Enhancement of electroluminescence properties of red diketopyrrolopyrrole-doped copolymers by oxadiazole and carbazole units as pendants

Article abstract of DOI:10.1016/j.polymer.2010.09.046

Two novel diketopyrrolopyrrole (DPP)-based copolymers P1-2 were prepared by doping red emitting DPP monomer (1mol%) into benzothiadiazole, alkoxybenzene and 9,9-dialkylfluorene-based copolymers through base-free Suzuki polymerization. P1 contained the pen

Full text of DOI:10.1016/j.polymer.2010.09.046

Polymer 51 (2010) 5726e5733
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Polymer
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Enhancement of electroluminescence properties of red diketopyrrolopyrrole-
doped copolymers by oxadiazole and carbazole units as pendants
,
Yi Jin a, Yanbin Xu a, Zhi Qiao a, Junbiao Peng b, Baozheng Wang b, Derong Cao a
*
a School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
b Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel diketopyrrolopyrrole (DPP)-based copolymers P1e2 were prepared by doping red emitting
DPP monomer (1 mol%) into benzothiadiazole, alkoxybenzene and 9,9-dialkylfluorene-based copolymers
through base-free Suzuki polymerization. P1 contained the pendants of electron-transport oxadiazole
and hole-transport carbazole, but P2 did not contain them. P1 had higher glass transition temperature
than P2. The electroluminescence (EL) devices of P1 and P2 (ITO/PEDOT:PSS/polymer/CsF/Al) exhibited
red emission with external quantum efficiency of 0.63% and 0.18%, and with brightness of 2681 and
885 cd/m2, respectively. The results show that the EL properties of P1 are much better than that of P2
due to the introduction of oxadiazole and carbazole as the pendants. The pendants could restrain
aggregation which might induce fluorescence quenching and are of benefit to keep high charge mobility.
The efficient energy transfer existed among the pendants, polymer backbone and DPP unit. These factors
should be responsible for the higher EL performance of P1.
Received 18 June 2010
Received in revised form
5 September 2010
Accepted 16 September 2010
Available online 1 October 2010
Keywords:
Diketopyrrolopyrrole
Conjugated polymer
Light-emitting diodes (LED)
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
In solid state, luminogenic molecules lie in close vicinity and
incline to form aggregates. Chromophore aggregation could usually
Diketopyrrolopyrrole (DPP) derivatives are a class of strongly
fluorescent heterocyclic pigments and their structures could be
easily optimized through variations of substituents at the 2,5- and
3,6-positions [1e3]. However, high content of DPP unit in the DPP-
based conjugated polymer decrease the quantum yield in solid
state and the electroluminescence (EL) efficiency of the polymeric
light-emitting diodes (PLED) [4,5]. The EL performance is enhanced
by doping low content of DPP unit (guest) in the main chain of
polyfluorene (host) but emission wavelength is blue shifted [4].
In the dopant/host system, carrier trapping and subsequent
direct carrier recombination on the guest molecule can adjust
charge balance in emitting layer and enhance the device efficien-
cies [6e10]. Besides this positive effect, charge trapping can lower
carrier mobility and often cause a high operating voltage [11e15],
even disturb the original charge balance and decrease the EL
performance [4,12,15]. This negative effect can be addressed by
using of a blend of carrier transport material with EL polymer [13].
The unavoidable drawback of the approach is the possible phase
separation between two different kinds of materials, which reduces
the lifetime and stability of the devices [16,17].
quench the emission of organic luminophores which have very high
luminescence efficiencies in the dilute solutions, due to formation of
detrimental species such as excimers [18e21]. This effect can be
mitigated by covalently attaching branched chains, bulky cyclics,
spiro kinks, and dendritic wedges to aromatic rings [22]. Moreover,
balanced charge injection and mobility and high charge mobility are
desirable to move the charge recombination zone away from the
near electrodes and improve the exciton generation rate in high
efficient electroluminescent devices [6,23,24]. However, in conju-
gated polymers, charge transport occurs via on-chain migration and
interchain hopping of carriers between adjacent chains which is the
rate limiting step. The interchain charge transport can occur at only
where there is sufficient overlap of wavefunctions betweenpolymer
chains. The bulky substituent should hinder the interchain interac-
tionandchainmovement. Asa result, chargemobilityof the polymer
with bulky pendant is lower than one without bulkyl pendant
[25e27]. This negativeeffect can be overcome bygraftingoxadiazole
and carbazole units as pendants into conjugated polymers, because
these substituents have good charge conduction property
[23,24,28e30]. That is to say, the grafted copolymers could facilitate
charge transport and injection, decrease operating voltage, and
enhance maximum brightness and EL efficiency in the emitting
diodes [16,30e35]. Furthermore, by incorporating benzothiadiazole
(BT) and dioctyloxybenzene into the fluorene polymer, the resulting
* Corresponding author. Tel./fax: þ86 20 87110245.
