Synthesis and photovoltaic studies on novel fluorene based cross-conjugated donor-acceptor type polymers

Article abstract of DOI:10.1016/j.orgel.2016.10.039

Direct arylation polymerization (DAP) is emerging as a promising green, cheap, simple, and efficient environment friendly method for synthesizing conjugated polymers without involving any organometallic reagent. We report fluorene based novel cross-conjugated alternate and random copolymers for polymer solar cells (PSCs), which were synthesized by DAP and/or Yamamoto polymerization under appropriate reaction conditions to obtain high molecular weight. These cross-conjugated polymers possess absorption maxima in the range of 490–520 nm and have narrow band gap (1.7–2.05 eV) which is suitable for bulk heterojuntion (BHJ) type organic solar cells. Among the synthesized polymers, the highest number average molecular weight (M_n) i.e. 43.1 kg mol^?1 was obtained for polymer P2b (poly((9H-fluoren-9-ylidene)methylene)bis((2-ethylhexyl)sulfane)-alt-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)), and so good polymeric films were formed for P2b. Thus, BHJ films were prepared for P2b for device performance studies and the morphology of these films was studied by atomic force microscopy (AFM). Polymer P2b was blended with the fullerene derivative [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) in different ratios and under the illumination of solar simulator with Air Mass global (AM 1.5G) irradiated at 100 mW cm^?2. Power conversion efficiency (PCE) of 1.4% has been achieved for BHJs in ratio of 1:2 of P2b: PC₇₁BM in simply processed devices. This result indicates that cross-conjugated polymers can be tapped as potential donors for BHJs as the PCE obtained is the highest among this type of cross-conjugated polymers.

Full text of DOI:10.1016/j.orgel.2016.10.039

Organic Electronics xxx (2016) 1e9
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Synthesis and photovoltaic studies on novel fluorene based
cross-conjugated donor-acceptor type polymers
a
b
b
a, *
Bhavna Sharma , Firoz Alam , Viresh Dutta , Josemon Jacob
a
Centre for Polymer Science and Engineering, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India
Photovoltaic Laboratory, Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 September 2016
Received in revised form
Direct arylation polymerization (DAP) is emerging as a promising green, cheap, simple, and efficient
environment friendly method for synthesizing conjugated polymers without involving any organome-
tallic reagent. We report fluorene based novel cross-conjugated alternate and random copolymers for
polymer solar cells (PSCs), which were synthesized by DAP and/or Yamamoto polymerization under
appropriate reaction conditions to obtain high molecular weight. These cross-conjugated polymers
possess absorption maxima in the range of 490e520 nm and have narrow band gap (1.7e2.05 eV) which
is suitable for bulk heterojuntion (BHJ) type organic solar cells. Among the synthesized polymers, the
24 October 2016
Accepted 25 October 2016
Available online xxx
Keywords:
Fluorene
Cross-conjugated polymers
Direct arylation
Yamamoto polycondensation
Polymer solar cells
ꢀ
1
highest number average molecular weight (M
n
) i.e. 43.1 kg mol
was obtained for polymer P2b
(
poly((9H-fluoren-9-ylidene)methylene)bis((2-ethylhexyl)sulfane)-alt-4,7-di(thiophen-2-yl)benzo[c]
[1,2,5]thiadiazole)), and so good polymeric films were formed for P2b. Thus, BHJ films were prepared for
P2b for device performance studies and the morphology of these films was studied by atomic force
microscopy (AFM). Polymer P2b was blended with the fullerene derivative [6,6]-phenyl C71 butyric acid
methyl ester (PC71BM) in different ratios and under the illumination of solar simulator with Air Mass
ꢀ
2
global (AM 1.5G) irradiated at 100 mW cm . Power conversion efficiency (PCE) of 1.4% has been ach-
ieved for BHJs in ratio of 1:2 of P2b: PC71BM in simply processed devices. This result indicates that cross-
conjugated polymers can be tapped as potential donors for BHJs as the PCE obtained is the highest
among this type of cross-conjugated polymers.
©
2016 Elsevier B.V. All rights reserved.
1. Introduction
polymers reported in literature are of donor-acceptor type (D-A)
alternating structures [2,7,8].
Thin-film PSCs have various attractive features over inorganic
based solar cells for the generation of renewable energy [1e3]. They
have various advantages such as light weight, flexibility, good
mechanical property, low cost fabrication, solution-processability
and possess promising applications in large-area flexible devices
Presently, such polymers are being synthesized by Kumada,
Stille and Suzuki type catalytic polycondensation coupling re-
actions which require the preparation of organometallic monomers
[9e14]. The highly toxic nature of the stannylated compounds for
Stille coupling, and lesser stability of the borylated compounds for
Suzuki cross-coupling strongly discourage the use of these cross-
coupling methods [10,14,15]. DAP has therefore attracted a great
deal of attention as an easy and non-toxic alternative to the above
mentioned traditional methods [9,13,16]. It is the dehy-
drohalogenative coupling of aromatic compounds and aryl halides,
which proceeds by CeH bond cleavage [13] of aromatic molecules
with an arylpalladium carboxylate intermediate [11,17]. As the
polycondensation reactions by DAP does not involve any organo-
metallic monomer, this coupling does not produce undesirable
metal-containing waste [12]. The catalyst used for DAP is phos-
phine free Pd-catalyst, which prevents the formation of defects in
the polymers due to covalently-bonded phosphine ligands [17].
[1e4]. BHJ solar cells have the active layer of conjugated organic
polymers as electron donors and fullerene acceptors, which are
blended together and is the most efficient cell design among all
PSCs [1,3]. The development of new polymer structures and the
optimization for PSCs have led to PCE as high as 10% [5]. Therefore,
the design and synthesis of highly efficient low band gap novel
conjugated polymers are of great importance to improve efficiency
in solar cells [6]. The most efficient low band gap conjugated
*
Corresponding author.
E-mail address: jacob@polymers.iitd.ac.in (J. Jacob).
http://dx.doi.org/10.1016/j.orgel.2016.10.039
566-1199/© 2016 Elsevier B.V. All rights reserved.
1
Please cite this article in press as: B. Sharma, et al., Synthesis and photovoltaic studies on novel fluorene based cross-conjugated donor-acceptor
type polymers, Organic Electronics (2016), http://dx.doi.org/10.1016/j.orgel.2016.10.039

2
B. Sharma et al. / Organic Electronics xxx (2016) 1e9
Moreover, DAP gives higher molecular weight [16], which reduces
concentration of end groups and so higher purity [12,17,18]. Also,
the high molecular weight of the polymer improves the device
performance [19,20].
