PEDOT:PSS, 10 nm)/host: 30% FIrpic
(65 nm)/1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi,
40 nm)/Ca:Ag. A layer of PEDOT:PSS with thickness of
10 nm was spin-coated directly onto the ITO glass and dried
at 120 °C for 20 min under vacuum to enhance the hole injec-
tion ability and to smooth the ITO substrate. The solution of the
polymer was prepared under a nitrogen atmosphere and spin-
coated on the PEDOT:PSS layer. The TPBI was used as an
tion of BF3 Et2O complex (3.73 mL, 29.37 mmol) in appro-
3
priate CH2Cl2 (40 mL) was added dropwise to a mix-
ture solution of 1 (6.45 g, 24.96 mmol) and carbazole (4.16 g,
24.96 mmol) in appropriate CH2Cl2 (200 mL). The reaction
mixture was stirred at 25 °C under nitrogen until starting material
is no longer detectable by TLC. After that, ethanol (100 mL) and
water (300 mL) were successively added to quench the reaction,
and then organic phases were separated and the aqueous phase
was extracted with dichloromethane. The combined organic
phases were washed and dried over MgSO4. After removal of
the solvent, the remaining crude product was purified by silicon
gel chromatography (petroleum ether/dichloromethane) to
yield products. Yield: 4.98 g (49%). MS (m/z): [Mþ] calcd for
C31H21N, 407.51; found 407.20. 1H NMR (400 MHz, DMSO-
d6) δ (ppm): 11.2 (s, 1H), 7.92ꢀ7.94 (d, J = 7.3 Hz, 2H),
7.84ꢀ7.86 (d, J = 8.0 Hz, 2H), 7.78 (s, 1H), 7.50ꢀ7.51 (d, J = 7.4
Hz, 2H), 7.29ꢀ7.42 (m, 7H), 7.21ꢀ7.25 (m, 3H), 7.12ꢀ7.17 (t,
3H), 7.02ꢀ7.05 (t, 1H). 13C NMR (100 MHz, DMSO-d6) δ
(ppm): 65.5, 111.2, 111.4, 118.9, 119.1, 120.4, 120.9, 122.5,
126.0, 126.2, 126.6, 127.0, 127.9, 128.1, 128.2, 128.7, 136.0,
139.0, 139.9, 140.5, 147.0, 151.8. Anal. Calcd for C31H21N: C,
91.37; H, 5.19; N, 3.44. Found: C, 91.65; H, 5.23; N, 3.39.
Synthesis of 9-(2-Chloroethyl)-3-(9-phenylfluoren-9-yl)car-
bazole (3). 2 (15 mmol) was added to an intensity stirred
mixture of 1,2-dichloroethylene (40 mL), tetrabutylammonium
bromide (0.13 g, 0.4 mmol), KOH (7 g, 125 mmol), and K2CO3
(5.5 g, 40 mmol). The stirring was continued at 45ꢀ50 °C for
3.5ꢀ5.5 h. After cooling, the inorganic material was filtered off,
and the organic solution was washed with water (2 ꢁ 25 mL).
The solution was then dried over anhydrous MgSO4 and filtered,
and the solvent was evaporated to dryness. The product was
purified by crystallization from ethanol. Yield: 2.68 g (38%). MS
1
(m/z): [Mþ] calcd for C33H24ClN, 470.00; found 470.20. H
NMR (400 MHz, DMSO-d6) δ (ppm): 7.93ꢀ7.95 (d, J = 7.8 Hz,
2H), 7.88ꢀ7.90 (d, J = 7.7 Hz, 2H), 7.83 (s, 1H), 7.59ꢀ7.61
(d, J = 8.1 Hz, 1H), 7.51ꢀ7.53 (d, J = 7.6 Hz, 3H), 7.38ꢀ7.43
(t, 3H), 7.08ꢀ7.41 (m, 9H). 13C NMR (100 MHz, DMSO-d6)
δ (ppm): 43.4, 44.5, 65.4, 109.9, 110.0, 119.3, 119.5, 120.5,
121.0, 122.3, 122.4, 126.2, 126.3, 126.7, 127.0, 128.0, 128.1,
128.3, 128.8,136.8, 139.3, 139.9, 140.9, 146.8, 151.7. Anal. Calcd
for C33H24ClN: C, 84.33; H, 5.15; N, 2.98. Found: C, 84.22; H,
5.30; N, 2.91.
Synthesis of N-Vinyl-3-(9-phenylfluoren-9-yl)carbazole (4).
