Efficient organic photovoltaic cells based on thiazolothiazole and benzodithiophene copolymers with π-conjugated bridges

Article abstract of DOI:10.1002/pola.29084

New donor–π–acceptor (D–π–A) type conjugated copolymers, poly[(4,8-bis((2-hexyldecyl)oxy)benzo[1,2-b:4,5-b′]dithiophene)-alt-(2,5-bis(4-octylthiophen-2-yl)thiazolo[5,4-d]thiazole)] (PBDT-tTz), and poly[(4,8-bis((2-hexyldecyl)oxy)benzo[1,2-b:4,5-b′]dithiophene)-alt-(2,5-bis(6-octylthieno[3,2-b]thiophen-2-yl)thiazolo[5,4-d]thiazole)] (PBDT-ttTz) were synthesized and characterized with the aim of investigating their potential applicability to organic photovoltaic active materials. While copolymer PBDT-tTz showed a zigzagged non-linear structure by thiophene π-bridges, PBDT-ttTz had a linear molecular structure with thieno[3,2-b]thiophene π-bridges. The optical, electrochemical, morphological, and photovoltaic properties of PBDT-tTz and PBDT-ttTz were systematically investigated. Furthermore, bulk heterojunction photovoltaic devices were fabricated by using the synthesized polymers as p-type donors and [6,6]-phenyl-C₇₁-butyric acid methyl ester as an n-type acceptor. PBDT-ttTz showed a high power conversion efficiency (PCE) of 5.21% as a result of the extended conjugation arising from the thienothiophene π-bridges and enhanced molecular ordering in the film state, while PBDT-tTz showed a relatively lower PCE of 2.92% under AM 1.5 G illumination (100 mW/cm²).

Full text of DOI:10.1002/pola.29084

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Efficient Organic Photovoltaic Cells Based on Thiazolothiazole and
Benzodithiophene Copolymers with p-Conjugated Bridges
Junhoo Park,1 Jong Baek Park,1 Jong-Woon Ha,1 Hea Jung Park,1 In-Nam Kang,2
1
Do-Hoon Hwang
1Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan, 46241, Republic of
Korea
2Department of Chemistry, The Catholic University of Korea, Gyeonggi-do, 420-743, Republic of Korea
Correspondence to: D.-H. Hwang (E-mail: dohoonhwang@pusan.ac.kr)
Received 6 May 2018; accepted 2 June 2018; published online 00 Month 2018
DOI: 10.1002/pola.29084
ABSTRACT: New donor–p–acceptor (D–p–A) type conjugated
copolymers, poly[(4,8-bis((2-hexyldecyl)oxy)benzo[1,2-b:4,5-b0]
dithiophene)-alt-(2,5-bis(4-octylthiophen-2-yl)thiazolo[5,4-d]thia-
zole)] (PBDT-tTz), and poly[(4,8-bis((2-hexyldecyl)oxy)benzo[1,2-
b:4,5-b0]dithiophene)-alt-(2,5-bis(6-octylthieno[3,2-b]thiophen-2-
yl)thiazolo[5,4-d]thiazole)] (PBDT-ttTz) were synthesized and
characterized with the aim of investigating their potential appli-
cability to organic photovoltaic active materials. While copoly-
mer PBDT-tTz showed a zigzagged non-linear structure by
thiophene p-bridges, PBDT-ttTz had a linear molecular struc-
ture with thieno[3,2-b]thiophene p-bridges. The optical, electro-
chemical, morphological, and photovoltaic properties of PBDT-
tTz and PBDT-ttTz were systematically investigated. Further-
more, bulk heterojunction photovoltaic devices were fabricated
by using the synthesized polymers as p-type donors and [6,6]-
phenyl-C71-butyric acid methyl ester as an n-type acceptor.
PBDT-ttTz showed a high power conversion efficiency (PCE) of
5.21% as a result of the extended conjugation arising from the
thienothiophene p-bridges and enhanced molecular ordering in
the film state, while PBDT-tTz showed a relatively lower PCE of
2.92% under AM 1.5 G illumination (100 mW/cm2).
2017
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2017, 00, 000–000
KEYWORDS:
thieno[3,2-b]thiophene;
thiazolo[5,4-d]thiazole;
benzo[1,2-b:4,5-b0]dithiophene;
wide-band gap polymer;
organic photovoltaic cells
energy levels of BDT-based conjugated polymers can be
finely tuned by introducing different substituents on the cen-
tral phenyl ring.9–11 Recently, conjugated polymers based on
thiazolo[5,4-d]thiazole (Tz) and various electron-donating
repeating units have been developed for polymer solar cell
applications.12–15 The Tz unit, consisting of an electron-
deficient fused heterocyclic ring is a good electron acceptor,
and exhibits high charge mobility, planarity, and intermolecu-
lar stacking.16–19
INTRODUCTION In recent years, research into high-efficiency
organic photovoltaic cells (OPVs) has expanded tremen-
dously and led to the development of various semiconduct-
ing materials, especially donor–acceptor (D–A) conjugated
polymers, which have achieved high power conversion effi-
ciencies (PCEs) of over 11% when blended with fullerene-
derivative acceptors.1–4 Conjugated polymers incorporating
electron donating and accepting (D–A) systems that exhibit
broad absorption in the UV–visible region and possess
appropriate low band gaps (<2 eV) and HOMO–LUMO
energy levels by the intra-molecular charge transfer (ICT),
have particularly attracted a lot of attention.5–7
In this study, we designed and synthesized two new D–p–A
type conjugated copolymers, namely, poly[(4,8-bis((2-hexyl-
decyl)oxy)benzo[1,2-b:4,5-b0]dithiophene)-alt-(2,5-bis(4-
octylthiophen-2-yl)thiazolo[5,4-d]thiazole)] (PBDT-tTz), and
poly[(4,8-bis((2-hexyldecyl)oxy)benzo[1,2-b:4,5-
b0]dithiophene)-alt-(2,5-bis(6-octylthieno[3,2-b]thiophen-2-
yl)thiazolo[5,4-d]thiazole)] (PBDT-ttTz), which consist of the
electron-rich BDT and electron-deficient Tz units bridged
with either alkyl-substituted thiophene (t) or thieno[3,2-
Among the various electron-donating units, benzo[1,2-b:4,5-
b0]dithiophene (BDT) is one of the most well-known building
blocks for organic solar cells with high PCE.4,8 The rigid pla-
nar structure with extended p-conjugation of BDT facilitates
charge-carrier mobility and improves the p–p interactions
between the polymer chains in the solid state. The HOMO
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Synthetic routes of tTz and ttTz monomers: (i) THF, n-BuLi, 1-formylpiperidine, room temperature, 2 h. (ii) Dithiooxa-
mide, DMF, 150 8C, 12 h. (iii) NBS, THF/DMF(1:2), 40 8C, 2 h. (iv) THF, n-BuLi, 1-formylpiperidine, room temperature, 2 h. (v) Dithioox-
amide, DMF, 150 8C, 12 h. (vi) NBS, DMF, 80 8C, 1.5 h.
