Intrinsically chemo- And thermostable electron acceptors for efficient organic solar cells

Full text of DOI:10.1246/BCSJ.20200231

Advance Publication Cover Page
Intrinsically chemo- and thermostable electron acceptors for efficient organic solar cells
Qian-Qian Zhang, Yaokai Li, Di Wang, Zeng Chen, Yuhao Li, Shuixing Li, Haiming Zhu,
Xinhui Lu, Hongzheng Chen, and Chang-Zhi Li*
Advance Publication on the web September 29, 2020
doi:10.1246/bcsj.20200231
©
2020 The Chemical Society of Japan
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Intrinsically chemo- and thermostable electron acceptors for efficient
organic solar cells
Dedicated to Professor Eiichi Nakamura on the occasion of his 70th birthday
1
1
1
2
3
1
2
Qian-Qian Zhang, Yaokai Li, Di Wang, Zeng Chen, Yuhao Li, Shuixing Li, Haiming Zhu,
3
1
1,*
Xinhui Lu, Hongzheng Chen, Chang-Zhi Li
1
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, State Key Laboratory of Silicon
Materials, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China.
2
Center for Chemistry of High-Performance & Novel Materials, Department of Chemistry, Zhejiang University,
Hangzhou, 310027, China.
3
Department of Physics, The Chinese University of Hong Kong, New Territories, Hong Kong 999077, P.R. China.
Email: czli@zju.edu.cn
Abstract:
The traditional excess of non-fullerene acceptors
NFAs) via Knoevenagel condensation reaction (KCR)
Besides, the condensed products from KCR
exhibit the reversible reactivity under acidic or
alkaline aqueous conditions, in which the vinyl group
decompose back to aldehyde and methylene
(
of aldehyde and active methylene leaves the
vulnerable and reversible exocyclic vinyl bonds in
structures, which undermine the intrinsic chemo- and
photostability of NFAs. In this work, we demonstrate
the new access of acceptor-donor-acceptor (A-D-A)
NFAs via Stille coupling between new electron
deficient group and classic donor core in over 90%
yield, wherein the robust carbon-carbon bonds,
replacing the exocyclic double bonds from traditional
KCR, resulting in the stable A-D-A acceptors, Q1-XF
3
3-35
species.
Researchers have also demonstrated that
two exocyclic vinyl groups of neighboring molecules
can yield a four-membered ring intermediate via [2+2]
3
6
cycloaddition mechanism. The mixture of ITIC-4F
and ITIC-0F can be observed in the unsymmetrical
ITIC-2F:PBDB-TF blend solution after storing in
glove box for 16 hours, due to the exchange and
redistribution of end group via cycloaddition of the
3
7
vinyl bond.
Thus, considering the long-term
(
X
representing 0,
2
and
4
fluorine atoms,
stabilities of OSCs, it is desirable to come up new
design for NFAs by avoiding the vulnerable exocyclic
vinyl groups stemming from the reversible KCRs,
meanwhile remaining the advantages of A-D-A
strategy in tuning optoelectronic properties of NFAs.
Herein, we have developed new access of A-D-A
acceptors via Stille coupling reaction (SCR), wherein
a new kind of electron deficient group, namely,
respectively). Among the three studied examples,
Q1-4F exhibits the improved optoelectronic and
electron transport properties, leading to the best
photovoltaic performance with optimal charge
kinetics for Q1-4F based OSCs. Overall, this strategy
can direct a new way for developing the stable
photovoltaic materials.
indeno([2,1-b]thiophen-8-ylidene)malononitrile
(A
1
. Introduction
unit, IT), is applied to couple with the typical electron
donating unit IDT (D unit). Therefore, Q1-XF (X
representing 0, 2, or 4 fluorine atoms) can be feasibly
obtained in over 90% yields. The robust
carbon-carbon bond linkages in the new A-D-A
acceptors, replacing exocyclic double bonds from
KCR, lead to the intrinsically chemo- and
thermostable NFAs. The photostability and
thermostability of Q1-XF films are much superior to
the classic IT-4F made from KCR. Furthermore, by
increasing the number of fluorine substituents at IT
unit, the optoelectronic properties and electron
transport of acceptors can be improved from Q1-0F,
Q1-2F to Q1-4F, resulting in the best photovoltaic
performance for Q1-4F:PBDT-T based OSCs.
