Thieno[3,4-b]thiophene ebenzo[1,2-b:4,5-b′]dithiophene-based polymers bearing optically pure 2-ethylhexyl pendants: Synthesis and application in polymer solar cells

Article abstract of DOI:10.1016/j.polymer.2014.11.033

Optically active narrow-bandgap polymers ((R,R)- and (S,S)-PTB5) consisting of alternating n-octyl thieno [3,4-b]thiophene-2-carboxylate and benzo[1,2-b:4,5-b′]dithiophene units bearing optically pure (R)- and (S)-2-ethylhexyl pendants, respectively, were synthesized for the first time. (R,R)- and (S,S)-PTB5 films showed apparent circular dichroism in their absorption regions of the polymer backbone due to the formation of a chirally ordered superstructure induced by the chirality of the branched alkyl pendants. Inverted-type bulk heterojunction polymer solar cells (PSCs) were fabricated using (R,R)- or (S,S )-PTB5 as electron donors and [6,6]-phenyl-C₆₁-butyric acid methyl ester as an electron acceptor. The photovoltaic properties of the PSCs were compared with those of the corresponding PSC containing optically inactive PTB5 bearing racemic 2-ethylhexyl pendants.

Full text of DOI:10.1016/j.polymer.2014.11.033

Accepted Manuscript
Thieno[3,4-b]thiophene–benzo[1,2-b:4,5-b']dithiophene-based polymers bearing
optically pure 2-ethylhexyl pendants: synthesis and application in polymer solar cells
Tomoyuki Ikai , Ryotaro Kojima , Sinji Katori , Tomoyuki Yamamoto , Takayuki
Kuwabara , Katsuhiro Maeda , Kohshin Takahashi , Shigeyoshi Kanoh
PII:
S0032-3861(14)01059-3
10.1016/j.polymer.2014.11.033
JPOL 17429
DOI:
Reference:
To appear in: Polymer
Received Date: 19 September 2014
Revised Date: 3 November 2014
Accepted Date: 11 November 2014
Please cite this article as: Ikai T, Kojima R, Katori S, Yamamoto T, Kuwabara T, Maeda K, Takahashi K,
Kanoh S, Thieno[3,4-b]thiophene–benzo[1,2-b:4,5-b']dithiophene-based polymers bearing optically pure
2-ethylhexyl pendants: synthesis and application in polymer solar cells, Polymer (2014), doi: 10.1016/
j.polymer.2014.11.033.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Thieno[3,4-b]thiophene–
benzo[1,2-b:4,5-b']dithiophene-based polymers
bearing optically pure 2-ethylhexyl pendants:
synthesis and application in polymer solar cells
Tomoyuki Ikai a,b,*, Ryotaro Kojima a, Sinji Katori a,
Tomoyuki Yamamoto a, Takayuki Kuwabara a,b, Katsuhiro Maeda a,b,*,
Kohshin Takahashi a,b, Shigeyoshi Kanoh a,b
a Graduate School of Natural Science and Technology, Kanazawa University,
Kakuma-machi, Kanazawa 920-1192, Japan
b Research Center for Sustainable Energy and Technology, College of Science and
Engineering, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan
*Corresponding authors. E-mail: ikai@se.kanazawa-u.ac.jp,
maeda@se.kanazawa-u.ac.jp
Tel.: +81 76 234 4781; Fax: +81 76 234 4781.
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ABSTRACT
Optically active narrow-bandgap polymers ((R,R)- and (S,S)-PTB5) consisting of
alternating
n-octyl
thieno[3,4-b]thiophene-2-carboxylate
and
benzo[1,2-b:4,5-b']dithiophene units bearing optically pure (R)- and (S)-2-ethylhexyl
pendants, respectively, were synthesized for the first time. (R,R)- and (S,S)-PTB5 films
showed apparent circular dichroism in their absorption regions of the polymer backbone
due to the formation of a chirally ordered superstructure induced by the chirality of the
branched alkyl pendants. Inverted-type bulk heterojunction polymer solar cells (PSCs)
were fabricated using (R,R)- or (S,S)-PTB5 as electron donors and
[6,6]-phenyl-C61-butyric acid methyl ester as an electron acceptor. The photovoltaic
properties of the PSCs were compared with those of the corresponding PSC containing
optically inactive PTB5 bearing racemic 2-ethylhexyl pendants.
