Acceptor-donor-acceptor small molecules containing benzo [1,2-b:4,5-b′]dithiophene and rhodanine units for solution processed organic solar cells

Article abstract of DOI:10.1016/j.dyepig.2015.01.006

Two novel solution-processed acceptor-donor-acceptor (A-D-A) structured organic molecules with 5,8-disubstituted benzo[1,2-b:4,5-b′]dithiophene (BDT or BDTT) as central and donor units, 2-(1,1-dicyanomethylene)rhodanine (DCRD) as acceptor unit and terminal group, and trithiophene (T₃) as π-bridges, D(T₃-DCRD)-BDT and D(T₃-DCRD)-BDTT, are designed and synthesized for the application as donor materials in organic solar cells (OSCs). Both compounds exhibit broad absorption covering the wavelength range 300-750 nm and the relatively lower HOMO energy levels from -5.39 to -5.46 eV. D(T₃-DCRD)-BDTT demonstrates strong absorbance and higher hole mobility than that of D(T₃-DCRD)-BDT. The power conversion efficiency (PCE) values of the OSCs based on the compounds/PC₆₁BM (1:1, w/w) are 1.10% for D(T₃-DCRD)-BDT and 1.94% for D(T₃-DCRD)-BDTT, under the illumination of AM. 1.5G, 100 mW/cm^-2. Furthermore, the high open-circuit voltage (V_oc) values are 0.93 V for D(T₃-DCRD)-BDT and 0.96 V for D(T₃-DCRD)-BDTT, respectively. It indicates that incorporating DCRD as an acceptor unit into D-A-D-type small molecule has proven to be promising candidates for high efficiency solution processed OSCs.

Full text of DOI:10.1016/j.dyepig.2015.01.006

Dyes and Pigments 116 (2015) 13e19
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Acceptor-donor-acceptor small molecules containing benzo
[1,2-b:4,5-b']dithiophene and rhodanine units for solution processed
organic solar cells
a
a
a
a
,
*
a
a
Qunping Fan , Min Li , Pingao yang , Yu Liu , Manjun Xiao , Xiangdong Wang ,
Hua Tan , Yafei Wang , Renqiang Yang , Weiguo Zhu a
College of Chemistry, Xiangtan University, Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education, Xiangtan 411105, China
Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China
a
a
b,
*
, *
a
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 November 2014
Received in revised form
Two novel solution-processed acceptor-donor-acceptor (A-D-A) structured organic molecules with 5,8-
disubstituted benzo[1,2-b:4,5-b']dithiophene (BDT or BDTT) as central and donor units, 2-(1,1-
dicyanomethylene)rhodanine (DCRD) as acceptor unit and terminal group, and trithiophene (T
3
) as p-
25 December 2014
bridges, D(T -DCRD)-BDT and D(T -DCRD)-BDTT, are designed and synthesized for the application as
3
3
Accepted 7 January 2015
Available online 14 January 2015
donor materials in organic solar cells (OSCs). Both compounds exhibit broad absorption covering the
wavelength range 300e750 nm and the relatively lower HOMO energy levels from ꢀ5.39 to ꢀ5.46 eV.
3 3
D(T -DCRD)-BDTT demonstrates strong absorbance and higher hole mobility than that of D(T -DCRD)-
Keywords:
BDT. The power conversion efficiency (PCE) values of the OSCs based on the compounds/PC61BM (1:1, w/
w) are 1.10% for D(T -DCRD)-BDT and 1.94% for D(T -DCRD)-BDTT, under the illumination of AM. 1.5G,
100 mW/cm . Furthermore, the high open-circuit voltage (Voc) values are 0.93 V for D(T -DCRD)-BDT
and 0.96 V for D(T -DCRD)-BDTT, respectively. It indicates that incorporating DCRD as an acceptor unit
Organic solar cells
Benzo[1,2-b:4,5-b']dithiophene
Rhodanin
Acceptor-donor-acceptor
Bulk-heterojunction
Small molecule
3
3
ꢀ2
3
3
into D-A-D-type small molecule has proven to be promising candidates for high efficiency solution
processed OSCs.
