Design, synthesis and photophysical properties of A-D-A-D-A small molecules for photovoltaic application

Article abstract of DOI:10.1016/j.dyepig.2015.05.009

A series of linear-conjugated small molecule compounds featured with the acceptor-donor-acceptor-donor-acceptor (A-D-A-D-A) structure were designed, synthesized and characterized. The films of the compounds showed a broad UV-Vis absorption range of 300-800 nm with high molar absorption coefficient of more than 1.0 × 10⁴ cm^-1. Their hole-mobility can be as high as 1.0 × 10^-3 cm² V^-1 s^-1. The compounds have a compatible HOMO energy level of -5.0 eV to PC₇₁BM and a high solubility up to 20 mg mL^-1 in chloroform. Therefore, compounds can blend with PC₇₁BM in the chloroform and form an active layer for a photovoltaic cell device by spin-coating of the blend solution. A maximum power conversion efficiency of 3.2% was achieved with the DERH3TT solar cell device. These results indicate that the A-D-A-D-A small molecules with electron-withdrawing dyes as terminals are promising candidates for the high efficiency solution processed organic photovoltaic cells.

Full text of DOI:10.1016/j.dyepig.2015.05.009

Dyes and Pigments 121 (2015) 99e108
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Design, synthesis and photophysical properties of A-D-A-D-A small
molecules for photovoltaic application
Weibo Yan a, Qian Zhang b, Qingsong Qin a, Senyun Ye a, Yuanwei Lin a, Zhiwei Liu a,
,
Zuqiang Bian a *, Yongsheng Chen b, Chunhui Huang a
a Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and
Molecular Engineering, Peking University, Beijing 100871, China
b State Key Laboratory for Functional Polymer Materials and Center for Nanoscale Science & Technology, Institute of Polymer Chemistry, College of
Chemistry, Nankai University, Tianjin 300071, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of linear-conjugated small molecule compounds featured with the acceptor-donor-acceptor-
donor-acceptor (A-D-A-D-A) structure were designed, synthesized and characterized. The films of the
compounds showed a broad UVeVis absorption range of 300e800 nm with high molar absorption co-
Received 1 April 2015
Received in revised form
25 April 2015
Accepted 7 May 2015
Available online 16 May 2015
efficient of more than 1.0 ꢀ 104 cmꢁ1. Their hole-mobility can be as high as 1.0 ꢀ 10ꢁ3 cm2 Vꢁ1
s
ꢁ1. The
compounds have a compatible HOMO energy level of ꢁ5.0 eV to PC71BM and a high solubility up to
20 mg mLꢁ1 in chloroform. Therefore, compounds can blend with PC71BM in the chloroform and form an
active layer for a photovoltaic cell device by spin-coating of the blend solution. A maximum power
conversion efficiency of 3.2% was achieved with the DERH3TT solar cell device. These results indicate that
the A-D-A-D-A small molecules with electron-withdrawing dyes as terminals are promising candidates
for the high efficiency solution processed organic photovoltaic cells.
Keywords:
Linear-conjugated small molecules
Acceptor-donor-acceptor-donor-acceptor
Hole mobility
© 2015 Elsevier Ltd. All rights reserved.
Low bandgap
Solution processed
Organic solar cell
1. Introduction
phenyl-C71-butyric acid methyl ester (PC71BM). All these unique
properties of the linear-conjugated small molecules including high
Linear-conjugated small molecules have attracted considerable
attentions recently due to their applications in many fields, such as
in light-emitting diodes [1e5], thin-film transistors [6,7], as well as
in organic photovoltaic (OPV) cells [8e25]. Generally, these con-
jugated small molecules prepared by rational molecular design
contain finite conjugated units and are highly soluble in organic
solvents. Therefore, their corresponding films can be formed simply
by solution processing without the vacuum evaporation process
that is required for the fabrications of most film devices [26]. These
small molecules have an acceptor-donor-acceptor (A-D-A) or
donor-acceptor-donor (D-A-D) structure, which is favorable for low
bandgap molecules and low HOMO energy level with broad
UVeVis absorption range. In addition, due to their rigid structures
and high mobility, the soluble small molecules can form a good
phase-separated mixture as a donor with the acceptors such as
[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,6]-
solubility, excellent light absorption ability, high hole-mobility and
the compatible energy level and good phase-separation with PCBM
contribute to their high performances in OPV cells.
The currently available single layer OPV devices with the
solution-processed small molecules as donor active materials can
show PCEs of >9% [25], which is comparable with that of the
polymer counterparts [27]. Therefore, novel donor materials are
very attractive for the preparation high efficiency solution pro-
cessed organic photovoltaic cells. In the present work, we re-
ported the design, synthesis and OPV performances five acceptor-
donor-acceptor-donor-acceptor (A-D-A-D-A) small molecules
including DERH3TT, DPRH3TT, DORH3TT, DPPR3TT and DBPPR3TT
(Chart 1). Thienothiophene-one unit was chosen as the central
building block due to its extendable conjugation and planar
structure. In addition, this unit can form donor-acceptor-donor
(D-A-D) structure with three thiophene units on both sides due
to its weak electron-drawing property. This construction can
lower the HOMO energy level and band-gap of the target mole-
cules. Rhodanine and pyrazolone derivatives were selected as the
terminal groups, which had been manifested to enhance the light
* Corresponding author.
E-mail address: bianzq@pku.edu.cn (Z. Bian).
http://dx.doi.org/10.1016/j.dyepig.2015.05.009
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

100
W. Yan et al. / Dyes and Pigments 121 (2015) 99e108
Chart 1. The chemical structures of DERH3TT, DPRH3TT, DORH3TT, DPPR3TT and DBPPR3TT.
absorption ability, to complete the A-D-A-D-A structure with its
electron-drawing group. This was able to lower the band-gap of
the molecules [11]. Moreover, different alkyl chains were intro-
duced into the rhodanine and pyrazolone skeletons to tune their
performances in the OPV devices. The thiophene and
thienothiophene-one units were also modified with long alkyl
chains to improve the solubility of the molecules in organic sol-
vents, which ensured the formation of the photoactive layer for
OPV devices. OPV cells with >3% efficiency was achieved under
preliminarily optimized conditions.
MALDI-TOF mass spectrometer (Bruker Daltonics Inc.). FTIR
spectra were obtained on the instrument Bruker, TENSOR 27.
Absorption spectra were observed with a JASCO V-570 spectro-
photometer. Thermo-gravimetric and differential thermal ana-
lyses were carried out on Q50TGA instruments at an elevation
temperature rate of 5 ꢂC minꢁ1 under nitrogen flow. Cyclic vol-
tammetry was performed with a computer controlled CHI600C
electrochemical workstation using a conventional three electrode
configuration consisting of a compartment electrolysis cell with a
platinum button as a working electrode, platinum wire as a
counter electrode, and Ag/AgCl reference electrode. Cyclic vol-
tammograms were obtained in dichloromethane (1 ꢀ 10ꢁ3 M)
using TBAPF6 (0.1 M) as the supporting electrolyte at scan rate of
0.1 Vsꢁ1 with Fc/Fcþ as internal reference. Atomic force micro-
scope (AFM) investigation was performed using SPA 400 in
contact-mode and transmission electron microscopy (TEM)
investigation was performed using JEM-2100F.
