High efficiency polymer solar cells based on alkylthio substituted benzothiadiazole-quaterthiophene alternating conjugated polymers

Article abstract of DOI:10.1016/j.orgel.2016.10.032

We have designed and synthesized two alkylthio substituted benzothiadiazole-quaterthiophene based conjugated polymers (P1 and P2) and investigated their photovoltaic performances. Theoretical simulation has demonstrated that the introduction of alkylthio substituents can increase the planarity of the resulted conjugated polymers. The fluorinated polymer P1 possesses a deeper HOMO energy level than the non-fluorinated polymer P2 and can form well-developed fibril networks when blended with PC₇₁BM. PSCs based on P1:PC₇₁BM (1:1.2, by weight) gave a PCE of 7.76% with a V_oc of 0.69?V, a J_sc of 16.30?mA?cm^?2 and an FF of 0.69. Our results have demonstrated that alkylthiothiophene could be a useful building block for the construction of high efficiency polymer donor materials used for PSCs.

Full text of DOI:10.1016/j.orgel.2016.10.032

Organic Electronics 40 (2017) 36e41
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
High efficiency polymer solar cells based on alkylthio substituted
benzothiadiazole-quaterthiophene alternating conjugated polymers
Zhe Zhang a, Zhen Lu b, Jicheng Zhang a, Yahui Liu a, Shiyu Feng a, Liangliang Wu a,
,
, **
Ran Hou a, Xinjun Xu a *, Zhishan Bo a
a Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Key Laboratory of Theoretical and Computational
Photochemistry, Ministry of Education, Beijing Normal University, Beijing 100875, China
b College of Chemistry and Chemical Engineering, Shanxi Datong University, Datong 037009, China
a r t i c l e i n f o
a b s t r a c t
Article history:
We have designed and synthesized two alkylthio substituted benzothiadiazole-quaterthiophene based
conjugated polymers (P1 and P2) and investigated their photovoltaic performances. Theoretical simu-
lation has demonstrated that the introduction of alkylthio substituents can increase the planarity of the
resulted conjugated polymers. The fluorinated polymer P1 possesses a deeper HOMO energy level than
the non-fluorinated polymer P2 and can form well-developed fibril networks when blended with
PC71BM. PSCs based on P1:PC71BM (1:1.2, by weight) gave a PCE of 7.76% with a Voc of 0.69 V, a Jsc of
16.30 mA cmꢀ2 and an FF of 0.69. Our results have demonstrated that alkylthiothiophene could be a
useful building block for the construction of high efficiency polymer donor materials used for PSCs.
© 2016 Elsevier B.V. All rights reserved.
Received 20 September 2016
Received in revised form
19 October 2016
Accepted 21 October 2016
Keywords:
Conjugated polymer
Organic solar cells
Donor and acceptor
Low bandgap polymer
Photovoltaic
1. Introduction
and their morphology in the active layer [37e39]. Recently, thienyl-
substituted or phenyl-substituted benzodithiophenes carrying
Polymer solar cells (PSCs) have received more and more atten-
tion in recent years because of their good solution processability
and mechanical flexibility, which potentially allow them to be
fabricated by more convenient and low-cost processes such as roll-
to-roll printing or inkjet printing [1e8]. To accomplish the goal of
application, the development of novel active layer materials has
played an important role in improving the device performance.
Within which, the progress of low band-gap (LBG) D-A conjugated
polymer donors has helped to significantly enhance the power
conversion efficiency (PCE) of PSCs because of their tunable band
gaps and energy levels [9e21]. Among the reported LBG polymers,
benzothiadiazole-quaterthiophene based polymers usually afford
high PCEs [22e36].
alkylthio chains have been used as building blocks for the synthesis
of polymer donors [20,21,25,36,40,41]. Compared with alkyl/alkoxy
chains, alkylthio chains have been demonstrated to be more
effective in tuning the HOMO levels of most conjugated polymers.
