An Unfused-Core-Based Nonfullerene Acceptor Enables High-Efficiency Organic Solar Cells with Excellent Morphological Stability at High Temperatures

Article abstract of DOI:10.1002/adma.201705208

Most nonfullerene acceptors developed so far for high-performance organic solar cells (OSCs) are designed in planar molecular geometry containing a fused-ring core. In this work, a new nonfullerene acceptor of DF-PCIC is synthesized with an unfused-ring core containing two cyclopentadithiophene (CPDT) moieties and one 2,5-difluorobenzene (DFB) group. A nearly planar geometry is realized through the F···H noncovalent interaction between CPDT and DFB for DF-PCIC. After proper optimizations, the OSCs with DF-PCIC as the acceptor and the polymer PBDB-T as the donor yield the best power conversion efficiency (PCE) of 10.14% with a high fill factor of 0.72. To the best of our knowledge, this efficiency is among the highest values for the OSCs with nonfullerene acceptors owning unfused-ring cores. Furthermore, no obvious morphological changes are observed for the thermally treated PBDB-T:DF-PCIC blended films, and the relevant devices can keep ≈70% of the original PCEs upon thermal treatment at 180 °C for 12 h. This tolerance of such a high temperature for so long time is rarely reported for fullerene-free OSCs, which might be due to the unique unfused-ring core of DF-PCIC. Therefore, the work provides new idea for the design of new nonfullerene acceptors applicable in commercial OSCs in the future.

Full text of DOI:10.1002/adma.201705208

CommuniCation
Organic Solar Cells
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An Unfused-Core-Based Nonfullerene Acceptor Enables
High-Efficiency Organic Solar Cells with Excellent
Morphological Stability at High Temperatures
Shuixing Li, Lingling Zhan, Feng Liu,* Jie Ren, Minmin Shi,* Chang-Zhi Li,
Thomas P. Russell, and Hongzheng Chen*
nonfullerene acceptors possess superior
optoelectronic properties, such as stronger
light-harvesting capability and more easily
Most nonfullerene acceptors developed so far for high-performance organic
solar cells (OSCs) are designed in planar molecular geometry containing
a fused-ring core. In this work, a new nonfullerene acceptor of DF-PCIC is
synthesized with an unfused-ring core containing two cyclopentadithiophene
(CPDT) moieties and one 2,5-difluorobenzene (DFB) group. A nearly planar
geometry is realized through the F···H noncovalent interaction between CPDT
and DFB for DF-PCIC. After proper optimizations, the OSCs with DF-PCIC as
the acceptor and the polymer PBDB-T as the donor yield the best power con-
version efficiency (PCE) of 10.14% with a high fill factor of 0.72. To the best of
our knowledge, this efficiency is among the highest values for the OSCs with
nonfullerene acceptors owning unfused-ring cores. Furthermore, no obvious
morphological changes are observed for the thermally treated PBDB-T:DF-
PCIC blended films, and the relevant devices can keep ≈70% of the original
PCEs upon thermal treatment at 180 °C for 12 h. This tolerance of such a
high temperature for so long time is rarely reported for fullerene-free OSCs,
which might be due to the unique unfused-ring core of DF-PCIC. Therefore,
the work provides new idea for the design of new nonfullerene acceptors
applicable in commercial OSCs in the future.
