Implication of side-chain fluorination on electronic properties, ordering structures, and photovoltaic performance in asymmetric-indenothiophene-based semiconducting polymers

Article abstract of DOI:10.1016/j.orgel.2019.04.010

Side-chain fluorination of conjugated polymers has been proved to be a highly effective approach for optimizing optical and electrical properties of the relative polymers. However, current studies on the fluorination are all based on symmetric molecular structures. In this work, two new D-A type photovoltaic polymers, namely PIT_Ph-DT_ffBT and PIT_Phf-DT_ffBT, based on asymmetric indenothiophene (IT) donor units with alkoxyphenyl or fluoroalkoxyphenyl substituents were designed and synthesized. Effects of the fluorine substitution in the asymmetric IT donor units on the electronic structure, ordering structure, photovoltaic properties, and charge generation and recombination dynamics were investigated. It is found that side-chain fluorination in the asymmetric donor units of the D-A polymers endowed the relative polymer with a deeper HOMO level, higher and more balanced charge mobilites, increased charge dissociation efficiency and reduced bimolecular recombination. As a result, the bulk heterojunction solar cell based on the blend film of PIT_Phf-DT_ffBT and PC₇₁BM demonstrated an efficiency of 7.03%, whereas the cell efficiency based on PIT_Ph-DT_ffBT was only 5.68%. These results indicate that our design strategy by introducing a fluoroalkoxyphenyl unit as side chain to fabricate asymmetric IT-based polymer is efficient in improving the photovoltaic performance. We believe that the results provide new insights into the design of high-performance semiconducting photovoltaic polymers.

Full text of DOI:10.1016/j.orgel.2019.04.010

Accepted Manuscript
Implication of side-chain fluorination on electronic properties, ordering structures,
and photovoltaic performance in asymmetric-indenothiophene-based semiconducting
polymers
Chenglin Jiang, Xiaofeng Chen, Mingzhi Zhao, Yongfang Li, Xiaoyu Li, Haiqiao Wang
PII:
S1566-1199(19)30175-2
DOI:
https://doi.org/10.1016/j.orgel.2019.04.010
ORGELE 5204
Reference:
To appear in: Organic Electronics
Received Date: 30 January 2019
Revised Date: 9 April 2019
Accepted Date: 9 April 2019
Please cite this article as: C. Jiang, X. Chen, M. Zhao, Y. Li, X. Li, H. Wang, Implication of side-
chain fluorination on electronic properties, ordering structures, and photovoltaic performance in
asymmetric-indenothiophene-based semiconducting polymers, Organic Electronics (2019), doi: https://
doi.org/10.1016/j.orgel.2019.04.010.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Implication of Side-Chain Fluorination on Electronic Properties, Ordering
Structures,
and
Photovoltaic
Performance
in
Asymmetric-Indenothiophene-Based Semiconducting Polymers.
a,b
a,b
a,b
c
b
a,b
Chenglin Jiang , Xiaofeng Chen , Mingzhi Zhao , Yongfang Li , Xiaoyu Li , Haiqiao Wang
a. State Key Laboratory of Organic-Inorganic Composite, Beijing University of Chemical
Technology, Beijing 100029, China.
E-mail: wanghaiqiao@mail.buct.edu.cn (H. Wang)
b. Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing
University of Chemical Technology, Beijing 100029, China
E-mail: lixy@mail.buct.edu.cn (X. Li)
c. Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy
of Sciences, Beijing 100190, China
Abstract
Side-chain fluorination of conjugated polymers has been proved to be a highly effective approach
for optimizing optical and electrical properties of the relative polymers. However, current studies
on the fluorination are all based on symmetric molecular structures. In this work, two new D-A
type photovoltaic polymers, namely PIT -DT BT and PIT -DT BT, based on asymmetric
Ph
ff
Phf
ff
indenothiophene (IT) donor units with alkoxyphenyl or fluoroalkoxyphenyl substituents were
designed and synthesized. Effects of the fluorine substitution in the asymmetric IT donor units on
the electronic structure, ordering structure, photovoltaic properties, and charge generation and
recombination dynamics were investigated. It is found that side-chain fluorination in the
asymmetric donor units of the D-A polymers endowed the relative polymer with a deeper HOMO
level, higher and more balanced charge mobilites, increased charge dissociation efficiency and
reduced bimolecular recombination. As a result, the bulk heterojunction solar cell based on the
blend film of PIT -DT BT and PC BM demonstrated an efficiency of 7.03%, whereas the cell
Phf
ff
71
efficiency based on PIT -DT BT was only 5.68%. These results indicate that our design strategy
Ph
ff
by introducing a fluoroalkoxyphenyl unit as side chain to fabricate asymmetric IT-based polymer
is efficient in improving the photovoltaic performance. We believe that the results provide new
insights into the design of high-performance semiconducting photovoltaic polymers.
Keywords: Asymmetric; Indenothiophene; Side-chain fluorination; Solar cells
Introduction
The past decade has witnessed the rapid improvement in polymer solar cells (PSCs) with
[
1,2]
advantages of low-cost, light weight, and flexibility.
heterojunction structure with a p-type conjugated polymer as donor and a n-type material
Generally, efficient PSCs adopt a bulk
[3-7,38]
(
fullerene derivatives, nonfullerene small-molecular, conjugated polymers) as acceptor.
Fullerene derivatives, such as PC BM ([6,6]-phenylC butyric acid methyl ester) and ICBA
7
1
71
(
indene-C bisadduct), are one of the most widely used acceptor materials in PSCs. The great
60
success of PC BM or ICBA could be ascribed to their fundamental properties, including high
7
1
[8,9]
electron affinity and electron mobility, and ability to form appropriate phase separation. A lot of

