Enhancement of photovoltaic performance by increasing conjugation of the acceptor unit in benzodithiophene and quinoxaline copolymers

Article abstract of DOI:10.1039/c4tc01440e

Copolymers based on benzodithiophene and quinoxaline, represented by 4,8-bis(5-(3,4,5-tris(octyloxy)phenyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (TOBDT) and 2,3-diphenyl-5,8-di(thiophen-2-yl)quinoxaline (TQ1) or 10,13-bis(4-(2-ethylhexyl)thiophen-2

Full text of DOI:10.1039/c4tc01440e

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PAPER
Enhancement of photovoltaic performance by
increasing conjugation of the acceptor unit in
benzodithiophene and quinoxaline copolymers†
Cite this: DOI: 10.1039/c4tc01440e
Tong Hu,‡ab Liangliang Han,‡b Manjun Xiao,b Xichang Bao,b Ting Wang,b
a
b
*
*
Mingliang Sun and Renqiang Yang
Copolymers based on benzodithiophene and quinoxaline, represented by 4,8-bis(5-(3,4,5-tris(octyloxy)-
phenyl)thiophen-2-yl)benzo[1,2-b:4,5-b0]dithiophene (TOBDT) and 2,3-diphenyl-5,8-di(thiophen-2-yl)-
quinoxaline (TQ1) or 10,13-bis(4-(2-ethylhexyl)thiophen-2-yl)dibenzo[a,c]phenazine (TQ2), were
synthesized via a Stille coupling reaction. By increasing the conjugation in the TQ2 unit, the polymer
based on TQ2 showed a narrower band gap (Eg), a lower highest occupied molecular orbital energy level
and enhanced interchain p–p interactions. Polymer solar cells based on TQ2 showed a simultaneous
enhancement of the open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF)
compared with polymer solar cells based on the TQ1 polymer. A good power conversion efficiency of
4.24% was achieved by solar cells based on the TQ2 polymer and [6,6]-phenyl-C71-butyric acid methyl
ester composite. These preliminary results indicate that increasing the acceptor unit (quinoxaline)
conjugation is an effective way of improving the performance of polymer solar cells.
Received 4th July 2014
Accepted 31st July 2014
DOI: 10.1039/c4tc01440e
www.rsc.org/MaterialsC
promising donor materials as a result of its easy synthesis, high
hole mobility and good solubility. The power conversion effi-
Introduction
ciency (PCE) of PSCs based on P3HT can reach 7%, with an
external quantum efficiency (EQE) of about 70%.5,9–12 However,
the large Eg value for P3HT and relatively high energy of the
highest occupied molecular orbital (HOMO) (about ꢀ5.0 eV)
lead to narrow absorption spectra and a low value for Voc.
Alternating donor–acceptor (D–A) conjugated copolymers
can be used to overcome these issues because the absorption
and energy levels of the copolymers can be easily tuned by
engineering the molecular structures. Using this method, many
different D–A copolymers have been synthesized and these
materials show excellent photovoltaic properties with PCEs as
high as 8–9%.13,14 Of these, the benzo[1,2-b:4,5-b0]dithiophene
(BDT) unit has been widely used as the donor unit in D–A
copolymers and shows promising photovoltaic effects.15–20
Further research has been directed towards designing polymers
based on BDT and high PCE values have been achieved21–25 by
copolymerizing BDT with 4,7-bis(thiophen-2-yl)benzo[c][1,2,5]-
thiadiazole,26 thieno[3,4-c]pyrrole-4,6-dione,27 diketopyrrolo-
pyrrole28 and thieno[3,4-b]-thiophene.29
Much progress has recently been made in the development of
polymer solar cells (PSCs), which are considered to be prom-
ising sustainable solar energy converters as a result of their
unique advantages of low cost, light weight and potential
application in exible devices with large surface areas.1–6 It is
widely recognized that the bulk heterojunction structure, which
is formed by blending an electron-rich conjugated polymer as
the donor and an electron-decient fullerene as the acceptor, is
the most efficient conguration for PSCs.7 The best fullerene
derivative currently available for use in PSCs is [6,6]-phenyl-C61
-
butyric acid methyl ester (PC61BM) or PC71BM.1,2,4,8 Most efforts
have focused on engineering the molecular structure to tune the
band gaps (Eg) and energy levels of the polymer donors. High-
performance PSCs require narrow band gaps and suitable
energy levels to maximize the short-circuit current (Jsc), open-
circuit voltage (Voc) and high carrier mobility to facilitate hole
transport. Poly(3-hexylthiophene) (P3HT) is one of the most
aInstitute of Material Science and Engineering, Ocean University of China, Qingdao
266100, People's Republic of China. E-mail: mlsun@ouc.edu.cn; Fax: +86-532-
66781927; Tel: +86-532-66781690
bCAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and
Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, People's
Republic of China. E-mail: yangrq@qibebt.ac.cn; Fax: +86-532-80662778; Tel: +86-
532-80662700
In addition to these acceptors, electron-decient quinoxa-
line-based units have also been studied and widely used as
narrow Eg polymers for PSC applications30–34 because quinoxa-
line can be easily modied at the 2 and 3 positions. The qui-
noxaline
derivative
2,3-diphenyl-5,8-di(thiophen-2-yl)-
quinoxaline (TQ1), which has two separated phenyl rings, has
the advantages of easy synthesis and versatility. However, most
of the polymers based on TQ1 have a large Eg and low charge
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4tc01440e
‡ These authors contributed equally to this work.