©
2015 Elsevier Ltd. All rights reserved.
1
. Introduction
allow for a clear understanding of structure-property relationships.
More recently, tremendous efforts have been dedicated to devel-
oping the small molecule organic solar cells (SM-OSCs) [12e16]. So
far, the highest PCE of 9.95% have been achieved for SM-OSCs [17],
which is comparable to those of the best PSCs. Obviously, in view of
the advantages of small molecules [8e11], it is believed that higher
performance could be obtained for SM-OSCs.
Over the past few years, for improving photovoltaic perfor-
mance of the BHJ SM-OSCs, a variety of small molecule donor
materials based on D-A combinations have been reported [18e23],
Solution-processed organic solar cells (OSCs), as a promising
alternative to traditional silicon solar cells, have attracted tremen-
dous interest and attention over the past two decades due to their
advantages of low cost, light weight, and flexibility in large-area
applications [1e4]. Recently, significant progress has been ach-
ieved for polymer solar cells (PSCs) with the most promising bulk-
heterojunction (BHJ) architecture and the power conversion effi-
ciency (PCE) above 10% have been achieved for PSCs [5e7]. Mean-
while, solution-processed small molecule donor materials based
OSCs have also stimulated great attention, due to their advantages
of well-defined chemical structure, monodisperse molecular
weight, and convenient purification methods [8e11]. These assets
such as D-A-D-type [18,19], A-D-A-type [20,21], D-p-A-type [22]
and star-shaped molecules architectures [23]. Besides, some
planar linear A-D-A-type small molecules grafting alkoxy-
substituted benzo[1,2-b:4,5-b']dithiophene (BDT) or dithienyl-
substituted benzo[1,2-b:4,5-b']dithiophene (BDTT) as the donor
(D) cores, and various acceptor unit as terminal groups (such as
alkyl cyanoacetate, 3-ethylrhodanine, thiobarbituric acid, inden-
edione), showed broad absorption, low HOMO energy level, high
hole mobility and good photovoltaic performance [17,24e27],
*
Corresponding authors. Tel.: þ86 731 58293377; fax: þ86 731 58292251.
E-mail addresses: liuyu03b@126.com (Y. Liu), yangrq@qibebt.ac.cn (R. Yang),
zhuwg18@126.com (W. Zhu).
http://dx.doi.org/10.1016/j.dyepig.2015.01.006
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

14
Q. Fan et al. / Dyes and Pigments 116 (2015) 13e19
which are benefited from the large and rigid planar conjugated BDT
unit [28] and the effect of two-dimensional (2D) conjugated BDTT
unit [29,30]. As we known, these A-D-A-type small molecules not
molecules, i.e., D(T
alkoxy- (or dithienyl-) substituted benzo[1,2-b:4,5-b']dithiophene
(BDT or BDTT) as the donor (D) core, terthiophene (T ) as -bridge,
3 3
-DCRD)-BDT and D(T -DCRD)-BDTT, in which
3
p
only can exhibit strong intermolecular
enhance their -conjugation degree and hole mobility, which is
beneficial for higher PCEs for the SM-OSCs.
Recently, novel electron accepter unit of 2-(1,1-
p
-
p
interaction, but also
and 2-(1,1-dicyanomethylene)rhodanine (DCRD) as terminal group
and acceptor (A) unit. BDT and BDTT was selected as the D group
core because of their relatively large and planar conjugated struc-
p
a
ture and could enhance the p-conjugation degree and increase hole
dicyanomethylene)rhodanine (DCRD) was reported [31], and
shown wide absorption spectra and strong electron affinities due to
its possess double electron affinity groups of rhodanine and
dicyanomethylene. Obviously, this peculiar double electron
acceptor groups not only can guarantee strong electron-accepting
ability, but also maintain excellent electron injection efficiency.