ACCEPTED MANUSCRIPT
literatures have focused on the design of highly efficient donors matched with fullerene acceptor,
[10-14]
leading to devices with power conversion efficiencies (PCEs) over 10%.
Recently, asymmetric structural materials, including polymer or small-molecule, have caused
increasing attention. Compared with symmetric units, asymmetric structures can induce higher
dipole moment which can strongly influence molecular self-assembly and resultantly improve the
[15]
order and crystallinity in BHJ films. With the increase of dipole moment, the intramolecular
charge transfer (ICT) effect will be enhanced, thus resulting in the higher charge carrier
[16,17]
mobility.
Furthermore, it has been proven that the asymmetric structures also efficiently
decrease the highest occupied molecular orbital (HOMO) values of the corresponding
[18,19]
polymers,
which would be beneficial for higher Voc achieved for the relative devices. For
example, in order to achieve a donor unit with a deep HOMO level and good J and FF values,
sc
[20]
Hoang et al. designed a new BDT monomer, bearing an alkoxy group on one side of the moiety
and an alkyl group on the other side. This strategy both reduced the HOMO energy level and
[21]
maintained J and FF compared to the dialkoxy-BDT polymer. Gu et al. synthesized a new D-A
sc
type polymer (PBDT -DT BT) based on alkylthienyl asymmetric BDT and the well-known
Th
ff
4
,7-di(thiophen-2-ethylhexyl)-5,6-difluoro-2,1,3-benzothiadiazole (DT BT), the optimized
ff
-2
PBDT -DT BT based device exhibited a V of 0.87 V, a J of 15.06 mA cm , FF of 70.4% and
Th
ff
oc
sc
a high PCE of 9.22%. The PCE is higher than the control polymers which have the same backbone
[22]
structure but have the 1D (8.3%) or 2D (6.2%) symmetric substituted BDT unit. Guo et al.
designed and synthesized a small molecule named DRBDTCO based on BDT with an asymmetric
side chain as donor material for solution-processed bulk heterojunction solar cells. The optimized
device exhibits a high PCE of 8.18% with an outstanding FF of 73%. These results demonstrated
that the asymmetric BDT unit could be a promising building block for photovoltaic materials with
high efficiencies and high fill factors. Indenothiophene (IT), an asymmetric counterpart of
[23]
indacenothiophene (IDT), was ultilized by Wang et al.
as the donor unit for building D-A
copolymers. These PSCs based on IT copolymers were found to exhibit the highest PCE of 9.14%,
[24]
meanwhile with a high Voc of 1.0 V. Kim et al.
synthesized a new asymmetric n-type
semiconducting molecule (PhITBD) with an indenothiophene core. The relative PSC with
asymmetric PhITBD displayed improved PCE of 6.57% as compared to those with the symmetric
molecular backbone.
In addition, it has been demonstrated that side-chain fluorination of conjugated polymers is a
highly effective approach for optimizing optical and electrical properties of the relative polymers,
[25-32,47]
thus improving the photovoltaic performance of fullerene-based PSCs.
Given the strong
electron-withdrawing nature, introduction of fluorine can effectively lower the HOMO and
LUMO energy levels while minimally changing energy band gap (E ), resulting in the
g
[
27,29,33-35,39-41]
enhancement of Voc.
In addition, it is also believed that the fluorine atom offers
…
…
noncovalent attractive interactions with the hydrogen or sulfur atoms (F H or F S), which can
promote the molecular planarity and intermolecular assembly, and therefore the crystallinity
without introducing any steric hindrance. This often leads to enhanced charge mobility of the
[36]
polymers.
Furthermore, fluorination can tune the polarity of the polymers and control the
interfacial interaction with fullerenes, resulting in improved BHJ morphology with smaller phase
domains and larger interfacial areas, thus increasing the exciton dissociation and enhancing J and
sc
[37]
FF values. All these features are expected to improve PSC performance. However, as far as we
know, current studies on the side-chain fluorination are all based on symmetric molecular