tuned energy levels, thus allowing OSCs
with nonfullerene acceptors to achieve
the improved current and voltage.[6–8]
To date, although various nonfullerene
acceptors are developed,[9–17] the acceptor–
donor–acceptor (A–D–A) architecture
with coplanar molecular geometry seems
to be the most successful design strategy
for high-performance nonfullerene accep-
tors,[18–21] since the bulky substituents
attached on the sp3 carbon atoms of
electron-donating core (D) can suppress
intermolecular aggregation, thus achieves
fine phase separation domains; in the
meantime, two electron-withdrawing end-
groups (A) are exposed to form efficient
π–π stacking.[22–24]
At present, nearly all fullerene-free
OSCs with the efficiencies surpassing
10% adopt A–D–A type planar acceptors
with an indacenodithiophene (IDT)[25–28]
Fullerene-free organic solar cells (OSCs) have gained great pro-
gresses in recent years with the enhanced efficiencies of over
13%.[1–3] Compared to traditional fullerene-based acceptors,[4,5]
or indacenodithieno[3,2-b]thiophene (IDTT) core, e.g., 3,9-bis(2-
methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-
tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-
b:5,6-b’]dithiophene (ITIC, Figure 1a).[29–32] The fully fused struc-
ture of IDT and IDTT is beneficial for efficient intramolecular
charge transfer and face-on molecular arrangement, which
broaden the absorption range and improve charge transport in
OSCs, respectively. In the meanwhile, the design of efficient
nonfullerene acceptors with new structures should be mean-
ingful for further advancing OSCs. We think that the introduc-
tion of unfused-ring core would lead to diversified nonfullerene
acceptors constructed by smaller building blocks. In addition, the
device stability is another issue that is to be solved for the com-
mercialization of OSCs.[33,34] Relative to fullerene derivatives, the
reduced crystallinity or amorphous nature of many nonfullerene
acceptors enables the improved morphological stability for the
relevant OSCs, thus potentially extending device lifetime.[13,35,36]
Based on the abovementioned considerations, we here design
and synthesize a new nonfullerene acceptor with a planar geo-
metry, 2,2’-((2Z,2’Z)-(((2,5-difluoro-1,4-phenylene)bis(4,4-bis(2-
ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b’]dithiophene-6,2-diyl))
bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-
diylidene))dimalononitrile (DF-PCIC, Figure 1b), which owns the
Dr. S. Li, L. Zhan, J. Ren, Prof. M. Shi, Prof. C.-Z. Li, Prof. H. Chen
MOE Key Laboratory of Macromolecular Synthesis and Functionalization
State Key Laboratory of Silicon Materials
Department of Polymer Science and Engineering
Zhejiang University
Hangzhou 310027, P. R. China
E-mail: minminshi@zju.edu.cn; hzchen@zju.edu.cn
Prof. F. Liu
Department of Physics and Astronomy
and Collaborative Innovation Center of IFSA (CICIFSA)
Shanghai Jiao Tong University
Shanghai 200240, P. R. China
E-mail: fengliu82@sjtu.edu.cn
Prof. T. P. Russell
Department of Polymer Science and Engineering
University of Massachusetts
Amherst, MA 01003, USA
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.201705208.
DOI: 10.1002/adma.201705208
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Figure 1. a) Chemical structure of ITIC. b) Chemical structure and geometry of DF-PCIC. c) Synthetic route of DF-PCIC.
same A terminal groups (2-(3-oxo-2,3-dihydroinden-1-ylidene)
malononitrile, IC) as ITIC, but a new D core constructed by con-
necting two cyclopentadithiophene (CPDT) moieties[37–42] with
one 2,5-difluorobenzene unit through simple and routine reac-
tions (Figure 1c). The fluorine atoms on 2,5-difluorobenzene
respectively, which outclass those based on the fullerene acceptor,
[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM).
The synthetic route of DF-PCIC is described in Figure 1c
and the detailed information can be found in the Supporting
Information. At first, ethylhexyl groups are attached to CPDT
unit easily at room temperature in a high yield of 77%. After
introducing trimethyl tin group on one side of CPDT unit,
Stille coupling reaction is used to synthesize intermediate 4,
followed by lithiation with n-butyllithium and quenching with
dimethylformamide to get intermediate 5. Finally, Knoevenagel
condensation between intermediate 5 and IC is conducted,
yielding the target molecule, DF-PCIC. Chemical structure
…
facilitates the F H noncovalent interaction with the hydrogen
atoms on adjacent CPDT units, therefore locking the molecular
conformation with a planar geometry[43–45] (Figure 1b) as indi-
cated by 1H NMR and 19F NMR spectra. And the fluorine atoms
on 2,5-difluorobenzene help deepen the highest occupied molec-
ular orbital (HOMO) of DF-PCIC, leading to a deep value of −5.49
eV. The bulky branched alkyl chains, ethylhexyl groups on CPDT
units, which are used to ensure the solubility in the solution and
hold back the formation of large aggregates in the film, can be
attached easily. The obtained DF-PCIC possesses similar absorp-
tion bands and energy levels as ITIC. Then, DF-PCIC is used as
an electron acceptor to pair with a medium bandgap polymer,
poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-
b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2-ethylhexyl)
benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione)] (PBDB-T),[46,47] and
a high power conversion efficiency (PCE) of 10.14% is achieved
with an excellent fill factor (FF) of 0.72 for the relevant OSCs. It
turns out that the PBDB-T:DF-PCIC blended films show excel-
lent morphological stability, and the related devices can also
bear high temperature treatment for 12 h with 85.5%, 86.2%
and 69.8% initial efficiencies remained at 130, 150 and 180 °C,
1
of DF-PCIC is fully characterized by H NMR, 13C NMR, 19F
NMR, and matrix-assisted laser desorption/ionization time of
filght (MALDI-TOF) Mass Spectroscopy. Besides, DF-PCIC
shows good solubility in common solvents, such as dichlo-
romethane, chloroform, and chlorobenzene.
