Vinylene-bridged difluorobenzo[: C] [1,2,5]-thiadiazole (FBTzE): A new electron-deficient building block for high-performance semiconducting polymers in organic electronics

Article abstract of DOI:10.1039/c8tc05764h

A new class of an acceptor unit, vinylene-bridged 5,6-difluorobenzothiadiazole FBTzE, has been developed. Palladium-catalyzed Migita-Kosugi-Stille coupling reactions of 1 with 2, yielding 3 and its sequential dehydrogenative coupling with 4, readily affor

Full text of DOI:10.1039/c8tc05764h

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Vinylene-bridged difluorobenzo[c][1,2,5]-
thiadiazole (FBTzE): a new electron-deficient
building block for high-performance semiconducting
polymers in organic electronics†
b
Cite this:DOI: 10.1039/c8tc05764h
Yuya Asanuma,a Hiroki Mori,b Ryosuke Takahashia and Yasushi Nishihara
*
A new class of an acceptor unit, vinylene-bridged 5,6-difluorobenzothiadiazole FBTzE, has been
developed. Palladium-catalyzed Migita–Kosugi–Stille coupling reactions of 1 with 2, yielding 3 and its
sequential dehydrogenative coupling with 4, readily afforded FBTzE-containing monomers 5a–5c that
have lower lowest unoccupied molecular orbital (LUMO) energy level and smaller energy gap than those
of 5,6-difluorobenzothiadiazole (DFBT). Subsequently, three types of FBTzE-containing copolymers 3T,
4T, and 2TTT were synthesized by Migita–Kosugi–Stille coupling of monomers 5b and 5c with distanny-
lated thiophene, bithiophene, and thienothiophene, respectively and their physicochemical properties
and solar cell performances were evaluated. As a result of cyclic voltammogram, the synthesized FBTzE-
based polymers have deeper highest occupied molecular orbital (HOMO) and LUMO energy levels, and
stronger intermolecular interactions than those of DFBT-based polymer PffBT4T-DT. Although 3T/PC61BM
blended film formed favorable face-on orientation with short dp of 3.57 Å, its solar cell showed poor PCE of
2.7% owing to the construction of large phase separation structure with a domain size over 100 nm. In a
sharp contrast, 2TTT/PC61BM formed unsuitable edge-on orientation with short dp of 3.49 Å, but its film
formed optimal nanoscale phase separation, leading to a good performance with PCE of up to 5.2%.
Received 16th November 2018,
Accepted 18th December 2018
DOI: 10.1039/c8tc05764h
rsc.li/materials-c
various donor and acceptor units.3–5 To date, many researchers
have developed the high-performance D–A polymers and
Introduction
Donor–acceptor (D–A) type semiconducting polymers have achieved high field-effect mobility (m) over 1 cm2 Vꢀ1 sꢀ1 6–11
been widely utilized for the development of high-performance and high power conversion efficiency (PCE) over 10%.12–17
p-type and n-type semiconductors in organic field-effect transistors
In order to obtain the high-performance low-bandgap p-type
(OFETs)1,2 and organic photovoltaic cells (OPVs).3,4 Main advan- polymers and air-stable n-type polymers for OFETs and OPVs,
tages of D–A polymers are electron delocalization and the con- the development of strong acceptor units is highly desirable.
struction of quinoidal structures through a mesomeric effect, Among the developed strong acceptors including diketopyrro-
leading to broad absorption with a small bandgap (Eg) and lopyrrole (DPP),7,18–20 isoindigo (IID),21–24 naphthalenediimide
strong intermolecular interactions owing to their electrostatic (NDI),25–27 naphthobisthiadiazole (NTz),13,28 and a double B’N
interaction between polymer mainchains.3,4 In addition, the bridged bipyridyl (BNBP),29–31 5,6-difluorobenzo[c][1,2,5]-
electronic states (highest occupied molecular orbital (HOMO) thiadiazole (DFBT, Fig. 1) is the well-known strong acceptor unit
and lowest unoccupied molecular orbital (LUMO) energy levels) for both high-performance OFETs and OPVs,15,32–40 because of
of D–A polymers can easily be controlled by combining the the following reasons. First, not only the electron-deficient
1,2,5-thidiazole ring and o-benzoquinoidal structure of a DFBT
core, but also two fluorine atoms can offer the low-lying LUMO
a Graduate School of Natural Science and Technology, Okayama University,
and HOMO energy level and narrow bandgap.15,32–40 Therefore,
3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530, Japan
b Research Institute for Interdisciplinary Science, Okayama University,
DFBT-containing polymers have high air-stability and high
light-harvesting ability. Second, since a DFBT core can facilitate
the intra- and intermolecular non-covalent interactions such as
Nꢁ ꢁ ꢁH, Fꢁ ꢁ ꢁS, and Fꢁ ꢁ ꢁH between neighboring aromatic p-spacer
or polymer backbones, DFBT-based copolymers have high
coplanarity with dihedral angles of nearly 01 and can promote
the p-orbital overlap between polymer backbones, which can
3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530, Japan.
E-mail: ynishiha@okayama-u.ac.jp
† Electronic supplementary information (ESI) available: Full characterizations,
and 1H, 13C{1H}, and 19F{1H} NMR spectra of all new compounds and polymers,
details of theoretical calculations, GPC traces, physicochemical properties, SCLC
hole mobility, photovoltaic properties, GIWAXS analysis. See DOI: 10.1039/
c8tc05764h
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copolymers containing thiophene, bithiophene, and thieno-
thiophene as the spacers (Fig. 1b). In addition, their physico-
chemical properties, thin-film structure analyses, and solar cell
characteristics were investigated to evaluate the potential of
FBTzE for high-performance semiconducting polymers. Here,
each FBTzE and bis(4-methylthienyl)FBTzE has four possible
conformations. From DFT calculations, conformers 1 and 5 is
the most thermodynamically stable structure (Fig. S2, ESI†).
Thus, we hereafter describe chemical structures of FBTzE-based
copolymers with these conformations.
Experimental
General
All the reactions were carried out under an Ar atmosphere using
standard Schlenk techniques. Glassware was dried in an oven
(130 1C) and heated under reduced pressure prior to use.
Dehydrated tetrahydrofuran (THF), dimethylsulfoxide (DMSO),
and toluene were purchased from Kanto Chemicals Co., Ltd.
