Narrow band gap benzodithiophene and quinoxaline bearing conjugated polymers for organic photovoltaic applications

Article abstract of DOI:10.1016/j.dyepig.2020.108479

The reasonable selection and optimization of donor-acceptor units allow the band gap tuning as well as the light absorption ability and the energy levels of the photoactive layer of Bulk Heterojunction (BHJ) solar cells. In this work, a series of new quin

Full text of DOI:10.1016/j.dyepig.2020.108479

Dyes and Pigments 180 (2020) 108479
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Narrow band gap benzodithiophene and quinoxaline bearing conjugated
polymers for organic photovoltaic applications
Meric Caliskana, Mert Can Erera, Sultan Taskaya Aslana, Yasemin Arslan Udumb,
,
c
,
d
,
,c,d,f,*
Levent Topparea
e, Ali Cirpana
a Department of Chemistry, Middle East Technical University, 06800, Ankara, Turkey
b Technical Sciences Vocational High School, Gazi University, 06374, Turkey
c Department of Polymer Science and Technology, Middle East Technical University, 06800, Ankara, Turkey
d The Center for Solar Energy Research and Application (GUNAM), Middle East Technical University, 06800, Ankara, Turkey
e Department of Biotechnology, Middle East Technical University, Ankara, 06800, Turkey
f Department of Micro and Nanotechnology, Middle East Technical University, 06800, Ankara, Turkey
A R T I C L E I N F O
A B S T R A C T
Keywords:
The reasonable selection and optimization of donor-acceptor units allow the band gap tuning as well as the light
absorption ability and the energy levels of the photoactive layer of Bulk Heterojunction (BHJ) solar cells. In this
work, a series of new quinoxaline and selenophene based benzo[1,2-b:4,5-b0] dithiophene (BDT) bearing con-
jugated polymers were synthesized via Pd (0) catalyzed Stille polycondensation reaction. The 2-(2-octyldodecyl)
selenophene ring was attached to 4th and 8th positions of phenyl ring in BDT to obtain donor moiety and three
different quinoxaline moieties as acceptors were introduced into the BDT backbone. After completion of syn-
thesis of desired polymers, their structures were identified with 1H NMR spectroscopy and their number and
weight average molecular weights were examined with gel permeation chromatography (GPC). The effects of
introducing fused structure and incorporation of an electron withdrawing F atom to the quinoxaline part on the
electrochemical, spectroelectrochemical, optical and morphological properties of the synthesized polymers were
examined and correlated to the photovoltaic performance. Both polymers namely P1, P2 and P3 were utilized as
donor segments in active layer. They were combined with PC71BM ([6,6]-phenyl C71-butyric acid methyl ester)
as the acceptor to fabricate bulk heterojunction solar cells. Highest power conversion efficiencies were found as
2.36% for P1, 2.07% for P2 and 2.45% for P3 under AM 1.5 G (100 mW/cm2) conditions.
Quinoxaline
Benzodithiophene
Bulk heterojunction solar cells
Stille polycondensation reaction
1. Introduction
sustainable renewable energy is known as solar energy. All around the
world solar energy is most commonly utilized for the generation of
The world power consumption has raised incrementally day by day
owing to increase in demand of energy, limited availability of reserves of
non-renewable energy, continuous rising human population in devel-
oped countries and global warming [1]. According to recent studies,
excessive release of exhaustible sources mainly fossil fuels cause envi-
ronmental pollution due to high quantity of carbon dioxide (CO2)
emission in the Earth’s atmosphere [2]. As the quality of human life is
strongly affected by environmental factors including climate change and
air pollution, generation of energy from alternative clean sources is the
most important topic in today’s world. The urgency of power production
allows scientist to search new future energy sources; The renewable
energy. Nowadays, the most environmentally friendly, clean and
electricity and the scientific were pointed organic material based pho-
tovoltaics to decrease production cost in a safe way [3,4]. Most specially,
in both industrial and scientific areas
π conjugated polymer based
organic photovoltaics play a crucial role for electricity production.
Organic photovoltaics can be basically defined as diode systems that
absorb light to generate current by collecting charges. As compared to
inorganic counterparts, conjugated organic semiconducting materials
exhibit properties like low cost fabrication, flexibility, high absorption
coefficient in the visible range and solution processability [5]. Also,
delocalized
π electron system of conjugated organic semiconductors
build electrical pathway for charge carriers through the polymer back-
bone [4]. For these reasons, they are preferred for utilizing active layer
* Corresponding author. Department of Chemistry, Middle East Technical University, 06800, Ankara, Turkey.
E-mail address: acirpan@metu.edu.tr (A. Cirpan).
https://doi.org/10.1016/j.dyepig.2020.108479
Received 18 January 2020; Received in revised form 4 March 2020; Accepted 23 April 2020
Available online 30 April 2020
0143-7208/© 2020 Elsevier Ltd. All rights reserved.

M. Caliskan et al.
Dyes and Pigments 180 (2020) 108479
of photovoltaics. Yu, Heeger and co-workers designed Bulk Hetero-
junction (BHJ) devices where the acceptor and donor semiconductors
are blended in a bulk forming morphology in the active layer. BHJ de-
vice overcomes the charge generation and exciton diffusion problems
since matrix of donor-acceptor provides long diffusion length and
effective charge separation and also prevents charge recombination at
the interface [6,7]. In the BHJ photovoltaics, electron rich conjugated
polymers or small molecules act as a donor and fullerene derivatives act
as an acceptor molecule owing to their special characteristics. The
design and synthesis of new low band gap conjugated polymers based on
donor-acceptor approach has been proven to be an effective strategy to
achieve high PCE for organic photovoltaics due to high absorption in
solar spectrum with a narrow band gap. Therefore, researchers try to
couple the appropriate moieties for the synthesis of donor and acceptor
subunits on the main chain of polymers [8]. Since the paper of Hou and
Yang et al. on the synthesis of benzo[1,2-b:4,5-b0]dithiophene (BDT)
with various conjugated structures was published, BDT moiety has
attracted great interest in the photovoltaic studies as the donor unity
[9]. BDT building block has structural symmetry with a rigid fused ar-
omatic system that contains benzene ring with two flanking thiophene.
This fused unit shows excellent electron donating property since large
atmosphere in order to get inert medium. The synthesized products were
chromatographically purified on column with Merck Silica Gel 60.
2.2. Measurements
Structures of synthesized molecules were characterized via Nuclear
Magnetic Resonance Spectroscopy technique (NMR) and both 1H NMR
and 13C NMR shifts were detected using Bruker Spectrospin Avance
DPX-400 Spectrometer in chloroform-d (CDCl3) solutions with respect to
internal reference tetramethyl silane (TMS). Gel permeation chroma-
tography (GPC) was utilized to calculate molecular weight distribution
and polydispersity index (PDI) of the synthesized polymers in a chlo-
roform solution (2.0 mg/mL) with polystyrene standard. Spectroelec-
trochemical studies were accomplished using Varian Cary 5000 UV–Vis
Spectrometer. Electrochemical studies of conjugated polymers were
performed via cyclic voltammetry (CV) at a constant scan rate (100 mV/
s) using Gamry Instrument Reference 600 Potentiostat. Notably, elec-
trochemical experiment set up was consisted of three electrodes where
Ag wire was the reference electrode (RE), a platinum wire was the
counter electrode (CE) and Indium Tin Oxide (ITO) coated glass was the
working electrode (WE). The synthesized polymers were spray coated
onto working electrode by Omni coating gun and then coated ITO was
immersed in acetonitrile (ACN) solution containing 0.1 M tetrabuty-
lammonium hexafluorophosphate (TBAPF6). Additionally, PerkinElmer
Pyris 1 Thermal Gravimetry Analysis and PerkinElmer Differential
Scanning Calorimetry were utilized for the thermal analysis of synthe-
sized three polymers.
