Development of a new diindenopyrazine-benzotriazole copolymer for multifunctional application in organic field-effect transistors, polymer solar cells and light-emitting diodes

Article abstract of DOI:10.1016/j.orgel.2012.04.033

A new donor-acceptor (D-A) copolymer (PIPY-DTBTA) containing 6,12-dihydro-diindeno[1,2-b;1′,2′-e]pyrazine donor and benzotriazole acceptor was synthesized and characterized for multifunctional applications in organic field-effect transistors (OFETs), polymer solar cells (PSCs) and polymer light-emitting diodes (PLEDs). The polymer exhibits high molecular weights, excellent film-forming ability, a deep HOMO energy level, and good solution processability. Solution-processed thin film OFETs based on this polymer revealed good p-type characteristic with a high hole mobility up to 0.0521 cm² V^-1 s^-1. Bulk-heterojunction PSCs comprising this polymer and PC₆₁BM gave a power conversion efficiency (PCE) of 0.77%. The single-layer PLEDs based on PIPY-DTBTA emitted a yellow-red light with a maximum brightness of 385 cd m^-2 at the turn-on voltage of 6 V.

Full text of DOI:10.1016/j.orgel.2012.04.033

Organic Electronics 13 (2012) 1671–1679
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Development of a new diindenopyrazine–benzotriazole copolymer
for multifunctional application in organic field-effect transistors,
polymer solar cells and light-emitting diodes
Xinping Liu a, Ji Zhang b, Peng Tang a, Gui Yu b, Zhiyong Zhang a, Huajie Chen b, Ya Chen a,
a,
Bin Zhao a, Songting Tan a, , Ping Shen
⇑
⇑
a College of Chemistry and Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan 411105,
PR China
b CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new donor–acceptor (DꢀA) copolymer (PIPY–DTBTA) containing 6,12-dihydro-diinde-
no[1,2-b;10,20-e]pyrazine donor and benzotriazole acceptor was synthesized and character-
ized for multifunctional applications in organic field-effect transistors (OFETs), polymer
solar cells (PSCs) and polymer light-emitting diodes (PLEDs). The polymer exhibits high
molecular weights, excellent film-forming ability, a deep HOMO energy level, and good
solution processability. Solution-processed thin film OFETs based on this polymer revealed
good p-type characteristic with a high hole mobility up to 0.0521 cm2 Vꢀ1 sꢀ1. Bulk-hetero-
junction PSCs comprising this polymer and PC61BM gave a power conversion efficiency
(PCE) of 0.77%. The single-layer PLEDs based on PIPY–DTBTA emitted a yellow–red light
with a maximum brightness of 385 cd mꢀ2 at the turn-on voltage of 6 V.
Received 16 February 2012
Received in revised form 18 April 2012
Accepted 27 April 2012
Available online 29 May 2012
Keywords:
Diindenopyrazine
Benzotriazole
Multifunctional application
Organic field-effect transistors
Polymer solar cells
Ó 2012 Elsevier B.V. All rights reserved.
Polymer light-emitting diodes
1. Introduction
bandgap of CPs allows variation in absorption in the visible
region, the type of charge carriers upon doping, and emis-
In the past decade, conjugated polymers (CPs) have at-
tracted extensive scientific interesting for organic elec-
tronic applications, including organic field-effect
transistors (OFETs) [1–4], polymer solar cells (PSCs) [2,5–
10], polymer light-emitting diodes (PLEDs) [11–13], and
electrochromic devices [14,15]. CPs that are applicable to
many fields are regarded as multifunctional materials
[16–20] offering great potential to lower the cost of active
layer production for organic electronics. The attractive
properties of CPs are mostly based on the ability to tune
the optical and electrical properties with chemical struc-
tural modifications, such as controlling of a functional
group, conjugation length or the backbone. Tailoring the
sion wavelength [21–24]. In recent years, some low band-
gap donor–acceptor (DꢀA) copolymers have been
developed and applied in organic electronics [6,9,10]. The
exciting power conversion efficiencies (PCEs) over 7% have
been achieved for BHJ PSCs [25,26]. OFETs based on DꢀA
copolymers with hole mobilities exceeding 1 cm2 Vꢀ1 sꢀ1
have also been demonstrated [4,17,27,28], which appear
to utilize intermolecular self-assembling properties en-
hanced by the dipoles along the DꢀA backbone.
In the previous report, organic small molecules based
on 6,12-dihydro-diindeno[1,2-b;10,20-e]pyrazine (Diinde-
nopyrazine, IPY, Fig. 1) were applied as an efficient deep
blue emission with CIE coordinates (0.154, 0.078) and ex-
hibit a high external quantum efficiency of ꢁ4.6% [29].
Yamashita’s group used a IPY derivative as the channel
material in an n-type OFET with excellent results (mobility,
0.17 cm2 Vꢀ1 sꢀ1; on/off ratio, ꢁ107) [30]. Therefore, we
⇑
Corresponding authors.
E-mail address: shenping802002@163.com (P. Shen).
1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2012.04.033

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X. Liu et al. / Organic Electronics 13 (2012) 1671–1679
Fig. 1. Molecular structures of diindenopyrazine (IPY) and related copolymer PIPY–DTBTA.
have great interesting on IPY containing materials for the
following reasons: The IPY core possessing rigid and planar
structure facilitates the high chemical stability and effec-
2.2. Analytical instruments
1H NMR and 13C NMR spectra were measured with Bru-
ker AVANCE 400 spectrometer. UV–vis spectra were mea-
sured on Perkin-Elmer Lamada 25 spectrometer and
photoluminescence (PL) spectra were measured on Per-
kin-Elmer LS-50 luminescence spectrometer. Molecular
mass was determined by matrix assisted laser desorp-
tion–ionization time-of-flight mass spectrometry (MALDI-
TOF MS) using a Bruker Aupoflex-III mass spectrometer.
