Synthesis and photovoltaic properties of organic small molecules containing triphenylamine and benzothiadiazole moieties with different terminal groups

Article abstract of DOI:10.1016/j.dyepig.2013.03.011

Three new organic small molecules (TPA-DTBT, TPA-DTBT-TH and TPA-DTBT-CAO) containing triphenylamine (TPA) and 4,7-bis(thiophen-2-yl)benzo[1,2,5] thiadiazole (DTBT) moieties with different terminal groups were designed and synthesized. Their thermal, phot

Full text of DOI:10.1016/j.dyepig.2013.03.011

Dyes and Pigments 98 (2013) 464e470
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and photovoltaic properties of organic small molecules
containing triphenylamine and benzothiadiazole moieties with
different terminal groups
Changwei Wang, Zhencai Cao, Bin Zhao, Ping Shen, Xunshan Liu, Haohao Li,
*
Tianpei Shen, Songting Tan
College of Chemistry, Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University,
Xiangtan 411105, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new organic small molecules (TPA-DTBT, TPA-DTBT-TH and TPA-DTBT-CAO) containing triphe-
nylamine (TPA) and 4,7-bis(thiophen-2-yl)benzo[1,2,5]thiadiazole (DTBT) moieties with different
terminal groups were designed and synthesized. Their thermal, photophysical, electrochemical and
photovoltaic properties were investigated. The introduction of 2-ethylhexyl cyanoacetate and
2-tetradecyl thiophene as terminal groups extended molecular p-conjugation length and broadened the
absorption spectra. Three small molecules exhibit good solubility in common organic solvents, good film-
Received 3 January 2013
Received in revised form
18 March 2013
Accepted 19 March 2013
Available online 6 April 2013
forming ability, and thermal stability. Bulk heterojunction (BHJ) solar cells based on the blend of small
molecule TPA-DTBT and PC61BM showed a power conversion efficiency (h) of 1.85%.
Keywords:
Triphenylamine
Benzothiadiazole
Terminal groups
Ó 2013 Elsevier Ltd. All rights reserved.
Power conversion efficiency
Bulk heterojunction solar cells
Synthesis
1. Introduction
adjust the molecular structure properties, easier purification and
control band structure [12,17]. Thus SM BHJ has become one of the
hottest research spots, when they emerged as organic solar cells.
With more detailed studies have been carried out, prominent ef-
In recent years, organic solar cells (OSCs) has become arguably
one of the hottest topics in academia and industry due to its low
cost, light weight, flexibility and steady advances in the effciencies
[1e6]. The bulk heterojunction (BHJ) has sandwiched structure
with bicontinuous interpenetrating network, which is composed of
a conjugated polymer or small molecule donor and a soluble
electron-deficient fullerene derivative acceptor [7e10]. Recently,
significant progress has been achieved for polymer based solar cells
(PSCs) with the most promising BHJ architecture, and the power
ficiencies with 7% have been achieved, which is close to the best h of
PSCs [12]. Through the molecules structure design and device
physics optimization, we have reason to believe that small
molecules-based solar cells could develop even higher efficiencies
in the near future.
It is generally known that triphenylamine (TPA) and its de-
rivatives are widely investigated and used as various opto- and
electro-optical materials such as dye-sensitized solar cells (DSSCs),
organic light emitting diodes (OLEDs), organic field-effect transis-
tors (OFETs), which attributes to its 3D propeller starburst molec-
ular structure, good electron donating ability and transporting
capability [18e22]. Among dyes under investigation, Zeng et al.
reported a triphenylamine dye C219 with an encouraging overall
efficiency over 10% [23]. No only that, organic small molecules
derived from TPA and its derivatives have been designed and syn-
thesized for photovoltaic application [24e26].
conversion efficiencies (h) of PSCs have reached 8% [11,12]. In earlier
study, 4,7-bis(thiophen-2-yl)benzo[1,2,5]thiadiazole (DTBT) unit
has been widely investigated and used as electron-deficient group
due to its relative low-lying HOMO energy level [13,14]. The PSCs
based on conjugated DTBT unit have shown promising photovoltaic
performance [15,16]. Meanwhile, small molecule bulk hetero-
junction (SM BHJ) solar cells have some promising advantages, for
example, well defined structure less batch-to-batch variation,
Here, we report three novel TPA-containing organic molecules
TPA-DTBT, TPA-DTBT-TH and TPA-DTBT-CAO (Fig. 1) for SM BHJ
solar cells, which combined with the advantages of TPA and DTBT
* Corresponding author.
E-mail address: tanst2008@163.com (S. Tan).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.03.011

C. Wang et al. / Dyes and Pigments 98 (2013) 464e470
465
2.2. Instruments and characterizations
1H NMR and 13C NMR spectra were measured with a Bruker
Avance 400 instrument. Elemental analysis was obtained on
PerkineElmer 2400 element analyzer. UVevisible spectra of the
small molecules were measured on a PerkineElmer Lamada 25
spectrometer. FT-IR spectra were obtained on a PerkineElmer
Spectra One spectrometer. MALDI-TOF mass spectrometric mea-
surements were performed on Bruker Autoflex III. Thermal ana-
lyses was performed on a Netzsch TG 209 analyzer under N2
atmosphere with a heating and cooling rate of 20 ꢀC minꢁ1
.
Electrochemical redox potentials were obtained by cyclic vol-
tammetry (CV) using a three-electrode configuration and an
electrochemistry workstation (ZAHNER ZENNIUM). CV was con-
ducted on an electrochemistry workstation with the thin film on a
Pt plate as the working electrode, Pt slice as the counter electrode,
and saturated calomel electrode (SCE) as the reference electrode
in a 0.1 M tetra-n-butylammonium hexafluorophosphate aceto-
nitrile solution at a scan rate of 50 mV sꢁ1. Current densitye
voltage (JeV) characteristics were measured by a Keithley 2602
Source Meter under 100 mW cmꢁ2 irradiation using a 500 W Xe
lamp equipped with a global AM 1.5 filter for solar spectrum
simulation, and the incident light intensity was calibrated using a
standard Si solar cell.
