Controlling blend film morphology by varying alkyl side chain in highly coplanar donor-acceptor copolymers for photovoltaic application

Article abstract of DOI:10.1021/ma201009n

A series of varied length alkyl substituted donor-acceptor (D-A) conjugated copolymers with benzo[1,2-b:4,5-b′]dithiophene (BDT) as donor and thiophene rings attached to both sides of the benzothiadiazole (TBT) moieties as acceptors were designed and synthesized. The optical and electrochemical properties showed that the absorption spectrum, the band gaps, and the energy levels of the copolymers were not affected by the varied substituted alkyls, and all these copolymers showed low band gaps around 1.75 eV. In addition, the morphologies of the blend film between copolymers and PCBM can be fine-tuned by increasing the length of substituted alkyl of the copolymers, changing from pea-like aggregation to interpenetrating network to grain-like aggregation. Bulk heterojunction photovoltaic devices were fabricated by using the copolymers as donors and (6,6)-phenyl C₆₁-butyric acid methyl ester (PC ₆₁BM) or (6,6)-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) as acceptors. The optimized photovoltaic performances showed the stable open-circuit voltage (V_oc) in the range of 0.68 to 0.74 eV, and dramatically increasing short circuit current density (J_sc) by optimizing the blending morphologies of copolymer and PCBM films. The optimized photovoltaic performance with a V_oc of 0.70 V, J _sc of 7.19 mA/cm², a fill factor (FF) of 0.52, and a power conversion efficiency (PCE) of 2.88%, was obtained by the copolymer PBDT-TBT-C8 (PBDT-TBT-C8:PC₆₁BM, 1:3 w/w, in CB solution). This is due to its low band gap and interpenetrating network morphology of PBDT-TBT-C8:PC ₆₁BM blend film. The photovoltaic device based on PBDT-TBT-C8:PC ₇₁BM showed a J_sc of 8.6 mA/cm² and a PCE of 3.15%.

Full text of DOI:10.1021/ma201009n

ARTICLE
pubs.acs.org/Macromolecules
Controlling Blend Film Morphology by Varying Alkyl Side Chain
in Highly Coplanar DonorꢀAcceptor Copolymers for Photovoltaic
Application
Yaowen Li, Yujin Chen, Xing Liu, Zhong Wang, Xiaoming Yang, Yingfeng Tu,* and Xiulin Zhu
Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering,
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, P. R. China
S Supporting Information
b
ABSTRACT: A series of varied length alkyl substituted do-
norꢀacceptor (DꢀA) conjugated copolymers with benzo[1,2-
b:4,5-b0]dithiophene (BDT) as donor and thiophene rings
attached to both sides of the benzothiadiazole (TBT) moieties
as acceptors were designed and synthesized. The optical and
electrochemical properties showed that the absorption spec-
trum, the band gaps, and the energy levels of the copolymers
were not affected by the varied substituted alkyls, and all these
copolymers showed low band gaps around 1.75 eV. In addition,
the morphologies of the blend film between copolymers and
PCBM can be fine-tuned by increasing the length of substituted alkyl of the copolymers, changing from pea-like aggregation to
interpenetrating network to grain-like aggregation. Bulk heterojunction photovoltaic devices were fabricated by using the
copolymers as donors and (6,6)-phenyl C61-butyric acid methyl ester (PC61BM) or (6,6)-phenyl C71-butyric acid methyl ester
(PC71BM) as acceptors. The optimized photovoltaic performances showed the stable open-circuit voltage (Voc) in the range of 0.68
to 0.74 eV, and dramatically increasing short circuit current density (Jsc) by optimizing the blending morphologies of copolymer and
PCBM films. The optimized photovoltaic performance with a Voc of 0.70 V, Jsc of 7.19 mA/cm2, a fill factor (FF) of 0.52, and a power
conversion efficiency (PCE) of 2.88%, was obtained by the copolymer PBDT-TBT-C8 (PBDT-TBT-C8:PC61BM, 1:3 w/w, in CB
solution). This is due to its low band gap and interpenetrating network morphology of PBDT-TBT-C8:PC61BM blend film. The
photovoltaic device based on PBDT-TBT-C8:PC71BM showed a Jsc of 8.6 mA/cm2 and a PCE of 3.15%.
’ INTRODUCTION
in the active layer must possess better intrinsic absorption
properties, desirable HOMO/LUMO energy levels, and higher
photocurrent generation. To conquer these problems, low-
bandgap (LBG) polymers have been utilized recently in PVCs
with fullerene derivatives, such as PC61BM or PC71BM, yielding
better PCE values up to 7.7%.8ꢀ11 It is notable that most of the
promising electron donor materials are composed of electron-
donating (donor) and electron-accepting (acceptor) units, which
possess low band gaps and deeper HOMO energy levels.12ꢀ14
Meanwhile, incorporation of wide ranges of donors and accep-
tors into LBG polymers can manipulate the electronic structures.
The interaction between electron-donating moiety and electron-
accepting moiety forms the intramolecular charge transfer (ICT)
transition leading to newly formed high-lying HOMO and low-
lying LUMO, which can also account for the reduced optical
absorption band gap.15,16
In recent years, considerable efforts have been directed toward
the development of new photovoltaic polymers, which are
becoming more and more attractive because they represent
low cost and flexible devices, tunable electronic properties, and
ease of processing.1ꢀ3 At present, polymer photovoltaic cells
(PVCs) with a bulk heterojunction (BHJ) active layer have been
proven to be the most successful. The fundamental concept of
BHJ polymer PVCs involves the self-assembly of nanoscale
heterojunctions by spontaneous phase separation of the donor
(polymer) and the acceptor (fullerene), resulting in the forma-
tion of heterojunctions, or an interpenetrating network through-
out the bulk of the donor and acceptor molecules.4 Over the past
decade, much progress has been made for BHJ PVCs based on
regioregular poly(3-hexylthiophene) (P3HT) and PCBM.5ꢀ7
However, the relatively large band gap of P3HT (∼1.9 eV)
limits the fraction of the solar spectrum that can be harvested, as
well the relatively small energy difference between the highest
occupied molecular orbital (HOMO) of P3HT and the lowest
unoccupied molecular orbital (LUMO) of the fullerene acceptor
results in a low open-circuit voltage (0.6 V). In order to get
improved performance of PVCs, the donor andacceptor molecules
BHJ solar cells based on semiconducting polymers are com-
plex systems. Their performances are not only affected by the
Received: May 3, 2011
Revised:
July 22, 2011
Published: August 02, 2011
r
2011 American Chemical Society
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|

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ARTICLE
electrical properties of donor polymers, but also by many other
factors, e.g., interaction of donor polymers and acceptors, mor-
phology of the blend films, interfacial properties, and elec-
trodes.17ꢀ19 All these factors are crucial for the charge transfer
and collection. By blending donor and acceptor materials to-
gether, an interpenetrating network with a large DꢀA interfacial
area, which is regarded as ideal morphology for BHJ solar cells,
can be achieved through controlling the phase separation be-
tween the two components in bulk. In this way, any absorbing
site in the composite is within 10ꢀ15 nm of the donorꢀacceptor
phase separation, and the formation of a bicontinuous network
creates two channels to transport holes in the donor domain and
electrons in the acceptor domain, leading to much enhanced
quantum efficiency of charge separation.20 In addition, the strong
driving force for the development of long-range order aided by
the coplanarity of the polymer backbone, which generally exhibit
better πꢀπ stacking and higher carrier mobility in solid state,
favors the exclusion of the fullerene and induces a large extent of
phase segregation in a blend film with the fullerene. Therefore,
among these factors, the morphology control of the blend film
plays an important role. To achieve high solar power conversion
efficiency (PCE), several effective methods have been developed
to optimize the phase separation formed by the electron donor
and acceptor molecules, including thermal annealing,21 and
solvent mixtures22 or additives.23 However, these methods are
not all-purpose because of their complicated mechanism in tuning
the morphology. Generally, all the above methods are adopted to
tune the morphology one after the other, which greatly increases
the cost and periods of the devices fabrication.
