Configurational Isomers Induced Significant Difference in All-Polymer Solar Cells

Article abstract of DOI:10.1002/adfm.202100877

The design of polymer acceptors plays an essential role in the performance of all-polymer solar cells. Recently, the strategy of polymerized small molecules has achieved great success, but most polymers are synthesized from the mixed monomers, which seriously affects batch-to-batch reproducibility. Here, a method to separate γ-Br-IC or δ-Br-IC in gram scale and apply the strategy of monomer configurational control in which two isomeric polymeric acceptors (PBTIC-γ-2F2T and PBTIC-δ-2F2T) are produced is reported. As a comparison, PBTIC-m-2F2T from the mixed monomers is also synthesized. The γ-position based polymer (PBTIC-γ-2F2T) shows good solubility and achieves the best power conversion efficiency of 14.34% with a high open-circuit voltage of 0.95?V when blended with PM6, which is among the highest values recorded to date, while the δ-position based isomer (PBTIC-δ-2F2T) is insoluble and cannot be processed after parallel polymerization. The mixed-isomers based polymer, PBTIC-m-2F2T, shows better processing capability but has a low efficiency of 3.26%. Further investigation shows that precise control of configuration helps to improve the regularity of the polymer chain and reduce the π–π stacking distance. These results demonstrate that the configurational control affords a promising strategy to achieve high-performance polymer acceptors.

Full text of DOI:10.1002/adfm.202100877

ReseaRch aRticle
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Configurational Isomers Induced Significant Difference
in All-Polymer Solar Cells
Hengtao Wang, Hui Chen, Weicheng Xie, Hanjian Lai, Tingxing Zhao, Yulin Zhu,
Lin Chen, Chunxian Ke, Nan Zheng, and Feng He*
1
. Introduction
The design of polymer acceptors plays an essential role in the performance
of all-polymer solar cells. Recently, the strategy of polymerized small
molecules has achieved great success, but most polymers are synthe-
sized from the mixed monomers, which seriously affects batch-to-batch
reproducibility. Here, a method to separate γ-Br-IC or δ-Br-IC in gram
scale and apply the strategy of monomer configurational control in which
two isomeric polymeric acceptors (PBTIC-γ-2F2T and PBTIC-δ-2F2T) are
produced is reported. As a comparison, PBTIC-m-2F2T from the mixed
monomers is also synthesized. The γ-position based polymer (PBTIC-γ-
Benefitting from the efforts and remark-
able works of many scientists, all-polymer
solar cells (all-PSC) have made great pro-
[
1–5]
gress over the last 25 years.
Recently,
the power conversion efficiencies (PCEs)
of all-PSCs have been improved to 14–15%.
A major contributor to this improvement
was the incorporation of the acceptor–
(
(
donor–acceptor–donor)–acceptor
A–DAD–A) electron-deficient type poly-
2
F2T) shows good solubility and achieves the best power conversion effi-
[6–10]
merization unit in the polymer acceptor.
ciency of 14.34% with a high open-circuit voltage of 0.95 V when blended
with PM6, which is among the highest values recorded to date, while
the δ-position based isomer (PBTIC-δ-2F2T) is insoluble and cannot be
processed after parallel polymerization. The mixed-isomers based polymer,
PBTIC-m-2F2T, shows better processing capability but has a low efficiency
of 3.26%. Further investigation shows that precise control of configura-
tion helps to improve the regularity of the polymer chain and reduce the
π–π stacking distance. These results demonstrate that the configurational
control affords a promising strategy to achieve high-performance polymer
acceptors.
In order to enrich the polymer accep-
tors for all-PSCs, many strategies have
been proposed and many polymers have
been synthesized. An important milestone
was the application of donor–acceptor
[
11]
(
D–A) polymerization in 2007 following
which, several polymer acceptors based on
[12–14]
naphthalene diimide (NDI)
ylenediimide (PDI)
or per-
[
15,16]
were synthesized.
