Two new medium bandgap asymmetric copolymers based on thieno[2,3-f]benzofuran for efficient organic solar cells

Article abstract of DOI:10.1016/j.dyepig.2017.01.058

Two new medium bandgap, alkoxyphenyl substituted thieno[2,3-f]benzofuran (TBFPO) and alkoxyl substituted thieno[2,3-f]benzofuran (TBFP)-based polymers were designed, synthesized and applied in polymer solar cells (PSCs), namely, TBFPO-BDD and TBFO-BDD, re

Full text of DOI:10.1016/j.dyepig.2017.01.058

Dyes and Pigments 140 (2017) 337e345
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Two new medium bandgap asymmetric copolymers based on thieno
[2,3-f]benzofuran for efficient organic solar cells
a
a
a
b
b
a, *
Lixia Qiu , Jun Yuan , Dingjun He , Zhi-Guo Zhang , Yongfang Li , Yingping Zou
a
College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 December 2016
Received in revised form
Two new medium bandgap, alkoxyphenyl substituted thieno[2,3-f]benzofuran (TBFPO) and alkoxyl
substituted thieno[2,3-f]benzofuran (TBFP)-based polymers were designed, synthesized and applied in
polymer solar cells (PSCs), namely, TBFPO-BDD and TBFO-BDD, respectively. Their thermal, optical,
electronic properties and photovoltaic performances were systematically investigated. The PSCs pre-
pared using o-dichlorobenzene as solvent, with the single layer device structure of ITO/PEDOT:PSS/
TBFPO-BDD:PC71BM (1:1; w/w)/perylene diimide (PDI) derivative (PDINO)/Al, exhibited a promising
20 January 2017
Accepted 21 January 2017
Available online 25 January 2017
2
power conversion efficiency (PCE) of 6.58% under the illumination of AM 1.5G, 100 mW/cm , without any
post-treatment. While the devices based on TBFO-BDD showed an enhanced PCE of 3.07% with 1 vol%
Keywords:
Medium bandgap polymer
Alkoxyphenyl
Alkoxy
1,8-diodooctane (DIO) as additive. This study demonstrates that TBFPO-BDD is a promising polymer for
organic electronics.
Interlayer
© 2017 Published by Elsevier Ltd.
Polymer solar cells
1. Introduction
regulating solubility, electronic and morphological properties
[8e11]. Therefore optimizing side chains is considered as an
Organic photovoltaics (OPVs) have been considered as one of
important issue for the molecular design for PSCs [12e16]. Gener-
ally, two-dimensional (2D) conjugated polymers not only have
broad and strong absorption bands due to extended side chains, but
also have larger conjugated planes, better inter-chain overlap and
higher charge mobility than one-dimensional conjugated polymers
[17]. Benzodichalcogenophene (BDC) derivatives have been widely
used in BHJ PSCs and deeply investigated by many groups owing to
their lower highest occupied molecular orbital (HOMO) level and
stronger absorption, which contributed to obtain high open-circuit
voltage (Voc) and short circuit current (Jsc) [18e20]. Many low
the most promising candidates for practical photovoltaic applica-
tions to solve the increasing energy problems worldwide due to
light weight, low-cost, flexible devices through roll-to-roll solution
processes and impressive conversion efficiency [1]. Over the past
several decades, significant efforts have been made to optimize the
performance of OPVs, as a result, the power conversion efficiencies
(
[
PCEs) of polymer solar cells (PSCs) have been achieved up to 11.7%
2] using conventional, single-layer bulk heterojunction (BHJ) OPVs
and over 12% for tandem cells [3], with the development of pho-
toactive materials, interface engineering and device modification.
Although the result is satisfactory, it is still a need to design and
synthesize ideal donor materials to improve PCEs by chemical
structure modifications and in-depth investigation of the rela-
tionship between structure and properties to push this technology
toward commercial applications [4e7].