Thermogravimetric analyses (TGA) were performed under
nitrogen at a heating rate of 20 °C/min with Netzsch TG
209 analyzer. The average molecular weight and polydis-
persity index (PDI) of the copolymers were determined
using Waters 1515 gel permeation chromatography (GPC)
analysis with THF as eluent and polystyrene as standard.
Electrochemical redox potentials were obtained by cyclic
voltammetry (CV) using a three-electrode cell and an elec-
trochemistry workstation (CHI830B, Chenhua Shanghai).
The working electrode was a Pt ring electrode; the auxil-
iary electrode was a Pt wire, and saturated calomel elec-
trode (SCE) was used as reference electrode.
Tetrabutylammonium hexafluorophosphate 0.1 M was
used as supporting electrolyte in dry acetonitrile.
tive
pꢀp stacking of molecular in the thin film state [31],
which is good for the charge carrier mobility. The alkyl
groups can be introduced at the 6- and 12-positions of
IPY which can easy tuned the solution processability of
the polymers. On the other hand, the benzotriazole (BTA)
unit can be provide an additional advantage of incorporat-
ing soluble alkyl chains onto the acceptor compared with
the typical and successful benzothiadizole (BT) unit
[32,33]. More important, the synthesis of highly soluble
IPY-containing copolymers and their applications in or-
ganic electronics, especially for PSCs, have not been re-
ported yet.
Expanding on the foregoing points, herein, a new DꢀA
copolymer PIPY–DTBTA (Fig. 1) containing IPY and BTA
units was designed and synthesized for multifunctional or-
ganic electronic application. The unique distribution of the
alkyl chains allows the polymer backbone to adopt a more
planar conformation, which can promote close packing of
the polymer chain and increase the hole mobility of the
resulting polymers. As a result, we confirmed a high
molecular weight, good solubility, film-forming ability,
and impressively high mobility for this new IPY-based
polymer. The preliminary performance of OFETs, PSCs
and PLEDs showed that PIPY–DTBTA could be use as a
promising multifunctional material.
2.3. Fabrication and characterization of PSCs
The photovoltaic cells were constructed in the tradi-
tional sandwich structure through several steps. The ITO
coated glass substrates were cleaned by successive ultra-
sonic treatment in acetone and isopropyl alcohol, then
treated with a nitrogen–oxygen plasma oven for 5 min.
Poly(3,4-ethylene dioxythiophene)–poly(styrenesulfonate)
(PEDOT:PSS, from Bayer AG) was spin-coated from an
aqueous solution on a cleaned indium tin oxide (ITO) glass
substrate giving a thickness of about 40 nm as measured
by Ambios Technology XP-2 surface profilometer, then it
was dried at 150 °C for 15 min. Subsequently, the photoac-
tive layer was prepared by spin-coating the dichloroben-
zene solution of polymer:PC61BM (1:2, w/w) with the
polymer concentration of 6 mg/mL on the top of the PED-
OT:PSS layer and the thickness of the active layers of
ca. 80 nm, and then annealed at 150 °C for 5 min in a
nitrogen-filled glove box. Finally, the substrates were
2. Experimental section
2.1. Materials and reagents
5-Bromo-indanone, amylnitrite, 2,1,3-benzothiazole,
isopropoxyboronic acid pinacol ester and tetrabutylammo-
nium hexafluorophosphate were purchased from commer-
cial suppliers (Pacific ChemSource and Alfa Aesar) in
analytical grade. Toluene was dried and distilled over so-
dium/benzophenone. DMF, CHCl3, and CH3CN were dried
over by accustomed methods and distilled before using.
All other solvents and chemicals used in this work were
analytical grade and used without further purification. All
chromatographic separations were carried out on silica
gel (200–300 mesh).

X. Liu et al. / Organic Electronics 13 (2012) 1671–1679
1673
transferred into an evaporator and pumped down to
5 ꢂ 10ꢀ4 Pa to deposit 10 nm of Ca and 100 nm of alumi-
num cathodes, producing an active area of 9 mm2 for each
cell. The cathode of devices, consisting of 10 nm of Ca and
100 nm of Al, was thermally deposited on the top of photo-
active layer at 5 ꢂ 10ꢀ4 Pa. The active area of the device is
4 mm2. Current density–voltage (JꢀV) characteristics were
thermal evaporator through a shadow mask, yielding ac-
tive areas of 9 mm2. For device characterizations, EL spec-
tra, CIE coordinates, current–voltage, and brightness-
voltage characteristics of devices were measured with a
Spectrascan PR650 spectrophotometer at the forward
direction and a computer-controlled Keithley 2602 under
ambient condition.
measured by
a Keithley 2602 Source meter under
100 mW cmꢀ2 irradiation using a 500 W Xe lamp equipped
with a global AM 1.5 filter for solar spectrum simulation,
and the incident light intensity was calibrated using a stan-
dard Si solar cell. The measurement of external quantum
efficiency (EQE) was performed using a Zolix DCS300PA
Data acquisition system.
2.6. Synthesis of the monomers and polymer
The synthetic route to the novel copolymer (PIPY–
DTBTA) is outlined in Scheme 1. The detailed synthetic
procedures are as follows.
2.6.1. 5-Bromo-2,3-dihydro-2-(hydroxyimino)inden-one (1)
In a 250 mL three-necked flask, 5-bromo-indanone
(5.0 g, 23.5 mmol) was dissolved in 100 mL toluene. After
bubbling dry HCl gas for 5 min, amylnitrite (3.8 mL,
28 mmol) was added dropwise. While dry HCl gas was
bubbled, the mixture was heated to 40 °C and stirred for
2 h. After it was again stirred for 12 h at room temperature,
the crude product was filtered and washed with MeOH to
get 4.4 g light yellow solid of 1 (yield:77%). 1H NMR
(400 MHz, DMSO, d/ppm): 12.72 (s, 1H), 7.90 (s, 1H), 7.68
(s, 2H), 3.79 (s, 2H)
2.4. Fabrication and characterization of OTFTs
OTFTs fabricated on OTS-modified SiO2/Si substrates.
