Design and synthesis of new ultra-low band gap thiadiazoloquinoxaline-based polymers for near-infrared organic photovoltaic application

Article abstract of DOI:10.1039/c5ra24364e

Two D-A copolymers, F1 and F2, with fluorene and thiazole units were substituted, respectively, on a thiadiazoloquinoxaline (TDQ) unit to enhance the electron-accepting strength of TDQ. The copolymers were synthesized by a cross-coupling Stille reaction and their optical and electrochemical properties were examined, which revealed that they have ultra-low band gaps and absorption in the near-infrared. These copolymers were employed as donors along with PC₇₁BM as an electron acceptor for the fabrication of solution-processed bulk heterojunction (BHJ) polymer solar cells. After the optimization of the donor-to-acceptor weight ratio and the solvent additive (4 v% DIO as solvent additive), devices with F1:PC₇₁BM and F2:PC₇₁BM displayed power conversion efficiencies (PCEs) of 5.80% and 3.32%, respectively. Although F2 possesses a broader absorption profile compared with F1, the lower value of PCE for the F2-based device was attributed to the low LUMO offset between F2 and PC₇₁BM, which limited the exciton dissociation. The abovementioned results indicate that these copolymers can be utilized for ternary BHJ and tandem solar cells to achieve a high PCE.

Full text of DOI:10.1039/c5ra24364e

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Design and synthesis new ultra low band gap thiadiazolouinoxaline based polymers for
near infrared organic photovoltaic application.
M. L. Keshtova*, S. A. Kuklina, N. A. Radychevb, A.Y. Nikolaeva, E. N. Koukarasc,,d
Abhishek Sharmae, G. D. Sharmaf*
aInstitute of Organoelement Compounds of the Russian Academy of Sciences, Vavilova St.,
28, 119991 Moscow, Russian Federation
bCarl von Ossietzky University of Oldenburg, 26129, Oldenburg, Germany
cInstitute of Chemical Engineering Sciences, Foundation for Research & Technology, Hellas
(FORTH/ICEꢀHT), Stadiou Str. Platani, Patras, 26504, Greece
dMolecular Engineering Laboratory, Department of Physics, University of Patras, Patras,
26500 GR, Greece
eDepartment of Electronics and communication Engineering, LNMIIT (Deemed University),
Jamdoli, Jaipur (Raj.) 302031 India
fMolecular Electronic and Optoelectronics Device research Laboratory, Department of
Physics, LNMIIT (Deemed University), Jamdoli, Jaipur (Raj.) 302031 India
Two DꢀA copolymers F1 and F2 with fluorene and thiazole units substituted respectively to
thiadiazoloquinoxaline (TDQ) unit to enhance the electronꢀaccepting strength of TDQ. The
copolymers were synthesized by crossꢀcoupling Stille reaction, and their optical and
electrochemical properties were examined revealing that they have ultra low bandgaps, and
absorption in the near infrared. These copolymers were employed as donor along with
PC71BM as electron acceptor for the fabrication of solution processed bulk heterojunction
(BHJ) polymer solar cells. After the optimization of donor to acceptor weight ratio and
solvent additive (4 % v DIO as solvent additive), the devices with F1:PC71BM and
F2:PC71BM showed power conversion efficiencies (PCEs) of 5.80 and 3.32 %, respectively.
Although F2 possesses a broader absorption profile as compared to F1, the low value of PCE
for the F2 based device has been attributed to the low LUMO offset between F2 and PC71BM
that limited the exciton dissociation. The above results indicate that these copolymers can be
utilized for ternary BHJ and tandem solar cells, to achieve the high PCE.
Key word: NIR conjugated coꢀpolymers, bulk heterojunction, polymer solar cells, solvent
additives
* corresponding authors
1

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Introduction
The development of solar cells based on polymeric materials with bulk heterojunction
(BHJ) active layer has become an impressive research area in recent years due to its potential
as an alternative source of energy [1]. The power conversion efficiency (PCE) of these solar
cells recently have been increased reaching the range of 9ꢀ11 % through the design of new
conjugated polymers and better control of the charge transport from careful adjustment of the
morphology of the BHJ active layer [2]. It is expected to approach a 15 % theoretical upper
limit as a result of the design and development new absorbing materials with reduced
bandgap offset along with high fill factor and incident photon to current efficiency (IPCE)
[3]. Achieving the high PCE in polymer solar cells requires the concurrent optimization of
absorption profile, good oxidation stability and band gap of polymer donor as well as
alignment of energy levels with the acceptor counter part used in the BHJ active layer. A
polymer with energy bandgap (1.8 < Eg < 1.10) has to be designed to harvest the photon
energy over a broad range, covering from the visible to the near infrared region of the solar
spectrum and complementary absorption to fullerene derivative. Low bandgap polymers
possess strong absorption in both UVꢀvisible and near IR regions and thus can achieve a high
photocurrent in solar cells, since the PCE of the PSCs is directly proportional to photon
absorption ability of the donor polymer used in active layer [4]. The polymers with narrow
bandgap with absorption band up to 1000 nm absorb a higher percentage of solar energy and
can provide short circuit current (Jsc) over 30 mA/cm2 [5]. The most successful strategy to
design the low bandgap polymers is the copolymerization of alternating electron rich and
electron acceptor units as intramolecular charge transfer (ICT) between these units. ICT can
be modified by adjusting the strength of two monomers, thereby enabling the tuning the
HOMO and LUMO levels [6, 7].
The design and synthesis of high performance low bandgap polymers is quite
challenging and there are limited low bandgap polymers with PCE higher than 8 % [4, 8],
since the open circuit is relative low about 0.7 V [9]. The reduction of the optical bandgap of
a polymer is generally accompanied with either rising the HOMO or lowering the LUMO
energy level. The rising of the HOMO energy level has a direct impact on Voc whereas deeper
a LUMO energy level causes reduction in the exciton dissociation efficiency that is directly
related to the Jsc of the photovoltaic device. Moreover, it is important to design low bandgap
polymers with both high thermal and air stability. The air stability can also be affected by the
HOMO level of the polymer. The polymers with deeper HOMO levels are generally more
2

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stable against oxidation in air. Additionally, low bandgap polymer should have high hole
mobility in order to achieve high PCE [10]. However, all the ultra low bandgap polymers do
not directly guarantee high PCE. Proper alignment of the HOMO and LUMO energy levels
are critical for efficient charge transfer to the acceptor and to ensure a large Voc of the device.
