Synthesis and photovoltaic properties of two-dimensional copolymers based on novel benzothiadiazole and quinoxaline acceptors with conjugated dithienylbenzothiadiazole pendants

Article abstract of DOI:10.1002/pola.27888

Two novel acceptors of benzo[c][1,2,5]thiadiazole and quinoxaline with conjugated dithienylbenzothiadiazole pendants were first designed and synthesized for building efficient photovoltaic copolymers. Based on benzo[1,2-b;3,4-b′]dithiophene donors and the

Full text of DOI:10.1002/pola.27888

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Synthesis and Photovoltaic Properties of Two-Dimensional Copolymers
Based on Novel Benzothiadiazole and Quinoxaline Acceptors with
Conjugated Dithienylbenzothiadiazole Pendants
1
1
1
1
1,2
1,2
Yuanshuai Huang, Linglong Ye, Fen Wu, Suli Mei, Huajie Chen, Songting Tan
1
Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, College of Chemistry,
Xiangtan University, Xiangtan 411105, People’s Republic of China
2
Key Laboratory of Polymeric Materials and Application Technology of Hunan Province, Xiangtan University, Xiangtan 411105,
People’s Republic of China
Correspondence to: S. Tan (E-mail: tanst2008@163.com)
Received 30 May 2015; accepted 20 August 2015; published online 16 September 2015
DOI: 10.1002/pola.27888
ABSTRACT: Two novel acceptors of benzo[c][1,2,5]thiadiazole
and quinoxaline with conjugated dithienylbenzothiadiazole
pendants were first designed and synthesized for building effi-
cient photovoltaic copolymers. Based on benzo[1,2-b;3,4-
levels of highest occupied molecular orbitals. The polymer
photovoltaic devices based on benzo[c][1,2,5]thiadiazole-based
copolymer/phenyl-C71-butyric acid methyl ester exhibited
a
power conversion efficiency of 2.42%, attributed to its rela-
tively better light-harvesting ability and active film morphol-
0
b ]dithiophene donors and the two acceptors, two new copoly-
ogy.
V
2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A:
C
mers have been prepared by Stille coupling polymerization.
1
Polym. Chem. 2016, 54, 668–677
The resulting copolymers were characterized by H NMR, gel
permeation chromatography, and thermogravimetric analysis.
UV–Visible absorption and cyclic voltammetry measurements
indicated that the two copolymers possessed strong and broad
absorption in the range of 300–700 nm, and deep-lying energy
KEYWORDS: benzodithiophene; benzothiadiazole; conjugated
polymers; functionalization of polymers; polymer solar cells;
synthesis
INTRODUCTION Polymer solar cells (PSCs) is a promising
photovoltaic technology for clean and renewable energy
sources due to its superior properties such as mechanical
flexibility, light weight, visible transparency, and large-area
electron-donating building blocks in PSCs application, due to
their excellent features including structural symmetry, pla-
narity as well as rigid and p-extended conjugation, which
can enhance electron delocalization and intermolecular inter-
1
6–19
1
–7
actions to improve charge mobility and PCEs.
Moreover,
manufacturing compatibility.
Recently, remarkable pro-
benzo[c][1,2,5]thiadiazole (BT) and quinoxaline (Q) were the
most promising blocks for building high-performance poly-
meric donor materials. In most cases, a great number of con-
jugated polymers were focused on alkoxyl- and fluorine-
modified BT and Q. Many PSCs based on BT and Q units
gress has been made in this field, and the power conversion
8
–10
efficiencies (PCEs) of PSCs have surpassed 10%.
How-
ever, the design and synthesis of novel polymeric donor
materials with unique structures still plays an important
role in understanding the basic rules between molecular
structure and performance, which can make great contribu-
tions to the commercial applications of PSCs in the future.
2
0–23
showed high PCE values of over 8%.
For instance, Yan
and coworkers reported the PSCs based on BT unit exhibited
a high PCE over 10%, which is the highest PCE from single
6
In recent years, the donor–acceptor (D–A) copolymers which
consist of electron-donating and electron-withdrawing build-
ing blocks have been widely used in a number of application,
such as PSCs, organic field-effect transistors, and organic
light-emitted diodes, due to their superiorities in realizing
tunable properties like absorption spectra, molecular energy
junction-based PSCs. While the BT- and Q-based copolymers
with the two-dimensional (2-D)-conjugated electron-deficient
pendants have not yet been explored. Compared with the lin-
ear polymers, this type of polymer possesses three distinc-
tive characteristics: (1) extension of the p-conjugation; (2)
good solubility owing to the overlapping of the conjugated
side chains interactions with the main chains; and (3) the
increase of electron affinity and the red-shift of absorption.