For instance, Li's group have reported a series of 5-alkylthiothienyl
substituted benzodithiophene based polymers. Compared with 5-
alkylthienyl or 5-alkoxythiothienyl substituted benzodithiophene
based analogues, the Voc of devices was enhanced from 0.78 or
0.74 Ve0.84 V [25]. Hou's group have reported a series of conju-
gated polymers, which yielded an enhancement of Voc in ~0.15 V in
comparison with the 4,5-bis(alkylthio)thienyl substituted benzo-
dithiophene based analogues [36]. Our group have reported syn-
thesized
a
series
of
4-(alkylthio)phenyl
substituted
Generally, a variety of molecular design strategies have been
used in tuning the band gap and energy level of polymer donors.
For example, altering the side chains of conjugated polymers can
tune their energy levels, their solubility in the processing solvent,
benzodithiophene based conjugated polymers, which gave an
impressive PCE of 7.7% when blended with PC71BM as the active
layer and a PCE of 10.4% in ternary PSCs with ITIC and PC71BM as the
acceptors [20,21]. However, up to now, the reported alkylthio
substituents are all confined to benzodithiophene groups. Although
benzodithiophene unit is a very attractive building block to
construct high-performance polymer donors, it suffers from rapid
degradation due to the photooxidation in air at the 4- and 8-
* Corresponding author.
** Corresponding author.
E-mail address: xuxj@bnu.edu.cn (X. Xu).
http://dx.doi.org/10.1016/j.orgel.2016.10.032
1566-1199/© 2016 Elsevier B.V. All rights reserved.

Z. Zhang et al. / Organic Electronics 40 (2017) 36e41
37
positions [42]. To the best of our knowledge, the photovoltaic
performance of 4-alkylthio substituted benzothiadiazole-
quaterthiophene based conjugated polymers has never been
exploited yet. Herein, we have synthesized two benzothiadiazole-
quaterthiophene based conjugated polymers (P1 and P2)
comprising 4-alkylthio substituents as side chains, and demon-
strated by theoretical simulation that the introduction of alkylthio
substituent can significantly increase the planarity of polymer
backbones in comparison with their alkyl/alkoxy substituted
counterparts. The fluorinated polymer P1 possesses a deeper
HOMO energy level and can form well-developed fibril network
morphology when blended with PC71BM. And P1 based PSCs gave a
PCE of 7.76% with a Voc of 0.69 V, a Jsc of 16.30 mA cmꢀ2 and an FF of
0.69.
weights and their distributions of P1 and P2 were obtained by gel
permeation chromatography (GPC) using CB as an eluent at 130 ꢁC
with narrowly distributed polystyrenes as calibration standards,
and the results are summarized in Table S1. Thermal properties of
P1 and P2 were investigated by thermogravimetric analysis (TGA)
under a nitrogen atmosphere. As shown in Fig. S2, P1 and P2 dis-
played a very good thermal stability with the 5% decomposition
temperature up to 345 and 310 ꢁC, respectively. The differential
scanning calorimetry (DSC) test was performed from 80 ꢁC to
250 ꢁC at the heating rate of 10 ꢁC/min under a nitrogen atmo-
sphere. The DSC traces in Fig. S3 showed no obvious glass transition
for these two polymers. The packing of polymer chains as film was
characterized using small angle X-ray diffraction (SAXRD) method.