E-mail address: drcao@scut.edu.cn (D. Cao).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.09.046

Y. Jin et al. / Polymer 51 (2010) 5726e5733
5727
copolymers possess high electron and hole transporting properties
and high EL performance [36].
24 mmol) in CH2Cl2 (50 ml) at 0 ꢁC. The mixture was stirred for 4 h at
0 ꢁC followed by stirring for 14 h at room temperature. After that
NaHSO3 aqueous solution was added into the mixture until brown
color disappeared. The mixture was extracted with CH2Cl2, washed
with water and dried over MgSO4. After filtration, the solvent was
evaporated under reduced pressure and the residue was recrystal-
lized with CH2Cl2 and ethanol to give 3 as white solid (12.207 g,
In this paper, lowcontent of red emitting DPP monomer (1 mol%)
was doped into the main chain of benzothiadiazole and alkoxy-
benzene-based polyfluorene through Suzuki polymerization to
obtain high efficient red emitting polymer. Oxadiazole as the elec-
tron-transport moiety, and N-carbazolyl as the hole-transport
moiety were introduced to the copolymers as pendants, in order to
restrain aggregation induced fluorescence quenching and the
negative impact of dopant, to obtain balanced charge injection and
mobility, and lower operating voltage. The photophysical, electro-
chemical and electroluminescence properties of the polymers were
evaluated and compared with the polymer without the pendants as
well as with the polymer without the dopant.
yield ¼ 86%). mp: 90e91 ꢁC. 1H NMR (400 MHz, CDCl3,
d, ppm):
7.09 (s, 2H, Ar), 3.96 (t, J ¼ 6.4 Hz, 4H, CH2), 3.43 (t, J ¼ 6.8 Hz, 4H.
CH2), 1.92e1.89 (m, 4H, CH2), 1.84e1.81 (m, 4H, CH2), 1.54e1.51 (m,
8H, CH2).
2.3. 4-tert-Butyl-N0-(4-hydroxybenzoyl)benzohydrazide (6)
A solution of DMF (100 ml), pyridine (20.8 g, 263 mmol) and
4-tert-butylbenzoyl chloride (25.78 g, 131 mmol) was added
dropwise to the solution of 4-hydroxybenzo hydrazide (20 g,
131 mmol) and DMF (450 ml) at 0 ꢁC. The mixture was stirred at
0 ꢁC for 4 h. It was poured into water and washed with water.
The crude product was redissolved in NaOH aqueous solution.
After filtration, the red solution was neutralized with HCl to give
6 as white solid (25.87 g, yield ¼ 63.1%). mp: 258e260 ꢁC. 1H
2. Experimental
2.1. Instrumentation and materials
NMR (1H and 13C) spectra were collected on a Bruker DRX 400
spectrometer (in CDCl3 or DMSO-d6, TMS as internal standard).
APCI-MS spectra were recorded with a Bruker Esquire HCT plus
mass spectrometer. Elemental analysis was performed using
a Vario EL III instrument. Fourier transform infrared (FT-IR) spec-
tra were recorded on an RFX-65A (Analect Co.) spectrometer.
The number-average molecular weight (Mn) and weight-average
molecular weights (Mw) were determined by Waters GPC 515ꢀ410
in tetrahydrofuran (THF) with a calibration curve of polystyrene
standards. Ultravioletevisible (UVevis) absorption spectra were
recorded on an HP 4803 instrument. Cyclic voltammetry (CV)
was carried out on an EG&G model 283 computer-controlled
potential/galvanostat (Princeton Applied Research) with platinum
electrodes at a scanning rate of 50 mV/s against a calomel reference
electrode with a nitrogen-saturated solution of 0.1 M tetrabutyl
ammonium hexafluorophosphate (Bu4NPF6) in acetonitrile. Ther-
mogravimetric analysis (TGA) was conducted on a Pyris 1 TGA
under a heating rate of 10 ꢁC/min and a N2 flow rate of 20 mL/min.
Differential scanning calorimetry (DSC) measurement was per-
formed on a Diamond DSC under N2 at a heating rate of 10 ꢁC/min.
PL and EL spectra were recorded on an Instaspec IV CCD spectro-
photometer (Oriel Co.). The current-luminance voltage (IꢀLꢀV) was
measured using a Keithley 236 source measurement unit and
a calibrated silicon photodiode. The luminance was calibrated using
a PR-705 Spectra Scan spectrophotometer (Photo research).
NMR (400 MHz, DMSO-d6, d, ppm): 10.33 (s, 1H, OH), 10.22 (s,
1H, NH), 10.12 (s, 1H, NH), 7.84 (d, J ¼ 8 Hz, 2H, Ar), 7.79 (d,
J ¼ 8 Hz, 2H, Ar), 7.52 (d, J ¼ 8 Hz, 2H, Ar), 6.83 (d, J ¼ 8 Hz, 2H,
Ar), 1.30 (s, 9H, CH3).
2.4. 4-(5-(4-tert-Butylphenyl)-1,3,4-oxadiazol-2-yl)phenol (7)
A
mixture of 6 (21.107 g, 67.5 mmol), pyridine (1.069 g,
13.5 mmol) and SOCl2 (215 ml) was stirred at 88 ꢁC for 6 h. The
solvent was removed and the residue was poured into ice water.
