Effect of Core Size on Performance of Fused-Ring Electron Acceptors

Abstract

We report 4 fused-ring electron acceptors (FREAs) with the same end-groups and side-chains but different cores, whose sizes range from 5 to 11 fused rings. The core size has considerable effects on the electronic, optical, charge transport, mor-phological and photovoltaic properties of the FREAs. Extending the core size leads to red-shift of absorption spectra, up-shift of the energy levels, enhancement of molecular packing and electron mobility. From 5 to 9 fused rings, the core size extension can simultaneously enhance open-circuit voltage (VOC), short-circuit current density (JSC) and fill factor (FF) of organic solar cells (OSCs). The best efficiency of the binary-blend devices increases from 5.6% to 11.7%, while the best efficiency of the ternary-blend devices increases from 6.3% to 12.6% as the acceptor core size extends.

Full text

Effect of Core Size on Performance of Fused-Ring Electron Acceptors
Shuixing Dai,
†
Yiqun Xiao,
‡
Peiyao Xue,
†
Jeromy James Rech,
⊥
Kuan Liu,
†
Zeyuan Li,
†
Xinhui Lu,
‡
Wei You,
⊥
and Xiaowei Zhan*
,†
†
Department of Materials Science and Engineering, College of Engineering, Key Laboratory of Polymer Chemistry and Physics of
Ministry of Education, Peking University, Beijing 100871, China
‡
Department of Physics, The Chinese University of Hong Kong, New Territories, Hong Kong, China
⊥
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States
*
SSupporting Information
ABSTRACT: We report 4 fused-ring electron acceptors (FREAs)
with the same end-groups and side-chains but different cores,
whose sizes range from 5 to 11 fused rings. The core size has
considerable effects on the electronic, optical, charge transport,
morphological, and photovoltaic properties of the FREAs.
Extending the core size leads to red-shift of absorption spectra,
upshift of the energy levels, and enhancement of molecular
packing and electron mobility. From 5 to 9 fused rings, the core
size extension can simultaneously enhance open-circuit voltage
(VOC), short-circuit current density (JSC), and fill factor (FF) of
organic solar cells (OSCs). The best efficiency of the binary-blend
devices increases from 5.6 to 11.7%, while the best efficiency of the ternary-blend devices increases from 6.3 to 12.6% as the
acceptor core size extends.
■INTRODUCTION
Organic solar cells (OSCs) have been proved to be a
promising candidate to utilize solar energy,
1−3
and the power
conversion efficiencies (PCEs) have reached over 11% in
fullerene-based OSCs
4,5
and over 13% in nonfullerene-based
OSCs in recent years.
6−9
Fullerene acceptors have been the
predominant choice in the acceptor materials of OSCs for two
decades; however, the limited tunability of electronic proper-
ties and weak absorption of fullerene derivatives in visible
range hinder further development of OSCs. In contrast,
electronic properties of nonfullerene acceptors can be readily
tuned by chemical tailoring, providing an effective avenue
toward higher performance of OSCs.
10−24
In 2015, we reported a new class of nonfullerene acceptors,
fused-ring electron acceptor (FREA). Generally, FREA consists
of an electron-donating fused-ring core and two electron-
withdrawing end-groups.
25
Recently, some high-efficiency
FREAs have been reported,
25−44
among them, different core
sizes, such as fused-4-ring,
26
fused-5-ring,
27−29
fused-6-
ring,
30−32
fused-7-ring,
25,33−40
fused-8-ring,
30,41,42
fused-9-
ring,
43
fused-10-ring,
30,44
and fused-11-ring,
40
have been used
individually, but systematic comparisons of different cores have
rarely been reported.
30
These FREAs based on different cores
usually contain different end-groups and side-chains, and they
are often paired with different donor materials to fabricate
devices with different performance. Under this situation, it is
impossible to rationally compare these FREAs with different
cores or properly understand how the chemical nature of the
core and its size would affect performance of the FREAs.
