Two-dimensional like conjugated copolymers for high efficiency bulk-heterojunction solar cell application: Band gap and energy level engineering

Article abstract of DOI:10.1007/s11426-011-4257-3

Three two-dimensional like conjugated copolymers PFSDCN, PFSDTA and PFSDCNIO, which consist of alternating fluorene and triphenylamine main chain, and different pendant acceptor groups (malononitrile, 1,3-diethtyl-2- thiobarbituric acid and 2-(1,2-dihydro

Full text of DOI:10.1007/s11426-011-4257-3

SCIENCE CHINA
Chemistry
April 2011 Vol.54 No.4: 685–694
doi: 10.1007/s11426-011-4257-3
• ARTICLES •
Two-dimensional like conjugated copolymers for high efficiency
bulk-heterojunction solar cell application: Band gap and
energy level engineering
DUAN ChunHui1, WANG ChuanDao2, LIU ShengJian1, HUANG Fei1*,
CHOY C. H. Wallace2* & CAO Yong1*
1Institute of Polymer Optoelectronic Materials & Devices, Key Laboratory of Specially Functional Materials of the Ministry of Education;
South China University of Technology, Guangzhou 510640, China
2Department of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, China
Received February 7, 2011; accepted February 22, 2011
Three two-dimensional like conjugated copolymers PFSDCN, PFSDTA and PFSDCNIO, which consist of alternating fluorene
and triphenylamine main chain, and different pendant acceptor groups (malononitrile, 1,3-diethtyl-2-thiobarbituric acid and
2-(1,2-dihydro-1-oxoinden-3-ylidene)malononitrile) with thiophene as -bridge, have been designed, synthesized and charac-
terized. The structure-property relationships of the two-dimensional like conjugated copolymers were systematically investi-
gated. The absorption spectra, band gaps, and energy levels of the polymers were effectively tuned by simply attaching differ-
ent acceptor groups. As the electron-withdrawing ability of the acceptors increased, the band gaps of the polymers were nar-
rowed from 2.05 to 1.61 eV; meanwhile, the LUMO energy levels of the polymers decreased from 3.27 to 3.75 eV, whereas
their relatively deep HOMO energy levels of ~5.35 eV were preserved. BHJ solar cells were fabricated and characterized by
using the three polymers as donor materials and the highest power conversion efficiency of 2.87% was achieved for the device
based on PFSDTA:(6,6)-phenyl-C71-butyric acid methyl ester blend.
conjugated polymers, two-dimensional like polymers, band gaps, bulk-heterojunction solar cells
tential of fabrication large area device by solution process-
1 Introduction
ing [1–5]. Nevertheless, the power conversion efficiency
(PCE) of polymeric BHJ solar cells is still incomparable
with other photovoltaic techniques, such as silicon solar
cells and dye-sensitized solar cells [6, 7], which limits their
practical application. Polymeric BHJ solar cells are usually
composed of an active layer of conjugated polymer donor
and fullerene derivative (such as (6,6)-phenyl-C61- butyric
acid methyl ester (PC61BM) or (6,6)-phenyl-C71- butyric
acid methyl ester (PC71BM)) acceptor blending composite,
which is sandwiched between a low work function metal
cathode and an ITO anode [1, 8]. Apparently, the as broad
as absorption spectra of the polymer donors with high
coefficients to harvest as much as solar radiation is critical
for high performance polymeric BHJ solar cells, especially
Photovoltaic techniques, such as solar cells, which can di-
rectly convert solar energy into electricity energy, have
been regarded as one of the most promising approaches to
solve future global energy crises and environmental pollu-
tion problems. Among all kinds of photovoltaic techniques,
polymeric bulk-heterojunction (BHJ) solar cells have at-
tracted a lot of attention recently from both academic com-
munity and application-oriented companies due to their ad-
vantages of low-cost, light weight, flexibility and their po-
*Corresponding author (email: msfhuang@scut.edu.cn; chchoy@eee.hku.hk,
yongcao@scut.edu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2011
chem.scichina.com
www.springerlink.com