Keywords: Chirality, π-Conjugated copolymer, Electron donor, Low bandgap, Polymer
solar cell
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1. Introduction
Organic photovoltaics (OPVs) show great potential for cost-effective solar
energy conversion and have been recognized as prospective alternatives to silicon-based
photovoltaics. In particular, polymer solar cells (PSCs) containing electron-donating
π-conjugated polymers and electron-accepting fullerene derivatives in their active layers
have attracted substantial attention because of their unique advantages including
flexibility, lightness, colorfulness, transparency and large-area device production by
roll-to-roll printing [1-10]. To improve the photovoltaic performance of PSCs, extensive
research effort has been devoted to developing π-conjugated polymers using rational
synthetic strategies, e.g., narrowing bandgap [11-14], expanding absorption range
[15-19], tuning highest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) energy levels [20-23] and increasing hole mobility [24, 25].
Since π-conjugated polymers consisting of thieno[3,4-b]thiophene (TT) and
benzo[1,2-b:4,5-b']dithiophene (BDT) units (PTB-based polymers) have been reported
as a promising donor for PSCs in 2009 [26], various structural modifications of the
PTB-based polymers have been investigated to improve the power conversion
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efficiency (PCE) of their PSCs. For example, replacements of the alkyl esters with
alkylketones [27, 28] or phenyl esters [22] and direct introduction of fluoro groups [29,
30] or thienyl units [31] onto the polymer backbone increase the PCE of the resulting
PSCs.
The solubility of donor polymers is an important concern in the fabrication of
PSC devices. The introduction of long or branched alkyl chains as pendants is the
simplest way to increase the solubility of donor polymers. However, alkyl pendants that
are too long or bulky inhibit the π–π stacking interactions between donor polymers,
which are essential for efficient hole transport, impairing the performance of donor
materials [32]. Therefore, the alkyl chains introduced onto polymer backbones need to
be appropriate. Considering the trade-off between polymer solubility and
hole-transporting ability, branched alkyl chains, particularly 2-ethylhexyl chains, have
been introduced onto π-conjugated polymers as solubilizing pendants. Because a
2-ethylhexyl chain possesses an asymmetric center, there are two enantiomeric
structures with different absolute configurations. Almost no attention has been paid to
the chirality of alkyl pendants, and racemic alkyl chains have been introduced onto
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donor polymers to increase their solubility. Recently, it was reported that the hole
mobility of small molecule-based donor materials was enhanced by controlling the
chirality of their branched alkyl chains [33], although use of the corresponding materials
in OPVs was not demonstrated. PSCs containing donor polymers bearing enantiopure
2-ethylhexyl pendants have not been reported previously. Because the chirality control
of 2-ethylhexyl pendants in donor polymers enhances a structural regularity of the
polymers, intermolecular interactions between optically active polymers is expected to
be more effective compared with conventional donor polymers randomly bearing (R)-
and (S)-2-ethylhexyl pendants. In addition, the introduction of enantiopure 2-ethylhexyl
pendants has a potential to provide characteristic chiral aggregate formations in the film
states [34-36]. Therefore, it is important to clarify the effect of the chirality of the
branched alkyl chains in donor polymers on the photovoltaic properties of their PSCs.
In the present study, we synthesized optically active PTB-based polymers
((R,R)- and (S,S)-PTB5) containing BDT units bearing optically pure (R)- and
(S)-2-ethylhexyl pendants, respectively. The influence of the pendant chirality on the
thermal stability, absorption and chiroptical properties, frontier orbital energy levels and
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photovoltaic performance of (R,R)- and (S,S)-PTB5 was investigated by direct
comparison with the properties of optically inactive PTB5 bearing racemic 2-ethylhexyl
pendants. To the best of our knowledge, this is the first report on the synthesis and
application of donor polymers bearing the enantiopure 2-ethylhexyl pendants to the
PSC devices. PTB5 was used to investigate the effect of pendant chirality on PSC
properties because it is the simplest structure bearing alkyl esters on TT and alkoxy
groups on BDT of PTB-based polymers reported to date.
2. Experimental
2.1. Materials
The anhydrous solvents (toluene, N,N-dimethylformamide (DMF), pyridine
and tetrahydrofuran (THF)) and the common organic solvents were purchased from
Kanto (Tokyo, Japan). Capstone® fluorosurfactant FS-31 was kindly supplied by Du
Pont
(Wilmington,
DE,
USA).
Trimethyltin
chloride
and
N-(p-toluenesulfonyl)-L-phenylalanyl chloride ((S)-6) were bought from Tokyo Kasei
(TCI, Tokyo, Japan). Zinc, 4-(dimethylamino)pyridine and n-butyllithium (1.6 M in
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n-hexane) were purchased from Wako (Osaka, Japan). p-Toluenesulfonyl chloride was
obtained from Kishida (Osaka, Japan). Tetrakis(triphenylphosphine)palladium(0)
(Pd(PPh3)4) and sodium hydroxide (NaOH) were purchased from Nacalai (Kyoto,
Japan). Tetrabutylammonium bromide and rac-2-ethylhexanol (rac-2-OH) were from
Aldrich (Milwaukee, WI, USA). Poly(3,4-ethylenedioxylenethiophene):poly(4-styrene
sulfonic acid) (PEDOT:PSS) 1.3 wt% dispersion in water was from H. C. Starck
(Munich, Germany). Chiralcel OJ (25 x 2.0 cm ID) and Chiralcel OJ-H (25 x 0.46 cm
ID) were purchased from Daicel Chemical Industries (Tokyo, Japan).
