Synthesis and photovoltaic properties of benzo[1,2-b:4,5-b′] dithiophene derivative-based polymers with deep HOMO levels

Article abstract of DOI:10.1039/c2jm32514d

Two benzo[1,2-b:4,5-b′]dithiophene (BDT) derivatives with conjugated substituents, triisopropylsilylethynyl (TIPS) and 4-octylphenylethynyl groups, were synthesized as donor units (D) and copolymerized with two acceptor units (A), 4,7-bis(4-octylthiophen-2-yl)-2,1,3-benzothiadiazole (BT) and 4,4′-diundecyl-2,2′-bithiazole (BTZ), respectively, using Stille coupling reaction to afford four new copolymers, PTBDT-BT, PTBDT-BTZ, POPEBDT-BT, POPEBDT-BTZ. All polymers exhibited highest occupied molecular orbital (HOMO) energy levels that were deeper than -5.4 eV due to the conjugated substituents. Small band gaps were successfully achieved for PTBDT-BT (1.67 eV) and POPEBDT-BT (1.67 eV) and were attributable to the strong intramolecular charge transfer within the D-A alternating structure. The resultant photovoltaic performances showed high open-circuit voltages (V_oc) ranging from 0.73 V to 0.92 V, whereas the power conversion efficiencies (PCEs) depended strongly on the blend morphologies. The polymer solar cell based on the blend of PTBDT-BT and PC₇₁BM gave the best photovoltaic performance among the series, with a high V_oc of 0.81 V and a PCE of 4.61%.

Full text of DOI:10.1039/c2jm32514d

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PAPER
Synthesis and photovoltaic properties of benzo[1,2-b:4,5-b0]dithiophene
derivative-based polymers with deep HOMO levels†
Heung Gyu Kim,‡a Sae Byeok Jo,‡a Chiyeoung Shim,a Jaewon Lee,a Jisoo Shin,a Eun Chul Cho,b
c
Soo-Ghang Ihn, Yeong Suk Choi, Yungi Kim and Kilwon Cho
c
c
a
*
Received 21st April 2012, Accepted 23rd June 2012
DOI: 10.1039/c2jm32514d
Two benzo[1,2-b:4,5-b0]dithiophene (BDT) derivatives with conjugated substituents,
triisopropylsilylethynyl (TIPS) and 4-octylphenylethynyl groups, were synthesized as donor units (D)
and copolymerized with two acceptor units (A), 4,7-bis(4-octylthiophen-2-yl)-2,1,3-benzothiadiazole
(BT) and 4,40-diundecyl-2,20-bithiazole (BTZ), respectively, using Stille coupling reaction to afford four
new copolymers, PTBDT-BT, PTBDT-BTZ, POPEBDT-BT, POPEBDT-BTZ. All polymers
exhibited highest occupied molecular orbital (HOMO) energy levels that were deeper than ꢀ5.4 eV due
to the conjugated substituents. Small band gaps were successfully achieved for PTBDT-BT (1.67 eV)
and POPEBDT-BT (1.67 eV) and were attributable to the strong intramolecular charge transfer within
the D–A alternating structure. The resultant photovoltaic performances showed high open-circuit
voltages (Voc) ranging from 0.73 V to 0.92 V, whereas the power conversion efficiencies (PCEs)
depended strongly on the blend morphologies. The polymer solar cell based on the blend of PTBDT-
BT and PC71BM gave the best photovoltaic performance among the series, with a high Voc of 0.81 V
and a PCE of 4.61%.
hindered further enhancements of the photovoltaic
1. Introduction
performances.
Conjugated polymer solar cells have attracted interest as possible
candidates for realizing lightweight flexible energy sources with
low costs and large-area fabrication.1–3 Until recently, bulk
heterojunction (BHJ)-based polymer solar cells prepared from
blends of conjugated donor polymers and (6,6)-phenyl-C61-
butyric acid methyl ester (PCBM) as the acceptor have yielded
the most successful enhancement in the power conversion effi-
ciency (PCE).4 Among the many electron-donating conjugated
polymers developed thus far, highly crystalline poly(3-hexyl-
thiophene) (P3HT) has been the most widely studied donor
polymer, reaching PCEs of up to 5% in the presence of an
optimized active layer morphology prepared via a variety of post-
treatment techniques.5–9 Despite the successful optimization of
the active layer characteristics, device PCEs must be enhanced to
at least 10% before practical applications become feasible. The
limited range (300–650 nm) of the visible light absorption profile
and the high HOMO energy level (ꢀ5.0 eV) of P3HT have
In this sense, tremendous efforts have been devoted toward
developing new small band gap polymers with deep HOMO
energy levels, and several design rules for this purpose have been
proposed thus far. An alternating copolymer structure with
electron-rich (donor, D) and electron-deficient (acceptor, A)
repeating units has been shown to provide the most effective
strategy for adjusting the optical and electrical properties of
copolymers via intramolecular charge transfer.10,11 These D–A
alternating copolymers were designed to achieve deep HOMO
energy levels (<ꢀ5.2 eV) toward a high open-circuit voltage (Voc)
and a small band gap (<1.9 eV) for efficient sunlight absorption.