b]thiophene (tt). The introduction of p-bridges into the D–A
copolymers was expected to have an effect on the molecular
ordering and consequently influence the photovoltaic proper-
ties of the OPV cells.20–22 The synthetic routes and chemical
structures of the synthesized polymers are shown in
Schemes 1 and 2.
Characterization
1H (300 MHz) and 13C NMR (75 MHz) spectra were mea-
sured using a Varian Mercury Plus spectrometer, and the
chemical shifts were recorded in units of parts per million
relative to chloroform as the internal standard. The number
average molecular weight (Mn) and polydispersity index
(PDI) of the polymer was determined by gel permeation
chromatography (GPC) analysis relative to a polystyrene
standard on an Agilent 1260 Infinity HPLC. The absorption
spectra were measured by a JP/UV-1800 UV/Vis spectropho-
tometer. Cyclic voltammetry (CV) was performed on a CH
Instruments Electrochemical Analyzer and the measurements
were carried out by using Ag/AgNO3 as the reference elec-
trode, a platinum wire as the counter electrode, and a plati-
num wire coated with the synthesized polymer as the
working electrode in acetonitrile solution containing 0.1 M
tetrabutylammonium tetrafluoroborate (TBABF4) as the sup-
porting electrolyte.
EXPERIMENTAL
Materials
All starting materials and reagents were purchased from
Aldrich, Alfa Aesar, and TCI, and used without further purifi-
cation unless stated otherwise. Tetrakis(triphenylphosphine)-
palladium(0) (Pd(PPh3)4) catalyst and [6,6]-phenyl-C71
-
butyric acid methyl ester (PC71BM) were purchased from
Strem Chemicals, Inc. and EM-index, respectively. Tetrahydro-
furan (THF) was purified by fractional distillation. 3-
Octylthiophene (1), 3-octylthieno[3,2-b]thiophene (4), and
(4,8-bis((2-hexyldecyl)oxy)benzo[1,2-b:4,5-b0]dithiophene-2,6-
diyl)bis(trimethylstannane) (BDTHD) were synthesized fol-
lowing previously reported procedures.23–25
Synthesis of 4-Octylthiophene-2-Carbaldehyde (2)
A vacuum-dried 250 mL flask was charged with compound 1
(8.41 g, 42.83 mmol) and dry THF (100 mL) under Ar
SCHEME 2 Synthetic routes of PBDT-tTz and PBDT-ttTz polymers.
2
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atmosphere. The solution was cooled to 278 8C and n-BuLi
(17.13 mL, 42.83 mmol, 2.5 M solution in hexane) was added
slowly to the flask. After 30 min of stirring the reaction mix-
ture at 278 8C, 1-formylpiperidine (5.33 g, 47.11 mmol) was
added in one portion. The solution was allowed to warm to
room temperature and stirred for 2 h. Subsequently, the mix-
ture was poured into water (200 mL) and the organic layer
was extracted with ethyl acetate three times. The combined
organic layer was washed with brine. After drying over anhy-
drous MgSO4, the solvent was evaporated using a rotary
evaporator and the residue was purified by silica gel column
chromatography with the solvent mixture of ethyl acetate
and hexane (1:6) as the eluent to obtain a yellow oil (5.00 g,
52%). 1H NMR (300 MHz, CDCl3, d): 9.86 (s, 1H), 7.60 (s,
1H), 7.36 (s, 1H), 2.63 (t, J 5 7.5 Hz, 2H), 1.67–1.59 (m, 2H),
1.29–1.24 (m, 10H), 0.87 (t, J 5 7.2 Hz, 3H). 13C NMR (75
MHz, CDCl3, d): 183.0, 144.8, 137.2, 134.4, 130.7, 31.9, 30.4,
29.7, 29.3, 29.2, 28.5, 22.7, 14.1. HRMS (LC/MS): Calcd. for
the flask contents were transferred to a separatory funnel.
The organic layer was washed with water (3 3 100 mL) and
dried over anhydrous MgSO4. The solvent was evaporated
using a rotary evaporator, and the residue was purified by
reprecipitation from methylene chloride and methanol to
obtain a yellow solid (0.91 g, 60%). 1H NMR (300 MHz,
CDCl3, d): 7.25 (s, 2H), 2.57 (t, J 5 7.5 Hz, 4H), 1.62–1.59 (m,
4H), 1.33–1.28 (m, 20H), 0.88 (t, J 5 6.9 Hz, 6H). 13C NMR
(75 MHz, CDCl3, d): 161.8, 149.7, 143.5, 136.7, 127.3, 113.4,
31.9, 29.6, 29.6, 29.5, 29.3, 29.2, 22.7, 14.2. HRMS (MALDI-
TOF): Calcd. for C28H36Br2N2S4, 688.67; Found, 689.03. Elem.