After filtration, filter cake was recrystallized with ethanol to give 7
as white solid (14.371 g, yield ¼ 72.3%). mp: 240e242 ꢁC. 1H NMR
(400 MHz, DMSO-d6,
2H, Ar), 7.94 (d, J ¼ 8 Hz, 2H, Ar), 7.62 (d, J ¼ 8 Hz, 2H, Ar), 6.97 (d,
J ¼ 8 Hz, 2H, Ar), 1.31 (s, 9H, CH3).
d
, ppm): 10.36 (s, 1H, OH), 8.00 (d, J ¼ 8 Hz,
2.5. 2-(4-6-(5-Bromo-4-(6-(4-(5-(4-(2-bromopropan-2-yl)
phenyl)-1,3,4-oxadiazol-2-yl)phenoxy)hexyloxy)-2-methylphenoxy)
hexyloxy)phenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadizaole (8)
A mixture of 3 (7.06 g, 11.9 mmol), 7 (7.0 g, 23.8 mmol), NaOH
(1.07 g, 26.2 mmol), tetrabutyl ammonium bromide (1.50 g,
4.65 mmol), DMSO (70 ml) and ethanol (70 ml) was heated to 90 ꢁC
for 6 h. After that the mixture was poured into water (500 ml) and
precipitates were separated. These precipitates were washed with
water then ethanol followed by recrystallization with CH2Cl2 and
ethanol to give 8 as white solid (12.14 g, yield ¼ 73.2%). mp:
4-Hydroxybenzo
hydrazide
[37],
2,5-dioctyl-3,6-bis(40-
formylphenyl)pyrrolo[3,4-c]pyrrole-1,4-dione [38],1,4-dibromo-2,5-
bi-octyloxybenzene [39], 4,7-dibromo-2,1,3-benzothiadia-zole [40],
9,9-bis(N-carbazolyl-hexyl)-2,7-dibromofluorene [41], 2,7-dibromo-
9,9-dioctyl fluorine [42] were synthesized according to the published
literature; other chemicals were obtained from commercially avail-
able resources.
169e172 ꢁC 1H NMR (400 MHz, DMSO-d6,
d, ppm): 8.06e8.02 (m,
8H, Ar), 7.52 (d, J ¼ 8.4 Hz, 4H, Ar), 7.07 (s, 2H, Ar), 7.00 (d, J ¼ 8.8 Hz,
4H, Ar), 4.04 (t, J ¼ 6.4 Hz, 4H, CH2), 3.96 (t, J ¼ 6.4 Hz, 4H, CH2),
2.2. 1,4-Dibromo-2,5-bis(6-bromohexyloxy)benzene (3)
1.85e1.84 (m, 8H, CH2),1.58e1.56 (m, 8H, CH2),1.35 (s,18H, CH3); 13
C
NMR (100 MHz, CDCl3, d, ppm): 161.9, 155.2, 150.0, 128.7, 126.7,
A mixture of hydroquinone (5.416 g, 49 mmol), NaOH (4.328 g,
108.2 mmol) and ethanol (105 ml) was added dropwise into the
solution of1,6-dibromohexane(60 g, 245 mmol)andethanol (75 ml)
at 90 ꢁC. After refluxing for 4 h, the mixture was extracted with
CH2Cl2 and the residue was removed. The solution was washed with
H2O and the solvent was evaporated under reduced pressure. To oily
product ethanol (200 ml) was added that gave precipitates of 1,4-bis
(6-bromohexyloxy)benzene (2) as white solid (9.43 g, yield ¼ 44%),
mp: 87e90 ꢁC. The product was used directly for the next step
without further purification. A mixture of Br2 (9.632 g, 60 mmol) and
CH2Cl2 (15 ml) was added dropwise into the solution of 2 (10.5 g,
126.0, 121.1, 118.5, 116.2, 115.0, 111.1, 70.0, 68.0, 35.1, 31.1, 29.0, 25.7,
25.7; MS (APCI) calcd for C54H60 Br2N4O6 1020.3, found 1021.3. Anal.
Calc. for C54H60Br2N4O6: C, 63.53; H, 5.92; N, 5.49. Found: C, 63.69; H,
6.15; N, 5.46.