2.3. Device fabrication of SM BHJ
The photovoltaic cells were constructed in the traditional
sandwich structure through several steps. The BHJ solar cells were
prepared on pre-patterned commercial indium tin oxide (ITO) glass
substrates. The thickness and sheet resistance of the ITO are 120 nm
and 12
U/square, respectively. The active area of each solar cell
device is 0.5 cm2. The ITO coated glass substrates were cleaned by
successive ultrasonic treatment in acetone and isopropyl alcohol,
then treated with a nitrogeneoxygen plasma oven for 5 min. The
photovoltaic cells were constructed in the conventional device
configuration with traditional sandwich structure through the
following several steps. First, a poly(3,4-ethylenedioxythiophene)-
poly(styrene sulfonate) (PEDOT-PSS, from Bayer AG) thin film was
spin-coated from an aqueous solution on a cleaned ITO glass sub-
strate giving a thickness of about 30 nm as measured by Ambios
Technology XP-2 surface profilometer, and then baked at 100 ꢀC for
15 min. Secondly, an active layer (ca.100 nm) was spin-coated on
top of the PEDOT-PSS from the a solution of chlorobenzene (20 mg/
mL in CB) of the compound:PC61BM blends (1:1.2e1:1.5 weight
ratio), and then annealed at 100 ꢀC for 15 min in a nitrogen-filled
glove box. Finally, 0.5 nm of LiF and 100 nm of aluminum (Al)
cathodes were deposited on the top of the active layer in a vacuum
of 7e8 ꢂ 10ꢁ4 Pa to complete the photovoltaic device fabrication.
Fig. 1. Molecular structures of TPA-DTBT, TPA-DTBT-TH and TPA-DTBT-CAO.
units. Three organic molecules contain different terminal groups,
such as hydrogen atom, 2-tetradecyl thiophene (TH) and 2-
ethylhexyl cyanoacetate (CAO) groups. We expect that the
different terminal groups could tune optoelectric properties of the
small molecules. Furthermore, the vinylene linkage between the
TPA central unit and DTBT arm group is introduced to enhance the
co-planarity of three organic molecules [27]. The effects of different
terminal groups on the thermal stability, photophysical, electro-
chemical and photovoltaic properties of the resultant small mole-
cules were studied in detail.
2. Experimental section
2.1. Materials
2.4. Synthesis
Tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) (98%)
was obtained from Puyang Huicheng Chemical Co, Ltd and used
without further purification. All other starting materials were
purchased from Pacific ChemSource or Alfa Aesar in analytical
grade. Tetrahydrofuran (THF) and toluene were dried and distilled
over sodium and benzophenone. DMF was dried and distilled un-
der reduced pressure. Phosphorus oxychloride, CHCl3, CH3CN and
1,2-dichloroethane were atmospheric distillation. All chromato-
graphic separations were carried out on silica gel (200e300 mesh).
All other solvents and chemicals used in this work were analytical
grade and used without further purification.
5-Bromo-1,3-bis(diethyl-phsophonatemethyl)-2-(ethylhexyl)
benzene was synthesized according to literature procedure with
some modification [21]. 4-Methyl-N-(4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl)-N-p-tolylbenzenamine, 3-hexyl-5-(7-
(4-hexylthiophen-2-yl)benzo[1,2,5]thiazdiazol-4-yl)thiophene-2-
carbaldehyde and 3-hexyl-5-(7-(4-hexyl-5-(5-tetradecylthiophen-
2-yl)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophene-2-
carbaldehyde [24,28] were synthesized according to already pub-
lished procedures. The synthetic routes to the three organic small
molecules are shown in Scheme 1. The detailed synthetic pro-
cedures are as follows.

466
C. Wang et al. / Dyes and Pigments 98 (2013) 464e470
Scheme 1. Synthetic routes of TPA-DTBT, TPA-DTBT-TH and TPA-DTBT-CAO.
2.4.1. Synthesis of 4-(5-((1E,7E)-5-bromo-2-(2-ethylhexyloxy)-3-((E)-
2-(3-hexyl-5-(7-(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)
thiophen-2-yl)vinyl)styryl)-4-hexylthiophen-2-yl)-7-(4-hexylthiophen-
2-yl)benzo[c][1,2,5]thiadiazole (1)
(0.23 g, 1.65 mmol) and toluene (70 mL) were placed in a 100 mL
three-necked flask and the ensuing mixture heated to 30 ꢀC.
Pd(PPh3)4 (100 mg, 0.08 mmol) and a little water were added
quickly and the mixture was stirred at 100 ꢀC for 72 h, cooled to
room temperature and the resulting solution was treated with
dilute aqueous HCl (10 mL) solution. This solution was stirred for
30 min and then extracted with dichloromethane. The combined
organic layer was washed with water and brine several times, dried
over anhydrous MgSO4, filtered and evaporated. The crude product
was purified by silica gel column chromatography using petroleum
ether/dichloromethane (2:1, v:v) as eluent to give 0.94 g TPA-DTBT
5-Bromo-1,3-bis(diethyl-phsophonate-methyl)-2-(ethylhexyl)
benzene (1.414 g, 2.4 mmol) and 3-hexyl-5-(7-(4-hexylthiophen-2-
yl)benzo[1,2,5]thiazdiazol-4-yl)thiophene-2-carbaldehyde (3.0 g,
6.0 mmol) were dissolved in THF (15 mL), and the solution was
stirred at room temperature for 30 min under nitrogen atmosphere.
