Regio-Regular Polymer Acceptors Enabled by Determined Fluorination on End Groups for All-Polymer Solar Cells with 15.2 % Efficiency

Article abstract of DOI:10.1002/anie.202016284

Polymerization sites of small molecule acceptors (SMAs) play vital roles in determining device performance of all-polymer solar cells (all-PSCs). Different from our recent work about fluoro- and bromo- co-modified end group of IC-FBr (a mixture of IC-FBr1 and IC-FBr2), in this paper, we synthesized and purified two regiospecific fluoro- and bromo- substituted end groups (IC-FBr-o & IC-FBr-m), which were then employed to construct two regio-regular polymer acceptors named PYF-T-o and PYF-T-m, respectively. In comparison with its isomeric counterparts named PYF-T-m with different conjugated coupling sites, PYF-T-o exhibits stronger and bathochromic absorption to achieve better photon harvesting. Meanwhile, PYF-T-o adopts more ordered inter-chain packing and suitable phase separation after blending with the donor polymer PM6, which resulted in suppressed charge recombination and efficient charge transport. Strikingly, we observed a dramatic performance difference between the two isomeric polymer acceptors PYF-T-o and PYF-T-m. While devices based on PM6:PYF-T-o can yield power conversion efficiency (PCE) of 15.2 %, devices based on PM6:PYF-T-m only show poor efficiencies of 1.4 %. This work demonstrates the success of configuration-unique fluorinated end groups in designing high-performance regular polymer acceptors, which provides guidelines towards developing all-PSCs with better efficiencies.

Full text of DOI:10.1002/anie.202016284

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 10137–10146
International Edition: doi.org/10.1002/anie.202016284
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Solar Cells
Hot Paper
Regio-Regular Polymer Acceptors Enabled by Determined
Fluorination on End Groups for All-Polymer Solar Cells with 15.2%
Efficiency
Han Yu+, Mingao Pan+, Rui Sun+, Indunil Agunawela, Jianquan Zhang,* Yuhao Li, Zhenyu Qi,
Han Han, Xinhui Zou, Wentao Zhou, Shangshang Chen, Joshua Yuk Lin Lai, Siwei Luo,
Zhenghui Luo,* Dahui Zhao, Xinhui Lu, Harald Ade, Fei Huang, Jie Min,* and He Yan*
Abstract: Polymerization sites of small molecule acceptors
(SMAs) play vital roles in determining device performance of
all-polymer solar cells (all-PSCs). Different from our recent
high-performance regular polymer acceptors, which provides
guidelines towards developing all-PSCs with better efficiencies.
work about fluoro- and bromo- co-modified end group of IC- Introduction
FBr (a mixture of IC-FBr1 and IC-FBr2), in this paper, we
synthesized and purified two regiospecific fluoro- and bromo-
substituted end groups (IC-FBr-o & IC-FBr-m), which were
then employed to construct two regio-regular polymer accept-
ors named PYF-T-o and PYF-T-m, respectively. In comparison
with its isomeric counterparts named PYF-T-m with different
conjugated coupling sites, PYF-T-o exhibits stronger and
bathochromic absorption to achieve better photon harvesting.
Meanwhile, PYF-T-o adopts more ordered inter-chain packing
and suitable phase separation after blending with the donor
polymer PM6, which resulted in suppressed charge recombi-
nation and efficient charge transport. Strikingly, we observed
a dramatic performance difference between the two isomeric
polymer acceptors PYF-T-o and PYF-T-m. While devices
based on PM6:PYF-T-o can yield power conversion efficiency
(PCE) of 15.2%, devices based on PM6:PYF-T-m only show
poor efficiencies of 1.4%. This work demonstrates the success
of configuration-unique fluorinated end groups in designing
Solution-processed polymer solar cells (PSCs) have been
developed as a sustainable technology for solar energy
harvesting, which attract significant research attention owing
to their outstanding advantages of mechanical flexibility, low
cost, light weight and facile large-area fabrication.[1–7] Re-
cently, the bulk-heterojunction (BHJ) PSCs based on polymer
donors and small molecule acceptors (SMAs) have pro-
gressed rapidly with impressive power conversion efficiencies
(PCEs) exceeding 17%, due to the in-depth investigations on
efficient photovoltaic materials, device optimization and
interfacial engineering.[8–17] Different from SMA-based PSCs,
all-polymer solar cells (all-PSCs), which comprise conjugated
