Molecular-level architectural design using benzothiadiazole-based polymers for photovoltaic applications

Article abstract of DOI:10.3762/bjoc.13.87

A series of low band gap, planar conjugated polymers, P1 (PFDTBT), P2 (PFDTDFBT) and P3 (PFDTTBT), based on fluorene and benzothiadiazole, was synthesized. The effect of fluorine substitution and fused aromatic spacers on the optoelectronic and photovoltaic performance was studied. The polymer, derived from dithienylated benzothiodiazole and fluorene, P1, exhibited a highest occupied molecular orbital (HOMO) energy level at -5.48 eV. Density functional theory (DFT) studies as well as experimental measurements suggested that upon substitution of the acceptor with fluorine, both the HOMO and lowest unoccupied molecular orbital (LUMO) energy levels of the resulting polymer, P2, were lowered, leading to a higher open circuit voltage and short circuit current with an overall improvement of more than 110% for the photovoltaic devices. Moreover, a decrease in the torsion angle between the units was also observed for the fluorinated polymer P2 due to the enhanced electrostatic interaction between the fluorine substituents and sulfur atoms, leading to a high hole mobility. The use of a fused π-bridge in polymer P3 for the enhancement of the planarity as compared to the P1 backbone was also studied. This enhanced planarity led to the highest observed mobility among the reported three polymers as well as to an improvement in the device efficiency by more than 40% for P3.

Full text of DOI:10.3762/bjoc.13.87

Molecular-level architectural design using benzothiadiazole-
based polymers for photovoltaic applications
Vinila N. Viswanathan1, Arun D. Rao1, Upendra K. Pandey2, Arul Varman Kesavan1
and Praveen C. Ramamurthy*1,2,§
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2017, 13, 863–873.
1Department of Materials Engineering, Indian Institute of Science,
doi:10.3762/bjoc.13.87
Bangalore, Karnataka, India and 2Interdisciplinary Centre for Energy
Research, Indian Institute of Science, Bangalore, Karnataka, India
Received: 31 December 2016
Accepted: 06 April 2017
Published: 10 May 2017
Email:
Praveen C. Ramamurthy* - onegroupb203@gmail.com
Associate Editor: H. Ritter
*
Corresponding author
§
Fax: +91-80-2360-0472; Tel: +91-80-2293-2627
© 2017 Viswanathan et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
bulk heterojunction; donor–acceptor–donor polymer; low band gap
polymer; organic photovoltaics
Abstract
A series of low band gap, planar conjugated polymers, P1 (PFDTBT), P2 (PFDTDFBT) and P3 (PFDTTBT), based on fluorene
and benzothiadiazole, was synthesized. The effect of fluorine substitution and fused aromatic spacers on the optoelectronic and
photovoltaic performance was studied. The polymer, derived from dithienylated benzothiodiazole and fluorene, P1, exhibited a
highest occupied molecular orbital (HOMO) energy level at −5.48 eV. Density functional theory (DFT) studies as well as experi-
mental measurements suggested that upon substitution of the acceptor with fluorine, both the HOMO and lowest unoccupied molec-
ular orbital (LUMO) energy levels of the resulting polymer, P2, were lowered, leading to a higher open circuit voltage and short
circuit current with an overall improvement of more than 110% for the photovoltaic devices. Moreover, a decrease in the torsion
angle between the units was also observed for the fluorinated polymer P2 due to the enhanced electrostatic interaction between the
fluorine substituents and sulfur atoms, leading to a high hole mobility. The use of a fused π-bridge in polymer P3 for the enhance-
ment of the planarity as compared to the P1 backbone was also studied. This enhanced planarity led to the highest observed
mobility among the reported three polymers as well as to an improvement in the device efficiency by more than 40% for P3.
Introduction
The great interest in organic photovoltaic (OPV) devices is acceptor architecture in the active layer of the OPV devices led
motivated by their ease of low-temperature solution processing, to a new type of device, the so-called bulk heterojunction (BHJ)
light weight, flexibility and potential to produce large area solar cells, with improved power-conversion efficiency (PCE)
devices [1]. The introduction of an interpenetrating donor and [2-4]. A large number of polymer semiconducting materials of
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Beilstein J. Org. Chem. 2017, 13, 863–873.
donor–acceptor–donor (D–A–D) architecture have been synthe- morphology [8]. Among them, the introduction of fluorine has
sized and used in OPV devices recently reaching remarkable attained great interest because of its small size and strong elec-
PCEs of up to 11.7% [5-7]. However, the diversity of tron-withdrawing nature, and fluorine substitution will amend
monomeric units and the numerous available reports on the both the HOMO and LUMO energy levels. In addition, substi-
structural complexity of D–A–D-conjugated p-type polymers tution along the backbone persuades more inter- and intramo-
indicate that there is still need for new materials which can lecular interactions [25-29]. Furthermore, the modification of
further improve the performance of OPV devices based on π-bridges between the donor and acceptor unit of p-type mole-
D–A–D polymers [8-13]. The properties of D–A–D-type mate- cules plays a significant role in increasing the efficiency for
rials such as band gap, structural planarity, charge carrier trans- OPVs [30,31]. However, fused π-bridges (such as thienothio-
port, etc., can be easily tailored by careful selection, combina- phene) having a larger molecular structure and higher degree of
tion, and position of the donor and acceptor moieties.
