Synthesis and electro-optical properties of fluorene-based copolymer for organic photovoltaic device application

Article abstract of DOI:10.1080/15421406.2011.564076

The following soluble π-conjugated polymer was synthesized by palladium-catalyzed Suzuki polymerization: poly[(9,9-dioctylfluorene)-alt-(3- hexylthiophen-5-yl)-(4'-hexylthiazol-5'-yl)] poly(FO-HThHTha) derived from poly(9,9-dioctylfluorene) (PFO) and poly[(3-hexylthiophen-5-yl)-2,2'-(4'- hexylthiazol-5'-yl)] poly(HThHTa). The polymer, poly(FO-HThHTa) was soluble in common organic solvents and easily spin coated onto indium-tin oxide (ITO) coated glass substrates. Bulk heterojunction photovoltaic cells with an ITO/PEDOT/poly(FO-HThHTa):PC₇₀BM/TiO_x/Al configuration were fabricated and the photovoltaic cell using poly(FO-HThHTa):PC ₇₀BM (1:4) showed the best photovoltaic performance compared with those using poly(FO-HThHTa):PC₇₀BM (1:2 and 1:6). The optimal bulk heterojunction photovoltaic cell exhibits a power conversion efficiency (PCE) of 0.39% (V_oc=0.44 V, J_sc=2.57, FF = 0.34) with poly(FO-HThHTa):PC₇₀BM (1:4) measured by using an AM 1.5G irradiation (100mW/cm²) on an Oriel Xenon solar simulator (Oriel 300W). Copyright Taylor & Francis Group, LLC.

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Synthesis and Electro-Optical Properties
of Fluorene-Based Copolymer for Organic
Photovoltaic Device Application
Jang Soo Kim a , Myungkwan Song a , Jin Su Park a , Mikyoung Yoon a ,
Hoe Won Yoon a , Ae Jin Kim a , Jae Wook Lee b , Yeong-Soon Gal c &
Sung-Ho Jin a
a Department of Chemistry Education , Interdisciplinary Program of
Advanced Information and Display Materials, and Center for Plastic
Information System, Pusan National University , Busan, Korea
b Department of Chemistry , Dong-A University , Busan, Korea
c Polymer Chemistry Lab., Kyungil University , Hayang, Korea
Published online: 16 May 2011.
To cite this article: Jang Soo Kim , Myungkwan Song , Jin Su Park , Mikyoung Yoon , Hoe Won Yoon ,
Ae Jin Kim , Jae Wook Lee , Yeong-Soon Gal & Sung-Ho Jin (2011) Synthesis and Electro-Optical
Properties of Fluorene-Based Copolymer for Organic Photovoltaic Device Application, Molecular
Crystals and Liquid Crystals, 538:1, 223-231, DOI: 10.1080/15421406.2011.564076
To link to this article: http://dx.doi.org/10.1080/15421406.2011.564076
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Mol. Cryst. Liq. Cryst., Vol. 538: pp. 223–231, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.564076
Synthesis and Electro-Optical Properties of
Fluorene-Based Copolymer for Organic
Photovoltaic Device Application
JANG SOO KIM,1 MYUNGKWAN SONG,1 JIN SU
PARK,1 MIKYOUNG YOON,1 HOE WON YOON,1
AE JIN KIM,1 JAE WOOK LEE,2 YEONG-SOON GAL,3
AND SUNG-HO JIN1
1Department of Chemistry Education, Interdisciplinary Program of
Advanced Information and Display Materials, and Center for Plastic
Information System, Pusan National University, Busan, Korea
2Department of Chemistry, Dong-A University, Busan, Korea
3Polymer Chemistry Lab., Kyungil University, Hayang, Korea
The following soluble p-conjugated polymer was synthesized by palladium-catalyzed
Suzuki polymerization: poly[(9,9-dioctylfluorene)-alt-(3-hexylthiophen-5-yl)-
(40-hexylthiazol-50-yl)] poly(FO-HThHTha) derived from poly(9,9-dioctylfluor-
ene) (PFO) and poly[(3-hexylthiophen-5-yl)-2,20-(40-hexylthiazol-50-yl)] poly
(HThHTa). The polymer, poly(FO-HThHTa) was soluble in common organic
solvents and easily spin coated onto indium-tin oxide (ITO) coated glass substrates.
Bulk heterojunction photovoltaic cells with an ITO=PEDOT=poly(FO-HThHTa):
PC70BM=TiOx=Al configuration were fabricated and the photovoltaic cell using
poly(FO-HThHTa):PC70BM (1:4) showed the best photovoltaic performance
compared with those using poly(FO-HThHTa):PC70BM (1:2 and 1:6). The opti-
mal bulk heterojunction photovoltaic cell exhibits a power conversion efficiency
(PCE) of 0.39% (Voc ¼ 0.44 V, Jsc ¼ 2.57, FF ¼ 0.34) with poly(FO-HThHTa):
PC70BM (1:4) measured by using an AM 1.5 G irradiation (100 mW=cm2) on an
Oriel Xenon solar simulator (Oriel 300 W).
Keywords Bulk heterojunction; low-band gap polymer; organic photovoltaics
Introduction
Since the initial discovery of p-conjugated polymers in the late 1970s, diverse
applications of these materials have emerged owing to their remarkable electronic
and photonic properties [1]. There are many reasons for the interest in organic
photovoltaics (OPVs) when comparing the technology to the silicon-based
Address correspondence to Sung-Ho Jin, Department of Chemistry Education, Interdis-
ciplinary Program of Advanced Information and Display Materials, and Center for Plastic
Information System, Pusan National University, Busan 609-735, Korea. E-mail: shjin@
pusan.ac.kr
223