polymers as both electron donors and acceptors, provide extra
merits of excellent morphological stability, remarkable me-
chanical flexibility and suitability for industrial fabrication
(e.g., roll-to-roll printing).[18–26] However, the major bottle-
neck that limits the development of all-PSCs is currently the
[*] H. Yu,[+] R. Sun,[+] Prof. J. Min
The Institute for Advanced Studies, Wuhan University
Wuhan 430072 (China)
Y. Li, Prof. X. Lu
Department of Physics, Chinese University of Hong Kong
New Territories, Hong Kong 999077 (China)
E-mail: min.jie@whu.edu.cn
H. Han, Prof. D. Zhao
H. Yu,[+] Dr. J. Zhang, Dr. Z. Luo, Prof. H. Yan
Hong Kong University of Science and Technology–Shenzhen Re-
search Institute
No. 9, Yuexing 1st RD, Hi-tech Park, Nanshan, Shenzhen 518057
(China)
Beijing National Laboratory for Molecular Sciences, Centre for Soft
Matter Science and Engineering, Key Lab of Polymer Chemistry &
Physics of the Ministry of Education, College of Chemistry, Peking
University, Beijing 100871 (China)
Prof. F. Huang, Prof. H. Yan
H. Yu,[+] M. Pan,[+] Dr. J. Zhang, Z. Qi, X. Zou, W. Zhou, Dr. S. Chen,
Dr. J. Y. L. Lai, S. Luo, Dr. Z. Luo, Prof. H. Yan
Department of Chemistry, Guangdong-Hong Kong-Macao Joint
Laboratory of Optoelectronic and Magnetic Functional Materials,
Energy Institute and Hong Kong Branch of Chinese National
Engineering Research Center for Tissue Restoration & Reconstruc-
tion, Hong Kong University of Science and Technology
Clear Water Bay, Kowloon, Hong Kong (China)
E-mail: jzhangbn@connect.ust.hk
Institute of Polymer Optoelectronic Materials and Devices, State Key
Laboratory of Luminescent Materials and Devices, South China
University of Technology, Guangzhou 510640 (China)
Prof. J. Min
Key Laboratory of Materials Processing and Mold, Zhengzhou
University, Ministry of Education, 450002 Zhengzhou (China)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under https://doi.org/10.
1002/anie.202016284.
luozheng@ust.hk
hyan@ust.hk
I. Agunawela, Prof. H. Ade
Department of Physics and Organic and Carbon Electronics
Laboratories (ORaCEL), North Carolina State University
Raleigh, NC 27695 (USA)
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lack of high-performance polymer acceptors with superior
photovoltaic properties.
coupling sites are generally asymmetric with the bromine
atoms locating at different positions. Historically, the regu-
larity of polymer chains has been thoroughly investigated
and emphasized in order to endow the materials with better
solid-state packing, synthetic repeatability and less energy
disorder, eventually leading to enhanced device perfor-
mance.[10,13,22,44–47] Considering these issues, the development
of terminal groups with the determined substitution positions
of fluorine and bromine atoms for regiospecific SMA subunits
and thus regular polymer acceptors would provide more
intuitive insights into interchain packing,[48,49] which can
further improve the performance of all-PSCs.
In this study, we report our further endeavors to enhance
the PCE of all-PSCs up to 15.2% by designing regio-regular
and fluorinated polymer acceptors. We successfully synthe-
sized two new isomeric end groups with determined substi-
tution positions of fluorine and bromine, named IC-FBr-o and
IC-FBr-m (i.e., ortho- and meta-positions of the two halogen
atoms). These end groups have different cross-coupling sites,
leading to two configurationally unique comonomers and
regular polymer acceptors (PYF-T-o & PYF-T-m, Figure 1).
The design principle is to decrease the conformational
disorder of the previous PYF-T-based acceptor. It is antici-
pated that reducing the number of configurational isomers of
the polymer can enhance the interchain packing, and thus
charge transport. In this way, we investigated the isomeric
effects on the optoelectronic properties and photovoltaic
performance of the polymer acceptors. PYF-T-o, which
couples with thiophene units at the ortho-positions of fluorine
on the end group moieties, shows a stronger intramolecular
charge transfer (ICT) effect relative to PYF-T-m coupling at
the meta-positions. This is revealed by theoretical calculations
that PYF-T-o shows better conjugation along the polymer
backbone and thus a bathochromic absorption. When blend-
Conventional all-PSCs typically employ the polymer
acceptors based on naphthalene diimide, perylene diimide,
!