4,8-Dihydrobenzo[1,2-b:4,5-b']dithiophen-4,8-dion
(1)
[37]
and
n-octyl
4,6-dibromothieno[3,4-b]thiophene-2-carboxylate (5) [22] were prepared according to
the procedures in the literature.
2.2. Measurements
1
13
The H and C NMR spectra were measured in CDCl3 at room temperature
and 50 °C with a JEOL ECA-500 spectrometer (JEOL, Tokyo, Japan). IR spectra were
obtained using a JASCO FT/IR-460Plus spectrometer (JASCO, Tokyo, Japan) as a KBr
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pellet. The molecular weights and distributions of the polymers were estimated using
size-exclusion chromatography (SEC) equipped with a Shodex KF-805L column
(Showa Denko, Tokyo, Japan), a JASCO CO-1560 column oven, a JASCO PU-2080
Plus HPLC pump and a JASCO UV-970 UV/VIS detector at 650 nm, where THF was
used as the eluent. The molecular weight calibration curve was obtained with
polystyrene standards (Tosoh). The optical rotation was measured at 25 °C with a
JASCO P-1030 polarimeter. Thermogravimetric analysis (TGA) was conducted using a
TG/DTA6200 (SII NanoTechnology Inc., Chiba, Japan) with a heating rate of
10 °C/min under a flow of nitrogen. The UV-vis-NIR absorption and circular dichroism
(CD) spectra were measured in chlorobenzene with 1.0 mm quartz cell and in
spin-coated films prepared on glass substrates from chlorobenzene solution (5 mg/mL)
using JASCO V-570 and JASCO J-720 spectrometers. The concentrations of the
chlorobenzene solutions were adjusted in a way that is approximately consistent with
the absorbance in the corresponding polymer films. Thin-film X-ray diffraction (XRD)
was recorded on an X-ray diffractometer RINT 2500 (Rigaku, Tokyo, Japan) with Cu
Kα radiation (λ = 1.540 Å) operated at 40 keV and 200 mA. The thickness of the
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polymer:PC61BM blend film was measured by a microfigure measuring instrument
Surfcorder ET200 (Kosaka Laboratory, Tokyo, Japan). The HOMO of the polymer was
measured using a photoelectron spectrometer AC-2 (Riken Keiki, Tokyo, Japan) by
measuring the ionization potential of polymer films in air. The ionization potential of
the polymers was estimated using the plots of the photoelectron quantum yield from the
polymer solid surface against the incident photon energy. The threshold of the
photoelectron quantum yield was correlated with the ionization potential.
2.3. Synthesis of optically active BDT-based monomers
2.3.1.
N-(p-Toluenesulfonyl)-L-phenylalanine 2-ethylhexyl ester ((S,R)-7 and
(S,S)-7)
To a solution of rac-2-OH (9.64 g, 74.0 mmol) in anhydrous toluene (74 mL)
was added (S)-6 (32.0 g, 94.7 mmol) and the mixture was stirred at 80 °C. After 12 h,
the reaction system was diluted with ethyl acetate, and the organic layer was washed
with saturated NaHCO3 aqueous solution and water, and then dried over Na2SO4. After
evaporating the solvent, the crude product was purified by silica gel chromatography
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using n-hexane—ethyl acetate (2/1, v/v) as the eluent to give a diastereomeric mixture
of (S,R)-7 and (S,S)-7. The diastereomers were separated by chiral high-performance
liquid chromatography (HPLC) on Chiralcel OJ (column dimensions 25 x 2.0 cm ID;
eluent methanol (MeOH); flow rate 8.0 mL/min; temperature 20 °C) to give (S,R)-7
(13.5 g, 31.3 mmol, 42% yield) and (S,S)-7 (13.7 g, 31.7 mmol, 43% yield) as a white
solid. The diastereomeric excess (de) of the isolated products was determined by HPLC
analysis on Daicel Chiralcel OJ-H (column dimensions 25 x 0.46 cm ID; eluent MeOH;
flow rate 0.5 mL/min; temperature 20 °C;
λ 254 nm; t(S,R)-7 = 17.9 min, t(S,S)-7 = 24.9
min) and we confirmed that each isomer possesses a high diastereomeric purity (>99%
1
de). (S,R)-7: H NMR (500 MHz, CDCl3, rt): δ 7.64 (d, J = 8.0 Hz, 2H), 7.25-7.21 (m,
5H), 7.10-7.06 (m, 2H), 5.03 (d, J = 9.2 Hz, 1H), 4.23-4.17 (m, 1H), 3.78-3.66 (m, 2H),
3.08-2.97 (m, 2H), 2.40 (s, 3H), 1.39-1.03 (m, 9H), 0.88 (t, J = 7.2 Hz, 3H), 0.78 (t, J =
1
7.5 Hz, 3H). (S,S)-7: H NMR (500 MHz, CDCl3, rt): δ 7.64 (d, J = 8.0 Hz, 2H),
7.25-7.21 (m, 5H), 7.11-7.06 (m, 2H), 5.03 (d, J = 9.2 Hz, 1H), 4.23-4.17 (m, 1H),
3.79-3.67 (m, 2H), 3.07-2.99 (m, 2H), 2.40 (s, 3H), 1.37-1.04 (m, 9H), 0.88 (t, J = 7.2
Hz, 3H), 0.78 (t, J = 7.5 Hz, 3H).