In view of the disadvantage of postprocessing method for the
morphology control of the active layer, polymer chemical modifi-
cation affords various useful strategies for changing its solubility,
coplanarity of the conjugated main chain and the miscibility
between the donor polymer and fullerene derivative acceptor,
which can adjust intermolecular interactions and self-assembly
ability of the composite to form the desirable morphology. To
our knowledge, there have been several reports on the morphol-
ogy control of the blend film by modification polymer donor and
the photovoltaic performance has been greatly improved. Most
recently, Yu et al.9 reported incorporating fluorine into the
various positions of the polymer backbone can significantly affect
the morphology of the polymer/PC71BM blend film by changing
from spherical domain to uniform and fine features, and the solar
cells’ power conversion efficiency were improved from 2.3% to
7.2%. Meanwhile, our recent work has succeeded in developing a
series of low band gap DꢀA copolymers with varied ratios of two
different electron-accepting moieties, which exhibit different self-
assembly characteristics when blend with PCBM. The morphol-
ogies of the blend film were also tuned from a grain-like aggrega-
tion to an interpenetrating network to a club-like structure, and
the corresponding devices showed an improvement PCEs from
0.68 to 2.89%.24 In addition, there are several reports about the
effect of varied donor copolymer side chains on the blend film
(polymer and PCBM) morphology, where it can significantly
impact structural order, orientation in polymer backbones and
πꢀπ stacking interactions in the polymer domains, and critically
affecting device performance.25ꢀ27 On all accounts, the blend
film morphology between donor and acceptor plays an important
role in the performance of PVCs.
obtain potential photovoltaic materials, we construct a series high
coplanarity DꢀA based low band gap copolymers. The benzo-
[1,2-b:4,5-b0]dithiophene (BDT) and benzothiadiazole (BT) are
generally considered highly coplanar electron-donating and
electron-accepting moieties, respectively. As a result of less steric
hindrance and easily formed πꢀπ stacking, the incorporation of
BDT and BT moiety not only induces more intense electronic
transitions in the entire visible range to broaden the absorption
bandwidth and reduce the band gap, but also improves the hole
mobility.28,29 In addition, the BDT and BT moieties were bridged
by a conjugated moiety, which can be expected to enhance the
conjugated length, broaden the absorption spectrum, and im-
prove the hole mobility of the copolymers. Because of the bad
solubility of copolymers with rigid backbone, flexible alkoxyl or
alkyl chains was introduced on the copolymers to improve the
solubility. Meanwhile, considering the fact that varied types and
attached position of flexible chain may cause the different solubility
of copolymers, the miscibility between copolymer and PCBM, and
the twist of the backbone, a better selection of the attached flexible
chain on the backboneof the copolymers may realizethe control of
the blend film morphology between copolymer and PCBM.
On the basis of the above idea, a series of high coplanar DꢀA
copolymers consisting of alkoxy substituted BDT (alkoxy-BDT)
with no substituent on 1, 3, 5, and 7 positions were designed as
the donor, which makes BDT an ideal conjugated unit for new
photovoltaic materials. The alkoxy substituted 2,1,3-benzothia-
diazoles were chosen as electron-accepting moiety, and it con-
nected with thiophene units to give compounds (TBT) from
M-1 to M-5 copolymerizing with alkoxy-BDT. In order to fine-
tune the πꢀπ stacking effect and self-assembly ability properties,
five copolymers with varied alkyl length attaching to thiophene
moiety were designed and synthesized. Both the UVꢀvisible
absorption spectra and electrochemical properties of the copo-
lymers exhibited low band gaps around 1.75 eV. Meanwhile, the
substituted varied alkyl length on the thiophene moiety can not
only keep their low band gap but also give a stable HOMO and
LUMO energy level. Furthermore, with the increasing substi-
tuted alkyl length, the morphology of the blend films between
copolymers and PC61BM exhibited different self-assembly char-
acteristics, changing from pea-like aggregation to interpenetrat-
ing network to grain-like aggregation. The optimized photo-
voltaic performance under AM1.5 conditions with a Voc of 0.70 V,
Jsc of 7.19 mA/cm2, a fill factor (FF) of 0.52, and a power
conversion efficiency (PCE) of 2.88%, was obtained by the
copolymer PBDT-TBT-C8 (PBDT-TBT-C8:PC61BM, 1:3 w/w,
in CB-solution) due to its interpenetrating network morphology
of PBDT-TBT-C8:PC61BM blend film. Additionally, the photo-
voltaic device based on PBDT-TBT-C8:PC71BM showed a Jsc of
8.6 mA/cm2 and PCE of 3.15%.
’ EXPERIMENTAL SECTION
Materials and Synthesis. 2,6-Bis(trimethyltin)-4,8-didodecylo-
xybenzo[1,2-b;3,4-b]dithiophene was synthesized according to Hou
et al. reported.30 All reagents and chemicals were purchased from
commercial sources (Aldrich, Across, Fluka) and used without further
purification unless stated otherwise. All solvents were distilled over
appropriate drying agent(s) prior to use and were purged with nitrogen.
1,2-Bis(octyloxy)benzene (1). To a solution of catechol (5.0 g,
45.0 mmol) in dry DMF (40 mL) were added 1-bromooctane (20.3 g,
18.2 mL, 105 mmol,) and K2CO3 (19 g, 135 mmol). The mixture was
stirred at 100 °C under a nitrogen atmosphere for 48 h. After cooling the
In order to further develop the chemical modification method
for controlling the blend film morphology, enhancing the πꢀπ
stacking interactions between polymer and PCBM, and hence
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mixture to room temperature (RT), 80 mL of water were added. The
aqueous layer was extracted with CH2Cl2 and the organic layer was
separated. The organic extracts were dried over anhydrous MgSO4,
evaporated and purified with column chromatography on silica gel with
CH2Cl2: petroleumether (1:10) as the eluent to give white solid 12.8 g
(38.3 mmol, yield 85%). 1H NMR (500 MHz, CDCl3, TMS): δ (ppm)
6.86 (s, 4H, ꢀPh), 3.98 (t, 4H, J = 6.75 Hz, ꢀOCHH2), 1.81 (m, 4H,
ꢀCH2), 1.47 (m, 4H, ꢀCH2), 1.34 (m, 16H, ꢀCH2), 0.89 (t, 16H,
J = 6.75, ꢀCH3). Anal. Calcd for C22H38O2: C, 78.99; H, 11.45. Found:
C, 78.95; H, 11.47.
1,2-Dinitro-4,5-bis(octyloxy)benzene (2). To a two neck
round-bottom flask containing dichloromethane (140 mL), acetic acid
(140 mL), and 1,2-bis(octyloxy)benzene (6.7 g, 19.9 mmol) cooled to
10 °C was added dropwise 65% nitric acid (20 mL). The reaction was
allowed to warm to room temperature and stirred for 1 h. The mixture
was again cooled to 10 °C and 100% nitric acid (50 mL) was added
dropwise. The mixture was allowed to warm to room temperature and
the mixture was stirred for 40 h. After completion of the reaction, the
reaction mixture was poured into iceꢀwater and the dichloromethane
layer was separated. The water phase was extracted with dichloro-
methane. The combined organic phase was washed with water, saturated
NaHCO3 (aq), and brine and dried over MgSO4. Concentration in
vacuum gave the crude product and was recrystallized from ethanol,
obtained yellow solid 7.8 g (18.3 mmol, yield 92%). 1H NMR (CDCl3,
300 MHz): δ(ppm) 7.30(s, 2H,ꢀPh), 4.10(t,J= 8.8 Hz, 4H, ꢀOCHH2),
1.88 (m, 4H, ꢀCH2), 1.48 (m, 4H, ꢀCH2) 1.294 (m, 16H, ꢀCH2),
0.88 (t, J = 6.6 Hz, 6H, ꢀCH3). Anal. Calcd for C22H38O2: C, 62.24; H,
8.55. Found: C, 62.21; H, 8.60.
was heated to reflux for 24 h under argon. The reaction mixture was
concentrated, then purified with column chromatography on silica gel
with CH2Cl2: petroleumether (1:3) as the eluent to give orange solid
1
473 mg (78%). H NMR (CDCl3, 400 MHz): δ (ppm) 8.48 (d, J =
3.6 Hz, 2H, ꢀTh), 7.51 (d, J = 4.8 Hz, 2H, ꢀTh), 7.24 (m, 2H, ꢀTh),
4.12 (t, J = 7.1 Hz, 4H, ꢀOCHH2), 1.93 (m, 4H, ꢀCH2), 1.45 (m, 4H,
ꢀCH2), 1.33 (br, 16H, ꢀCH2), 0.91 (m, 6H, ꢀCH2). 13C NMR
(CDCl3, 100 MHz): δ (ppm) 152.0, 151.1, 134.2, 130.7, 127.4, 126.8,
117.7, 74.4, 32.0, 30.5, 29.7, 29.4, 26.1, 22.8, 14.3.
