Synthesis and optoelectronic properties of amino-functionalized carbazole-based conjugated polymers

Article abstract of DOI:10.1007/s11426-013-4864-2

A series of alcohol soluble amino-functionalized carbazole-based copolymers were synthesized via Suzuki coupling reaction. The pendent amino groups endow them high solubility in polar solvents, as well as efficient electron injection capability from high work-function metals. The relationships between the photophysical and electrochemical properties and the polymer backbone structure were systematically investigated. These alcohol-soluble carbazole-based copolymers were used as cathode interlayers between the high work-function metal Al cathode and P-PPV emissive layer in polymer light-emitting diodes with device structure of ITO/PEDOT:PSS/P-PPV/interlayer/Al. The resulting devices exhibited improved performance due to the better electron injection/transporting ability of the designed copolymers from Al cathode to the light-emitting layer.

Full text of DOI:10.1007/s11426-013-4864-2

SCIENCE CHINA
Chemistry
August 2013 Vol.56 No.8: 1119–1128
doi: 10.1007/s11426-013-4864-2
• ARTICLES • Progress of Projects Supported by NSFC
Synthesis and optoelectronic properties of amino-functionalized
carbazole-based conjugated polymers†
LIU ShengJian, ZHANG ZhiPeng, CHEN DongCheng, DUAN ChunHui, LU JunMing,
ZHANG Jie, HUANG Fei*, SU ShiJian*, CHEN JunWu & CAO Yong
State Key Laboratory of Luminescent Materials and Devices; Institute of Polymer Optoelectronic Materials & Devices,
South China University of Technology, Guangzhou 510640, China
Received December 21, 2012; accepted January 4, 2013; published online April 17, 2013
A series of alcohol soluble amino-functionalized carbazole-based copolymers were synthesized via Suzuki coupling reaction.
The pendent amino groups endow them high solubility in polar solvents, as well as efficient electron injection capability from
high work-function metals. The relationships between the photophysical and electrochemical properties and the polymer
backbone structure were systematically investigated. These alcohol-soluble carbazole-based copolymers were used as cathode
interlayers between the high work-function metal Al cathode and P-PPV emissive layer in polymer light-emitting diodes with
device structure of ITO/PEDOT:PSS/P-PPV/interlayer/Al. The resulting devices exhibited improved performance due to the
better electron injection/transporting ability of the designed copolymers from Al cathode to the light-emitting layer.
polymer light-emitting diodes, amino-functionalized conjugated polymers, electron injection/transporting material, in-
terfacial material, carbazole
face between the EML and the cathode [14]. In addition,
1 Introduction
metal ions generated at the metal/organic interface tend to
migrate into the EML, thus affecting the long term stability
of devices. To circumvent these problems, air-stable high
WF metals (such as Al, Ag, and Au) are applied to improve
stability. In order to minimize the energy barrier between
EML and high WF metals, it is necessary to introduce an
interfacial layer between the active layer and the metal elec-
trode, such as LiF [15], Cs2CO3 [16] , TiOx [17], LiN3 [18],
ZnO [19, 20], and organic surfactants [21–26].
Water/alcohol soluble conjugated polymers (WSCPs)
have attracted considerable attention in past decade due to
their potential application in PLEDs and polymer solar cells
(PSCs) as cathode interlayers [13, 27–31]. Their solubility
in high polar solvents, which is orthogonal to many com-
monly used conjugated polymer active materials, allows for
fabrication of multilayer devices through solution pro-
cessing. Most importantly, WSCPs with polar pendant
groups can dramatically improve charge injection from me-
Since the demonstration of the first polymer light-emitting
diodes (PLEDs) [1], considerable attention has been paid to
this area due to their unique characteristics, such as low cost,
light weight, and possible flexibility and large-area coverage
[2–6]. PLEDs usually adopt a basic architecture composed
of an emitting layer (EML) of organic semiconducting ma-
terial sandwiched between two electrodes. To achieve
highly efficient PLEDs, interfacial layers with good
hole/electron transporting/injection properties are required
to optimize charge injection, transport, and recombination
[7–13]. Low work function (WF) metals (such as Ca and Ba)
are widely used as a cathode to facilitate electron injection.
However, these metals are very sensitive to moisture and
oxygen, forming detrimental quenching sites near the inter-
*Corresponding authors (email: msfhuang@scut.edu.cn; mssjsu@scut.edu.cn)
†Recommended by QIU Yong (Tsinghua University)
© Science China Press and Springer-Verlag Berlin Heidelberg 2013
chem.scichina.com www.springerlink.com

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Liu SJ, et al. Sci China Chem August (2013) Vol.56 No.8
tallic electrodes into organic active layers, resulting in sig-
nificant enhancement in the device performance [3139].