High carrier mobility as well as favorable morphology when blended with electron acceptor
material is required to enhance the device performance. Recently a number of NIR
conjugated polymers have been designed and achieved PCE of 6% for single junction PSCs
[11]. These NIR narrow band gap polymers are also very promising for applications in
tandem solar cells in combination with medium band gap polymers. Quite recently it has
been reported that tandem solar cell, based on NIRꢀconjugated polymer as one of the donor
polymer, reached 10.6% PCE [12]. Thus, the development of NIRꢀpolymers with strong
absorption in the range above 700 nm in combination with medium bandgap is very
important for obtaining promising tandem solar cells with high PCE. These recent
achievements inspired us to develop novel NIRꢀabsorption polymers for PSC applications.
Strong electronꢀdonor and strong electronꢀaccepting blocks are generally used for
development of donorꢀacceptor (DꢀA) polymers with ultraꢀnarrow band gap. Common strong
electronꢀdonating monomers include pyrrole, thiophene, bithiophene derivatives etc., and
strong electronꢀaccepting blocks used in NIR DꢀA polymers are diketopyrrolopyrrole [13],
benzobisthiadiazole [14], pyrazinoquinoxaline [15], thiadiazoloquinoxaline (TDQ) [16],
benzotriazole [17], thienoisoindigo [18], tetraazabenzodifluoranthene [19], diimides [20] etc.
One of the promising and less studied electronꢀdonating monomers for NIR conjugated
polymer synthesis are TDQ derivatives possessing such unique properties as high extinction
coefficient, intensive absorption spectrum up to the NIR range and decent charge mobility.
Previous TDQꢀcontaining polymers were beneficial as active materials in organic solar cells
[21,5] and organic fieldꢀeffect transistors [16a, 22]. However, ultraꢀnarrow band gap
polymers based on TDQꢀderivatives are relatively poor studied and at the same time are
highly demanded. TDQꢀcontaining NIR polymers with Eg ~1.2 eV may extend absorption
band to near infrared range (over 1000 nm). Fused heteroaromatic blocks with extended πꢀ
conjugation and rigid planar structures promote formation of πꢀπ stacking of backbones
exhibiting enhanced absorption properties and high charge mobility. TDQ is a promising
construction block for synthesis of narrow band gap polymers due to strong electronꢀ
accepting properties of four imine groups in TDQ. In our efforts to develop novel NIRꢀ
absorbing DꢀA polymers we started from TDQꢀderivatives as a central accepting block.
3

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We prepared two novel electronꢀaccepting TDQꢀbased monomers by introducing
fluorene and thiazole units into TDQ blocks to enhance the electronꢀaccepting strength of
TDQ. On its basis and in crossꢀcoupling Stille reaction, we synthesized and characterized two
new NIR DꢀA copolymers F1 and F2 with ultraꢀlow band gap (Eg = 1.24 and 1.08 eV). New
synthesized F1 and F2 copolymers possess broad (panchromatic) absorption spectra in 350ꢀ
1200 nm wavelength range and demonstrate good solubility due to alkyl sideꢀsubstitutes.
These copolymers were employed as donor along with PCBM as electron acceptor for the
solution processed BHJꢀPSCs. After the optimization, i.e. donor to acceptor ratio, and solvent
additive, F1:PC71BM (1:2) and F2:PC71BM (1:2) active layer processed with solvent additive
(SA) i.e. (4 v% DIO/CF) solvent based bulk heterojunction PSC showed PCE of 5.80 % and
3.32 %, respectively. Our results show that after careful structural design such copolymers
demonstrate attractive photovoltaic properties and can be used for high ternary BHJ and
tandem PSCs.
Experimental details
Synthesis of copolymers
Synthesis of intermediate compounds and their characterization were described in
supplementary information.
2.4.15.8,12ꢀbis(5ꢀbromoꢀ4ꢀdodecylthiophenꢀ2ꢀyl)ꢀ2,5ꢀdi(nonadecanꢀ3ꢀyl) [1,2,5 ] thiadiꢀ
azolo [3,4ꢀi]bis[1,3] thiazolo[4,5ꢀa:5',4'ꢀc]phenazine (M1). NBS (99.2 mg, 0.5575 mmol)
was added to a solution of 8,12ꢀbis(4,5ꢀdidodecythiophenꢀ2ꢀyl)ꢀ2,5ꢀdi(nonadecanꢀ3ꢀyl)
[1,2,5] thiadiazolo [3,4ꢀi]bis[1,3]thiazolo[4,5ꢀa:5',4'ꢀc]phenazine (15) (339 mg, 0.2445 mmol)
in THF (60 ml), the reaction mixture was stirred at room temperature for 2 h. Then the
solvent was evaporated and the residue was purified by column chromatography on SiO2
(hexaneꢀCHCl3=3:1). The target compound M1 was obtained with the yield of 295 mg
1
(78%). H NMR (400 MHz, CDCl3, δ ppm): 8.89 (s, 2H), 3.33 (m, 2H), 2.66 (m, 4H), 2.25ꢀ
1.90 (m, 8H), 1.8ꢀ1.1 (m, 102H), 0.9 (m, 12H). 13C NMR (100 MHz, CDCl3, δ ppm): 176.14,
150.47, 146.39, 141.50, 137.29, 137.20, 134.89, 134.24, 130.77, 120.01, 119.95, 46.34,
35.07, 31.95, 31.93, 29.99, 29.85ꢀ29.66, 29.51, 29.40, 29.37, 28.03, 27.29, 22.70, 14.12,
11.91. Anal. calculated (%) for C84H130Br2N6S5: C, 65.34; H, 8.49; N, 5.44. Found: С, 65.49;
H, 8.40; N, 5.35.