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2.3.2.
(R)-2-Ethylhexanol ((R)-2-OH)
(S,R)-7 (6.9 g, 16 mmol) was dissolved in THF (29 mL) and then an aqueous
solution of NaOH (5 N, 29 mL) was added to the solution. The mixture was stirred at
60 °C for 24 h. After cooling to room temperature, the solvent was removed under
reduced pressure. The residue was diluted with hexane, and the solution was washed
with saturated NaHCO3 aqueous solution and water, and dried over Na2SO4. After
filtration, the solvent was removed by evaporation to give the desired product as a
25
colorless oil (2.0 g, 96% yield). [
α
]
= −3.5 (neat); lit. [α]25 = −3.4 (c 5.3, CHCl3)
D D
[38]. 1H NMR (500 MHz, CDCl3, rt): δ 3.55 (t, J = 5.4 Hz, 2H), 1.45-1.20 (m, 8H), 1.16
(t, J = 5.5 Hz, 1H), 0.90 (t, J = 7.2 Hz, 6H).
2.3.3.
(S)-2-Ethylhexanol ((S)-2-OH)
The title compound was prepared from (S,S)-7 in the same way as (R)-2-OH
25
and obtained in 90% yield. [
α
]
= +3.9 (neat); lit. [α]25 = +3.4 (c 5.3, CHCl3) [38].
D D
1H NMR (500 MHz, CDCl3, rt): δ 3.56 (t, J = 5.4 Hz, 2H), 1.45-1.20 (m, 8H), 1.15 (t, J
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= 5.7 Hz, 1H), 0.90 (t, J = 6.9 Hz, 6H).
2.3.4.
(R)-2-Ethylhexyl p-toluenesulfonate ((R)-2-OTs)
To a solution of (R)-2-OH (1.0 g, 7.7 mmol) in anhydrous pyridine (3.5 mL)
was
added
p-toluenesulfonyl
chloride
(2.2
g,
11.5
mmol)
and
4-(dimethylamino)pyridine (5.6 mg, 0.05 mmol). The mixture was stirred at 0 °C. After
12 h, the reaction system was diluted with hexane, and the organic layer was washed
with 1 N HCl aqueous solution, saturated NaHCO3 aqueous solution and water, and
then dried over Na2SO4. After evaporating the solvent, the crude product was purified
by silica gel chromatography using hexane–ethyl acetate (20/1, v/v) to yield (R)-2-OTs
1
as a colorless oil (1.7 g, 80%). H NMR (500 MHz, CDCl3, rt): δ 7.79 (d, J = 8.6 Hz,
2H), 7.35 (d, J = 8.0 Hz, 2H), 3.95-3.89 (m, 2H), 2.45 (s, 3H), 1.61-1.49 (m, 1H),
1.37-1.09 (m, 8H), 0.84 (t, J = 7.2 Hz, 3H), 0.79 (t, J = 7.5 Hz, 3H).
2.3.5.
(S)-2-Ethylhexyl p-toluenesulfonate ((S)-2-OTs)
The title compound was prepared from (S)-2-OH in the same way as
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(R)-2-OTs and obtained in 72% yield. 1H NMR (500 MHz, CDCl3, rt): δ 7.79 (d, J = 8.3
Hz, 2H), 7.35 (d, J = 8.3 Hz, 2H), 3.95-3.89 (m, 2H), 2.45 (s, 3H), 1.59-1.48 (m, 1H),
1.37-1.08 (m, 8H), 0.84 (t, J = 7.1 Hz, 3H), 0.79 (t, J = 7.4 Hz, 3H).