As is well-known, using fusion of aromatic rings, optimization of
side chains and extending p-conjugation systems could improve
PCEs. Side chains in conjugated polymers play a crucial role in
0
bandgap benzo[1,2-b:4,5-b ]dithiophene (BDT) [21,22] based con-
jugated polymers with different side chains were studied by several
groups. Hou et al. have synthesized 2D-conjugated polymers
PBDTTT-E-T and PBDTTT-C-T by introducing conjugated thiophene
side chains instead of alkoxy substituents on BDT unit. Hou and Li
et al. introduced alkylthiol side chain in PPV to tune the optoelec-
tronic properties of PPV derivatives in 2006 [23]. Later, Li and Huo
et al. have reported a series of 2D-BDT conjugated copolymers with
the alkylthienyl side chains in comparison to alkyl and alkoxyl
group to explore the effects of different side groups on PCEs [24].
Our group has also reported several 2D-conjugated BDT and BDF-
based polymers with substituted alkoxyphenyl or alkylthienyl
groups as the conjugated side chains, which have shown desirable
*
Corresponding author.
E-mail address: yingpingzou@csu.edu.cn (Y. Zou).
http://dx.doi.org/10.1016/j.dyepig.2017.01.058
143-7208/© 2017 Published by Elsevier Ltd.
0

338
L. Qiu et al. / Dyes and Pigments 140 (2017) 337e345
photovoltaic performance [25e27]. Recently, Huo et al. reported a
D-conjugated polymer PBDF-T1 delivering a high PCE of 9.43%
applied in PSCs, named TBFPO-BDD and TBFO-BDD, as shown in
Scheme 1, which possess good solubility, broad absorption in the
wavelength range from 300 nm to 700 nm and balanced charge
mobility. In order to enhance the photovoltaic performance, we
used two type of interlayers, zirconium acetylacetonate (ZrAcac)
and perylene diimide (PDI) derivatives (PDINO), shown in Fig. S2.
The PSCs with the device structure of ITO/PEDOT:PSS/TBFPO-
BDD:PC71BM(1:1;w/w)/PDINO/Al, exhibited a promising PCE of
6.58% without any post-treatment. While the devices based on
TBFO-BDD showed an enhanced PCE of 3.07% with 1% 1,8-
diodooctane (DIO) as additive.
2
with additive treatment [28]. However, the asymmetric structure of
BDC derivatives, thieno[2,3-f ]benzofuran (TBF), has not been
widely used and explored in PSCs. Some copolymers based on TBF
unit have been synthesized and investigated by our group, such as
PTBFTDTBT, PTBFP-BT, PTBFP-BO, TBFBT, TBFPF-BT and TBFPF-BO
(
shown in Fig. S1) with PCEs of 6.40%, 6.02%, 4.41%, 6.10%, 6.80%
and 5.98%, respectively [29e31]. Except for the asymmetric struc-
ture TBF, the difluoro-benzothiadiazole (DFBT) [32] unit and the
0
0
dithieno[3,2-b:2 ,3 -d]pyran (DTPa) building block [33] have been
also investigated to explore the effects of asymmetric structure on
the photovoltaic performances.
2. Experimental section
To further explore the copolymers based on TBF unit with or
without 2D-cojugated side chains, in this work, two new medium
bandgap, alkoxyphenyl and alkoxyl substituted thieno[2,3-f]
benzofuran (TBF)-based polymers were designed, synthesized and
2.1. Materials
3 4 3 3
Pd(PPh ) , tetrahydrofuran (THF) and Sn(CH ) Cl were obtained
Scheme 1. The Synthetic route of the monomers and polymers.

L. Qiu et al. / Dyes and Pigments 140 (2017) 337e345
339
from J&K and Alfa Asia Chemical Co., and they were used as
received. Toluene was dried over Na/benzophenone and freshly
distilled prior to use. All other reagents and solvents were pur-
chased commercially as ACS-grade quality and used without
further purification.
(Ambios Tech. XP-2). All of these fabrications and characterizations
after cleaning of ITO substrates were conducted in a glove box.
2.4. Synthesis
2,6-Bis(trimethyltin)-4,8-bis(4-ethylhexyloxy-1-phenyl)-
2
.2. Characterization
thieno-[2,3-f]benzofuran (M1) [30] and 2,6-Bis(trimethyltin)-4,8-
bis(2-ethylhexyloxy)- thieno[2,3-f]benzofuran (M2) [34] were
synthesized according to our published procedures; 1,3-
bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo-[1,2-c:4,5-c]dithio-
phene-4,8-dione (M3) [10,14,35,36], was prepared according to the
literature procedures. The detailed synthetic route to polymers
were shown in Scheme 1. The monomer's detailed synthetic pro-
cedures were shown in SI.