Here, we designed and synthesized a new fused-11-ring core,
7,7,15,15-tetrakis(4-hexylphenyl)-indacenobis(quadra-thienoa-
cene) (IBQT), which was end-capped with 2-(5,6-difluoro-3-
oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (2FIC)
43
to afford a new FREA, F11IC. We then synthesized another
new FREA, F5IC, based on fused-5-ring core end-capped with
2FIC. These two new FREAs, together with our previously
reported F7IC
40
and F9IC,
43
construct a library of structurally
closely related FREAs. Specifically, F5IC, F7IC, F9IC, and
F11IC (Chart 1) have the same end-groups and side-chains
but different core sizes. With this small library, we are able to
probe the effects of the core size on electronic, optical, charge-
transport, morphological, and photovoltaic properties of the
FREAs. We discover that the increase in the core size leads to
upshift of energy levels, red-shift of absorption spectra,
reduction of bandgaps, and increase in electron mobility of
the acceptors. OSCs based on binary blends of these electron
acceptors and a large-bandgap polymer donor FTAZ (Chart 1)
exhibit PCEs of 5.6 −11.7%; OSCs based on ternary blends of
FTAZ/F5IC/F11IC, FTAZ/F7IC/F11IC, and FTAZ/F9IC/
F11IC exhibit PCEs of 6.3−12.6%.
■RESULTS AND DISCUSSION
Synthesis and Characterization. The synthetic routes for
F5IC and F11IC are shown in Scheme S1, and the synthesis
Received: May 26, 2018
Revised: July 1, 2018
Published: July 2, 2018
Article
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details and characterization data are presented in Supporting
Information. F5IC has good solubility in common organic
solvents such as chloroform and o-dichlorobenzene. F11IC has
low solubility in chloroform (ca. 0.5 mg mL−1) and does not
dissolve in o-dichlorobenzene at room temperature. The
thermal stability was investigated using thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC)
(Figure S1). The decomposition temperatures (Td, 5% weight
loss) of F5IC and F11IC, measured by TGA, are 344 and 400
°C, respectively, indicating good thermal stability. The DSC
Chart 1. Chemical Structures of F5IC, F7IC, F9IC, F11IC, and FTAZ
Figure 1. (a) Absorption spectra in chloroform and (b) as a thin film; (c) cyclic voltammograms and (d) energy levels for F5IC, F7IC, F9IC, and
F11IC.
Table 1. Absorption and Energy Levels of the Acceptors
λmax (nm)
compound solution film ε
a
(M−1cm−1)Egopt
b
(eV) Eox
c
(V) Ered
c
(V) HOMO (eV) LUMO (eV) EgCV
d
(eV) μe(cm2V−1s−1)
F5IC 666 694 1.9 ×1051.64 1.02 −0.75 −5.82 −4.05 1.77 8.1 ×10−5
F7IC 690 730 2.0 ×1051.56 0.94 −0.79 −5.74 −4.01 1.73 1.5 ×10−4
F9IC 710 746 2.5 ×1051.49 0.72 −0.83 −5.52 −3.97 1.55 1.7 ×10−4
F11IC 716 740 2.4 ×1051.47 0.64 −0.86 −5.44 −3.94 1.50 1.4 ×10−3
a
Molar absorptivity at λmax in solution.
b
Estimated from the absorption edge in film.
c
The onset oxidation and reduction potentials vs FeCp2+/0.
d
HOMO/LUMO gap from CV.
Chemistry of Materials Article
DOI: 10.1021/acs.chemmater.8b02222
Chem. Mater. XXXX, XXX, XXX−XXX
B

curves reveal that F5IC, F7IC, and F9IC have phase
transitions, while F11IC shows no phase transition below
300 °C.
The absorption spectra of these four FREAs were carried out
in dilute chloroform solution and thin film, respectively. F5IC,
F7IC, F9IC, and F11IC show gradually red-shifted absorption
spectra with absorption peaks from 666 to 716 nm in solution
(Figure 1a) as the core size increases. The maximum molar
extinction coefficients are 1.9 ×105to 2.5 ×105M−1cm−1
(Table 1). The thin films of all four molecules show red-shifted
and broadened absorption spectra with peaks at 694−746 nm
relative to their solutions (Figure 1b). F11IC shows absorption
at 550−740 nm stronger than that of F9IC due to the larger
conjugation backbone. However, F11IC and F9IC show
similar absorption peak and absorption edge, implying that
further extending the core does not further red-shift the
absorption spectra.
Cyclic voltammetry (CV) was used to measure the
electrochemical properties of the four compounds (Figure
1c). Because each of the four compounds exhibits irreversible
reduction and oxidation waves, the highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) energy levels are estimated from the onset oxidation
and reduction potentials, respectively, assuming the absolute
energy level of FeCp2+/0 to be 4.8 eV below vacuum. The
LUMO level up-shifts from −4.05 to −3.94 eV, while the
HOMO level up-shifts from −5.82 to −5.44 eV (Figure 1d) as
the core size extends from 5 to 11 fused rings.