Anal. Calcd. for C28H36Br2N2S4: C, 48.83; H, 5.27; N, 4.07; S,
18.62; Found, C, 49.05; H, 5.13; N, 3.98; S, 18.57. Melting
point 5 104 8C (760 mmHg). FTIR (KBr, neat, cm21): 3055
(sp2 CAH stretch, w), 2952, 2922, 2847 (sp3 CAH stretch,
m), 1475 (aromatic C@C stretch, s), 1392, 1291, 1182, 629
(CABr stretch, s).
C
13H20OS, 224.36; Found [M 1 H]1 225.13. Elem. Anal. Calcd.
Synthesis of 6-Octylthieno[3,2-b]Thiophene-2-
Carbaldehyde (5)
for C13H20OS: C, 69.6; H, 9.0; O, 7.1; S, 14.3; Found, C, 69.6;
H, 9.3; O, 8.4; S, 13.2. FTIR (KBr, neat, cm21): 3096 (sp2
CAH stretch, m), 2961, 2923 (sp3 CAH stretch, w), 2853,
2694 (aldehyde CAH stretch, w), 1673 (conjugated C@O
stretch, s), 1467 (aromatic C@C stretch, m), 1434, 1239,
1188.
A vacuum-dried 250 mL flask was charged with compound
4 (3.69 g, 14.62 mmol) and dry THF (80 mL) under Ar
atmosphere. The solution was cooled to 278 8C and 2.5 M
solution of n-BuLi in hexane (5.85 mL, 14.62 mmol) was
slowly added into the flask. After stirring the mixture at
278 8C for 20 min, 1-formylpiperidine (1.98 g, 17.54 mmol)
was added in one portion. The solution was allowed to
slowly warm to room temperature and then stirred for 2 h.
The reaction mixture was subsequently poured into water
and extracted with ethyl acetate (3 3 100 mL). After drying
over anhydrous MgSO4, the solvent was removed and the
crude product was purified by silica gel chromatography
using the solvent mixture of methylene chloride and hexane
Synthesis of 2,5-Bis(4-Octylthiophen-2-Yl)Thiazolo[5,4-
d]Thiazole (3)
Dithiooxamide (1.07 g, 8.91 mmol) was added into a solution
of compound 2 (5.00 g, 22.29 mmol) dissolved in N,N-dime-
thylformamide (DMF) (80 mL) under N2 atmosphere. The
mixture was stirred overnight at 150 8C. Subsequently, the
solution was cooled, poured into water (100 mL), and
extracted with methylene chloride (3 3 100 mL). The organic
extracts were combined and washed with brine
(3 3 100 mL). After drying over anhydrous MgSO4, the sol-
vent was evaporated using a rotary evaporator and the resi-
due was purified by silica gel chromatography using the
solvent mixture of methylene chloride and hexane (1:2) as
the eluent to obtain a yellow solid (3.55 g, 30%). 1H NMR
(300 MHz, CDCl3, d): 7.41 (s, 2H), 7.05 (s, 2H), 2.62 (t,
J 5 7.5 Hz, 4H), 1.64–1.57 (m, 4H), 1.35–1.25 (m, 20H), 0.87
(t, J 5 7.2 Hz, 6H). 13C NMR (75 MHz, CDCl3, d): 162.7, 149.6,
144.5, 137.1, 128.0, 123.5, 31.9, 30.4, 30.4, 29.4, 29.2, 22.7,
14.1. HRMS (MALDI-TOF): Calcd. for C28H38N2S4, 530.87;
Found, 531.18. Elem. Anal. Calcd. for C28H38N2S4: C, 63.4; H,
7.2; N, 5.3; S, 24.2; Found, C, 63.8; H, 7.3; N, 5.1; S, 24.5.
Melting point 5 93 8C (760 mmHg). FTIR (KBr, neat, cm21):
3053 (sp2 CAH stretch, w), 2960, 2921, 2850 (sp3 CAH
stretch, m), 1486 (aromatic C@C stretch, m), 1380, 1310,
1189.
1
(1:2) as the eluent to obtain an orange oil (3.68 g, 90%). H
NMR (300 MHz, CDCl3, d): 9.94 (s, 1H), 7.90 (s, 1H), 7.30
(s, 1H), 2.73 (t, J 5 7.5 Hz, 2H), 1.77–1.72 (m, 2H), 1.32–
1.27 (m, 10H), 0.87 (t, J 5 6.6 Hz, 3H). 13C NMR (75 MHz,
CDCl3, d): 183.5, 146.3, 144.7, 138.6, 135.5, 129.7, 128.5,
31.8, 29.6, 29.4, 29.3, 29.2, 28.4, 22.7, 14.1. HRMS (LC/MS):
Calcd. for C15H20OS2, 280.45; Found [M 1 H]1 281.10. Elem.
Anal. Calcd. for C15H20OS2: C, 64.2; H, 7.2; O, 5.7; S, 22.9;
Found, C, 64.8; H, 7.2; O, 7.0; S, 22.9. FTIR (KBr, neat,
cm21): 3086 (sp2 CAH stretch, m), 2959 (sp3 CAH stretch,
m), 2852, 2736 (aldehyde CAH stretch, w), 1668 (conju-
gated C@O stretch, s), 1464 (aromatic C@C stretch, w),
1416, 1379, 1227, 1176.