The polymer structures were confirmed by nuclear magnetic
resonance (NMR) spectroscopy which was recorded using a Bruker
DMX-400 spectrometer or DMX-500 spectrometer in deuterated
chloroform solution at 298 K. Chemical shifts were reported in ppm
with tetramethylsilane (TMS) as the internal reference (0.00 ppm).
Molecular weight and polydispersity of the polymers were deter-
mined by gel permeation chromatography (GPC) analysis with
polystyrene as standard. Thermogravimetric analysis (TGA) was
conducted on a Perkin-Elmer TGA-7 with a heating rate of 10 K/min
under inert atmosphere. UVeVis absorption spectra were recorded
on the Hitachi U-3010 UVevis spectrophotometer. For the solid
state measurements, polymer in chloroform solution was spin-
coated on quartz plates. Cyclic voltammetry was recorded with a
computer controlled Zahner IM6e electrochemical workstation
2.5. Synthesis of TBFPO-BDD
A solution of M1 (0.136 g, 0.15 mmol), M3 (0.115 g, 0.15 mmol) in
dry toluene (10 mL) was put into a two-necked flask. The solution
was flushed with argon for 20 min and Pd(PPh ) (15 mg) was
3 4
added into the flask. The solution was flushed with argon again for
ꢀ
20 min. The oil bath was carefully heated to 110 C and the reactant
2
using polymer films on a platinum disk (1.0 cm ) as the working
electrode, a platinum wire as the counter electrode and Ag/AgCl
was stirred for 24 h at this temperature under an argon atmo-
sphere. After the reaction mixture was cooled to room temperature,
the mixture was poured into methanol (150 mL) slowly, allowing
the crude polymer to precipitate. After filtration, the crude product
was subjected to Soxhlet extractions with methanol, hexane and
chloroform in sequence. The chloroform fraction was evaporated to
dryness and further dried under vacuum for 1 day to get the final
product as a blue black solid (150 mg, yield: 81%). Elemental
(
0.1 mol/L) as the reference electrode in an anhydrous and argon-
saturated solution of 0.1 mol/L tetrabutyl ammonium hexa-
fluorophosphate (Bu NPF ) in acetonitrile solution at a scan rate of
0 mV/s. Electrochemical onsets were determined at the position
4
6
5
where the current starts to differ from the baseline. The morphol-
ogies of the polymer/PC71BM blend films were investigated by a SPI
3
3
800N atomic force microscope (AFM) in contacting mode with a
mm scanner.
analysis calculated for C72
analysis: C,72.85; H, 6.95, S, 13.51.
GPC (THF): M
¼ 32 kDa; M
H
82
O
5
S
5
: C, 72.81; H, 6.96; S,13.50: Actual
n
w
¼ 65 kDa; PDI ¼ 1.83.
2.3. Fabrication and characterization of polymer solar cells
2.6. Synthesis of TBFOeBDD
The PSCs were fabricated in the configuration of ITO/PEDOT: PSS
(
40 nm)/active layer/cathode. Patterned ITO glass with a sheet
TBFOeBDD was obtained by a similar procedure with the syn-
resistance of 10 /square was purchased from CSG HOLDING Co.
U
thesis of TBFPO-BDD starting from M2 (0.114 g, 0.15 mmol) and M3
LTD. (China). The ITO glass was cleaned by sequential ultrasonic
treatment in detergent, deionized water, acetone, isopropanol and
then treated in an ultraviolet-ozone chamber (Jelight Company,
USA) for 20 min. Then PEDOT:PSS (poly(3,4-ethylen
edioxythiophene):poly(styrene-sulfonate)) (Baytron P VP AI 4083
(0.115 g, 0.15 mmol). The final product was a purple blue solid.
(145 mg, yield: 79%). Elemental analysis calculated for C60
C, 60.59; H, 7.20; S, 15.48; Actual analysis: C,60.54; H, 7.24; S, 15.42.
GPC (THF): M ¼ 36 kDa; PDI ¼ 1.62.