conjugation are less explored with respect to thiophene and
furan spacers. Thienothiophene ensures a highly delocalized
For OPV systems, it is desirable that p-type polymers should electron system and higher charge carrier mobility due to its
have a low band gap for a broad absorption area of the solar rigid and coplanar fused structure. Also, some thienothiophene-
spectrum to harvest a maximum number of photons [14]. Simul- based polymers show a noticeable bathochromic shift in their
taneously, these compounds should also be soluble in common absorption spectra in comparison with thiophene-substituted
organic solvents and have good film forming properties. How- polymers [32-34]. Herein, keeping all these criteria in mind, we
ever, these are not the only parameters to consider for the endeavored to obtain a series of low band gap polymers, P1, P2,
design of a new donor polymer system. In OPV devices, a and P3, with matching HOMO–LUMO energy levels with the
bicontinuous layer of a donor and an acceptor material is sand- acceptor moiety, without sacrificing the planarity of the mole-
wiched between two electrodes. After the absorption of light, cule. Benzothiadiazole and fluorene, which are commonly used
excitons are generated which dissociate towards the interface of moieties in D–A–D-type polymers, have been chosen as the
the donor–acceptor layer and are separated into free carriers. acceptor and donor, respectively [35,36]. The acceptor motif
These free charge carriers are then collected at the electrode for was further coupled with thiophene to increase the conjugation
current generation [15]. The driving force for this charge sepa- length and absorption. The same acceptor moiety was substi-
ration originates from the energy offset between the frontier tuted with fluorine and the effect of this substitution on the
molecular energy levels of the donor and acceptor material, polymeric and photovoltaic properties was studied. Further-
broadly known as the binding energy [15]. While reducing the more, the effect of planarity and conjugation extension on the
band gap by adjusting the highest occupied molecular orbital polymer backbone was also studied by coupling with a fused
(
HOMO) and lowest unoccupied molecular orbital (LUMO) thienothiophene moiety.
energy levels, a downhill driving force for exciton dissociation
should be maintained for optimum performance of the OPV. If Results and Discussion
not, the total exciton dissociation will decrease, and hence, the For the synthesis of polymers with alternating
overall device efficiency too.
donor–acceptor–donor architecture, suitable monomers were
first prepared (Scheme 1). The synthesis of monomer M1
Moreover, for efficient OPV systems, a moderately high charge started with the cyclization of o-phenylenediamine with thionyl
carrier mobility is required, which is attainable by increasing chloride in the presence of triethylamine, a strong base and
the crystallinity of polymers with firmly packed polymer dichloromethane as the solvent at 0 °C. Compound 1 was then
chains. However, an increase in polymer crystallinity will si- treated with bromine and HBr to obtain 4,7-dibromo-
multaneously decrease their processability in solution. This will benzo[c][1,2,5]thiadiazole (2). The latter compound was then
result in the reduced formation of the desired bicontinuous mor- coupled with trimethyl(thiophene-2-yl)stannane through a Stille
phology with the acceptor [16]. Hence, when designing new reaction using tris(dibenzylideneacetone)dipalladium(0) and tri-
molecules for OPV applications, a subtle balance between o-tolylphosphine as the catalyst system. The dithiophenylated
lowering the band gap, crystallinity, and solubility should be product 3 was washed several times with methanol to remove
maintained. Extensive studies have been reported for the tuning the palladium catalyst and other impurities and subsequently
of optoelectronic and photovoltaic properties by architectural brominated using N-bromosuccinimide to produce the desired
design at the molecular level [17-19], such as quinoidation of monomer M1.
the polymer backbone [20], alternate D–A–D architectures
[
21,22], and substitution with electron-withdrawing or electron- The synthesis of fluorinated monomer M2 started from 1,2-
donating groups [23,24]. Substitutions can be used to tune the difluorobenzene. However, the direct bromination of this com-
band gap, energy levels, solubility, packing of material and pound in the 1,4-position is hindered due to the electronegative
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Beilstein J. Org. Chem. 2017, 13, 863–873.
Scheme 1: Synthetic route towards monomers M1, M2 and M3.