bithiophene imide derivatives or B N bridge bipyridine
moieties (Figure S1), but these polymer acceptors suffer from
several inherent drawbacks including narrow absorption
range, deficient absorptivity, low-lying energy levels and
excessive crystallinity, which limit the further improvement of
their PCEs.[27–31] Regarding these issues, Li and co-workers
first proposed an effective approach by adopting SMAs as the
acceptor units to construct high-performance donor-acceptor
(D-A) polymer acceptors,[32] which significantly improves the
PCEs of all-PSCs owing to the higher absorption coefficient,
broader photon response range and better complementary
spectra with donor polymers in multiple cases.[33–35] Subse-
quently, encouraged by the rapid progress of Y-series SMAs,
the efficiencies of all-PSCs consisting of a novel polymer
acceptor, PY-T, have been optimized to over 13%.[36,37] These
results demonstrate that SMA-polymerization is an effective
approach, but most research has concentrated on the
modifications of the D units and the side chains of the SMAs
as the comonomers (Figure S2).[38–41] Fluorination on polymer
chain is an effective method to enhance the photon harvesting
ability and aggregation property. Recently, we reported that
by implementing a polymer acceptor called PYF-T (Figure 1)
with fluorinated end groups on the SMA sub-units, a remark-
ably enhanced PCE of 14.1% can be achieved.[42,43] However,
one of the intrinsic disadvantages of this type of polymer
acceptors lies in the uncertain configurations of the polymer
chains (i.e., random polymers), as the SMA sub-units of these
polymer acceptors are the mixture of several constitutional
isomers. The origin of this phenomenon is that the terminal
groups used to end-cap the SMA sub-units and provide cross-
Figure 1. The chemical structures of PYF-T, IC-FBr, IC-FBr-o, IC-FBr-m, PYF-T-o and PYF-T-m.
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ing with the donor polymer PM6, the PYF-T-o-based devices
as, it is a different story for the dihalogenated terminal groups,
showed both enhanced open-circuit voltage (VOC, 0.901 V)
and fill factor (FF, 72.4%) compared with the PYF-T-based
ones (14.0%), thus a higher PCE of 15.2%, which is among
the best efficiencies for reported all-PSCs (Table S1). Surpris-
ingly, the PM6:PYF-T-m device presented an extremely low
performance of 1.40%, which results from deficient exciton
dissociation, poor charge mobility and severe charge recom-
bination. Morphological studies elucidated that the higher
performance of the PYF-T-o-based devices originated from
the stronger interchain packing and more suitable phase
separation. Based on these findings, we further investigate the
adaptability of the PM6:PYF-T-o in large-scale manufacture
that a decent PCE of 13% was maintained in 1-cm2 device
through blade-coating method. Our work sheds light on the
structure-performance relationships of the fluorine-substitut-
ed Y-series polymer acceptors and highlights the importance
of the design of regular polymer acceptors for efficient all-
PSCs.
because the similar dipole moments between the isomers lead
to comparable solubility in different solvents, hindering the
separation of IC-FBr isomers by the previous method.
Therefore, we intuitively design synthetic routes that can
yield only one regiospecific product of dihalogenated termi-
nal groups. The synthetic routes to IC-FBr-o and IC-FBr-m
are illustrated in Scheme 1.[50,51] By introducing the fluorine
atom adjacent to one of the carbonyl groups of the precursor,
the following condensation reaction will only occur at the
other carbonyl group with less steric hindrance, resulting in
configuration-unique terminal group moieties and the corre-
sponding SMA comonomers (Y-OD-FBr-o and Y-OD-FBr-
m). Notably, the synthetic routes of these two regiospecific
terminal groups are even easier and more efficient compared
to that of the previous IC-FBr containing regio-isomers.
Finally, Y-OD-FBr-o or Y-OD-FBr-m was copolymerized
with 2,5-bis(trimethylstannyl)thiophene, generating two nov-
el Y-series polymer acceptors, PYF-T-o and PYF-T-m, with
different cross-coupling sites between the thiophene units and
the terminal group moieties. Structure characterizations
including 1H NMR, 13C NMR and mass spectra of all the
intermediate compounds are summarized in the Supporting
Information. Both polymer acceptors can dissolve well in
common organic solvents, such as chloroform and chloroben-
zene. High-temperature gel permeation chromatography
measurements (GPC, Table 1) showed that all polymer
acceptors possess comparable molecular weights. The thermal
decomposition temperature (Td) of PYF-T, PYF-T-o and
PYF-T-m are 3378C, 3308C and 3438C, respectively, obtained
Results and Discussion
The key to constructing regular fluorinated polymer
acceptors based on Y-series SMAs is to synthesize the
terminal groups with determined positions of fluorine and
bromine atoms. As we know, terminal groups are strong
nucleophiles that would even condense itself during column
chromatography, so that it is a typical method to purify the
isomeric terminal groups through recrystallization.[47] Where-
Scheme 1. Synthetic routes to IC-FBr-o, IC-FBr-m, PYF-T-o and PYF-T-m.
Table 1: Molecular weight, optical and electrochemical properties of PYF-T, PYF-T-o and PYF-T-m.
[a]
Mn (PDI)
[kDa]
Average
Monomer units
lmax,sol
[nm]
lmax,film
[nm]
amax,film
[cmÀ1
lonset,film
[nm]
Eg
LUMO/HOMO[b]
[eV]
]
[eV]
PYF-T
PYF-T-o
PYF-T-m
11.0 (2.22)
11.1 (1.79)
10.5 (2.18)
6.0
6.0
5.7
798
800
758
821
824
805
1.40”105
1.45”105
1.36”105
900
896
880
1.38
1.38
1.41
À3.80/À5.71
À3.81/À5.73
À3.77/À5.73
[a] calculated from the absorption onset of the films. [b] estimated from the reduction/oxidation onset of the CV curves.