The electron mobility of the four FREAs, estimated from the
space charge-limited current (SCLC) method, increases from
8.1 ×10−5to 1.4 ×10−3cm2V−1s−1, as the core size extends
from 5 to 11 fused rings (Table 1,Figure S2).
Photovoltaic Properties. We fabricated OSCs with a
structure of indium tin oxide (ITO)/ZnO/PFN-Br/FTAZ:-
FREA/MoOx/Ag. FTAZ is a widely used wide-bandgap
polymer donor with strong absorption at 400−620 nm (Figure
S3) and a high hole mobility of 1.2 ×10−3cm2V−1s−1.
45
The
absorption spectra of the donor and acceptors are comple-
mentary, and their energy levels and mobilities fit well. The
optimized donor/acceptor (D/A) weight ratio is 1:1.5 (w/w)
(Table S1); the optimized content of additive 1,8-diiodooctane
(DIO) is 0.25% (v/v) (Table S2). Figure 2a shows the current
Figure 2. (a) J−Vcharacteristics (a) and EQE spectra (b) of the optimized binary-blend OSCs under illumination of an AM 1.5 G at 100 mW
cm−2, (c) Jph versus Veff characteristics, and (d) JSC versus light intensity of the optimized devices.
Table 2. Performance of the Optimized OSCs
acceptor VOC (V)
a
JSC (mA cm−2)
a
calcd JSC (mA cm−2) FF (%)
a
PCE (%)
a
F5IC 0.703 ±0.012 (0.704) 14.49 ±0.39 (14.88) 14.32 52.0 ±1.7 (53.7) 5.3 ±0.3 (5.6)
F7IC 0.741 ±0.011 (0.742) 18.30 ±0.19 (18.43) 17.60 57.5 ±2.6 (59.7) 7.8 ±0.4 (8.2)
F9IC 0.856 ±0.017 (0.873) 19.72 ±0.52 (20.20) 19.35 67.7 ±1.3 (66.4) 11.4 ±0.3 (11.7)
F11IC
b
95% F5IC + 5% F11IC 0.700 ±0.010 (0.709) 14.98 ±0.39 (15.32) 14.70 57.2 ±1.6 (57.7) 6.0 ±0.3 (6.3)
95%F7IC+5%F11IC 0.744 ±0.012 (0.747) 18.61 ±0.18 (18.77) 18.16 61.4 ±2.8 (63.9) 8.5 ±0.5 (9.0)
95%F9IC+5%F11IC 0.870 ±0.019 (0.883) 20.34 ±0.61 (20.90) 20.30 69.9 ±1.9 (68.0) 12.4 ±0.2 (12.6)
a
Average values with standard deviation were obtained from 20 devices; the values in parentheses are the parameters of the best device.
b
No entries
due to limited solubility.
Chemistry of Materials Article
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Chem. Mater. XXXX, XXX, XXX−XXX
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density−voltage (J−V) curves of the best binary-blend OSCs.
Due to the low solubility of F11IC, no photovoltaic effect was
observed from FTAZ/F11IC-based OSCs.
The open circuit voltage (VOC) of three binary-blend OSCs
increases from 0.704 to 0.873 V as the aromatic core size of the
acceptor extends, which is due to the upshift of LUMO levels
in F5IC, F7IC, and F9IC (Table 2). The short circuit current
density (JSC) increases from 14.88 to 20.20 mA cm−2as the
core size of the acceptor extends, which is related to the red-
shift of absorption spectra. The fill factor (FF) increases from
53.7 to 66.4% as the core size of the acceptor extends, which
benefits from enhancement of electron mobility. Finally, the
best PCE increases from 5.6 to 11.7% as the core size of the
acceptor extends from 5 to 9 fused rings. The photon energy
loss (Eloss)(Eloss =Eg−eVOC)
46
of the F5IC, F7IC, and F9IC-
based OSCs is 0.94, 0.76, and 0.61 eV, respectively; Eloss
decreases as the core size of the acceptor increases. The
external quantum efficiency (EQE) spectra of three OSCs are
broadened gradually in near-infrared wavelength from F5IC to
F9IC (Figure 2b), which is consistent of the neat film
absorption profile of these three acceptors (Figure 1b). The
integrated JSC of the devices calculated from the EQE spectra
with the AM 1.5G reference spectrum are consistent with the
JSC measured from J−V(the error is <5%, Table 2).