For thin layer chromatography (TLC) analyses throughout this
work, Merck precoated TLC plates (silica gel 60 GF254, 0.25 mm)
were used. Silica gel column chromatography was carried out
using Silica gel 60 N (spherical, neutral, 40–100 mm) from Kanto
Fig. 1 Chemical structures of DFBT, FBTzE, and its copolymers.
provide a strong aggregation.35,41,42 Indeed, it is reported that Chemicals Co., Ltd. The 1H, 13C{1H}, 19F{1H} NMR spectra were
simple DFBT-quaterthiophene copolymers (PffBT4T-R, Fig. 1a) recorded on a Varian 400-MR (400 MHz) and Varian INOVA-600
exhibited strong temperature-dependent aggregation.15,33,34,43 (600 MHz) spectrometer. Infrared spectra were recorded on a
Moreover, this strong aggregation behavior can facilitate the Shimadzu IRPrestige-21 spectrophotometer. Elemental analyses
high crystalline thin films, resulting in a high field-effect hole were carried out with a Perkin-Elmer 2400 CHN elemental analyzer
mobility of nearly 2 cm2 Vꢀ1 sꢀ1 33
In addition, since DFBT at Okayama University. Polymerizations were performed with a
.
copolymers also formed the long-range ordered structure even Biotage initiator microwave reactor. Molecular weights of polymers
in the blended films with soluble fullerene, space-charge limited were determined by gel-permeation chromatography (GPC) with a
current (SCLC) hole mobility reached 10ꢀ2 cm2 Vꢀ1 sꢀ1, and thus TOSOH HLC-8321GPC/HT and TSKgel GMHHR-H HT using a
the fabricated OPV exhibited excellent PCE of 12%.15,43
polystyrene standard and o-dichlorobenzene (o-DCB) as the eluent
However, to the best our knowledge, despite these excellent at 140 1C. Recycling preparative high performance liquid chromato-
features, new type of DFBT-containing acceptor units have not graphy (HPLC) was performed on a Shimadzu LC-20AP instrument
been developed. Therefore, the development of a new class equipped with Shodex GPC K-4001L and -4002L columns, and
of acceptor unit bearing a DFBT unit is highly important to Shimadzu RID-10 refractive index detector. Chloroform was
develop the novel high-performance semiconducting polymers used as the mobile phase at room temperature with a flow rate
for OFETs and OPVs.
14 mL minꢀ1
4-Bromo-5,6-difluorobenzo[c][1,2,5]thiadiazole (1),44 2-bromo-
.
In terms of the new high-performance p-type and n-type
semiconductors based on a DFBT unit, we newly designed and 3-(2-hexyldecyl)thiophene (4a),45 2-bromo-3-(2-octyldodecyl)-
synthesized vinylene-bridged DFBT unit (FBTzE, Fig. 1b). The thiophene (4b),45 2-bromo-3-(2-decyltetradecyl)thiophene (4c),46
presence of two DFBT units in the FBTzE core might enhance 2,5-bis(trimethylstannyl)thiophene (6),47 5,50-bis(trimethylstannyl)-
the electron affinity, which can lower HOMO and LUMO energy 2,20-bithiophene (7),48 2,5-bis(trimethylstannyl)thieno[3,2-b]-
levels. In fact, the DFT calculation revealed that model compound thiophene (8),49 5,6-difluorobenzo[c][1,2,5]thiadiazole (DFBT),38
of FBTzE–quaterthiophene copolymer has lower HOMO and LUMO 5,6-difluoro-4,7-bis(5-bromo-4-(2-decyltetradecyl)thiophen-2-yl)-
energy levels than those of a DFBT counterpart (Fig. S1, ESI†). This benzo[c][1,2,5]thiadiazole (DFBT2T-DT-Br),38 and PffBT4T-DT43
feature is highly advantageous for high-performance OPVs and were synthesized according to the reported procedures. All
n-type semiconductors. In addition, more rigid and p-extended other chemicals were used without further purification unless
structure of the FBTzE core owing to various intramolecular otherwise indicated.
non-covalent interaction can enhance the effective p-orbital
(E)-1,2-Bis(5,6-difluorobenzo[c][1,2,5]thiadiazol-4-yl)ethene
overlaps, compared to those of DFBT-based polymers, which (FBTzE) (3). To a deaerated solution of 4-bromo-5,6-difluoro-
may lead to the construction of high crystalline thin-film with benzo[c][1,2,5]thiadiazole (1, 1.18 g, 4.7 mmol) and trans-1,2-
longer-range ordered structure.28 Herein, we report the synthesis bis(tributylstannyl)ethylene (2, 1.30 g, 2.1 mmol) in anhydrous
of three bis(4-alkylthienyl)FBTzE monomers and the three toluene (105 mL) in a 200 mL two-necked round-bottomed
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1
flask were added tris(dibenzylideneacetone)dipalladium(0)- 536 (m). H NMR (600 MHz, CDCl3, rt): d 0.87 (t, J = 6.9 Hz,
chloroform adduct (Pd2(dba)3ꢁCHCl3, 110 mg, 0.11 mmol) 12H), 1.20–1.40 (m, 64H), 1.71 (m, 2H), 2.50 (d, J = 7.2 Hz, 4H),
and tri(o-tolyl)phosphine (P(o-tolyl)3, 130 mg, 0.43 mmol). 7.82 (s, 2H), 8.43 (s, 2H). 13C{1H} NMR (100 MHz, CDCl3, rt):
The reaction mixture was heated to a gentle reflux for 12 h. d 14.15, 14.33, 14.49, 22.74, 22.92, 23.08, 26.65, 29.62, 29.64,
The resulting mixture was cooled to room temperature and 29.92, 29.99, 30.34, 31.98, 32.15, 32.32, 33.35, 33.82, 38.60,
1 M potassium fluoride (KF) aqueous solution (100 mL) was 111.56 (d, J = 11.5 Hz), 112.98 (d, J = 10.7 Hz), 116.17 (d, J =
added. The crude mixture was extracted with chloroform 9.2 Hz), 122.02 (br), 131.01, 131.79 (d, J = 10.7 Hz), 141.33,
(100 mL ꢂ 5) and washed with brine and dried over MgSO4. 148.02 (d, J = 9.3 Hz), 148.34 (d, J = 8.3 Hz), 148.62 (dd, J =
After the removal of the solvent under reduced pressure, 235.0, 19.1 Hz), 151.22 (dd, J = 235.8, 18.3 Hz). 19F{1H} NMR
obtained solid was washed with hexane and purified by passing (376 MHz, CDCl3, rt): d ꢀ130.98 (d, J = 14.7 Hz), ꢀ129.29 (d, J =
through a pad of Florisil with chloroform as the eluent to afford 3 14.7 Hz). Anal. calcd for C62H86Br2F4N4S4: C, 59.51; H, 6.93;
(772 mg, 2.1 mmol), quantitatively, as a yellow solid. Mp 255–256 1C. N, 4.48%. Found: C, 59.51; H, 6.96; N, 4.39%.
