Overcoming the trade-off between Voc and Jsc: Asymmetric chloro-substituted two-dimensional benzo[1,2-b:4,5-b′]dithiophene-based polymer solar cells

Article abstract of DOI:10.1016/j.dyepig.2018.10.071

To achieve the high PCE and simultaneously overcome the trade-off between open circuit voltage (V_OC) and short-circuit current density (J_SC), the reasonable design of the donor material is still a great challenge in non-fullerene polymer solar cells (PSCs). Herein, a pair of asymmetric 2D BDT-based wide band gap polymers, named PBPTBz-1Cl and PBPTBz-2Cl, respectively, were designed and synthesized through simultaneously introducing a chloro-substituted thiophene and another alkoxylphenyl groups as side chains onto the same BDT unit. As expected, the polymers PBPTBz-1Cl and PBPTBz-2Cl exhibit the deeper HOMO levels of ?5.32 and ?5.41 eV, respectively, compared with the HOMO value of ?5.27 eV for chlorine-free substituted polymer PBPTBz-0Cl. Therefore, the PBPTBz-2Cl:IT-M based PSCs exhibited a higher V_oc of 0.99 V, with an enhanced J_sc of 14.92 mA cm^?2 and PCE of 7.18%. The V_oc, J_sc and PCE were simultaneously enhanced from PBPTBz-0Cl:IT-M to PBPTBz-1Cl:IT-M, then to PBPTBz-2Cl:IT-M based PSCs successively. These results demonstrate that the challenge of the trade-off between V_oc and J_sc could be overcome through the asymmetric strategy of the chloro-substituted thiophene and alkylphenyl groups as different side chains onto the same BDT unit. Hence, considering the low cost of starting materials and convenient introduction of chlorine atoms, the asymmetric chlorination of the side chain of BDT unit would be the great potential for achieving the high-performance PSCs.

Full text of DOI:10.1016/j.dyepig.2018.10.071

Accepted Manuscript
Overcoming the trade-off between V and J : Asymmetric chloro-substituted two-
oc
sc
dimensional benzo[1,2-b:4,5-b′]dithiophene-based polymer solar cells
Pengjie Chao, Longzhu Liu, Jianfei Qu, Qiming He, Shenglong Gan, Hong Meng, Wei
Chen, Feng He
PII:
S0143-7208(18)32163-6
https://doi.org/10.1016/j.dyepig.2018.10.071
DYPI 7137
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 29 September 2018
Revised Date: 25 October 2018
Accepted Date: 30 October 2018
Please cite this article as: Chao P, Liu L, Qu J, He Q, Gan S, Meng H, Chen W, He F,
Overcoming the trade-off between V and J : Asymmetric chloro-substituted two-dimensional
oc
sc
benzo[1,2-b:4,5-b′]dithiophene-based polymer solar cells, Dyes and Pigments (2018), doi: https://
doi.org/10.1016/j.dyepig.2018.10.071.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
Graphical Abstract:

ACCEPTED MANUSCRIPT
Overcoming the Trade-off between Voc and Jsc: Asymmetric
Chloro-Substituted
Two-Dimensional
Benzo[1,2-b:4,5-b′]dithiophene-based Polymer Solar Cells
Pengjie Chao a, Longzhu Liu b, Jianfei Qu b, Qiming He d, Shenglong Gan c, Hong
Meng a, Wei Chen c,d, , Feng He b,
* *
a
b
c
School of Advanced Materials, Peking University Shenzhen Graduate School,
Peking University, Shenzhen, 518055, P.R. China
Department of Chemistry and Shenzhen Grubbs Institute, Southern University of
Science and Technology, Shenzhen, 518055, P. R. China
Materials Science Division, Argonne National Laboratory, 9700 Cass Avenue,
Lemont, Illinois, 60439, United States
d Institute for Molecular Engineering, The University of Chicago, 5640 South Ellis
Avenue, Chicago, Illinois, 60637, United States
Corresponding Author
*E-mail: hef@sustc.edu.cn (F. H.)
*E-mail: wchen@anl.gov (W. C.)
1

ACCEPTED MANUSCRIPT
Abstract: To achieve the high PCE and simultaneously overcome the trade-off
between open circuit voltage (VOC) and short-circuit current density (JSC), the
reasonable design of the donor material is still a great challenge in non-fullerene
polymer solar cells (PSCs). Herein, a pair of asymmetric 2D BDT-based wide band
gap polymers, named PBPTBz-1Cl and PBPTBz-2Cl, respectively, were designed and
synthesized through simultaneously introducing a chloro-substituted thiophene and
another alkoxylphenyl groups as side chains onto the same BDT unit. As expected,
the polymers PBPTBz-1Cl and PBPTBz-2Cl exhibit the deeper HOMO levels of
-5.32 and -5.41 eV, respectively, compared with the HOMO value of -5.27 eV for
chlorine-free substituted polymer PBPTBz-0Cl. Therefore, the PBPTBz-2Cl:IT-M
based PSCs exhibited a higher Voc of 0.99 V, with an enhanced Jsc of 14.92 mA cm-2
and PCE of 7.18%. The Voc, Jsc and PCE were simultaneously enhanced from
PBPTBz-0Cl:IT-M to PBPTBz-1Cl:IT-M, then to PBPTBz-2Cl:IT-M based PSCs
successively. These results demonstrate that the challenge of the trade-off between Voc
and Jsc could be overcome through the asymmetric strategy of the chloro-substituted
thiophene and alkylphenyl groups as different side chains onto the same BDT unit.
Hence, considering the low cost of starting materials and convenient introduction of
chlorine atoms, the asymmetric chlorination of the side chain of BDT unit would be
the great potential for achieving the high-performance PSCs.
keywords: Asymmetric chloro-substitution, side-chain engineering, low photon
energy loss, polymer solar cells,
2