OTFT devices were fabricated in a top-contact configura-
tion. Before the deposition of organic semiconductors,
octyltrichlorosilane (OTS) treatment was performed on
the gate dielectrics which were placed in a vacuum oven
with OTS at a temperature of 120 °C to form an OTS self-
assembled monolayer. A layer of polymer film was then
deposited on the OTS-treated SiO2 surface by spin-coating
from a PIPT-DTBTA solution in chloroform (8 mg/mL) at a
speed of 3000 rpm for 40 s followed by Au deposition
through a shadow mask to define the source and drain
electrodes. For annealed OTFTs (annealing at 100 °C for
5 min), the samples were further placed on a hotplate in
air for 5 min before cooling down to room temperature.
The OTFTs characteristics of the devices were determined
at room temperature in air by using a Keithley 4200 SCS
semiconductor parameter analyzer. The mobility of the de-
vices was calculated in the saturation regime. The equation
is listed as follows:
2.6.2. 2,8-Dibromo-6,12-dihydrodiindeno[1,2-b:1,2-
e]pyrazine (2)
Under a nitrogen atmosphere, the synthesized 1 (3.0 g,
12.5 mmol), sodium dithionite (6.5 g, 37.5 mmol) and
30 mL EtOH were added in a 250 mL three-necked flask,
then 14.5% ammonia solution (25 mL) was added drop-
wise. The mixture was stirred at room temperature with
no roomlight for 3 days. After distilled water (30 mL) was
added, the solution was heated to reflux for 24 h in the
air. When it was cooled, more distilled water (60 mL)
was added. The solution was stirred for another 2 h. The
mixture was filtered and washed with MeOH, diethyl ether
to get 1.86 g orange solid of 2 (yield:72%). 1H NMR
(400 MHz, CDCl3, d/ppm): 7.99 (d, J = 8.10 Hz, 2H), 7.82
(s, 1H), 7.66 (d, J = 7.80 Hz, 2H), 4.07 (s, 4H). MALDI-TOF
MS (C18H10Br2N2) m/z: calcd for 414.093; found 414.692
(M+).
2
IDS ¼ ðW=2LÞCi ðVGS ꢀ VthÞ
l
where W/L is the channel width/length, Ci is the insulator
capacitance per unit area, and VGS and Vth are the gate volt-
age and threshold voltage, respectively.
2.5. Fabrication and characterization of PLEDs
2.6.3. 2,8-Dibromo-6,6,12,12-tetraoctyl-6,12-dihydro-
diindeno[1,2-b;10,20-e]pyrazine (3)
The single-layered PLEDs with the configuration ITO/
PEDOT:PSS/polymer/Ca/Al were fabricated. ITO-coated
glass substrates were first thoroughly cleaned in an ultra-
sonic solvent bath and then dried in a heating chamber
at 120 °C. PEDOT doped with PSS was spin-coated on the
ITO glasses at a speed of 2000 rpm for 30 s and then baked
for 30 min at 120 °C to give a thin layer film of about 30 nm
thick. The light-emitting polymer layer was then deposited
onto the film by spin coating a polymer solution in chloro-
benzene (10 mg/mL) at a speed of 2000 rpm for 30 s, after
which the device was thermally annealed at 80 °C for
30 min in a glove box before cathode deposition. Uniform
and pinhole-free films with thicknesses around 70–80 nm
could be readily obtained. Finally, a thin layer of calcium
(about 10 nm) and a layer of aluminum (about 300 nm)
were deposited in sequence in a vacuum (5 ꢂ 10ꢀ4 Pa)
Under a nitrogen atmosphere, to a 100 mL three-necked
flask, the synthesized 2 (1.86 g, 4.49 mmol), KOH (2.5 g,
44.9 mmol), and n-Bu4NBr (0.29 g, 0.9 mmol) were dis-
solved in DMSO/H2O (45 mL/5 mL). The mixture was
heated to 50 °C, then 1-bromooctane (7.8 mL, 44.9 mmol)
was added dropwise. The mixture was heated to 65 °C for
24 h. Finally, the mixture was poured into water, and ex-
tracted with chloroform. The organic layer was dried over
anhydrous MgSO4, and the solvent was evaporated under
reduced pressure. The crude product was purified by col-
umn chromatography (hexane) on silica gel to get 2.0 g
yellow solid of 3 (yield: 52%). 1H NMR (400 MHz, CDCl3,
d/ppm): 7.94 (d, J = 8.54 Hz, 2H), 7.55 (m, 4H), 2.27–2.00
(m, 4H), 1.99–1.91 (m, 4H), 1.17–1.10 (m, 48H), 0.80–
0.69 (m, 12H), 13C NMR (100 MHz, CDCl3, d/ppm):

1674
X. Liu et al. / Organic Electronics 13 (2012) 1671–1679
Scheme 1. Synthetic route of the monomers and the copolymer PIPY–DTBTA.
162.94, 152.40, 151.14, 137.73, 130.57, 126.46, 123.48,
122.60, 53.23, 38.36, 31.70, 29.70, 29.04, 29.01, 23.79,
22.50, 13.93. MALDI-TOF MS (C50H74Br2N2) m/z: calcd for
862.944; found 861.223 (M+).
evaporated under reduced pressure. The crude product
was purified by column chromatography on silica gel to
get 2.33 g compound 4 as a colorless oil (yield: 40%). 1H
NMR (400 MHz, CDCl3, d/ppm): 7.87 (dd, J = 6.50, 3.06 Hz,
2H), 7.37 (dd, J = 6.52, 3.00 Hz, 2H), 4.63 (d, J = 7.09 Hz,
2H), 2.25–2.22 (m, 1H), 1.57–1.03 (m, 8H), 0.93 (t,
J = 7.45 Hz, 3H), 0.87 (t, J = 6.70 Hz, 3H).
