Thiophene-rich fused-aromatic thienopyrazine acceptor for donor-acceptor low band-gap polymers for OTFT and polymer solar cell applications

Article abstract of DOI:10.1039/c0jm00903b

Thiophene enriched fused-aromatic thieno[3,4-b]pyrazine systems were designed and employed to produce low band gap polymers (E_g = 1.0-1.4 eV) when copolymerized with fluorene and cyclopentadithiophene. The copolymers are mainly investigated for organic thin film transistor and organic photovoltaic applications. Molecular packing in the thin films of these polymers was investigated using Grazing incidence X-ray Scattering. Although both fluorene and cyclopentadithiophene polymers follow similar face to face π-π stacking, the latter polymers show much smaller lamellar d-spacings due to side-chain interdigitation between the lamellae. This lead to the higher charge carrier mobilities in cyclopentadithiophene polymers (up to 0.044 cm2/V.s) compared to fluorene polymers (up to 8.1 × 10^-3 cm²/V.s). Power conversion efficiency of 1.4% was achieved using fluorene copolymer in solar cells with a fullerene derivative as an acceptor. Although the cyclopentadithiophene polymers show lower band gaps with higher absorption coefficients compared to fluorene copolymers, but the power conversion efficiencies in solar cells of these polymers are low due to their low ionization potentials. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0jm00903b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Thiophene-rich fused-aromatic thienopyrazine acceptor for donor–acceptor
low band-gap polymers for OTFT and polymer solar cell applications†
Rajib Mondal,a Hector A. Becerril,a Eric Verploegen,ad Dongwook Kim,‡b Joseph E. Norton,b Sangwon Ko,a
a
Nobuyuki Miyaki, Sangjun Lee, Michael F. Toney, Jean-Luc Bredas, Michael D. McGehee
c
d
b
c
ꢀ
a
*
and Zhenan Bao
Received 31st March 2010, Accepted 22nd April 2010
First published as an Advance Article on the web 4th June 2010
DOI: 10.1039/c0jm00903b
Thiophene enriched fused-aromatic thieno[3,4-b]pyrazine systems were designed and employed to
produce low band gap polymers (Eg ¼ 1.0–1.4 eV) when copolymerized with fluorene and
cyclopentadithiophene. The copolymers are mainly investigated for organic thin film transistor and
organic photovoltaic applications. Molecular packing in the thin films of these polymers was
investigated using Grazing incidence X-ray Scattering. Although both fluorene and
cyclopentadithiophene polymers follow similar face to face p–p stacking, the latter polymers show
much smaller lamellar d-spacings due to side-chain interdigitation between the lamellae. This lead to
the higher charge carrier mobilities in cyclopentadithiophene polymers (up to 0.044 cm2/V.s) compared
to fluorene polymers (up to 8.1 ꢀ 10ꢁ3 cm2/V.s). Power conversion efficiency of 1.4% was achieved
using fluorene copolymer in solar cells with a fullerene derivative as an acceptor. Although the
cyclopentadithiophene polymers show lower band gaps with higher absorption coefficients compared
to fluorene copolymers, but the power conversion efficiencies in solar cells of these polymers are low
due to their low ionization potentials.
In addition, it is important for the polymer to have good charge-
carrier mobility for efficient charge dissociation.9
1. Introduction
Development of new polymer semiconducting materials has
become an active area of research in recent years due to their
potential uses in light weight and flexible organic photovoltaics
(OPVs) and organic thin-film transistors (OTFTs).1–5 However,
in many instances, the lower efficiency and limited air stability of
polymers has limited their commercial use.1 A combination of
donor polymer and fullerene derivative (as acceptor) is
commonly used in bulk heterojunction (BHJ) solar cells. In most
of these solar cells, a small region of the solar spectrum is
absorbed, mainly in the visible region. For example, poly-
(phenylene vinylene) (PPV)6 and poly(3-hexyl thiophene)
(P3HT)7 have band gaps larger than 1.9 eV, and thus absorb only
30% from the AM1.5 (air mass) solar photon flux. Lower band
gap polymers can achieve absorption of a greater fraction of light
from the solar spectrum and enable a higher efficiency solar cell.8
Recently, we have reported electron-deficient fused aromatic
thienopyrazine (TP) units for donor–acceptor type conjugated
polymers.10,11 The fused TP unit provides a planar and electron
rich p-face that enhances p–p stacking between the polymeric
chains.12,13 It was shown that a field-effect mobility as high as
0.2 cm2/Vs can be achieved using acenaphthylthienopyrazine
type copolymers.10 However, the power conversion efficiency
(PCE) was limited to only 1.4% for BHJs with the polymers in
this class, which may be due to higher band gaps (ꢂ1.65 eV) and
lower absorption coefficients (3–6 ꢀ 104 cmꢁ1).11 Interestingly,
the band-gap can be significantly lowered by replacing ace-
naphthyl with phenanthrene in the TP unit. However, the
absorption coefficient in the latter polymer became rather low
(3.4 ꢀ 104 cmꢁ1) and the charge carrier mobility was also
reduced. We conjectured that the lower mobility could be due to
lower molecular weight of the phenanthrene-based polymer,14
which was again limited by poor solubility. This was also
responsible for the poor device performance.
aDepartment of Chemical Engineering, Stanford University, Stanford, CA,
94305. E-mail: zbao@stanford.edu
bSchool of Chemistry and Biochemistry and Center for Organic Photonics
and Electronics, Georgia Institute of Technology, Atlanta, GA, 30332
cDepartment of Materials Science, Stanford University, Stanford, CA,
94305
dStanford Synchrotron Radiation Lightsource, Menlo Park, CA, 94025
† Electronic supplementary information (ESI) available: Transistor and
solar cell fabrication, HOMO and LUMO wavefunctions of the
monomers, absorption spectra of the polymers in solution and thin
films, I–V and output curves of p-type OTFTs of BDTup-FL and
BDTup-CPDT. See DOI: 10.1039/c0jm00903b
In this study, we describe the design and synthesis of fused TP
units enriched with thiophene groups in the form of benzodithio-
phene (BDT) – the dithiophene analog of phenanthrene. Detailed
quantum-chemical characterization was carried out using density
functional theory (DFT). Thiophene-rich fused aromatic units in
conjugated polymers are of particular importance dueto in general
their lower reorganization energies and improved device perfor-
mance compared to the corresponding benzene analogs.15 Thio-
phenederivativesarealsowellknowntoprovidebettersolubilityin
organic solvents compared to their benzene analogs. Copolymers
of these thiophene enriched fused-aromatic TP with fluorene (FL)
‡ Current address: Department of Chemistry, Kyonggi University,
Suwon, 443-760, Korea
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ꢃ
reaction mixture was cooled to 0 C and a solution of stannous
chloride dihydrate (17.5 g, 77 mmol) in 25 mL ethanol was added
slowly. The resulted reaction mixture was vigorously stirred at 30
ꢃC for overnight (ꢂ18 h). The reaction mixture was poured into
50 mL of cold 25% sodium hydroxide solution. The resulted
basic solution was passed through a tightly packed celite pad,
followed by washing with toluene. The aqueous layer was
extracted with toluene. Drying in a rotary evaporator under
1
reduced pressure yielded 1.02 g (100%) of diamine 4. H NMR
(300 MHz, CDCl3) d 0.8 (m, 12 H), 1.2 (m, 16 H), 1.604 (m, 2H),
2.536 (d, J ¼ 7.2 Hz, 4 H), 3.436 (s, 4H), 6.940 (d, J ¼ 5.4 Hz, 2
H), 7.280 (d, J ¼ 5.1 Hz, 2 H).