The most representative of those polymer
acceptors was N2200, which is widely
[
17]
used in organic solar cells (OSC). Other
[
18]
A-type units, such as isoindigo, B→N
[
19]
bridged bipyridine, and cyanobenzothia-
diazole[20] were also applied and contributed significantly to the
forward progress of all-PSCs. In 2017, Zhang et al. proposed
a strategy of polymerizing small molecular acceptors (PSMA)
with an appropriate proper linker and obtained PZ1, a high-
Dr. H. Wang, Dr. H. Chen, W. Xie, H. Lai, Dr. T. Zhao, Y. Zhu, L. Chen,
C. Ke, Prof. F. He
Shenzhen Grubbs Institute and Department of Chemistry
Southern University of Science and Technology
Shenzhen 518055, China
E-mail: hef@sustech.edu.cn
Dr. H. Chen
Academy for Advanced Interdisciplinary Studies
Southern University of Science and Technology
Shenzhen 518055, China
[21]
performance polymeric acceptor. Devices based on PZ1 at
that time achieved a PCE of 9.19%. It should be mentioned that
polymers based on this strategy will have the properties of both
SMAs and polymer acceptors. Accordingly, a series of PSMAs
[
22–26]
[27–29]
[5–9,30,31]
based on IDIC,
ITIC,
and BTIC
were synthe-
sized, and a remarkable advance in all-polymer solar cells was
achieved.
Dr. N. Zheng
Institute of Polymer Optoelectronic Materials and Devices
State Key Laboratory of Luminescent Materials and Devices
South China University of Technology
Guangzhou 510640, China
Although great progress has been made in recent years, it
should be noted that most of the high-performance n-type poly-
mers are synthesized from mixtures of two monomers with
γ-Br-IC or δ-Br-IC end functional groups. The monomers of
these n-type polymers are a mixture of three components, and
this negatively affects the batch-to-batch reproducibility. Further-
more, the randomly distributed units in the polymer discourage
the molecular planarity which affects intermolecular packing
and decreases the charge transfer. Our previous work also con-
firmed that the isomerism of the bromine end groups led to
Prof. F. He
Guangdong Provincial Key Laboratory of Catalysis
Southern University of Science and Technology
Shenzhen, Guangdong 518055, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.202100877.
DOI: 10.1002/adfm.202100877
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notable structural differences in small-molecular non-fullerene
used as the linker in the polymerization in an effort to tune the
acceptors.[
32,33]
Recently, Yang and coworker utilized thiophene
energy gap
interactions
[36–38]
and enhance the intermolecular non-covalent
of the final polymeric acceptors. In order to
[
39,40]
as linker and synthesized two configuration-confirmed polymer
acceptors, PY-IT and PY-OT. The following tests showed that
PY-IT significantly improved the fill factor (FF) in the devices and
gain deeper insight in this system, we also used the mixed
monomers m-Br-BTIC in the polymerization and thus achieved
the polymer acceptor PBTIC-m-2F2T as a comparison. Interest-
ingly, those three polymers PBTIC-γ-2F2T, PBTIC-δ-2F2T, and
PBTIC-m-2F2T have significantly different physicochemical
properties. PBTIC-γ-2F2T dissolves easily in common solvents
such as dichloromethane (DCM), chloroform (CF), and chlo-
robenzene (CB). In contrast, PBTIC-δ-2F2T shows very poor
solubility in these solvents, and it results in almost opposite
photovoltaic properties in their devices. PBTIC-γ-2F2T shows
the best photovoltaic performance among these materials and
achieves a PCE of 14.34% with a high Voc of 0.95 V. It was very
hard to process PBTIC-δ-2F2T in solution due to its poor solu-
bility, which resulted in almost zero efficiency in devices, and
hindered the further investigation of the polymer's photovoltaic
properties. The bad solubility of PBTIC-δ-2F2T mainly results
from the strong aggregation because of the “V”-type polymer
chain in PBTIC-δ-2F2T, and leads to smaller intramolecular
hindrance and higher planarity. PBTIC-m-2F2T showed modest
solubility but the photovoltaic performance is only about 3.26%
in efficiency, much lower than that of PBTIC-γ-2F2T. The dimin-
ished efficiency is mainly caused by the electron mobility which
is almost 2 orders of magnitude lower than that of PBTIC-γ-
2F2T. In order to further explore the isomerism effect in this
system, 2,2-bithiophene was also incorporated as a linker to
deliver PBTIC-γ-2T, a non-fluorinated polymer. Unsurprisingly,
delivered an extremely high performance in all-polymer organic
solar cells.[
34]
Hence, the development of configurationally
defined polymer would be a promising strategy to understand
the anomeric effect in all-polymer solar cells and lead to the high
performance n-type polymer acceptors. Apparently, the isolation
of γ-Br-IC and δ-Br-IC was the key factor to obtain the pure mono-
mers for the further polymerization. Recrystallization from chlo-
roform and ethanol proved to be an effective way to separate the
Cl-substituted isomers,[ but it is not very efficient to separate
Br-substituted isomers, especially to obtain the δ-Br-IC isomer.