Thermal properties of DF-PCIC are characterized by ther-
mogravimetric analysis and differential scanning calorimetry
(DSC). It is found that DF-PCIC possesses good thermal sta-
bility with a decomposition temperature (Td, 5% weight loss)
at 347 °C under nitrogen atmosphere (Figure S1a, Supporting
Information). In the DSC curve, we observe one melting
peak at 291 °C and one crystallization peak at 225 °C under a
heating or cooling rate of 10 °C min−1 (Figure S1b, Supporting
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b)
a)
10
5
DF-PCIC
Fc/Fc+
1.0
0.8
0.6
0.4
0.2
0.0
DF-PCIC sol
DF-PCIC film
0
-5
-10
-15
-20
300
400
500
600
700
800
900
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Wavelength (nm)
Potential (V, vs. SCE)
Figure 2. a) UV–vis absorption spectra of DF-PCIC in chloroform solution and DF-PCIC thin film. b) Cyclic voltammograms of DF-PCIC and Fc/Fc+ in
dichloromethane solutions.
1
Information), implying that DF-PCIC can crystallize only upon
thermal annealing at high temperatures.
existence of hydrogen bonds, we compare H NMR spectra of
DF-PCIC at 120 °C and room temperature (Figure S5, Sup-
porting Information) and 19F NMR spectra of DF-PCIC at
100 °C and room temperature (Figure S6, Supporting Informa-
tion). The results show that a large shift of around 0.16 ppm
happens for the CPDT units’ hydrogen atoms that close to the
fluorine atoms and a comparable shift of around 0.23 ppm hap-
pens for the fluorine atoms, indicating that there indeed exist
Optical properties of DF-PCIC are investigated by UV–vis spec-
troscopy and relevant results are displayed in Figure 2a. A sharp
peak appears at 671 nm for DF-PCIC solution. DF-PCIC film
shows broader absorptions, and two peaks are observed at 645 and
696 nm, respectively. These changes indicate the intermolecular
interaction or π–π stacking of DF-PCIC is enhanced in the con-
densed state.[48–50] Thus, from the film absorption onset at around
781 nm, the optical bandgap is calculated to be 1.59 eV. The narrow
bandgap of DF-PCIC allows us to pick a wide or medium bandgap
polymer donor to pair with. Besides, we measure the absorption
coefficients of DF-PCIC both in solution and film (Figure S2, Sup-
porting Information) and find that DF-PCIC solution shows a
…
F H noncovalent interaction between 2,5-difluorobenzene
and CPDT units, since F H noncovalent interaction would be
…
broken at high temperature.
