Improving photovoltaic properties of the linear A-Ar-A type small molecules with rhodanine by extending arylene core

Article abstract of DOI:10.1016/j.dyepig.2016.12.015

In order to efficiently tune photovoltaic performance, a series of linear A-Ar-A type small molecules (SMs) of (DRCN3T)₂Ar were designed and synthesized, which contain the same terminal of 2-(1,1-dicyanomethylene) rhodanine (DRCN) and π-bridged space of 5-vinyl-trithiophene (3T), but different central arylene (Ar) unit, respectively. Significantly extending film absorption and increasing hole mobility were obtained in these SMs with enlarging Ar units from phenylene (Ph), naphthylene (Nap) to anthrylene (Ant). As a result, photovoltaic properties were remarkably improved in these SM/PC₇₁BM based solution-processing organic solar cells (OSCs) by enlarging Ar units in (DRCN3T)₂Ar. The highest power conversion efficiency of 5.15% with a short-circuit current density of 11.34 mA cm^?2 was obtained in the (DRCN3T)₂Ant based device, which is three times of that in the (DRCN3T)₂Ph-based device. Our work further indicates that properly extending Ar core could be beneficial to improve photovoltaic properties for the A-Ar-A type SMs.

Full text of DOI:10.1016/j.dyepig.2016.12.015

Dyes and Pigments 139 (2017) 42e49
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Improving photovoltaic properties of the linear A-Ar-A type small
molecules with rhodanine by extending arylene core
a
a
,
c
a
a
a
a
Bin Liu , Linrui Duan , Jianhua Chen , Xiongwei Duan , Ting Lei , Yufeng Cai ,
a
a
,
**
, Renqiang Yang c , Weiguo Zhu a
,
***
, b, *
Qiong Wang , Hua Tan
College of Chemistry, Xiangtan University, Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education, Xiangtan 411105, PR
a
China
b
School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Changzhou University,
Changzhou 213164, PR China
c
Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 October 2016
Received in revised form
In order to efficiently tune photovoltaic performance, a series of linear A-Ar-A type small molecules
(
SMs) of (DRCN3T)
2
Ar were designed and synthesized, which contain the same terminal of 2-(1,1-
-bridged space of 5-vinyl-trithiophene (3T), but different
dicyanomethylene) rhodanine (DRCN) and
p
8
December 2016
central arylene (Ar) unit, respectively. Significantly extending film absorption and increasing hole
mobility were obtained in these SMs with enlarging Ar units from phenylene (Ph), naphthylene (Nap) to
anthrylene (Ant). As a result, photovoltaic properties were remarkably improved in these SM/PC71BM
Accepted 8 December 2016
Available online 11 December 2016
2
based solution-processing organic solar cells (OSCs) by enlarging Ar units in (DRCN3T) Ar. The highest
Keywords:
A-Ar-A type small molecules
Arylene
ꢀ
2
power conversion efficiency of 5.15% with a short-circuit current density of 11.34 mA cm was obtained
in the (DRCN3T) Ant based device, which is three times of that in the (DRCN3T) Ph-based device. Our
2
2
Dicyanomethylene-rhodanine
Photovoltaic property
Organic solar cells
work further indicates that properly extending Ar core could be beneficial to improve photovoltaic
properties for the A-Ar-A type SMs.
© 2016 Elsevier Ltd. All rights reserved.
1. Introduction
comparable photovoltaic performance in contrast to polymeric
photovoltaic materials. The development of new photovoltaic SMs
Organic solar cells (OSCs) with bulk heterojunction (BHJ) ar-
chitectures have been considered as a promising solar energy
conversion technology because of their some advantages of solu-
tion processablility, lightweight, low-cost and flexibility in large-
area applications [1e9]. Their photovoltaic performances have
been rapidly improved by development of novel photovoltaic ma-
terials and optimization of device processing technology in the past
few years. Much progress with a power conversion efficiency (PCE)
about 10% was achieved in the single-junction polymer-based OSCs
is still needed for high-performance SM-OSCs [13e16].
Generally, spectral response, absorption intensity, molecular
orbital energy levels, charge mobility and film morphology have an
important influence on photovoltaic performance of donor mate-
rials, which are mostly dominated by molecular structure [16e21].
To optimize these properties, alkyls in side chains, and the bridge
length, central building blocks, end groups in skeleton, as well as
the linkage positions of functional groups were tuned in the
photovoltaic SMs [6,22e28]. On the other hand, a class of A-p-D-p-
(
PSCs) and small/oligomer molecule-based OSCs (SM/OM-OSCs),
A type SMs was constructed to realize the above goal, which con-
tains a central electron-donating (D) unit, two terminal electron-
respectively [10e13]. However, only a few SMs exhibited a
accepting (A) units and two
several advantages for this kind of SMs applied in SM-OSCs, such as
i) high mobility with planar structure and efficient in-
teractions, (ii) a low bandgap, which is beneficial for intramolecular
charge transfer, (iii) good film quality due to a long conjugated
backbone and dispersed alkyl chains similar to polymers [29e31].
p-conjugated bridges. There are
(
p-p
*
Corresponding author. College of Chemistry, Xiangtan University, Key Lab of
Environment-Friendly Chemistry and Application in Ministry of Education, Xiang-
tan 411105, PR China.
*
*
* Corresponding author.
** Corresponding author.
As a result, these A-p-D-p-A type SMs exhibited a significantly
E-mail addresses: yangrq@qibebt.ac.cn (R. Yang), zhuwg18@126.com (W. Zhu).
http://dx.doi.org/10.1016/j.dyepig.2016.12.015
143-7208/© 2016 Elsevier Ltd. All rights reserved.
0

B. Liu et al. / Dyes and Pigments 139 (2017) 42e49
43
increasing PCE of 10.08% in SM-OSCs [13].
