Synthesis and characterization of thieno[3,2-b]thiophene-isoindigo-based copolymers as electron donor and hole transport materials for bulk-heterojunction polymer solar cells

Article abstract of DOI:10.1002/pola.26400

A novel class of thieno[3,2-b]thiophene (TT) and isoindigo based copolymers were synthesized and evaluated as electron donor and hole transport materials in bulk-heterojunction polymer solar cells (BHJ PSCs). These π-conjugated donor-acceptor polymers were derived from fused TT and isoindigo structures bridged by thiophene units. The band-gaps and the highest occupied molecular orbital (HOMO) levels of the polymers were tuned using different conjugating lengths of thiophene units on the main chains, providing band-gaps from 1.55 to 1.91 eV and HOMO levels from -5.34 to -5.71 eV, respectively. The corresponding lowest unoccupied molecular orbital (LUMO) levels were appropriately adjusted with the isoindigo units. Conventional BHJ PSCs (ITO/PEDOT:PSS/active layer/interlayer/Al) with an active layer composed of the polymer and PC ₇₁BM were fabricated for evaluation. Power conversion efficiency from a low of 1.25% to a high of 4.69% were achieved with the best performing device provided by the D-π-A polymer with a relatively board absorption spectrum, high absorption coefficient, and more uniform blend morphology. These results demonstrate the potential of this class of thieno[3,2-b]thiophene-isoindigo- based polymers as efficient electron donor and hole transport polymers for BHJ PSCs.

Full text of DOI:10.1002/pola.26400

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Synthesis and Characterization of Thieno[3,2-b]thiophene-isoindigo-based
Copolymers as Electron Donor and Hole Transport Materials for
Bulk-Heterojunction Polymer Solar Cells
Xiaofeng Xu,1 Ping Cai,2 Yong Lu,1 Ng Siu Choon,1 Junwu Chen,2 Xiao Hu,3 Beng S. Ong1,4
1School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459
2State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices,
South China University of Technology, Guangzhou 510640, China
3School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798
4Institute of Creativity and Department of Chemistry, Hong Kong Baptist University, Hong Kong, China
Correspondence to: Ng. S. Choon (E-mail: NgSC@ntu.edu.sg) or B. S. Ong (E-mail: bong@hkbu.edu.hk)
Received 19 August 2012; accepted 30 September 2012; published online 19 October 2012
DOI: 10.1002/pola.26400
ABSTRACT: A novel class of thieno[3,2-b]thiophene (TT) and
isoindigo based copolymers were synthesized and evaluated
as electron donor and hole transport materials in bulk-hetero-
junction polymer solar cells (BHJ PSCs). These p-conjugated
donor-acceptor polymers were derived from fused TT and iso-
indigo structures bridged by thiophene units. The band-gaps
and the highest occupied molecular orbital (HOMO) levels of
the polymers were tuned using different conjugating lengths of
thiophene units on the main chains, providing band-gaps from
1.55 to 1.91 eV and HOMO levels from ꢀ5.34 to ꢀ5.71 eV,
respectively. The corresponding lowest unoccupied molecular
orbital (LUMO) levels were appropriately adjusted with the iso-
indigo units. Conventional BHJ PSCs (ITO/PEDOT:PSS/active
layer/interlayer/Al) with an active layer composed of the poly-
mer and PC71BM were fabricated for evaluation. Power conver-
sion efficiency from a low of 1.25% to a high of 4.69% were
achieved with the best performing device provided by the
DꢀpꢀA polymer with a relatively board absorption spectrum,
high absorption coefficient, and more uniform blend morphol-
ogy. These results demonstrate the potential of this class of
thieno[3,2-b]thiophene-isoindigo-based polymers as efficient
electron donor and hole transport polymers for BHJ PSCs.
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2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem
51: 424–434, 2013
KEYWORDS: bulk-heterojunction polymer solar cells; charge
transport; conjugated polymers; isoindigo; optical properties;
thieno[3,2-b]thiophene
INTRODUCTION During the past decade, polymer solar cells
(PSCs) have attracted growing attention from academic and
industry communities as a potentially viable solar energy
harvesting medium by virtue of their ability in providing
light weight and large-area devices at low manufacturing
costs.1,2 The low manufacturing cost is made possible via
conventional low-cost solution-based fabrication techniques
such as coating and printing. From this perspective, bulk-het-
erojunction polymer solar cells (BHJ PSCs), which utilize a
solution-processed active layer composed of an electron
donor/hole transport component and an electron acceptor/
electron transport counterpart, sandwiched between an ITO
anode and a low work-function metal cathode, have been
intensively investigated.3–6 For optimum cell performance,
broad-absorption and narrow band-gap polymers would be
highly desirable as they are more efficient in capturing solar
energy.7–12 In general, narrow band-gap polymers can be
constructed from a polymer framework with an alternating
electron-rich donor (D) and an electron-deficient acceptor
(A) repeating unit, The conjugated DꢀA polymer system
offers an opportunity for tuning the band-gaps and energy
levels through the donor and acceptor strengths and nature
of p-conjugation. An ideal electron donor/hole transport
polymer for BHJ PSC would require a low-lying HOMO level
to ensure a high open-circuit voltage (Voc) and an intense
broad absorption spectrum to maximize the short circuit
current (Jsc). In addition, the polymer should enable forma-
tion of proper morphology with continuous nanostructures
of hole and electron transport components with maximum
interfacial areas as the morphology has a profound influence
on exciton dissociation, and thus the performance of BHJ
PSC.13 Likewise, interfacial modification between the active
layer and electrodes can also facilitate extraction of exciton
into respective electrodes, which would lead to further
improvements in photovoltaic performance.14–16 To date,
many structurally diverse systems of narrow band-gap
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polymers have been synthesized for use in BHJ PSCs, with the
power conversion efficiency (PCE) reaching up to 8%.17–27
Nevertheless, the rational design of novel DꢀA conjugated
copolymers with good photovoltaic property remains a key in-
terest in synthesis. Driving organic photovoltaic PCE up to an
economically viable level would require further development
in electron donor and hole transport materials.
on polymers P1, P2, and P3 were 1.25, 4.69, and 3.24%,
respectively. The film morphology of all the blend layers and
the hole mobility of polymer P2 were also investigated. Com-
parison of polymer main chains based on thieno[3,2-b]thio-
phene and isoindigo bridged with diverse n-alkylthiophene
units will provide some useful information on the influence
of DꢀpꢀA structure for photovoltaic properties of isoindigo-
based copolymers.