With the development of non-fullerene acceptors
NFAs), organic solar cells (OSCs) have rapidly
reached over 17 % power conversion efficiency (PCE)
in recent years. Though promising for the tunable
(
1
-5
optoelectronic properties of acceptor-donor-acceptor
6
-13
(
A-D-A) NFAs,
synthetic strategy exclusively
condensation of aldehyde and active methylene
through Knoevenagel condensation reaction
leaving a vulnerable exocyclic vinyl group
between the donor core and terminal acceptor group in
it is worth noting that their
employs the
1
4-19
(KCR),
2
0-24
A-D-A NFAs.
These structural factors potentially
undermine the intrinsic chemo- and photostability of
2
5-26
NFAs,
wherein oxygen and water can attack the
vulnerable exocyclic vinyl groups between D and A
units, hence accelerating the degradation of
photoexcited acceptors. Meanwhile, the practice of
OSCs will not be achieved without the desirable
long-term photochemical stability of active
Femtosecond
transient
absorption
(fs-TA)
measurements reveal that Q1-4F exhibits the
improved charge transfer and mitigated recombination
in blends, hence, enhancing photovoltaic performance.
Overall, this work provides a new design strategy that
allows accessing stable photovoltaic materials.
2
7-32
components,
which are critical for determining the
overall photo-to-charge conversion and stability of
devices.

Scheme 1. a) The synthetic procedures for A-D-A NFAs from traditional methods and this work; b) synthesis of
Q1-XF. c) Single crystal structure and stacking model of IT-1F. d) Side-views of molecular geometry, e) isosurface
of electrostatic potential, f) LUMO and g) HOMO of Q1-4F based on the DFT calculation. Reagents and
conditions: (i) 3-thienyl boronic acid, Na
then aqueous HCl; (iii) oxalyl chloride in DMF/CH
ºC; (v) Malononitrile and CH COONa in EtOH at reflux; (vi) IDT-SnMe
2
CO
3
and Pd(PPh
Cl , and then with AlCl
3 4
, Pd(PPh )
3
)
4
in THF/H
2
O at 70 ºC; (ii) NaOH in EtOH/H
in CH Cl ; (iv) Br /CHCl , Na
in toluene at 110 ºC.
2
O, and
CO , 0
2
2
3
2
2
2
3
2
3
3
3
2
. Results and Discussion
(IT0F, X
1
2 1 2
= X = H; IT1F, X = H and X = F; IT2F
2
.1 Synthesis and characterization
X
1
= X
2
= F). Finally, we employed Stille coupling to
Note that the traditional method for accessing
A-D-A NFAs extensively employs the KCR of
aldehyde and active methylene, which yields the
reactive and reversible exocyclic vinyl bonds to
conjugate the terminal groups to the donor core. In
this work, we develop effective and new access for
A-D-A NFAs, wherein the robust carbon-carbon
bonds via SCR, replacing the exocyclic double bonds,
are employed to construct A-D-A NFAs, leading to the
intrinsically chemo- and thermostable acceptors
obtain Q1-XF between IT and IDT with excellent
yields of ~ 90%. The characterization of the chemical
1
13
19
structure included H, C and F nuclear magnetic
resonance (NMR) and matrix-assisted laser
desorption/ionization-time of flight (MALDI-TOF)
Mass Spectroscopy (Figure S1). Single crystals of
IT-1F reveal the planar structure with F-substituent at
meta-position (Scheme 1c). A - distance of 3.35 Å
is measured for the “brick-like” stacking of IT units,
which would be helpful for the intermolecular charge
transportation.
(
Scheme 1a). The synthetic route of Q1-XF is
described in Scheme 1b and the detailed information
can be found in the Supporting Information (SI).