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Beilstein J. Org. Chem. 2017, 13, 863–873.
fluorine substituents. Hence, 1,2-difluorobenzene was reacted quently treated with N,N-diethyl phenylazothioformamide in
with trimethylsilyl chloride in the presence of lithium diiso- chloroform to remove any palladium impurities. Furthermore,
propylamide to afford the 1,4-disilylated intermediate 4 and unreacted reagents and oligomers were removed by successive
bromination of the latter compound in neat bromine afforded Soxhlet extraction with methanol, hexane, and chloroform, re-
the desired 1,4-dibromo-2,3-difluorobenzene (5). Nitration of 5 spectively. The chloroform-soluble fraction of the polymers was
by treatment with fuming nitric acid and acetic acid gave dinitro concentrated and precipitated from methanol. The number-aver-
compound 6. The nitro groups in 6 were then reduced by treat- aged molecular weight (Mn), weight-averaged molecular weight
ment with iron powder and acetic acid. The cyclization of the (Mw) and polydispersity index (PDI) of the polymers were
diamino compound 7 (as described for compound 1) afforded calculated and are summarized in Table S1 (see Supporting
the difluorinated benzothiadiazole 8. The monomer M2 was ob- Information File 1). All polymers showed excellent thermal
tained by coupling 8 with stannylated thiophene, followed by stability with onset decomposition temperature of >410 °C.
bromination using NBS. For the synthesis of monomer M3, the
required thienothiophene substrate 10 was prepared by the Computational modeling of the three polymers was performed
butyllithium-mediated reaction of thieno[3,2-b]thiophene with to gain insight into the molecular energy levels along with band
chlorotrimethylstannane. The latter compound was then coupled gaps using density functional theory (DFT) at the B3LYP/6-
with benzothiadiazole 2 affording compound 11. Finally, bro- 31G (d,p) level as implemented in Gaussian-09 software [37].
mination of 11 using NBS afforded the desired monomer M3. For computational modeling of the polymers, the alkyl chains
were replaced by methyl groups and only two repeating units of
With the monomers M1–M3 at hand, the corresponding poly- monomers were used to keep the calculations simple. The
mers of D–A–D architecture were then synthesized. Thus, the ground-state potential energy of all optimized structures in their
brominated acceptor moieties M1–M3 were reacted with 9,9- stable local minima was obtained to find the HOMO and
dihexylfluorene-2,7-diboronic acid bis(1,3-propanediol)ester LUMO. The isodensity surface plots of the frontier molecular
under Suzuki coupling reaction conditions in the presence of so- orbitals and optimized geometries of the polymers are shown in
dium bicarbonate solution with tris(dibenzylideneacetone)dipal- Figure 1.
ladium(0) and tri-o-tolylphosphine as the catalyst (Scheme 2).
For experimental details, see Supporting Information File 1. The HOMO orbitals show a good delocalization of charge along
The crude polymers were precipitated with methanol and subse- the polymer backbone, while the LUMO orbitals are localized
Scheme 2: Synthesis of polymers P1, P2, and P3.
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Beilstein J. Org. Chem. 2017, 13, 863–873.
Figure 1: Isodensity surface plots of frontier molecular orbitals and optimized molecular geometries of P1, P2 and P3 and their HOMO–LUMO orbitals
obtained from DFT calculations.
at the acceptor moieties. In the case of the fluorinated analogue substituents are replaced by a fused thienothiophene bridge, a
P2, the LUMO energy decreased by 0.10 eV compared to P1 torsion angle of 179° is found between the benzothiadiazole and
(
indicating an increased electron affinity upon fluorination) and thienothiophene part. This clearly indicates an increased
the corresponding HOMO level stabilized by 0.24 eV. Hence, planarity of the polymer P3 compared to P1 due to the pres-
both molecular energy levels could be tailored by the introduc- ence of the fused π-bridge. Moreover, time-dependent density
tion of fluorine. In the case of P3, the HOMO level remained functional theory (TDDFT) calculations were also performed to
unchanged and the energy of the LUMO slightly decreased as allow an estimation of the wavelengths at which electronic tran-
compared to P1.
sitions take place upon excitation [37,38]. TDDFT calculations
have been carried out using the B3LYP/6-31G (d,p) functional
Furthermore, to check the planarity of the polymers, we calcu- basis set to identify the first 30 electronic transitions in the
lated the torsion angles of each unit of the polymers from the polymers. The calculation shows that the first and most feasible
optimized structures and they were found to be close to 180°. singlet-to-singlet transition occurs at a wavelength of 656 nm,
The calculated torsion angles of the polymers are collected in 601 nm, and 674 nm for P1, P2, and P3, respectively. In
Table 1 and the angles are pictured in Figure S1 of Supporting polymer P1 this signal corresponds to the two electronic transi-
Information File 1.
tions from the HOMO to LUMO and HOMO−1 to LUMO
energy levels. On the other hand, the transitions of polymer P2
It is observed from the torsion angles for P2 that the introduc- are attributed to HOMO to LUMO and HOMO to LUMO+1.
tion of fluorine on the polymer backbone does not hinder the The λmax value of 674 nm calculated in P3 includes three elec-
planarity. In contrast, it decreased the torsion angle between the tronic transitions: HOMO to LUMO, HOMO−1 to LUMO, and
fluorinated benzothiadiazole and the thiophene unit. This obser- HOMO to LUMO+1. The calculated absorption spectra for the
vation is attributed to the attractive electrostatic interaction be- polymers P1–3 are shown in Figure 2 and are reliable with the
tween the positively charged sulfur atom and the negatively experimental values obtained by UV–vis spectroscopy. Here it
charged fluorine atom. In the case of P3, where the thiophene is observed that both the HOMO–LUMO values and the elec-
Table 1: Dihedral angles between all units of the polymer backbones in polymers P1, P2, and P3.