2.3.6.
4,8-Bis((R)-2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene ((R,R)-3)
To an aqueous solution of NaOH (5 N, 9.0 mL) was added 1 (0.45 g, 2.0
mmol) and metallic zinc (sandy, 0.29 g, 4.5 mmol) and the mixture was stirred under
reflux for 1 h, After tetrabutylammonium bromide (0.13 g, 0.41 mmol) and (R)-2-OTs
(1.73 g, 6.08 mmol) were added to the mixture, the reaction mixture was further
refluxed for 12 h. The reaction mixture was then extracted with dichloromethane, and
the organic layer was washed with brine and dried over anhydrous Na2SO4. After
evaporating the solvent, the crude product was purified by silica gel chromatography
using hexane–dichloromethane (10/1, v/v) to yield (R,R)-3 as a pale yellow oil (0.648 g,
1.46 mmol, 73%). 1H NMR (500 MHz, CDCl3, rt): δ 7.47 (d, J = 5.2 Hz, 2H), 7.36 (d, J
= 5.7 Hz, 2H), 4.20-4.15 (m, 4H), 1.85-1.76 (m, 2H), 1.74-1.30 (m, 16H), 1.01 (t, J =
7.5 Hz, 6H), 0.94 (t, J = 7.2 Hz, 6H).
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2.3.7.
4,8-Bis((S)-2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene ((S,S)-3)
The title compound was prepared from (S)-2-OTs in the same way as (R,R)-3
and obtained in 87% yield. 1H NMR (500 MHz, CDCl3, rt): δ 7.47 (d, J = 5.8 Hz, 2H),
7.37 (d, J = 5.2 Hz, 2H), 4.20-4.15 (m, 4H), 1.85-1.76 (m, 2H), 1.74-1.30 (m, 16H),
1.01 (t, J = 7.5 Hz, 6H), 0.94 (t, J = 7.2 Hz, 6H).
2.3.8. 2,6-Bis(trimethyltin)-4,8-bis((R)-2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene
((R,R)-4)
To a solution of (R,R)-3 (0.64 g, 1.4 mmol) in anhydrous THF (2.5 mL) was
added dropwise n-butyllithium (1.6 M in hexane, 2.3 mL, 3.7 mmol) via syringe at
-78 °C under nitrogen atmosphere. The mixture was stirred at -78 °C for 30 min and
then at room temperature for 30 min. After the mixture was cooled to -78 °C again,
trimethyltin chloride (0.87 g, 4.4 mmol) in anhydrous THF (0.9 mL) was added. The
mixture was warmed to room temperature and stirred for 12 h. After adding a small
amount of water to quench the reaction, the solvent was removed under reduced
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pressure. The residue was diluted with hexane, and the organic layer was washed with
water, dried over anhydrous Na2SO4 and concentrated. The crude product was purified
by recrystallization from ethanol to yield (R,R)-4 as colorless crystals (0.68 g, 0.88
25
1
mmol, 63%). Mp 74.7–75.2 °C. [
α
]
= +1.0 (c 1.0, CHCl3). H NMR (500 MHz,
D
CDCl3, rt): δ 7.51 (s, 2H), 4.18 (d, J = 5.2 Hz, 4H), 1.86-1.76 (m, 2H), 1.75-1.33 (m,
13
16H), 1.02 (t, J = 7.4 Hz, 6H), 0.94 (t, J = 6.6 Hz, 6H), 0.44 (s, 18H). C NMR (125
MHz, CDCl3, rt): δ 143.59, 140.73, 134.21, 133.24, 128.31, 75.97, 41.00, 30.86, 29.58,
24.23, 23.54, 14.55, 11.70, -8.01. Anal. Calcd for C32H54O2S2Sn2: C, 49.77; H, 7.05.
Found: C, 49.53; H, 6.93.
2.3.9. 2,6-Bis(trimethyltin)-4,8-bis((S)-2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene
((S,S)-4)
The title compound was prepared from (S,S)-3 in the same way as (R,R)-4 and
25
obtained in 68% yield. Mp 75.1–75.3 °C. [
α
]
D = –0.93 (c 1.0, CHCl3). 1H NMR (500
MHz, CDCl3, rt): δ 7.51 (s, 2H), 4.19 (d, J = 5.2 Hz, 4H), 1.86-1.76 (m, 2H), 1.75-1.33
13
(m, 16H), 1.02 (t, J = 7.4 Hz, 6H), 0.94 (t, J = 6.6 Hz, 6H), 0.44 (s, 18H). C NMR
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(125 MHz, CDCl3, rt): δ 143.59, 140.73, 134.21, 133.24, 128.31, 75.98, 41.00, 30.86,
29.58, 24.23, 23.54, 14.56, 11.71, -8.00. Anal. Calcd for C32H54O2S2Sn2: C, 49.77; H,
7.05. Found: C, 49.60; H, 6.93.
2.4.