2.6.4. 2,8-Bis-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
6,6,12,12-tetraoctyl-6,12-dihydrodiindeno[1,2-b;10,20-e]
prazine (Monomer 1)
Under a nitrogen atmosphere, the synthesized 3 (2.0 g,
2.3 mmol) and 40 mL THF were added in a 100 mL three-
necked flask. n-Butyllithium (3.7 mL, 2.5 M in hexane,
9.25 mmol) was added dropwise at ꢀ78 °C, and the solu-
tion was stirred for 2 h at ꢀ78 °C, then isopropoxyboronic
acid pinacol ester (2.4 mL, 11.5 mmol) was added. The
mixture was allowed up to room temperature naturally
and stirred for another 24 h. Finally, the mixture was
poured into water, and extracted by chloroform. The or-
ganic layer was dried over anhydrous MgSO4, and the sol-
vent was evaporated under reduced pressure. The crude
product was recrystallised by hexane to get 1.00 g white
solid Monomer 1 (yield: 45%). 1H NMR (400 MHz, CDCl3,
d/ppm): 8.10 (d, J = 7.36 Hz, 2H), 7.90 (d, J = 7.46 Hz, 2H),
7.84 (s, 2 H), 2.28–2.23 (m, 4H), 2.05–2.00 (m, 4H), 1.41
(s, 24H), 1.14–1.01 (m, 48H), 0.78–0.75 (m, 12H). 13C
NMR (100 MHz, CDCl3, d/ppm): 163.87, 151.93, 149.53,
141.64, 133.98, 129.09, 120.45, 83.89, 53.05, 38.44, 31.74,
29.78, 29.10, 29.06, 24.95, 23.83, 22.53, 13.98. MALDI-
TOF MS (C62H98B2N2O4) m/z: calcd for 957.070; found
957.718 (M+).
2.6.6. 4,7-Dibromo-2-ethylhexyl-2,1,3-benzotriazole (5)
Under a nitrogen atmosphere, to a 100 mL two-necked
flask, Compound 4 (2.33 g, 10.07 mmol) and aqueous HBr
solution (10.26 mL, 70.00 mmol) were added. And the mix-
ture was stirred for 1 h at 100 °C. Bromine (1.55 mL,
30.21 mmol) was added dropwise. The reaction mixture
was stirred for 12 h at 135 °C. After the mixture was cooled
to room temperature, an aqueous solution of NaHSO3 was
added and the product was extracted by CH2Cl2. The or-
ganic layer was dried over anhydrous MgSO4 and the sol-
vent was evaporated under reduced pressure. The crude
product was purified by column chromatography on silica
gel to get 2.82 g compound 5 as a light yellow oil (yield:
72%). 1H NMR (400 MHz, CDCl3, d/ppm): 7.44 (s, 2H),
4.68 (d, J = 7.29 Hz, 2H), 2.36–2.25 (m, 1H), 1.37–1.28 (m,
8H), 0.94–0.85 (m, 6H)
2.6.7. 4,7-Bis(thiophen-2-yl)-2-ethylhexyl-2,1,3-benzotriazole
(6)
Under a nitrogen atmosphere, to a 100 mL three-necked
flask, compound
5
and 2-(tri-butystannyl)thiophene
(6.77 g, 18.13 mmol) were dissolved in 40 mL anhydrous
toluene, the solution was purged with argon for 30 min
and Pd(PPh3)4 (0.17 g, 0.145 mmol) was added quickly.
Then the mixture was stirred for 72 h at 85 °C. After evap-
orating the solution under reduced pressure, the product
was extract with CH2Cl2. The organic layer was dried over
anhydrous MgSO4 and the solvent was evaporated under
reduced pressure. The crude product was purified by col-
umn chromatography on silica gel to get 2.44 g compound
6 as a light green solid (yield: 85%). 1H NMR (400 MHz,
2.6.5. 2-Ethylhexyl-2,1,3-benzotriazole (4)
Under a nitrogen atmosphere, to a 100 mL three-necked
flask, 2,1,3-benzotriazole (3.00 g, 25.18 mmol) and potas-
sium hydroxide (3.00 g, 26.68 mmol) were dissolved in
40 mL DMSO, then 2-ethylhexyl bromide (4.74 mL,
26.68 mmol) was added dropwise. The reaction mixture
was stirred for 24 h at 75 °C and monitors by TLC.
The crude product was extracted by CH2Cl2. The organic
was dried over anhydrous MgSO4 and the solvent was

X. Liu et al. / Organic Electronics 13 (2012) 1671–1679
1675
CDCl3, d/ppm): 8.10 (d, J = 3.08 Hz, 2H), 7.63 (s, 2H), 7.38
(d, J = 4.84 Hz, 2H), 7.26–7.09 (m, 2H), 4.76 (d, J = 6.68 Hz,
2H), 2.49–2.08 (m, 1H), 1.62–1.24 (m, 8H), 1.02–0.98 (t,
J = 7.36 Hz, 3H), 0.92–0.88 (t, J = 7.98 Hz, 3H). 13C NMR
(100 MHz, CDCl3, d/ppm): 142.05, 140.05, 127.99, 126.90,
125.46, 123.63, 122.63, 59.77, 40.48, 30.69, 28.52, 24.10,
22.89, 13.95, 10.55.
CDCl3, d/ppm): 8.19–8.13 (br, 4H), 7.81–7.67 (br, 6H),
7.57 (br, 2H), 4.86 (br, 2H), 2.35 (br, 4H), 2.09 (br, 4H),
1.47–0.65 (m, 75H).