4,7-Bis(4-hexylthiophen-2-yl)-5,6-bis(octyloxy)benzo[c]-
[1,2,5]thiadiazole (M-2). This compound was prepared with the
1
same procedure according to M-1. H NMR (CDCl3, 400 MHz): δ
(ppm) 8.31 (s, 2H, ꢀTh), 7.09 (s, 2H, -Th), 4.10 (t, J = 7.0 Hz, 4H,
ꢀOCHH2), 2.72 (t, J = 7.6 Hz, 4H, ꢀCH2), 1.92 (m, 4H, ꢀCH2), 1.72
(m, 4H, ꢀCH2), 1.43 (m, 8 H, ꢀCH2), 1.34 (br, 24 H, ꢀCH2), 0.91
(m, 12 H, ꢀCH3). 13C NMR (CDCl3, 100 MHz): δ (ppm) 152.1,
151.1, 143.0, 134.0, 132.1, 122.4, 117.7, 74.4, 32.0, 31.9, 30.8, 30.7, 30.6,
29.8, 29.5, 29.3, 26.2, 22.9, 22.8, 14.3.
5,6-Bis(octyloxy)-4,7-bis(4-octylthiophen-2-yl)benzo[c]-
[1,2,5]thiadiazole (M-3). This compound was prepared with the
same procedure according to M-1. 1H NMR (CDCl3, 400 MHz):
δ(ppm) 8.31 (s, 2H, ꢀTh), 7.10 (s, 2H, ꢀTh), 4.10 (t, J = 7.0 Hz,
4H, ꢀOCHH2), 2.72 (t, J = 7.8 Hz, 4H, ꢀCH2), 1.93 (m, 4H, ꢀCH2),
1.72 (m, 4H, ꢀCH2), 1.65 (m, 4 H, ꢀCH2), 1.34 (br, 36 H, ꢀCH2),
0.90 (m, 12 H, ꢀCH3).
4,7-Bis(4-decylthiophen-2-yl)-5,6-bis(octyloxy)benzo[c]-
[1,2,5]thiadiazole (M-4). This compound was prepared with the
same procedure according to M-1. 1H NMR (CDCl3, 400 MHz):
δ (ppm) 8.34 (s, 2H, ꢀTh), 7.12 (s, 2H, -Th), 4.12 (t, J = 7.0 Hz, 4H,
ꢀOCHH2), 2.73 (t, J = 7.6 Hz, 4H, ꢀCH2), 1.94 (m, 4H, ꢀCH2), 1.73
5,6-Bis(octyloxy)benzo[c][1,2,5]thiadiazole (3). A mixture of
1,2-dinitro-4,5-bis(octyloxy)benzene (1.43 g, 3.37 mmol) and SnIICl2
3
2H2O (6.07 g, 26.9 mmol) in ethanol (50 mL) and concentrated HCl
(20 mL) was heated to 85 °C overnight. After cooling to room tempe-
rature, the product was filtered and washed with water and methanol.
Finally it was dried at RT under a stream of argon and used directly. To a
mixture of 4,5-bis(octyloxy)benzene-1,2-diaminium chloride (1.11 g,
2.54 mmol) and triethylamine (25.1 mmol, 3.5 mL) in 40 mL of dichlo-
romethane was slowly added a solution of thionyl chloride (4.83 mmol,
352 μL) in 5 mL of dichloromethane. After addition, the mixture was
heated to reflux for 6 h. The cooled solution was concentrated in vacuum
followed by trituration in water. After the solution was stirred for 30 min,
the product was filtered off and recrystallized from ethanol and gave off-
white solid 0.63 g (yield 62%). 1H NMR (CDCl3, 300 MHz): δ (ppm)
7.14 (s, 2H, -Ph), 4.09 (t, J = 6.6 Hz, 4H, ꢀOCHH2), 1.92 (m, 4H,
ꢀCH2), 1.52 (m, 4H, ꢀCH2), 1.30 (m, 16H, ꢀCH2), 0.89 (t, J = 9.0 Hz,
ꢀCH3) 13C NMR (CDCl3, 75 MHz): δ (ppm) 154.1, 151.3, 98.4, 69.1,
31.8, 29.3, 29.2, 28.7, 25.1, 22.6, 14.1.
(m, 4H, ꢀCH2), 1.30 (m, 48 H, ꢀCH2), 0.92 (m, 12 H, ꢀCH3). 13
C
NMR (CDCl3, 100 MHz): δ (ppm) 152.0, 151.1, 143.0, 133.9, 132.1,
122.4, 117.7, 74.4, 32.1, 32.0, 30.8, 30.6, 29.8, 29.7, 29.6, 29.5, 29.5, 28.4,
26.9, 26.2, 22.9, 17.4, 14.3, 13.8.
4,7-Bis(4-dodecylthiophen-2-yl)-5,6-bis(octyloxy)benzo-
[c][1,2,5]thiadiazole (M-5). This compound was prepared with the
same procedure according to M-1. 1H NMR (CDCl3, 400 MHz):
δ(ppm) 8.34 (s, 2H, ꢀTh), 7.12 (s, 2H, ꢀTh), 4.12 (t, J = 7.0 Hz,
4H, ꢀOCHH2), 2.73 (t, J = 7.8 Hz, 4H, ꢀCH2), 1.95 (m, 4H, ꢀCH2),
1.74 (m, 4H, ꢀCH2), 1.66 (m, 4H, ꢀCH2), 1.28 (m, 52 H, ꢀCH2), 0.92
(m, 12 H, ꢀCH3). 13C NMR (CDCl3, 100 MHz): δ (ppm) 152.0,
151.1, 143.0, 133.9, 132.1, 122.4, 117.7, 74.4, 32.1, 32.0, 30.8, 30.6, 29.9,
29.8, 29.7, 29.7, 29.6, 29.5, 29.5, 28.4, 26.9, 26.2, 22.9, 17.4, 14.3, 13.8.
4,7-Bis(5-bromothiophen-2-yl)-5,6-bis(tetradecyloxy)-
benzo[c][1,2,5]thiadiazole (BM-1). To a solution of 5 (760 mg,
1.37 mmol) in CHCl3 (48 mL) and glacial acetic acid (48 mL) was
added NBS (2.73 mmol, 485 mg) in one portion. The mixture was
stirred at room temperature for 20 h in the dark. The reaction mixture
was concentrated directly on Celite in vacuum and then purified with
column chromatography on silica gel with CH2Cl2:petroleum ether
(1:3) as the eluent to give an orange solid in 832 mg (85%) yield. 1H
NMR (CDCl3): δ = 8.41 (s, 2H, -Th), 7.22 (s, 2H, ꢀTh), 4.12 (br, 4H,
ꢀOCHH2), 2.00 (m, 4H, ꢀCH2), 1.51 (br, 20H, ꢀCH2), 0.96 (m, 6H,
ꢀCH2). 13C NMR (CDCl3, 100 MHz): δ (ppm)151.6, 150.5, 135.8,
131.1, 129.8, 117.1, 115.6, 74.7, 32.0, 30.4, 29.6, 29.4, 26.1, 22.8, 14.3.
4,7-Bis(5-bromo-4-hexylthiophen-2-yl)-5,6-bis(octyloxy)-
benzo[c][1,2,5]thiadiazole (BM-2). This compound was prepared
with the same procedure according to BM-1. 1H NMR (CDCl3,
400 MHz):δ(ppm) 8.35 (s, 2H, -Th), 4.15 (t, J =6.6 Hz, 4H, ꢀOCHH2),
2.70 (t, J = 7.4 Hz, 4H, ꢀCH2), 1.99 (m, 4H, ꢀCH2), 1.72 (m, 4H,
ꢀCH2), 1.38 (br, 32H, ꢀCH2), 0.94 (m, 12 H, ꢀCH3). 13C NMR
(CDCl3, 100 MHz): δ (ppm) 151.6, 150.5, 141.8, 133.9, 131.7, 117.0,
112.8, 74.5, 32.0, 31.9, 30.5, 30.0, 29.8, 29.7, 29.5, 29.2, 26.2, 22.9,
22.9, 14.3.