However, as a strong electron-deficient group, DCRD has been re-
ported only one on the DCRD-based small molecules donor mate-
rials for application in dye-sensitized solar cells (DSSCs), and shown
excellent photoelectric properties in DSSCs [32]. But so far, OSCs
that utilize DCRD as acceptor unit in small molecules donor ma-
terials is no investigated.
mobility. DCRD was chosen as the A unit due to its relatively strong
electron-accepting ability and can maintain excellent electron in-
jection efficiency. At the same time, the nitrogen of 3-position in
DCRD was functionalized by alkyl chain octane to improve solubi-
3 3
lity. The synthetic routes of the D(T -DCRD)-BDT and D(T -DCRD)-
BDTT are shown in Scheme 1. As expected, both small molecules
show a strong and broad optical absorption in the wavelength
range from 300 to 750 nm due to the rhodanine based molecules
attribute to its stronger accepter. Solution processed OSCs based on
D(T
1.94% with an open circuit voltage (Voc) of up to 0.96 V, while OSCs
based on D(T -DCRD)-BDT/PC61BM (1:1, w/w) yield a maximum
PCE of 1.10% with a high Voc of 0.93 V, respectively. Furthermore, the
3
-DCRD)-BDTT/PC61BM (1:1, w/w) exhibit a maximum PCE of
3
Inspired by those works, we reported the synthesis and photo-
voltaic performances of two novel linear A-D-A-type small
ꢀ
4
2
ꢀ1 ꢀ1
s was obtained in the D(T
hole mobility of 6.22 ꢁ 10 cm V
3
-
3 3
Scheme 1. Synthetic routes of D(T -DCRD)-BDT and D(T -DCRD)-BDTT by Stille coupling reaction.

Q. Fan et al. / Dyes and Pigments 116 (2015) 13e19
15
DCRD)-BDTT-based device, which was about twelve times higher
than that in the D(T -DCRD)-BDT-based device. The results indicate
2.4. Synthesis of target molecules
3
that the solution-processible organic molecules with DCRD as
acceptor (A) group are promising A unit for the BHJ OSCs.
2.4.1. Synthesis of D(T
To a mixture of BrT
ethylhexyloxy)- benzo[1,2-b:4,5-b']dithiophene (BDT, 55 mg,
0.07 mmol) and tetrakis(triphenyl-phosphine)palladium
Pd(PPh , 6.0 mg) was added a degassed mixture of toluene
3
-DCRD)-BDT
3
-DCRD (120 mg, 0.16 mmol), 5,8-bis(2-
2
. Experimental section
(
3 4
)
2.1. Materials
(50 mL). The mixture was refluxed for 24 h under nitrogen pro-
tection. After cooled to room temperature, the mixture was poured
All reagents and chemicals were purchased from commercial
into water (50 mL) and extracted with CHCl
3
(3 ꢁ 30 mL). The
sources (Aldrich, Acros, TCI) and used without further purification
unless stated otherwise. Tetrahydrofuran (THF) was distilled over
sodium and benzophenone under an inert nitrogen atmosphere. All
of organic small molecules were prepared according to several
classical reactions. Compound 1 was prepared according to the
literature [33]. Compound 2 was prepared by substituted reaction
combined organic layer was dried over anhydrous magnesium
sulfate and the solvent was removed off by rotary evaporation. The
residue was purified by silica gel column chromatography using
petroleum ether-dichloromethane (PE-DCM) (V/V, 3/2) as eluent to
1
yield a black solid (85 mg, 62%). H NMR (400 MHz, CDCl
3
, TMS),
d
(ppm): 7.96 (s, 2H), 7.45 (s, 2H), 7.28-7.26 (m, 4H), 7.18-7.16 (m,
of thiophene and NBS in mixing solution of chloroform (CHCl
acetic acid with a yield of 57%. Compound 3 was synthesized by
Grignard reaction with anhydrous THF as solvent. Compound 4 and
3
) and
4H), 4.20 (d, J ¼ 4.0 Hz, 8H), 2.89-2.82 (m, 8H), 1.88-1.86 (m, 2H),
1.86-1.74 (m, 16H), 1.58-1.28 (m, 72H), 1.11-1.02 (m, 12H), 0.90-0.88
13
(m, 18H). C NMR (100 MHz, CDCl
141.19, 141.08, 140.59, 138.19, 138.15, 136.09, 135.66, 134.27, 133.88,
32.46, 130.39, 129.12, 128.27, 128.13, 127.82, 126.09, 116.09, 113.50,
113.31, 112.31, 75.96, 55.70, 45.28, 40.82, 31.93, 31.90, 31.71, 29.70,
9.67, 29.52, 29.48, 29.34, 29.31, 29.07, 29.04, 25.99, 23.98, 23.28,
22.70, 22.68, 22.60, 14.08, 14.03, 11.45. Anal. Calcd. For
10: C 68.53, H 7.60, N 4.28, S 16.33. Foumd: C 68.13, H
3
, dppm): 165.86, 165.42, 144.04,
5
were prepared according to the literature [34].