82 2 6
0.90e0.86 (m, 12H). MS (MALDI-TOF) m/z: calcd for C64H O S
þ
[M] , 1710.68; found, 1710.66.
In recent years, rhodanine was reported as a promising type of
acceptor terminal unit in photovoltaic SMs owing to its strong
electron-withdrawing property and effectively inducing intra-
molecular charge transfer [31e35]. By modifying rhodanine, tuning
the D and p-bridge blocks, the A-p-D-p-A type SMs have showed a
high PCE of 9%e10% [13,36]. While the arylene (Ar) hydrocarbon
replaces those conjugated electron-rich D unit with heteroatom, it
was further found that the A-p-Ar-p-A type SMs with a weak
electron-donating Ar unit (phenylene or naphthylene) exhibited a
deeper HOMO energy level than those analogues with stronger
electron-donating D units (thiophene or thieno-thiophene), which
presented higher Voc value [37,38]. Our group recently obtained a
2
.2.2. Synthesis of Nap(3TCHO)
Nap(3TCHO) was prepared according to the synthetic proced-
ure of Ph(3TCHO) . A red solid of 200 mg was obtained with a yield
of 68.0%. H NMR (400 MHz, CDCl ): 9.84 (s, 2H), 8.01 (s, 2H), 7.85
d, J ¼ 8 Hz, 2H), 7.75 (d, J ¼ 8 Hz, 2H), 7.61 (s, 2H), 7.32 (s, 2H), 7.28
s, 2H), 7.18 (d, J ¼ 4 Hz, 2H), 2.85e2.83 (q, 8H), 1.75e1.68 (m, 8H),
.45e1.38 (m, 8H), 1.35e1.25 (m, 32H), 0.90e0.88 (m, 12H). MS
2
2
2
1
3
d
(
(
1
þ
(MALDI-TOF) m/z: calcd for C68
84 2 6
H O S [M] , 1124.48; found,
1124.679.
similar type SM of DPP
of 5.44% and higher hole mobility of 4.02 ꢁ 10 cm
DPP Ph and DPP Nap [39]. It indicates that the terminal A and
central Ar units play an important role in improving photovoltaic
properties for their resulting SMs.
In order to efficiently tune photovoltaic performance and
further reveal influence of the central Ar and terminal A units on
2
properties, a series of linear A-Ar-A type SMs of (DRCN3T) Ar was
primarily designed and synthesized. An Ar unit and 2-(1,1-
dicyanomethylene) rhodanine (DRCN) were respectively used as
central core and terminal acceptor, 5-vinyl-trithiophene (3T) was
employed as space and insetted between Ar and A units in these
SMs. The optophysical, electrochemical and photovoltaic properties
were systematically investigated. Significant effect of the central Ar
unit on these opto-electronic properties was observed in these SMs
2
An(2,6), which exhibited an increasing PCE
2
.2.3. Synthesis of Ant(3TCHO)
Ant(3TCHO) was prepared according to the synthetic proced-
ure of Ph(3TCHO) . A red solid of 200 mg was obtained with yield of
0.0%. H NMR (400 MHz, CDCl ): 9.84 (s, 2H), 8.38 (s, 2H), 8.17 (s,
2
ꢀ
4
ꢀ2 ꢀ1 ꢀ1
v
s
than
2
2
2
2
1
7
2
2
3
d
H), 8.00 (d, J ¼ 8 Hz, 2H), 7.74 (d, J ¼ 8 Hz, 2H), 7.61 (s, 2H), 7.36 (s,
H), 7.28 (d, J ¼ 4 Hz, 2H), 7.19 (d, J ¼ 4 Hz, 1H), 2.86e2.84 (q, 8H),
1.76e1.70 (m, 8H), 1.48e1.42 (m, 8H), 1.35e1.25 (m, 32H),
0
.89e0.87 (m, 12H). MS (MALDI-TOF) m/z: calcd for C72
H
86
O
2
S
6
þ
[M] , 1174.5; found, 1174.884.
2
.2.4. Synthesis of (DRCN3T)
2
Ph
(
3TCHO) Ph (177 mg,
2
0.165
mmol)
and
2-(1,1-
dicyanomethylene) rhodanine (318 mg, 1.65 mmol) was dissolved
in a solution of dry chloroform (20 mL), then five drops of trie-
thylamine were added. The mixture was stirred overnight under
argon atmosphere at room temperature. The solvent was then
removed off with a rotating evaporator. The residue was dissolved
in 8 mL of chloroform and precipitated from methanol. The pre-
cipitate was filtered off and purified by silica gel chromatography
using a mixture of PE and chloroform (1:1) as eluent to produce
black solid (180 mg, 77.0%). H NMR (400 MHz, CDCl
2
of (DRCN3T)
improved in these SMs/PC71BM-based solution-processing OSCs by
enlarging Ar units in (DRCN3T) Ar. The best photovoltaic properties
with a PCE of 5.15% and a short-circuit current density of
2
Ar. The photovoltaic properties were remarkably
2
ꢀ2
1
1.34 mA cm were obtained in the (DRCN3T) Ant based OSCs.
2
1
3
):
d
8.00 (s,
2
. Experimental section
H), 7.61 (s, 4H), 7.32 (s, 2H), 7.31 (s, 2H), 7.22 (s, 2H), 7.18 (d,
J ¼ 4 Hz, 2H), 4.35e4.30 (q, 4H), 2.88e2.81 (q, 8H), 1.73e1.69 (m,
2
.1. Materials
8H), 1.44e1.42 (m, 14H), 1.33e1.25 (s, 32H), 0.91e0.88 (s, 12H). MS
þ
(MALDI-TOF) m/z: calcd for C80
H
92
N
6
O
2
S
8
, [M] , 1424.50; found,
All starting materials, unless otherwise indicated, were pur-
1424.658. Elemental analysis for C80
H
92
N
6
O
2
S
8
: calcd. C, 67.37; H,
chased from commercial suppliers and used without further puri-
fication. Compounds 1, 2, 3, 4 and 5 were prepared according to the
reported methods [40e42]. Three new photovoltaic SMs of
6.50; N, 5.89; S, 17.99; found C, 67.10; H, 6.32; N, 5.73; S, 18.12.