FT-IR (KBr, cmꢀ1): 3107 (w), 3074 (w), 1528 (m), 1466 (s), 1350 (m),
(E)-1,2-Bis(7-(5-bromo-4-(2-decyltetradecyl)thiophen-2-yl)-5,6-
1308 (s), 1182 (s), 974 (s), 937 (m), 872 (s), 843 (s), 438 (m). 1H NMR difluorobenzo[c][1,2,5]-thiadiazol-4-yl)ethene (5c). Red solid.
(600 MHz, CDCl3, rt): d 7.71 (dd, J = 8.8, 7.2 Hz, 2H), 8.88 (s, 2H). Yield: 45% (hexane: dichloromethane = 5 : 1 as the eluents,
13C{1H} NMR (151 MHz, CDCl3, rt): d 105.49 (d, J = 19.8 Hz), 116.74 Rf = 0.80). Mp 62–64 1C. FT-IR (KBr, cmꢀ1): 2924 (s), 2851 (s),
(d, J = 11.5 Hz), 125.46 (dd, J = 6.9, 3.5 Hz), 149.81 (d, J = 8.1 Hz), 1543 (m), 1489 (m), 1443 (s), 1354 (m), 1342 (m), 1005 (m), 982
1
150.99 (d, J = 12.7 Hz), 151.45 (dd, J = 262.4, 18.4 Hz), 154.10 (m), 851 (m), 721 (m), 536 (m). H NMR (300 MHz, CDCl3, rt):
(dd, J = 257.9, 18.5 Hz). 19F{1H} NMR (376 MHz, CDCl3, rt): d 0.87 (t, J = 6.8 Hz, 12H), 1.20–1.40 (m, 80H), 1.73 (s, 2H), 2.54
d ꢀ131.27 (d, J = 17.3 Hz), ꢀ128.28 (d, J = 17.3 Hz). Anal. calcd (d, J = 7.2 Hz, 4H), 7.89 (s, 2H), 8.61 (s, 2H). 13C{1H} NMR
for C14H4F4N4S2: C, 45.65; H, 1.09; N, 15.21%. Found: C, 45.70; (100 MHz, CDCl3, rt): d 14.32, 14.48, 19.80, 20.99, 22.74, 22.91,
H, 0.88; N, 15.20%.
23.08, 26.67, 29.62, 29.63, 29.92, 29.24, 29.98, 30.01, 30.36,
Typical procedure for the synthesis of (E)-1,2-bis(7-(5-bromo- 31.99, 32.16, 32.17, 32.33, 33.37, 33.85, 38.61, 43.29, 111.62
4-(2-hexyldodecyl)thiophen-2-yl)-5,6-difluorobenzo[c][1,2,5]- (d, J = 10.7 Hz), 113.08 (d, J = 10.7 Hz), 116.17 (d, J = 9.2 Hz),
thiadiazol-4-yl)ethene (5a). In a 50 mL Schlenk tube, 2-bromo-3- 122.16 (br), 131.04, 131.86 (d, J = 10.8 Hz), 141.38, 148.08 (d, J =
(2-hexyldecyl)thiophene (4a, 1.79 g, 4.6 mmol) was added to a 9.2 Hz), 148.42 (d, J = 8.4 Hz), 148.68 (dd, J = 240.5, 19.1 Hz),
mixture of compound 3 (423 mg, 1.15 mmol), palladium(II) 151.28 (dd, J = 237.2, 18.3 Hz). 19F{1H} NMR (376 MHz, CDCl3, rt):
trifluoroacetate (Pd(tfa)2, 77 mg, 0.23 mmol), and silver(I) d ꢀ130.97 (d, J = 14.7 Hz), ꢀ129.27 (d, J = 14.7 Hz). Anal. calcd for
carbonate (Ag2CO3, 2.54 g, 9.2 mmol) in DMSO (11.5 mL). C70H102Br2F4N4S4: C, 61.66; H, 7.54; N, 4.11%. Found: C, 61.67;
The reaction mixture was stirred at 140 1C for 24 h. After H, 7.76; N, 4.09%.
cooling to room temperature, the mixture was extracted with
Typical procedure for the synthesis of polymers (PFBTzE3T-
dichloromethane (50 mL ꢂ 3), washed with 1 M HCl aqueous OD, 3T). Monomers 5b (62.6 mg, 0.05 mmol), 2,5-bis(trimethyl-
solution and brine, and then dried over MgSO4. After the stannyl)thiophene (6, 20.5 mg, 0.05 mmol), tetrakis(triphenyl-
removal of the solvent under reduced pressure, obtained solid phosphine)palladium(0) (Pd(PPh3)4, 1.2 mg, 1 mmol), copper
was purified by silica gel column chromatography with hexane iodide(I) (CuI, 1.0 mg, 5 mmol) and toluene (2.5 mL) were added
and hexane–dichloromethane (5 : 1) as the eluents (Rf = 0.41) to to a reaction vessel, which was sealed and refilled with argon.