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structures. Therefore, whether fluorination on asymmetric structure has the same effect mentioned
above still needs to be further studied.
Herein, we designed and synthesized two novel D-A copolymers, named PIT -DT BT and
Phf
ff
PIT -DT BT,
based
on
the
indenothiophene
donor
unit
and
Ph
ff
4
,7-di(thiophen-2-ethylhexyl)-5,6-difluoro-2,1,3-benzothiadiazole (DT BT),
a well-known
ff
acceptor unit. These two polymers feature asymmetric indenothiophene core with lower numbers
of alkyl chains and different numbers of fluorine atoms (0 and 1) in the side chains of the donor
units. The effect of the side–chain fluorination on the performance of the PSCs was investigated
systematically. We found that, side chain fluorination has a remarkable impact on the properties of
the polymers and the relative device performance. First, the fluorinated PIT -DT BT polymer
Phf
ff
shows lower HOMO and LUMO levels, thus improving the V values (0.81 V for PIT -DT BT
oc
Ph
ff
device and 0.84 V for PIT -DT BT device). In addition, fluorination is beneficial for enhancing
Phf
ff
charge mobility and promoting exciton dissociation with suppressed recombination process,
thereby significantly improving both Jsc and FF values. Furthermore, compared with
nonfluorinated PIT -DT BT, the side-chain fluorination of PIT -DT BT facilitates the
Ph
ff
Phf
ff
formation of favorable BHJ morphology with small phase domains. As a result, the PIT -DT BT
Ph
ff
cell exhibited moderate PCE of up to 5.68%, whereas the PIT -DT BT cell yielded PCE as high
Phf
ff
as 7.03%.
Results and discussion
The two polymers were synthesized via Stille coupling reaction between the IT-based donor unit
and the DT BT acceptor unit with Pd (dba) /P(o-tol) as the catalyst. The synthetic routes to the
ff
2
3
3
monomers and the corresponding copolymers are showed in Scheme 1. Because of the asymmetry
of IT structure, Stille coupling reaction could take place on two different sites. Thus, both the two
polymers are random polymers. The resulting copolymers were precipitated in methanol and
filtered off. The collected polymers were subjected to consecutive soxhlet extractions with
methanol, acetone, n-hexane and chloroform in turn, and final the chloroform fractions were
recovered. The copolymers are readily soluble in common organic solvents such as chloroform
(CF), tetrahydrofuran (THF), toluene, and 1,2-dichlorobenzene (o-DCB). Gel permeation
chromatography (GPC) analysis using THF as eluent at room temperature gave a number average
molecular weight (Mn) of 67.5 kDa with a polydispersity index (PDI) of 2.29 for PIT -DT BT
Ph
ff
and an Mn of 70.5 kDa with a PDI of 2.58 for PIT -DT BT with respect to a polystyrene
Phf
ff
standard. Obviously, the differences between the two polymers in Mn and PDI are very small,
which is beneficial for the comparative study on the effect of side-chain fluorination.

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Scheme 1 Synthetic routes to the monomers and the corresponding copolymers.
Optical properties
Fig.1 (a) and (b) show the absorption spectra of PIT -DT BT and PIT -DT BT in chloroform
Ph
ff
Phf
ff
solution and as thin films coated on quartz glass, and the relative optical data are summarized in
Table 1. The absorption spectra in both the solution state and the thin film state exhibited dual
absorption bands. In each spectrum, the absorption band in the 350-500 nm wavelength range
could be assigned to the localized π−π* transition, whereas the relatively broad absorption in the
5
00-700 nm range can be attributed to the intramolecular charge transfer (ICT) between the
acceptor units and donor units. In solution, PIT -DT BT has an absorption maximum peak at 553
Ph
ff
nm, while PIT -DT BT is slightly blue shifted with a maximum at 548 nm. In thin film, the
Phf
ff
absorption peaks for PIT -DT BT and PIT -DT BT bathochoromically shift to 589 and 584 nm,
Ph
ff
Phf
ff
respectively, indicative of some ordered structure and π−π stacking interactions in the film due to
their coplanar backbones. The absorption onsets of PIT -DT BT and PIT -DT BT are at 721
Ph
ff
Phf
ff
and 713 nm in thin film, which correspond to optical bandgaps of 1.72 and 1.74 eV, respectively.
It can be found that the spectrum of PIT -DT BT is slightly blue-shifted in thin film compared
Phf
ff
with that of PIT -DT BT, whether the absorption peak or onset, which may be due to the weak
Ph
ff
[42-44]
steric hindrance of F atom lead to the weakened π−π stacking effect of polymer backbone.

