Precisely Controlling the Position of Bromine on the End Group Enables Well‐Regular Polymer Acceptors for All‐Polymer Solar Cells with Efficiencies over 15%

Abstract

Recent advances in the development of polymerized A–D–A‐type small‐molecule acceptors (SMAs) have promoted the power conversion efficiency (PCE) of all‐polymer solar cells (all‐PSCs) over 13%. However, the monomer of an SMA typically consists of a mixture of three isomers due to the regio‐isomeric brominated end groups (IC‐Br(in) and IC‐Br(out)). In this work, the two isomeric end groups are successfully separated, the regioisomeric issue is solved, and three polymer acceptors, named PY‐IT, PY‐OT, and PY‐IOT, are developed, where PY‐IOT is a random terpolymer with the same ratio of the two acceptors. Interestingly, from PY‐OT, PY‐IOT to PY‐IT, the absorption edge gradually redshifts and electron mobility progressively increases. Theory calculation indicates that the LUMOs are distributed on the entire molecular backbone of PY‐IT, contributing to the enhanced electron transport. Consequently, the PM6:PY‐IT system achieves an excellent PCE of 15.05%, significantly higher than those for PY‐OT (10.04%) and PY‐IOT (12.12%). Morphological and device characterization reveals that the highest PCE for the PY‐IT‐based device is the fruit of enhanced absorption, more balanced charge transport, and favorable morphology. This work demonstrates that the site of polymerization on SMAs strongly affects device performance, offering insights into the development of efficient polymer acceptors for all‐PSCs.

Full text

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CommuniCation
Precisely Controlling the Position of Bromine on the End
Group Enables Well-Regular Polymer Acceptors for
All-Polymer Solar Cells with Eciencies over 15%
Zhenghui Luo, Tao Liu,* Ruijie Ma, Yiqun Xiao, Lingling Zhan, Guangye Zhang,
Huiliang Sun,* Fan Ni, Gaoda Chai, Junwei Wang, Cheng Zhong, Yang Zou,
Xugang Guo, Xinhui Lu, Hongzheng Chen, He Yan,* and Chuluo Yang*
Dr. Z. Luo, Dr. F. Ni, Dr. Y. Zou, Prof. C. Yang
Shenzhen Key Laboratory of Polymer Science and Technology
College of Materials Science and Engineering
Shenzhen University
Shenzhen 518060, P. R. China
E-mail: clyang@szu.edu.cn
Dr. Z. Luo, Dr. T. Liu, R. Ma, Dr. H. Sun, G. Chai, Prof. H. Yan
Department of Chemistry and Hong Kong Branch of Chinese National
Engineering Research Center for Tissue Restoration & Reconstruction
Hong Kong University of Science and Technology (HKUST)
Clear Water Bay, Kowloon, Hong Kong, P. R. China
E-mail: liutaozhx@ust.hk; hyan@ust.hk
Y. Xiao, Prof. X. Lu
Department of Physics
Chinese University of Hong Kong
New Territories, Hong Kong 999077, P. R. China
Dr. L. Zhan, Prof. H. Chen
State Key Laboratory of Silicon Materials
MOE Key Laboratory of Macromolecular
Synthesis and Functionalization
Department of Polymer Science and Engineering
Zhejiang University
Hangzhou 310027, P. R. China
Dr. G. Zhang
eFlexPV Limited (China)
Plant B701
Guofu Cultural Creative Industry Plant Area
No. 16, Lanjing Middle Road, Zhukeng Community, Longtian Street,
Pingshan District, Shenzhen 518057, P. R. China
Dr. G. Zhang
eFlexPV Limited
Flat/RM B, 12/F, Hang Seng Causeway Bay BLDG, 28 Yee Wo Street,
Causeway Bay, Hong Kong 999077, P. R. China
DOI: 10.1002/adma.202005942
Thanks to their light weight, flexibility,
semitransparency, and potential for
low-cost large-area fabrication, polymer
solar cells (PSCs) have attracted enor-
mous research interest in recent years.[]
Benefiting from the emergence of the
acceptor–donor–acceptor (A–D–A)-type
small-molecule acceptors (SMAs),[] the
PSCs based on polymer donors and A–D–
A-type SMAs have realized power con-
version eciencies (PCEs) over %.[]
Unlike SMAs-based PSCs, all-polymer
solar cells (all-PSCs), with their photoac-
tive layers composed of polymer donors
and polymer acceptors, display unique
features such as outstanding thermal
and morphological stability, and excel-
lent stretchability and mechanical dura-
bility.[] However, the PCEs of all-PSCs
lag behind those of SMAs-based PSCs in
general, which is mainly due to the lack
of high-performance polymer acceptors.