To develop novel strategies for further improving
1
1–15
levels, mobilities, and so forth.
Among them, alternating
copolymers based on 4,8-bis(alkoxyl)-benzo[1,2-b;4,5-
0
b ]dithiophene have attracted considerable interest as
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ducted on a Perkin-Elmer Lamada 25 UV–Vis–NIR spectrom-
eter. The average molecular weight and polydispersity index
(
PDI) of the polymers were determined using Waters 1515
gel permeation chromatography (GPC) analysis with THF as
eluent and polystyrene as standard. TGA measurement was
conducted on a Netzsch TG 209 analyzer under nitrogen at a
2
1
heating rate of 20 8C min . Electrochemical redox potentials
were obtained by CV using a three-electrode configuration
and an electrochemistry workstation (ZAHNER ZENNIUM) at
2
1
a scan rate of 100 mV s . CV was conducted on an electro-
chemistry workstation with the thin film on a Pt plate as the
working electrode, Pt slice as the counter electrode, and sil-
ver electrode as the reference electrode. The supporting elec-
trolyte is 0.1 M tetra-n-butylammonium hexafluorophosphate
FIGURE 1 Structures of the copolymers PBDT–BT and PBDT–Q.
(
4 6
Bu NPF ) in anhydrous acetonitrile solution. All AFM meas-
photovoltaic performance, the effect of different acceptor
units on PSCs performance is worthy of being studied.
urements were performed on a Digital Instruments Veeco
Multimode 8 in a tapping mode.
Herein, we report the design and synthesis of two novel 2-D
copolymers, PBDT–BT and PBDT–Q (Fig. 1), with novel ben-
zo[c][1,2,5]thiadiazole (BT) and quinoxaline (Q) acceptor
units modified by conjugated dithienylbenzothiadiazole
Fabrication of PSCs
The structure of bulk heterojunction (BHJ) PSCs was indium
tin oxide (ITO)/MoO (20 nm)/polymers:phenyl-C (or C )-
3
61
71
(
DTBT) pendants via vinylene groups, which were used as
butyric acid methyl ester (PC BM or PC BM) (ꢀ100 nm)/
6
1
71
the donor materials in PSCs for the first time. Moreover, the
comparative studies would be carried out to study the effect
of different acceptor on their thermal stabilities, optical
properties, molecular energy levels, and film morphologies of
the two polymers by thermogravimetric analysis (TGA), UV–
Visible (UV–Vis) spectroscopy, cyclic voltammetry (CV), and
atomic force microscope (AFM), respectively. The results
indicated that, the two copolymers exhibited high thermal
stability, strong and broad absorption, and deep-lying energy
levels of highest occupied molecular orbitals (HOMO). The
polymer photovoltaic devices based on PBDT–BT/phenyl-
C -butyric acid methyl ester (PC BM) exhibited a PCE of
LiF (0.5 nm)/Al (100 nm). The PSCs devices were fabricated
with ITO glass as a positive electrode and LiF/Al as a nega-
tive electrode. The ITO glass was precleaned and then modi-
fied by a thin layer of MoO , which was deposited on the
3
24
surface of ITO by vacuum evaporation under 5 3 10 Pa.
The photoactive layer was prepared by spin-coating a blend
solution of polymer and PC BM (or PC BM) in chloroben-
6
1
71
zene (CB) on the surface of ITO/MoO3 substrate. Then, the
LiF/Al cathode was deposited on the polymer layer by vac-
2
4
uum evaporation under 5 3 10 Pa. The accurate area of
2
every device is 3.8 mm , defined by the overlap of the ITO
7
1
71
and metal electrode. The current density–voltage (J–V)
2
.42%, attributed to the better light-harvesting ability and
curves were measured by a Keithley 2602 Source Meter
2
2
active film morphology.
under 100 mW cm standard AM 1.5 G spectrum using a
Sol 3A Oriel solar simulator. The incident light intensity was
calibrated using a standard Si solar cell. The measurement of
monochromatic incident photon-to-current conversion effi-
ciencies (IPCE) was performed using a Zolix Solar Cell Scan
EXPERIMENTAL
Materials
All the chemicals were purchased from Alfa Aesar and Chem
Greatwall Chemical Company (Wuhan, China). Tetrahydrofu-
ran (THF) and toluene were dried and distilled from
sodium/benzophenone. All other solvents and chemicals
used in this work were analytical grade and used without
further purification. 2,6-Di(trimethyltin)-4,8-di((2-ethylhexy-
l)oxy) benzo[1,2-b;4,5-b ]dithiophene (M3)
hexylthhiophen-2-yl)benzo[c][1,2,5] thiadiazole-4-yl)-3-hex-
ylthiophene-2-carbaldehyde were prepared according to lit-
erature procedures, respectively.