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4,9ꢀDibromoꢀ6,7ꢀbis(9,9ꢀdidodecylꢀ9Hꢀfluorenꢀ2ꢀyl)ꢀ[1,2,5]thiadiazolo[3,4ꢀg] quinoxaline
(M2). Diketone 17 (1.560 g, 14.7 mmol), diamine 16 (0.573 g, 17.7 mmol) and acetic acid
(50 ml) were placed into 100 ml flask, and the mixture was refluxed for 10 h. After
evaporation, the residue was purified by column chromatography using a mixture of hexaneꢀ
dichloromethane = 10:1 as eluent. Target monomer M2 was obtained as red oil. The yield is
1
1.6 g (80%). H NMR (400 MHz, CDCl3, δ ppm): 7.89 – 7.81 (m, 4H), 7.73 – 7.65 (m, 4H),
7.37 – 7.30 (m, 6H), 1.94 – 1.80 (m, 8H), 1.31 – 0.93 (m, 72H), 0.91 (t, J=7.0 Hz, 12H),
13
0.87ꢀ0.85 (m, 8H). C NMR (100 MHz, CDCl3, δ ppm): 156.6, 152.5, 151.7, 151.0, 143.8,
140.2, 138.3, 136.6, 129.6, 128.2, 127.1, 125.2, 123.1, 120.6, 119.8, 113.8, 77.5, 77.2, 76.8,
55.3, 40.4, 32.1, 30.2, 29.8, 29.8, 29.8, 29.6, 29.5, 24.1, 22.8, 14.3. Anal. calculated (%) for
C82H114Br2N4S: C, 73.08; H, 8.53; N, 4.16; S, 2.38; Br, 11.86. Found: C, 73.00; H, 8.44; N,
4.14; S, 2.17; Br, 11.65.
Synthesis of polymer F1: 8,12-bis(5-bromo-4-dodecylthiophen-2-yl)-2,5-di(nonadecan-3-yl)
[1,2,5] thiadiazolo [3,4-i]bis[1,3] thiazolo[4,5-a:5',4'-c]phenazine (0.6738 g, 0.5 mmol)
compound M2, 2,7ꢀ bis (trimethyltin)ꢀ4,5ꢀbis(undecyl)benzo[2,1ꢀb;3,4ꢀb’] dithio phene
(0.4122 g, 0.5 mmol), Pd(Ph3P)4 (0.027 g, 0.0234 mmol) and toluene (20 ml) were heated
under argon at 110˚C for 48 h. Then 2ꢀ bromothiophene (0.02 g) was added and the mixture
was stirred at 110˚C for another 5 h. After cooling to r.t. reaction mixture was poured into
methanol (200 ml) and filtrated. Polymer was dissolved in CHCl3 and precipitated with
methanol. Then it was purified by extraction with methanol, hexane and chloroform in
Soxlett apparatus. Yield is 88%. Calc. for C114H162N4S3,%: C,81.27; H,9.69; N,3.33; S,5.71.
1
Found: C, 80.96; H,9.48; N,2.85; S, 5. 41. H NMR (400 MHz, CDCI3, δ, ppm): 9.60ꢀ7.30
(m,16H), 3.43 (s,8H), 2.06ꢀ0.51(m,138H)
Synthesis of copolymer F2: F2 was prepared similarly as F1 using 8,12ꢀbis(5ꢀbromoꢀ4ꢀ
dodecylthiophenꢀ2ꢀyl)ꢀ2,5ꢀdi(nonadecanꢀ3ꢀyl)
[1,2,5]
thiadiazolo
[3,4ꢀi]bis
[1,3]
thiazolo[4,5ꢀa:5',4'ꢀc]phenazine compound ( M1) and 2,7ꢀ bis (trimethyltin)ꢀ4,5ꢀbis (undecyl)
benzo[2,1ꢀb;3,4ꢀb’]dithiophene with 79% yield. Calc. for C116H182N6S7, %:, C73.91; H,9.73;
1
N, 4.46; S,11.91; F, 20.91 Found: C, 73.49; H,9.58; N, 4.14; S,11.54. H NMR (400 MHz,
CDCI3, δ, ppm): 9.90ꢀ7.11 (m, 4H), 3.49ꢀ0.80 (m,178H).
Device fabrication and characterization:
The PSCs were fabricated with a structure of ITO/PEDOT:PSS/F1 or F2:PC71BM/Al
using a conventional solution processing method. The indium tin oxide (ITO) coated glass
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substrates were cleaned subsequently by ultrasonic treatment in detergent, deionized water,
acetone, and isopropyl alcohol for 20 min each and than dried in ambient condition. A layer
of PEDOT:PSS was spin cast (3000 rpm, 40 nm thick) onto ITO glass substrate and backed at
120° C for 10 min. The active layer was spin coated from the different weight ratios blends of
F1 or F2 and PC71BM in chloroform and solvent additive (different concentration of 1,8
diiodoctane (DIO/chloroform (total concentration of 20 mg/mL) at 1500 rpm for 30s on the
top of PEDOT:PSS layer and dried for 30 min in ambient condition. Finally, 60nm aluminum
(Al) layer was deposited on the top of active layer under high vacuum using a shadow mask
of 20 mm2. All the devices were fabricated and characterized under ambient atmosphere
without encapsulation. The holeꢀonly and electronꢀonly devices with ITO/PEODT:PSS/active
layer/Au and ITO/Al/active layer:PC71BM/Al architectures were also fabricated in an
analogous way, in order to measure the hole and electron mobility, respectively. The current–
voltage characteristics of the BHJ organic solar cells were measured using a computer
2
ꢀ
controlled Keithley 238 source meter under simulated AM1.5G, 100 mW/cm . A xenon
light source coupled with optical filter was used to give the stimulated irradiance at the
surface of the devices. The incident photon to current efficiency (IPCE) of the devices was
measured illuminating the device through the light source and monochromator and the
resulting current was measured using a Keithley electrometer under short circuit condition.
Results and discussion
Synthesis and characterization
The synthetic routes of the monomers M1 and M2 are illustrated in Scheme 1. To
obtain M1 monomer a new αꢀdiketone 8 was synthesized in seven reaction steps via
following route, comprising: synthesis of αꢀethylstearic acid 2 by treatment of compound 1
with LDA, tetramethylenediamine (TMEDA) and ethyl iodide; interaction of αꢀethylstearic
acid with carbonyldimidazole (CDI) and ammonia aqueous solution with subsequent
generation of αꢀethylstearic acid amide 3; following by Lawesson's reagent treatment with
subsequent obtaining of thioamideꢀethylstearic acid 4. 2ꢀ(1ꢀethylheptadecyl)ꢀ1,3ꢀthiazole 5
was synthesized by compound 4 bromehtylacetal treatment under hydrochloric acid catalysis.