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from thermogravimetric analysis (TGA, at 5% weight loss,
Figure S3), which indicates that all these fluorinated polymer
acceptors have comparable thermal stability for all-PSC
fabrication.
while the 4th-position-coupling thiophene can conjugate with
both malononitrile and central backbone, forming larger
conjugation and resulting in bathochromic absorption (Fig-
ure S5f). Apart from absorption properties, the energy levels
of the three polymer acceptors were then measured by cyclic
voltammetry (CV, Figure 2e) in solid state using ferrocene/
ferrocenium (À4.80 eV) as the external standard. As illus-
trated in Figure 2 f, PYF-T-o demonstrated comparable
HOMO/LUMO levels (À5.73/À3.81 eV) to PYF-T (À5.71/
À3.80 eV), while the LUMO level of PYF-T-m was slightly
upshifted to À3.77 eV. The wider electrochemical band gap of
PYF-T-m is consistent with its optical band gap, which may be
due to the weaker conjugation along the polymer chains.[53,54]
Therefore, the cross-coupling sites of polymer acceptors
indeed have great impacts on the conjugation and thus
optoelectronic properties.
Conventional devices with the structure of ITO/PE-
DOT:PSS/PM6:polymer acceptor/PNDIT-F3N/Ag were then
fabricated to investigate the photovoltaic performances of all-
PSCs based on the three fluorinated polymer acceptors.
Figure 3a displays the current density versus voltage (J–V)
curves of the PM6:PYF-T, PM6:PYF-T-o and PM6:PYF-T-m
devices, and Table 2 summarizes the photovoltaic parameters
of the three optimized all-PSCs. The PYF-T-based devices as
the reference yielded a decent PCE of 14.0% with a VOC of
0.891 V, a short-circuit current density (JSC) of 23.1 mAcmÀ2
and a FF of 68.0%, which is comparable to the previous
report.[42,43] The PYF-T-o-based all-PSCs produced a similar
VOC of 0.901 V, while the PYF-T-m-based ones generated
a higher VOC of 0.949 V that can be attributed to its higher-
lying LUMO level. Besides the enhancement in VOC, the
PYF-T-o-based devices delivered a simultaneously improved
JSC of 23.3 mAcmÀ2 and a FF of 72.4%, leading to a superior
PCE of 15.2% relative to the PYF-T-based ones. It is notable
that the JSC and PCE of the PYF-T-o-based devices are among
the highest values for the reported all-PSCs (Figure 3b,c).
Despite the similar chemical compositions, PYF-T-m behaved
The UV/Vis absorption spectra of PYF-T, PYF-T-o and
PYF-T-m were obtained in both dilute solutions and thin-film
states. In chloroform solution, PYF-T and PYF-T-o show
comparable maximum absorption peaks (lmax,sol) centered at
798 and 800 nm, respectively, while PYF-T-m exhibits an
obviously hypochromic absorption peak centered at 758 nm
(Figure 2a). In thin-film state, all fluorinated polymer accept-
ors exhibit bathochromic absorption relative to their solution
states, showing the maximum absorption peaks (lmax,film) at
821 nm, 824 nm and 805 nm for PYF-T, PYF-T-o and PYF-T-
m, respectively. The corresponding optical band gaps (Eg) of
PYF-T, PYF-T-o and PYF-T-m in film state are 1.38, 1.38 and
1.41 eV, determined from their absorption onsets of 900, 896
and 880 nm, respectively (Figure 2b). Besides, the absorption
coefficients are measured to be 1.40 ” 105, 1.45 ” 105 and 1.36 ”
105 cmÀ1 for the PYF-T, PYF-T-o and PYF-T-m neat films,
respectively. Consequently, all the polymer acceptors can
form complementary absorptions when blended with PM6,
covering the solar spectrum in the range of 300–900 nm
(Figure S4). To reveal the relationship between absorption
and conjugation, density-functional theory (DFT) calcula-
tions were carried out. As shown in Figure 2c, the hypochro-
mic absorption of PYF-T-m is owing to the weaker conjuga-
tion between the thiophene comonomer and the end group
moieties (Figure S5) and a smaller dipole moment (Fig-
ure 2d) that leads to the weakened ICT effect.[52] Besides, the
rising absorption peaks of PYF-T and PYF-T-o in the region
of 450–550 nm can be attributed to the stronger transition
dipole moment (e.g., HOMO!LUMO + 3), which is also an
indication of better orbital overlapping, namely better back-
bone conjugation. Such a phenomenon is not obvious for
PYF-T-m (Figure S6), because the 5th-position-coupling thio-
phene can only conjugate with ketone and central backbone,
Figure 2. a) Normalized UV/Vis absorption spectra of PYF-T, PYF-T-o and PYF-T-m in dilute chloroform solution (concentration: 1.0”10À5 M).
b) UV/Vis absorption spectra of PYF-T, PYF-T-o and PYF-T-m in thin films. c) Simulated absorption spectra of each repeating unit (PYF-T-1 and
PYF-T-2 are two possible repeating units of PYF-T with different coupling sites) and d) dipole moments of half backbone of each repeating unit
calculated at the B3LYP/6-31G (d, p) level. TT-IC-FT-1 and TT-IC-FT-2 refer to PYF-T, TT-IC-FT-o refers to PYF-T-o and TT-IC-FT-m refers to PYF-T-
m. e) Cyclic Voltammetry curves of PYF-T, PYF-T-o and PYF-T-m. f) Architecture and energy alignment of the bulk heterojunction devices.