3. Results and discussion
3.1. Synthesis and characterization
2.6.8. 4,7-Bis(5-bromothiophen-2-yl)-2-ethylhexyl-2,1,3-
benzotrizole (Monomer 2)
The synthetic route to the novel copolymer (PIPY–
DTBTA) is outlined in Scheme 1. Compounds 1 [29], 2
[29], 4 [33] and 5 [15] were synthesized according to liter-
ature methods. Then compound 2 was alkylated with 1-
bromooctane to afford 3. The important intermediate of
Monomer 1 was prepared by a modified procedure with
2.5 equiv. of n-BuLi followed by 2-isopropyl-4,4,5,5-tetra-
methyl-1,3,2-dioxaborolane. The other key intermediate
Monomer 2 was prepared from compound 5 by two suc-
cessive reactions. The novel D–A polymer PIPY–DTBTA
was obtained through a Suzuki coupling reaction between
Monomer 1 and 2 using Pd(PPh3)4 as catalyst in toluene/
Na2CO3 (2 M) mixture end-capped with phenyl boronic
acid pinacol ester and bromobenzene in sequence. The four
n-octyl chains into 6- and 12-position of the IPY moiety
and the ethylhexyl into BTA unit were present to obtain
balanced properties including solubility, interchain pack-
ing, and chain planar conformation of the polymer. PIPY–
DTBTA has excellent film-forming property and solubility
in common organic solvents such as chloroform, chloro-
benzene (CB) and tetrahydrofuran (THF), providing conve-
nience for characterization and device processing.
Under a nitrogen atmosphere, to a 50 mL two-necked
flask, 4,7-bis(20-thienyl)-2-ethylhexyl-2,1,3-benzotriazole
(1.00 g, 2.53 mmol) was dissolved in 25 mL DMF, then N-
bromosuccinimide (NBS) (0.90 g, 5.06 mmol) added slowly
in darkness. The mixture was stirred at room temperature
overnight, and water was added. The product was ex-
tracted with CH2Cl2. The organic layer was dried over
anhydrous MgSO4 and the solvent was evaporated under
reduced pressure. The crude product was purified by col-
umn chromatography on silica gel to get 1.12 g Monomer
2 as a yellow solid (yield: 80%). 1H NMR (400 MHz, CDCl3,
d/ppm): 7.79 (d, J = 3.84 Hz, 2H), 7.51 (s, 2H), 7.12 (d,
J = 3.72 Hz, 2H), 4.74 (d, J = 6.64 Hz, 2H), 2.49–2.05 (m,
1H), 1.64–0.94 (m, 8H), 0.99 (t, J = 7.36 Hz, 3H), 0.91 (t,
J = 7.04 Hz, 3H). 13C NMR (100 MHz, CDCl3, d/ppm):
141.67, 141.32, 130.84, 126.87, 123.03, 122.10, 113.17,
59.83, 40.48, 30.65, 28.48, 24.08, 22.91, 13.98, 10.56.
2.6.9. The copolymer PIPY–DTBTA
Under a nitrogen atmosphere, Monomer 1 (260 mg,
0.27 mmol), Monomer
2
(120 mg, 0.270 mmol), n-
Bu4N+Brꢀ (8.72 mg, 0.270 mmol) were dissolved in a mix-
ture of toluene (12 mL) and aqueous 2 M Na2CO3 (4 mL).
The mixture was bubbled nitrogen for 30 min to remove
O2, Pd(PPh3)4 (10 mg, 0.008 mmol) was added. The reac-
tion was refluxed for 3 days. At the end of polymerization,
phenyl boronic acid pinacol ester (50 mg in 1 mL toluene)
was added, and after 2 h, bromobenzene (1 mL) was added,
the mixture was allowed to reflux for 2 h. Then the reac-
tion mixture was cooled to room temperature and dropped
into methanol. The precipitated was filtered and Soxhlet
extracted with methanol, acetone, hexane, and chloroform.
The chloroform extracts were then concentrated and pre-
cipitated into methanol and then filtered and wash with
methanol. The residual solvent was then removed under
vacuum, affording PIPY–DTBTA as light-red solid (yield:
60%). GPC: Mn = 10.8 kDa, PDI = 2.11. 1H NMR (400 MHz,
The number-averaged molecular weight (Mn) of PIPY–
DTBTA is 108.2 kDa with a polydispersity index of 2.11
(Table 1), as determined through gel permeation chroma-
tography (GPC) using a polystyrene standard with THF sol-
vent. The desired high molecular weight means that the
polymer would obtain a high hole mobility because the
molecular weight is a very important potential factor influ-
encing polymer FET behavior [4,34–36]. PIPY–DTBTA pos-
sesses good thermal stability, with the decomposition
temperature (5% weight loss, Td) higher than 440 °C, as
measured using thermogravimetric analysis (TGA, see Sup-
plementary information (SI), Fig. S1). Such high value is ex-
pected because the polymer backbone owns rigid and
coplanar merits due to the fused-ring system of IPY unit.
Obviously, the thermal stability of this polymer is good en-
ough for the applications in all optoelectronic devices.
Table 1
Properties of the copolymer PIPY–DTBTA.
PDIa
2.11
Td (°C)
kmax soln.c (nm)
kmax filmd (nm)
kedge (nm)
EHOMO (eV)
ELUMO (eV)
a
b
e
g
h
Eogpt (eV)
f
Mn (kDa)
108.2
446
515
522
569
2.18
ꢀ5.56
ꢀ3.19
a
b
c
Mn and PDI determined by GPC using polystyrene standards in THF.
Decomposition temperature (5% weight loss) determined by TGA under N2.
Measured in chloroform solution.
d
e
f
Cast from chloroform solution.
Estimated from the absorption edge of thin films.
Eogpt = 1240/kedge
.
g
h
EHUMO ¼ ꢀeðEox þ 4:40Þ (eV).
Determined from ELUMO ¼ EHUMO þ Eogpt (eV).

1676
X. Liu et al. / Organic Electronics 13 (2012) 1671–1679
that a higher open-circuit voltage (Voc) could be obtained
PL (Soln.)
PL (Film)
UV-vis (Soln.)
UV-vis (Film)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
than that of poly(3-hexylthiophene) (P3HT)-based devices
(ꢁ0.6 V) [38]. At the same time, the relatively low HOMO
energy level of the polymer indicates that it possesses
the enough good air stability against oxidation in ambient
condition, which is very important for the application in
organic electronic devices [39].