2. Experimental section
2.1. Materials and methods
3-octylthiophene, NBS, Mg, BrCH2CH2Br, BrC5H11-n, BrC8H17-n
and BuLi, were purchased from J&K Company. NiCl2 (ppp), TsOH,
glycol, Me3SnCl, Pd(PPh3)4, pyridine, rhodanine, and 3-ethyl-
rhodanin were purchased from Alfa Aesar Company. All solvents
were dried and purified by standard methods prior to use. The
procedure for synthesis of compounds 2, 5-dibromothiophene
and 3-methyl-1-phenyl-2-pyrazolin-5-one were according to the
literature [11]. The 1H NMR and 13C NMR spectra were measured
on a Bruker AC-400 and a Bruker AC-500 using TMS as internal
standard. HRMS for small molecules (molecular weight <800)
were recorded on a Fourier transform ion cyclotron resonance
mass spectrometer (Bruker, APEX IV FTMS) with ESI resource. MS
for macromolecules (molecular weight >800) were recorded on a
2.2. Synthesis
2.2.1. Synthesis of DCHO3TT
Compound 11 (1.70 g, 2.0 mmol), compound
5 (0.34 g,
0.8 mmol) and Pd(PPh3)4 (70 mg, 0.06 mmol) were added into dry
toluene (100 mL) under protection of argon flow, and the mixture
was refluxed under argon for 48 h under darkness. Then the
toluene was removed and the crude black product was dissolved in
CH2Cl2/THF (40/10 mL). Then HCl (4 M, 10 mL) was added and
stirred at room temperature for 12 h. Subsequently, the mixture

W. Yan et al. / Dyes and Pigments 121 (2015) 99e108
101
was extracted by CH2Cl2, dried by anhydrous Na2SO4 and purified
by silica column with eluent as petroleum ether:AcOEt ¼ 40:1
firstly, then petroleum ether:CH2Cl2 ¼ 1:1. At last, red solid (0.81 g,
0.64 mmol) was obtained with two-step yield 64.1%, from com-
pound 11 to DCHO3TT.
(1H, m), 2.83e2.75 (8H, m), 1.93e1.82 (2H, m), 1.71e1.69 (14H, m),
1.44e1.25 (52H, m), 0.94e0.87 (24H, m).13C NMR (100 MHz, CDCl3):
d
[ppm] ¼ 199.27, 192.21, 192.17, 167.48,151.71, 142.41, 141.24, 141.19,
141.17, 141.03, 139.44, 137.43, 137.21, 137.18, 137.06, 136.26, 135.41,
135.20, 135.19, 134.79, 133.54, 133.46, 130.94,130.16,127.96,127.84,
127.18, 126.90, 126.40, 126.08, 124.72, 124.66, 122.18, 121.54, 120.74,
120.54, 49.77, 44.85, 32.32, 31.94, 31.91, 30.44, 30.28, 30.26, 29.89,
29.85, 29.73, 29.71, 29.59, 29.53, 29.49, 29.35, 29.31, 28.92, 26.69,
25.95, 22.96, 22.72, 22.28, 14.14, 14.13, 14.02, 13.93, 12.14. MS
(MALDI-TOF) calcd. For [C82H116N2O3S12]þ: 1634.6; found: 1634.8.
1H NMR (400 MHz, CDCl3):
d
[ppm] ¼ 9.85 (1H,s), 9.84 (1H,s),
7.87 (1H, s), 7.62 (1H, s), 7.61 (1H,s), 7.29 (1H, d, J ¼ 4.0 Hz), 7.27 (1H,
d, J ¼ 4.0 Hz), 7.21 (1H, d, J ¼ 4.0 Hz), 7.17 (1H, d, J ¼ 4.0 Hz), 7.16 (1H,
s), 7.12 (1H,s), 3.29e3.20 (1H, m), 2.87e2.80 (8H, m), 1.88e1.82 (2H,
m), 1.78e1.66(8H, m), 1.43e1.25 (46H, m), 0.96e0.86 (18H, m).13
C
NMR (100 MHz, CDCl3):
d
[ppm] ¼ 199.38, 182.47, 151.86, 142.57,
Melting point [oC]
¼
201e208. FTIR (KBr): wavenumber
141.52, 141.50, 140.90, 140.66, 140.63, 140.58, 140.51, 140.40, 138.97,
138.96, 137.76, 137.72, 137.41, 136.36, 135.27, 134.86, 133.72, 133.65,
130.84, 129.95, 128.15, 127.90, 127.12, 127.09, 126.67, 126.38, 126.35,
122.18, 49.77, 34.72, 34.17, 32.35, 31.93, 31.91, 30.54, 30.36, 29.89,
29.74, 29.67, 29.66, 29.55, 29.49, 29.46, 29.32, 29.28, 26.00, 22.94,
22.72, 22.36, 14.12, 14.06, 13.98, 12.12. MS (MALDI-TOF) calcd. for
[C72H94O3S8]þ: 1262.5; found: 1263.2.
[cmꢁ1] ¼ 2955, 2921, 2854, 1693, 1659, 1571, 1533, 1416, 1382, 1324,
1257, 1193, 1135, 1101, 1025, 777, 731.
2.2.4. Synthesis of DORH3TT
Compound DCHO3TT (0.20 g, 0.158 mmol), excess 3-octyl-rho-
danin (2.00 g, 8.15 mmol) and 3 drops pyridine were dissolved in
dry CHCl3 (40 mL). After being degassed for 5 times with argon, the
mixture was refluxed for 48 h under argon and darkness. Then the
solvent was removed and the crude solid was obtained. It was
recrystallized in mixture of CH2Cl2 and methanol (Vol [CH2Cl2:
MeOH] ¼ 1:10) for 3 times and then in mixture of CH2Cl2 and
hexane [CH2Cl2: hexane ¼ 1:20] for 3 times. At last, obtained solid
was further purified by Al2O3 column (eluent as petroleum ether:
CH2Cl2 ¼ 6:1), to get product (0.19 g, 0.11 mmol) with metal gloss
with yield as 69.9%.
2.2.2. Synthesis of DERH3TT
Compound DCHO3TT (0.12 g, 0.095 mmol) and excess 3-ethyl-
rhodanin (0.12 g, 7.45 mmol) were dissolved in dry CHCl3 (50 mL),
followed by adding 3 drops pyridine. After being degassed for 5
times by argon, the mixture was refluxed for 48 h under argon
and darkness. After remove of the solvent, the crude product
was obtained. And then it was recrystallized in the mixture of
CH2Cl2 and CH3OH (Vol [CH2Cl2]/[CH3OH] ¼ 1:10) for 3 times,
followed in the mixture of CH2Cl2 and hexane (Vol [CH2Cl2]/
[hexane] ¼ 1:20 for 3 times. Then obtained solid was further
purified by Al2O3 column with eluent as petroleum ether:
CH2Cl2 ¼ 3:1 to get golden brown product (0.10 g, 0.064 mmol)
with yield as 68.0%.