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carrier mobility33,35 as a result of the less planar structure caused
by the two separated phenyl rings.36
We report here a more planar dibenzo[a,c]phenazine that
facilitates intermolecular packing and charge transport of the
acceptor unit. In addition, the extended p-conjugation of the
fused quinoxaline functions as a stronger electron acceptor.36 3-
(2-Ethylhexyl)thiophene was attached to both sides of dibenzo
[a,c]phenazine to build a D–A–D structure with the purpose of
obtaining a narrow Eg. Using this method, 10,13-bis[4-(2-ethyl-
hexyl)thiophen-2-yl]dibenzo[a,c]phenazine (TQ2) was synthe-
sized. TQ2 can be easily puried by column chromatography
because of the improved solubility when two 2-ethylhexyl chains
are introduced. Interestingly, when a side chain is introduced to
the 4-position of thiophene, there is almost no inuence on Eg
and sometimes Voc is improved. 4,8-Bis(2-(3,4,5-tris(octyloxy)-
phenyl)thiophene-2-yl)benzo[1,2-b:4,5-b0]dithiophene (TOBDT)
with six alkoxylated substituents has recently been designed by
our group and this has improved the processing of the polymers
due to its excellent solubility.37 Therefore the two polymers
PTOBDTTQ1 and PTOBDTTQ2 were synthesized by copoly-
merizing TOBDT with TQ1 or TQ2, respectively.
Scheme
copolymers.
2
Synthetic routes used to obtain the monomers and
o-dichlorobenzene at room temperature. The molecular weight
of the polymers and their polydispersity index (PDI) were
measured by gel permeation chromatography (GPC) using
monodispersed polystyrene as the standard and THF as the
eluent (Table 1). The number average molecular weight (Mn)
and PDI of both polymers are 20 kDa, 1.08 and 20 kDa, 1.09,
respectively. This may prove that the introduction of 2-ethyl-
hexyl as a side chain to the 4-position of thiophene is helpful
when the monomer has solubility problems. The decomposi-
tion temperatures (Td) of the polymers are 356 and 379 ꢁC,
respectively, indicating sufficient thermal stability for applica-
tion in PSCs.
Results and discussion
Synthesis and characterization
Scheme 1 shows the molecular structures of PTOBDTTQ1 and
PTOBDTTQ2. Scheme 2 shows the detailed synthetic routes
used to obtain the TQ1 and TQ2 monomers.
