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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.
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Eect 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 dierent cores,
whose sizes range from 5 to 11 fused rings. The core size has
considerable eects 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 ll factor (FF) of
organic solar cells (OSCs). The best eciency of the binary-blend
devices increases from 5.6 to 11.7%, while the best eciency 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,
13
and the power
conversion eciencies (PCEs) have reached over 11% in
fullerene-based OSCs
4,5
and over 13% in nonfullerene-based
OSCs in recent years.
69
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 eective avenue
toward higher performance of OSCs.
1024
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-eciency
FREAs have been reported,
2544
among them, dierent core
sizes, such as fused-4-ring,
26
fused-5-ring,
2729
fused-6-
ring,
3032
fused-7-ring,
25,3340
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 dierent cores have
rarely been reported.
30
These FREAs based on dierent cores
usually contain dierent end-groups and side-chains, and they
are often paired with dierent donor materials to fabricate
devices with dierent performance. Under this situation, it is
impossible to rationally compare these FREAs with dierent
cores or properly understand how the chemical nature of the
core and its size would aect 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-diuoro-3-
oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (2FIC)
43
to aord 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. Specically, F5IC, F7IC, F9IC, and
F11IC (Chart 1) have the same end-groups and side-chains
but dierent core sizes. With this small library, we are able to
probe the eects 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 6.312.6%.
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 mL1) and does not
dissolve in o-dichlorobenzene at room temperature. The
thermal stability was investigated using thermogravimetric
analysis (TGA) and dierential 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 lm; (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 lm ε
a
(M1cm1)Egopt
b
(eV) Eox
c
(V) Ered
c
(V) HOMO (eV) LUMO (eV) EgCV
d
(eV) μe(cm2V1s1)
F5IC 666 694 1.9 ×1051.64 1.02 0.75 5.82 4.05 1.77 8.1 ×105
F7IC 690 730 2.0 ×1051.56 0.94 0.79 5.74 4.01 1.73 1.5 ×104
F9IC 710 746 2.5 ×1051.49 0.72 0.83 5.52 3.97 1.55 1.7 ×104
F11IC 716 740 2.4 ×1051.47 0.64 0.86 5.44 3.94 1.50 1.4 ×103
a
Molar absorptivity at λmax in solution.
b
Estimated from the absorption edge in lm.
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, XXXXXX
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 lm, 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 coecients are 1.9 ×105to 2.5 ×105M1cm1
(Table 1). The thin lms of all four molecules show red-shifted
and broadened absorption spectra with peaks at 694746 nm
relative to their solutions (Figure 1b). F11IC shows absorption
at 550740 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 ×105to 1.4 ×103cm2V1s1, 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 400620 nm (Figure
S3) and a high hole mobility of 1.2 ×103cm2V1s1.
45
The
absorption spectra of the donor and acceptors are comple-
mentary, and their energy levels and mobilities t 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) JVcharacteristics (a) and EQE spectra (b) of the optimized binary-blend OSCs under illumination of an AM 1.5 G at 100 mW
cm2, (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 cm2)
a
calcd JSC (mA cm2) 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
DOI: 10.1021/acs.chemmater.8b02222
Chem. Mater. XXXX, XXX, XXXXXX
C
densityvoltage (JV) curves of the best binary-blend OSCs.
Due to the low solubility of F11IC, no photovoltaic eect 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 cm2as the
core size of the acceptor extends, which is related to the red-
shift of absorption spectra. The ll factor (FF) increases from
53.7 to 66.4% as the core size of the acceptor extends, which
benets 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 =EgeVOC)
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 eciency (EQE) spectra of three OSCs are
broadened gradually in near-infrared wavelength from F5IC to
F9IC (Figure 2b), which is consistent of the neat lm
absorption prole 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 JV(the error is <5%, Table 2).
Among the four acceptors, F11IC has the highest LUMO
level, which is benecial 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 lm, 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 eective 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 ecient 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 lms (Figure S5, Table S4).
For the binary blends, the hole mobilities are similar (3.0 ×
104to 2.1 ×104cm2V1s1), while the electron mobility
increases from 3.4 ×105to 1.5 ×104cm2V1s1as the
acceptor core size extends from 5 to 9 fused rings, and the
Figure 3. (a) 2D GIWAXS patterns. (b) The corresponding GIWAXS intensity proles along the in-plane (dashed line) and out-of-plane (solid
line) directions. (c) GISAXS intensity proles and best ttings along the in-plane direction.