100 QE/IPCE measurement system.
Synthesis
-Methyl-benzo[C][1,2,5]thiadiazole (1)
5
Triethylamine (14.00 mL, 101.00 mmol) was added to a
solution of 3,4-diaminotoluene (5.00 g, 40.31 mmol) and
0
24
and 5-(7-(4-
CHCl (150 mL). Then thionyl chloride (7.51 mL, 103 mmol)
3
2
4
was added with vigorous stirring in an ice–water bath. After
the dropwise addition, the solution was refluxed for 8 h.
Evaporation of the solvent and purification by column chro-
matography on silica gel with petroleum ether/dichlorome-
thane (6:1, v/v) as eluent was performed. Compound 1 was
Instrumentation
Nuclear magnetic resonance spectra were measured with
Bruker AVANCE 400 MHz spectrometer. Molecular mass of
the compounds was determined by Agilent 5975 GC-MS
instrument or Bruker Autoflex III MALDI-TOF measurement.
UV–Visible absorption spectra of the polymers were con-
1
finally obtained as a light yellow oil (4.50 g, 73.2% yield). H
NMR (400 MHz, CDCl , d, ppm): 7.89–7.87 (d, J 5 8.8 Hz,
3
1H), 7.76 (s, 1H), 7.44–7.42 (d, J 5 8.8 Hz, 1H), 2.55 (s, 3H);
GC-MS (C H N S) m/z: calcd. for 150.0; found, 150.1.
7
6 2
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1
4
,7-Dibromo-5-methyl-benzo[C][1,2,5]thiadiazole (2)
yield). H NMR (400 MHz, CDCl , d, ppm): 8.13 (s, 1H),
3
A solution of compound 1 (2.00 g, 13.31 mmol) in 47% HBr
8.01–8.00 (d, J 5 6.8 Hz, 2H), 7.88–7.82 (m, 2H), 7.45 (s,
2H), 7.06 (s, 1H), 2.85–2.82 (t, J 5 7.6 Hz, 2H), 2.72–2.68 (t,
(
(
10 mL) was heated to 120 8C, and then bromine (Br2)
15 mL) in HBr (3 mL) was slowly added dropwise. The mix-
1
3
J 5 7.6 Hz, 2H), 1.76–1.35 (m, 16H), 0.94–0.91 (m, 6H);
C
ture was stirred for 8 h and then cooled to room tempera-
ture. A saturated sodium bisulfite aqueous solution was then
added to remove the excess bromine. The precipitate was fil-
tered and purified by column chromatography on silica gel
with petroleum ether/dichloromethane (8:1, v/v) as eluent
NMR (400 MHz, CDCl , d, ppm): 154.04, 152.58, 152.49,
3
151.90, 145.32, 144.47, 139.15, 138.84, 138.32, 136.66,
130.58, 130.10, 129.34, 126.68, 125.88, 125.78, 125.24,
124.97, 124.00, 121.95, 113.49, 113.04, 31.74, 31.72, 31.00,
30.67, 30.47, 29.70, 29.17, 29.07, 28.85, 22.66, 22.64, 14.12,
14.09; MALDI-TOF MS (C H Br N S ) m/z: calcd. for
to give compound 2 as a white solid (3.11 g, 75.6% yield).
3
4
34
2 4 4
1
H NMR (400 MHz, CDCl
3
, d, ppm): 7.78 (s, 1H), 2.62 (s,
786.001; found, 786.068.
3
H); MALDI-TOF MS (C H Br N S) m/z: calcd. for 307.844;
7
4
2 2
1
-Bromo-2,3-butanedione (5)
A solution of 2,3-butanedione (2.01 g, 23.23 mmol) in CHCl3
(8 mL) was cooled to 0 8C, and then Br (3.97 g, 24.85
found, 307.861.
4
,7-Dibromo-5-bromomethyl-benzo[C][1,2,5]thiadiazole (3)
2
A mixture of compound 2 (1.51 g, 4.87 mmol), benzoyl per-
oxide (BPO) (0.10 g), and N-bromosuccinimide (NBS)
mmol) in CHCl3 (7 mL) was slowly added dropwise. The
mixture was stirred for 3 h. A saturated sodium bisulfite
aqueous solution was then added to remove the excess bro-
mine. The reaction mixture was extracted with dichlorome-
thane and washed with water. The organic phase was dried
(1.04 g, 5.84 mmol) in tetra-chloromethane (50 mL) was
heated at reflux for 24 h under an argon atmosphere. The
solution was filtered and the filtrate concentrated by rotary
evaporation. The residues were purified by column chroma-
tography on silica gel with petroleum ether/dichloromethane
over anhydrous MgSO , and then the solvent was removed
4
by rotary evaporation to give compound 5 as a light yellow
solid (2.01 g, 52.2% yield). GC-MS (C H BrO ) m/z: calcd. for
(
(
8:1, v/v) as eluent to give compound 3 as a white solid
4
5
2
1
1.20 g, 63.7% yield). H NMR (400 MHz, CDCl
3
, d, ppm):
163.9; found, 163.9.