Considering the difficulty of purifying the target monomer
TQ2, which consists of a more planar structure, 3-(2-ethylhexyl)
thiophene was attached to both sides of monomer 2 via a Stille
coupling reaction to increase the solubility of the TQ2 mono-
mer. This may be benecial for the molecular weight, which
could affect the PCE of the device. The target monomers
DBrTQ1 and DBrTQ2 were obtained aer the bromination of
the monomers TQ1 and TQ2, respectively, with N-bromosucci-
nimide (NBS) in chloroform/acetic acid solution. The Stille
coupling polymerization between the monomer TOBDTSn and
DBrTQ1 or DBrTQ2 with a tris(dibenzylideneacetone)dipalla-
dium [Pd2(dba)3/P(o-tol)3] catalyst in toluene gave the polymers
PTOBDTTQ1 and PTOBDTTQ2. The nal polymers were puri-
ed using Soxhlet extraction and silica gel column chromatog-
raphy and showed good solubility in organic solvents such as
tetrahydrofuran (THF), chloroform, chlorobenzene and
Optical properties
Fig. 1 shows the UV-visible absorption spectra of the polymers
in chloroform solution and in thin lms. Both of the polymers
showed two absorption peaks in chloroform solution, which is a
common feature of D–A polymers and is caused by the strong
intramolecular charge transfer (CT) state. The absorption peak
red-shied for both the polymers in their thin lm forms. The
absorption around the CT band became stronger and wider,
which is ascribed to intermolecular CT between the molecular
backbones. PTOBDTTQ2 has a lower energy absorption than
PTOBDTTQ1 in both solution and as a thin lm. The maximum
CT absorption (lmax) of PTOBDTTQ2 as a thin lm was about
696 nm, i.e. bathochromically shied about 59 nm from that of
Table 1 Molecular weight and thermal data of the polymers
Polymer
Mwa (kDa)
Mna (kDa)
PDIa
Td (ꢁC)
PTOBDTTQ1
PTOBDTTQ2
21.6
21.8
20
20
1.08
1.09
356
379
a
Scheme 1 Polymer structures of PTOBDTTQ1 and PTOBDTTQ2 (EH
¼ 2-ethylhexyl).
The Mw, Mn and PDI of the polymers were estimated by GPC using
polystyrene as the standard and THF as the eluent.
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Fig. 1 UV-visible absorption spectra of the polymers in chloroform
solution and as thin films.
PTOBDTTQ1, indicating the stronger electron-withdrawing
ability of TQ2 compared with TQ1. In addition, PTOBDTTQ2
showed more structural absorption than PTOBDTTQ1 in the
thin lm form. This is because the higher planarity of TQ2
compared with TQ1 led to strong aggregation aer spin-coating.
The strong absorption around 350–500 nm of both polymers are
attributed to the p–p* transition of the conjugated backbone,
which may help to absorb more sunlight as a result of the
extended p–p conjugation of TOBDT. The absorption onsets of
PTOBDTTQ1 and PTOBDTTQ2 in the lm form are 739 and 778
nm, corresponding, respectively, to optical band gaps (Eogpt) of
1.68 and 1.59 eV. The Eogpt of PTOBDTTQ2 is lower than reported
previously.33,35,36,38 These phenomena indicate that the more
planar TQ2 unit not only effectively broadens the absorption
spectra, but also exhibits good p–p stacking in the solid state,
consistent with the design of the polymer.
Fig. 2 Cyclic voltammetry curves of the polymer films.
LUMO ¼ HOMO + Eogpt
Electrochemical properties
The high LUMO energy levels of the two polymers ensure a
sufficient driving force for CT from the polymer to the PCBM.
Moreover, the LUMO energy level of PTOBDTTQ2, which has a
more planar quinoxaline unit, is lower than that of
PTOBDTTQ1. This is consistent with a previous report36 which
suggested that the LUMO delocalization of the TQ1-based
polymer is mainly limited to the quinoxaline unit while it is
extended onto the fused ring system as a result of the enhanced
planarity of the TQ2-based polymer.