According to the absorptions and energy levels of DF-PCIC,
we select a medium bandgap polymer with the lower-lying
HOMO level, PBDB-T, as the donor to match DF-PCIC, and
evaluate its photovoltaic performances while used as an electron
acceptor. An inverted device structure of indium tin oxide (ITO)/
ZnO/poly[9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7-fluorene)-
alt-2,7-(9,9-ioctylfluorene] (PFN)/PBDB-T:DF-PCIC/MoO3/Ag
is adopted to fabricate the OSCs (Figure 3a,b).[16,52] Relevant
J–V curves are displayed in Figure 3c and the photovoltaic
parameters of the OSCs are summarized in Table 1. We first
vary the weight ratios of the donor (D) and acceptor (A) from
1:1 to 1:1.5, and find 1:1.2 of D/A weight ratio is the best com-
position. Under this condition, a PCE of 6.21% is obtained with
an open-circuit voltage (Voc) of 0.92 V, a short-circuit current
(Jsc) of 14.22 mA cm−2 and an FF of 0.48. To further improve
the device performance, two post-treatment methods, thermal
annealing and 1-chloronaphthalene (CN) additive, are per-
formed, respectively. The annealing temperature is changed
from 130 to 200 °C, and the best performance is achieved when
an annealing temperature of 180 °C is used. Under this condi-
tion, the PCE is improved to 9.10% with a Voc of 0.85 V, a Jsc
of 15.86 mA cm−2, and an FF of 0.68. Obviously, the improve-
ment of PCE is mainly attributed to the enhancement of Jsc and
FF. We also observe that thermal annealing would cause the
decrease of Voc. As for the CN additive processing, we change
the amounts of CN additive from 0.2% to 0.8% and find that
0.5% amount of CN additive works best. When 0.5% CN is
added, the PCE can be elevated to 8.10% with a Voc of 0.95 V, a
Jsc of 13.38 mA cm−2, and an FF of 0.63. Thus CN additive can
help improve the Voc and FF, but the Jsc is reduced to some
maximum absorption coefficient of 1.96 × 105
m
−1 cm−1 at 670 nm,
which is comparable to that of ITIC solution (2.07 × 105
m
−1 cm−1
at 679 nm). The relative relation between the absorption coeffi-
cients of DF-PCIC and ITIC films is similar to those of DF-PCIC
and ITIC solutions. Cyclic voltammetry measurements (Figure 2b)
are used to check the energy levels of DF-PCIC. We use the equa-
tion ELUMO/HOMO = −e(Ered/ox + 4.36) (eV) to calculate the lowest
unoccupied molecular orbital (LUMO) and HOMO levels, since
the redox potential of Fc/Fc+ is measured as 0.44 V (the energy
level of Fc/Fc+ as 4.8 eV below vacuum[51]) in our lab. Then, the
onset reduction potential (Ered) and onset oxidation potential (Eox)
of DF-PCIC are found to be −0.59 and 1.13 V, respectively. Thus,
the LUMO and HOMO are calculated as −3.77 and −5.49 eV,
respectively, which are close to those of ITIC (−3.82 and −5.54 eV,
Figure S3, Supporting Information). The deep HOMO of DF-
PCIC allows us to choose a polymer donor with the deep HOMO
level to pair with.
To investigate the geometry of DF-PCIC, density functional
theory calculation is carried out at the B3LYP/6-31G level, and
the ethylhexyl alkyl chains are replaced by methyl for simpli-
fying the calculation. The results are displayed in Figure 1b. It
shows that DF-PCIC possesses a nearly planar geometry and
the dihedral angle between the 2,5-difluorobenzene and CPDT
units is found to be around 5.15°. The almost planar geom-
⋯
etry is formed with the help of F H noncovalent interaction
between the fluorine atoms on 2,5-difluorobenzene and the
hydrogen atoms on CPDT units. To experimentally prove the
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a)
b)
d)
c)
80
0
70
60
50
40
30
20
10
0
1:1.2 as-cast
1:1.2 180 oC
1:1.2 CN
-4
-8
1:1.2 CN+180 oC
1:1.2 CN+130 oC
1:1.2 as-cast
1:1.2 180 oC
1:1.2 CN
-12
-16
1:1.2 CN+180 oC
1:1.2 CN+130 oC
0.0
0.2
0.4
0.6
0.8
1.0
300
400
500
600
700
800
Voltage (V)
Wavelength (nm)
Figure 3. a) Diagram of the device structure. b) Energy level diagram of ITO, ZnO, PFN, DF-PCIC, PBDB-T, MoO3, and Ag. c) J–V curves of the OSCs
fabricated under different conditions. d) EQE spectra of the OSCs fabricated under different conditions.
extent. So, we combine thermal annealing and CN additive
together, expecting that high Voc can remain with the help of
CN additive and high Jsc can increase with the help of thermal
annealing. We fix the amount of CN additive at 0.5% and
change the annealing temperature from 120 to 200 °C. If the
annealing temperature is 180 °C, the PCE is found to be 9.60%.
The best device performance is reached when 130 °C annealing
temperature is applied. Thus, we achieve the highest PCE of
10.14% with a Voc of 0.91 V, a Jsc of 15.66 mA cm−2, and an FF
of 0.72, under the condition of 0.5% CN additive and 130 °C
annealing temperature. More information about device perfor-
mances under different conditions can be found in Figure S7
in the Supporting Information.