1H NMR (400 MHz, CDCl3):
d
[ppm] ¼ 7.85 (1H, s), 7.74 (1H, s),
7.73 (1H, s), 7.20e7.19 (4H, m), 7.16 (1H, d, J ¼ 3.6 Hz), 7.12 (1H, d,
J ¼ 3.6 Hz), 7.10 (1H, s), 7.06 (1H, s), 4.10e4.05 (4H, m), 3.30e3.23
(1H, m), 2.83e2.79 (8H, m), 1.92e1.84 (2H, m), 1.75e1.62 (14H, m),
1.48e1.22 (64H, m), 0.94e0.85 (24H, m).13C NMR (100 MHz, CDCl3):
d
[ppm] ¼ 199.32, 192.25, 192.20, 167.51, 151.76, 142.48, 141.33,
1H NMR (400 MHz, CDCl3):
d
[ppm] ¼ 7.86 (1H, s), 7.78 (1H, s),
141.28, 141.24, 141.12, 139.40, 139.12, 137.44, 137.22, 137.19, 137.06,
136.31, 135.48, 135.29, 135.24, 134.84, 133.60, 133.53, 130.93, 130.12,
128.05, 127.85, 127.27, 127.25, 126.97, 126.52, 126.21, 124.77, 124.70,
122.18,121.57,120.80,120.61, 49.77, 44.88, 32.33, 31.93, 31.80, 30.47,
30.30, 30.29, 29.88, 29.80, 29.70, 29.68, 29.60, 29.55, 29.51, 29.47,
29.33, 29.30, 29.15, 27.01, 26.82, 25.97, 22.94, 22.71, 22.69, 22.65,
14.13, 14.11, 14.08, 14.00, 12.13. MS (MALDI-TOF) calcd. for
[C94H128N2O3S12]þ: 1718.8; found: 1719.6. Melting point
[oC] ¼ 162e170. FTIR (KBr): wavenumber [cmꢁ1] ¼ 2955, 2921,
2853, 1693, 1656, 1572, 1534, 1458, 1416, 1382, 1358, 1324, 1261,
1231, 1168, 1135, 1102, 1013, 812, 778, 727.
7.77 (1H, s), 7.25 (1H, d, J ¼ 3.2 Hz), 7.24e7.22 (3H, m), 7.19 (1H, d,
J ¼ 3.2 Hz), 7.16 (1H, d, J ¼ 3.2 Hz), 7.14 (1H, s), 7.11 (1H, s),
4.20e4.18 (4H, m), 3.30e3.21(1H, m), 2.85e2.81 (8H, m),
1.90e1.82 (2H, m), 1.75e1.64 (8H, m), 1.45e1.26 (52H, m),
0.90e0.86 (18H, m).13C NMR (100 MHz, CDCl3):
d
[ppm] ¼ 199.36,
192.07, 192.02, 167.32, 151.81, 142.54, 141.42, 141.37, 141.32, 141.21,
139.41, 139.13, 137.46, 137.28, 137.23, 137.08, 136.35, 135.50, 135.32,
135.26, 134.87, 133.65, 133.59, 130.92, 130.09, 128.10, 127.85,
127.36, 127.33, 127.03, 126.62, 126.33, 124.86, 124.78, 122.17,
121.59, 120.88, 120.70, 49.78, 39.96, 39.94, 32.35, 31.92, 31.89,
30.49, 30.31, 29.88, 29.76, 29.72, 29.66, 29.58, 29.52, 29.49, 29.45,
29.32, 29.29, 26.95, 25.99, 22.94, 22.70, 14.12, 13.99, 12.30, 12.12.
MS (MALDI-TOF) calcd. for [C82H104N2O3S12]þ: 1550.5; found:
1550.5. Melting point [oC] ¼ 215e226. FTIR (KBr): wavenumber
[cmꢁ1] ¼ 2953, 2920, 2849, 1693, 1657, 1570, 1456, 1412, 1383,
1345, 1319, 1235, 1127, 1101, 1080, 877, 816, 779, 732.
2.2.5. Synthesis of DPPR3TT
Compound DCHO3TT (0.10 g, 0.079 mmol), excess 1-phenyl-3-
methyl-1H-pyrazol-5(4H)-one (1.00 g, 5.74 mmol) and 3 drops tri-
ethylamine were dissolved in dry CHCl3 (50 mL). After it was
degassed for 5 times with argon, the mixture was refluxed for 48 h
under argon and darkness. Then the mixture was recrystallized in
the mixture of CH2Cl2 and CH3OH, (Vol [CH2Cl2:CH3OH] ¼ 1:10)
for 3 times, followed in the mixture of CH2Cl2 and hexane (Vol
[CH2Cl2: hexane] ¼ 1:20) for 3 times. At last, the obtained solid
was further purified by silica column (eluent as Vol [petroleum
ether: CH2Cl2] ¼ 4:1) to get product (0.051 g, 0.032 mmol) with
yield as 40.9%.
2.2.3. Synthesis of DPRH3TT
Compound DCHO3TT (0.20 g, 0.158 mmol), excess 3-pentyl-
rhodanin (2.00 g, 9.85 mmol) and 3 drops pyridine were dissolved
in dry CHCl3 (70 mL). After it was degassed for 5 times with argon,
the mixture continued to reflux for 48 h under argon and darkness.
The solvent was removed and the crude product was recrystallized
in mixture of CH2Cl2 and methanol (Vol [CH2Cl2: MeOH] ¼ 1:10) for
3 times and then in mixture of CH2Cl2 and hexane (Vol [CH2Cl2:
hexane ¼ 1:20]) for 3 times. Then obtained solid was further pu-
rified by neutral Al2O3 column with eluent as petroleum ether:
CH2Cl2 ¼ 4:1 to get product (0.12 g, 0.11 mmol) with metal gloss
with yield as 69.6%.