Polymer
θ1 °
θ2 °
θ3 °
θ4 °
θ5 °
θ6 °
θ7 °
P1
P2
P3
156
149
154
173.8
175
175.4
179.8
179.8
155
155.9
149.3
154
174.6
175.5
177.4
168.4
178.3
179.5
154.1
154.7
179.2
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Beilstein J. Org. Chem. 2017, 13, 863–873.
tronic transition wavelengths of the polymers showed promis- tive degradation of the material. Also, the reduction of the
ing results for OPV applications.
HOMO levels will further increase the open circuit voltage
Voc), since it is calculated from the difference of
(
HOMO–LUMO energy levels of the donor and acceptor. In
case of polymer P3 the HOMO energy increased by 0.13 eV
and the LUMO level remained almost the same for P1. This
higher HOMO energy is due to the incorporation of the elec-
tron-donating thienothiophene bridge in the polymer backbone.
The experimentally obtained values of frontier molecular
energy levels follow the same trends with respect to the theoret-
ical calculations. All polymers have deep-lying HOMO energy
levels that are lower than the threshold for air oxidation
(
approximately −5.2 eV) [40,41] leading to a good ambient
stability. The electrochemical band gap of the polymers, P1, P2
and P3 were determined as 1.9 eV, 1.78 eV, and 1.83 eV, re-
spectively.
Figure 2: Theoretical absorption spectra of the polymers P1–P3 calcu-
lated using TDDFT.
The HOMO–LUMO energy levels of the polymers were deter- Next, the optical properties of the polymers were studied by
mined by cyclic voltammetry, using non-aqueous Ag/AgCl as UV–vis spectroscopy. The absorption spectra were obtained in
the reference electrode in acetonitrile with 0.1 M tetrabutylam- chlorobenzene solution (Figure 4). All polymers showed a
monium hexafluorophosphate as electrolyte at a scan rate of broad absorption in the lower energy region (450–650 nm) due
1
00 mV/s. The instrument was calibrated with ferrocene/ to the intramolecular charge transfer (ICT) through the back-
ferrocenium and was found to be ≈0.11 V. The HOMO–LUMO bone of polymers and another broad peak in the higher energy
energy levels were calculated using the following equation region due to π–π* transitions. A bathochromic shift in the λmax
based on the onset of oxidation and reduction obtained from the of all thin film spectra of the polymers was observed, as com-
electrochemical spectra (Figure 3).
pared to the solution spectra. This is due to the enhanced inter-
chain stacking and ordered structural organization of the poly-
mers in the thin film. The intensity of the ICT band of P1 and
P3 is less than that of the π–π* transition band, whereas in P2,
the ICT band is more pronounced due to the increased charge
separation in the D–A–D polymer backbone due to the elec-
tronegative fluorine substituents in the acceptor moiety. As an-
ticipated, the spectrum of P3 is wider than the spectra of the
other polymers since it has an extended conjugation along the
backbone. The peak corresponding to the ICT of P2 displays a
red shift (≈33 nm) compared to the other polymers. This is
caused by the increased electrostatic interaction between fluo-
rine and sulfur in the solid state. The UV–vis spectra of
annealed films of the polymers at 130 °C show an apparent red
shift in the onset absorption because of an increased aggrega-
tion of the polymer chains upon heating. The optical band gaps
of P1, P2, and P3 were calculated from the onset of the absorp-
tion as 1.95 eV, 1.93 eV and 1.87 eV, respectively. Again, a
lower band gap is observed for P3 owing to the extended conju-
gation over the other polymers. The combined optical, electro-
EHOMO = −[Eox (onset) − EFc/Fc2+ + 4.8] eV
ELUMO = −[Ered (onset) − EFc/Fc2+ + 4.8] eV [39].
Figure 3: Electrochemical spectra of polymers P1–P3.
The effect of fluorine substitution at the polymer backbone is chemical, and theoretical calculations are summarized in
apparent on the frontier molecular orbitals of polymer P2. Both Table 2.
the HOMO and LUMO levels of the polymer have decreased
energy as compared to polymer P1 (from −5.48 eV to −5.53 eV Photoluminescence (PL) quenching studies were performed
and from −3.58 eV to −3.75 eV, respectively). This reduction of with pure donor polymers and with different ratios (by weight)
the HOMO energy level improves the resistance towards oxida- of polymers with PC70BM ([6,6]-phenyl C71 butyric acid
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Beilstein J. Org. Chem. 2017, 13, 863–873.