afford 5a (663 mg, 0.58 mmol) in 52% yield as a red solid. Mp The reaction mixture was heated at 180 1C for 2 h in a
88–90 1C. FT-IR (KBr, cmꢀ1): 2924 (s), 2853 (s), 1541 (m), 1489 microwave reactor. After being cooled to room temperature,
(m), 1445 (s), 1354 (m), 982 (m), 851 (m), 536 (m). 1H NMR the reaction mixture was poured into 100 mL of methanol
(600 MHz, CDCl3, rt): d 0.89 (t, J = 7.2 Hz, 12H), 1.20–1.40 containing 5 mL of concentrated hydrochloric acid and stirred
(m, 48H), 1.67 (m, 2H), 2.42 (d, J = 7.2 Hz, 4H), 7.70 (s, 2H), 8.10 for 3 h. The precipitate was then subjected to sequential Soxhlet
(s, 2H). 13C{1H} NMR (150 MHz, CDCl3, rt): d 14.30, 14.31, extraction with methanol, hexane, and chloroform to remove
22.87, 22.90, 26.67, 26.70, 29.57, 29.86, 29.93, 30.26, 32.11, low molecular-weight fractions. The residue was extracted with
32.12, 33.47, 33.48, 34.09, 38.67, 112.15 (d, J = 11.5 Hz), chlorobenzene, and concentrated solution was poured into
113.83 (d, J = 10.4 Hz), 116.06 (d, J = 9.2 Hz), 123.40 (br), 50 mL of methanol. The formed precipitates were collected by
131.24, 132.44 (d, J = 11.6 Hz), 141.75, 148.55 (d, J = 9.2 Hz), filtration and dried in vacuo to afford 3T (44.7 mg, 76%) as a
148.99 (d, J = 8.1 Hz), 149.18 (dd, J = 260.1, 19.6 Hz), 151.58 metallic purple solid. GPC (o-DCB, 140 1C): Mn = 46.9 kDa, Mw =
(dd, J = 262.4, 18.4 Hz). 19F{1H} NMR (376 MHz, CDCl3, rt): 93.1 kDa, PDI = 1.98. Anal. calcd for C66H90F4N4S5: C, 67.42;
d ꢀ130.98 (d, J = 14.7 Hz), ꢀ129.22 (d, J = 14.7 Hz). Anal. calcd H, 7.72; N, 4.77%. Found: C, 66.86; H, 7.52; N, 4.61%.
for C54H70Br2F4N4S4: C, 56.93; H, 6.19; N, 4.92%. Found: C,
56.98; H, 6.15; N, 4.87%.
PFBTzE4T-DT, 4T. Monomers 5c (68.2 mg, 0.05 mmol), 5,50-
bis(trimethylstannyl)-2,20-bithiophene (7, 24.6 mg, 0.05 mmol),
(E)-1,2-Bis(7-(5-bromo-4-(2-octyldodecyl)thiophen-2-yl)-5,6- tetrakis(triphenylphosphine)palladium(0) (1.2 mg, 1 mmol), and
difluorobenzo[c][1,2,5]-thiadiazol-4-yl)ethene (5b). Red solid. toluene (2.5 mL) were subjected to the polymerization procedure,
Yield: 45% (hexane : dichloromethane = 5 : 1 as the eluents, and the reaction mixture was heated at 140 1C for 30 min in a
Rf = 0.70). Mp 73–75 1C. FT-IR (KBr, cmꢀ1): 2924 (s), 2853 (s), microwave reactor. Sequential Soxhlet extraction with the same
1543 (m), 1489 (m), 1443 (s), 1354 (m), 984 (m), 851 (m), solvents used for 3T was employed to obtain 4T (40.6 mg, 59%) as
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a metallic purple solid. GPC (o-DCB, 140 1C): Mn = 133.8 kDa, Mw = rinsed with acetone and isopropanol at room temperature,
319.8 kDa, PDI = 2.39. Anal. calcd for C78H108F4N4S6: C, 68.38; then in hot isopropanol for 5 min. The substrates were dried
H, 7.95; N, 4.09%. Found: C, 68.37; H, 8.01; N, 3.94%.
and immediately transferred into a nitrogen-filled glove box.
PFBTzE2TTT-DT, 2TTT. Monomers 5c (68.2 mg, 0.05 mmol), The active layers with PC61BM were deposited by spin-coating
2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene (8, 23.3 mg, (at 600 rpm for 60 s for 3T, and at 600 rpm for 30 s for 4T and 2TTT)
0.05 mmol), tetrakis(triphenylphosphine)palladium(0) (1.2 mg, from a solution containing a polymer sample (10.0 mg mLꢀ1 for 3T,
1 mmol), and toluene (2.5 mL) were subjected to the polymeriza- 3.3 mg mLꢀ1 for 4T, and 6.0 mg mLꢀ1 for 2TTT) and a
tion procedure, and the reaction mixture was heated at 140 1C for respective amount of PC61BM in anhydrous chlorobenzene
30 min in a microwave reactor. Sequential Soxhlet extraction (CB). The solution was kept at 100–140 1C (100 1C for 3T, and
with the same solvents used for 3T was employed to obtain 140 1C for 4T and 2TTT) for 30 min, and the hot solution was
2TTT (39.9 mg, 59%) as a metallic purple solid. GPC (o-DCB, spin-coated on the substrate (at room temperature for 3T, and
140 1C): Mn = 76.4 kDa, Mw = 166.7 kDa, PDI = 2.18. Anal. calcd preheated at 140 1C for 4T and 2TTT before spin-coating).
for C76H106F4N4S6: C, 67.91; H, 7.95; N, 4.17%. Found: C, 67.65; The p/n ratio denotes weight ratios of polymers and PC61BM.
H, 7.95; N, 3.92%.
1,8-Diiodooctane (DIO, 1 vol% for 4T) or diphenyl ether
(DPE, 2.5 vol% for 3T and 4 vol% for 2TTT) were used as the
solvent additives. The active layer thickness of 3T and 2TTT was
Instrumentation and theoretical calculation
UV-vis absorption spectra were measured using a Shimadzu 100 and 130 nm, respectively. The uniform 4T/PC61BM blended
UV-2450 UV-vis spectrometer. Cyclic voltammograms (CVs) were film could not be obtained, due to too strong aggregation
recorded on Electrochemical Analyzer CHI-600B in acetonitrile con- tendency of 4T. MoO3 (6 nm) as an anode interlayer and an Ag
taining tetrabutylammonium hexafluorophosphate (TBAP, 0.1 M) as (50 nm) layer were deposited under high vacuum (B6 ꢂ 10ꢀ5 Pa)
supporting electrolyte at a scan rate of 100 mV sꢀ1. A Pt through a shadow mask. The active area of all devices was
electrode (surface area: A = 0.071 cm2, BAS), an Ag/Ag+ (Ag wire 0.16 cm2. The thickness of the active layer was measured with
in 0.01 M AgNO3/0.1 M TBAPF6/CH2Cl2 or CH3CN), and a Pt an AlphaSteps IQ surface profiler (KLA Tencor).