1H NMR (400 MHz, CDCl3):
d
[ppm] ¼ 8.00 (4H, d, J ¼ 6.4 Hz),
7.87 (1H, s),7.79 (2H, d, J ¼ 8.8 Hz),7.46e7.39 (8H, m),7.21e7.17
(4H, m), 7.14(1H, s), 7.11 (1H, s), 3.29e3.24 (1H, m), 2.89e2.82 (8H,
m),2.35 (3H, s), 2.34 (3H, s), 1.77e1.71 (8H, m), 1.34e1.26 (52H, m),
0.90e0.87 (18H, m).13C NMR (100 MHz, CDCl3):
d
[ppm] ¼ 198.39,
161.47,150.81, 148.82, 143.30, 143.09, 141.49, 141.08, 140.42, 139.97,
139.50, 137.56, 137.52, 137.11, 136.77, 136.72, 135.33, 134.48,
134.00, 133.25, 133.11, 132.64, 132.59, 127.78, 127.20, 127.13,
126.90, 126.02, 125.60, 125.58, 125.31, 123.70, 123.64, 121.18,
1H NMR (400 MHz, CDCl3):
d
[ppm] ¼ 7.83 (1H, s), 7.72 (1H, s),
7.70 (1H, s), 7.20e7.17 (4H, m), 7.13 (1H, d, J ¼ 4.0 Hz), 7.10 (1H, d,
J ¼ 4.0 Hz), 7.06 (1H, s), 7.10 (1H, s), 4.10e4.04 (4H, m), 3.30e3.23

102
W. Yan et al. / Dyes and Pigments 121 (2015) 99e108
120.76, 120.62, 118.04, 118.02,48.72, 31.36, 30.89, 29.43, 29.25,
28.84, 28.78, 28.69, 28.64, 28.60, 28.49, 28.46, 28.42, 28.30, 28.27,
25.00, 21.92, 21.67, 13.09, 12.96, 12.02, 11.09. MS (MALDI-TOF)
calcd. for [C92H110N4O3S8]þ: 1574.6; found: 1575.4. Melting point
[oC] ¼ 153e165. FTIR (KBr): wavenumber [cmꢁ1] ¼ 2956, 2924,
2849, 1728, 1677, 1658, 1588, 1555, 1499, 1457, 1397, 1368, 1299,
1177, 1139, 1093, 1023, 995, 785, 748.
34.54, 32.40, 31.92, 31.60, 30.60, 30.48, 30.30, 29.88, 29.80, 29.71,
29.67, 29.66, 29.63, 29.51, 29.45, 29.32, 29.14, 29.08, 26.95, 25.31,
23.94, 23.09, 22.95, 22.70, 22.67, 22.63, 14.12, 13.99, 13.07, 12.12,
11.42, 11.14. MS (MALDI-TOF) calcd. for [C120H150N4O5S8]þ:
1985.0; found: 1984.2. Melting point [oC] ¼ 205e212. FTIR (KBr):
wavenumber [cmꢁ1] ¼ 2957, 2919, 2848, 1721, 1681, 1656, 1595,
1497, 1464, 1388, 1361, 1296, 1263, 1241, 1175, 1138, 1029, 996,
816, 794, 729.
2.2.6. Synthesis of DBPPR3TT
Compound DCHO3TT (0.20 g, 0.152 mmol), excess compound 20
(2.00 g, 5.28 mmol) and 3 drops tri-ethylamine were dissolved in
dry CHCl3 (80 mL). After the mixture was degassed for 5 times with
argon, it was refluxed for 48 h under argon and darkness. Then the
mixture was recrystallized in the mixture of CH2Cl2 and CH3OH (Vol
[CH2Cl2: MeOH] ¼ 1:10) for 3 times, followed in the mixture of
CH2Cl2: hexane (Vol [CH2Cl2: hexane] ¼ 1:20) for 3 times. The
obtained solid was further purified by silica column (eluent as Vol
3. Results and discussion
3.1. Material synthesis and characterization
The synthetic methods are outlined in Schemes 1e3. 1-(4,6-
dibromothieno[3,4-b]thiophen-2-yl)-2-ethylhexan-1-one
(3)
and 5-bromo-3,3-dioctyl-2,200,2000-terthiophene-2-carbaldehyde
(9) were synthesized according to the reported procedures,
respectively [11,28]. It is noteworthy to mention that the syn-
thesis of compound 4 was optimized by using 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ) instead of m-chloroperox-
ybenzoic acid to oxide compound 3 to form the aromatic thio-
phene ring [28]. This one-step method we proposed in the
present work significantly simplified the synthesis process with
improved yields of the target compound. Due to their active re-
actions with BuLi, the aldehyde-groups of compound 9 were first
protected with glycol to form a five-ring diether for the synthesis
[petroleum ether: CH2Cl2]
0.051 mmol) with yield as 33.6%.
1H NMR (400 MHz, CDCl3):
¼
4:1) to get product (0.102 g,
d
[ppm] ¼ 8.04 (4H, d, J ¼ 8.4 Hz),
7.88 (1H, s), 7.81 (1H, s), 7.19 (1H, s), 7.61 (4H, d, J ¼ 8.8 Hz), 7.54
(4H, d, J ¼ 8.4 Hz), 7.48 (1H, s), 7.47 (1H, s),7.43 (1H, d, J ¼ 4.0 Hz),
7.42 (1H, d, J ¼ 4.0 Hz), 7.22 (1H, d, J ¼ 4.0 Hz), 7.20 (1H, d,
J
¼
4.0 Hz), 7.16 (1H, s), 7.12 (1H, s), 6.98 (4H, d,
J ¼ 8.8 Hz),3.88e3.89 (4H, m), 3.30e3.23 (1H, m), 2.91e2.82 (8H,
m), 2.37(3H, s), 2.36 (3H, s), 1.91e1.82 (2H, m), 1.78e1.72 (8H, m),
1.35e1.25 (67H, m), 1.90e0.84 (35H, m).13C NMR (100 MHz,
of
5-(1,3-dioxolan-2-yl)-3,3-dioctyl-2,200,2000-terthiophene-2-
CDCl3):
d
[ppm] ¼ 199.43, 162.47, 158.93, 158.91, 149.94, 145.04,
tributylstannane (11) [29]. The protective groups can be easily
removed under acidic condition after the Stille-coupling reaction.
The reaction of precursor DCHO3TT (see Scheme 3), catalyzed by
piperidine, with 3-ethyl-rhodanin produced DERH3TT compound
in a yield of 68.0%. DPRH3TT and DORH3TT were then synthe-
sized with 3-pentyl-rhodanin and 3-octyl-rhodanin as terminal
144.66, 144.61, 144.34, 144.11, 142.56, 141.50, 141.48, 140.58,
138.14, 137.75, 137.31, 137.26, 137.24, 137.18, 135.50, 135.45, 135.07,
134.33, 133.70, 133.67, 133.65, 133.12, 133.08, 128.28, 128.20,
127.89, 127.09, 127.08, 126.96, 126.66, 126.39, 121.84, 121.71,
119.25, 114.87, 70.69, 53.40, 41.38, 39.47, 36.09, 35.69, 34.70,
Scheme 1. Synthetic routes for the monomers and precursor DCHO3TT of the target molecules.

W. Yan et al. / Dyes and Pigments 121 (2015) 99e108
103
Scheme 2. Synthetic routes for the terminal groups 13, 14 and 20.