Methyl benzoate derivatives 1 was employed, via
Suzuki coupling with 3-thienyl boronic acid, to
synthesize ester 2, then hydrolyzed to benzoic acid 3.
Through the intramolecular Friedel-Crafts cyclization
of compound 3, the ketone 4 is feasibly obtained. And
followed by bromination of the thiophene ring, the
compound 5 is synthesized and then condensed with
malononitrile to yield electron deficient IT units 6
Density functional theory (DFT) calculation was
employed to study the molecular geometries and
frontier orbitals (Scheme 1d-g and Figure S2). Planar
molecular backbones are observed for Q1-0F, Q1-2F
and Q1-4F, with the dihedral angle of 2.33º, 1.27º, and
4.39º between IT and IDT in side-view of molecular
geometries, respectively. In addition, the electrostatic
potential (ESP) of NFAs indicates the terminal groups
with strong electronegative atoms like fluorine atoms
and cyano groups have positive ESP while the

conjugated thiophene surfaces have negative ESP.
These results are consistent with the frontier orbital
results, showing the spatial separation of HOMO and
LUMO orbitals, with the down-shifted energies in the
sequence of Q1-0F, Q1-2F, and Q1-4F. These results
suggest that the new A-D-A acceptors with C-C bonds
linkages remain planar backbones and effective
conjugation among aromatic units.
Figure 1. UV-vis absorption spectra and the inserted photo images (left and right: before and after illumination) of
a) Q1-4F and b) IT-4F thin films; c) The change of absorption intensity of Q1-XF and IT-4F under different aging
time; d) The TGA curves of Q1-XF and IT-4F.
Table 1. The basic photophysical and electrochemical data of Q1-XF.
sol
max
film
max
film
opt
Q1-XF
λ
λ
λ
onset

sol
E
g
E
HOMO/LUMO
E
g
4
-1
-1
(eV) [a]
(eV) [b]
(eV) [b]
(
nm)
(nm)
727
757
766
(nm)
817
868
888
( 10 M cm )
Q1-0F
Q1-2F
Q1-4F
699
716
716
5.90
6.05
7.42
1.52
-5.40/-3.78
-5.43/-3.90
-5.54/-3.99
1.62
1.43
1.53
1.40
1.55
[
a]
Optical bandgaps are from film UV-vis absorption spectra. [b] Evaluated by electrochemical CV measurements.
The ultraviolet-visible (UV-vis) absorption
the energy levels of the three acceptors. The lowest
unoccupied molecular orbital (LUMO) and highest
occupied molecular orbital (HOMO) were determined
to be -3.78/-5.40 eV, -3.90/-5.43 eV, and -3.98/-5.54
eV for Q1-0F, Q1-2F, and Q1-4F, respectively. This
experimental evidence indicates that the new
acceptors connected with single C-C bonds possess
the effective ICT between A and D units, and the
spectra of the three acceptors in chloroform and thin
film are displayed in Figure S3, and the
corresponding data are summarized in Table 1. All of
them possess two distinct peaks, and the small one
locating at 350-550 nm may stem from the -*
transition of the end-group. Another intense peak
locating at 550-900 nm represents the intramolecular
charge transfer (ICT) between the electron deficient
IT and electron rich IDT units. The absorption onsets
fluorination of
A units leads to the tunable
optoelectronic properties of acceptors.