Numerous electron-rich monomers, such as carbazole,12 dithie-
nosilole,13 benzo[1,2-b:4,5-b0]dithiophene (BDT),14 etc., and
electron-deficient monomers, such as benzothiadiazole,13 thieno
[3,4-b]thiophene-2-carboxylate,15 bithiazole,16 and thieno[3,4-c]
pyrrole-4,6-dione,17–19 etc., were synthesized, and their combi-
nations toward D–A alternating copolymers have been investi-
gated. More comprehensive and systematic investigations that
could provide an understanding of the relationship between the
molecular structure and properties of D–A alternating copoly-
mers would be required for the further development of finely
tuned materials for use in highly efficient polymer solar cells.
Among the recently synthesized D–A copolymers, BDT-based
copolymers have attracted the most interest with their promising
photovoltaic performances (>7%).20 BDT units display strong
p–p stacking and good hole mobilities due to their high planarity
aDepartment of Chemical Engineering, Pohang University of Science and
Technology, Pohang, 790-784, Korea. E-mail: kwcho@postech.ac.kr
bDepartment of Chemical Engineering, Hanyang University, Seong-dong
gu, Seoul, 133-791, Korea
cSamsung Advanced Institute of Technology Samsung Electronics, Yongin,
446-712, Korea
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2jm32514d
‡ Heung Gyu Kim and Sae Byeok Jo contributed equally to this work.
This journal is ª The Royal Society of Chemistry 2012
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and small steric hindrance between adjacent molecules.14 Alkyl
or alkoxy groups are commonly introduced as BDT substituents
to increase the solubility of the polymers in organic solvents. The
choice of the substituent plays a critical role in controlling the
optical and electrical properties of the conjugated polymers;
therefore, the electron affinity of a substituent must be carefully
considered.20 The triisopropylsilylethynyl (TIPS) and alkyl phe-
nylethynyl substituents have been used previously to modify
acene molecules in organic field effect transistors (OFETs).21,22
TIPS substituents are more effective in lowering the lowest
unoccupied molecular orbital (LUMO) and HOMO energy
levels than conventional alkoxy substituents.23 Moreover, TIPS
substituents improve the solubility, p–p stacking, and stability of
acene molecules.21 Another conjugated substituent, the alkyl
phenylethynyl group, has been tested in the context of BDT
homopolymers and has yielded significant reductions in the
HOMO energy level (ꢀ5.61 eV) of the polymers. Consequently,
high Voc values (0.85 eV) have been obtained from polymer solar
cell devices that include PCBM.24 Yang and coworkers utilized
an alkylthiophene substituent on BDT-based copolymers to
obtain relatively deep HOMO energy levels (ꢀ5.31 eV), a high
Voc (0.92 eV), and a PCE of 5.66%.25 A handful of other studies
have reported on the modification of the BDT unit to prepare D–
A copolymers;26,27 however, additional studies are required to
optimize the properties of BDT-based copolymers.
Scheme 1 Synthetic route for the monomers. Reagents and conditions:
(a) 1: ((triisopropylsilyl)ethynyl)magnesium chloride, THF, 60 ꢁC;
2: SnCl2$2H2O, 10% HCl, 60 ꢁC, 57%; (b) 1: n-BuLi, THF, ꢀ78 ꢁC; 2:
Me3SnCl, RT, 80%; (c) 1: ((4-octylphenyl)ethynyl)magnesium chloride,
THF, 60 ꢁC; 2: SnCl2$2H2O, 10% HCl, 60 ꢁC, 72%; and (d) 1: n-BuLi,
THF, ꢀ78 ꢁC; 2: Me3SnCl, RT, 80%.
N,N-dimethylformamide (DMF), chlorobenzene (CB), and 1,2-
dichlorobenzene (DCB), were purchased from Aldrich. The
monomers,
4,7-bis(4-octylthiophen-2-yl)-2,1,3-benzothiadi-
azole28 and 5,50-dibromo-4,40-diundecyl-2,20-bithiazole,29 were
synthesized according to modified literature procedures.
Measurements
1H and 13C NMR spectra were recorded on a Bruker AVANCE
400 spectrometer operated at 400 MHz and 100 MHz in
chloroform-d solutions. The number-average (Mn) and weight-
average (Mw) molecular weights were measured by gel perme-
ation chromatography (GPC, Shimadzu) using chloroform as
the eluent and a calibration curve of polystyrene standards at
40 ꢁC. UV-Visible absorption spectra were measured using a
Varian CARY-5000 UV/Visible spectrophotometer. Differential
scanning calorimetric (DSC) and thermogravimetric analysis
(TGA) measurements of the polymers were performed using
DSC 2910 (TA Instruments) and TGA 2050 (TA Instruments)
instruments under a nitrogen atmosphere at heating and cooling
rates of 10 ꢁC minꢀ1. Cyclic voltammograms were recorded using
a SP-200 (BioLogic). A 0.1 M tetrabutylammonium hexa-
fluorophosphate (Bu4NPF6) solution in acetonitrile was used as
In this work, we modified a BDT unit by introducing two
substituents with different conjugation lengths, TIPS and 4-
octylphenylethynyl groups, into the central benzene ring, with
the aim of lowering the HOMO energy levels of the BDT unit
and, consequently, the HOMO energy levels of the D–A
copolymers comprising the modified BDT unit. Because the
HOMO energy levels of D–A copolymers are mainly determined
by the HOMO energy level of the donor unit, a significant
reduction in the HOMO energy levels of the modified BDT-
based copolymers was expected. As shown in Scheme 1, 4,8-
bis((triisopropylsilyl)ethynyl)benzo[1,2-b:4,5-b0]dithiophene
(TBDT) and 4,8-bis((4-octylphenyl)ethynyl)benzo[1,2-b:4,5-b0]
dithiophene (OPEBDT) were synthesized as donor monomers.