This work focuses on the development of synthetic procedure of
planarized cross-conjugated fluorene-type monomer and polymers
[23,27]. These polymer chains are anticipated to exhibit strong solid
state interaction by
p-stacking due to the planar geometry of the
The main chain of the low band gap polymers has electron-rich
donor (D) and electron -deficient acceptor (A) moieties [21]. The
band gap in the polymers is reduced by intramolecular charge
transfer and their physical properties can be tuned by modifying
the structures of D and A or by linking ethylene bridge [21]. The
conjugated polymers possess distinct optical and electronic prop-
erties, which usually arise from a chemically tailored structure
repeat unit [23]. The effect of introduction of thioalkyl group by
ethylene bridge on conjugation, solubility, band gap and device
performance has been studied. Cross-conjugated D-A polymers
(structure of target polymers is shown in Scheme 2 and Scheme 3),
are synthesized under varied reaction conditions by polymerization
of cross-conjugated fluorene derivative (M1) as donor with DTBT
(M4 and M5) and alkylated DTBT (M2 and M3) as acceptor by DAP
and Yamamoto polymerization [17]. We report on the optical, elec-
trochemical and device properties of this novel class of materials.
[1,8,22]. Preferably, for an ideal donor, the absorption spectra of the
polymers must be broad with high intensity, so as to effectively
absorb the solar radiation [3,7]. For high open-circuit voltage (VOC),
the donor polymer must have low HOMO level and also a good
compatibility with PCBM to form a BHJ with a large interface area
2. Experimental section
[4,22]. The BHJ so formed must have high hole mobility for active
2.1. General characterization
transportation of photo-generated charge carriers [4,21,22].
Alkylated fluorene is widely used as a donor monomer or
comonomer in conjugated polymer synthesis [17]. The side alkyl
chains control solubility and crystallinity of a polymer by influ-
encing the stacking of the main chain [1,21]. In most of the reported
1HNMR spectra were recorded on a Bruker DPX-300 spec-
trometer operating at 300 MHz in deuterated chloroform with
tetramethylsilane (TMS) as a reference for chemical shifts. UVevis
spectra were measured on a T90 þUV/visible spectrophotometer in
THF solution. Photoluminiscence (PL) spectra were measured on a
SHIMADZU spectrofluorophotometer RF5301PC. Cyclic voltammo-
grams (CV) were recorded on a Zehnar Zehnnium, potentiostat/
galvanostat at a constant scan rate of 20 mV s . For GPC mea-
surements, THF was used as the mobile phase. GPC analysis was
done on a Perkin Elmer Series 200 GPC system equipped with a
refractive index detector using monodispersed polystyrene as a
standard. The morphology of the active layers was investigated
using Bruker AFM in tapping mode under ambient conditions.
3
conjugated polymers, alkyl chains are bonded to sp hybridized
carbon atom which hinders close packing of polymer chains
[21,23,24]. By inserting an ethylene bridging unit between the side
ꢀ1
chain and the main chain of D-A polymers synthesized in this study,
the unwanted steric hindrance can be effectively eliminated [21].
2
The carbon atom in ethylene bridge has sp hybridisation [23e25],
due to which the side chains and main chain are coplanar and
hence due to close intermolecular
p-p stacking [1,24e26] main
chains are closely packed which leads to smooth charge carrier
hoping between the polymer chains, and high hole mobility
[1,21,24]. Modified fluorene monomer (M1) is designed with thia-
2.2. Synthetic procedures
vinylidene structure at 9-position, where sulphur atom has been
introduced in the alkyl chains by ethylene bridge i.e. the alkyl group
of alkylated fluorene is replaced by thioalkyl group (structure of the
monomer is shown in Scheme 1). The polymers with ethylene
bridge are cross-conjugated as conjugation is present in main chain
as well as side chain.
2.2.1. Monomer synthesis
2.2.1.1. Synthesis of ((2, 7-dibromo-9H-fluoren-9-ylidene)methylene)
bis((2-ethylhexyl)sulfane) (M1). 2, 7-Dibromofluorene (1 g,
3.086 mmol) was dissolved in 30 mL of dry DMSO in a dried schlenk
flask under nitrogen atmosphere and the solution was cooled to
Scheme 1. Synthetic procedure for monomer M1 and structures of M2, M3, M4 and M5.
Please cite this article in press as: B. Sharma, et al., Synthesis and photovoltaic studies on novel fluorene based cross-conjugated donor-acceptor
type polymers, Organic Electronics (2016), http://dx.doi.org/10.1016/j.orgel.2016.10.039

B. Sharma et al. / Organic Electronics xxx (2016) 1e9
3
Scheme 2. Synthetic routes for polymers P1a, P1b and P1c. The reaction conditions for the polymer synthesis are mentioned in Table 1.
Scheme 3. Synthetic routes for polymers P2a, P2b and P2c. The reaction conditions for the polymer synthesis are mentioned in Table 1.
ꢁ
ꢁ
0
C. NaH (0.2 g, 8.33 mmol) was added very slowly to the solution
as the reaction is highly exothermic followed by addition of CS
0.3 mL, 3.47 mmol). As CS is volatile, it was added in a little larger
cooled to 0 C. NBS (0.67 g, 0.38 mmol) was added and the reaction
2
mixture was stirred overnight at room temperature in dark by
covering the flask with aluminium foil. The reaction was quenched
(
2
quantity (0.9 mL). The reaction mixture was stirred for 1 h, followed
by addition of 2-ethylhexyl bromide (1.5 mL, 8.55 mmol) and stir-
red for further 4 h. The mixture was poured into 5% HCl solution
3
with 2 M NaOH solution and then was extracted with CHCl . The
crude product was purified by silica gel column chromatography
using 2% ethyl acetate/hexane as eluent to obtain pure product as
ꢁ
1
and extracted with ethyl acetate which was dried over Na
2
SO
4
. The
bright orange solid (0.12 g, 96%). Melting point ¼ 152 C. HNMR
(300 MHz, CDCl : 7.72(s, 2H), 7.70(s, 2H), 2.61(t, 4H), 1.62(m, 4H),
1.31(m, 20H), 0.83(t, 6H) [28] (see Scheme 4).
solvent was evaporated and the crude product was purified by silica
3
) d
gel column chromatography using hexane as eluent to obtain yel-
1
low viscous liquid as product (1.04 g, 54%). HNMR (300 MHz,
3
CDCl ) d: 9.09(s, 2H) 7.55(d, 2H), 7.45(d, 2H), 3.06(d, 4H), 1.64(m,
2
.2.1.4. Synthesis of 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazo-
2
H), 1.43e1.48(m, 16H), 0.92(t, 12H) [26].
le(M4). 4,7-Dibromo-2,1,3-benzothiadiazole (0.2 g, 0.68 mmol) and
thiophene (0.272 mL, 3.4 mmol) were dissolved in 40 mL dry DMF.