Based on these unique characteristics, highly efficient all
printable PLEDs were recently realized [40].
AV-300. Molecular weights of the polymers were deter-
mined by a Waters GPC 2410 in THF using a calibration
curve with polystyrene standards. Thermogravimetric anal-
yses (TGA) were conducted on a NETZSCH TG 209 under
a heating rate of 10 °C min1 and a nitrogen flow rate of 20
mL min1. Cyclic voltammetry (CV) data were measured on
a CHI600D electrochemical workstation by using Bu4NPF6
(0.1 M) in acetonitrile as electrolyte, and platinum and sat-
urated calomel electrode (SCE) as the working and refer-
ence electrode, respectively. UV-vis spectra were recorded
on a HP 8453 spectrophotometer. Photoluminescence (PL)
and electroluminescence (EL) spectra were recorded on an
Instaspec IV CCD spectrophotometer (Oriel Co.).
All manipulations involving air-sensitive reagents were
performed under an atmosphere of dry argon. All reagents,
unless otherwise specified, were obtained from Alfa Aesar
or Sigma-Aldrich and used as received. Some of the sol-
vents used were further purified before use (THF from so-
dium/benzopheone, acetonitrile from CaH2, toluene was
washed with H2SO4 and then treated with MgSO4).
Amino-functionalized conjugated polymers (AFCPs) can
be dissolved in alcohol in the presence of trace acetic acid
because of the weak interaction between the nitrogen atoms
in pendent amino groups (PAGs) and acetic acid [41]. By
inserting a layer of AFCPs or cross-linked AFCPs between
high WF metal cathode and EML in PLEDs, the electron
injection from high WF metal cathode could be significantly
benefited from the interfacial dipole formation between the
PAGs in AFCPs and the metal cathode [33–35, 38–43].
Therefore, AFCP becomes one of the most important
WSCPs used as cathode interlayer in PLEDs, PSCs, and so
on [44, 45].
Organic material with carbazole skeleton has been wide-
ly used in EML and hole transporting layer in PLEDs [46],
due to the following advantages: (i) the nitrogen atom can
be easily functionalized with a large variety of substituents
to modulate the carbazole properties; (ii) carbazole units can
be linked at different positions; (iii) excellent luminescent
properties and hole transport properties. Polycarbazoles
bearing an electron-rich main chain and a large band gap are
known as typical p-type semiconductors [46]. Compared
with the large family of fluorene-based AFCPs, construction
of carbazole-based interlayer polymers for optoelectronic
devices receives much less attention [13]. Chen and
coworkers reported that the PAGs in polycarbazoles can
induce interfacial dipole formation and improve electron
injection from high WF metal cathode into EML [47, 48].
Although a large number of AFCPs with different pendent
polar groups have been developed, AFCPs with different
main chain structures remains very rare [13, 38]. Moreover,
the relationship between material structure and properties is
of great significance for the development of novel efficient
WSCPs interfacial material.
2.2 Monomer synthesis
1′-((6′ -Bromohexyl)oxy)-4-methylbenzene (1)
A mixture of p-cresol (28 g, 255 mmol), 1,6-dibromohexane
(220 mL, 1426 mmol), tetrabutylammonium bromide (3.8 g,
12 mmol), and potassium carbonate (60 g, 435 mmol) in
acetone (150 mL) was refluxed under argon for 48 h. The
solid was filtered off and the filtrate was evaporated to dry-
ness. The residue was distilled under reduced pressure to
obtain the desired product as a colorless liquid (53.5 g, yield:
76%).
1H NMR (300 MHz, CDCl3): δ (ppm) 7.10 (d, 2H, J
= 8.7 Hz), 6.80 (d, 2H, J = 8.6 Hz), 3.90 (t, 2H, J = 7.1 Hz),
3.45 (t, 2H, J = 6.9 Hz), 2.28 (s, 3H), 1.85~1.75 (m, 8H).
1,13-Bis(p-tolyloxy)tridecan-7-ol (2)
In this work, the amino-functionalized polycarbazoles
(PCNs) with branched alkyl were designed, synthesized and
used as cathode interlayers in PLEDs (Scheme 1). The ter-
tiary amino groups in branched alkyl substitution not only
endows PCNs excellent solubility in polar solvents, but also
improves electron injection from high WF metal cathode
into EML. The relationships between chemical structure and
optoelectronic properties were also systematically investi-
gated. It was found that by using PCNs as an ETL in PLEDs,
the device performance was significantly enhanced. The
main chain structures not only influence the photophysical
properties and electrochemical properties, but also have
significant impact on the resulting device performance.