The oxidation potentials of the polymers were determined
electrochemically by cyclic voltammetry (CV) with the polymer
coated on glassy carbon electrodes (Fig. 2); the electrochemical
data are summarized in Table 2. The HOMO energy levels of
the polymers were calculated using the equation: HOMO ¼
(ferrocene)
ꢀ[Eox ꢀ E
+ 4.8] eV, where Eox is the onset oxidation
1/2
potential. Thus the HOMO energy levels of PTOBDTTQ1 and
PTOBDTTQ2 were calculated to be ꢀ5.33 and ꢀ5.42 eV,
respectively. It is widely acknowledged that the Voc of PSCs is
directly proportional to the difference between the HOMO
energy level of the polymer donor and the lowest unoccupied
Photovoltaic properties
molecular orbital (LUMO) energy level of the PCBM electron Solar cells were fabricated by blending PTOBDTTQ1 or
acceptor. Therefore it is not difficult to infer that PTOBDTTQ2 PTOBDTTQ2 with PC71BM. The photoactive layer was sand-
will have a higher Voc because of its deeper lying HOMO. In wiched between transparent ITO/PEDOT:PSS and reective Ca/
addition, the rotational angles caused by the 2-ethylhexyl side Al back electrodes with
a structure of ITO/PEDOT:PSS/
chain in TQ2 do not seem to affect the Voc and may even facil- PTOBDTTQ1 or PTOBDTTQ2:PC71BM(1 : 2 w/w)/Ca/Al. The
itate the performance of the device based on the HOMO energy active layers of the devices were spin-coated from o-dichloro-
level. The LUMO energy levels of the two polymers were calcu- benzene solutions of the polymer and PC71BM at a concentra-
lated from the HOMO energy levels and Eogpt by the equation:
tion of 30 mg mLꢀ1. PC71BM was selected as the electron
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Table 2 Optical and electrochemical properties of the polymers
lmax, lm
(nm)
lonset, lm
(nm)
Eogpt
(eV)
HOMO
(eV)
LUMO
(eV)
Polymer
PTOBDTTQ1
PTOBDTTQ2
388, 637
473, 696
739
778
1.68
1.59
ꢀ5.33
ꢀ5.42
ꢀ3.65
ꢀ3.83
acceptor because it has similar electronic properties to PC61BM,
but a considerably higher absorption coefficient in the visible
region. As the ratio of polymer:PC71BM is an important
parameter, the weight ratio between the polymer and PC71BM
was optimized to 1 : 2. Table 3 lists the photovoltaic properties
of the devices.
The devices were measured under the simulated AM 1.5G
illumination condition (100 mW cmꢀ2) aer annealing at 110
ꢁC for 10 min. Fig. 3a shows the current–voltage (J–V curves) of
the PSCs. The PTOBDTTQ1 device has a Voc of 0.76 V, a Jsc of
7.49 mA cmꢀ2 and an FF of 47%, resulting in a PCE of 2.66%.
The PTOBDTTQ2 device shows an increased PCE of 4.24% with
a Voc of 0.83 V, a Jsc of 9.40 mA cmꢀ2 and an FF of 54%. The
results are in close agreement with previous assumptions about
the PSCs. The slightly increased Voc of PTOBDTTQ2 is consis-
tent with its deeper HOMO level compared with that of
PTOBDTTQ1. This is further proof that the 2-ethylhexyl side
chain in TQ2 facilitates the performance of the device. The
increased Jsc value may contribute to the signicantly improved
PCE of PTOBDTTQ2. With respect to the Jsc value, the difference
can be inferred to be due to two reasons: rst, PTOBDTTQ2 has
a broader absorption spectrum than PTOBDTTQ1, as previously
shown in Fig. 1; and, second, PTOBDTTQ2 may have a higher
charge mobility than PTOBDTTQ1 due to the enhanced p–p
stacking in the more planar structure of TQ2.
Fig.
3
(a)
J–V
curves
of
PTOBDTTQ1:PC71BM
and
PTOBDTTQ2:PC71BM (1 : 2 w/w). (b) External quantum efficiency
(EQE) spectra of the PSCs.
To conrm this speculation, the hole mobility was measured
by the space charge limited current (SCLC) J–V curves as
obtained in the dark using hole-only devices (ITO/PEDOT:PSS/
polymer:PC71BM/Au). The SCLC behavior in the trap-free region
can be characterized using the Mott–Gurney square law.37 The
hole mobility of PTOBDTTQ2 was calculated to be 5.42 ꢂ 10ꢀ5
cm2 Vꢀ1 sꢀ1, higher than the 2.44 ꢂ 10ꢀ5 cm2 Vꢀ1 sꢀ1 of
PTOBDTTQ1 (Fig. 4). The higher hole mobility of PTOBDTTQ2
is expected to contribute in part to the higher Jsc and FF values
observed in the solar cell devices.39
To conrm the PCE, the EQE of the devices was measured
under illumination with monochromatic light. As shown in
Fig. 3b, the PTOBDTTQ1 device shows a photo response
between 300 and 750 nm, although the photo response of the
PTOBDTTQ2 device extends to around 800 nm. The device
based on PTOBDTTQ2 shows higher EQE values, along with
broader absorption spectra. This result again indicates that the
more planar TQ2 is helpful in improving the Jsc performance of
the device. Both the current density (Jsc) values, calculated by
the integration of the EQE of the devices, are close to the Jsc
values obtained from the J–V curves.