These monomers were each copolymerized with two electron-
deficient monomers, 4,7-bis(4-octylthiophen-2-yl)-2,1,3-benzo-
thiadiazole (BT, strong acceptor) and 4,40-diundecyl-2,20-bithia-
zole (BTZ, weak acceptor). Four new copolymers, PTBDT-BT,
PTBDT-BTZ, POPEBDT-BT, and POPEBDT-BTZ, were
successfully synthesized by Stille coupling reaction, as shown in
Scheme 2. We demonstrated that the HOMO energy levels of the
polymers were successfully lowered via modification of the BDT
unit. A high Voc for photovoltaic devices comprising the synthe-
sized polymers was achieved, up to 0.92 V. The synthesis, char-
acterization, and photovoltaic properties of these polymers were
thoroughly investigated.
2. Experimental
Materials
All reagents were purchased from Aldrich, TCI, and Acros,
and were used without further purification. All anhydrous
organic solvents for the synthesis and device fabrication steps,
including tetrahydrofuran (THF), chloroform, toluene,
Scheme 2 Synthesis of BDT-based D–A polymers.
17710 | J. Mater. Chem., 2012, 22, 17709–17717
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the electrolyte solution with a scan rate of 50 mV sꢀ1. A three-
electrode system was used, consisting of an Ag/AgCl reference
electrode, a glassy carbon working electrode, and a platinum
counter electrode. Polymer thin films were formed by drop-
casting of the polymer solutions in chloroform onto the working
electrode. The potential of the Ag/AgCl reference electrode was
internally calibrated using a ferrocene/ferrocenium redox couple
(Fc/Fc+). The energy levels were estimated according to the
equations: HOMO ¼ ꢀ(4.80 + Eonset, ox) and LUMO ¼ ꢀ(4.80 +
Eonset, red).30,31
adding n-BuLi (1.6 M in hexane, 1.5 mL, 2.4 mmol). After stir-
ring at ꢀ78 ꢁC for 1 h, trimethyltin chloride (1.0 M in THF,
2.4 mL, 2.4 mmol) was added in one portion. The solution was
stirred overnight and allowed to reach RT. After the reaction, the
solution was diluted with dichloromethane (200 mL), washed
with water (300 mL ꢂ 3), and dried over MgSO4. The organic
phase was concentrated to 10 mL, and methanol was poured over
the solution to yield a yellow solid. Yield: 0.77 g (80%). 1H NMR
(400 MHz, CDCl3), d (ppm): 7.68 (s, 2H), 1.24 (m, 42H), 0.46 (s,
18H). 13C NMR (100 MHz, CDCl3), d (ppm): 144.68, 143.50,
139.63, 131.10, 110.36, 103.34, 100.64, 18.80, 11.38, ꢀ8.37.
Elemental analysis: calc. for C38H62S2Si2Sn2: C, 52.06; H, 7.13; S,
7.32. Found: C, 52.34; H, 7.07; S, 7.38%.
4,8-Bis((triisopropylsilyl)ethynyl)benzo[1,2-b:4,5-b0]dithiophene
(1). Isopropylmagnesium chloride (2.0 M in THF, 5.5 mL, 11.0
mmol) was added dropwise to a solution of (triisopropylsilyl)
acetylene (1.98 g, 10.86 mmol) in dry THF (10 mL). The solution
was stirred at 60 ꢁC for 2 h and cooled to RT. Benzo[1,2-b:4,5-b0]
dithiophene-4,8-dione (0.6 g, 2.72 mmol) was added to the
solution, and the mixture was stirred at 60 ꢁC for 8 h. After
cooling to RT, SnCl2$2H2O (2.0 g, 8.86 mmol) and a 10% aq.
HCl solution (2 mL) were added carefully to the solution. The
reaction mixture was refluxed at 60 ꢁC for 5 h. After the reaction,
the solution was poured into water (300 mL), and dichloro-
methane (300 mL) was added. The organic layer was washed with
water (300 mL ꢂ 3) and dried over MgSO4. After removal of the
solvent, the residue was purified by column chromatography on
silica gel using hexane as the eluent to afford a greenish solid.
Yield: 0.86 g (57%). 1H NMR (400 MHz, CDCl3), d (ppm): 7.61
(d, 2H), 7.56 (d, 2H), 1.24 (m, 42H). 13C NMR (100 MHz,
CDCl3), d (ppm): 140.87, 138.51, 128.28, 123.15, 112.19, 102.64,
101.62, 18.78, 11.33. Elemental analysis: calc. for C32H46S2Si2: C,
69.75; H, 8.41; S, 11.64. Found: C, 69.98; H, 8.17; S, 11.44%.