A mixture of 1 (16.1 g, 60 mmol) and magnesium turnings
(2.17 g, 90 mmol) in THF (60 mL) was refluxed under ar-
gon for 3 h to afford the Grignard reagent, which was
cooled to room temperature (rt). In a three neck flame-dried
500 mL round-bottom flask, ethyl formate (1.48 g, 20 mmol)
was dissolved in THF (100 mL) and cooled to –78 ℃. The
freshly prepared Grignard reagent was then added dropwise
and the resulting mixture was allowed to stir overnight at rt.
The reaction was quenched by methanol and then saturated
aqueous NH4Cl. The mixture was extracted with dichloro-
methane (DCM), and the organic phase was washed with
brine, and dried over MgSO4. The crude product was dis-
tilled under reduced pressure to obtain pure product as a
1
white solid (21.07 g, yield: 82%). H NMR (300 MHz,
CDCl3): δ (ppm) 7.10 (d, 4H, J = 8.5 Hz), 6.75 (d, 4H, J =
8.6 Hz), 3.95 (t, 4H, J = 7.3 Hz), 3.67~3.57 (m, 1H), 2.26 (s,
6H), 1.55~1.45 (m, 21H). 13C NMR (75 MHz, CDCl3)
157.00, 129.84, 129.66, 114.40, 76.60, 71.91, 68.00, 37.42,
29.44, 29.28, 26.05, 25.57, 20.43.
2 Experimental section
2.1 Measurements and material
1H and 13C NMR spectra were measured using a Bruker

Liu SJ, et al. Sci China Chem August (2013) Vol.56 No.8
1121
Scheme 1 Synthetic route of the monomer and target polymers. Reagents and conditions: a) 1,6-dibromohexane, tetrabutylammonium bromide, potassium
carbonate, acetone, refluxed, 48 h; b) magnesium turnings, THF, refluxed, 3 h, and then ethyl formate, THF, –78 °C, 2 h; c) p-toluenesulfonyl chloride,
DCM, Et3N, Me3N·HCl, 5–10 °C, 90 min; d) fuming nitric acid, glacial acetic acid, 100 °C, 6 h; e) triethyl phosphite, refluxed, 24 h; f) DMSO, KOH, rt, 8 h;
g) hydrobromide, glacial acetic acid, refluxed, 48 h; h) DMF, diethylamine, refluxed, 12 h; i) Pd(PPh3)4, 20% aq. tetraethylammonium hydroxide, toluene,
THF, 90 °C, 48 h.
1′,13-Bis(p-tolyloxy)tridecan-7′-yl-4-methylbenzenesulfonate
(3)
mmol), Et3N (5.1 mL) and Me3N·HC1 (1.38 g, 14.55 mmol)
in DCM (50 mL) in a 250 mL flask at 0–5 °C. The mixture
was stirred for 90 min, then quenched by water. The mix-
ture was extracted three times with DCM. The organic
p-Toluenesulfonyl chloride (4.16 g, 21.8 mmol) in DCM
(30 mL) was added to a stirred solution of 2 (6.03 g, 14.55

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Liu SJ, et al. Sci China Chem August (2013) Vol.56 No.8
phase was washed with water, brine, dried over Na2SO4,
and concentrated under reduced pressure. The residue was
purified by silica-gel column chromatography (petroleum
ether/ethyl acetate as eluent, v/v = 9/1) to obtain pure prod-
N-(1′,13′-dibromotridecan-7′-yl)-2,7-dibromo-carbazole (7)
A mixture of 6 (11.0 g, 15.22 mmol), hydrobromide (60 g,
wt 50%) in glacial acetic acid (150 mL) was refluxed under
argon for 48 h. The reaction mixture was poured into water
(300 mL), extracted with DCM (3 × 100 mL), the combined
organic fractions were dried over Na2SO4 and concentrated.
The white residue was purified by silica gel column chro-
matography with petroleum ether to yield pure product
1
uct as a white solid (6.52 g, yield: 78%). H NMR (300
MHz, CDCl3): δ (ppm) 7.80 (d, 2H, J = 8.4 Hz), 7.30 (d, 2H,
J = 7.9 Hz), 7.10 (d, 4H, J = 8.6 Hz), 6.80 (d, 4H, J = 8.4
Hz), 4.61~4.51 (m, 1H), 3.95 (t, 4H, J = 7.5), 2.46 (s, 3H),
2.30 (s, 6H), 1.46~1.36 (m, 20H). 13C NMR (75 MHz,
CDCl3): δ (ppm) 156.93, 144.39, 134.70, 129.87, 129.70,
129.67, 127.72, 144.32, 84.37, 67.85, 34.09, 29.16, 29.04,
25.86, 24.64, 21.60, 20.47.