ACCEPTED MANUSCRIPT
1. Introduction
Polymer solar cells (PSCs) have drawn considerable attention because of the recent
great breakthrough, and the power conversion efficiencies (PCEs) have reached up to
14% in single-junction devices as a few new wide band gap (WBG) donor polymers
and narrow band gap acceptor-donor-acceptor (A-D-A) type small molecular
acceptors were gradually developed.1-5 PSCs as a promising and potential renewable
energy technology possess many unique advantages, including light-weight,
mechanical flexibility, large-area preparation through the solution processing.6-15
However, it is still a great challenge in the materials design and device engineering,
and to further push the PCEs of PSCs toward industrial application with a simple
17
device structure.16,
Lately, great progress in a series of narrow band gap
non-fullerene acceptors has been achieved, such as IT-4F and IT-4Cl, making the
photovoltaic performance exceed a PCE of 14% paired with a wide band gap donor
material, and opening up a new window for PSCs.1, 2, 18, 19 Meanwhile, it is well
known that the intrinsic properties, for instance, the complementary absorption
spectra, the suitable energy levels and good miscibility of the active layer comprised
of the donor and acceptor dominantly influence the device performance in PSCs.20, 21
Therefore, with regard to a specific narrow band gap acceptor material, the design of
the corresponding donor materials would be of great importance.19 Recently, great
efforts have focused on the development of the WBG donor polymers forming a good
absorption and suitable energy levels which can boost the short-circuit current density
(JSC) and achieve a high open circuit voltage (VOC) when matched with the narrow
3

ACCEPTED MANUSCRIPT
band gap acceptor materials.22-26 Donor-acceptor (D-A) type conjugated polymers
based on a two-dimensional (2D) benzodithiophene (BDT) moiety have been widely
developed for design and synthesis of the donor materials in PSCs, because the energy
levels can be readily modified through tuning the polymer backbone and side chain
substitution units.27-29 The 2D-BDT based polymer could strengthen the
intermolecular π–π interactions through extending the effective region of π–π
conjugation, contributing to enhance light-harvesting ability, exciton diffusion and
charge transport.30-32 In addition, the highest occupied molecular orbitals (HOMO)
level of those polymers could be decreased when introducing the phenyl type of
substitutions due to its high ionization potential and low electron densities, which was
helpful to achieve high Voc in PSCs.33 At the same time, introducing the halogen,
especially fluorine (F), was regarded as a quite effective method to downshift the
HOMO level and obtain high Voc without sacrificing the light-harvesting ability and
causing the evident steric hindrance used in the donor and acceptor materials of
PSCs.17, 34-36 However, except fluorine, the other halogen, such as chlorine which
possesses strong electronegativity after the fluorine in the halogen family, has not
been widely explored in the field of PSCs.28 It has been demonstrated that although
the electronegativity of F is stronger than Cl, the dipole moment of the
carbon-chlorine (C-Cl) bond is a little larger than carbon-fluorine (C-F), which could
contribute to modulate the ICT effect in comparison with F.1 In addition, the empty 3d
orbitals in chlorine endow their capability to accept lone pair electrons and π-electrons
from the conjugated system, which would form the intermolecular non-covalent
4

ACCEPTED MANUSCRIPT
interactions of Cl⋯S and Cl⋯π. Such delocalization is even more beneficial to adjust
the energy level and ordered molecular stacking in films.37, 38 Our recent results
demonstrated that the chloro-substitution of 4-position of conjugated thiophene as a
side chain of BDT core could efficiently decrease the HOMO level without causing
the steric hindrance, and push the Voc to beyond 1.0 V with reasonable energy loss
(Eloss) of 0.63 eV.37 Eloss is an important factor hindering the device performance, and
the empirical threshold is believed to be around 0.6 eV.39 In order to overcome the
large Eloss as far as possible, the acceptor with narrow band gap and the donor with the
deeper HOMO level would be simultaneously and reasonably selected to fabricate the
bulk heterojunction (BHJ) PSCs.40
Herein, inspired by our previous investigations, three novel asymmetric 2D-2D
BDT-based donor units with alkoxylphenyl and alkylthiophene/chloro-substituted
thiophene as the different side chain on the same BDT unit were designed and
prepared, named PBPTBz-0Cl, PBPTBz-1Cl and PBPTBz-2Cl, respectively (Scheme
1), in which the advantages of those two kinds of side chains were anticipated to be
combined. The phenyl group has a high ionization potential and low electron density
which can downshift the HOMO level keeping a high Voc, and the chlorinated thienyl
group could enhance the intermolecular non-covalent interaction because of the empty
3d orbitals of chlorine, resulting in a good molecular stacking and charge transfer in
vertical direction.33, 41-43 In order to obtain the better complementary absorption of
these three WBG polymers, the small molecular acceptor IT-M with a narrow optical
band gap was selected to optimize the photovoltaic performance.26 As a result, among
5

ACCEPTED MANUSCRIPT
the three polymers, the Voc, Jsc and PCEs were boosted as the increasing of the amount
of chlorine atoms, in particular, the PSCs based on PBPTBz-2Cl:IT-M show a
maximum Voc of 0.99 V with a simultaneously enhanced Jsc of 14.92 mA cm-2. This
non-fullerene device exhibits an excellent PCE of 7.18% with a very low Eloss of 0.58
eV, which was below the typical threshold of 0.6 eV.44 In other words, the strategy of
simultaneous substitution of phenyl and chloro-substituted thiophene into the same
BDT unit could simultaneously improve the Voc and Jsc with improved Eloss. Thus, the
strategy of asymmetric BDT building block seems to be a promising method and
accelerate the development of the photovoltaic materials in PSCs.
6