Benzo[1,2-b:4,3-b0]dithiophene-4,5-dione (5). 3,30-Bithiophene
(10 g, 60 mmol) was taken in a dry 500 mL round bottom flask
and 175 mL of dry 1,2-dichloroethane was added. Three
portions of oxalyl chloride (3 mL, 34.5 mmol) were added in
each 5 days and the reaction mixture was continuously refluxed
for 15 days under Ar atmosphere. The reaction mixture was
cooled to rt and then kept in refrigerator for overnight. The
resulted red precipitate was filtered and washed with hexanes
Chart 1 Structures of the thienopyrazine copolymers containing fused
aromatic units discussed in this work. Synthesis and device performances
of AC-FL and PH-FL were previously reported.11
and cyclopentadithiophene (CPDT) derivatives (Chart 1) are
synthesized. 3-(2-Ethylhexyl)thiophene groups were introduced
to both sides of a fused aromatic TP to enhance the solubility in
common organic solvents. 2-Ethylhexyl groups (branched alkyl
chains) were chosen for this study as this class of polymers with
branched alkyl chains have shown better efficiency in BHJ solar
cells compared to the ones with linear alkyl chains.11 The
synthesized polymers were used to fabricate OTFT and BHJ solar
cell devices and the performance of these devices is discussed.
Packing in the thin film was investigated using Grazing incidence
X-ray Scattering. The observed differences in device performance
appear to be related to variations in interchain p–p interactions
and lamellar d-spacing. The CPDT polymers show lower band
gaps, higher absorption coefficients, and higher charge-carrier
mobility compared to the FL copolymers; they also form
smoother thin films. However, the FL copolymers present higher
power conversion efficiencies in solar cells.
1
and warm ethanol. Yield of dione 5 was 10.1g (76%). H NMR
(300 MHz, CDCl3) d 7.282 (d, J ¼ 7.8 Hz, 2 H), 7.818 (d,
J ¼ 7.8 Hz, 2 H); HRMS (EI) m/z ¼ 220.9726 (M + H+), calcd
m/z ¼ 220.9726.
Benzo[2,1-b:3,4-b0]dithiophene-4,5-dione (6). 50 mL of ethanol
and 12 mL of water were added to 2,20-bithiophene-3,30-dicar-
boxaldehyde (1.2 g, 5.4 mmol). Sodium cyanide (0.1 g, 2 mmol)
was added to the reaction mixture and refluxed for 2 h. The
reaction flask was then cooled slowly and kept under ambient
condition for overnight to promote aerial oxidation of the
reaction mixture. The resulted black precipitate (0.3 mg) was
filtered out and washed with hexane and ethanol. The filtrate was
extracted with CH2Cl2, the solvent was dried with Na2SO4; and
evaporated in a rotary evaporator. The crude redish product was
subjected to column chromatography (silica gel, 50% CH2Cl2 in
hexanes) to yield another 0.15 g of dione 6. The overall yield was
0.45 g (38%). 1H NMR (300 MHz, CDCl3) d 7.206 (d, J ¼ 5.1 Hz,
2 H), 7.498 (d, J ¼ 5.1 Hz, 2 H); HRMS (EI) m/z ¼ 220.9722
(M + H+), calcd m/z ¼ 220.9726.
2. Experimental
Synthesis of monomers
2,5-Bis(3-(2-ethylhexyl)thiophen-2-yl)-3,4-dinitrothiophene (3).
3-(2-Ethylhexyl)-2-tributylstannylthiophene (1) and 2,5-dibromo-
3,4-dinitrothiophene (2) were synthesized following literature
reported procedures. Compounds 1 (5.4 g, 12 mmol), 2 (2.0 g, 6
mmol), and catalytic amount PdCl2(PPh3)2 (44 mg, 0.06 mmol)
were placed in dry 35 mL microwave proof thick walled glass vial
equipped with a stir bar and a snap cap. 20 mL of dry THF was
added and degassed by bubbling Ar gas for 30 min. The solution
8,10-Bis(3-(2-ethylhexyl)thiophen-2-yl)trithieno[3,4-b:20,30-
f:300,200-h]quinoxaline (7). Diamine 4 (0.99 g, 1.97 mmol) and
dione 5 (0.42 g, 1.90 mmol) were suspended in 75 mL of dry
chloroform. Catalytic amount of p-toluenesulfonic acid (38 mg,
0.2 mmol) was added. The pH of the solution should be little
acidic at this stage. The resulted solution was stirred overnight
under Ar atm to yield a deep green solution. The chloroform
solution was washed with dil. NaHCO3 (aq.) and dried using
anhydrous Na2SO4. After removing the solvent in rotary evap-
orator, the crude product was subjected to column chromatog-
raphy (silica gel, 25% CH2Cl2 in hexanes) to yield compound 7 as
green powder (0.77 g, 57%). 1H NMR (300 MHz, CDCl3) d 0.820
(m, 12 H), 1.2–1.4(m, 16 H), 1.78 (m, 2H), 3.002 (d, J ¼ 7.5 Hz,
4 H), 7.042(d, J ¼ 5.1 Hz, 2 H), 7.459(d, J ¼ 5.1 Hz, 2 H), 7.63(d,
J ¼ 5.1 Hz, 2 H), 7.73(d, J ¼ 5.1 Hz, 2 H); 13C NMR (125 MHz,
CDCl3) d 11.696, 15.086, 24.063, 26.792, 29.685, 33.613, 35.825,
40.558, 124.275, 124.615, 127.633, 130.166, 131.200, 131.725,
ꢃ
was heated at 70 C using 200 W of microwave power for 2 h.
After the reaction, THF was evaporated in a rotary evaporator
and the oily product was subjected to column chromatography
(silica gel, 20% CH2Cl2 in hexane). The compound 3 was isolated
1
yellow viscous liquid (3.3 g, 98%). H NMR (300 MHz, CDCl3)
d 0.8 (m, 12 H), 1.193 (m, 16 H), 1.52 (m, 2H), 2.517 (d, J ¼ 7.2
Hz, 4 H), 6.988 (d, J ¼ 5.1 Hz, 2 H), 7.482 (d, J ¼ 5.1 Hz, 2 H);
HRMS (EI) m/z ¼ 563.2070 (M + H+), calcd m/z ¼ 563.2067.
2,5-Bis(3-(2-ethylhexyl)thiophen-2-yl)thiophene-3,4-diamine
(4). Compound 3 (1.15 g, 2.04 mmol) was dissolved in 35 mL of
ethanol and 35 mL of conc. HCl was added carefully. The
5824 | J. Mater. Chem., 2010, 20, 5823–5834
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136.878, 137.864, 139.327, 140.677, 140.930; HRMS (EI) m/z ¼
687.2021 (M + H+), calcd m/z ¼ 687.2024.
capped with phenyl group by adding excess of phenylboronic
acid and bromobenzene. Crude polymers were precipitated in
methanol and washed with hexane and acetone. These polymers
were further redissolved in 5 mL of chloroform along with Pd(0)
scavenger16 and stirred at rt for overnight. The solution filtered
through a 0.45 mm PTFE syringe filter and precipitated in
methanol. The polymer was filtered and washed with acetone and
hexane.