8
.42 (s, 1H), 7.27 (s, 2H); MALDI-TOF MS (C H Br N S) m/z:
7
3
3 2
5,8-Dibromo-2-bromomethyl-3-methyl-quinoxaline (6)
calcd. for 385.755; found, 385.870.
To a suspension of zinc (6.72 g, 102.80 mmol) and 2,7-
dibromo-benzo[c][1,2,5]thiadiazole (1.51 g, 5.14 mmol) in
acetic acid (70 mL), a few drops of water was added. The
mixture was stirred at 60 8C for 6 h and the solid residue
was removed by filtration. To the filtrate, compound 5
(1.02 g, 6.22 mmol) was added and the resulting solution
was stirred at 60 8C overnight under an argon atmosphere.
(
4,7-Dibromo-benzo[C][1,2,5]thiadiazole-5-
ylmethyl)Phosphonic Acid Diethyl Ester (4)
A solution of compound 3 (1.20 g, 3.10 mmol) in triethyl
phosphite (30 mL) was heated at reflux for 24 h under an
argon atmosphere. Excess triethyl phosphite was removed by
vacuum distillation. The residue was purified by column
chromatography on silica gel with petroleum ether/acetic
The mixture was transferred to
a separatory funnel,
ether (2:1, v/v) as eluent to give compound 4 as a white
extracted with dichloromethane, and then washed with
water. The dichloromethane phase was dried over MgSO4.
Removal of the solvent gave a crude product which was
purified by column chromatography on silica gel with petro-
leum ether/dichloromethane (3:1, v/v) as eluent to give
1
solid (0.71 g, 51.2% yield). H NMR (400 MHz, CDCl , d,
3
ppm): 7.96 (s, 1H), 4.17–4.10 (m, 4H), 3.60–3.54 (d, J 5 22.4
Hz, 2H), 1.33–1.29 (t, J 5 7.0 Hz, 6H); MALDI-TOF MS
(
C H Br N O PS) m/z: calcd. for 443.873; found, 443.961.
11 13 2 2 3
1
compound 6 as a white solid (0.80 g, 39.4% yield). H NMR
4
-(4-Hexyl-2-thienyl)-7-[2-(4,7-dibromo-5-
(
400 MHz, CDCl , d, ppm): 7.92–7.87 (m, 2H), 4.82 (s, 2H),
3
benzo[C][1,2,5]thiadiazole-vinyl)-4-hexyl-2-thienyl]-
benzo[C][1,2,5]thiadiazole (M1)
13
2
1
3
3
.97 (s, 3H); C NMR (400 MHz, CDCl , d, ppm): 155.09,
3
52.41, 140.41, 139.35, 133.68, 132.90, 123.63, 122.99,
5-(7-(4-Hexylthhiophen-2-yl)benzo[c][1,2,5] thiadiazole-4-yl)-
0.77, 22.30; MALDI-TOF MS (C H Br N ) m/z: calcd. for
1
0
7
3 2
3-hexylthiophene-2-carbaldehyde (0.84 g, 1.70 mmol) and
93.814; found, 393.830.
compound 4 (0.62 g, 1.40 mmol) were dissolved in THF
40 mL), and the solution was stirred for 30 min under an
(
(5,8-Dibromo-3-methyl-quinoxaline-2-ylmethyl)Phosphonic
Acid Diethyl Ester (7)
argon atmosphere. Then the solution of potassium tert-but-
oxide (0.28 g, 2.50 mmol) and THF (15 mL) was added
dropwise. The reaction mixture was stirred at room temper-
ature for 1 h, and then heated to reflux for 15 h. After cool-
ing to room temperature, the reaction mixture was extracted
with dichloromethane and washed with dilute aqueous HCl
solution. The organic phase was dried over anhydrous
By following the similar method as for compound 4, com-
pound 7 was synthesized from compound 6 (0.80 g, 2.03
mmol) and triethyl phosphite (20 mL). The crude product
was purified by column chromatography on silica gel with
petroleum ether/acetic ether (6:1, v/v) as eluent to give
1
compound 7 as a light yellow solid (0.81 g, 88.4% yield). H
MgSO , and then the solvent was removed by rotary evapo-
NMR (400 MHz, CDCl , d, ppm): 8.00 (s, 2H), 4.18–4.12 (m,
4
3
ration. The crude product was purified by column chroma-
tography on silica gel with petroleum ether/dichloromethane
4H), 3.80–3.74 (d, J 5 22.8 Hz, 2H), 2.97 (s, 3H), 1.27–1.24
(m, 6H); MALDI-TOF MS (C H Br N O S) m/z: calcd. for
1
4
17
2 2 3
(
1:1, v/v) as eluent to give M1 as a red solid (0.78 g, 70.8%
451.932; found, 452.002.