ACCEPTED MANUSCRIPT
Scheme 1. The structures and synthetic route of monomers M1, M2, M3 and polymers
PBPTBz-0Cl, PBPTBz-1Cl and PBPTBz-2Cl.
2. Results and discussion
2.1 Synthesis and characterization
The structure and synthetic routes of the M1, M2, M3 molecules and polymers PBPTBz-0Cl,
PBPTBz-1Cl and PBPTBz-2Cl are shown in Scheme 1. All products were characterized
through 1H NMR, 13C NMR and high-resolution mass spectroscopy (HRMS). The key
intermediates monomer M1, M2 and M3 were synthesized through Grignard reagent,
and then n-butyllithium (n-BuLi) with a high yield. The detailed synthetic procedure
is provided in the Supporting Information. Polymer PBPTBz-0Cl, PBPTBz-1Cl and
PBPTBz-2Cl were synthesized through Stille coupling reaction catalyzed by
Tetrakis(triphenylphosphine)palladium (Pd(PPh3)4) in toluene with a yield of 64.55,
64.29 and 34.48%, respectively, as displayed in Table 1. In order to remove the
remaining monomers, oligomers and catalytic impurities, soxhlet extraction was
conducted successively with methanol, acetone, and hexane solvents. The structures
of the polymers were characterized by 1H NMR spectroscopy. Polymers PBPTBz-0Cl,
PBPTBz-1Cl and PBPTBz-2Cl all exhibit good solubility in chloroform,
chlorobenzene and o-dichlorobenzene, which is favorable for solution processing. The
7

ACCEPTED MANUSCRIPT
average molecular weight was measured via gel permeation chromatography (GPC)
using 1,2,4-trichlorobenzene as the eluent at high temperature (160 °C), as displayed
in Table 1, the corresponding Mn values were 44.93, 21.49 and 23.66 K Da with the
PDI of 1.33, 1.57 and 1.99 for PBPTBz-0Cl, PBPTBz-1Cl and PBPTBz-2Cl,
respectively. The thermal properties of the polymers PBPTBz-0Cl, PBPTBz-1Cl and
PBPTBz-2Cl were estimated through thermogravimetric analysis (TGA) in nitrogen
atmosphere shown in Figure S1 and the relevant results were summarized in Table 1.
The TGA results demonstrate that the decomposition temperature (Td) with 5% weight
o
loss are determined to be 428, 412 and 426 C for polymers PBPTBz-0Cl,
PBPTBz-1Cl and PBPTBz-2Cl, respectively. These Td values demonstrate that these
three polymers all possess good thermal stability for future PSCs applications.
Table 1. The yield, molecular weights and thermal properties of the donor polymers.
Polymer
Yield (%)
64.55
Mn (K Da)
44.93
Mw (K Da)
60.04
PDI
1.33
1.57
1.99
Td (oC)
428
PBPTBz-0Cl
PBPTBz-1Cl
PBPTBz-2Cl
64.29
21.49
33.78
412
34.48
23.66
47.11
426
2.2. Theoretical calculations
8

ACCEPTED MANUSCRIPT
Figure 1. Optimized molecular backbone conformations of polymers using DFT calculations at
the B3LYP/6-31g*(d, p) level: top views and side views, and the HOMO and LUMO electron
distributions based on two repeat units.
In order to gain deeper insight into the effects of the side chain of phenyl and
chloro-substituted conjugated thiophene on the molecular geometry and orbital
electron cloud density distribution, density functional theory (DFT) calculations with
a the B3LYP/6-31G* (d, p) level were carried out. For the simulation shown in Figure
1, two repeating units were chosen, and a methyl group was used to replace the alkyl
substituent to simplify the calculations. The changes of the HOMO levels with the
phenyl and chloro-substituted thiophene side chain into one BDT unit with the
number of substituted chlorine atoms were studied. As shown in Figure 1, for the
polymer PBPTBz-1Cl and PBPTBz-2Cl, The HOMO and the lowest unoccupied
molecular orbital (LUMO) levels were gradually decreased with increasing the
chlorine atoms from one to two, compared to the polymer PBPTBz-0Cl. The orbital
distribution of those three polymers is similar because of the same backbone. The
calculated HOMO levels of the model of PBPTBz-0Cl, PBPTBz-1Cl and
9

ACCEPTED MANUSCRIPT
PBPTBz-2Cl were -4.74, -4.83 and -4.86 eV, respectively. It is noted that the HOMO
energy levels of PBPTBz-1Cl and PBPTBz-2Cl were 0.09 and 0.12 eV lower,
respectively, as compared with the HOMO level of PBPTBz-0Cl, demonstrating that
the HOMO level is decreased by 0.09 eV through introducing one chlorine atom.
Besides, the band gap of the three polymers were almost the same, revealing that the
planarity of the backbones did not change upon the chlorination of conjugated
thiophene side chain of asymmetric BDT unit. Moreover, the molecular polarity was
enhanced and the corresponding dipole moment (δ) was 1.56, 5.47 and 6.69 for
PBPTBz-0Cl, PBPTBz-1Cl and PBPTBz-2Cl, respectively, revealing that the dipole
moment of polymer is boosted through chlorination of side chain, which in
consequence contributes to the ICT and intermolecular dipole-dipole effect. The
results from the present theoretical calculations indicate that the chlorination of
conjugated thiophene side chains of asymmetric BDT is a promising strategy to
decrease the HOMO level of donor materials.
2.3. Optical and electrochemical properties
10

ACCEPTED MANUSCRIPT
Figure 2. (a) Normalized absorption spectra of four polymers in diluted chlorobenzene (CB)
solution at 20 oC; (b) normalized UV−vis absorption spectra of thin films; (c) cyclic
voltammogram of the donor polymers; and (d) energy levels diagram of the corresponding donor
polymers and acceptor IT-M in this work.
The UV-vis absorption spectra of the polymers in diluted chlorobenzene (CB)
solution (10-5 M) and in thin films at room temperature are displayed in Figure 2a and
2b, respectively, and the corresponding absorption data are collected in Table 2. As
shown in Figure 2a, the polymers PBPTBz-0Cl, PBPTBz-1Cl and PBPTBz-2Cl show
the relatively similar hump-shaped absorption curves from 400 to 650 nm with a
shoulder peak for each polymer. The maximum absorption peaks of PBPTBz-0Cl,
PBPTBz-1Cl and PBPTBz-2Cl are located at 541, 539 and 533 nm with the
corresponding molar extinction coefficients of 1.35×105, 1.26×105 and 1.19×105 cm−1
L mol-1, indicating that they all possess a strong light-harvesting ability at the same
order of magnitude, respectively. Figure 2b displays the absorption in solid thin films,
11