A mixture of 3 (5.0 mmol), KOH (1.12 g, 20 mmol), and
hydroquinone (25ꢀ30 mg, 0.22ꢀ0.27 mmol) was placed in
2-propanol (30 mL) and refluxed for 3 h. Next the alcohol was
evaporated under reduced pressure, and the organic material was
extracted from the reaction mixture by means of CH2Cl2 (3 ꢁ
20 mL). The extract was then dried over anhydrous MgSO4 and
filtered, and the solvent was evaporated to dryness. The product
was purified by crystallization from methanol. Yield: 2.12 g
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COMMUNICATION TO THE EDITOR
Scheme 1. Synthetic Route of PVKPFa
a Reagents and conditions: (a) carbazole, BF3 E2O, CH2Cl2; (b) 1,2-dichloroethylene, KOH, K2CO3, 45ꢀ50 °C; (c) isopropanol, KOH,
3
hydroquinone, refluxed, 3 h; (d) AIBN, toluene, 85 °C, 2 days.
(98%). MS (m/z): [Mþ] calcd for C33H23N, 433.54; found
1
433.15. H NMR (400 MHz, DMSO-d6) δ (ppm): 7.93ꢀ7.95
(d, J = 7.6 Hz, 3H), 7.88 (s, 1H), 7.77ꢀ7.79 (d, J = 8.3 Hz, 1H),
7.71ꢀ7.74 (d, J = 8.7, 1H), 7.48ꢀ7.55 (p, 3H), 7.38ꢀ7.45 (m,
3H), 7.29ꢀ7.33 (m, 2H), 7.14ꢀ7.25 (m, 7H). 13C NMR (100
MHz, DMSO-d6) δ (ppm): 65.4, 79.6, 101.5, 111.3, 111.4, 119.4,
120.7, 121.2, 123.4, 123.7, 126.6, 127.1, 127.2, 128.1, 128.3,
130.1, 138.5, 139.5, 140.0, 146.6, 151.4. Anal. Calcd for
C33H23N: C, 91.42; H, 5.35; N, 3.23. Found: C, 91.15; H,
5.52; N, 3.13.
Synthesis of Poly(N-vinyl-3-(9-phenylfluoren-9-yl)carbazole)
(PVPFK). The monomer 4 (0.433 g, 1.0 mmol) was allowed to
polymerize carried out in toluene (10 mL) with 10 wt % 2,20-
azobis(isobutyronitrile) (AIBN) (0.0433 g) as initiator and
refluxed under nitrogen at 85 °C for 2 days. The polymerization
was stopped by pouring the reaction mixture into ethanol. The
obtained polymer was purified by repeated reprecipitations
followed by drying under vacuum. Yield: 0.251 g (58%). Mn =
Figure 1. TGA curve of PVPFK recorded at a heating rate of 20 °C/min
(decomposition temperature from the position of 5% weight loss). Inset:
DSC trace of PVPFK recorded at a heating rate of 20 °C/min.
1
9047, PDI = 1.79. H NMR (400 MHz, CDCl3) δ (ppm):
the target polymer PVPFK in a 58% yield. PVPFK is readily
soluble in common organic solvents such as chloroform, tetra-
hydrofuran (THF), toluene, and chlorobenzene. The chemical
structures of monomer and PVPFK were confirmed by 1H NMR
and 13C NMR and element analysis. The number-average
molecular weight (Mn) and polydispersity index (PDI) of PVPFK,
as determined by GPC against a polystyrene standard in THF,
were around 9 ꢁ 103 and 1.79, respectively. Thermal properties
of PVPFK have been estimated by TGA and DSC. PVPFK has
outstanding thermal stability with the decomposition tempera-
ture (Td) of up to 434 °C. No glass phase transition was observed
in the temperature range from 50 to 300 °C. The high thermal
stability of PVPFK probably is attributed to the PFM in the 3D
cardo structure, suppressing its crystallization process, which is
greatly favorable for the stable polymer host materials in PPLEDs.
Photophysical Properties. Figures 2a,b compare the UVꢀvis
absorption and PL spectra of PVPFK with the pristine PVK in
diluted chlorobenzene solutions as well as in the thin film spin-
coated from chlorobenzene solution. The normalized UVꢀvis
absorption spectra of PVPFK in solution shows three strong
absorption bands at 299, 336, and 349 nm, similar to that of PVK
with the bands at 293, 330, and 343 nm, respectively. A
5.28ꢀ8.03 (br, ArH), 2.48 (s, CH), 2.31(s, CH2). 13C NMR
(100 MHz, CDCl3) δ (ppm): 45.7, 57.3, 65.4, 112.1, 120.2,
123.8, 124.3, 125.3, 126.6, 127.0, 127.6, 128.2, 129.0, 129.8,
130.0, 130.5, 131.5, 140.0, 146.4, and 152.0. Anal. Calcd
for C33H23N: C, 91.42; H, 5.35; N, 3.23. Found: C, 90.98; H,
5.78; N, 3.32.