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Figure 3. a) J–V characteristic curves, b, c) the plot of PCE against VOC and PCE against JSC for all-PSCs reported previously with PCEs over 8%
and in this study. d) EQE spectra. e) TR-PL decay spectra of the PYF-T, PYF-T-o, PYF-T-m neat films, and the PM6:PYF-T, PM6:PYF-T-o and
PM6:PYF-T-m blend films. The pump wavelength and probe wavelengths are 750 nm and 850 nm, respectively. f) TPC and g) TPV measurements
of the PM6:PYF-T, PM6:PYF-T-o and PM6:PYF-T-m all-PSCs.
Table 2: Photovoltaic parameters of the solar cell devices based on
PM6:PYF-T, PM6:PYF-T-o and PM6:PYF-T-m with the conventional
To study the differences in charge dissociation process of
the three all-PSCs, time-resolved photoluminescence (TRPL)
technique was employed and the decay dynamics of both neat
and blend films are shown in Figure 3e.[55] The photolumi-
nescence decay dynamics of the three neat polymer acceptors
are comparable with the lifetime of ꢀ 0.36 ns. For the blend
films, both the PM6:PYF-T and PM6:PYF-T-o blend films
exhibited very fast charge dissociation (lifetime ꢀ 0.09 ns).
However, it is obvious that the PM6:PYF-T-m blend pre-
sented almost identical photoluminescence decay as the neat
PYF-T-m, suggesting the very low extent of charge dissoci-
ation (lifetime ꢀ 0.32 ns). This is further supported by the
measurements of exciton dissociation probability P(E, T) that
the PYF-T-m-based device showed a P(E, T) of only 74.2%
(Figure S7 and Table S3). In contrast, the PYF-T- and PYF-T-
o-based devices presented efficient charge dissociation with
P(E, T) of 97.5% and 98.3% respectively, thus partially
explaining the dramatically enhanced JSC in PYF-T and PYF-
T-o-based all-PSCs with stronger conjugated polymer accept-
ors.[56]
Apart from the variations in JSC, it is also intriguing to
reveal the reasons for the enormous differences in FF of the
all-PSCs based on these fluorinated polymer acceptors from
the perspective of charge extraction and recombination.
Hence, transient photocurrent (TPC) and transient photo-
voltage (TPV) techniques were conducted for the three all-
PSCs.[57] First, we performed the TPC measurements under
short circuit condition to evaluate the charge extraction
capability of the three all-PSCs. As depicted in Figure 3 f, the
photocurrent decay times of PM6:PYF-T, PM6:PYF-T-o and
PM6:PYF-T-m are 0.39, 0.34 and 0.58 ms, respectively. There-
fore, the PYF-T-o-based device exhibits a superior charge
architecture under the illumination of 100 mWcmÀ2
.
Material
combinations
VOC
[V]
JSC
FF
[%]
PCE (PCEavg
[%]
)
[mAcmÀ2
]
PM6:PYF-T
PM6:PYF-T-o
PM6:PYF-T-m
0.891
0.901
0.949
23.1
23.3
4.60
68.0
72.4
32.7
14.0 (13.7Æ0.2)
15.2 (14.9Æ0.2)
1.40 (1.20Æ0.2)
Average values from 15 devices with the highest values shown in
parentheses.
differently in photovoltaic devices in comparisons with PYF-
T and PYT-F-o, yielding a much lower JSC of 4.60 mAcmÀ2,
a drastically dropped FF of 32.7%, and thus an inferior PCE
of only 1.4%. To unveil the differences in JSC among the three
all-PSCs, the external quantum efficiency (EQE) spectra
were recorded accordingly. As presented in Figure 3d, the
PYF-T- and PYF-T-o-based devices display almost identical
EQE edges and maximum EQE response approaching 80%.
The PYF-T-o-based device exhibits slightly higher EQE
values in the range of 400–500 and 650–820 nm, contributing
to the enhanced JSC. Whereas, the photon response of the
PYF-T-m-based device is much weaker and narrower with the
EQE values of only 10–15%. As a result of the huge
differences in photon response, the integrated JSC calculated
from the EQE spectra are 22.2, 22.6 and 4.40 mAcmÀ2 for the
PYF-T, PYF-T-o and PYF-T-m based all-PSCs, respectively,
which match well with the values obtained from the J-V
curves. The PYF-T-o with medium molecular weight exhibits
better performance (Table S2), presenting a similar trend as
the previous reported PYF-T based all-PSC.[42]
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extraction capability, leading to a faster charge sweep-out.