Then potassium tertbutoxide (0.673 g, 6 mmol) was dissolved in
THF (20 mL) and added dropwise to above solution. The reaction
mixture was stirred for 5 h at room temperature, and then the
mixture was heated to 45 ꢀC for 12 h. After cooling to room tem-
perature, the reaction mixture was extracted with dichloromethane
and washed with dilute aqueous HCl solution. The organic phase
was dried over anhydrous MgSO4, and then the solvent was
removed by rotary evaporation. The crude product was purified by
silica gel column chromatography using petroleum ether/
dichloromethane (1:1, v:v) as eluent to give 1.58 g 1 as a dark red
as a dark red solid (Yield: 55.3%). FT-IR (KBr, cmꢁ1): 950 (
n
C]C). 1H
/ppm): 8.03e8.00 (d, J ¼ 6.7 Hz, 4H); 7.85
NMR (400 MHz, CDCl3,
d
(s, 4H); 7.68 (s, 2H); 7.52e7.50 (d, J ¼ 8.2 Hz, 2H); 7.40 (s, 4H); 7.15e
7.05 (m,12H); 3.83e3.82 (d, J ¼ 4.0 Hz, 2H); 2.81e2.68 (m, 8H); 2.34
(s, 6H); 1.94 (m, 1H); 1.73e1.69 (m, 8H); 1.25e1.44 (m, 20H); 1.11e
1.09 (t, J ¼ 7.0 Hz, 2H); 0.91e0.89 (m, 28H). 13C NMR (CDCl3,
100 MHz, d/ppm): 154.25, 152.73, 152.71, 152.66, 152.64, 147.77,
145.38, 144.36, 142.57, 139.15, 138.51, 136.75, 134.08, 132.69, 131.60,
130.70, 129.99, 129.03, 127.70, 125.95, 125.71, 125.47, 125.28, 124.79,
123.58, 123.42, 122.81, 121.54, 121.10, 78.32, 41.16, 31.80. 31.77,
31.01, 30.99, 30.72, 30.49, 29.71, 29.23, 29.12, 28.73, 24.17, 23.38,
22.69, 22.68, 20.84, 14.28, 14.12, 14.11, 11.78, 11.76. Anal. Calcd for
C90H103N5O2S6: C, 73.88; H, 7.10; N, 4.79; S, 13.15. Found: C, 73.84;
H, 7.21; N, 4.82; S, 13.19. MALDI-TOF MS (C90H103N5OS6) m/z: calcd
for 1462.646; found 1462.126.
solid (Yield: 83.2%). FT-IR (KBr, cmꢁ1): 944 (
n
C]C). 1H NMR
/ppm): 8.14e8.12 (d, J ¼ 8.0 Hz, 4H), 7.96 (s, 4H),
(400 MHz, CDCl3,
d
7.74 (s, 2H), 7.41e7.39 (d, J ¼ 8.0 Hz, 4H), 7.17 (s, 2H), 3.90e3.89 (d,
J ¼ 4.0 Hz, 2H), 2.94e2.80 (m, 8H), 2.04e1.99 (m, 8H), 1.87e1.80 (m,
8H), 1.56e1.48 (m, 28H), 1.22e1.19 (t, J ¼ 6.0 Hz, 4H), 1.05e1.00 (m,
18H). MALDI-TOF MS (C70H85BrN4OS6) m/z: calcd for 1270.744;
found 1270.443.
2.4.2. Synthesis of TPA-DTBT
2.4.3. Synthesis of compound 2
Under an argon atmosphere, compound 1 (1.50 g, 1.18 mmol), 4-
methyl-N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-
N-p-tolylbenzenamine (0.71 g, 1.77 mmol), anhydrous K2CO3
In ice bath conditions, compound TPA-DTBT (0.3 g, 0.21 mmol)
was dissolved in 40 mL of 1,2-dichloroethane and this solution was
stirred for 30 min. Vilsmeier reagent that was prepared from 0.5 mL

C. Wang et al. / Dyes and Pigments 98 (2013) 464e470
467
of DMF and 0.4 mL of phosphorus oxychloride were added quickly
and then the mixture was kept at 85 ꢀC for 24 h. After neutralization
with 25% NaOH solution, the mixture was extracted with
dichloromethane and washed with water. The organic solution was
dried over anhydrous MgSO4, and then the solvent was removed by
rotary evaporation. The crude product was purified by silica gel
column chromatography using petroleum ether/dichloromethane
(1:1, v:v) as eluent and dried under vacuum to give 0.18 g 2 as a
(5-tetradecylthiophen-2-yl)thiophen-2-yl)benzo[c][1,2,5]thiadia-
zol-4-yl)thiophene-2-carbaldehyde (0.80 g, 1.03 mmol) and po-
tassium tertbutoxide (0.12 g, 1.03 mmol). The crude product was
purified by silica gel column chromatography using petroleum
ether/dichloromethane (3:2, v:v) as eluent to give 0.35 g 3 as a red
n
solid (Yield: 47.9%). FT-IR (KBr, cmꢁ1): 945( C]C). 1H NMR
(400 MHz, CDCl3,
d
/ppm): 8.00e7.97 (d, J ¼ 11.6 Hz, 4H); 7.80 (s,
4H), 7.61 (s, 2H), 7.26 (s, 4H), 7.04e7.03 (d, J ¼ 3.1 Hz, 2H); 6.77e6.76
(d, J ¼ 2.7 Hz, 2H); 3.78e3.77 (m, 2H); 2.85e2.77 (m, 12H); 1.92e
1.89 (m, 1H); 1.73e1.72 (m, 12H); 1.45e1.27 (m, 72H); 1.11e1.08
(t, J ¼ 7.3 Hz, 4H); 0.93e0.86 (m, 24H). 13C NMR (CDCl3, 100 MHz,
purple-red solid (Yield: 56.3%). FT-IR (KBr, cmꢁ1): 1655 (
C]C). 1H NMR (400 MHz, CDCl3,
/ppm): 10.10 (s, 2H), 8.08e8.07
nC]O), 952
(n
d
(d, J ¼ 3.6 Hz, 4H); 7.98e7.97 (m, 2H); 7.87e7.86 (d, J ¼ 7.6 Hz, 2H);
7.68 (s, 2H); 7.52e7.50 (d, J ¼ 8.4 Hz, 2H); 7.41e7.39 (d, J ¼ 5.7 Hz,
2H); 7.15e7.06 (m,10H); 3.84e3.83 (m, 2H); 3.06e3.02 (t, J ¼ 7.6 Hz,
4H); 2.82e2.78 (t, J ¼ 7.6 Hz, 4H); 2.34e2.31 (m, 6H); 1.96e1.93 (m,
1H); 1.82e1.72 (m, 12H); 1.57e1.26 (m, 22H); 1.12e1.09 (t,
d/ppm): 153.70,152.48,146.31,142.97,139.73,137.78,137.22,136.30,
133.61, 133.42, 133.00, 130.50, 130.31, 126.89, 125.57, 125.39, 125.15,
125.10, 124.73, 124.41, 121.32, 121.08, 117.41, 100.03, 78.27, 41.13,
31.95, 31.83, 31.75, 31.61, 31.00, 30.91, 30.53, 30.21, 29.72, 29.68,
29.63, 29.57, 29.43, 29.40, 29.37, 29.32, 29.25, 28.70, 24.05, 23.40,
22.73, 22.70, 14.35, 14.16, 14.10, 11.89, 1.02. MALDI-TOF MS
(C106H145BrN4OS8) m/z: calcd for 1827.736; found 1827.219.