’ RESULTS AND DISCUSSION
Synthetic routes of the monomer 4 and target PVPFK are
depicted in Scheme 1 according to a reported route in the
previous literature.39 PFM-functionalized cabazole 2 was synthe-
sized through FriedelꢀCrafts reaction using BF3 Et2O complex
3
as a Lewis acid catalyst in the yield of 49% according to our
previous works.40 The following nucleophilic substitution of
compound 2 with dichloroethylene was carried out to give
compound 3, followed by the vinylization under the condition
of hydroquinone/isopropanol/KOH to give monomer 4 in the
yield of 98%. Free radical polymerization of monomer 4 was
conducted in toluene using AIBN as initiator in the weight ratio
of 10% at 85 °C for 2 days under nitrogen to smoothly prepare
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Figure 2. (a) UVꢀvis absorption and PL spectra of PVPFK and PVK in diluted chlorobenzene solution. (b) UVꢀvis absorption and PL spectra of
PVPFK and PVK in the solid state. (c) Normalized UVꢀvis spectra of FIrpic and PL spectra of PVPFK in thin film. (d) PL spectra of thin films of PVPFK
doped with various concentrations of FIrpic.
bathochromic shift of 6 nm is probably due to the hyperconjugation
effect of bulky PFMs. However, the PL spectrum of PVPFK in
solution is obviously different from that of PVK whose major
emission peak at 367 nm and a shoulder emission peak at 400 nm.
But PVPFK exhibits only one emission peak at 376 nm without any
excimer emission peaks in solution; for details see Figure 2. Further-
more, PVPFK in solution have an obviously narrow emission with
9 nm red shift with respect to that of PVK. Two emission peaks of
PVK in solution come from intrachain partial and full face-to-face
overlaps of carbazole groups. These results suggest that no full
overlapping conformation formed in PVPFK solution.41 The first
emission peak exhibits a red shift of 13 nm when transferred from the
solution to thin film and the intensity of emission peak at 410 nm
becomes stronger, which suggest that full overlapping components
Figure 3. Cyclic voltammograms of PVPFK and PVK film on a glassy
carbon electrode (0.1 MBu4NBF4 in acetonitrile, scan rates 20 mV/s).
increased. In contrast, the PVPFK in the thin film exhibits emission
peak at 379 nm, which is slightly red-shifted (ca. 3 nm) compared
with that of the solution spectrum. These results indicate that πꢀπ
stacking alignments of carbazole groups in PVPFK are effectively
blocked by the bulky PFMs owing to their steric hindrance effects.
The optical band gap of PVPFK was estimated to be 3.34 eV from
the absorption edge of its UVꢀvis spectrum in thin solid film, a little
smaller than that of PVK film (3.44 eV).
Figure 2c shows the absorption spectrum of FIrpic and the PL
spectrum of pure PVPFK in the solid state. There is a good
overlap between the PL spectrum of PVPFK and the absorption
spectrum of FIrpic, which guarantee efficient F€orster energy
transfer from singlet-excited state in the host (PVPFK) to
metalꢀligand (FIrpic) charge-transfer absorption band in the
guest. In order to estimate the potential of PVPFK as a polymer
host for blue phosphorescent emitters and test the efficiency of
F€orster energy transfer, the PL spectra of PVPFK films doped
with various contents of FIrpic were investigated. Figure 2d
shows the PL spectra of pure PVPFK film and the doped PVPFK
films. The doping concentrations of FIrpic in PVPFK varied from
0.1%, 0.5%, 1.0%, and 3.0% to 5.0% by weight. The PL profile
contains two emission peaks: one peak at 379 nm comes from
the PVPFK emission, and the other peak at 473 nm comes from
The phosphorescence spectra of PVPFK and PVK were also
measured in CH2Cl2 glass at 77 K under the same conditions
(Figure S1). The value of ET level of PVPFK was estimated from
the highest energy 0ꢀ0 phosphorescent emission located at
442 nm to be 2.80 eV, which was lower than that of PVK (ET =
2.95 eV, 422 nm). This result is consistent with the previous
reports that the introduction of fluorene into carbazole-based
groups has an reduction of ET level.8 However, the ET level of
2.80 eV is high enough for PVPFK to serve as host materials for
blue triplet emitters, such as FIrpic (ET = 2.65 eV).