The external quantum efficiency (EQE) spectra of PBDB-
T:DF-PCIC-based OSCs are illustrated in Figure 3d. We can
observe that all curves exhibit good photovoltaic responses in
the broad range of 450–750 nm, which can be ascribed to the
complementary absorptions of PBDB-T and DF-PCIC. And
the calculated Jscs from EQE curves are perfectly consistent
with the measured values within 4% error (Table 1). More
specifically, when compared to those of the as-cast device,
thermal annealing induces higher EQE values and extend
EQE spectra to 780 nm; on the contrary, CN additive leads to
minor decreasing of EQE values in the band of 500–600 nm,
which agree well with the measured Jsc, too. To explain the
phenomena observed in EQE spectra, we also check the
absorptions of the blended films treated
Table 1. Photovoltaic parameters of the OSCs based on PBDB-T:DF-PCIC under the
under different thermal annealing condi-
tions (Figure S8, Supporting Information).
The blend absorptions are enhanced and
redshifted slightly after thermal annealing
at 130 °C, while thermal annealing at 180 °C
can promote the crystallization of DF-PCIC,
giving an intense absorption peak centered
at 740 nm (Figure S9, Supporting Informa-
tion). This treatment with 180 °C thermal
annealing also decreases the 500–650 nm
absorption band, in which DF-PCIC con-
tributes in amorphous feature. The newly
formed absorptions in near-infrared range
contribute largely to the currents in devices,
illumination of AM 1.5 G, 100 mW cm−2.
CN [%] Annealing temp.a) Voc [V]
[°C]
Jsc
FF
PCEmax [%] PCEavg.c) [%]
b)
D/A [w/w]
[mA cm−2
]
w/o
w/o
0.5
w/o
180
w/o
180
130
0.92
0.85
0.95
0.88
0.91
14.22 (13.95)
15.86 (15.23)
13.38 (13.32)
15.35 (14.83)
15.66 (15.37)
0.48
0.68
0.63
0.70
0.72
6.21
9.10
8.10
9.60
10.14
6.19
9.07
8.07
9.55
10.12
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
0.5
0.5
a)Annealing for 10 min; b)Values in the parentheses are calculated from the corresponding EQE curves;
c)Average PCEs for ten devices.
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as seen from the enhanced Jsc and EQE responses beyond
700 nm.
corresponding to a distance of 3.63 Å. These structure features
remain in BHJ blends. As seen from the as-cast thin film, the
polymer lamellae diffraction is observed in both in-plane and out-
of-plane patterns with a broad azimuthal angle spreading. The in-
plane crystal coherence length (CCL) is estimated to be 115.2 Å.
The π–π stacking peak area is dominated by that of DF-PCIC,
centered at 1.73 A−1. Thermal annealing can effectively change the
structure order of the BHJ thin film, leading to great changes of
scattering features. As seen from the corresponding image and
profiles, 180 °C annealing enhances the crystallization of both
donor and acceptor materials. The in-plane CCL for PBDB-T is
enhanced to be 152.1 Å with the pronounced highly ordered reflec-
tions. A new peak at 0.50 Å−1 in both in-plane and out-of-plane pro-
files is seen, which comes from the lamellae stacking of DF-PCIC,
and their corresponding π–π stacking peak can also be allocated.
Quite sharp π–π stacking is recorded for DF-PCIC in the 180 °C
annealed BHJ film, with the peak centered at 1.70 Å−1 (3.69 Å),
which is slightly away from that of the neat thin film. The CCL of
π–π stacking in out-of-plane direction is estimated to be 58.7 Å,
which is much larger than that in the neat film (14.6 Å). This
obvious crystallinity enhancement for both donor and acceptor
leads to a morphology change of forming bicontinuous crystalline
networks, thus electron and hole mobilities undergo an order of
magnitude improvement. In a consequence, Jsc and FF increase
dramatically for the related OSC. On the other hand, the crystal-
linity improvement narrows the bandgap of DF-PCIC, thereby, its
LUMO energy level would become lower, which may be respon-
sible for the reduced Voc of the device annealed at 180 °C.
Charge transport properties of PBDB-T:DF-PCIC blended
films are investigated by the space charge limited current
method (Figure S10, Supporting Information). The struc-
tures of the hole-only devices and electron-only devices are
ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS)/PBDB-T:DF-PCIC/MoO3/Ag and ITO/ZnO/
PFN/PBDB-T:DF-PCIC/PFN/Al, respectively. We find that
thermal annealing at 180 °C can greatly improve both hole
and electron mobilities. The hole mobility is improved from
0.15 × 10−4 (the as-cast device) to 1.10 × 10−4 cm2 V−1 s−1, while
the electron mobility is improved from 0.10 × 10−4 (the as-cast
device) to 1.00 × 10−4 cm2 V−1 s−1. However, CN additive has com-
pletely different effects on the hole and electron mobility. For
the hole mobility, CN additive can enhance the hole mobility to
0.85 × 10−4 cm2 V−1 s−1. But for the electron mobility, CN addi-
tive reduces the electron mobility to 0.06 × 10−4 cm2 V−1 s−1.