Atomic force microscopy (AFM) was used to investigate the
morphology of the polymer and PC71BM blend lms. Fig. 5
shows AFM images of PTOBDTTQ1:PC71BM (1 : 2 w/w) and
PTOBDTTQ2:PC71BM (1 : 2 w/w) lms aer annealing at 110 ꢁC
for 10 min. Both lms show a close surface roughness of 1.65
and 1.93 nm, respectively, and a similar degree of phase sepa-
ration, which relates to the charge separation. A 1 ꢂ 1 mm AFM
(Fig. S1†) image was also obtained that shows a similar height
image and RMS value to the 5 ꢂ 5 mm AFM images.
Table 3 Photovoltaic properties of the devices under the simulated
AM 1.5G illumination condition
Experimental section
Reagents and measurements
Polymer:PC71BM
(1 : 2 w/w)
Voc
(V)
Jsc
FF
(%)
PCE
(%)
(mA cmꢀ2
)
Common solvents were purchased as analytical-reagent grade
and used as received. Deuterated chloroform (CDCl3) and o-
dichlorobenzene (o-DCB) were purchased from Sigma-Aldrich.
THF and toluene were dried over sodium-benzophenone under
PTOBDTTQ1
PTOBDTTQ2
0.76
0.83
7.49
9.40
47
54
2.66
4.24
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platinum wire counter electrode and saturated calomel refer-
ence electrode; the working electrode was a glassy carbon
electrode coated with the two polymer lms. The molecular
weights of the polymers were measured by GPC using an ELEOS
system with polystyrene as the standard (room temperature)
and THF as the eluent. Thermogravimetric analysis measure-
ments were made using an SDT Q600 V20.9 Build 20 instrument
at a heating rate of 10ꢁC minꢀ1. AFM images were acquired with
an Agilent-5400 scanning probe microscope with a Nano drive
controller in the tapping mode with MikroMasch NSC-15 AFM
tips with resonant frequencies of about 300 kHz.
Both of the PSCs were fabricated with the same conventional
device
structure
(ITO/PEDOT:PSS/PTOBDTTQ1
or
PTOBDTTQ2:PC71BM(1 : 2 w/w)/Ca/Al). ITO-coated glass
substrates were sequentially cleaned ultrasonically in detergent,
water, acetone and isopropanol, followed by treatment in an
oxygen plasma (Plasma Preen II-862 Cleaner). A thin layer
(ꢃ40 nm) of PEDOT:PSS (Heraeus Clevios TM P AI 4083) was
spin-coated onto the pre-cleaned ITO-coated glass substrates
Fig. 4 Dark J–V characteristics of polymer:PC71BM with hole-only
devices. The solid lines are the best linear fit of the data points.
ꢁ
and annealed in an oven at 150 C for 20 min. The substrates
were then transferred into an argon-lled glove-box.
PTOBDTTQ1 or PTOBDTTQ2 and PC71BM (American Dye
Source) were dissolved in o-DCB without DIO solvent. The
solution was then stirred overnight at room temperature. The
solution of polymer:PC71BM was then spin-coated to form the
active layer (ꢃ100 nm) on the PEDOT:PSS modied ITO-coated
glass. The devices were completed aer the deposition of Ca
(10 nm)/Al (100 nm) as the cathode through a shadow mask
under a high vacuum (ꢃ10ꢀ4 Pa). The thickness of the active
layer was controlled by changing the spin speed during the spin-
coating process and was estimated using a Veeco Dektak150
surface proler in our laboratory. The effective area of the device
was measured to be 0.1 cm2. The current density–voltage (J–V)
characteristics of the PSCs were recorded using a Keithley 2420
source measurement unit under AM 1.5G illumination (100 mW
cmꢀ2, Newport solar simulator). The light intensity was cali-
brated with a standard silicon solar cell. The EQE of the solar
cells was analyzed using a certied Newport incident photon
conversion efficiency (IPCE) measurement system.
Fig.
5
Atomic
force
microscopy
images:
(a
and
c)
PTOBDTTQ1:PC71BM and PTOBDTTQ2:PC71BM height image
respectively; (b and d) PTOBDTTQ1 and PTOBDTTQ2 phase image,
respectively. All images are 5 ꢂ 5 mm.