Specifically, the shortage of eective elec-
tron-deficient building blocks has been the
Recent advances in the development of polymerized A–D–A-type small-
molecule acceptors (SMAs) have promoted the power conversion eciency
(PCE) of all-polymer solar cells (all-PSCs) over 13%. However, the monomer
of an SMA typically consists of a mixture of three isomers due to the regio-
isomeric brominated end groups (IC-Br(in) and IC-Br(out)). In this work, the
two isomeric end groups are successfully separated, the regioisomeric issue
is solved, and three polymer acceptors, named PY-IT, PY-OT, and PY-IOT,
are developed, where PY-IOT is a random terpolymer with the same ratio of
the two acceptors. Interestingly, from PY-OT, PY-IOT to PY-IT, the absorp-
tion edge gradually redshifts and electron mobility progressively increases.
Theory calculation indicates that the LUMOs are distributed on the entire
molecular backbone of PY-IT, contributing to the enhanced electron transport.
Consequently, the PM6:PY-IT system achieves an excellent PCE of 15.05%,
significantly higher than those for PY-OT (10.04%) and PY-IOT (12.12%).
Morphological and device characterization reveals that the highest PCE for
the PY-IT-based device is the fruit of enhanced absorption, more balanced
charge transport, and favorable morphology. This work demonstrates that the
site of polymerization on SMAs strongly aects device performance, oering
insights into the development of ecient polymer acceptors for all-PSCs.
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.202005942.
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bottleneck throughout the development of polymer acceptors.
The most common electron-deficient building blocks before
 include perylene diimide (PDI),[] naphthalene diimide
(NDI),[] bithiophene imide (BTI),[] and B←N-bridged bipy-
ridine (BN-Py).[] However, the polymer acceptors based on
these electron-deficient units more or less have some defects
such as the weak absorption intensity for PDI-, NDI-, and BTI-
based polymer acceptors, and the low electron mobility for the
BN-Py-based polymer acceptors, limiting their performance in
PSC devices.[] In , Li and Zhang etal. employed the “‘SMA
polymerization”’ strategy to develop a novel narrow-bandgap
polymer acceptor, namely PZ, which simultaneously pos-
sessed high mobility and large absorption coecients, leading
to a remarkable all-PSC with a PCE of .%.[a] Subsequently, a
series of high-performance polymerized SMAs were developed
through screening the copolymerization units, for example,
thiophene, benzodithiophene, dithieno[,-b:′,′-d]silole, or
manipulating the central core of SMAs (Figure S, Supporting
Information).[]
Despite the great progress in polymerized SMAs, there is still
an important issue to be addressed regarding the performance
and consistency of polymer acceptors: the regioisomeric issue
of brominated ,-dicyanomethylene--indanone (IC-Br), where
IC is the most widely adopted terminal unit in the state-of-the-
art polymerized SMAs. It is known that IC-Br is a mixture of
two isomers with similar polarity, which is dicult to separate.
Consequently, the SMAs with IC-Br as terminal units are a mix-
ture of three isomers, which severely aects the batch-to-batch
reproducibility of the polymerized SMAs, induces a dierence
in physicochemical and morphological properties for dierent
material batches, and eventually results in a large deviation in
device eciency of all-PSCs. Additionally, there have already been
reports showing that the pure SMAs yielded better device perfor-
mance than the mixture of isomers due to the improvement in
morphology and charge transport,[] which inspires us to develop
pure brominated SMAs for polymer acceptor and all-PSCs.