Chemistry of Materials Article
DOI: 10.1021/acs.chemmater.8b02222
Chem. Mater. XXXX, XXX, XXXXXX
D
corresponding hole/electron mobility ratio (μh/μe) decreases
from 8.8 to 1.4. Eects 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 benecial for high FF.
Film Morphology. We used atomic force microscopy
(AFM) to characterize the surface morphology of the four neat
acceptor lms, the binary and ternary blends (Figure S6). The
root-mean-square roughness (Rq) of F11IC neat lm is larger
than that of other three acceptor neat lms; Rqof FTAZ/
F11IC binary blend lm is larger than that of other three
acceptor binary blend lms, and Rqof the ternary blends
(0.891.20 nm) is larger than that of the corresponding binary
blends (0.800.99 nm), which is related to the high
crystallinity of F11IC.
Grazing incidence wide-angle X-ray scattering (GIWAXS)
measurements were employed to investigate the lm
crystallinity and molecular packing information.
48,49
Figures
3,S7, and S8 present the two-dimensional (2D) GIWAXS
patterns and the corresponding intensity proles of the binary-
blend, ternary-blend, and pure acceptor lms. The pure F5IC
shows no obvious scattering peaks, indicative of its amorphous
nature. The pure F7IC lm shows a very weak lamellar peak
along the qraxis (q= 0.330 Å1,d= 19.0 Å), demonstrating
that it has weak face-on orientation. The pure F9IC lm
presents a notably higher crystallinity with a strong and sharp
lamellar peak along the qraxis (q= 0.30 Å1,d= 20.9 Å) and a
distinctive ππpeak along the qzaxis (q= 1.77 Å1,d= 3.55
Å). The pure F11IC lm exhibits the highest crystallinity
compared with the other three, however, not much preferential
orientation, as reected 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 dierent
molecular packing behaviors are observed. For the FTAZ/
F5IC lm, a strong lamellar peak appears at qz= 0.350 Å1(d
= 17.9 Å), and a relatively weaker lamellar peak appears at qr=
0.320 Å1. Because the lamellar peak of FTAZ was reported to
appear at qr= 0.320 Å1,
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 ππpeak
appears at qz= 1.65 Å1(d= 3.76 Å), possibly due to the face-
on oriented F5IC domains or the synergistic contribution of
both F5IC and FTAZ because the ππpeak of FTAZ was
reported to appear at qr= 1.7 Å1.
43
The scattering pattern of
FTAZ/F7IC lm appears to be similar to that of FTAZ/F5IC
lm except for a much weaker edge-on lamellar peak (qz=
0.340 Å1,d= 18.5 Å), and a more distinctive face-on ππ
peak that appeared at relatively larger q(qz= 1.67 Å1,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 lm. Due to the strong face-on ordering of
both F9IC and FTAZ, the FTAZ/F9IC lm demonstrates the
highest face-on ordering among the binary lms with a lamellar
peak at qr= 0.290 Å1and a ππpeak at qz= 1.70 Å1,
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 proles along in-
plane direction of the pure and blended lms are presented in
Figures 3c and S9 to estimate the nanoscale phase separation
information. We adopt the DebyeAndersonBrumberger
(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 tted
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 lms manifest a scattering shoulder at
0.07 Å1, which is attributed to the scattering from pure
FTAZ domains of an averaged size of 9 nm. Because the
scattering of the pure F7IC lm is very weak, we ignore its
scattering contribution in the FTAZ/F7IC lm. Although the
pure F11IC lm shows the strongest scattering, it contributes
no distinctive scattering features in the FTAZ/F9IC/F11IC
lm. Therefore, we consider F9IC and F11IC indistinguishable
upon GISAXS for the ternary blend lm. The sizes of acceptor
domains are tted 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 lms, respectively. Compared
to other lms, the FTAZ/F9IC/F11IC lm 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 dierent sizes ranging from 5
to 11 fused rings. The core size has considerable eects 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 lms
exhibit ca. 40 nm red-shift of absorption peaks relative to those
of F5IC. However, F11IC shows an absorption prole very
similar to that of F9IC, implying that further extending the
core size with fused-thiophene is not able to further extend the
lm 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 ×105
to 1.4 ×103cm2V1s1in pure acceptor lms. 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
cm2, which benets from the red-shift of absorption spectra,
more ecient charge dissociation and collection, and reduced
bimolecular charge recombination. FF increases from 53.7 to
66.4%, which benets 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, XXXXXX
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
ecient 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 nancial interest.