J ¼ 7.2 Hz, 6H); 0.91e0.89 (m, 18H). 13C NMR (CDCl3, 100 MHz,
d/
ppm): 181.96, 181.88, 154.23, 153.24, 152.50, 152.39, 147.79, 147.45,
145.29, 142.63, 139.57, 137.35, 136.90, 136.15, 133.81, 132.73, 132.69,
131.51, 131.37, 130.85, 130.83, 130.30, 129.98, 129.93, 128.86, 128.82,
127.72, 127.67, 127.61, 127.15, 124.80, 124.73, 124.59, 123.96, 123.70,
123.69, 123.49, 123.47, 122.72, 122.68, 120.66, 78.37, 74.79, 74.74,
74.68, 68.53, 47.73, 47.68, 47.64, 47.62, 47.58, 41.14, 31.93, 31.77,
31.61, 31.42, 30.95, 29.70, 29.35, 29.21, 29.10, 28.71, 28.66, 24.13,
23.39, 22.66, 22.59, 21.14, 20.82, 20.78, 19.25, 19.23, 14.31, 14.10,
14.04, 11.83. MALDI-TOF MS (C92H103N5O3S6) m/z: calcd for
1518.641; found 1518.725.
2.4.6. Synthesis of TPA-DTBT-TH
TPA-DTBT-TH was synthesized according to compound TPA-
DTBT, instead using compound 3 (0.30 g, 0.16 mmol), 4-methyl-
N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-N-p-
tolylbenzenamine (0.098 g, 0.25 mmol) and anhydrous K2CO3
(0.032 g, 0.23 mmol). The crude product was purified by silica gel
column chromatography using petroleum ether/dichloromethane
(3:1, v:v) as eluent to give 0.15 g TPA-DTBT-TH as a dark red solid
2.4.4. Synthesis of compound TPA-DTBT-CAO
n
(Yield: 45.3%). FT-IR (KBr, cmꢁ1): 951 ( C]C). 1H NMR (400 MHz,
Compound 2 (0.10 g, 0.066 mmol), 2-ethylhexyl cyanoacetate
(0.52 g, 2.63 mmol), and piperidine (1.0 mL) were dissolved in
CHCl3 (20 mL) and CH3CN (40 mL), and the solution was refluxed at
80 ꢀC for 12 h under nitrogen atmosphere. Then the solution was
poured into a diluted aqueous HCl (10 mL) solution and extracted
with dichloromethane. The organic phase was dried over anhy-
drous MgSO4, and the solvent was removed by rotary evaporation.
The crude product was purified by silica gel column chromatog-
raphy using petroleum ether/dichloromethane (1:1, v:v) as eluent
to give 0.07 g TPA-DTBT-CAO as a purple-red solid (Yield: 56.6%).
CDCl3, /ppm): 8.03 (s, 2H), 7.98 (s, 2H), 7.82 (s, 4H), 7.67 (s, 2H),
d
7.52e7.50 (d, J ¼ 8.3 Hz, 2H); 7.39 (s, 4H), 7.15e7.04 (m, 12H); 6.77e
6.76 (d, J ¼ 3.0 Hz, 2H); 3.84e3.83 (m, 2H); 2.85e2.77 (m, 12H);
2.34 (s, 6H), 1.96e1.93 (m, 1H); 1.74e1.74 (m, 12H); 1.45e1.27 (m,
72H); 1.12e1.08 (t, J ¼ 7.3 Hz, 4H); 0.91e0.88 (m, 24H). 13C NMR
(CDCl3, 100 MHz, d/ppm): 154.22, 154.20, 152.67, 152.64, 147.73,
147.72, 146.49, 146.44, 145.36, 142.58, 142.55, 139.89, 139.85, 138.47,
136.99, 136.92, 136.80, 136.47, 134.08, 133.59, 133.56, 133.10, 133.07,
132.67, 131.56, 130.68, 130.63, 129.98, 127.68, 125.72, 125.69, 125.49,
125.45,125.29,125.05,125.02,124.77,124.47,123.45,123.30,122.79,
120.96, 78.30, 41.15, 31.97, 31.81, 31.79, 31.76, 31.74, 31.63, 31.01,
30.97, 30.60, 30.23, 29.72, 29.71, 29.70, 29.63, 29.61, 29.58, 29.44,
29.41, 29.39, 29.38, 29.34, 29.24, 29.21, 28.72, 24.15, 23.40, 23.36,
22.70, 22.67, 20.85, 14.32, 14.27, 14.12, 11.82, 11.76, 1.04. Calcd for
C90H103N5O2S6: C, 74.91; H, 8.13; N, 3.47; S, 12.70. Found: C, 74.84;
H, 8.28; N, 3.62; S,12.79. MALDI-TOF MS (C126H163N5OS8) m/z: calcd
for 2019.065; found 2019.181.