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4
-(4-Hexyl-2-thienyl)-7-[2-(5,8-dibromo-3-methyl-2-
adopted for the monomer M1 involved several simple steps.
The first was ring cyclized with thionyl chloride in good
yield. Compound 2 was prepared by bromination of com-
pound 1 with Br2 and HBr (45%) according to the proce-
dure used for the synthesis of 4,7-dibromo-benzo[c]
[1,2,5]thiadiazole. Compound 3 was synthesized from com-
pound 2 by further bromination with NBS using BPO as the
radical initiator. Compound 4 was subsequently treated with
triethyl phosphite to give phosphonic acid diethyl ester 5.
Monomer M1 was synthesized by the Wittig–Horner reaction
quinoxaline-vinyl)-4-hexyl-2-thienyl]-
benzo[C][1,2,5]thiadiazole (M2)
By following the similar method as for M1, M2 was synthe-
sized from compound 7 (0.50 g, 1.11 mmol), 5-(7-(4-Hex-
ylthhiophen-2-yl)benzo[c][1,2,5]thiadiazole-4-yl)-3-hexylthiop
hene-2-carbaldehyde (0.66 g, 1.33 mmol), THF (40 mL), and
potassium tert-butoxide (0.22 g, 2.00 mmol). The crude
product was purified by column chromatography on silica
gel with petroleum ether/dichloromethane (2:1, v/v) as elu-
2
5
ent to give M2 as a purple–red solid (0.62 g, 70.3% yield).
by coupling
4
with 5-(7-(4-hexylthiophen-2-yl)benzo[c]
1
H NMR (400 MHz, CDCl , d, ppm): 8.57–8.53 (d, J 5 14.8 Hz,
[1,2,5]thiadiazol-4-yl)23-hexylthiophe-2-carbaldehyde. Com-
pound 6 was synthesized by reduction reaction and ring
cyclized from 4,7-dibromo-benzo[c][1,2,5]thiadiazole. The
synthesis of monomer M2 involved two steps from com-
pound 6 according to the procedure used for the synthesis
of monomer M1. Two copolymers (PBDT–BT and PBDT–Q)
were synthesized by Stille coupling polymerization using
Pd(0) as catalyst under argon atmosphere. Crude copolymers
were purified by extracting with methanol, petroleum ether
and chloroform in this order. The structures of the copoly-
3
1
7
2
1
H), 8.01–7.99 (d, J 5 6.8 Hz, 2H), 7.90–7.53 (m, 4H), 7.28–
.24 (d, J 5 14.8 Hz, 1H), 7.07 (s, 1H), 2.97 (s, 3H), 2.93–
.90 (t, J 5 7.6 Hz, 2H), 2.72–2.68 (t, J 5 7.6 Hz, 2H), 1.80–
1
3
.68 (m, 4H), 1.49–0.92 (m, 16H), 0.92–0.90 (m, 6H);
C
NMR (400 MHz, CDCl , d, ppm): 154.20, 152.52, 150.77, 147,
3
28, 144.48, 139.52, 138.83, 136.97, 132.38, 131.87, 130.45,
129.34, 126.68, 125.88, 125.22, 123.51, 122.76, 122.00,
119.47, 31.74, 31.37, 30.67, 30.51, 29.28, 29.10, 28.96,
22.85, 22.74, 22.68, 14.18, 14.15; MALDI-TOF MS
1
(
C H Br N S ) m/z: calcd. for 794.060; found, 794.057.
mers were confirmed by H NMR.
3
7
38
2 4 3
Polymerization for PBDT–BT
,6-Bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b;3,4-
The copolymers were found to be highly soluble in most of
organic solvents such as chloroform, toluene, chlorobenzene,
and o-dichlorobenzene at room temperature. The number-
2
0
b ]dithiophene (102.2 mg, 0.130 mmol) (M3) and M1
100.1 mg, 0.130 mmol) were dissolved in toluene (10 mL).