planar conjugated backbone provide easy formation of π–π stacking
[10]. Hence charge carrier mobility of BDT containing polymers is ex-
pected to be high in solar applications owing to continuous electron
delocalization on the polymer axis. Also, by attaching several sub-
stituents (alkyl or alkoxy) at the 4 and 8 positions of BDT, some prop-
erties of this donor moiety such as solubility, absorption maxima, steric
hindrance and band gap can be controlled [11,12]. It also demonstrates
substantial PCEs when coupled with various acceptors such as benzo-
thiadiazole (BT), thienopyrroledione (TPD) and quinoxaline [13]. On
the other hand, the stable quinoid structures and electron deficient
characters of quinoxaline unities make them promising candidates for
utilizing as the acceptor part of the conjugated D-A polymers. Strong
electron withdrawing nature comes from the imine bonds of the pyr-
azole ring and this property provides high electron transporting ability
to the quinoxaline derivatives [14,15]. These moieties have been
commonly employed to construct low band gap copolymers with high
solubility since they can be easily functionalized from 2nd and 3rd po-
sitions. Hence the control of solubility becomes easy with this func-
tionalization characteristic of strong acceptor unities. Moreover,
quinoxalines have led high efficiencies when copolymerized with
various electron donor materials, including carbazole and benzodithio-
phene [16]. The first BDT and quinoxaline containing polymer was
synthesized by Hou and coworkers with a very low PCE of 0.23% in
2008 [9]. In this work, in order to explore the relationship between
chemical structure and photovoltaic performance of the solar devices
that contain BDT and quinoxalines moieties, three conjugated polymers
with alkylselenophene based BDT as the donor unit and 3-hexylthio-
phene substituted three different quinoxaline building blocks as the
acceptor units were synthesized via Stille coupling polycondensation
reaction. In order to minimize steric interactions between D and A
2.3. Synthesis
Tributyl(4-hexylthiophen-2-yl)stannane, 4,7-dibromobenzo [c] [1,2,
5]thiadiazole (1), 3,6-dibromo-1,2-benzenediamine (2) and 3,6-dibro-
mo-4-fluorobenzene-1,2-diamine (5) were synthesized according to
the previous literature procedures [14,18–20]. Synthetic route adopted
for the preparation of monomers and polymers were shown in Scheme 1
and Scheme 2 respectively.
2.3.1. Synthesis of 5,8-dibromo-2,3-diphenylquinoxaline (3)
Compound 2 (0.50 g, 1.88 mmol) and benzil (0.43 g, 2.07 mmol)
were put in a 100-mL single neck flask and 25 mL glacial acetic acid
(AcOH) were added carefully. The solution mixture was refluxed for 15
h. The reaction was controlled by thin layer chromatography (TLC).
After cooling to room temperature, the solution was transferred into
cold distilled water and washed several times to get rid of excess AcOH.
The participation was filtered and recrystallized in ethanol to obtain a
1
pure product as pale yellow solid (0.73 g, 87%). H NMR (400 MHz,
CDCl3) δ (ppm): 7.92 (s, 2H), 7.70–7.62 (m, 4H), 7.44–7.33 (m, 6H). 13
C
NMR (100 MHz, CDCl3) δ (ppm): 154.2, 139.4, 138.0, 133.1, 130.2,
129.5, 128.3, 123.7.
2.3.2. Synthesis of 10,13-dibromodibenzo[a,c]phenazine (4)
moieties 3-hexylthiophene as an π bridge was inserted between BDT and
Phenanthrene-9,10-dione (0.30 g, 1.13 mmol) and compound 2
(0.26 g, 1.24 mmol) were placed in a single neck round bottom flask and
dissolved in 25 mL acetic acid and the mixture was stirred for 16 h under
reflux. After the reaction was completed, the solution was poured into
cold distilled water (200 mL). The precipitate was filtered and recrys-
quinoxaline unities. The reason of introduction of selenophene side
chain to the BDT unit is achieving high conductivity and mobility due to
intermolecular Se–Se interactions [17,18].
1
2. Experimental
tallized in ethanol to afford dark yellow solid (0.37 g, 76%). H NMR
(400 MHz, CDCl3) δ (ppm): 9.46 (dd, J ¼ 5.9, 2.0 Hz, 2H), 8.56 (d, J ¼
7.9 Hz, 2H), 8.03 (s, 2H), 7.87–7.75 (m, 4H).
2.1. Materials
All starting materials and reagents used in the synthesis of monomers
and polymers were supplied by Sigma-Aldrich except selenophene
which was purchased from TCI Korea. Tetrahydrofuran and toluene
were dried over Na/benzophenone ketyl prior to use in reactions and
other solvents were utilized without any drying process. Moisture and
air sensitive reactions were carried out in the presence of nitrogen
2.3.3. Synthesis of 5,8-dibromo-6-fluoro-2,3-diphenylquinoxaline (6)
A mixture of compound 5 (0.30 g, 1.06 mmol), benzil (0.24 g, 1.16
mmol) and 20 mL acetic acid was refluxed for 20 h. After cooling, the
solution was poured into ice-water and the product was extracted three
times with chloroform. The organic layer was dried over Na2SO4 and the
solvent was removed under vacuum using rotary evaporator. The
2

M. Caliskan et al.
Dyes and Pigments 180 (2020) 108479
Scheme 1. Synthetic routes for monomers.
3

M. Caliskan et al.
Dyes and Pigments 180 (2020) 108479
Scheme 2. Synthetic routes for polymers.
compound was purified by column chromatography eluting with CHCl3/
to reach room temperature and stirred for 20 h. The product was poured
into NaHCO3-water and extracted with chloroform to get rid of excess
NBS. Organic layer was filtered, dried over Na2SO4 and lastly concen-
trated on rotary evaporator. Recrystallization with cold methanol yiel-
1
hexane (1:1) and a pale-yellow solid was obtained (0.27 g, 56%). H
NMR (400 MHz, CDCl3) δ (ppm): 7.94 (d, J ¼ 8.0 Hz, 1H), 7.68–7.62 (m,
4H), 7.43–7.34 (m, 6H). 13C NMR (100 MHz, CDCl3) δ (ppm): 154.6,
137.8, 137.7, 130.2, 130.1, 129.7, 129.5, 128.3, 123.3, 123.0.
1
ded the crude product as a red-orange solid. (0.39 g, 88%). H NMR
(400 MHz, CDCl3) δ (ppm): 8.03 (s, 2H), 7.70 (dd, J ¼ 7.6, 1.8 Hz, 4H),
7.53 (s, 2H), 7.45–7.36 (m, 6H), 2.63 (t, 4H), 1.72–1.61 (m, 4H),
1.43–1.30 (m, 12H), 0.91 (t, J ¼ 7.0 Hz, 6H). 13C NMR (100 MHz,
CDCl3) δ (ppm): 151.9, 141.3, 138.3, 137.6, 136.8, 130.5, 130.4, 129.1,
128.2, 126.5, 125.6, 114.1, 77.3, 77.0, 76.6, 31.6, 29.8, 29.5, 29.0, 22.6,
14.1.
