Novel conjugated copolymers based on dithiafulvalene moiety for bulk heterojunction solar cells

Article abstract of DOI:10.1002/pola.25998

New conjugated copolymers, P1-P3, based on dithiafulvalene-fused entity and different conjugated segments have been synthesized. Incorporation of electron-deficient conjugated segments into the conjugated copolymers results in red shifting the absorption

Full text of DOI:10.1002/pola.25998

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Novel Conjugated Copolymers Based on Dithiafulvalene Moiety for Bulk
Heterojunction Solar Cells
Yung-Chung Chen, Chih-Yu Hsu, Jiann T. Lin
Institute of Chemistry, Academia Sinica, Nankang 11529, Taipei, Taiwan
Correspondence to: J. T. Lin (E-mail: jtlin@chem.sinica.edu.tw)
Received 12 December 2011; accepted 3 February 2012; published online 28 February 2012
DOI: 10.1002/pola.25998
ABSTRACT: New conjugated copolymers, P1-P3, based on
dithiafulvalene-fused entity and different conjugated segments
have been synthesized. Incorporation of electron-deficient con-
jugated segments into the conjugated copolymers results in
red shifting the absorption band and lowering the hole mobil-
ity. Bulk heterojunction solar cells using on these polymers as
the donor and [6,6]-phenyl-C61-butyric acid methyl ester
(PC61BM) as the acceptor were fabricated by solution process.
The cells based on the blend of P1-P3/PC61BM (1:1, w/w) have
power conversion efficiencies (PCEs) ranging from 0.53 to
0.93%. Among these, the cell of P1/PC61BM exhibited the high-
est open-circuit voltage at 0.85 V, and the cell of P3/PC61BM
exhibited the best PCE at 0.93% with the short-circuit current
(JSC) of 4.88 mA/cm2.
2012 Wiley Periodicals, Inc. J Polym
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KEYWORDS: bulk
heterojunction;
dithiafulvalene;
organic
photovoltaics
INTRODUCTION Due to limited natural reserves and continu-
ally increasing consumption rate of fossil fuel, and concerns
over global warming, renewable energy has become an im-
portant issue of policy maker and subject of researchers.1,2
Among these, the polymer solar cells (PSCs) have been
attractive because of their low cost, large area manufacture,
and flexibility.3,4 PSCs normally have a bulk heterojunction
(BHJ) architectures which consists of the fullerene deriva-
tives of [6,6]-phenyl C61-butyric acid methyl ester (PC61BM)
as electron acceptor and conjugated polymers as the donor
part.5 Recently, high power conversion efficiencies (PCEs) of
>7% were reported for the blend of some conjugated poly-
mers, such as PBnDT-DTffBT,6 PTBF1,7 PBDTTT-CF,8a PBnDT-
FTAZ,8b PCDTBT9 and so forth, with fullerene derivatives.
The design rules for donor polymers in BHJ devices include
low HOMO energy level for higher open-circuit voltage, low
band gap for good solar light harvesting, appropriate crystal-
linity and morphology for high carrier mobility, and well-
matched HOMO and LUMO energy levels between polymers
and fullerenes for efficient charge separation.3,10 Conjugated
polymers of alternating donor–acceptor (D–A) type have
attracted considerable attention because their band gaps,
energy levels, and carriers mobilities can be easily tuned.11
The desired photophysical, electrochemical, and other prop-
erties are normally achievable via appropriate choice of
donor or acceptor.
storage capacity of conjugated polymer may be significantly
increased upon grafting with TTF units.12 Moreover, large
number of sulfur atoms in TTF-fused polymers increase the
polarizability of the molecules and reduce intramolecular
and intermolecular Coulomb repulsions between the charged
species. Consequently, there is increased dimensionality of
charge transport.13 Gautier et al. investigated a TTF deriva-
tive in which two quinones were connected by TTF back-
bone and found that the quinone units could readily commu-
nicate with each other through the TTF backbone. Therefore,
the charge transport of the conjugated polymers can be
increased via incorporation of TTF units.14 Solar cells based
on TTF derivatives with TTF units either in the main chain
or side chain of the conjugated polymers have been studied
by several groups.15,16 Chen et al. prepared a novel TTF-
fused poly(aryleneethynylene) with an acceptor main chain
and donor side chains. Intramolecular charge transfer (CT)
exists between the electron-rich TTF side chains and the
electron-deficient main chain. The PCE was reported to be
0.25% under AM 1.5 sunlight.15 Skabara et al. development
a series of polymers derived from thiophene and TTF moi-
eties. Despite very low band gap (1.44 eV) of the polymer,
the PCE only reached 0.13%.16 Nonetheless, the excellent
intramolecular CT and good p-stacking characteristics of this
kind of TTF-fused polymers render them promising materials
for photovoltaic and other organic electronic devices.