Alkylthiazole 5 was then converted into the corresponding bromothiazole 6 by sequential
treatment with nꢀBuLi and CBr4 at reduced temperature. After ionization of derivative 6 with
LDA at ꢀ78 °C, the migration of halogen to a position adjacent to the nitrogen atom has been
occurred. The resulting organolithium intermediate undergoes oxidative dimerization after
addition of an equimolar quantity of copper chloride (II) to form a bithiazolo derivative 7
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with
91%
yield,
which
was
converted
to
the
desired
2,7ꢀbis(1ꢀ
ethylheptadecyl)[1,3]thiazolo[4,5ꢀg][1,3]benzothiazolꢀ4,5ꢀdione (8) with 37% yield in oneꢀ
pot reaction by sequential treatment with nꢀBuLi and diethyl oxalate. Further to obtain dinitro
compound 13 the Grignard reagent was prepared from dodecyl bromide and then converted
into compound 11 by alkyl chain introduction into 3ꢀbromthiophene via Kumada coupling
with
following
stannylation.
4,7ꢀbis(4ꢀdodecylthiopheneꢀ2ꢀyl)ꢀ5,6ꢀ
dinitrobenzo[c][1,2,5]thiadiazole 13 was prepared by compound 11 treatment with equimolar
quantity of dinitro derivative 12 under Stille reaction. Diamine 14 was synthesized by
reduction of dinitro compound 13 in acetic acid with Fe, which in reaction with αꢀdiketone 8
was subsequently converted into 8,12ꢀbis(4,5ꢀdidodecylthiopheneꢀ2ꢀyl)ꢀ2,5ꢀdi(nonadecaneꢀ3ꢀ
yl)[1,2,5]thiadiazolo[3,4ꢀi]bis[1,3]thiazolo[4,5ꢀa:5`,4`ꢀc]phenazine15.
Targeted
M1
monomer was obtained by compound 15 treatment with bromosuccinimide with 78% yield.
Monomer M2 was synthesized from a wellꢀknown amine 16 [22b,c] and αꢀdiketone ꢀ 1,2ꢀbis
(9',9'ꢀdidodecylꢀ9'Hꢀfluorenꢀ2'ꢀyl)ethaneꢀ1,2ꢀdione (17) [23] according to Scheme 1. The
reaction was conducted in acetic acid under reflux conditions. Monomer M-2 was obtained
after purification by column chromatography with a yield of over 80%.
The structure of all intermediates and final (M1 and M2) compounds were established
1
1
by H and 13C NMR spectroscopy, and elemental analysis. In the aromatic part of the H
spectrum of the compound 15, there are two singlet signals of equal intensity at δ 9.26 and
7.40 ppm, related to protons of the thiophene fragments. The aliphatic part of the spectrum
contains signals of the alkyl substituent at the thiazole and thiophene fragments, the ratio of
the integrated intensity values of the individual signals confirms the proposed structure of 15.
After treatment of intermediate 15 with two equivalents of Nꢀbromosuccinimide and the
formation of the monomer M1 in the aromatic part of the spectrum only one singlet at δ 9.02
ppm was observed, indicating that the bromination reaction was complete. The aliphatic part
of the spectrum of the monomer M1 also contains signals of alkyl substituent, and integrated
intensity values consistent with the proposed structure. The 13C NMR spectrum of the
monomer M1 has 11 signals in the aromatic region, and most downfield signal at δ 176.14
ppm which corresponds to a carbon atom of SꢀCꢀN moiety of the thiazole cycle (Figure S12,
supplementary information).
1
The H NMR spectrum of the monomer M2 in the downfield region at δ 7.89ꢀ7.81;
7.73ꢀ7.65; 7.37ꢀ7.30 ppm there are three multiplets belonging to fourteen different aromatic
hydrogens; in the ranges of δ 1.94 – 1.80 ppm there are signals characteristic for the CH2ꢀ
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groups directly connected to the fluorene ring; signals belonging to the terminal CH3 groups
of alkyl chains are located at δ 0.91 ppm. Signals corresponding to the remaining hydrogen
atoms of the alkyl chains are detected in the ranges of δ 1.85ꢀ1.20 and 0.87ꢀ0.85 ppm.
Although the proton spectrum of M2 is complex but the ratio of the integrated intensity
between aromatic and aliphatic part of the spectrum is consistent with the proposed structure.
In the 13C spectrum of M2 there are 16 signals in the downfield region belonging to 16
different aromatic carbon atoms, and at δ 55.3 and 14.3 ppm there are signals which are
characteristic for cyclopentane fluorene moiety and the terminal CH3ꢀgroups of the alkyl
chains, respectively. In the interval of δ 40.4ꢀ22.8 ppm signals relating to the other aliphatic
carbon atoms were detected, which confirms the proposed structure 9 (Figure S13,
supplementary information).
The copolymers F1 and F2 were prepared via Pdꢀmediated Stille coupling reaction
between bis(trimethylthin)ꢀ4,5ꢀbis(undecyl)benzo[2,1ꢀb;3,4ꢀb’]dithiophene monomer and
brominated monomers (M1 or M2) with yields ranging from 88% to 79% (Scheme 2).
After polymerization, crude polymers were washed using Soxhlet extraction with methanol,
hexane and chloroform in sequence. The title polymers were obtained by reprecipitation of
their concentrated chloroform solutions from methanol. Both F1 and F2 can be readily
dissolved in common solvents such as chloroform, chlorobenzene, and tetrahydrofuran,
which is attributed to the incorporation of solubilizing alkyl chains adjoined to thiophene
flank of polymer backbone. The structures of the obtained copolymers were confirmed by
elemental analysis, 1H NMR and UVꢀvisꢀNIR spectroscopy. 1H NMR spectrum of F1 and F2
is shown in Figure S14 (supplementary information). The peaks at 7.30ꢀ9.60 ppm are
assigned to the proton response of aromatic units. There is a peak at δ 3.40ꢀ3.50 ppm which is
attributed to methylene group. The peaks in the ranges of δ1.90ꢀ0.77 ppm arise from alkyl
substituent. All of the integral ratios of peak areas between the aromatic and aliphatic signals
agree with corresponding molecular structure of the polymers.
The molecular weight and polydispersity index (PDI) of the polymers were measured
by gel permeation chromatography (GPC) using polystyrene as the standard and THF as the
eluent. The GPC results indicated that these copolymers have the numberꢀaverage molecular
weight (Mn) of 14.2 and 8.5kDa with a narrow PDI of 1.73 and 1.63 for F1 and F2,
respectively. F2 showed a low Mn, probably due to the steric hindrance resulted by the too
rigid planarity structure of acceptor units. The thermal properties of polymers were
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characterized by differential scanning calorimetry (DSC) and thermogravimetric analyses
(TGA) (Figure S15, supplementary information). DSC did not show obvious glass transition
for F1 and F2 under test conditions, indicating the amorphous nature of these copolymers.