Scheme 3. Synthetic routes for the target molecules under the condition: peridine (3 drops), CHCl3, Ar, darkness, reflux.
groups in similar yields [30]. Both DPRH3TT and DORH3TT
showed high solubility in organic solvents including CHCl3,
CH2Cl2 and o-dichlorobenzene (ODCB). The molecular energy
level, absorption ability and hole-mobility of the synthesized A-
D-A-D-A small molecules were tuned by using pyrazolone de-
rivatives as the terminal groups. The yields of the pyrazolone
terminated DPPR3TT and DBPPR3TT are in the range of 30e40%,
slightly lower than those of the rhodanin derivatives terminated
small molecules. This can be attributed to the high hindrance of
the pyrazolone groups.
The performance of the A-D-A-D-A small molecule based
photovoltaic device is highly dependent on the solubility, film-
forming property and stability of the small molecule. The solu-
bility of DERH3TT, DPPR3TT and DBPPR3TT is 10 mg mLꢁ1 in
CHCl3 and those of DPRH3TT and DORH3TT in CHCl3 are
15 mg mLꢁ1 and 20 mg mLꢁ1, respectively. The decomposition
temperatures of DERH3TT, DPRH3TT, DORH3TT, DPPR3TT
and DBPPR3TT are 395, 393, 394, 314 and 322 ꢂC, respectively
(Fig. 1). Therefore, these compounds can be applied in the op-
toelectronic devices at the annealing temperatures between 100
and 200 ꢂC. DSC analysis under nitrogen atmosphere from 20 to
250 ꢂC indicates no obvious glass transition occurred in these
compounds. The rhodanin derivatives terminated DERH3TT,
DPRH3TT and DORH3TT displayed obvious melting temperatures.
The melting temperature (Tm) of DORH3TT is 167 ꢂC, which is
39 ꢂC or 56 ꢂC lower than those of DERH3TT and DPRH3TT,
respectively, indicating that the octyl-terminal chains can
affect the melting temperature of the compounds. The pyr-
azolone derivatives terminated DPPR3TT and DBPPR3TT showed
no obvious melting temperature below their decomposition
temperatures.
3.2. XRD characterization and electrochemical properties
The packing of the molecules in the solid state was charac-
terized with X-ray powder diffraction (XRD) [31]. As shown

104
W. Yan et al. / Dyes and Pigments 121 (2015) 99e108
Fig. 1. a) TGA plots of DERH3TT, DPRH3TT, DORH3TT, DPPR3TT andꢁD1BPPR3TT obtained at a heating rate of 10 ꢂC minꢁ1 under N2 atmosphere; b) DSC curves of the small molecule
compounds obtained under N2 flow at a heating rate of 10 ꢂC min
.
Fig. 2. a) Powder XRD patterns of the small molecule films; b) cyclic voltammograms of the five compounds.
in Fig. 2a, DERH3TT, DPRH3TT, DORH3TT, DPPR3TT and
DBPPR3TT exhibited diffraction peaks at 2
of 4.61ꢂ, 4.56ꢂ,
lower HOMO levels than the rhodanin derivatives terminated
DERH3TT, DPRH3TT and DORH3TT.
q
4.42ꢂ, 4.49ꢂ and 3.66ꢂ, respectively, corresponding to the back-
bone distances of 19.15, 19.35, 19.97, 19.67 and 24.10 Å, respec-
tively. This indicates that the long terminal alkyl chain and big
biphenyl terminal groups can prevent the molecules from
approaching.
3.3. Photophysical properties
Fig. 3 shows the UVeVis absorption spectra of the five synthe-
sized A-D-A-D-A compounds in solution and film. Due to the
Fig. 2b shows the CV curves of DERH3TT, DPRH3TT, DORH3TT,
DPPR3TT and DBPPR3TT, which can be used to determine
their HOMO energy levels. Their onset oxidation potentials are
0.19 V, 0.15 V, 0.21 V, 0.35 V and 0.34 V, respectively. Their
HOMO levels were calculated with EHOMO ¼ ꢁ(Eox þ 4.80)
as ꢁ4.99, ꢁ4.95,ꢁ5.01, ꢁ5.15 and ꢁ5.14 eV, respectively [32]. Pyr-
azolone derivatives terminated DPPR3TT and DBPPR3TT exhibited
thienothiophene-one
p-bridges of the A-D-A-D-A structures, all
five compounds exhibited a strong absorption band ranging from
250 to 700 nm in chloroform (Fig. 3a). When rhodanin or pyr-
azolone derivatives were introduced onto the terminal of the pre-
cursor DCHO3TT, its absorption coefficient was significantly
enhanced and the absorption peaks was red-shifted about
25e30 nm in solution. This indicates that the electron-withdrawing
Fig. 3. (a) UVeVis absorption spectra of the compounds in chloroform; (b) UVeVis absorption spectra of the compound films.

W. Yan et al. / Dyes and Pigments 121 (2015) 99e108
105
Table 1
DPPR3TT and DBPPR3TT films are at ca. 807, 817, 815, 812 and
814 nm, corresponding to the optical band gaps (Eg,opt) of 1.54, 1.52,
1.52, 1.52, and 1.52 eV, respectively (Table 1). These results indicate
that the small molecules are good light absorbers and possess
compatible energy levels to the acceptor PCBM.
Electrochemical and optical properties and hole mobility of the compounds.
HOMOb
(eV)
LUMOb
(eV)
mc
a
Small
molecules
lmax
lmax
Eg,opt
(eV)
(nm)
(nm)
(cm2 Vꢁ1 sꢁ1
)
Solution Film
DERH3TT
DPRH3TT
DORH3TT
DPPR3TT
DBPPR3TT
559
556
554
567
562
642
652
657
662
630
1.54
1.52
1.52
1.52
1.52
ꢁ4.99
ꢁ4.95
ꢁ5.01
ꢁ5.15
ꢁ5.14
ꢁ3.45
ꢁ3.43
ꢁ3.49
ꢁ3.63
ꢁ3.62
8.51 ꢀ 10ꢁ4
5.04 ꢀ 10ꢁ4
1.12 ꢀ 10ꢁ4
2.38 ꢀ 10ꢁ3
5.33 ꢀ 10ꢁ4
3.4. Hole mobility of the target molecular films
Compatible hole mobility to PCBM is usually required for donors
to achieve the balanced electron and hole mobility of the small
molecules and PCBM blends, which is very crucial for the high
performance solar cells. To measure the carrier mobility of the
compounds, bottom-gate and bottom-contact field effect transis-
tors (FETs) were fabricated [31]. The source-drain electrodes with
8 nm Cr and 60 nm Au layers were prepared by thermal evaporation
with p-doped Si/300 nm SiO2 as a substrate. The thin small mole-
cule films were spin-coated on the electrodes with their chloroform
solutions (5 mg mLꢁ1) at 1700 rpm for 1 min Fig. 4 shows the
transfer characteristic curve of DERH3TT device. The characteristic
curves of the devices prepared with other A-D-A-D-A small mole-
cules compounds can be found in the Supporting Information
a
Calculated from the absorption band edge of these compounds, Eg,opt ¼ 1240/
ledge
.
b
Calculated by the equation ELUMO ¼ EHOMO þ Eg,opt [29].
c
Hole Mobility of these molecular films were measured by field effect
transistors [28].
groups can increase the light absorption of the small molecules.