8,10-Bis(3-(2-ethylhexyl)thiophen-2-yl)trithieno[3,4-b:30,20-
f:200,300-h]quinoxaline (8). Compound 8 was synthesized from
dimaine 4 and dione 6 similarly as described for the synthesis of
compound 7. Yield ¼ 78%. 1H NMR (400 MHz, CDCl3) d 0.822
(m, 12 H), 1.2–1.38 (m, 16 H), 1.812 (m, 2H), 3.004 (d, J ¼ 7.6 Hz,
4 H), 7.036 (d, J ¼ 5.2 Hz, 2 H), 7.423(d, J ¼ 5.2 Hz, 2 H), 7.444
(d, J ¼ 5.2 Hz, 2 H), 8.279 (d, J ¼ 5.2 Hz, 2 H); 13C NMR (100
MHz, CDCl3) d 11.692, 15.089, 24.068, 26.790, 29.687, 33.607,
35.860, 40.576, 124.599,125.160, 126.738, 127.451, 130.241,
131.151, 136.406, 137.036, 139.425, 140.729, 140.888; HRMS
(EI) m/z ¼ 687.2018 (M + H+), calcd m/z ¼ 687.2024.
BDTdn-FL: Yield ¼ 90%, GPC (THF): Mw ¼ 12844, Mn
¼
7510, PDI ¼ 1.71; 1H-NMR (300MHz, CDCl3) d 0.5–1.5 (m, 58
H), 1.9–2.1 (m, 6H), 3.1 (br, 4H), 7.34 (br, 4H), 7.6–7.9 (m, 8H);
anal. calcd for C67H80N2S5: C 74.95, H 7.51, N 2.61, S 14.93;
found C 73.53, H 6.92, N 2.39, S 13.87. BDTup-FL: Yield ¼ 92%,
1
GPC (THF): Mw ¼ 33783, Mn ¼ 12895, PDI ¼ 2.62; H-NMR
(300MHz, CDCl3) d 0.5–1.5 (m, 58 H), 1.85–2.15 (m, 6H), 3.15
(br, 4H), 7.35 (br, 4H), 7.76 (br, 6H), 8.7 (br, 2H); anal. calcd for
C67H80N2S5: C 74.95, H 7.51, N 2.61, S 14.93; found C 74.14, H
7.33, N 2.41, S 13.91.
8,10-Bis(5-bromo-3-(2-ethylhexyl)thiophen-2-yl)trithieno[3,4-
b:20,30-f:300,200-h]quinoxaline (9). N-Bromosuccinimide (0.302 g,
1.7 mmol) in 25 mL of DMF was added dropwise to a suspension
of the compound 7 (0.53 g, 0.77 mmol) in 50 mL of DMF
at ꢁ20 ꢃꢃC for 30min. The reaction mixture was stirred for 2h
at ꢁ20 C, warmed to room temperature. The resulted solution
was poured into water and extracted with CH2Cl2. The crude
product was then subjected to column chromatography (silica
General procedure for microwave assisted Stille poly-
condensation reaction. Compound 9 or 10 (84.5 mg, 1 mmol),
4,4-bis(2-ethylhexyl)-2,6-bis(trimethylstannyl)-4H-cyclopenta[2,
1-b:3,4-b0]dithiophene (12, 1 mmol) and catalytic amount tetra-
kis(triphenylphosphine)palladium(0) (5 mg, 4.3 mol%) were
placed in dry 7 mL microwave proof thick walled glass vial
equipped with a stir bar and a snap cap. 2 mL of dry toluene was
added and degassed by bubbling Ar gas for 30 min. The solution
was heated using fixed power (300 W) for 1 h. Maximum
1
gel, 15% CH2Cl2 in hexanes). Yield ¼ 0.3 g, 46%. H NMR
(300 MHz, CDCl3) d 0.850 (m, 12 H), 1.2–1.4(m, 16 H), 1.77 (m,
2H), 2.91 (d, J ¼ 7.5 Hz, 4 H), 6.98(s, 2 H), 7.645(d, J ¼ 5.1 Hz,
2 H), 7.751(d, J ¼ 5.1 Hz, 2 H); 13C NMR (125 MHz, CDCl3)
d 11.692, 15.142, 24.087, 26.7, 29.641, 33.453, 36.294, 40.233,
115.434, 123.137, 124.130, 131.941, 132.298, 133.051, 136.441,
137.852, 138.594, 140.124, 140.261; HRMS (EI) m/z ¼ 843.0249
(M + H+), calcd m/z ¼ 843.0235.
ꢃ
temperature of 180 C and internal pressure of 30 PSI reached
during the reaction. The polymer was precipitated in methanol.
1 mL of conc. HCl was added. The crude polymer was filtered
and washed with acetone and hexanes. Polymers were further
purified as described for the Suzuki polycondensation reaction.
BDTdn-CPDT: Yield ¼ 85%, GPC (THF): Mw ¼ 13702, Mn ¼
8620, PDI ¼ 1.59; 1H-NMR (300MHz, CDCl3) d 0.6–1.5 (m, 58
H), 1.8–2.2 (m, 6H), 3.05 (br, 4H), 7.15–7.3 (m, 4H), 7.5–7.8 (m,
4H); anal. calcd for C63H76N2S7: C 69.69, H 7.06, N 2.58, S
20.67; found C 67.69, H 6.60, N 2.48, S 19.24; BDTup-CPDT:
Yield ¼ 89%, GPC (THF): Mw ¼ 17250, Mn ¼ 10150, PDI ¼
8,10-Bis(5-bromo-3-(2-ethylhexyl)thiophen-2-yl)trithieno[3,4-
b:30,20-f:200,300-h]quinoxaline (10). N-Bromosuccinimide (0.230 g,
1.3 mmol) in 30 mL of DMF was added dropwise to a suspension
of the compound 8 (0.4 g, 0.59 mmol) in 15 mL of DMF
at ꢁ20 ꢃC for 30min. The reaction mixture was stirred for 2h
at ꢁ20 ꢃC, warmed to room temperature. After stirring the
reaction mixture for overnight, the resulted solution was
extracted with chloroform and washed with water. Finally, the
crude product was subjected to column chromatography (silica
gel, 15% CH2Cl2 in hexanes). Yield ¼ 49%. 1H NMR (400 MHz,
CDCl3) d 0.890 (m, 12 H), 1.2–1.43 (m, 16 H), 1.77 (m, 2H), 2.757
(d, J ¼ 7.2 Hz, 4 H), 6.809 (s, 2 H), 7.276 (d, J ¼ 5.2 Hz, 2 H),
7.921 (d, J ¼ 5.2 Hz, 2 H); 13C NMR (100 MHz, CDCl3) d 11.692,
15.172, 24.113, 26.707, 29.657, 33.456, 36.413, 40.159, 115.272,
123.022, 125.100, 126.518, 132.410, 132.835, 135.754, 136.907,
138.371, 139.713, 139.993; HRMS (EI?) m/z ¼ 845.0199 (M +
H+), calcd m/z ¼ 845.0214.
1
1.70; H-NMR (300MHz, CDCl3) d 0.6–1.5 (m, 58 H), 1.8–2.1
(m, 6H), 3.0 (br, 4H), 6.9–7.3 (m, 4H), 7.48 (br, 2H), 8.4 (b, 2H);
anal. calcd for C63H76N2S7: C 69.69, H 7.06, N 2.58, S 20.67;
found C 69.42, H 6.88, N 2.51, S 19.32.