Decomposition temperatures (Td) (5% weight loss) were 334° C and 377° C for F1 and F2,
respectively. This indicated that the thermal stability of the copolymers is enough for PSCs
applications.
Optical and electrochemical properties
The UVꢀvisible optical absorption spectra of copolymers F1 and F2 in dilute
chloroform solution and thin film are displayed in Figure 1 and optical parameters are
summarized in table 1. As shown in Figure 1, both F1 and F2 copolymers show two
characteristics bands. The higher energy band in the wavelength range of 350ꢀ600 nm arises
from the localized πꢀπ transition and relatively broad and weak absorption in 600 ꢀ1300 nm
can be attributed to the intramolecular charge transfer between the donor and acceptor units.
The ICT absorption band of F2 is significantly redshift relative to F1 may be attributed to
improved backbone coplanarity leading to enhanced intermolecular charge transfer in
solution of F2 after introducing fused thiazole rings into F2. Compared to F1, F2 with
conjugated fused thiazole could further increase spectrum absorption at longer wavelengths.
This phenomenon reveals the larger conjugated fused acceptor units could reduce the band
gap of copolymers. In thin film, the absorption band corresponds to a lower energy band that
is significantly redshifted and broadened as compared to the absorption band in solution, that
is attributed to the strong intermolecular interactions and close packing of the polymer
backbone, in the solid state. The optical bandgap of F1 and F2 is 1.24 and 1.08 eV,
respectively, determined by the absorption onset in thin film. The fused heteroꢀaromatic units
in backbone of F2 show more broad absorption and narrower band gap than F1. These results
indicate that the optical bandgap of copolymers depends on their electronꢀ withdrawing
ability of the polymers backbone. These results demonstrate that the strong acceptor TDQ is
favorable to develop narrow band gap copolymers, and different combination between donor
and TDQ core allow for efficient tuning of the optical properties. Compare with other
reported TDQꢀbased polymers F1 and F2 presented narrower band gap than values reported
in the literature on TDQ containing alternating polymers with thiophene [5, 24], fluorine
[24b, 25] carbazole [24b, 26]. The TDQ based polymers all show an extended absorption
towards nearꢀinfrared region (beyond 1100nm). However, both polymers show a weak
absorption in the 599ꢀ700 nm region of the active solar spectrum, which may result in the low
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photon harvesting in PSCs. The use of organic photovoltaic material with absorption in NIR
spectral region and even beyond 1000 nm, could enhance the PCE of organic BHJ solar cells.
Cyclic voltammetry (CV) was employed to investigate the electrochemical properties
and determine the HOMO and LUMO energy levels. The experimentally measured cyclic
voltammograms are shown in Figure 2 and the results are summarized in table 1. The HOMO
and LUMO energy levels of the copolymers were estimated according to the expressions:
oxd
onset
HOMO = ꢀ(
E
ꢀ
EF1/c2/ Fc+ + 4.8) (eV),
ꢀ
EF1/c2/ Fc+ + 4.8) (eV)
onset
red
LUMO = ꢀ(
E
Egec = HOMOꢀLUMO (eV)
oxd
potential of ferrocene EF1/c2/ Fc+ = 0.40 eV) vs. SCE is used as an internal standard. The
E
onset
was observed to be 0.96 and 1.00 V for F1 and F2, respectively. Accordingly, the HOMO
energy levels were calculated to be −5.36 and −5.40 eV for F1 and F2, which are in good
agreement with the ideal HOMO energy levels to ensure good air stability and high open
circuit voltage (Voc) in PSCs [27]. Deep HOMO energy levels of copolymers were desired for
achieving a higher Voc in the BHJ PSCs and made these copolymers promising candidates for
use as polymer donor materials. The HOMO level of F1 (ꢀ5.34 eV) was found to be 0.04 eV
red
higher than that of F2 (ꢀ5.40 eV). The
E
was observed to be ꢀ0.56 and ꢀ0.44 V for F1 and
onset
F2, respectively. The LUMO levels were thus calculated to be _3.84 eV, and _3.96 eV for F1
and F2, respectively, which are higher than that of PC61BM (−4.2 eV) to guarantee an
energetically favorable electron transfer. F2 has the lower lying LUMO level due to the
stronger electron deficiency of benzodithiazolo substitute. These results suggested that the
adjustment of the substitution positions and architectures has a weak influence on HOMO
and LUMO of the DꢀA copolymers. This result shows that extension of πꢀconjugation of
TDQ is a viable strategy to obtain stronger acceptors with lower band gap of the polymers
thus possessing deep LUMO and HOMO levels. The electrochemical band gaps ( Egelc ) are
1.52 and 1.44 eV for F1 and F2, respectively. The difference between the optically and
electrochemically measured energy gaps can be explained by the exciton binding energy of
the conjugated copolymers [28] and electron and hole remain electrostatically bound to one
another on the excited state [29]. The introduction of two donor units in F2 leads to lowering
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of its LUMO level relative to F1 and as a consequence the LUMO off set (ꢀELL) between F2
and PC71BM is only 0.24 eV and significantly less than the ~0.3 eV threshold for charge
separation. This problem often occurs in conjugated polymers with very low optical bandgap
[30]. Whereas, copolymer F1 with DꢀA structure exhibits a less deep LUMO energy level to
give ꢀELL about 0.36 eV which is significantly higher than the threshold value for charge
separation. Recently, it was reported that the charge separation also occurs when LUMO
offset is below 0.3 eV [31] indicating that the F2 can also potentially be used as an electron
donor for polymer solar cells.