UV-vis
PL
3.3. Performance in OFETs
To understand whether incorporation of largely ring-
fused IPY unit into the polymer backbones can enhance
hole transporting ability of the polymer, the OTFT devices
with a conventional bottom-contact and bottom-gate con-
figuration were fabricated using PIPY–DTBTA as a semi-
conductor. Fig. 3 shows the representative transfer and
output current–voltage plots for OTFTs fabricated on an
octadecyltrichlorosilane-treated Si/SiO2 (300 nm) sub-
strate. Six devices were measured and well reproduced at
room temperature and the annealed condition (100 °C)
with the small standard deviations (Supplementary infor-
mation, Table S1). Without thermal annealing, the polymer
300
400
500
600
700
800
Wavelength (nm)
Fig. 2. UV–vis absorption spectra and PL spectra of PIPY–DTBTA in dilute
CHCl3 solution and as a solid film.
3.2. Optical and electrochemical properties
The UV–vis absorption spectra and photoluminescence
(PL) emission spectra of the copolymer in dilute chloro-
form solution and the solid film are shown in Fig. 2. In di-
lute chloroform solution, the polymer exhibits an obvious
broad absorption in the region of 350–600 nm with an
absorption maximum (kmax) at 515 nm and a left shoulder
peak at 493 nm. The solid film absorption of the polymer
delineates a similar trend to the solution, only broadened
and red-shifted compared with the corresponding spec-
trum in solution, indicating the presence of intermolecular
interactions in the solid state. It should be noted that the
absorption behavior of PIPY–DTBTA is quite similar to
the already reported BTA-based DꢀA alternating copoly-
mers with fluorene and carbazole as the donating units
[33]. The optical bandgap (Eogpt) of PIPY–DTBTA calculated
from the onset of the solid film absorption (569 nm) is
2.18 eV. As for PSCs application, this relatively wider band-
gap is a disadvantage in that less light is harvested from
the solar spectrum, which will be not good for short-circuit
current. However, this problem can be solved effectively by
changing the copolymerized unit with IPY. The related re-
search will be reported in the near future. As shown in PL
emission spectra (Fig. 2), the maximum emission peaks of
PIPY–DTBTA are observed at about 537 and 603 nm for the
solution and film state, respectively.
Cyclic voltammetry (CV) of the polymer thin film coated
on a platinum electrode indicates that the onset oxidation
potential (Eox) is about 1.16 V (see Supplementary infor-
mation, Fig. S2). The corresponding HOMO energy level
of PIPY–DTBTA is calculated to be ꢀ5.56 eV from the equa-
tion EHUMO ¼ ꢀeðEox þ 4:40Þ eV, where the unit of Eox is volt
vs. SCE [37]. Since we could not obtain reliable onset
reduction potential for the polymer, the optical band gap
(Egopt) of the polymer film was utilized to estimate the
LUMO energy level of the polymer based on the equation
of ELUMO ¼ ðEogpt þ EHOMOÞ eV. The calculated LUMO energy
level is ꢀ3.38 eV. The HOMO and LUMO energy levels are
also outlined in Table 1. Obviously, the material displays
HOMO energy level over 0.4 eV lower than the currently
favored, wide bandgap polymer, P3HT (ꢀ5.1 eV), implying
films
showed
a
maximum
hole
mobility
of
0.0191 cm2 Vꢀ1 sꢀ1 (average of 0.0158 for six devices, on/
off ratio of ꢁ106). Annealing of the polymer films at
100 °C resulted in an little increase in the maximum hole
mobility to 0.0521 cm2 Vꢀ1 sꢀ1 (average of 0.0440 for six
devices, on/off ratio of ꢁ107). Obviously, the annealing de-
vices showed relatively better hole mobility than those of
the unannealed. This fairly high hole mobility may be as-
cribed to the following three aspects. The first reason
should be attribute to the high molecular weight
(Mn = 108.2 kDa). As we known, the molecular weight is a
key factor influencing polymer FET behavior, which has
been particularly well demonstrated for regioregular
P3HT [34–36]. Usually, higher molecular weights, ex-
pressed in number average molecular weight Mn, lead to in-
crease macroscopic order and therefore elevate hole
mobilities as compared to the lower Mn case [27,28,40].
The polymer thin film surface morphologies were studied
with close-contact atomic force microscopy (AFM) (Supple-
mentary information, Fig. S3) and X-ray diffraction (XRD)
(Supplementary information, Fig. S4). From the AFM
images, it can be seen that the thin film without annealing
has a nearly homogeneous surface with a RMS surface
roughness of 1.07 nm. After annealing, the film exhibits
more homogeneous with a decreased RMS surface rough-
ness of 0.959 nm. This result shows that the polymer has
an inherent advantage of good film-forming ability, which
is beneficial to the carrier transfer. The molecular packing
of the copolymer in thin films cast from solution (onto a
Si wafer as substrate) was studied by X-ray diffraction
(XRD) analysis. However, no scattering patterns are ob-
served, suggesting a macroscopically disordered, amor-
phous solid. This result gave a reasonable explain why
there is not a very obvious improvement (just 2.7 times)
of the mobility after the annealing. The last reason probably
arises from the larger
which can enhance the intermolecular interaction and
stacking between polymer backbones. This excellent
p-conjugated system of the IPY unit
p
–
p

X. Liu et al. / Organic Electronics 13 (2012) 1671–1679
1677
Fig. 3. The (a) output and (b) transfer curves of the PIPY–DTBTA-based OTFT devices after annealing at 100 °C for 5 min.