As shown in Fig. 1, P1 exhibited three diffraction peaks in the XRD
curves. The first peak, which reflected the distance of polymer
backbones separated by the flexible side chains, is located at 2
7.91ꢁ, corresponding to a distance of 11.17 Å. The peak located at 2
of 24.75ꢁ in the wide angle region reflects the
stacking distance
q of
q
2. Results and discussion
p-p
2.1. Material synthesis and characterization
of 3.59 Å between polymer backbones [26]. In contrast to the
polymer with the same length of alkyl side chain reported in the
literature which has a lamellar stacking distance of 19.0 Å and a p-p
The syntheses of M1, M2, P1 and P2 are outlined in Scheme 1
and the concrete procedures are provided in the Supporting
Information. Compound 1 was synthesized according to the re-
ported procedure [43]. Compounds 4, 5 and 8 were purchased from
Suna.Tech. Inc.. Starting from commercially available 3-
bromothiophene, its treatment with n-butyllithium (n-BuLi) was
followed by quenching the formed 3-thienyl anion with sublimed
sulfur to afford thiophene-3-thiol (1) in a yield of 88%. The reaction
of compound 1 and 1-bromo-2-octyldodecane with potassium tert-
butoxide as the base in anhydrous ethanol gave compound 2 in a
yield of 96%. The subtraction of the proton at the 2-position of
thiophene ring in compound 2 with lithium dimethylamide (LDA)
was followed by reaction with tri(n-butyl)tin chloride to give
compound 3, which was directly used for the next step without
further purification. Stille cross-coupling of compound 3 and 4,7-
dibromo-5,6-difluoro-2,1,3-benzothiadiazole (4) using Pd2(dba)3
and P(o-tol)3 as the catalyst precursors and THF as the solvent
furnished compound 6 in a yield of 77%. Bromination of compound
6 with N-bromosuccinimide (NBS) in CH3Cl3 gave M1 in almost
quantitative yield. Polymer P1 was synthesized in a yield of 70% by
Stille cross-coupling reaction of M1 and compound 8 using
Pd(PPh3)4 as the catalyst precursor in a solvent mixture of toluene
and N,N-dimethylformamide (DMF) at 110 ꢁC. Similarly, compound
7, M2 and polymer P2 were synthesized. P1 is insoluble in
commonly used organic solvents such as chlorobenzene (CB), o-
dichlorobenzene (DCB) at room temperature, but can be fully sol-
uble at elevated temperature; whereas P2 is fully soluble in the
above mentioned organic solvents at room temperature. Molecular
stacking distance of 3.70 Å, our alkylthio substituted polymer
shows a more closely packing [19]. This result may be attributed to
the F/S interaction which could improve inter/intramolecular
interaction of polymer chains [44]. As for P2, two diffraction peaks
were observed in the XRD curve. The first peak located at 2q
of 7.53ꢁ
in the small angle region reflected the distance of polymer back-
bones separated by the flexible side chains, which is corresponding
to a distance of 11.72 Å. The second peak, which reflected the
p
-p
stacking distance between polymer backbones, are located at 2
q
of
8000
P1
P2
6000
4000
2000
0
5
10
15
20
25
2 Theta(Degree)
Fig. 1. Film XRD curves of P1 and P2.
Scheme 1. Synthetic route to polymers P1 and P2. (a) BuLi, S8, Et2O; (b) (CH3)3COK, RBr, EtOH; (c) LDA, Tri(n-butyl)tin chloride, THF; (d) 4,7-dibromo-5,6-difluorobenzothiadiazoel
(4) or 4,7-dibromobenzothiadiazole (5), Pd2(dba)3, P(o-tol)3,THF; (e) NBS, CH3Cl3; (f) Pd(PPh3)4, toluene/DMF.

38
Z. Zhang et al. / Organic Electronics 40 (2017) 36e41
24.57ꢁ, corresponding to a distance of 3.62 Å. In the wide angle
region, the diffraction peak of P1 become more acute than P2,
which could reflect that crystallinity of polymers is improved when
F atom is incorporated into the polymer backbone. The appropriate
increase of crystallinity contributes to the improvement of the
specific nanoscale morphology of the active layer. This is also
consistent with the results of morphology characterizations
[45,46]. Density functional theory (DFT) calculations at the B3LYP/
6-31G(d) level were also used to investigate the molecular simu-
lation of P1 and P2 are shown in Fig. 2. For simplicity, methyl
substituents were used for the calculation instead of 2-octyldodecyl
chains. Compared with the alkyl substituted polymer in Fig. 2 (c),
alkylthio substituted polymers (P1 and P2) showed better planar
a
1.0
0.8
0.6
0.4
0.2
0.0
P1 solution under 100oC
P2 solution under 100oC
400 500 600 700 800
Wavelength (nm)
conformation, which would lead to extended
enhanced intermolecular interactions.