ACCEPTED MANUSCRIPT
polymer PBPTBz-2Cl exhibits a redshift (5 nm) in the maximum absorption from
solution to thin film, but PBPTBz-0Cl and PBPTBz-1Cl almost does not change. The
Eogpt of PBPTBz-0Cl, PBPTBz-1Cl and PBPTBz-2Cl are calculated to be 1.96, 1.97
and 1.98 eV according to Egopt =1240/λonset. The HOMO energy levels are evaluated
through electrochemical cyclic voltammetry (CV) shown in Figure 2c and the
corresponding energy diagrams is depicted in Figure 2d. The HOMO and LUMO
level values are summarized in Table 2. The HOMO level values of PBPTBz-0Cl,
PBPTBz-1Cl and PBPTBz-2Cl are -5.27, -5.32 and -5.41eV, respectively.
PBPTBz-0Cl exhibits deeper HOMO than J52 (-5.21 eV), which is attributed to the
relatively low electron densities and high ionization potential of phenyl group
compared with thienyl group.24, 33 The lower HOMO levels of PBPTBz-1Cl and
PBPTBz-2Cl are attributed to the synergistic effect of the phenyl groups and
chloro-substituted thiophene into the BDT units, which is beneficial to achieve a high
VOC in PSCs.45 The variation trend of HOMO level from CV results agrees well with
the DFT calculation.
Table 2. Physical, Electronic, and Optical Properties of the Donors Materials
Eogpt
Solution
Film
HOMO LUMO
Ecgv
Polymer
(eV)c
(eV)b
(eV)
(eV)
ε (M-1 cm-1)
λmax (nm)
λmax (nm)
λonset (nm)a
1.35 × 105
1.26 × 105
1.19 × 105
PBPTBz-0Cl
PBPTBz-1Cl
PBPTBz-2Cl
541, 584
539, 582
532, 571
542, 588
540, 584
537, 578
631
629
626
1.96
1.97
1.98
-5.27
-5.32
-5.41
-3.56
-3.55
-3.54
1.71
1.77
1.87
a Absorption edge of the polymer films spin-coated from CB solution. b Calculated from the absorption edge of the
polymer films: Egopt = 1240/ λedge
.
c Electrochemical bandgap obtained from ELUMO − EHOMO
.
12

ACCEPTED MANUSCRIPT
2.4. Temperature-dependent aggregation
Figure 3. Temperature-dependent absorption spectra of the solutions of (a) PBPTBz-0Cl, (b)
PBPTBz-1Cl, (c) PBPTBz-2Cl in CB solution (10-5 M ) and (d) the blueshift variation trend of
absorption edge λedge of polymers with increasing temperature from 20 to 110 °C.
In order to investigate the influence of the two different side chains of asymmetric
BDT units on the aggregation behaviors of polymers PBPTBz-0Cl, PBPTBz-1Cl and
PBPTBz-2Cl, the temperature-dependent absorption measurements were conducted in
the temperature range from 20 to 110 °C as shown in Figure 3. All three polymers
exhibit distinct redshift as the temperature decreases, indicating a strong molecular
aggregation at room temperature. In addition, the polymers PBPTBz-0Cl and
PBPTBz-1Cl exhibit the obvious shoulder at about 570 nm in the high temperature
regime (100-110 °C), however, the shoulder peak of polymer PBPTBz-2Cl has
disappeared at 90 °C, revealing that the chains of polymer PBPTBz-2Cl are
completely disaggregated at high temperature of 100-110 °C beneficial to the
13

ACCEPTED MANUSCRIPT
morphology controlling of the active layer. The two polymers PBPTBz-0Cl and
PBPTBz-1Cl maintain their strongly aggregated behaviors at 20-110 °C which could
result in the severe phase separation. In addition, as shown in Figure 1d, the
absorption edge of PBPTBz-2Cl is very sensitive to the increase of temperature from
40 to 90 °C with the gradually disappeared shoulder peak, but PBPTBz-0Cl and
PBPTBz-1Cl exhibit the weaker response to the change of temperature. These results
demonstrate that the side chain of 3,4-dichloro-substituted thiophene for PBPTBz-2Cl
provides a sufficient steric twist to allow PBPTBz-2Cl to fully disaggregate at high
temperatures but tend to more coplanar orientation upon the cooling process, which is
consistent with the absorption spectra in solid thin film at room temperature.45
However, the steric hindrance of the bare and 4-chloro-substituted thiophene for
PBPTBz-0Cl and PBPTBz-1Cl is not sufficient to overcome the strong aggregation at
high temperature.
2.5. Photovoltaic properties
14

ACCEPTED MANUSCRIPT
Figure 4. (a) The J−V curves of PSCs based on polymer:IT-4M (1:1, wt/wt) under the illumination
of AM 1.5G, 100 mW cm−2, (b) EQE curves based on corresponding PSCs, (c) Photocurrent (Jph)
against effective voltage (Veff) curves of the polymer:IT-4M blends and (d) Short-circuit current
density (JSC ) versus light intensity plots of the three polymer devices.
The photovoltaic performance of the three polymers was evaluated with a
conventional architecture of ITO/PEDOT:PSS/polymer:IT-M/PDINO/Al under the
illumination of AM 1.5 G (100 mW cm−2).46 Considering the complementary
absorption spectra, IT-M was selected as the acceptor material. The optimum D/A
weight ratios were found to be 1:1. CB was used as processing solvent without any
solvent additives and post treatment according to our previous research.28 The
optimum current density−voltage (J−V) curves, external quantum efficiency (EQE)
curves, and the relevant photovoltaic results are displayed in Figure 4a-4b, and Table
3, respectively. PBPTBz-0Cl:IT-M based device exhibited a PCE of 6.15%, with a Voc
of 0.81 V, a Jsc of 14.26 mA cm-2 and a fill factor (FF) of 52.95%. As for device based
15