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Duan CH, et al. Sci China Chem April (2011) Vol.54 No.4
when considering that the commonly used acceptor materi-
als (PC61BM and PC71BM) show poor light absorbing abil-
ity [9–14]. In addition, it is well known that high hole mo-
bility and suitable energy levels of the donor polymers are
also important to achieve high PCE [9, 15]. Therefore, lin-
ear donor-acceptor (D-A) narrow-band-gap (< 2.0 eV) con-
jugated polymers with alternating electron-rich (donor) and
electron-deficient (acceptor) units along their backbone
were extensively explored as the donor materials for poly-
meric BHJ solar cells [16–35], because their absorbance and
energy levels can be tuned by controlling the intramolecular
charge transfer (ICT) from the donor to the acceptor units.
Excitingly, some of them exhibited excellent photovoltaic
properties with the PCE values as high as 6%–7% when
used as donors in polymeric BHJ solar cells [28, 30, 33–36].
With respect to the exploration of linear D-A type nar-
row-band-gap conjugated polymers, the extension of ab-
sorption spectrum of polythiophene by attaching conjugated
side chains to extend its conjugation length is an another
straightforward approach. For example, BHJ solar cells
based on a two-dimensional like conjugated polythiophene,
which was developed in Li’s group, exhibited much higher
PCE as well as higher open circuit voltage (Voc) than those
of poly(3-hexylthiophene) (P3HT) under the same condi-
tions [37]. However, in many cases, it is very hard to obtain
a high short circuit current (Jsc) and Voc simultaneously
among those linear D-A narrow-band-gap polymers or
two-dimensional like conjugated polythiophenes. To over-
come this, Huang et al. recently reported a new molecular
engineering strategy of attaching the strong electron-with-
drawing acceptor groups onto the end of the side chains and
connecting with the electron-donating conjugated main chain
through a -bridge, which resulted in a two-dimensional
like D-A conjugated polymer [38]. By this way, the band
gaps of the resulted D-A polymers can be effectively nar-
rowed by changing the pendant acceptor groups on the side
chains, while the relatively deep highest occupied molecular
orbital (HOMO) energy levels can be maintained by using
fluorene, silafluorene or carbazole units to build the main
chains [38–44]. And the resulted polymeric BHJ solar cells
with this kind of two-dimensional like conjugated polymers
as donor materials exhibited high Voc more than 0.9 V and a
highest PCE value of 4.74% [38–41, 43].
two-dimensional like conjugated polymers. And the rela-
tionships between the structures and properties of the re-
sulted polymers (including absorption spectra, band gaps,
energy levels, charge transport ability and consequently
photovoltaic performance) were systematically investigated.
2 Experimental
2.1 General
All starting chemicals, unless otherwise specified, were
purchased from Alfa Aesar or Sigma-Aldrich and used
without further purification. All the solvents used were fur-
ther purified prior to use. The other materials were of the
1
common commercial level and used as received. H NMR
and 13C NMR spectra were recorded on a Bruker AV-300
(300 MHz) in deuterated chloroform solution using tetrame-
thylsilane (TMS;  = 0 ppm) as internal standard. The MS of
the compound was recorded on a LCQ DECA XP liquid
chromatography-mass spectrometer. Elemental analyses
were performed on a Vario EL elemental analysis instru-
ment (Elementar Co.). Number-average (Mn) and weight-
average (Mw) molecular weights were determined by a Wa-
ters GPC 2410 in THF using a calibration curve with stan-
dard polystyrene as a reference. Differential scan calo-
rimetry (DSC) measurements were performed on a Netzsch
DSC 204 under N2 flow at heating and cooling rates of 10 °C
min1. Thermogravimetric analyses (TGA) were performed
on a Netzsch TG 209 under N2 flow at a heating rate of 10 °C
min1. UV-vis absorption spectra were recorded on a HP
8453 spectrophotometer. Cyclic voltammetry (CV) was per-
formed on a CHI600D electrochemical workstation with a
platinum working electrode and a Pt wire counter electrode
at a scan rate of 50 mV s1 against an Ag/Ag+ (0.1 M of
AgNO3 in acetonitrile) reference electrode with a nitro-
gen-saturated anhydrous solution of 0.1 mol L1 tetrabu-
tylammonium hexafluorophosphate in acetonitrile. The
polymer films for electrochemical measurements were
coated from a polymer-THF dilute solution.
2.2 Synthetic procedures
Herein, a series of new two-dimensional like D-A con-
jugated polymers PFSDCNIO, PFSDCN and PFSDTA, with
2-(1,2-dihydro-1-oxoinden-3-ylidene) malononitrile, malononi-
trile and 1,3-diethtyl-2-thiobarbituric acid as the acceptors
respectively, were developed for polymeric BHJ solar cell
application. Thiophene, instead of styrylthiophene which
was widely used as -bridge in the previously reported
two-dimensional like conjugated polymers, was used as the
-bridge to enhance the resulted polymers’ photo- oxidation
stability [45, 46]. Moreover, the shortened -bridge may be
beneficial to the more efficient molecular packing in solid
state and consequently better hole transport ability of the
Tris(4-bromophenyl)amine (1) [47], bis(4-bromophenyl)4-
(thienyl)phenylamine (2) [48], 2,7-bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (M2) [49] and
2-(1,2-dihydro-1-oxoinden-3-ylidene)malononitrile [50] were
prepared according to the reported methods.
Synthesis of 5-(4-(Bis(4-bromophenyl)amino)phenyl)thio-
phene-2-ca-rbaodehyde (M1)
Compound 2 (3.5 g, 7.2 mmol) was dissolved in 30 mL of
dry N,N-dimethylformamide. POCl3 (20 mL, 215 mmol)
was added dropwise to the solution at 0 °C. The reaction
mixture was allowed to warm to 50 °C and stirred for 2 h.