(
λ
onset) of Q1-2F and Q1-4F are 868 nm and 878 nm,
To check the stabilities of new A-D-A acceptors,
a classic acceptor IT-4F with exocyclic vinyl linkage
was chosen as control molecule. Photostability of neat
films for Q1-XF, IT-4F and their blend films with
PBDB-T polymer were tested under one sun
equivalent illumination in ambient. Note that a metal
halide lamp without UV filtration is employed as a
opt
leading to narrow optical bandgaps (E
g
) of 1.43 eV
and 1.41 eV, respectively. With the increased number
of fluorine atoms, the redshift of main absorption
peaks of acceptors is observed in the film samples due
to the enhanced molecular aggregation. Cyclic
voltammetry measurements were used to determine

light source and its light density is monitored with a
silicon cell. The absorption profiles of these samples
have been recorded upon the extension of illumination
times. The results are depicted in Figures 1a-1c and
S4-6. For neat films illuminated in ambient, the
absorption profile of IT-4F decreased sharply under
this harsh condition. After 10 hours, its film faded to
nearly transparent, indicating the destroying of
conjugation system of IT-4F. Interestingly, little
change of absorption ability can be observed for
Q1-XF acceptors under the same condition, indicating
that Q1-XF is generally more robust than IT-4F under
the same photo-aging test. Considering the major
difference of molecules is at the linkage between
electron deficient unit and IDT, the significant
improvement of photostability for Q1-XF, can be
ascribed to the merits of single C-C linkages, instead
of the exocyclic C=C bonds of IT-4F. In addition, the
introduction of fluorine substituents can enhance the
photostability of Q1-XF. After illumination for 22
hours, the maximum absorption peak of Q1-2F and
Q1-4F remained unchanged, while that of Q1-0F
decreased to 70 % of its original absorption intensity.
Moreover, thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) were applied
to detect the thermal properties of Q1-XF and IT-4F.
According to Figure 1d, the decomposition
o
temperature (Td) (5% weight loss) of IT-4F is 350 C,
while for Q1-XF, the Td are generally higher than 400
o
C, indicating Q1-XF exhibiting better thermal
stability than IT-4F. The improvement of
thermostability for Q1-XF can also be ascribed to the
robust single C-C linkages, compared with the
reversible exocyclic C=C bonds of ID-4F.
Figure 2. a) Schematic illustration of OSC device structure; b) Energy level diagram of Q1-XF and PBDB-T; c) J–
V curves and d) EQE spectra, e) Jsc and (f) Voc versus light intensity characteristics of the corresponding OSCs.
2
.2 Photovoltaic properties
According to the optoelectronic data of Q1-XF,

we selected a wide-bandgap polymer PBDB-T to
match with these acceptors, in a conventional device
structure of ITO/PEDOT:PSS/active layer/PFN/Ag.
The schematic device structure and energy level are
shown in Figure 2a and 2b. Their photovoltaic
performance and device parameters are summarized in
Table 2 and Figure 2c. Q1-0F based devices exhibit
relatively low PCEs of 2.75 %, with an open-circuit
voltage (Voc) of 0.89 V, a short-circuit current (Jsc) of
illumination for 250 hours, the PCE of IT-4F based
cells decreased to 30% of its original efficiency while
Q1-4F based cells still maintained over 60% of its
original efficiency (SI). Thus, Q1-XF with robust
carbon-carbon bond linkage also show better device
stability than that of IT-4F.
To understand the cause of varied photovoltaic
performance of these acceptors, we investigated the
recombination of Q1-XF based solar cells, and the
-2
7
.20 mA cm and a fill factor (FF) of 0.44. And
function of J-V properties versus light density (Plight)
devices based on Q1-2F acceptor achieve a PCE of
is shown in Figure 2e and 2f. By fitting the curve
with power-law equation Jsc  (Plight) , we found that
-2

6
.05 %, a Voc of 0.76 V, a Jsc of 12.41 mA cm and an
FF of 0.64. When increasing the number of fluorine
atoms from 2 to 4, Q1-4F based devices reach the best
PCE of 7.93 % among three new acceptors, wherein
the slightly decreased Voc from 0.76 V to 0.72 V can