224
J. S. Kim et al.
photovoltaics (PVs) and interestingly the two very different technologies are comp-
lementary in many ways. The OPVs offer low cost, solution processing, flexible
substrates, and a very high speed of processing. In the late 1990s, when the OPVs
were in their infancy they showed low power conversion efficiency (PCE) and short
lifetime [2]. In recent years, this has changed dramatically and both the PCE and
lifetime of OPVs have improved significantly with efficiencies of around 6% and
estimated operational lifetimes of 20,000 h [3]. During the last decade several
research groups have reported the synthesis and use of materials which absorb
light with wavelengths above 600 nm in OPVs. Among the different concepts which
have been proposed in OPVs, the bulk heterojuction approach is an attractive and
significant progress is being made in improving the PCE of OPVs in recent times.
One of the possible ways to improve the PCE of photovoltaic cells is the
implementation of new materials absorbing the part of the solar spectrum. So it
is important to design and synthesis of low-band gap polymers, which improved
overlap of the polymer absorption spectrum with the standard solar spectrum
under AM 1.5 global [4]. Low-band gap polymers are loosely defined as a polymer
with a band gap below 2.0 eV, i.e., absorbing light with wavelengths longer than
620 nm [5]. One successful approach to synthesis of low-band gap polymer is
arranging the electron-donor and electron-acceptor repeating units alternatively
along the polymeric backbone [6]. From this synthetic design rule, a number of
low-band gap polymers have been synthesized, which have lower band gaps
around 0.3–0.5 eV compared to the conventional p-conjugated polymers [7]. OPVs
made of low-band gap polymers have been reported previously [8].
In the present article, we report the synthesis and characterization of a new
p-conjugated polymer with electron-donating fluorene and electron-withdrawing
thiophene-thiazole units and its characterization of photovoltaic properties of bulk
heterojunction photovoltaic cells.
Experimental
3-Hexylthiophene, copper cyanide, N-bromosuccinimide (NBS), O,O0-diethyl
dithiophosphate, 2-octanone, palladium(II) acetate, tricyclohexylphosphine,
and tetraethylammonium hydroxide, were purchased from Aldrich Chemical
Co. Solvents were dried and purified by fractional distillation over sodium=ben-
zophenone and handled in moisture free atmosphere. 1H-NMR spectra were
recorded in CDCl3 on a Varian Mercury 300 and chemical shifts were recorded
in ppm. The absorption and photoluminescence (PL) spectra were measured
using a Jasco V-570 UV-visible spectrometer and a Hitachi F-4500 fluorescence
spectrophotometer, respectively. The molecular weight and polydispersity of
polymer were determined with gel permeation chromatography (GPC) analysis
relative to polystyrene calibration (Agilent 1100 series GPC, Plgel 5 mm
MIXED-C, refractive index detector) in THF solution. The thermal analysis
were carried out on a Mettler Toledo TGA=SDTA 851e, DSC 822e analyzer
under an N2 atmosphere at a heating rate of 10ꢀC=min. Cyclic voltammetry
(CV) was carried out with a Bioanalytical Systems CV-50 W voltammetric ana-
lyzer at a potential scan rate of 50–100 mV=s in a 0.1 M solution of tetrabutyl-
ammonium tetrafluoroborate (Bu4NBF4) in anhydrous acetonitrile. Each
polymer film was coated onto a Pt disc electrode (0.2 cm2) by dipping the
electrode into the polymer solution.

