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Unique Energy Alignments of a Ternary Material System toward High‐Performance Organic Photovoltaics

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CommuniCation
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Unique Energy Alignments of a Ternary Material System
toward High-Performance Organic Photovoltaics
Pei Cheng, Jiayu Wang, Qianqian Zhang, Wenchao Huang, Jingshuai Zhu, Rui Wang,
Sheng-Yung Chang, Pengyu Sun, Lei Meng, Hongxiang Zhao, Hao-Wen Cheng,
Tianyi Huang, Yuqiang Liu, Chaochen Wang, Chenhui Zhu, Wei You,* Xiaowei Zhan,*
and Yang Yang*
DOI: 10.1002/adma.201801501
Research on OPV has begun in as early
as 1950s, when the active layer consisted
of only a single organic semiconductor
(p-type or n-type).[8] Considering the large
binding energy of an organic semicon-
ductor (0.3–1 eV),[9] the single component
OPV yielded a very low PCE (1–3%) due to
the insufficient exciton dissociation by the
electric field. To overcome this problem,
the active layer consisted of donor/acceptor
binary blend (bulk heterojunction (BHJ)
structure) was reported in 1995.[10,11] In
this binary blend OPV, the exciton dissocia-
tion became much more efficient than that
in the single component solar cells due to
the increased donor/acceptor interfaces
in the active layer. In recent five years, a
new type of active layer with ternary blend
(donor, acceptor, and third component)
was developed to further enhance the performance of binary
blend OPV.[12–15] These ternary blend OPV presented some
advantages via various design strategies: broader and stronger
absorption,[16–25] more efficient charge transfer,[26–29] more effi-
cient charge transport pathways,[30–33] better charge extraction at
the electrodes,[34–37] and improved stability.[38–42]
Among the design strategies for ternary blend
OPV, employing a near-infrared (NIR) absorber as the third
component is the most successful one to boost the PCE of
Incorporating narrow-bandgap near-infrared absorbers as the third component
in a donor/acceptor binary blend is a new strategy to improve the power conver-
sion efficiency (PCE) of organic photovoltaics (OPV). However, there are two
main restrictions: potential charge recombination in the narrow-gap material
and miscompatibility between each component. The optimized design is to
employ a third component (structurally similar to the donor or acceptor) with a
lowest unoccupied molecular orbital (LUMO) energy level similar to the acceptor
and a highest occupied molecular orbital (HOMO) energy level similar to the
donor. In this design, enhanced absorption of the active layer and enhanced
charge transfer can be realized without breaking the optimized morphology of
the active layer. Herein, in order to realize this design, two new narrow-bandgap
nonfullerene acceptors with suitable energy levels and chemi cal structures are
designed, synthesized, and employed as the third component in the donor/
acceptor binary blend, which boosts the PCE of OPV to 11.6%.
Organic Solar Cells
Photovoltaic (PV) technology is one of the promising renew-
able energy technologies for the human future. Especially,
the solution-processable organic photovoltaics (OPV) have
attracted considerable attention in the last decade due to some
potential advantages, such as light-weight, flexibility, possible
semitransparence, low energy consumption, and fast and
large-area fabrication.[1–6] To date, OPV have shown the best
performance with certified power conversion efficiency (PCE)
exceeding 11%.[7]
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.201801501.