From this point, it is very necessary to find out a more effective
way to collect the γ-Br-IC and δ-Br-IC in large scale, such as in
gram scale. Based on that, the researchers in this field can have
the capability to make a deep insight of the isomeric effect in the
all-polymer solar cells. What's more important, a deep explora-
tion of the polymerization of these isomer-free units and other
conjugated linking moieties could deliver better understanding
of the configurational control in polymer solar cell, and further
pump the performance blooming in this area.
35]
Herein, we applied the strategy of configurational control
in the design of acceptors with isolated γ-Br-IC and δ-Br-IC
at large scale, and successfully produced the two polymers,
PBTIC-γ-2F2T and PBTIC-δ-2F2T, shown in Scheme 1. In the
formation of these polymers, fluorinated 2,2′-bithiophene was
Scheme 1. a) Synthetic route to m-Br-IC, γ-Br-IC, and δ-Br-IC; b) Structures of PBTIC-γ-2F2T, PBTIC-δ-2F2T, PBTIC-m-2F2T, and PBTIC-γ-2T.
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this polymer also showed good performance with a PCE of
5,5′-bis(trimethylstannyl)-3,3′-difluoro-2,2′-bithiophene
and
5,5′-bis(trimethylstannyl)-2,2′-bithiophene to afford the target
polymer acceptors. PBTIC-γ-2F2T, PBTIC-m-2F2T, and PBTIC-
γ-2T exhibit good solubility in CF and CB, but PBTIC-δ-2F2T
with really strong aggregation showed insolubility in these sol-
vents. The reaction times were adjusted to lower the molecular
weight of the polymeric product, but the PBTIC-δ-2F2T that was
produced still exhibited strong aggregation which hindered fur-
ther investigation of its properties. Thermogravimetric analysis
11.92%. The performance of PBTIC-γ-2T, which is slightly lower
than that of PBTIC-γ-2F2T is mainly due to the decreased pla-
narity and charge mobility in these polymer acceptors. Hence,
configurational control of monomers offers an opportunity to
enhance the charge mobility in polymer acceptors and eventu-
ally elevate the device performance of all-polymer solar cells.
(
TGA) showed that all of these polymers exhibit good thermal
2
. Results and Discussion
stability with 5% weight-loss at temperatures above 340 °C as
shown in Figure S1, Supporting Information, and the results
are summarized in Table S1, Supporting Information. PBTIC-
γ-2F2T showed the highest stability and achieved a 5% weight-
loss temperature of 362 °C. Differential scanning calorimetry
(DSC) showed that all these materials have high morphological
stability. No obvious melting temperature and crystallization
temperature were observed when the materials were heated up
to 250 °C or cooled down to 30 °C (Figure S2, Supporting Infor-
mation), which means a stable state in the blend films and this
is very important for the morphological stability in the devices.