Synthesis of polymers
Polymerization was carried out in a dry Schlenk flask under a nitrogen
atmosphere in a similar manner to that previously reported [28]. Each monomer was
purified by recrystallization just before use. A typical polymerization procedure is
described below.
(R,R)-4 (400 mg, 0.52 mmol), 5 (235 mg, 0.52 mmol) and Pd(PPh3)4 (24 mg,
21 µmol) were placed in a Schlenk flask under a nitrogen atmosphere. Anhydrous
toluene (8.3 mL) and DMF (2.1 mL) were added with a syringe and the mixture was
heated to 120 °C. After 12 h, the reaction mixture was cooled to room temperature and
poured into MeOH. The resulting polymer was collected by centrifugation and washed
with MeOH for 12 h and then with hexane for 12 h in a Soxhlet apparatus to remove
any by-products and oligomers. After the polymer was collected with chlorobenzene,
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the chlorobenzene solution was passed through Celite to remove the metal catalyst. The
solution was concentrated and poured into hexane. The precipitate was collected by
centrifugation and dried in vacuo to yield (R,R)-PTB5 as a dark purple solid (351 mg,
1
92%). IR (KBr): 1712 cm-1 (νC=O). H NMR (500 MHz, CDCl3, 50 °C): δ 8.3-6.3 (3H,
br), 4.8-3.5 (6H, br), 2.6-0.7 (45H, br). Anal. Calcd for (C41H54O4S4)n: C, 66.63; H, 7.36.
Found: C, 66.34; H, 7.48.
In the same way, (S,S)-PTB5 and PTB5 were prepared from the corresponding
monomers. The polymerization results are summarized in Table 1 and 1H NMR spectra
and SEC curves of (R,R)-PTB5, (S,S)-PTB5 and PTB5 are shown in Figs. S1 and S2.
Spectroscopic data for (S,S)-PTB5. IR (KBr): 1712 cm-1 (νC=O). 1H NMR (500
MHz, CDCl3, 50 °C): δ 8.3-6.3 (3H, br), 4.8-3.5 (6H, br), 2.6-0.7 (45H, br). Anal. Calcd
for (C41H54O4S4)n: C, 66.63; H, 7.36. Found: C, 66.37; H, 7.57.
Spectroscopic data for PTB5. IR (KBr): 1712 cm-1 (νC=O). 1H NMR (500 MHz,
CDCl3, 50 °C): δ 8.3-6.3 (3H, br), 4.8-3.5 (6H, br), 2.6-0.7 (45H, br). Anal. Calcd for
(C41H54O4S4·0.7H2O)n: C, 65.51; H, 7.43. Found: C, 65.61; H, 7.41.
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2.5. Fabrication of hole-only devices
ITO electrode was ultrasonicated in 2-propanol, cleaned in boiling 2-propanol,
and then dried in air. A dispersion of PEDOT:PSS (0.65 wt%) in water containing
Capstone® fluorosurfactant FS-31 (0.5 wt%) was spin-coated onto the clean ITO
electrode at 2000 rpm, and then dried at 150 °C for 5 min. A solution of the polymers in
chlorobenzene (25 mg/mL) was spin-coated onto the ITO/PEDOT:PSS substrate at 700
rpm. The above aqueous dispersion of PEDOT:PSS was spin-coated onto the
polymer:PC61BM layer at 2000 rpm. The Au electrode was vacuum deposited onto the
PEDOT:PSS layer under a pressure of 2 × 10-5 Torr. The structure of the hole-only
device is ITO/PEDOT:PSS/polymer:PC61BM/PEDOT:PSS/Au, whose effective area is
0.04 cm2.
2.6. Fabrication of polymer solar cells
ITO-glass electrodes were washed by sonication in 2-propanol, rinsed with
boiling 2-propanol, and then dried in air. Zinc oxide film (ZnO) of about 60 nm
thickness was prepared on the clean ITO substrate using a solꢀgel method as described
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in our previous paper [39]. Solutions of o-dichlorobenzene containing polymer (12.5
mg/mL) and PC61BM (18.75 mg/mL) were spin-coated onto the ITO/ZnO substrate at
700 rpm. Dispersions of PEDOT:PSS (0.65 wt%) in water containing Capstone®
fluorosurfactant FS-31 (0.5 wt%) were spin-coated onto the polymer:PC61BM layer at
2000 rpm. An Au back electrode was vacuum deposited onto the PEDOT:PSS layer
under a pressure of 2 × 10-5 Torr. The effective area of the solar cell was restricted to
1.0 cm2 by depositing the Au electrode through a shadow mask. All spin-coated films
were prepared under a relative humidity of less than 35% at room temperature in air.