Interestingly, compared with rhodanin-terminal derivatives, the
pyrazolone-terminal derivatives showed ~5 nm red-shifts absorp-
tion peak. This may be attributed to the stronger electron-drawing
property of pyrazolone derivatives. As expected, extending the
conjugated terminal group increased the absorption intensity in UV
region (Fig. 3a).
Compared with those of their solutions, the absorption bands of
DERH3TT, DPRH3TT, DORH3TT, DPPR3TT and DBPPR3TT films are
stronger, broader and red-shifted by ca. 100 nm (Fig. 3b).
In addition, a new vibronic peak at ca. 715 nm, close to absorption
edge, was observed, indicating the ordered structure and strong
(Fig. S19). The hole mobility (
lated with the equation IDS ¼ (W/2L)·
m
) of the solar cell device was calcu-
m
·Ci·(VG-VT)2, where IDS, W, L,
Ci, VG and VT were the source-drain current, the channel width, the
channel length, the gate dielectric capacitance per unit area (SiO2,
300 nm, Ci ¼ 11 nF cmꢁ2), the gain voltage and threshold voltage,
respectively. The results indicate that DERH3TT, DPRH3TT,
DORH3TT, DPPR3TT and DBPPR3TT have
a
hole-mobility of
2.38
10ꢁ3 and
s
ꢁ1, respectively, in the saturated regime,
p p stacking interaction between the molecular backbones at solid
ꢁ
8.51
ꢀ
10ꢁ4
,
5.04
ꢀ
10ꢁ4, 1.12
ꢀ
10ꢁ4
,
ꢀ
status. The absorption edges of DERH3TT, DPRH3TT, DORH3TT,
5.33 ꢀ 10ꢁ4 cm2 Vꢁ1
which are consistent with that of P3HT [33] and the previous report
[11]. These results indicate that shorter alkyl terminal chain is
favorable for hole-mobility and the phenyl-pyrazolone terminal
groups increase the hole-mobility of the molecule.
3.5. Theoretical computations
To further understand the structure and physicochemical
properties of the compounds, the molecular geometries and elec-
tronic wave functions of frontier orbital were investigated with
theoretical computation. Since the computations of both geometry
optimizations and excited-state were conducted on their chloro-
form solutions, the solvent effects were included in the framework
of the polarizable continuum model (PCM). The detailed compu-
tation is described in the Supporting Information. Due to the slight
effects of the long side alkyl chain on the energy level of the whole
molecule, n-octyl groups on the thiophene were simplified as the
methyl groups in the model [34]. Fig. 5 shows the theoretical
electronic wave functions of HOMO and LUMO of DERH3TT,
DPRH3TT, DORH3TT, DPPR3TT and DBPPR3TT. The electronic wave
functions of the frontier orbital were delocalized on the whole main
chain. Relatively high HOMO densities were observed on the cen-
tral thienothiophene-one group and high LUMO densities were
found on the terminal rhodanine or pyrazolone groups, which were
consistent with the previous reports [11,16]. Furthermore, the in-
crease of the alkyl chain on N-rhodanine and the substitutions of N-
pyrazolone showed almost no effect on the HOMO and LUMO of the
compounds (Table 2). Theoretically, the less hindrance around the
electron-withdrawing terminal groups such as in DERH3TT would
be favorable for the electron transfer from the donor molecules to
the fullerene acceptors in the bulk hetero-junction OPV [16]. In all,
the theoretical computation results are in good agreement with
experimental results (Table 1).
Fig. 4. a) Transfer and b) output characteristics of the DERH3TT solar cell device.

106
W. Yan et al. / Dyes and Pigments 121 (2015) 99e108
Table 2
B3LYP/6-31G* calculated HOMO and LUMO energies, excitation energies and oscil-
lator strengths of the lowest singlet excited states (S1). (Energies are in unit of eV).
Small molecules
HOMO (eV)
LUMO (eV)
E(S1) (eV)
Oscillator (S1)
DERH3TT
DPRH3TT
DORH3TT
DPPR3TT
DBPPR3TT
ꢁ4.85
ꢁ4.86
ꢁ4.85
ꢁ4.91
ꢁ4.92
ꢁ3.00
ꢁ2.99
ꢁ2.99
ꢁ3.05
ꢁ3.04
1.55
1.57
1.57
1.57
1.58
3.00
2.97
3.02
3.21
3.34
3.6. Applications of the A-D-A-D-A small molecule compounds in
solar cells
To determine the photovoltaic properties of these compounds,
bulk-heterojunction OSCs were fabricated with them as the
donor and PC70BM as the acceptor in the ITO/PEDOT:PSS/pho-
toactive layer/PFN/Al device. The photoactive layer was prepared
by the conventional spin-coating with the chloroform solutions
of the compounds. The best JꢁV curves of the devices are
shown in Fig. 6a and the corresponding photovoltaic perfor-
mance is summarized in Table 3. Since the solubility of DERH3TT,
DPPR3TT and DBPPR3TT in chloroform is ~10 mg mLꢁ1, their
solutions with concentrations of 10 and 8 mg mLꢁ1 were used
for the preparation of the photoactive layer (Table S1 and S2).
The DERH3TT device prepared with a D/A weight ratio of 1:0.8 at
a donor concentration of 10 mg mLꢁ1 showed the best photo-
voltaic performance with a high open circuit voltage (Voc) of
0.81 V, a short-circuit current density (Jsc) of 7.51 mA cmꢁ, a
fill factor (FF) of 52.8%, and a power conversion efficiency (PCE)
of 3.19%.
In view of the high solubility of DPPR3TT and DOPPR3TT in
chloroform, respectively, therefore, higher concentrations of the
donors were used for the fabrication of their solar cells. As shown in
Table 3 and S3, the PCE of the devices prepared with 12 mg mLꢁ1
donor solutions dramatically dropped due to the deceased Jsc. It can
be explained that the thickness of the active layer is increased with
the increase of the donor concentration under the same film-
formation condition, which increased the recombination proba-
bility of the excitation, leading to lower Jsc.
The external quantum efficiency (EQE) spectra of the opti-
mized DERH3TT, DPRH3TT, DORH3TT, DPPR3TT and DBPPR3TT
devices are shown in Fig. 6b. The DERH3TT/PC71BM device
exhibited high photo conversion efficiency in the wavelength
range from 300 to 800 nm, with a highest EQE value of 38% at
430 nm. The theoretical Jsc of DERH3TT integrated from EQE
spectrum is 7.25 mAcmꢁ2 and has about 5% deviation from the
experimental result. Although the PCEs of the A-D-A-D-A small
molecule solar cells are relatively low, their efficient photo con-
version efficiency ranges are broader than those of the similar
small molecule devices previously reported [11e13]. In addition,
despite the similar absorption spectra, band-gap and hole-
mobility of the compounds, the DPPR3TT and DBPPR3TT devices
showed lower EQEs and higher Jsc than the DERH3TT, DPRH3TT
and DORH3TT devices. This can be explained that DPPR3TT and
DBPPR3TT compounds can not form a good phase separation with
PC71BM as shown in the AFM and TEM images of the their blends
with PC71BM (Fig. S21 and S22).