2
.1. Materials Design and Synthesis
The synthesis of the three end caps, m-Br-IC, γ-Br-IC, and
δ-Br-IC is shown in Scheme 1. m-Br-IC was produced in two
steps from 4-bromoisobenzofuran-1,3-dione. γ-Br-IC was crys-
tallized from CF with minor modifications of our reported
method.[ δ-Br-IC was obtained by crystallization from the
CF/hexane. It deserved to mention that both brominated iso-
mers could be obtained in gram scales, which brought great
convenience for the further investigation of the isomeric effect
in the OSCs. Detailed information can be found in Supporting
Information (SI), in which Scheme S1 describes the synthetic
route to the polymer acceptors, PBTIC-γ-2F2T, PBTIC-δ-2F2T,
PBTIC-m-2F2T, and PBTIC-γ-2T. The three end caps were
connected with BT-CHO through a Knoevenagel condensa-
tion to achieve the corresponding monomers, γ-Br-BTIC,
δ-Br-BTIC, and m-Br-BTIC, and the monomers obtained
in this way underwent Pd catalyzed polymerization with
33]
2.2. Physicochemical Properties
The optical properties of PBTIC-γ-2F2T, PBTIC-m-2F2T, and
PBTIC-γ-2T were assessed both in solution and as thin films,
and are shown in Figure 1a with the parameters summarized
in Table 1. We also attempted to measure the UV–vis absorption
Figure 1. a) Normalized absorption spectra of PBTIC-γ-2F2T, PBTIC-m-2F2T, and PBTIC-γ-2T in CF solution and in thin films. b) Energy level diagrams
of PM6, BTIC-γ-2F2T, PBTIC-m-2F2T, and PBTIC-γ-2T series. c) J-V characteristics of device based on PBTIC-γ-2F2T, PBTIC-m-2F2T, and PBTIC-γ-2T.
d) EQE spectra of PBTIC-γ-2F2T, PBTIC-m-2F2T, and PBTIC-γ-2T based devices.
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Table 1. UV–vis absorption, and electrochemical properties of PBTIC-γ-2F2T, PBTIC-m-2F2T, and PBTIC-γ-2T.
sol
film
film
opt
EHOMOb) [eV]
ELUMOc) [eV]
Acceptors
λ
[nm]
λ
[nm]
λ
[nm]
Eg a)
[eV]
max
max
onset
PBTIC-γ-2F2T
PBTIC-m-2F2T
PBTIC-γ-2T
758
805
882
1.41
−5.56
−5.59
−5.51
−5.58
−3.80
−3.78
−3.77
−3.83
758
759
747
790
796
802
866
871
895
1.43
1.42
1.39
PBTIC-δ-2F2T
a)E go pt = 1240/λonset; b)ELUMO = −(Ered + 4.46); c)EHOMO = −(Eox + 4.46).
spectrum of PBTIC-δ-2F2T and details can be found in Figures S5
and S6, Supporting Information. The parameters were also
summarized in Table 1. PBTIC-γ-2F2T, PBTIC-m-2F2T, and
PBTIC-γ-2T showed similar absorption in the 500–800 nm range
and similar maximum absorption peaks at ≈758 nm in solution,
while PBTIC-δ-2F2T at 747 nm. The apparent different absorp-
tion peaks of PBTIC-γ-2F2T and PBTIC-δ-2F2T might be caused
by the side effects of substitution on electronic properties of
the backbone. PBTIC-γ-2F2T shows stronger absorption with
Figure S8, the fluorinated molecule, PBTIC-γ-2F2T, showed
higher planarity than the non-fluorinated polymer, PBTIC-γ-2T.
It was found that introduction of a fluorine atom into the bith-
iophene unit decreased the dihedral angle between the two
thiophene units from 15.41° to 0.34°, significantly improving
the planarity of PBTIC-γ-2F2T. This was attributed to the intra-
molecular F-S interaction, the measured F⋅⋅⋅S distance being
2.94 Å. The improved molecular planarity might result from a
smaller steric hindrance when attached to the BTIC unit and
could improve the intermolecular packing and charge mobility,
which will benefit the photovoltaic performance. Polymerization
at different sites also affected the molecular configuration as the
δ-position based polymer, PBTIC-δ-2F2T, showed higher flex-
ibility with a “V”-shaped backbone. This special configuration
could be expected to improve the intermolecular π–π stacking
between BTIC units and enhance the intermolecular electron
transport. However, the resulting strong aggregation would
significantly lower the solubility and make PBTIC-δ-2F2T diffi-
cult to process in solution. Figure S9, Supporting Information,
shows the calculated HOMO and LUMO energy levels and the
electron distribution across the molecular backbones of PBTIC-
γ-2F2T, PBTIC-δ-2F2T, and PBTIC-γ-2T. It was found that all
three of these polymers hold the same calculated HOMO energy
levels of −5.43 eV, and the calculated LUMO of PBTIC-γ-2F2T
(−3.39 eV) and PBTIC-δ-2F2T (−3.39 eV) were slightly deeper
than that of PBTIC-γ-2T (−3.38 eV), which is consistent with the
cyclic voltammetry data.