2.6. 2,5-Dioctyl-3,6-bis{40-[2-(4-bromophenyl)-2-cyano-vinyl]-
phenyl}pyrrolo[3,4-c] pyrrole-1,4-dione (11)
A mixture of 2-(4-bromophenyl)acetonitrile (122 mg, 0.625
mmol), 2,5-dioctyl-3,6-bis(40-formylphenyl)pyrrolo[3,4-c]pyrrole-
1,4-dione (142 mg, 0.25 mmol) and potassium carbonate (863 mg,

5728
Y. Jin et al. / Polymer 51 (2010) 5726e5733
6.25 mmol) in absolute ethanol (4 ml) was refluxed for 2.5 h. After
cooling, the mixture was poured into H2O (5 ml) and washed with
water. Precipitates were separated and recrystallized with CH2Cl2
and methanol which gave 11 as red solid (100 mg, yield ¼ 43%). mp:
and anhydrous toluene (10 ml) was stirred at 120 ꢁC for 60 h. 9,9-
Dihexylfluorene-2,7-bis(trimethyleneboronate) (30.0 mg, 0.060
mmol)wasadded, andagainstirredfor6 h. Bromobenzene(31.0 mg,
0.200 mmol) was added, and further stirred for 6 h. The mixture was
cooled to room temperature, and washed with water. The organic
layer was collected, and methanol was added. The precipitate was
collected and dissolved in CHCl3, and then acetone was added. The
precipitate was collected and extracted with acetone in Soxhlet
apparatus for 24 h to give P1 as orange solid (349 mg, yield ¼ 78%).
FT-IR (film, cmꢀ1): 3051, 2928, 2856, 1612, 1495, 1462, 1377, 1256,
1013, 817, 749.
263e264 ꢁC. 1H NMR (400 MHz, CDCl3,
d, ppm): 7.96 (d, J ¼ 8.0 Hz,
4H, Ar), 7.87 (d, J ¼ 8.0 Hz, 4H, Ar), 7.52e7.47 (m, 10 H, Ar and
CH ¼ C), 3.76 (t, J ¼ 7.2 Hz, 4H, NCH2), 1.52 (m, 4H, CH2), 1.17 (m,
20H, CH2), 0.81 (t, J ¼ 6.8 Hz, 6H, CH3); 13C NMR (100 MHz, CDCl3,
d,
ppm): 162.4, 147.5, 140.6, 135.5, 132.8, 132.3, 129.8, 129.7, 129.4,
127.4, 124.0, 117.3, 111.8, 110.6, 41.9, 31.7, 29.4, 29.1, 29.0, 26.7, 22.6,
14.1; MS (APCI) calcd for C52H52N4O2Br2 924.2, found 925.7. Anal.
Calc. for C52H52N4O2Br2: C, 67.52; H, 5.67; N, 6.06. Found: C, 67.59;
H, 5.70; N, 6.04.
P0 was prepared as yellow solid (333 mg, yield ¼ 75%) by using
the same synthetic procedure of P1. FT-IR (film, cmꢀ1): 3051, 2928,
2856, 1613, 1495, 1459, 1377, 1255, 1013, 817, 750.
P2 was prepared as orange solid (176 mg, yield ¼ 56%) by using
the same synthetic procedure of P1. FT-IR (film, cmꢀ1): 2927, 2855,
1608, 1460, 1377, 1252, 816.
2.7. Synthesis of P1
A mixture of 11 (9.3 mg, 0.01 mmol), 8 (168.4 mg, 0.163 mmol),
9,9-bis(N-carbazolyl-hexyl)-2,7-dibromofluorene (135.7 mg, 0.163
mmol), 4,7-dibromo-2,1,3-benzothiadiazole (48.5 mg, 0.163 mmol),
2.8. Light-emitting diode (LED) fabrication and characterization
9,9-dihexylfluorene-2,7-bis(trimethyleneboronate)
(258.9 mg,
0.500 mmol), tetrakis(triphenylphosphine)-palladium(0) (18.0 mg,
The polymers were dissolved in toluene and THF and filtered
0.015 mmol), tetrabutyl ammonium fluoride (523 mg, 2.00 mmol)
through a 0.45-mm filter. Patterned indium tin oxide (ITO) coated
Scheme 1. Syntheses of monomers.

Y. Jin et al. / Polymer 51 (2010) 5726e5733
5729
Scheme 2. Syntheses and composition of polymers.
glass substrates were cleaned with acetone, detergent, distilled
water, and 2-propanol and subsequently in an ultrasonic bath. After
treatment with oxygen plasma, 40 nm of poly(3,4-ethylenedi-
oxythiophene) (PEDOT) doped with poly(styrenesulfonic acid)
(PSS; Batron-P 4083, Bayer AG) was spin-coated onto the ITO
substrate, followed by drying in a vacuum oven. A thin film of the
polymer was coated onto the anode via spin casting inside a dry
box. The film thickness of the active layers was 80 nm, as measured
with Alfa Step 500 surface profiler (Tencor). A thin layer of CsF
(1.5 nm) and subsequently 120 nm layers of Al were vacuum-
evaporated subsequently on the top of an EL polymer layer under
a vacuum of 1 ꢂ 10ꢀ4 Pa.
Table 1
Structural, thermal, and pohotophysical data of the polymers.
a
b
Polymer Mw
PDI
Td
Tg
labs (nm)
lPL (nm)
(ꢁC) (ꢁC)
P0
P1
P2
25848 2.55 407 101 296, 370, 438
(296, 370, 457)
28138 2.90 403 103 296, 370, 438
(296, 370, 457)
416, 438, 542
(546)
416, 438, 542
(544, 618)
3. Results and discussion
3.1. Synthesis and characterization
17296 2.08 412
85 370, 438 (370, 457) 416, 438, 542
(534, 626)
The synthetic route of the monomers and polymers are shown
in Schemes 1 and 2. Anhydrous and base-free Suzuki polymeriza-
tion [43] was used for preparation of the polymers to suppress the
a
The absorption peaks in CHCl3 and film (in brackets).