A tetrathiafulvalene (TTF) moiety has three stable oxidation
states including the TTF0, TTFþ, and TTF2þ. Thus, the charge
Similar to TTFs, dithiafulvalene (DTFs) also have good charge
transport property because of p-p and S…S interactions.
Additional Supporting Information is present in the online version of this article.
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SCHEME 1 Synthesis of monomer 1.
However, monomeric and polymeric DTF derivatives17 were
not explored as intensively as TTF congeners. To our knowl-
edge, no OPVs using DTF-based polymers have been
reported. Therefore, we chose 2-(9H-fluoren-9-ylidene)-4,5-
bis(hexylthio)-1,3-dithiole entity (Scheme 1; monomer 1) an
the electron-donating motif for D-A alternating copolymer
based on the following reasons: (1) bis(hexylthio)-1,3-
dithiole entity (BTDT) may have a lower HOMO level com-
pared with its TTF congener, and a higher open-circuit volt-
age is expected; (2) BTDT may function as the hole-hopping
sites; (3) the two hexylthio substituents may not only
enhance the solubility of the polymer for device fabrication
but also help intermolecular stacking; (4) large number of
sulfur atoms along the structure may facilitate molecular
aggregation. In this article, conjugated polymers constructed
from 2-(9H-fluoren-9-ylidene)-4,5-bis(hexylthio)-1,3-dithiole
and different acceptor moieties will be reported. Their
photophysical, electrochemical, and thermal properties, as
well as the BHJ type photovoltaic cells are also included.
EXPERIMENTAL
General Information
Infrared spectra were recorded on a Perkin–Elmer FT-IR
spectrometer spectrum 100. 1H NMR and 13C NMR spectra
were taken on a Bruker AMX-400 or Bruker AV-400 spec-
trometer using CDCl3 as the solvent. Fast atom bombardment
mass spectrometry (FABMS) analysis was performed on a
JEOL Tokyo Japan JMS-700 mass spectrometer equipped with
the standard FAB source. Elemental analyses were performed
on a Perkin–Elmer Model 2400 analyzer. Absorption spectra
were recorded on a Dynamica DB-20 UV–Vis spectrophotom-
eter. Low-energy photoelectron spectra were taken from a
photoelectron spectrometer (AC-2, Riken-Keiki PT5-0210).
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Gel permeation chromatography (GPC) was performed with
a Waters apparatus equipped with Waters Stygel columns
and a refractive index detector using tetrahydrofuran (THF)
as the eluent (polystyrene calibration). Glass transition tem-
perature (Tg) and thermal decomposition temperature (Td)
of the copolymer were determined by differential scanning
calorimetry (DSC) and thermogravimetric analysis (TGA)
using Seiko DSC 6220 SII Extra 6000 and Thermo Cahn
Versa Therm analyzer systems, respectively. The photoelec-
trochemical characterizations on the solar cells were carried
out using an Oriel Class A solar simulator (Oriel 91195A,
Newport). Photocurrent–voltage characteristics of the OPVs
were recorded with a potentiostat/galvanostat (CHI650B, CH
Instruments) at a light intensity of 100 mW/cm2 calibrated
by an Oriel reference solar cell (Oriel 91150, Newport). The
monochromatic quantum efficiency was recorded at short-
circuit condition using a monochromator (Oriel 74100, New-
port). The morphology of the thin films was analyzed by
atomic force microscopy (AFM) (Digital instrument NS 3a
controller with D3100 stage).
freshly distilled over appropriate drying agents before use
and were purged with nitrogen. 2,7-Dibromo-fluoren-9-one
(1a),19 bis(tetraethylammonium)bis(1,3-dithiole-2-thione-4,5-
dithiolato)zincate,20
2,5-bis(trimethylstannyl)-thiophene,21
4,7-dibromo-benzo[1,2,5]thiadiazole,22 1,3-dibromo-5-ethyl-
hexylthieno[3,4-c]pyrrole-4,6-dione23 were synthesized accord-
ing to the literature methods.
Synthesis of Monomer 1b24
The
bis(tetraethylammonium)bis(1,3-dithiole-2-thione-4,5-
dithiolato)zincate (zincate) (0.83 g, 1.15 mmol) and 1-bro-
mohexane (0.58 mL, 4.18 mmol) in MeCN (12 mL) under N2.
The mixture was stirred at reflux for 24 h. After the reaction
was complete, the solvent was removed under vacuum and
1
1b was obtained as a brown viscous liquid (0.70 g, 82%). H
NMR: (400 MHz, CDCl3, d, ppm): 2.86 (t, J ¼ 7.2, 4H), 1.67–
1.62 (m, 4H), 1.43–1.36 (m, 4H), 1.28-1.26 (m, 8H), 0.89–
0.86 (m, 6H).