FT-IR (KBr, cmꢁ1): 2214 (
nC^N), 1720 (nC]OOR), 949(n
C]C). 1H NMR
(400 MHz, CDCl3, /ppm): 8.46 (s, 2H), 8.20 (s, 2H), 8.07 (s, 2H),
d
8.02e8.01 (d, J ¼ 7.8 Hz, 2H); 7.87e7.85 (d, J ¼ 7.7 Hz, 2H); 7.68 (s,
2H); 7.52e7.50 (d, J ¼ 8.4 Hz, 2H); 7.41e7.39 (m, 2H); 7.15e7.05 (m,
12H); 4.25e4.24 (m, 4H); 3.85e3.83 (d, J ¼ 4.9 Hz, 2H); 2.91e2.87
(t, J ¼ 7.5 Hz, 4H); 2.81e2.77 (t, J ¼ 7.5 Hz, 4H); 2.34e2.30 (m, 6H);
1.97e1.94 (m, 1H); 1.73e1.72 (m, 10H); 1.45e1.35 (m, 32H); 1.14e
1.10 (t, J ¼ 7.4 Hz, 6H); 0.98e0.89 (m, 30H). 13C NMR (CDCl3,
100 MHz,
d
/ppm): 163.56, 155.65, 154.28, 152.47, 152.39, 147.78,
3. Results and discussion
147.57, 145.30, 143.62, 142.63, 139.83, 136.82, 136,16, 133.85, 132.72,
131.61, 131.35, 130.85, 130.75, 129.97, 129.89, 129.86, 128.86, 127.86,
127.67, 127.60, 124.80, 124.63, 124.57, 123.75, 123.49, 123.42, 123.00,
122.70, 120.52, 116.20, 110.02, 96.93, 78.46, 69.51, 68.78, 68.52,
65.53, 49.05, 41.14, 38.97, 38.77, 35.20, 31.77, 31.59, 31.46, 31.34,
30.95, 30.86, 30.63, 30.50, 30.32, 30.30, 29.68, 29.27, 29.23, 29.14,
29.06, 28.88, 28.67, 24.10, 23.94, 23.71, 23.34, 22.98, 22.92, 22.66,
22.59, 22.55, 20.82, 20.77, 19.20, 14.32, 14.10, 14.01, 13.68, 11.88,
11.45, 11.07, 10.89. Calcd for C90H103N5O2S6: C, 72.92; H, 7.35; N,
5.22; S, 10.25. Found: C, 72.84; H, 7.48; N, 5.62; S, 10.79. MALDI-TOF
MS (C114H137N7O5S6) m/z: calcd for 1877.742; found 1877.004.
3.1. Synthesis and characterization
The detailed synthetic routes of TPA-DTBT, TPA-DTBT-TH and
TPA-DTBT-CAO are shown in Scheme 1. All of three organic small
molecules were prepared according to several classical reactions.
First, the compound 1 and 3 were obtained from 5-bromo-1,3-
bis(diethyl-phsophonate-methyl)-2-(ethylhexyl)benzene by Wit-
tigeHorner reaction with (4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)phenyl)-N-p-tolylbenzenamine or 3-hexyl-5-(7-(4-hexyl-5-
(5-tetradecylthiophen-2-yl)thiophen-2-yl)benzo[c][1,2,5]-thiadia-
zol-4-yl)thiophene-2-carbaldehyde in THF, respectively. The TPA-
DTBT and TPA-DTBT-TH were synthesized by Suzuki coupling re-
action, which were synthesized from 4-methyl-N-(4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-N-p-tolylbenzen-amine
and compound 1 or 3 with Pd(PPh3)4 as the catalyst in a biphasic
mixture of aqueous K2CO3 and toluene with yields of 55.3% and
45.3%, respectively. Then the compound 2 was obtained from
2.4.5. Synthesis of 3-hexyl-5-(7-(4-hexyl-5-(5-tetradecylthiophen-
2-yl)thiophen-2-yl) benzo[c][1,2,5]thiadiazol-4-yl)thiophene-2-
carbaldehyde (3)
Compound
3 was synthesized according to compound 1,
instead using 5-bromo-1,3-bis(diethyl-phsophonate-methyl)-2-
(ethylhexyl)benzene (0.24 g, 0.4 mmol), 3-hexyl-5-(7-(4-hexyl-5-

468
C. Wang et al. / Dyes and Pigments 98 (2013) 464e470
TPA-DTBT by VilsmeiereHaack reaction with phosphorus oxy-
chloride in DMF and 1,2-dichloroethane. Subsequently, Knoevena-
gel condensation reactions with 2-ethylhexyl cyanoacetate in the
presence of piperidine gave the target molecular TPA-DTBT-CAO in
56.6% yield. The structures of the three organic molecules (TPA-
DTBT, TPA-DTBT-TH and TPA-DTBT-CAO) were verified by 1H NMR,
13C NMR, MALDI-TOF mass spectra and elemental analysis. All of
synthesized organic molecules show good solubility in common
solvent, such as chloroform and DMF at room temperature to satisfy
the requirement on fabrication of photovoltaic devices.
The thermal stability of the three organic molecules (TPA-DTBT,
TPA-DTBT-TH and TPA-DTBT-CAO) has been investigated by ther-
mogravimetric analysis (TGA), which revealed high decomposition
temperatures (Td, 5% weight loss) of 352 ꢀC, 318 ꢀC and 301 ꢀC,
respectively (Fig. 2), indicating that thermal stability is good
enough for long-term photovoltaic devices application.
3.2. Photophysical properties
Fig. 3. UVevis absorption spectra of TPA-DTBT, TPA-DTBT-TH and TPA-DTBT-CAO in
CHCl3 solution.
The photophysical properties of TPA-DTBT, TPA-DTBT-TH and
TPA-DTBT-CAO were investigated by UVevis absorption spectros-
copy in diluted (10ꢁ5 M) chloroform solution, and the spectra and
corresponding data are shown in Fig. 3 and Table 1. In CHCl3 so-
lution, TPA-DTBT gives three distinct absorption bands (318 nm,
370 nm, 504 nm) covering the wavelength range from 300 to
600 nm. The former two absorption bands in the near-UV region
which are ascribed to the strong electron-withdrawing effect of
CAO moiety [28,31].
The adsorption spectra of three organic molecules on thin film
were measured and showed in Fig. 4. The corresponding data are
also summarized in Table 1. Compared with the solution absorption
spectra, it can be seen that the maximum absorption of three
organic molecules are obviously broader and red-shifted, which is
correspond to the
p-p* electron transitions of the conjugated
molecule, whereas the visible absorption band could be assigned to
the intramolecular charge transfer (ICT) transition between the TPA
moiety and DTBT unit [24,28]. The TPA-DTBT-TH and TPA-DTBT-
CAO exhibit two distinct absorption bands: one absorption band is
due to the strong p-p packing between the molecule backbones at
solid state [12,32,33]. The optical band gaps ðEgoptÞ of TPA-DTBT,
TPA-DTBT-TH and TPA-DTBT-CAO are 1.93, 1.83, and 1.75 eV,
respectively, calculated from the onset of the film absorptions.
in the UV region (300e430 nm) corresponding to the p-p* electron
transitions of the conjugated molecules; and the other is in the
visible region (480e650 nm) that can be assigned to an intra-
molecular charge transfer (ICT) [29,30]. TPA-DTBT, TPA-DTBT-TH
and TPA-DTBT-CAO exhibit absorption maxima at 370, 371 and
539 nm with extinction coefficients of 4.81 ꢂ 104, 1.04 ꢂ 105 and
8.45 ꢂ 104 Mꢁ1 cmꢁ1 in chloroform solution (Table 1), respectively.