1
2
2.2.5. Synthesis of (DRCN3T) Nap
(
DRCN3T)
2
Ar were characterized by MS, H NMR, and elemental
(DRCN3T)
2
Nap was prepared according to the synthetic pro-
Ph. A black solid of 120 mg was obtained with
a yield of 80.0%. H NMR (400 MHz, CDCl ):
8.01 (d, J ¼ 8 Hz, 4H),
.86 (d, J ¼ 8 Hz, 2H), 7.76 (d, J ¼ 8 Hz, 2H), 7.33 (s, 4H), 7.31 (s, 2H),
analysis, which are consistent with their molecular structures.
cedure of (DRCN3T)
2
1
d
2
2
.2. Synthesis
3
7
7
.20 (d, J ¼ 4 Hz, 2H), 4.35e4.30 (q, 4H), 2.89e2.84(q, 8H),1.80e1.70
.2.1. Synthesis of Ph(3TCHO)
2
(m, 8H), 1.46e1.40 (m, 14H), 1.34e1.25 (m, 32H), 0.92e0.88 (m,
A solution of 1,4-benzenediboronic acid bis(pinacol) ester
þ
0
0
00
0
0
00
12H). MS (MALDI-TOF) m/z: calcd for C84H94N O S [M] , 1474.52;
(
78 mg, 0.235 mmol) and 5 -bromo-3,3 -dioctyl-[2,2 :5 ,2 -ter-
6
2 8
found, 1474.890. Elemental analysis for C84
94
H N
6
O
2
8
S : calcd. C,
thiophene]-5-carbaldehyde (300 mg, 0.517 mmol) in toluene
6
8.34; H, 6.42; N, 5.69; S, 17.38; found C, 68.10; H, 6.35; N, 5.73; S,
(
(
8 mL) and 1 M aqueous sodium carbonate (Na
2 mL) was degassed twice with argon. Then tetrakis(triphenyl-
, 10 mg, 0.026 mmol) and Aliquat
36 (0.05 mL) were added and the resulting mixture was stirred at
2 3
CO ) solution
17.15.
3 4
phosphine)palladium (Pd(PPh )
3
8
2.2.6. Synthesis of (DRCN3T)
(DRCN3T) Ant was prepared according to the synthetic pro-
cedure of (DRCN3T) Ph. A black solid of 180 mg was obtained with
a yield of 82.0%. H NMR (400 MHz, CDCl ): 8.34 (s, 2H), 8.14 (s,
2
Ant
ꢂ
0
C for 24 h under argon atmosphere. Cooled down to room
2
temperature, the mixture was poured into water (60 mL), and
extracted with chloroform (CHCl
, 3 ꢁ 10 mL). The organic layer
was dried over anhydrous sodium sulphate (Na SO ). The solvent
was removed off by a rotating evaporator and the residue was
purified by silica gel chromatography using a mixture of petroleum
2
1
3
3
d
2
4
2H), 7.98 (d, J ¼ 12 Hz,4H), 7.72 (d, J ¼ 8 Hz, 2H), 7.35 (s, 2H), 7.31 (d,
J ¼ 2 Hz, 2H), 7.28 (s, 2H), 7.20 (d, J ¼ 2 Hz, 2H), 4.31e4.27 (q, 4H),
2.88e2.83 (q, 8H), 1.78e1.69 (m, 8H), 1.45e1.35 (m, 14H), 1.35e1.25
ether (PE) and dichloromethane (DCM) (2:1) as eluent to provide
(m, 32H), 0.91e0.87 (m, 12H). MS (MALDI-TOF) m/z: calcd for
1
þ
red solid (151 mg, 60.0%). H NMR (400 MHz, CDCl
3
):
d
9.84 (s, 2H),
C
88
H
96
N
6
O
2
S
8
[M] , 1524.54; found, 1524.834. Elemental analysis
7
.61 (s, 4H), 7.26 (s, 4H), 7.22 (s, 2H), 7.16 (d, J ¼ 4Hz, 2H), 2.87e2.76
for C88
H
96
N
6
O
2
S
8
: calcd. C, 69.25; H, 6.34; N, 5.51; S, 16.81; found C,
(
q, 8H), 1.74e1.66 (m, 8H), 1.47e1.40 (m, 8H), 1.34e1.26 (m, 32H),
69.08; H, 6.12; N, 5.23; S, 16.40.

4
4
B. Liu et al. / Dyes and Pigments 139 (2017) 42e49
2
.3. Measurements and characterization
(PEDOT:PSS) was spin-coated onto the precleaned ITO substrate,
the photosensitive layer was subsequently prepared by spin-
coating a solution of the SM/PC71BM (1:3, w/w) in CHCl on the
1
All H NMR spectra were recorded on a Bruker DRX-400 spec-
3
ꢀ1
trometer using CDCl
on a Bruker Daltonics BIFLEX III MALDI-TOF analyzer. Thermogra-
vimetric analyses (TGA) were conducted under a dry nitrogen gas
flow at a heating rate of 10 C min on a PerkineElmer TGA 7. The
differential scanning calorimetry (DSC) was measured on a TA
DSCQ-10 instrument at a heating/cooling rate of 10 C min under
a nitrogen atmosphere. 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 at room temperature, which was dipped in a 0.1 M
3
as solvent at 298 K. Mass spectra were made
PEDOT:PSS layer with a typical concentration of 10 mg mL . The
resulting substrates were dried under nitrogen at room tempera-
ture 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 (J-V) charac-
teristics of the devices were carried out on a computer controlled
Keithley 236 source measurement system under simulated
ꢂ
ꢀ1
ꢂ
ꢀ1
ꢀ
2
100 mW cm (AM 1.5 G) irradiation from a Newport solar simu-
lator. Light intensity was calibrated with a standard silicon solar
ꢀ
2
cell. The active area was 0.1 cm each cell. The thicknesses of the
spun-cast films were recorded by a profilometer (Alpha-Step 200,
Tencor Instruments). The external quantum efficiency (EQE) was
measured with a Stanford research systems model SR830 DSP lock-
in amplifier coupled with WDG3 monochromator and a 150 W
xenon lamp.