Synthesis of 2,5-Bis(6-Octylthieno[3,2-b]Thiophen-2-
Yl)Thiazolo[5,4-d]Thiazole (6)
Dithiooxamide (0.72 g, 5.96 mmol) was added to a solution
of compound 5 (3.68 g, 13.12 mmol) in DMF (30 mL) under
N2 atmosphere. The mixture was stirred overnight at 150 8C
and subsequently cooled, poured into water (100 mL), and
extracted with methylene chloride (3 3 100 mL). The
organic extracts were combined and washed with brine
(3 3 100 mL). After drying over anhydrous MgSO4, the sol-
vent was removed, and the residue was purified by silica
gel chromatography (methylene chloride:hexane 5 1:2) to
Synthesis of 2,5-Bis(5-Bromo-4-Octylthiophen-2-
Yl)Thiazolo[5,4-d]Thiazole (tTz)
N-Bromosuccinimide (NBS, 0.86 g, 4.83 mmol) was added to
a solution of compound 3 (1.17 g, 2.20 mmol) in THF/DMF
(1:2) (60 mL) and the mixture was stirred at 40 8C for 2 h.
Subsequently, water was added to the reaction mixture and
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obtain an orange solid (3.37 g, 40%). 1H NMR (300 MHz,
CDCl3, d): 7.74 (s, 2H), 7.11 (s, 2H), 2.75 (t, J 5 6.9 Hz, 4H),
1.80–1.77 (m, 4H), 1.34–1.28 (m, 20H), 0.88 (t, J 5 7.2 Hz,
6H). 13C NMR (75 MHz, CDCl3, d): 162.7, 149.8, 141.5,
139.0, 138.1, 135.2, 124.5, 119.4, 31.9, 29.8, 29.4, 29.2,
28.5, 22.7, 14.1. HRMS (MALDI-TOF): Calcd. for C32H38N2S6,
643.05; Found, 643.36. Elem. Anal. Calcd. for C32H38N2S6: C,
59.8; H, 6.0; N, 4.4; S, 29.9; Found, C, 59.8; H, 6.0; N, 4.2; S,
30.5. Melting point 5 168 8C (760 mmHg). FTIR (KBr, neat,
cm21): 3102 (sp2 CAH stretch, w), 2923, 2848 (sp3 CAH
stretch, w), 1464 (aromatic C@C stretch, m), 1438, 1406,
1297, 1155.
Polymerization of Poly[(4,8-Bis((2-
Hexyldecyl)Oxy)Benzo[1,2-b:4,5-B0]Dithiophene)-Alt-(2,5-
Bis(4-Octylthiophen-2-Yl)Thiazolo[5,4-d]Thiazole)]
(PBDT-tTz)
BDTHD (130.0 mg, 0.13 mmol), tTz (89.82 mg, 0.13 mmol),
and Pd(PPh3)4 (4.52 mg, 3.91 lmol) were mixed together.
The polymer was obtained as a dark-red solid (141 mg,
90%). Mn 5 49 kg/mol, PDI 5 1.8, Td 5 327 8C.
Polymerization of Poly[(4,8-Bis((2-
Hexyldecyl)Oxy)Benzo[1,2-b:4,5-B0]Dithiophene)-Alt-(2,5-
Bis(6-Octylthieno[3,2-b]Thiophen-2-Yl)Thiazolo[5,4-
d]Thiazole)] (PBDT-ttTz)
BDTHD (130.0 mg, 0.13 mmol), ttTz (104.5 mg, 0.13 mmol),
and Pd(PPh3)4 (4.52 mg, 3.91 lmol) were mixed together.
The polymer was obtained as a dark-green solid (161 mg,
94%). Mn 5 21 kg/mol, PDI 5 1.5, Td 5 323 8C.
Synthesis of 2,5-Bis(5-Bromo-6-Octylthieno[3,2-
b]Thiophen-2-Yl)Thiazolo[5,4-d]Thiazole (ttTz)
N-Bromosuccinimide (0.76 g, 4.27 mmol) was slowly added
into a stirred solution of compound 6 (1.24 g, 1.93 mmol) in
DMF (30 mL). The mixture was heated at 80 8C for 1.5 h and
then cooled to room temperature. The reaction was
quenched with water and the flask contents were transferred
to a separatory funnel. The organic layer was extracted with
chloroform (3 3 100 mL) and then washed with distilled
water (3 3 100 mL). After drying over anhydrous MgSO4, the
solvent was evaporated. The residue was then purified by
reprecipitation from methylene chloride and methanol to
obtain an orange solid (0.69 g, 45%). 1H NMR (300 MHz,
CDCl3, d): 7.65 (s, 2H), 2.76 (t, J 5 7.8 Hz, 4H), 1.75–1.72 (m,
4H), 1.36–1.28 (m, 20H), 0.89 (t, J 5 6.3 Hz, 6H). 13C NMR
(125 MHz, C6D6, d): 162.2, 150.3, 140.1, 137.9, 137.4, 136.9,
134.6, 119.0, 113.1, 31.8, 29.2, 28.8, 28.4, 27.8, 22.6, 13.9.
HRMS (MALDI-TOF): Calcd. for C32H36Br2N2S6, 800.84;
Found, 801.16. Elem. Anal. Calcd. for C32H36Br2N2S6: C,
47.99; H, 4.53; N, 3.50; S, 24.02; Found, C, 48.55; H, 4.56; N,
3.46; S, 24.28. Melting point 5 233 8C (760 mmHg). FTIR
(KBr, neat, cm21): 3086 (sp2 CAH stretch, w), 2953, 2922,
2852 (sp3 CAH stretch, s), 1524, 1405 (aromatic C@C
stretch, s), 1296, 1156, 611 (CABr stretch, m).