As expected, both the absorption spectra of TPA-DTBT-TH and TPA-
DTBT-CAO are broadened and red-shifted compared to TPA-DTBT
3.3. Electrochemical properties
To evaluate the highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) energy levels of the
TPA-DTBT, TPA-DTBT-TH and TPA-DTBT-CAO, cyclic voltammetry
(CV) measurement (Fig. 5) was performed in anhydrous CH3CN
with 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6)
as a supporting electrolyte at scan rate of 50 mV sꢁ1. The redox
potential of ferrocene is 0.57 V vs SCE. On the basis of 4.8 eV below
vacuum for the energy level of Fc/Fcþ, the HOMO and LUMO energy
levels of as well as the electrochemical energy gaps ðEgecÞ of three
organic molecules were calculated according to the following
equations:
due to the expansion of
of 2-tetradecyl thiophene (TH) and 2-ethylhexyl cyanoacetate
(CAO) groups in the -conjugated molecules. In comparison with
p-conjugated systems by the introduction
p
TPA-DTBT, the maximum absorption peaks of ICT of TPA-DTBT-TH
and TPA-DTBT-CAO are red shift 18 nm and 35 nm, respectively,
EHOMO ¼ ꢁeðEOX þ 4:23ÞðeVÞ
ELUMO ¼ ꢁeðEred þ 4:23ÞðeVÞ
ec
Eg ¼ eðEOX ꢁ EredÞðeVÞ
Table 1
Photophysics properties of three organic molecules.
opt
Compounds
Film
Solution
Film
3
Eg
ledge (nm)b ls,max (nm)a lf,max (nm)b (Mꢁ1 cmꢁ1
)
(eV)d
c
TPA-DTBT
TPA-DTBT-TH
TPA-DTBT-CAO 708
643
677
367
371
539
389
377
548
4.81 ꢂ 104
1.04 ꢂ 105
8.45 ꢂ 104
1.93
1.83
1.75
a
Measured in chloroform solution.
Measured on quarte plate by cast from chloroform solution.
Extinction coefficient at lmax in chloroform solution.
Band gap estimated from the edge wavelength of optical film absorption,
b
c
d
opt
Fig. 2. TGA curves of TPA-DTBT, TPA-DTBT-TH and TPA-DTBT-CAO.
Eg
¼ 1240=ledge
.

C. Wang et al. / Dyes and Pigments 98 (2013) 464e470
469
Table 2
Electrochemical properties of three organic molecules.
Eox (V)a Ered (V)a EHOMO (eV)b ELUMO (eV)b Eg (eV)c
ec
Compounds
TPA-DTBT
TPA-DTBT-TH
TPA-DTBT-CAO 1.14
1.20
1.10
ꢁ0.66
ꢁ0.61
ꢁ0.60
ꢁ5.43
ꢁ5.33
ꢁ5.37
ꢁ3.57
ꢁ3.62
ꢁ3.63
1.86
1.71
1.74
a
Potential measured by cyclic voltammetry in 0.1 M Bu4NPF6/CH3CN.
b
c
EHOMO/LUMO ¼ ꢁe(4.23 þ Eox/red) (eV).
ec
Eg ¼ ꢁðEHOMO ꢁ ELUMOÞðeVÞ.
Al(100 nm). We found that no further improvement can be
observed by using 1,8-diiodooctane (DIO) as the additive. There
may be two reasons, firstly, the triphenylamine moiety is 3D star-
burst molecular structure and will not positively contribute to the
piepi interaction between neighboring molecules. Secondly, there
are no strong aggregations occurred because the thiophene units of
all three organic molecules contain long hexyl groups, especially,
the introduction of CAO and TH containing long alkyl as terminal
groups, it will not have a positive effect. The JeV curves for the
corresponding three organic molecules are presented in Fig. 6, and
the device parameters of the respective photovoltaic data of the
devices have been collected in Table 3. Under the same condition,
SM BHJ solar cells based on TPA-DTBT gave a short circuit photo-
Fig. 4. UVevis absorption spectra of TPA-DTBT, TPA-DTBT-TH and TPA-DTBT-CAO
on film.
The electrochemical data of the compounds are summarized in
Table 2. All three organic molecules exhibit lower-lying HOMO
energy levels than ꢁ5.2 eV, indicating that good air stability in
ambient conditions [34]. It can be seen that the value of HOMO of
TPA-DTBT is sufficiently more negative, which predicts its higher
Voc based on the molecules as donor materials in solar cell devices
[35,36]. In addition, we also estimated the LUMO energy levels of
(6,6)-phenyl-C61-butyric acid methyl ester (PC61BM) by using the
same condition. Then the LUMO energy level is estimated to
be ꢁ4.02 eV for PC61BM. By comparison, the LUMO energy levels of
TPA-DTBT (ꢁ3.57 eV), TPA-DTBT-TH (ꢁ3.62 eV) and TPA-DTBT-
CAO (ꢁ3.63 eV) are higher above 0.3 eV than PC61BM, which in-
dicates the charge transfer from the small molecules to PC61BM
would be allowed.
current density (Jsc) of 5.66 mA/cmꢁ2, an open circuit voltage (Voc
of 0.92 V, and a fill factor (FF) of 0.35, corresponding to the most
promising power conversion efficiency ( ) of 1.85%. The corre-
)
h
sponding device parameters based on TPA-DTBT-TH with electron-
donating terminal group of TH and TPA-DTBT-CAO with electron-
withdraw terminal group of CAO show value of 0.83%
(Jsc
¼
4.3 mA/cmꢁ2
,
Voc
¼
0.76 V, FF
¼
0.25) and 1.02%
(Jsc ¼ 4.71 mA/cmꢁ2, Voc ¼ 0.84 V, FF ¼ 0.26), respectively.