Synthesis of Monomer 2-(2,7-Dibromo-fluoren-
9-ylidene)-4,5-bis-hexylsulfanyl-[1,3]dithiole (1)
Triethylphosphite (15 mL) was added slowly to a solution of
1a (1.40 g, 4.14 mmol) and 1b (0.76 g, 2.07 mmol) in tolu-
ene (20 mL) under N2. The mixture was stirred at 120ꢀC for
3 h. After the reaction was complete, the excess 1a was
filtered off. The solvent of the filtrate was removed under
vacuum. Addition of methanol to the resulting viscous solu-
tion led to precipitation of monomer 1. The precipitate was
separated by filtration and washed with methanol thoroughly
to provide pure 1 in 46% yield (0.62 g). FT-IR: 1683 (C¼¼C
stretch), 1587 (SAC¼¼C stretch), 1448, 1276 cmꢁ1 (SAC
stretch). 1H NMR: (400 MHz, CDCl3, d, ppm): 7.82 (s, 2H),
7.61 (d, J ¼ 8.0 Hz, 2H), 7.41 (dd, J ¼ 8.4, 1.6 Hz, 2H), 2.97
(t, J ¼ 7.2, 4H), 1.74–1.67 (m, 4H), 1.50–1.42 (m, 4H), 1.33–
1.29 (m, 8H), 0.90–0.87 (m, 6H). 13C NMR (100 MHz, CDCl3,
d, ppm): 142.01, 138.28, 135.58, 129.52, 128.36, 125.74,
121.07, 120.89, 36.97, 31.57, 29.96, 28.48, 22.76, 14.25.
(FAB): m/z 656 (Mþ). Anal. Calcd: C, 51.22; H, 4.91. Found:
C, 51.34; H, 4.71.
Device Fabrication
The fabricated BHJ device has a configuration of ITO/
PEDOT:PSS/active layer/Ca/Al. They were prepared accord-
ing to the following procedures: (1) Glass/ITO substrates (8
X/&) were sequentially patterned by hydrochloric acid
(HClaq), followed by cleaning with acetone, cleanser, deion-
ized water, and isopropyl alcohol, dried and treated with
oxygen plasma for 5 min; (2) PEDOT:PSS (Clevios P-VP
AI4083) was passed through a 0.45-lm filter before being
deposited on ITO through spin-coating at 4000 rpm in air;
(3) the sample was then dried at 100ꢀC for 1 h and 130ꢀC
for 30 min; (4) A blend of fullerene derivatives (PCBM) and
the polymers (P1-P3) [ratios of (w/w), 1.2 wt % in o-
dichlorobenzene (o-DCB)] were stirred overnight in o-DCB,
filtered through a 0.22-lm poly(tetrafluoroethylene) filter,
and then spin-coated with 800 rpm for 40 s on top of the
PEDOT:PSS layer. Subsequently, the devices were thermal
annealed at 130ꢀC for 10 min. Then, the device was com-
pleted by depositing a 30-nm thick layer of Ca and a 120-nm
thick layer of Al at pressures of less than 10ꢁ6 torr. Finally,
the devices were transferred to the glove box and encapsu-
lated using UV-curing glue (Lumtec, Taiwan). The active area
of the device was 0.1 cm2.
Synthesis of P1
Compound 1 (0.66 g, 1.00 mmol), 2,5-bis(trimethylstannyl)-
thiophene (0.41 g, 1.00 mmol), Pd2(dba)3 (18.3 mg), and
P(o-tol)3 (48.7 mg) were placed in a 25 mL round-bottomed
flask. The degassed chlorobenzene (8 mL) was added to the
flask as the solvent. The resulting mixture was heated at
110ꢀC for 3 days under N2. After cooling to room tempera-
ture, the mixture was poured into methanol. The precipitate
formed was collected by filtration through a frit and then
purified by Soxhlet extraction first with hexane and acetone
to remove soluble impurities, and then with chloroform to
extract the polymer. The chloroform solution was concen-
trated by rotary evaporation followed by precipitation with
methanol. The solid was dried under vacuum and obtained
as a red solid in 50% yield. FT-IR: 1595 (C¼¼C stretch), 1530
(SAC¼¼C stretch), 1456, 1261 cmꢁ1 (SAC stretch). 1H NMR:
(400 MHz, CDCl3, d, ppm): 8.05–6.90 (br, 8H), 2.95 (br, 4H),
1.80–1.23 (br, 16H), 0.86 (br, 6H). GPC (THF) polydispersity
index (PDI) ¼ 3.18, Mw ¼ 17155.