Fig. 5. The electronic wave functions of the lowest unoccupied (LUMO) and the
highest occupied (HOMO) in the DERH3TT (a), DPRH3TT (b), DORH3TT (c), DPPR3TT
(d) and DBPPR3TT (e) chloroform solutions, depicted at isodensity surface of 0.025 au
and calculated at B3LYP/6-31G* level.

W. Yan et al. / Dyes and Pigments 121 (2015) 99e108
107
Fig. 6. (a) JꢁV curves of the solar cells with the photoactive layer composed of DERH3TT, DPRH3TT, DORH3TT, DPPR3TT and DBPPR3TT/PC71BM (1:0.8, w/w) without additives and
annealing treatment; (b) EQE plots of the solar cells with an active layer composed of DERH3TT, DPRH3TT, DORH3TT, DPPR3TT and DBPPR3TT/PC71BM (1:0.8, w/w).
Table 3
Device performance parameters for BHJ solar cells based on DERH3TT, DPRH3TT,
DORH3TT, DPPR3TT and DBPPR3TT with donor concentration of 10 mg mLꢁ1
.
Donor molecular D/A ratio D (mg mLꢁ1) Voc (V) Jsc (mA cmꢁ2) FF (%) PCE (%)
DERH3TT
1:0.5
1:0.8
1:1
1:0.8
1:0.8
1:0.8a
1:0.8
1:0.8
10
10
10
10
10
10
10
10
0.82
0.81
0.77
0.81
0.79
0.82
0.73
0.71
6.72
7.51
5.92
6.01
5.28
6.03
4.80
4.26
50.8 2.80
52.8 3.19
30.4 1.39
55.9 2.72
52.3 2.19
50.3 2.49
42.3 1.48
40.0 1.21
DPRH3TT
DORH3TT
DPPR3TT
DBPPR3TT
a
devices were annealed at 160 ꢂC.
3.7. Morphologies of the A-D-A-D-A small molecule photoactive
layers
The surface morphologies of DERH3TT/PC71BM, DPRH3TT/
PC71BM, DORH3TT/PC71BM, DPPR3TT/PC71BM and DBPPR3TT/
PC71BM (1:0.8, w/w) blend films spin-coated from their chloroform
solutions were imaged with AFM. Their average roughness are 4.63,
4.90, 7.42, 25.3, 29.4 nm, respectively, indicating that the DERH3TT,
DPRH3TT and DORH3TT with alkyl chain terminal groups have
better miscibility with PC71BM molecules in the blend film than
DPPR3TT and DBPPR3TT with large phenyl groups.
Fig. 7 shows the TEM images of the A-D-A-D-A small molecule
photoactive layers of the solar cell devices. As shown in Fig. 7a and
b, DERH3TT and DPRH3TT are continuous interpenetrating net-
works with the domain sizes more than 100 nm. Blending PC71BM
with DORH3TT distributed the domains with an obvious aggrega-
tion, resulting in an irregular morphology (Fig. 7c). The blending of
PC71BM with DPPR3TT and DBPPR3TT even broke their continuous
interpenetrating networks, leading to low efficient excitation
dissociation, charge transport, and thus lower Jsc. Therefore, the
dissatisfactory solar cell performance is at least in some degree
attributed to irregular morphology of the blend film of the com-
pounds with PC71BM and fine design of the A-D-A-D-A small
molecule structure is critical to obtain desired solar cell
performances.
Fig. 7. TEM images of DERH3TT (a), DPRH3TT (b), DORH3TT (c), DPPR3TT (d) and
DBPPR3TT (e)/PC71BM (1:0.8, w/w) photoactive layers.
4. Conclusions
high hole-mobility and compatible energy level with PCBM, as well
as good photoelectric response in the range from 300 to 800 nm.
The solar cell device prepared with the short-alkyl-chain termi-
nated DERH3TT as the photoactive layers showed a highest Jsc and a
highest PCE of 3.19%. The high molar absorption coefficient, broad
photoelectric response range and the high hole-mobility of these
compounds make them the promising candidates for the high ef-
ficiency solution processed organic photovoltaic cells.
In summary, DERH3TT, DPRH3TT, DORH3TT, DPPR3TT and
DBPPR3TT molecules with the A-D-A-D-A structure and
thienothiophene-one as the central building block have been
designed, synthesized and well characterized. They showed broad
UVeVis absorption spectrum, high molar absorption coefficient
and good solubility in most organic solvents. In addition, they have

108
W. Yan et al. / Dyes and Pigments 121 (2015) 99e108
[15] Liu Y, Yang Y, Chen C, Chen Q, Dou L, Hong Z, et al. Solution-processed small
Acknowledgments
molecules using different electron linkers for high-performance solar cells.
Adv Mater 2013;25(33):4657e62.
The authors gratefully acknowledge the financial support from
the National Basic Research Program (2011CB933303) and the Na-
tional Natural Science Foundation of China (NSFC) (21321001,
21371012).
[16] Shen S, Jiang P, He C, Zhang J, Shen P, Zhang Y, et al. Solution-processable
organic molecule photovoltaic materials with bithienyl-benzodithiophene
central unit and indenedione end groups. Chem Mater 2013;25(11):2274e81.
[17] Zhou J, Zuo Y, Wan X, Long G, Zhang Q, Ni W, et al. Solution-processed and
high-performance organic solar cells using small molecules with a benzodi-
thiophene unit. J Am Chem Soc 2013;135(23):8484e7.
[18] Chen Y, Wan X, Long G. High performance photovoltaic applications using
solution-processed small molecules. Acc Chem Res 2013;46(11):2645e55.
[19] Bredas J. When electrons leave holes in organic solar cells. Science
2014;343(6170):492e3.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2015.05.009.
[20] Kim Y, Baek J, Ha J, Chung DS, Kwon S, Park C, et al. A high-performance
solution-processed small molecule: alkylselenophene-substituted benzodi-
thiophene organic solar cell. J Mater Chem C 2014;2(25):4937e46.
References
€
€
[21] Lobert M, Mishra A, Uhrich C, Pfeiffer M, Bauerle P. Synthesis and character-
ization of benzo- and naphtho[2,1-b:3,4-b0]dithiophene-containing oligo-
mers for photovoltaic applications. J Mater Chem C 2014;2(24):4879e92.
[22] Sun Y, Welch G, Leong W, Takacs CJ, Bazan GC, Heeger AJ. Solution-processed
small-molecule solar cells with 6.7% efficiency. Nat Mater 2012;11(1):44e8.