ACCEPTED MANUSCRIPT
[40,45,46]
The similar blue shifts have been observed in some other fluorinated D-A polymer systems.
Thus, the introduction of fluoroalkoxyphenyl side chains into indenothiophene unit will increase
the band gap, which would favor its high V in PSCs.
oc
Fig.1 Normalized UV-vis absorption spectra of the polymers in chloroform solution (a) and in thin
film (b).
Table 1 Optical and electrochemical properties of the polymers
Experimental
Calculated
sol
film
film
opt
e
c
Polymer
λmax
nm)
416,553 426,589 721
λmax
λedge
Eg
HOMO LUMO
Eg
HOMO
(eV)
LUMO
(eV)
Eg
(
(nm)
(nm)
(eV)
1.72
(eV)
(eV)
(eV)
1.78
(eV)
2.21
PIT -DT BT
-5.28
-3.50
-4.92
-2.71
Ph
ff
PIT -DT BT 418,548 424,584 713
1.74
-5.38
-3.52
1.86
-5.00
-2.77
2.23
Phf
ff
Electrochemical properties
As we know, energy level matching of donor and acceptor materials is an important factor to
achieve high-performance PSCs. Cyclic voltammetry (CV) was performed to measure the HOMO
energy level (EHOMO) and LUMO energy level (ELUMO) of the polymers. The measured cyclic
voltammograms curves are illustrated in Fig.S1, and the results are summarized in Table 1. The
onset oxidation/reduction potentials (E /E ) of the polymers, as shown in Fig.S1, are 0.57/-1.21
ox red
+
+
V vs. Ag/Ag for the alkoxyphenyl chain substituted PIT -DT BT and 0.67/-1.19 V vs Ag/Ag
Ph
ff
for the fluoroalkoxyphenyl chain substituted PIT -DT BT. The EHOMO/ELUMO levels of
Phf
ff
PIT -DT BT and PIT -DT BT are calculated to be -5.28/-3.50 eV and -5.38/-3.52 eV,
Ph
ff
Phf
ff
respectively, according to the equations: EHOMO/LUMO=-e (Eox/red+4.71) (eV). The EHOMO of
PIT -DT BT is 0.10 eV deeper than that of PIT -DT BT, which will benefit for higher V of
Phf
ff
Ph
ff
oc
the PSCs with the polymer as donor. Comparing to the alkoxyphenyl substitute,
fluoroalkoxyphenyl substitute lowered the HOMO level of the D-A copolymer. Moreover, the
change of ELUMO is not very obvious. This is probably because the F atom is located on the donor
unit, which would have great influence on the EHOMO and little effect on the ELUMO. Notably, the
HOMO energy offset (∆EHOMO) and LUMO energy offset (∆ELUMO) between the two polymers

ACCEPTED MANUSCRIPT
and PC BM are both larger than the empirical threshold of 0.3 eV for effective exciton
7
1
dissociation to overcome the binding energy of the excitons.
Theoretical calculation
To demonstrate the optimal molecular geometry and electronic properties, theoretical calculations
were performed by using density functional theory (DFT) B3LYP with 6-31 G(d, p) basic set. To
simplify the calculations, we investigated monomer model (the number of repeat units is one) of
each polymer. At the same time, all of the alkoxy side chains were replaced by methoxy groups to
save the calculation time. Fig.2 shows the frontier molecular orbitals, the corresponding LUMO
and HOMO levels, and the calculated dipole moments of PIT -DT BT and PIT -DT BT. As can
Ph
ff
Phf
ff
be observed, the HOMO wave functions are distributed entirely over the planar conjugated
backbone, while the LUMO wave functions are mainly concentrated on the benzothiadiazole
acceptor units. These images revealed the well-designed D-A structures and ICT behaviors of the
two polymers. For PIT -DT BT, the calculated HOMO, LUMO and E are -4.92, -2.71, and 2.21
Ph
ff
g
eV, respectively, while for PIT -DT BT, the calculated HOMO, LUMO and E are -5.00, -2.77
Phf
ff
g
and 2.23 eV, respectively. The calculated HOMO level of PIT -DT BT is 0.08 eV lower than that
Phf
ff
of PIT -DT BT, which is consistent with the CV results. Though the LUMO difference between
Ph
ff
the two polymers is 0.06 eV from theoretical calculation, which is bigger than that of 0.02 eV
from CV method, considering the calculation error, it still suggests that the influence of fluorine
substitute on the energy level is mainly manifested on the HOMO level because
fluoroalkoxyphenyl substitute is located on the donor unit. The difference of calculated E is also
g
opt
e
g
consistent with that of Eg and E . Furthermore, the dipole moments (µ ) calculated by DFT are
g
2
.27 D and 2.45 D for PIT -DT BT and PIT -DT BT, respectively. The higher µ of
Ph ff Phf ff g
PIT -DT BT indicates that introducing a fluorine atom to the side chain of donor unit can
Phf
ff
improve the molecular polarizability, further enhance the molecular interaction. Some reports have
mentioned that a repeating polymer unit with a high dipole moment might exert a considerable
[17]
influence over the molecular assembly and the consequent morphology in film state. Especially,
such unit might produce abundant donor-acceptor interfaces, thereby resulting in the suppression
of charge recombination in bulk heterojunction PSCs. Therefore, the PIT -DT BT polymer with
Phf
ff
a higher dipole moment is expected to produce a PSC device with higher Jsc and FF than
PIT -DT BT.
Ph
ff

ACCEPTED MANUSCRIPT
Fig.2 Theoretical calculated frontier molecular orbital and the dipole moment, obtained from
density functional theory (DFT) calculations on the polymers with a chain length n=1 at the
B3LYP/6-31 G(d,p) basic set.
Photovoltaic properties
To investigate the effect of fluorine atom on the photovoltaic properties, the PSC devices
comprising of the polymer PIT -DT BT and PIT -DT BT as the donors and PC BM as the
Phf
ff
Ph
ff
71
acceptor were fabricated with the configuration of ITO/PEDOT:PSS/polymer:PC BM/PDINO/Al,
7
1
where PEDOT:PSS and PDINO are the abbreviations of poly(3,4-ethylenedioxythiophene):
poly(styrenesulfonate) and perylene diimide (PDI) derivative. Chloroform is used as the
processing solvent and the optimized weight ratios for both two polymer blend systems are 1:1.2.
To further optimize the morphology of the blend films and thus the photovoltaic performance, we
adopted thermal annealing process and additive DPE (diphenyl ether) treatment to obtain the
optimal devices. The optimal annealing temperature and time are at 120 ℃ for 2 min. All the
photovoltaic measurements were carried out under AM 1.5G illumination at an intensity of 100
-
2
mW cm . Fig.3 shows the current density–voltage (J-V) curves of the PSC devices based on the
two polymers. The short-circuit currents (J ), open-circuit voltages (V ), fill factors (FF) and
sc
oc
PCEs of these PSCs are summarized in Table 2. As shown in Table 2, the as-cast device based on