1
(7.72 g, yield: 76%). H NMR (300 MHz, CDCl3): δ (ppm)
7.90 (t, 2H, J = 8.0 Hz), 7.65 (d, 2H, J = 8.2 Hz), 7.37 (d,
2H, J = 7.9 Hz), 4.51~4.38 (m, 1H), 3.35 (t, 4H, J = 7.6 Hz),
2.18~2.06 (m, 4H), 1.82~1.68 (m, 4H), 1.30~1.22 (m, 4H),
1.20~0.86 (m, 8H). 13C NMR (75 MHz, CDCl3): δ (ppm)
122.50, 121.59, 121.34, 119.86, 119.25, 114.42, 112.03,
56.83, 33.79, 33.33, 32.49, 28.37, 27.79, 26.52.
2,7-Dibromo-carbazole (5)
A mixture of fuming nitric acid (70%, 132 mL) was added
dropwise to a stirred solution of 4,4′-dibromobiphenyl (20 g,
64 mmol) in glacial acetic acid (300 mL) at 100 °C. The
reaction was allowed to proceed for 6 h followed by cooling
to rt. The yellow precipitate was collected by filtration, fol-
lowed by recrystallization from ethanol to obtain crude
product 4 as a white solid. The crude product 4 was used in
next step without further purification.
To a 250 mL round bottomed flask equipped with a re-
flux condenser were added 4 (16.52 g, 46.1 mmol) and tri-
ethyl phosphite (60 mL). The resulting mixture was re-
fluxed for 24 h under argon. The excess triethyl phosphite
was then distilled off and the residue was purified by silica
gel column chromatography using a gradient of solvents
ranging from 5–20% ethyl acetate in petroleum ether to
yield pure product [49] (9.28 g, yield: 62%). 1H NMR (300
MHz, CDCl3): δ (ppm) 8.25~8.15 (br, 1H, NH), 7.90 (d, 2H,
J = 8.2 Hz), 7.56 (d, 2H, J = 8.1 Hz), 7.25 (dd, 2H, J = 7.7
Hz).
N-(1′,13′-(N,N-diethylamino)tridecan-7′-yl)-2,7-dibromo-
carbazole (M1)
To a solution of 7 (15 g, 22.6 mmol) in DMF (50 mL) under
argon, diethylamine (30 mL) was added in one portion. The
mixture was refluxed with vigorously stirring for 12 h under
argon atmosphere. After the reaction was cooled to rt, the
mixture was poured into ice water, and the aqueous layer
was extracted with DCM for several times. The combined
organic layers were washed with water and brine, dried over
anhydrous MgSO4. After removal of the solvent by rotary
evaporation, the residue was purified by silica gel column
chromatography (petroleum/ethyl acetate/triethylamine,
v:v:v = 20:1:0.5) to afford M1 as pale-yellow viscous liquid
[39] (12.52 g, 85%).1H NMR (300 MHz, CDCl3): δ (ppm)
7.85 (t, 2H, J = 7.8 Hz), 7.68 (d, 2H, J = 8.2 Hz), 7.35 (d,
2H, J = 8.3 Hz), 4.51~4.32 (m, 1H), 2.50 (q, 8H, J = 6.9
Hz), 2.30 (t, 4H, J = 7.4 Hz), 2.10~1.98 (m, 4H), 1.28~1.16
(m, 6H), 1.11~0.68 (m, 22H). 13C NMR (75 MHz, CDCl3):
δ (ppm) 122.10, 121.50, 121.40, 119.45, 114.44, 112.10,
56.98, 52.60, 46.90, 33.55, 29.31, 29.20, 27.30, 26.90,
11.60.