2.3.4. Synthesis of 5,8-bis(4-hexylthiophen-2-yl)-2,3-diphenylquinoxaline
(7)
Compound 3 (0.50 g, 1.14 mmol), tributyl(4-hexylthiophen-2-yl)
stannane (1.30 g, 2.84 mmol) and 35 mL freshly distilled THF were
added in a 100 mL two neck flask under inert atmosphere. Bubbling
process were performed for 1 h and Pd(PPh3)2Cl2 (0.02 mg, 0.02 mmol)
was added to the reaction mixture quickly. The reaction was allowed to
2.3.6. Synthesis of 10,13-bis(4-hexylthiophen-2-yl)dibenzo[a,c]phenazine
(9)
�
stir at 90 C for 2 days under argon atmosphere. After evaporation of
solvent under vacuum, the product was extracted with chloroform and
water. The organic layer was dried over anhydrous Na2SO4. Removal of
organic solvent by rotary evaporation was performed and crude product
was purified by column chromatography with chloroform/hexane (3:7)
to obtain faint orange solid (0.43 g, 61%). 1H NMR (400 MHz, CDCl3) δ
(ppm): 8.10 (s, 2H), 7.75 (d, J ¼ 10.0 Hz, 6H), 7.38 (d, J ¼ 6.2 Hz, 6H),
7.11 (s, 2H), 2.69 (t, J ¼ 7.7 Hz, 4H), 1.75–1.67 (m, 4H), 1.44–1.30 (m,
12H), 0.91 (t, J ¼ 6.4 Hz, 6H). 13C NMR (100 MHz, CDCl3) δ (ppm):
151.4, 142.8, 138.7, 138.4, 137.3, 131.2, 130.4, 128.9, 128.2, 128.1,
126.9, 123.8, 76.7, 31.7, 30.6, 29.0, 22.6, 14.1.
Compound 4 (0.50 g, 1.14 mmol) and tributyl(4-hexylthiop hen-2-
yl)stannane (1.30 g, 2.85 mmol) were dissolved in 40 mL freshly
distilled THF in a 100 mL two neck flask. Before the addition of Pd
(PPh3)2Cl2 catalyst (16.02 mg, 0.03 mmol), the solution was purged
several times with argon to get inert atmosphere for 1 h. The reaction
was stirred at 90 �C for 48 h. After evaporation of solvent under vacuum,
the product was partitioned between chloroform and water. The organic
layer was dried by anhydrous Na2SO4. Removal of organic solvent by
rotary evaporation was performed and crude product was purified by
column chromatography on silica gel with chloroform/hexane (1:5) to
obtain light red solid (0.43 g, 62%). 1H NMR (400 MHz, CDCl3) δ (ppm):
9.54–9.49 (m, 2H), 8.53 (d, J ¼ 7.2 Hz, 2H), 8.16 (s, 2H), 7.78–7.73 (m,
6H), 7.23 (s, 2H), 2.75 (t, 4H), 1.82–1.73 (m, J ¼ 15.0, 7.6 Hz, 6H),
1.43–1.34 (m, J ¼ 7.3 Hz, 12H), 0.94 (t, J ¼ 6.8 Hz, 6H). 13C NMR (100
MHz, CDCl3) δ (ppm): 142.8, 141.6, 138.6, 138.5, 132.3, 131.5, 130.5,
130.3, 128.0, 128.0, 127.7, 126.9, 123.9, 122.8, 31.8, 30.6, 30.6, 29.1,
2.3.5. Synthesis of 5,8-bis(5-bromo-4hexylthiophen-2-yl)-2,3-diphenylqui-
noxaline (8)
In a single neck round bottom compound 7 (0.35 g, 0.56 mmol) was
dissolved in 25 mL chloroform, to which NBS (0.21 g, 1.20 mmol) was
added portion by portion in dark at 0 �C. Then the solution was allowed
4

M. Caliskan et al.
Dyes and Pigments 180 (2020) 108479
22.6, 14.1.
1H), 7.13 (dd, J ¼ 5.6, 3.6 Hz, 1H), 6.92 (d, J ¼ 2.7 Hz, 1H), 2.83 (d, J ¼
6.5 Hz, 2H), 1.64–1.51 (m, 1H), 1.44–1.16 (m, 32H), 0.90 (t, J ¼ 6.7 Hz,
6H). 13C NMR (100 MHz, CDCl3) δ (ppm): 151.9, 128.9, 128.1, 127.2,
40.8, 36.9, 33.9, 33.1, 31.9, 31.6, 30.8, 29.9, 29.8, 29.6 29.5, 29.3, 29.3,
26.7, 26.6, 22.7, 14.1.
2.3.7. Synthesis of 10,13-bis(5-bromo-4-hexylthiophen-2-yl)dibenzo[a,c]
phenazine (10)
In a single neck round bottom flask compound 9 (0.30 g, 0.50 mmol)
was dissolved in 25 mL chloroform, to which NBS (0.17 g, 0.09 mmol)
was added portion by portion in dark at 0 �C. Then the temperature of
mixture was maintained at room temperature and reaction was stirred
for overnight. After, the product was poured into NaHCO3-water and
extracted with chloroform to get rid of excess NBS. Organic layer was
filtered, dried over Na2SO4 and lastly condensed on rotary evaporator.
Through recrystallization of crude product with methanol a shiny
brown-red solid was afforded (0.27 g, 72%). 1H NMR and 13C NMR re-
sults could not be reported for this molecule since the fused structure
hamper the solubility of monomer in the common solvents.
2.3.11. Synthesis of 4,8-bis(5(2-octyldodecyl)selenophen-2-yl)benzo[1,2-
b:4,5-b’]dithiophene (14)
In a 100 mL two neck round bottom flask, compound 13 (1.68 g,
4.09 mmol) was dissolved with 50 mL degassed THF. Slow addition of
�
2.5 M n-BuLi (2.18 mL, 5.45 mmol) was done at À 78 C under inert
atmosphere. After that, temperature was increased to 50 �C and the
mixture was stirred for 1.5 h. In the next step, the benzo[1,2-b:4,5-b’]
dithiopene-4,8-dione (0.30 g, 1.36 mmol) was added to the solution in
one portion and the solution was stirred for another 1.5 h before being
�
cooled to 25 C. After the completion of cooling process, SnCl2⋅2H2O
2.3.8. Synthesis of 5,8-bis(4-hexylthiophen-2yl)-6-fluoro-2,3-diphenylqui-
noxaline (11)
(2.35 g, 11.30 mmol) was dissolved in 10 mL HCl (10%) and the mixture
was added directly into the reaction medium and mixture was further
stirred for overnight. The mixture was quenched with ice water and
extraction was performed by using diethyl ether as an organic solvent.
The extracted organic phase was washed with brine, dried over Na2SO4
and the evaporation of solvent under reduced pressure was done. Puri-
fication step was achieved by silica gel column chromatography eluting
with hexane. Purified crude product was obtained as a sticky brownish
yellow oil (0.85 g, 61%). 1H NMR (400 MHz, CDCl3) δ 7.66 (d, J ¼ 5.7
Hz, 2H), 7.44 (d, J ¼ 5.6 Hz, 2H), 7.40 (d, J ¼ 3.6 Hz, 2H), 7.03 (d, J ¼
3.6 Hz, 2H), 2.91 (d, J ¼ 6.5 Hz, 4H), 1.73–1.66 (m, 2H), 1.41–1.24 (m,
64H), 0.91–0.86 (m, 12H). 13C NMR (100 MHz, CDCl3) δ 153.8, 142.7,
138.8, 136.2, 130.0, 127.5, 127.3, 126.2, 123.5, 40.8, 37.4, 33.3, 31.9,
30.0, 29.7, 29.6, 29.4, 26.6, 22.7, 14.1.