Furthermore, the charge carrier lifetimes were estimated by
the TPV technique under open circuit condition. The
extracted carrier lifetimes of PM6:PYF-T, PM6:PYF-T-o
and PM6:PYF-T-m are 1.96, 4.58 and 1.48 ms, respectively
(Figure 3g). The longest carrier lifetime indicates the slowest
recombination rate of the PM6:PYF-T-o device. In contrast,
PYF-T-m based device suffers from severe charge recombi-
nation as well as poor charge extraction. These results are
further evidenced by the light-intensity-dependent JSC/VOC
experiments of the three all-PSCs, where the PM6:PYF-T-o
device expressed both effectively suppressed bimolecular and
trap-assisted charge recombination (Figure S8) that guaran-
tee a high FF.[58,59]
out-of-plane directions. The (010) peaks of the pristine PYF-
T, PYF-T-o and PYF-T-m films appeared at qz = 1.65, 1.64 and
1.67 ŠÀ1, demonstrating the p-p stacking distances (dp-p) of
3.82, 3.84 and 3.77 Š, respectively. However, PYF-T-m shows
a smaller (010) coherence length of 17 Š than those of PYF-T
(19 Š) and PYF-T-o (19 Š), indicative of a slightly poor inter-
chain packing of PYF-T-m. Besides, Figure 4b desplays that
À1
the higher scattering peaks of PYF-T and PYF-T-o at 0.3 Š
À1
and 0.6 Š in the in-plane (001) direction represent the
backbone peak (001) and (002), which suggest better polymer
packing in the horizontal direction of the substrate and
explain the better charge transport properties of these two
materials. After blending with PM6, all three blends exhibit
predominant face-on orientation with the p-p stacking peaks
À1
Besides charge recombination, we characterized the hole
(mh) and electron (me) mobility of the three all-PSCs by the
space-charge-limit current (SCLC, Figure S9) method. In this
study, the hole- and electron-only devices were fabricated
with the structures of ITO/PEDOT:PSS/active layer/MoO3/
Al and ITO/ZnO/active layer/PNDIT-F3N/Ag, respective-
ly.[60] As tabulated in Table S4, the measured mh and me of
PM6:PYF-T-o are 8.4 ” 10À4 and 7.8 ” 10À4 cm2 VÀ1 sÀ1 respec-
tively, which are higher than those of PM6:PYF-T (mh = 7.3 ”
10À4 cm2 VÀ1 sÀ1 and me = 6.5 ” 10À4 cm2 VÀ1 sÀ1). However,
PM6:PYF-T-m exhibits a much poorer charge mobilities
(mh = 1.2 ” 10À4 cm2 VÀ1 sÀ1, me = 2.7 ” 10À4 cm2 VÀ1 sÀ1). In ad-
dition, the more balance mh/me value of PM6:PYF-T-o is also
beneficial for charge extraction and collection, thereby the
better FF of the devices. Overall, these results illuminate the
reduced charge recombination and excellent charge mobility
in the PM6:PYF-T-o blend, which may be attributed to the
better conjugation along the polymer backbone and the more
ordered interchain-packing that facilitate three-dimensional
charge transport in the BHJ.
located at 1.68 ŠÀ1,1.70 ŠÀ1, 1.70 Š with the coherence
lengths of 19, 26 and 26 Š for PM6:PYF-T, PM6:PYF-T-o and
PM6:PYF-T-m, respectively. The almost identical p-p stack-
ing distances may be due to the fact that the blend film
morphology is mostly dominated by PM6 (qz = 1.72 ŠÀ1, dp-
p = 3.65 Š, Table S5). Nevertheless, the scattering volume
normalized integrated intensity of PM6:PYF-T-m is indeed
lower than the other two blends, which could explicate the
inferior carrier mobility of the device. In this way, it can be
concluded that the ordered fluorinated backbone with better
conjugation leads to more compact interchain packing and
thus the enhanced charge transport of the devices.
Moreover, we performed atomic force microscopy (AFM,
Figure 5a) to characterize the surface morphology of the
three blends. The height images show that all the blends form
compatible surface morphology with suitable root-mean-
square roughness (1.01 nm for PM6:PYF-T, 0.891 nm for
PM6:PYF-T-o and 1.05 nm for PM6:PYF-T-m, respectively).