The solution was purged with nitrogen for 10 min followed by
2.2.1.2. Synthesis of 4,7-bis(4-octylthiophen-2-yl)benzo[c][1,2,5]
thiadiazole(M2).
,4,5,5-tetramethyl-2-(4-octlythiophen-2-yl)-1,3,2-dioxaborolane
addition of pivalic acid (0.104 g, 1.02 mmol), K
2
CO
3
(0.47 g,
4
3.4 mmol) and catalyst Pd(OAc)
2
(0.076 g, 0.34 mmol). The reaction
ꢁ
(
(
0.918 g, 2.856 mmol) and 4,7-dibromo-2,1,3-benzothiadiazole
0.3 g, 1.02 mmol) were dissolved in a mixture of toluene (7.5 mL)
was conducted for 4 h at 80 C under nitrogen atmosphere. The
product was extracted with ethyl acetate and the solvent was
evaporated to obtain crude product which was purified by silica gel
column chromatography using 3% ethyl acetate/hexane as eluent to
obtain M4, a bright orange colour solid (81.7 g, 40%). Melting
and THF (7.5 mL) in a schlenk flask under nitrogen followed by the
addition of Na CO (0.648 g dissolved in 1 mL H O) and a pinch of
2
3
2
phase transfer catalyst tetrabutylammonium bromide (TBAB). The
reaction mixture was purged with nitrogen for another 10 min and
ꢁ
1
point ¼ 128 C. HNMR (300 MHz, CDCl
3
) d: 8.13(d, 2H), 7.90(s, 2H),
then Pd(PPh
3
)
4
(0.47 g, 0.04 mmol) was added. The reaction was
7.46(d, 2H), 7.23(m, 2H) [29].
ꢁ
conducted for 24 h at 110 C. The product was extracted with
2 4
chloroform and dried over Na SO . The product was purified by
2
.2.1.5. Synthesis of 4,7-bis(5-bromothiophen-2-yl)benzo[c][1,2,5]
using silica gel column chromatography using 2% ethyl acetate/
hexane as eluent to obtain M2 (406.39 g, 76%) as bright orange
thiadiazole(M5). M4 (0.07 g, 0.233 mmol) was dissolved in 3.3 mL
dry CHCl
ꢁ
3
followed by addition of NBS (0.87 g, 0.489 mmol) at 0 C.
ꢁ
1
solid. Melting point ¼ 79 C. HNMR (300 MHz, CDCl
3
)
d
: 7.98(s,
The reaction was conducted in dark by covering the flask with
aluminium foil. The product was extracted with CHCl and the
solvent was evaporated to obtain crude product which was purified
by silica gel column chromatography using 5% ethyl acetate/hexane
as eluent to obtain M5 as bright orange solid (0.101 g, 95%). Melting
2
0
H), 7.83(s, 2H), 7.04(s, 2H), 2.72(t, 4H), 1.71(m, 4H), 1.35(m, 20H),
3
.89(t, 6H) [28].
2.2.1.3. Synthesis of 4,7-bis(5-bromo-4-octylthiophen-2-yl)benzo[c]
ꢁ
1
[1,2,5]thiadiazole(M3). M2 (0.1 g, 0.19 mmol) was dissolved in
point ¼ 258 C. HNMR (300 MHz, CDCl
3
) d: 7.85(s, 2H), 7.80(d, 2H),
mixture of dry CHCl (3 mL) and AcOH (3 mL) and the solution is
3
7.13(d, 2H) [29] (see Scheme 5).
Please cite this article in press as: B. Sharma, et al., Synthesis and photovoltaic studies on novel fluorene based cross-conjugated donor-acceptor
type polymers, Organic Electronics (2016), http://dx.doi.org/10.1016/j.orgel.2016.10.039

4
B. Sharma et al. / Organic Electronics xxx (2016) 1e9
Scheme 4. Synthetic routes for M2 and M3.
Scheme 5. Synthetic routes for M4 and M5.
2
2
.2.2. Polymer synthesis
remove traces of catalyst and then purified by soxhlet extraction
with methanol, acetone and chloroform. The polymer soluble in
.2.2.1. Synthesis of P1a (Poly((9H-fluoren-9-ylidene)methylene)
chloroform was concentrated and filtered in cold methanol to
bis((2-ethylhexyl)sulfane)-ran-4,7-bis(4-octylthiophen-2-yl)benzo[c]
1,2,5]thiadiazole). M1 (0.165 g, 0.264 mmol) and M3 (0.18 g,
.264 mmol) were dissolved in mixture of dry DMF (1 mL) and dry
1
obtain P1b as dark purple powder. (Yield-72.7%). HNMR (300 MHz,
[
0
CDCl
.7(s br, 2H), 3.14(d br, 2H), 2.51(d br,2H), 1.61(s br, 20H), 0.98(m br,
0H).
3
) d: 9.51(s br, 2H), 9.11(s br, 2H), 8.22(d br, 2H), 7.83(s br, 2H),
7
3
toluene (1 mL) in a dried schlenk tube inside a glove box. The so-
lution was continuously purged with nitrogen for 20 min followed
by addition of 2,2 -bipyridyl (0.103 g, 0.66 mmol) and cyclo-
0
octadiene (COD) (0.8 mL, 0.66 mmol). The reaction mixture was
2.2.2.3. Synthesis of P1c (Poly((9H-fluoren-9-ylidene)methylene)
bis((2-ethylhexyl)sulfane)-alt-4,7-bis(4-octylthiophen-2-yl)benzo[c]
[1,2,5]thiadiazole). M1 (0.120 g, 0.192 mmol) and M2 (0.1 g,
0.192 mmol) were dissolved in 1.92 mL of toluene in a dried schlenk
tube under nitrogen pressure. Pivalic acid (0.0058 g, 0.0576 mmol),
stirred for 30 min and then Ni(COD)
The reaction mixture was further heated with stirring at 95 C for
2
(0.33 g, 1.2 mmol) was added.