2,6-(Trimethyltin)-4,8-bis((4-octylphenyl)ethynyl)benzo[1,2-b:
4,5-b0]dithiophene (4). Compound 4 was prepared using a
procedure similar to that described for the synthesis of
compound 3. Yield: 0.61 g (80%). 1H NMR (400 MHz, CDCl3), d
(ppm): 7.75 (s, 2H), 7.62 (d, 4H), 7.24 (d, 4H), 2.65 (t, 4H), 1.62
(quintet, 4H), 1.30 (m, 20H), 0.91 (t, 6H), 0.49 (s, 18H). 13C NMR
(100 MHz, CDCl3), d (ppm): 144.53, 143.95, 143.36, 138.99,
131.75, 130.95, 128.58, 120.30, 110.21, 98.76, 85.75, 36.01, 31.89,
31.32, 29.48, 29.27, 22.68, 14.12, ꢀ8.24. Elemental analysis: calc.
for C48H62S2Sn2: C, 61.29; H, 6.64; S, 6.82. Found: C, 61.47; H,
6.78; S, 6.75%.
Poly{4,8-bis((triisopropylsilyl)ethynyl)benzo[1,2-b:4,5-b0]dithio-
phene-alt-4,7-bis(4-octylthiophen-2-yl)-2,1,3-benzothiadiazole-5,
50-diyl} (PTBDT-BT). Compound 3 (0.3 g, 0.34 mmol), 4,7-bis-
(5-bromo-4-octylthiophen-2-yl)-2,1,3-benzothiadiazole (0.23 g,
0.34 mmol), and Pd(PPh3)4 (15 mg) were dissolved in a mixture
of toluene (15 mL) and DMF (2 mL). The reaction flask was
purged with nitrogen for 2 h. The solution was refluxed for 3 days
at 120 ꢁC, and then cooled to RT. The polymer solution was
poured into methanol (200 mL). The resulting blue-black solid
was filtered and Soxhlet extracted with methanol, acetone,
hexane, and chloroform until the wash solution of each extrac-
tion was colorless. The chloroform fraction was concentrated
and poured into methanol, filtered, and dried under vacuum.
Yield: 0.28 g (76%). 1H NMR (400 MHz, CDCl3), d (ppm): 8.12–
7.25 (m, 6H), 2.98–2.72 (br, 4H), 1.82–1.10 (br, 66H), 0.90 (br,
4,8-Bis((4-octylphenyl)ethynyl)benzo[1,2-b:4,5-b0]dithiophene
(2). Isopropylmagnesium chloride (2.0 M in THF, 5.0 mL, 10.0
mmol) was added dropwise to a solution of (4-octylphenyl)
acetylene (2.0 g, 9.33 mmol) in dry THF (10 mL). The solution
was stirred at 60 ꢁC for 2 h and cooled to RT. Benzo[1,2-b:4,5-b0]
dithiophene-4,8-dione (0.5 g, 2.27 mmol) was added to the
solution, and the mixture was stirred at 60 ꢁC for 8 h. After
cooling to RT, SnCl2$2H2O (2.0 g, 8.86 mmol) and a 10% aq.
HCl solution (2 mL) were added carefully to the solution. The
reaction mixture was refluxed at 60 ꢁC for 5 h. After the reaction,
the solution was poured into water (300 mL), and dichloro-
methane (300 mL) was added. The organic layer was washed with
water (300 mL ꢂ 3) and dried over MgSO4. After removal of the
solvent, the residue was purified by column chromatography on
silica gel using hexane–chloroform (1 : 1, v/v) as an eluent to
afford an orange solid. Yield: 1.0 g (72%). 1H NMR (400 MHz,
CDCl3), d (ppm): 7.72 (d, 2H), 7.61 (d, 4H), 7.59 (d, 2H), 7.24 (d,
4H), 2.65 (t, 4H), 1.62 (quintet, 4H), 1.30 (m, 20H), 0.89 (t, 6H).
13C NMR (100 MHz, CDCl3), d (ppm): 144.18, 140.28, 138.19,
131.71, 128.63, 128.02, 123.28, 120.01, 112.09, 99.49, 85.13,
36.02, 31.89, 31.30, 29.47, 29.28, 29.27, 22.68, 14.12. Elemental
analysis: calc. for C42H46S2: C, 82.03; H, 7.54; S, 10.43. Found:
C, 81.86; H, 7.60; S, 10.21%.
6H). GPC (CHCl3, 40 C): Mn ¼ 15.4 kg molꢀ1, Mw ¼ 25.7 kg
ꢁ
molꢀ1, PDI ¼ 1.67.
Poly{4,8-bis((triisopropylsilyl)ethynyl)benzo[1,2-b:4,5-b0]
dithiophene-alt-4,40-diundecyl-2,20-bithiazole}
(PTBDT-BTZ).
PTBDT-BTZ was synthesized using the procedure described for
the synthesis of PTBDT-BT. Compound 3 (0.4 g, 0.46 mmol) and
5,50-dibromo-4,40-diundecyl-2,20-bithiazole (0.29 g, 0.46 mmol)
1
were used as the monomers. Yield: 0.43 g (92%). H NMR (400
MHz, CDCl3), d (ppm): 7.83–7.72 (br, 2H), 3.10–2.77 (br, 4H),
1.89–1.76 (br, 4H), 1.47–1.22 (br, 74H), 0.87 (br, 6H). GPC
(CHCl3, 40 ꢁC): Mn ¼ 13.6 kg molꢀ1, Mw ¼ 27.5 kg molꢀ1, PDI ¼
2.02.