Compound 6 (0.26 g, 0.57 mmol), tributyl(4-hexylt hiophen-2-yl)
stannane (0.65 g, 1.42 mmol) were put into a 100 mL two neck round
bottom flask and then they were dissolved in degassed THF. After the
addition of catalyst Pd(PPh3)2Cl2 (8.00 mg, 0.01 mmol), the solution
was bubbled with nitrogen gas and heated to 90 �C under inert atmo-
sphere for 46 h. The resulting reaction was extracted with DCM and
water. The combined organic phases were filtered and dried over
Na2SO4. Later, solvent removal process was performed under reduced
pressure, the product was purified by silica based column chromatog-
raphy eluting with hexane/CH2Cl2 (5:1) to afford orange crystals (0.28
g, 78%). 1H NMR (400 MHz, CDCl3) δ (ppm): 7.95 (d, J ¼ 13.4 Hz, 1H),
7.86 (s, 1H), 7.81–7.71 (m, 5H), 7.53–7.31 (m, 7H), 7.17 (d, 1H), 2.69
(dd, J ¼ 15.9, 8.3 Hz, 4H), 1.76–1.68 (m, 4H), 1.45–1.31 (m, 12H), 0.91
(t, J ¼ 6.9 Hz, 6H). 13C NMR (100 MHz, CDCl3) δ (ppm): 140.6, 140.3,
136., 136.17, 128.3, 128.1, 126.8, 126.7, 126.3, 125.9, 122.6, 121.9,
29.5, 28.3, 28.2, 26.9, 26.8, 25.9, 24.5, 20.3, 15.1, 11.8, 11.3.
2.3.12. Synthesis of 4,8-bis(5(2-octyldodecyl)selenophen-2-yl)benzo[1,2-
b:4,5-b’]dithiophene-2,6-diyl)bis (trimethylstannane) (15)
In a dry 100 mL round bottom flask, compound 14 (0.80 g, 0.79
mmol) was dissolved in degassed THF (30 mL) under inert atmosphere.
�
2.3.9. Synthesis of 5,8-bis(5-bromo-4-hexylthiophen-2yl)-6-fluoro-2,3-
diphenyquinoxaline (12)
The temperature of medium was maintained at À 78 C using dry ice-
acetone, and 2.5 M n-butyllithium (0.9 mL, 2.38 mmol) was added
dropwise to the reaction intermediate under same conditions. The so-
lution was heated up to room temperature while stirring for 20 min.
Before the green color disappears, trimethyltin chloride (2.54 mL, 2.54
mmol) was added in one portion and the mixture was turned into pale
yellow color. After that, the solution was quenched with ice water and it
was partitioned between diethyl ether and brine. The combined organic
phase was separated, dried (Na2SO4) and the solvent was removed under
reduced pressure. Isolated donor compound was obtained as viscous
yellowish-brown liquid (0.96 g, 90%). 1H NMR (400 MHz, CDCl3) δ 7.74
(s, J ¼ 14.8 Hz, 2H), 7.46 (d, J ¼ 3.6 Hz, 2H), 7.07 (d, J ¼ 3.6 Hz, 2H),
2.95 (d, J ¼ 6.5 Hz, 4H), 1.82–1.69 (m, 2H), 1.35–1.28 (m, 64H),
0.85–0.80 (m, 12H), 0.50–0.37 (m, 18H)⋅13C NMR (100 MHz, CDCl3) δ
153.5, 143.6, 142.1, 138.9, 137.0, 131.3, 129.8, 127.5, 124.6, 40.7,
37.4, 33.4, 31.9, 30.0, 29.7, 29.4, 29.3, 29.1, 27.4, 27.2, 26.7, 22.7,
14.1, 13.7, 9.4, 8.7.
Compound 11 (0.25 g, 0.4 mmol) was dissolved in 15 mL of CHCl3.
N-bromosuccinimide (NBS) (0.15 g, 0.8 mmol) was added slowly to the
reaction intermediate in dark at 0 �C. After that, the mixture was stirred
at room temperature for 24 h. The product was extracted four times with
chloroform: water. The organic phases were filtered and dried over
Na2SO4 and condensed by using rotary evaporator under reduced
pressure. The product was chromatographically purified on column
eluting with ethyl acetate/hexane (1:30). This procedure gave a product
as a shiny orange solid (0.24 g, 76%). 1H NMR (400 MHz, CDCl3) δ
(ppm): 7.74 (d, J ¼ 13.8 Hz, 1H), 7.67–7.53 (m, 5H), 7.45–7.24 (m, 7H),
2.63–2.48 (m, 4H), 1.63–1.54 (m, 4H), 1.39–1.21 (m, 12H), 0.83 (t, J ¼
6.6 Hz, 6H). 13C NMR (100 MHz, CDCl3) δ (ppm): 152.4, 151.1, 141.3,
137.9, 136.3, 136.0, 130.5, 130.4, 129.3, 128.2, 116.2, 115.7, 115.6,
115.1, 77.3, 77.0, 76.7, 31.6, 29.8, 29.6, 29.0, 22.6, 14.1.
2.3.10. Synthesis of 2-(2-octyldodecyl)selenophene (13)
Selenophene (2.50 g, 15.26 mmol) was dissolved in dry THF (60 mL)
under inert atmosphere. The solution was cooled down to À 78 �C before
the addition of 2.5 M (in hexane) n-butyllithium (6.11 mL, 15.26 mmol).
After the dropwise addition of n-BuLi was completed, the solution was
stirred for 1.5 h. The alkyl chain 9-(bromomethyl) nonadecane (5.52 g,
15.26 mmol) was added dropwise slowly under same conditions. Then,
temperature was raised to 80 �C and stirred for 15 h. The solution was
hydrolyzed with saturated NH4Cl and extraction was performed with
diethyl ether. The organic phase was combined, filtered and dried over
anhydrous Na2SO4. The solvent removal was done via rotary evaporator
and the residue was purified by column chromatography technique
eluting with hexane. Isolated product was obtained as a clear yellow oil
(1.82 g, 57%). 1H NMR (400 MHz, CDCl3) δ (ppm): 7.81 (d, J ¼ 4.6 Hz,
2.3.13. Synthesis of P1
In a two neck 25 mL flask, compound 15 (0.17 g, 0.13 mmol) and
compound 8 (0.10 g, 0.13 mmol) were charged and they were dissolved
in freshly distilled toluene 10 mL. The reaction mixture was deaerated
under vacuum and purged with nitrogen gas. The bubbling process was
performed for 1 h to the reaction medium prior to addition of two
different
catalysts
tris(dibenzylideneacetone)dipalladium
(0)
(Pd2(dba)3) (0.006 g, 0.007 mmol) and tris(o-tolyl)phosphine (P(o-tol)3)
(0.01 g, 0.045 mmol). After the addition of catalyst completed, the re-
action mixture was heated up to 120 �C and stirred for 46 h under inert
atmosphere. Then, 2-bromothiophene (0.04 mg, 0.26 mmol) and 2-trib-
utylstanylthiophene (0.10 g, 0.26 mmol) were added to the reaction to
end-cap the polymer chain. After cooling to room temperature (25 �C),
5

M. Caliskan et al.
Dyes and Pigments 180 (2020) 108479
the resulting solution was poured into cold methanol. The precipitate
was subjected to Soxhlet extraction with methanol, acetone, hexane, and
chloroform respectively. The small amount of quadrasil (10 mg) was
added to the chloroform extract as a scavenger and the extract was
purified by the silica gel column. Lastly, CHCl3 fraction was condensed
by evaporation, precipitated into 250 mL methanol and filtered by
vacuum to afford the desired polymer as dark red solid (0.12 g, 56%).