Synthesis and Electro-Optical Properties
Synthesis of 2-Bromo-3-hexylthiophene
225
N-Bromosuccinimide (5.8 g, 3.27 mmol) and DMF (10 mL) were placed in a three
neck round bottom flask (100 mL). 3-Hexylthiophene (5 g, 29.7 mmol) was added
flask under N2 atmosphere. The solution was stirred overnight at room temperature.
The solution was extracted with ethyl acetate and water (2–3 times) and dried with
anhydrous MgSO4. Flash column chromatography using hexane as an eluent to give
2-bromo-3-hexylthiophene (3.48 g, 47%).
1H-NMR (CDCl3, d ppm): 7.19 (d, 1H, aromatic protons on thiophene), 6.89
(d, 1H, aromatic protons on thiophene), 2.5–2.6 (t, CH2), 1.59 (m, CH2), 1.32
(m, 3CH2), 0.85 (m, CH3).
Synthesis of 2-Cyano-3-hexylthiophene
2-Bromo-3-hexylthiophene (4 g, 16.2 mmol) and copper cyanide (8.11 g, 90.6 mmol)
were stirred in DMF (30 mL) in a three neck round bottom flask (100 mL) under
N2 atmosphere. The reaction mixture was stirred 24 h at 130ꢀC. The reaction mixture
was extracted with toluene and water (2–3 times). The obtained organic phase was
dried with anhydrous MgSO4 and the product was obtained by column chromato-
graphy (eluent: hexane:ethyl acetate ¼ 8:2) to give 2-cyano-3-hexylthiophene
(2.43 g, 77%).
1H-NMR (CDCl3, d ppm): 7.48 (d, 1H, aromatic protons on thiophene), 6.99
(d, 1H, aromatic protons on thiophene), 2.7–2.8 (t, CH2), 1.62 (m, CH2), 1.25
(m, 3CH2), 0.82 (m, CH3).
Synthesis of 2-Thioamino-3-hexylthiophene
2-Cyano-3-hexylthiophene (1 g, 5.17 mmol), THF and water (10:1) (15 mL) were
stirred in a three neck round bottom flask (100 mL) under N2 atmosphere. Then
O,O0-diethyl dithiophosphate (4 mL, 25.8 mmol) was added into the flask. After
reaction was carried out at 70ꢀC for 20 h, the cooled reaction mixture was quenched
with water and then extracted 2–3 times with ethyl acetate and water. The obtained
organic phase was dried with anhydrous MgSO4 and the product was purified using
column chromatography using hexane and ethyl acetate (8:2) as an eluent to give
2-thioamino-3-hexylthiophene (1 g, 85%).
1H-NMR (CDCl3, d ppm): 7.27 (d, 1H, aromatic protons on thiophene), 6.9
(d, 1H, aromatic protons on thiophene), 2.8–2.9 (m, CH2), 2.0 (s, NH2), 1.6
(m, CH2), 1.3 (m, 3CH2), 0.82 (m, CH3).
Synthesis of 2-Thiazole (30-hexyl)-3-hexylthiophene
1-Bromo-2-octanone (1.36 g, 6.59 mmol) was added to a DMF solution of the
obtained 2-thioamino-3-hexylthiophene (0.5 g, 2.19 mmol). After reaction was car-
ried out at 130ꢀC for about 1.5 h, the reaction mixture was quenched with water
and extraction with ethyl acetate, water (2–3 times) and brine. The obtained organic
phase was dried with anhydrous MgSO4 and the product was purified by column chro-
matography using hexane and dichloromethane (3:2) as an eluent to give 2-thiazole
(30-hexyl)-3-hexylthiophene (0.65 g, 89%).