In addition, from the phase images, it is clearly observed that
the PM6:PYF-T-o and PM6:PYF-T-m blends exhibit obvi-
ously fiber-like surface morphology, which is not observed
from the PM6:PYF-Tone. This could originate from the more
ordered polymer chain packing induced by the regular
backbone of PYF-T-o and PYF-T-m. To provide more in-
depth understanding about the huge differences among the
three blends, grazing-incidence small-angle X-ray scattering
Grazing incidence wide-angle X-ray scattering (GI-
WAXS) experiments were conducted to investigate the
morphology features in the pristine and blend films.[61] The
two-dimensional GIWAXS patterns are shown in Figure 4a,
and Figure 4b,c display the corresponding one-dimensional
sector-averaged (Æ 108) scattering profiles in the in-plane and
Figure 4. a) 2D GIWAXS patterns of pure PYF-T, PYF-T-o and PYF-T-m films and PM6:PYF-T, PM6:PYF-T-o and PM6:PYF-T-m blend films. The
corresponding 1D GIWAXS line-cuts of the in-plane and out-of-plane directions of b) neat films and c) blend films.
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lead to oversized domains in blend. Therefore, the balance
between interchain packing and phase separation through
ingenious chemical method is the key to achieving the high
device performance of PM6:PYF-T-o.
The energy loss (Eloss) plays a crucial role in determining
the efficiency limit of a solar cell, and the detailed energy
losses were investigated for the three all-PSCs subsequently
(Figure 6 and Table S6).[64] Theoretically, Eloss for any types of
solar cells contains three parts. The first part DE1 (EgÀqVoSQc
)
Figure 5. a) The AFM height (top) and phase (bottom) images, and
b) the GISAXS intensity profiles and best fittings along the in-plane
direction of the PM6:PYF-T, the PM6:PYF-T-o and the PM6:PYF-T-m
blend film.
from the radiative recombination loss above Eg exists in all
types of solar cells according to the detailed balance theory,
and the three all-PSCs yielded almost identical DE1 of
SQ
rad
oc
0.26 eV.[65] The second part DE2 (qV ÀqV ) from the
oc
radiative recombination loss depends on the absorption
below Eg, and both PYF-T- and PYF-T-o-based devices show
smaller DE2s of 0.03 and 0.04 eV, respectively, than that of
PYF-T-m (0.06 eV). As for the third part DE3 (the non-
radiative recombination loss) is attained through the equa-
tion: DE3 = ÀkTln(EQEEL). A small energy offset between
donor and acceptor can effectively suppress the charge-
transfer (CT) state recombination through the hybridization
of the local excited (LE) state and the CT state, resulting in
reduced non-radiative recombination loss.[66] Due to the
smallest offset between PM6 and PYF-T-m, PM6:PYF-T-m
was found to exhibit a smaller DE3 of 0.20 eV relative to those
of the other two devices (0.24 eV for PM6:PYF-Tand 0.23 eV
for PM6:PYF-T-o). According to these results, the total Eloss
were determined to be 0.53, 0.52 and 0.52 eV for PM6:PYF-T,
PM6:PYF-T-o and PM6:PYF-T-m, respectively. The slightly
smaller Elosss of the two devices based on regular polymer
acceptors may be partially attributed to the decreased
conformational disorder of polymer chains, leading to regular
packing, less vibration states and reduced non-radiative
recombination.
(GISAXS) experiments were conducted to study the phase
segregation of these three material systems (Figure 5b).[62]
The intermixing domain spacings are determined to be 42.9,
33.9 and 63.5 nm for the PYF-T, PYF-T-o, and PYF-T-m-
based blend, respectively. Furthermore, the domain sizes of
the acceptor phase are calculated to be 18.7, 13.9, 24.2 nm,
respectively. This is consistent with the trend in the exciton
dissociation of the corresponding devices. The excessively
large domain size of the PM6:PYF-T-m blend deviates from
the ideal exciton diffusion length of 10–20 nm,[63] while those
of the other two blends lie in this range. Therefore, the
PM6:PYF-T-m blend suffers from the oversized domain that
limits the area of the D-A interface, which could be the major
reason for the inferior exciton dissociation, poor carrier
transportation and severe charge recombination of the
devices as demonstrated above. In contrast, the smallest
domain size of the PM6:PYF-T-o blend is more desirable for
the exciton dissociation and thus beneficial for the following
charge transport and suppressed charge recombination. We
proposed that the weaker conjugated coupling site of PYF-T-
m owns less steric hindrance in polymer chain which could
Figure 6. Semi-logarithmic plots of normalized EL, measured EQE and EQE calculated by FTPS (EQEFTPS) as a function of energy for devices
based on a) PM6:PYF-T, b) PM6:PYF-T-o and c) PM6:PYF-T-m. The ratio of fEL/fbb was used to plot the EQE in the low-energy regime (black
line), where fEL and fbb represent the emitted photon flux and the room-temperature blackbody photon flux, respectively. Of note is that fEL/fbb
follows experimental EQEFTPS faithfully at higher energies as expected from reciprocity. d) Schematic diagram for energy losses of PSCs according
to the detailed balance theory. e) Eloss and its detailed three parts of DE1, DE2 and DE3 values of the three all-PSCs.