Synthesis
The monomers (4,8-bis(5-(3,4,5-tris(octyloxy)phenyl)thiophen-
2-yl)benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl)bis(trimethyl-
stannane) (TOBDTSn),37 tributyl(thiophen-2-yl)stannane,40 5,8-
dibromo-2,3-diphenylquinoxaline (1),41 tributyl(4-(2-ethylhexyl)-
thiophen-2-yl)stannane42 and 10,13-dibromodibenzo[a,c]phen-
azine (2)41 were synthesized according to reference
methods.38,43–45
Synthesis of 2,3-diphenyl-5,8-di(thiophen-2-yl)quinoxaline
(TQ1). Pd(PPh3)2Cl2 (0.05 g, 0.075 mmol) was added to a solu-
tion of tributyl(thiophen-2-yl)stannane (1.68 g, 4.5 mmol) and
5,8-dibromo-2,3-diphenylquinoxaline (1) (0.66 g, 1.5 mmol) in
dry toluene (20 mL) under an argon atmosphere. The mixture
was then reuxed overnight. Aer cooling to room temperature
the solvent was removed under reduced pressure and the
residue was puried by silica gel column chromatography
nitrogen and were freshly distilled before use. Bis-
(triphenylphosphine)palladium(II)chloride [Pd(PPh3)2Cl2], tris-
(dibenzylideneacetone)dipalladium [Pd2(dba)3], tri(o-tolyl)-
phosphine
[P(o-tol)3]
and
tetrabutylammoniumhexa-
uorophospate (Bu4NPF6) were from Aladdin or Sigma-Aldrich.
Nuclear magnetic resonance (NMR) spectra were measured in
CDCl3 on a Bruker AVANCE 600 MHz Fourier transform NMR
spectrometer; the chemical shis were quoted relative to the
internal standard tetramethylsilane. UV-visible absorption
spectra were measured using a Hitachi U-4100 UV-visible-NIR
scanning spectrophotometer. CV measurements were made on
a CHI660D electrochemical work station. The CV was carried
out in a 0.1 M anhydrous acetonitrile solution of Bu4NPF6 with a
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(eluent petroleum ether–CH2Cl2, 3 : 1 v/v) to yield an yellow mL of toluene in a ask protected by argon. The mixture was
1
ꢁ
solid (yield 85%, 0.57 g). H-NMR (600 MHz, CDCl3): d (ppm) then heated at 110 C for 24 h. Aer cooling to room tempera-
8.15 (s, 2H), 7.89–7.86 (m, 2H), 7.74 (dd, J ¼ 7.6, 1.7 Hz, 4H), ture, the mixture was poured into methanol. The precipitated
7.52 (dd, J ¼ 5.0, 0.6 Hz, 2H), 7.39 (t, J ¼ 6.2 Hz, 6H), 7.18 (dd, J ¼ solid was collected and puried by Soxhlet extraction and silica
5.0, 3.8 Hz, 2H).
gel column chromatography with chloroform as the eluent.
Synthesis of 10,13-bis(4-(2-ethylhexyl)thiophen-2-yl)dibenzo Aer drying overnight under vacuum, the target polymer was
[a,c]phenazine (TQ2). Pd(PPh3)2Cl2 (0.05 g, 0.075 mmol) was obtained as a dark solid, yield 80%, 276 mg. Mn ¼ 20 kDa, PDI ¼
1
added to a solution of tributyl(4-(2-ethylhexyl)thiophen-2-yl) 1.08. H-NMR (600 MHz, CDCl3): d (ppm) 8.65–5.84 (br, 26H),
stannane (2.18 g, 4.5 mmol) and 10,13-dibromodibenzo[a,c] 4.51–3.25 (m, 12H), 2.00–0.18 (m, 90H).