3. Results and discussion
3.1. Synthesis
The synthetic routes to the optically active BDT-based monomers ((R,R)- and
(S,S)-4) and polymers ((R,R)- and (S,S)-5) are summarized in Scheme 1. Reaction of 1
with alkyl tosylates, which can be easily derivatized from the corresponding alkyl
alcohols, has been used to prepare BDT-based monomers bearing alkoxy pendants [30].
According to this strategy, enantiopure (R)- and (S)-2-OH were needed to prepare
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(R,R)- and (S,S)-4, respectively. Several methods to prepare (R)- and (S)-2-OH, such as
catalytic asymmetric synthesis [40-43], asymmetric induction [34, 44-46] and
enzymatic reactions [38, 47-51], have been reported. However, none of these methods
satisfy all of the requirements of high enantiomeric purity, high scalability and
simplicity. Therefore, we used the following approach to synthesize (R)- and (S)-2-OH
that included a reliable, scalable and simple diastereomer separation as an important
step (Scheme 2). Optically inactive rac-2-OH was first converted to a diastereomeric
mixture of (S,R)- and (S,S)-7 using chiral derivatizing agent (S)-6. We then successfully
isolated (S,R)- and (S,S)-7 (de > 99%) by preparative HPLC separation on Daicel
Chiralcel OJ using MeOH as an eluent. Alkaline hydrolysis of the isolated (S,R)- and
(S,S)-7 gave optically pure (R)- and (S)-2-OH, respectively. The absolute configurations
of (R)- and (S)-2-OH were determined by comparison of the signs of optical rotation
with literature data [38]. Finally, the BDT-based monomers (R,R)- and (S,S)-4 were
synthesized via reaction of 1 with (R)- and (S)-2-OTs derived from (R)- and (S)-2-OH,
respectively, followed by stannylation at the 2- and 6-positions of (R,R)- and (S,S)-3.
Copolymerization of (R,R)- or (S,S)-4 and 5 by Stille cross-coupling was performed
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using Pd(PPh3)4 as the catalyst in a mixture of toluene/DMF (4/1, v/v) at 120 °C (Table
1). For comparison, we also prepared the previously reported optically inactive PTB5
(run 3 in Table 1) [30]. The molecular weight (Mn) and polydispersity index (PDI) of the
polymers were determined by SEC analysis in THF. The obtained polymers possessed
similar Mn within experimental variation. The thermal stabilities of (R,R)- and
(S,S)-PTB5 were investigated by thermogravimetric analysis under a nitrogen
atmosphere (Supporting Information, Fig. S3). The 5% weight-loss temperatures (Td5)
of (R,R)- and (S,S)-PTB5 are 326 and 323 °C, respectively, which are comparable to
that of conventional PTB5 (326 °C). The slightly smaller Td5 of (S,S)-PTB5 may be
attributed to low-molecular-weight components present in (S,S)-PTB5. These results
indicate that controlling the chirality of the branched alkyl pendants has little effect on
the polymerizability and thermal stability of these polymers.
3.2. Absorption and chiroptical properties
Absorption and CD spectra of the polymers in chlorobenzene and films are
shown in Fig. 1. The optical bandgaps (Egopt) of the polymers were calculated from the
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absorption onset wavelength (λonset) in the film state (Table 1). As expected because
opt
(R,R)-PTB5 and (S,S)-PTB5 are enantiomers, the absorption spectrum and Eg of
(R,R)-PTB5 were coincident with those of (S,S)-PTB5. In addition, the absorption
spectrum of PTB5 was also similar to those of the optically active polymers both in
solution and film states. These results suggest that controlling the chirality of the
2-ethylhexyl pendants hardly affects the absorption properties of the PTB-based
polymers, which is consistent with a previous report using small molecule-based donor
materials [33]. The (R,R)- and (S,S)-PTB5 films exhibited apparent Cotton effects in the
polymer backbone region located around 500–800 nm and their CD spectral patterns
were almost mirror images of each other. The optically inactive PTB5 film did not show
a CD signal, which suggests that the observed CD signals for (R,R)- and (S,S)-PTB5 in
the film state are not artificial peaks. In contrast, (R,R)- and (S,S)-PTB5 hardly exhibited
apparent Cotton effects in chlorobenzene. This indicates that (R,R)- and (S,S)-PTB5
form chirally ordered superstructures induced by the chirality of the optically pure
pendants in the film state. Similar chiral aggregate formation has been observed for
various π-conjugated polymers bearing optically active pendants in the film state as well
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as in poor solvents [34-36].