p-conjugation and
b
1.0
0.8
0.6
0.4
0.2
0.0
P1 film
P2 film
2.2. Optical properties and electrochemical properties
Optical properties of P1 and P2 were investigated by UVevis
absorption spectroscopy. As shown in Fig. 3a, P1 and P2 in DCB
solutions at 100 ꢁC displayed similar absorption spectra with two
absorption peaks located at 450 and 629 nm for P1 and 433 and
600 nm for P2. In going from solution to film, the two absorption
peaks are red-shifted to 451 and 649 nm with a new should peak at
691 nm as shown in Fig. 3b, indicating that P1 form stronger
interchain interactions in the solid state. For P2 as film, the two
absorption peaks were red-shifted to 449 and 629 nm as shown in
Fig. 3b. No should peak was observed in the long wavelength region
indicated that P2 film is more amorphous. The onsets of film ab-
sorption for P1 and P2 were determined to be 780 and 775 nm,
respectively. The band gaps of P1 and P2 were therefore calculated
to be 1.59 and 1.60 eV, respectively, according to the equation:
Eg,opt ¼ 1240/lonset. Electrochemical properties of P1 and P2 were
investigated by cyclic voltammetry (CV) using a standard three-
electrode electrochemical cell and the CV curves are shown in
Fig. 4. The highest occupied molecular orbital (HOMO) energy
levels of P1 and P2 were determined using the equation EHOMO ¼ -
e(Eox þ 4.71) to be ꢀ5.28 and ꢀ5.25 eV, respectively, indicating that
the fluorine atom substitution can lower the HOMO energy level of
polymers. Consequently, devices fabricated with P1 are expected to
show higher Voc than those fabricated with P2. The lowest unoc-
cupied molecular orbital (LUMO) energy levels of P1 and P2 were
calculated according to the equation ELUMO ¼ EHOMO þ Eg,opt to
be ꢀ3.69 and ꢀ3.65 eV, respectively. The optical data, energy levels
and band gaps are also summarized in Table 1.
300
400
500
600
700
800
Wavelength (nm)
Fig. 3. UVevisible absorption spectra of P1 and P2 in DCB solutions at 100 ꢁC (a) and
as films (b).
0.8
PI
P2
0.6
0.4
0.2
0.0
-0.2
-0.4
0.0
0.2
0.4
0.6
0.8
1.0
Potential (V vs Ag/Ag+)
Fig. 4. Cyclic voltammograms of copolymers P1 and P2 in films on a platinum elec-
trode in 0.1 mol/L Bu4NPF6 acetonitrile solution at a scan rate of 100 mV/s.
2.3. Photovoltaic properties
To evaluate the photovoltaic performance of P1 and P2, PSCs
were fabricated with the conventional device configuration of in-
poly(styrenesulfonate) (PEDOT:PSS)/polymer:PC71BM/LiF/Al. The
weight ratio of polymer to PC71BM, the morphology of the active
dium
tin
oxide
(ITO)/poly(3,4-ethylenedioxythiophene):
Fig. 2. Molecular geometry of the polymers with methyl groups replacing the alkyl substituents to simplify the calculations: a, P1; b, P2; c, alkylthio side chains of P1 was replaced
by the same alky side chains (yellow, sulfur; green, fluorine; blue, nitrogen). (For interpretation of the references to colour in this figure legend, the reader is referred to the web
version of this article.)