ꢁ
4
8 h. Bromobenzene (0.041 g, 0.264 mmol) was added to the re-
action mixture and reaction was stirred for additional 6 h at same
temperature. The mixture was cooled to room temperature and
poured into cold methanol (300 mL); filtered and washed with
EDTA solution to remove traces of Ni metal. The polymer was then
purified by soxhlet extraction and reprecipitated in cold methanol
Cs
mixture while stirring followed by addition of ligand P(o-OCH
Ph) (0.0108 g, 0.0307 mmol). The mixture was purged with ni-
trogen for 20 min and then Pd(OAc)
2 3
CO (0.25 g, 0.768 mmol), were then added to the reaction
3
-
3
2
(0.0034 g, 0.0153 mmol) was
ꢁ
and dried after filtration to obtain purple solid. (Yield-55.5%).
added. The reaction was conducted at 80 C for 48 h, which was
then cooled and poured in cold methanol. The polymer was filtered,
dried and washed with EDTA solution. Purification of polymer is
done by soxhlet extraction with acetone and chloroform, the
polymer soluble in chloroform was concentrated and reprecipitated
1
HNMR (300 MHz, CDCl
H), 7.93(s br, 2H), 7.7(s br, 2H), 3.05(d br, 2H), 2.21(d br,2H), 1.4(s
br, 20H), 0.98(m br, 30H).
3
) d: 9.1(s br, 2H), 8.41(d br, 2H), 8.22(d br,
2
in cold methanol to obtain a purple powder. (Yield-79.3%). %).
2
.2.2.2. Synthesis of P1b (Poly((9H-fluoren-9-ylidene)methylene)
bis((2-ethylhexyl)sulfane)-alt-4,7-bis(4-octylthiophen-2-yl)benzo[c]
1,2,5]thiadiazole). A mixture of K CO (0.11 g, 0.8 mmol), pivalic
acid (0.098 g, 0.096 mmol), M1 (0.2 g, 0.32 mmol), M2 (0.168 g,
.32 mmol) was dissolved in 3.2 mL of NMP in a dried schlenk tube
1
3
HNMR (300 MHz, CDCl ) d: 9.51(s br, 2H), 9.11(s br, 2H), 8.22(d br,
2H), 7.83(s br, 2H), 7.7(s br, 2H), 3.14(d br, 2H), 2.51(d br,2H), 1.61(s
[
2
3
br, 21H), 0.98(m br, 32H).
0
in nitrogen atmosphere followed by addition of Pd(OAc)
2
(0.007 g,
2.2.2.4. Synthesis of P2a (Poly((9H-fluoren-9-ylidene)methylene)
bis((2-ethylhexyl)sulfane)-ran-4,7-di(thiophen-2-yl)benzo[c][1,2,5]
thiadiazole). Polymer P2a was synthesized according to the pro-
cedure followed for P1a using M1 (0.2 g, 0.436 mmol), M5 (0.272 g,
ꢁ
0
.032 mmol). The reaction mixture was stirred at 80 C for 48 h
under nitrogen atmosphere. The polymer was then precipitated in
cold methanol, filtered, dried, washed with EDTA solution to
Please cite this article in press as: B. Sharma, et al., Synthesis and photovoltaic studies on novel fluorene based cross-conjugated donor-acceptor
type polymers, Organic Electronics (2016), http://dx.doi.org/10.1016/j.orgel.2016.10.039

B. Sharma et al. / Organic Electronics xxx (2016) 1e9
5
0
ꢀ2
0
1
.436 mmol), 2,2 -bipyridyl (0.17 g, 1.09 mmol), COD (0.13 mL,
(intensity ¼ 100 mW cm ) from a Xenon lamp (150 W Oriel) using
an aperture to define the illuminated area. The J-V characteristics of
the solar cells were measured by testing six cells.
.068 mmol) and Ni(COD)
DMF (2 mL) and dry toluene (2 mL) at 95 C for 48 h. The product
2
(0.3 g, 1.09 mmol) in a mixture of dry
ꢁ
was obtained as dark purple solid. (Yield-48%).
d: 9.1(s br, 2H),
8
2
.41(d br, 2H), 8.22(d br, 2H), 7.93(s br, 2H), 7.7(s br, 2H), 7.04(s, br,
H), 3.05(d br, 2H), 2.21(d br,2H), 1.4(s br, 20H), 0.98(m br, 30H).
3. Results and discussion
3.1. Design and synthesis
2.2.2.5. Synthesis of P2b (Poly((9H-fluoren-9-ylidene)methylene)
bis((2-ethylhexyl)sulfane)-alt-4,7-di(thiophen-2-yl)benzo[c][1,2,5]
thiadiazole). Polymer P2b was synthesized according to the pro-
cedure followed for polymer P1c, using M1 (0.2 g, 0.32 mmol), M4
The cross-conjugated monomer M1 of fluorene ((2, 7-dibromo-
9H-fluoren-9-ylidene)methylene)bis((2-ethylhexyl)sulfane), was
synthesized from 2,7-dibromofluorene. The fluorenyl anion was
generated by using a strong base sodium hydride, which then
reacted with carbon disulfide to give a ketene dithiolate, which was
subjected to alkylation by ethylhexyl bromide to give monomer M1.
4,7-Bis(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT) is one
of the most widely used acceptor monomer with high planarity for
designing low band gap D-A polymers. DTBT is a D-A-D type with
electron donating thiophene and electron withdrawing benzo[c]
(
0.096 g, 0.32 mmol), Pd(OAc)
Ph) (0.018 g,. 0.0512 mmol), pivalic acid (0.01 g, 0.096 mmol) and
Cs CO
product was obtained as purple solid. (Yield-86.1%). HNMR
300 MHz, CDCl : 9.40(s br, 2H), 9.00(s br, 2H), 8.095(d br, 2H),
.83(s br, 2H), 7.61(s br, 2H), 7.44(s br, 2H), 3.14(d br, 2H), 1.65(s br,
0H), 0.90(m br, 20H).