be well compensated with the largely increased FF
and Jsc values. These results indicate the relatively
fine electron accepting characteristics for these robust
Q1-XF molecules. In addition, the introduction of
fluorine atom into these acceptors helps effectively
tuning the photovoltaic parameters and stability of
acceptors. The external quantum efficiency (EQE)
spectra of PBDB-T:Q1-XF based OSCs are illustrated
in Figure 2d. A good photoelectrical response can be
observed in the brand range of 350-860 nm, which can
be ascribed to the complementary absorptions of
PBDB-T and Q1-XF. The calculated Jsc were 7.95,
the value of α were 0.906, 0.976, and 0.982 for
PBDB-T:Q1-0F, PBDB-T:Q1-2F, and PBDB-T:Q1-4F
blends correspondingly. The closer to 1 for the value
of α represents the lower probability of bimolecular
recombination, which demonstrates the bimolecular
recombination of PBDB-T:Q1-2F and PBDB-T:Q1-4F
is less than that of PBDB-T:Q1-0F. On the other hand,
V
oc is subject to Plight with a slope (n) of kT/q (k is the
Boltzmann constant, T is the temperature, and q is the
elementary charge). As shown in Figure 2f, slopes of
1.78, 1.23, and 1.19 were obtained for the
PBDB-T:Q1-0F, PBDB-T:Q1-2F, and PBDB-T:Q1-4F
devices, respectively, indicating the monomolecular or
trap-assisted recombination is severe in Q1-0F based
devices. It could be due to the non-optimal charge
generation and transportation in the Q1-0F based
blend. Whereas, the slopes of 1.23 and 1.19 for
PBDB-T:Q1-2F and PBDB-T:Q1-4F represent the
mitigated recombination in these devices, comparing
to that of PBDB-T:Q1-0F. These results suggest the
fluorination of Q1-XF allow mitigating the charge
recombination in the respective OSCs, hence
exhibiting the improved photovoltaic performances.
-2
1
0.75, and 14.97 mA cm for Q1-0F, Q1-2F, and
Q1-4F respectively, exhibiting good consistency with
the experimental values from the J-V curves. The
photostability of OSCs based on IT-4F and Q1-4F
have been compared under continuous
1 sun
equivalent illumination provided by a metal halide
lamp without UV-filtration. Upon the continous
Figure 3. a) 2D color plot of fs-TA spectra of PBDB-T:Q1-4F blend film. b) TA kinetics of hole transfer process

-2
for PBDB-T:Q1-4F at indicated delay time under 725 nm excitation with a fluence below 7.25 μJ cm . c)
Representative TA spectra at selected delay times. Grayline: TA spectra of the neat Q1-4F film excited at 725 nm at
1
ps. d) Normalized TA kinetics of Q1-XF GSB in the blend films.
Table 2. Photovoltaic parameters of OSCs under illumination of AM 1.5 G at 100 mW cm-2
PCE[a]
τ
1
[b]
τ
2
[b]
Active layer
V
oc
J
sc
FF
-2
(
V)
(mA cm )
7.20
(%)
(ps)
0.12
0.14
0.12
(ps)
1.45
4.37
6.01
PBDB-T:Q1-0F
PBDB-T:Q1-2F
PBDB-T:Q1-4F
0.89
0.76
0.73
0.44
0.64
0.69
2.75 (2.55 ± 0.16)
6.05 (5.83 ± 0.12)
7.93 (7.82 ± 0.10)
12.41
15.63
[
[
a]
Average PCEs with the standard deviations in parentheses from 20 devices.
b]
τvalues are from the hole transfer process of respective blends under fs-TA measurements.
2
.3 Charge transportation, separation and
recombination.
The charge transport properties of these blends
Figure 3c, the decay of Q1-4F matches well with the
rising of PBDB-T GSB in blend film, confirming the
hole transfer process from the photoexcited Q1-4F to
PBDB-T. Here, the kinetics of ~608 nm was chosen to
represent GSB kinetics of donor because the
photoexcited QI-4F shows no signal there (Figure 3b).
The rising kinetics of PBDB-T GSB in the blend
reflect the hole transfer process, while the decay on a
ps-ns time scale corresponding to the charge
were studied by space charge limited current method
and the results are shown in Figure S7 and Table 2.
Hole- and electron-only devices are fabricated with
the structure of ITO/PEDOT:PSS/blend/MoO
ITO/ZnO/blend film/PFN-Br/Al, respectively. The
hole mobilities ( ) of Q1-0F, Q1-2F, Q1-4F were
.04×10 , 1.41×10 , 1.43×10 cm V s , while the
electron mobilities (
.57×10 , 3.63×10 cm V s . These results suggest
the fluorinated Q1-XF indeed help to improve the
3
/Ag and
h
-4
-4
-4
2
-1 -1
38
2
recombination process.