In this work, two monobrominated terminal units, namely
IC-Br(in) and IC-Br(out) (where “in” and “out” indicate that
the bromine and carbonyl groups are on the same side and the
opposite side, respectively) (Figure1a), have been developed
from IC-Br by recrystallization of dierent solvents. Afterward,
two brominated SMAs with certain molecular structures are
synthesized, which have a backbone similar to Y, an ecient
SMA. Ultimately, two polymer acceptors named PY-IT and
PY-OT with dierent polymerization sites have been obtained
via the typical Stille coupling polycondensation (Figureb). To
gain insight into the eect of mixed isomers on photoelectric
properties and photovoltaic performance, we have also devel-
oped the polymer PY-IOT via a random ternary copolymeriza-
tion strategy with two acceptors at the same ratio. Interestingly,
from PY-OT, PY-IOT to PY-IT, the absorption edges gradually
redshift and the lowest unoccupied molecular orbital (LUMO)
values slightly decrease, and the electron mobility increases
steadily. As a result, the best all-PSC device based on PM:PY-IT
demonstrates a PCE of .% with a short-circuit current den-
sity (JSC) of .mA cm−, and an outstanding fill factor (FF)
of ., which are significantly higher than the devices based
on PY-OT (PCE = .%; JSC= . mA cm−; FF = .)
and PY-IOT (PCE = .%; JSC= .mA cm−; FF = .).
The higher JSC and FF for PY-IT-based devices are mainly attrib-
uted to the higher charge mobilities, more balanced charge
transport, and the more favorable blend morphology with
Dr. H. Sun, J. Wang, Prof. X. Guo
Department of Materials Science and Engineering and The Shenzhen
Key Laboratory for Printed Organic Electronics
Southern University of Science and Technology (SUSTech)
No. 1088, Xueyuan Road, Shenzhen, Guangdong 518055, P. R. China
E-mail: Sunhl@sustech.edu.cn
Dr. C. Zhong, Prof. C. Yang
Department of Chemistry
Wuhan University
Wuhan 430072, P. R. China
Figure 1. a) The synthetic routes of IC-Br(in) and IC-Br(out). b) Chemical structures of PY-IT, PY-OT and PY-IOT.
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appropriate domain size. Our work demonstrates that the puri-
fication of the monobrominated A–D–A SMAs is an eective
method to develop high-performance polymer acceptors for
ecient all-PSCs.
As illustrated in Scheme S, Supporting Information, IC-Br(in)
and IC-Br(out) can be purified by recrystallization of IC-Br with
chloroform (CF) and ethanol, respectively. The three polymer accep-
tors were obtained via a Stille coupling reaction between dierent
monobrominated A–D–A SMAs and ,-bis(trimethylstannyl)
thiophene. The number-average molecular weight (Mn) and poly-
dispersity index (PDI) of these polymer acceptors are determined
to be about kg mol− and .. All the three polymer acceptors
exhibit good solubility in common organic solvents such as CF
and chlorobenzene (CB). In addition, thermal decomposition tem-
peratures (at % weight loss) were recorded over  °C for the
polymers (Figure Sa, Supporting Information).
The optical properties of the three polymer acceptors were
surveyed in both dilute CF solution as well as solid films
(Figure2a,b), and the related results are outlined in Table1.
In the CF solution, from PY-OT, PY-IOT to PY-IT, the absorp-
tion peaks are gradually redshifted, which could be the result
of the enhanced LUMO delocalization (as discussed below). In
addition, PY-IT in dilute chloroform solution (− ) shows a
maximum extinction coecient (εmax) of . ×  − cm− at
nm (Figure Sb, Supporting Information), slightly higher
than those of PY-OT (. ×  − cm− at  nm) and
PY-IOT (. ×   − cm− at nm). Similarly, neat PY-IT
film exhibits a redshifted absorption spectrum with a max-
imum absorption coecient of . ×  cm− at nm com-
pared with PY-OT (λmaxfilm= nm; εmax= . ×  cm−)
and PY-IOT (λmaxfilm=  nm; εmax= . ×  cm−). The
optical bandgaps of PY-IT, PY-OT, and PY-IOT are around
.eV. To gain insight into the eect of polymerization site
on the frontier molecular orbitals, density functional theory
(DFT) calculation was conducted. We used DFT to investigate
the optimal molecular geometries and molecular energy levels
of Y-IT and Y-OT, which are subunits of PY-IT and PY-OT,
respectively. As shown in Figure S, Supporting Information,
the LUMOs are distributed over the entire molecule of Y-IT,
which contributes to the enhanced electron transport in PY-IT.