5
–1
–1
an absorption coefficient of over 1.24 × 10 ꢀ cm (Figure S3,
Supporting Information) compared with PBTIC-m-2F2T (1.15 ×
5
–1
–1
5
–1
–1
1
0 ꢀ cm ) and PBTIC-γ-2T (1.02 × 10 ꢀ cm ). This is attrib-
uted to the fluorination and the regular arrangement along the
polymer chain. In the thin film, the absorption peaks of PBTIC-
γ-2F2T and PBTIC-δ-2F2T red shifted to 805 and 802 nm which
were 15 nm and 12 nm higher than that of PBTIC-m-2F2T,
respectively. This proved that precise control of configuration
could enhance the intermolecular packing and broaden the
absorption spectra, benefitting the Jsc in the devices. It was inter-
esting to find that the non-fluorinated polymer, PBTIC-γ-2T also
exhibited 6 nm redshift in the film when compared to PBTIC-m-
2F2T, which again demonstrates that precise control of the mole-
cular structure can enhance the intermolecular π–π stacking and
thus benefit the intermolecular charge transportation.
Cyclic voltammetry (CV) was used to determine the energy
levels of the polymer acceptors, and the results are shown in
Figure 1b, and Figures S4 and S5, Supporting Information. A
+
ferrocene–ferrocenium (Fc/Fc ) redox couple was chosen as
the calibrating potential, and the onset oxidation/reduction
+
potentials versus Ag/Ag were 1.10/−0.66 eV for PBTIC-γ-2F2T,
2.3. Photovoltaic Performance
1
.12/−0.63 eV for PBTIC-δ-2F2T, 1.13/−0.68 eV for PBTIC-m-
F2T, and 1.05/−0.69 eV for PBTIC-γ-2T. Hence, the HOMO/
LUMO energy levels of PBTIC-γ-2F2T, PBTIC-δ-2F2T, PBTIC-
m-2F2T, and PBTIC-γ-2T were −5.56/−3.80, −5.58/−3.83,
5.59/−3.78, and −5.51/−3.77 eV, respectively, and the corre-
2
In order to explore the photovoltaic performance, devices based
on these materials with the structure of ITO/TEDOT:PSS/
PM6:acceptor/PNDIT-F3N/Ag were fabricated. The current
density versus voltage (J–V) curves are shown in Figure 1c,
and the corresponding parameters are summarized in Table 2.
The device based on PBTIC-γ-2F2T showed the highest PCE
−
sponding results are summarized in Table 1. The LUMO energy
levels of PBTIC-γ-2F2T and PBTIC-δ-2F2T were lower than the
other two, and this will enhance the exciton dissociation at the
interlayer of the donor and acceptor. The bandgap of PBTIC-
γ-2F2T and PBTIC-δ-2F2T were smaller than that of PBTIC-m-
–
2
of 14.34% with a current density (J ) of 22.56 mA cm and
sc
fill factor (FF) of 66.89%, which is much higher than that of
PBTIC-m-2F2T whose PCE is 3.26%. Apparently, the improved
efficiency of PBTIC-γ-2F2T mainly comes from the improved Jsc
and FF, which in turn is attributed to the red-shifted absorption
and higher charge mobility. However, the process of PBTIC-δ-
2F2T proved to be really difficult due to the terrible solubility,
and the resulted blend film seemed to be apparently different
from others. Not surprisingly, the corresponding device exhib-
ited almost zero efficiency (0.02%) under the test conditions
(Table 2). The device based on the non-fluorinated polymer,
PBTIC-γ-2T, also exhibited good performance with a PCE of
2
F2T, which might imply a better intermolecular packing and
higher charge transport in devices based on these polymers.
