A Twisted Thieno[3,4‐b]thiophene‐Based Electron Acceptor Featuring a 14‐π‐Electron Indenoindene Core for High‐Performance Organic Photovoltaics

Abstract

With an indenoindene core, a new thieno[3,4-b]thiophene-based small-molecule electron acceptor, 2,2′-((2Z,2′Z)-((6,6′-(5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydroindeno[2,1-a]indene-2,7-diyl)bis(2-octylthieno[3,4-b]thiophene-6,4-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (NITI), is successfully designed and synthesized. Compared with 12-π-electron fluorene, a carbon-bridged biphenylene with an axial symmetry, indenoindene, a carbon-bridged E-stilbene with a centrosymmetry, shows elongated π-conjugation with 14 π-electrons and one more sp3 carbon bridge, which may increase the tunability of electronic structure and film morphology. Despite its twisted molecular framework, NITI shows a low optical bandgap of 1.49 eV in thin film and a high molar extinction coefficient of 1.90 × 105m−1 cm−1 in solution. By matching NITI with a large-bandgap polymer donor, an extraordinary power conversion efficiency of 12.74% is achieved, which is among the best performance so far reported for fullerene-free organic photovoltaics and is inspiring for the design of new electron acceptors.

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A Twisted Thieno[3,4-b]thiophene-Based Electron
Acceptor Featuring a 14-
π
-Electron Indenoindene
Core for High-Performance Organic Photovoltaics
Sheng jie Xu, Zichun Zhou, Wuyue Liu, Zhongbo Zhang, Feng Liu, Hongping Yan,
and Xiaozhang Zhu*
Dr. S. Xu, Z. Zhou, W. Liu, Z. Zhang, Prof. X. Zhu
Beijing National Laboratory for Molecular Sciences
CAS Key Laboratory of Organic Solids
Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190, China
E-mail: xzzhu@iccas.ac.cn
Z. Zhou, W. Liu, Z. Zhang, Prof. X. Zhu
School of Chemistry and Chemical Engineering
University of Chinese Academy of Sciences
Beijing 100049, China
Prof. F. Liu
Department of Physics and Astronomy
Shanghai Jiaotong University
Shanghai 200240, China
Dr. H. Yan
Department of Chemical Engineering
Stanford University
443 Via Ortega, Stanford, CA 94305-4125, USA
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.201704510.
DOI: 10.1002/adma.201704510
utilization regarding the advantages in low
cost, flexibility, and large-area fabrication.[1]
Emerging as a new dawn in OPV field, non-
fullerene electron acceptors[2] have caught
intense interests to achieve further break-
throughs of power conversion efficiency
(PCE). Especially, acceptor (A)–donor (D)–
acceptor (A)-type small-molecule electron
acceptors[3] featuring an electron-rich core
flanked with two electron-deficient moieties
have exhibited great potential, which can be
attributed to their excellent electronic tun-
ability via judicious molecular design.
Beyond functioning as an electron-rich
moiety to induce intramolecular charge
transfer, the bulky “D” component with
sp3 carbon bridges actually plays a critical
role in providing an adequate solubility
for solution processability and realizing
a suitable molecular packing and phase
separation for charge separation and trans-
port. Besides being successfully utilized to
reduce strong self-aggregation of perylene diimide (PDI) motifs
such as in 9,9′-spirobi[fluorene] (SF)-PDI2,[4] fluorene, a polycyclic
aromatic hydrocarbon (PAH) with weak electron-donating ability,
is one of the firstly used “D” building block. In 2013, Watkins
and co-workers reported electron acceptors F8IDT and FEHIDT
with a fluorene core.[5] By combining poly(3-hexylthiophene)
(P3HT) donor, FEHIDT-based OPVs delivered a maximum PCE
of 2.12%. In 2015, Holliday and co-workers designed and synthe-
sized an electron acceptor (5Z,5′Z)-5,5′-{(9,9-dioctyl-9H-fluorene-
2,7-diyl)bis[2,1,3-benzothiadiazole-7,4-diyl(Z)methylylidene]}
bis(3-ethyl-2-thioxo-1,3-thiazolidin-4-one) (FBR) consisting of a
fluorene core flanked by an electron-withdrawing benzothiadia-
zole and rhodanine combination, which showed a much higher
PCE of up to 4.1% with P3HT.[3c] In 2016, Chen and co-workers
developed 2,2′-((2Z,2′Z)-((5,5′-(9,9-dioctyl-9H-fluorene-2,7-diyl)
bis(thiophene-5,2-diyl))bis(methanylylidene))bis(3-oxo-2,3-di-
hydro-1H-indene-2,1-diylidene))dimalononitrile (DICTF) by
replacing 2,3-dihydro-1H-indene-1,3-dione of F8IDT with more
electron-deficient 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene)pro-
panedinitrile (INCN).[6] By combining a low-bandgap polymer
donor, PTB7-Th, DICTF-based OPVs gave a PCE of up to 7.93%
that is comparable to that of PC71BM. Quite recently, Peng and co-
workers developed a wide-bandgap electron acceptor, SFBRCN,
with a 3D spirobifluorene core, which showed a high PCE of
With an indenoindene core, a new thieno[3,4-b]thiophene-based small-
molecule electron acceptor, 2,2′-((2Z,2′Z)-((6,6′-(5,5,10,10-tetrakis(2-
ethylhexyl)-5,10-dihydroindeno[2,1-a]indene-2,7-diyl)bis(2-octylthieno[3,4-b]
thiophene-6,4-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-
1H-indene-2,1-diylidene))dimalononitrile (NITI), is successfully designed and
synthesized. Compared with 12-
π
-electron fluorene, a carbon-bridged biphe-
nylene with an axial symmetry, indenoindene, a carbon-bridged E-stilbene
with a centrosymmetry, shows elongated
π
-conjugation with 14
π
-electrons
and one more sp3 carbon bridge, which may increase the tunability of elec-
tronic structure and film morphology. Despite its twisted molecular frame-
work, NITI shows a low optical bandgap of 1.49 eV in thin film and a high
molar extinction coefficient of 1.90 × 105 m−1 cm−1 in solution. By matching
NITI with a large-bandgap polymer donor, an extraordinary power conversion
efficiency of 12.74% is achieved, which is among the best performance so
far reported for fullerene-free organic photovoltaics and is inspiring for the
design of new electron acceptors.
Organic Solar Cells
Bulk-heterojunction-organic photovoltaics (BHJ-OPVs) con-
sisting of electron donor and acceptor blends have been recog-
nized as one of the most promising technique in sunlight energy
Adv. Mater. 2017, 1704510