4,7-Dibromo-5,6-bis(octyloxy)benzo[c][1,2,5]thiadiazole
(4). To a solution of 3 (4.6 g, 11.7 mmol) in a mixture of dichloro-
methane (328 mL) and acetic acid (144 mL) was added bromine
(4.1 mL, 80 mmol), and the resulting mixture was stirred in the dark for
ca. 48 h at room temperature. The mixture was then poured into water
(400 mL), extracted with dichloromethane, sequentially washed with
water, saturated NaHCO3 (aq), and 1 M Na2SO3 (aq), and the solvents
were evaporated off under reduced pressure. The crude product was
purified by recrystallization from ethanol twice to give needle-like
1
crystals 4.76 g (yield 74%). H NMR (CDCl3, 300 MHz): δ (ppm)
4.16 (t, J = 6.75 Hz, 4H, ꢀOCHH2), 1.89 (m, 4H, ꢀCH2), 1.56 (m, 4H,
ꢀCH2), 1.36 (m, 16H, ꢀCH2), 0.91 (t, J = 6.9 Hz, 6H, ꢀCH3). 13C
NMR (CDCl3, 75 MHz): δ (ppm) 154.2, 150.1, 106.0, 74.9, 31.6, 30.1,
29.2, 29.1, 25.8, 22.5, 13.9.
5,6-Bis(octyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-
thiadiazole (M-1). To a solution of 4 (0.6 g, 1.09 mmol), Pd-
(PPh3)Cl2 (38 mg, 0.05 mmol) in dry THF (20 mL) was added
2-tributylstannylthiophene (1.23 g, 3.3 mmol) and the reaction mixture
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Scheme 1. Synthesis of Monomers and Copolymers
4,7-Bis(5-bromo-4-octylthiophen-2-yl)-5,6-bis(octyloxy)-
benzo[c][1,2,5]thiadiazole (BM-3). This compound was prepared
with the same procedure according to BM-1. H NMR (CDCl3, 400
4,7-Bis(5-bromo-4-dodecylthiophen-2-yl)-5,6-bis(octyloxy)-
benzo[c][1,2,5]thiadiazole (BM-5). This compound was prepared
with the same procedure according to BM-1. H NMR (CDCl3, 400
1
1
MHz): δ (ppm) 8.32 (s, 2H, -Th), 4.13 (t, J = 6.2 Hz, 4H, ꢀOCHH2),
2.67 (t, J = 6.8 Hz, 4H, ꢀCH2), 1.96 (m, 4H, ꢀCH2), 1.70 (m, 4H,
ꢀCH2), 1.31 (br, 40H, ꢀCH2), 0.91 (m, 12 H, ꢀCH3). 13C NMR
(CDCl3, 100 MHz): δ (ppm) 151.6, 150.5, 141.8, 133.9, 131.7, 117.0,
112.8, 74.5, 32.1, 32.0, 30.5, 30.1, 29.8, 29.7, 29.6, 29.6, 29.5, 26.2,
22.9, 14.3.
MHz): δ (ppm) 8.33 (s, 2H, ꢀTh), 4.13 (t, J = 7 Hz, 4H, ꢀOCHH2),
2.67 (t, J = 7.6 Hz, 4H, ꢀCH2), 1.96 (m, 4H, ꢀCH2), 1.69 (m, 4H,
ꢀCH2), 1.27 (br, 56H, ꢀCH2), 0.89 (m, 12 H, ꢀCH3). 13C NMR
(CDCl3, 100 MHz): δ (ppm) 151.6, 150.6, 141.9, 133.9, 131.7, 117.0,
112.7, 74.6, 32.1, 32.0, 30.5, 30.0, 29.9, 29.8, 29.7, 29.6, 29.5, 29.4, 29.1,
29.0, 28.4, 26.9, 26.1, 22.9, 14.1, 13.6.
4,7-Bis(5-bromo-4-decylthiophen-2-yl)-5,6-bis(octyloxy)-
benzo[c][1,2,5]thiadiazole (BM-4). This compound was prepared
with the same procedure according to BM-1. 1H NMR (CDCl3,
400 MHz): δ (ppm) 8.33 (s, 2H, -Th), 4.13 (t, J = 7 Hz, 4H, ꢀOCHH2),
2.67 (t, J = 7.6 Hz, 4H, ꢀCH2), 1.96 (m, 4H, ꢀCH2), 1.70 (m, 4H,
ꢀCH2), 1.29 (br, 48H, ꢀCH2), 0.89 (m, 12 H, ꢀCH3). 13C NMR
(CDCl3, 100 MHz): δ (ppm) 151.5, 150.4, 141.8, 133.9, 131.7, 116.9,
112.8, 74.5, 53.6, 32.1, 32.1, 30.6, 30.1, 29.9, 29.8, 29.8, 29.7, 29.6, 29.5,
26.2, 22.9, 14.3.
General Procedures of Polymerization. Stille cross-coupling
reaction was used to synthesize the copolymers as shown in Scheme 1.
The 5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole
with different alkyl chains, 2,6-bis(trimethyltin)-4,8-didodecyloxybenzo-
[1,2-b;3,4-b]dithiophene (BDT) and (PPh3)4Pd(0) (2 mol % with
respect to the monomer BM) as the catalyst precursor were dissolved in
a mixture of toluene (15 mL). The solution was stirred under Ar atmos-
phere and refluxed with vigorous stirring for 24 h. The resulting solution
was then poured into methanol and followed by washing with water. The
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precipitated solid was extracted with methanol and acetone for 24 h in a
Soxhlet apparatus to remove the oligomers and catalyst residues,
respectively. The soluble fraction was then collected via extraction with
chloroform for 24 h. The chloroform solution was then concentrated to
afford the copolymers.
Poly{4,8-didodecyloxybenzo[1,2-b;3,4-b]dithiophene-
alt-5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-
thiadiazole} (PBDT-TBT). The resulting copolymer PBDT-TBT was
1
obtained as a dark powder with a yield of 72%. H NMR (400 MHz,
CDCl3): δ (ppm) 8.56 (br, 2H, ꢀTh), 7.43 (br, 4H, ꢀTh), 4.33 (br, 4H,
ꢀOCHH2), 4.21 (br, 4H, ꢀOCHH2), 1.96 (br, 8H, ꢀCH2), 1.27
(br, 56H, ꢀCH2), 0.86 (br, 12 H, ꢀCH3). Anal. Calcd for PBDT-TBT:
C, 69.15; H, 8.10; N, 2.52. Found: C, 68.51; H, 8.01; N, 2.51.
Poly{4,8-didodecyloxybenzo[1,2-b;3,4-b]dithiophene-
alt-4,7-bis(4-hexylthiophen-2-yl)-5,6-bis(octyloxy)benzo-
[c][1,2,5]thiadiazole} (PBDT-TBT-C6). The resulting copolymer
PBDT-TBT-C6 was obtained as a dark powder with a yield of 81%. 1H
NMR (400 MHz, CDCl3): δ (ppm) 8.52 (br, 2H, ꢀTh), 7.60 (br,
2H, ꢀTh), 4.37 (br, 4H, ꢀOCHH2), 4.22 (br, 4H, ꢀOCHH2), 3.04
(br, 4H, ꢀCH2), 1.96 (br, 12H, ꢀCH2), 1.27 (br, 68H, ꢀCH2), 0.86
(br, 18H, ꢀCH3). Anal. Calcd for PBDT-TBT-C6: C, 71.25; H, 8.91; N,
2.19. Found: C, 70.78; H, 8.57; N, 2.41.
Poly{4,8-didodecyloxybenzo[1,2-b;3,4-b]dithiophene-
alt-5,6-bis(octyloxy)-4,7-bis(4-octylthiophen-2-yl)benzo[c]-
[1,2,5]thiadiazole} (PBDT-TBT-C8). The resulting copolymer PBDT-
TBT-C8 was obtained as a dark powder with a yield of 85%. 1H NMR (400
MHz, CDCl3):δ(ppm) 8.52(br, 2H,ꢀTh), 7.60(br, 2H,ꢀPM), 4.37(br,
4H, ꢀOCHH2), 4.22 (br, 4H, ꢀOCHH2), 3.03 (br, 4H, ꢀCH2), 1.95 (br,
12H, ꢀCH2), 1.29 (br, 76 H, ꢀCH2), 0.87 (br, 18 H, ꢀCH3). Anal. Calcd
for PBDT-TBT-C8: C, 71.85; H, 9.13; N, 2.10. Found: C, 71.66; H, 9.04;
N, 2.22.