wire electrode were used as working, reference, and counter
The characteristics of the solar cell devices were measured
electrodes, respectively. Samples of the polymer films were through a 4 ꢂ 4 mm photo-mask, with a Keithley 2401 semi-
prepared by drop-casting on a working electrode from their conductor analyzer, using a Xe lamp (Bunkokeiki OTENTO-SAN
chloroform solutions. All the potentials were calibrated with III type G2) as the light source, under AM 1.5 G simulated solar
the standard ferrocene/ferrocenium redox couple (Fc/Fc+: E1/2
=
irradiation at 100 mW cmꢀ2 at room temperature under a
+0.52 V for CH2Cl2, and E1/2 = +0.01 V for CH3CN measured nitrogen atmosphere. The light intensity was determined by a
under identical conditions). Dynamic force-mode atomic force calibrated standard silicon solar cell (Bunkokeiki, BS-520BK).
microscopy was carried out using an SPA 400-DFM (SII Nano External quantum efficiency (EQE) was measured using a
Technologies). Grazing incidence wide-angle X-ray diffraction Bunkokeiki SM-250 Hyper Monolight System.
(GIWAXS) analyses were carried out at SPring-8 on beamline
BL46XU. The samples were irradiated at a fixed angle on the Fabrication and characterization of hole-only devices
order of 0.121 through a Huber diffractometer with an X-ray
energy of 12.39 keV (l = 1 Å), and the GIWAXS patterns were
recorded on a 2D image detector (Pilatus 300K). Films of the
Hole-only devices were fabricated as follows. ITO substrates
were washed and treated with UV-ozone in the same manner
as described above. Then, ITO substrates were spin-coated
polymers and blended films with PC61BM were fabricated by
with poly(3,4-ethylene-dioxythiophene):poly(styrene sulfonate)
spin-coating on the ZnO treated ITO substrate. Geometry
(PEDOT:PSS) (Clevios P VP AI 4083) through a 0.45 mm PVDF
syringe filter at 5000 rpm for 30 s, and dried at 120 1C for
10 min in air. After being dried, the substrates were immedi-
ately transferred into a nitrogen-filled glove box. Thin films of
optimizations and normal-mode calculations were performed at
the B3LYP/6-31G(d) or M06-2X/6-31G** level using the Gaussian 09,
Revision D.01, program package.50
an active layer were deposited by same procedure as described
Fabrication of inverted bulk-heterojunction solar cells
above (high concentration solution was used, 13.3 mg mLꢀ1 for
3T and 8.0 mg mLꢀ1 for 2TTT). The active layer thickness of 3T
The inverted bulk-heterojunction solar cells were fabricated as
follows. ZnO precursor solution was prepared by hydrolysis
and 2TTT was 240 and 220 nm, respectively. After the thin films
of Zn(OAc)2.51 The ITO substrates (ITO, Geomatec Co. Ltd,
were dried, MoO3 (6 nm) and Al (80 nm) layers with an active
thickness = 150 nm, sheet resistance o 12 O sqꢀ1, transmittance
(l = 550 nm) Z 85%) were successively washed using ultrasonica-
tion in a neutral detergent, deionized water, acetone, and
isopropanol at room temperature, and in hot isopropanol, for
10 min, respectively. Then ITO substrates were treated with
UV-ozone for 20 min. Pre-cleaned ITO substrates were spin-
coated with 0.4 M ZnO precursor solution at 4000 rpm for 30 s,
and then immediately baked at 200 1C for 30 min in air. After
gradual cooling to room temperature, the substrates were
cathode area of 0.16 cm2 were deposited under high vacuum
(B6 ꢂ 10ꢀ5 Pa) through a shadow mask.
Current density–voltage ( J–V) characteristics of the fabri-
cated devices were measured using a Keithley 2401 Source
Meter in the dark. Voltage sweeps were performed in the range
of 0–8 V, and hole mobilities were estimated from the J–V curve
of the Mott–Gurney space charge limited current (SCLC) law:52
J = (9/8)e0erm(V2/L3)
(1)
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where e0 is the permittivity of free space, er is the dielectric high and low retention time region, and Mn of the polymer
constant of the polymer, m is the hole mobility, L is the obtained from a low-concentration solution of 4T and 2TTT
thickness of the active layer, and V is the voltage drop across become a lower value than that from high-concentration solution
the device (V = Vappl – Vbi). er is assumed to be 3, which is a (Fig. S5 and Table S2, ESI†). Therefore, polymers 4T and 2TTT
typical value for semiconducting polymers.
have stronger intermolecular interaction, likely due to their higher
symmetry of polymer backbones.55
Results and discussion
Computational study of FBTzE and its derivatives
Synthesis of FBTzE derivatives and FBTzE-based copolymers
PFBTzEAr
In order to evaluate the non-covalent intermolecular interaction,
the optimized structures of several model compounds were calcu-
Scheme 1 shows synthetic routes of FBTzE 3, monomers 5a–5c, lated by density functional theory (DFT) using M06-2X/6-31G**
and three copolymers. Migita–Kosugi–Stille coupling of 4-bromo- level.35 Fig. 2 shows the optimized structures and dihedral angles
5,6-difluorobenzo[c][1,2,5]thiadiazole (1)44 and trans-1,2-bis(tributyl- of non-substituted and fluorinated trans-stilbenes, vinylene-
stannyl)ethylene (2) afforded FBTzE (3) quantitatively. Then, bridged benzothiadiazoles, and FBTzE derivatives. trans-Stilbene
FBTzE-containing monomers 5a–5c were synthesized by (I) has a large dihedral angle of 18.21 between benzene rings and a
dehydrogenative coupling of 3 with 2-bromo-3-(2-alkyl)thiophenes vinylene moiety owing to a steric repulsion of each hydrogen atom
4a–4c.44,53,54 After optimization of the palladium catalyst, oxidant, (Fig. 2a). On the other hand, by introducing fluorine atoms or a
and additive, we could obtain compound 5a in 51% isolated yield fused thiadiazole ring instead of two hydrogen atoms, the dihedral
(Table S1, ESI†). Accordingly, with the same reaction conditions angles of fluorinated trans-stilbene (II) and vinylene-bridged
in hand, monomers 5b and 5c with different side chains were benzothiadiazole (III) were drastically decreased to 0.671 and
also synthesized. When 5a was used for copolymerization with 0.051, respectively (Fig. 2b and c). This suggests that Nꢁ ꢁ ꢁH or
distannylated bithiophene 7, soluble polymers were not obtained Fꢁ ꢁ ꢁH interaction can increase the coplanarity of a FBTzE
due to its insufficient solubility. Therefore, Migita–Kosugi–Stille framework. Especially, NꢁꢁꢁH can provide the higher coplanarity
coupling reactions of 5b with distannylated thiophene 6, and of 5c than that of FꢁꢁꢁH, likely due to the existence of stronger hydrogen
with distannylated bithiophene 7 and thienothiophene 8 yielded bond. Furthermore, FBTzE (3) with both thiadiazole ring has a
three copolymers PFBTzEAr in 76% (Ar = 3T), 59% (Ar = 4T), and completely coplanar structure with a dihedral angle of 01 (Fig. 2d).