Not surprisingly, we have recently witnessed intense activ-
ities in exploring new electron donor and acceptor structures
for building more efficient hole transport polymers. In this
respect, the isoindigo-based acceptors have been utilized
with different electron-rich moieties (e.g., thiophene, bithio-
phene, terthiophene, and benzodithiophene) for the con-
struction of DꢀA polymers.28–33 One of these copolymers
exhibited promising photovoltaic results in BHJ PSCs with
PCE reaching as high as 6.3%,34 In addition, this class of
DꢀA polymers as stable active materials have also shown
high hole mobility in organic field-effect transistors.35 None-
theless, several research about isoindigo-based DꢀA poly-
mers exhibited limited PCE below 3%. One of the challenges
besetting the isoindigo-based DꢀA polymers is poor solubil-
ity due to the rigid ketopyrrole cores. This difficulty in
attaining either highly ordered molecular assembly or favor-
able layer morphology has adversely affected the device
performance.
EXPERIMENTAL
Materials
All reagents and solvents, unless otherwise specified, were
obtained from Aldrich, Alfa Aesar, and TCI Chemical and
were used as received. Anhydrous diethyl ether, tetrahydro-
furan (THF) and toluene were distilled over sodium/benzo-
phenone under N2 prior to use. All manipulations involving
air-sensitive reagents were performed under an argon
atmosphere.
3,6-Dioctylthieno[3,2-b]thiophene,37 2,5-bis(trimethylstannyl)
thieno[3,2-b]thiophene (5),42 (E)-6,60-dibromo-1,10-bis(2- butyloc-
tyl)-[3,30-biindolinylidene]-2,20-dione (6),34 and (E)-6,60-dibromo-
1,10-bis(2-octyldodecyl)-[3,30-biindolinylidene]-2,20- dione (7)34
were prepared according to the literatures.
Measurements
1H and 13C NMR spectra were recorded on a Bruker AV 300
spectrometer with tetramethylsilane as the internal refer-
ence. Molecular weights of the polymers were obtained on a
Waters 2695 GPC using a calibration curve of polystyrene
standards, with THF as the eluent. Elemental analyses were
performed on a Vario EL elemental analysis instrument (Ele-
mentar). UV–vis absorption spectra were recorded on a Shi-
madzu UV 2450 spectrophotometer. Thermogravimetric anal-
ysis (TGA) was carried out on a Perkiꢀn1 Elmer Diamond
TGA/DTA, at a heating rate of 10 ꢁC min from 50 to 650
ꢁC under a nitrogen atmosphere. Cyclic voltammetry was
carried out on a CHI660A electrochemical workstation with
platinum electrodes at a scan rate of 50 mV secꢀ1 against
the Ag/Agþ reference electrode with an argon saturated so-
lution of 0.1 M tetrabutylammonium hexafluorophosphate
(Bu4NPF6) in acetonitrile (CH3CN). The deposition of a poly-
mer on the electrode was done by the evaporation of its
dilute chlorobenzene (CB) solution.
In this article, we report the synthesis of four novel
thieno[3,2-b]thiophene (TT) and isoindigo based copolymers,
P1, P2, P3, and P4 via Pd-catalyzed Stille coupling polymer-
ization. The use of fused TT unit affords a strong electron-
rich moiety, which may also enhance hole transporting
properties of the resulting polymers.36,37 Introduction of
alkyl pendant chains into TT unit would serve to further
increase the polymer solubility. The relatively planar struc-
tures of 2,5-bis(3-alkylthiophen-2-yl)-TT and 3,6-alkyl-2,5-
di(thiophen-2-yl)-TT moieties on the polymer chains would
be expected to facilitate pꢀp stacking to attain more ordered
molecular assembly of conjugated main chains, which can
also extend the coplanarity of polymer backbones and would
be conducive to intermolecular charge-carrier hopping.38–40
In addition, the TT units can modify the effective conjugation
length of polymer chains, thus suppressing the HOMO levels
of some high-mobility polythiophenes.38,41
A low-lying
HOMO level is a precondition for high Voc achievement for
BHJ solar cells. The alternating TT and isoindigo moiety
bridged by diverse n-alkylthiophene units afforded four nar-
row band-gap copolymers with different conjugated length
of backbones. Their thermal stability, UV–vis absorption and
electrochemical properties were investigated to understand
their structure-property relationships. Polymers P1, P2, and
P3 exhibited good solubility characteristics and could be
used in fabrication of BHJ PSCs. Conventional BHJ PSCs
(ITO/PEDOT:PSS/active layer/interlayer/Al) with PC71BM as
the electron acceptor/electron transport material and the
conjugated polymer as electron donor/hole transport mate-
rial were fabricated. In addition, various processing solvents
and additives were also investigated to further improve on
the device performance. The observed PCE of devices based
Monomer Synthesis
2,5-Bis(trimethylstannyl)-3,6-dioctylthieno
[3,2-b]thiophene (1)
To a mixture of 3,6-dioctylthieno[3,2-b]thiophene (0.55 g,
1.50 mmol) and N,N,N,0N0-tetramethylethylenediamine
(TMEDA) (0.35 g, 3.0 mmol) in 20 mL of anhydrous diethyl
ether was added a 1.6 M solution of n-BuLi in hexane (2.81
mL, 4.50 mmol) at 0 ꢁC under an argon atmosphere. The
reaction mixture was warmed to ambient temperature and
stirred for 3 h. Then the reaction mixture was cooled to 0 C
again and Me3SnCl (1.20 g, 6.00 mmol) was added in one
portion. The reaction mixture was stirred at 0 C for 30 min
and then warmed to room temperature and stirred
overnight. The resulting mixture was poured into water. The
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organic layer was extracted with hexane (50 mL), washed
successively with water, and then dried over anhydrous
MgSO4. The solution was concentrated to afford the final
compound as a light yellow liquid (0.99 g, 96%). 1H NMR
(300 MHz, CDCl3), d (ppm): 2.68 (t, J ¼ 7.9 Hz, 4H), 1.55 (m,
4H), 1.28 (m, 20H), 0.89 (m, 6H), and 1.40 (s, 18H). 13C
NMR (75 MHz, CDCl3), d (ppm): 146.8, 141.8, 134.5, 32.4,
31.9, 30.1, 29.9, 29.5, 29.2, 22.7, 14.1, and ꢀ7.9. Anal. Calcd
(%) for C28H52S2Sn2: C, 48.72; H, 7.59; S, 9.26. Found: C,
48.61; H, 7.75; S, 9.31.