2 3
(0.0057 g, 0.0256 mmol), P(o-OCH -
3
ꢁ
3
(0.417 g, 1.28 mmol) in 3.2 mL toluene at 100 C for 48 h. The
2
1
(
3
) d
7
1
[1,2,5]thiadiazole group [2]. Additional alkyl chains are introduced
2
.2.2.6. Synthesis of P2c (Poly((9H-fluoren-9-ylidene)methylene)
on thiophene moiety to further improve the solubility of these
polymers (alkylated DTBT) [8]. Although the alkyl chains provide
solubility to the polymer, the charge transport properties are
changed due to disturbance in intermolecular charge transfer be-
tween donor and acceptor units [30]. Monomer M2 (alkylated
DTBT) was synthesized by Suzuki coupling of boronic ester of 3-
octylthiophene and 4,7-dibromo-2,1,3-benzothiadiazole using
bis((2-ethylhexyl)sulfane)-alt-4,7-di(thiophen-2-yl)benzo[c][1,2,5]
thiadiazole). Polymer P2c was synthesized according to the pro-
cedure followed for polymer P1c, using M1 (0.106 g, 0.17 mmol), M4
(
0.051 g, 0.17 mmol), Cs
2
CO
3
(0.166 g, 0.51 mmol), pivalic acid
-Ph) (0.0024 g, 0.0068 mmol),
(
0.017 g, 0.17 mmol), P(o-OCH
3
3
Herrmann catalyst (0.003 g, 0.0034 mmol) in 0.34 mL toluene at
ꢁ
100 C for 48 h. The product was obtained as purple solid. (Yield-
Pd(PPh
was then brominated to obtain monomer M3 by using n-bromo-
succinimide in dry CHCl . Monomer M4 (DTBT) was synthesized by
direct arylation one pot synthesis using thiophene and 4,7-
dibromo-2,1,3-benzothiadiazole as reactants with Pd(OAc) as
catalyst, K CO as base and pivalic acid in DMF. This was bromi-
nated using n-bromosuccinimide in CHCl to obtain M5. The
3 4 2 3 2
) as catalyst and Na CO as base in THF/toluene/H O. M2
6
8
2
2%). 1HNMR (300 MHz, CDCl
3
)
d: 9.49(s br, 2H), 9.10(s br, 2H),
.19(d br, 2H), 7.93(s br, 2H), 7.71(s br, 2H), 7.54(s br, 2H), 3.20(d br,
H), 1.65(s br, 10H), 0.91(m br, 20H).
3
2
2
.3. Device fabrication and evaluation
2
3
3
Photovoltaic devices were fabricated according to the following
structures of all the monomers are depicted in Scheme 1.
procedures. The device configuration used for fabrication was glass/
The target polymers are shown in Schemes 2 and 3. Random
copolymers P1a and P2a were synthesized by Yamamoto poly-
merization in good yields. Polymers P1b, P1c, P2b and P2c were
synthesized by DAP polymerization. The varied reaction conditions
are mentioned in Table 1. The polymers were purified by soxhlet
indium
tin
oxide
(ITO)/poly(3,4-
(PEDOT:PSS)/
ethylenedioxythiphene):poly(styrenesulphonate)
active layer/Al with a pixel area of ~0.1 cm . Firstly, the patterned
2
ꢀ1
ITO-coated glass substrates with a conductivity of 20
U square ,
were washed with detergent solution. Then, these substrates were
cleaned by sequential ultrasonic treatments with de-ionized water,
acetone, and isopropyl alcohol for 20 min at every step followed by
vapour treatment by isopropyl alcohol vapours. The cleaned ITO-
3
extraction with methanol, acetone, and CHCl and then were
washed with EDTA solution to remove traces of catalyst left in the
product. All the polymers showed good solubility in common
organic solvents such as CHCl , THF, toluene, and chlorobenzene.
3
The gel permeation chromatography (GPC) measurements were
done against monodisperse polystyrene standards with THF as
ꢁ
coated glass substrates were dried in a vacuum oven at 100 C for
3
0 min to remove any remaining solvents. A hole transporting layer,
PEDOT:PSS (Heraeus Clevios PH-1000) was deposited on the ITO
surface by spin coating at 3000 rpm for 90 s and then it was dried in
a vacuum oven at 120 C for 20 min. The thickness of PEDOT:PSS
eluent. The number average molecular weights (M
n
) were found to
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
be 10.3 kg mol , 8.1 kg mol , 6.0 kg mol , 8.1 kg mol
,
ꢁ
ꢀ1
ꢀ1
43.1 kg mol , and 8.6 kg mol for polymer P1a, P1b, P1c, P2a, P2b
and P2c, respectively. Table 2 summarizes the molecular weight of
all the polymers.
layer was ~40 nm under these conditions. The substrates were then
moved to a glove box under nitrogen atmosphere. Active layer
solutions were dissolved in 1,2,4-trichlorobenzene (TCB) with
ꢀ1
polymer concentration of 30 mg mL and then were ultra soni-
cated for 30 min. The solution was filtered and the active layer was
spin coated on the ITO/PEDOT:PSS electrode at 1000 rpm for 90 s in
the glove box with thickness of active layer of 120 nm. The active
layer films were immediately annealed on the hot plate kept inside
3.2. Absorption studies
UVevisible absorption spectra of the synthesized random and
alternate copolymers measured in dilute THF solution and as thin
films are shown in Fig. 1 and the corresponding optoelectronic
properties are summarized in Table 2. The solution absorption
maxima in THF ranged from 490 to 520 nm. Two distinct absorption
ꢁ
the nitrogen-filled glove box at 120 C for 20 min. The samples were
then
transferred
into
the
vacuum
chamber
6
(
(
pressure < 2 ꢂ 10ꢀ Torr) for the deposition of the Al electrode
bands were observed at around 350 nm and 500 nm, the one at
*
thickness 100 nm) on the top of the photoactive layer by thermal
350 nm can be attributed to
pꢀ
p
transitions of the conjugated
2
evaporation. The area of device was 0.1 cm and the electrical
properties of the device thus obtained were measured using the
light from solar simulator equipped with a Keithley 2635A source
measurement unit. The J-V characteristics of the devices were
measured under Air Mass global (AM 1.5G) irradiation
backbones, whereas the one around 500 nm is due to internal
charge transfer between donor and acceptor units. The solution
spectra of all the polymers have almost same absorption maxima
because of same chemical structure of the random copolymers and
alternate copolymers. While moving from solution to film, the
Please cite this article in press as: B. Sharma, et al., Synthesis and photovoltaic studies on novel fluorene based cross-conjugated donor-acceptor
type polymers, Organic Electronics (2016), http://dx.doi.org/10.1016/j.orgel.2016.10.039

6
B. Sharma et al. / Organic Electronics xxx (2016) 1e9
Table 1
Polymerization results of P1 (a,b and c) and P2 (a,b and c) under varied conditions.
ꢁ
S.No.