1
2.2. Measurements and characterization
2
All H NMR and 13C NMR spectra were recorded on a Bruker
1
DRX-400 spectrometer using CDCl
3
as solvent at 298 K. Elemental
112 148 6 4
C H N O S
analyses were carried out with a Harrios elemental analysis in-
strument. Thermogravimetric analyses (TGA) were conducted un-
7.70, N 4.12, S 16.54%.
ꢂ
ꢀ1
der a dry nitrogen gas flow at a heating rate of 20 C min on a
PerkineElmer TGA 7. UVeVis absorption spectra were recorded on
a HP-8453 UV visible system. Cyclic voltammetry (CV) was carried
out on a CHI660A electrochemical work station in a three-electrode
cell dipped in a 0.1 M tetrabutyl-ammonium hexafluorophosphate
3
2.4.2. Synthesis of D(T -DCRD)-BDTT
D(T
procedure of compound D(T
was obtained with yield of 68%. H NMR (400 MHz, CDCl
(ppm): 7.96 (s, 2H), 7.62 (s, 2H), 7.33-7.26 (m, 6H), 7.14-7.12 (m,
3
-DCRD)-BDTT was prepared according to the synthetic
3
-DCRD)-BDT. A black solid of 97 mg
1
3
, TMS),
d
(
Bu
4
NPF
6
) acetonitrile solution under nitrogen protection at a scan
4H), 6.95 (d, J ¼ 4.9 Hz, 2H), 4.19 (t, J ¼ 8.0 Hz, 4H), 2.91 (d,
rate of 100 mV/s and room temperature. In this three-electrode cell,
a platinum rod, platinum wire and Ag/AgCl (0.1 M) electrode were
used as a working electrode, counter electrode and reference
electrode, respectively. Surface morphologies were recorded by
atomic force microscopy (AFM) on a Veeco-DI Multimode NS-3D
apparatus in a trapping mode under normal air condition at room
temperature.