Among the four acceptors, F11IC has the highest LUMO
level, which is beneficial for achieving high VOC in OSCs.
Moreover, F11IC has the highest electron mobility, which is
similar to the hole mobility of FTAZ; balanced charge
mobilities of the donor and acceptor are required for high
FF in devices. Although F11IC cannot be used for binary-
blend devices due to its limited solubility, it can be used in
ternary-blend devices as a third component if only a small
amount is required. Thus, we added F11IC into FTAZ/F5IC,
FTAZ/F7IC, and FTAZ/F9IC blends as a third component to
fabricate ternary-blend OSCs (Figure S4a,Table 2). The
optimized weight ratio of F11IC in the acceptor component is
5%. When the F11IC weight ratio is increased to 10%,
insoluble particles can be observed in the blended film, leading
to decrease in device performance (Table S3). The addition of
F11IC increases the VOC of three ternary OSCs due to the
higher LUMO level of F11IC relative to other three acceptors.
The JSC and FF are also enhanced with the addition of 5%
F11IC. Finally, all three ternary blends yield higher PCEs
relative to their binary-blend counterparts. When the F11IC
ratio is 5%, the F9IC-based ternary devices yield the best PCE
of 12.6%, higher than those of F5IC and F7IC-based ternary
devices (6.3 and 9.0%, respectively). The maximum EQE and
integrated JSC values of F5IC, F7IC, and F9IC-based ternary
devices are higher than those of binary devices (Figure S4b,
Table 2).
To investigate the charge generation, dissociation, and
extraction properties, we carried out measurements of
photocurrent density (Jph) versus the effective voltage (Veff)
of OSCs (Figures 2candS4c). Generally, the JSC/Jsat
characterizes the charge extraction under short-circuit
condition when all the photogenerated excitons are dissociated
into free charge carriers and collected by electrodes at high Veff
(>2 V). The calculated JSC/Jsat of F5IC-, F7IC-, and F9IC-
based binary OSCs and F5IC/F11IC, F7IC/F11IC, and F9IC/
F11IC-based ternary OSCs are 87.4, 89.8, 94.2, 88.6, 92.3, and
95.5%, respectively. It appears that the increase in core size
leads to more efficient charge dissociation and collection, and
the addition of F11IC further enhances the charge dissociation
and collection.
Charge recombination is investigated from the JSC versus
light density (P) curves, which describes as JSC ∝Pα.
47
There is
only negligible charge recombination occurred in OSCs if the
αvalue is 1. The αvalues of F5IC-, F7IC-, and F9IC-based
binary OSCs and F5IC/F11IC-, F7IC/F11IC-, and F9IC/
F11IC-based ternary OSCs are 0.900, 0.928, 0.950, 0.953,
0.960, and 0.977, respectively. Apparently, the increase in core
size leads to decrease in bimolecular charge recombination,
and the addition of F11IC further reduces bimolecular charge
recombination (Figures 2d and S4d).
SCLC method was employed to investigate the hole and
electron mobilities of the blended films (Figure S5, Table S4).
For the binary blends, the hole mobilities are similar (3.0 ×
10−4to 2.1 ×10−4cm2V−1s−1), while the electron mobility
increases from 3.4 ×10−5to 1.5 ×10−4cm2V−1s−1as the
acceptor core size extends from 5 to 9 fused rings, and the
Figure 3. (a) 2D GIWAXS patterns. (b) The corresponding GIWAXS intensity profiles along the in-plane (dashed line) and out-of-plane (solid
line) directions. (c) GISAXS intensity profiles and best fittings along the in-plane direction.
Chemistry of Materials Article
DOI: 10.1021/acs.chemmater.8b02222
Chem. Mater. XXXX, XXX, XXX−XXX
D

corresponding hole/electron mobility ratio (μh/μe) decreases
from 8.8 to 1.4. Effects of the acceptor core size on charge
transport in ternary blends are similar to that in the binary
blends. The addition of F11IC leads to faster and more
balanced charge transport relative to the binary blends, which
is beneficial for high FF.