N-(1′,13′-bis(p-tolyloxy)tridecan-7′-yl)-2,7-dibromo-
carbazole (6)
In a flame-dried 250 mL round-bottom flask under argon, a
solution of 3 (17.01 g, 30 mmol) in DMSO (50 mL) was
added dropwise to a stirred solution of 5 (6.52 g, 20.0 mmol)
and KOH (5.61 g, 100 mmol) in DMSO (60 mL). The reac-
tion mixture was stirred at rt for 8 h, then poured into water
(200 mL), extracted with DCM (3 × 100 mL). The com-
bined organic fractions were dried over Na2SO4 and con-
centrated. The residue was purified by silica gel column
chromatography with petroleum ether [49] (11.13 g, yield:
77%). 1H NMR (300 MHz, CDCl3): δ (ppm) 7.98 (t, 2H, J =
8.4 Hz), 7.60 (d, 2H, J = 7.9 Hz), 7.35 (dd, 2H, J = 8.1 Hz),
7.08 (d, 4H, J = 7.8 Hz), 6.80 (d, 4H, J = 8.0 Hz), 4.47 (m,
1H, J = 7.4), 3.80 (t, 4H, J = 6.9 Hz), 2.42 (s, 6H),
1.58~1.08 (m, 20H). 13C NMR (75 MHz, CDCl3): δ (ppm)
156.92, 129.81, 129.65, 122.43, 121.53, 121.29, 119.82,
119.22, 114.36, 112.08, 68.04, 67.81, 56.99, 33.44, 29.11,
29.04, 26.70, 25.77, 20.43.
2.3 General procedure to prepare PCNs
Polymerization reactions were carried out by palladium(0)-
catalyzed Suzuki cross-coupling reactions. Diboronic es-
ter/acid monomer (0.5 mmol) and dibromo monomer M1
(0.5 mmol) were mixed with Pd(PPh3)4 (10 mg) in a mix-
ture of 20% aqueous tetraethylammonium hydroxide (1 mL),
toluene (10 mL) and THF (5 mL) under argon. After degas-
sing, the reaction mixture was vigorously stirred at 90 °C
for 48 h. The end groups were capped by refluxing 6 h with
phenylboronic acid and then bromo-benzene, respectively.
After the mixture was cooled to rt, it was poured into meth-
anol (200 mL). The precipitate was collected by filtration
through a funnel. The resulting solid material was washed
for 24 h using acetone in a Soxhlet apparatus to remove
oligomers and catalyst residues. The remaining material was

Liu SJ, et al. Sci China Chem August (2013) Vol.56 No.8
1123
dissolved in 30 mL of toluene, and filtered through a 0.45
3 Results and discussion
μm PTFE filter. The filtrate was concentrated, and the solid
residue was triturated with methanol, and dried under vac-
uum to give the anticipated polymers, yield: 6585%.
3.1 Synthesis and characterization
The chemical structures and synthetic route are shown in
Scheme 1. Mono-alkylation of 1,6-dibromohexane with
p-cresol through Williamson reaction afforded the ether 1.
The resulting Grignard reagent from 1 reacted with ethyl
formate, leading to the alcohol 2 with a good yield and high
purity after distillation under reduced pressure. The tosylate
derivative 3 was obtained from alcohol 2 and p-toluenesul-
fonyl chloride in 78% yield. The alkylation reaction of
2,7-dibromo-carbazole 5 with compound 3 was carried out
following the procedure described by Marzoni and Gar-
brecht [49]. The bromide 7 was obtained by treating 6 with
aq. HBr and acetic acid, which was further converted to M1
by displacement with excess diethylamine in DMF under
Poly[N-9′-heptadecanyl-2,7-carbazole-alt-N-(1′,13′-(N,N-di
ethylamino)tridecan-7′-yl)-2,7-carbazole] (PCCN)
1H NMR (300 MHz, CDCl3): δ (ppm) 8.22~7.70 (br, 12H),
4.90~4.45 (m, 2H), 2.60~2.40 (m, 8H), 2.35~2.00 (m, 8H),
1.88~0.62 (br, 60H). Mn = 36500, MW/Mn = 1.62, Td(5%)
=
425 °C.
Poly[9,9-dioctyl-2,7-fluorene-alt-N-(1′,13′-(N,N-diethylami
no)tridecan-7′-yl)-2,7-carbazole] (PFCN)
1H NMR (300 MHz, CDCl3): δ (ppm) 8.12~7.24 (br, 12H),
4.56~4.45 (m, 1H), 2.62 (q, 8H, J = 6.8 Hz), 2.36 (t, 4H, J =
7.3 Hz), 2.18~1.98 (m, 8H) 1.92~1.80 (m, 8H), 1.82~0.61
(br, 50H). Mn = 64900, MW/Mn = 1.69, Td(5%) = 427 °C.
1
argon. Their chemical structures were confirmed by H and
13C NMR spectroscopy.
With the key intermediate M1 in hand, copolymerization
with either M2, M3 or M4 was performed using the stand-
ard Pd-catalyzed Suzuki cross-coupling conditions to afford
PCNs. All the polymers display excellent solubility not only
in common organic solvents (such as THF, chloroform, tol-
uene) due to their long branched alkyl substitutions in the
side chains of the polymers, but also in alcohol solvents in
the present of trace acetic acid due to the weak interaction
between the amino groups in the side chains and acetic acid.