Duan CH, et al. Sci China Chem April (2011) Vol.54 No.4
687
After cooling to room temperature, the reaction mixture was
poured into ice water, neutralized with sodium carbonate,
and extracted with dichloromethane. The combined organic
phases were washed with water and then brine several times.
After drying over magnesium sulfate, the solvent was re-
moved under reduced pressure. The crude product was puri-
fied using column chromatography (silica gel, petroleum
ether/dichloromethane (2/1)) to yield M1 as yellow needles
2.06 (m, 4H), 1.20–1.09 (m, 20H), 0.82–0.80 (t, J =7.0 Hz,
10H), 0.77 (m, 4H). GPC (THF, polystyrene standard) Mn =
18.5 kg mol1, Mw = 37.5 kg mol1, PDI = 2.03.
Synthesis of PFSDTA
To a solution of PFSCHO (130 mg, 0.175 mmol of the re-
peating unit) and 1,3-diethyl-2-thiobarbituric acid (700 mg,
3.5 mmol) in 15 mL of chloroform was added 0.5 mL of
pyridine. The mixture solution was stirred in the dark for 20 h
at room temperature after which the resulted mixture was
poured into methanol and the precipitate was filtered off. The
resulted polymer was dissolved in 15 mL of chloroform.
The solution was filtered through a 0.45 m PTFE filter,
and precipitated from methanol to yield the product as a
1
(2.7g, 5.3 mmol, 74%). H NMR (300 MHz, CDCl3, ppm,
): 9.86 (s, 1H), 7.73–7.72 (d, J=4.0 Hz, 1H), 7.56–7.52 (m,
2H), 7.42–7.36 (m, 4H), 7.33–7.31 (d, J = 4.0 Hz, 1H),
7.08–7.03 (m, 2H), 7.01–6.98 (m, 4H). 13C NMR (75 MHz,
CDCl3, ppm, ): 182.68, 153.93, 148.08, 145.75, 141.76,
137.68, 132.66, 127.52, 127.42, 126.28, 123.31, 123.26,
116.69. APCIMS: m/z 513.3 (M+). Anal. calcd For
(C23H15Br2NOS)n (%): C, 53.82; H, 2.95; N, 2.73; S, 6.25,
found (%): C, 53.60; H, 3.06; N, 2.64; S, 6.18.
1
purple solid (150 mg, 93%). H NMR (300 MHz, CDCl3,
ppm, ): 8.66 (s, 1H), 7.89–7.88 (d, J = 4.0 Hz, 1H),
7.81–7.72 (q, 4H), 7.69–7.60 (m, 8H), 7.49–7.48 (d, J=4.0
Hz, 1H), 7.33–7.31 (d, J = 8.0 Hz, 4H), 7.24–7.21 (d, J=9.0
Hz, 2H), 4.64–4.61 (m, 4H), 2.06 (m, 4H), 1.39–1.30 (m,
6H), 1.20–1.09 (m, 20H), 0.82–0.80 (t, J = 7.0 Hz, 10H),
0.78 (m, 4H). GPC (THF, polystyrene standard) Mn = 17.7
kg mol1, Mw = 35.8 kg mol1, PDI = 2.02.
Synthesis of PFSCHO
2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-
dioctylfluorene (M2) (0.642 g, 1.0 mmol), and 5-(4-(bis(4-
bromophenyl)amino)phenyl)thiophene-2-ca-rbaodehyde (M1)
(0.513 g, 1.0 mmol) and Pd(PPh3)4 (10 mg) were dissolved
in a mixture of toluene (10 mL), tetrahydrofuran (THF)
(10 mL) and sodium carbonate (2 M in water, 4 mL) in a
50 mL two-necked round-bottomed flask under argon. The
mixture was refluxed with vigorous stirring in the dark for
24 h under argon atmosphere. After cooling to room tem-
perature, the mixture was poured into methanol. The pre-
cipitated material was collected by filtration through a fun-
nel. After washing with acetone for 24 h in a Soxhlet appa-
ratus to remove oligomers and catalyst residues, the resulted
material was dissolved in 30 mL of chloroform. The solu-
tion was filtered with a 0.45 m PTFE filter, concentrated
and precipitated from methanol to yield PFSCHO as a yel-
low solid (650 mg, 88%). 1H NMR (300 MHz, CDCl3, ppm,
): 9.88–9.87 (s, 1H), 7.80–7.73 (m, 3H), 7.67–7.59 (m,
10H), 7.36–7.29 (m, 5H), 7.24–7.13 (d, J = 9.0 Hz, 2H),
2.06 (m, 4H), 1.26–1.08 (m, 20H), 0.82–0.80 (t, J =7.0 Hz,
6H), 0.77 (m, 4H). GPC (THF, polystyrene standard) Mn =
17.4 kg mol1, Mw = 37.3 kg mol1, PDI = 2.14.
Synthesis of PFSDCNIO
To a solution of PFSCHO (130 mg, 0.175 mmol of the repeat-
ing unit) and 2-(1,2-dihydro-1-oxoinden-3-ylidene)malononi-
trile (700 mg, 3.6 mmol) in 15 mL of chloroform was added
0.5 mL of pyridine. The mixture solution was stirred in the
dark for 20 h at room temperature after which the resulted
mixture was poured into methanol and the precipitate was
filtered off. The resulted crude polymer was dissolved in
15 mL of chloroform, and then precipitated from methanol.
This procedure was repeated for several times to completely
remove 2-(1,2-dihydro-1-oxoinden-3-ylidene)malononitrile.
After being dissolved in 15 mL of chloroform again, the
solution was filtered through a 0.45 m PTFE filter, and
precipitated from methanol to yield the product as a black
1
solid (82 mg, 51%). H NMR (300 MHz, CDCl3, ppm, ):
8.87 (s, 1H), 8.71–8.68 (d, J = 7.0 Hz, 1H), 7.92 (d, J = 7.0
Hz, 1H), 7.86 (s, 1H), 7.81–7.60 (m, 14H), 7.45 (s, 1H),
7.34–7.31 (d, J = 8.0 Hz, 4H), 7.22–7.16 (m, 2H), 2.06 (m,
4H), 1.25–1.09 (m, 20H), 0.82–0.80 (t, J = 7.0 Hz, 6H), 0.78
(m, 4H). GPC (THF, polystyrene standard) Mn = 7.6 kg
mol1, Mw = 10.8 kg mol1, PDI = 1.42.
Synthesis of PFSDCN
To a solution of PFSCHO (150 mg, 0.202 mmol of the re-
peating unit) and malononitrile (688 mg, 10.1 mmol) in 15 mL
of chloroform was added 0.5 mL of pyridine. The mixture
solution was stirred in the dark for 20 h at room temperature
after which the resulted mixture was poured into methanol
and the precipitate was filtered off. The resulted polymer was
dissolved in 15 mL of chloroform. The solution was filtered
through a 0.45 m PTFE filter, and precipitated from
methanol to yield the product as a deep red solid (150 mg,
94%). 1H NMR (300 MHz, CDCl3, ppm, ): 7.81–7.78 (m,
3H), 7.71–7.59 (m, 11H), 7.38–7.37 (d, J = 4.0 Hz, 1H),
7.33–7.30 (d, J = 8.0 Hz, 4H), 7.23–7.20 (d, J=9.0 Hz, 2H),
2.3 Solar cell device fabrication and characterization
ITO-coated glass substrates (15 /□) were cleaned sequen-
tially by sonication in detergent, de-ionized water, acetone,
and ethanol, and dried in a nitrogen stream, followed by UV
ozone treatment as the reported procedures [51]. To fabricate
photovoltaic devices, a thin layer (ca. 30 nm) of PEDOT:PSS
(Baytron P VP AI 4083, filtered at 0.45 m) was first
spin-coated on the pre-cleaned ITO-coated glass substrates
at 4000 r/min and baked at 140 °C for 10 min under ambient