ACCEPTED MANUSCRIPT
on PBPTBz-1Cl:IT-M under the same condition, a higher PCE of 6.46% with an
enhanced Jsc of 14.78 mA cm-2, an increased Voc of 0.87 V and a slightly decreased FF
of 50.20% were obtained. As anticipated from the lowest HOMO level of
PBPTBz-2Cl among the three polymers, the PBPTBz-2Cl:IT-M based device exhibits
a much higher Voc of 0.99 V. The PSCs based on PBPTBz-2Cl:IT-M exhibit a higher
PCE of 7.18% with a quite low Eloss of 0.58 eV, and a further enhanced Jsc of 14.86
mA cm-2 compared with PBPTBz-0Cl and PBPTBz-1Cl. It is noted that the PCEs, Jsc
and Voc are simultaneously enhanced for PBPTBz-0Cl, PBPTBz-1Cl and
PBPTBz-2Cl based PSCs, respectively, which we attribute to the synergistic effect
from the side chains of phenyl group and 3,4-dichloro-substituted thiophene into
asymmetric BDT units. We believe that there is still a possibilities to further boost the
PCEs for PBPTBz-2Cl based PSCs. As shown in Figure 4b, the PBPTBz-2Cl based
device exhibits a broad, and stronger photoresponse than that of PBPTBz-0Cl and
PBPTBz-1Cl in the range of 300–800 nm, which is ascribed to the better
complementary absorption. In addition, the EQE value stays above the line of 60-70%
in the range of 500–720 nm, contributing to achieve the high JSC of the PSCs.
Furthermore, as displayed in Table 3, the integrated current densities from the EQE
spectra are 13.69, 14.71 and 14.86 cm2 v-1 s-1 for PBPTBz-0Cl, PBPTBz-1Cl and
PBPTBz-2Cl based PSCs, respectively, which agrees well with the JSC from the J–V
measurements within a mismatch of 4%. The hole mobility as an important factor of
PSCs were also measured by the space-charge limited current (SCLC) method. As
shown in Figure S2, the hole mobility was calculated to be 1.4×10-4, 1.6×10-4 and
16

ACCEPTED MANUSCRIPT
1.3×10-4 cm2 v-1 s-1 at the same order of magnitudes for the optimal PBPTBz-0Cl,
PBPTBz-1Cl and PBPTBz-2Cl based PSCs, respectively.
Table 3. Photovoltaic parameters of the PSCs based on polymer:IT-M (1:1, wt/wt), under the
illumination of AM 1.5G, 100 mW cm-2
a
b
Thickness
Voc
Jsc
Jsc
FF
PCEmax
(%)
µh
Polymer
(nm)
(V)
(mA cm-2)
(mA cm-2)
13.69
(%)
(cm2 v−1 s−1)
1.4×10-4
1.6×10-4
95
0.81
0.87
0.99
14.26
14.78
14.92
52.95
50.20
48.61
6.15
6.46
7.18
PBPTBz-0Cl
PBPTBz-1Cl
PBPTBz-2Cl
105
100
14.71
14.86
1.3×10-4
aThe Jsc calculated from the EQE spectrum, bµh measured by the SCLC model.
To further explore the photocurrent behaviors of three devices based on
PBPTBz-0Cl, PBPTBz-1Cl and PBPTBz-2Cl under the optimal conditions, the
exciton dissociation probability P (E,T) was carried out shown in Figure 4c. Jph is
defined as the difference between the light and dark current densities, and Veff as the
difference of the applied voltage and the voltage Va (when Jph is zero). Jph saturates at
sufficiently high reverse voltage and the Jph/Jsat ratio (P (E,T)= Jph/Jsat) can be
measured to account for the exciton dissociation and charge collection ability. As
displayed in Figure 4c, the calculated P (E,T) for optimal PBPTBz-0Cl:IT-M,
PBPTBz-1Cl:IT-M and PBPTBz-2Cl:IT-M based PSCs are 0.97, 0.98 and 0.98,
respectively, revealing that the P (E,T) of the three devices are equally efficient, and
PBPTBz-1Cl:IT-M and PBPTBz-2Cl:IT-M device exhibits slightly better exciton
dissociation and charge collection. Besides, the process of bimolecular recombination
17

ACCEPTED MANUSCRIPT
was investigated. Figure 4d exhibits the linear relation between log (Jsc) and incident
light power log(Plight) described by Jsc∝(Plight)S, the slope (S) suggests the extent of
the bimolecular recombination and the recombination is weak and negligible in the
device when the value of the slope is close to 1. As displayed in Figure 4d, the
relatively high S values were calculated to be 0.90, 0.91 and 0.92 for optimal
PBPTBz-0Cl:IT-M, PBPTBz-1Cl:IT-M and PBPTBz-2Cl:IT-M based PSCs,
respectively, demonstrating the bimolecular recombination is distinctly suppressed in
the corresponding devices which contributes to achieve a higher Jsc.
2.6. Blended film morphology
Figure 5. Tapping mode AFM height images (5 × 5 um) of blend films (polymer:IT-M, 1 : 1,
wt/wt), (a) PBPTBz-0Cl:IT-M, (b) PBPTBz-1Cl:IT-M and (c) PBPTBz-2Cl:IT-M; TEM images of
polymer:IT-M blend films (polymer:IT-M, 1 : 1, wt/wt): (d) PBPTBz-0Cl:IT-M, (e)
PBPTBz-1Cl:IT-M and (f) PBPTBz-2Cl:IT-M.
It is well known that the surface and the bulk morphology of the active layers plays
a significant role for the photovoltaic performance. Thus, atomic force microscopy
18