ACCEPTED MANUSCRIPT
-
2
PIT -DT BT exhibits a V of 0.81 V, J of 10.66 mA cm , and FF of 0.467, which yields the
Ph
ff
oc
sc
PCE of 4.06%. When the alkoxyphenyl is replaced by fluoroalkoxyphenyl, the PIT -DT BT
Phf
ff
based as-cast device exhibits a relatively higher PCE of 5.56%, with a V of 0.84 V, J of 11.99
oc
sc
-
2
mA cm , and FF of 0.549. As expected from the deeper E
, the cells based on PIT -DT BT
Phf ff
HOMO
exhibit higher Voc than the PIT -DT BT. The J and FF are increased significantly after the
Ph
ff
sc
introduction of F atom. For the two polymers, the thermal annealing at 120 ℃ for 2 min
dramatically improved the device performance. The PCE values of the PSCs based on
PIT -DT BT and PIT -DT BT increased to 5.12% and 6.55%, respectively. The notably
Ph
ff
Phf
ff
improved J and FF account for the increased PCE values after thermal annealing. When DPE
sc
was incorporated into the active layer with the thermal annealing in the meantime, the
-
2
corresponding PSC shows the highest efficiency of 5.68% with a J of 13.19 mA cm and a FF of
sc
-2
0
0
.533 for PIT -DT BT, and the highest efficiency of 7.03% with a J of 13.21 mA cm and FF of
Ph ff sc
.633 for PIT -DT BT. The adding of DPE further enhances the J and FF values, resulting in
Phf
ff
sc
even higher efficiencies for the two polymers.
Fig.3 Typical J-V characteristics of the PSCs based on (a) PIT -DT BT (1:1.2, w/w) and (b)
Ph
ff
PIT -DT BT (1:1.2, w/w) with (120 ℃ )or without (none) thermal annealing at 120 ℃ for 2 min
Phf
ff
and further additive DPE added with thermal annealing (120 ℃ , 1% DPE), under the illumination
2
of AM1.5G, 100 mW/cm .
Table 2. Photovoltaic performance parameters of the PSCs based on polymer/PC BM (1:1.2 w/w),
7
1
2
under the illumination of AM1.5G, 100 mW/cm .
-2
b
Polymer
Treatment
none
V (V)
J (mA cm )
FF (%)
46.7
52.2
53.3
54.9
62.1
63.3
PCE (%)
oc
sc
PIT -DT BT
0.81
0.81
0.81
0.84
0.84
0.84
10.66
12.06
13.19
11.99
12.53
13.21
4.06 (3.83±0.23)
5.12 (4.94±0.18)
5.68 (5.53±0.15)
5.56 (5.36±0.20)
6.55 (6.29±0.26)
7.03 (6.90±0.13)
Ph
ff
a
TA
a
TA , 1%DPE
PIT -DT BT
none
Phf
ff
a
TA
a
TA , 1%DPE
a
b
Thermal annealing at 120 ℃ for 2 min. In parentheses are average values based on eight
devices.
The optimal external quantum efficiency (EQE) curves of the two polymer devices are shown in