226
J. S. Kim et al.
1H-NMR (CDCl3, d ppm): 7.25 (d, 1H, aromatic protons on thiophene), 6.95
(d, 1H, aromatic protons on thiophene), 6.82 (d, 1H, aromatic protons on thiazole),
2.8 (t, CH2, hexyl group of thiazole), 2.89 (t, CH2, hexyl group of thiophene), 1.65
(m, 2CH2, hexyl group of thiophene and thioazole), 1.35 (m, 6CH2, hexyl group
of thiophene and thiazole), 0.9 (m, 2CH3, hexyl group of thiophene and thiazole).
Synthesis of 5-Bromo-2-thiazole (20-bromo-30-hexyl)-3-hexylthiophene
NBS (0.58 g, 3.27 mmol) was added to a DMF (40 mL) solution of 2-thiazole
(30-hexyl)-3-hexylthiophene (0.5 g, 1.49 mmol) in a three neck round bottom flask
(100 mL) under N2 atmosphere. After reaction was carried out at 40ꢀC for overnight,
the reaction mixture was extracted with ethyl acetate and water (2–3 times) and
obtained organic phase was dried with anhydrous MgSO4. Column chromatography
using hexane and dichloromethane (5=1) as an eluent gave the product (0.6 g, 83%).
1H-NMR (CDCl3, d ppm): 6.88 (d, 1H, aromatic protons on thiophene), 2.75
(m, 2CH2, hexyl group of thiophene and thiazole), 1.63 (m, 2CH2, hexyl group of
thiophene and thiazole), 1.35 (m, 6CH2, hexyl group of thiophene and thioazole),
0.9 (m, 2CH3, hexyl group of thiophene and thiazole).
Synthesis of Poly[(9,9-dioctylfluorene)-alt-(3-hexylthiophen-5-yl)-2,20-(40-
hexylthiazol-50-yl)] poly(FO-HThHTha)
5-Bromo-2-(5-bromo-3-hexylthiophen-2-yl)-4-hexylthiazole (0.13 g) and 2,7-bis
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (0.169 g) were dis-
solved in anhydrous toluene (2 mL) with stirring under N2 atmosphere. After
addition of palladium(II) acetate (8.5 mg) and tricyclohexyl phosphine (2.5 mg),
the reaction mixture was stirred at 90ꢀC for 5 min under N2 condition. Tetraethyl-
ammonium hydroxide (0.37 mL) was added and then stirred at 90ꢀC for 1 h. After
the reaction was completed, the reaction solution was dark brown solid. The
obtained polymer (0.14 g) was concentrated and reprecipitated into excess amount
of methanol (250 mL).
Fabrication and Measurement of Bulk Heterojunction Photovoltaic Cells
Each glass substrate was coated with a transparent ITO electrode (110 nm thick,
10–20 X=sq sheet resistance). The ITO-coated glass substrates were ultrasonically
cleaned with detergent, deionized water, acetone, and isopropyl alcohol. Bulk het-
erojunction photovoltaic cells was fabricated by spin-coating a 40 nm thick layers
of PEDOT:PSS (CLEVIOS P VP AI 4083) on a ultrasonically cleaned 110 nm thick
ITO coated patterned glass substrate, followed by baking on a hot plate at 150ꢀC for
30 min. For the fabrication of the active layer composed of interconnected networks
of electron donor, poly(FO-HThHTha) (10 mg=mL) and electron acceptor, PC70BM
(15 mg=mL) were dissolved in 1,2-dichlorobenzene and chloroform (1:1 wt%),
respectively, followed by shaking at room temperature for 12 h. The poly(FO-
HThHTha) and PC70BM blend was then prepared by mixing the two solutions
and subsequent shaking for 12 h was performed to obtain a homogeneously mixed
blend system. The weight ratio of the active layer as an electron donor, poly
(FO-HThHTha) to electron acceptor, PC70BM was 1:2, 1:4 and 1:6, respectively.
The TiOx precursor solution (1 wt%) was spin cast (4000 rpm) onto the active layer

Synthesis and Electro-Optical Properties
227
with thickness around 10 nm and heated at 80ꢀC for 10 min in air. During this pro-
cess the precursor converts to TiOx by hydrolysis. Subsequently the devices were
pumped down in vacuum (5 ꢁ 10ꢂ6 torr) and then Al electrodes with thickness
around 100 nm were deposited. The top metal electrode area, determining active area
of the devices, was evaluated to be 4 mm2. Photovoltaic performance was measured
using a A.M. 1.5 G solar simulator (Oriel 300 W) at 100 mW=cm2 light illumination
after adjusting the light intensity using Oriel power meter (model No. 70260 which is
calibrated using laboratory standards that are traceable to the National Institute of
Standards and Technologies USA). Current density-voltage (J-V) curves were
recorded using a standard source measurement unit (Keithley 236). All fabrication
steps and characterization measurements were performed in an ambient environment
without a protective atmosphere. Thickness of the thin films was measured using a
KLA Tencor Alpha-step IQ surface profilometer with an accuracy of ꢃ1 nm.
Results and Discussion
Scheme 1 shows the synthetic routes for the monomer and the polymer via Suzuki
polymerization method. 2-Bromo-3-hexylthiophene, 2-cyano-3-hexylthiophene [9],
Scheme 1. Synthetic routes for poly(FO-HThHTha).