Compared to the two others, TPA-DTBT shows the highest Voc of
0.92 eV, because of the relatively lowest HOMO energy levels. Also,
the best
h of the TPA-DTBT can be explained by the highest Jsc and
FF. TPA-DTBT-CAO with electron-withdraw terminal group exhibits
the higher
and Jsc.
h than TPA-DTBT-TH, due to the relatively high Voc
3.4. Photovoltaic properties
To understand the influence of charge carrier mobility in the
blend films on the Jsc and FF, the hole mobility of the blend target
small moleculars is measured by using the spacecharge limit cur-
rent (SCLC) method, which is extensively used to investigate the
charge transport properties of the OSCs active layer. Hole mobilities
values of 1.8 ꢂ 10ꢁ2, 1.5 ꢂ 10ꢁ5 and 2.3 ꢂ 10ꢁ4 cmꢁ2 Vꢁ1 mꢁ1 are
observed for TPA-DTBT, TPA-DTBT-TH and TPA-DTBT-CAO,
respectively (Table 3). It should be pointed out that the hole
SM BHJ solar cells using the three organic molecules TPA-DTBT,
TPA-DTBT-TH and TPA-DTBT-CAO as the donors and PC61BM as the
acceptor were investigated with the device configuration of ITO/
PEDOT:PSS(30 nm)/Compounds:PC61BM(w80 nm)/LiF(0.5 nm)/
Fig. 5. Cyclic voltammogram of TPA-DTBT, TPA-DTBT-TH and TPA-DTBT-CAO films
(Ferrocene as the reference).
Fig. 6. JeV curves of the SM BHJ solar cells composed of Compounds:PC61BM.

470
C. Wang et al. / Dyes and Pigments 98 (2013) 464e470
Table 3
[12] Zhou J, Wan X, Liu Y, Zuo Y, Li Z, He G, et al. Small molecules based on benzo
[1,2-b:4,5-b0]dithiophene unit for high-performance solution-processed
organic solar cells. J Am Chem Soc 2012;134:16345e51.
Photovoltaic performance parameters of three organic molecules.
Compounds/PC61BM Ratio Jsc
(mA/cm2) (V)
3.43
Voc
FF
h
(%)a Hole mobility
(cm2 Vꢁ1 mꢁ1
[13] Hsu SL, Lin Y, Lee R, Sivakumar C, Chen J, Chou W. Synthesis and character-
ization of new low bandgap polyfluorene copolymers for bulk heterojunction
solar cells. J Polym Sci Part A Polym Chem 2009;47:5336e43.
[14] Uy RL, Price SC, You W. Structure-property optimizations in donor polymers
via electronics, substituents, and side chains toward high efficiency solar cells.
Macromol Rapid Comm 2012;33:1162e77.
[15] Wang M, Hu X, Liu P, Li W, Gong X, Huang F, et al. Donor-acceptor conjugated
polymer based on naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole for high-
performance polymer solar cells. J Am Chem Soc 2011;133:9638e41.
[16] Chang H, Tsai C, Lai Y, Chiou D, Hsu S, Hsu C, et al. Synthesis, molecular and
photovoltaic properties of donor-acceptor conjugated polymers incorporating
a new heptacylic indacenodithieno[3,2-b]thiophene arene. Macromolecules
2012;45:9282e91.
b
)
TPA-DTBT
1:1
1.00 0.26 0.88
0.92 0.35 1.85
1.02 0.26 1.01
0.76 0.25 0.83
0.84 0.26 1.02
e
1:1.2 5.66
1:1.5 3.80
1:1.2 4.30
1:1.2 4.71
1.8 ꢂ 10ꢁ2
e
TPA-DTBT-TH
TPA-DTBT-CAO
1.5 ꢂ 10ꢁ5
2.3 ꢂ 10ꢁ4
a
Devices structure: ITO/PEDOT:PSS (30 nm)/Compounds:PC61BM (w80 nm)/LiF
(0.5 nm)/Al (100 nm).
b
To measure hole-only devices structure: ITO/PEDOT:PSS (30 nm)/Com-
pounds:PC61BM/MoO3 (10 nm)/Al (100 nm).
[17] Shang H, Fan H, Liu Y, Hu W, Li Y, Zhan X. A solution-processable star-shaped
molecule for high-performance organic solar cells. Adv Mater 2011;23:
1554e7.
[18] Ning Z, Tian H. Triarylamine: a promising core unit for efficient photovoltaic
materials. Chem Commun 2009:5483e95.
mobility of TPA-DTBT is higher 2e3 orders than TPA-DTBT-TH and
TPA-DTBT-CAO, which may be the other reason of the highest
h in
three organic molecules.
[19] Shirota Y. Photo- and electroactive amorphous molecular materials-molecular
design, syntheses, reactions, properties, and applications. J Mater Chem 2005;
15:5483e93.
4. Conclusions
[20] Song Y, Di C, Yang X, Li S, Xu W, Liu Y, et al. A cyclic triphenylamine dimer for
organic field-effect transistors with high performance. J Am Chem Soc 2006;
128:15940e1.
[21] 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;114:3280e6.
In this study, three new organic molecules containing triphe-
nylamine (TPA) and 4,7-bis(thiophen-2-yl)benzo[1,2,5]thiadiazole
(DTBT) moieties, TPA-DTBT, TPA-DTBT-TH and TPA-DTBT-CAO
were successfully designed and synthesized. The organic molecules
(TPA-DTBT-TH and TPA-DTBT-CAO) with terminal groups 2-
tetradecyl thiophene and 2-ethylhexyl cyanoacetate showed
broad and red-shifted absorption spectra. However, The organic
molecule (TPA-DTBT) without terminal group exhibited a relatively
higher hole mobility and best photovoltaic properties.
[22] Shen P, Sang G, Lu J, Zhao B, Wan M, Zou Y, et al. Effect of 3d
pep stacking on
photovoltaic and electroluminescent properties in triphenylamine-containing
poly(p-phenylenevinylene) derivatives. Macromolecules 2008;41:5716e22.
[23] Zeng W, Cao Y, Bai Y, Wang Y, Shi Y, Zhang M, et al. Efficient dye-sensitized
solar cells with an organic photosensitizer featuring orderly conjugated
ethylenedioxythiophene and dithienosilole blocks. Chem Mater 2010;22:
1915e25.