Poly{4,8-didodecyloxybenzo[1,2-b;3,4-b]dithiophene-
alt-4,7-bis(4-decylthiophen-2-yl)-5,6-bis(octyloxy)benzo[c]-
[1,2,5]thiadiazole} (PBDT-TBT-C10). The resulting copolymer
PBDT-TBT-C10 was obtained as a dark powder with a yield of 81%.
1H NMR (400 MHz, CDCl3): δ (ppm) 8.52 (br, 2H, -Th), 7.59 (br, 2H,
ꢀTh), 4.37 (br, 4H, ꢀOCHH2), 4.22 (br, 4H, ꢀOCHH2), 3.03 (br,
4H, ꢀCH2), 1.97 (br, 12H, ꢀCH2), 1.29 (br, 84 H, ꢀCH2), 0.87 (br, 18
H, ꢀCH3). Anal. Calcd for PBDT-TBT-C10: C, 72.40; H, 9.34; N, 2.01.
Found: C, 72.09; H, 9.52; N, 2.20.
Poly{4,8-didodecyloxybenzo[1,2-b;3,4-b]dithiophene-
alt-4,7-bis(4-dodecylthiophen-2-yl)-5,6-bis(octyloxy)benzo-
[c][1,2,5]thiadiazole} (PBDT-TBT-C12). The resulting copolymer
PBDT-TBT-C12 was obtained as a dark powder with a yield of 76%. 1H
NMR (400 MHz, CDCl3): δ (ppm) 8.52 (br, 2H, ꢀTh), 7.60 (br, 2H,
ꢀTh), 4.37 (br, 4H, ꢀOCHH2), 4.22 (br, 4H, ꢀOCHH2), 3.03 (br,
4H, ꢀCH2), 1.97 (br, 12H, ꢀCH2), 1.29 (br, 92 H, ꢀCH2), 0.87 (br, 18
H, ꢀCH3). Anal. Calcd for PBDT-TBT-C12: C, 72.91; H, 9.53; N, 1.93.
Found: C, 73.01; H, 9.76; N, 2.10.
Figure 1. 1H NMR spectrum and chemical structure of monomer BM-2
and copolymer PBDT-TBT-C6 in CDCl3 solution.
microscope operated at 200 kV. Thin film X-ray diffraction (XRD) was
recorded on a Bruker D8 Discover thin-film diffractometer with Cu KR
radiation (λ = 1.54056 Å) operated at 40 keV and 40 mA.
Device Fabrication and Characterization. The active layer
contained a blend of P3HT as electron donor and C60 derivatives as
electron acceptor, which was prepared from varied weight ratios by
solution (10 mg/mL of P3HT) in chlorobenzene. After spin coating the
blend from solution at 600 rpm, the devices were completed by
evaporating a 0.8 nm LiF layer protected by 100 nm of Al at a base
pressure of 4 ꢁ 10ꢀ4 Pa. The effective photovoltaic area as defined by
the geometrical overlap between the bottom ITO electrode and the top
cathode was 12 mm2. Currentꢀvoltage characteristics of the solar cells
in the dark and under illumination of 100 mW/cm2 white light from a
HgꢀXe lamp filtered by a Newport 81094 Air Mass Filter, using a
GWinstek SFG-1023 source meter. Monochromatic light from a
HgꢀXe lamp (Newport 67005) in combination with monochromator
(Oriel, Cornerstone 260) was modulated with a mechanical chopper.
The response was recorded as the voltage over a 50 Ω resistance, using a
lock-in amplifier (Newport 70104 Merlin). A calibrated Si cell was used
as reference. All the measurements were performed under ambient
atmosphere at room temperature. The optimized photovoltaic perfor-
mance characteristics are given in Table 3.
’ RESULTS AND DISCUSSION
Characterization. The elemental analysis was carried out with a
Thermoquest CHNS-Ovelemental analyzer. The gel permeation chro-
matographic (GPC) analysis was carried out with a Waters 410 instru-
ment with tetrahydrofuran as the eluent (flow rate: 1 mL/min, at 35 °C)
and polystyrene as the standard. Thermogravimetric analysis were
performed on a Perkin-Elmer Pyris 1 analyzer under nitrogen atmo-
Synthesis and Characterization of Copolymers. We
adopted the high coplanar and strong electron-donating ability
BDT moiety as electron donor. The BDT moiety not only has a
large planar conjugated structure and easily formed πꢀπ stack-
ing, but also possesses low steric hindrance between adjacent
units. This is because the 4,9-bis-alkoxy-BDT has no substituent
on the 1, 3, 5, and 7 positions, which makes BDT an ideal con-
jugated unit for new photovoltaic material design.30 The alkoxy-
substituted 2,1,3-benzothiadiazoles was chosen as electron-ac-
cepting moiety, and connected with thiophene units containing
different alkyl length to give compounds (TBT) from M-1 to
M-5, which is then brominated with NBS to give monomer from
BM-1 to BM-5. Finally, we used Pd(0)-catalyzed Stille coupling
in toluene under heating to synthesize copolymers consisting of
sphere (100 mL/min) at a heating rate of 10 °C/min. 1H NMR and 13
C
NMR spectra were measured using Inova nuclear magnetic resonance
instruments. UVꢀvisible absorption spectra were measured using a
Shimadzu UV-3100 spectrophotometer. Electrochemical measurements
of these derivatives were performed with a Bioanalytical Systems BAS
100 B/W electrochemical workstation. Atomic force microscopy
(AFM) images of blend films were carried out using a Nanoscope IIIa
Dimension 3100. The transmission electron microscopy (TEM) mea-
surements were conducted on a HITACHI H-800 transmission electron
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Table 1. Polymerization Results for All the Copolymers
Mn (kg/mol)a
Mw (kg/mol)a
PDI
Tdb (°C)
PBDT-TBT
62.9
43.5
23.8
28.3
32.3
95.7
113.6
50.4
1.52
2.62
2.12
2.78
2.65
312
327
331
321
329
PBDT-TBT-C6
PBDT-TBT-C8
PBDT-TBT-C10
PBDT-TBT-C12
93.1
85.8
a Calculated from GPC (eluent: CHCl3; polystyrene standards). b Tem-
perature at 5% weight loss by a heating rate of 10 °C/min under nitrogen.
TBT as electron-accepting moieties and BDT as an electron-
donating moiety. The synthetic procedure for momoners and
copolymers are shown in Scheme 1. The 1H NMR spectrum of
monomer BM-2 and copolymer PBDT-TBT-C6 in CDCl3
solution are shown in Figure 1. All the hydrogens of monomer
and copolymer have been given good adscriptions according to
the corresponding chemical structure, which confirmed the
success of copolymerization.
All the copolymers except PBDT-TBT exhibited good solu-
bility in common organic solvents such as chloroform, tetrahy-
drofuran, and chlorobenzene. The worse solubility of PBDT-
TBT may be caused by the high molecular weight and less flexible
chains on the backbone. The molecular weights and polydisper-
sities of the resulting polymers were determined by GPC analysis
with number-average molecular weight (Mn) of 23.8ꢀ62.9 kg/
mol and PDI (polydispersity index, Mw/Mn) of 1.5ꢀ2.8. The
long alkoxy and alkyl chain on the monomers, which increases
the solubility of intermediates during the polymerization, should
be the main reason for the relative high polymerization degree.
All the polymers exhibited good thermal stability with 5% weight-
loss temperatures (Td) higher than 312 °C under N2, as revealed
by thermogravimetric analysis (TGA) (see Figure S1, Supporting
Information). The high thermal stability of the resulting poly-
mers prevents the deformation of the polymer morphology and
the degradation of the active layer in PVCs. Table 1 summarized
the polymerization results including molecular weights, PDI and
thermal stability of the copolymers.