In addition, the added acetic acid could be completely re-
moved from the polymers after dried in vacuum, which is
beneficial for the fabrication of multilayer PLEDs [41].
The number molecular weight (Mn) estimated by gel
permeation chromatography (GPC) against the polystyrene
standard with THF as an eluent ranged from 29300 to 64
900, with a polydispersity index (Mw/Mn) from 1.53 to 1.69.
The thermal properties of the polymers were studied by
thermogravimetric analysis (TGA). Figure 1 shows the
Poly[1,4-phenylene-alt-N-(1′,13′-(N,N-diethylamino)tridecan-
7′-yl)-2,7-carbazole] (PPCN)
1H NMR (300 MHz, CDCl3): δ (ppm) 8.12~7.20 (m, 10H),
4.56~4.36 (m, 1H), 2.58~2.41 (m, 8H), 2.38~2.22 (m, 4H),
2.17~1.98 (m, 4H), 1.20~0.60 (m, 28H). Mn = 29300,
MW/Mn = 1.53, Td(5%) = 419 °C.
2.4 PLEDs fabrication and characterization
For the PLED devices, a thickness of 40 nm PEDOT:PSS
(Baytron P4083, BayerAG) layer was spin-coated on a
pre-cleaned ITO substrate and dried by baking at 200 °C
for 10 min to remove residual water. Then, 90 nm
poly[2-(4-(3′,7′-dimethyloctyloxy)-phenyl)-p-phenylenevin
ylene] (P-PPV) (the structure shown in Scheme 1) layer was
spin-casted onto PEDOT:PSS from p-xylene solution. The
electron injection materials PCNs were spin-coated onto
P-PPV layer from methanol/acetic acid as cathode interlayer.
Finally, an 80 nm thick aluminum was thermally deposited
as cathode through a shadow mask (defined active area of
0.19 cm2) in a chamber with a base pressure of <5×10−4 Pa.
The cathode thickness was monitored upon deposition by
using a crystal thickness monitor (Sycon). Profilometry
(Veeco Dektak150) was used to determine the thickness of
the polymer films. Device fabrication was carried out in a
N2 atmosphere dry-box (Vacuum Atmosphere Co.). JVL
data were collected using a Keithley 236 source meter and a
calibrated silicon photodiode in the N2 atmosphere dry-box.
After typical encapsulation of the devices with UV epoxy
and cover glass, the devices were taken out from dry-box
and the external electroluminescence quantum efficiencies
(EQEs) were obtained by measuring the total light output
in all directions in an integrating sphere (IS-080, Lab-
sphere).
Figure 1 Thermogravimetric analysis plots of PCNs.

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Liu SJ, et al. Sci China Chem August (2013) Vol.56 No.8
thermograms of these copolymers, in the temperature range
from 25 to 800 °C in the nitrogen atmosphere. The copoly-
mers exhibited excellent thermal stability with 5%
weight-loss temperature (Td(5%)) over 400 °C, indicating the
polymers can resist the degradation at the operating temper-
atures of the resulting PLEDs.
3.2 Photophysical properties
Figures 2 and 3 show the UV-vis absorption and PL spectra
of the polymers in chloroform and in solid film, respectively.
Relevant optical parameters are summarized in Table 1. The
absorption peak of PPCN in chloroform is observed at 373
nm, while the absorption in film exhibits a little red-shift
and peaks at 385 nm. PFCN and PCCN exhibit red-shift of
about 15 nm in the absorption spectra as compared with that
of PPCN both in chloroform and thin films. This could be
attributed to the more effective  electron delocalization in
the polymer main chain. The absorption edges of the result-
ing polymers in thin films were almost identical and located
at 423, 426 and 424 nm for PCCN, PFCN and PPCN, re-
spectively, from which the optical band gaps (Eg) were cal-
culated to be 2.93 eV for PCCN, 2.91 eV for PFCN, and
2.92 eV for PPCN.
As shown in Figures 2 and 3, all the PCNs copolymers
exhibit blue fluorescence under 370 nm excitation from
HeCd laser [46–51]. The PL spectra of PFCN and PPCN are
red-shifted compared with that of PPFN in chloroform,
which are consistent with those observed in the absorption
spectra. The PL spectra of all PCNs copolymers show
maximum of emission at around 430 nm followed by vi-
bronic sidebands at 450 nm in the solid state. The PFCN
and PCCN exhibit broader PL spectra in thin film compared
to that of PPCN, suggesting more pronounced aggregation
of polymer chains as would be expected on the basis of their
more rigid backbone structures.