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Duan CH, et al. Sci China Chem April (2011) Vol.54 No.4
condition. The substrates were then transferred into a nitro-
gen-filled glove-box. Subsequently, the polymer: PC61BM
(or PC71BM) active layer (ca. 60 nm) was spin-coated on
the PEDOT:PSS layer at 750 r/min from a homogeneously
blended solution for 40 s. The solution was prepared by
dissolving the polymer (2 mg mL1) and PC61BM (or
PC71BM) (8 mg mL1) in chloroform/chlorobenzene (1/1, v/v)
mixture and filtered with a 0.2 m PTFE filter. The sub-
strates were annealed at 100 °C for 10 min prior to elec-
trode deposition. At the final stage, the substrates were
pumped down to high vacuum (< 2 × 106 Torr), and LiF
(1 nm) topped with aluminum (100 nm) was thermally
evaporated onto the active layer through shadow masks to
define the active area of the devices. The effective devices
area was measured to be 5.77 mm2. Hole mobility was
measured in hole-only devices by using the space charge
limited current (SCLC) method [52, 53] with a device
configuration of ITO/PEDOT:PSS/blend layer/Au (20 nm)/Al
(100 nm). Details of the device characterization were reported
elsewhere [51].
polymers is presented in Scheme 1. The intermediate com-
pounds 1 and 2 were prepared according to the reported
methods [47, 48]. The aldehyde-bearing monomer of M1
was prepared from compound 2 via a Vilsmeier-Haack
formylation under mild conditions to afford a moderate
yield of 74%. The aldehyde-bearing copolymer PFSCHO,
which is the precursor polymer for the target polymers was
successfully obtained via the Suzuki coupling reaction with
the equimolar amounts of the monomer M1 and monomer
M2 under the common conditions (Catalyzed by Pd(PPh3)4
in the two-phase system of toluene/tetrahydrofuran (1/1, v/v)
and 2.0 M of aqueous Na2CO3 under refluxing). Thus, the
band gap and energy level engineering of this class of poly-
fluorene-derived two-dimensional like conjugated polymers
could be manipulated via attaching different electron-deficient
acceptor groups onto the end of the side chain of PFSCHO.
The three target polymer PFSDCN, PFSDTA and
PFSDCNIO were prepared through the Knoevenagel con-
densation by reacting the aldehyde-bearing precursor poly-
mer PFSCHO with malononitrile, 1,3-diethylthiobarbituric
acid and 2-(1,2-dihydro-1-oxoinden-3-ylidene)malononitrile,
respectively, with the catalysis amount of pyridine under
mild conditions. One of the advantages of this precursor
polymer approach is to avoid the possible decomposition of
the final Donor--Acceptor organic dyes under the Suzuki
coupling reaction conditions. More importantly, all of the
3 Results and discussion
3.1 Synthesis and characterization
The synthetic route of making new monomer M1 and target
Scheme 1 Synthetic route of the monomer and target polymers.