J ¼ 4.0 Hz, 4H), 2.83-2.76 (m, 8H), 1.71-1.68 (m, 18H), 1.56-1.28 (m,
13
3
72H), 1.11-0.86 (m, 30H). C NMR (100 MHz, CDCl , dppm): 165.87,
165.44, 146.07, 141.14, 141.09, 140.64, 138.64, 138.26, 138.20, 137.31,
136.72, 135.72, 134.21, 133.78, 130.46, 128.28, 128.18, 127.97, 127.84,
126.07, 125.54, 123.31, 119.15, 113.40, 113.30, 112.31, 55.73, 45.27,
41.56, 31.92, 31.89, 31.70, 29.63, 29.50, 29.47, 29.32, 29.29, 29.06,
29.04, 29.03, 25.99, 23.11, 22.69, 22.67, 22.60, 14.23, 14.08, 14.03,
152 6 2
11.02. Anal. Calcd. For C120H N O S12: C 68.79, H 7.31, N 4.01, S
2.3. Device fabrication and characterization
18.36. Foumd: C 68.48, H 7.41, N 3.94, S 18.63%.
OSCs were fabricated using indium tin oxide (ITO) glass as an
3. Result discussion
anode, Ca/Al as a cathode, and a blend film of the small molecule/
PC61BM as a photosensitive layer. After a 30 nm buffer layer of
poly(3,4-ethylenedioxy-thiophene) and polystyrene sulfonic acid
3.1. Synthesis and thermal property
(
PEDOT:PSS) was spin-coated onto the precleaned ITO substrate,
The synthetic routes of D(T
BDTT were outlined in Scheme 1. The detailed procedures of the
monomer (BrT -DCRD) are shown in the electronic supporting
information (ESI). Both small molecules of D(T -DCRD)-BDT
and D(T -DCRD)-BDTT were synthesized by Stille coupling re-
action in the presence of Pd(PPh with yields of 62% and 68%,
respectively. Their structures are consistent with molecular
3 3
-DCRD)-BDT and D(T -DCRD)-
the photosensitive layer was subsequently prepared by spin-
coating a solution of the small molecule/PC61BM (1:1, w/w) in
3
CHCl
5 mg mL , followed the substrates were dried under nitrogen at
room temperature in a nitrogen-filled glove-box. Ca (10 nm) and Al
100 nm) were successively deposited on the photosensitive layer
in vacuum and used as top electrodes. The current density-voltage
JeV) characteristics of the devices was carried out on a computer-
controlled Keithley 236 source measurement system under simu-
3
on the PEDOT:PSS layer with a typical concentration of
3
ꢀ
1
1
3
3 4
)
(
1
13
formulas characterized by H NMR, C NMR and elemental
analysis (ESI).
(
The thermogravimetric analyses (TGA) curves of D(T
BDT and D(T -DCRD)-BDTT are depicted in Fig. 1, and the corre-
sponding data are summarized in Table 1. The decomposition
3
-DCRD)-
ꢀ
2
lated 100 mW cm (AM 1.5 G) irradiation from a Newport solar
simulator. Light intensity was calibrated with a standard silicon
solar cell. The active area was 0.1 cm for each cell. The thicknesses
3
2
ꢂ ꢂ
d 3
) level of 350 C for D(T -DCRD)-BDT and 396 C for
temperature (T
D(T -DCRD)-BDTT are observed at a 5% weight loss under nitrogen
of the spun-cast films were recorded by a profilometer (Alpha-Step
3
2
00, Tencor Instruments). The external quantum efficiency (EQE)
protection, respectively. The results show that both small mole-
cules here have high thermal stability and good enough for the
was measured by a Stanford Research Systems model SR830 DSP
lock-in amplifier coupled with WDG3 monochromator and a 150 W
xenon lamp.
application in optoelectronic devices. Furthermore, D(T
3
-DCRD)-
BDTT with dithienyl-substituted side chains shows much better

16
Q. Fan et al. / Dyes and Pigments 116 (2015) 13e19
3 3
Fig. 2. Normalized UVeVis absorption spectra of D(T -DCRD)-BDT and D(T - DCRD)-
Fig. 1. TGA curves of D(T
2
3
-DCRD)-BDT and D(T
ꢂ
0 C min under nitrogen atmosphere.
3
-DCRD)-BDTT at a heating rate of
ꢀ1
BDTT in dilute CHCl
3
solution and its neat film at room temperature.
þ
potential of ferrocene/ferrocenium (Fc/Fc ) as calibrated standard.
thermal stability than that of the corresponding alkoxy-substituted
D(T - DCRD)-BDT [27].