Fabrication of Organic Photovoltaic Devices
The bulk heterojunction (BHJ) OPV devices were fabricated
with the indium tin oxide (ITO)/ZnO/active layer/MoO3/Ag
inverted structure. ITO substrate was ultrasonically washed
in sequence with detergent, distilled water, acetone, and iso-
propyl alcohol. It was then treated with UV–ozone plasma
for 20 min. The ZnO layer was spin-coated on ITO glass at a
speed of 2000 rpm for 30 s. The substrate was then baked
at 200 8C for 60 min in air to obtain a thin film with a thick-
ness of 30 nm. The active layer materials with various poly-
mer:PC71BM blend ratios (1.0:1.0, 1.0:1.5, 1.0:2.0, and
1.0:3.0, w/w) were first fully dissolved in chlorobenzene
solution (30 mg/mL, active area of 0.09 cm2). Then, the
active layer was spin-coated on the ZnO layer at a speed of
700–1600 rpm for 30 s in a N2-filled glove box. After drying
for 1 h, the MoO3 layer (8 nm) and Ag metal anode (100 nm)
were deposited by vacuum evaporation at pressures below
3 3 1026 Torr.
Mobility Measurements
The hole mobilities of the polymers were measured by
applying the space-charge limited current (SCLC) model to J-
V measurements of the devices with ITO/PEDOT:PSS/active
layer/MoO3/Ag configuration. The ITO substrate was washed
in sequence with detergent, distilled water, acetone, and iso-
propyl alcohol and then treated with UV–ozone plasma for
20 min. The PEDOT:PSS (Clevios P Al 4083) was spin-coated
on the ITO substrate at a speed of 2000 rpm for 30 s and
baked at 150 8C for 15 min to obtain a thin film with a thick-
ness of 40 nm. After baking, the active layer materials were
fully dissolved in chlorobenzene solution and spin-coated on
the PEDOT:PSS layer at a speed of 700–1300 rpm for 30 s.
The substrate was dried for 1 h, and then MoO3 (8 nm) and
Ag (100 nm) cathode were finally coated by vacuum evapora-
tion at a pressure below 3 3 1026 Torr.
Polymerization Procedure
The two polymers were synthesized by palladium-catalyzed
Stille coupling polymerization. To a mixture of tTz or ttTz,
precursor BDTHD, Pd(PPh3)4 catalyst, anhydrous toluene
(10 mL), and anhydrous DMF (1 mL) were added under Ar
atmosphere and stirred at 110 8C for 1 day. A small amount
of 2-bromothiophene was added as an end-capper into the
mixture. After stirring for 2 h, a small amount of 2-(tributyl-
stannyl)thiophene was added as an end-capper into the mix-
ture. The reaction mixture was stirred for an additional 2 h
and then poured into methanol (200 mL). The crude poly-
mers were filtered and purified successively by Soxhlet
extraction with acetone, hexane, methanol, and chloroben-
zene for 12 h. After collecting the filtrate dissolved in chloro-
benzene, the reprecipitation process was again performed
with chlorobenzene and methanol.
The hole mobilities were calculated from the SCLC model
using the Mott–Gurney equation.26
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TABLE 1 Molecular and thermal properties of polymers
a
b
b
Mn
Mw
Td
Tg
Tc
Polymer
(kg/mol)
(kg/mol)
PDI
(8C)
(8C)
(8C)
PBDT-tTz
PBDT-ttTz
49
21
90
31
1.8
1.5
327
323
–
–
70
240
a
5% weight loss temperature with a heating rate of 10 8C/min at N2
atmosphere.
b
Measured by differential scanning calorimetry (DSC) within the range
of 220 to 300 8C.
J
SCLC5ð9=8Þere0l ðV2=L3Þ
where, er is the dielectric constant, e0 refers to vacuum per-
mittivity (8.85 3 10214 C/Vꢀcm), l stands for hole mobility,
V 5 Vappl 2 Vbi (Vappl is the applied potential, Vbi is the built-
in voltage that results from the difference in the work func-
tions of the cathode and anode), and L refers to the film
thickness. The J–V curves of the hole-only devices and field-
dependent hole mobilities in the SCLC regime are shown in
Supporting Information Figure S4.
FIGURE 1 TGA curves of the synthesized polymers with a heat-
ing rate of 10 8C/min under N2 atmosphere. Heating range was
20–500 8C. [Color figure can be viewed at wileyonlinelibrary.
com]
calorimetry (DSC). The thermal decomposition temperatures
(Td, 5% weight loss temperature) of PBDT-tTz and PBDT-ttTz
were 327 and 323 8C, respectively (Fig. 1 and Table 1). There
Two-Dimensional (2D) Grazing-Incidence X-Ray
Diffraction (GIXD) Analysis
The 2D grazing-incidence X-ray diffraction (GIXD) measure-
ments were performed at the 3C beamline in the Pohang
Accelerator Laboratory, South Korea. To obtain information
about the crystallinity of materials in the solid thin film
state, the materials were spin-coated on the ZnO-modified Si
wafer (15 3 15 mm) under the same conditions as those
used for the OPV device fabrication. The X-ray wavelength
was 1.290 Å and the incidence angle (the angle between the
irradiation beam and the Si wafer) was 0.12 8. The crystallin-
ity of the materials was decoded using a 2D CCD detector
(Rayonix SX165), and the X-ray irradiation time was fixed at
30 s. The results obtained from the detector were analyzed
according to the relationship between the scattering vector q
and the d spacing, q 5 2p/d.
RESULTS AND DISCUSSION
Synthesis and Characterization of PBDT-tTz and PBDT-
ttTz
The two polymers, PBDT-tTz and PBDT-ttTz, were synthe-
sized by Stille coupling polymerization using Pd(PPh3)4 as a
catalyst in anhydrous toluene/DMF (10/1, v/v) solution. The
detailed synthetic procedures are described in the Experi-
mental section. PBDT-tTz and PBDT-ttTz were found to be
soluble in common organic solvents such as chlorobenzene,
o-dichlorobenzene, and hot chloroform. The Mn and PDI of
the synthesized polymers PBDT-tTz and PBDT-ttTz were
measured by gel permeation chromatography (GPC) using
chloroform as an eluent at 35 8C and found to be 49,000 and
21,000 g/mol, and 1.8 and 1.5, respectively (Table 1). The
thermal properties of the two polymers were investigated by
thermogravimetric analysis (TGA) and differential scanning
FIGURE 2 UV–vis absorption spectra of PBDT-tTz and PBDT-
ttTz (a) in chlorobenzene solution and (b) thin film state. [Color
figure can be viewed at wileyonlinelibrary.com]
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TABLE 2 Optical and electrochemical properties of the polymers
Polymer
Solution
Film
Egopt (eV)a
Eox (eV)
EHOMO (eV)b
ELUMO (eV)c
kmax (nm)
kmax (nm)
PBDT-tTz
PBDT-ttTz
527, 570
549, 595
544, 584
549, 595
1.97
1.96
0.65
0.58
25.35
25.28
23.38
23.32
a
b
Calculated from the absorption kedge of polymer films: Egopt 5 1240/
.