(
n
average molecular weight (M ) and PDI are 13.6 KDa and
The solution was flushed with argon for 30 min and
Pd(PPh ) (15 mg, 0.01 mmol) was added to the flask. The
1.85 for PBDT–BT, 18.2 KDa and 1.99 for PBDT–Q (Table 1),
determined by GPC with THF as eluent.
3
4
flask was purged five times by successive vacuum and argon
filling cycles. The polymerization reaction mixture was
heated at 110 8C with stirring for 48 h under an argon
atmosphere. It was then cooled to room temperature and
then slowly poured into methanol (200 mL). The precipitate
was filtered and washed with methanol and hexane in a
Soxhlet apparatus to remove oligomers and catalyst residue.
Thermal Properties
The copolymers were analyzed by TGA and the results were
summarized in Table 1. The polymers revealed high de-
composition temperatures (T
d
, 5 wt % loss) of 370 8C and
359 8C for PBDT–BT and PBDT–Q, respectively (Fig. 2).
These results indicated that the copolymers possessed a
good thermal and morphology stability, matching well with
the requirements of device fabrications.
Finally, the polymer was extracted with CHCl . The solution
3
was condensed by evaporation and precipitated into metha-
nol. The polymer PBDT–BT was collected as purple–red
Optical Properties
Figure 3 shows the UV–Vis absorption spectra of the copoly-
mers in dilute chloroform solutions (0.01 mg mL ) (a) and
in solid films (b), respectively. In chloroform solutions,
PBDT–Q showed one broad absorption band covering the
wavelength range from 300 to 700 nm, and the maximum
1
solid (87.1 mg, 62.8% yield). H NMR (400 MHz, CDCl , d,
3
ppm): 8.96–7.00 (br, 10H), 4.43–3.22 (br, 4H), 2.90–2.65 (br,
21
6
H), 2.04–0.87 (br, 50H); GPC: M 5 13.6 KDa, M 5 25.3
n w
KDa.
Polymerization for PBDT–Q
The polymerization process was the same as that for PBDT–
BT, except that M1 was used instead of M2 (103.0 mg,
4
absorption wavelength (ks,max) was at 517 nm [4.7 3 10
2
1
21
(
g/mL)
cm ], which was attributed to intramolecular
charge transfer (ICT) interaction. Note that there was a
shoulder peak located around at 420 nm, which should be
ascribed to the localized p–p* transition of the polymer back-
0
.130 mmol) and the polymer PBDT–Q was collected as pur-
1
ple–red solid (79.2 mg, 55.8% yield). H NMR (400 MHz,
CDCl , d, ppm): 8.73–6.98 (br, 11H), 4.39–4.12 (br, 4H),
.08–2.67 (br, 9H), 2.24–0. 56 (br, 50H); GPC: M 5 18.2
3
2
6
bone. While PBDT–BT had two obvious absorption peaks
3
n
4
21
21
at 381 and 521 nm [5.6 3 10 (g/mL) cm ]. Moreover,
PBDT–BT exhibited a slightly broader absorption which was
derived from the stronger ICT compared to PBDT–Q. In com-
parison with the absorptions in solutions, similar profile and
red-shift in the solid films were observed, probably due to
the increase of intermolecular p–p stacking in the solid state.
In addition, compared with the absorption in solutions, the
KDa, M 5 36.2 KDa.
w
RESULTS AND DISCUSSION
Syntheses and Characterization
The synthetic routes of monomers M1, M2, and target
copolymers were outlined in Scheme 1. The synthetic route
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SCHEME 1 Synthetic routes of monomers and copolymers.
maximum absorption peaks of PBDT–BT and PBDT–Q were
red-shifted 23 and 38 nm, respectively. The absorption of
PBDT–Q was red-shifted more obviously than PBDT–BT,
onset wavelengths of the film absorption spectra, which
were 1.71 and 1.66 eV, respectively (Table 2).
Electrochemical Properties
probably due to its better planarity. The optical band gaps
opt
The electrochemical properties of the copolymers were stud-
ied by CV (Fig. 4 and Table 2). The potentials were refer-
(E
) for PBDT–BT and PBDT–Q were calculated from the
g
1
enced to the ferrocene/ferrocenium redox couple (Fc/Fc ).