-5
e
2
) of them were 1.36×10 ,
Hole transfer processes were also observed in
other blends from TA spectra (Figure S8&9). The
hole transfer kinetics of blend films (see Figure 3d)
can be fitted by a biexponential function: i =
-5
-5
-1 -1
2
electron transport properties of acceptors. Hence, the
PBDB-T:Q1-4F showed a 
smaller than that of PBDB-T:Q1-2F (5.49) and
PBDB-T:Q1-2F (14.99). As results, due to the
h
/
e
ratio of 3.94, which is
A
1
ꢀexp(−t/τ
lifetimes of τ
previous studies of domain size characteristics on the
1
) + A
2
ꢀexp(−t/τ
2
), with fast and slow
1
and τ
2
and prefactors of A
1
and A ꢀ. As
2
1
9, 39-40
relatively
balanced
charge
transportation,
organic BHJ blends,
consists of an ultrafast hole-transfer process at the
interface, as characterized by the fast lifetime τ , as
the hole transfer process
PBDB-T:Q1-4F blend achieved the improved FF and
Jsc values, as well as PCEs, over other two blends.
1
Femtosecond transient absorption (fs-TA)
well as the diffusion of excitons towards interfaces
before dissociation, as characterized by the slow
measurement was employed to further investigate hole
transfer and recombination of blend films. The results
are shown in Figure 3 and Figure S8&9. Since the
primary steady-state absorption peak for different
acceptors and donor are well separated in spectral
domain, both spectral and kinetics characteristics of
hole transfer process can be extracted. A 725 nm
4
1-42
lifetime τ
in Table S1. All the kinetics exhibit similar the fast
rise account for ~ 40% with characteristic times (τ
for PBDB-T:Q1-0F, PBDB-T:Q1-2F, and
1
.
All of the fitting parameters are listed
1
)
PBDB-T:Q1-4F of 0.119, 0.141, and 0.121 ps,
respectively, indicating high hole transfer rate at the
polymer donor and Q1-XF acceptor interface.
However, the slow rising dynamics are quite different
-2
pump pulse (below 7.5 μJ cm ) was used to
selectively pump acceptors without exciting donor in
blends. And we, after a delay time, probe the relative
transmittance change (ΔT/T) using a white light
continuum. Figure 3a-b show the 2D color plot of TA
spectra of PBDB-T:Q1-4F blend film and a few
representative TA spectra at selected delay times, and
the neat Q1-4F TA spectrum (black dashed line) for
comparison. The bleach peaks at 700-800 nm appear
in both neat Q1-4F film and blend film, corresponding
to the combined contributions from stimulated
emission (SE) and ground state bleach (GSB) of
Q1-4F after photoexcitation. With the decreasing of
Q1-4F bleach peak in the first 100 ps, two bleach
peaks (at ~ 595 nm and ~ 645 nm), corresponding to
the of PBDB-T GSB signal, emerge in the TA
spectrum of PBDB-T:Q1-4F blend. As plotted in
2
for the three blends. The characteristic times (τ ) are
1.446, 4.371, and 6.014 ps for PBDB-T:Q1-0F,
PBDB-T:Q1-2F, and PBDB-T:Q1-4F, respectively. By
comparison, the longest hole transfer time in
PBDB-T:Q1-4F is probably related to the extension of
the photoexcited exciton diffusion length, which is
consistent with the observed fibrous aggregates in
blends (discussed hereafter). More importantly, in
comparison with the charge recombination rate of
PBDB-T:Q1-0F, the decay dynamics in the
PBDB-T:Q1-2F and PBDB-T:Q1-4F are slower,
suggesting the slow hole transfer kinetics are
favorable for suppressing charge recombination.
These trends well agree with the improved Jsc and FF
values in representative OSCs.

Figure 4. AFM height (a, b and c) and phase (d, e and f) images of the PBDB-T:Q1-0F film (a and d),
PBDB-T:Q1-2F film (b and e), PBDB-T:Q1-4F film (c and f).