4 6
tetrabutyl-ammonium hexa-fluorophosphate (Bu NPF ) acetoni-
trile solution under nitrogen protection at a scan rate of 100 mV/s.
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. The morphology of
active layers was examined by transmission electron microscope
(
TEM), which was carried out on a FEI Tecnai T20 with LaB6 oper-
3
. Results and discussion
ated at 200 kV. The active layer used as TEM measurement was
placed onto a copper grid after dissolving the PEDOT: PSS in water
3.1. Synthesis, thermal property and crystallinity
[43,44].
The general synthetic routes for these SMs of (DRCN3T)
2
Ph,
2.4. Device fabrication and characterization
2 2
(DRCN3T) Nap and (DRCN3T) Ant are outlined in Scheme 1. They
were obtained with a yield about 80%. Their molecular structures
1
OSCs were fabricated using indium tin oxide (ITO) glass as an
are confirmed by the H NMR and MALDI-MS measurement, which
anode, Ca/Al as a cathode, and a blend film of the SM/PC71BM as a
photosensitive layer, respectively. After a 30 nm buffer layer of
poly(3,4-ethylenedioxy-thiophene) and polystyrene sulfonic acid
are consistent with molecular formulas.
The thermal stability of these SMs was investigated by TGA
under the nitrogen atmosphere. The corresponding TGA curves and
2 2 2
Scheme 1. Synthetic routes of (DRCN3T) Ph, (DRCN3T) Nap and (DRCN3T) Ant.

B. Liu et al. / Dyes and Pigments 139 (2017) 42e49
45
ꢂ
ꢀ1
ꢂ
ꢀ1
Fig. 1. (a) TGA curves at a heating rate of 10 C min under nitrogen atmosphere and (b) DSC curves at a heating/cooling rate of 10 C min under nitrogen atmosphere for
DRCN3T) Ph, (DRCN3T) Nap and (DRCN3T) Ant.
(
2
2
2
Table 1
Optical and electrochemical data of (DRCN3T)
2
Ph, (DRCN3T)
2
Nap and (DRCN3T)
max (nm)b
2
Ant.
ꢂ
ꢀ
1
ꢀ1
max (nm)a
onset (nm)c
opt
elec
Molecules
ε(
l
max) (M cm
)
l
l
l
E
g
(eV)
EHOMO (eV)
ELOMO (eV)
E
g
(eV)
T
d
( C)
4
(DRCN3T)
(DRCN3T)
(DRCN3T)
2
2
2
Ph
Nap
Ant
7.8 ꢁ 10
7.6 ꢁ 10
6.2 ꢁ 10
521
520
523
579
601
585,625
706
741
763
1.76
1.67
1.63
ꢀ5.27
ꢀ5.25
ꢀ5.22
ꢀ3.57
ꢀ3.56
ꢀ3.57
1.70
1.69
1.65
354
387
394
4
4
a
b
c
Measured in CHCl
3
.
Measured in the thin film.
opt
E
g
¼ 1240/
l
onset.
data were depicted in Fig. 1 (a) and Table 1, respectively. The
2
films. It is noteworthy that the (DRCN3T) Ant films shows a larger
red-shifted by 103 nm and a distinct shoulder at 625 nm, which
result from a more effective molecular packing between molecular
ꢂ
thermal decomposition temperatures (T
exhibited for (DRCN3T) Ph, (DRCN3T) Nap, (DRCN3T)
weight loss, respectively. It implies that all of three SMs here have
good thermal stability. Further-more, the (DRCN3T) Ant with
bigger Ar rings shows better thermal stability than (DRCN3T) Ph
and (DRCN3T) Nap. Therefore, properly extending the central Ar
d
) of 354, 387, 394 C are
2
2
2
Ant at 5%
backbones by the effect of a lager conjugated anthracene. Based on
opt
2
the onset of the pure film absorption, the optical band gaps (E
(DRCN3T) Ph, (DRCN3T) Nap, (DRCN3T)
1.76, 1.67 and 1.63 eV, respectively.
g
) of
2
2
2
2
Ant are calculated to be
2
ring could enhance thermal stability, which is similar to the phe-
nomena reported in our previous work. Fig. 1 (b) depicts the dif-
ferential scanning calorimetry (DSC) plots of SMs in solid state. The
Compared to the pure films, it is found that the SM/PC71BM
blend films shows an increasing absorption in the range of
400e500 nm, as showed in Fig. 2 (b), which is assigned to the
contribution of PC71BM. While those blend films are processed by
ꢂ
typical endothermal peaks at 245, 247 and 237 C are observed for
(
DRCN3T)
2
Ph, (DRCN3T)
2
Nap, (DRCN3T)
2
Ant during heating pro-
), respectively.