Theoretical Calculations
We have additionally performed a theoretical study on the F1 and F2 molecular
structures within the framework of density functional theory (DFT) and timeꢀdependent
density functional theory (TDꢀDFT). The initial geometry optimization calculations were
performed employing the gradient corrected functional PBE [32] of Perdew, Burke and
Ernzerhof. The defꢀSVP basis set [33] was used for all of the calculations. At this stage of the
calculations, to increase the computational efficiency (without loss in accuracy), the
resolution of the identity method [34] was used for the treatment of the twoꢀelectron
integrals. Subsequent geometry optimization were further performed using the hybrid
exchange–correlation functional B3LYP [35] as well as Truhlar’s metaꢀhybrid exchange–
correlation functional M06 [36], and the same basis set. Tight convergence criteria were
placed for the SCF energy (up to 10–7 Eh) and the oneꢀelectron density (rms of the density
matrix up to 10–8) as well as for the norm of the Cartesian gradient (residual forces both
average and maximum smaller than 1.5x10–5 a.u.) and residual displacements (both average
and maximum smaller than 6x10–5 a.u.). Solvent effects were included for chloroform (CF)
using the integral equation formalism variant of the Polarizable Continuum Model
(IEFPCM), as implemented in the Gaussian package [37]. TDꢀDFT excited state calculations
were performed to calculate the optical gaps of F1 and F2 using the same functionals and
basis set on the corresponding ground state structures. The UV/Vis spectra were calculated
using the B3LYP and M06 functionals. The first round of geometry optimization was
performed using the Turbomole package [38]. All of the follow up calculations were
performed using the Gaussian package [37].The first round of calculations was the geometry
optimizations of the F1 and F2 structures. To increase the computational efficiency the alkyl
groups were truncated to ethyl groups. Vibrational analysis on all of the optimized structures
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did not reveal any vibrational modes with imaginary eigenfrequencies, i.e. the final optimized
structures are true local (if not global) minima.
In the F1 structure the TDQ and benzodithiophene (BDT) moieties (along which the
polymeric chains grow) form a planar configuration with small dihedral angles (up to 6°).
The fluorine (FL) moieties form dihedral angles with TDQ in the range of of 37° – 44°
functional (depending on the functional used and the presence of solvent). For the F2
structure the thiadiazoloꢀdithiazoloꢀphenazine (TDTAP) forms small dihedral angles (up to
5°) with the linking thiophenes. Steric effects lead to moderate dihedral angles between
TDTAP and BDT, up to 27°. We have calculated the HOMO and LUMO energy levels and
the optical gaps, defined here as the energetically lowest allowed vertical electronic
excitation, employing the PBE, M06, and B3LYP functionals. In Table 2, in addition to the
frontier orbitals’ energy levels, we also provide the optical gap the main contributions to the
first excitation as well as the wavelength of the first excitation and of the excitations with the
largest oscillator strengths.
In addition to the B3LYP functional we have also performed our calculations
employing the M06 functional. The M06 metaꢀhybrid functional was chosen since it provides
leveled performance over transition types [39, 40]. We provide results using all three
functionals, which can additionally be used for comparison with the literature. The HOMO–
LUMO (HL) gap of each structure calculated using the hybrid B3LYP functional is slightly
smaller than that using the metaꢀhybrid M06 functional (by ~0.3 eV), however, the calculated
optical gaps are only marginally smaller. All of the calculated optical gaps are low (less than
2 eV), with the lowest one corresponding to the F2 structure, which also exhibits the larger
oscillator strengths. In Table 2, we also provide the character of the first allowed excitations
only for contributions larger than 4%. The first excitation, as calculated by each of the
functional for all three structures, clearly exhibits a singleꢀconfiguration character.
In Figure 3, we have plotted the isosurfaces (isovalue=0.02) of the HOMO and
LUMO for both of the structures. For both structures the HOMO is extended over the main
body linked groups that form the polymeric chain. In the case of F1 there is minimum
delocalization over the FL moieties. In the case of F2 there is minimum delocalization over
the benzo(bis)thiazole moiety of TDTAP. The LUMO of each structure displays a higher
degree of localization; for F1 the LUMO extends over TDQ and for F2 over TDTAP.
Detailed contributions of transitions to electronic excitations are given in the Supporting
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Information for all structures. To quantify the contributions of the moieties to the frontier
orbitals we have calculated the total and partial density of states (PDOS). The PDOSs for F1
and F2 are shown in Figure S16 (Supplementary information). We partition the F1 structure
into TDQ, BDT, FL, and alkyl moieties, and the F2 structure to TDTAP and BDT, linking
thiophenes and alkyl moieties. The two structures have similar contributions to the HOMO
from the respective characteristic moieties, specifically, for F1 the contribution from the
TDQ moiety is 34.7% and for F2 the contribution from TDTAP 35.2%. However, the
contribution of BDT to the HOMO at 61.4% is much higher compared to the case of F2
which is at 18.4%. For F2 the linking thiophenes contribute significantly to HOMO, at
45.0%. The LUMO of both structures has dominant contributions from the characteristic
moieties, specifically for F1 the TDQ contributes 83.3% and for F2 the TDTAP moiety
contributes 81.0%. Secondary contribution to LUMO for F1 is from the BDT moiety at
10.5% and for F2 from the thiophene moieties at 15.6%. These are in agreement with our
earlier observations on the orbital delocalizations.
In Figure 4, we show the UV/visible absorption spectra of the F1 and F2 structures
calculated at the TDꢀDFT/M06 level of theory, both accounting for solvent effects for CF and
in gas phase. The spectra have been produced by convoluting Gaussian functions with
HWHM = 0.22 eV centered at the excitation wavenumbers. In Figure S17 (see Supporting
Information) we also provide the corresponding spectra calculated using the B3LYP
functional, which only slightly overestimates (by ~15 nm in the small wavelength region and
by ~50 nm in the large wavelength region) the wavelengths and retains all the main
characteristics with that using M06 and the experimental spectra. The absorption spectra of
F1 and F2 exhibit two and three high absorbance bands, respectively. For F1, the two bands
are centered at 660 nm and 300 nm, with low intensity absorption peaks intermediate of the
two main bands. For F2, the three bands are located at 1015 nm and at the low wavelength
region at ~420 nm and ~320 nm. In the region intermediate of the low and high wavelengths
(500ꢀ 650 nm) there is an obvious lack of absorbance even of moderate intensity.