In order to investigate the configuration of these molecules
and the distribution of the electronic atmosphere, density func-
tional theory (DFT) was employed with B3LYP/6-31G (d,p), in
which the undecyl and octyldodecyl chains were simplified and
represented as methyl and isobutyl groups. Since PBTIC-m-
2
F2T was randomly polymerized from the mixed isomers m-Br-
BTIC, theoretic calculations were not performed. As shown in
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Table 2. Photovoltaic parameters of the polymer acceptor based OSC
In order to further understand the effect of configuration
control on the charge transport properties, the space-charge-
limited current (SCLC) measurements were made and the
hole- and electron-only diodes with device structures of ITO/
devices.
Donor: Acceptora)
PM6: PBTIC-γ-2F2T
PM6: PBTIC-m-2F2T
PM6: PBTIC-δ-2F2T
PM6: PBTIC-γ-2T
Voc [V]
0.95
0.99
0.08
0.95
Jsc [mA cm–2
22.56
]
FF [%]
66.89
33.84
24.83
60.22
PCE [%]
14.34
3.26
PEDOT:PSS/PBDBT-2F:acceptor/MoO /Ag and ITO/ZnO/
3
PBDBT-2F: acceptor/PDINO/Al were fabricated. The cor-
responding hole and electron mobility curves are shown in
Figure S10, Supporting Information, and the results are sum-
marized in Table S3, Supporting Information. It was found that
both hole mobility and electron mobility of the PM6: PBTIC-γ-
9.72
0.91
0.02
20.85
11.92
a)1-chloronaphthalene (CN) (volume ratio: 0.5%) was added into solutions as
additives.
–
4
2
–1 –1
–4
2 –1 –1
2F2T blend (μ = 2.6 × 10 cm v s , μ = 1.8 × 10 cm v s )
h
e
–5
2
–1 –1
and the PM6: PBTIC-γ-2T blend (μ = 1.3 × 10 cm v s , μ =
h
e
5
.7 × 10–5 cm v s ) are higher than that of the PM6: PBTIC-m-
2
–1 –1
–2
–5
2
–1 –1
–6
2 –1 –1
1
1.92% (Voc = 0.95, J = 20.85 mA cm , FF = 60.22%). Hence,
2F2T blend (μ = 8.9 × 10 cm v s , μ = 6.4 × 10 cm v s ).
sc
h
e
the strategy of precise configurational control appears to be an
effective strategy to improve the performance of all-PSCs.
The external quantum efficiency (EQE) of these polymer
acceptors based on all-PSCs are shown in Figure 1d. It was
found that both PBTIC-γ-2F2T and PBTIC-γ-2T based devices
showed high photo response from 450 to 850 nm with high
EQE values, approaching 80% and the corresponding integrated
Further, the charge transport of the PM6: PBTIC-γ-2F2T blend
(μ /μ = 1.4) and the PM6: PBTIC-γ-2T blend (μ /μ = 1.6 for
h
e
h
e
PBTIC-γ-2T) were more balanced than that of the PM6:
PBTIC-m-2F2T blend (μ /μ = 2.0). The better carrier trans-
h
e
port properties of the γ-position based polymers are due to the
improved intermolecular interactions, increased molecular
planarity, and optimized morphology which helps to suppress
the accumulation and recombination of charges. Accordingly,
an investigation of charge recombination in the devices was
performed by measurement of the change in the photocurrent
with light intensity (Plight). The relationships between Jsc and
Plight for PBTIC-γ-2F2T, PBTIC-γ-2T, and PBTIC-m-2F2T were
used as an exponential factor to reflect the bimolecular recom-
–2
current Jcal values are 22.14 and 20.45 mA cm , well matched
with the J-V test (Figure 1c). However, PBTIC-m-2F2T based
devices showed much weaker photo response and the EQE was
lower than 40%, which resulted in a significant decrease of the
–2
integrated Jcal of 9.53 mA cm . The higher photo response of
γ-position based polymers over that of the mixed polymers indi-
cated that precise control of the molecular configuration helps
to advance the Jsc in the devices.
[
41]
bination. As shown in Figure 2a, Jsc and Plight, plotted with
dual logarithmic coordinates, obtained slopes (α) of the three
Figure 2. a) Double logarithmic plots of Jsc versus Plight. b) EL quantum efficiencies of the PBTIC-γ-2F2T, PBTIC-m-2F2T, and PBTIC-γ-2T based devices.
c) The FTPS-EQE curves of PBTIC-γ-2F2T, PBTIC-m-2F2T, and PBTIC-γ-2T based devices. d) Energy loss of PBTIC-γ-2F2T, PBTIC-m-2F2T, and PBTIC-
γ-2T based devices.