Dr. P. Cheng, Dr. W. Huang, R. Wang, S.-Y. Chang, P. Sun, Dr. L. Meng,
H. Zhao, H.-W. Cheng, T. Huang, Y. Liu, C. Wang, Prof. Y. Yang
Department of Materials Science and Engineering
University of California
Los Angeles, CA 90095, USA
E-mail: yangy@ucla.edu
J. Wang, J. Zhu, Prof. X. 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, P. R. China
E-mail: xwzhan@pku.edu.cn
Q. Zhang, Prof. W. You
Department of Chemistry
University of North Carolina at Chapel Hill
Chapel Hill, NC 27599, USA
E-mail: wyou@unc.edu
Dr. W. Huang, H. Zhao, H.-W. Cheng, Y. Liu, Prof. Y. Yang
California NanoSystems Institute
University of California
Los Angeles, CA 90095, USA
Dr. C. Zhu
Advanced Light Source
Lawrence Berkeley National Laboratory
Berkeley, CA 94720, USA
Adv. Mater. 2018, 30, 1801501
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OPV. These NIR absorbers, including donors[16–22,26] and accep-
tors,[23,43–46] can utilize photons in the NIR region, which will
potentially contribute to the short circuit current density (JSC)
of solar cells. On the other hand, the highest occupied mole-
cular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) energy levels of NIR absorbers can be designed to be
positioned between the HOMOs and LUMOs of the donor and
acceptor, which can provide more routes for charge transfer
by forming a cascade energy heterojunction (Figure 1a).[47]
However, there are two main restrictions for the application
Adv. Mater. 2018, 30, 1801501
Figure 1. a) Schematic diagrams of different models for charge transfer. b) Molecular structures of donor (FTAZ), acceptor (IDIC), and third compo-
nents (ITIC-Th, ITIC-Th-S, ITIC-Th-O). c) Energy levels and optical bandgaps of FTAZ, IDIC, ITIC-Th, ITIC-Th-S, and ITIC-Th-O.
Figure 2. The synthetic routes of ITIC-Th-S and ITIC-Th-O.
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of NIR absorbers as the third component: potential charge
recombination in the narrow gap material and miscompatibility
between each component.[12,14,15] The charge recombination
traps can be formed if the bandgap of the third component
is too narrow (Figure 1a). The miscompatibility occurs if the
chemical structures of donor/acceptor and the third compo-
nent are mainly different, which will break the optimized mor-
phology of the donor/acceptor binary blend active layer.[48] Thus,
the optimized design is to employ a third component (structur-
ally similar to donor or acceptor) with a LUMO energy level
similar to the acceptor and a HOMO energy level similar to the
donor (Figure 1a). In this design, enhanced absorption of active
layer and enhanced charge transfer can be realized without the
breaking of the optimized morphology of active layer.
Herein, in order to realize this optimized design of ternary
blend OPV, we employed the third component in the active
layer with a similar chemical structure to the acceptor and
suitable energy levels. On the basis that 2,7-bis(3-dicyanometh-
ylene-2Z-methylene-indan-1-one)-4,4,9,9-tetrakis(5-hexylth-
iophen-2-yl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b]di(thieno[3,2-
b] thiophene) (ITIC-Th)[49,50] shows a similar backbone and
bandgap (1.60 eV), but up-shifted LUMO and HOMO energy
levels to 2,7-bis(3-dicyanomethylene-2Z-methylene-indan-
1-one)-4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b]-
dithiophene (IDIC), ITIC-Th was chosen as the third com-
ponent in the blend of donor FTAZ[51]/acceptor IDIC.[52,53]
In order to achieve optimized energy level alignments of
ITIC-Th-based third component with the donor FTAZ/acceptor
IDIC, 2,7-bis(5-(3-dicyanomethylene-2Z-methylene-indan-1-
one)-3-hexylthio-thiophene-2-yl)-4,4,9,9-tetrakis(5-hexylthio-
phen-2-yl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b]di(thieno[3,2-
b] thiophene) (ITIC-Th-S), and 2,7-bis(5-(3-dicyanomethylene-
2Z-methylene-indan-1-one)-3-hexyloxy-thiophene-2-yl)-4,4,9,9-
tetrakis(5-hexylthiophen-2-yl)-4,9-dihydro-s-indaceno[1,2-b:5,6-
b]di(thieno[3,2-b]thiophene) (ITIC-Th-O) (Figure 1b) were
designed and synthesized through the incorporation of thio-
phene with alkylthio/alkoxy side chains as pi-bridge. This
approach could down-shift the LUMO energy level and up-shift
the HOMO energy level of the ITIC-Th counterpart and, there-
fore, redshift the absorption into the NIR region (bandgaps:
1.45 eV for ITIC-Th-S and 1.40 eV for ITIC-Th-O, as shown
in Figure 1c). Through this chemical modification, the HOMO
Adv. Mater. 2018, 30, 1801501
Figure 3. a) Cyclic voltammograms for ITIC-Th, ITIC-Th-S, and ITIC-Th-
O. b) Absorption spectra of pure FTAZ, IDIC, ITIC-Th, ITIC-Th-S, and
ITIC-Th-O films.