Poly{4,8-bis((4-octylphenyl)ethynyl)benzo[1,2-b:4,5-b0]dithio-
phene-alt-4,7-bis(4-octylthiophen-2-yl)-2,1,3-benzothiadiazole-5,
50-diyl} (POPEBDT-BT). POPEBDT-BT was synthesized using
the procedure described for the synthesis of PTBDT-BT.
2,6-(Trimethyltin)-4,8-bis((triisopropylsilyl)ethynyl)benzo[1,2-
b:4,5-b0]dithiophene (3). Compound 1 (0.6 g, 1.09 mmol) was
dissolved in dry THF (40 mL) and cooled to ꢀ78 ꢁC before
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Compound 4 (0.3 g, 0.32 mmol) and 4,7-bis(5-bromo-4-octylth-
iophen-2-yl)-2,1,3-benzothiadiazole (0.22 g, 0.32 mmol) were
used as the monomers. Yield: 0.27 g (70%). 1H NMR (400 MHz,
CDCl3), d (ppm): 8.11–6.85 (br, 14H), 3.02–2.44 (br, 8H), 1.88–
1.65 (br, 8H), 1.28 (br, 40H), 0.92–0.85 (m, 12H). GPC (CHCl3,
40 ꢁC): Mn ¼ 9.2 kg molꢀ1, Mw ¼ 14.4 kg molꢀ1, PDI ¼ 1.56.
Oriel) with monochromatic light from a xenon lamp. The power
density of the monochromatic light was calibrated using a Si
photodiode certified by the National Institute for Standards and
Technology (NIST).
3. Results and discussion
Poly{4,8-bis((4-octylphenyl)ethynyl)benzo[1,2-b:4,5-b0]dithio-
Synthesis and thermal stability
phene-alt-4,40-diundecyl-2,20-bithiazole}
(POPEBDT-BTZ).
The general synthetic route for the monomers (compounds 3 and
4) is described in Scheme 1. Compounds 1 and 2, with triiso-
propylsilylethynyl (TIPS) and 4-octylphenylethynyl substituents,
were synthesized by their corresponding Grignard reactions,
followed by reduction with SnCl2$2H2O, in good yield (57% and
72%). The resulting compounds were easily lithiated by n-BuLi,
and then reacted with trimethyltin chloride to afford compounds
3 and 4 in 80% yield. The Stille coupling reaction with a
Pd(PPh3)4 catalyst in toluene/DMF yielded the polymers, as
shown in Scheme 2, in which compounds 3 or 4 were used as
donor units, and 4,7-bis(4-octylthiophen-2-yl)-2,1,3-benzothia-
diazole (BT) or 4,40-diundecyl-2,20-bithiazole (BTZ) were used as
acceptor units. The resulting polymers were carefully purified
through Soxhlet extraction using methanol, acetone, hexane, and
chloroform. The chemical structures of the synthesized mono-
mers and polymers were confirmed by NMR spectroscopy. All
polymers exhibited good solubility in common organic solvents,
such as chloroform, toluene, CB, and DCB. The number-average
molecular weights (Mn) and polydispersity indices (PDIs) of the
four polymers were measured by gel permeation chromato-
graphy (GPC) using chloroform as the eluent and polystyrene as
the internal standard. The results are summarized in Table 1. The
Mn values for PTBDT-BT, PTBDT-BTZ, POPEBDT-BT, and
POPEBDT-BTZ were 15.4 kg molꢀ1, 13.6 kg molꢀ1, 9.2 kg
molꢀ1, and 9.4 kg molꢀ1, with PDIs of 1.67, 2.02, 1.56, and 1.67,
respectively. PTBDT-BT and PTBDT-BTZ exhibited higher Mn
than POPEBDT-BT and POPEBDT-BTZ because the TBDT
molecules containing bulky triisopropylsilylethynyl groups dis-
played better solubility than OPBDT, with the rigid 4-octyl-
phenylethynyl substituents.
POPEBDT-BTZ was synthesized using the procedure described
for the synthesis of PTBDT-BT. Compound 4 (0.28 g, 0.30
mmol) and 5,50-dibromo-4,40-diundecyl-2,20-bithiazole (0.19 g,
0.30 mmol) were used as the monomers. Yield: 0.26 g (80%). 1H
NMR (400 MHz, CDCl3), d (ppm): 7.93–6.73 (br, 10H), 3.15–
2.68 (br, 8H), 1.31 (br, 60H), 0.91 (br, 12H). GPC (CHCl3,
40 ꢁC): Mn ¼ 9.4 kg molꢀ1, Mw ¼ 15.7 kg molꢀ1, PDI ¼ 1.67.
Fabrication and characterization of the polymer solar cells
An ITO patterned glass (15 U sqꢀ1) was cleaned by sonication with
warm acetone and isopropyl alcohol, followed by UV-ozone
treatment. After cleansing, a 10 nm MoO3 film was formed by
depositing and annealing Mo films in the presence of oxygen.