mmol), and P(o-tol)3 (0.04 mg, 0.12 mmol) were dissolved
in 80 mL anhydrous THF. The reaction mixture was heated
to 80 ꢁC for 12 h under an argon atmosphere. After cooling
to room temperature, the organic layer was extracted with
dichloromethane (100 mL), washed successively with water
and then dried over anhydrous MgSO4. The residue was
purified by column chromatography with 1:10 (v/v)
dichloromethane/hexane as the eluent to give the final com-
pound as a purple solid (11.48 g, 73%). 1H NMR (300 MHz,
CDCl3), d (ppm): 9.15 (d, J ¼ 8.4 Hz, 2H), 7.25 (m, 4H), 6.94
(d, J ¼ 1.2 Hz, 4H), 3.68 (d, J ¼ 6.9 Hz, 4H), 2.63 (t, J ¼ 7.7
Hz, 4H), 1.91 (s, 2H), 1.65 (m, 4H), 1.28 (m, 92H), 0.88 (m,
18H). 13C NMR (75 MHz, CDCl3), d (ppm): 168.6, 145.6,
144.7, 143.7, 137.9, 131.9, 130.1, 125.5, 120.9, 120.8, 119.0,
104.8, 44.4, 36.4, 31.93, 31.91, 31.8, 30.7, 30.5, 30.0, 29.9,
29.69, 29.67, 29.63, 29.60, 29.5, 29.4, 29.39, 29.37, 29.33,
28.3, 26.8, 26.7, 22.71, 22.69, 17.3, 14.1, and 13.6. Anal.
Calcd (%) for C84H134N2O2S2: C, 79.56; H, 10.65; N, 2.21; S,
5.06. Found: C, 79.67; H, 10.78; N, 2.29; S, 5.12.
3,6-Dioctyl-2,5-di(thiophen-2-yl)thieno[3,2-b]thiophene (3)
2,5-Dibromo-3,6-dioctylthieno[3,2-b]thiophene (1.34 g, 2.56
mmol), 2-(tributylstannyl)thiophene (2.39 g, 6.40 mmol),
Pd(PPh3)4 (0.09 g, 0.08 mmol) were dissolved in 50 mL
anhydrous THF. The reaction mixture was heated to 80 ꢁC
for 12 h under an argon atmosphere. After cooling to room
temperature, the organic layer was extracted with dichloro-
methane (100 mL), washed successively with water and then
dried over anhydrous MgSO4. The residue was purified by
column chromatography with hexane as the eluent to give
(E)-6,60-bis(5-bromo-4-decylthiophen-2-yl)-1,10-bis
1
(2-octyldodecyl)-[3,30-biindolinylidene]-2,20-dione (9)
N-Bromosuccinimide (NBS) (0.89 g, 5.02 mmol) in 10 mL
N,N-dimethylformamide (DMF) was added dropwise into
compound (8) (3.17 g, 2.5 mmol) in 50 mL chloroform over
30 min under an argon atmosphere. The reaction mixture
was stirred at room temperature for another 30 min. The
resulting mixture was poured into water. The organic layer
was extracted with dichloromethane (100 mL), washed suc-
cessively with water, and then dried over anhydrous MgSO4.
The residue was purified by column chromatography with
1:9 (v/v) dichloromethane/hexane as the eluent to give the
final compound as a dark purple solid (3.30 g, 92%). 1H
NMR (300 MHz, CDCl3), d (ppm): 9.15 (d, J ¼ 8.4 Hz, 2H),
7.18 (d, J ¼ 6.6 Hz, 2H), 7.08 (s, 2H), 6.84 (s, J ¼ 1.5 Hz,
2H), 3.67 (d, J ¼ 7.2 Hz, 4H), 2.68 (t, J ¼ 7.7 Hz, 4H), 1.90
(s, broad, 2H), 1.62 (m, 4H), 1.26 (m, 92H), 0.85 (m, 18H).
13C NMR (75 MHz, CDCl3), d (ppm): 168.5, 145.7, 143.6,
143.3, 137.0, 131.8, 130.3, 124.9, 121.2, 118.5, 110.0, 104.4,
44.4, 36.4, 31.94, 31.93, 31.90, 31.8, 30.0, 29.8, 29.75, 29.70,
29.66, 29.64, 29.5, 29.4, 29.36, 29.35, 26.7, 22.7, and 14.1.