59% (Ar = 2TTT), respectively. In the case of 3T, the addition of In thiophene-containing compounds, bisthienyl-trans-stilbene
CuI and longer time were needed to obtain high-molecular-weight (IV) also has a large dihedral angle of 27.21 between benzene
polymer. From high-temperature gel-permeation chromatography and adjacent thiophene rings (Fig. 2e). On the other hand, model
(GPC) analyses, all three polymers have high molecular weight compounds V and VI have largely decreased dihedral angles
over 40 kDa. Among them, the number-average molecular weights of 18.81 and 13.91, respectively, but they do not have completely
(Mn) of 4T and 2TTT polymers (Mn = 133.8 and 76.4 kDa) are planar structures (Fig. 2f and g). In contrast, the dihedral angle of
significantly higher than that of 3T (Mn = 46.9 kDa). This result bisthienyl FBTzE (VII) is less than 11, indicating that both NꢁꢁꢁH
might be attributed to the high aggregation in the solution. In and FꢁꢁꢁS interactions are required to afford the completely planar
fact, GPC curves of 4T and 2TTT showed obvious dual peaks at structure (Fig. 2h). These NꢁꢁꢁH and FꢁꢁꢁS interactions between
Scheme 1 Synthesis of FBTzE 3, monomers 5a–5c, and copolymers.
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Fig. 2 The optimized molecular structures and calculated dihedral angles of model compounds by DFT using M06-2X/6-31G** level.
benzothiadiazole derivatives and neighboring thiophene ring is
well-known and evaluated by single-crystal structure analyses.42,56,57
These calculation results are consistent with previously reported
results. From these results, highly coplanar structure of FBTzE
and its derivative can be expected to give the densely packing
structure in the solid state.
Physicochemical properties of FBTzE derivatives
UV-vis absorption spectra and cyclic voltammograms of 3 and
monomers 5b and 5c are shown in Fig. 3a and b, respectively, and
the results are summarized in Table 1, comparing the physico-
chemical properties of the standard DFBT and DFBT-containing
monomer (DFBT2T-DT-Br, Fig. 1a) (Fig. S6a and b, ESI†). The
UV-vis absorption spectrum of 3 showed the absorption max-
imum at 385 nm, which is 78 nm red-shifted absorption than that
of DFBT. In addition, the optical energy gap (Eg) of 3 (2.80 eV) is
significantly smaller than that of DFBT (3.71 eV), indicating the
effective p-extension of a FBTzE core. In monomers 5b and 5c, the
31 nm red-shifted absorption (483 nm) and about 0.2 eV smaller
Eg were observed, compared to those of DFBT2T-DT-Br. Further-
more, the absorption coefficients of 5b and 5c are ca. 3.5 times
higher (B55 000 Mꢀ1 cmꢀ1) than that of DFBT2T-DT-Br. This
results indicates that FBTzE derivatives 5b and 5c have stronger
absorption, which is beneficial for solar cells.
Fig. 3 (a) UV-vis absorption spectra and (b) cyclic voltammograms of 3
and monomers 5b and 5c in CH2Cl2 solution.
The electrochemical properties of 3 and monomers 5b and
5c were investigated (Fig. 3b and Table 1). FBTzE (3) exhibited
clear one oxidation and reduction waves. The estimated HOMO
and LUMO energy levels are ꢀ6.20 and ꢀ3.26 eV, respectively,
both of which are lower than those of DFBT (HOMO = ꢀ6.03 eV On the other hand, 5b and 5c have significantly higher HOMO
and LUMO = ꢀ2.98 eV). This indicates that 3 has a larger electron energy levels around ꢀ5.7 eV, and their LUMO energy level are
affinity due to the existence of two DFBT moieties in its core. slightly lower than that of 3. From DFT calculations, the
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Table 1 Physicochemical properties of FBTzE and DFBT derivatives
Compound
lmax,sola/nm
lmax,filmb/nm
Eogpt (ECgV)c/eV
EHOMOd/eV
ELUMOe/eV
3
5b
5c
3T
385
—
—
—
632, 685
636, 698
644, 707
—
—
637, 698
2.80 (2.94)
2.16 (2.29)
2.13 (2.28)
1.56 (1.74)
1.55 (1.64)
1.54 (1.64)
3.71 (3.05)
2.34 (2.40)
1.58 (1.63)
ꢀ6.20
ꢀ5.69
ꢀ5.68
ꢀ5.45
ꢀ5.33
ꢀ5.35
ꢀ6.03
ꢀ5.71
ꢀ5.25
ꢀ3.26
ꢀ3.40
ꢀ3.40
ꢀ3.71
ꢀ3.69
ꢀ3.71
ꢀ2.98
ꢀ3.31
ꢀ3.62
483, 506
483, 506
629, 693
643, 704
648, 708
307
4T
2TTT
DFBT
DFBT2T-DT-Br
317, 452
599, 695
PffBT4T-DT
a
b
c
Absorption maxima in dichloromethane or chlorobenzene solution at rt. Absorption maxima in thin film. Optical energy gap estimated from
d
absorption edge (ledge) (out of parentheses) and electrochemical gap (in parentheses). All the potentials were calibrated with the standard
ferrocene/ferrocenium redox couple (Fc/Fc+: E1/2 = +0.52 V for CH2Cl2 (small molecules), and E1/2 = +0.01 V for CH3CN (polymer) measured under
identical conditions). Estimated with the oxidation onset vs. Ag/Ag+; EHOMO = –4.28 or ꢀ4.79 – E
. Estimated with the reduction onset vs.