The PL emission peaks in CHCl3 and film (in brackets).
b

5730
Y. Jin et al. / Polymer 51 (2010) 5726e5733
Fig. 3. Absorption spectra of polymers in film.
Fig. 1. 1H NMR spectra of copolymers in CDCl3.
Moreover, all polymers showed broad absorption bands centered at
457 nm in film (Fig. 3).
hydrolysis of amide configuration of DPP ring. The resulting
copolymers could be readily dissolved in common organic solvents,
such as chloroform, toluene, and THF. Their chemical structures
were verified by 1H NMR (Fig. 1) and FT-IR. Gel-permeation
chromatography (GPC) analysis with polystyrene standards
showed that these copolymers had weight-average molecular
weights (Mw) of 28138e17296 with polydispersity indices (PDI,
Mw/Mn) of 2.90e2.08 (Table 1). Decomposition temperatures of the
polymers were almost similar, with 5% weight loss temperatures
ranging from 403 to 412 ꢁC, which indicated their good thermal
stability. Glass transition temperatures of P0 and P1 were 101 and
103 ꢁC, respectively. The values were higher than that of P2 (85 ꢁC),
showing that the introduction of oxadiazole and carbazole side
chains could improve the morphological stability of the polymers.
The PL spectra of all polymers were similar and the emission
band of DPP unit (lem ¼ 604 nm) was hardly observed, due to the
low DPP content in the dilute solution (Fig. 4). In contrast, the PL
spectra of P1 and P2 in film exhibited a strong emission peak from
DPP at 618 and 626 nm, respectively. The emission peak of polymer
backbone became weak, indicating that energy transferred to the
DPP unit via Förster energy transfer because of the efficient overlap
of the absorption spectrum of DPP with PL spectrum of P0 (Figs 2
and 5). The emission peak of the DPP unit was red shifted 8 nm
from P1 to P2. The reason for this might be that the carbazole and
oxadiazole in the side chains of P1 could more efficiently restrict
pep aggregation and intramolecular planarization of DPP than
octyl of P2 [22,34,35].
3.2. Spectroscopic properties
3.3. Electrochemical properties
P0 and P1 had identical UVeVis absorption spectra in CHCl3 and
film (Table 1). The absorptions at 370, 438 nm could be attributed to
the polyfluorene part and benzothiadiazole part of the polymer,
respectively. The absorption of DPP unit (lmax ¼ 519 nm) was not
observed in the absorption spectra of the polymers (Fig. 2).
Cyclic voltammetry measurements on drop-cast all polymers
and DPP monomer films were conducted in acetonitrile with
Bu4NPF6 as the electrolyte, calomel electrode as reference elec-
trode. The HOMO and LUMO levels were determined from onset
Fig. 2. Absorption spectra of DPP monomer and polymers in CHCl3.
Fig. 4. PL spectra of DPP monomer and polymers in CHCl3.

Y. Jin et al. / Polymer 51 (2010) 5726e5733
5731
Fig. 5. PL spectra of polymers in film.
Fig. 6. EL spectra of polymers.
potential of first oxidation and reduction peak during anodic and
cathodal sweep in the cyclic voltammetry measurements, respec-
tively (EHOMO ¼ ꢀ(Eoxd þ 4.4) eV, ELUMO ¼ ꢀ(Ered þ 4.4) eV) [23]. As
the 1,3,4-oxadiazole and carbazole units were introduced into
copolymer, the HOMO energy level increased. The HOMO energy
levels of P0 and P1 (ꢀ5.66 eV and ꢀ5.68) were closer to that of
PEDOT:PSS (ꢀ5.2 eV) [44] than that of P2 (ꢀ5.74 eV). This means
that the hole-injection barriers for P1 and P0 decreased relative to
P2. However, the LUMO levels of P0, P1 and P2 were similar. The
HOMO and LUMO levels of DPP monomer (ꢀ5.58 and ꢀ3.57 eV)
were above and below the corresponding values of P0 (ꢀ5.66 and
ꢀ3.44 eV), respectively. Considering the extended conjugated
length of the DPP unit in conjugation with neighboring conjugated
segments in the polymer backbone, the actual HOMO level of the
DPP unit in the backbone should be above ꢀ5.58 eV and the actual
LUMO level should be below ꢀ3.57 eV. This indicated possible
charge trapping of the DPP unit in the electroluminescence process
[45] (Table 2).