OFET performance is sufficiently good for manufacturing
low-cost display backplane electronics.
potentials to obtain higher PCEs by copolymerizing with
stronger electron-deficient units to broaden the absorption
band. In addition, to find other reasons of low Jsc, we have
study the morphology of polymer/PC61BM active layer
using AFM (Supplementary information, Fig. S5). As shown
in AFM figure, we can find there is almost not phase sepa-
3.4. Performance in PSCs
Since PIPY–DTBTA has a deep-lying HOMO energy level
and a respectably high mobility, this new polymer could be
expected to be a suitable donor material in PSCs. Therefore,
we carried out a preliminary study on the photovoltaic
properties of PSCs using PIPY–DTBTA as an electron donor
and PC61BM as an electron acceptor with a device structure
of ITO/PEDOT:PSS/polymer:PC61BM (1:2, w/w)/Ca/Al. Fig. 4
shows the current–voltage (J–V) curve of the PSCs based on
2
PIPY-DTBTA
1
0
the copolymer under the illumination of AM1.5
G
(100 mW/cm2). As expected, PSCs based on PIPY–DTBTA
showed a higher Voc (0.78 V) than that of P3HT (ꢁ0.6 V),
which could be readily understood from the low HOMO
energy level (ꢀ5.56 eV) of the copolymer. However, a low
short-circuit current density (Jsc) of 2.05 mA cmꢀ2 and fill
factor (FF) of 0.48 were obtained, which result a non-ideal
PCE of 0.77% eventually. The low Jsc value can be reflected
from low external quantum efficiency (EQE) value of PI-
PY–DTBTA (Fig. 5). As shown in Fig. 5, the highest EQE va-
lue is about 12%. And in the main absorption region (350–
600 nm), there is a rather low EQE with an average value
under 10%, this means that it is impossible to give a high
Jsc. Considering a relatively wide bandgap of the copoly-
mer, we believe that IPY-based copolymers have great
-1
-2
-3
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (V)
Fig. 4. The typical J–V curve of the photovoltaic cells based on PIPY–
DTBTA.

1678
X. Liu et al. / Organic Electronics 13 (2012) 1671–1679
15
10
5
de l’Eclairage (CIE) coordinates x, y are (0.48, 0.51). The ini-
PIPY-DTBTA
tial result shows that the maximum brightness and current
efficiency of reach 385 cd mꢀ2 and 0.05 cd Aꢀ1
respectively.
,
4. Conclusions
In conclusion, a novel diindenopyrazineꢀbenzotriazole
based DꢀA copolymer PIPY–DTBTA was synthesized and
well characterized by 1H NMR, GPC, TGA, UV–vis and CV.
The OTFT devices based on PIPY–DTBTA exhibited impres-
sive field-effect performance with a hole mobility up to
0.0521 cm2 Vꢀ1 sꢀ1 and a on/off current ratio of 107, which
should be ascribe to the advantages of high molecular
weight as well as large ring-fused IPY system in the poly-
mer backbone. The preliminary investigation of the PSCs
based on a PIPY–DTBTA:PC61BM (1:2 w/w) BHJ exhibits a
PCE of 0.77%. Considering the relatively narrow light
absorption (the onset wavelength at ca. 569 nm), its per-
formance in PSCs could be further improved through mol-
ecule structural and device optimization. Additionally, a
simple PLED was fabricated using the copolymer as the
only emitting layer, and gave a maximum brightness up
to 385 cd mꢀ2. Our results demonstrated that IPY unit is
a promising build block to develop solution-processable
polymer semiconductors for multifunctional materials,
especially for high-performance OTFT application. Detailed
investigations on PIPY–DTBTA and further synthesis of
new IPY-based polymers are currently in progress and will
be reported in the near future.
0
300
400
500
600
700
Wavelength (nm)
Fig. 5. EQE plot for the polymer solar cell based on the PIPY–DTBTA.
ration of active layer with a very small RMS of 0.621, which
is not good for the effective transfer of carriers and unfa-
vorable to the production of Jsc. So, this non-ideal phase
separation should be also responsible for the low Jsc to a
certain extent.
3.5. Performance in PLEDs
In view of the good luminescence properties of IPY-
based materials [29,31], PIPY–DTBTA was also utilized as
the only emissive active layer in PLEDs to evaluate the
electroluminescent (EL) characteristics. The device struc-
ture was ITO/PEDOT:PSS/polymer/LiF/Al. It should be
noted that we used the simplest device to investigate the
preliminary EL properties of the polymer. At a voltage of
6 V, PIPY–DTBTA displayed saturated yellow–red emission
with the maximum peak at 596 nm (Fig. 6). Compared with
that of PL in the solid state (Fig. 2), the EL spectrum of the
copolymer exhibits the similar emission, which indicates
that the same excitation mechanisms were involved in
both cases. The corresponding Commission International
Acknowledgements
We gratefully acknowledge the financial support of Na-
tional Natural Science Foundation of China (Nos.
21004050, 50973092, 51003089), National Natural Science
Foundation of Hunan Province (Nos. 10JJ4007), and Under-
graduate Investigated-Study and Innovated Experiment
Plan of Xiangtan University.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.orgel.2012.04.033.
6 V (0.56, 0.43)
1.0
0.8
0.6
0.4
0.2
0.0
References
[1] A. Pron, P. Gawrys, M. Zagorska, D. Djurado, R. Demadrille, Chem.
Soc. Rev. 39 (2010) 2577.
[2] A. Facchetti, Chem. Mater. 23 (2011) 733.
[3] P.M. Beaujuge, J.M.J. Fréchet, J. Am. Chem. Soc. 133 (2011) 20009.
[4] H.N. Tsao, D.M. Cho, I. Park, M.R. Hansen, A. Mavrinskiy, D.Y. Yoon, R.
Graf, W. Pisula, H.W. Spiess, K. Müllen, J. Am. Chem. Soc. 133 (2011)
2605.
[5] Y. Li, Acc. Chem. Res. (2012), (Articles ASAP), http://dx.doi.org/
10.1021/ar2002446.
[6] B.C. Thompson, J.M.J. Fréchet, Angew. Chem. Int. Ed. 47 (2008) 58.
[7] Y.-J. Cheng, S.-H. Yang, C.-S. Hsu, Chem. Rev. 109 (2009) 5868.
[8] C.J. Brabec, S. Gowrisanker, J.J.M. Halls, D. Laird, S. Jia, S.P. Williams,
Adv. Mater. 22 (2010) 3839.
400
500
600
700
800
Wavelength (nm)
[9] P.-L.T. Boudreault, A. Najari, M. Leclerc, Chem. Mater. 23 (2011) 456.