ox
onset
e
Ag/Ag+; ELUMO = –4.28 or ꢀ4.79 – E
.
red
onset
coefficient of LUMO in 5b and 5c strongly localized on the central Physicochemical properties of FBTzE-based copolymers, 3T, 4T,
FBTzE core, but a HOMO coefficient delocalized on the entire the and 2TTT
molecules (Fig. S3, ESI†). Therefore, the difference of LUMO
UV-vis absorption spectra of PFBTzEAr polymers are depicted in
Fig. 4a and b, and the extracted parameters are also summarized
in Table 1. Fig. S6c (ESI†) also showed absorption spectra of DFBT-
based polymer PffBT4T-DT for the comparison. 4T has the almost
similar Eg (1.55 eV) to that of the representative reported polymer
PffBT4T-DT (1.58 eV) in the solid state (Fig. 4b and Fig. S6c, ESI†).
energy level between 3 and monomers 5b and 5c is rather
small.58 Such tendency is in a good agreement with DFT calcula-
tions (Fig. S1, ESI†). Compared to DFBT2T-DT-Br, 5b and 5c
exhibited lower LUMO and similar HOMO energy levels, indicat-
ing the p-extension of with two FBTzE cores, which is consistent
with the result of UV-vis absorption spectra.
Fig. 4 UV-vis absorption spectra in solution (a) and in thin film (b). (c) Cyclic voltammograms in thin film of polymers 3T, 4T, and 2TTT and (d) their
energy diagrams.
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One possible reason for such similar Eg may be a similar effective structure owing to their higher regioregularity (Fig. S4, ESI†).
p-conjugation length. In fact, DFT calculations of their model Such coplanar structure can enhance the effective p–p over-
compounds revealed that the difference of Eg between 4T and laps, resulting in a stronger aggregation ability. On the other hand,
PffBT4T-DT decreased with an increase of repeating units (Fig. S1, polymer 3T has 0.1 eV deeper HOMO and same LUMO energy
ESI†). Interestingly, both 4T and PffBT4T-DT showed strong levels as those of polymers 4T and 2TTT (Fig. 4c, d and Table 1).
temperature-dependent aggregation, but quite different behaviors This may be attributed to its twisted backbone to diminish the
were observed (Fig. S6c and S7b, ESI†). Upon heating at ca. 80 1C in effective p–p overlap59,60 or arising from the terthiophene unit
solution, 4T exhibited a slight blue-shifted spectra and the having the reduced electron-donating ability.61
decrease of an intensity, while PffBT4T-DT showed significantly
Photovoltaic properties of polymer/PC61BM-based solar cells
hypochromic shifted absorption spectrum and one broad absorp-
tion at longer wavelength region, indicating that 4T formed partial
aggregate in the high temperature solution, whereas PffBT4T-DT
showed completely disaggregated behavior. These results
suggest that 4T has stronger intermolecular interaction than
that of PffBT4T-DT due to its extended p-electron system.
Cyclic voltammograms of polymers revealed that 4T have lower
HOMO and LUMO energy levels (ꢀ5.33 and ꢀ3.69 eV) than
those of PffBT4T-DT (ꢀ5.25 and ꢀ3.62 eV) (Table 1, Fig. 4c, d
and Fig. S6d, ESI†). This may be attributed to a larger electron
affinity of the FBTzE core, which are consistent with DFT
calculations. Such low-lying HOMO and LUMO energy levels
of FBTzE-based polymers are beneficial for the development of
high-performance p-type polymers and n-type semiconductors
in OPVs.
To evaluate the potential of FBTzE for high-performance electronics,
typical inverted solar cells with device structure of ITO/ZnO/(polymer:
PC61BM)/MoO3 (6 nm)/Ag (50 nm) were fabricated and characterized.
Typical current density ( J)–voltage (V) characteristics of the
fabricated solar cells under AM 1.5 G simulated solar irradia-
tion at 100 mW cmꢀ2 are depicted in Fig. 5a, and the extracted
solar cell parameters are summarized in Table 2 and Table S4
(ESI†). The solar cell based on 4T did not show photovoltaic
All the three polymers 3T, 4T, and 2TTT exhibited similar
absorption spectra with almost same Eg (Fig. 4a, b and Table 1).
The intensity in absorption spectra of 4T and 2TTT was
significantly decreased (Fig. S7 and Table S3, ESI†). This
indicates that polymers 4T and 2TTT have partial aggregation
in the high-temperature solution, and thus have a strong
intermolecular interaction. In thin film, the absorption
spectrum of 2TTT is identical to that of its room-temperature
solution, indicating that 2TTT formed the highly ordered
packing structure in room-temperature solution. On the other
hand, the absorption spectrum of 4T-based film showed
slightly blue-shifted compare to room-temperature solution.
One possible reason for such difference may be attributed to
the different packing motif in the solid state. Such tendency is
also observed in high molecular-weight PffBT4T-DT,33 but
actual reason has not been described. Polymer 3T has a
similar absorption before and after heating. In addition, the
spectrum of 3T in solution at room temperature is almost
identical to that in its thin film. This suggests that 3T seems to
form large aggregate even in the high-temperature solution,
which is inconsistent with the result of GPC trace. Polymer 3T
has a higher solubility and lower viscosity in high concen-
tration solution than that of the other polymers 4T and 2TTT.
In addition, 3T showed 10–20 nm blue-shifted spectra than
that of 4T and 2TTT. From these results, we concluded that 3T
did not form large aggregate in the room-temperature solution
and has weaker intermolecular interactions. One possible
reason for such difference among three polymers is their
symmetry of the polymer backbones. From DFT calculations
of a dimer structure, 3T formed a twisted backbone due to the
steric hindrance between two alkyl side chains onto terthio-
Fig. 5 (a) J–V curves and (b) EQE spectra of 3T and 2TTT/PC61BM-based
solar cells.
Table 2 Solar cell performances of polymer/PC61BM-based devicesa
Deviceb
Jsc/mA cmꢀ2
Voc/V
FF
PCE (PCEavg)/%
3T
2TTT
7.00
11.47
0.88
0.80
0.43
0.57
2.66 (2.60)
5.23 (5.09)
a
b
Average values are shown in parentheses. For 3T, the polymer/
PC61BM blend ratio is 1 : 1 (w/w), solvent = chlorobenzene (CB) + 2.5 vol%
diphenyl ether (DPE); for 2TTT, the polymer/PC61BM blend ratio is 1 : 2 (w/w),
phene unit, whereas 4T and 2TTT have a relatively coplanar solvent = CB + 4 vol% DPE.