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Figure 4. (a) Current densityꢀvoltage (JꢀV) curves of PVPFK and PVK devices. (b) Luminanceꢀvoltage (LꢀV) curves of PVPFK and PVK devices.
the FIrpic triplet emission. The emission intensity of the host
material at 379 nm decreased with increasing doping concentra-
tion of FIrpic and was completely quenched at a doping
concentration of about 5 wt %, indicating that there was an
efficient F€orster energy transfer from PVPFK to FIrpic.
Electrochemical Properties. CV measurement was carried
out to investigate the oxidation and reduction behavior of
PVPFK and estimate its HOMO and LUMO energy levels.
Figure 3 displays the CV spectra of PVK and PVPFK measured
under the same condition. The oxidation onset potentials were
recorded at 0.80 V for PVPFK and 0.83 V for PVK versus
Ag/Agþ, respectively. As a result, HOMO energy levels were esti-
mated to be ꢀ5.58 eV for PVPFK and ꢀ5.61 eV for PVK according
Figure 5. Luminance and power efficiency characteristic curves as a
function of luminance of PVPFK and PVK devices.
to the empirical formula EHOMO = ꢀ(Eox ꢀEFOC) ꢀ 4.8 eV, where
the value of EFOC for ferrocene is 0.02 V vs Ag/Agþ and 4.8 eV is
the energy level of ferrocene below vacuum. The result indicated
that the introduction of the bulky PFMs into PVK does not affect
the HOMO energy level. The reduction onset potentials were
measured to be ꢀ2.70 V for PVPFK and ꢀ2.88 V for PVK, from
which the LUMO energy levels of PVPFK and PVK were
calculated to be ꢀ2.08 and ꢀ1.90 eV, respectively, according to
the empirical formula ELUMO = ꢀ(Ered ꢀ EFOC) ꢀ 4.8 eV, where
the value of EFOC for ferrocene is 0.02 V vs Ag/Agþ and 4.8 eV
is the energy level of ferrocene below vacuum. These results suggest
that PVPFK have better electron-transporting ability than that of
PVK owing to the introduction of fluorene groups. In general, PVK
has poor electron-transporting ability.42 Electron-transporting
materials, such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-ox-
adizaole (PBD) or 1,3-bis[(4-tert-butylphenyl)-1,3,4- oxadiazolyl]-
phenylene (OXD), were required to dope into PVK to balance
the hole and electron transportation in PPLEDs. However, the
risk of phase separation occurring over time could not be
avoided in PVK and PBD/OXD blends based PPLEDs.43,44
The electrochemical characterization studies of PVPFK indi-
cate that PVPFK could be applied as potential host material
for PPLEDs directly with no need to dope with electron-
transporting materials.
Electroluminescence Properties of PPLEDs. From the fore-
going analysis, we known that PVPFK possesses overall better
physicochemical properties and carrier transporting ability than
that of pristine PVK in terms of the thermal and morphological
stability, photophysical properties, and electronic structures,
except for the ET level (2.80 eV), making it as a potential polymer
host for blue PPLEDs. To testify our assumption, blue PPLEDs
were fabricated using PVPFK as blue phosphorescent host
material and FIrpic as phosphorescent emitter in a simply
three-layered device structure. For comparison, PVK-based
devices were also fabricated by using PVK to replace PVPFK
Figure 6. EL spectra of PPLEDs made from PVPFK device (dash line)
and PVK device (solid line) doped with FIrpic at a mass ratio of 30 wt %.
matrix in the same device configuration. For all devices the
doping concentration was controlled at 30%. The emitting layer
was spin-coating from chlorobenzene solution on the PEDOT:
PSS smoothed ITO glass substrate, and the electron-transporting
layer of TPBi was vapor deposited under vacuum. The device
architecture was ITO/PEDOT:PSS (10 nm)/host: 30% FIrpic
(65 nm)/TPBI (40 nm)/Ca:Ag, where the host material was
PVPFK or PVK.