This agrees well with the observation that thermal annealing
can improve the current, and CN additive will reduce the cur-
rent. When both thermal annealing and CN additive are applied,
the hole mobilities are measured as 2.05 × 10−4 cm2 V−1 s−1
for the CN + 180 °C device and 1.28 × 10−4 cm2 V−1 s−1 for
the CN + 130 °C device, the electron mobilities are measured
as 1.00 × 10−4 cm2 V−1 s−1 for the CN + 180 °C device and
0.26 × 10−4 cm2 V−1 s−1 for the CN + 130 °C device. The changes
of hole and electron mobilities are in accord with the variation
tendency of the currents observed in J–V curves.
The self-assembly of pure materials and bulk heterojunction
(BHJ) blends treated under different processing conditions are
studied by grazing incidence X-ray diffraction (GIXD). Presented
in Figure 4 are the diffraction patterns and line-cut profiles.
PBDB-T polymer shows a (100) lamellae stacking at 0.30 Å−1
both in in-plane and out-of-plane directions. A π–π stacking at
1.66 Å−1 is seen in out-of-plane direction, giving a stacking dis-
tance of 3.78 Å. DF-PCIC exhibits low crystallinity, however, a π–π
stacking in out-of-plane direction is found to locate at 1.73 Å−1,
Blends processed from CN additive shows a polymer in-plane
(100) CCL of 133.3 Å with highly ordered reflections. The π–π
stacking is located at 1.70 Å−1, with good intensities in both in-
plane and out-of-plane profiles. However, the peak from the
lamellae stacking of DF-PCIC is not observed, implying that CN
additive can only improve the crystallization of the donor. More-
over, CN can induce edge-on orientation of the materials, which is
proved by (200) lamellae stacking at 0.60 Å−1 for PBDB-T. These
findings can account for the enhanced hole transport and the
Figure 4. The 2D GIXD images of a) PBDB-T, b) DF-PCIC, c) BHJ as-cast, d) BHJ annealed 180 °C, e) BHJ CN + as-cast, and f) BHJ CN + annealed
130 °C. g) 1D out-of-plane (solid lines) and in-plane (dot lines) X-ray profiles extracted from GIXD patterns.
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reduced electron transport. When thermally annealed at 130 °C,
the PBDB-T (100) in-plane CCL is calculated to be 134.8 Å, which
is similar as that of nonannealed thin film. Yet the π–π stacking
of the sample shifts to 1.73 Å−1, with the enhanced intensity and
a more preferred face-on orientation. The lower temperature
treatment can thus fine-tune the material self-assembly to form
a tighter π–π stacking and more favorable orientation to boost
transport properties. In the meanwhile, the crystalinity of DF-
PCIC is not improved greatly. Therefore, high Voc remains, and
Jsc and FF increase for the related device, providing the best PCE
of 10.14%. When annealed at 180 °C for CN processed thin films,
similar results are obtained as those in 180 °C annealed thin film
without CN. These results suggest that only thermal annealing at
high temperatures, e.g., 180 °C, can induce the obvious crystal-
lization of DF-PCIC, which agrees well with the findings from
DSC and absorption measurements (Figures S1b and S9, Sup-
porting Information).
Thin film morphology is investigated by transmission elec-
tron microscopy (TEM). As seen in Figure S11 in the Supporting
Information, the as-cast thin film shows a homogeneous phase
image. Thermal annealing at 180 °C induces the crystallization
of both components, and in the corresponding TEM picture,
quite tiny fibers with the length of several tens of nanometer
are observed, which gives rise to the enhanced Jsc and FF. When
CN is used, BHJ film shows a large phase separation. Annealed
at 180 °C, the phase separation size of the thin film is reduced,
which is attributed to the formation of material crystallites. The
reduced phase separation domains and enhanced crystalliza-
tion can reduce Voc and improve Jsc and FF, and thus PCE is
improved. A more balanced improvement is obtained through the
annealing under 130 °C, which leads to the finer phase separation
domains. This morphology feature help balance Voc and Jsc/FF,
and the champion device performance is obtained.