GPC analysis: The number-average molecular weight (Mn) of P1 is
found as 12 kDa and the weight average molecular weight (Mw) is
detected as 16 kDa with 1.3 of polydispersity index (PDI).
sequentially with toluene, detergent (Hellmanex), distilled water and
isopropyl alcohol. Each solvent was sonicated for 15 min in ultrasonic
bath. Prior to oxygen plasma treatment all substrates were dried with N2
gun. Plasma treatment was carried out by utilizing Harrick Plasma
Cleaner for 5 min to the substrates. Indeed, this step was necessary for
the enhancement of work function of ITO via removing organic impu-
rities and for reducing the surface tension of ITO. Then, the glass sub-
strates were covered by PEDOT:PSS via spin coating technique at
different rpm values. Thereafter, to remove water residues on the coated
substrates, annealing was done at 140 �C for 15 min on the hot plate.
Active layer processing was performed by blending Polymer:PC71BM
(purchased from Solenne) with different weight ratios and different
blend concentrations. Also, active layer was treated with different sol-
vent additive concentrations. Then, after preparation of photoactive
layer, they were spin coated onto hole transport layer inside the N2 filled
glove box. Lastly, the device was completed by thermal evaporation of
LiF and Al metal in a vacuum evaporation chamber. Current density (J)-
voltage (V) characteristics of polymers under dark and illumination (AM
1.5G, 100 mW/cm2) were investigated by using Keithley 2400 source-
meter inside the nitrogen filled glove box.
2.3.14. Synthesis of P2
Compound 15 (0.16 g, 0.12 mmol) and compound 10 (0.09 g, 0.12
mmol) were put in a two neck 25 mL flask and compounds were dis-
solved in dry toluene (10 mL). The reaction mixture was deaerated
under vacuum, backfilled with nitrogen gas and bubbled for 1 h before
the addition of tris(dibenzylideneacetone)dipalladium (0) (Pd2(dba)3)
(0.005 g, 0.006 mmol) and tris(o-tolyl)phosphine (P(o-tol)3) (0.01 g,
0.045 mmol). After the addition of the two catalysts, the reaction tem-
perature was set as 120 �C and the mixture was stirred for 50 h under
inert atmosphere. Subsequently, two different end-cappers 2-bromothio-
phene (0.04 mg, 0.26 mmol) and 2-tributylstanylthiophene (0.10 g,
0.26 mmol) were added to the reaction medium respectively. After
cooling to room temperature (25 �C), the resulting polymer was poured
into cold methanol and precipitated. Soxhlet extraction was done for
purification of precipitate eluting with methanol, acetone, hexane, and
chloroform. Then, the quadrasil (10 mg) was added to the chloroform
solution and the mixture was stirred for 2 h. After stirring process, the
product was purified through silica gel. The chloroform fraction was
condensed by evaporation, precipitated into 250 mL methanol and
filtered by vacuum in order to obtain the corresponding polymer as dark
green solid (0.10 g, 51%). GPC analysis: The number-average molecular
weight (Mn) of P2 is found as 7 kDa and the weight average molecular
weight (Mw) is detected as 9 kDa with 1.3 of polydispersity index (PDI).
3. Results and discussions
3.1. Optical studies
Normalized UV–Vis absorption spectra of synthesized polymers in
dilute CHCl3 solutions and as thin-films spin-coated from CHCl3 are
shown in Fig. 1. These polymers which were designed based on the
intrachain D-A polymer theme, were expected to demonstrate the strong
and broad absorption in the visible and near IR region. Hence, all
compounds exhibited two distinct absorption bands in both long
wavelength zone and short wavelength zone. The absorption in the
shorter wavelength or high energy band is attributed as the localized
π-π* transition, whereas absorption at longer wavelength in other words
lower energy band is assigned as the intra molecular charge transfer
among donor and accepter moieties [21]. Absorption at shorter wave-
lengths can be assigned as shoulders and the thin film peaks at 450 nm
and 445 nm are the shoulders of P1 and P3 respectively. On the other
hand, the green polymer namely P2 demonstrated two peaks both in the
red region at 460 nm and blue region at 660 nm in thin film absorption
spectrum of P2. Maximum wavelength (λmax) values of all polymers both
in solution and thin film are summarized in Table 1. In the thin film of
corresponding polymers, absorption becomes broader and both of them
are red shifted with three peaks at 600, 715 and 610 nm respectively.
Particularly, red shifts and obvious broadening peaks in the thin film
absorption spectra of the polymers signalizes that polymer chains are
highly aggregated in their solid state. The P2 possesses larger red shift
compared to P1 and P3, the reason of this situation can be explained
from the phenanthrene unit in the P2, which induces fused planarity by
cyclization of two phenyl rings on the quinoxaline structure with an
enhancement of intermolecular electronic interactions in the solid state
[22]. Moreover, it is expected that P2 should demonstrate effective
conjugation length with a narrower band gap owing to highest planarity.
2.3.15. Synthesis of P3
Compound 15 (0.18 g, 0.14 mmol) and compound 12 (0.11 g, 0.14
mmol) were charged in a two neck 25 mL flask and compounds were
dissolved in degassed toluene (10 mL). The reaction mixture was dea-
erated under vacuum, backfilled with nitrogen gas and bubbled for 1 h
prior to addition of two different catalysts tris (dibenzylideneacetone)
dipalladium(0) (Pd2(dba)3) (0.006 g, 0.007 mmol) and tris(o-tolyl)
phosphine (P(o -tol)3) (0.02 g, 0.05 mmol). After the addition of catalyst
�
was completed, the reaction mixture warmed to 120 C in an oil bath
and stirred for 2 days under inert atmosphere. Two different end cappers
namely 2-bromothiophene (0.04 mg, 0.26 mmol) and 2-tributylstanylth-
iophene (0.1 g, 0.26 mmol) were added to the reaction medium. The
resulting mixture was precipitated into cold methanol. The precipitate
was subjected to Soxhlet extraction with methanol, acetone, hexane, and
chloroform respectively. After the addition of little amount of quadrasil,
the chloroform fraction was purified by passing through silica gel and
then it was condensed by evaporation, precipitated into 300 mL meth-
anol and filtered by vacuum to afford the corresponding polymer as red
solid (0.14 g, 63%). GPC analysis: The number-average molecular
weight (Mn) of P3 is found as 10 kDa and the weight average molecular
weight (Mw) is detected as 15 kDa with 1.5 of polydispersity index
(PDI).
3.2. Electrochemical studies
Cyclic voltammetry was conducted for the electrochemical charac-
terization of the synthesized three polymers. CV measurements were
carried out in order to get information on the redox behaviors of the P1,
P2 and P3 at a constant 100 mV/s scan rate. Onset oxidation, onset
reduction and doping-dedoping potentials of polymers were investi-
2.4. Device fabrication
In order to evaluate photovoltaic performances of synthesized
polymers, traditional ITO/PEDOT:PSS/Polymer: PC71BM/LiF/Al device
architecture was constructed. Typical BHJ devices were fabricated as
follows; ITO etching-cleaning, PEDOT:PSS coating, processing active
layer and metal evaporation. ITO glass substrates were etched by zinc
powder and hydrochloric acid and then ultrasonically cleaned
gated from the cyclic voltammograms which were depicted in Fig. 2.
onset
Briefly, the E
of P1, P2 and P3 were detected as 0.60, 0.57, 0.63
of polymers were found as À 1.20, À 1.13 and À 1.10
ox
onset
while the E
red
respectively. According to voltammograms, all three polymers have p-
doping and n-doping properties in other words both polymers possess
6

M. Caliskan et al.
Dyes and Pigments 180 (2020) 108479
Fig. 1. Normalized UV–Vis absorption spectra of synthesized polymers a) P1, b) P2, c) P3 in CHCl3 solutions (in red) and thin-films spin-coated from CHCl3 (in
black). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
HOMO levels for P1, P2 and P3 were evaluated as À 5.35 eV, À 5.32
Table 1
eV, À 5.38 eV whereas LUMO energy levels were found as À 3.55 eV,
Maximum wavelength (λmax) values of all polymers both in solution and thin
À 3.62 eV, À 3.65 eV, correspondingly. The electronic band gaps of P1
film.
and P3 were estimated to be 1.80 eV and 1.73 eV respectively, while the
Polymer
Solution λmax (nm)
Thin film λmax (nm)
lowest electronic band gap was found as 1.70 eV for P2.