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As discussed in the introduction, all-PSCs are expected to
exhibit robust morphology that is promising for large-scale
manufacturing. Hence, we fabricated the 1.0 cm2 devices of
PM6:PYF-T-o by spin-coating as well as blade-coating (Fig-
ure 7a) to further investigate the potential of PM6:PYF-T-o
in scalable processing methods. Figure 7b depicted that the
spin-coated device with an active area of 1.0 cm2 shows
a decreased PCE of 14.5% relative to the small-area device
above (0.07 cm2). It is noted that this value is higher than the
previous reports (12–13%) with similar active areas.[67]
Surprisingly, when blade-coated under ambient condition,
the resulting devices showed very similar VOC and JSC to the
spin-coated ones while a reasonably dropped FF of 63.4%,
leading to a decent PCE of 13.0% for large-area all-PSCs.
Therefore, these results highlight that the scalable processing
of PM6:PYF-T-o have great potential in the large-scale
production of highly efficient all-PSCs.
the resultant regular polymer acceptors (PYF-T-o & PYF-T-
m) were prepared and compared with a reported random
polymer (PYF-T). Theoretical calculations indicated the
better conjugation and the stronger ICT effect of PYF-T-o
relative to the other two polymers. When blended with PM6,
morphology studies revealed that the PM6:PYF-T-o blend
formed ordered interchain packing and suitable fiber-like
phase separation relative to the PM6:PYF-T one, while the
PM6:PYF-T-m blend suffers from oversized phase segrega-
tion that leads to inferior exciton dissociation, slow charge
transport and severe charge recombination. Consequently,
the PM6:PYF-T-o all-PSC achieved the highest PCE of
15.2%, with a VOC of 0.901 V and a remarkably enhanced JSC
of 23.3 mAcmÀ2, which was superior to PM6:PYF-T (14.0%)
and PM6:PYF-T-m (1.40%), and this is one of the highest
efficiencies for all-PSCs. Our work demonstrates the struc-
ture-performance relationships of the fluorine-substituted Y-
series polymer acceptors and paves the way towards regular
fluorinated polymer acceptors for efficient all-PSCs.
Acknowledgements
The work described in this paper was partially supported by
the National Key Research and Development Program of
China (No. 2019YFA0705900) funded by MOST, the Basic
and Applied Basic Research Major Program of Guangdong
Province (No. 2019B030302007), Guangdong-Hong Kong-
Macao Joint Laboratory of Optoelectronic and Magnetic
Functional Materials (project number 2019B121205002), the
Shen Zhen Technology and Innovation Commission (project
number JCYJ20170413173814007, JCYJ20170818113905024),
the Hong Kong Research Grants Council (Research Impact
Fund R6021-18, collaborative research fund C6023-19G,
project numbers 16309218, 16310019, and 16303917), Hong
Kong Innovation and Technology Commission for the support
through projects ITC-CNERC14SC01 and ITS/471/18), Na-
tional Natural Science Foundation of China (NSFC, No.
91433202). NCSU gratefully acknowledges the support of
ONR grant N000141712204. X-ray data were acquired at
beamlines 11.0.1.2 and 7.3.3 at the Advanced Light Source,
which is supported by the Director, Office of Science, Office
of Basic Energy Sciences, of the U.S. Department of Energy
under Contract No. DE-AC02-05CH11231.
Figure 7. Schematics of the spin-coating and blade-coating processes,
and the photograph of the 1-cm2 device. d) The J–V curves of the spin-
coated and blade-coated devices with a device area of a 1.0 cm2.
Steady performance under long-term photo-irradiation
should be ensured since exposure to light is inevitable during
the device operation. As shown in Figure S10a, the
PM6:PYF-T-o and PM6:PYF-T devices showed light-induced
losses of ca. 20% and 25% after 250 hours irradiation,
respectively, while the performance of PYF-T-m-based devi-
ces dropped by more than 30% over the same time period.
The thermal stability of the devices was also tested (Fig-
ure S10b), where the PYF-T-o-based device demonstrated
robust performance with ꢀ 10% decrease after 250-hour
thermal aging at 808C. In contrast, the PYF-T- and PYF-T-m-
based devices displayed decreased thermal stability, retaining
80% and 65% of their initial efficiencies under the same
condition. Both stability experiments clearly demonstrate
that the stability of the PYF-T-o-based devices is better than
the other two systems, probably because of the denser
intermolecular packing rising from fluorine-assisted interac-
tions and stronger conjugation that stabilize the chemical
structure and blend morphology.
Conflict of interest
The authors declare no conflict of interest.
Keywords: all-polymer solar cells ·fluorinated end group ·
isomeric effect ·organic solar cells ·polymer acceptors
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Manuscript received: December 8, 2020
Accepted manuscript online: January 27, 2021
Version of record online: March 17, 2021
10146 www.angewandte.org
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Angew. Chem. Int. Ed. 2021, 60, 10137 –10146