Photovoltaic properties of F1 and F2
In order to balance the absorbance and charge transporting network of photoactive
layer, the weight ratio of copolymer and PC71BM were varied from 1:0.5 to 1:2.5 and the 1:2
ratio showed the best photovoltaic performance for both F1 and F2 based photovoltaic
devices. Further increase of PC71BM content in the active layer yielded a negative effect on
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both Jsc and FF, which resulted in a decrease in overall PCE. Initially, we used chloroform as
solvent of solution casting of active layer. The current –voltage characteristics under
illumination and incident photon to current conversion efficiency (IPCE) spectra of solar cells
based on optimized F1:PC71BM and F2:PC71BM with weight ratio 1:2 are presented in
Figure 5a and the corresponding photovoltaic parameters are complied in Table 3. The
polymer solar cell based on F1:PC71BM showed overall PCE of 2.71 % with Jsc = 8.36
mA/cm2, Voc= 0.60 V and FF = 0.54. Conversely the device based on optimized F2:PC71BM
acquires relatively low PCE of 1.68% with a Jsc of 5.16 mA/cm2, Voc of 0.64 V and a FF of
0.51. Although both the copolymers posses deeper HOMO energy levels, but the LUMO
energy levels that are close to PC71BM may lead to voltage loss in Voc [41], resulting in low
values of Voc. The lower value of PCE for the device based F2:PC71BM might be mainly due
to the low Jsc attributed to the insufficient exciton dissociation efficiency as evidenced by the
low value of ꢀELL. The device based on F2:PC71BM showed a Voc of 0.64 V attributed to the
relatively deeper HOMO energy level of F2 as compared to F1. The photon energy loss Eloss
= Egoptꢀ qVoc equals to 0.44 eV, is below the threshold value of 0.6 eV for efficient charge
transport [42] and resulting low Jsc. On the other hand, the device based on F1:PC71BM
showed higher PCE with Jsc = 8.36 mA/cm2, but lower Voc (0.60 V) as a consequence of
relatively higher lying HOMO energy level and the photon energy loss to 0.64 eV, which is
higher than the threshold value. The difference in the Jsc is also reflected in the IPCE spectra
(Figure 5b) of the devices. Both devices show a broad IPCE response from 350 to 1050 nm
and 1200 nm for F1 and F2, respectively, covering the absorption spectra of PC71BM and
copolymer. The IPCE spectra of these devices closely resembles with the absorption spectra
of active layers of the corresponding active layers (Figure 6), indicating that both PC71BM
and copolymers contributes to the photocurrent generation. As can be seen from Figure 5b,
the F2 copolymer shows a relatively low IPCE values (maximum 20.4 %) as compared to F1
(maximum 26 %). The Jsc value estimated from the integration of IPCE spectra for the
devices based on F1:PC71BM and F2:PC71BM are 8.25 mA/cm2 and 5.07 mA/cm2,
respectively, and are consistent with the values observed in the JꢀV characteristics under
illumination. These results indicate that the exciton dissociation is more efficient in the
F1:PC71BM as compared to F2:PC71BM, in spite of the IPCE spectra of the later being
broader as compared to the former.
Here we have tried to improve the PCE of the devices by employing a solvent additive
method to prepare the active layer [43]. 1,8ꢀdiiodooctane (DIO) was added as a processing
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additive to explore its effect on the overall PCE of the devices. For the solvent additive
treatment, we have varied the concentration of DIO from 0.5 to 4.5 % and found that 4% DIO
in the host CF solution provides the best photovoltaic performance. The JꢀV characteristics
under illumination and corresponding IPCE spectra of the devices are shown in Figure 5a and
5b, respectively. The overall PCE of the devices based on F1:PC71BM and F2:PC71BM
enhanced up to 5.79 % ( Jsc = 14.71 mA/cm2, Voc= 0.58 V and FF=0.68) and 3.32 % (
Jsc=8.92 mA/cm2, Voc =0.60 V and FF=0.62), respectively. The increase in PCE is associated
with enhancement in Jsc and FF with a slight decline of Voc. The increase in Jsc and FF is
mainly because DIO possesses a high boiling point and is able to solvate PC71BM, leading to
significant effect on the morphology for facilitating charge carrier transportation [44].
However, the slight loss in Voc can be attributed to the lowering of charge separated and
charge transfer state energies upon incorporation of DIO [45]. The DIO additive significantly
improved the performance of the BHJ polymer solar cells, by reducing the nanoꢀsize and
increasing the DꢀA interfacial area within the active layer [46] and help to enhance the Jsc and
FF. The values of IPCE also increased significantly for the devices based on DIO/CF cast
active layer (Figure 5b). The Jsc value estimated from the integration of IPCE spectra of the
devices based on F1 and F2 is 14.62 mA/cm2 and 8.76 mA/cm2, respectively.
The PCE of the PSC depends upon the light harvesting ability of the device which is
directly related with the absorption molar coefficient and response. In order to get
information about the effect of the solvent additive on absorption properties, the absorption
spectra of the active layer processed with and without were measured and shown in Figure 6.
As shown in Figure 6, in comparison to the spectra of active layer films cast from CF, the
spectra of the films cast from DIO/CF, the absorption peak corresponds to the ICT band of
copolymers are redshifted and also the absorption intensity was enhanced. These effects are
related to the enhanced πꢀπ stacking. The increase in the absorption of active layer produces
more excitons, thereby resulting higher Jsc.
In order to get information about the improved PCE of the PSCs, the morphology of
the active layer was investigated using transmission electron microscopy (TEM). As shown
in Figure 7, the copolymer:PC71BM blended films without SA shows bigger domains. This
morphology may be the factor for insufficient exciton diffusion and dissociation and
associated with the low Jsc values. The morphology of the active layer spin cast with SA was
different. The large crystallites and phase separated domains reduced, resulting in favorable
exciton diffusion and dissociation and charge transport. Among all the films, F1:PC71BM cast
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with SA showed much finer and denser texture and nanophase separation between the
polymer and PC71BM, which could result in high PCE with high Jsc value [47].
Besides the absorption, energy levels, the charge carrier transport ability within the
active layer is also a critical factor for the resulting photovoltaic performance of PSC,
especially FF. We have measured the hole mobility in the active layers processed with and
without DIO, by space charge limited current (SCLC) method [48] using hole only devices.
The SCLC could be estimated using the MottꢀGurney expression: J= (9/8) εoεrꢁ(V2/d3),
where J is current density, εr is the dielectric constant of active layer, εo is the free space
permittivity (8.85x10ꢀ12 F/m), d is the thickness of the active layer (90 nm), V= Vapplꢀ Vbi, V
is effective voltage, Vappl is the applied voltage, Vbi is the builtꢀin voltage that arises from the
work function difference between anode and cathode. The JꢀV characteristics of hole only
devices (ITO/PEDOT:PSS/active layer/Au) for optimized F1:PC71BM and F2:PC71BM active
layers processed with and without SA are shown in Figure 8. The hole mobility estimated
from Figure 8 and fitting with the MottꢀGurney equations are 3.67x10ꢀ5cm2/Vs and 1.86 x10ꢀ5
cm2/Vs for F1:PC71BM and F2:PC71BM, respectively, spin cast from CF only. When the
active layer was deposited using SA, the hole mobility increased to 8.76 x10ꢀ5 cm2/Vs and
6.75 x10ꢀ5 cm2/Vs for F1:PC71BM and F2:PC71BM, respectively. The electron mobilities
were also estimated using the electron only devices (ITO/Al/active layer/Al) for optimized
F1:PC71BM and F2:PC71BM active layers processed with and without SA using SCLC JꢀV
characteristics in dark (as shown in Figure 9 for the devices based on F1:PC71BM only) and
are 2.34 x10ꢀ4 cm2/Vs, 2.42 x10ꢀ4 cm2/Vs, 2.44x10ꢀ4 cm2/Vs and 2.48x10ꢀ4 cm2/Vs for
F1:PC71BM (as cast), F1:PC71BM (SA), F2:PC71BM (as cast) and F2:PC71BM (SA),
respectively. The higher hole mobility and slight change in electron mobility results more
balanced charge transport which is beneficial for the improvement in FF [49].