Figure 3 Absorption and PL spectra of PCNs in films.
3.3 Electrochemical properties
The electrochemical properties of the polymers were inves-
tigated by CV in argon-saturated anhydrous solutions of 0.1
M Bu4NPF6 in acetonitrile at a scan rate of 50 mV s1 at rt.
A platinum electrode coated with a polymer thin film was
used as the working electrode. A Pt wire and SCE were
used as the counter and reference electrode. The p-doping
waves were recorded, but the n-doping process could not be
detected in these copolymers. Figure 4 shows C-V curves of
the PCNs films measured in 0.1 M Bu4NPF6 versus ferro-
cene/ferrocenium (Fc/Fc+) in acetonitrile. Relevant electro-
chemical and energy level parameters were also summa-
rized in Table 1.
The highest occupied molecular orbital (HOMO) energy
levels were calculated according to the empirical formula
EHOMO = -(Eox + 4.80) (eV) using ferrocene value of 4.8 eV
Figure 4 CV curves of the PCNs films measured in 0.1 M Bu4NPF6
versus Fc/Fc+ in acetonitrile.
Figure 2 Absorption and PL spectra of PCNs in CHCl3.

Liu SJ, et al. Sci China Chem August (2013) Vol.56 No.8
1125
Table 1 Optical and electrochemical properties of the polymers
a)
b)

abs (nm)
em (nm)
EHOMO
(eV)
ELUMO
(eV)
Polymer
Eg (eV)
Eox (eV)
CHCl3
389
film
398
395
385
CHCl3
419
film
430
433
428
PCCN
PFCN
PPCN
2.93
2.91
2.92
0.48
0.58
0.63
5.28
5.38
5.43
2.35
2.47
2.52
391
421
373
414
a) EHOMO = (Eox + 4.80) eV; b) ELUMO = (EHOMO + Eg) eV
as the internal standard [52]. With increasing electron den-
sity of the main chain, the HOMO energy levels of PCNs
increased progressively. The HOMO energy levels of PPCN,
PFCN, and PCCN were calculated to be 5.43, 5.38, and
5.28 eV, respectively. The lowest unoccupied molecular
orbital (LUMO) levels were estimated from HOMO levels
and the Eg. The LUMO levels of PPCN, PFCN, and PCCN
were calculated to be 2.52, 2.47, and 2.35 eV, respec-
tively.
3.4 Cathode interlayer function in PLEDs
PCNs with polar pendant amine groups can not only dra-
matically improve charge injection from high WF metal
electrode into EML, but also, due to their excellent solubil-
ity in polar solvents, can prevent interfacial mixing between
the EML and the adjacent electron transporting layer (ETL).
These characteristics suggest that PCNs can be promising
new materials for cathode interfacial engineering in PLEDs
and improve the overall performance. Herein, PCNs were
used as cathode interfacial materials in PLEDs to systemat-
ically investigate the relationships between chemical struc-
ture and optoelectronic properties.
Figure 5 EL spectra of the PLEDs.
To evaluate the electron injection properties of PCNs in
PLEDs, P-PPV was used as the EML in PLEDs, with device
configuration of indium tin oxide (ITO)/poly(3,4-ethyl-
enedioxythiophene):poly(styrene sulfonic acid) (PEDOT:
PSS)/P-PPV/interlayer/Al. The PCNs were spin-coated
from methanol/acetic acid solution onto the P-PPV layer,
followed by evaporation of a 100 nm thick Al cathode. For
comparison, the devices using Al and Ba/Al as the cathode
were also fabricated.
Figure 5 shows the electroluminescence (EL) spectra of
the PLEDs. Figure 6 shows the current density (J) and lu-
minance (L) versus voltage (V) characteristics of different
devices. Figure 7 shows the luminance efficiency (LE) ver-
sus J characteristics of different devices. Table 2 summa-
rizes device performance at J = 30 mA cm−2. As shown in
Figure 5, all devices give similar EL spectra, indicating that
their exciton recombination zones are all in the P-PPV lay-
er.