were close to the standard red (0.66, 0.34) demanded by the
National Television System Committee. The maximum external
quantum efficiency (EQE) of P1 and P2 were 0.63%, 0.18%, which
were higher than that of the red emission polymer PF-DPP50
(EQEmax ¼ 0.13%, CIE (0.62, 0.37)) [4]. The higher electrolumines-
cence performance of P1 might be due to several reasons. The bulky
substituents of carbazole and oxadiazole could mitigate chromo-
phore aggregation and restrict formation of detrimental species
such as excimers [22,34,35]. The lower hole-injection barrier is of
benefit to holes injection, which might be of benefit to balanced
charge injection. Oxadiazole and carbazole substituents have
good charge conduction property [23,24,29,30], which could facil-
itate charge transport. High charge mobility is desirable to move
the charge recombination zone away from the near electrodes and
improve the exciton generation rate in high efficient electrolumi-
nescent devices [6,23,24]. Carbazole group acted as a hole trapping
site for efficient electron-hole recombination to yield blue-emitting
excitons [46] and the energy could transfer to polymer backbone
and then to DPP unit. Energy transfer also existed among oxadia-
zole, polymer backbone and DPP unit [33]. These factors could
enhance EL performance.
3.4. Electroluminescence properties
Carbazole has high HOMO level which is of benefit to hole
injection and good hole conduction property [30]. Oxadiazole has
good electron mobility [24,28]. Thus grafting oxadiazole and
carbazole units as pendants into conjugated polymers should
enhance current density. However, the current density of P1 was
slightly lower than that of P2 below 9.7 V.
The EL spectra of polymers were collected at current density
12 mA/cm2. The EL spectra of P1 and P2 exhibited a strong emission
peak from DPP at 620 and 624 nm, respectively, while the EL
emission P0 at 548 nm was from polymer backbone (Fig. 6). The
dramatic difference between the PL and EL spectra reveals that
Förster energy transfer does not account solely for the observed EL.
Another dominant mechanism for exciting DPP would be direct
charge trapping at the dopant unit, followed by recombination with
opposite charges [7e10] (Table 3).
During charge transport process of the conjugated polymer,
charge carriers will move along the backbone and hop at only
The maximum luminance efficiency of the polymer LED with
the configuration of ITO/PEDOT:PSS/P1/CsF/Al was 2.21 cd/A, while
that of P0 and P2 were 5.86 and 0.29 cd/A, respectively. The CIE
coordinates of P1 and P2 were (0.61, 0.37) and (0.64, 0.35) which
Table 3
Performances of Double-Layer LED Devices (ITO/PEDOT:PSS/Polymer/CsF/Al).
CIE (x, y)b
Von LEmax EQE Bmax
VBmax CDBmax
c
d
e
f
g
Polymer lem
(nm)a
548
(V) (cd/A) (%)
(cd/m2) (V)
(mA/cm2)
P0
P1
P2
(0.42, 0.56) 3.1 5.86
(0.61, 0.37) 5.0 2.21
(0.64, 0.35) 5.0 0.29
1.26 5940
0.63 2681
0.18 885
8.0
10.3
12.0
227
236
289
Table 2
620
624
Electrochemical Data of the Copolymers and DPP monomer.
a
a
a
Emission maximum of EL spectrum.
Calculated from the EL spectrum.
Turn-on voltage (defined as the voltage required to give
Polymer/Monomer
Eoxd (V)
Ered (V)
HOMOb (eV)
LUMOb (eV)
b
c
P0
P1
P2
6
1.26
1.28
1.34
1.18
ꢀ0.96
ꢀ0.96
ꢀ0.93
ꢀ0.83
ꢀ5.66
ꢀ5.68
ꢀ5.74
ꢀ5.58
ꢀ3.44
ꢀ3.44
ꢀ3.47
ꢀ3.57
a
luminance of
1 cd/m2).
d
Maximum luminous efficiency.
Maximum brightness.
Operating voltage at the maximum brightness.
Current density at maximum brightness.
e
f
a
Onset potential vs calomel electrode.
Estimated from the onset potential.
b
g

5732
Y. Jin et al. / Polymer 51 (2010) 5726e5733
Fig. 8. EQE of OLEDs with the configuration of ITO/PEDOT:PSS/polymer/CsF/Al.
considerable reduction of efficiency with increasing current
density. According to log is the carrier mobility
1/2, where
m
f
b
F
m
and F is the electric field strength [25,48], the hole and electron
mobilities of P0 and P1 might be more different than that of P2 in
the high applied electric field. The recombination zone of carriers
might extend to hole transporting layer or cathode with
increasing current [49,50] and the degree of the extension was
much larger in P0 and P1 than that in P2. Therefore, the sharp
reduction of EL quantum efficiency in P0 and P1 might relate to
the imbalanced electron and hole mobilities.