ACKNOWLEDGMENTS
X.Z. thank NSFC (Grants 21734001 and 51761165023). X.L.
thanks the nancial 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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... 33,34 Despite the fact that fullerene acceptors have been the most popular materials for two decades, the restricted electrical characteristics and moderate absorbing capacity of fullerene derivatives in the visible region has influenced the evolution of organic photovoltaic devices. 35 In the past decade, non-fullerene acceptors (NFAs) have received a great deal of attention, primarily for research into the improvement of organic solar cells. To a large extent, in this area they have taken the spotlight away from fullerene derivatives. ...
Article
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Organic compounds having significant nonlinear optical (NLO) applications are being employed in the optoelectronics field. In the current work, a series of non-fullerene acceptor (NFA) based compounds are designed by modifying the acceptors with different substituents using DTS(FBTTh 2 ) 2 R1 as a reference compound. To study the NLO responses to the tuning of various acceptors, DFT and TD-DFT based parameters were calculated at the M06 level along with the 6-31G(d,p) basis set. The designed compounds (MSTD2-MSTD7) showed smaller values of the energy gap in comparison to the reference compound. The energy gaps of the title compounds were linked to global reactivity insights; MSTD7 provided a lower band gap, with smaller and larger quantities for hardness and softness characteristics, respectively. Further, UV-vis analyses were performed for all of the designed compounds, displaying wavelengths red-shifted from that of DTS(FBTTh 2 ) 2 R1 . The intraelectron transfer (ICT) process and stability of the title compounds were explored via frontier molecular orbital (FMO) and natural bond orbital (NBO) studies, respectively. Out of all the designed compounds, the highest value of linear polarizability ⟨α⟩ of 3.485 × 10-22 esu, first hyperpolarizability (βtotal) of 13.44 × 10-27 esu and second-order hyperpolarizability ⟨γ⟩ of 3.66 × 10-31 esu were exhibited by MSTD7. In short, all of the designed compounds exhibited promising NLO properties because of their low charge transport resistance. These NLO properties may be useful for experimental researchers to uncover NLO materials for modern applications.
... ITIC, a landmark NFA developed by Zhan, et al., that takes multiple conjugated heteroaromatic rings as the central electron-donating core, 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (INCN) as the terminal electron-accepting unit, drew great attention to the design of the current state-of-the-art acceptor-donor-acceptor (A-D-A)-type acceptors. Design strategies such as conjugation expansion, 10,23 heteroatom incorporation, 20,[24][25][26][27][28][29] terminal fluorination, 30,31 and sidechain engineering 16,32,33 have effectively contributed to the performance promotion, but also led to the increased synthesis complexity and brought a great challenge to the large-scale preparation of NFA materials. Taking Y6 as an example, its synthesis consists of 16 steps when starting from commercially available materials, resulting in a low total yield of less than 5%. ...
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Despite the development of nonfullerene acceptors (NFAs) that have made a breakthrough in the photovoltaic performance, large-scale preparation of NFAs that is prerequisite for commercial application has never been explored. Herein, we designed two dodecacyclic all-fused-ring electron acceptors, F11 and F13, and develop a whole set of synthetic procedures, achieving unprecedented scalable preparation of NFAs in the lab at a 10-g scale notably within one day. The single-crystal structures of F11 reveals the 3D network packing. F11 and F13 display the lowest costs among reported NFAs, even comparable with the classical donor material, P3HT. By matching a medium-bandgap polymer donor, F13 delivers power conversion efficiencies of over 13%, which is an efficiency record for non-INCN acceptors. Benefiting from the intrinsically high stability, OSCs based on F11 and F13 show device stability superior to the typical ITIC- and Y6-based OSCs as evidenced by the tiny burn-in losses. The current work presents a first example for large-scale preparation of low-cost NFAs with good efficiency and high device stability, which is significant for OSC commercialization in near future.