1,8-diiodooctane (DIO) additive, the (DRCN3T)
further demonstrates a little increasing absorption intensity. In
contrast, the (DRCN3T) Ph and (DRCN3T) Nap blend films display a
little decreasing absorption intensity. It indicates that adding the
DIO solvent additive in the (DRCN3T) Ant blend films is in favour of
2
Ant blend film
cess, which correspond to melting temperatures (T
Moreover, (DRCN3T)
display an exothermal peak at 220, 201, 190 C during cooling
process, respectively. It indicates that three SMs possess good
crystallinity.
m
2
Ph, (DRCN3T)
2
Nap and (DRCN3T)
2
Ant also
2
2
ꢂ
2
improving intermolecular interaction.
3.2. Optical properties
3.3. Electrochemical properties
The normalized UVeVis absorption spectra of (DRCN3T)
DRCN3T) Nap and (DRCN3T) Ant in dilute CHCl solution and in
2 2 3
2
Ph,
The electrochemical properties were characterized by cyclic vol-
tammetry (CV) method, in which oxidation and reduction potentials
were calibrated using the ferrocene/ferrocenium (Fc/Fc ) redox
(
þ
their pure/blend films are shown in Fig. 2 (a). Their corresponding
absorption data are summarized in Table 1. Strong and broad ab-
sorption bands in the visible to near-infrared region are observed
for these three SMs. In the solution states, all of three SMs exhibit
the same absorption peak at ~521 nm in the low-lying region, that
are independent of the central Ar units. The similar absorption may
be contributed to the similar conjugation length of these series of
molecules mainly governed by the six thiophenes units together
with the two conjugated end units [13]. In the solid states, all of
them display a broader and obvious red-shifted absorption peak by
couple (4.8 eV below the vacuum level). The energy levels of the
highest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) are calculated from the onset oxidation
and reduction potentials, respectively. The recorded CV curves are
shown in Fig. 3 and the relevant data are summarized in Table 1. It is
found that the HOMO energy levels (EHOMO) have a little increase
from ꢀ5.27, ꢀ5.25 to ꢀ5.22 eV with the increasing Ar rings from Ph,
2
Nap to Ant in (DRCN3T) Ar. But their LUMO energy levels (ELUMO) are
almost similar values at ꢀ3.56 ~ ꢀ3.57 eV, which are largely domi-
5
0e100 nm in the low-lying region in comparison to those in their
solution absorption profiles. It indicates that ordered structure and
strong stacking effect, as well as intermolecular interaction
should exist between the molecular backbones in the solid pure
nated by the same electron-deficient ending groups. The resulting
electrochemical band gaps of (DRCN3T)
(DRCN3T) Ant are estimated to be 1.70, 1.69, 1.65 eV respectively,
which are consistent with their optical band gaps.
2 2
Ph, (DRCN3T) Nap and
p-
p
2

46
B. Liu et al. / Dyes and Pigments 139 (2017) 42e49
Fig. 4. J-V curves of three SM-based OSCs at optimized processing conditions under
ꢀ
2
the illumination of AM 1.5 G, 100 mW cm
.
Fig. 2. UVevis absorption spectra of (DRCN3T)
2 2 2
Ph, (DRCN3T) Nap and (DRCN3T) Ant
in CHCl solutions and in pure films (a), blend films without DIO additive and blend
3
films with 2% DIO additive (b).
Fig. 5. EQE curves of the (DRCN3T)
2 2 2
Ph, (DRCN3T) Nap and (DRCN3T) Ant based de-
vices at the optimized processing conditions.
3.4. Photovoltaic properties
The photovoltaic properties of these linear SMs were investi-
gated in their bulk heterojunction OSCs with a structure of ITO/
PEDOT:PSS/SM:PC71BM/Ca/Al. The device fabrication processes are
described in detail in the experimental section. The ratios between
SM and PC71BM were changed from 1:2, 1:3 to 1:4, as well as the
doping concentrations of the DIO additive were tuned from 0%, 1%,
2%e3% in order to optimize processing technology. The measured
photovoltaic data of the SM/PC71BM based cells are listed in Table
S1 and S2. It demonstrates that the optimized SM/PC71BM ratio is
the same at 1:3, but the DIO additive has played different rule in
different devices. For the (DRCN3T)
cells, adding DIO additive destroys device performance. In contrast,
adding 2% DIO additive makes the (DRCN3T) Ant based cell exhibit
the best device performance. The optimized photovoltaic perfor-
2 2
Ph and (DRCN3T) Nap based
2
2 2 2
Fig. 3. Cyclic voltammetry curves of (DRCN3T) Ph, (DRCN3T) Nap and (DRCN3T) Ant.
mances for the (DRCN3T)
2
Ph, (DRCN3T)
2
Nap and (DRCN3T)
2
Ant
Table 2
Photovoltaic performance of the SM/PC71BM OSCs and the hole mobilities of the SM/PC71BM blend films.
sc (mA/cm2)
FF (%)
PCE (%)
m
(cm2
v
ꢀ1
s
ꢀ1
)
m
(cm2
v
ꢀ1 ꢀ1
s
)
SM
D:A (w/w)
V
oc (V)
J
h
e
6
4
ꢀ4
ꢀ4
(
(
(
(
DRCN3T)
2
2
2
2
Ph
1:3
1:3
1:3
1:3
0.97 ± 0.01
0.98 ± 0.01
0.95 ± 0.01
0.87 ± 0.01
4.03 ± 0.11
4.15 ± 0.18
4.50 ± 0.11
11.08 ± 0.30
43 ± 1
48 ± 2
51 ± 1
53 ± 2
1.64 ± 0.05 (1.74)
1.97 ± 0.03 (2.01)
2.18 ± 0.06 (2.29)
5.11 ± 0.02 (5.15)
6.56 ꢁ 10ꢀ
2.80 ꢁ 10
2.52 ꢁ 10
e
ꢀ
DRCN3T)
DRCN3T)
DRCN3T)
Nap
Ant
Anta
1.44 ꢁ 10
e
ꢀ4
ꢀ4
2.74 ꢁ 10
2.86 ꢁ 10
a
2% DIO additive. The average PCEs was obtained from over 5 devices. The best PCEs are provided in parentheses.