¼ 17 kDa; M
74 5 5
H O S :
n
w
from H. C. Starck) was filtered through a 0.45
mm poly(tetra-
3. Results and discussion
fluoroethylene) (PTFE) filter and spin coating at 4000 rpm for 40 s
on the ITO substrate. Subsequently, PEDOT: PSS film was baked at
3.1. Thermal stability and polymerization result
ꢀ
150 C for 15 min in the air. The polymer and PC71BM (10 mg/mL for
polymer) were dissolved in ODCB overnight, and spin-cast onto the
PEDOT:PSS layer. DIO was added to the blend solutions before spin-
coating. Then methanol solutions of PDINO and ZrAcac at a con-
centration of 1.0 mg/mL were then deposited atop the active layer
at 3000 rpm for 30 s to afford a thickness of 10 nm. Lately, top Al
The thermal stability of the polymers were investigated by TGA
in an inert atmosphere, and the TGA plots of the two polymers are
shown in Fig. S2. The TGA profiles reveal that two polymers possess
similar thermal stabilities. Although the onset points of the 5%
ꢀ
weight loss (T
d
) of the polymers are under 300 C, the thermal
(
~100 nm) electrode was thermally evaporated under a shadow
stability of TBFPO-BDD and TBFO-BDD is still good enough for PSCs.
Molecular weights determined by GPC using THF as eluent were
ꢁ5
mask with a base pressure of ca. 10 Pa. The active area of the PSCs
is 4.7 mm . Device characterizations were carried out under AM
1
2
32 kDa (M
n
w n
) and 65 kDa (M ) for TBFPO-BDD and 17 kDa (M ) and
2
.5G irradiation with the intensity of 100 mW/cm by a Newport
36 kDa (M
w
) for TBFO-BDD with corresponding polydispersity
Oriel 91150V reference cell. J-V curves were recorded on a
computer-controlled Keithley 2450 Source-Measure Unit. Oriel
Sol3A Class AAA Solar Simulator (model, Newport 94023A) with a
indices (PDI) of 1.83 and 1.62, respectively. The TGA and GPC data
are shown in Table 1.
4
50 W xenon lamp and an air mass (AM) 1.5 filter was used as the
3.2. Optical properties
light source. The EQE measurements were performed by Solar Cell
Spectral Response Measurement System QE-R3-011 (Enli Technol-
ogy Co., Ltd., Taiwan). The light intensity at each wavelength was
calibrated with a standard single-crystal Si photovoltaic cell. The
thickness of the interlayer was determined by a profilometer
The spectral absorption of two polymers could reflect their
ability to utilize solar spectrum, which usually have a direct impact
on Jsc. Fig. 1a shows the UVeVis absorption spectra of the conju-
gated polymers in chloroform solution and as a thin film on a

340
L. Qiu et al. / Dyes and Pigments 140 (2017) 337e345
Table 1
Molecular weights and thermal properties of the polymers.
a
a
PDIa
b
ꢀ
Polymers
M
w
(KDa)
M
n
(KDa)
d
T ( C)
TBFPO-BDD
TBFO-BDD
65
36
32
17
1.83
1.62
287
255
a
n w
M , M and PDI of the polymers were determined by GPC using polystyrene
standard in THF.
b
The 5% weight-loss temperatures under inert atmosphere.
Fig. 2. (a) Cyclic voltammograms of polymer films on a glassy carbon electrode in
.1 M Bu NPF , CH CN solution. (b) The energy diagram of the materials involved in
PSCs.
0
4
6
3
TBFPO-BDD caused red shifts in absorption maximum in the solu-
tion compared to that of TBFO-BDD, and similar phenomenon was
observed in the solid films. For example, the TBFPO-BDD solution
exhibited the broader absorption with the red shifted absorption
peak at 581 nm which contributed to higher Jsc compared with
TBFO-BDD peaking at 550 nm. Moreover, the film spectra exhibited
broad absorption over the range from 300 to 750 nm and had two
strong absorption peaks [37]. From Fig. 1b, the pure TBFPO-BDD
Fig. 1. (a) UVeVis absorption spectra of two polymers in solution and film states; (b)
Absorption coefficient spectra of the pure polymer film and polymer:PC71BM blend
film.
4
4
quartz substrate cast from chloroform solution, the related ab-
sorption data are listed in Table 2 for comparison. As expected, the
introduction of a conjugated benzene ring on TBF moiety from
(6.17 ꢂ 10 ) and the blend film (4.01 ꢂ 10 ) have higher absorp-
4
tion coefficient than the TBFO-BDD (5.61 ꢂ 10 ) and the TBFO-BDD/
4
PC BM blend (3.51 ꢂ 10 ) in the maximum absorption wavelength.