the extension of treatment time. Such different morphological
changes may result in different device stability. We then fabricate
PBDB-T:DF-PCIC and PBDB-T:PC71BM devices, and test the sta-
bility of PCEs after thermal treatment at 130, 150, or 180 °C for
various times (Figure 5b). It is found that PCEs of the devices
based on PBDB-T:DF-PCIC blended films are insensitive to
thermal treatment at high temperatures and 85.5% and 86.2%
initial efficiencies can remain after thermal treatment at 130 and
150 °C for 12 h, respectively. Even the temperature is increased to
180 °C, 69.8% initial efficiency can still be kept after 12 h thermal
treatment. But for PBDB-T:PC71BM-based devices, the situations
are bad. After 12 h thermal treatment, only 52.5% and 50.8% ini-
tial efficiencies can remain for 130 and 150 °C, respectively. If
the temperature increases to 180 °C, the PCE quickly decreases
to 11.9% of initial efficiency after 12 h. Furthermore, we also test
the device stability of PBDB-T:ITIC-based OSCs at high tem-
peratures and find that the devices based on PBDB-T:ITIC are
defeated by PBDB-T:DF-PCIC-based ones in device stability, too
(FigureS12,SupportingInformation).Withoutdoubts,OSCsbased
on PBDB-T:DF-PCIC blended films can exhibit excellent thermal
stability. As for the reasons of the excellent morphological sta-
bility for PBDB-T:DF-PCIC films and the related devices, we
believe that high temperatures lead to the stronger aggregation
…
of DF-PCIC. However, since the F H noncovalent interactions
between CPDT units and 2,5-difluorobenzene groups would be
destroyed under thermal treatment at high temperatures, e.g.,
180 °C, DF-PCIC may adopt a twisted molecular geometry, and
thus over aggregation might be avoided at high temperatures.
Meanwhile, the twisted D core has minor effects on the intra-
molecular charge transfer from CPDT units to IC end-groups as
well as on the π–π stacking between IC terminals of DF-PCIC,
which is proved by the improved crystallinity of the thermally
annealed films. Therefore, good absorptions and efficient charge
transport can remain even the blended films or devices are ther-
mally treated at 180 °C for 12 h, which demonstrates that our
molecular designing strategy is successful for achieving ther-
mally stable OSCs. What is more, we further test the device sta-
bility of PBDB-T:DF-PCIC-based OSCs under continuous AM
1.5 illumination at 100 mW cm−2 for various times (Figure S13,
Supporting Information). It turns out that 88% initial PCE is
remained after 3 h continuous illumination, which is mean-
ingful for the practical applications of OSCs.
The current material and processing recipe does not only
enhance the device performance, but also improve the device
stability. We investigate the morphological stability of PBDB-
T:DF-PCIC blended films, and compared them with those of
PBDB-T:PC71BM blended films. Through thermal treatment at
180 °C for various times (Figure 5a), PBDB-T:DF-PCIC blended
films remains uniform even after 3 h thermal treatment. In con-
trast, PBDB-T:PC71BM blended films under similar condition
shows granular agglomerations, which is much severer with
Figure 5. a) Optical microscopy images of PBDB-T:DF-PCIC and PBDB-T:PC71BM blended films after thermal treatment at 180 °C for various times.
b) The PCEs for the devices based on PBDB-T:DF-PCIC and PBDB-T:PC71BM blended films after thermal treatment at 130, 150, or 180 °C for various times.
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…
molecular geometry can be constructed for DF-PCIC without
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by the National Natural Science Foundation of China (Nos. 21734008,
21474088, 51473142, 51561145001, 51620105006, and 61721005), the
973 Program (2014CB643503), and Zhejiang Province Science and
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by the Fundamental Research Funds for the Central Universities
(2016FZA4007). T.P.R. thanks the support from the N00014-15-1-
2244 under contract N00014-15-1-2244. Portions of this research were
carried out at beamline 7.3.3 and 11.0.1.2 at the Advanced Light Source,
Molecular Foundry, and National Center for Electron Microscopy,
Lawrence Berkeley National Laboratory, which was supported by the
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Conflict of Interest
The authors declare no conflict of interest.
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Received: September 11, 2017
Revised: October 23, 2017
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