B. Liu et al. / Dyes and Pigments 139 (2017) 42e49
47
conditions, the PCE and short-circuit current density (Jsc) values are
obviously improved from the (DRCN3T) Ph and (DRCN3T) Nap
based cells to the (DRCN3T) Ant based cell, whereas open-circuit
voltage (Voc) values have a little decrease. The best photovoltaic
2
2
2
ꢀ2
performances with a PCE of 5.15% and Jsc of 11.38 mA cm were
obtained in the (DRCN3T) Ant based cell at the optimized pro-
2
cessing conditions. The results here further indicate that properly
extending the central Ar ring could be beneficial to improve
photovoltaic properties for the A-Ar-A type SMs [33e35].
2
To further understand why the (DRCN3T) Ant based cell
exhibited the highest Jsc value among these cells, the external
quantum efficiency (EQE) curves of these devices under the opti-
mized processing conditions were measured and shown in Fig. 5. A
broad photo-response region with different EQE values in the re-
gion of 330e700 nm is analogously observed for these devices. The
(
DRCN3T)
2
Ant based device displays a significantly improved EQE
Ph and the (DRCN3T) Nap
of 73% in comparison to the (DRCN3T)
2
2
based cells. It indicates that introducing central Ant ring is available
to increase EQE value for its SM, which is available to promote the
increase of the Jsc value.
2
In order to explain why the (DRCN3T) Ant based cell exhibited
the highest FF value, the hole and electron mobilities of three SMs
were measured in their hole and electron-only devices using the
space charge limited current (SCLC) method, respectively. The J-V
characteristics in the dark are depicted in Fig. 6 (a) and (b) for these
optimized hole- and electron-only devices, respectively. An
ꢀ
6
increasing hole mobility is observed from 6.56
ꢁ
10
to
ꢀ
4
ꢀ4
2 ꢀ1 ꢀ1
1
.44 ꢁ 10
and 2.74 ꢁ 10 in the hole-only
cm v
s
(DRCN3T) Ph, (DRCN3T)
2
2
Nap and (DRCN3T) Ant based devices,
2
respectively. However, the electron mobilities of three SMs have a
ꢀ
4
ꢀ4
2 ꢀ1 ꢀ1
s . It is
little change in a range of 2.52 ꢁ 10 ~ 2.86 ꢁ 10 cm v
clear that (DRCN3T) Ant presents much higher hole mobility and
more balanced carrier mobility than (DRCN3T) Ph and
DRCN3T) Nap, which is consistent with the corresponding FF and
sc results. It firmly demonstrates that properly extending the
Fig. 6. J-V characteristics of the optimized hole-only (a) and electron-only (b) devices
based on (DRCN3T) Ph, (DRCN3T) Nap and (DRCN3T) Ant.
2
2
2
2
2
(
J
2
based cells are listed in Table 2. The corresponding current density
vs voltage (J-V) curves are shown in Fig. 4 measured under AM 1.5G
irradiation at an intensity of 100 mW cm . Under the optimized
central Ar ring can increase hole mobility and balance carrier
mobility due to better molecular packing of the enlarged Ar ring.
ꢀ2
2 2 2
Fig. 7. TEM images of the SM:PC71BM (1:3, w/w) blend films for (DRCN3T) Ph (a,d), (DRCN3T) Nap (b,e) and (DRCN3T) Ant (c,f) without/with 2%DIO additive.

4
8
B. Liu et al. / Dyes and Pigments 139 (2017) 42e49
efficiency and high fill factor via multiple fluorine substituents and thiophene
The morphologies of the SM/PC71BM blend films under the
bridge. Adv Funct Mater 2015;25:3514e23.
optimized processing conditions were recorded with transmission
electron microscopy (TEM) and shown in Fig. 7. The observed dark
phases are assigned to the PC71BM domains because of its relatively
higher electron scattering density [39,45]. The fibrillar structure
with a width around 30e60 nm are exhibited in these TEM images
for the SM/PC71BM blend films without DIO additive (Fig. 7a, b and
c). While the 2% DIO additive is added, the (DRCN3T) Ant blend film
2
show a continuous acicular fibrous structure with a decreased size
of 14 nm (Fig. 7f). It means that a continuously interpenetrated
network is formed in the (DRCN3T)
DIO additive, which is favorable for the charge transportation.
However, the (DRCN3T) Ph and (DRCN3T) Nap blend films show a
significantly dispersed phase structures after processed by 2% DIO
additive (Fig. 7d and e). It implies that the DIO additive has pro-
[8] Wang JL, Xiao F, Yan J, Wu Z, Liu KK, Chang ZF, et al. Difluorobenzothiadiazole-
based small-molecule organic solar cells with 8.7% efficiency by tuning of
conjugated spacers and solvent vapor annealing. Adv Funct Mater 2016;26:
803e12.
p-
1
[
9] Wang JL, Liu KK, Yan J, Wu Z, Liu F, Xiao F, et al. Series of multifluorine
substituted oligomers for organic solar cells with efficiency over 9% and fill
factor of 0.77 by combination thermal and solvent vapor annealing. J Am
Chem Soc 2016;138:7687e97.
[
[
10] You JB, Dou LT, Yoshimura K, Kato T, Ohya K, Moriarty T, et al. A polymer
tandem solar cell with 10.6% power conversion efficiency. Nat Commun
2013;4:66e78.
11] Liu YH, Zhao JB, Li ZK, Mu C, Ma W, Hu HW, et al. Aggregation and morphology
control enables multiple cases of high-efficiency polymer solar cells. Nat
Commun 2014;5. 5293e5293.