ACCEPTED MANUSCRIPT
(AFM) and transmission electron microscopy (TEM) were adopted to further
investigate the surface and the bulk morphology in the corresponding devices. As
shown in Figure 5a-c, the three blends exhibit smooth surfaces with the small
root-mean-square (RMS) roughness of 0.70, 0.74 and 0.89 nm, respectively. The
smooth surface of the active layer is beneficial to achieve a better contact behavior
between the hole or electron transport layers and the two electrodes, which
contributes to boost the photovoltaic performance. In the TEM images (Figure 5d-5e),
the overlarge dark regions were observed for PBPTBz-0Cl:IT-M blend and the scales
of phase separation were too small for PBPTBz-1Cl:IT-M blend. However, the blend
film of PBPTBz-2Cl:IT-M exhibits a uniform fibrillar and interpenetrating network
structure with a suitable scale (Figure 5f), which contributes to achieve better exciton
dissociation and charge transport, resulting in a higher Jsc.
2.7. GIWAXS
19

ACCEPTED MANUSCRIPT
Figure 6. (a), (b) and (c) Two-dimensional GIWAXS patterns of PBPTBz-0Cl, PBPTBz-1Cl and
PBPTBz-2Cl blend films with IT-M. (d) The out-of-plain and (e) In-plain line-cut profiles of
GIWAX patterns for the three optimized blends.
In order to further explore the molecular packing and microstructure of the
corresponding blends, grazing incidence wide-angle X-ray scattering (GIWAXS) was
carried out. As displayed in Figure 6, similar GIWAXS patterns and the obvious (010)
diffraction peaks in the out-of-plane (OOP) direction are observed for the three blends,
suggesting that the existence of face-on oriented π−π stacking in blends, which could
contribute to the charge transport. As shown in Figure 6d, the (010) diffraction peaks
in the OOP direction located at qz= 1.71, 1.71 and 1.70 Å-1, corresponding to the π−π
stacking distances are 3.67, 3.67 and 3.69 Å for PBPTBz-0Cl, PBPTBz-1Cl and
PBPTBz-2Cl based blend films, respectively. In addition, the lamellar distances were
calculated to be 19.69, 19.36 and 19.36 Å for PBPTBz-0Cl, PBPTBz-1Cl and
PBPTBz-2Cl based blends according to the (100) diffraction peaks in the Figure 6e,
20

ACCEPTED MANUSCRIPT
respectively. The results above demonstrate that the multiple-substitution of the larger
chlorine atoms has no distinct impact on the π−π stacking distance, which could be
attributed to the strong intermolecular non-covalent interactions of Cl⋯S and Cl⋯π.
3. Conclusions
In summary, two asymmetric chloro-substituted 2D BDT-based wide band gap
polymers, named PBPTBz-1Cl and PBPTBz-2Cl, respectively, were designed and
prepared through simultaneously introducing the alkoxylphenyl groups and the
chloro-substituted thiophene as the two different side chains onto the same BDT unit.
On the one hand, the introduction of the chlorine atoms could enhance the ICT effect
due to the large dipole moment of C-Cl bond while efficiently downshift the HOMO
energy level of the donor polymers. On the other hand, the introduction of the phenyl
group could lower the HOMO level because of the high ionization potential and low
electron density. As a result, the polymers PBPTBz-1Cl and PBPTBz-2Cl exhibit the
deeper HOMO levels of -5.32 and -5.41 eV, respectively, compared with the HOMO
value of -5.27 eV for chlorine-free substituted polymer PBPTBz-0Cl. Therefore, a
high Voc of 0.99 V was achieved for the PBPTBz-2Cl based PSCs when matched with
a small molecular acceptor IT-M. This device exhibited a much higher PCE of 7.18%
with a further enhanced Jsc of 14.92 mA cm-2 compared to other two polymer based
devices. It’s noted that the asymmetric design of chloro-substituted thiophene and the
alkySlphenyl group onto the same BDT unit could simultaneously enhance the Voc,
PCE and Jsc, indicating the great challenge of the trade-off between Voc and Jsc has
21

ACCEPTED MANUSCRIPT
been overcome using the above method, which would be an efficient way for next
high-performance polymer solar cell investigation.
Author information
Corresponding Author
*E-mail: hef@sustc.edu.cn (F. H.) wchen@anl.gov (W. C.)
Notes
The authors declare no competing financial interest.
Acknowledgements
The authors acknowledge financial support from SUSTech, the Recruitment Program
of Global Youth Experts of China, the National Natural Science Foundation of China
(51773087, 21733005, 51603100). And Shenzhen Fundamental Research program
(JCYJ20160504151731734, JCYJ20150630145302237, JCYJ20170817111214740)
and Shenzhen Nobel Prize Scientists Laboratory Project (C17783101).
Supplementary data
Supplementary data related to this article can be found at *********
References
[1] Hao, Z.; Huifeng, Y.; Junxian, H.; Jie, Z.; Jianqi, Z.; Wanning, L.; Runnan, Y.;
Bowei, G.; Shaoqing, Z.; Jianhui, H. Over 14% efficiency in organic solar cells
enabled by chlorinated nonfullerene small-molecule acceptors. Adv Mater 2018; 30:
22

ACCEPTED MANUSCRIPT
1800613.
[2] Zhang, S.; Qin, Y.; Zhu, J.; Hou, J. over 14% efficiency in polymer solar cells
enabled by a chlorinated polymer donor. Adv Mater 2018; 30: 1800868.
[3] Wang, Y.; Zhang, Y.; Qiu, N.; Feng, H.; Gao, H.; Kan, B.; Ma, Y.; Li, C.; Wan, X.;
Chen, Y. A halogenation strategy for over 12% efficiency nonfullerene organic solar
cells. Adv Energy Mater 2018; 8: 1702870.
[4] Fan, B.; Zhu, P.; Xin, J.; Li, N.; Ying, L.; Zhong, W.; Li, Z.; Ma, W.; Huang, F.;
Cao, Y. High-performance thick-film all-polymer solar cells created via ternary
blending of a novel wide-bandgap electron-donating copolymer. Adv Energy Mater
2018; 8: 1703085.
[5] Chen, S.; Yao, H.; Li, Z.; Awartani, O. M.; Liu, Y.; Wang, Z.; Yang, G.; Zhang, J.;
Ade, H.; Yan, H. Surprising effects upon inserting benzene units into a
quaterthiophene-based d-a polymer–improving non-fullerene organic solar cells via
donor polymer design. Adv Energy Mater 2017; 7: 1602304.
[6] Chen H.; Guo Y.; Chao P.; Liu L.; Chen W.; Zhao D.; He F. A Chlorinated
Polymer Promoted Analogue Co-Donors for Efficient Ternary All-Polymer Solar
Cells. Sci. China Chem 2018; DOI: 10.1007/s11426-018-9371-0.
[7] Chen, H.; Hu, Z.; Wang, H.; Liu, L.; Chao, P.; Qu, J.; Chen, W.; Liu, A.; He, F. A
Chlorinated π-Conjugated Polymer Donor for Efficient Organic Solar Cells. Joule
2018; 2: 1-12.
[8] Wang, H.; Chao, P.; Chen, H.; Mu, Z.; Chen, W.; He, F. Simultaneous Increase in
Open-Circuit Voltage and Efficiency of Fullerene-Free Solar Cells through
23