Figure 4. a) J–V curves and b) EQE spectra of devices based on FTAZ/
IDIC without or with different third components under illumination of an
AM 1.5G solar simulator, 100 mW cm2.
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and the LUMO energy levels of ITIC-Th-O were respectively
similar to the HOMO energy level of donor FTAZ (5.35 eV)
and the LUMO energy level of acceptor IDIC (3.91 eV). After
the device optimization, the best PCE of FTAZ/ITIC-Th-O/
IDIC ternary blend OPV was measured to be 11.6%, which
is higher than that of the controlled FTAZ/IDIC binary blend
OPV (10.4%).
The synthetic routes of ITIC-Th-S and ITIC-Th-O
are shown in Figure 2. The intermediate compounds
5,5-(4,4,9,9-tetrakis(5-hexylthiophen-2-yl)-4,9-dihydro-s-
indaceno[1,2-b:5,6-b] di(thieno[3,2-b]thiophene)-2,7-diyl)bis(4-
(hexylthio)thiophene-2-carbaldehyde) (ITS-CHO)/5,5-(4,4,9,9-
tetrakis(5-hexylthiophen-2-yl)-4,9-dihydro-s-indaceno[1,2-b:5,6-
b] di(thieno[3,2-b]thiophene)-2,7-diyl)bis(4-(hexyloxy)thiophene-
2-carbaldehyde) (ITO-CHO) were synthesized in 83%/71%
yield through the Stille coupling reaction using Pd(PPh3)4 as a
catalyst. A straightforward reaction of ITS-CHO/ITO-CHO with
1,1-dicyanomethylene-3-indanone afforded ITIC-Th-S/ITIC-Th-
O in 76%/95% yield. The compounds ITS-CHO/ITO-CHO and
ITIC-Th-S/ITIC-Th-O were fully characterized by MALDI-TOF
MS, 1H NMR, 13C NMR, and elemental analysis. ITIC-Th-S and
ITIC-Th-O were readily soluble in common organic solvents
such as chloroform and o-dichlorobenzene at room tempera-
ture. The details of synthesis and related characterization are
shown in the Supporting Information.
The cyclic voltammetry (CV) method was used to investi-
gate the electrochemical properties of ITIC-Th, ITIC-Th-S,
and ITIC-Th-O films and further estimate their energy levels
(Figure 3a). The HOMO/LUMO energy levels of ITIC-Th-S
and ITIC-Th-O were estimated to be 5.41 eV/3.86 eV and
5.36 eV/3.91 eV, respectively (calculation details were shown
in the Supporting Information). Due to electron-donating
nature of alkylthio/alkoxy side chains and high electronega-
tivity of sulfur (2.58) and oxygen atoms (3.44), ITIC-Th-S and
ITIC-Th-O exhibited an up-shifted HOMO energy levels and
down-shifted LUMO energy levels compared with ITIC-Th
(5.61 eV/3.81 eV). Through this simple chemical modifica-
tion, the LUMO energy level of the third component can be
down-shifted from 3.81 to 3.91 eV, which is similar to that of
the acceptor IDIC (3.91 eV[49]); and the HOMO energy level of
the third component can be up-shifted from 5.61 to 5.36 eV,
which is similar to that of the donor FTAZ (5.35 eV, Figure S1,
Supporting Information). Meanwhile, the down-shifted LUMO
energy level and up-shifted HOMO energy level of the third
component can narrow the bandgap and broaden the absorp-
tion of active layer into the NIR region. The absorption spectra
of FTAZ, IDIC, ITIC-Th, ITIC-Th-S, and ITIC-Th-O films are
shown in Figure 3b. Compared with ITIC-Th, ITIC-Th-S, and
ITIC-Th-O exhibit redshifted absorptions (855 and 885 nm) and
reduced bandgaps (1.45 and 1.40 eV). The redshifted absorption
of the third component can potentially benefit the absorption of
the ternary blend active layer, which is only cut-off at 775 nm
for the FTAZ/IDIC binary blend (1.60 eV).