Anal. Calcd (%) for C84H132Br2N2O2S2: C, 70.76; H, 9.33; N,
1.96; S, 4.50. Found: C, 70.85; H, 9.51; N, 2.05; S, 4.59.
the final compound as a yellow solid (1.12 g, 83%). H NMR
(300 MHz, CDCl3), d (ppm): 7.35 (d, J ¼ 7.8 Hz, 2H), 7.17 (d,
J ¼ 4.8 Hz, 2H), 7.09 (m, 2H), 2.88 (t, J ¼ 7.9 Hz, 4H), 1.75
(m, 4H), 1.28 (m, 20H), 0.89 (t, J ¼ 7.5 Hz 6H). 13C NMR (75
MHz, CDCl3), d (ppm): 138.5, 136.6, 132.0, 131.3, 127.5,
126.2, 125.7, 31.9, 29.7, 29.4, 29.2, 29.1, 28.8, 22.7, and 14.1.
Anal. Calcd (%) for C30H40S4: C, 68.13; H, 7.62; S, 24.25.
Found: C, 68.25; H, 7.75; S, 24.28.
2,5-Bis(5-trimethylstannyl-thienyl-2yl)-3,6-dioctylthieno
[3,2-b]thiophene (4)
To a mixture of 3,6-dioctyl-2,5-di(thiophen-2-yl)thieno[3,2-
b]thiophene (3) (0.40 g, 0.76 mmol) and TMEDA (0.18 g,
1.52 mmol) in 15 mL of anhydrous diethyl ether was added
a 1.6 M solution of n-BuLi in hexane (1.43 mL, 2.28 mmol)
at 0 ꢁC under an argon atmosphere. The reaction mixture
was warmed to ambient temperature and stirred for 2 h.
Then the reaction mixture was cooled to 0 ꢁC again and
Me3SnCl (0.61 g, 3.04 mmol) was added in one portion. The
reaction mixture was stirred at 0 ꢁC for 30 min and then
warmed to room temperature and stirred overnight. The
resulting mixture was poured into water. The organic layer
was extracted with diethyl ether (50 mL), washed succes-
sively with water, and then dried over anhydrous MgSO4.
The solution was concentrated to afford the final compound
General Procedure for Polymerization
The four polymers were synthesized by the same procedure
of the palladium(0)-catalyzed Stille coupling reactions with a
dibromide monomer and a bis(trimethylstannyl)-substituted
monomer under an argon atmosphere.
1
as a yellow liquid (0.60 g, 93%). H NMR (300 MHz, CDCl3),
d (ppm): 7.26 (m, 2H), 7.16 (d, J ¼ 3.6 Hz, 2H), 2.88 (t, J ¼
7.9 Hz, 4H), 1.75 (m, 4H), 1.28 (m, 20H), 0.89 (m, 6H), 1.40
(s, 18H). 13C NMR (75 MHz, CDCl3), d (ppm): 142.4, 138.6,
138.5, 135.5, 131.6, 131.5, 127.1, 31.9, 29.7, 29.3, 29.2, 29.0,
28.8, 22.7, 14.2, and ꢀ8.2. Anal. Calcd (%) for C36H56S4Sn2:
C, 50.60; H, 6.61; S, 15.01. Found: C, 50.71; H, 6.78; S, 15.09.
Dibromide monomer 0.25 mmol of bis(trimethylstannyl)-sub-
stituted monomer, tris(dibenzylideneacetone)dipalladium(0)
(Pd2(dba)3) (7 mg) and tri(o-tolyl)phosphine (P(o-Tol)3) (9
mg) were dissolved in 15 mL of anhydrous toluene. The mix-
ture was refluxed with vigorous stirring for 24 h under an
argon atmosphere. After the mixture was cooled to room
temperature, it was poured into 250 mL of methanol. The
precipitated material was redissolved and filtrated through a
funnel and precipitated again. The polymer was washed in a
(E)-1,10-bis(2-octyldodecyl)-6,60-bis(4-decylthiophen-2-yl)-
[3,30-biindolinylidene]-2,20-dione (8)
(E)-6,60-dibromo-1,10-bis(2-octyldodecyl)-[3,30-biindolinylidene]-
2,20-dione (7) (12.17 g, 12.40 mmol), 3-decyl-5-tributylstan-
nylthiophene (15.92 g, 31.00 mmol), Pd2(dba)3 (0.03 g, 0.03
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Soxhlet extractor with acetone and hexane, and then Soxhlet-
extracted with hot chloroform. The chloroform fraction was
concentrated and poured into 250 mL of methanol. Finally,
the precipitate was collected and dried in vacuum to afford
the title polymer as a dark fiber.
the resulting PSCs were measured under 1 sun, AM 1.5 G
(air mass 1.5 global) spectrum from a solar simulator (Oriel
model 91192). The current densityꢀvoltage (JꢀV) character-
istics were recorded with a Keithley 2410 source unit. The
external quantum efficiencies of PSCs were measured with a
commercial photomodulation spectroscopic setup (DSR
100UV-B) including a xenon lamp, an optical chopper, a
monochromator, and a lock-in amplifier operated by a PC
computer (Zolix Instruments), and a calibrated Si photodiode
was used as a standard.
Polymer P1. 260 mg. Yield: 72%. GPC: Mn ¼ 28.5 kDa; Mw/
1
Mn ¼ 2.3. H NMR (CDCl3, 300 MHz), d (ppm): 9.22 (br, 2H),
7.20–6.65 (m, 4H), 3.73 (br, 4H), 2.75 (br, 4H), 2.42 (br, 2H),
1.80–1.55 (m, 4H), 1.30–1.26 (m, 52H), 0.88 (br, 18H). Anal.