ACCEPTED MANUSCRIPT
Chlorinated Thieno[3,4-b]thiophene Polymer Donor. ACS Energy Letters 2017, 2:
1971-77.
[9] Shin, J.; Kim, M.; Kang, B.; Lee, J.; Kim, H. G.; Cho, K. Impact of side-chain
fluorination on photovoltaic properties: fine tuning of the microstructure and energy
levels of 2D-conjugated copolymers. J Mater Chem A 2017; 5: 16702-11.
[10]Lin, Y.; He, Q.; Zhao, F.; Huo, L.; Mai, J.; Lu, X.; Su, C.-J.; Li, T.; Wang, J.; Zhu,
J.; Sun, Y.; Wang, C.; Zhan, X. A facile planar fused-ring electron acceptor for as-cast
polymer solar cells with 8.71% efficiency. J Am Chem Soc 2016; 138: 2973-76.
[11]Ding, G.; Yuan, J.; Jin, F.; Zhang, Y.; Han, L.; Ling, X.; Zhao, H.; Ma, W.
High-performance all-polymer nonfullerene solar cells by employing an efficient
polymer-small molecule acceptor alloy strategy. Nano Energy 2017; 36: 356-65.
[12] Zhang, Y.; Xu, Y.; Ford, M. J.; Li, F.; Sun, J.; Ling, X.; Wang, Y.; Gu, J.; Yuan, J.;
Ma, W. Thermally stable all-polymer solar cells with high tolerance on blend ratios.
Adv Energy Mater 2018; 8: 1800029.
[13] Yuan, J.; Ford, M. J.; Xu, Y.; Zhang, Y.; Bazan, G. C.; Ma, W., Improved tandem
all-polymer solar cells performance by using spectrally matched subcells. Adv Energy
Mater 2018; 8: 1703291.
[14] Yuan, J.; Zhai, Z.; Li, J.; Lu, J.; Huang, X.; Xu, Z.; Ma, W., Correlation between
structure and photovoltaic performance of a series of furan bridged donor–acceptor
conjugated polymers. J Mater Chem A 2013; 1: 12128-36.
[15] Yue, W.; Huang, X.; Yuan, J.; Ma, W.; Krebs, F. C.; Yu, D., A novel
benzodipyrrolidone-based low band gap polymer for organic solar cells. J Mater
24

ACCEPTED MANUSCRIPT
Chem A 2013; 1: 10116-19.
[16]Uddin, M. A.; Kim, Y.; Younts, R.; Lee, W.; Gautam, B.; Choi, J.; Wang, C.;
Gundogdu, K.; Kim, B. J.; Woo, H. Y. Controlling energy levels and blend
morphology for all-polymer solar cells via fluorination of a naphthalene
diimide-based copolymer acceptor. Macromolecules 2016; 49: 6374-83.
[17] Jung, J. W.; Jo, J. W.; Chueh, C. C.; Liu, F.; Jo, W. H.; Russell, T. P.; Jen, A. K.
Fluoro-substituted n-type conjugated polymers for additive-free all-polymer bulk
heterojunction solar cells with high power conversion efficiency of 6.71%. Adv Mater
2015; 27: 3310-7.
[18] Cui, Y.; Zhang, S.; Liang, N.; Kong, J.; Yang, C.; Yao, H.; Ma, L.; Hou, J.
Toward efficient polymer solar cells processed by a solution-processed layer-by-layer
approach. Adv Mater 2018; 1802499.
[19] Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang, B.; Hou, J. Molecular
optimization enables over 13% efficiency in organic solar cells. J Am Chem Soc 2017;
139: 7148-51.
[20] Xia, D.; Wu, Y.; Wang, Q.; Zhang, A.; Li, C.; Lin, Y.; Colberts, F. J. M.; van
Franeker, J. J.; Janssen, R. A. J.; Zhan, X.; Hu, W.; Tang, Z.; Ma, W.; Li, W. Effect of
alkyl side chains of conjugated polymer donors on the device performance of
non-fullerene solar cells. Macromolecules 2016; 49: 6445-54.
[21] Li, Z.; Lin, H.; Jiang, K.; Carpenter, J.; Li, Y.; Liu, Y.; Hu, H.; Zhao, J.; Ma, W.;
Ade, H.; Yan, H., Dramatic performance enhancement for large bandgap thick-film
polymer solar cells introduced by a difluorinated donor unit. Nano Energy 2015; 15:
25