4. Conclusions
In conclusion, a series of benzothiadiazole, alkoxybenzene and
fluorene-based copolymers were synthesized through base-free
Suzuki polymerization. Introduction of oxadiazole and carbazole as
the pendants enhanced glass transition temperature. The electro-
chemical measurements revealed that the pendants could affect
electrochemical characteristics, and DPP unit and carbazole pendant
could act as charge trapping sites in the polymers. The competition
effects of restricted interchain carriers hopping, charge trapping,
lower hole-injection barrier and enhanced charge mobility by oxa-
diazole and carbazole might be responsible for the comparable
current density between P1 and P2. The roll-off of the quantum
efficiencies for P1 and P0 were much larger than that of P2, which
might relate to the imbalanced electron and hole mobilities in the
high applied electric field. The maximum luminescences of P0, P1,
and P2 were 5940, 2681, and 885 cd/m2 and the maximum EL effi-
ciencies were 1.26%, 0.63%, and 0.18%, respectively. The CIE coordi-
nates of P1andP2were (0.61, 0.37) and(0.64, 0.35) which wereclose
to the standard red (0.66, 0.34) demanded by the National Television
System Committee. P1 possessed higher electroluminescence
performance due to the introduction of oxadiazole and carbazole
pendants compared to P2. The pendants could restrain aggregation
which might induce fluorescence quenching and are of benefit to
keep high charge mobility. The efficient energy transfer existed
among the pendants, polymer backbone and DPP unit. These factors
should be responsible for the higher EL performance of P1. It is worth
mentioning that the EL quantum efficiency of P1 was reduced in
comparison to P0. The main reason may be a low quantum efficiency
of DPP monomer which acts as a quench center for fluorescence.
Structure modification of DPP monomers and optimization of EL
devices should increase their EQE.
Fig. 7. (a) Current densityevoltage (JeV), and (b) luminescenceevoltage (LeV) char-
acteristics of PLEDs of the polymers with the configuration of ITO/PEDOT:PSS/polymer/
CsF/Al.
where two polymer chains are packed closely and form sufficient
overlap of wave functions. Interchain hopping of carriers between
adjacent chains is the rate limiting step. The bulky substituents of
P1 should restrict interchain interaction and chain movement and
lower the mobility of carriers [25e27]. Furthermore, the incor-
porated carbazole pendant acted as a trap for the holes [46].
However, the high content (33 mol%) of carbazole in P1 should be
enough to open additional charge hopping channel among the
carbazole units [12,47]. Moreover, the lower hole-injection barrier
is of benefit to holes injection and oxadiazole group could enhance
electron mobility [28,35]. These competition effects might be
responsible for the comparable current density between P1 and
P2. The current density of P1 increased more sharply with the
enhanced voltage than that of P2. The possible reason is that the
charge mobility of pendants might have larger dependence of the
electric field than that for polymer backbone [48]. The driving
voltage at 100 cd/m2 was 3.9, 6.6, and 7.6 V for P0, P1, and P2,
respectively (Fig. 7b).
EQE of the devices are displayed in Fig. 8. Although the initial
quantum efficiency of P0 and P1 was higher than that of P2, the
roll-off of the quantum efficiency for P2 was much smaller than
that of P0 and P1. The quantum efficiency of P2 was maintained
almost the same with 0.17%e0.18% between 250 cd/m2 and
885 cd/m2 (EQE ¼ 0.17% at 250 cd/m2, 100 mA/cm2; EQE ¼ 0.18% at
885 cd/m2, 300 mA/cm2). In contrast, P0 and P1 exhibited

Y. Jin et al. / Polymer 51 (2010) 5726e5733
5733
[21] Zhao ZJ, Wang ZM, Lu P, Chan CYK, Liu DD, Lam JWY, et al. Angew Chem Int Ed
2009;48:7608e11.
Acknowledgments
[22] Hong YN, Lam JWY, Tang BZ. Chem Commun; 2009:4332e53.
[23] Kulkarni AP, Tonzola CJ, Babel A, Jenekhe SA. Chem Mater 2004;16:4556e73.
[24] Huang J, Hou W-J, Li J-H, Li G, Yang Y. Appl Phys Lett 2006;89:133509e12.
[25] Laquai F, Wegner G, Im C, Bassler H, Heun S. J Appl Phys 2006;99:023712e8.
[26] Laquai F, Wegner G, Im C, Bassler H, Heun S. J Appl Phys 2006;99:033710e6.
[27] Hoofman R, de Haas MP, Siebbeles LDA, Warman JM. Nature 1998;392:54e6.
[28] Huang SP, Liao JL, Tseng HE, Jen TH, Liou JY, Chen SA. Synth Met
2006;156:949e53.
This work was supported by the National Natural Science
Foundation of China (20872038), Science and Technology Planning
Project of Guangdong Province, China (2007A010500011).
References
[29] Xiang HF, Xu ZX, Roy VAL, Che CM, Lai PT, Zeng PJ, et al. Appl Phys Lett
2008;92:073305e7.
[1] Rabindranath AR, Zhu Y, Heim I, Tieke B. Macromolecules 2006;39:8250e6.
[2] Zhang K, Tieke B. Macromolecules 2008;41:7287e95.
[3] Zhu Y, Rabindranath AR, Beyerlein T, Tieke B. Macromolecules 2007;40:
6981e9.