3.3. Energy levels
The HOMO energy levels of the polymers were estimated by photoelectron
spectroscopy by measuring the ionic potential of the polymer films in the atmosphere
(Supporting Information, Fig. S4). LUMO energy levels were estimated as the sum of
the HOMO energy levels and Egopt values. The HOMO and LUMO energy levels of the
polymers are summarized in Table 1. The three polymers exhibit almost identical
HOMO and LUMO energy levels and the frontier orbital energy levels of the
PTB-based polymers are almost independent on the chirality of the 2-ethylhexyl
pendants.
3.4. Photovoltaic properties of PSCs
The photovoltaic properties of the polymers were evaluated in inverted-type
bulk
heterojunction
(BHJ)
PSCs
with
a
configuration
of
ITO/ZnO/Polymer:PC61BM/PEDOT:PSS/Au [39]. Optimized photovoltaic devices
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were obtained by spin-coating a solution of polymer:PC61BM (1:1.5, w/w) in
o-dichlorobenzene. The thickness of the active layers was around 100 nm. The current–
voltage (J–V) characteristics of the devices measured under AM 1.5 G irradiation (100
mW/cm2) from a solar simulator are depicted in Fig. 2. The corresponding device
parameters including short-circuit current density (Jsc), open-circuit voltage (Voc), fill
factor (FF) and PCE are summarized in Table 2. The three kinds of polymers with
different chiral information showed approximately similar device behavior, although
slight deviation was found in each device. In addition, no clear differences were
observed in the hole mobilities of the polymer films (ꢀh = ca. 3.0 × 10−4 cm2·V−1·s−1,
Supporting Information, Fig. S5), which were measured in accordance with the
space-charge-limited current model, and the π–π stacking distances between the
polymers (d = 3.91 Å) determined by XRD (Supporting Information, Fig. S6). In light
of the overall results for the optically active and inactive polymers, the differences in
the parameters of the PSCs seem to be caused by the batch-to-batch variability in the
devices or the difference in molecular weight of the polymers. These results mean that
controlling the chirality of the 2-ethylhexyl pendants does not lead to obvious
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improvement in the performance of our inverted-type PSC devices using BDT-based
polymers as a donor material. Recently, Castellano and coworkers reported OPV
devices containing small molecule-based donor materials bearing optically active
2-ethylhexyl groups [52]. They also did not achieve meaningful differences in device
performance through chirality control of the branched alkyls. The performance of PSCs
is determined not only by the inherent nature of donor polymers but also by
complicating factors such as the morphology of polymer/PC61BM blend films and
interfacial adhesion between the active layers and electrodes [53]. Further device
engineering may be required to elicit the effect of the chirality of alkyl pendants on the
performance of photovoltaic devices.
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4. Conclusion
We have successfully synthesized the novel optically active narrow-bandgap
polymers (R,R)- and (S,S)-PTB5 containing the BDT units bearing (R)- and
(S)-2-ethylhexyl pendants, respectively, as electron donors for PSCs. The effect of the
pendant chirality on the thermal, optical and photovoltaic properties of the polymers
was investigated by direct comparison with the properties of optically inactive PTB5
bearing racemic 2-ethylhexyl pendants. In terms of thermal stability, absorption
behavior and frontier orbital energy levels, optically active (R,R)- and (S,S)-PTB5
showed almost identical characteristics to the conventional optically inactive PTB5. The
formation of a chirally ordered superstructure in the (R,R)- and (S,S)-PTB5 films was
confirmed using CD spectroscopy. The photovoltaic properties of the donor polymers
were evaluated using preliminary inverted-type BHJ solar cells consisting of PC61BM
as an acceptor. The (R,R)- and (S,S)-PTB5-based PSCs showed comparable
performance to that of the PTB5-based PSC. However, it might be premature to
conclude that we can ignore the chirality of the branched alkyl chain in PSCs because
the performance of PSCs cannot be determined solely by the inherent nature of donor
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polymers. The latent potential of polymers bearing optically active branched alkyls may
be realized through innovation in device engineering, which includes a formation of a
nanoscale percolating network in the BHJ structures and a high interfacial adhesion
between active layers and electrodes, and lead to unexpected improvement in PSC
performance including efficiency and stability.
Acknowledgments
We thank Prof. T. Yamagishi (Kanazawa University) for allowing us to use
the TGA instrument. This work was supported by Yazaki Memorial Foundation for
Science and Technology and the Japan Society for the Promotion of Science (JSPS)
KAKENHI Grants-in-Aid for Scientific Research (B), (Grant No. 24350092), Japan.
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