Polymer
Base
Catalyst
Ni(COD)
Solvent
Ligand
Temperature ( C)
Yield (%)
1
2
3
4
5
6
P1a
P1b
P1c
P2a
P2b
P2c
e
2
DMF/Toluene
NMP
Toluene
DMF/Toluene
Toluene
Toluene
e
95
80
80
95
100
100
55.5
72.7
79.3
48
86.1
62
K
2
CO
3
Pd(OAc)
Pd(OAc)
2
2
e
Cs
e
2
CO
3
P(o-OCH
e
3
-Ph)
3
Ni(COD)
Pd(OAc)
Herrmann catalyst
2
Cs
Cs
2
CO
CO
3
2
P(o-OCH
P(o-OCH
3
-Ph)
-Ph)
3
3
2
3
3
Table 2
Optical and electrochemical properties a) THF solution, b) Film, c) Optical band gap, d) Electrochemical band gap, M
average molecular weight.
n w
is number average molecular weight and M is weight
c
d
S.No.
Polymer
Absorption (nm)
Emission (nm)
HOMO
(eV)
LUMO
E
g(opt)
E
g
M
n
M
w
a
b
b
a
ꢀ1
(kg mol )
l
max
l
max
l
edge
l
max
Band gap (eV)
1
2
3
4
5
6
P1a
P1b
P1c
P2a
P2b
P2c
490
500
520
520
520
500
522
534
534
506
546
514
676
730
710
720
715
682
645
659
634
648
638
625
ꢀ5.75
ꢀ5.75
ꢀ5.60
ꢀ5.70
ꢀ5.70
ꢀ5.75
ꢀ3.90
ꢀ4.10
ꢀ3.70
ꢀ3.85
ꢀ3.70
ꢀ3.70
1.85
1.65
1.74
1.85
1.73
1.81
1.83
1.69
1.90
1.72
2.00
2.05
10.3
8.1
6.0
8.1
43.1
8.6
15.4
10.5
6.3
10.2
67.2
10.2
Fig. 1. Absorption spectra of polymers in (A) THF solution, (B) Film on glass. (C) Emission spectra of polymers in THF solution.
absorption maxima became slightly broader and red shifted owing
to the increased inter-chain interactions and stacking in the
3.3. Electrochemical properties
p-p
solid state. The optical band gap (Eg(opt)) of these polymers was
estimated from the onset absorption in the thin film and found to
be in the range of 1.65e1.85 eV. The narrower Eg(opt) can be
attributed to ordered and planar structure of the conjugated
structure. The emission spectra were also measured in THF solution
and the maxima range from 625 to 659 nm and are summarized in
Table 2.
The electrochemical properties were studied using cyclic vol-
tammetry. The highest occupied molecular orbital (HOMO) en-
ergies were calculated from the first oxidation onset of the cyclic
voltammogram according to the equation EHOMO ¼ -e (Eox þ 4.4) eV.
The lowest unoccupied molecular orbital (LUMO) energy levels
were calculated by using the equation ELUMO ¼ -e (Ered e 4.4) eV
from the onset reduction potential. The deep lying HOMO level for
Please cite this article in press as: B. Sharma, et al., Synthesis and photovoltaic studies on novel fluorene based cross-conjugated donor-acceptor
type polymers, Organic Electronics (2016), http://dx.doi.org/10.1016/j.orgel.2016.10.039

B. Sharma et al. / Organic Electronics xxx (2016) 1e9
7
the polymers ranges from ꢀ5.60 eV to ꢀ5.75eV. This is an ideal
range to ensure better air stability and greater obtainable open
circuit voltage (Voc) in the photovoltaic cell. The LUMO level ranges
from ꢀ3.70 eV to ꢀ4.10 eV which is again fits with the required
electronic levels for better device performance using PC71BM as the
acceptor. The band gap ranges from 1.69 eV to 2.05 eV. Table 2
summarizes the HOMO, LUMO and electrochemical band gap
values of all the polymers. The cyclic voltammograms are shown in
Fig. 4.
P2b as donor. P2b was chosen for the device fabrication because it
formed good quality films due to its high molecular weight. The
photovoltaic performance was optimized in different P2b:PC71BM
weight ratios to achieve best PCE. Three different binary composi-
tions of P2b and PC71BM i.e. 1:1, 1:2 and 1:3 were used. The blends
were prepared by mixing a solution of P2b:PC71BM in (TCB) which
were then used for BHJ solar cells by spin coating. After optimiza-
tion studies, a weight ratio of P2b to PC71BM of 1:2, a concentration
ꢀ
1
ꢁ
of 30 mg mL , thermal annealing at 120 C for 20 min and with a
thickness of the blend films of about 100 nm gave the best device
properties. For the binary blend (1:2), a PCE of 1.4% was attained
3.4. Photovoltaic properties
ꢀ2
with Voc of 0.49 V, short circuit current (Jsc) of 9.6 mA cm and fill
factor (FF) of 0.29. To verify the consistency of the best efficiency, six
devices were fabricated on all conditions. The current densities
versus voltage (J-V) characteristics of the fabricated device are
displayed in Fig. 2 and their key features are summarized in Table 3.
The FF values observed are quite low and the PCE can be further
increased by improving the FF. The lower FFs can be attributed to
BHJ solar cells were fabricated for studying the photovoltaic
properties of the blends. The device configuration is glass/ITO/
PEDOT:PSS/P2b:PC71BM/Al. PEDOT:PSS works as an electrical
connector to accelerate the recombination of photogenerated
charge carriers (electron and hole). The photoactive layers were
composed of PC71BM as acceptor and newly synthesized polymer
Fig. 2. Cyclic voltammograms of the polymers.
Please cite this article in press as: B. Sharma, et al., Synthesis and photovoltaic studies on novel fluorene based cross-conjugated donor-acceptor
type polymers, Organic Electronics (2016), http://dx.doi.org/10.1016/j.orgel.2016.10.039

8
B. Sharma et al. / Organic Electronics xxx (2016) 1e9
Table 3
roughness values are 2.84 nm, 1.94 nm and 2.22 nm respectively.
The film with 1:2 ratio of P2b:PC71BM blend exhibited relatively
smooth surface without large phase separation with lowest rms
value of 1.94 nm which indicates a good miscibility between P2b
and PC71BM, leading to an improved device performance. The films
with ratio 1:1 and 1:3 have higher rms values (2.84 and 2.22
respectively) and so they show a little larger phase separation and
hence a lower device efficiency. Hence ratio of 1:2 for P2b:PC71BM
blend film is most desirable for better performance in PSCs.
Summary of photovoltaic properties of polymer blend P2b:PC71BM.