3 3
The CV curves of D(T -DCRD)-BDT and D(T -DCRD)-BDTT films
were shown in Fig. 3, and the corresponding data summarized in
Table 1. The HOMO and LUMO energy levels of small molecules
were estimated from the onset of oxidation (Eox) and reduction
3
3.2. Optical properties
(Ered) values according to the following empirical equations [35]:
E
HOMO ¼ - (Eox þ 4.35) eV and ELUMO ¼ - (Ered þ 4.35) eV. As shown
3
The normalized UVeVis absorption spectra of D(T -DCRD)-BDT
and D(T -DCRD)-BDTT in DCM and in their neat films are shown in
3
Fig. 2. Their corresponding absorption data are summarized in
Table 1. In DCM solution, benefited from D-A-D-type molecular
structures, the absorption spectra of these compounds exhibit a
strong and broad absorption band in the wavelength range from
3
in Fig. 3, the onset value of oxidation potentials for D(T -DCRD)-
BDT and D(T
.11 eV, corresponding to the HOMO energy levels of ꢀ5.39 for
D(T -DCRD)-BDTT, respectively.
-DCRD)-BDT and ꢀ5.46 eV for D(T
Meanwhile, the LUMO energy levels of D(T -DCRD)-BDT and D(T
3
-DCRD)-BDTT were determined to be 1.04 eV and
1
3
3
3
3
-
DCRD)-BDTT were calculate to be ꢀ2.84 eV and ꢀ2.83 eV, respec-
tively, from the onset reduction potentials (Ered ¼ ꢀ1.51 eV
and ꢀ1.52 eV). The electrochemical bandgaps of two small mole-
3
00 to 650 nm. The absorption band at approximately 450 nm is
assigned to the * transition and the band at long wavelength
p-p
from 523 nm can be attributed to the strong intramolecular charge
transfer (ICT) in DCM solutions. Furthermore, absorption spectra of
the neat films are significantly broadened and red-shifted by ca.
3
cules are estimated to be 2.55 and 2.63 eV for D(T -DCRD)-BDT and
3
D(T -DCRD)-BDTT, respectively. This discrepancy between the op-
tical and electrochemical band gaps might be induced by the
presence of an energy barrier at the interface between the small
molecule film and the electrode surface [36].
7
0 nm in comparison with their solutions, indicating that ordered
structure and strong stacking effect as well as intermolecular
interaction exist between the molecular backbones in the solid
films. The absorption edges of D(T -DCRD)-BDT and D(T -DCRD)-
BDTT in their neat films are at ca. 766 nm and 770 nm, corre-
p-p
3
3
3.4. Photovoltaic properties
opt
sponding to the optical band gaps (E
g
) of 1.62 and 1.61 eV,
BHJ OSCs were fabricated by solution processed with a general
device structure of ITO/PEDOT:PSS/active layer/Ca/Al to investigate
respectively, as listed in Table 1. Obviously, a broad absorption
range and a low band gap would be in favor of light-harvesting for
these donor materials in OSCs.
3.3. Electrochemical properties
Owing to the energy levels of small molecules could highly
affect their photovoltaic performance, cyclic voltammetry (CV) was
employed to investigate the electrochemical properties of small
molecules by using Ag/AgCl electrode as a reference, and the redox
Table 1
Thermal, optical and electrochemical properties of D(T
DCRD)-BDTT.
3 3
-DCRD)-BDT and D(T -
opt
cv
Small molecules
l
(
abs
l
abs
l
onset
E
g
HOMO LUMO
(eV)
E
g
T
d
nm)a (nm)
b
(nm) (eV) (eV)
b
(eV) ( C)
o
D(T
D(T
3
3
-DCRD)-BDT 446,
23
-DCRD)-BDTT 452,
23
480, 582, 766
618
492, 594 770
1.62 ꢀ5.39 ꢀ2.84 2.55 350
1.61 ꢀ5.46 ꢀ2.83 2.63 396
5
5
a
3
Measured in CHCl .
Measured in the neat film.
b
3 3
Fig. 3. Cyclic voltammetry curves of D(T -DCRD)-BDT and D(T -DCRD)-BDTT.