2
.4 Morphology characterization.
3. Conclusion
Atom force microscopy (AFM) was conducted
to examine the surface morphology of neat and blend
films. The root-mean-square (RMS) surface roughness
In summary, we have demonstrated herein the
new access of A-D-A non-fullerene acceptors via
Stille coupling reaction in excellent yield, wherein the
robust carbon-carbon bonds, replacing the exocyclic
double bonds from traditional Knoevenagel
condensation, are employed to conjugate electron
deficient end group IT to IDT, leading to the
intrinsically chemo- and thermostable acceptors. New
acceptors Q1-XF (X representing 0, 2, and 4 fluorine
atoms, respectively) showed better thermal stabilities
of
PBDB-T:Q1-0F,
PBDB-T:Q1-2F,
and
PBDB-T:Q1-4F were 1.16 nm, 0.916 nm, and 1.40 nm
respectively. As displayed in Figure 4, relatively fine
fibril-structures are observed in Q1-4F based blend
films. Whereas, Q1-0F and Q1-2F based blend films
exhibit large features. The characteristic results
indicate that, compared with Q1-0F and Q1-2F based
blend films, Q1-4F based blend film possesses more
continuous carrier transport channels, resulting in
better optoelectronic properties. Grazing-incidence
wide-angle X-ray scattering (GIWAXS) has been
performed to probe the molecular stacking and
orientation of neat and blend films. The
two-dimensional (2D) scattering patterns and intensity
profiles in the out-of-plane (OOP) and in-plane (IP)
directions are shown in Figure S10&11. All three
acceptors show face-on orientation with a - peak in
the OOP direction and lamellar peaks in the IP
direction. The lamellar peak and - peak of Q1-4F
o
upon to decomposition temperature of 400 C, which
o
is over 50 C higher than that of classic A-D-A
acceptor IT-4F. The photostability of Q1-XF films is
also significantly improved over that of IT-4F. While
IT-4F sharply decreased to transparent, little change
can be found in Q1-4F films under one-sun
illumination in ambient. As results, among the three
studied examples, Q1-4F exhibits the improved
optoelectronic and electron transport properties,
leading to the best photovoltaic performance with
optimal charge kinetics for Q1-4F based OSCs.
Overall, we believe this design strategy can direct a
new way for the development of stable photovoltaic
materials.
-
1
neat film locate at q
r
= 0.330 Å (d =19.0 Å) in IP
direction, and q
z
=1.79 Å-1 (d = 3.51 Å) in OOP
direction, respectively. In binary films, all the three
blends exhibit lamellar peaks in IP direction and π-π
stacking signal in OOP direction. Among them, the
PBDB-T:Q1-4F blends display the closest π-π
Conflicts of interest
There are no conflicts to declare.
-1
stacking of q
direction, with a lamellar peak at q
9.6 Å) in IP direction, which would be benifitial for
charge transport in such blend based devices.
z
= 1.79 Å (d = 3.51 Å) in OOP
Acknowledgements
-1
r
= 0.320 Å (d =
This research was funded by the National Key
Research and Development Program of China (No.
2019YFA0705900), National Natural Science
1

Foundation of China (Nos. 21722404, 21674093,
1721005 and 21734008). C.-Z. Li thanks the support
of the Zhejiang Natural Science Fund for
Distinguished Young Scholars (LR17E030001).
Xiao, L. Ding, Y. Zou, X. Zhan, C. Yang, Z. Li,
C. J. Brabec, Y. Li, J. Min, J. Mater. Chem. A
2019, 7, 25088.
6
21. S. Park, H. J. Son, J. Mater. Chem. A 2019, 7,
2
5830.
Dedicated to Professor Eiichi Nakamura on the occasion of his 70th
birthday.
2
2. J. Hou, W. Ma, P. Bi, C. An, B. Gao, K. Xian, B.
Lin, Y. Tang, T. Zhang, L. Hong, H. Yao, Y.
Cui, Natl. Sci. Rev. 2020, 7, 1239.
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