S.No
P2b:PC71BM
J
sc (mA cmꢀ2)
V
oc (V)
FF (%)
PCE (%)
1
2
3
1:1
1:2
1:3
4.3
9.6
5.4
0.48
0.49
0.53
28.9
29.3
28.9
0.6
1.4
0.8
4. Conclusions
Novel fluorene and benzothiadiazole based cross-conjugated D-
A polymers with low band gap have been developed. These poly-
mers possess ethylene bridging units between main chain and side
thioalkyl chains. Ethylene bridge gives a coplanar configuration
between the side chains and main chain and electron rich sulphur
provides electron donating properties to the polymer. The syn-
thesized polymers show good solubility in common organic sol-
vents with a broad absorption band centered around 500 nm The
electrochemical properties were studied and the polymers were
found to have low band gap in the range of 1.7 eVe2.05 eV. Among
the synthesized polymers, P2b formed a good film because of its
high molecular weight and hence was used for device fabrication
for solar cells. P2b:PC71BM based BHJs were fabricated in different
ratios by solution processing using elevated temperatures and it
Fig. 3. J-V characteristics of devices measured under the illumination of AM 1.5 G (100
mW cm ) from a solar simulator.
ꢀ
2
Fig. 4. AFM images (10
m
m ꢂ 10
mm) of P2b:PC71BM films spin coated from 1,2,4-trichlorobenzene solutions in three different ratios of (A)1:1, (B)1:2 and (C)1:3.
inappropriate morphology and improper thickness of the active
layer [31,32]. The FF values may also be low because of chemical
degradation of the metal-polymer interface [31,32] The FF values
can be improved by smoothening the morphology of the active
layer by thermal treatment or by blending with small molecules
like 1,8-diiodooctane, 1,8-octanedithiol or chloronaphthalene with
active layer as reported in recent literature [33]. These additives
have been found to enhance the miscibility of the two components
of the active layer, thereby leading to the formation of a better
interpenetrating network. Variation in the thickness of the active
layer can also increase FF values [32] and these will be the focus of
future studies.
was found that the ratio of 1:2 is the most promising system and
exhibited 1.4% PCE. The limited performance of the devices pre-
sented can be because of inefficient exciton dissociation or
enhanced charge recombination. Further modifications in the de-
vice structure and optimization studies to smoothen the surface
morphology of the photoactive layers are expected to give
increased photocurrent for the devices with better efficiency.
Acknowledgements
The authors gratefully acknowledge partial financial support
from the Department of Electronics and Information Technology
and the Department of Science and Technology (DST grant no. SB/
S1/OC-12/2013). One of us (BS) acknowledges a graduate fellowship
from UGC.
3.5. Morphological studies
The morphology of P2b:PC71BM blend films, is closely related to
References
the device performance of BHJ PSCs, and was investigated by AFM
using tapping mode. Fig. 3 shows the AFM images of the
P2b:PC71BM blend films in 1,2,4-trichlorobenzene solution in three
ratios i.e. 1:1, 1:2 and 1:3 with the root-mean-square (rms)
[
1] C. Cui, W.-Y. Wong, Effects of alkylthio and alkoxy side chains in polymer
donor materials for organic solar cells, Macromol. Rapid Commun. 37 (2016)
287e302.
Please cite this article in press as: B. Sharma, et al., Synthesis and photovoltaic studies on novel fluorene based cross-conjugated donor-acceptor
type polymers, Organic Electronics (2016), http://dx.doi.org/10.1016/j.orgel.2016.10.039

B. Sharma et al. / Organic Electronics xxx (2016) 1e9
9
[
2] L. Han, X. Bao, T. Hu, Z. Du, W. Chen, D. Zhu, Q. Liu, M. Sun, R. Yang, Novel
donoreacceptor polymer containing 4,7-Bis(thiophen-2-yl)benzo[c][1,2,5]
thiadiazole for polymer solar cells with power conversion efficiency of 6.21%,
Macromol. Rapid Commun. 35 (2014) 1153e1157.
3] X. Gong, C. Li, Z. Lu, G. Li, Q. Mei, T. Fang, Z. Bo, Anthracene-containing wide-
band-gap conjugated polymers for high-open-circuit-voltage polymer solar
cells, Macromol. Rapid Commun. 34 (2013) 1163e1168.
4] W. Shin, M.Y. Jo, D.S. You, Y.S. Jeong, D.Y. Yoon, J.-W. Kang, J.H. Cho, G.D. Lee,
S.-S. Hong, J.H. Kim, Improvement of efficiency of polymer solar cell by
incorporation of the planar shaped monomer in low band gap polymer, Synth.
Met. 162 (2012) 768e774.
for improving the performance of organic photovoltaics, Adv. Funct. Mater. 24
(2014) 3226e3233.
[18] J. Kuwabara, K. Yamazaki, T. Yamagata, W. Tsuchida, T. Kanbara, The effect of a
solvent on direct arylation polycondensation of substituted thiophenes,
Polym. Chem. 6 (2015) 891e895.
[19] D. Mori, H. Benten, H. Ohkita, S. Ito, K. Miyake, Polymer/Polymer blend solar
cells improved by using high-molecular-weight fluorene-based copolymer as
electron acceptor, ACS Appl. Mater. Interfaces 4 (2012) 3325e3329.
[20] Z. Xiao, K. Sun, J. Subbiah, T. Qin, S. Lu, B. Purushothaman, D.J. Jones,
A.B. Holmes, W.W.H. Wong, Effect of molecular weight on the properties and
organic solar cell device performance of a donor-acceptor conjugated poly-
mer, Polym. Chem. 6 (2015) 2312e2318.
[21] J. Liu, H. Choi, J.Y. Kim, C. Bailey, M. Durstock, L. Dai, Highly crystalline and low
bandgap donor polymers for efficient polymer solar cells, Adv. Mater. 24
(2012) 538e542.
[
[
[
[
5] B. Sharma, Y. Sarothia, R. Singh, Z. Kan, P.E. Keivanidis, J. Jacob, Synthesis and
characterization
of
light-absorbing
cyclopentadithiophene-based
donoreacceptor copolymers, Polym. Int. 65 (2016) 57e65.
6] S.-W. Chang, H. Waters, J. Kettle, Z.-R. Kuo, C.-H. Li, C.-Y. Yu, M. Horie, Pd-
catalysed direct arylation polymerisation for synthesis of low-bandgap con-
jugated polymers and photovoltaic performance, Macromol. Rapid Commun.