Figure 5: Photoluminescence spectra of polymers P1–P3 and
polymer:PC70BM blends.
solar cell was ITO/PEDOT:PSS (40 nm)/polymer:PC70BM
(
120 nm)/Ca (15 nm)/Al (100 nm). Figure 6 shows the favor-
able energy alignments for both electrons and holes for the
collection at the electrodes once generated after absorption of
sunlight.
Figure 4: Normalized absorption spectra of the polymers in solution,
film and annealed film (130 °C) forms.
methyl ester) as an acceptor to evaluate the suitability of the Figure 7 shows the current–voltage characteristics in the dark
polymers for photovoltaic devices. A significant reduction in and after illumination with AM 1.5G (100 mW cm−2) for
the PL emission intensity of the donor when mixed with an devices with P1, P2, and P3. The photovoltaic parameters of
acceptor is a good indication of an efficient charge transfer be- the devices are summarized in Table 3. Optimized blend ratios
tween the donor and acceptor when excited at the wavelength of of 1:3, 1:1.5 and 1:3 were observed for polymers P1, P2, and
the absorption maximum of the donor. An efficient charge P3, respectively.
transfer between donor and acceptor is essential for good solar
cell devices. The PL spectra of the polymers, consisting of The blend P1:PC70BM 1:3 ratio shows a current density of
PC70BM with various blend ratios, are shown in Figure 5. Poly- 1.63 mA/cm2 and Voc of 0.60 V. However, the device suffers
mers P1, P2, and P3 were excited at their absorption maxima of from a moderate fill factor of 0.29. The corresponding device
≈
377 nm, 543 nm and ≈395 nm, respectively. It is evident from with the fluorinated polymer P2 (having a deeper HOMO
the spectra that all polymers show significant quenching in their energy level) shows an improved Voc (0.62 V) and JSC
emission, indicating their suitability for OPVs.
(2.8 mA/cm2). Moreover, the device prepared with polymer P3
shows a Jsc of 2.3 mA/cm2 and Voc of 0.69 V (Figure 7). the
Next, we fabricated OPVs based on bulk heterojunction (BHJ) hole mobilities (Table 3) of all the polymers were calculated
solar cells of polymers P1, P2 and P3 and tested them with using the space charge limited current method (SCLC, see Sup-
PC70BM as an acceptor. The device architecture of the BHJ porting Information File 1). Hole mobility values of
Table 2: Electrochemical and optical properties along with theoretical calculations.
Polymer
λmax
λonset
(solution)
(nm)
λmax
(film)
(nm)
λonset
(film)
(nm)
Ega
(film)
(eV)
EHOMO
(eV)
ELUMO
(eV)
Egb
(eV)
(solution)
nm)
(
P1
P2
P3
372,
98
376,
10
372,
15
598
596
617
377,
520
633
642
662
1.95
1.93
1.87
−5.48
(−4.9)c
−3.58
(−2.6)c
1.9
(2.2)c
4
386,
543
−5.53
(−5.1)c
−3.75
(−2.7)c
1.78
(2.3)c
5
382,
533
−5.35
(−4.9)c
−3.52
(−2.7)c
1.83
(2.1)c
5
aOptical band gap calculated from the absorption onset. bElectrochemical band gap calculated from the cyclic voltammogram. cEnergy levels and
band gap obtained from DFT calculations.
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Beilstein J. Org. Chem. 2017, 13, 863–873.
Figure 6: Bulk heterojunction solar cells device architecture, illustrating favorable conditions for absorption of sunlight.
the fluorine-substituted polymer P2, an enhanced electrostatic
interaction between the units was observed, and for P3, the
fused thienothiophene moiety improves the planarity of the
molecules.
To understand the photovoltaic results, we have also investigat-
ed the morphology of the active layers of the devices by tapping
mode AFM. With this method, information about the topogra-
phy as well as phase images as shown in Figure S2 (Supporting
Information File 1) can be obtained. The active layer morpholo-
gy in polymer solar cells is crucial and can drastically affect the
performance of the devices. For optimum performance of an
OPV device, the phase-separated donor and acceptor domain
sizes should be twice the exciton diffusion length, which is typi-
cally of the order of 7–12 nm. From the images, a well-spaced,
uniform phase contrast of donor polymer and acceptor was ob-
served, indicating the uniform spatial distribution of the
Figure 7: J–V Spectra in chlorobenzene (CB) for which the ratio of
polymer:PC70BM was optimized as follows: P1:PC70BM, 1:3;
P2:PC70BM, 1:1.5; P3:PC70BM, 1:3.
1
3
.32 × 10−6 cm2/V·s, 4.49 × 10−5 cm2/V·s, and polymer:PC70BM in the matrix for all three polymers. Lower
.98 × 10−5 cm2/V·s were observed for blends based on P1, P2, values of Jsc indicate that not all the photons absorbed are sepa-
and P3, respectively. Compared to the device fabricated using rated into free carriers, which could be attributed to the larger
P1, the corresponding devices fabricated with P2 and P3 show domain sizes of phase separated polymer and PC70BM. The
high hole mobilities. This increase in the mobility can be attri- roughness of P1, P2, and P3 was obtained as 0.234 nm,
buted to the enhanced planarity of the molecules. In the case of 0.826 nm, and 0.914 nm, respectively.