Figure 4 shows the current densityꢀvoltageꢀluminance
(JꢀVꢀL) curves of PVPFK and PVK devices. Clearly, the
PVPFK device exhibited lower operation voltage and higher
luminance than PVK device under the same applied voltage. For
example, the turn-on voltage (recorded at a brightness of 1 cd/m2)
was 6.1 V for PVPFK device vs 7.4 V for PVK device, and the
luminance of PVPFK device is higher than the PVK device in the
operational voltage range. For example, the brightness of PVPFK
device reached 12418 cd/m2 at 17 V, while PVK device only
exhibited 4875 cd/m2 at the same applied voltage. Moreover, our
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Macromolecules
COMMUNICATION TO THE EDITOR
Table 1. Blue PPLEDs Performance Based on PVK or PVPFK Host Materials
host
Von [V]a
Lmax [cd/m2]
LEmax [cd/A]
PEmax [lm/W]
LEb [cd/A]
PEb [lm/W]
CIEc [x, y]
LEc [cd/A]
PEc [lm/W]
PVK
PVPFK
7.4
6.1
10 968
13 287
13.3
14.2
2.4
2.8
6.1
1.5
2.7
0.161, 0.398
0.152, 0.379
12.9
14.1
2.2
2.6
12.0
a Recorded at 1 cd/m2. b Obtained at a brightness of 100 cd/m2. c Obtained at a brightness of 1000 cd/m2.
device showed higher maximum brightness with maximum
luminance of 13287 cd/m2 compared with 10968 cd/m2 of PVK
device. Lower operation voltage and higher luminance make our
device more efficient, which was witnessed in the luminance
efficiency curves as depicted in Figure 5. Our device exhibited
higher luminance efficiency than PVK device in the whole
measuring voltages range from 0 to 21 V. The maximum lumi-
nance efficiencies were 14.2 and 13.3 cd/A for PVPFK and PVK
device, respectively, and the maximum power efficiencies were
2.8 and 2.4 lm/W for PVPFK and PVK device, respectively (for
details, see Table 1). Another thing worth mentioning is that the
maximum efficiency of our device was obtained in the practical
luminance range of 100ꢀ1000 cd/m2, which means more useful
for commercial application.
As for the EL spectra of the blue PPLEDs, only blue emission
was observed with peak at 476 nm and a shoulder at around
500 nm, and no high-energy shortwave emission from PVPFK or
PVK host was observed, suggesting efficient energy from the host
to the dopant phosphor emitter (for details, see Figure 6). Similar
emission spectra from both PVPFK and PVK devices indicate the
same mechanism involved in the EL process. But our device
showed a little bluer emission compared with the PVK device,
which was mirrored in the weaker shoulder and smaller full
width at half-maximum (fwhm). The fwhm was 60.9 nm for
the PVPFK device while the PVK device was 65.5 nm. And
thus the corresponding Commission of International de
l’Eclairage (CIE) coordinates x, y were (0.152, 0.379) and
(0.161, 0.398) for the PVPFK and PVK devices, respectively.
Blue emission spectrum of the blue PPLEDs indicates that
PVPFK was more suitable than PVK as a blue phosphorescent
host material.
material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: iamlhxie@njupt.edu.cn (L.-H.X.); wei-huang@njupt.edu.cn
(W.H.).
’ ACKNOWLEDGMENT
The project was supported by the National Key Basic Re-
search Program of China (973) (2009CB930600), National
Natural Science Foundation of China (60876010, 20704023,
20974046, 21003076), Key Project of the Ministry of Educa-
tion of China (104246, 208050), and Natural Science Founda-
tion of Jiangsu Province (BK2008053, 200805005, 200907003,
10KJB510013, 09KJB150009, SJ209003).
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’ CONCLUSIONS
We have proposed a strategy of steric hindrance functionaliza-
tion of π-stacked polymers to control their electronic structures
and phase behaviors. A solution-processable π-stacked homo-
polymer host with a congested conformation, PVPFK, has been
successfully designed and synthesized through the typical vinyl
polymerization of vinylcarbazole monomers containing cardo-
type bulky frameworks. Photophysical and electrochemical stud-
ies show that PVPFK has the ET level of 2.80 eV with the better
electron-transporting ability than PVK. Proof-of-concept PPLEDs
have been fabricated using PVPFK as host and FIrpic as blue
guest. The comparative study on PVPFK device and PVK device
shows that this hindrance-functionalized π-stacked polymer,
PVPFK, is a more effective host material for blue PPLEDs than
PVK. The PVPFK-based device exhibited the lower operation
voltage, higher luminance efficiency, and better CIE coordinate
and EL efficiency than the PVK device under the same conditions.
’ ASSOCIATED CONTENT
S
Supporting Information. Structural characterizations
b
and phosphorescence spectra for the new compounds. This
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