E
HOMO 5 2(4.7 1 Eox).
c
kedge
E .
LUMO 5 Egopt 1 EHOMO
FIGURE 3 Top views, side views, and HOMO and LUMO orbitals of two repeating groups of polymers with minimum alkyl chains
calculated by DFT using the B3LYP, 6-31G(d). [Color figure can be viewed at wileyonlinelibrary.com]
was no significant difference in the thermal stabilities of
PBDT-tTz and PBDT-ttTz. Furthermore, the TGA data of
PBDT-tTz and PBDT-ttTz indicated that these polymers are
suitable for the fabrication of polymer solar cell devices.26
PBDT-ttTz exhibited a glass transition temperature (Tg) at
whereas PBDT-tTz showed no detectable peaks in the DSC
thermogram (Supporting Information Fig. S1).
Optical and Electrochemical Properties
The UV–Visible absorption spectra of the two polymers in
chlorobenzene solution and thin solid film states are shown
70 8C and
a crystallization temperature (Tc) at 240 8C,
TABLE 3 Best photovoltaic performances and hole mobilities of the OPV devices based on polymer:PC71BM 1.0:2.0 and 1.0:2.0
with 1 vol % 1-CN under AM 1.5 G illumination, 100 mW/cm2
Polymer
PBDT-tTz
Additive
VOC (V)a
JSC (mA/cm2)a
FF (%)a
PCEmax (avg) (%)a
lh (cm2/Vs)b
Without
With
0.73
0.76
0.79
0.81
4.40 [3.96]c
6.09 [5.85]c
10.15 [9.89]c
8.49 [8.18]c
46.5
63.1
64.8
58.2
1.50 (1.35)
2.92 (2.66)
5.21 (5.14)
3.99 (3.89)
2.21 3 1024
5.04 3 1024
9.61 3 1024
7.07 3 1024
PBDT-ttTz
Without
With
a
b
Photovoltaic properties of the polymer:PC71BM devices spin-coated
from chlorobenzene solution. The devices consist of ITO/ZnO/active
layer/MoO3/Ag.
Measured by applying the SCLC model. The devices consist of ITO/
PEDOT:PSS/active layer/MoO3/Ag.
c
JSC value calculated by EQE.
6
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conjugation of polymer backbone. Additionally, owing to the
strong intermolecular interaction by enhanced p–p stacking
in the solid state, the PBDT-ttTz film exhibited an increase in
the absorption shoulder at 595 nm. In the film state, PBDT-
tTz exhibited absorption kmax at 544 nm, which was red-
shifted by 17 nm compared with the peak in the solution
spectrum. This was attributed to the enhanced p–p stacking
in the solid state.27 Interestingly, the positions of the absorp-
tion peaks of PBDT-ttTz film (i.e., 549 and 595 nm) were the
same as those of the polymer solution. This suggested that
the thieno[3,2-b]thiophene bridge in the polymer backbone
enhanced the p-stacking between the polymer chains even in
the solution state. For detailed structural information of the
polymers, density functional theory (DFT) calculations were
performed. The optical energy band gaps (Egopt) of the PBDT-
tTz and PBDT-ttTz polymer thin films were measured from
their onset absorption wavelengths and determined to be
1.97 and 1.96 eV, respectively.
The HOMO energy levels (EHOMO) of the synthesized poly-
mers were measured by cyclic voltammetry (CV) with a plat-
inum wire as a counter electrode and Ag/Ag1 as a reference
electrode in anhydrous acetonitrile with 0.1 M TBABF4 as the
supporting electrolyte at a scan rate of 50 mV/s.
The CV curves of polymers are shown in Supporting Infor-
mation Figure S2. EHOMO levels were determined to be
25.35 and 25.28 eV for PBDT-tTz and PBDT-ttTz, respec-
tively, by the following equation, EHOMO52ðEooxnset14:70Þ
(eV). The EHOMO of PBDT-ttTz was slightly higher than that
of PBDT-tTz because the thieno[3,2-b]thiophene group has a
stronger electron-donating ability than the thiophene
group.28 The LUMO energy levels (ELUMO) were determined
to be 23.38 and 23.32 eV for PBDT-tTz and PBDT-ttTz,
respectively, by the following equation, ELUMO5Egopt1EHOMO
(eV). The ELUMO levels of the two synthesized polymers were
higher than that of PC71BM (ca., 24.30 eV), which is very
promising for their application in BHJ OPV cells as donor
materials.29 The optical and electrochemical properties of
the polymers are summarized in Table 2.