In contrast, there are no LUMOs distribution on linking unit
(thiophene) for Y-OT. Furthermore, the HOMOs are local-
ized on the central core of Y for both of Y-IT and Y-OT.
The similar HOMOs and more delocalized LUMOs could
enhance the charge transfer of PY-IT, and thereby causes a
redshifted absorption spectrum compared with PY-OT. In
addition, DFT results indicate that the polymerization site has
Figure 2. a) Normalized absorption spectra of PY-IT, PY-OT, and PY-IOT in CHCl3 solution. b) Absorption spectra of PY-IT, PY-OT, PY-IOT, and PM6
in film. c) CV curves of PY-IT, PY-OT and PY-IOT. d) Photoluminescence spectra of PY-IT, PY-OT, and PY-IOT (excited at 630nm) films as well as the
blend films of PM6:PY-IT, PM6:PY-OT, and PM6:PY-IOT (excited at 630nm).
Table 1. Optical and electrochemical properties of PY-IT, PY-OT, and
PY-IOT.
Acceptor λmaxsol
[nm]
λmaxfilm
[nm]
λonsetfilm
[nm]
Egopta)
[eV]
HOMOb)
[eV]
LUMOb)
[eV]
PY-IT 789 808 894 1.39 −5.68 −3.94
PY-OT 763 791 874 1.42 −5.69 −3.90
PY-IOT 777 796 882 1.41 −5.68 −3.92
a)Calculated from Egopt= 1240/λonset; b)Measured with CV.
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no impact on energy levels of Y-IT and Y-OT. Cyclic voltam-
metry method was utilized to measure the highest occupied
molecular orbital (HOMO) levels and LUMO levels of the
three polymer acceptors (Figure c). The assumed vacuum
energy level and measured relative energy level of Fc/Fc+ were
−. and .eV, respectively. The onset oxidation and reduc-
tion potentials versus Ag/Ag+ were .eV/−.eV for PY-IT,
.eV/−.eV for PY-OT, and .eV/−.eV for PY-IOT,
respectively. Therefore, The HOMO/LUMO values are calcu-
lated to be −./−.eV, −./−.eV, −./−.eV for
PY-IT, PY-OT, and PY-IOT, respectively. The small dierences
between the three acceptors in the HOMO/LUMO values
further verify that the polymerization site has little eect on
molecular energy levels.
Photoluminescence (PL) quenching experiment under
 nm photo-excitation was performed to investigate the
charge transfer and exciton dissociation in the blend films
(Figured). The PM:PY-IT blend yields the highest quenching
eciency of .%, compared to the blends of PY-OT (.%)
and PY-IOT (.%), indicating that the hole transfer from
PY-IT to the donor polymer (PM) is the most eective among
the three polymer acceptors. In addition, the highest quenching
eciency in the PM:PY-IT blend is beneficial to the exciton
dissociation compared to those of blend based PM:PY-OT and
PM:PY-IOT.
To evaluate the photovoltaic performance of three polymer
acceptors, all-PSCs with a conventional configuration of ITO/
PEDOT:PSS/PM:Polymer acceptor/Zracac (zirconium acetyl-
acetonate)/Al were fabricated. The details of device prepara-
tion can be found in the SI. The current density–voltage (J–V)
curves of the optimal all-PSC devices are displayed in Figure3a,
and the photovoltaic parameters are summarized in Table2.