The J–V curves and external quantum efficiency (EQE)
spectra of devices based on FTAZ/IDIC without or with dif-
ferent third components (ITIC-Th, ITIC-Th-S, and ITIC-Th-O)
under the illumination of an AM 1.5 G solar simulator
(100 mW cm2) are shown in Figure 4. The average (calculated
from 20 individual devices) and best device characteristics are
summarized in Table 1. The device characteristics of FTAZ/
ITIC-Th, FTAZ/ITIC-Th-S, and FTAZ/ITIC-Th-O binary
blend solar cells are also summarized in Table S1 (Supporting
Information). After addition of third component (10% of the
donor and acceptor by weight), the ternary blend OPV exhibited
an enhanced JSC with maintaining an open circuit voltage (VOC)
(0.85 V) and fill factor (FF) (0.71) comparable to controlled
FTAZ/IDIC binary blend device. The average JSC of devices
increased from 16.6 to 17.8 and 18.5 mA cm2 by employing
ITIC-Th-S and ITIC-Th-O as the third component, respectively.
As a result, the best PCE of the devices increased from 10.4% to
11.1% and 11.6% by incorporation of ITIC-Th-S and ITIC-Th-
O as the third component, respectively. The best ternary blend
OPV was based on ITIC-Th-O as a third component, which
showed similar LUMO energy level to acceptor IDIC (3.91 eV)
and similar HOMO energy level to donor FTAZ (5.35 eV).
In addition, PCE (11%) and FF ( 70%) of ternary blend
device can be mainly maintained by continuous increasing the
Adv. Mater. 2018, 30, 1801501
Table 1. Average and best device data based on FTAZ/IDIC without or with different third components.
Active layer VOC [V] JSC [mA cm2] Calculated JSC [mA cm2]FF [%] PCE [%]
Average Best
FTAZ/IDIC 0.85 ± 0.01 16.6 ± 0.5 15.9 71.0 ± 0.4 10.0 ± 0.3 10.4
FTAZ/ITIC-Th/IDIC 0.85 ± 0.01 17.1 ± 0.4 16.5 70.9 ± 0.5 10.3 ± 0.2 10.6
FTAZ/ITIC-Th-S/IDIC 0.85 ± 0.01 17.8 ± 0.5 17.1 71.2 ± 0.4 10.8 ± 0.2 11.1
FTAZ/ITIC-Th-O/IDIC 0.85 ± 0.01 18.5 ± 0.4 18.0 71.9 ± 0.4 11.3 ± 0.2 11.6
Figure 5. PL spectra of FTAZ/IDIC blend films without or with different
third components.
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amount of third component (up to 30%, Table S2, Supporting
Information).
As shown in Figure 4b and Figure 3b, the EQE in 400–
600 nm range is attributed mainly to FTAZ, and the EQE in
600–750 nm range is attributed mainly to IDIC. After the addi-
tion of ITIC-Th-S or ITIC-Th-O as the third component, the
EQE can be broadened to 850 or 880 nm, which is consistent
with the absorption spectra of blend films (Figure S2, Sup-
porting information). From integration of the EQE spectra
with the AM 1.5G reference spectrum, the calculated JSC was
obtained, which was similar to J–V measurement (the average
error is 3.6%, Table 1).