PC71BM (99.5%, Solenne Inc.) was used as an acceptor material in
thisstudy. The polymer:PC71BM blend solutions were prepared in
DCB with various composition ratios and were blended above
60 ꢁC for 2 days with severe agitation. Several devices were
fabricated in such a way that 1,8-diiodooctane (DIO) was added
to the solution 1 h before film deposition. The blend solutions were
then spin-cast onto the MoO3-coated glasses and dried for 1 h
under a N2 environment, giving a total thickness of 70–120 nm,
depending on the concentration of each solution. The ZnO–PEG
solutions32,33 were spin-coated onto the active layers to a thickness
of 45 nm and dried under high vacuum (<10ꢀ6 Torr) for 1 h. On
top of these films, Al cathodes were formed by thermal evapora-
ꢀ
tion. In the case of the devices without ZnO–PEG layers, the 6 A
LiF layer was deposited on the active layers by thermal evapo-
ration before the formation of Al electrode. The current–voltage
(J–V) characteristics were measured using a Keithley 4200 power
source under AM 1.5G illumination at an intensity of 100 mW
cmꢀ2 (Oriel 1 kW solar simulator) using a PVM 132 reference cell
certified by NREL. All electrical measurements and fabrication
processes were performed under an inert N2 environment. The
incident photon-to-current conversion efficiency (IPCE) was
measured using a photo-modulation spectroscopy setup (Merlin,
The thermal stabilities of the synthesized polymers were
measured by TGA, as shown in Fig. 1. The thermal decompo-
sition temperatures (Td, at 5% weight loss) of PTBDT-BT,
PTBDT-BTZ, POPEBDT-BT, and POPEBDT-BTZ were 421,
364, 316, and 398 ꢁC, indicating good thermal stability for
applications in polymer solar cells, as shown in Table 1. The DSC
Table 1 Intrinsic properties of the synthesized polymers
UV-vis absorption spectra
Cyclic voltammetry
Solution
Film
onset
ox
onset
red
LUMOd
(V/eV)
Hole
Surface E mobilityb
E
/
E
/
Mn
(kg molꢀ1
lmax
(nm)
lmax
(nm)
lonset Eogpt
HOMOd
a
Polymer
)
PDIa Td (ꢁC) (mJ mꢀ2
)
(cm2 Vꢀ1 sꢀ1
)
(nm) (eV)c (V/eV)
EEgC (eV)
PTBDT-BT
PTBDT-BTZ
POPEBDT-BT
15.4
13.6
9.2
1.67 421
2.02 364
1.56 316
1.67 398
33.69
20.86
21.31
28.06
1.93 ꢂ 10ꢀ3
4.34 ꢂ 10ꢀ5
4.82 ꢂ 10ꢀ4
5.66 ꢂ 10ꢀ4
544
485
554
531
616 742
528, 565 612
1.67 0.62/ꢀ5.42 ꢀ1.28/ꢀ3.52 1.90
2.03 1.11/ꢀ5.91 ꢀ1.28/ꢀ3.52 2.39
1.67 0.78/ꢀ5.58 ꢀ1.27/ꢀ3.53 2.05
1.95 0.84/ꢀ5.64 ꢀ1.29/ꢀ3.51 2.13
598
543
742
635
POPEBDT-BTZ 9.4
a
Determined by GPC using polystyrene standards in chloroform as the eluent. b Calculated from SCLC measurement. c Estimated from the onset of the
d
UV-vis spectra measured from thin films. Measured by cyclic voltammetry in a 0.1 M Bu4NPF6/CH3CN solution vs. Ag/AgCl.
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respectively, whereas those of thin films were observed at 616,
565, 598, and 543 nm. The absorption spectra of the polymers
were red-shifted in the thin films due to the increased p–p
interchain interactions upon aggregation. The optical band gaps
(Eogpt, eV) of PTBDT-BT, PTBDT-BTZ, POPEBDT-BT, and
POPEBDT-BTZ were calculated from an extrapolation of the
absorption edges in the thin film states and were found to be 1.67,
2.03, 1.67, and 1.95 eV. The small band gap of 1.67 eV was
successfully achieved for PTBDT-BT and POPEBDT-BT by the
strong intramolecular charge transfer effects between the donor
units and the acceptor unit (BT), whereas the two conjugated
substituents showed only minor effects. The effects of the
conjugation length were stronger in the presence of a weak
acceptor unit (BTZ). POPEBDT-BTZ (1.95 eV) exhibited a
smaller band gap than PTBDT-BTZ (2.03 eV). On the other
hand, PTBDT-BT and POPEBDT-BTZ displayed a hint of
shoulders adjacent to the absorption maxima in the thin film
state, possibly due to the formation of intermolecular p–p
stacking and the planarization of the backbone.
Fig. 1 TGA plots of the polymers with a heating rate of 10 ꢁC minꢀ1
under a N2 atmosphere.
measurements did not reveal endo- or exothermic behavior
between 50 ꢁC and 260 ꢁC.