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acceptor-based devices of 0.97, 0.93, and 0.90, respectively.
In principle, the closer of the gradient, α to 1 means better
exciton transport property and lower photocurrent loss, which
enables a higher FF. Apparently, both PBTIC-γ-2F2T and
PBTIC-γ-2T based device showed higher slopes (α) than that
of PBTIC-m-2F2T, and this was consistent with their higher
Jsc and FF.
∆
E = E − qVoc
g
SQ
SQ
rad
rad
=
(
E
g
− qVOC
)
+
(
qVOC − qVOC
)
+
(
qVOC −Voc
)
(1)
= ∆E + ∆E + ∆E
1
2
3
Eg is the estimated bandgap based on the intersection of the
maximum and the edge of the EQE spectrum of each device.
The obtained bandgaps of the PBTIC-γ-2F2T, PBTIC-m-2F2T,
PBTIC-γ-2T based devices are 1.51, 1.52, and 1.52 eV respec-
tively (Table S4, Supporting Information). According to the
2
.4. Investigation of Voltage Loss
[
43]
To achieve insight into the effects of configurational control on
Shockley–Queisser theory, the three devices present a similar
the voltage loss, we performed the FTPS-EQE and EQE meas-
ΔE of ≈0.272 eV. We measured ΔE from the FTPS-EQE spectra
EL
1
2
urements. Based on the previous reports,[ the total energy
42]
of the three devices shown in Figure 2b, and the calculated
values were determined to be 0.060, 0.061, and 0.052 eV, respec-
tively. For the third part of the voltage loss, the EQEEL experi-
ments of the solar cells were carried out and the values of each
device were evaluated with the equation: ΔE = −kTln(EQE )
loss (ΔE) originated from the three sources: (1) ΔE , radiative
1
recombination loss above the bandgap; (2) ΔE , radiative recom-
2
bination loss below the bandgap; (3) non-radiative energy loss.
The equation governing this is:
3
EL
Figure 3. a–c) AFM height images of PM6:PBTIC-γ-2F2T, PM6:PBTIC-m-2F2T, and PM6:PBTIC-γ-2T. d–f) 2D GIWAXS patterns of PM6:PBTIC-γ-2F2T,
PM6:PBTIC-m-2F2T ,and PM6:PBTIC-γ-2T. g–i) 1D line-cuts in the in-plane and out-of-plane directions of PM6:PBTIC-γ-2F2T, PM6:PBTIC-m-2F2T, and
PM6:PBTIC-γ-2T.
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as shown in Figure 2c. Interestingly, the PBTIC-m-2F2T based
device showed the highest EQEEL value of 9.97 × 10– which cor-
It was found that the polymerization at different sites has
significant effects on the properties of the materials. The
γ-position based polymer, PBTIC-γ-2F2T, has proper crystal-
linity and good miscibility with polymer donors, resulting in
an excellent photovoltaic performance with a PCE of 14.34%.
This is one of the highest values delivered by all-polymer
solar cells to date. The δ-position based polymer, PBTIC-δ-
2F2T exhibits strong aggregation and is almost insoluble in
common solvents. This hinders subsequent studies of physic-
ochemical, and results in almost zero (0.02%) in photovoltaic
efficiency. The mix-position based polymer, PBTIC-m-2F2T,
shows a relatively low photovoltaic performance with a PCE
of 3.26%, which proves that precise control of the position
for the polymerization has a significant effect on the perfor-
mance of OSCs. PBTIC-γ-2T, obtained from the polymeriza-
tion of non-fluorinated 2,2,-bithiophene and γ-position based
monomer, also showed good performance with a high PCE,
approaching 12%. These results show that the precise con-
trol of molecular configuration can enhance charge carrier
mobility, decrease π–π stacking distance, and thus afford a
promising strategy for providing high performance all-polymer
solar cells.