Impressively, the PM:PY-IT-based device yields a PCE as
high as .%, with a JSC of .mA cm−, an open-circuit
voltage (VOC) of .V along with an FF of .. To the best
of our knowledge, .% is the highest PCE for the all-PSCs
to date. Compared to the PM:PY-IT-based device, the PY-OT-
based device shows a slightly enhanced VOC (.V), signifi-
cantly reduced FF (.) and JSC (. mA cm−), and thus
a much lower PCE of .%. The PY-IOT-based device gives
a moderate PCE of .%, and the VOC, JSC, and FF are all
between those of the PY-OT and PY-IT based devices. As shown
in Figure b, the PM:PY-IT-based device exhibits higher
incident photon-to-current eciency (IPCE) values in the entire
absorption spectrum relative to those device based on PY-OT
and PY-IOT, and the integrated JSC values of PY-IT, PY-OT, and
PY-IOT-based devices are ., ., and . mA cm−,
respectively, matching well with the JSC values from the J-V
curves. To further confirm the high PCE of the PM:PY-IT-
based device, we also fabricated devices at Zhejiang University
(Figure S, Supporting Information, and Table) in the conven-
tional device structure, ITO/PEDOT:PSS/PM:PY-IT/PFN-Br/
Ag, and a similar PCE (.%) was realized.
The photocurrent density (Jph)-eective voltage (Ve) curves
are plotted to investigate the charge extraction and generation
properties (Figure S, Supporting Information), and related data
are summarized in Table S, Supporting Information, including
the saturated current (Jsat), exciton dissociation eciency (Pdiss)
and charge collection eciency (Pcoll). The Pcoll and Pdiss were
obtained according to the formula of P= Jph/Jsat under the max-
imum power output and short-circuit conditions, respectively.
The Pdiss/Pcoll values are .%/.% for PY-IT-based device,
.%/.% for the PY-OT-based device, and .%/.% for
the PYIOT-based device. The highest Pdiss and Pcoll values imply
the most eective exciton dissociation and charge collection in
the PY-IT-based device. Additionally, the relationship between
light intensity (Plight) and JSC was investigated to understand the
charge recombination.[] The fitted slope (S values) in the equa-
tion of JSC∝ PlightS for PY-IT, PY-OT, and PY-IOT-based device
are ., ., and ., respectively, indicating the weakest
bimolecular recombination in PY-IT-based device.
To estimate the charge transport properties of the three devices,
the space-charge-limited current (SCLC) model was employed and
Figure 3. a) J–V characteristics of the best PSCs under the illumination of AM 1.5G, 100mW cm−2. b) IPCE spectra of the best PSC devices based on
PM6:PY-IT, PM6:PY-OT, and PM6:PY-IOT.
Table 2. The optimized photovoltaic performances of the all-PSCs based
on PM6/acceptors. The average values and standard deviations were
obtained from 20 devices.
Acceptor VOC
[V]
JSC
[mA cm−2]
FF% PCEmax
(PCEavg)%
PY-IT 0.933 22.30 72.3 15.05 (14.83± 0.21)
PY-OT 0.954 16.82 62.6 10.04 (9.82± 0.19)
PY-IOT 0.939 19.71 65.6 12.12 (11.97± 0.21)
PY-ITa) 0.924 22.96 70.6 14.93
a)Solar cells are fabricated and measured at Zhejiang University.
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charge carrier mobilities are obtained for both neat acceptor films
and blend films (Figures S and S, and Table S, Supporting
Information). The electron (μe) mobilities of neat PY-IT, PY-OT,
and PY-IOT films from  devices are (. ± .) × −, (.±
.) × −, and (.± .) × − cm V− s−, respectively. Sim-
ilarly, the μe of the PY-IT, PY-OT, and PY-IOT-based blend films
are (.± .) × −, (.± .) × −, and (. ± .) ×
− cm V− s−, respectively; for the hole (μh) mobilities of the
blends, the corresponding values are (.± .) × −, (.±
.) × −, and (.± .) × − cm V− s−, respectively. The
higher and more balanced carrier mobilities could contribute to
the better JSC and FF in the PY-IT-based device.