Conclusion
In summary, two DꢀA conjugated copolymers F1 and F2 with an ultra low bandgap
1.24 eV and 1.08 eV, were designed and applied as donor along with PC71BM as electron
acceptor for solution processed polymer solar cells. After the optimization of the active layer,
the F1:PC71BM and F2:PC71BM processed with 4% DIO/CF (SA) solution showed over all
PCE of 5.80 % and 3.32 %. The device based on F2 showed relatively low PCE as compared
to F1, in spite of the low optical bandgap and broader absorption profile relative to F1, that is
mainly attributed to insufficient driving force for exciton dissociation due to low value of the
LUMO offset between F2 and PC71BM, and also from low photon energy loss is below the
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threshold value of 0.6 eV for efficient charge transport. Since the HOMO energy level of
both copolymers is quite deep, there is an energy barrier with PEDOT:PSS for hole
extraction, which limits the Jsc. This can be overcome by employing a hole transport layer
with high work function that matches with the HOMO energy level of copolymer. The work
in under progress and will be reported when completed. The above investigation also
demonstrates that these copolymers have the potential for ternary BHJ and tandem solar cells
to achieve high PCE. The research work in this direction is under progress.
Acknowledgements
M.L.K, S.A.K., N.A.R, A.Y.N and G.D.S are thankful to the Russian Science Foundation
(grant number 14ꢀ13ꢀ01444) for financial assistance.
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Table 1 Optical and electrochemical properties of F1 and F2
a
b
ox
onset
red
onset
Egopt (eV)
E
E
Egelec (eV)
Copolymer
EHOMO ELUMO
(eV) (eV)
λmax
λmax
(V)
(V)
406,
844
418, 956 1.24
0.96
ꢀ0.56
ꢀ5.36 ꢀ3.84 1.52
ꢀ5.40 ꢀ3.96 1.44
F1
F2
426,
964
430,
1.08
1.0
ꢀ0.44
1026
ain chloroform solution, bthin film cast from chloroform, EHOMO/ELUMO = ꢀ(
E
red +4.40)eV
ox
onset
/
Egopt = 1240/λonset(film)
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Table 2 Calculated properties of F1 and F2. Specifically HOMO and LUMO energies (eV),
HOMO–LUMO gap (eV), HL, Optical gap (eV), OG, with corresponding oscillator strengths,
f
, the wavelengths of the first excitation and excitations with the largest oscillator strengths,
the main contributions to the first excited state, and the dipole moment (D),
µ
.
HOM
O
(eV)
λ1st/max
(nm)
LUM HL OG
O (eV) (eV) (eV)
ꢁ
(D)
f
Main Contributions
F1
–4.75 –3.67 1.08 1.46
–4.90a –3.79 a 1.11 a 1.41 a
846
0.20
3.21
H→L (93%), H–2→L
(5%)
PBE
882 a
0.04
4.13 a
H–1→L (84%), H→L
(15%)a
a
–5.33 –3.23 2.10 1.82 681/419/366/327/316/295 0.25
2.79
H→L (99%)
B3L
YP
–5.48 a –3.34 a 2.14 a 1.80 a 687/432/361/329/319/295 a 0.32
3.45 a
H→L (99%)a
a
–5.60 –3.17 2.44 1.89 658/398/350/322/307/292 0.24
3.00
H→L (99%)
H→L (99%)a
M06
PBE
–5.77 a –3.28 a 2.49 a 1.88 a 660/508/404/321/306/296 a 0.31
3.83 a
a
F2
–4.28 –3.75 0.53 0.92
–4.47 a –3.92 a 0.55 a 0.89 a
1353
1392 a
0.23
3.37
4.87 a
H→L (100%)
H→L (100%)a
0.33
a
–4.80 –3.42 1.38 1.19 1043/435/400/350/337/320 0.33
2.95
H→L (100%)
H→L (100%)a
B3L
YP
–4.99 a –3.57 a 1.42 a 1.17 a 1063/437/407/352/327/320a 0.45
3.89 a
a
–5.07 –3.38 1.69 1.24 1000/419/388/335/320/308 0.34
3.23
4.44 a
H→L (100%)
H→L (100%)a
M06
–5.27 a –3.54 a 1.73 a 1.22 a 1015/420/344/336/319/309a 0.46
a
a Values when solvent effects are taken into account for chloroform.
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Table 3 Photovoltaic parameters of BHJ organic solar cells based on F1 and F2 as donor and
PC71BM as acceptors processed with CF and DIO/CF
Active layer
Jsc (mA/cm2)
Voc (V) FF
PCE (%)
PCE (%)
(average)c
F1:PC71BMa
F2:PC71BMa
F1:PC71BMb
F2:PC71BMb
8.36
5.16
14.71
8.92
0.60
0.64
0.58
0.60
0.54
2.71
1.68
5.80
3.32
2.65
1.60
5.74
3.24
0.51
0.68
0.62
acast from CF
bcast from DIO/CF
caverage of 10 devices
25

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Scheme 1. Synthetic route for M1 and M2 monomers
26

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Scheme 2. Stille coupling synthesis of F1 and F2 copolymers
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Figure 1 Optical absorption spectra of F1 and F2 in chloroform solution and thin film cast
from chloroform
28

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Figure 2 Cyclic voltammograms of the copolymers F1 and F2 films cast on platinum
electrode in 0.1 mol/L Bu4NOCI4/ CH3CN at scan rate 50 mVsꢀ1
Figure 3 Frontier orbitals of F1 and F2 copolymers.
29