ACCEPTED MANUSCRIPT
Fig.4. Both the two devices exhibited photo response ranging from 300 to 700 nm, suggesting that
both the donor and acceptor contribute to EQE and thus the J . Therein, the PIT -DT BT blend
sc
Phf
ff
showed a higher EQE value than the PIT -DT BT blend, reaching a maximum value of 75% and
Ph
ff
over 70% across the range 350-650 nm. This result indicates the more highly efficient
photoelectron conversion process in the PIT -DT BT device, which is in consistent with the
Phf
ff
enhanced Jsc of PIT -DT BT blend device. For the two polymers, both the J values of the
Phf
ff
sc
relative devices were in good agreement with the EQE spectra within an experimental error of 5%.
-2
The J values calculated from the EQE spectra were 12.80 and 12.95 mA cm for the devices
sc
based on PIT -DT BT and PIT -DT BT, respectively, which are in good agreement with the J
sc
Ph
ff
Phf
ff
values obtained from the J-V curves.
Fig.4 The EQE spectra of the PSCs based on polymer/PC BM blends with 1% DPE and thermal
7
1
annealing at 120 ℃ for 2 min.
To further understand the additive DPE and thermal annealing effect on the device performance,
the bulk transport properties of the polymer/PC BM blends were investigated using the space
7
1
charge-limited current (SCLC) method with the hole only device (ITO/PEDOT:PSS/active
1
/2
layer/Au) and electron only device (ITO/ZnO/active layer/PDINO/Al). The plots of J vs
Vappl-V ) characteristics for the devices are showed in Fig.S2, and the results are summarized in
(
bi
Table 3. The hole (µ )/electron (µ ) mobilities of the PIT -DT BT/PC BM films are calculated to
h
e
Ph
ff
71
be 0.15 for the as-cast film, 0.29 for the annealed film and 0.33 for the annealed film with DPE.
The hole (µ )/electron (µ ) mobilities of the PIT -DT BT/PC BM films are calculated to be 0.18
h
e
Phf
ff
71
for the as-cast film, 0.36 for the annealed film and 0.37 for the annealed film with DPE.
Apparently, after thermal annealing, charge mobilities are increased for some extent, and further
enhanced by adding the additive DPE. Meanwhile, more balanced hole and electron
mobilities(µ /µ ) are also observed with thermal annealing and additive DPE. These observations
h
e
are consistent with the corresponding PCE changes shown in Tabel 1. Around two or three times
higher and more balanced hole and electron mobilities of the polymer blends with thermal
annealing and DPE compared to the as-cast blends is one of the reasons for the enhanced J of the
sc
corresponding devices. In addition, for the two polymers, in comparison with the charge carrier
mobilities of as-cast, thermal annealed or annealed with DPE devices, it can be found that the
devices based on the fluoroalkoxyphenyl substituted copolymers (PIT -DT BT) demonstrated
Phf
ff
higher and more balanced hole and electron mobilities, which should be one reason for their

ACCEPTED MANUSCRIPT
higher J and PCE values.
sc
Tabel 3 Hole and electron mobilities of PIT -DT BT and PIT -DT BT blends measured via the
Ph
ff
Phf
ff
SCLC method
Polymer
2
-1 -1
2
-1 -1
Treatment
none
µ [cm V s ]
µ [cm V s ]
µh/µe
0.15
0.29
0.33
0.18
0.36
0.37
h
e
-4
-4
PIT -DT BT
0.34×10
1.14×10
2.47×10
0.52×10
1.84×10
3.37×10
2.26×10
Ph
ff
a
-4
-4
-4
TA
3.95×10
a
-4
TA , 1%DPE
7.38×10
-4
-4
PIT -DT BT
none
2.84×10
Phf
ff
a
-4
-4
-4
TA
5.18×10
a
-4
TA , 1%DPE
9.21×10
a
Thermal annealing at 120 ℃ for 2 min.
To gain deeper insight into the charge generation and dissociation behavior, the photocurrent
density (J ) as a function of effective applied voltage (V ) were measured for the two polymer
ph
eff
devices. J =J -J , where J is the current density under illumination and J is the current density
ph
L
D
L
D
in the dark. V =V -V , where V is the applied voltage and V is the voltage at which J is zero.
eff
0
a
a
0
ph
As shown in Fig.5, when Veff arrives at ~2V, Jph values for both devices reach saturation (Jsat),
suggesting that charge recombination is minimized at higher voltage due to the high internal
electric field in the devices. The charge dissociation efficiency can be evaluated by using J /Jsat
ph
ratio. Under the short-circuit and maximal power output conditions, the J /J values of
ph sat
PIT -DT BT are 91.6%, 64.1% for the as-cast device, 92.6%, 64.8% for the device with thermal
Phf
ff
annealing, and 93.8%, 68.6% for the device with additive DPE and thermal annealing, respectively.
The increased J /J values indicate that the thermal annealing and additive DPE treated devices
ph sat
exhibited higher exciton dissociation and more efficient charge collection efficiency compared to
the as-cast device. In addition, the J /Jsat values of PIT -DT BT are 93.2%, 60.5% for the device
ph
Ph
ff
with additive DPE and thermal annealing, which are smaller than those of PIT -DT BT. The
Phf
ff
results indicate that the change in the structure of fluoroalkoxyphenyl substitute can improve the
charge dissociation and collection as well.
Fig.5 Photocurrent density versus effective voltage (J -V ) characteristics for devices based on
ph
eff
PIT -DT BT and PIT -DT BT blends.
Ph
ff
Phf
ff