Optical properties
Electrochemical properties
The UV-visible absorption spectra of the polymers were obtained
in chloroform solutions and in the thin film state, as shown in
Fig. 2. The maximum absorption wavelengths (lmax) and optical
band gaps (Eogpt) are summarized in Table 1. The lmaxs of
PTBDT-BT, PTBDT-BTZ, POPEBDT-BT, and POPEBDT-
BTZ in solution were observed at 544, 485, 544, and 531 nm,
Cyclic voltammetry (CV) was used to investigate the HOMO and
LUMO energy levels of the synthesized polymers using a grassy
carbon working electrode, a platinum wire counter electrode,
and a Ag/AgCl reference electrode in a 0.1 M tetrabutylammo-
nium hexafluorophosphate (Bu4NPF6) solution in acetonitrile at
a scan rate of 50 mV sꢀ1, and the results are shown in Fig. 3 and
Table 1. All polymers exhibited good reversible oxidation and
reduction peaks. The HOMO and LUMO energy levels were
calculated according to the procedure described in the
Fig. 3 (a) Cyclic voltammograms of the polymers in the thin films and
Fig. 2 Normalized absorption spectra of the polymers (a) in chloroform
(b) the energy level diagrams of the polymers.
solution and (b) in thin films.
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literature.30,31 The HOMO energy levels of PTBDT-BT, PTBDT-
BTZ, POPEBDT-BT, and POPEBDT-BTZ were calculated to
be ꢀ5.42, ꢀ5.91, ꢀ5.58, and ꢀ5.64 eV, whereas the LUMO
energy levels were ꢀ3.52, ꢀ3.52, ꢀ3.53, and ꢀ3.51 eV, respec-
tively, with ferrocene as the reference. All synthesized polymers
exhibited deep HOMO energy levels (<ꢀ5.4 eV) to ensure good
air stability and a high Voc in the polymer solar cell applications.
PTBDT-BTZ and POPEBDT-BTZ presented deeper HOMO
levels than PTBDT-BT and POPEBDT-BT. PTBDT-BTZ
exhibited an especially deep HOMO energy level (ꢀ5.91 eV) in
comparison with the other synthesized polymers. The LUMO
energy levels of the synthesized polymers were about 0.7 eV
higher than the LUMO level (ꢀ4.2 eV) of PCBM, generating a
downhill driving force for the energetically favorable electron
transfer reactions.
Photovoltaic properties
The photovoltaic characteristics of the synthesized polymers
were investigated by fabricating devices with the structure of
ITO/MoO3/polymer:PC71BM/LiF/Al. Fig. 4 shows the J–V
characteristics of devices under AM 1.5 conditions with an
intensity of 100 mW cmꢀ2. The best electrical properties achieved
under various conditions are summarized in Table 2. The
PTBDT-BT:PC71BM 1 : 1 (w/w) blend device with a thickness of
120 nm showed the best performance, with a Voc of 0.81 V, a Jsc
of 8.86 mA cmꢀ2, a FF of 47.9%, and a PCE of 3.44%. The
PTBDT-BTZ:PC71BM,
POPEBDT-BT:PC71BM,
and
POPEBDT-BTZ:PC71BM devices showed the best perfor-
mances at 1 : 4 (w/w) blend ratios, thicknesses of 75–85 nm, and
without any post-treatments.
As shown in Fig. 4, all devices comprising the polymers
exhibited high Voc values of 0.73–0.92 V. Although the origin of
the Voc remains the subject of debate, it is widely accepted that
the energy difference between the HOMO of the donor and the
LUMO of the acceptor determines the built-in potential of a
device and, consequently, the values of Voc.34 As shown in Fig. 3,
the synthesized polymers possessed deep HOMO energy levels
relative to P3HT (ꢀ5.0 eV). Moreover, the highest values of Voc
achieved in the devices displayed a similar trend as a function of
the HOMO levels of the donor polymers. The values of Voc
achieved in this study were attributed to the deep HOMO energy
levels of the polymers, possibly due to the extended delocaliza-
tion of BDT units upon the introduction of conjugated
substituents. The slight differences between the properties of
PTBDT-BT and POPEBDT-BT could be explained in terms of
the different optimal compositions of the photoactive layers, 1 : 1
for PTBDT-BT and 1 : 4 for POPEBDT-BT.35
The generation of photocurrent from organic photovoltaic
(OPV) cells using the synthesized polymers was investigated by
comparing the external quantum efficiencies (EQE) of the
devices. The measured EQE values agreed well with the
measured Jsc values from the J–V curves, as shown in Fig. 4(b).
The calculated Jsc values from the EQE obtained under AM 1.5
solar illumination differed only slightly (<5%) from the measured
values. In principle, photocurrent generation depends directly on
the product of light absorption, exciton separation, and charge
collection. Because all synthesized polymers showed similar
LUMO energy levels (Fig. 3), the exciton dissociation at the
polymer–PC71BM interface should have minor effects on the
generation of charge carriers.36 In this regard, variations in
Fig. 4 (a) J–V characteristics of the polymer:PC71BM solar cells (AM
1.5G, 100 mW cmꢀ2) and (b) EQE curves of the corresponding polymer
solar cells.
Table 2 Photovoltaic properties of polymer solar cells incorporating polymer:PC71BM blends
Interfacial
layer
Polymer : PC71BM
(weight ratio)
Polymer
VOC (V)
JSC (mA cmꢀ2
)
FF (%)
PCE (%)
PTBDT-BTa
PTBDT-BTZ
POPEBDT-BT
POPEBDT-BTZ
LiF
ZnO-PEG
LiF
ZnO-PEG
LiF
ZnO-PEG
LiF
ZnO-PEG
1 : 1
1 : 1
1 : 4
1 : 4
1 : 4
1 : 4
1 : 4
1 : 4
0.81
0.81
0.92
0.74
0.73
0.74
0.83
0.83
8.86
9.62
0.04
0.68
4.16
6.49
2.39
3.71
47.9
59.2
35.5
27.9
51.3
54.9
57.1
58.2
3.44
4.61
0.013
0.14
1.56
2.91
1.14
1.82
a
With 3 vol% DIO.