In order to study the charge or energy transfer
between different components, we carry out the same method
as literatures (investigating the J–V curves of solar cells based
on two acceptors as active layer (without donor).[20,54] J–V curves
of devices based on pure IDIC, pure ITIC-Th-O, and IDIC/
ITIC-Th-O blend under illumination of an AM 1.5G solar simu-
lator, 100 mW cm
2 are shown in Figure S3 (Supporting infor-
mation). The JSC of solar cells based on IDIC/ITIC-Th-O blend
Adv. Mater. 2018, 30, 1801501
Figure 6. a–d) 2D GIWAXS patterns of FTAZ/IDIC film (a), FTAZ/ITIC-Th/IDIC film (b), FTAZ/ITIC-Th-S/IDIC film (c), and FTAZ/ITIC-Th-O/IDIC film (d).
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Adv. Mater. 2018, 30, 1801501
(6:1, same as the value in the best FTAZ/ITIC-Th-O/IDIC ter-
nary device) is much larger than that of the pure IDIC or pure
ITIC-Th-O based cells, which should be attributed to the effec-
tive charge transfer between IDIC and ITIC-Th-O. The steady
state photoluminescence (PL) was carried out to investigate
the charge transfer in active layer. The PL intensities of FTAZ/
IDIC films without or with different third components (ITIC-
Th, ITIC-Th-S, and ITIC-Th-O) are shown in Figure 5 (excite
at 660 nm). Considering the absorption edge of FTAZ is at
630 nm, the PL peak at 760 nm of FTAZ/IDIC blend film is
assigned to IDIC. Compared with FTAZ/IDIC binary blend
film, the PL intensity of ternary blend films decreased by 70%
(the PL intensities of different films were compared at their
PL peaks). The stronger PL quenching of IDIC can mainly
be caused by extra photoinduced hole transfer from IDIC to
donor FTAZ or to third components, which suggests a more
efficient charge transfer in the active layer. The more efficient
charge transfer in ternary blend active layer is attributed to the
formation of the optimized energy level alignment, which con-
sequently enhances the JSC and PCE of OPV.
In order to compare the film morphology of FTAZ/IDIC
active layer without or with third component, grazing-incidence
wide-angle X-ray scattering (GIWAXS) and transmission
electron microscopy (TEM) measurements were carried out.
GIWAXS provides molecular-level structural information such
as lattice constant and orientation of molecular packing, while
TEM provides larger length-scale phase separation information
up to hundreds of nanometers. Figure 6 shows 2D GIWAXS
patterns of FTAZ/IDIC films without or with different third
components (ITIC-Th, ITIC-Th-S, and ITIC-Th-O). The out-
of-plane and in-plane line cuts are shown in Figure S4 (Sup-
porting information). The molecular packing behavior in binary
blend film and all ternary blend films was similar in terms of
molecular crystallite orientation and aggregation. At the same
time, these binary blend and ternary blend films exhibited
similar scale of phase separation, which is apparent in TEM
images (Figure S5, Supporting information). Thus, the film
morphology of FTAZ/IDIC active layer without or with these
ITIC-Th based third components was shown to be similar at
both molecular-level and hundreds of nanometers scale. This
suggests that the employment of third component with good
compatibility (structurally similar to acceptor) can maintain the
optimized film morphology of active layer.
The space charge limited current (SCLC)[55] method was
employed to measure the hole and electron mobility of binary
and ternary blend films. Hole-only and electron-only diodes
were fabricated using the architectures of indium tin oxide
(ITO)/poly(3,4-ethylenedioxythiophene): poly(styrene sul-
fonate) (PEDOT: PSS)/active layer/gold (Au) for holes and
aluminum (Al)/active layer/Al for electrons. The dark J–V
curves of the devices are plotted as ln[Jd3/(Vappl – Vbi)2] versus
[(VapplVbi)/d]0.5 in Figure 7. The average hole and electron
mobilities are calculated and listed in Table 2. Thanks to pre-
served optimized film morphology of active layer, the average
hole and electron mobilities of the binary blend and all ternary
blend films were shown to be similar (1.0–1.4 × 104 cm2 V1 s1
for hole; 2.6–3.4 × 105 cm2 V1 s1 for electron).