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Fused-ring electron acceptors (FREAs) have a donor–acceptor–donor structure comprising an electron-donating fused-ring core, electron-accepting end groups, π-bridges and side chains. FREAs possess beneficial features, such as feasibility to tailor their structures, high property tunability, strong visible and near-infrared light absorption and excellent n-type semiconducting characteristics. FREAs have initiated a revolution to the field of organic solar cells in recent years. FREA-based organic solar cells have achieved unprecedented efficiencies, over 20%, which breaks the theoretical efficiency limit of traditional fullerene acceptors (~13%), and boast potential operational lifetimes approaching 10 years. Based on the original studies of FREAs, a variety of new structures, mechanisms and applications have flourished. In this Review, we introduce the fundamental principles of FREAs, including their structures and inherent electronic and physical properties. Next, we discuss the way in which the properties of FREAs can be modulated through variations to the electronic structure or molecular packing. We then present the current applications and consider the future areas that may benefit from developments in FREAs. Finally, we conclude with the position of FREA chemistry, reflecting on the challenges and opportunities that may arise in the future of this burgeoning field. Fused-ring electron acceptors are excellent n-type organic semiconductors with outstanding optoelectronic conversion and electron transport abilities. This Review highlights the fundamental principles, design strategies and versatile applications of fused-ring electron acceptors in photovoltaics, electronics and photonics. As part of the Springer Nature Content Sharing Initiative, a view-only version of this paper can be accessed and publicly shared through the following SharedIt link: https://rdcu.be/cSNO4
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Two polycyclic non-fullerene acceptors BTMe-OEH-2F and BTMe-OEH-2Cl were constructed by connecting the fused ring core with 4-(2-ethylhexyloxy) phenyl side chain to two kind of end groups 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (2F-IC) and 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (2Cl-IC), respectively. Compared with BTMe-OEH-2F, BTMe-OEH-2Cl showed a more red-shifted absorption and lower energy level. When the two non-fullerene acceptors were blended with the donor PM6, a fiber distribution and continuous nanophase separation can be observed in the PM6:BTMe-OEH-2Cl blend film, and more balanced charge transport and less charge recombination were exhibited in the organic solar cell (OSC) device based on BTMe-OEH-2Cl, which achieved a higher PCE of 13.23% with a Voc of 0.98 V, a Jsc of 18.76 mA cm⁻² and a FF of 72.23%. These results proved that the synergistic effect between side chains and end groups on the properties of fused-ring electron acceptors (FREAs) can be exerted by choosing appropriate end groups to match with alkoxy side chains.
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Designing electron acceptors with fully non-fused chemical structures is an effective method to solve the cost issue in the application of organic solar cells (OSCs). In this work, two A-D-A type non-fullerene acceptors (NFAs) with easy synthesis processes, namely A3T-2 and A3T-5, were designed and synthesized by using non-fused terthiophene as the D-skeleton and “open mode” or “closed-loop mode” substituted functional groups as the side chain modifiers, respectively. The two NFAs show similar molecular energy levels, while the calculation results and the absorption spectra indicate that A3T-5 has a more stable and planar conformation, which is beneficial to realize efficient charge transport capability. The OSCs based on A3T-2 and A3T-5 were fabricated by using PBDB-TF as the electron donor in parallel, and the power conversion efficiencies are 6.20% and 7.03%, respectively, which are high values among the OSCs based on non-fused NFAs with terthiophene as D-cores. The more stable planar molecular conformation of A3T-5 and the lower miscibility in the PBDB-TF: A3T-5 blend enabled the higher current and the PCE obtained in PBDB-TF: A3T-5-based OSC than that of PBDB-TF: A3T-2-based one. This work shows great potential in realizing efficient photovoltaic properties by employing non-fused acceptors via appropriate molecular design.
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Fullerene derivatives are classic electron acceptor materials for organic solar cells (OSCs) but possess some intrinsic drawbacks such as weak visible light absorption, limited optoelectronic property tunability, difficult purification and photochemical/morphological instability. Fullerene acceptors are a bottleneck restricting further development of this field. Our group pioneered the exploration of novel nonfullerene acceptors in China in 2006, and initiated the research of two representative acceptor systems, rylene diimide polymer and fused‐ring electron acceptor (FREA). FREA breaks the theoretical efficiency limit of fullerene‐based OSCs (~13%) and promotes the whole field to an unprecedented prosperity with efficiency of 20%, heralding a nonfullerene era for OSCs. In this review, we revisit 15‐year nonfullerene exploration journey, summarize the design principles, molecular engineering strategies, physical mechanisms and device applications of these two nonfullerene acceptor systems, and propose some possible research topics in the near future. This article is protected by copyright. All rights reserved.