Calcd (%) for (C62H90N2O2S2)n: C, 77.54; H, 9.38; N, 2.92; S,
6.67. Found: C, 77.22; H, 10.23; N, 2.10, S, 7.33.
RESULTS AND DISCUSSION
Polymer P2. 359 mg. Yield: 76%. GPC: Mn ¼ 23.5 kDa; Mw/
1
Synthesis of Monomers and Polymers
Mn ¼ 1.9. H NMR (CDCl3, 300 MHz), d (ppm): 9.02 (br, 2H),
The synthetic routes for monomers are shown in Scheme 1.
In the presence of excess n-BuLi and (CH3)3SnCl, 3,6-dio-
ctylthieno[3,2-b]thiophene can be transformed to a bis(tri-
methylstannyl)-substituted monomer 1 in a 96% yield.
TMEDA was used as a ligand, which can convert n-BuLi into
a cluster with the higher reactivity during lithiation of 3-
alkylthiophene. The 2,5-dibromo compound 2 was synthe-
sized by bromination of compound 1 with NBS, with a yield
of 89%. Compound 3 with a total yield of 83% was synthe-
sized by Stille coupling of compound 2 and 2-(tributylstan-
nyl)thiophene with Pd(PPh3)4 as the catalyst. A similar
lithiation reaction of compound 3 afforded the target bis
(trimethylstannyl)-substituted monomer 4 in a high yield of
93%. The isoindigo derivative 8 was synthesized by Stille
coupling of compound 7 and 3-decyl-5-tributylstannylthio-
phene using Pd2(dba)3 and P(o-tol)3 as the catalyst, with a
yield of 73%. The 2,5-dibromo monomer 9 was synthesized
by bromination of compound 8 with 2.01 equivalent of NBS
in a 92% yield. The chemical structures of compounds 1ꢀ9
were confirmed using 1H NMR, 13C NMR and elemental
analysis.
7.24–6.91 (m, 6H), 6.60 (br, 2H), 3.68 (br, 4H), 2.80 (br, 4H),
2.61 (br, 2H), 1.69 (br, 4H), 1.30–1.26 (m, 92H), 0.84 (br,
18H). Anal. Calcd (%) for (C90H134N2O2S4)n: C, 76.91; H,
9.54; N, 1.99; S, 9.11. Found: C, 75.71; H, 10.15; N, 2.24; S,
9.54.
Polymer P3. 388 mg. Yield: 68%. GPC: Mn ¼ 21.8 kDa; Mw/
1
Mn ¼ 2.1. H NMR (CDCl3, 300 MHz), d (ppm): 9.14 (br, 2H),
7.19–6.85 (m, 10H), 3.70 (br, 4H), 2.70 (m, 8H), 2.58 (br,
2H), 1.74–1.24 (m, 120H), 0.87 (br, 24H). Anal. Calcd (%) for
(C114H170N2O2S6)n: C, 76.30; H, 9.48; N, 1.56; S, 10.71.
Found: C, 74.98; H, 10.45; N, 1.91; S, 10.45.
Polymer P4. 234 mg. Yield: 51%. Anal. Calcd (%) for
(C86H126N2O2S4)n: C, 76.55; H, 9.35; N, 2.08; S, 9.49. Found:
C, 75.76; H, 10.15; N, 2.30; S, 9.25.
PSC Fabrication and Characterization
Patterned indium tin oxide (ITO) coated glass with a sheet
resistance of 15–20 ohm/square was cleaned by a surfactant
scrub and then underwent a wet-cleaning process inside an
ultrasonic bath, beginning with deionized water followed by
acetone and isopropanol. After oxygen plasma cleaning for 5
min, a 40 nm thick poly(3,4-ethylenedioxythiophene):poly(s-
tyrenesulfonate) (PEDOT:PSS) (Bayer Baytron 4083) anode
buffer layer was spin-cast onto the ITO substrate and then
dried by baking in a vacuum oven at 80 ꢁC for overnight.
The active layer, with a thickness in the range of 80–90 nm,
was then deposited on top of the PEDOT:PSS layer by spin-
coating from a CB or dichlorobenzene (DCB) solution. The
interlayer solution in methanol was spin-coated on the top
of the active layer to form a thin interlayer of 3–5 nm. The
thickness of the PEDOT:PSS and active layer was verified by
a surface profilometer (Tencor, Alpha-500). Determination of
the thickness of the interlayer followed a previously pub-
lished article.43 At last, a 100 nm aluminum layer was ther-
mally evaporated with a shadow mask at a base pressure of
3 ꢂ 10ꢀ4 Pa. The overlapping area between the cathode and
anode defined a pixel size of 0.15 cm2. The thickness of the
evaporated cathodes was monitored by a quartz crystal
thickness/ratio monitor (model: STM-100/MF, Sycon). Except
the deposition of the PEDOT:PSS layers, all the fabrication
processes were carried out inside a controlled atmosphere of
nitrogen drybox (Vacuum Atmosphere) containing <10 ppm
oxygen and moisture. The power conversion efficiencies of
Scheme 2 shows the polymerization routes for the four tar-
get polymers. The preparations of the polymers P1, P2, P3,
and P4 were accomplished via Pd2(dba)3-catalyzed Stille
reaction of the corresponding monomers. After polymeriza-
tion, the crude polymers were purified by extracting with ac-
etone, hexane, and chloroform in the Soxhlet extractor for
hours. The chemical structures of P1, P2, and P3 were con-
1
firmed by H NMR and elemental analysis. Polymers P1, P2,
and P3 exhibited good solubility in many organic solvents,
such as THF, chloroform, toluene, CB, and DCB. Polymer P2
depicted better solubility in CB and DCB than P1 and P3.