3. Results and discussion
3.1 Synthesis of monomers and polymers
2,5-Bis(3-(2-ethylhexyl)thiophen-2-yl)thiophene-3,4-diamine (4)
was synthesized almost quantitatively by reducing 2,5-bis(3-(2-
ethylhexyl)thiophen-2-yl)-3,4-dinitrothiophene (3) using SnCl2/
HCl in ethanol (Scheme 1).17 Dinitro compound 3 was prepared
following a microwave assisted Stille coupling reaction between
General procedure for Suzuki polycondensation reaction.
0.5 mL of potassium carbonate solution (aq., 2M) was added
to a mixture of compound 9 or 10 (0.1 mmol), 2,7-bis-
(trimethyleneborate)-9,9-bis(2-ethylhexyl)fluorene
(11,
0.1
mmol), tetrakis(triphenylphosphine)palladium (0) (3.5 mg, 3
mol%), and Aliquatꢀ336 (0.02 g) in 2 mL of o-dichlorobenzene
under argon. The reaction mixture was frozen and degassed
under vacuum for three times. Then the reaction mixture was
heated to 95 ꢃC and stirred for 5 days. These polymers were end
Scheme 1
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in o-dichlorobenzene (ODCB) afforded relatively higher Mw
polymers, since conjugated polymers are usually more soluble in
chlorinated solvents and higher temperature can be used for
polymerization due to the higher boiling point of ODCB. For
example, the number average molecular weight (Mn) of BDTup-
FL was increased to 13000 g/mol from 6000 g/mol by changing
the solvent from toluene to ODCB for polymerization.
Microwave assisted Stille coupling reactions were carried out
between 9 and 10 with 4,4-bis(2-ethylhexyl)-2,6-bis(trimethyl-
stannyl)-4H-cyclopenta[2,1-b:3,4-b0]dithiophene (12) in toluene
to synthesize the CPDT copolymers BDTdn-CPDT and BDTup-
CPDT, respectively. Any tin complex remaining in these poly-
mers were hydrolyzed with dilute HCl in MeOH to yield bromo
or hydrogen end-capped polymers. Microwave assisted poly-
merization has advantages over the traditional thermal poly-
merization reactions.23 High Mw polymers can be synthesized
within a very short time period using microwave. It is believed
that decomposition of any reactive intermediates are minimized
by the very fast microwave-assisted heating. In our case, we
carried out the polymerization for one hour. Mws are increased
during the successive irradiation in the first 20–30 mins, as
observed during the optimization processes. A relatively low Mw
polymer was synthesized following the traditional thermal
polymerization reaction, even though the reaction was carried
out for more than 5 days. The Suzuki coupling reaction could not
be optimized as it uses a heterogeneous aqueous reaction mixture
and the solution boils vigorously under microwave radiation.
After both Suzuki and Stille coupling reactions, polymers were
precipitated from the reaction mixtures into MeOH. The
precipitate was filtered and redissolved in chloroform with
palladium scavenger in order to remove any residual palladium
catalyst.24 Precipitation in methanol followed by filtration and
washing with acetone and hexane afforded pure polymers. These
polymers are soluble in hot tetrahydrofuran (THF), chloroform,
chlorobenzene, and ODCB.
Scheme 2
two equivalent of 3-(2-ethylhexyl)-2-tributylstannylthiophene
(1) and one equivalent of 2,5-dibromo-3,4-dinitrothiophene
(2). Condensation between diamine 4 and benzo[1,2-b:4,3-
b0]dithiophene-4,5-dione (5) and benzo[2,1-b:3,4-b0]dithiophene-
4,5-dione (6) in chloroform yielded fused aromatic TPs;
8,10-bis(3-(2-ethylhexyl)thiophen-2-yl)trithieno[3,4-b:20,30-
f:300,200-h]quinoxaline (BDTdn, 7; 57%) and 8,10-bis(3-(2-ethyl-
hexyl)thiophen-2-yl)trithieno[3,4-b:30,20-f:200,300-h]quinoxaline
(BDTup, 8; 78%), respectively. Quinone 5 was synthesized with
76% yield by a reported Friedel–Crafts acylation reaction of 3,30-
bithiophene with oxalyl chloride.18 Synthesis of quinone
6 was achieved by a intramolecular benzoin condensation of
2,-20-bithiophene-3,30-dicarboxaldehyde with 38% yield.19
Bromination of 7 and 8 with N-bromosuccinimide (NBS) in
DMF afforded 8,10-bis(5-bromo-3-(2-ethylhexyl)thiophen-2-
yl)trithieno[3,4-b:20,30-f:300,200-h]quinoxaline (9, 46%) and
8,10-bis(5-bromo-3-(2-ethylhexyl)thiophen-2-yl)trithieno[3,4-
b:30,20-f:200,300-h]quinoxaline (10, 49%), respectively (Scheme
2).11 These compounds contain the bromo polymerizable sites
that were used for the polymerization. They are also relatively
stable and readily soluble in chlorinated organic solvents.
Donor–acceptor type copolymers of these fused TPs with
fluorene (FL) and cyclopentadithiophene (CPDT)15,20 were
synthesized via Suzuki and Stille polycondensation reactions,
respectively (Scheme 3). The TP units are electron poor and act
as acceptor units, whereas FL and CPDT are donor units in the
polymer chain. The Suzuki coupling between 9 and 10 with 2,7-
bis(trimethyleneborate)-9,9-bis(2-ethylhexyl)fluorene (11) yiel-
ded FL copolymers BDTdn-FL and BDTup-FL, respectively,
with good yields.21,22 These polymers were end-capped with
phenyl groups. When toluene was used as solvent for polymeri-
zation reactions, the molecular weights (Mw) of the polymers
were very low. We assumed that the lower solubility of the
polymers in toluene was responsible for low Mw. Polymerization
3.2 Optical and electrochemical properties
The optical and electrochemical properties of the polymers were
investigated in ODCB and in thin films and are summarized in
Table 1. The absorption spectra of the thin films of the FL
copolymers (BDTdn-FL and BDTup-FL) are compared with
acenaphthyl- and phenanthrene-based FL copolymers11 in Fig. 1.
The absorption peaks are mainly located in two regions; around
300–500 nm and around 525–775 nm for acenaphthyl and 600–
950 nm for the others. The absorption spectra of the BDT-based
FL copolymers are very similar to those of the phenanthrene-
based FL copolymers with significant absorption in the near-
infrared region of the solar spectrum. This can help in generating
higher photocurrent in photovoltaic devices by absorbing more
sunlight. The fused aromatic unit used for TP strongly affects the
band gap of the resulting polymer. It was shown that the band
gap or the absorption edge could be efficiently reduced by more
than 0.2 eV by changing the aromatic unit from acenaphthyl to
phenanthrene.11 Band gaps can be further reduced by ꢂ0.1 eV by
introducing BDT units, the thiophene analogs of phenanthrene.