[22] A.P. Zoombelt, S.G.J. Mathijssen, M.G.R. Turbiez, M.M. Wienk, R.A.J. Janssen,
Small band gap polymers based on diketopyrrolopyrrole, J. Mater. Chem. 20
(2010) 2240e2246.
33 (2012) 1927e1932.
[
7] C. Zuo, J. Cao, L. Ding, A fused-ring acceptor unit in dea copolymers benefits
photovoltaic performance, Macromol. Rapid Commun. 35 (2014) 1362e1366.
8] M.E. Foster, B.A. Zhang, D. Murtagh, Y. Liu, M.Y. Sfeir, B.M. Wong, J.D. Azoulay,
Solution-processable donoreacceptor polymers with modular electronic
properties and very narrow bandgaps, Macromol. Rapid Commun. 35 (2014)
[23] T.W. Bünnagel, F. Galbrecht, U. Scherf,
A
novel poly[2,6-(4-
dialkylmethylenecyclopentadithiophene)] with “in-Plane” alkyl substituents,
Macromolecules 39 (2006) 8870e8872.
[24] M. Heeney, C. Bailey, M. Giles, M. Shkunov, D. Sparrowe, S. Tierney, W. Zhang,
I. McCulloch, Alkylidene fluorene liquid crystalline semiconducting polymers
for organic field effect transistor devices, Macromolecules 37 (2004)
5250e5256.
[25] R. Grisorio, P. Mastrorilli, G. Ciccarella, G.P. Suranna, C.F. Nobile, Synthesis and
optical behaviour of monodispersed oligo(fluorenylidene)s, Tetrahedron Lett.
49 (2008) 2078e2082.
[26] K. Takagi, S. Sugimoto, M. Mitamura, Y. Yuki, S.-i. Matsuoka, M. Suzuki,
Polymerization of fluorene-based monomers modified with thiavinylidene
structure at 9-position and their optical properties, Synth. Met. 159 (2009)
228e233.
[
1516e1521.
[
9] A.E. Rudenko, C.A. Wiley, J.F. Tannaci, B.C. Thompson, Optimization of direct
arylation polymerization conditions for the synthesis of poly(3-
hexylthiophene), J. Polym. Sci. Part A Polym. Chem. 51 (2013) 2660e2668.
[
10] X. Wang, K. Wang, M. Wang, Synthesis of conjugated polymers via an
exclusive direct-arylation coupling reaction: a facile and straightforward way
to synthesize thiophene-flanked benzothiadiazole derivatives and their co-
polymers, Polym. Chem. 6 (2015) 1846e1855.
[
[
[
[
[
[
11] M. Wakioka, Y. Kitano, F. Ozawa, A highly efficient catalytic system for
polycondensation
of
2,7-Dibromo-9,9-dioctylfluorene
and
1,2,4,5-
[27] W.-Y. Wong, G.-L. Lu, K.-H. Choi, Z. Lin, Functionalization of 9-(Dicyano-
methylene)fluorene derivatives with substituted acetylenes, Eur. J. Org. Chem.
2003 (2003) 365e373.
[28] Y. Chen, Z. Du, W. Chen, S. Wen, L. Sun, Q. Liu, M. Sun, R. Yang, New small
molecules with thiazolothiazole and benzothiadiazole acceptors for solution-
processed organic solar cells, New J. Chem. 38 (2014) 1559e1564.
[29] M.L. Keshtov, D.V. Marochkin, V.S. Kochurov, A.R. Khokhlov, E.N. Koukaras,
G.D. Sharma, New conjugated alternating benzodithiophene-containing co-
polymers with different acceptor units: synthesis and photovoltaic applica-
tion, J. Mater. Chem. A 2 (2014) 155e171.
[30] W. Li, R. Qin, Y. Zhou, M. Andersson, F. Li, C. Zhang, B. Li, Z. Liu, Z. Bo, F. Zhang,
Tailoring side chains of low band gap polymers for high efficiency polymer
solar cells, Polymer 51 (2010) 3031e3038.
[31] D. Gupta, S. Mukhopadhyay, K.S. Narayan, Fill factor in organic solar cells, Sol.
Energy Mater. Sol. Cells 94 (2010) 1309e1313.
Tetrafluorobenzene via direct arylation, Macromolecules 46 (2013) 370e374.
12] A.E. Rudenko, B.C. Thompson, Influence of the carboxylic acid additive
structure on the properties of poly(3-hexylthiophene) prepared via direct
arylation polymerization (DArP), Macromolecules 48 (2015) 569e575.
13] X. Zhang, Y. Gao, S. Li, X. Shi, Y. Geng, F. Wang, Synthesis of poly(5,6-difluoro-
2,1,3-benzothiadiazole-alt-9,9-dioctyl-fluorene) via direct arylation poly-
condensation, J. Polym. Sci. Part A Polym. Chem. 52 (2014) 2367e2374.
14] M. Kuramochi, J. Kuwabara, W. Lu, T. Kanbara, Direct arylation poly-
condensation of bithiazole derivatives with various acceptors, Macromole-
cules 47 (2014) 7378e7385.
15] S. Kowalski, S. Allard, K. Zilberberg, T. Riedl, U. Scherf, Direct arylation poly-
condensation as simplified alternative for the synthesis of conjugated (co)
polymers, Prog. Polym. Sci. 38 (2013) 1805e1814.
16] M.-m. Sun, W. Wang, L.-y. Liang, S.-h. Yan, M.-l. Zhou, Q.-d. Ling, Substituent
effects on direct arylation polycondensation and optical properties of alter-
nating fluorene-thiophene copolymers, Chin. J. Polym. Sci. 33 (2015)
[32] B. Qi, J. Wang, Fill factor in organic solar cells, Phys. Chem. Chem. Phys. 15
(2013) 8972e8982.
7
83e791.
[33] H.-C. Liao, C.-C. Ho, C.-Y. Chang, M.-H. Jao, S.B. Darling, W.-F. Su, Additives for
morphology control in high-efficiency organic solar cells, Mater. Today 16
(2013) 326e336.
[
17] J. Kuwabara, T. Yasuda, S.J. Choi, W. Lu, K. Yamazaki, S. Kagaya, L. Han,
T. Kanbara, Direct arylation polycondensation: a promising method for the
synthesis of highly pure, high-molecular-weight conjugated polymers needed
Please cite this article in press as: B. Sharma, et al., Synthesis and photovoltaic studies on novel fluorene based cross-conjugated donor-acceptor
type polymers, Organic Electronics (2016), http://dx.doi.org/10.1016/j.orgel.2016.10.039