71
Table 2
The data of absorption spectra and cyclic voltammetry.
Polymers
Absorption spectra
Solutionaa
Cyclic voltammetry
p-doping
Filmb
max (nm)
n-doping
red/LUMO (V)/(eV)
opt
g
ox/HOMOd (V)/(eV)
lmax (nm)
l
ledge (nm)
E
E
E
c
c
TBFPO-BDD
TBFO-BDD
581
550
628
607
706
683
1.76
1.80
0.89/ꢁ5.30
0.86/ꢁ5.27
ꢁ0.76/ꢁ3.65
ꢁ0.95/ꢁ3.46
a
Measured in chloroform solution.
Cast from chloroform solution.
Bandgap estimated from the onset of the optical absorption.
b
c
d
HOMO ¼ ꢁe(Eox þ 4.41) (eV); LUMO ¼ ꢁe(Ered þ 4.41) (eV).

L. Qiu et al. / Dyes and Pigments 140 (2017) 337e345
341
Fig. 3. (a) The hole and electron mobilities of the TBFPO-BDD:PC71BM (b) The hole and
electron mobilities of the TBFO-BDD:PC71BM.
The optical band gaps (E go pt) estimated from the UVeVis absorption
onsets in solid state of TBFPO-BDD and TBFO-BDD are determined
to be 1.76 eV, 1.81 eV, respectively.
3.3. Electrochemical properties
Cyclic voltammetry is used to measure the HOMO and LUMO
energy levels of the polymers, which were calculated from the
onset oxidation and reduction potentials. As shown in Fig. 2a and b,
the onset reduction potentials (Ered) of TBFPO-BDD and TBFO-BDD
are ꢁ0.76 V and ꢁ0.95 V, while the onset oxidation potentials (Eox
)
are 0.89 V and 0.86 V, respectively. It is well known that the HOMO
and LUMO levels of the polymers were calculated according to the
equations, HOMO ¼ ꢁe(Eox þ 4.41) (eV); LUMO ¼ ꢁe(Ered þ 4.41)
Fig. 4. (a) Current densityevoltage (JeV) characteristics; (b) External quantum effi-
ciency (EQE) spectra of TBFPOeBDD and TBFOeBDD based solar cells; (c) Light in-
tensity dependence of the short-circuit current density.
(
eV), where the unit of Eox and Ered is in V vs. Ag/AgCl. The HOMO
and LUMO levels of TBFPO-BDD and TBFO-BDD are ꢁ5.30 eV/
ꢁ
3.65 eV and ꢁ5.27 eV/ꢁ3.46 eV, respectively. The investigation
3
.4. Mobility measurements
shows that the two polymers possess low lying HOMO level, which
offers the potential to obtain high Voc [38]. Moreover, their LUMO
energy level offset from PCBM acceptor is enough for charge
transfer and separation [39]. The related CV data are summarized in
Table 2.
The charge mobility has an important impact upon the PCE.
Charge mobilities were measured by space-charge-limited current
SCLC) method with a structure of ITO/PEDOT: PSS/polymer:
(

342
L. Qiu et al. / Dyes and Pigments 140 (2017) 337e345
Table 3
Summary of the photovoltaic characteristics of the polymer:PC71BM blend films.
sc (mA/cm2)
FF (%)
PCEmax (%)
PCEavg (%)
d
e
s
R (U
cm2)
Ratio
V
oc (V)
J
TBFPO-BDD:PC71BM
TBFO-BDD:PC71BM
1:1a
0.82
0.82
0.84
0.79
0.80
0.83
0.83
0.84
0.81
0.79
12.81
12.10
8.59
12.07
11.56
5.36
5.41
3.63
8.52
8.28
62
63
68
65
67
51
49
56
46
46
6.58
6.26
4.87
6.22
6.21
2.29
2.18
1.71
3.21
3.05
6.32
6.14
4.76
6.20
6.18
2.23
2.15
1.70
3.07
3.02
11.29
11.70
13.85
11.43
11.77
52.58
55.94
59.03
31.32
33.49
b
1
1
1
1
:1
:2
:1
:1
b
a,c
b,c
a
1:1
b
1
1
1
1
:1
:2
:1
:1
b
a,c
b,c
a
Device structure of ITO/PEDOT:PSS/polymer:PC71BM/PDINO/Al.