4
responded with the lowest ΔE of 0.179 eV. This apparently low
3
voltage loss accounts for the highest Voc of the PBTIC-m-2F2T
based solar cell. PBTIC-γ-2F2T based devices showed an EQE
EL
–
4
value of 4.10 × 10 slightly higher than that of PBTIC-γ-2T
–
4
(
3.01 × 10 ). The ΔE was calculated to be 0.202 and 0.210 eV
3
for the two polymer acceptors. The non-radiative energy loss
ΔE ) of the three polymers was among the lowest values
(
3
reported to date, and contribute to the lower total voltage loss
ΔE) and the higher Voc.
(
2
.5. Morphology Study of Blend Films
In order to investigate the effects of configurational control
on the morphologies, atomic force microscopy (AFM) and
grazing incidence wide-angle X-ray scattering (GIWAXS) were
employed and the results are shown in Figure 3. As shown in
the height images, all of the blend films exhibited a smooth
surface with a root-mean-square roughness (RMS) value of
no more than 1.0 nm. Among the three blend films, the PM6:
PBTIC-γ-2F2T blend film showed a RMS value of 0.630 nm
which is slightly smaller than that of PM6: PBTIC-m-2F2T and
PM6: PBTIC-γ-2T whose RMS values are 0.680 and 0.942 nm.
The lower RMS value means a smoother surface of the blend
film, which suggests better carrier transport properties in the
devices. GIWAXS was applied to measure the intermolecular
packing properties and the crystallinity.[ The 2D GIWAXS
patterns and 1D line cuts of the blend films are shown in
Figure 3d–i, and the corresponding parameters are summa-
rized in Table S5, Supporting Information. It was found that
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
44]
Acknowledgements
H.W. and H.C. contributed equally to this work. This work was financially
supported by the National Natural Science Foundation of China
PM6: PBTIC-γ-2F2T blend film showed a lamellar stacking
peak (100) at 0.304 Å–1 (d ≈ 20.67 Å) in the q axis direction
L
xy
(
51773087, 21733005, 21975115, and 51903116), Shenzhen Fundamental
and is similar to PM6: PBTIC-m-2F2T and PM6: PBTIC-γ-2T
Research Program (JCYJ20170817111214740, JCYJ20180302180238419,
KQJSCX20180319114442157), Guangdong Provincial Key Laboratory of
Catalysis (2020B121201002), Guangdong Innovative and Entrepreneurial
Research Team Program (2016ZT06G587) and Shenzhen Sci-Tech Fund
(KYTDPT20181011104007). The authors also thank the SUSTech Core
Research Facilities for the AFM and TEM measurements.
–
1
blend films which have peaks at 0.289 Å (d ≈ 21.74 Å) and
L
–
1
0
.306 Å (d ≈ 20.53 Å), respectively. This suggests a face-on
orientation for the three acceptors in the blend films. In the
L
qz axis direction, the lamellar packing peaks (010) of PM6:
–1
PBTIC-γ-2F2T and PBTIC-γ-2T were observed at 1.724 Å and
–
1
1
3
.721 Å with calculated π-π stacking distances of 3.64 and
.65 Å respectively, which were slightly smaller than the
PBTIC-m-2F2T based blend films (1.714 Å , d ≈ 3.67 Å). The
–1
π
Conflict of Interest
The authors declare no conflict of interest.
smaller π–π stacking distance implies a higher intermolecular
charge transport, which is consistent with the higher mobili-
ties obtained from the photovoltaic tests. Hence, the results
obtained from the AFM and GIWAXS measurements imply
that the γ-position based polymers blend films exhibit proper Data Availability Statement
orientation, smaller π-π stacking distance, and smoother sur-
Research data are not shared.
face which is beneficial for carrier transport in the vertical
direction and improves the photovoltaic performance.
Keywords
3
. Conclusion
all-polymer solar cells, bromination, configurational isomers, polymer
acceptors, polymerization
In summary, we successfully separated γ-Br-IC and δ-Br-IC in
gram scale and synthesized four polymer acceptors, namely
PBTIC-γ-2F2T, PBTIC-δ-2F2T, PBTIC-m-2F2T, and PBTIC-γ-2T
based on the strategy of configurational control of monomers.
Received: January 26, 2021
Revised: March 24, 2021
Published online:
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2100877 (7 of 8)
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