Film Morphology. We used atomic force microscopy
(AFM) to characterize the surface morphology of the four neat
acceptor films, the binary and ternary blends (Figure S6). The
root-mean-square roughness (Rq) of F11IC neat film is larger
than that of other three acceptor neat films; Rqof FTAZ/
F11IC binary blend film is larger than that of other three
acceptor binary blend films, and Rqof the ternary blends
(0.89−1.20 nm) is larger than that of the corresponding binary
blends (0.80−0.99 nm), which is related to the high
crystallinity of F11IC.
Grazing incidence wide-angle X-ray scattering (GIWAXS)
measurements were employed to investigate the film
crystallinity and molecular packing information.
48,49
Figures
3,S7, and S8 present the two-dimensional (2D) GIWAXS
patterns and the corresponding intensity profiles of the binary-
blend, ternary-blend, and pure acceptor films. The pure F5IC
shows no obvious scattering peaks, indicative of its amorphous
nature. The pure F7IC film shows a very weak lamellar peak
along the qraxis (q= 0.330 Å−1,d= 19.0 Å), demonstrating
that it has weak face-on orientation. The pure F9IC film
presents a notably higher crystallinity with a strong and sharp
lamellar peak along the qraxis (q= 0.30 Å−1,d= 20.9 Å) and a
distinctive π−πpeak along the qzaxis (q= 1.77 Å−1,d= 3.55
Å). The pure F11IC film exhibits the highest crystallinity
compared with the other three, however, not much preferential
orientation, as reflected by the appearance of several strong
ring-likes scattering peaks. As the acceptor core size extends
from 5 to 11 fused rings, the molecular packing is stronger,
leading to higher electron mobility. When these acceptors are
mixed with FTAZ, which has shown preferentially face-on
orientation in its blend with other FREAs,
43
quite different
molecular packing behaviors are observed. For the FTAZ/
F5IC film, a strong lamellar peak appears at qz= 0.350 Å−1(d
= 17.9 Å), and a relatively weaker lamellar peak appears at qr=
0.320 Å−1. Because the lamellar peak of FTAZ was reported to
appear at qr= 0.320 Å−1,
43
we attribute the former to the edge-
on oriented F5IC domains and the latter to the face-on
oriented FTAZ domains. Furthermore, a broadened π−πpeak
appears at qz= 1.65 Å−1(d= 3.76 Å), possibly due to the face-
on oriented F5IC domains or the synergistic contribution of
both F5IC and FTAZ because the π−πpeak of FTAZ was
reported to appear at qr= 1.7 Å−1.
43
The scattering pattern of
FTAZ/F7IC film appears to be similar to that of FTAZ/F5IC
film except for a much weaker edge-on lamellar peak (qz=
0.340 Å−1,d= 18.5 Å), and a more distinctive face-on π−π
peak that appeared at relatively larger q(qz= 1.67 Å−1,d= 3.76
Å). This indicates a slight enhancement of face-on ordering of
the acceptor domains, consistent with the increased electron
mobility in this film. Due to the strong face-on ordering of
both F9IC and FTAZ, the FTAZ/F9IC film demonstrates the
highest face-on ordering among the binary films with a lamellar
peak at qr= 0.290 Å−1and a π−πpeak at qz= 1.70 Å−1,
originating from the face-on oriented F9IC domains and FTAZ
domains, respectively. With the addition of the high-crystalline
F11IC, the crystallinity and face-on ordering is further
strengthened for all the ternary devices (Figures S7),
consistent with the obtained higher FF and PCE.
Two-dimensional grazing incidence small-angle X-ray
scattering (GISAXS) patterns and intensity profiles along in-
plane direction of the pure and blended films are presented in
Figures 3c and S9 to estimate the nanoscale phase separation
information. We adopt the Debye−Anderson−Brumberger
(DAB) model, a polydispersed hard sphere model, and a
fractal-like network model to account for the scattering
contribution from intermixing amorphous phases, FTAZ
domains, and acceptor domains, respectively.