Q. Fan et al. / Dyes and Pigments 116 (2015) 13e19
17
Fig. 4. JeV curves of the small molecule-based OSCs at optimized conditions under the
2
illumination of AM 1.5 G, 100 mW/cm .
Fig. 5. EQE curves of the optimized small molecule/PC61BM-based devices.
the photovoltaic properties. The device fabrication processes are
described in detail in the Experimental section. The active layer
15% at 436 nm and 22% at 430 nm are exhibited in the D(T -DCRD)-
3
BDT and D(T -DCRD)-BDTT based devices, respectively. According
3
consisted of the donor of D(T
and the acceptor of PC61BM. In this work, optimal fabrication
conditions were obtained from a CHCl solution with a concen-
3
-DCRD)-BDT (or D(T
3
-DCRD)-BDTT)
to the EQE curves and the solar irradiation spectra, the integral
current density values of the OSCs are calculated to be 2.43 and
ꢀ
2
3
3.67 mA cm
3 3
for D(T -DCRD)-BDT and D(T -DCRD)-BDTT,
tration of 15 mg/mL at room temperature. As we known, the per-
formances of OSCs could be strongly affected by the processing
parameters, including the small molecule/PC61BM ratio (w/w) and
thicknesses of the photoactive layer. In order to understand the
influence of ratios between the donor small molecule and acceptor,
as well as thicknesses of the photoactive layer on the photovoltaic
properties, the current density-voltage (JeV) characteristics of the
respectively, consistent with the Jsc values obtained from JeV
measurement.
In order to get some ideas about the influence of the chemical
structures on their charge transporting properties, hole mobilities
(mh) of the hole-only devices were measured by the space-charge-
limited-current (SCLC) method with a device structure of ITO/
PEDOT:PSS/active layer/Au. The SCLC is estimated by the Mott
2
3
D(T
3
-DCRD)-BDTT:PC61BM-based devices at different ratios and
Gurney equation [14,37]: J ¼ 9ε ε
0 r h
m V /8L , in which J is current
thicknesses were shown in Fig. S1, and S2, and their corresponding
data were summarized in Table S1 and Table S2 (ESI). The optimized
ratio of 1:1 and thickness of 85 nm were obtained. At these opti-
mized conditions, Fig. 4 shows the current density-voltage (JeV)
density, L is film thickness of active layer (85 nm),
h
m is hole
mobility, ε is relative dielectric constant of the transport medium,
r
ꢀ
12
ꢀ1
ε is permittivity of free space (8.85 ꢁ 10
0
F m ), V is internal
voltage in the device and V ¼ V ꢀV ꢀV , where V is applied
appl
a
bi
appl
characteristics of these devices using D(T
D(T -DCRD)-BDTT) as donor and PC61BM as acceptor, and their
corresponding data were listed in Table 2.
As a result, a maximal PCE of 1.10% with a Voc of 0.93 V, a Jsc of
3
-DCRD)-BDT (and/or
a bi
voltage, V is voltage drop, and V is the built-in voltage due to the
3
relative work function difference of the two electrodes. As depicted
in Fig. 6, the hole-only mobilities of D(T -DCRD)-BDT and D(T -
3
3
and
ꢀ
5
2
ꢀ1 ꢀ1
DCRD)-BDTT are calculated to be 5.07 ꢁ 10 cm
V
s
ꢀ
2
ꢀ4
2
ꢀ1 ꢀ1
2
.44 mA cm and FF of 49% was obtained in the D(T
PC61BM-based device, while the D(T -DCRD)-BDTT/PC61BM-based
device also exhibited a PCE of 1.94% with a Voc of 0.96 V, a Jsc of
3
-DCRD)-BDT/
6.22 ꢁ 10 cm V
s in the hole-only small molecule/PC61BM-
3
based devices, respectively. It is obvious that D(T -DCRD)-BDTT
3
presents a relatively higher hole mobility than D(T -DCRD)-BDT,
3
ꢀ
2
3
.69 mA cm and FF of 55%. Obviously, compared to the D(T
DCRD)-BDT/PC61BM-based device, all the three parameters (Voc, Jsc
and FF) of D(T -DCRD)-BDTT-based device were enhanced, resulted
a maximum PCE up to 1.94%, which is 1.76 times higher than that of
D(T -DCRD)-BDT based OSCs. This impressive photovoltaic perfor-
mance for D(T -DCRD)-BDTT was probably caused by its much
3
-
3
3
3
broader absorption and higher hole mobility by introducing the
conjugated side chain [17,27].