Clearly, the bare Al device exhibits very poor device
performance due to the high WF of Al (~4.3 eV), with a
large barrier for the electron injection from Al cathode into
the P-PPV EML. As shown in Figure 7 and Table 2, the
bare Al cathode device shows a LE of 0.27 cd A1 with a
Figure 6 JLV characteristics of the green devices with different cath-
odes.
luminance of 97 cd m2 at the J = 30 mA cm2, together
with maximum LE (LEmax) of 0.28 cd A1 and maximum L
(Lmax) of 264 cd m2. The device with the low WF metal Ba
as the cathode shows much improved device performance
due to the greater electron injection from Ba cathode into
EML. The turn on voltage (Von) decreased from 5.4 V for
the Al cathode device to 2.8 V for Ba cathode device. The

1126
Liu SJ, et al. Sci China Chem August (2013) Vol.56 No.8
Table 2 Device performance of PLEDs using different cathodes in device configuration ITO/PEDOT/P-PPV/ETL/Al
a)
Von
Voltage b)
(V)
Brightness b)
(cd/m2)
LE b)
(cd/A)
EQE b)
(%)
Brightness (max)
(cd/m2)
LE (max)
(cd/A)
EQE (max)
(%)
Cathode
(V)
Al
5.4
2.8
3.4
3.2
3.6
9.2
5.4
8.6
8.6
8.8
97
0.27
14.32
11.13
12.01
10.04
0.10
5.51
4.43
4.75
3.96
264
0.28
0.11
6.23
4.44
4.77
3.97
Ba/Al
5460
3792
3900
3400
26759
9805
16.21
11.16
12.07
10.06
PCCN/Al
PFCN/Al
PPCN/Al
12267
13145
a) The driving voltage at a brightness of 1 cd m2; b) device performance at a current density of 30 mA cm2.
tion barrier at the interface [13, 33–35].
Although all the above devices with PCNs as ETL show
significantly improved device performance, their individual
properties are slightly different. Since PCNs have the same
pendent amino groups, the differences in device perfor-
mance are most reasonably attributed to the backbones.
Among them, the PPCN device shows slightly lower per-
formance with higher Von of 3.6 V and lower LEmax of 10.06
cd A1. One plausible reason for the lower performance of
the PPCN device, compared to those of the PFCN and
PCCN devices, is that the incorporation of phenylene unit
into polymer main chain could induce lesser conjugation
length [38].
4 Conclusions
In summary, a series of alcohol soluble amino-functionalized
carbazole-based copolymers with different backbone were
designed and synthesized via Suzuki coupling reaction. Due
to the more effective  electron delocalization, PFCN and
PCCN exhibit red-shift in the absorption spectra as com-
pared with that of PPCN both in chloroform and thin films.
All the PCNs exhibit blue fluorescence under 370 nm exci-
tation. With increasing electron density of the main chain,
the HOMO energy levels of PCNs go up progressively. By
using PCNs as ETL, the devices show significantly im-
proved device performance attributed to the interfacial di-
pole formation between the pendent amino groups and the
high WF metal cathode. These results support that PCNs are
promising candidates as highly efficient electron injection
materials for high-efficiency PLEDs.
Figure 7 LE-J characteristics of the green devices with different cath-
odes.
Ba cathode device shows a LE of 14.32 cd A1 with a lumi-
nance of 5460 cd m2 at the J = 30 mA cm2. The Ba cath-
ode device achieved high efficiency at the expense of sensi-
tivity toward moisture and oxygen which is detrimental to
long-term stability of device [14].
The devices with PCNs as ETL also demonstrate signifi-
cantly improved device performance. By using PPCN as
ETL, the device’s LE reaches 10.04 cd A1 with a L of 3400
cd m2 at J = 30 mA cm2, and Von decreases from 5.4 V for
the Al device to 3.6 V for PPCN device. The PCCN de-
vice’s LE reaches 11.13 cd A1 with a L of 3792 cd m2 at J
= 30 mA cm2, and Von of 3.4 V. Device with PFCN/Al
cathode exhibits better performance and lower Von. Further
increase was observed for the PFCN device performance,
with a LE of 12.01 cd A1 and a L of 3900 cd m2 at J = 30
mA cm2. The Von decreases from 5.4 V for the Al device to
The work was financially supported by the National Basic Research Pro-
gram of China (2009CB623601, 2009CB930604, 2011AA03A110), the
National Natural Science Foundation of China (21125419, 50990065,
51073057, 91233116), the Guangdong Natural Science Foundation
(S2012030006230) and the Research Fund for the Doctoral Program of
Higher Education of China (20120172140001).
3.2 V for the PFCN device, the maximum LE (LEmax
)
reaches 12.07 cd A1, which is more than forty times higher
than that of the bare Al device. The improved performance
by using PCNs as ETL, indicates that electron injection
from Al was facilitated. This may be attributed to the inter-
facial dipole formation between the pendent amino groups
and the high WF metal cathode, which can lower the injec-
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