228
J. S. Kim et al.
2-thioamino-3-hexylthiophene, 2-thiazole (30-hexyl)-3-hexylthiophene [10], and
2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene [11] were
synthesized according to literature procedures. The monomer, 5-bromo-2-thiazole
(20-bromo-30-hexyl)-3-hexylthiophene was successfully prepared by the bromination
of 2-thiazole (30-hexyl)-3-hexylthiophene using NBS=DMF. Polymerization was
carried out with an excess of palladium(II) acetate in dried toluene under N2 atmos-
phere for 1 h. The polymer, poly[(9,9-dioctylfluorene)-alt-(3-hexylthiophen-5-yl)-
2,20-(40-hexylthiazol-50-yl)] poly(FO-HThHTha) was found to be completely soluble
in various organic solvents such as acetone, chloroform, 1,2-dichloroform, and
THF, etc. The good solubility of poly(FO-HThHTha) is a very important require-
ment for device fabrication. To improve device performance, poly(FO-HThHTha)
was further purified by soxhlet extraction with different solvents including methanol,
acetone and finally chloroform to remove the unreacted monomers, impurities and
oligomers. By carrying out these processes, the highly purified poly(FO-HThHTha)
was obtained. Table 1 summarizes the polymerization results, molecular weight,
polydispersity, and thermal characteristics of poly(FO-HThHTha). The weight-
average molecular weight (Mw) and the polydispersity (PDI) of the poly(FO-
HThHTha) was 48,100, 2.45, respectively. The glass transition temperature of
poly(FO-HThHTha) was 58ꢀC as determined by DSC. The TGA thermogram of
the poly(FO-HThHTha) revealed a 5% weight loss at 415ꢀC under a N2 atmosphere.
Figure 1 shows the normalized UV-visible and photoluminescence (PL) spectra
of the poly(FO-HThHTha) in thin film state. The absorption spectrum of the
poly(FO-HThHTha) has broad peaks at 423 nm in solution state and at 424 nm in
thin film state. The absorption band in thin film state was slightly red-shift compared
to the solution state because of the interchain interactions between the polymer
chains. From the onset of the absorption spectrum, the band gap of poly(FO-
HThHTha) was calculated to be 2.4 eV. The PL spectrum of the poly(FO-
HThHTha) has at 527 nm in solution state and at 579 nm in thin film state. These
results indicate that more efficient energy transfer from the wide-band gap of fluor-
ene segment to the low-band gap thiophene-thiazole segment has occurred in the
poly(FO-HThHTha), due to the completely disappearance of PL emission from
the fluorene unit. The transfer of the exciton energy from fluorene segment to the
thiophene-thiazole segment occurs mainly along the poly(FO-HThHTha) chain
and=or intermolecular energy transfer occurs in poly(FO-HThHTha).
We carried out redox measurements using cyclic voltammetry (CV). The HOMO
energy level of poly(FO-HThHTha) for the standard ferrocene=ferrocenium
Table 1. Polymerization results, thermal and electro-optical properties of
poly(FO-HThHTha)
a
e
Mw
DSCb TGAc
Abs (nm)
solution filmd
PL (nm)
Eg
(ꢁ104) PDIa (ꢀC)
(ꢀC)
solution filmd (eV)
Polymer
4.81
2.45
58
415
423
424
527 579 2.45
a Mw and PDI of the polymers were determined by GPC using polystyrene standard.
bDetermined by DSC at a heating rate of 10ꢀC=min under N2 atmosphere.
cTGA was measured at temperature of 5% weight loss for the polymer.
dMeasured in the thin film onto the quartz substrate.
eBand gap estimated from the onset wavelength of the optical absorption.