phenazine (2) (0.66 g, 1.5 mmol) in dry toluene (20 mL) under an
Synthesis of polymer PTOBDTTQ2. PTOBDTTQ2 was
argon atmosphere. The mixture was reuxed overnight. Aer synthesized by the same procedure as PTOBDTTQ1. TOBDTSn
cooling to room temperature, the solvent was removed under (320.3 mg, 0.2 mmol), DBrTQ2 (168.6 mg, 0.2 mmol), Pd2(dba)3
reduced pressure and the residue was puried by silica gel (1.8 mg, 0.002 mmol) and P(o-tol)3 (3.6 mg, 0.012 mmol) were
1
column chromatography (eluent petroleum ether–CH2Cl2. 4 : 1 used. Yield 75%, 292 mg. Mn ¼ 20 kDa, PDI ¼ 1.09. H-NMR
1
v/v) to yield a red solid (yield 80%, 0.81 g). H-NMR (600 MHz, (600 MHz, CDCl3): d (ppm) 9.07–5.42 (br, 22H), 4.61–3.41 (m,
CDCl3): d (ppm) 9.50 (dd, J ¼ 7.6, 1.6 Hz, 2H), 8.50 (d, J ¼ 7.4 Hz, 16H), 2.28–0.21 (m, 120H).
2H), 8.15 (s, 2H), 7.77–7.70 (m, 6H), 7.20 (s, 2H), 2.68 (dd, J ¼
6.8, 3.3 Hz, 4H), 1.73 (s, 2H), 1.44–1.30 (m, 16H), 0.99–0.91 (m,
12H).
Conclusions
Synthesis of 5,8-bis(5-bromothiophen-2-yl)-2,3-diphenylqui- The new polymer PTOBDTTQ2, based on TQ2 and the 2D-BDT
noxaline (DBrTQ1). NBS (0.39 g, 2.2 mmol) was added slowly in unit, was designed, synthesized and characterized. Compared
the dark to a solution of compound TQ1 (0.44 g, 1 mmol) in with TQ1, which has two separated phenyl rings, TQ2 has a
CHCl3 and acetic acid (12 mL, CHCl3–acetic acid, 5 : 1 v/v). The more planar structure because the two phenyl rings are con-
mixture was stirred for 1 h and then poured into water, nected by a single bond; this facilitates p–p stacking and charge
extracted with CH2Cl2 and washed with NaHCO3 saturated transport. With the help of the highly soluble TOBDT with six
solution. The organic phase was dried over anhydrous Na2SO4. alkoxylated substituents, the polymer could be processed in
Aer removing the solvent under reduced pressure, the residue solution. The PSCs fabricated from the blend of the polymer
was puried by silica gel column chromatography (eluent and PC71BM have PCEs of 2.66 and 4.24%, respectively. These
petroleum ether–CH2Cl2, 4 : 1 v/v) and then recrystallized from results show that the quinoxaline derivative TQ2 is a promising
isopropanol to yield a deep orange uffy solid (yield 80%, 0.49 acceptor unit and further research and improvement of this
g). 1H-NMR (600 MHz, CDCl3): d (ppm) 8.04 (s, 2H), 7.69 (d, J ¼ derivative are indicated.
6.8 Hz, 4H), 7.55 (d, J ¼ 4.0 Hz, 2H), 7.44–7.38 (m, 6H), 7.11 (d, J
¼ 4.0 Hz, 2H). 13C-NMR (150 MHz, CDCl3): d (ppm) 152.23,
Acknowledgements
139.60, 138.21, 136.70, 130.66, 130.50, 129.24, 129.17, 128.33,
125.80, 125.65, 117.18.
The authors are deeply grateful to the National Natural Science
Synthesis of 10,13-bis(5-bromo-4-(2-ethylhexyl)thiophen-2- Foundation of China (Projects 21274134, 51173199, 51303197
yl)dibenzo[a,c]phenazine (DBrTQ2). NBS (0.39 g, 2.2 mmol) was and 61107090), New Century Excellent Talents in University
added slowly in the dark to a solution of compound TQ2 (0.67 g, (NCET-11-0473) and the Qingdao Municipal Science and Tech-
1 mmol) in CHCl3 and acetic acid (12 mL, CHCl3–acetic acid, nology Program (13-1-4-200-jch) for nancial support.
5 : 1 v/v). The mixture was stirred for 1 h and then poured into
water, extracted with CH2Cl2 and washed with NaHCO3 satu-
rated solution. The organic phase was dried over anhydrous
Notes and references
Na2SO4. Aer removing the solvent under reduced pressure, the
residue was puried by silica gel column chromatography
(eluent petroleum ether–CH2Cl2, 5 : 1 v/v) then washed in hot
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