Table 3: Photovoltaic properties of the devices based on polymers P1, P2 and P3.
Polymer
Open circuit voltage
V)
Fill factor
(%)
Short circuit current
(mA/cm2)
η
(%)
µh
(
(cm2/V·s)
P1:PC70BM
1.63
(1.41)a
1.32 × 10−6
4.49 × 10−5
3.98 × 10−5
(
1:3)
P2:PC70BM
1:1.5)
P3:PC70BM
1:3)
0.6
29
35
25
0.28
0.61
0.41
2.8
(2.32)a
(
0.62
0.69
2.36
(2.17)a
(
aaverage Jsc.
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Beilstein J. Org. Chem. 2017, 13, 863–873.
The relatively low external quantum efficiency (EQE) values in better stacking between aromatic units as compared to
obtained for the reported polymers also explain the lower Jsc polymer P1. A bathochromic shift in the absorption spectra
values. Basically, EQE spectra reveal the photon–current along with a higher hole mobility in the resulting polymer was
response of the devices, providing information about the num- observed. This resulted in an increase in the short circuit cur-
ber of charges contributing to the overall device current com- rent from 1.63 mA/cm2 to 2.36 mA/cm2 with an increase in the
pared to the total number of incident photons at a particular overall efficiency by ≈46%. These studies suggest that planar-
wavelength. Figure 8 shows the EQE spectra for devices with conjugated polymers based on flourene and benzothiadiazole
P1, P2, and P3. The device based on P2 shows an excellent (when substituted with appropriate groups) can play a vital role
photocurrent response over the absorption range of in attaining higher efficiency for D–A–D-based OPV systems.
3
20–700 nm, with a maximum at around 620 nm. Similarly, the
devices fabricated with P1 and P3 show two distinct peaks at Experimental
50 and 470 nm (P1) and at 320 nm and 523 nm (P2), respec- The syntheses of the monomers and polymers are provided in
3
tively. This implies that the overall photocurrent generation is the Supporting Information File 1. The fabrication of the photo-
contributed by the full polymer absorption range. By inte- voltaic devices has been carried out using the following proce-
grating the EQE spectra with the AM1.5G spectrum, the calcu- dure. Patterned ITO-coated glass substrates (Xinyan Technolo-
lated Jsc values were obtained as 1.41 mA cm−2, 2.32 mA cm−2 gy Limited, Taiwan) were sequentially cleaned in deionized
and 2.17 mA cm−2 for blends with P1, P2, and P3, respectively (DI) water with soap (Hellmanex III), DI water, isopropyl
(
Table 3).
alcohol, acetone and water under sonication for ≈30 min each.
The cleaned substrates were then dried with nitrogen gas.
UV–ozone was performed on cleaned substrates for 30 min to
remove residual impurities. PEDOT:PSS (Baytron VP Al 4083)
was spin-coated on the UV–ozone-treated ITO substrate at
3
000 rpm, for 60 s followed by annealing at 130 °C for 20 min
to remove residual solvents. The PEDOT:PSS-coated sub-
strates were then immediately transferred into the glove box
(
H2O <1 ppm, O2 <1 ppm) for active layer coating. The solu-
tions with different blend ratios of polymers and PC70BM were
prepared in chlorobenzene by weight ratio and stirred overnight
at 60 °C in the dark. The concentrations of the solutions were
kept at 25 mg/mL. The blend solution was then spin-coated on
the PEDOT:PSS-coated substrate at 700 rpm for 60 s to obtain a
film thickness of 120 nm. The substrates were then annealed at
Figure 8: External quantum efficiency spectra of optimized devices
fabricated with polymers P1, P2 and P3 and PC70BM as an acceptor.
1
50 °C for 15 min before being placed in the thermal evapo-
Conclusion
rator for the cathode (Ca/Al) evaporation. Finally, 15 nm Ca (at
In conclusion, we have synthesized three polymers with a rate of 0.2 Å/s) and 100 nm Al (at the rate of 1 Å/s) were
D–A–D architecture based on benzothiadiazole and fluorene. subsequently evaporated at a base pressure of 3 × 10−6 mbar to
The effect of substitution with electron-withdrawing fluorine complete the devices. The thickness of the different layers was
substituents and the incorporation of fused thienothiophene measured using a Dektak XT surface profiler.
groups in the polymer backbone and their effect on the opto-
electronics and photovoltaic performances has been demon-
Supporting Information
strated. It was observed that the incorporation of fluorine, a
strong electron-withdrawing group, resulted in deeper HOMO
Supporting Information File 1
energy levels for the polymer P2 as compared to polymer P1.