49
The fitted
parameters are summarized in Table S5. The correlation
lengths of the intermixing phase are 49, 28, 39, 38, 40, and 21
nm for FTAZ/F5IC, FTAZ/F7IC, FTAZ/F9IC, FTAZ/F5IC/
F11IC, FTAZ/F7IC/F11IC, and FTAZ/F9IC/F11IC, respec-
tively. All the blend films manifest a scattering shoulder at
∼0.07 Å−1, which is attributed to the scattering from pure
FTAZ domains of an averaged size of ∼9 nm. Because the
scattering of the pure F7IC film is very weak, we ignore its
scattering contribution in the FTAZ/F7IC film. Although the
pure F11IC film shows the strongest scattering, it contributes
no distinctive scattering features in the FTAZ/F9IC/F11IC
film. Therefore, we consider F9IC and F11IC indistinguishable
upon GISAXS for the ternary blend film. The sizes of acceptor
domains are fitted to be 73, 23, 50, 77, and 23 nm for FTAZ/
F5IC, FTAZ/F9IC, FTAZ/F5IC/F11IC, FTAZ/F7IC/F11IC,
and FTAZ/F9IC/F11IC blend films, respectively. Compared
to other films, the FTAZ/F9IC/F11IC film demonstrates
relatively smaller intermixing and acceptor domains, which
facilitates the exciton dissociation contributing to a higher JSC
of the devices.
■CONCLUSION
These four FREAs in this study have the same end-groups and
side-chains, but their cores have different sizes ranging from 5
to 11 fused rings. The core size has considerable effects on
their electronic, optical, charge transport, morphological, and
photovoltaic properties. Extending the acceptor core size leads
to red-shift of absorption spectra; F7IC, F9IC, and F11IC films
exhibit ca. 40 nm red-shift of absorption peaks relative to those
of F5IC. However, F11IC shows an absorption profile very
similar to that of F9IC, implying that further extending the
core size with fused-thiophene is not able to further extend the
film absorption. Increasing core size from 5 to 11 fused rings
up-shifts the HOMO level from −5.82 to −5.44 eV and the
LUMO level from −4.05 to −3.94 eV, leading to the bandgap
lowering from 1.64 to 1.47 eV. As the acceptor core size
extends from 5 to 11 fused rings, the molecular packing is
stronger, and the electron mobility increases from 8.1 ×10−5
to 1.4 ×10−3cm2V−1s−1in pure acceptor films. In binary-
blend devices, as the acceptor core size extends from 5 to 9
fused rings, the best PCE increases from 5.6 to 11.7%. This
core engineering approach can simultaneously enhance VOC,
JSC, and FF. VOC increases from 0.704 to 0.873 V due to the
upshift of LUMO level. JSC increases from 14.88 to 20.20 mA
cm−2, which benefits from the red-shift of absorption spectra,
more efficient charge dissociation and collection, and reduced
bimolecular charge recombination. FF increases from 53.7 to
66.4%, which benefits from enhancement of electron mobility,
more balanced charge transport, and reduced bimolecular
charge recombination.
Furthermore, we used F11IC as the third component to
fabricate ternary-blend devices. As the acceptor core size
extends from 5 to 9 fused rings, the best PCE increases from
6.3 to 12.6%. The ternary-blend OSCs yield higher VOC,JSC,
Chemistry of Materials Article
DOI: 10.1021/acs.chemmater.8b02222
Chem. Mater. XXXX, XXX, XXX−XXX
E

FF, and PCE than their binary counterparts. The higher VOC is
attributed to the higher LUMO level of F11IC in comparison
with F5IC, F7IC, and F9IC. The higher JSC is related to more
efficient charge dissociation and collection and reduced
bimolecular charge recombination. The higher FF is due to
higher electron mobility, more balanced charge transport, and
reduced bimolecular charge recombination.
■ASSOCIATED CONTENT
*
SSupporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.chemma-
ter.8b02222.
Materials synthesis and characterization; TGA and DSC
curves, SCLC, AFM images, GIWAXS, and GISAXS
data; device fabrication, optimization, and character-
ization (PDF)
■AUTHOR INFORMATION
Corresponding Author
*E-mail: xwzhan@pku.edu.cn.
ORCID
Xiaowei Zhan: 0000-0002-1006-3342
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
X.Z. thank NSFC (Grants 21734001 and 51761165023). X.L.
thanks the financial support from NSFC/RGC Joint Research
Scheme (Grant N_CUHK418/17), Research Grant Council of
Hong Kong (General Research Fund 14314216 and Theme-
based Research Scheme T23-407/13-N), the beam time and
technical supports provided by 23A SWAXS beamline at
NSRRC, Hsinchuand BL19 beamline at SSRF, Shanghai. J.J.R.
and W.Y. were supported by the National Science Foundation
(Grant CBET-1639429).
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