Density Functional Theory (DFT) Calculations
DFT calculations were carried out to gain a deeper under-
standing of the electronic distributions of the polymers using
the Gaussian 09W program with B3LYP functional and 6-
31G(d) basis sets. The calculations were performed on two
repeating groups and minimum alkyl chains (methyl, isobu-
tyl) to reduce excessive computation demand. While these
simple model calculations sometimes fail to capture the
important physical characteristics of polymers, they can still
provide useful information about the trends in electronic
properties.24 The optimized geometries and the HOMO and
LUMO orbitals of the two repeating groups of the backbones
of PBDT-tTz and PBDT-ttTz are shown in Figure 3. PBDT-tTz
demonstrates a zigzagged non-planar geometry with the
thiophene p-bridge acting as a bent and twisted connector in
the polymer backbone. The steric hindrance between the
substituted alkyl chain on the 4-position of the thiophene
FIGURE 4 J-V curves of (a) PBDT-tTz:PC71BM (1.0:2.0) and (b)
PBDT-ttTz:PC71BM (1.0:2.0) blend films with or without the
additive (1 vol % 1-CN) measured under AM 1.5 G illumination,
100 mW/cm2; (c) EQE curves of the OPV devices. [Color figure
can be viewed at wileyonlinelibrary.com]
in Figure 2. The maximum absorption peaks (kmax) of the
polymers in chlorobenzene solution were observed at 527
and 549 nm for PBDT-tTz and PBDT-ttTz, respectively (Table
2). The absorption maximum of PBDT-ttTz was shifted by
22 nm toward the longer wavelength region than that of
PBDT-tTz in solution because of greater extended p-
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FIGURE 5 Tapping-mode AFM topography images of (a) PBDT-tTz:PC71BM (1.0:2.0), (b) PBDT-ttTz:PC71BM (1.0:2.0), (c) PBDT-
tTz:PC71BM (1.0:2.0 with 1 vol % 1-CN), and (d) PBDT-ttTz:PC71BM (1.0:2.0 with 1 vol % 1-CN) blend films. [Color figure can be
viewed at wileyonlinelibrary.com]
and sulfur atom of the BDT unit led to distortion in the poly-
mer and a dihedral angle of 25 8. The replacement of p-
bridges from the thiophene units to thieno[3,2-b]thiophene
units resulted in a change in the backbone conformation of
the polymer from zigzagged to a linear conformation. Even
though PBDT-ttTz had a dihedral angle of 35 8 between BDT
and thieno[3,2-b]thiophene units, it adopted a linear back-
bone conformation as a result of the parallel bonds on the 2-
and 5-positions of thieno[3,2-b]thiophene. The thieno[3,2-
b]thiophene p-bridge favored the linear backbone conforma-
tion in this case because both BDT and Tz are linear linkers.
The HOMOs of the two polymers were delocalized along
their entire backbones, while the LUMOs were largely local-
ized on the electron accepting Tz unit.
a consequence of its distorted structure.30 The J–V curves of
the OPV devices fabricated with the different poly-
mer:PC71BM blend weight ratios are shown in Supporting
Information Figure S3. As shown in Table 3, the optimized
weight ratio for both PBDT-tTz and PBDT-ttTz in the poly-
mer:PC71BM blend, that is, at which the best photovoltaic
performances were achieved, was 1.0:2.0. The OPV devices
exhibited maximum PCE values of 1.50% and 5.21% for
PBDT-tTz and PBDT-ttTz, respectively. PBDT-ttTz showed sim-
ilar VOC but higher JSC and FF values compared with those of
PBDT-tTz [Fig. 4(a,b)]. Additionally, PBDT-ttTz had a higher
EQE than PBDT-tTz in the range of 350–600 nm [Fig. 4(c)].
The PBDT-ttTz-based device showed an excellent JSC value
that exceeded 10 mA/cm2 and an FF of approximately 65%.
The JSC values are affected by many factors including the
absorption properties, charge carrier mobility, morphology,
and crystallinity of the blend layer.1,24 The higher JSC of the
PBDT-ttTz-based device compared with that of the PBDT-tTz-
based device was likely a result of the different polymer
crystallinities arising due to the introduction of thieno[3,2-
b]thiophene p-bridges, which affected the charge carrier
mobility and p–n heterojunction morphology of the active
layer. Interestingly, addition of 1 vol % chloronaphthalene
(1-CN) as a processing additive to the polymer:PC71BM
Photovoltaic Performances and Hole Mobilities
To investigate the photovoltaic performances of PBDT-tTz
and PBDT-ttTz, BHJ OPV devices were fabricated with an
inverted configuration of ITO/ZnO/polymer:PC71BM/MoO3/
Ag. The weight ratios of polymer:PC71BM blend films were
varied as 1.0:1.0, 1.0:1.5, 1.0:2.0, and 1.0:3.0 to optimize the
morphologies of the active layer. PC71BM was selected as the
electron acceptor because of its higher absorption coefficient
in the visible region compared with that of PC61BM, which is
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FIGURE 6 2D GIWAXS patterns of (a) PBDT-tTz, (b) PBDT-ttTz, (c) PBDT-tTz:PC71BM (1.0:2.0, w/w), (d) PBDT-ttTz:PC71BM (1.0:2.0,
w/w), (e) PBDT-tTz:PC71BM (1.0:2.0, w/w, 1 vol % 1-CN), and (f) PBDTttTz: PC71BM (1.0:2.0, w/w, 1 vol % 1-CN) blend films. [Color
figure can be viewed at wileyonlinelibrary.com]
blends affected the PBDT-tTz- and PBDT-ttTz-based devices
differently. The PBDT-tTz-based device showed an improve-
ment in all parameters such as VOC, JSC, FF, and EQE, and
consequently the PCE was increased from 1.50% to 2.92%
(Fig. 4). On the contrary, the PBDT-ttTz-based device showed
a decrease in JSC, FF, and EQE values, and the PCE was
reduced from 5.21% to 3.99% upon addition of the additive
in the blend film. The effect of the additive is discussed in
more detail in the film morphology section.
of recombination processes between holes and electrons.31
The hole mobilities of the polymer:PC71BM blend films with
1-CN additives were 5.04 3 1024 cm2/Vꢀs and 7.07 3 1024
cm2/Vꢀs for PBDT-tTz and PBDT-ttTz, respectively. While the
hole mobility of the PBDT-tTz:PC71BM blend film was
increased, that of the PBDT-ttTz:PC71BM blend film was
decreased upon addition of 1-CN. This indicated that the 1-
CN additive impeded charge-carrier mobility in the PBDT-
ttTz blend film.