Fabrication of Hole- and Electron-Only Devices
The hole- and electron-only devices in this study were fabri-
cated according to the literature with modification.18 The
hole only device has a configuration of ITO/PEDOT:PSS/
active layer/MoO3/Al, and MoO3 was thermally evaporated
to a thickness of 20 nm. The electron-only device has a con-
figuration of ITO/CS2CO3/active layer/Ca/Al, in which
CS2CO3 (0.2 wt % in methoxyethanol) was deposited by
spin-coating process.
Materials
All the starting materials were used as received without any
further purification. All the solvents such as dichlorome-
thane, THF, and dimethylformamide, and toluene were
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SCHEME 2 Structure and synthesis of DTF-based copolymers (P1–P3).
Synthesis of P2
monomer 1a was prepared according to the literature proce-
dures,19 while monomer 1b24 was obtained from zincate
metal complex20 and 1-bromohexane. Condensation reaction
of 1a and 1b mediated by P(OEt)3 was then carried out in
refluxing toluene to afford 1. Elemental analysis, FT-IR, and
NMR and mass spectra (FAB–MS) were used to identify
structures of the target monomer 1. The monomer has
three characteristic bands at 1587, 1448, and 1276 cmꢁ1
(Supporting Information Fig. S1). The first band is attributed
to SAC¼¼C stretching, and the latter two are attributable to
SAC stretching. Both the NMR spectra (Supporting Informa-
tion Fig. S2) and elemental analysis of the monomer 1 agree
well with the proposed molecular structure.
Compound 1 (0.33 g, 0.50 mmol), 2,5-bis(trimethylstannyl)-
thiophene (0.41 g, 1.00 mmol), 4,7-dibromo-benzo[1,2,5]thia-
diazole (0.15 g, 0.50 mmol), Pd2(dba)3 (18.3 mg), and P(o-
tol)3 (48.7 mg) were placed in a 25 mL round-bottomed
flask. The degassed chlorobenzene (8 mL) was added to the
flask as the solvent. The resulting mixture was heated to
110ꢀC for 3 days under N2. After being cooled to room tem-
perature, the mixture was poured into methanol. The precip-
itate was collected by filtration through a frit and then puri-
fied by Soxhlet extraction, first with hexane and acetone to
remove soluble impurities, and then with chloroform to
extract the polymer. The chloroform solution was concen-
trated by rotary evaporation followed by precipitation with
methanol. The solid was dried under vacuum and obtained
as a black solid in 15% yield. FT-IR: 1595 (C¼¼C stretch),
1527 (SAC¼¼C stretch), 1456, 1261 cmꢁ1 (SAC stretch). 1H
NMR: (400 MHz, CDCl3, d, ppm): 8.05–6.90 (br, 12H), 2.97
(br, 4H), 1.78–1.23 (br, 16H), 0.87 (br, 6H). GPC (THF) PDI ¼
3.20, Mw ¼ 10149.
Scheme 2 illustrates the synthetic route of the polymers P1-
P3. They were obtained via Stille coupling reaction between
a distannyl reagent and a dibromide. 2,5-Bis(trimethyl-
stannyl)-thiophene was selected due to the reluctance of 1
in double stannylation. Only very low-molecular weight
copolymers were obtained when Pd2(PPh3)2Cl2 was used as
the catalyst. Consequently, Pd2(dba)3 and P(o-tol)3 was used
instead. The desired polymers could be obtained in 15-50%
yields from the reaction of the monomers 1, an acceptor
moiety (for P2 and P3) and 2,5-bis(trimethylstannyl)-thio-
phene in chlorobenzene as solvent. The crude polymers
were purified by precipitating in methanol and washing off
residual small molecules by hexane and acetone via Soxhlet
extraction. The polymers were further purified by extraction
with chloroform followed by precipitation with methanol.
Typical IR spectra for all the copolymers are shown in
Supporting Information Figure S1. All the copolymers
showed characteristic IR absorption bands of the SAC¼¼C
and the SAC groups. The characteristic IR absorption bands
P3 was obtained as a black solid following the procedure
described for P2. The yield was 16%. FT-IR: 1743, 1701
(imide C¼¼O stretch), 1595 (C¼¼C stretch), 1531 (SAC¼¼C
stretch), 1456, 1259 cmꢁ1 (SAC stretch). 1H NMR: (400
MHz, CDCl3, d, ppm): 8.05–6.90 (br, 10H), 3.35 (br, 2H), 2.94
(br, 4H), 1.65–1.24 (br, 25H), 0.86 (br, 12H). GPC (THF) PDI
¼ 3.14, Mw ¼ 13233.
RESULTS AND DISCUSSION
Synthesis and Characterization
The synthetic protocol of the novel electron-donating mono-
mer bearing DTF moiety (1) is illustrated in Scheme 1. The
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