2
Ant blend film processed by
2
2
[12] Liu C, Yi C, Wang K, Yang YL, Bhatta R, Tsige M, et al. Single junction polymer
solar cells with over 10% efficiency by novel two-dimensional donor-acceptor
conjugated copolymer. ACS Appl Mater Interfaces 2015;7:4928e35.
[
[
[
[
13] Kan B, Li MM, Zhang Q, Liu F, Wan XJ, Wang YC, et al. A series of simple
oligomer-like small molecules based on oligothiophenes for solution-
processed solar cells with high efficiency. J Am Chem Soc 2015;137:3886e93.
14] Gao K, Li LS, Lai TQ, Xiao LG, Huang Y, Huang F, et al. Deep absorbing
porphyrin small molecule for high performance organic solar cells with very
low energy losses. J Am Chem Soc 2015;137:7282e5.
motes molecular self-aggregation for (DRCN3T)
2
Ph and
Nap blend films. It further explains why the
Ant based cells exhibited higher FF values than the
Ph and (DRCN3T) Nap based cells.
(DRCN3T)
(DRCN3T)
(DRCN3T)
2
2
2
2
15] Mishra A, B a€ uerle P. Small molecule organic semiconductors on the move:
promises for future solar energy technology. Angew Chem Int Ed 2012;51:
4
. Conclusions
2020e67.
16] Kan B, Zhang Q, Liu F, Wan XJ, Wang YC, Ni W, et al. Small molecules based on
0
Three linear A-Ar-A type SMs of (DRCN3T)
2
Ph, (DRCN3T)
2
Nap
alkyl/Alkylthio-thieno [3,2-b] thiophene-substituted benzo [1,2-b:4,5-b ]
dithiophene for solution-Processed solar cells with high performance. Chem
Mater 2015;27:8414e23.
and (DRCN3T) Ant were obtained. The influence of the central Ar
unit on optical, electrochemical and photovoltaic properties was
presented. With enlarging the central Ar ring from phenylene,
naphthalene to anthracene, the A-Ar-A type SMs of (DRCN3T) Ar
2
exhibited the much improved PCE and Jsc values. The best photo-
2
[
[
17] Su WY, Fan QP, Xiao MJ, Chen JH, Zhou P, Liu B, et al. Improved photovoltaic
performance of a side-chain D-A polymer in polymer solar cells by shortening
the phenyl spacer between the
014;215:2075e83.
D and A units. Macromol Chem Phys
2
18] Zhang MJ, Guo X, Ma W, Zhang SQ, Huo LJ, Ade H, et al. An easy and effective
method to modulate molecular energy level of the polymer based on ben-
zodithiophene for the application in polymer solar cells. Adv Mater 2014;26:
voltaic performance with a PCE of 5.15% was obtained in the
(
DRCN3T)
2
Ant/PC71BM based cell, which is three times of that in
Ph/PC71BM based device.
2
089e95.
the (DRCN3T)
2
[
[
[
19] Zhang MJ, Guo X, Ma W, Ade H, Hou JH. A large-bandgap conjugated polymer
for versatile photovoltaic applications with high performance. Adv Mater
2015;27:4655e60.
Acknowledgements
20] Fan QP, Su WY, Guo X, Guo B, Li WB, Zhang YD, et al. A new polythiophene
derivative for high efficiency polymer solar cells with PCE over 9%. Adv Energy
Mater 2016;6:1600430.
21] Zhang MJ, Guo X, Zhang SQ, Hou JH. Synergistic effect of fluorination on
molecular energy level modulation in highly efficient photovoltaic polymers.
Adv Mater 2014;26:1118e23.
Thanks to the financial supports from the Major Cultivation and
General Programs of the National Natural Science Foundation of
China (51673031, 91233112, 21172187, 51403178), the Program for
Innovative Research Cultivation Team in University of Ministry of
Education of China (1337304), the Natural Science Foundation of
Hunan (14JJ4019, 2015JJ3113), Open Project for the National Key
Laboratory of Luminescent Materials and Devices (2014-skllmd-
[22] Li WW, Hendriks KH, Furlan A, Roelofs WSC, Meskers SCJ, Wienk MM, et al.
Effect of the fibrillar microstructure on the efficiency of high molecular weight
diketopyrrolopyrrole-based polymer solar cells. Adv Mater 2014;26:1565e70.
[
[
[
23] Zhang MJ, Guo X, Ma W, Ade H, Hou JH. A polythiophene derivative with
superior properties for practical application in polymer solar cells. Adv Mater
2014;26:5880e5.
24] Yan WB, Zhang Q, Qin QS, Ye SY, Lin YW, Liu ZW, et al. Design, synthesis and
photophysical properties of A-D-A-D-A small molecules for photovoltaic
application. Dyes Pigm 2015;121:99e108.
25] Bai HT, Wang YF, Cheng P, Li YF, Zhu DB, Zhan XW. Acceptor-donor-acceptor
small molecules based on indacenodithiophene for efficient organic solar
cells. ACS Appl Mater Interfaces 2014;6:8426e33.
1
0), the Hunan Postgraduate Science Foundation for Innovation
(
CX2014B257, CX2015B201).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2016.12.015.
[26] Liu YS, Zhou JY, Wan XJ, Chen YS. Synthesis and properties of acceptor-donor-
acceptor molecules based on oligothiophenes with tunable and low band gap.
Tetrahedron 2009;65:5209e15.
[
27] He GR, Li Z, Wan XJ, Liu YS, Zhou JY, Long GK, et al. Impact of dye end groups
on acceptor-donor-acceptor type molecules for solution-processed photo-
voltaic cells. J Mater Chem 2012;22:9173e80.
References
[
1] Cheng YJ, Yang SH, Hsu CS. Synthesis of conjugated polymers for organic solar
cell applications. Chem Rev 2009;109:5868e923.