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response due to the current leakage, because uniform thin-film GIWAXS measurement and surface morphology of pure
was not fabricated due to its strong aggregation behavior. polymer and polymer/PC61BM blended films
The best p/n ratio of 3T and 2TTT-based devices were found
In order to evaluate their structure–property relationships, grazing
to be 1 : 1 and 1 : 2. The best solar cell performances were
incidence wide-angle X-ray scattering (GIWAXS) and atomic force
obtained, when the substrate temperatures were room tem-
microscopy (AFM) measurements of polymer/PC61BM blended
perature for 3T and 140 1C for 2TTT, respectively (Table S5,
films were carried out (Fig. 6, 7 and Table S6, ESI†). In pure
ESI†). Both 3T- and 2TTT-based devices without optimization
exhibited poor solar cell performances with PCE of 0.87% for
3T and 3.09% for 2TTT, respectively, due to their low short-
circuit current density ( Jsc) and fill factor (FF). When 2.5% and
4% of diphenyl ether (DPE) were used as the solvent additive
in 3T and 2TTT-based solar cells, respectively, the best OPV
performances were observed. Since 3T has a deeper HOMO
energy level, its solar cell exhibited higher open-circuit voltage
(Voc) (0.88 V) than that of 2TTT-based solar cell (0.80 V).
However, 2TTT-based cell showed significantly higher Jsc
(11.47 mA cmꢀ2) and FF (0.57) than those of 3T-based cell
( Jsc = 7.00 mA cmꢀ2, FF = 0.43), and thus higher PCE of 5.23%.
From EQE spectra, 2TTT-based solar cell has a higher photo-
current conversion with maximum EQE of 48% in all regions,
compared to the 3T-based device (Fig. 5b), leading to a high
Jsc. In order to understand the difference of solar cell perfor-
mances between 3T and 2TTT, the hole-only devices with the
device configuration of ITO/(PEDOT:PSS)/(polymer:PC61BM)/
MoO3 (6 nm)/Al (80 nm) were fabricated and characterized to
estimate their SCLC hole mobility (Fig. S8, ESI†). However,
despite higher FF, SCLC hole mobility of the 2TTT-based hole-
only device (mh = 5.40 ꢂ 10ꢀ5 cm2 Vꢀ1 sꢀ1) is lower than that of
3T-based device (mh = 7.66 ꢂ 10ꢀ5 cm2 Vꢀ1 sꢀ1). We speculated
Fig. 7 Topological (left) and error-signal (right) images of polymer/
that poor mobility balance of 3T-based device may lead to low
PC61BM blended films on ITO/ZnO substrate; (a) 3T/PC61BM (p/n = 1 : 1,
FF, because such mobility balance strongly affects FF of solar
cells.62
CB + 2.5 vol% DPE), and (b) 2TTT/PC61BM (p/n = 1 : 2, CB + 4 vol% DPE).
Scale bar is 500 nm.
Fig. 6 2D GIWAXS images of (a and b) polymer and (c and d) polymer/PC61BM blended films on ITO/ZnO substrate; (a) 3T, (b) 2TTT, (c) 3T/PC61BM
(p/n = 1 : 1, CB + 2.5 vol% DPE), and (d) 2TTT/PC61BM (p/n = 1 : 2, CB + 4 vol% DPE). 1D cross-sectional profiles of (e and f) pure polymer films and
(h and g) polymer/PC61BM blended films on ITO/ZnO substrate; (e and g) out-of-plane and (f and h) in-plane.
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polymer films, 3T film showed weak lamellar diffraction (100) at (No. 2401)’’, MEXT, Grant 15H00751, Japan, Electric Technology
0.300 Åꢀ1 in qxy axis and p-stacking diffraction (010) at 1.777 Åꢀ1 Research Foundation of Chugoku, Chugoku Regional Innovation
in qz axis (Fig. 6a, e and f). From these diffractions, 3T formed a Research Center, and JKA through its promotion funds from
favorable face-on orientation with short p-stacking distance (dp) KEIRIN RACE (2018M-184). The GIWAXS experiments were
of 3.54 Å. In addition, 3T/PC61BM blended film also formed performed at BL46XU of SPring-8 with the approval of the
predominantly face-on orientation with almost same lamellar Japan Synchrotron Radiation Research Institute (JASRI) (Proposals
distance (dlm) of 21.1 Å and short dp of 3.57 Å (Fig. 6c, g and h). 2017A1771 and 2017B1831). We gratefully thank Prof. Itaru Osaka
Although 3T has favorable molecular orientation in both pure and Dr Masahiko Saito (Hiroshima University) and Dr Tomoyuki
polymer and blended film with PC61BM, it formed large-scale Koganezawa (JASRI) for the measurement of GIWAXS images,
phase separation with the domain size of 300–500 nm (Fig. 7a). Prof. Koichi Mitsudo and Prof. Seiji Suga (Okayama University) for
Such unfavorable phase separation structure must prevent the the CV measurements, and Prof. Naoshi Ikeda (Okayama Uni-
effective photocurrent generation, leading to a poor Jsc.63,64 versity) for the AFM images, and Prof. Yoshihiro Kubozono
In contrast, 2TTT exhibited strong lamellar diffraction (100) at (Okayama University) for the EQE and thickness measurement,
0.298 Åꢀ1 in qz axis and p-stacking diffraction (010) at 1.805 Åꢀ1 in and Ms Megumi Kosaka and Mr Motonari Kobayashi at the
qxy axis, which are completely opposite direction compared to 3T Department of Instrumental Analysis, Advanced Science Research
film (Fig. 6b, e and f). This indicates that 2TTT formed unsuitable Center, Okayama University, for the measurements of elemental
edge-on orientation with a short dp of 3.48 Å. Moreover, 2TTT/ analyses and the SC-NMR Laboratory of Okayama University for
PC61BM blended film also exhibited almost same diffraction the NMR spectral measurements.
patterns with almost same lamellar distance (dlm) of 21.5 Å and
short dp of 3.49 Å (Fig. 6d, g and h). Such unfavorable orientation
may limit the efficient carrier transport, leading to low hole mobility.
However, 2TTT/PC61BM blended film formed well-separated
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