However, no dramatic changes in the absorption coefficients
were observed. Absorption coefficients are in the range of
Scheme 3
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Table 1 Optical and Electrochemical Properties of the Polymers
Solution (in o-dichlorobenzene)
Thin Films
d
Polymers
PH-FL
M
w (PDI)a
A
max(nm)
HOMOb(V) LUMOb(V) Eg(eV) Amax(onset)(nm)
a(cmꢁ1
)
Eg(eV)c HOMOe(V) LUMOe(V)
7710 (1.57) 445, 730
ꢁ4.85
ꢁ3.45
ꢁ3.52
ꢁ3.45
ꢁ3.53
ꢁ3.46
1.40
1.41c
1.33
1.29c
1.39
1.32c
1.07
1.11c
1.14
1.14c
464, 742 (888)
3.4 ꢀ 104 1.40
ꢁ4.93
ꢁ3.53
BDTdn-FL
BDTup-FL
12844 (1.71) 349, 447, 774 ꢁ4.85
33783 (2.62) 336, 393, 746 ꢁ4.84
329, 447, 777 (964) 3.0 ꢀ 104 1.29
349, 357, 779 (955) 3.5 ꢀ 104 1.30
368, 505, 904(1204) 4.0 ꢀ 104 1.03
356, 509, 870 (1117) 6.2 ꢀ 104 1.11
ꢁ5.16
ꢁ3.87
ꢁ5.00
ꢁ3.70
BDTdn-CPDT 13702 (1.59) 368, 515, 893 ꢁ4.60
BDTup-CPDT 17250 (1.70) 354, 499, 839 ꢁ4.60
-
-
-
-
a
Determined from GPC using the THF soluble part using polystyrene as standard and THF as eluent, Weight average molecular weight (Mw) in
b
Daltons, and PDI ¼ Polydispersity index. Measured from cyclic voltammetry in o-dichlorobenzene and calibrated based on the oxidation
c
d
e
potential of Fc/Fc+. Estimated from the onset of the absorption spectra. (O.D.max ꢀ ln10)/thickness of the film. HOMO energy levels were
estimated from PES.
display similar characteristic and have energies within a narrow
range of ꢁ4.3–ꢁ4.4 eV (Table 6). The LUMO levels are more
affected by the nature of the electron-accepting group and exhibit
a larger energy variation ranging from ꢁ2.50 eV for AC-FL
to ꢁ2.83 eV for BDTdn-FL. The lowest-energy LUMOs and
corresponding smaller HOMO–LUMO gap energies (which we
denote here as Eg) occur in the systems containing BDT groups.
The Eg values extrapolated for BDTdn-FL and BDTup-FL are
1.51 and 1.59 eV, respectively, and are as much as 0.4 eV lower
than that of the AC-FL copolymer and 0.1 to 0.2 eV lower than
that of the PH-FL copolymer. A similar effect is observed from
the calculated IP, EA, and transition energies, where the S1
transition energies of the BDT polymers are noticeably lower
than those of the AC-FL and PH-FL polymers; the S1 transition
energy (optical gap) for the BDTdn-FL polymer is calculated to
be 1.24 eV.
Fig. 1 Absorption spectra of the thin films of acenaphthyl, phenan-
The differences in the electronic energy levels calculated for the
threne, and benzodithiophene-based FL copolymers. The general struc-
copolymers are attributed to shifts in the LUMO levels origi-
nating from the nature of the aromatic group fused onto the
electron-accepting moieties. The HOMO and LUMO wave-
functions of the PH-FL and BDT copolymers are shown in
Fig. 2. For each copolymer, the LUMO wavefunction localizes
onto the fused aromatic groups and thus is more affected by the
electron-accepting nature of the acceptor, while the HOMO
wavefunction is more delocalized along the polymer backbone
and only marginally affected by the acceptor characteristics. The
thiophene-enriched BDT groups enhance the electron-accepting
nature of the thienopyrazine group of the acceptor.
ture of the polymers is given in the inset.
3–3.5 ꢀ 104 cmꢁ1 in the thin films of BDT and phenanthrene-
based FL copolymers.
The electronic properties of the monomer units and oligomer
structures of the BDT copolymers were calculated (as isolated
molecules) at the Density Functional Theory (DFT) B3LYP/6-
31G(d,p) level. The frontier energy levels (HOMO and LUMO)
were characterized for the neutral electronic states. The vertical
and adiabatic ionization potentials (IP) and electron affinities
(EA) were computed using the neutral and ionic potential energy
surfaces, and the lowest singlet excited state transitions were
calculated using Time Dependent (TD) DFT methods
(Table S3). The properties of the polymers were extrapolated
from oligomer calculations and Kuhn fits of energy versus 1/N
where N is the number of formal double bonds in the molecule.25
Note that the calculations were performed on model systems of
BDT copolymers where all side-chain substituents were replaced
with methyl groups (this has only minimal effect on the electronic
properties).
The HOMO (ionization potential) and LUMO (electron
affinity) energies were determined from cyclic voltammetry,
performed under argon atmosphere with a supporting electrolyte
of 0.05 M n-Bu4NPF6 in anhydrous ODCB. The HOMO of the
CPDT copolymers (4.6 eV) is located higher than in the FL
copolymers (ꢂ4.85 eV) in solution. However, the LUMO energy
of both the FL or CPDT copolymers remains constant, since they
share the same electron-accepting units. For example, the
LUMO energy of both BDTdn-FL and BDTdn-CPDT polymer is
ꢂ3.52 eV. The ionization potentials (IPs) of the polymer films
were measured by photoelectron spectroscopy (PES). IPs of
BDTdn-FL and BDTup-FL are 5.16 and 5.0 eV, respectively. The
The calculated HOMO levels among the fluorene-containing
copolymers (AC-FL, PH-FL, BDTdn-FL, and BDTup-FL)
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Fig. 3 Absorption spectra of the thin films of BDTup-FL and BDTup-
CPDT.
copolymers (BDTdn-CPDT and BDTup-CPDT) becomes rela-
tively broader compared to FL copolymers (BDTdn-FL and
BDTup-FL) and extends up to 1210 and 1125 nm with band gaps
of 1.03 and 1.11 eV for polymers BDTdn-CPDT and BDTup-
CPDT, respectively.
Comparing the polymer absorption of thin films and in solu-
tion, the BDTdn copolymers are red shifted by only 5–10 nm,
whereas the BDTup copolymers are red-shifted by ꢂ30 nm. This
suggests stronger intermolecular interactions and hence tighter
inter-chain packing for BDTup polymers compared to BDTdn
polymers (vide infra). Absorption coefficients of the BDTup
copolymers are also 1.2–1.5 times higher compared to BDTdn
copolymers due to tighter inter-chain packing.
3.3 Thermal properties and molecular packing
Fig. 2 HOMO and LUMO wavefunctions of the (a) PH-FL, (b) BDTdn-
FL, and (c) BDTup-FL oligomer (n ¼ 3) model systems calculated at the
B3LYP/6-31G(d,p) level of theory.
The thermal properties of the polymers were investigated by
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC). All of these polymers showed thermal
decomposition temperature in the range of 400–420 ꢃC under
nitrogen by TGA, indicating good thermal stability. No thermal
transition signals were observed from 0 ꢃC to 350 ꢃC in the DSC
for these four polymers. DSC signals could be observed only for
the polymers in this class that are substituted with linear alkyl
chains;11 polymers with branched alkyl chains usually do not
show any DSC peaks due to poor ordering.
PES measurement in air becomes erroneous for CPDT copoly-
mers; this is probably due to oxygen doping25 in the polymer
films with lower IPs. In the present instance, the LUMO energies
in the thin films can be estimated from the HOMO energies and
optical band gap of the polymer films (lonset of the absorption
spectra); that the optical band gaps can be used for this purpose
Grazing incidence X-ray Scattering (GIXS) was employed to
investigate crystallinity and packing of the benzodithiophene-
based polymers. The data are summarized in Table 2 and room
is due to the fact that the electrochemical band gaps (Eg
¼
ELUMO - EHOMO) are very similar with the optical band gaps
determined from solution. For the PH-FL and the BDT copoly-
mers in solution and in thin films, the trends observed from the
experimental Eg and optical gap values reported in Table 1 are
supported by the calculation results. In agreement with the
calculations, the experimental Eg values for the BDT copolymers
are also ꢂ0.1 eV lower than those of the PH-FL copolymer.