P1
P2
P3
542
637
542
600
715
610
P3 had low lying HOMO and LUMO energy levels than those of P1
and P2 owing to introduction of strong electron withdrawing fluorine
atom on the polymer backbone. Substitution of F atom attained deeper
levels for HOMO/LUMO, at the same time lowered the electronic band
gap by diminishing electron density of the polymer backbone and by
minimizing unexpected steric interactions [25]. However, the narrower
electronic band gap was observed for the P2 which can be explained by
the planar and rigid structure of phenanthrene unit with fused BDT unit
and easy generation of quinoid structure [26]. Moreover, low resonance
stabilization and enhanced planarity eases electron delocalization on
conjugation main chain via lowering band gap. In comparison to non-
fluorinated P1, fluorinated P3 exhibited a lower band gap which may
result from the presence of F atom since it causes planarization of the
conjugated chain through intramolecular and intermolecular in-
teractions [27]. Representative electrochemical results of polymers as
well as the energy level parameters of them derived from the cyclic
voltammograms were given in Table 2.
ambipolar character. HOMO and LUMO energy levels as well as the
electronic band gap of the polymers in the thin films were calculated
from the corresponding onset oxidation and onset reduction potentials
recorded from the CV.
The curves obtained from CV were internally calibrated by utilizing
standard reference of ferrocene/ferrocenium redox couple (Fc/Fcþ)
with 0.05 eV half wave reduction potential relative to the reference
electrode (Ag wire) potential [23]. The absolute energy level of ferro-
cene was assumed as 4.80 eV below the vacuum level and after cali-
bration of reference electrode, it was assigned as 4.75 eV. Furthermore,
following equations were used to calculate HOMO-LUMO energy values
þ
of polymers where the E1/2
,
Fc,Fc was the half wave reduction potential of
ferrocene/ferrocenium redox couple, the Eox onset was the onset oxida-
onset
tion potential and E
was the onset reduction potential and also the
red
energy difference between calculated HOMO and LUMO gave electronic
band gap (Eegl) of all polymers recorded from the cyclic voltammograms
relative to the silver reference electrode [24].
3.3. Spectroelectrochemical studies
The synthesized polymers were spectrochemically characterized
using a three electrode system. An external potential was applied to the
polymers films stepwise and spectra were obtained in the UV–Vis–NIR
regions. Experimental procedure was initiated with the solvation pro-
cess of polymers in CHCl3, after that they were deposited onto the ITO
þ
onset
onset
HOMO ¼ - (4.80 - E1/2
,
þ E
) ¼ - (4.75 þ E
) eV
) eV
(1)
(2)
(3)
Fc,Fc
ox
ox
þ
onset
onset
LUMO ¼ - (4.80 - E1/2
Eegl ¼ HOMO-LUMO eV
,
Fc,Fc þ E
) ¼ - (4.75 þ E
red
red
coated glass slides at room
T
via spin coating. Then,
7

M. Caliskan et al.
Dyes and Pigments 180 (2020) 108479
Fig. 2. Cyclic voltammograms of polymers a) P1 b) P2 c) P3 in the thin films at a scan rate 100 mV/s and in 0.1 M supporting electrolyte/solvent couple
(TBAPF6/ACN).
Table 2
Representative electrochemical results of polymers derived from the cyclic voltammograms.
Egel (eV)
onset
onset
red
Ep-doping (V)
Ep-dedoping (V)
En-doping (V)
En-dedoping (V)
E
(V)
E
(V)
HOMO (eV)
LUMO (eV)
ox
P1
P2
P3
1.10
0.85
1.18
0.99
0.69
1.03
À 1.34/-2.00
À 1.26/-1.74
À 1.29/-1.85
À 1.25/-1.50
À 1.15/-1.37
À 1.28/-1.50
0.60
0.57
0.63
À 1.20
À 1.13
À 1.10
À 5.35
À 5.32
À 5.38
À 3.55
À 3.62
À 3.65
1.80
1.70
1.73
spectroelectrochemical results gave the optical absorption (a.u.) as a
function of wavelength (nm). As shown in Fig. 3, responses were
recorded in the absorption spectra upon oxidative doping potentials, all
polymers demonstrated two distinct absorption peaks in the UV–Vis
region and they possess absorption in the NIR region. With the applied
stepwise oxidation, as it was expected, the intensity of detected ab-
sorption bands of polymers began to reduce and absorption in the NIR
region was observed owing to the formation of polarons [20]. This
depletion and increment of the intensity of peaks can be explained with
the concentration changes of the polymers where neutral state concen-
tration of polymer decreases and polaron concentration increases upon
3.4. Kinetic studies
Kinetic features of polymers were achieved utilizing UV–Vis–NIR
spectrophotometer. P1, P2 and P3 coated ITO thin films were subjected
to square wave potential with 5 s time intervals in 0.1 M TBAPF6/ACN
and both optical contrasts and switching times of polymers were
determined. Notably, optical contrast shows the difference between
percent transmittance (ΔT%) of two extreme states of polymers which
are reduced and oxidized states at a 95% contrast since the human eye is
sensitive up to 95% of the full contrast [28]. Transmittance changes at
dominant wavelengths were depicted in Fig. 4. Values of optical contrast
(ΔT%) and switching times (s) at these wavelengths for P1, P2 and P3
were shown in Table 4. From the absorption versus time spectra of ki-
netic studies, it can be deduced that optical contrasts of three polymers
in the visible region were lower than the optical contrasts in the NIR
region. For the longer wavelength absorption band of P1, P2 and P3, the
higher optical contrasts were recorded as 34%, 28% and 47% with
smaller 1.0 s, 1.0 s and 0.9 s switching times respectively. On the other
hand, the optical contrast at shorter wavelength were found for P1; 13%,
23% for P2; 22%, 18% and for P3; 18%, 25% with longer switching
times.
applied potential. By taking tangent line of the π-π* interband transition
where the maximum absorption process was started for the polymers in
other words from the onset point, optical band gaps (Eogp) were found as
1.67 eV, 1.45 eV and 1.64 eV for P1, P2 and lastly P3 respectively.
onset
Table 3 represents the values of Eogp (eV) and λ
(nm) for each
max
polymer.
It is worth noting that the electronic gap mostly is larger than the
optical gap. The reason behind this difference can be explained by the
presence of free electrons on the chain during cyclic voltammetry
measurements. Therefore, synthesized polymers namely P1, P2 and P3
exhibited higher Eegl than Egop in this study. As to the Eogp values, P2 had
the lowest optical band gap. This phenomenon like in the case of elec-
3.5. Photovoltaic studies
tronic band gap, can be clarified by the degree of interchain π- π stacking
or/and planar rigid polymer chain of P2.