Optical Properties. The normalized UVꢀvis absorption
spectra of the copolymers in dilute chloroform solution (concen-
tration 10ꢀ5 M) are shown in Figure 2a, and the main optical
properties are listed in Table 2. The copolymer PBDT-TBT with
alternating electron-donating BDT moiety and electron-accept-
ing alkoxy-BT based TBT moiety showed three absorption peaks
at 344, 412, and 549 nm in dilute solution UVꢀvis absorption
spectra (Figure 2a), which can be assigned to the intrinsic
absorption of the BDT moiety, the πꢀπ* transition between
BDT and TBT moiety of the conjugated polymer backbone, and
ICT interaction between the BDT donor and TBT-based
acceptor, respectively. Similarly, the absorption spectra of copo-
lymers PBDT-TBT-C6, PBDT-TBT-C8, PBDT-TBT-C10, and
PBDT-TBT-C12 with different alkyl chain length attaching to
thiophene moiety showed almost the same, and it exhibited three
absorption peaks around 342, 393, and 521 nm, respectively,
which gives the similar assignments to the copolymer PBDT-
TBT. It has to be noted that PBDT-TBT showed red-shifted
absorption spectrum in chloroform solutions comparing other
copolymers. Since there are less bulky groups in the PBDT-TBT
copolymer, the coplanar property of BDT and TBT unit in the
copolymer backbone is better than in other four copolymers, and
results in stronger πꢀπ interactions between intermolecular
Figure 2. (a) Normalized absorption spectra of the copolymers in
chloroform solutions with the concentration of 10ꢀ5 mol/L. (b) Films
spin-coated from a 8ꢀ10 mg/mL chloroform solution with the thick-
ness around 60 nm.
copolymers. This is proved by PBDT-TBT copolymer’s increa-
sed crystallinity in solid state (as shown in Figure S2, Supporting
Information) and decreased solubility in solvents. The blue-
shifted absorption spectra of copolymers (PBDT-TBT-C6 to
PBDT-TBT-C12) also indicated a more twisted backbone
because of the long alkyl chain.31 The twisted backbone can
significantly reduce the conjugation length along the polymer
backbone, resulting in a decrease in the ICT strength and thus
electronic delocalization.32 Meanwhile, the optical gap (absorp-
tion onset) and the absorption coefficients are not significantly
affected by the length of alkyl chains. Moreover, the relatively
high absorption coefficients (εmax) could be calculated from the
Beer law equation with the same dilute concentration of the
copolymers in chloroform. The absorption spectra of the similar
thickness copolymer films (around 60 nm) also showed similar
absorption intensity, and the highest intensity reached 0.45,
which assured the copolymers absorb enough photons.
Figure 2b shows the optical absorption spectra of thin films of
the copolymers, and the optical properties are summarized in
Table 2. The thin film absorption spectra are generally similar in
shapes and trend to those in dilute solution. The maximum
absorption peaks and absorption edge (λedge) in film were red-
shifted by 57ꢀ67 nm and 17ꢀ99 nm, respectively, compared
with those in solution, suggesting that intermolecular interactions
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Table 2. Optical and Electrochemical Data of the Copolymers
in solutiona
in filmb
opticalc
Eg,ec
abs
max
abs
max
onset
onset
λ
[nm]
λedge
λ
λedge
E
(V)/
E (V)/
red
electrochem
ox
copolymer
(εmax [Mꢀ1 cmꢀ1
]
[nm]
[nm]
[nm]
HOMO(eV)
HOMO(eV)
Eg,ec (eV)
(eV)
PBDT-TBT
549 (40041)
521 (49766)
524 (45158)
521 (49766)
524 (42926)
691
609
611
609
608
606
588
589
584
588
708
704
700
700
707
0.52/ꢀ5.23
0.55/ꢀ5.26
0.58/ꢀ5.29
0.54/ꢀ5.25
0.52/ꢀ5.23
ꢀ1.16/ꢀ3.57
ꢀ1.16/ꢀ3.56
ꢀ1.16/ꢀ3.53
ꢀ1.15/ꢀ3.53
ꢀ1.12/ꢀ3.53
1.66
1.70
1.76
1.72
1.70
1.75
1.76
1.77
1.77
1.75
PBDT-TBT-C6
PBDT-TBT-C8
PBDT-TBT-C10
PBDT-TBT-C12
a 1 ꢁ 10ꢀ5 M in anhydrous chloroform. b Spin-coated from a 10 mg/mL chloroform solution. The optical band gap (Eg,opt) was obtained from
c
absorption edge.
were present in the solid state and were probably as strong as
those of regioregular P3HT. Especially for the copolymers
(PBDT-TBT-C6 to PBDT-TBT-C12) with different length
alkyl chains attaching to thiophene moiety, the twisted backbone
were extended and formed a higher coplanar backbone due
to the πꢀπ stacking in film.33 The optical band gaps Eg,opt of
the copolymers derived from the absorption edge of the thin
film spectra are around 1.75 eV (Table 2), the low band gap can
improve light harvesting and enhance the photocurrent of
the PVCs.
Electrochemical Properties. The cyclic voltammetry (CV)
diagrams of the copolymers were obtained by using (TBA)PF6 as
supporting electrolyte in acetonitrile solution with a platinum
button working electrode, a platinum wire counter electrode, and
a Ag/AgNO3 reference electrode under the N2 atmosphere.
Ferrocene was used as the internal standard. The results of the
Figure 3. Cyclic voltammetry curves of copolymers films on platinum
electrochemical measurements are listed in Table 2. Because of
the similar cyclic voltammetry curves of copolymers PBDT-
TBT-C6, PBDT-TBT-C8, PBDT-TBT-C10, and PBDT-TBT-
C12, Figure 3 shows only the cyclic volatammetry curves of
copolymer PBDT-TBT and PBDT-TBT-C12. It can be seen
from Figure 3 that there are irreversible p-doping/dedoping
(oxidation/reduction) processes in the positive potential range
for the copolymers. However, there are reversible n-doping/
dedoping (reduction/reoxidation) processes in the negative
electrode in 0.1 mol/L n-Bu4NPF6 in CH3CN solution, at a scan rate of
100 mV/s.
onset
red
LUMO ðeVÞ ¼ ꢀ eðE
þ 4:72Þ ðeVÞ
ð2Þ
Eg, ec ¼ ꢀ ðHOMO ꢀ LUMOÞ ðeVÞ
ð3Þ
onset
onset
red
where E
and E
are the measured potentials relative to Ag/
ox
potential range for the copolymers. Meanwhile, the onset oxida-
red
Ag+. The electrochemical properties as well as the energy level
parameters of polymers are list in Table 2.
onset
tion (Eoonxset) and reduction potential (E ) of all the copolymers
are almost the same.
The estimated HOMO and LUMO energy levels of the
copolymers are all around ꢀ5.25 and ꢀ3.53 eV, respectively. It
indicated that the HOMO and LUMO energy levels of the
copolymers are more dependent on the electron-donating and
electron-accepting moiety, which can dominate the electronic
delocalization intensity. In addition, the weak electron-donating
alkyl chain attached to the adjacent thiophene of BT-alkoxy
moieties did not affect the electron density distribution for both
HOMO and LUMO energy level.
The energy levels of the donor polymers are very important for
a high performance photovoltaic cell. First, the polymers should
have good air stability with a HOMO energy level being below
the air oxidation threshold (ca. ꢀ5.2 eV).36 Second, the relatively
low HOMO level of the polymers can allow a high open circuit
voltage (Voc) for the photovoltaic cell.37 Third, the LUMO level
of the donor has to be at least 0.3 eV higher than that of the
acceptor (e.g., PCBM) to guarantee the formation of a downhill
driving force for the energetically favorable electron transfer and
Since reduction potential of the DꢀA structure copolymer is
mainly determined by the electron-accepting moiety,34 all the
copolymers, which have a same alkoxy-BT moieties with stronger
electron-accepting ability, showed similar Eorendset. Meanwhile, the
copolymers also exhibited the nearly unchanged onset oxidation
potential (Eoonxset) (in the range of 0.52 to 0.58 V), which is caused
by unchangeable electron-donating BDT and thiophene moi-
eties. Therefore, it can be concluded that the alkyl with different
length attached to the adjacent thiophene of BT-alkoxy moieties
onset
showed no effect on both the Eorendset and E . The redox potential
ox
of Fc/Fc+ which has an absolute energy level of ꢀ4.8 eV relative
to the vacuum level for calibration is located at 0.08 V in 0.1 M
(TBA)PF6/acetonitrile solution.35 So the evaluation of HOMO
and LUMO levels, as well as the band gap (Eg,ec), could be done
according to the following equations:
onset
ox
HOMO ðeVÞ ¼ ꢀ eðE
þ 4:72Þ ðeVÞ
ð1Þ
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Table 3. Characteristic CurrentꢀVoltage Parameters From Device Testing at AM 1.5G White Light Conditions, Blend Films
Roughness of AFM Measurement
copolymer/PCBM (w/w ratio)
solvent
Voc (V)
Jsc(mA/cm2)
FF
PCE(%)
rmsc (nm)
PBDT-TBTa
1:3
1:3
1:3
1:3
1:3
1:3
CB
0.68
0.74
0.70
0.72
0.71
0.71
3.48
4.50
7.19
3.95
2.19
8.60
0.47
0.44
0.52
0.37
0.33
0.51
1.11
1.92
2.88
1.04
0.51
3.15
2.33
0.83
1.73
4.86
5.13
1.32
PBDT-TBT-C6a
PBDT-TBT-C8a
PBDT-TBT-C10a
PBDT-TBT-C12a
PBDT-TBT-C8b
CB
CB
CB
CB
o-DCB
a The active layer based on the blend film of copolymers and PC61BM. b The active layer based on the blend film of copolymers and PC71BM. c Root
mean-square (rms) roughness from AFM measurement.
to overcome the binding energy of the intrachain exciton.38,39
The HOMO energy levels of the copolymers (around ꢀ5.25 eV)
showed good air stability. Meanwhile, the LUMO energy levels
difference between donor and acceptor is more than 0.3 eV,
which guarantees the sufficient driving force for exciton dissocia-
tion. It is worth noting that the values of electrochemical band
gap (Eg,ec) of copolymers are well consistent with that of optical
band gap (Eg,opt).