Duan CH, et al. Sci China Chem April (2011) Vol.54 No.4
689
final polymers would have similar degrees of polymeriza-
tion and molecular weight distributions, which is desirable
for exploring their structure-property relationships. The
chemical structures of the three target polymers were con-
2.03, 2.02 and 1.42, respectively. Clearly, PFSDCN and
PFSDTA possessed similar Mn and PDI with their precursor
polymer PFSCHO, whereas PFSDCNIO had a pronounced
lower Mn than its precursor polymer PFSCHO. The possible
reason of this variation is the strong dipole-dipole interac-
tion in PFSDCNIO between the organic dye units which
resulted in a strong packing of the side chains and conse-
quently disturbed the GPC results [40, 42]. Benefiting from
the long alkyl side chains connecting on the fluorene units
and the propeller-like feature of triphenylamine group, all
the polymers exhibited good solubility in common organic
solvents, such as THF, chloroform, toluene, chlorobenzene
and 1,2-dichlorobenzene, and good film-forming ability,
which are desirable for the device fabrication.
1
firmed by H NMR. After chemical transformation, the sig-
nal at 9.88–9.87 ppm for aromatic aldehyde proton com-
1
pletely disappeared in the H NMR spectra of all the three
target polymers; meanwhile, new signals of olefinic protons
at 7.71, 8.66 and 8.87 ppm appeared, which confirmed the
complete conversion of the precursor polymer [39, 40].
Moreover, the ratios of integrated peak areas of olefinic
protons to the methane group adjacent to the fluorene 9-
positioning carbon atom protons are in good agreement with
the proposed structures of PFSDCN, PFSDTA and
PFSDCNIO. Clearly, the gradual increase of chemical shift
of olefinic protons from 7.71 ppm for PFSDCN, via 8.66
ppm for PFSDTA to 8.87 ppm for PFSDCNIO is consistent
with the electron-withdrawing strength of the three acceptor
groups, which also indicates that the optical band gaps (Eg)
of the three target polymers would decrease as the above
subsequence. Molecular weight and distributions of both the
precursor polymer and three target polymers were deter-
mined by gel permeation chromatography (GPC) using the
monodisperse polystyrene as standard and THF as an eluent.
As listed in Table 1, PFSCHO possessed an Mn of 17.4
kg mol1 and a polydispersity index (PDI) of 2.14. After
postfunctionlization, the values of Mn of PFSDCN,
PFSDTA and PFSDCNIO were determined to be 18.5, 17.7
and 7.6 kg mol1, respectively, with corresponding PDIs of
3.2 Thermal properties
The thermal properties of PFSDCN, PFSDTA and PFSDCNIO
were investigated by TGA and DSC under a nitrogen at-
mosphere, and the results are presented in Figure 1 and Ta-
ble 1. As shown in Figure 1(a), all of the resulted polymers
exhibited excellent thermal stability with 5% weight-loss
temperatures (Td) of 422, 361 and 367 °C for PFSDCN,
PFSDTA and PFSDCNIO, respectively, which is adequate
to resist the degradation of the polymers at the resulting device
operating temperatures. In addition, PFSDCN and PFSDTA
exhibited glass-transition temperature (Tg) values as high as
165 and 175 °C, respectively. However, no any clear glass
transition process was observed for PFSDCNIO up to 250 °C
Table 1 Molecular weight and thermal properties of the polymers
Mn (kg mol1
17.4
)
Mw (kg mol1
37.3
)
Polymers
PDI
2.14
2.03
2.02
1.42
Tg (°C)