However, after polymerization and precipitation in methanol,
the crude polymer P4 was found insoluble in most organic
solvents like THF, chloroform and toluene. This polymer was
only partially soluble in warm CB. In addition, since the 2,5-
bis(3-decylthiophen-2-yl)-TT moiety affords better solubility
than 3,6-octyl-2,5-di(thiophen-2-yl)-TT moiety, the polymer
P2 exhibited improved solubility in CB and DCB, in compari-
son to P3 and P4. 1H NMR measurement of polymer P4 in
CDCl3 only showed weak and broad signals, indicating the
polymer chains formed aggregates in the solution. We failed
to obtain a thick and smooth film from the P4 solution, thus
hampering investigation of its absorption spectrum,
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SCHEME 1 Monomer Synthesis.
electrochemical property and final device fabrication. The
molecular weights of P1, P2, and P3 as measured by GPC
with calibration using polystyrene standards and THF as the
eluent are listed in Table 1. The number-averaged molecular
weights (Mn) of P1, P2, and P3 are 28,500, 23,500, and
21,800 g molꢀ1 with polydispersity index (Mw/Mn) of 2.3,
1.9, and 2.1, respectively.
Thermal Stability
Thermal stability of the polymers was investigated using
TGA, as shown in Figure 1. The TGA analysis shows the
onset temperatures with 5% weight loss (Td) of P1, P2, P3,
and P4 between 320 and 390 ꢁC. These polymers are thus
of sufficient stability for applications in optoelectronic
devices.
SCHEME 2 Polymer Synthesis.
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TABLE 1 Molecular Weights and Thermal Properties of
Polymers
PDIa
Td
a
a
b
Polymers
Mn
Mw
P1
P2
P3
P4
a
28.5K
23.5K
21.8K
65.5K
44.6K
45.7K
2.3
1.9
2.1
389
325
359
374
c
c
c
ꢀ
ꢀ
ꢀ
Mn, Mw, and PDI of the polymers were determined by GPC using poly-
styrene standards with THF as the eluent.
b
The 5% weight loss temperatures under an inert atmosphere.
c
No molecule weight received from GPC because of its limited solubil-
ity in THF.
Absorption Spectra
The UV–vis absorption spectra of polymers P1, P2, and P3
in chloroform solutions and in film states are shown in Fig-
ure 2. The solution of P1 shows one absorption band in the
400–750 nm range with absorption maximum (kmax) at 610
nm (Table 2). When using alkylthiophene and alkylbithio-
phene as p-bridges between the TT and isoindigo units such
as P2 and P3, two distinct absorption bands in the wave-
length ranges of 350–500 and 500–800 nm appear. This
would be attributable to stronger DꢀA interaction and intra-
molecular charge transfer. The absorption maxima of P2 and
P3 in chloroform are 630 and 625 nm, respectively. In com-
parison to P2, polymer P3 containing n-alkylbithiophene p-
bridges exhibit similar absorption spectrum in the 600–800
nm regions. However, P3 has an obviously stronger absorp-
tion between 400 and 500 nm, which may be ascribable to
the bithiophene moiety improving conjugation in the seg-
ment. In films, the kmax value of P1 is nearly the same with
that in its solution, which exhibits a weak aggregation effect
in the absorption spectrum. Its absorption edge for the
absorption spectra of the film is at 765 nm. The kmax value
for the solid films of P2 and P3 are red-shifted to 644 and
647 nm, respectively, which is typically ascribable to the
improved stacking and aggregation of the conjugated main
FIGURE 2 Normalized UV–vis absorption spectra of polymers:
(a) in chloroform solutions; (b) in solid films.
chains in film phases. In comparison to the absorption
curves in solutions, P2 and P3 films exhibit a distinct
absorption shoulder from 700 to 750 nm, respectively, which
is indicative of stronger interchain interactions of conjugated
backbones in the condensed phase. The absorption edge of
P2 and P3 are at 830 and 817 nm in solid films, respec-
tively. The absorption coefficient of a solid film is also an im-
portant parameter for a narrow band gap polymer. A rela-
tively large absorption coefficient means the incident beam
is quickly attenuated on passage through a polymer film,
which translates to more light being absorbed. The polymers
P1 and P3 exhibit similar absorption coefficients of 0.35 ꢂ
10ꢀ2 nmꢀ1 and 0.36 ꢂ 10ꢀ2 nmꢀ1, respectively. The absorp-
tion coefficient of polymer P2 is higher than those of P1 and
P3, which is up to 0.61 ꢂ 10ꢀ2 nmꢀ1. The larger absorption
coefficient may be attributable to its more favorable pꢀp
stacking of polymer main chain in solid state.
Energy Level
Cyclic voltammetry (CV) measurements were performed for
determining the highest occupied molecular orbital (HOMO)
FIGURE 1 TGA plots of the polymers with a heating rate of 10
ꢁC minꢀ1 under an inert atmosphere.
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TABLE 2 Optical and Electrochemical Properties of Polymers
UV–vis absorption spectra
Cyclic voltammetry
n-Doping
red/LUMO (V)/(eV)f
Solution
Film
p-Doping
ox/HOMO (V)d/(eV)e
polymer
kabs (nm)
kabs (nm)
konset (nm)
Eg
(eV)c
u
u
Eg (eV)g
ec
a
b
opt
P1
P2
P3
a
610
630
625
609
644
647
765
830
817
1.62
1.49
1.52
1.16/ꢀ5.71
0.79/ꢀ5.34
0.88/ꢀ5.43
ꢀ0.75/ꢀ3.80
ꢀ0.76/ꢀ3.79
ꢀ0.74/ꢀ3.81
1.91
1.55
1.62
e
Absorption peak in chloroform solution.
Absorption peak in film.
Calculated according to HOMO ¼ ꢀe(Eox þ 4.55) (eV).
LUMO ¼ ꢀe(Ered þ 4.55) (eV).
b
c
f
g
Optical band gap.