Photovoltaic Properties. In order to investigate the photo-
voltaic properties of the copolymers, the BHJ photovoltaic cells,
with a structure of ITO/PEDOTꢀPSS/Copolymers:PCBM/
LiF/Al, were fabricated, where the copolymers were used as
donors and PC61BM or PC71BM as acceptor. The active layers of
the photovoltaic cells were fabricated from CB or o-DCB
solutions of copolymers and PC61BM or PC71BM by spin-
coating. Different conditions including the weight ratio between
Figure 4. Currentꢀvoltage characteristics of optimized photovoltaic
donor and acceptor and the thickness of the active layer were
optimized. The optimized performances were achieved with the
weight ratio of copolymers:PC61BM at 1:3 (w/w). The thickness
of the active layer is 70ꢀ90 nm, which could be controlled by
changing both the spin-coating rate and the concentration of the
solution. Because of the high performance of PBDT-TBT-C8, we
also fabricated the photovoltaic cell with the active layer (around
100 nm) prepared by spin-coating o-DCB solutions consisting of
PBDT-TBT-C8 and PC71BM with the weight ratio of at 1:3
(w/w). All devices were characterized in the dark and under
AM1.5 (100 mW/cm2) white light with simultaneous recording of
their currentꢀvoltage characteristics. The optimized photovoltaic
performance characteristics of each PVCs are given in Table 3.
The obtained currentꢀvoltage curves are presented in
Figure 4. The cell based on PBDT-TBT:PC61BM (1:3 w/w in
CB solution) showed an open circuit voltage (Voc) of 0.68 V, a
short circuit current density (Jsc) of 3.48 mA/cm2, a fill factor
(FF) of 0.47, giving a PCE of 1.11%. Another two cells based on
PBDT-TBT-C6:PC61BM and PBDT-TBT-C8:PC61BM showed
Voc of 0.74 and 0.70 V, significantly increasing Jsc to 4.50 and 7.16
mA/cm2, FF to 0.44 and 0.52, and PCEs to 1.92% and 2.88%,
respectively. The PCEs of the above two copolymer based devices
were greatly improved by increasing the length of alkyl chains
attached to thiophene moiety. However, with the length of alkyl
chains continuously increased, the performances of the cells based
on PBDT-TBT-C10:PC61BM and PBDT-TBT-C12:PC61BM
were dropped, and showed Jsc of 3.95 and 2.19 mA/cm2, Voc of
0.72 and 0.71 V, PCEs of 1.04% and 0.51%, respectively.
cells based on PBDT-TBT, PBDT-TBT-C6, PBDT-TBT-C8, PBDT-
TBT-C10, and PBDT-TBT-C12 under illumination of AM 1.5, 100
mW/cm2 white light. Key: (a) the active layer of copolymer and
PC61BM; (b) the active layer of copolymer and PC71BM.
Because of the high electron-donating ability and excellent hole
transporting properties of the two thiophene units bridged by
benzene (BDT) moiety and the adjacent thiophene moieties of
alkoxy-BT, the copolymers possessed relatively deeper HOMO
energy level, which lead to the moderate Voc (in the range of 0.68
to 0.74 V). Noticeably, with the increasing length of alkyl chains
attached to thiophene moiety, the HOMO energy levels of the
copolymers are almost the same, which may be caused by
the weak electron-donating ability of alkyl chains. Meanwhile,
the invariable HOMO energy levels also lead to the similar
Voc. Therefore, the well consistent tendency between HOMO
energy level and Voc of copolymers with varied length of alkyl
chains further confirmed that the HOMO energy levels can
effectively affect the Voc. At the same time, the PBDT-TBT-C8:
PC61BM based device also showed higher Jsc than the other four
copolymer/PC61BM based devices, the reason for which is not
as simple as that in the case of the Voc. The Jsc of polymer/
fullerene BHJ solar cells are influenced by their absorption
spectrum, the carrier mobility and also the interpenetrating
nanostructure formed by the blends of the copolymer and
fullerene.
Film morphology of the active layer has been found to be one
of the key elements in determining the PCE of polymer photo-
voltaic cell.40,41 To gain better insight into what might control
the short circuit current, and hence lead to the varied perfor-
mances of PVCs, atomic force microscopy (AFM) and transmission
It is generally considered that the Voc is mainly determined by
the difference between the HOMO energy level of the donor
(copolymer) and LUMO energy level of the acceptor (PCBM).
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Figure 5. Topography image obtained by tapping-mode AFM showing the morphology of the blend films spin-coated from chlorobenzene for (a)
PBDT-TBT/PC61BM (w/w, 1:4) (size 5 μm ꢁ 5 μm); (b) PBDT-TBT-C6/PC61BM (w/w, 1:3) (size 5 μm ꢁ 5 μm); (c) PBDT-TBT-C8/PC61BM
(w/w, 1:3) (size 5 μm ꢁ 5 μm); (d) PBDT-TBT-C10/PC61BM (w/w, 1:3) (size 5 μm ꢁ 5 μm); and (e) PBDT-TBT-C12/PC61BM (w/w, 1:3)
(size 5 μm ꢁ 5 μm).
electron microscopy (TEM) were used to examine the topogra-
phy of the active layer. Here, the AFM height topography image
were mainly used to investigate the surface morphology of the
active layer, which can obtain the roughness and aggregation of
the blend film, hence it reflect the solubility of the copolymers
and the miscibility between the copolymer and PC61BM.
Figure 5a shows the AFM height images of PBDT-TBT:PC61BM
blend film with the optimized weight ratio (1:3 w/w). As is
clearly evidenced by AFM, the PBDT-TBT:PC61BM blend film
showed a relative higher coarse surface with the root-mean-
square (rms) of 2.33 nm (Figure 5a and Table 3), and larger
PCBM aggregation compared with that obtained at a blend film
of PBDT-TBT-C6:PC61BM (rms = 0.83 nm). The rougher
surfaces of PBDT-TBT:PC61BM blend film presumably arose
from poor solubility of the copolymer PBDT-TBT (no alkyl
chains attached to thiophene moiety), which leads to aggregation
of PCBM that might limit the degree of charge dissociation and
reduce the photocurrent. In the case of copolymer PBDT-TBT-
C6 (Figure 5b), where the alkyl chains (six carbons) attached to
thiophene moiety, the smooth surface of the blend film PBDT-
TBT-C6:PC61BM indicates the good solubility of the PBDT-
TBT-C6 and better miscibility between PBDT-TBT-C6 and
PC61BM, which leads to the improved Jsc of PBDT-TBT-C6:
PC61BM cell (4.5 mA/cm2). This result implies that there is still
room for improvement in terms of the charge separation inter-
face in copolymer/PC61BM. With the continuous increase
length of alkyl chains (eight carbon) attached to thiophene
moiety, the morphology of PBDT-TBT-C8:PC61BM blend film
(Figure 5c) shows no obvious aggregation of PCBM and the rms
increases to 1.73 nm, which leads to the great increase of Jsc (7.19
mA/cm2). Interestingly, when the length of alkyl chains con-
tinually increased to ten and 12 carbons, the grain-aggregation in
the blend film PBDT-TBT-C10:PC61BM and PBDT-TBT-C12:
PC61BM appeared again (as shown in Figure 5d,e), and the
aggregated PCBM was almost homogeneously dispersed in the
copolymers matrix (rms is 4.86 and 5.13 nm, respectively). This
resulted in a large-scale phase separation between copolymers
and PC61BM, and hence decreased Jsc to 3.95 and 2.19 mA/cm2
based on the devices of PBDT-TBT-C10:PC61BM and PBDT-
TBT-C12:PC61BM, respectively. The degenerated device per-
formances may be caused by the decreased diffusional escape
probability for the mobile charge carriers and increased recombi-
nation. It is important to note that even though the PBDT-TBT-
C8:PC61BM blend film shows a relatively rough morphology in
the AFM height image, among all the material combinations, the
solar cells of PBDT-TBT-C8:PC61BM gave the highest Jsc,
suggesting the appropriate coarse phase separation between
the copolymers and PC61BM in these cells. Meanwhile, the FF
show the decreased values from 0.33 to 0.52 for the increasing
aggregation of PC61BM. From these results, it is clear that the
dramatic varied Jsc and FF observed in the copolymer:PCBM
photovoltaic devices is mostly due to the morphology problem
coming from poor miscibility and different self-assembly prop-
erty between the copolymers and PC61BM.