2] Li ZF, Dong QF, Li YW, Xu B, Deng M, Pei JN, et al. Design and synthesis of
solution processable small molecules towards high photovoltaic performance.
J Mater Chem 2011;21:2159e68.
3] Chen YS, Wan XJ, Long GK. High performance photovoltaic applications using
solution-processed small molecules. Acc Chem Res 2013;46:2645e55.
4] Coughlin JE, Henson ZB, Welch GC, Bazan GC. Design and synthesis of mo-
lecular donors for solution-processed high-efficiency organic solar cells. Acc
Chem Res 2014;47:257e70.
[28] Li S, He ZC, Yu J, Chen SA, Zhong AS, Tang RL, et al. How the linkage positions
affect the performance of bulk-heterojunction polymer solar cells. J Mater
Chem 2012;22:12523e31.
[
[29] Kumara CV, Cabau L, Koukaras EN, Sharmab GD, Palomares E. Synthesis, op-
tical and electrochemical properties of the A-p-D-p-A porphyrin and its
application as an electron donor in efficient solution processed bulk hetero-
junction solar cells. Nanoscale 2015;7:179e89.
[
[
[30] Kumar CV, Cabau L, Koukaras EN, Sharma A, Sharmab GD, Palomares E. A-
-A based porphyrin for solution processed small molecule bulk hetero-
junction solar cells. J Mater Chem 2015;3:16287e301.
p-D-
p
[
5] Walker B, Kim C, Nguyen TQ. Small molecule solution-processed bulk hetero-
junction solar cells. Chem Mater 2010;23:470e82.
6] Zhou P, Dang DF, Wang Q, Duan XW, Xiao MJ, Tao Q, et al. Enhancing the
photovoltaic performance of triphenylamine based star-shaped molecules by
tuning the moiety sequence of their arms in organic solar cells. J Mater Chem
A 2015;3:13568e76.
[31] Kan B, Zhang Q, Li MM, Wan XJ, Ni W, Long GK, et al. Solution-processed
organic solar cells based on dialkylthiol-substituted benzodithiophene unit
with efficiency near 10%. J Am Chem Soc 2014;136:15529e32.
[32] Zhou JY, Yi Z, Wan XJ, Long GK, 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;135:8484e7.
[
[7] Wang JL, Yin QR, Miao JS, Wu Z, Chang ZF, Cao Y, et al. Rational design of small
[33] Zhou JY, Wan XJ, Liu YS. Small molecules based on benzo[1,2-b:4,5-b']
dithiophene unit for high-performance solution-processed organic solar cells.
molecular donor for solution processed organic photovoltaics with 8.1%

B. Liu et al. / Dyes and Pigments 139 (2017) 42e49
49
J Am Chem Soc 2012;134:16345e51.
[40] Fan QP, Li M, Yang PG, Liu Y, Xiao MJ, Wang XD, et al. Acceptor-donor-
0
[
[
34] Lim N, Cho N, Paek S, Kim C, Lee JK, Ko J. High-performance organic solar cells
with efficient semiconducting small molecules containing an electron-rich
benzodithiophene derivative. Chem Mater 2014;26:2283e8.
acceptor small molecules containing benzo[1,2-b:4,5- b ] dithiophene and
rhodanine units for solution processed organic solar cells. Dyes Pigm
2015;116:13e9.
35] Ni W, Li MM, Liu F, Wan XJ, Feng HR, Kan B, et al. Dithienosilole-based small-
molecule organic solar cells with an efficiency over 8%: investigation of the
relationship between the molecular structure and photovoltaic performance.
Chem Mater 2015;27:6077e84.
[41] Zhou JY, Wan XJ, Liu YS, Long GK, 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.
[42] Higashijima S, Miura H, Fujita T, Kubota Y, Funabiki K, Yoshida T, et al. Highly
efficient new indoline dye having strong electron-withdrawing group for zinc
oxide dye-sensitized solar cell. Tetrahedron 2011;67:6289e93.
[43] Su WY, Fan QP, Guo X, Guo B, Li WB, Zhang YD, et al. Efficient ternary blend
all-polymer solar cells with a polythiophene derivative as a hole-cascade
material. J Mater Chem A 2016;4:14752e60.
[
[
36] Zhang Q, Kan B, Liu F, Long GK, Wan XJ, Chen XQ, et al. Small-molecule solar
cells with efficiency over 9%. Nat Phot 2015;9:35e41.
37] Lee JW, Choi YS, Jo WH. Diketopyrrolopyrrole-based small molecules with
simple structure for high Voc organic photovoltaics. Org Electron 2012;13:
3060e6.
[
[
38] Choi YS, Jo WH. A strategy to enhance both Voc and Jsc of A-D-A type small
molecules based on diketopyrrolopyrrole for high efficient organic solar cells.
Org Electron 2013;14:1621e8.
39] Duan XW, Xiao MJ, Chen JH, Wang XD, Peng WH, Duan LR, et al. Improving
photovoltaic performance of the linear A-Ar-A-type small molecules with
diketopyrro-pyrroles arms by tuning the linkage position of anthracene core.
ACS Appl Mater Interfaces 2015;7:18292e9.
[44] Wan Q, Guo X, Wang ZY, Li WB, Guo B, Ma W, et al. 10.8% Efficiency polymer
solar cells based on PTB7-Th and PC71BM via binary solvent additives treat-
ment. Adv Funct Mater 2016;26:6635e40.
[45] Burkhart B, Khlyabich PP, Thompson BC. Influence of the ethylhexyl side-
chain content on the open-circuit voltage in rr-poly (3-hexylthiophene-co-
3-(2-ethyl hexyl) thiophene) copolymers. Macromolecules 2012;45:3740e8.