Device structure of ITO/PEDOT:PSS/polymerPC71BM/ZrAcac/Al.
With 1% DIO as additive.
PCEavg is the data obtained from 20 devices.
Calculated from the inverse slope at V ¼ 0 in J-V curves under illumination.
b
c
d
e
PC71BM (1:1; w/w) with or without 1% DIO/Au for holes and Al/
photovoltaic properties can be further improved via higher mo-
lecular weight by optimizing the polymerization conditions and
different processing conditions such as thermal annealing and
different additives. Recently, non-fullerene n-type organic semi-
conductors consists of n-type polymers and small molecules
polymer: PC71BM (1:1; w/w) with or without 1% DIO/Al for elec-
1
/2
trons. The results are plotted as J ~ (Vappl -Vbi), as shown in Fig. 3a
and b, the hole and electron mobilities are calculated by MOTT-
Gurney equation [40,41]. The SCLC is described by
[
42e45] have attracted extensive attention, especially n-type small
9
8
V2
d3
acceptor ITIC [46], in this work, we also fabricated the non-
fullerene devices based on TBFPO-BDD/ITIC and TBFO-BDD/ITIC.
The result indicates that when TBFPO-BDD blending with ITIC
JSCLC
¼
0
r
ε ε m
where J is the current density, d stands for the thickness of the
device, ε is the relative dielectric constant of active layer material
usually 2e4 for organic semiconductor, herein we use a relative
dielectric constant of 4, ε is the permittivity of empty space, is the
mobility of hole or electron, V is the internal voltage in the device,
and V ¼ Vappl -Vbi, where Vappl is the applied potential and Vbi is the
built-in potential (in the hole-only and the electron-only devices,
(
with or without DIO), there is no any improvement of the photo-
r
voltaic performance than those of PC71BM (6.58%). However,
compared with the TBFPO-BDD:PC71BM, the PCEs of devices based
on TBFO-BDD: ITIC (with and without DIO) both have lager
improvement as shown in Table S1. EQEs of PSCs under the best
condition with PDINO as cathode layer for TBFPO-BDD:PC71BM
without any treatment and TBFPO-BDD: PC71BM with 1% DIO as
solvent additive are shown in Fig. 4b. The EQE curves are essentially
in accordance with the absorption wavelength range from Fig. 1,
BHJ solar cells exhibit a spectral response from 300 nm to 700 nm
with a broad and flat peak value around 60%e70% at approximately
0
m
Vbi values are 0.2 V and 0 V, respectively). The hole and electron
mobilities of the TBFPO-BDD/PC71BM blend without any treatment
ꢁ
4
2
ꢁ1
ꢁ5
ꢁ1
s ,
can be calculated to be 1.84
ꢂ
10
cm
V
and
s ,
ꢁ
4
2
ꢁ1 ꢁ1
2
ꢁ1 ꢁ1
1
4
.21 ꢂ 10 cm V
s
s
, respectively, while 5.10 ꢂ 10 cm V
ꢁ
5
2
ꢁ1 ꢁ1
.17 ꢂ 10 cm V
with 1% DIO treatment. This result suggests
3
3
50e650 nm for TBFPO-BDD and 30%e40% at approximately
70e650 nm for TBFO-BDD without any treatment, respectively.
DIO may suppress exciton separation and transport in TBFPO-BDD/
PC71BM blend system However, the hole and electron mobilities of
the TBFO-BDD/PC71BM blend with 1% DIO as additive, which is
conducive to form the interpenetrating networks to realize the
Additionally, a less than 5% deviation between Jsc and the integral of
the EQE is observed. Moreover, light-intensity dependence of
photovoltaic performance with the optimized condition is further
used to investigate the charge recombination behavior in the de-
vices, which is regarded as one of the determining factors to affect
the value of Jsc [47]. Generally, Jsc follows a power-law dependence
ꢁ
5
2
ꢁ1 ꢁ1
separation and transport of exciton, are 2.62 ꢂ 10 cm
V
s ,
ꢁ
5
2
ꢁ1
ꢁ1
s ,
3
1
.86
.19 ꢂ 10 cm V
DIO.