Grazing-incidence wide-angle X-ray scattering (GIWAXS)
was employed to probe the morphology of the films.[] As
shown in Figure S, Supporting Information, neat PY-IT,
PY-OT, and PY-IOT films adopt a preferential face-on orienta-
tion and their () π–π stacking peaks are found at qz≈ . Å−.
Similar to neat films, all blend films exhibit strong π–π ()
stacking intensity at qz≈ . Å− in the out-of-plane direc-
tion and the lamellar () diraction peaks at qz≈ . Å−,
indicating that the face-on orientation is also dominant in the
blends, which is beneficial for charge transport. Additionally,
atomic force microscopy (AFM) was utilized to investigate the
surface morphology of active layers (Figure4a). In AFM height
images, all three blends exhibit uniform and smooth surface
morphologies, and the root-mean-square (RMS) roughness
for PY-IT, PY-OT, PY-IOT are ., ., .nm, respectively.
As shown in AFM phase images, in comparison with the
PY-OT and PY-IOT-based blends, the PY-IT-based blend shows
smaller domains. To further study the domain size, grazing-
incidence small-angle X-ray scattering (GISAXS) experiments
were conducted (Figureb). The intermixing domain sizes are
calculated to be ., ., . nm for the PY-IT, PY-OT, and
PY-IOT-based blend, respectively, coinciding with the results of
AFM phase images, which is also consistent with the trend in
the PL quenching experiment. The corresponding pure phase
domain sizes are estimated to be , ., .nm, respectively.
The larger pure phase domain sizes for PM:PY-IT blend is
beneficial for charge transport and is still within the commonly
accepted exciton diusion length,[] which could therefore con-
tribute to the large FF in the PY-IT-based device.
In summary, we successfully separated two monobromi-
nated isomeric end groups, namely IC-Br(in) and IC-Br(out),
and developed two well-regular polymer acceptors (PY-IT and
PY-OT) with dierent polymerization sites. In addition, we
also synthesized PY-IOT, a random ternary copolymer with
the same ratio of two acceptors for comparison. We found that
from PY-OT, PY-IOT to PY-IT, the absorption edges become
more and more redshifted, the LUMO energy level deepened,
and electron mobility improved. DFT calculations indicate that
the LUMOs are distributed on the entire molecular backbone
of PY-IT, but not on the linking unit of PY-OT, indicating the
stronger charge transfer characteristics of PY-IT. Consequently,
the PY-IT-based all-PSCs achieved a significantly higher PCE
(.%) than those device based on PY-OT (.%) and
PY-IOT (.%), which was mainly ascribed to the mono-
tonic increase of JSC and FF from the PY-OT-, PY-IOT- to the
PY-IT-based device. The highest FF and JSC in PY-IT-based
devices were consistent with the enhanced absorption intensity,
more eective hole transfer from PY-IT to PM, higher charge
mobility, more balanced charge transport, and more favorable
morphology with suitable domain size. This work demonstrates
that developing well-regular polymer acceptor is an eective
strategy for high-performance all-PSCs.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
C.Y. thanks the support from the National Natural Science Foundation
of China (NSFC) (No. 21572171) and the Innovative Research Group
of Hubei Province (No. 2015CFA014). H.Y. thanks the support from
the National Key Research and Development Program of China
(No. 2019YFA0705900) funded by MOST, Hong Kong Research Grants
Council (HK-RGC grants: R6021-18, 16305915, 16322416, 606012, and
16303917).
Figure 4. a) AFM height images (top) and phase images (bottom), and b) GISAXS intensity profiles and best fittings along the in-plane direction of
PM6:PY-IT, PM6:PY-OT, and PM6:PY-IOT blend films.
Adv. Mater. 2020, 32, 2005942

© 2020 Wiley-VCH GmbH
2005942 (6 of 6)
www.advmat.dewww.advancedsciencenews.com
Conflict of Interest
The authors declare no conflict of interest.
Keywords
all-polymer solar cells, isomeric end groups, polymer acceptors, power
conversion eciency, small-molecule acceptors
Received: August 31, 2020
Revised: September 24, 2020
Published online: October 29, 2020
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