ACCEPTED MANUSCRIPT
To further probe the charge recombination behavior in the devices, which is one of the most
important factors influencing the FF values of PSCs, the light-intensity (P) dependence of current
density (J ) was then measured. The relationship between J and P can be represented by the
sc
sc
α
power-law equation J ∝P , where α would be unity (α=1), when the bimolecular recombination
sc
can be negligible under short circuit condition. Fig.6 shows the log-log plot of J as a function of
sc
the light intensiy (P). As presented in Fig.6, the α value for the as-cast PIT -DT BT blend is 0.94,
Phf
ff
which is higher than 0.92 for the as-cast PIT -DT BT blend. This suggests that less bimolecular
Ph
ff
recombination occurred in the PIT -DT BT blend, supporting its higher FF and PCE.
Phf
ff
Considering the significant impact of thermal annealing and additive DPE on the FF value, we
chose the polymer PIT -DT BT as the research object. The α values were measured as 0.96 for
Phf
ff
the device with thermal annealing and 0.97 for the device with thermal annealing and additive
DPE. In contrast to as-cast PIT -DT BT device, the devices via thermal annealing and further
Phf
ff
additive treatment possess higher α values, implying reduced bimolecular recombination, leading
ultimately to relatively higher FF values and consequently higher PCEs.
Fig.6 Double logarithmic polts of photocurrent density as a function of incident light intensity for
devices based on PIT -DT BT and PIT -DT BT blends.
Ph
ff
Phf
ff
Grazing incident wide-angle X-ray scattering (GIWAXS) is further used to investigate the
molecular packing and crystallinity of the blended PIT -DT BT and PIT -DT BT films with
Ph
ff
Phf
ff
and without solvent additives and thermal annealing treatment. Fig.7 (a) and (b) shows the
line-cuts of the blended PIT -DT BT/PC BM films in the in-plane and out-of-plane directions,
Phf
ff
71
respectively. Fig.7 (c) and (d) shows the line-cuts of the blended PIT -DT BT/PC BM films in
Ph
ff
71
the in-plane and out-plane directions, respectively. As shown in Fig.7, for all the blend films, the
lamellar (100) peaks in the in-plane and out-of-plane directions could be observed. This indicates
that the polymers have relatively strong lamellar molecular packing, which corresponds to the
semi-crystalline nature of the copolymers. However, the stronger intensity of the (100) reflection
in the out-of-plane profile than in the in-plane profile indicates that the lamellar packing of both
PIT -DT BT and PIT -DT BT based polymers preferentially stacked out of the film plane
Ph
ff
Phf
ff
(
edge-on rich orientation), though with some randomness. For the additive-free PIT -DT BT
Phf ff
-1
/PC BM film, the broad peak at q=1.32 Å is characteristic of PC BM aggregation, and the
7
1
7
1

ACCEPTED MANUSCRIPT
-1
out-of-plane (010) π-π stacking peak located at q=1.78 Å (d-spacing of 3.53 Å) is attributed to
face-on orientation of PIT -DT BT. After thermal annealing treatment, the blend film shows a
Phf
ff
-1
slight enhanced (010) π-π stacking peak located at q=1.81 Å (d-spacing of 3.47 Å). When DPE
was used as a solvent additive, the corresponding film shows a (010) π-π stacking peak located at
-1
q=1.83 Å (d-spacing of 3.43 Å). At the same time, the out-of-plane (100) peak slightly enhanced,
which is corresponding to the weakly increased in-plane (010) peak. Notably, the facial π-π
stacking distances are decreased. Similarly, for the PIT -DT BT/PC BM film, the location of
Ph
ff
71
-
1
-1
out-of-plane (010) peak increases from 1.75 Å (d-spacing of 3.59 Å) to 1.77 Å (d-spacing of
-1
3
.55 Å) and then 1.79 Å (d-spacing of 3.51 Å) after thermal annealing treatment and further
adding of additive DPE. The decreased π-π stacking distances and relatively strong face-on
orientation for PIT -DT BT/PC BM and PIT -DT BT/PC BM film are beneficial to charge
Phf
ff
71
Ph
ff
71
transport and reducing bimolecular recombination. As a result, the corresponding PSCs with
thermal annealing treatment and DPE exhibit enhanced PCEs with increased J and FF values.
sc
These results suggest that the thermal annealing and DPE additive treatment may help to
crystallize PIT -DT BT and PIT -DT BT, which are beneficial for improving the solar cell
Phf
ff
Ph
ff
performance. In addition, the out-of-plane (010) π-π stacking distances of PIT -DT BT is shorter
Phf
ff
than that of PIT -DT BT. This phenomenon indicates that the fluoroalkoxyphenyl substitute
Ph
ff
promotes the π-π stacking interaction, which can improve the inter-molecular exciton transfer. The
tighter packing of PIT -DT BT, associated well with its higher crystalline behavior of the blend
Phf
ff
film, corresponds well with the higher value of hole mobility and FF. These results suggest that
the presence of large amounts of fluoroalkoxyphenyl substitutes contribute to highly ordered
structure and effective charge transport, which are desirable for higher carrier mobility thereby
higher photovoltaic efficiencies.
Fig.7 GIWAXS of PIT -DT BT blend film: (a) in-plane line-cuts, (b) out-of-plane line-cuts; and
Phf
ff
PIT -DT BT blend film: (c) in-plane line-cuts, (d) out-of-plane line-cuts.
Ph
ff
In order to determine the effect of morphology of polymer/PC BM blend film on the device
7
1
performance, atomic force microscopy (AFM) was performed. The surface morphology was
recorded by tapping mode AFM and is illustrated in Fig.8. Among them, Fig.8 (a), (b) and (c) are
of PIT -DT BT/PC BM blend films; Fig.8 (d), (e) and (f) are of PIT -DT BT/PC BM blend
Ph
ff
71
Phf
ff
71
films. Fig.8 (a) and (d) are of casting films. Fig.8 (b) and (e) are of films after thermal annealing.
Fig.8 (c) and (f) are of films both treated with 1% DPE and thermal annealing. From the AFM