The fluorination also enhances the intramolecular interaction
between the polymer chains, which is reflected in the higher
hole mobility of P2 over P1. Though the photovoltaic parame-
ter values are very low for these polymers, it was observed that
Experimental details and characterization data.
http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-13-87-S1.pdf]
[
fluorination could increase the overall device performance by Acknowledgements
110%. The effect of increased planarity along the polymer The authors gratefully acknowledge the financial support from
≈
backbone was further explored by introducing the thienothio- the Department of Science and Technology, the Government of
phene motif, a fused aromatic π-bridge in polymer P3, resulting India (DSTO 1307, DSTO1286, SR/S3/ME/51/2012) and the
871

Beilstein J. Org. Chem. 2017, 13, 863–873.
2
1.Kitamura, C.; Tanaka, S.; Yamashita, Y. Chem. Mater. 1996, 8,
Partnership to Advance Clean Energy-Research (PACE-R) for
the Solar Energy Research Institute for India and the United
States (SERIIUS), funded jointly by the U.S. Department of
Energy (Office of Science, Office of Basic Energy Sciences,
and Energy Efficiency and Renewable Energy, Solar Energy
Technology Program, under subcontract DE-AC36-08GO28308
to the National Renewable Energy Laboratory, Golden,
Colorado) and the Government of India, through the Depart-
ment of Science and Technology under subcontract IUSSTF/
JCERDC-SERIIUS/2012 dated Nov. 22, 2012.
570–578. doi:10.1021/cm950467m
2
2.Yamamoto, T.; Zhou, Z.-h.; Kanbara, T.; Shimura, M.; Kizu, K.;
Maruyama, T.; Nakamura, Y.; Fukuda, T.; Lee, B.-L.; Ooba, N.;
Tomaru, S.; Kurihara, T.; Kaino, T.; Kubota, K.; Sasaki, S.
J. Am. Chem. Soc. 1996, 118, 10389–10399. doi:10.1021/ja961550t
23.Sotzing, G. A.; Reynolds, J. R.; Steel, P. J. Adv. Mater. 1997, 9,
95–798. doi:10.1002/adma.19970091005
7
2
2
2
2
4.Zhang, Q. T.; Tour, J. M. J. Am. Chem. Soc. 1998, 120, 5355–5362.
doi:10.1021/ja972373e
5.Lu, Y.; Xiao, Z.; Yuan, Y.; Wu, H.; An, Z.; Hou, Y.; Gao, C.; Huang, J.
J. Mater. Chem. C 2013, 1, 630–637. doi:10.1039/C2TC00327A
6.Dang, D.; Chen, W.; Yang, R.; Zhu, W.; Mammo, W.; Wang, E.
Chem. Commun. 2013, 49, 9335–9337. doi:10.1039/c3cc44931a
7.Kularatne, R. S.; Taenzler, F. J.; Magurudeniya, H. D.; Du, J.;
Murphy, J. W.; Sheina, E. E.; Gnade, B. E.; Biewer, M. C.;
Stefan, M. C. J. Mater. Chem. A 2013, 1, 15535–15543.
doi:10.1039/c3ta13686h
References
1.
2.
3.
Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater. 2009, 21,
323–1338. doi:10.1002/adma.200801283
Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992,
58, 1474–1476. doi:10.1126/science.258.5087.1474
1
2
2
8.Fei, Z.; Boufflet, P.; Wood, S.; Wade, J.; Moriarty, J.; Gann, E.;
Ratcliff, E. L.; McNeill, C. R.; Sirringhaus, H.; Kim, J.-S.; Heeney, M.
J. Am. Chem. Soc. 2015, 137, 6866–6879. doi:10.1021/jacs.5b02785
9.Gohier, F.; Frère, P.; Roncali, J. J. Org. Chem. 2013, 78, 1497–1503.
doi:10.1021/jo302571u
Kraabel, B.; Hummelen, J. C.; Vacar, D.; Moses, D.; Sariciftci, N. S.;
Heeger, A. J.; Wudl, F. J. Chem. Phys. 1996, 104, 4267.
doi:10.1063/1.471154
2
3
3
4.
Brabec, C. J.; Zerza, G.; Cerullo, G.; De Silvestri, S.; Luzzati, S.;
Hummelen, J. C.; Sariciftci, S. Chem. Phys. Lett. 2001, 340, 232–236.
doi:10.1016/S0009-2614(01)00431-6
0.Roncali, J. Acc. Chem. Res. 2009, 42, 1719–1730.
doi:10.1021/ar900041b
5
.
.
Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H.
Nat. Energy 2016, 1, No. 15027. doi:10.1038/nenergy.2015.27
Chen, C.-C.; Chang, W.-H.; Yoshimura, K.; Ohya, K.; You, J.; Gao, J.;
Hong, Z.; Yang, Y. Adv. Mater. 2014, 26, 5670–5677.
doi:10.1002/adma.201402072
1.Zhu, Z.; Waller, D.; Gaudiana, R.; Morana, M.; Mühlbacher, D.;
Scharber, M.; Brabec, C. Macromolecules 2007, 40, 1981–1986.
doi:10.1021/ma062376o
6
3
3
3
2.Li, Y.; Chang, C.-Y.; Chen, Y.; Song, Y.; Li, C.-Z.; Yip, H.-L.;
Jen, A. K.-Y.; Li, C. J. Mater. Chem. C 2013, 1, 7526–7533.
doi:10.1039/c3tc31600a
7.