Z. Zhang et al. / Organic Electronics 40 (2017) 36e41
39
Table 1
Physical, electronic, and optical properties of P1 and P2.
a
b
Polymer
Td (ꢁC)
l
(nm) solution
l
(nm) film
Eg,opt (eV)
HOMO (eV)
LUMO (eV)
P1
P2
345
310
450, 629
433, 600
451, 649, 691
449, 629
1.59
1.60
ꢀ5.28
ꢀ5.25
ꢀ3.69
ꢀ3.65
a
Td is the 5% weight-loss temperature under N2 atmosphere.
b
Calculated from the onset absorption of P1 and P2, Eg,opt ¼ 1240/lonset
.
layer and thickness of the active layer were optimized. Since ad-
ditive can influence the morphology of active layer, the content of
1,8-diiodooctane (DIO) was also screened. The optimal devices
were fabricated from a solution of polymer and PC71BM in CB and
DCB (1:1, v/v) containing 2.5% of DIO as the additive, and the
polymer concentration is 9 mg mlꢀ1 and the D/A ratio is 1:1.2. In
order to completely dissolve the polymer, the blend solution was
stirred at 110 ꢁC for at least 5 h. Both the blend solutions and the ITO
substrates were preheated to 110 ꢁC in a glove box before spin-
coating the active layer. And the active layers were annealed at
80 ꢁC for 10 min before being transferred to the vacuum chamber of
thermal evaporator inside the same glove box.
inter/intra molecular interactions, which usually lead to well-
developed fibril structure (vide infra) in the active layer [19,42]. In
addition, P1 based devices also exhibited slightly higher Voc than P2
based devices because of its slightly deeper HOMO energy level.
Consequently, P1 exhibited higher PCE than P2. Compared with the
donor polymer with the same backbone but substituted by alkyl
groups (polymer FH) reported in the literature, under the condition
of a similar device structure alkylthio substituted polymer (P1)
showed a better PCE than FH (6.4% reported in the reference) [19].
Although Yan's group has reported that alkyl substituted polymer
(FH) shows a higher PCE than this work, it should be noted that the
device structure is different with this work [10].
The current density-voltage (J-V) curves of the best performance
devices fabricated with and without DIO (2.5%) under AM 1.5G
illumination are shown in Fig. 5a. For P1, devices fabricated from
CB þ DCB solutions with and without 2.5% DIO gave PCEs of 7.76%
and 5.22%, respectively; whereas for P2, devices fabricated from
CB þ DCB solutions with and without 2.5% DIO gave PCEs of 2.02%
and 1.75%, respectively. The device parameters are also summarized
in Table 2. As expected, the fluorinated polymer P1 exhibited larger
Jsc and higher FF than non-fluorinated polymer P2. The fluorinated
polymers usually exhibited higher charge carrier mobility than the
corresponding non-fluorinated polymers because of their enhanced
External quantum efficiencies (EQEs) of optimized devices
measured under monochromatic light are shown in Fig. 5 b. When
using 2.5% DIO as the processing additive, P1 exhibited higher EQEs
(above 50%) than P2 in the range of 400e730 nm, indicating a more
efficient photo conversion process and a more balanced charge
transportation in devices. The Jsc values obtained from the inte-
gration of EQE curves are consistent with the Jsc obtained from J-V
measurements. Hole mobilities of blend films fabricated under the
optimized conditions were characterized by space charge limited
current (SCLC) method, the curve shown in Fig. S4. The P1:PC71BM
and P2:PC71BM blend films exhibited hole mobilities of 4.73 ꢂ 10ꢀ4
and 4.40 ꢂ 10ꢀ5 cm2 vꢀ1 sꢀ1, respectively. The polymer P1 shows a
higher hole mobility than P2, which can be attributed to the F atom,
because F atom can suppress bimolecular recombination as well as
improve specific nanoscale morphology of active blends [44,46].