In summary, two new nonfullerene acceptors ITIC-Th-S
and ITIC-Th-O were designed, synthesized and employed as a
third component in the blend of donor FTAZ/acceptor IDIC.
Through chemical modification, the HOMO energy level of the
third component was up-shifted from 5.61 eV (ITIC-Th) to
5.41 eV (ITIC-Th-S)/ 5.36 eV (ITIC-Th-O), which is similar
to that of the donor FTAZ (5.35 eV); and the LUMO energy
level of the third component was down-shifted from 3.81 eV
(ITIC-Th) to 3.86 eV (ITIC-Th-S)/ 3.91 eV (ITIC-Th-O), which
is similar to that of the acceptor IDIC (3.91 eV). The up-shifted
HOMO energy level and down-shifted LUMO energy level of the
third component was able to narrow the bandgaps and broaden
the absorption of active layer into the NIR region. Meanwhile,
Figure 7. a) Hole-only and b) electron-only devices based on FTAZ/IDIC
blend films without or with different third components.
Table 2 . Hole and electron mobilities of FTAZ/IDIC blend films without
or with different third components (average by five devices).
Active layer
µ
h [cm2 V1 s1]
µ
e [cm2 V1 s1]
FTAZ/IDIC 1.2 ± 0.4 × 1043.4 ± 1.5 × 105
FTAZ/ITIC-Th/IDIC 1.0 ± 0.3 × 1043.4 ± 1.4 × 105
FTAZ/ITIC-Th-S/IDIC 1.4 ± 0.4 × 1042.6 ± 1.2 × 105
FTAZ/ITIC-Th-O/IDIC 1.2 ± 0.4 × 1042.9 ± 1.4 × 105
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Adv. Mater. 2018, 30, 1801501
the optimized energy level alignment induces a more efficient
charge transfer. In addition, due to similar chemical structure
of these third components to the acceptor, these materials exhib-
ited a good compatibility which can maintain the optimized
film morphology of active layer at both molecular-level and hun-
dreds of nanometers-scale. As a result, the broadened absorp-
tion and enhanced charge transfer can contribute to JSC which
are the reasons for the higher performance of FTAZ/ITIC-Th-O/
IDIC ternary system compared with that of FTAZ/IDIC binary
system. The best PCE of FTAZ/ITIC-Th-O/IDIC ternary blend
OPV was measured to be 11.6%, which is higher than that of the
controlled FTAZ/IDIC binary blend OPV (10.4%).
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
P.C. and J.W. contributed equally to this work. Y.Y. acknowledges the Air
Force Office of Scientific Research (AFOSR) (Nos. FA2386-15-1-4108,
FA9550e15-1e0610, and FA9550-15-1-0333), Office of Naval Research
(ONR) (Nos. N00014-14-181-0648 and N00014-04-1-0434), National
Science Foundation (NSF) (No. ECCS-1509955), and UC-Solar Program
(No. MRPI 328368) for their financial support. Part of this research
was performed in beamline 7.3.3 in Advanced Light Source Lawrence
Berkeley National Laboratory. X.Z. acknowledges the National Science
Foundation China (NSFC) (Nos. 21734001 and 51761165023) for
financial support. W.Y. acknowledges NSF Grant (No. DMR-1507249)
and CBET-1639429 for financial support. The authors also thank Selbi
Nuryyeva for her insights on the manuscript.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
energy level alignments, fullerene-free, nonfullerene, organic solar cells,
ternary blends
Received: March 6, 2018
Revised: April 3, 2018
Published online: May 21, 2018
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... [170] mitigated the FF loss against J sc while keeping V oc constant. They used energy levels modulation in ITIC-Th by introducing an adjoining thiophene ring, with an extra S atom or O atom into its side chain, between the central core of ITIC-Th and each of the outer dangling units, to form ITIC-Th-S and ITIC-Th-O. ...
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