Td (°C)

PFSCHO
PFSDCN
PFSDTA
PFSDCNIO
18.5
37.5
165
175

422
361
367
17.7
35.8
7.6
10.8
Figure 1 (a) TGA and (b) DSC plots of PFSDCN, PFSDTA and PFSDCNIO.

690
Duan CH, et al. Sci China Chem April (2011) Vol.54 No.4
(Figure 1(b)). These high Tg values indicate that the resulted
polymers would have good stability to retard the defor-
mation of the active layer morphology at relatively high
temperatures, which is beneficial for PSCs applications [54].
the ICT absorption band was successfully red-shifted to
643 nm and the optical band gap was narrowed to 1.61 eV
through attaching a much stronger pendant acceptor of 2-
(1,2-dihydro-1-oxoinden-3-ylidene) malononitrile group for
PFSDCNIO. The variation of the ICT absorption bands and
optical band gaps of the resulted polymers clearly proved
that the photophysical properties of the resulted polymers
could be readily tuned by simply manipulating the strength
of the pendant acceptors in the side chains.
3.3 Optical properties
The absorption spectra of all three resulted polymers in thin
films are demonstrated in Figure 2. All of PFSDCN,
PFSDTA and PFSDCNIO exhibited two distinct absorption
bands. The first absorption bands with peaks of 374 nm for
PFSDCN, 379 nm for PFSDTA and 388 nm for PFSDCNIO
should be attributed to the -* transition of the polymers
conjugated backbone [38]. Notably, the resulted polymers
show slightly red-shifted -* transition absorption peaks
from the dicyano group, via 1,3-diethyl-2-thiobarbituric acid
group, to 2-(1,2-dihydro-1-oxoinden-3-ylidene)malononi-
trile group. This red-shift phenomenon is possible due to the
increased polarizabilities and consequently interchain inter-
actions of the polymers in films with the increase of elec-
tron-withdrawing ability of the pendant acceptor groups.
The absorption bands of the polymers in the low energy region
are corresponding to the ICT interaction between their di-
phenyl amino moieties on conjugated backbone and the
pendant acceptor groups [38–40]. Obviously, these ICT
absorption bands are progressively red-shifted, broadened
and also enhanced accompanied by the increase of the elec-
tron affinity of the pendant acceptor groups. The peaks of
ICT absorption bands are red-shifted from 518 nm for
PFSDCN, via 561 nm for PFSDTA to 643 nm for PFSDCNIO.
Based on the onset of the absorption spectra at 606, 645 and
772 nm for PFSDCN, PFSDTA and PFSDCNIO, respec-
tively, the optical band gap of them was calculated to be
2.05, 1.92 and 1.61 eV, respectively. It is note-worthy that
the ICT absorption bands of PFSDCN and PFSDTA are
obviously blue-shifted compared to their styrylthiophene--
bridge based analogues polymers PFDCN and PFPDT be-
cause of the absence of the absorption contribution from
vinylene groups in styrylthiophene--bridge [38]. However,
3.4 Electrochemical properties and energy levels
To insightfully understand the influence of the pendant ac-
ceptor groups on the energy levels of the resulted polymers,
CV experiments were carried out to estimate the HOMO
and the lowest unoccupied molecular orbital (LUMO) en-
ergy levels of PFSDCN, PFSDTA and PFSDCNIO. From
the oxidation curves presented in Figure 3(a), we can see
that all three resulted polymers show two clear oxidation
processes: the first reversible one can be ascribed to the
Figure 3 (a) CV curves of the polymers films measured in 0.1 M
Bu4NPF6 versus Ag/AgNO3 in acetonitrile. (b) Energy level diagrams for
PFSDCN, PFSDTA and PFSDCNIO.
Figure 2 UV-vis absorption spectra of the resulted polymers in films.