[24] Zhao G, Wu G, He C, Bai F, Xi H, Zhang H, et al. Solution-processable multi-
armed organic molecules containing triphenylamine and dcm moieties:
synthesis and photovoltaic properties. J Phys Chem C 2009;113:2636e42.
[25] Roquet S, Cravino A, Leriche P, Alévêque O, Frère P, Roncali J. Triphenylamine-
thienylenevinylene hybrid systems with internal charge transfer as donor
materials for heterojunction solar cells. J Am Chem Soc 2006;128:3459e66.
[26] He C, He Q, Yang X, Wu G, Yang C, Bai F, et al. Synthesis and photovoltaic
properties of a solution-processable organic molecule containing triphenyl-
amine and dcm moieties. J Phys Chem C 2007;111:8661e6.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (51173154, 51003089), and Open Fund of Key
Laboratory of Advanced Functional Polymeric Materials, College of
Hunan Province (12K049).
[27] Gu Z, Shen P, Tsang S, Tao Y, Zhao B, Tang P, et al. Development of a new
benzo(1,2-b:4,5-b0)dithiophene-based
copolymer
with
conjugated
References
dithienylbenzothiadiazole-vinylene side chains for efficient solar cells. Chem
Commun 2011;47:9381e3.
[28] Shen P, Liu Y, Huang X, Zhao B, Xiang N, Fei J, et al. Efficient triphenylamine
[1] Zhang Y, Zou J, Yip H, Chen K, Zeigler DF, Sun Y, et al. Indacenodithiophene
and quinoxaline-based conjugated polymers for highly efficient polymer solar
cells. Chem Mater 2011;23:2289e91.
[2] Chen J, Cao Y. Development of novel conjugated donor polymers for high-
efficiency bulk-heterojunction photovoltaic devices. Acc Chem Res 2009;42:
1709e18.
dyes for solar cells: effects of alkyl-substituents and
p-conjugated thiophene
unit. Dye Pigm 2009;83:187e97.
[29] Tamayo A, Kent T, Tantitiwat M, Dante MA, Rogers J, Nguyen T. Influence of
alkyl substituents and thermal annealing on the film morphology and per-
formance of solution processed, diketopyrrolopyrrole-based bulk hetero-
junction solar cells. Energy Environ Sci 2009;2:1180e6.
[3] Lin Y, Li Y, Zhan X. Small molecule semiconductors for high-efficiency organic
photovoltaics. Chem Soc Rev 2012;41:4245e72.
[30] Kylberg W, Sonar P, Heier J, Tisserant J, Muller C, Nuesch F, et al. Synthesis
thin-film morphology and comparative study of bulk and bilayer hetero-
junction organic photovoltaic devices using soluble diketopyrrolopyrrole
molecules. Energy Environ Sci 2011;4:3617e24.
[31] Tian Z, Huang M, Zhao B, Huang H, Feng X, Nie Y, et al. Low-cost dyes based on
methylthiophene for high-performance dye-sensitized solar cells. Dye Pigm
2010;87:181e7.
[32] Matsuo Y, Sato Y, Niinomi T, Soga I, Tanaka H, Nakamura E. Columnar struc-
ture in bulk heterojunction in solution-processable three-layered p-i-n
organic photovoltaic devices using tetrabenzoporphyrin precursor and silyl-
methyl[60]fullerene. J Am Chem Soc 2009;131:16048e50.
[33] Zhou J, Wan X, Liu Y, Long G, Wang F, Li Z, et al. A planar small molecule with
dithienosilole core for high efficiency solution-processed organic photovoltaic
cells. Chem Mater 2011;23:4666e8.
[4] He Z, Zhong C, Huang X, Wong W, Wu H, Chen L, et al. Simultaneous
enhancement of open-circuit voltage, short-circuit current density, and fill
factor in polymer solar cells. Adv Mater 2011;23:4636e43.
[5] Kaake LG, Jasieniak JJ, Bakus RC, Welch GC, Moses D, Bazan GC, et al. Photo-
induced charge generation in a molecular bulk heterojunction material. J Am
Chem Soc 2012;134:19828e38.
[6] Zhang F, Wu D, Xu Y, Feng X. Thiophene-based conjugated oligomers for
organic solar cells. J Mater Chem 2011;21:17590e600.
[7] Yang C, Kim JY, Cho S, Lee JK, Heeger AJ, Wudl F. Functionalized meth-
anofullerenes used as n-type materials in bulk-heterojunction polymer solar
cells and in field-effect transistors. J Am Chem Soc 2008;130:6444e50.
[8] Zhang Y, Yip H, Acton O, Hau SK, Huang F, Jen AKY. A simple and effective way
of achieving highly efficient and thermally stable bulk-heterojunction poly-
mer solar cells using amorphous fullerene derivatives as electron acceptor.
Chem Mater 2009;21:2598e600.
[34] Gu Z, Tang P, Zhao B, Luo H, Guo X, Chen H, et al. Synthesis and photovoltaic
properties of copolymers based on benzo[1,2-b:4,5-b0]dithiophene and thio-
phene with different conjugated side groups. Macromolecules 2012;45:
2359e66.
[9] Liu B, Chen X, Zou Y, Xiao L, Xu X, He Y, et al. Benzo[1,2-b:4,5-b0] difuran-
based donor-acceptor copolymers for polymer solar cells. Macromolecules
2012;45:6898e905.
[35] Gu Z, Deng L, Luo H, Guo X, Li H, Cao Z, et al. Synthesis and photovoltaic
properties of conjugated side chains polymers with different electron-
withdrawing and donating end groups. J Polym Sci Part A Polym Chem
2012;113:3848e58.
[36] Chen H, Hou J, Zhang S, Liang Y, Yang G, Yang Y, et al. Polymer solar cells with
enhanced open-circuit voltage and efficiency. Nat Photon 2009;3:649e53.
[10] Lin L, Chen Y, Huang Z, Lin H, Chou S, Lin F, et al. A low-energy-gap organic
dye for high-performance small-molecule organic solar cells. J Am Chem Soc
2011;133:15822e5.
[11] Small CE, Chen S, Subbiah J, Amb CM, Tsang S, Lai T, et al. High-efficiency
inverted dithienogermole-thienopyrrolodione-based polymer solar cells. Nat
Photon 2012;6:115e20.