1
The redox potential of Fc/Fc was assumed an absolute
energy level of 24.80 eV relative to vacuum, which was
measured under the same condition as polymer sample to
be 0.38 V related to the silver electrode. The corresponding
HOMO and LUMO energy levels of polymers were calculated
according to the equations: EHOMO 5 –e (Eox 1 4.42) (eV) and
TABLE 1 Molecular weights and thermal properties of the
copolymers
21 a
)
21 a
)
b
Polymers
M
n
(kgꢁmol
M
w
(kgꢁmol
PDI
d
T (8C)
PBDT–BT
PBDT–Q
13.6
18.2
25.3
36.2
1.85
1.99
370
359
ELUMO 5 –e (Ered 1 4.42) (eV). The HOMO levels of PBDT–BT
and PBDT–Q were 25.51 and 25.48 eV, respectively. The
a
deep and similar HOMO levels were mainly affected by the
donor unit of polymers and DTBT pendants, which were
beneficial for a high Voc in PSCs and good air stability. The
Determined by GPC in THF based on polystyrene standards.
2
7
b
Decomposition temperature, determined by TGA in nitrogen, based
on 5 wt % loss.
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the films of copolymers/PCBM (1:2, w/w) blends recorded
in a tapping mode. All the films exhibited typical cluster
structures with many aggregated domains, and root-mean-
square (RMS) roughness of 0.45, 0.73, and 1.12 nm were
obtained for PBDT–BT/PC61BM, PBDT–BT/PC71BM and
PBDT–Q/PC BM, respectively. The film surface with higher
6
1
roughness and nanoscaled texture also benefited the internal
3
0
light scattering and enhanced light absorption. Further-
more, a suitable phase separation was observed in Figure
5(f). Hence, it can be tentatively speculated that the more
proper phase separation for both exciton dissociation and
charge transporting in PBDT–BT/PC BM blend than those
7
1
of the other two blend systems contributed to its relatively
better photovoltaic performance.
Hole Mobility
FIGURE 2 TGA curves of the copolymers with the scan rate of
21
The charge carrier transport properties of conjugated poly-
20 8C min . [Color figure can be viewed in the online issue,
3
1
mers play a key role in the performance of PSCs. To under-
stand the influence of charge carrier mobility of the
copolymers/PC BM blend films on the photovoltaic proper-
ties, the hole mobility of the target copolymer was measured
using a space charge limit current method. Hole-only devices
were fabricated with the configuration of ITO/PEDOT:PSS
which is available at wileyonlinelibrary.com.]
6
1
LUMO levels of PBDT–BT and PBDT–Q were 23.40 and
2
3.43 eV, respectively. Note that their LUMO values were
0.3 eV higher than the LUMO energy level of PCBM
PC BM and PC BM), which suggested that the energy off-
>
(
6
1
71
(
(
30 nm)/polymers:PC BM (1:2, w/w)/MoO (20 nm)/Al
61 3
100 nm). The calculated hole mobilities of PBDT–BT and
sets between the LUMO energy levels of the polymer and
PCBM were sufficient to facilitate efficient charge transfer/
separation at the interfaces of copolymer donors and PCBM
acceptors. The electrochemical band gaps of the copolymers
were estimated to be 2.11 eV for PBDT–BT and 2.05 eV for
PBDT–Q, which were larger than their optical band gaps.
25
26
2
PBDT–Q blend films were 1.2 3 10 and 3.5 3 10 cm
21 21
V
s
, respectively (Table 3). It is worth noting that these
polymers may have good isotropic charge transport ability
due to their 2-D conjugated structures, which is desirable for
5
PSCs. The relatively higher mobility of PBDT–BT was
enhanced mainly due to the improved intermolecular inter-
actions and ordered alignment of the blend film, which
would benefit for the exciton separation and the transport.
Therefore, the device based on PBDT–BT would show rela-
tively higher Jsc and FF compared to the other device.
Film Morphology
The surface morphology of the active layer was one of the
2
8
key factors in determining the performance of PSCs. Ideal
domain size of 10–20 nm of polymer and PCBM with an
interpenetrating bicontinuous network has been found to be
important for achieving high-performance devices. How-
ever, both smaller (<10–20 nm) and larger (>10–20 nm)
domain sizes of the blend films will limit charge transfer and
separation. Figure 5 shows the AFM topographic images of
2
9
Photovoltaic Properties
To investigate whether the different acceptors of the two
copolymers make a contribution to the photoelectric conver-
sion in the PSCs, BHJ solar cells using the copolymers as the
21
FIGURE 3 UV–Vis absorption spectra of the copolymers (a) in chloroform solution (0.01 mg mL ) and (b) in solid films. [Color fig-
ure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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TABLE 2 Photophysical and electrochemical properties of the polymers
Film
Solution
4
21
21 a
)
b
b
opt
g
c
d
d
ec
e
e
Polymers
kmax (nm) (10 (g/mL) cm
kmax (nm)
k
edge (nm)
E
(eV)
HOMO (eV)
LUMO (eV)
E
(eV)
PBDT–BT
PBDT–Q
381 (5.2), 521 (5.9)
420 (3.2), 517 (4.7)
544
555
724
745
1.71
1.66
25.51
25.48
23.40
23.43
2.11
2.05
a
d
e
Dilute chloroform solution.