[4] Cao DR, Liu QL, Zeng WJ, Han SH, Peng JB, Liu SP. Macromolecules 2006;39:
8347e55.
[5] Qiao Z, Peng JB, Jin Y, Liu QL, Weng JN, Chen ZC, et al. Polymer 2010;51:
1016e23.
[6] Grimsdale AC, Chan KL, Martin RE, Jokisz PG, Holmes AB. Chem Rev 2009;109:
897e1091.
[7] Liu J, Cheng Y, Xie Z, Geng Y, Wang L, Jing X, Wang F. Adv Mater 2008;20:
1357e62.
[8] Liu J, Guo X, Bu L, Xie Z, Cheng Y, Geng Y, et al. Adv Funct Mater 2007;17:
1917e25.
[9] Liu J, Zhou Q, Cheng Y, Geng Y, Wang L, Ma D, et al. Adv Funct Mater
2006;16:957e65.
[10] Tu G, Zhou Q, Cheng Y, Wang L, Ma D, Jing X, Wang F. Appl Phys Lett
2004;85:2172e4.
[30] Yuan MC, Shih PI, Chien CH, Shu CF.
J Polym Sci Part A Polym Chem
2007;45:2925e37.
[31] Chen X, Liao J-L, Liang Y, Ahmed MO, Tseng H-E, Chen S-A. J Am Chem Soc
2003;125:636e7.
[32] Jin Y, Song S, Kim JY, Kim H, Lee K, Suh H. Thin Solid Films; 2008:7373e80.
[33] Wu CW, Tsai CM, Lin HC. Macromolecules 2006;39:4298e305.
[34] Sung HH, Lin HC. J Polym Sci Part A Polym Chem 2005;43:2700e11.
[35] Lee Y-Z, Chen X, Chen S-A, Wei P-K, Fann W-S. J Am Chem Soc 2001;
123:2296e307.
[36] Herguch P, Jiang XZ, Liu MS, Jen AKY. Macromolecules 2002;35:6094e100.
[37] Peng YQ, Song GH. Green Chem 2001;3:302e4.
[38] Qiao Z, Xu YB, Lin SM, Peng JB, Cao DR. Synth Met 2010;160:1544e50.
[39] Wu CW, Lin HC. Macromolecules 2006;39:7985e97.
[40] Ma Z, Lu S, Fan QL, Qing CY, Wang YY, Wang P, Huang W. Polymer 2006;
47:7382e90.
[41] Deng Y, Liu SJ, Fan QL, Fang C, Zhu R, Pu KY, et al. Synth Met 2007;157:813e22.
[42] Grisorio R, Mastrorilli P, Nobile CF, Romanazzi G, Suranna GP, Gigli G, et al.
Macromolecules 2007;40:4865e73.
[43] Brookins RN, Schanze KS, Reynolds JR. Macromolecules 2007;40:3524e6.
[44] Yan H, Lee P, Armstrong NR, Graham A, Evmenenko GA, Dutta P, Marks TJ.
J Am Chem Soc 2005;127:3172e83.
[45] Zhen HY, Xu W, King W, Chen QL, Xu YH, Jiang JX, et al. Macromol Rapid
Commun 2006;27:2095e100.
[46] Liao JL, Chen X, Liu CY, Chen SA, Su CH. J Phys Chem B 2007;111:10379e85.
[47] Laquai F, Hertel D. Appl Phys Lett 2007;90:142109e11.
[48] Bettenhausen J, Strohriegl P, Brutting W, Tokuhisa H, Tsutsui T. J Appl Phys
1997;82:4957e61.
[11] Pai DM, Yanus JF, Stolka M. J Phys Chem 1984;88:4714e7.
[12] Campbell AJ, Bradley DDC, Virgili T, Lidzey DG, Antoniadis H. Appl Phys Lett
2001;79:3872e4.
[13] Yang X, Neher D, Hertel D, Daeubler TK. Adv Mater 2004;16:161e6.
[14] Tseng SR, Chen YS, Meng HF, Lai HC, Yeh CH, Horng SF, et al. Synth Met
2009;159:137e41.
[15] Liu J, Tu GL, Zhou QG, Cheng YX, Geng YH, Wang LX, et al. J Mater Chem
2006;16:1431e8.
[16] Jin SH, Kim MY, Kim JY, Lee K, Gal YS. J Am Chem Soc 2004;126:2474e80.
[17] Xia Y, Friend RH. Adv Mater 2006;18:1371e6.
[18] An BK, Kwon SK, Jung SD, Park SY. J Am Chem Soc 2002;124:14410e5.
[19] Crenshaw BR, Weder C. Chem Mater 2003;15:4717e24.
[20] Yuan CX, Tao XT, Ren Y, Li Y, Yang JX, Yu WT, et al. J Phys Chem C 2007;
111:12811e6.
[49] Kang JW, Lee SH, Park HD, Jeong WI, Yoo KM, Park YS, Kim J. J Appl Phys Lett
2007;90:223508e10.
[50] Giebink NC, Forrest SR. Phys Rev B 2008;77:235215e23.