ꢂ
10
cm
V
respectively, higher than
ꢁ5
2
ꢁ1 ꢁ1
ꢁ5
2
ꢁ1 ꢁ1
s without 1%
s
and 0.20 ꢂ 10 cm V
on Plight (JscfPlight
when the bimolecular recombination of the charge carriers is
weak. As shown in Fig. 4c, the value of the TBFO-BDD as donor
material is 0.93, while the value of the TBFPO-BDD is 0.97, which
a), where power-law component (a) should be
1
3.5. Photovoltaic properties
a
a
In order to investigate the photovoltaic properties of two
suggests that the TBFPO-BDD as donor material in PSCs can sup-
press the bimolecular recombination well in correlation well with
the higher charge carrier mobilities of the devices mentioned
above.
different polymers, PSC devices with a configuration of ITO/
PEDOT:PSS/polymer:PC71BM/PDINO(ZrAcac)/Al were prepared and
measured by blending with PC71BM in ODCB at different weight
ratios (w/w; 1:1 and 1:2), DIO as additive. Fig. 4a shows the typical
current density-voltage (J-V) curves under the illumination of AM
3.6. Morphology
1
.5G, 100 mW/cm2 and the corresponding device performance
including Voc,
also summarized in Table 3. The highest PCE of TBFPO-BDD up to
.58% was obtained at a weight ratio of 1:1 without any treatment,
while the PCE of TBFO-BDD at 1:1 wt ratio was increased from
.29% to 3.21% with the addition of 1% DIO. Using ZrAcac and PDINO
as the different cathode buffer layer, the data of photovoltaic per-
formances are shown in Table 3. The different PCEs of TBFPO-BDD
and TBFO-BDD are mainly originated from Jsc and FF. Also the
J
sc, FF and PCE at different fabrication conditions is
The nanoscale morphology of the photoactive layer is also
important for device performances compared with absorption,
energy level and mobility, which is related to exciton dissociation
and charge transport. Therefore, we investigated the surface
morphology of the active layer by AFM (shown in Fig. 5aeh). The
surface roughness (RMS, root-mean-square) of TBFPO-BDD:PC71BM
(1:1) blend (without or with 1% DIO as additive) decreases from
1.23 nm (Fig. 5a) to 0.60 nm (Fig. 5c). Moreover, comparing with 1%
6
2

L. Qiu et al. / Dyes and Pigments 140 (2017) 337e345
343
Fig. 5. Tapping mode AFM height images: (a) TBFPO-BDD without DIO, (c) TBFPO-BDD with DIO, (e) TBFO-BDD without DIO, (g) TBFO-BDD with DIO and phase images: (b) TBFPO-
2
BDD without DIO, (d) TBFPO-BDD with DIO, (f) TBFO-BDD without DIO, (h) TBFO-BDD with DIO of the surface of the blend of polymer with PC71BM (1:1) (size: 3 ꢂ 3
m
m ). Root-
mean-square (RMS) roughness values are given to describe the smoothness of the morphology.

344
L. Qiu et al. / Dyes and Pigments 140 (2017) 337e345
DIO treatment, the phase images (Fig. 5b without DIO treatment)
shows better crystalline grains. Together with the photovoltaic data
in Table 3, the addition of DIO has decreased PCE of TBFPO-BDD. By
contrast, the TBFO-BDD has the opposite results, the RMS of TBFPO-
BDD: PC71BM(1:1) (without or with 1% DIO as solvent additive)
increases from 0.63 nm to 0.93 nm from Fig. 5e and h. The blend
with 1% DIO as shown in Fig. 5h has more surface aggregates with
no large phase separation, the addition of DIO can improve PCE
from 2.15% to 3.07% for of TBFO-BDD/PC71BM blend.
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. Conclusion
[
In summary, we have designed and synthesized two new me-
dium bandgap alkoxyphenyl and alkoxyl substituted thieno[2,3-f]
benzofuran (TBF)-based polymers to further explore the co-
polymers without or with 2D-cojugated side chains. The 2D-coju-
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ITO/PEDOT:PSS/TBFPO-BDD:PC71BM(1:1; w/w)/PDINO/Al, exhibi-
[
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2
100 mW/cm , without any post-treatment, with higher Jsc and FF
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Acknowledgements
This work was supported by NSFC (No. 51673205, 51173206),
Project of innovation-driven Plan in Central South University, China
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