Duan CH, et al. Sci China Chem April (2011) Vol.54 No.4
691
oxidation of the triphenylamine moieties, and the second
3.5 Photovoltaic properties
one corresponds to the oxidation of the fluorene rings [55].
However, the defined reduction curves of the resulted
polymers could be hardly obtained despite various CV
measurements being conducted. The HOMO energy levels
of PFSDCN, PFSDTA and PFSDCNIO were thus calcu-
lated to be 5.32, 5.39 and 5.36 eV, respectively, using
the redox potential of ferrocene/ferrocenium (4.8 eV to
vacuum) as the internal standard [56]. The LUMO energy
levels were estimated to be 3.27, 3.47 and 3.75 eV for
PFSDCN, PFSDTA and PFSDCNIO, respectively, from the
HOMO energy levels and the optical band gaps of them.
The energy level parameters of the three resulted polymers
are summarized in Figure 3(b) and Table 2. Clearly, the
HOMO energy levels of the polymers had little changes due
to the identical conjugated backbone of the polymers. The
LUMO levels of the resulted polymers, however, were sub-
stantially decreased from 3.27 eV for PFSDCN, to 3.47
eV for PFSDTA, and further to 3.75 eV for PFSDCNIO.
The relatively low HOMO energy levels of the resulted
polymers would endow them good chemical stability in
ambient conditions and ensure the high Voc of the resulted
solar cells based on them [9, 57, 58]. Meanwhile, the gaps
of larger than 0.4 eV between the LUMO energy levels of
PFSDCN or PFSDTA and fullerene-derived electron ac-
ceptors PC61BM or PC71BM would ensure energeticcally
favorable electron transfer [59, 60]. However, the offset
between PFSDCNIO and PC71BM is only 0.15 eV, which
may be detrimental to the electron transfer in donor- accep-
tor interface and increase the possibility of charge carrier
recombination. Anyway, it is clear that the photophysical
properties and electronic structures of the polymers were
successfully manipulated by the two-dimensional like Do-
nor--Acceptor molecule design, and a series of promising
donor materials were developed for BHJ solar cell applica-
tions.
To investigate the photovoltaic properties of PFSDCN,
PFSDTA and PFSDCNIO, BHJ solar cells were fabricated
using fullerene derivatives as the electron acceptor with a
convenient sandwich configuration of ITO/PEDOT:PSS/
polymer:acceptor blend/LiF/Al. The optimized blending
ratio of 1:4 between the donor polymer and fullerene ac-
ceptor was chosen to obtain the best device performance of
the BHJ solar cells. Besides PC61BM, PC71BM was also
used as an acceptor to fabricate BHJ solar cells, becausing
the latter can complement the absorption valley of the nar-
row- band-gap polymers in the visible range from 440 to
530 nm and consequently afford higher Jsc of the resulted
solar cells [61]. The J-V curves and device parameters of
the BHJ solar cells based on PFSDCN, PFSDTA and
PFSDCNIO are presented in Figure 4 and Table 3, respec-
tively. The best performing device is based on PFSDTA:
PC71BM blend, which showed a PCE of 2.87% along with a
Voc of 0.97 eV, a Jsc of 7.56 mA cm2 and a FF of 39.2%.
Figure 4 J-V characteristics of the BHJ solar cells with the device configura-
tion of ITO/PEDOT:PSS/polymer:donor/LiF/Al.
Table 2 Optical and electrochemical properties of the polymers
Polymers
PFSDCN
Egopt (eV)
2.05
Eox (V)
0.91
EHOMO (eV)
5.32
ELUMO (eV)
3.27

abs (nm)
374,518,606
379,561,645
388,643,772
PFSDTA
1.92
0.98
5.39
3.47
PFSDCNIO
1.61
0.95
5.36
3.75
Table 3 Photovoltaic performance of the BHJ solar cells with the device configuration of ITO/PEDOT:PSS/polymer:acceptor/LiF/Al and hole mobility of
the polymers estimated by SCLC method
Jsc (mA cm2)
3.91
µ0 (cm2 V1 s1)
5.45×105
1.29×104
4.31×105
2.42×104
2.05×105
3.77×105
Polymers
PFSDCN
Acceptors
PC61BM
PC71BM
PC61BM
PC71BM
PC61BM
PC71BM
Voc(V)
0.97
0.99
1.00
0.97
0.93
0.97
FF (%)
42.3
39.6
46.5
39.2
36.5
37.2
PCE (%)
1.60
5.42
2.12
PFSDTA
5.06
2.35
7.56
2.87
PFSDCNIO
3.52
1.19
4.54
1.65