The external quantum efficiency (EQE) curves of the devices
under the optimized conditions were also investigated. As shown in
Fig. 5, the EQE curves for both small molecules exhibited very broad
wavelength range from 310 nm to 720 nm. The maximum EQE of
Table 2
Photovoltaic performance of OSCs based on the small molecule/PC61BM photoactive
layer and the hole mobilities of the small molecule/PC61BM blend films.
Active layers
Ratio
V
oc
J
sc
FF PCE Hole mobility
2
2
ꢀ1
ꢀ1
(
w/w) (V) (mA/cm ) (%) (%) cm
v
m
49 1.10 5.07 ꢁ 10ꢀ
5
D(T
D(T
3
3
-DCRD)-BDT:PC61BM 1:1
-DCRD)-BDTT:PC61BM 1:1
0.93 2.44
0.96 3.69
55 1.94 6.22 ꢁ 10ꢀ4
3 3
Fig. 6. JeV curves of the optimized hole-only D(T -DCRD)-BDT and D(T -DCRD)-BDTT
devices.

18
Q. Fan et al. / Dyes and Pigments 116 (2015) 13e19
which is also responsible for the higher FF of D(T
3
-DCRD)-BDTT and
morphology would lead to higher exciton dissociation efficiency
the better performance of the devices using this molecule [17]. It
and thus improve device performance.
firmly verifies that introduction of two-dimensional moiety with
p-stacking potential can increase hole mobility due to better mo-
4. Conclusions
lecular packing [29].
The morphology of blend film under the optimized condition
was tested using atomic force microscopy (AFM), and is shown in
3
In summary, two novel small molecules of D(T -DCRD)-BDT and
D(T -DCRD)-BDTT with DCRD as acceptor (A) moiety and end
3
Fig. 7. The root mean square (RMS) roughness of 0.748 nm for D(T
3
-
groups were designed and synthesized. Both small molecules
exhibited broad absorption spectra and low HOMO energy levels,
DCRD)-BDT and 0.967 nm for D(T -DCRD)-BDTT are observed in
3
their blend films, respectively. Obviously, these blend films
3 3
and the photovoltaic cells based on D(T -DCRD)-BDT and D(T -
exhibited good surface morphology. In general, good surface
DCRD)-BDTT presented higher Voc of 0.93e0.96 V. The highest PCE
3 3
Fig. 7. AFM height images of the D(T -DCRD)-BDT/PC61BM (a) and D(T -DCRD)- BDTT/PC61BM (b) blend films under optimal ratios.

Q. Fan et al. / Dyes and Pigments 116 (2015) 13e19
19
of 1.94% was obtained in the optimized D(T
3
-DCRD)-BDTT/PC61BM
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Acknowledgments
The authors thank the Major Program for cultivation of the
National Natural Science Foundation of China (91233112), the Na-
tional Natural Science Foundation of China (51273168, 21172187
and 21202139), the Ministry of Science and Technology of the
People's Republic of China (2010DFA52310), the Innovation Group
and Xiangtan Joint Project of Hunan Natural Science Foundation
[20] Loser S, Bruns CJ, Miyauchi H, Ortiz RP, Facchetti A, Stupp SI, et al.
A naphthodithiophene-diketopyrrolopyrrole donor molecule for efficient so-
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(
12JJ7002 and 12JJ8001), the key project of Hunan Provincial Edu-
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Foundation for Innovation in Hunan Province (CX2012B249,
CX2013B268). Qunping Fan and Min Li contributed equally to this
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
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1