Onset voltage of oxidation process during cyclic voltammetry with
Electrochemical band gap.
d
Ag/Agþ reference electrode.
and the lowest unoccupied molecular orbital (LUMO) energy
levels and the electrochemical band gaps of the conjugated
polymers. Figure 3 shows the cyclic voltammograms of the
polymer films on Pt disk electrode in a 0.1M Bu4NPF6 aceto-
nitrile solution. The onset oxidation potentials (uox) of P1,
P2, and P3 are 1.16, 0.79, and 0.88V versus Ag/Agþ, respec-
tively. The onset reduction potentials (ured) are ꢀ0.75,
ꢀ0.76, and ꢀ0.74V versus Ag/Agþ, respectively. The HOMO
and LUMO energy levels of the polymers were calculated
according to the equations, HOMO ¼ ꢀe(Eox þ 4.55) (eV),
LUMO ¼ ꢀe(Ered þ 4.55) (eV).44 The results of the electro-
chemical measurements are summarized in Table 2. The
HOMO levels of polymer P1 is around ꢀ5.71 eV. After intro-
ducing thiophene bridges between the TT and isoindigo
units, P2 and P3 shows relatively narrow band gaps and
low Eox of 0.79 and 0.88 V, giving a higher HOMO level of
ꢀ5.34 and ꢀ5.43 eV, respectively. Compared with P2, poly-
mer P3 depicted a lower-lying HOMO resulting in the latter
polymer’s slightly larger band gap. All the three polymers
depicted relatively deep HOMO levels, which may lead to
high Voc of the BHJ PSCs. The LUMO levels of P1, P2, and P3
are all around ꢀ3.80 eV, which indicate that the LUMO levels
of these copolymers are determined by the presence of isoin-
digo units. The electrochemical band gaps of polymers P1,
P2, and P3 are calculated to be 1.91, 1.55, and 1.62 eV,
ec
respectively, according to the equation, Eg ¼ e(Eox ꢀ Ered
)
(eV).44 The optical band gaps of polymers P1, P2, and P3
estimated from the absorption edges of the solid films are
1.62, 1.49, and 1.52 eV, respectively. The energy level diagrams
of polymer P1, P2, P3, and PCBM are shown in Figure 4.
Photovoltaic Properties
The photovoltaic properties of polymers P1, P2, and P3
were investigated using BHJ PSCs with configuration ITO/
PEDOT:PSS (40 nm)/polymers:PC71BM (85 nm)/PCP-EP
interlayer (4 nm)/Al (100 nm). [Fig. 5(a)]. The CB and
FIGURE 3 Cyclic voltammograms of polymer films on a plati-
num plate electrode measured in a 0.1M Bu4NPF6 acetonitrile
FIGURE 4 Energy level diagrams for the polymers as the do-
solution at a scan rate of 100 mV secꢀ1
.
nor materials in BHJ photovoltaic cells.
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TABLE 3 Photovoltaic Performances of the PSCs Based on
Polymer:PC71BM Under the Illumination of AM1.5, 100 mW
cm22
FF
PCE
Polymers Solvent Ratioa Voc (V) Jsc (mA cmꢀ2
)
(%) (%)
P1
P2
P2
P3
P3
a
CB
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1.03
0.77
0.77
0.73
0.72
2.97
4.16
9.34
7.25
7.55
40.9 1.25
58.4 1.87
65.2 4.69
59.7 3.16
59.7 3.24
DCB
DCBb
CB
CBc
FIGURE 5 (a) The device configuration of the PSC. (b) Chemi-
cal structures of interlayer polymer PCP-EP.
Polymer : PC71BM weight ratio.
2.5% (v/v) DIO in 1,2-DCB.
2.5% (v/v) CI-naph in CB.
b
c
1,2-dichlorobenzene (o-DCB) were used as the processing
solvents for device fabrication. The alcohol-soluble conju-
gated polymer PCP-EP was used as the interlayer between
the active layer and Al cathode. The polymer PCP-EP has
shown good electron-injection ability in combination with
high work-function Al cathode in light-emitting diodes
(PLEDs).45 These conjugated polymers with alkylphospho-
nates and diethanolamino side groups were used as an or-
ganic interlayer to improve interfacial contacts between the
organic layer and inorganic cathode in efficient conventional
and inverted solar cells.46,47 The chemical structure of poly-
mer PCP-EP is shown in Figure 5(b).
from its low HOMO level of ꢀ5.71 eV. However, the Jsc of P1
was low and the PCE was 1.25%. Several solvent additives
were used to optimize the morphology of the blend layer, to
form favorable grain boundaries and nanostructures, thus
enhancing the Jsc and FF for higher PCE achievement of BHJ
PSCs.32,33,48,49 Unfortunately, the PSCs based on P1 were
unresponsive to any solvent additives used. Meanwhile, in
the course of device fabrication and optimization, polymer
P2 was found to afford better solubility and device perform-
ance when using o-DCB as the processing solvent. For a
P2:PC71BM ratio of 1:1.5, The Voc, Jsc, and FF of PSCs were
0.77 V, 4.16 mA cmꢀ2, and 58.4%, respectively. The calcu-
lated PCE was 1.87%. Using 1,8-diiodooctane (DIO) as the
additive was effective for the PSCs based on P2, showing
improvements of Jsc and FF. For a P2:PC71BM ratio of 1:1.5
with 2.5% (v/v) DIO in o-DCB, the Jsc and FF of PSCs exhib-
ited an obvious increase to 9.34 mA cmꢀ2 and 65.2%,
respectively. The calculated PCE attained 4.69%. Compared
to P1, the higher Jsc of P2 can be attributed to its broad and
red-shifted absorption spectrum and larger absorption coeffi-
cient. The lower Voc of P2 was consistent with its higher
HOMO level. The good FF (0.65) of the device may indicate
improved balanced charge transport of the polymer and
PC71BM. For polymer P3, when 1-chloronaphthalene (CI-
naph) was mixed in the P3/PC71BM blend layer, better pho-
tovoltaic performances were obtained than those devices
using the DIO additive. The CI-naph additive can reduce the
solvent evaporation rate and improve the self-organization
and crystallization of P3HT/PCBM blend systems.50 For a
P3:PC71BM ratio of 1:1.5 in CB, the Voc, Jsc, and FF of PSCs
were 0.73 V, 7.25 mA cmꢀ2, and 59.7%, respectively, corre-
sponding to the PCE of 3.16%. After using 2.5% (v/v) CI-
naph additive, the Jsc of PSCs depicted marginal increase to
7.55 mA cmꢀ2 and the calculated PCE was up to 3.24%. As
we revealed below, though polymer P3 depicted broader and
stronger absorption spectrum than P2 due to its bithio-
phene-bridged DꢀA backbone, the morphology of P3/
PC71BM blend layer may have a negative impact on the de-
vice performance.