The SCLC mobility results are quite consistent with the measured Jsc
and PCEs.
a
P1
P2
P1(2.5% DIO)
P2(2.5% DIO)
0
-5
2.4. Film morphologies
-10
-15
-20
The morphology of blend films fabricated under optimized
conditions was investigated by atomic force microscopy (AFM) in
tapping mode and transmission electron microscope (TEM) and the
AFM and TEM images are shown in Figs. 6 and 7, respectively. As
shown in Fig. 6, the morphologies of P1:PC71BM and P2:PC71BM
blend films were of some difference. P1 in the blend film formed
fibrils and showed a good miscibility with PC71BM; whereas P2 in
the blend film formed larger aggregates and exhibited apparent
phase separation with PC71BM. For the P1:PC71BM blend film, the
formation of more homogeneous morphology with well-developed
interconnected network structure can be more easily observed in
the TEM images as shown in Fig. 7. For the P2:PC71BM blend film, it
can be clearly seen that larger aggregates were formed from the
TEM images as shown in Fig. 7. The morphology difference was
probably caused by the solubility difference of these two polymers
in the processing solvent. For P1, the poor solubility made it
precipitated firstly to form nanofibrils during the drying of the
blend films. The formation of nanofibrils can further prevent the
formation of large aggregates by PC71BM during the drying of the
blend films; whereas for P2, PC71BM molecules might precipitate
firstly to form larger spherical aggregates as shown in Fig. 7 due to
the good solubility of P2 in the processing solvent. As reported in
the literature, the formation of phase separation in nanoscale for
the donor and acceptor in the blend film is beneficial for charge
0.0
0.2
Voltage / V
0.4
0.6
0.8
70
60
50
40
30
20
10
0
b
P1
P1(2.5% DIO)
P2
P2(2.5% DIO)
300 400 500 600 700 800
Wavelength (nm)
Fig. 5. J-V curves (a) and EQE spectra (b) of polymer/PC71BM solar cells.

40
Z. Zhang et al. / Organic Electronics 40 (2017) 36e41
Table 2
Photovoltaic performances of Polymer:PC71BM blend films.
Polymer
Solvent
Voc (V)
Jsc (mA cmꢀ2
)
FF
PCE (%) best/aveb
Thickness (nm)
P1
CB þ DCB
CB þ DCBa
CB þ DCB
CB þ DCBa
0.69
0.69
0.64
0.63
11.30
16.30
4.88
0.67
0.69
0.56
0.58
5.22/5.01
7.76/7.61
1.75/1.67
2.02/1.98
/
136
/
105
P2
5.52
a
With 2.5% DIO, by volume.
The average PCEs were based on ten devices.
b
Fig. 6. AFM images of P1:PC71BM and P2:PC71BM blend films fabricated under optimized conditions.
substituted thiophene units in each repeating unit. As demon-
strated by molecular simulation, the alkylthio substituted polymers
(P1 and P2) showed a more planar conformation than those alkyl
substituted polymers. PSCs with the blend of P1:PC71BM (1:1.2, by
weight) as the active layer showed a PCE of 7.76% with a Voc of
0.69 V, a Jsc of 16.30 mA cmꢀ2 and an FF of 0.69 under AM 1.5G
illumination. In comparison with the non-fluorinated counterpart
P2, the fluorinated polymer P1 possesses a deeper HOMO energy
level and can form well-developed fibril network morphology
when blended with PC71BM. And fluorinated polymer P1 based
PSCs showed higher PCEs than non-fluorinated polymer P2. Our
results have demonstrated that 3-alkylthiothiophene could be a
useful building block for the construction of high efficiency poly-
mer donor materials used for PSCs.
Fig. 7. TEM images of P1:PC71BM and P2:PC71BM blend films fabricated under opti-
mized conditions, the scale bar is 0.2 mm.
Acknowledgements
carrier generation and transportation. The marked morphology
difference of the active layer showed a significant influence on the
performance of photovoltaic devices. The device results can be well
illustrated by the investigation of morphology of blend films.
Financial support from the NSF of China (91233205 and
21574013), Program for Changjiang Scholars and Innovative
Research Team in University and the Fundamental Research Funds
for the Central Universities are gratefully acknowledged.
3. Conclusions
Appendix A. Supplementary data
In summary, we have designed and synthesized two novel
conjugated polymers (P1 and P2) comprising two alkylthio
Supplementary data related to this article can be found at http://

Z. Zhang et al. / Organic Electronics 40 (2017) 36e41
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