The purported ideal morphology is comprised of an nanoscale
interpenetrating network between donor and acceptor, which
enables a large interface area for exciton dissociation and con-
tinuous percolating paths for hole and electron transport to the
corresponding electrodes.42 In order to further investigate the
relation between the blend film morphology and Jsc, the copo-
lymers:PCBM blend films were observed by TEM (Figure 6),
which can directly determine the composition and more real
morphology of the blend film comparing with the AFM mor-
phology. The polymer networks and porous regions are imaged
in the TEM as darker (PCBM) and lighter (copolymer) colored,
respectively.43 From the TEM images of the PBDT-TBT:
PC61BM (1:3 w/w) blend films (Figure 6a), which have no alkyl
chains attached to thiophene moiety, we can observe substantial
pea-like PCBM aggregation with obvious phase separation. In
the case of PBDT-TBT-C6:PC61BM blend film (Figure 6b), the
alkyl chain with six carbon was attached to thiophene moiety, the
PCBM aggregation disappeared completely and the blend film
exhibited very smooth surface morphology and uniform mixing,
which can be further confirmed by the low rms value of its
corresponding AFM image. In contrast, the TEM images of
PBDT-TBT-C8:PC61BM blend film showed nanoscale interpe-
netrating network with a finer porous and nanoscale phase separa-
tion structure (Figure 6c), which can facilitate better percolation of
the carriers and hence lead to the highest Jsc of 7.19 mA/cm2.
Accordingly, the proper nanoscale phase separation with no
obvious aggregation between electron donor and acceptor mol-
ecules allows for large areas of interfaces for better photogenerated
charges and desirable Jsc. Meanwhile, the sufficient carrier separa-
tion on interface can also decrease the possibility of recombination
of holes and electrons in the network with high interfacial areas,
which lead to a reduced series resistance. This might be the reason
why the PBDT-TBT-C8:PC61BM based devices possess higher
FF values (0.52).44 Noticeably, with the increasing length of
alkyl chains, the TEM images of the PBDT-TBT-C10:PC61BM
and PBDT-TBT-C12:PC61BM (Figure 6d,e) showed a larger size
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Figure 6. TEM image obtained by the blend films spin-coated from chlorobenzene for (a) PBDT-TBT/PC61BM (w/w, 1:3); (b) PBDT-TBT-C6/
PC61BM (w/w, 1:3); (c) PBDT-TBT-C8/PC61BM (w/w, 1:3); (d) PBDT-TBT-C10/PC61BM (w/w, 1:3); and (e) PBDT-TBT-C12/PC61BM
(w/w, 1:3).
Figure 8. External quantum efficiencies (EQE) spectra of devices based
on copolymers:PC61BM.
Figure 7. XRD patterns of blend films spin-coated from chlorobenzene
for copolymers (PBDT-TBT, PBDT-TBT-C6, PBDT-TBT-C8, PBDT-
TBT-C10, and PBDT-TBT-C12)/PC61BM (w/w, 1:3).
studies revealed the crystallinity and π-stacking of the copoly-
mers in blend film. Only the PBDT-TBT/PC61BM film showed a
peak at q = 0.28 Å, which indicates that the copolymer PBDT-
TBT without alkyl chain substitution on the thiophene moiety
crystallized in the blend film.25 The crystallization of copolymer
can increase the ordering in blend film which can facilitate the
electronic communication across the interface and enhance hole
extraction and block undesired electron leakage. Therefore,
device based on the PBDT-TBT/PC61BM lead to the relative
high FF (0.47). All the blend films showed the same πꢀπ
stacking peaks at around q = 1.38 Å, which indicates that the
varied alkyl chain substitution attached to the thiophene unit may
not affect the πꢀπ stacking (d spacing with 4.55 Å) interaction
significantly in this polymer series, but presumably affects PCBM
interactions with the varied alkyl chain of copolymers, which
have been confirmed in the morphology characterization. The
unchanged πꢀπ stacking d spacing values (4.55 Å) of copoly-
mers with varied alkyl chain are well consistent with the Chen’s
report of the πꢀπ stacking spacing in films.26
grain-like aggregation. Because of the limited diffusion length of
excitons (10 nm), which is much smaller than the size grain-like
aggregation (50ꢀ200 nm) seen in the blend film images of
PBDT-TBT-C10:PC61BM and PBDT-TBT-C12:PC61BM, photo-
generated excitons will mainly recombine before reaching the
interfaces of the donor and acceptor, resulting in reduced charge
carrier generation and a concomitant loss of photocurrent
(reduce to 3.95 mA/cm2 and 2.19 mA/cm2). The nonoptimized
morphology can reduce hole or electron drift length (or both of
holes and electrons). The space charge accumulation caused by
sluggish charge carriers subsequently induces a nonuniform
electric field, which gives rise to a decreased photocurrent with
strong electric field dependence, resulting in low FF (0.37 and
0.33).45 These results showed that solar cell properties strongly
depend on the solid-state structures, and also fully demonstrated
that the morphology of the blend film can be well controlled
by molecular engineering of the copolymers. The different
miscibility and self-assembly structure between copolymer and
PC61BM can significantly influence transport properties, Jsc and
FF of the device to a large extent. Control of the blend film
morphology between copolymer and PC61BM and the formation
of interpenetrating network structure domains may improve
charge transport, depress leakage current, and, in turn, increase
Jsc, FF, and even Voc.
The external quantum efficiencies (EQE) of PVCs based on all
the copolymers blended with PC61BM under monochromatic
illumination are shown in Figure 8. The shape of the EQE curve
of the devices based on copolymer:PCBM are very similar to
their absorption spectrum, indicating that most of the absorptive
photons of the copolymer contributed to the photovoltaic con-
version. The spectral responses of the five copolymer:PC61BM
based devices reveal significant contributions of the EQE at
wavelengths between 300 and 700 nm, with the maximum EQE
Besides the miscibility with PC61BM, the varied crystallinity
and molecular packing of the copolymers in the blend films may
also influence the device efficiency. As shown in Figure 7, XRD
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in the range of 57% to 29% at around 460 nm. Meanwhile, the Jsc
calculated from the EQE are well consistent with the measured Jsc
in trend. It is worth noting that the maximum spectral responses
wavelength of the blend film morphologies with larger phase
separation (PBDT-TBT-C10 and PBDT-TBT-C12) are generally
blue shift about 20 nm comparing with that obtained from the
smooth or uniform blend film (PBDT-TBT, PBDT-TBT-C6, and
PBDT-TBT-C10), which may be caused by the reduced charge
carrier generation and a concomitant loss of photocurrent. Because
of the high performance of the device based on the PBDT-TBT-
C8/PC61BM and the better absorption spectrum of PC71BM as an
acceptor, the device based on the PBDT-TBT-C8:PC71BM in
o-DCB solution was prepared. The device shows a Voc of 0.71 V, a
Jsc of 8.60 mA/cm2, a FF of 0.51, and a PCE of 3.15%.
& Technology Innovation Cultivation Program of Soochow
University, a Project Funded by the Priority Academic
Program Development of Jiangsu Higher Education Institutions,
and National “973” Project (2011CB606004) is gratefully
acknowledged.
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Corresponding Author
*E-mail: tuyingfeng@suda.edu.cn. Telephone: +86512 65882130.
Fax: +86512 65882130.
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