Determined by CV.
b
c
ec
Thin film spin-cast from chloroform solution.
Optical band gap determined from the onset of absorption in the solid
E
5 2 (EHOMO 2 ELUMO) (eV).
e
state.
FIGURE 4 (a) Cyclic voltammograms of the copolymers and (b) Energy levels of the system components. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
FIGURE 5 AFM height images and phase images of the polymers/PCBM blend films (1:2, w/w). (a) and (d): PBDT–BT/PC61BM, (b)
and (e): PBDT–BT/PC71BM, (c) and (f): PBDT–Q/PC61BM. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
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TABLE 3 Photovoltaic properties and hole mobilities of the two copolymers
22
2
21 21
s
Polymers
PCBM
Polymers/PCBM
J
sc (mA cm
)
V
oc (V)
FF
PCE (%)
l
h
(cm V
)
2
5
PBDT–BT
PC61BM
PC61BM
PC71BM
PC61BM
PC61BM
PC61BM
PC61BM
PC71BM
PC61BM
PC61BM
1:1
1:2
1:2
1:3
1:4
1:1
1:2
1:2
1:3
1:4
4.31
5.36
6.65
5.05
3.95
3.38
4.40
4.71
2.89
2.58
0.88
0.88
0.88
0.87
0.87
0.57
0.60
0.60
0.55
0.55
0.32
0.36
0.42
0.31
0.29
0.30
0.30
0.41
0.29
0.27
1.21
1.70
2.42
1.36
1.00
0.57
0.80
1.16
0.46
0.38
1.2 3 10
26
PBDT–Q
3.5 3 10
donors and PCBM as the acceptor were investigated with the
conventional device configuration. The weight ratio of the
polymers/PC BM was optimized from 1:1 to 1:4 and
PC71BM has commonly led to performance enhancements in
BHJ donor–acceptor devices using semiconducting polymers
as the electron-donating component due to its stronger visi-
6
1
3
2
the optimized weight ratio for the two polymers to PC BM
ble absorption. Compared with the device with PBDT–BT/
PC BM (1:2, w/w), the device with PBDT–BT/PC BM (1:2,
6
1
was 1:2 (Table 3). Figure 6(a) displays J–V curves of the
devices incorporating both Jsc and FF of the PSCs. It was
observed that an increase in PC BM content resulted in no
6
1
71
22
w/w) show the higher Jsc (5.36 mA cm ) and FF (0.42),
6
1
due to an optimized blend morphology. In addition, there
obvious change in Voc. However, the Jsc and FF decreased
continuously, when the ratio of PBDT–BT to PC BM
was no change of V , and the resulting PCE was raised to
2.42%. This trend has also been observed for PBDT–Q/
oc
6
1
changed from 1:2 to 1:4. Therefore, the optimal blend ratio
for this polymer was determined to be 1:2. It was found that
the changes of the photovoltaic properties for PBDT–Q and
optimal blend ratio was the same as PBDT–BT. When the
mixing ratio of PBDT–Q to PC BM changed from 1:1 to 1:2,
PC BM (1:2, w/w), and the PCE was raised to 1.16%. Addi-
7
1
tionally, no further improvement was observed when using
,8-diiodooctane (DIO) as an additive.
1
To better understand the difference of the photovoltaic per-
6
1
the PCE of the device increased slightly from 0.57% to
.80% (Table 3). However, when the ratio of PBDT–Q and
formance, the shunt resistances (Rsh) and series resistances
0
(R
) were calculated according to the J–V curves. Usually, a
s
PC61BM changed from 1:2 to 1:4, the Jsc and FF decreased
slightly. PBDT–BT obtained a high Voc of 0.88 eV, which was
attributed to its deep-lying HOMO level. The highest Jsc of
the devices incorporating the PBDT–BT blends (5.36 mA
cm ) was higher than that of PBDT–Q (4.40 mA cm ).
This trend has also been observed for FF.
high-performance PSC requires not only low series resistance
(R ) but also high-shunt resistance (Rsh). A large Rsh indi-
s
cates the device possesses low electron-hole recombination
3
3
and efficient free carriers collection, which is beneficial for
2
2
22
34
obtaining a high Jsc and Voc
.
Additionally, a smaller R
is
s
3
5
helpful to get a higher Jsc and FF.
According to the
FIGURE 6 (a) J–V curves of the photovoltaic cells and (b) IPCE curves of the photovoltaic cells based on the polymers/PC61BM (or
PC71BM) (1/2, w/w). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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calculated results, the R and R values were 247 and 12.4
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