Absorption coefficients (a) are significantly enhanced by
copolymerizing these BDT monomers with CPDT units. For
example, a is increased by almost a factor of two from the FL
copolymer BDTup-FL (3.5 ꢀ 104 cmꢁ1) to the CPDT copolymer
BDTup-CPDT (6.2 ꢀ 104 cmꢁ1), Fig. 3. Absorption of CPDT
Table 2 d-spacing and p–p stacking distances of the BDTdn-FL,
BDTup-FL, BDTdn-CPDT, and BDTup-CPDT
d-spacing/layer-layer
ꢁ
spacing (A)
ꢁ
p–p stacking (A)
Polymer
X
BDTdn-FL
BDTup-FL
BDTdn-CPDT
BDTup-CPDT
19.9
22.6
12.8
13.7
4.39
3.85
4.05
4.01
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Fig. 4 GIXS images of polymer films (a) BDTdn-FL, (b) BDTup-FL, (c) BDTdn-CPDT, and (d) BDTup-CPDT. (e) Cartoon representing d-spacing and
p–p stacking distances.
temperature GIXS images of polymer films spin casted on bare
silicon (in situ annealing temperature ꢂ200 ꢃC) are shown in
Fig. 4. The sulfur atoms in the benzodithiophene unit face
towards the polymer main chain in BDTdn-FL and BDTdn-
CPDT, while they point away from the polymer main chain in
BDTup-FL and BDTup-CPDT. In addition, alkyl side chains on
the donor units and the fused aromatic rings are very likely on
the same side for the CPDT copolymers while they are expected
to point in opposite directions in the FL copolymers. These
structural differences have a marked impact on the film
morphology of the FL copolymers while their influence in the
CPDT copolymers appears to be only marginal; we found
texture (crystallographic orientation) of all these polymers is
different than the usual (100) orientation. The GIXS images in
Fig. 4 show that the p–p stacking direction is nominally oriented
normal to the substrate surface and the (100) reflections
predominate in the substrate plane. We suspect that this is due to
the two dimensional like structure of the polymers.
In order to shed some light on the nature of the interchain
packing in these polymers, four possible dimer forms of each
donor–acceptor monomer unit were optimized (Fig. 5) with the
ꢁ
ꢁ
differences on the order of 2.7 A and more than 0.5 A in lamellar
d-spacing and p–p stacking distances for FL copolymers,
respectively, while for the CPDT copolymers the lamellar d-
ꢁ
spacings differ by less than 1 A and the p–p stacking distances
are nearly equal.
With regards to interlayer d(100) lamellar spacings, they are
ꢁ
significantly smaller in CPDT copolymers (12.8ꢂ13.7 A) than in
ꢁ
FL copolymers (19.9ꢂ22.6 A). This is likely due to interdigita-
Fig. 5 Four different orientations of BDTup-CPDT obtained from
DFTB calculations (tilt view, see Figure S1 for additional views); similar
orientations of the monomer units were considered for BDTdn-FL,
BDTup-FL, and BDTdn-CPDT. Note that for orientations 1 and 3,
accepting units are on the same side while for orientations 2 and 4, they
are pointing in opposite directions.
tion of the side chains in the CPDT copolymers (vide infra). It is
also apparent from the images shown in Fig. 4 that the BDTdn-
FL polymer has better order than the other polymers. The other
polymers have only two or three broad reflections, showing that
they are poorly ordered. It is interesting to note that the film
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Density Functional Tight Binding (DFTB) methodology,26,27
which is a semi-empirical type of approach able to describe well
intermolecular interactions. Note that in these calculations all
the long alkyl side chains are replaced with methyl groups; this
simplification is justified by the fact that we are mainly interested
here in evaluating the p–p interactions between the monomers,
to which the longer alkyl chains contribute little (the long alkyl
chains are, however, expected to impact significantly the inter-
layer d-spacings, see below). In all cases, the orientations
involving antiparallel configurations between the fused TP units
(orientation 2, Fig. 5) are calculated to be the most stable.
The calculations and GIXD data concur in providing very
similar packing structures for BDTdn-CPDT and BDTup-CPDT.
For the FL polymers the calculations suggest similar orientations
(orientation 2) with similar p–p stacking distances for BDTdn-
FL and BDTup-FL. However, the experimental results are very
different. The interchain interactions in type-2 orientations
the alkyl chains on the thiophene rings can ‘‘open up’’ to
accommodate in between them the alkyl chains of a CPDT unit
on a neighboring polymer. For the FL polymers, on the other
hand, given that all the alkyl side chains of the thiophene and FL
units are located on the same side, the steric hindrance among
these chains prevents the same type of interdigitation as in the
CPDT polymers. This feature could provide the origin for the
longer d-spacing in the FL polymers vs. the CPDT polymers.
However, more work, including classical molecular dynamics
simulations, is warranted to reach a more definitive picture.28
3.4 Field-effect transistor characterization
The mobility of charge carriers (holes) in semiconducting poly-
mers determines their applicability to transistors and OPV
devices. High charge-carrier mobilities can lead to increased on-
currents and faster switching speeds in transistors, and to
increased photocurrent and power conversion efficiency in solar
cells. Top-contact organic thin-film field-effect transistors
(OTFT) were fabricated to measure the field-effect mobilities of
the polymers. Although the diode mobility is more pertinent for
PV applications, FET mobilities usually tend to correlate well
with diode mobilities.29 Polymer solutions in ODCB were drop-
cast on octadecyltrimethoxysilane (OTS)-treated, heavily-doped,
n-type SiO2 (300 nm)/Si wafers. The average thickness of these
polymer films was ꢂ30 nm. The thin films were annealed at
different temperatures and gold electrodes (40 nm) were depos-
ited through shadow masks. Electrical performance was tested
under dry N2 atmosphere (glovebox). The results from optimized
OTFTs for each polymer are summarized in Table 3. Repre-
sentative I–V characteristics of OTFT devices fabricated from
BDTdn-FL are shown in Fig. 7.
ꢁ
between CPDT units (p–p distance ꢂ3.9 A) are stronger than
ꢁ
those between of FL units (p–p distance ꢂ4.7 A). Thus, it is
possible that packing of the CPDT polymers is dominated
mainly by the interchain CPDT interactions irrespective of the
nature of acceptor units including the position of the sulfur
atoms in the fused TP units. In addition, the effects of side alkyl
chains are practically the same for both polymers. However, due
to the weaker interactions between the FL units, the positions of
the sulfur atoms in the TP units and the side chains might play
a more significant role in the thin-film packing of BDTdn-FL and
BDTup-FL, which could explain that these polymers follow very
different packing schemes.
In order to shed some light on the origin of the interlayer d-
spacings in the polymers, plausible packing structures were
constructed on the basis of the type-2 orientations in Fig. 5, using
monomer units including the full alkyl side chains, see Fig. 6. The
DFTD results suggest that, in the case of the CPDT polymers,
All of these polymers showed p-channel transport (h+) with
mobilities in the range of 10ꢁ5 to 10ꢁ2 cm2/(Vs) with a moderate
Fig. 6 Sketch of possible packing structures for (a) CPDT and (b) FL polymers; dimer configurations (top panel) and tetramer configurations (bottom
panel).