Article
Two isomeric fluorene-based heteroundecenes of bis(thienocyclopenthieno[3,2-b]thieno) fluorene (BT2T-F) and bis(dithieno[3,2-b:2’,3’-d]thiophene)cyclopentafluorene (B3T-F) have been designed and synthesized. The side chains of 4-hexylphenyl anchor on the 5th and 8th positions in B3T-F while on the 4th and 9th positions in BT2T-F, in which the former is closer to the center of the fused ring. The corresponding acceptor-donor-acceptor (A-D-A) type small molecule acceptors (SMAs) of BT2T-FIC and B3T-FIC were prepared by linking BT2T-F and B3T-F as fused ring donor units with the acceptor unit of 2-(5,6-difluoro-3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (IC-2F), respectively. B3T-FIC presents a superior crystallinity with intense face-on π-π stacking in its neat film while BT2T-FIC is more disordered. When blended with PBDB-T-2Cl as a polymer donor, the optimized PBDB-T-2Cl:BT2T-FIC device exhibits an averaged power conversion efficiency (PCE) of 10.56% while only 7.53% in the PBDB-T-2Cl:B3T-FIC device. The improved short-circuit current (Jsc) and fill factor (FF) of the PBDB-T-2Cl:BT2T-FIC device are the main contribution of its higher performance, which is attributed to its more efficient and balanced charge transport and better carrier recombination suppression. Given that BT2T-FIC blend and B3T-FIC blend films both take a preferential face-on orientated π-π stacking with comparable distances, the suitable SMA domain size obtained in the BT2T-FIC blend could account for its more efficient photovoltaic performance. These results highlight the importance of side-chain strategy in developing efficient SMAs with huge fused ring cores.
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During past several years, the photovoltaic performances of organic solar cells (OSCs) have achieved rapid progress with power conversion efficiencies (PCEs) over 18%, demonstrating a great practical application prospect. The development of material science including conjugated polymer donors, oligomer-like organic molecule donors, fused and nonfused ring acceptors, polymer acceptors, single-component organic solar cells and water/alcohol soluble interface materials are the key research topics in OSC field. Herein, the recent progress of these aspects is systematically summarized. Meanwhile, the current problems and future development are also discussed.
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A new synthetic route, to prepare an alkylated indacenodithieno[3,2‐b]thiophene‐based nonfullerene acceptor (C8‐ITIC), is reported. Compared to the reported ITIC with phenylalkyl side chains, the new acceptor C8‐ITIC exhibits a reduction in the optical band gap, higher absorptivity, and an increased propensity to crystallize. Accordingly, blends with the donor polymer PBDB‐T exhibit a power conversion efficiency (PCE) up to 12.4%. Further improvements in efficiency are found upon backbone fluorination of the donor polymer to afford the novel material PFBDB‐T. The resulting blend with C8‐ITIC shows an impressive PCE up to 13.2% as a result of the higher open‐circuit voltage. Electroluminescence studies demonstrate that backbone fluorination reduces the energy loss of the blends, with PFBDB‐T/C8‐ITIC‐based cells exhibiting a small energy loss of 0.6 eV combined with a high JSC of 19.6 mA cm−2
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Fluorine‐contained polymers, which have been widely used in highly efficient polymer solar cells (PSCs), are rather costly due to their complicated synthesis and low yields in the preparation of components. Here, the feasibility of replacing the critical fluorine substituents in high‐performance photovoltaic polymer donors with chlorine is demonstrated, and two polymeric donors, PBDB‐T‐2F and PBDB‐T‐2Cl, are synthesized and compared in parallel. The synthesis of PBDB‐T‐2Cl is much simpler than that of PBDB‐T‐2F. The two polymers have very similar optoelectronic and morphological properties, except the chlorinated polymer possess lower molecular energy levels than the fluorinated one. As a result, the PBDB‐T‐2Cl‐based PSCs exhibit higher open circuit voltage (Voc) than the PBDB‐T‐2F‐based devices, leading to an outstanding power conversion efficiency of over 14%. This work establishes a more economical design paradigm of replacing fluorine with chlorine for preparing highly efficient polymer donors.
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Full-text available
Over the past three years, a particularly exciting and active area of research within the field of organic photovoltaics has been the use of non-fullerene acceptors (NFAs). Compared with fullerene acceptors, NFAs possess significant advantages including tunability of bandgaps, energy levels, planarity and crystallinity. To date, NFA solar cells have not only achieved impressive power conversion efficiencies of ~13–14%, but have also shown excellent stability compared with traditional fullerene acceptor solar cells. This Review highlights recent progress on single-junction and tandem NFA solar cells and research directions to achieve even higher efficiencies of 15–20% using NFA-based organic photovoltaics are also proposed. This Review describes how non-fullerene electron acceptor materials are bringing improvements in the power conversion efficiency and stability of organic solar cells.
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