Synthesis and Electro-Optical Properties
229
Figure 1. UV-visible absorption and PL spectra of poly(FO-HThHTha).
(ꢂ4.8 eV) was ꢂ5.5 eV. The LUMO energy level was ꢂ3.1 eV, which was calculated
from the value of HOMO energy and the optical band gap. The energy band dia-
grams of poly(FO-HThHTha), PC70BM and TiOx are shown in Figure 2. Photo-
generation of charge in most of p-conjugated polymers is not very efficient
because there is always the recombination of two charge carriers involved. However,
from these energy band diagrams, we predict that the required transfer of charges
can be realized at the interface between poly(FO-HThHTha) as an electron donor
and PC70BM as an electron acceptor due to the higher LUMO energy levels of
poly(FO-HThHTha) and their lower HOMO energy level of PC70BM.
Bulk heterojunction photovoltaic cells were fabricated using the poly(FO-
HThHTha) as an electron donor and PC70BM as an electron acceptor. The photo-
voltaic cell structure was ITO=PEDOT:PSS (40 nm)=poly(FO-HThHTha):PC70BM
(80 nm)=TiOx (10 nm)=Al (120 nm) using a different ratio of electron donor to elec-
tron acceptor materials at a fixed photoactive layer thickness of 80 nm. We used
TiOx layer for electron transfer from active layer to Al. The energy level of TiOx
matches well with the energy level of Al. Many research groups obtained their best
Figure 2. Energy band diagram of poly(FO-HThHTha), PC70BM and TiOx.

230
J. S. Kim et al.
Figure 3. Current density-voltage (J-V) characteristics of the poly(FO-HThHTha):PC70BM
blend of photovoltaic cells.
photovoltaic performance by optimization of different ratio of electron donor to
electron acceptor. The charge balance depends on overall active layer thickness
and an appropriate amount of PC70BM in the blended active layer. Here, we study
the effect of active layer compositions on photovoltaic performance using 1,2-
dichlorobenzene and chloroform cosolvent system. The current density-voltage
(J-V) characteristics of bulk heterojunction photovoltaic cells fabricated with 1:2,
1:4 and 1:6 ratios of poly(FO-HThHTha) to PC70BM under AM 1.5 G illumination
are shown in Figure 3 and these photovoltaic performance are summarized in
Table 2. The open-circuit voltage (Voc) of bulk heterojunction photovoltaic cell is
closely related to the energy difference between the HOMO energy level of the elec-
tron donor and the LUMO energy level of the electron acceptor. The Voc of the bulk
heterojunction photovoltaic cell are almost identical, but a higher short-circuit cur-
rent density (Jsc) were observed with increment of the amount of PC70BM in the
blended active layer. The bulk heterojunction photovoltaic cell fabricated with 1:4
ratio of poly(FO-HThHTha) to PC70BM with TiOx exhibited higher photovoltaic
performance when compared with the poly(FO-HThHTha) to PC70BM with TiOx
giving a Voc of 0.44 V, a Jsc of 2.57 mA=cm2, a FF of 34% and a PCE of 0.39% as
Table 2. Photovoltaic performance of poly(FO-HThHTha):PC70BM
Device
Ratio
Voc (V)a
Jsc (mA=cm2)b
FF (%)c
PCE (%)d
1
2
3
1:2
1:4
1:6
0.48
0.44
0.41
1.60
2.57
2.68
31.25
34.49
28.21
0.24
0.39
0.31
aVoc: open-circuit voltage.
bJsc: short-circuit current density.
cFF: fill factor.
dPCE: power conversion efficiency.

Synthesis and Electro-Optical Properties
231
opposed to 0.31% for the poly(FO-HThHTha):PC70BM (1:6) bulk heterojunction
photovoltaic cell.
Conclusions
We have synthesized a new class of high molecular weight poly(FO-HThHTha) by
Suzuki polymerization. Poly(FO-HThHTha) exhibited a high glass transition tem-
perature and can be easily spin casted onto the ITO substrates. Bulk heterojunction
photovoltaic cells with an ITO=PEDOT:PSS=poly(FO-HThHTha):PC70BM=TiOx=
Al configuration were fabricated with poly(FO-HThHTha) as an electron donor
and PC70BM as an electron acceptor. The bulk heterojunction photovoltaic cells
were fabricated with 1:2, 1:4 and 1:6 ratios of poly(FO-HThHTha) to PC70BM.
The maximum PCE of 0.39% (under an AM 1.5 simulated solar light at 100 mW=
cm2) (Jsc: 2.57 mA=cm2, Voc: 0.44 V, FF: 34%) was achieved with poly
(FO-HThHTha):PC70BM (1:4 wt%).
Acknowledgment
This work was supported by the New & Renewable Energy program of the Korea
Institute of Energy Technology Evaluation and Planning (KETEP) grant (No.
20103020010050) funded by the Ministry of Knowledge Economy, Republic of
Korea.
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& Sons: New York.
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