Hu, H.; Jiang, K.; Yang, G.; Liu, J.; Li, Z.; Lin, H.; Liu, Y.; Zhao, J.;
Zhang, J.; Huang, F.; Qu, Y.; Ma, W.; Yan, H. J. Am. Chem. Soc. 2015,
3.Xu, Y.-X.; Chueh, C.-C.; Yip, H.-L.; Ding, F.-Z.; Li, Y.-X.; Li, C.-Z.;
Li, X.; Chen, W.-C.; Jen, A. K.-Y. Adv. Mater. 2012, 24, 6356–6361.
doi:10.1002/adma.201203246
1
37, 14149–14157. doi:10.1021/jacs.5b08556
Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S. Chem. Rev. 2009, 109,
868–5923. doi:10.1021/cr900182s
8
9
1
1
1
1
1
1
1
1
1
1
2
.
5
4.Son, H. J.; Lu, L.; Chen, W.; Xu, T.; Zheng, T.; Carsten, B.; Strzalka, J.;
Darling, S. B.; Chen, L. X.; Yu, L. Adv. Mater. 2013, 25, 838–843.
doi:10.1002/adma.201204238
.
Bessette, A.; Hanan, G. S. Chem. Soc. Rev. 2014, 43, 3342–3405.
doi:10.1039/c3cs60411j
0.Zhan, X.; Zhu, D. Polym. Chem. 2010, 1, 409–419.
doi:10.1039/b9py00325h
3
5.Chen, J.; Cao, Y. Acc. Chem. Res. 2009, 42, 1709–1718.
doi:10.1021/ar900061z
1.Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S. Chem. Rev. 2009, 109,
3
6.Ying, L.; Zou, J.; Yang, W.; Zhang, A.; Wu, Z.; Zhao, W.; Cao, Y.
Dyes Pigm. 2009, 82, 251–257. doi:10.1016/j.dyepig.2009.01.009
7.Gaussian 09, Revision C.01; Gaussian, Inc.: Wallingford, CT, 2010.
8.McCormick, T. M.; Bridges, C. R.; Carrera, E. I.; DiCarmine, P. M.;
Gibson, G. L.; Hollinger, J.; Kozycz, L. M.; Seferos, D. S.
Macromolecules 2013, 46, 3879–3886. doi:10.1021/ma4005023
9.Gedefaw, D.; Tessarolo, M.; Zhuang, W.; Kroon, R.; Wang, E.;
Bolognesi, M.; Seri, M.; Muccini, M.; Andersson, M. R. Polym. Chem.
5868–5923. doi:10.1021/cr900182s
2.Deng, P.; Zhang, Q. Polym. Chem. 2014, 5, 3298–3305.
doi:10.1039/c3py01598j
3
3
3.Zhou, H.; Yang, L.; You, W. Macromolecules 2012, 45, 607–632.
doi:10.1021/ma201648t
4.Bundgaard, E.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2007, 91,
3
4
4
9
54–985. doi:10.1016/j.solmat.2007.01.015
5.Brédas, J.-L.; Norton, J. E.; Cornil, J.; Coropceanu, V. Acc. Chem. Res.
009, 42, 1691–1699. doi:10.1021/ar900099h
6.Huang, Y.; Kramer, E. J.; Heeger, A. J.; Bazan, G. C. Chem. Rev.
014, 114, 7006–7043. doi:10.1021/cr400353v
2014, 5, 2083–2093. doi:10.1039/c3py01519j
2
0.Sun, J.; Zhu, Y.; Xu, X.; Lan, L.; Zhang, L.; Cai, P.; Chen, J.; Peng, J.;
Cao, Y. J. Phys. Chem. C 2012, 116, 14188–14198.
2
doi:10.1021/jp3009546
7.Chochos, C. L.; Choulis, S. A. Prog. Polym. Sci. 2011, 36, 1326–1414.
doi:10.1016/j.progpolymsci.2011.04.003
1.De Leeuw, D. M.; Simenon, M. M. J.; Brown, A. R.; Einerhand, R. E. F.
Synth. Met. 1997, 87, 53–59. doi:10.1016/S0379-6779(97)80097-5
8.Murali, M. G.; Rao, A. D.; Ramamurthy, P. C. RSC Adv. 2014, 4,
4
4902–44910. doi:10.1039/C4RA08214A
9.Kottokkaran, R.; Rao, A. D.; Ramamurthy, P. C. MRS Online Proc. Libr.
013, 1500. doi:10.1557/opl.2013.263
2
0.Hoogmartens, I.; Adriaensens, P.; Vanderzande, D.; Gelan, J.;
Quattrocchi, C.; Lazzaroni, R.; Bredas, J. L. Macromolecules 1992, 25,
7347–7356. doi:10.1021/ma00052a043
872

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