ACCEPTED MANUSCRIPT
images in Fig.8, it can be found that the casting polymer blends exhibit relatively large
root-mean-square (RMS) value of 3.73 nm for PIT -DT BT and 2.97 nm for PIT -DT BT due
Ph
ff
Phf
ff
to the formation of large-sized aggregates. After thermal annealing, reduced surface roughness
values of 2.48 and 2.35 nm are observed from the surface AFM images of PIT -DT BT and
Ph
ff
PIT -DT BT blends, respectively. Furthermore, both the addition of DPE and thermal annealing
Phf
ff
treated to the blends lead to improved morphology with the disappearance of relatively large-sized
aggregations, especially for PIT -DT BT blend with a fairly homogeneous surface in size and
Phf
ff
feature. The RMS values of PIT -DT BT and PIT -DT BT blends are reduced to 1.74 and 1.78
Ph
ff
Phf
ff
nm, respectively. As the morphology with small-scale phase separation is beneficial to efficient
exciton diffusion, exciton separation, and charge transport, resulting in an increased J and FF for
sc
the PSC devices, the post-treatment of thermal annealing and addition of DPE gradually reduce
the phase separation size, therefore significantly improve the J and FFs, further the PCEs of the
sc
two polymer based devices. These results seem to be directly linked to the above-observed
dependency of PCEs on the application of various processing techniques, such as addition of DPE
and thermal annealing. This morphology difference between the blend films with and without
additive and thermal annealing is one of the main reasons for the photovoltaic performance
variation of the two polymer based devices. What’s more, the Rms values of PIT -DT BT are
Phf
ff
smaller than those of PIT -DT BT (except for the similar Rms values of Fig.8(c) and (f)), which
Ph
ff
is owing to the stronger inter-molecular interactions of fluoroalkoxyphenyl substitute in polymer
PIT -DT BT. Base on the stronger inter-molecular interactions, smaller phase separation size can
Phf
ff
be formed in PIT -DT BT. This result may help to explain why PIT -DT BT tends to obtain
Phf
ff
Phf
ff
higher FFs and J , and thus higher PCEs than PIT -DT BT.
sc
Ph
ff
2
Fig.8 Topographic AFM images (5×5 um ) of the blend films of PIT -DT BT (a, b, c) and
Ph
ff
PIT -DT BT (d, e, f). Panels (a) and (d) are of as-cast films; (b) and (e) are of films with thermal
Phf
ff
annealing at 120 ℃ for 2min; (c) and (f) are of films with additive DPE and thermal annealing at
20 ℃ for 2min.
1
Conclusion

ACCEPTED MANUSCRIPT
We synthesized two novel IT-based asymmetric polymers with nonfluorinated and fluorinated side
chain, namely, PIT -DT BT and PIT -DT BT. The fluorination preferentially influences the
Ph
ff
Phf
ff
HOMO of the polymer, resulting in deeper HOMO, slightly deeper LUMO, and slightly wider Eg
for PIT -DT BT relative to the nonfluorinated PIT -DT BT counterpart. As expected from the
Phf
ff
Ph
ff
deep HOMO level, Voc of the PIT -DT BT cell was improved to 0.84 V from 0.81 V of the
Phf
ff
PIT -DT BT cell. The optimized PIT -DT BT cell exhibited 5.68% PCE and lower J and FF,
Ph
ff
Ph
ff
sc
whereas the PIT -DT BT cell had a 7.03% PCE with higher J and FF. The charge transport
Phf
ff
sc
study revealed that higher and more balanced hole and electron mobilities of PIT -DT BT
Phf
ff
should be one reason for the higher J , which could also be evidenced by the increased charge
sc
dissociation efficiency of PIT -DT BT relative to that of PIT -DT BT. The bimolecular
Phf
ff
Ph
ff
recombination study demonstrated that side-chain fluorination can decrease the recombination,
and this was likely the cause of the higher FF of the PIT -DT BT. The tighter π-π stacking and
Phf
ff
smaller phase domain of PIT -DT BT were also be detected by GIWAXS and AFM
Phf
ff
measurements, these behaviors were well-correlated with the results mentioned above.
Furthermore, thermal annealing and additive DPE are beneficial to gain high performance PSCs,
associated with the great improvement on J and FF. These findings and detailed studies provide
sc
an important guidance to tune and optimize the morphology and properties of
indenothiphene-based copolymers.
Experimental Section
Experimental details and additional characterization data are included in the Supporting
Information.
Acknowledgements
The 2D-GIXRD data were obtained at 1W1A, Beijing Synchrotron Radiation Facility. The authors
gratefully acknowledge the assistance of scientists of the Diffuse X-ray Scattering Station during
the experiments. The authors gratefully acknowledge the Chemcloud computing of the Beijing
University of Chemical Technology for the theoretical calculation of molecular orbitals.
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Highlights
1
. Two new D-A type asymmetric-indenothiophene-based polymers, PIT -DT BT and
Ph ff
PIT -DT BT were obtained.
Phf
ff
2
. Side-chain fluorination in the asymmetric donor units has a remarkably positive impact.
. PIT -DT BT shows enhanced charge mobility, better crystallinity and surface morphology.
3
Phf
ff