BHJ photovoltaic solar devices based on three polymers with an
8

M. Caliskan et al.
Dyes and Pigments 180 (2020) 108479
Fig. 3. The change in the electronic absorption spectra of corresponding polymers a) P1, b) P2 and c) P3 upon oxidative doping potentials in supporting electrolyte/
solvent couple (0.1 M TBAPF6/ACN).
light absorption ability of and result in thicker active layer respectively
Table 3
and these optimizations provides better photovoltaic performances [29,
The recorded values of Egop and λ
for each polymer.
(nm)
onset
max
30]. However, compare to 3% blend concentration of P1:PC71BM (1:3)
at 500 rpm, 2% blend concentration of P1:PC71BM (1:3) at 350 rpm
exhibited higher PCE. This unexpected situation could be clarified by a
rise in the FF values from 35% to 41%. For P2 containing solar devices,
blend ratio was changed from 1:2 to 1:3 and this optimization reduced
the efficiency of devices from 2.07% to 1.38%. In general, decreasing
concentration of polymers via increasing amount of PCBM can cause
reduction of JSC value but in this case JSC value did not change signifi-
cantly. However, the FF of devices were reduced from 55% to 37% in
chlorobenzene solution. This huge decrease of FF can be explained by
the active layer morphology and higher PCE device were achieved
owing to good morphology in active layer with an enhancement of FF for
the P2 based devices with 1:3 blend ratio and 2% blend concentration.
Fig. 6 represents J-V curve of PSCs based on P2:PC71BM with different
D/A ratios. Lastly, P3:PC71BM ratio optimizations proved that, ratio of
1:4 was found to be the optimum performance for P3:PC71BM contain-
ing solar cells. Although decreased polymer concentration should
possess lower JSC since the JSC is primarily determined by absorption
ability of the BHJ blend layer, P3:PC71BM with lowest polymer con-
centration (1:4 ratio) was achieved highest JSC values as compared to 1:2
and 1:3 ratios [31]. Blend concentration optimizations proved that 3%
weight/volume (w/v) blend concentration of P3: PC71BM solution in
o-DCB yielded a more efficient device than that of 2% weight/volume
(w/v) blend concentration. The reason behind this enhancement of the
PCE could be explained by the increase in current density values up to
7.59 mA/cm2 for the P3 based solar device. As the current density in-
creases harvesting incoming light becomes easier at the same time ef-
ficiency increases [32]. Fig. 7 represents J-V curve of PSCs based on P3:
PC71BM with different D/A ratios.
onset
Polymer
λ
Egop (eV)
max
P1
P2
P3
745
858
760
1.67
1.45
1.64
architecture of ITO/PEDOT:PSS/Polymer:PC71BM/LiF/Al were fabri-
cated and photovoltaic parameters of these devices were evaluated
under illumination (AM 1.5G, 100 mW/cm2). Herein, in order to get
most efficient solar cells, various optimizations were done such as blend
ratio of polymer and PC71BM (w:w) and blend concentration optimiza-
tions. In addition to BHJ blend ratio and blend concentration optimi-
zation, thickness optimizations were also carried out by altering spin
coating rate. Notably solvent additives had no positive effect on the
device photovoltaic performance in this study. For clear comparison of
device performance of synthesized three polymers namely P1, P2 and
P3, their photovoltaic data were shown in Table 5, Table 6 and Table 7
respectively.
Among all three polymers, fluorinated P3 with 1:4, w/w blend ratio
P3:PC71BM revealed the highest performance with a PCE of 2.45%, a JSC
of 7.59 mA/cm2, a VOC 0.77 V and a FF of 41%. Whereas, P1:PC71BM
(1:3, w/w) exhibited the best PCE as 2.36% and the P2:PC71BM (1:2, w/
w) exhibited the best PCE as 2.07%.
Fig. 5 illustrates the J-V curves of P1 based solar cells with different
blend ratios and different spin coating rates. When the blend ratio
changed from 1:2 to 1:3 short circuit current values were enhanced. The
best performing P1 based device was detected as 1:3 blend ratio with 2%
blend concentration (20 mg/mL Polymer: PC71BM solution) in o-DCB at
350 rpm. Generally, the increase in the polymer concentration inside the
active layer and decrease in the spin coating rate cause enhancement of
In order to confirm the accuracy of the characterized photovoltaic
9

M. Caliskan et al.
Dyes and Pigments 180 (2020) 108479
Fig. 4. Transmittance changes at dominant wavelengths for a) P1 b) P2 c) P3.
Table 4
Table 6
The values of optical contrasts (ΔT%) and switching times (s) at dominant
Photovoltaic parameters of P2 containing OPV cells.
wavelengths for polymers.
Polymer
P2:
JSC
VOC
(V)
JMAX
(mA/
cm2)
3.96
3.62
VMAX(V)
FF
%
PCE
%
Rpm
PC71BM
(mA/
cm2)
5.27
5.30
Wavelength (nm)
Optical contrast
Switching time (s)
(w:w)
1:2 (2%)
1:3 (2%)
ΔT%
0.71
0.52
55
2.07
500
P1
450
600
840
460
660
875
445
610
845
13
23
34
22
18
28
18
25
47
1.8
1.6
1.0
1.9
1.8
1.0
1.6
1.0
0.9
0.70
0.38
37
1.38
500
P2
P3
Table 7
Photovoltaic parameters of P3 containing OPV cells.
P3:PC71BM
(w:w)
JSC
VOC
(V)
JMAX
(mA/
cm2)
VMAX
(V)
FF
%
PCE
%
Rpm
(mA/
cm2)
1:1 (2%)
1:2 (2%)
1:3 (2%)
1:4 (2%)
1:4 (2.5%)
1:4 (3%)
1:4 (3%)
1:4 (3%)
2.50
3.95
3.85
5.14
7.13
7.63
7.59
5.90
0.75
0.75
0.75
0.76
0.74
0.75
0.77
0.74
1.73
2.44
2.35
3.22
3.38
4.29
4.36
3.51
0.53
0.57
0.61
0.51
0.54
0.47
0.56
0.42
49
46
49
42
34
35
41
33
0.92
1.38
1.43
1.65
1.83
2.02
2.45
1.48
500
500
500
500
500
500
350
750
Table 5
Photovoltaic parameters of P1 containing OPV cells.
P1:PC71BM
(w:w)
JSC
VOC
(V)
JMAX
(mA/
cm2)
VMAX(V)
FF
%
PCE
%
Rpm
(mA/
cm2)
1:2 (2%)
1:3 (2%)
1:4 (2%)
1:3 (2%)
1:3 (3%)
1:3 (2%)
1:3 (2%)
5.76
7.32
7.10
3.07
8.19
7.29
6.94
0.77
0.77
0.77
0.58
0.77
0.79
0.79
3.89
4.96
4.19
1.57
4.96
4.52
3.94
0.49
0.44
0.47
0.35
0.44
0.52
0.48
43
39
39
30
35
41
35
1.92
2.21
1.97
0.55
2.21
2.36
1.92
500
500
500
500
500
350
750
were possessed photo response at the range of 400–700 nm, which was
in agreement with red shifted spectrum of polymers that was obtained
from the optical studies of the thin films. Also, maximum EQE values
were detected as 62%, 35% and 48% for P1, P2 and P3, correspond-
ingly. After the integration of EQE values, JSC of the devices were
deduced as 6.95 mA/cm2 for P1, 6.95 mA/cm2 for P2 and 7.01 mA/cm2
for P3 and these current densities are reasonably close to the JSC of cells
obtained from the J-V curves.
cells the external quantum efficiencies of the devices were evaluated
[33]. For the EQE analysis, devices were constructed under same con-
ditions as those utilized for the photovoltaic measurement to achieve
representative J-V curves with AM 1.5 G spectrum. The corresponding
EQE curves of the polymers were depicted in Fig. 8 Clearly all devices
10

M. Caliskan et al.
Dyes and Pigments 180 (2020) 108479
Fig. 5. J-V curve of PSCs based on P1:PC71BM a) with different D/A ratios b) with different spin coating rates.