[23] Li G, Zhao Y, Li J, Cao J, Zhu J, Sun X, et al. Synthesis, characterization, physical
properties, and OLED application of single BN-fused perylene diimide. J Org
Chem 2015;80(1):196e203.
[1] Leung M, Chang C, Wu M, Chuang K, Lee J, Shieh S, et al. 6eN,N-Diphenyla-
minobenzofuran-derived pyran containing fluorescent dyes: a new class of
high-brightness red-light-emitting dopants for OLED. Org Lett 2006;8(12):
2623e6.
[2] Wei Y, Chen C. Doubly ortho-Linked cis-4,40-bis(diarylamino)stilbene/fluorene
hybrids as efficient nondoped, sky-blue fluorescent materials for optoelec-
tronic applications. J Am Chem Soc 2007;129(24):7478e9.
[24] Zhang Q, Xiao J, Yin Z, Duong HM, Qiao F, Boey F, et al. Synthesis, character-
ization, and physical properties of
a conjugated heteroacene: 2-methyl-
[3] Yao Y, Xiao J, Wang X, Deng Z, Zhang B. Starburst DCM-type red-light-emitting
materials for electroluminescence applications. Adv Funct Mater 2006;16(5):
709e18.
[4] Xiao J, Yang B, Wong J, Liu Y, Wei F, Tan K, et al. Synthesis, characterization,
self-Assembly, and physical properties of 11-methylbenzo[d]pyreno[4,5-b]
furan. Org Lett 2011;13(12):3004e7.
1,4,6,7,8,9-hexaphenylbenz(g)isoquinolin-3(2H)-one (BIQ). Chem Asian
J
2011;6(3):856e62.
[25] Kan B, Zhang Q, Li M, Wan X, Ni W, Long G, et al. Solution-processed organic
solar cells based on dialkylthiol-substituted benzodithiophene unit with ef-
ficiency near 10%. J Am Chem Soc 2014;136(44):15529e32.
€
[26] Mishra A, Bauerle P. Small molecule organic semiconductors on the move:
[5] Xiao J, Liu S, Liu Y, Ji L, Liu X, Zhang H, et al. Synthesis, structure, and physical
promises for future solar energy technology. Angew Chem Int Ed 2012;51(9):
2020e7.
properties
of
5,7,14,16-tetraphenyl-8:9,12:13-bisbenzo-hexatwistacene.
Chem Asian J 2012;7(3):561e4.
[27] Ye L, Zhang S, Zhao W, Yao H, Hou J. Highly efficient 2D-conjugated
benzodithiophene-based photovoltaic polymer with linear alkylthio side
chain. Chem Mater 2014;26(12):3603e5.
[28] Hou J, Chen H, Zhang S, Chen RI, Yang Y, Wu Y, et al. Synthesis of a low band
gap polymer and its application in highly efficient polymer solar cells. J Am
Chem Soc 2009;131(43):15586e7.
[29] Chen H, Huang H, Huang X, Clifford JN, Forneli A, Palomares E, et al. High
molar extinction coefficient branchlike organic dyes containing di(p-tolyl)
phenylamine donor for dye-sensitized solar cells applications. J Phys Chem
C 2010;114(7):3280e6.
[30] Ni W, Li M, Wan X, Feng H, Kan B, Zuo Y, et al. A high-performance photo-
voltaic small molecule developed by modifying the chemical structure and
optimizing the morphology of the active layer. RSC Adv 2014;4:31977e80.
[31] Li G, Kang C, Gong X, Zhang J, Li W, Li C, et al. 5,6-Difluorobenzothiadiazole
and silafluorene based conjugated polymers for organic photovoltaic cells.
J Mater Chem C 2014;2(26):5116e23.
[6] Komori T, Nakanotani H, Yasuda T, Adachi C. Light-emitting organic field-
effect transistors based on highly luminescent single crystals of thiophene/
phenylene co-oligomers. J Mater Chem C 2014;2(25):4918e21.
[7] Wang C, Zhang J, Long G, Aratani N, Yamada H, Zhao Y, et al. Synthesis,
structure, and air-stable N-type field-effect transistor behaviors of function-
alized octaazanonacene-8,19-dione. Angew Chem Int Ed 2015. http://
dx.doi.org/10.1002/anie.201500972.
[8] Walker B, Tamayo AB, Dang X, Zalar P, Seo J, Garcia A, et al. Nanoscale phase
separation and high photovoltaic efficiency in solution-processed, small-
molecule bulk heterojunction solar cells. Adv Funct Mater 2009;19(19):
3063e9.
[9] Liu Y, Wan X, Wang F, Zhou J, Long G, Tian J, et al. High-performance solar cells
using a solution-processed small molecule containing benzodithiophene unit.
Adv Mater 2011;23(45):5387e91.
[10] Cho N, Song K, Lee J, Ko J. Facile synthesis of fluorine-substituted benzothia-
diazole-based organic semiconductors and their use in solution-processed
small-molecule organic solar cells. Chem Eur J 2012;18(36):11433e9.
[11] Li Z, He G, Wan X, Liu Y, Zhou J, Long G, et al. Solution processable rhodanine-
based small molecule organic photovoltaic cells with a power conversion
efficiency of 6.1%. Adv Energy Mater 2012;2(1):74e7.
[32] Yan W, Hong C, Long G, Yang Y, Liu Z, Bian Z, et al. Synthesis, crystal structures
and photophysical properties of novel boron-containing derivatives of phe-
nalene with bright solid-state luminescence. Dyes Pigments 2014;106:
197e204.
[33] Mihailetchi VD, Xie HX, de Boer B, Koster LJA, Blom PWM. Charge transport
and photocurrent generation in poly(3-hexylthiophene):methanofullerene
bulk-heterojunction solar cells. Adv Funct Mater 2006;16(5):699e708.
[34] Mei C, Liang L, Zhao F, Wang J, Yu L, Li Y, et al. A family of donorꢁacceptor
photovoltaic polymers with fused 4,7-dithienyl-2,1,3-benzothiadiazole units:
effect of structural fusion and side chains. Macromolecules 2013;46(19):
7920e31.
[12] He G, Li Z, Wan X, Liu Y, Zhou J, Long G, et al. Impact of dye end groups on
acceptoredonoreacceptor type molecules for solution-processed photovol-
taic cells. J Mater Chem 2012;22(18):9173e80.
[13] Chen W, Yang X, Long G, Wan X, Chen Y, Zhang Q. A perylene diimide (PDI)-
based small molecule with tetrahedral configuration as
a non-fullerene
acceptor for organic solar cells. J Mater Chem C 2015. http://dx.doi.org/
10.1039/C5TC00865D.
[14] Sharenko A, Proctor CM, van der Poll TS, Henson ZB, Nguyen T, Bazan GC.
A high-performing solution-processed small molecule: perylene diimide bulk
heterojunction solar cell. Adv Mater 2013;25(32):4403e6.