692
Duan CH, et al. Sci China Chem April (2011) Vol.54 No.4
Clearly, PFSDTA-based solar cell show much higher Jsc
than those of the other two polymers. The increased absorp-
tion in long wavelength region of PFSDTA compared with
that of PFSDCN may account for the improved Jsc for the
the former compared to the latter. Nevertheless, the de-
creased offset between the LUMO energy levels of
PFSDCNIO and PC71BM and consequently higher recom-
bination losses of charge carriers is possibly responsible to
the lower Jsc of PFSDCNIO than those of the other two
polymers [15]. In addition, a significant drop in Jsc was ob-
served for all PC61BM devices, which is consistent with the
decreased absorption of PC61BM in the visible region with
respect to that of PC71BM, as the absorption spectra of the
blend films shown in Figure 5. Noticeably, all solar cells
presented Voc values larger than 0.9 V, which is consistent
with the relatively low HOMO energy levels of their active
donor polymers and proved the advantage of the two-dimen-
sional like Donor--Acceptor narrow-band-gap polymers
compared to the linear D-A narrow-band-gap copolymers
[18, 62, 63]. The incident-photon-to-converted current effi-
ciency (IPCE) spectra of BHJ solar cells based on three re-
sulting polymers, as presented in Figure 6, were in good
accordance with the blend films absorption spectra and the
Jsc values measured from the devices.
3.6 Hole mobility
The charge carrier transport abilities of conjugated polymers
have great influence on the resulted performance, especially
on Jsc of BHJ solar cells [15]. To insightfully understand the
difference of the photovoltaic properties of the resulted
polymers, the hole mobility of the polymers was thus meas-
ured by using the SCLC method, which can most accurately
evaluate the charge transport ability of the BHJ solar cells
active layer [52, 53], and the J-V plots of the devices are
shown in Figure 7. The mobility values of the polymers are
listed in Table 3. The hole mobility of PFSDCN, PFSDTA
and PFSDCNIO was calculated to be 5.45 × 105, 4.31 ×
105 and 2.05 × 105 cm2 V1 s1 respectively for poly-
mer:PC61BM blend, and 1.29 × 104, 2.42 × 104 and 3.77 ×
105 cm2 V1 s1, respectively for polymer: PC71BM blend.
Apparently, PFSDTA shows much better hole transport
ability than the other two polymers (about 2 times and 7
times than that of PFSDCN and PFSDCNIO, respectively)
when blending with PC71BM. In addition, all three polymers
show significantly increased hole transport ability for
polymer:PC71BM blend with respective to polymer:PC61BM
blend. Clearly, the differences of charge carrier transport
abilities of the resulted polymers can be also used to explain
the variation of Jsc and PCE of their resulted BHJ solar cells.
Anyway, the hole-transporting properties of all resulted
polymers are comparable with many fluorene-based linear
D-A narrow-band-gap copolymers [18, 63, 64]. Moreover,
when taking into account the two-dimensional conjugated
features, the polymers may have good isotropic charge
transport properties, which are desired for BHJ solar cells.
4 Conclusion
Figure 5 UV-vis absorption spectra of polymer:fullerene derivative (1:4)
blend films.
In conclusion, three two-dimensional like conjugated copoly-
Figure 7 J-V plots of devices in ITO/PEDOT:PSS/blend layer/Au (20 nm)/
Figure 6 The incident-to-converted-current efficiency (IPCE) spectra of
Al (100 nm) configuration. The solid lines represent the fitting curves.
the BHJ solar cells based on PFSDCN, PFSDTA and PFSDCNIO.

Duan CH, et al. Sci China Chem April (2011) Vol.54 No.4
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postfunctionalizing their aldehyde-bearing precursor poly-
mer PFSCHO, which were synthesized by the common Su-
zuki polycondensation. The postfunctionalization approach
provides the convenience to investigate the structure-property
relationships of the resulted polymers which have the same
degree of polymerization. The three polymers have good
solubility in common organic solvents and outstanding
thermal stability. The band gaps, photophysical properties
and electronic structures of the polymers were effectively
tuned by simply attaching acceptor groups which have dif-
ferent electron-withdrawing ability. Mentionable, the rela-
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successfully preserved while their band gaps were de-
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using PFSDTA and PC71BM as the active layer of BHJ so-
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polymers show high Voc of more than 0.9 V, which is larger
than those of most fluorene-based linear narrow-band-gap
copolymers.
This work was financially supported by the National Natural Science
Foundation of China (50990065, 51010003, 51073058 and 20904011), the
National Basic Research Program of China (973 Program) (2009CB623601)
and the Fundamental Research Funds for the Central Universities, South
China University of Technology (2009220012, 2009220043). Choy ac-
knowledges the supported UGC grant (#400897) of the University of Hong
Kong and Hong Kong Research Grants Council (HKU#712108 and
HKU#712010) from the Research Grants Council of the Hong Kong Spe-
cial Administrative Region, China.
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