ACCEPTED MANUSCRIPT
607-15.
[22] Li, W.; Li, G.; Guo, X.; Guo, B.; Bi, Z.; Guo, H.; Ma, W.; Ou, X.; Zhang, M.; Li,
Y. Efficient non-fullerene polymer solar cells based on a wide bandgap polymer of
meta-alkoxy-phenyl-substituted benzodithiophene and difluorobenzotriazole. J Mater
Chem A 2017; 5: 19680-86.
[23] Xue, L.; Yang, Y.; Xu, J.; Zhang, C.; Bin, H.; Zhang, Z.-G.; Qiu, B.; Li, X.; Sun,
C.; Gao, L.; Yao, J.; Chen, X.; Yang, Y.; Xiao, M.; Li, Y. Side chain engineering on
medium bandgap copolymers to suppress triplet formation for high-efficiency
polymer solar cells. Adv Mater 2017; 29: 1703344.
[24] Bin, H.; Zhang, Z.-G.; Gao, L.; Chen, S.; Zhong, L.; Xue, L.; Yang, C.; Li, Y.
Non-fullerene polymer solar cells based on alkylthio and fluorine substituted
2d-conjugated polymers reach 9.5% efficiency. J Am Chem Soc 2016; 138: 4657-64.
[25] Li, Y.; Zhong, L.; Gautam, B.; Bin, H.-J.; Lin, J.-D.; Wu, F.-P.; Zhang, Z.; Jiang,
Z.-Q.; Zhang, Z.-G.; Gundogdu, K.; Li, Y.; Liao, L.-S. A near-infrared non-fullerene
electron acceptor for high performance polymer solar cells. Energy Environ Sci 2017;
10: 1610-20.
[26] Li, S.; Ye, L.; Zhao, W.; Zhang, S.; Mukherjee, S.; Ade, H.; Hou, J. Energy-level
modulation of small-molecule electron acceptors to achieve over 12% efficiency in
polymer solar cells. Adv Mater 2016; 28: 9423-29.
[27] Feng, K.; Yang, G.; Xu, X.; Zhang, G.; Yan, H.; Awartani, O.; Ye, L.; Ade, H.; Li,
Y.; Peng, Q. High-performance wide bandgap copolymers using an edot modified
benzodithiophene donor block with 10.11% efficiency. Adv Energy Mater 2018; 8:
26

ACCEPTED MANUSCRIPT
1602773..
[28] Chao, P.; Wang, H.; Mo, D.; Meng, H.; Chen, W.; He, F. Synergistic effects of
chlorination and a fully two-dimensional side-chain design on molecular energy level
modulation toward non-fullerene photovoltaics. J Mater Chem A 2018; 6: 2942-51.
[29] Huang, G.; Jiang, H.; Zhang, J.; Liu, F.; Zhu, M.; Tan, H.; Wang, Y.; Chen, W.;
Yang,
R.;
Zhu,
W.
Two-dimensional
copolymers
based
on
an
alkylthionaphthyl-substituted benzo[1,2-b:4,5-b′]dithiophene for high-efficiency
polymer solar cells. ACS Appl Energy Mater 2018; 1: 1506-11.
[30] Ye, L.; Zhang, S.; Huo, L.; Zhang, M.; Hou, J. Molecular design toward highly
efficient photovoltaic polymers based on two-dimensional conjugated
benzodithiophene. Acc Chem Res 2014; 47: 1595-603.
[31] Huo, L.; Liu, T.; Fan, B.; Zhao, Z.; Sun, X.; Wei, D.; Yu, M.; Liu, Y.; Sun, Y.
Organic solar cells based on a 2d benzo[1,2-b:4,5-b′]difuran-conjugated polymer with
high-power conversion efficiency. Adv Mater 2015; 27: 6969-75.
[32] Liao, S.-H.; Jhuo, H.-J.; Cheng, Y.-S.; Chen, S.-A. Fullerene derivative-doped
zinc oxide nanofilm as the cathode of inverted polymer solar cells with low-bandgap
polymer (ptb7-th) for high performance. Adv Mater 2013; 25: 4766-71.
[33] Huo, L.; Liu, T.; Fan, B.; Zhao, Z.; Sun, X.; Wei, D.; Yu, M.; Liu, Y.; Sun, Y.
Organic solar cells based on a 2d benzo[1,2-b:4,5-b']difuran-conjugated polymer with
high-power conversion efficiency. Adv Mater 2015; 27: 6969-75.
[34] Zhang, Q.; Yan, L.; Jiao, X.; Peng, Z.; Liu, S.; Rech, J. J.; Klump, E.; Ade, H.; So,
F.; You, W. Fluorinated thiophene units improve photovoltaic device performance of
27

ACCEPTED MANUSCRIPT
donor–acceptor copolymers. Chem Mater 2017; 29: 5990-6002.
[35] Jo, J. W.; Jung, J. W.; Wang, H.-W.; Kim, P.; Russell, T. P.; Jo, W. H.,
Fluorination of polythiophene derivatives for high performance organic photovoltaics.
Chem Mater 2014; 26: 4214-4220.
[36] Chao, P.; Wang, H.; Qu, S.; Mo, D.; Meng, H.; Chen, W.; He, F. From semi- to
full-two-dimensional conjugated side-chain design: a way toward comprehensive
solar energy absorption. Macromolecules 2017; 50: 9617-25.
[37] Chao, P.; Mu, Z.; Wang, H.; Mo, D.; Chen, H.; Meng, H.; Chen, W.; He, F.
Chlorination of side chains: a strategy for achieving a high open circuit voltage over
1.0 v in benzo[1,2-b:4,5-b′]dithiophene-based non-fullerene solar cells. ACS Appl
Energy Mater 2018; 1: 2365-72.
[38] Hu, Z.; Chen, H.; Qu, J.; Zhong, X.; Chao, P.; Xie, M.; Lu, W.; Liu, A.; Tian, L.;
Su, Y.-A.; Chen, W.; He, F. design and synthesis of chlorinated
benzothiadiazole-based polymers for efficient solar energy conversion. ACS Energy
Lett 2017; 2: 753-58.
[39] Yao, Z.; Liao, X.; Gao, K.; Lin, F.; Xu, X.; Shi, X.; Zuo, L.; Liu, F.; Chen, Y.; Jen,
A. K. Y. Dithienopicenocarbazole-based acceptors for efficient organic solar cells
with optoelectronic response over 1000 nm and an extremely low energy loss. J Am
Chem Soc 2018; 140: 2054-57.
[40] Li, Y.; Lin, J.-D.; Che, X.; Qu, Y.; Liu, F.; Liao, L.-S.; Forrest, S. R. High
efficiency near-infrared and semitransparent non-fullerene acceptor organic
photovoltaic cells. J Am Chem Soc 2017; 139: 17114-19.
28