[10] H. Zhou, L. Yang, W. You, Macromolecules 45 (2012) 607.
[11] H.B. Wu, L. Ying, W. Yang, Y. Cao, Chem. Soc. Rev. 38 (2009) 3391.
Fig. 6. EL spectrum and the CIE coordinate of PIPY–DTBTA at the voltage
of 6 V.

X. Liu et al. / Organic Electronics 13 (2012) 1671–1679
1679
[12] H. Jiang, Macromol. Rapid Commun. 31 (2010) 2007.
[13] M.C. Gather, A. Köhnen, K. Meerholz, Adv. Mater. 23 (2011) 233.
[14] G.E. Gunbas, A. Durmus, L. Toppare, Adv. Mater. 20 (2008) 691.
[15] D. Baran, A. Balan, S. Celeb, B.M. Esteban, H. Neugebauer, N.S.
Sariciftci, L. Toppare, Chem. Mater. 22 (2010) 2978.
[16] H.-G. Jeong, B. Lim, S.-I. Na, K.-J. Baeg, J. Kim, J.-M. Yun, D.-Y. Kim,
Macromol. Chem. Phys. 212 (2011) 2308.
[27] W. Zhang, J. Smith, S.E. Watkins, R. Gysel, M. McGehee, A. Salleo, J.
Kirkpatrick, S. Ashraf, T. Anthopoulos, M. Heeney, I. McCulloch, J. Am.
Chem. Soc. 132 (2010) 11437.
[28] H.N. Tsao, D. Cho, J.W. Andreasen, A. Rouhanipour, D.W. Breiby, W.
Pisula, K. M€ullen, Adv. Mater. 21 (2009) 209.
[29] Y.I. Park, J.H. Son, J.S. Kang, S.K. Kim, J.H. Lee, J.W. Park, Chem.
Commun. (2008) 2143.
[17] H. Bronstein, Z. Chen, R.S. Ashraf, W. Zhang, J. Du, J.R. Durrant, P.S.
Tuladhar, K. Song, S.E. Watkins, Y. Geerts, M.M. Wienk, R.A.J. Janssen,
T. Anthopoulos, H. Sirringhaus, M. Heeney, I. McCulloch, J. Am.
Chem. Soc. 133 (2011) 3272.
[30] T. Nakagawa, D. Kumaki, J. Nishida, S. Tokito, Y. Yamashita, Chem.
Mater. 20 (2008) 2615.
[31] Y.I. Park, S. Kim, J.-H. Lee, D.H. Jung, C.-C. Wu, J. Park, Org. Electron.
11 (2010) 864.
[18] B. Liu, S. Ye, Y. Zou, B. Peng, Y. He, K. Zhou, Macromol. Chem. Phys.
212 (2011) 1489.
[32] J. Min, Z.G. Zhang, S.Y. Zhang, M.J. Zhang, J. Zhang, Y.F. Li,
Macromolecules 44 (2011) 7632.
[19] H. Bronstein, R.S. Ashraf, Y. Kim, A.J.P. White, T. Anthopoulos, K.
Song, D. James, W. Zhang, I. McCulloch, Macromol. Rapid Commun.
32 (2011) 1664.
[20] H. Chen, C. He, G. Yu, Y. Zhao, J. Huang, M. Zhu, H. Liu, Y. Guo, Y. Li, Y.
Liu, J. Mater. Chem. 22 (2012) 3696.
[21] D. Sainova, S. Janietz, U. Asawapirom, L. Romaner, E. Zojer, N. Koch,
A. Vollmer, Chem. Mater. 19 (2007) 1472.
[22] X. Zhan, S. Barlow, S.R. Marder, Chem. Commun. 15 (2009) 1948.
[23] B. Lim, Y.-C. Nah, J.-T. Hwang, J. Ghim, D. Vak, J.-M. Yun, D.-Y. Kim, J.
Mater. Chem. 19 (2009) 2380.
[33] L.J. Zhang, C. He, J.W. Chen, P. Yuan, L. Huang, C. Zhang, W.Z. Cai, Z.T.
Liu, Y. Cao, Macromolecules 43 (2010) 9771.
[34] R.J. Kline, M.D. McGehee, E.N. Kadnikova, J. Liu, M.J. Fréchet, Adv.
Mater. 15 (2003) 1519.
[35] A. Zen, J. Pflaum, S. Hirschmann, W. Zhuang, F. Jaiser, U. Asawapirom,
J.P. Rabe, U. Scherf, D. Neher, Adv. Funct. Mater. 14 (2004) 757.
[36] R. Zhang, B. Li, M.C. Iovu, M. Jeffries-El, G. Sauvé, J. Cooper, J.S. Jia, S.
Tristram-Nagle, D.N. Smilgies, R.D. Lambeth, T. Kowalewski, J. Am.
Chem. Soc. 128 (2006) 3480.
[37] P. Shen, G. Sang, J. Lu, B. Zhao, M. Wan, Y. Zou, Y. Li, S.T. Tan,
Macromolecules 41 (2008) 5716.
[38] S.C. Price, A.C. Stuart, L. Yang, H. Zhou, W. You, J. Am. Chem. Soc. 133
(2011) 4625.
[24] M. Manceau, E. Bundgaard, J.E. Carlé, O. Hagemann, M. Helgesen, R.
Søndergaard, M. Jørgensen, F.C. Krebs, J. Mater. Chem. 21 (2011)
4132.
[25] Y. Liang, L. Yu, Acc. Chem. Res. 43 (2010) 1227.
[26] H. Zhou, L. Yang, A.C. Stuart, S.C. Price, S. Liu, W. You, Angew. Chem.
Int. Ed. 50 (2011) 2995.
[39] B.C. Thompson, Y.G. Kim, J.R. Reynolds, Macromolecules 38 (2005)
5359.
[40] Y. Li, S.P. Singh, P. Sonar, Adv. Mater. 22 (2010) 4862.