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Table 3 OTFT Characterization of the Polymers in N2 environment.a
mmax
cm2/Vs
mave*
cm2/Vs
Polymer
Ion/off
Vth V
PH-FL
1.9 ꢀ 10ꢁ4
1.6 ꢀ 10ꢁ5
8.1 ꢀ 10ꢁ3
3.1 ꢀ 10ꢁ4
4.4 ꢀ 10ꢁ2
1.6 ꢀ 10ꢁ4
6.8 ꢀ 10ꢁ6
1.8 ꢀ 10ꢁ3
1.6 ꢀ 10ꢁ4
3.8 ꢀ 10ꢁ3
2.1 ꢀ 103
64
3.53 ꢀ 103
1.7 ꢀ 103
1.01 ꢀ 103
8.8
17
BDTdn-FL
BDTup-FL
BDTdn-CPDT
BDTup-CPDT
ꢁ25
ꢁ4
ꢁ13
a
Average parameters extracted from at least 5 devices each. Mobilities
reported are in the saturation regime. Top contact devices fabricated
by drop-cast on octadecyltrimethoxysilane-treated SiO2 (300 nm)/Si
wafers. Device channel length was 50 mm and W/L ¼ 20.
on/off ratio (in the order of 103) and low threshold voltages.
A high hole carrier mobility of 0.2 cm2/(Vs) was obtained using
the FL copolymer (AC-FL) with all linear alkyl chains.10 The
hole carrier mobility decreased to 0.07 cm2/(Vs) when linear alkyl
chains were replaced by branched 2-ethylhexyl alkyl chains in the
polymer.11 This was rationalized based on differences between
film morphology. Mainly, the polymer with linear alkyl chains
was more crystalline than that with branched alkyl chains.11
Compared to AC-FL, its phenanthrene analog PH-FL showed
significantly reduced carrier mobility (mmax ꢂ 10ꢁ4 cm2/(Vs)) and
its threshold voltage (Vth) for p-channel transport increases from
1.4 V to 8.8 V. Furthermore, replacing the aromatic unit by
benzo[1,2-b:4,3-b0]dithiophene in polymer BDTdn-FL further
reduces the carrier mobility (ꢂ10ꢁ5 cm2/(Vs)) and Vth shifts to
17 V. This is consistent with the lower oxidation potential of the
electron-rich phenanthrene- and benzodithiophene-type poly-
mers. However, Vth (ꢁ23 V) shifts to a high negative voltage in
the case of the benzo[2,1-b:3,4-b0]dithiophene-based polymer,
BDTup-FL, and much higher carrier mobility (ꢂ10ꢁ2 cm2/(Vs)) is
achieved, which we attribute to the larger p–p stacking distance
in BDTdn-FL. This leads to a small overlap between p orbitals
on adjacent molecules and thus reduces the charge carrier
mobility, despite the better order in this polymer compared to the
others (see Fig. 4). In addition, BDTdn-FL produces a very rough
film compared to BDTup-FL as indicated by AFM (Fig. 8).
The side chain interdigitation helps registration between the
adjacent lamellae of CPDT polymers,29 and is probably leading
to high charge carrier mobilities compared to FL polymers.
A hole carrier mobility of 0.044 cm2/Vs is found for BDTup-
Fig. 8 Tapping mode AFM images of polymer films (a) BDTdn-FL, (b)
BDTup-FL, (c) BDTdn-CPDT, and (d) BDTup-CPDT.
CPDT. The mobility of BDTdn-CPDT is still lower and on the
order of ꢂ10ꢁ4 cm2/Vs. The thin film morphologies CPDT
copolymers (BDTdn-CPDT and BDTup-CPDT) are also
smoother compared to their FL analogs (BDTdn-FL and
BDTup-FL) as indicated by the AFM images (Fig. 8).
3.5 Photovoltaic characterization
Solar cell devices were fabricated using blends of each of these
polymers and [6,6]-phenyl C61 butyric acid methyl ester
Table 4 Photovoltaic properties of the bulk heterojunction solar cellsa
Polymer
Jsc mA/cm2
Voc
V
FF
PCE %
PH-FL
2.3
3.2
3.2
0.64
3.1
0.48
0.40
0.59
0.375
0.345
0.52
0.45
0.42
0.60
0.37
0.57
0.57
0.79
0.14
0.40
BDTdn-FL
BDTup-FL
BDTdn-CPDT
BDTup-CPDT
a
polymers/PC61BM (1 : 4) blend as active layer.
Fig. 7 Representative I–V and output curves of p-type OTFTs of BDTdn-CPDT (a, b) tested in the inert and dry atmosphere (glove box).
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HOMO energies. However, the Voc (0.4 V) was significantly
reduced in the case of the BDTdn-FL based solar cell, even
though a similar Jsc was obtained (3.2 mA/cm2). AFM of thin
films of the blends indicates that BDTdn-FL tends to phase
separate from PC[61]BM on long (100 nm) length scales
(Fig. 10a). Since this is much longer than the exciton diffusion
length, this contributes towards its poor performance. In addi-
tion, film quality of BDTdn-FL-based devices was bad due to its
poor solubility in organic solvents. In contrast, BDTup-FL
blends forms a very smooth films with PC[61]BM or with
PC[71]BM irrespective of blending ratios (Fig. 10b–d), suggest-
ing a much finer phase separation.
Solar cell efficiency of BDTup-FL was further optimized using
different blending ratio of polymer with acceptor PC[61]BM and
Fig. 9 I–V characteristics under AM1.5 illumination for the solar cell
fabricated from AC-FL, PH-FL, BDTdn-FL, and BDTup-FL with
PC[61]BM (1 : 4 weight ratio).
PC[71]BM (Table 5, Fig. 11). An optimized PCE of 1.38% (Jsc
-
5.45 mA/cm2, Voc - 0.605 V, FF - 0.42) could be achieved using
this polymer with PC[61]BM (1 : 3 blend ratio). Solar cell devices
respond well in the visible to NIR region up to 980 nm, with
a maximum external quantum efficiency of 14% at 750 nm and
30% at ꢂ480 nm (Fig. 12). The previously reported FL copoly-
mer without ring fusion showed maximum PCE of 2.2%.31 For
this copolymer, a higher Jsc (8.2 mA/cm2) was achieved in opti-
mized devices, while Voc and FFs remain comparable to our
devices made of FL copolymers, such as the BDTup-FL devices.
Higher current is usually obtained by replacing PC[61]BM with
PC[71]BM as an acceptor in the active layer, as the latter has
higher absorption in the visible region.32 However, the PCE
could not be improved for BDTup-FL solar cells by using
Table 5 Optimization of PCE of solar cell containing BDTup-FL
Weight Thickness
(nm)
Acceptor Ratio
Jsc mA/cm2 Voc
V
FF
PCE %
PC61BM 1 : 1
1 : 2
1 : 3
1 : 4
PC71BM 1 : 1
1 : 2
100
65
85
150
62
64
87
4.38
4.475
5.45
3.2
0.625 0.35 0.96
0.585 0.41 1.07
0.605 0.42 1.38
0.585 0.42 0.79
0.575 0.39 0.83
0.595 0.44 1.07
0.585 0.43 1.16
0.595 0.44 1.26
Fig. 10 Tapping mode AFM images of thin films of the blends of
polymer and fullerene derivatives: (a) BDTdn-FL 1 : 1 with PC61BM, (b)
BDTup-FL 1 : 1 with PC61BM, (c) BDTup-FL 1 : 4 with PC61BM, and (d)
BDTup-FL 1 : 1 with PC71BM.