Fig. 6. J-V curve of PSCs based on P2:PC71BM with different D/A ratios.
Fig. 8. EQE curves of P1, P2 and P3 based best performing solar cells.
The surface roughness values measured from these images were 0.28
nm, 3.29 nm and 1.03 nm and active layer thicknesses were detected as
70 nm, 80 nm and 84 nm for P1, P2 and P3 correspondingly.
4. Conclusions
Three new conjugated polymers with electron accepting segments of
different quinoxalines and electron donating segments of BDT were
successfully designed and synthesized via Pd(0) catalyzed Stille cross
coupling reaction. From the cyclic voltammograms, it was observed that
all polymers have ambipolar characters and highest oxidation potential
was calculated for P3 owing to introduction of fluorine atom onto the
quinoxaline part. Corresponding LUMO levels were determined to be
À 3.55 eV, À 3.62 eV, À 3.65 eV while the HOMO levels were estimated as
À 5.35 eV, À 5.32 eV, À 5.38 eV for P1, P2 and P3, respectively.
Benefiting from the strong electronegativity of fluorine atom, the P3 has
deeper HOMO-LUMO levels and it possesses little enhancement on the
JSC value on the photovoltaic measurements in comparison with P1 and
P2. With respect to spectroelectrochemical results in thin film form, all
P1, P2 and P3 had broad absorption spectra which were located ab-
sorption edges at 745, 858 and 760 nm corresponding to optical band
gaps of 1.67, 1.45 and 1.64 eV respectively. For further characterization,
kinetic features of resulted D-A polymers were tested and it was found
that all polymers demonstrated the highest optical contrasts for P1 with
34%, for P2 with 28% and for P3 with 47% with the small switching
times 1.0 s, 1.0 s and 0.9 s respectively in the NIR region. The maximum
optical contrast and broadened absorption of the polymers at the longer
wavelengths eventuate the improvement of the sunlight harvesting for
photovoltaic characterizations. More pronounced red shifted absorption
spectra of P2 in these three polymers signalizing that P2 has the highest
aggregation in the solid state and the highest roughness value as also
Fig. 7. J-V curve of PSCs based on P3:PC71BM with different D/A ratios.
3.6. Morphology
The surface morphologies of photoactive layers were investigated
with Atomic Force Microscopy (AFM) to gain deeper information about
devices. The resulted AFM images of the best performing films of syn-
thesized three polymers are provided in Fig. 9. The AFM images of
photoactive films indicated that there was no large aggregation for P1:
PC71BM and P3:PC71BM films and these good film morphologies could
be the result of high JSC of these polymers [34]. Since the thin film of P2
which showed more pronounced red-shifted absorption spectra
compared to P1 and P3, the P2 containing devices were presumably
revealed an aggregation in the solid state of photoactive layer.
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Dyes and Pigments 180 (2020) 108479
Fig. 9. The surface morphologies of photoactive layers obtained from AFM measurements; a) P1:PC71BM (1:3, w:w), b) P2:PC71BM (1:2, w:w) c) P3:PC71BM (1:4, w:
w). Scale bar represents 200 nm.
[8] Li S, et al. Synthesis, characterization and photovoltaic performances of D-A
observed in the AFM image. Electrochemical, spectroelectrochemical
copolymers based on BDT and DBPz: the largely improved performance caused by
and optical experiments revealed that corresponding polymers are
additional thiophene blocks. J Mater Chem A 2013;1(14):4508–15.
applicable for the solar applications owing to proper HOMO-LUMO
energy levels, broad absorption spectra in the visible region and most
importantly their low band gaps. Moreover, the tunable band gap of
organic materials from 1.0 to 2.0 eV is an important parameter for the
photovoltaic studies in order to harvest as much as solar spectrum [35].
Photovoltaic characterization of the blends of three conjugated poly-
mers with a fullerene derivative as an acceptor reveal that the best
performing device was achieved 2.45% PCE in the P3 bearing device
owing to strong electron withdrawing fluorine atom substitution and
enhanced morphology as compared to nonfluorinated counterparts.
Notably, these results indicate that resulting polymers are promising
materials for BHJ photovoltaic applications.
[9] Hou J, et al. “Bandgap and molecular energy level control of conjugated polymer
photovoltaic materials based on benzo[l,2-b:4,5-b0]dithiophene. Macromolecules
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with deep low-lying HOMO levels and high mobilities. J Mater Chem A 2014;2
(33):13580–6.
[12] Zhang D, Wang M, Liu X, Zhao J. Synthesis and characterization of donor-acceptor
type conducting polymers containing benzotriazole acceptor and
benzodithiophene donor or: S -indacenodithiophene donor. RSC Adv 2016;6(96):
94014–23.
[13] Chen HC, Chen YH, Liu CC, Chien YC, Chou SW, Chou PT. Prominent short-circuit
currents of fluorinated quinoxaline-based copolymer solar cells with a power
conversion efficiency of 8.0%. Chem Mater 2012;24(24):4766–72.
[14] Sun Y, et al. Side chain engineering and conjugation enhancement of
benzodithiophene and phenanthrenequnioxaline based conjugated polymers for
photovoltaic devices. J Polym Sci Part A Polym Chem 2015;53(16):1915–26.
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polymers and their cyclized derivatives for bulk-heterojunction polymer solar cell
applications. Macromolecules 2011;44(4):894–901.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
[16] Xu X, et al. Side-chain engineering of benzodithiophene-fluorinated quinoxaline
low-band-gap Co-polymers for high-performance polymer solar cells. Chem - A Eur
J 2014;20(41):13259–71.
CRediT authorship contribution statement
[17] Chakravarthi N, Kranthiraja K, Song M. “New alkylselenyl substituted
benzodithiophene-based solution-processable 2D π-conjugated polymers for bulk
heterojunction polymer solar cell applications. Sol Energy Mater Sol Cells 2014;
122:136–45.
Meric Caliskan: Investigation. Mert Can Erer: Investigation. Sultan
Taskaya Aslan: Supervision. Yasemin Arslan Udum: Writing - original
draft. Levent Toppare: Resources. Ali Cirpan: Writing - review &
editing.
[18] Chang WH, et al. A selenophene containing benzodithiophene- alt
-thienothiophene polymer for additive-free high performance solar cell.
Macromolecules 2015;48(3):562–8.
[19] Kitamura C, Tanaka S, Yamashita Y. Design of narrow-bandgap polymers.
Syntheses and properties of monomers and polymers containing aromatic-donor
and o-quinoid-acceptor units. Chem Mater 1996;8(2):570–8.
[20] Kutkan S, Goker S, Hacioglu SO, Toppare L. Syntheses, electrochemical and
spectroelectrochemical characterization of benzothiadiazole and
benzoselenadiazole based random copolymers. J Macromol Sci Part A Pure Appl
Chem 2016;53(8):475–83.
Acknowledgements
Meric Caliskan would like to thank the Scientific and Technological
Research Council of Turkey (TUBITAK) (Project number: 115F604) for
financial support.
[21] Heeger AJ. 25th anniversary article: bulk heterojunction solar cells: understanding
the mechanism of operation. Adv Mater 2014;26(1):10–28.
[22] He R, et al. Narrow-band-gap conjugated polymers based on 2,7-dioctyl-substituted
dibenzo[a,c]phenazine derivatives for polymer solar cells. Macromolecules 2014;
47(9):2921–8.
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