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the photovoltaic performance resulted from differences in the
optical absorption, phase separation, and charge transport
properties of the polymers.
from the obvious increase in the surface roughness (Fig. 5),
micron-scale aggregation and dewetting behavior prevailed upon
DIO addition (Fig. 6). As a result, photovoltaic properties were
not observed.
The charge transport properties of the synthesized polymers
affected the solar cell efficiency. These effects were investigated
by measuring the space charge limited current (SCLC) mobility
for each polymer. As shown in Fig. S6,† the measured hole
mobilities were 1.93 ꢂ 10ꢀ3 cm2 Vꢀ1 sꢀ1 for PTBDT-BT, 4.34 ꢂ
10ꢀ5 cm2 Vꢀ1 sꢀ1 for PTBDT-BTZ, 4.82 ꢂ 10ꢀ4 cm2 Vꢀ1 sꢀ1 for
POPEBDT-BT and 5.66 ꢂ 10ꢀ4 cm2 Vꢀ1 sꢀ1 for POPEBDT-
BTZ. In general, an imbalance between electron and hole
mobilities can promote the accumulation of space charge in BHJ
structures, which results in the insufficient extraction of photo-
generated charge.37 The devices would then exhibit a low fill
factor (FF) (<40%) with a higher series resistance and leakage
current.8,38–40 The devices prepared from each polymer except for
PTBDT-BTZ showed relatively high FFs (>55%) under optimal
conditions. These results indicated that within the range of the
measured hole mobilities, the balance between each type of
carrier transport did not play an important role in the photo-
generation of current.
Finally, the effects of interfacial layers on device performance
were investigated. A solution of PEGylated ZnO nanoparticle
The effects of the optical absorption were investigated by
measuring the absorption coefficients (a) of the synthesized
polymers. PTBDT-BT displayed the highest maximum among
the polymers, an a of 650 nm, which was one order of magnitude
higher than the value for PTBDT-BTZ and twice the values of a
for POPEBDT-BT and POPEBDT-BTZ (Fig. S7†). These
results agreed well with the measured current densities of the
devices, as shown in Table 2. Although POPEBDT-BTZ dis-
played a higher maximum a than POPEBDT-BT, the current
density was higher for POPEBDT-BT. This discrepancy could be
explained in terms of the development of phase separation and
morphological variations within the photoactive layers. As
shown in Table 1, the difference of the surface energies of the
polymers to the PC71BM (34.2 mJ mꢀ2) is higher in the case of
POPEBDT-BT. Since the polymers exhibited no particular
crystallinity (Fig. S4†), the weak interactions between the poly-
mer chains may have provided a well-mixed D–A phase in the
blend films without the need for post-treatments. The differences
between the surface energies of the donor and acceptor phases
could have provided a strong driving force for spontaneous
phase separation during the film-forming process, possibly by
providing well-developed bicontinuous carrier transport path-
ways other than well-mixed blend structures.8,41,42
The photovoltaic performance of the PTBDT-BT:PC71BM
blend could be optimized by adding 1,8-diiodooctane (DIO), a
solvent additive with selective solubility for PC71BM, to the
blend solution prior to film deposition. PTBDT-BT has a surface
energy (33.69 mJ mꢀ2) similar to that of PC71BM; therefore, the
materials initially formed a well-mixed blend structure. By
contrast, the presence of the additive induced the chains of the
donor polymer to self-aggregate, resulting in the spontaneous
separation of donor and acceptor phases.42–44 The photovoltaic
performance of the PTBDT-BT:PC71BM blend in the presence of
DIO exhibited a 20% increase in the photocurrent with a slight
decrease (0.04 V) in Voc; however, because the other polymers
also displayed low surface energies, the presence of DIO in the
blend solutions produced dramatic phase separation in the blend
films, especially in PTBDT-BTZ and POPEBDT-BT. Aside
Fig. 5 AFM topography images of polymer:PC71BM blend films.
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for POPEBDT-BT, and ꢀ5.64 eV for POPEBDT-BTZ) were
achieved by introducing TIPS and 4-octylphenylethynyl
substituents onto the BDT unit. The photovoltaic performances
of the synthesized polymers were investigated using the device
configuration ITO/MoO3/polymer:PC71BM/ZnO/Al. PTBDT-
BT exhibited the best photovoltaic performance in the series,
with a Jsc of 9.62 mA cmꢀ2, a Voc of 0.81 V, a FF of 59.2%, and a
PCE of 4.61% due to the highest hole mobility (1.93 ꢂ 10ꢀ3 cm2
Vꢀ1 sꢀ1), the high absorption coefficients, and an excellent BHJ
morphology.
Acknowledgements
This work was supported by a grant (code no. 2011-0031628)
from the Center for Advanced Soft Electronics under the Global
frontier Research Program of the Ministry of Education, Science
and Technology, Korea.
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