3.7
4.075
4.625
4.825
1 : 3
1 : 4
100
(PC[61]BM) with a device structure of ITO/PEDOT-PSS/poly-
mer:PCBM/Al. Though several weight ratios of these polymers
and PC[61]BM were tested, blends with a 1 : 4 ratio usually
produced higher short-circuit current densities (Jsc), possibly due
to formation of stable bimolecular structures of fullerene inter-
calated between the side-chains of the semiconducting polymer.30
The solar cell data collected under simulated 1 sun AM 1.5G
radiation are summarized in Table 4 and the representative I–V
curves are shown in Fig. 9. The phenanthrene-based FL copoly-
mer (PH-FL) was reported earlier11 and its solar cell data are
compared with the BDT copolymers. Polymer BDTup-FL shows
the best power conversion efficiency (PCE) among these FL
copolymers; this is likely due to its higher hole carrier mobility
compared to PH-FL and BDTdn-FL. A slight increase in the Voc
was observed in BDTup-FL cells (ꢂ0.6 V) compared to PH-FL
(ꢂ0.48 V) due to its higher ionization potential. Higher Voc is
expected in BDTdn-FL compared to BDTup-FL based on their
Fig. 11 I–V characteristics under AM1.5 illumination for the solar cell
fabricated from BDTup-FL with different weight ratio of PC[61]BM
(1 : 1, 1 : 2, 1 : 3, and 1 : 4).
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Table 6 B3LYP/6-31G(d,p) Energy Values of the Polymers Extrapo-
lated from Oligomer Calculations.a
Eg
(HOMO–
HOMO LUMO LUMO)
(V)b
IP
EA
S1
(eV)d
Compound (V)b
(eV)
(eV)c (eV)c
AC-FL
PH-FL
BDTdn-FL
BDTup-FL
ꢁ4.40
ꢁ2.50
ꢁ2.73
ꢁ2.83
ꢁ2.82
1.90
1.69
1.51
1.59
4.86
4.86
4.79
4.86
ꢁ2.04 1.62
ꢁ2.27 1.41
ꢁ2.37 1.24
ꢁ2.36 1.32
ꢁ4.42
ꢁ4.34
ꢁ4.41
a
Energies were extrapolated from Kuhn fits of oligomer (n ¼ 1–3)
b
c
calculations. Gas-phase HOMO and LUMO levels. Adiabatic
values calculated from optimized structures on the neutral and ionic
potential energy surfaces. Vertical transition to the lowest S1 state
d
calculated at the TDDFT/B3LYP/6-31G(d,p) level.
Fig. 12 EQE spectrum of a solar cell fabricated using blend of BDTup-
FL and PC61BM (1 : 3 wt. ratio) in the active layer.
analogous polymers based on acenaphthyl and phenanthrene
fused aromatic units. It can be concluded that introduction of
fused aromatic units within thienopyrazine is a convenient
approach to tune the optoelectronic properties of donor–
acceptor polymers (Eg ¼ 1.3 to 1.6 eV, HOMO ¼ ꢁ4.9 to
ꢁ5.2 V). We demonstrate that the improved p–p stacking
afforded by the fused thienopyrazine units results in high hole
mobilities (up to 0.2 cm2/Vs). These polymers also show
moderate power conversion efficiencies, up to 1.4% (BDTup-FL -
Jsc ¼ 5.45 mA/cm2, Voc ¼ 0.605 V, FF ¼ 0.42), in bulk hetero-
junction devices. The band-gap of these fused thienopyrazine
polymers could be further lowered by co-polymerizing with
cyclopentadithiophene (Eg ꢂ 1 eV). Although both fluorene and
cyclopentadithiophene polymers afford similar face-to-face p–p
stacking cyclopentadithiophene polymers show higher charge
carrier mobility (mFET ¼ 0.044 cm2/Vs) in thin film transistor
devices compared to the analogous fluorene polymers (mFET ¼
8.1 ꢀ 10ꢁ3 cm2/Vs). This result can be ascribed to decreased
lamellar d-spacings of cyclopentadithiophene polymers as
compared to the fluorene polymers yielding from side
chain interdigitation between lamellae. However, the power
conversion efficiencies of these cyclopentadithiophene polymers
are limited due to low open circuit voltages caused by low
ionization potentials. Maintaining all the properties of these
cyclopentadithiophene polymers along with high ionization
potential (>5.2 eV) will be necessary to achieve high efficiency
solar cells.
a PC[71]BM acceptor; this may be due to morphological changes
in BDTup-FL. A similar effect was observed in some other
recently reported TP copolymers.33
Power conversion efficiencies of CPDT copolymers (0.14–
0.40%) are low due to low open circuit voltages (0.35–0.38 V). As
indicated earlier, ionization potentials of the CPDT polymers are
lower compared to FL copolymers and thus reduce the Voc and
limit the efficiency. These CPDT polymers are among very few
low band-gap polymers with band gap approaching 1 eV, some
of which have been successfully used as donor material in the
bulk heterojunction solar cell structure with fullerene derivatives
as acceptor, but with very low PCE (<0.1%).34
We were able to achieve the desired low band gap for BHJ
solar cells (<1.5 eV) and to induce a favorable packing of the
donor–acceptor polymers by introducing novel fused aromatic
units in the thienopyrazine structure as evidenced by the
enhancement of charge carrier mobilities. Despite this achieve-
ment, low absorption coefficients (3–6.2 ꢀ 104 cmꢁ1) and high
HOMO energies in these polymers limit their PCE. DFT calcu-
lations confirm that the electron density in the LUMO wave
function is more localized on the acceptor fused TP unit of the
polymers, while the electron density associated with the HOMO
wave function is delocalized over both the acceptor and donor
units (Fig. 3). The localization of the LUMO decreases orbital
overlap with the HOMO, which can be detrimental to the
strength of the lowest S0 / S1 absorption. We have recently
shown that the absence of the adjacent thiophenes around the
acceptor enhances the HOMO/LUMO overlap and thus
enhances the absorption coefficient for the lowest optical tran-
sition (which is dominated by HOMO-to-LUMO electron exci-
tation).35 This also raises the ionization potential. As
a consequence, significant enhancement in PCE was observed in
solar cell devices using the fused TP based polymer without
adjacent thiophenes, due to increased open-circuit voltage and
short-circuit current. Further investigation is underway to ach-
ieve higher efficiency solar cells.
Acknowledgements
This publication was partially based on work supported by the
Center for Advanced Molecular Photovoltaics, Award No KUS-
C1-015-21, made by King Abdullah University of Science and
Technology (KAUST). We also acknowledge support from the
Global Climate and Energy Program (GCEP) and the facility
usage at the Stanford Center for Polymer Interfaces and
Macromolecular Assemblies (CPIMA). Portions of this research
were carried out at the Stanford Synchrotron Radiation Light-
source, a national user facility operated by Stanford University
on behalf of the U.S. Department of Energy, Office of Basic
Energy Sciences. R.M thanks Jack E. Parmer, George Margulis,
and Eric Hoke for their help.
4. Summary and conclusions
In summary, two isomers of benzodithiophene were fused into
the aromatic thieno[3,4-b]pyrazine system and their fluorene
copolymers are compared with the previously reported
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