The measurements of photovoltaic performances of polymers
P1, P2, and P3 as electron donor/hole transport materials
in BHJ PSCs were carried out under illumination of AM1.5G
simulated solar light at 100 mW cmꢀ2. The weight ratio of
polymer: PC71BM and thickness of the active layer was opti-
mized. Figure 6 shows the current densityꢀvoltage (JꢀV)
characteristics of PSCs. The open-circuit voltage (Voc), short-
circuit current (Jsc), fill factor (FF), and PCE of the PSCs are
summarized in Table 3. For a P1:PC71BM weight ratio of
1:1.5 in CB, the PSC showed Voc, Jsc, and FF of 1.03 V, 2.97
mA cmꢀ2, and 40.9%, respectively. The high Voc was derived
To verify the measurements of Jsc, We measured the external
quantum efficiency (EQE) of the PSCs based on poly-
mer:PC71BM blends under monochromatic light (Fig. 7). The
FIGURE 6 Current densityꢀvoltage (JꢀV) characteristics of
PSCs based on polymer:PC71BM blends under illumination of
AM1.5G, 100 mW cmꢀ2
.
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work in the P2/PC71BM blend layer. Compared to P2, the
blend layer of P3 exhibited larger domain size and the rms
roughness is up to 4.8 nm, which may be ascribed to its
more rigid and inflexible backbone when using n-alkylthio-
phene as p-bridge between isoindigo and dioctylthieno[3,2-
b]thiophene moieties.
Hole Mobility
Since polymer P2 exhibited the best photovoltaic property
among these four polymers, the hole mobility of the D/A
blend film based on polymer P2 was also investigated using
the space charge limited current (SCLC) method. The struc-
ture of hole-only device is ITO/PEDOT:PSS/polymer
P2:PC71BM (85 nm)/MoO3/Al. The mobility was determined
by fitting the dark current to the model of a single carrier
SCLC, which is described by the equation:
V2
h d3
FIGURE 7 EQE curves of the PSCs with (polymer:PC71BM ¼
9
J ¼ e0erl
1:1.5) as the active layer.
8
where J is the current, lh is the charge mobility at zero field,
e0 is the free-space permittivity, er is the relative permittivity
of the material, d is the thickness of the active layer, and V is
the effective voltage. V ¼ Vappl ꢀ Vbi ꢀ Vs, Vappl is the applied
potential, Vbi is the built-in potential and Vs is the voltage
drop due to the substrate series resistance. The hole-mobil-
ity can be calculated from the slope of the J1/2ꢀV curve
(Fig. 9). The calculated mobility of the polymer P2 is 1.9 ꢂ
maxima of the EQE are 19, 43 and 40% for P1, P2, and P3,
respectively. The EQE curves generally correspond with UV–
vis absorptions of the polymers depicting a higher value in
the range from 400 to 800 nm. The enhanced quantum effi-
ciency in the region of 400–500 nm is attributable to the
high absorption intensity of PC71BM in the visible region,
which can lead to accentuated Jsc.51 The Jsc calculated from
integration of the EQE with an AM 1.5G reference spectrum
are 2.70, 8.00, and 6.68 mA cmꢀ2, respectively, which corre-
spond to the spectral mismatch factors between 1.10 and
1.85. The mismatch factors may arise from spectral differen-
ces between the simulated solar spectrum and the AM1.5G
standard spectrum.
10ꢀ4 cm2 Vꢀ1 sꢀ1
.
CONCLUSIONS
In summary, we have synthesized a novel class of thieno[3,2-
b]thiophene and isoindigo-based D–A polymers, which can
be used as electron donor and hole transport materials in
BHJ PSCs. The polymers exhibited good thermal stability and
their absorption spectral properties as well as energy levels
can be tuned and optimized via thieno[3,2-b]thiophene-isoin-
digo repeating units and conjugated length of thiophene p-
bridges. Polymer P1 with direct DꢀA moieties in the back-
bone depicted a low lying HOMO level of ꢀ5.71 eV. In con-
trast, polymer P2 containing alkylthiophene-bridged DꢀA
backbone and corresponding alkylbithiophene-bridged poly-
mer P3 exhibited higher HOMO levels of around ꢀ5.4 eV
Film Morphology
The morphology of the active layers was investigated by
atomic force microscopy (AFM) (Fig. 8). The blend layers
were fabricated under the respective optimized process
according to the device performance. All the blend layers
depicted smooth and uniform films. The root-mean-square
(rms) of surface roughness was 2.1 and 2.3 nm for P1 and
P2, respectively. The better photovoltaic performance of P2
was likely ascribable to the uniform interpenetrating net-
FIGURE 8 Tapping mode AFM topography images (5 ꢂ 5 lm2) of the 1:1.5 (weight ratio) composite film of (a) P1/PC71BM (b) P2/
PC71BM with 2.5% (v/v) DIO (c) P3/PC71BM with 2.5% (v/v) CI-naph.
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€
18 Wang, E. G.; Hou, L. T.; Wang, Z. Q.; Hellstrom, S.; Zhang,
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