The hole mobilities of the polymer:PC71BM (1.0:2.0) blend
films were 2.21 3 1024 cm2/Vꢀs and 9.61 3 1024 cm2/Vꢀs for
PBDT-tTz and PBDT-ttTz, respectively (Table 3). The latter
blend film showed a higher hole mobility than the former,
resulting in a higher JSC value because of the reduced degree
Film Morphologies
Tapping-mode atomic force microscope (AFM) was used to
investigate the film surface morphology of the polymers
according to their structures. AFM topography images of the
polymer:PC71BM blend films are shown in Figure 5. As
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shown in Figure 5(a,b), the PBDT-ttTz:PC71BM blend film
exhibited a smaller domain size and a smoother surface than
the PBDT-tTz:PC71BM blend film. The uniform and bi-
continuous network of the PBDT-ttTz:PC71BM blend implied
good miscibility between PBDT-ttTz and PC71BM. This con-
tributed to an efficient charge separation and transport in
the active layer, and consequently the PBDT-ttTz:PC71BM
blend film could achieve a high JSC value. However, the
PBDT-tTz:PC71BM blend film exhibited a rough surface and a
high root-mean-square (RMS) roughness value over 20 nm
impeding smooth current flow. As shown in Figure 5(c,d),
the PBDT-tTz:PC71BM blend film with the additive showed a
remarkable improvement in morphology, while the PBDT-
ttTz:PC71BM blend film with the additive showed an
increased roughness of film surface.
showed decreased polymer crystallinity upon addition of the
1-CN additive, as evidenced by the disappearing (010) peak
[Fig. 6(f)]. The decreased molecular ordering of the polymer
film affected the JSC value and led to decreased OPV perform-
ances of the PBDT-ttTz:PC71BM blend film with 1-CN
additive.
CONCLUSIONS
In this study, two new D–p–A type conjugated wide-band
gap copolymers, PBDT-tTz and PBDT-ttTz, were synthesized
and BHJ organic photovoltaic cells with these two polymers
as acceptor materials in the photoactive layers were fabri-
cated. PBDT-tTz and PBDT-ttTz showed different molecular
ordering behavior in solid thin film state depending on the
presence of either 4-alkyl thiophene or 6-alkyl thieno[3,2-
b]thiophene p-bridges between the BDT and Tz units. PBDT-
ttTz, which possessed 6-alkyl thieno[3,2-b]thiophene p-
bridges in the polymer, exhibited strong p–p molecular stack-
ing even in the solution state owing to its linear molecular
structure. The PBDT-ttTz:PC71BM (1.0:2.0, w/w) blend film
showed higher crystallinity than the PBDT-tTz:PC71BM
(1.0:2.0, w/w) blend film, and consequently led to better
OPV performances. The PBDT-ttTz:PC71BM blend film also
showed a high PCE of 5.21% with a JSC of 10.15 mA/cm2,
while the PBDT-tTz:PC71BM blend film showed a relatively
low PCE of 2.92% and a JSC of 6.09 mA/cm2. The PBDT-ttTz-
based film exhibited higher hole mobility and JSC value
because of attributes such as improved molecular ordering
and well-organized p–n heterojunction morphology of the
active layer as compared with the PBDT-tTz-based film.
To gain a deeper understanding of the characteristics of the
polymer nanostructures in the blend films including crystal-
lite orientation, crystal coherence length, and intermolecular
distance, GIWAXS measurements were performed on pristine
and blend films.24 The 2D-GIWAXS patterns and 1D out-of-
plane plots of all films are shown in Figure 6 and Supporting
Information Figure S5, respectively. The PBDT-tTz pristine
film showed a face-on orientation in the out-of-plane direc-
tion with a strong (010) reflection peak at qz 5 1.60 Å21
(d(010) 5 3.93 Å) while the pristine PBDT-ttTz film showed
an edge-on orientation with (100), (200), (300), and (400)
reflection peaks with high crystallinity [Fig. 6(a,b)]. The
thieno[3,2-b]thiophene p-bridges in PBDT-ttTz improved the
crystallinity of the polymer in the solid thin film state as
compared with thiophene p-bridges in PBDT-tTz. The strong
(010) peak of the face-on structure of PBDT-tTz disappeared
on blending with PC71BM in the PBDT-tTz:PC71BM blend
film. This indicated that the PBDT-tTz film lost its crystallin-
ity and adopted an amorphous-like orientation when it was
blended with PC71BM [Fig. 6(c)]. On the other hand, the
PBDT-ttTz:PC71BM blend film showed a bimodal structure of
edge-on orientations with (100) and (200) peaks and face-
on orientation with the (010) reflection peak at qz 5 1.58
Å21 (d(010) 5 3.98 Å) [Fig. 6(d)]. PBDT-ttTz conserved its
crystallinity even though the original polymer orientation
was partially changed from edge-on to face-on when it was
blended with PC71BM. The higher molecular ordering of the
PBDT-ttTz:PC71BM blend film compared with that of PBDT-
tTz:PC71BM resulted in higher JSC and FF values, and conse-
quently higher PCE because of efficient charge-carrier trans-
portation of holes and electrons.32,33 Furthermore, the
partial face-on structure of PBDT-ttTz:PC71BM possibly
helped to improve the OPV device performances.
ACKNOWLEDGMENTS
This work was supported by the R&D Convergence Program of
NST (National Research Council of Science & Technology) of
Republic of Korea (CAP-15-04-KITECH), and National Research
Foundation (NRF) grants, funded by the Korean government
(MSIT) (NRF-2017R1A2A2A05001345 and No. 2011-0030013
through GCRC SOP).
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