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10.26% with PTB7-Th.[7] With indenofluorene, a
π
-extended
fluorene derivative, Baran et al. reported an electron acceptor,
IDFBR, that combined with PTB7-Th donor and IDTBR acceptor
delivered high PCEs of up to 11% in a ternary solar cell.[8]
Recently, a series of new
π
-conjugated PAHs, carbon-bridged
oligo(p-phenylenevinylene)s (COPVs), were developed and
exhibit excellent photoelectric properties, e.g., unit fluorescence
quantum yield and highly reversible electrochemical prop-
erty.[9] COPV series has identified its importance with applica-
tions such as molecular wire,[10] thin-film organic laser,[11] dye-
sensitized solar cell,[12] and etc. Indenoindene is the smallest
repeating unit of COPV series. Compared with 12-
π
-electron
fluorene, a carbon-bridged biphenylene with an axial symmetry,
indenoindene, a carbon-bridged E-stilbene with a centrosym-
metry, shows elongated
π
-conjugation with 14
π
-electrons and
one more sp3 carbon bridge, which increases the tunability of
electronic structure and film morphology. Thus, we consider
indenoindene as a new promising building block for the design
of small-molecule electron acceptors. We report herein the
design and synthesis of a new thieno[3,4-b]thiophene (TbT)-
based[13] electron acceptor NITI that possesses an indenoin-
dene core as shown in Figure 1a. The combination of TbT and
INCN has been proved to be effective for the design of small-
bandgap electron acceptor, (Z)-dioctyl 6,6′-(4,4,9,9-tetrakis(4-
hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-
2,7-diyl)bis(4-((Z)-(1-(dicyanomethylene)-3-oxo-1H-inden-2(3H)-
ylidene)methyl)thieno[3,4-b]thiophene-2-carboxylate) (ATT-2 ),
that enables efficient single-junction semi-transparent OPVs.[14]
As indicated in Figure 1b, NITI has a twisted molecular frame-
work with a dihedral angle of 25° between the planar TbT–
INCN combination and indenoindene core according to theo-
retical calculation at B3LYP/6-31G** level, which may benefit
phase separation. Despite of the twisted molecular structure,
NITI in dichloromethane shows a very high molar extinction
coefficient of 1.90 × 105 m−1 cm−1 at the maximum absorption
(719) nm that is bathochromically shifted to 762 nm with an
absorption onset at 832 nm in thin film, corresponding to a
low optical bandgap (Egopt) of 1.49 eV. By matching NITI with a
wide-bandgap polymer donor 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),[15] we achieved an extraordinary PCE of
12.74% by device fabrication and optimization.
The synthetic route toward NITI is shown in Figure 2. The
key intermediate 3 was synthesized by Suzuki coupling reaction
of 2,2′-(5,5,10,10-tetra(2-ethylhexyl)-5,10-dihydroindeno[2,1-a]
indene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) 1[16]
and 6-bromo-2-octylthieno[3,4-b]thiophene-4-carbaldehyde 2[13c]
in 66% yield as an orange solid. Target molecule NITI was
then synthesized by Knoevenagel reaction of dialdehyde 3 and
2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononi-
trile in 95% yield as a black solid and was fully characterized
by conventional NMR, mass, and elemental analyses. NITI is
soluble in common solvents such as chloroform, chloroben-
zene, and o-dichlorobenzene (>30 mg mL−1), which is enough
to fabricate solution-processed OPV devices. Thermogravimetric
analysis indicated that NITI is thermally stable at up to 329 °C
(5 wt% loss, Figure S2, Supporting Information). From the differ-
ential scanning calorimetry curve (Figure S3, Supporting Infor-
mation), only an obvious melting peak was observed in 233 °C.
Photophysical properties of NITI in dichloromethane solu-
tion and thin film were investigated by UV–vis–NIR absorp-
tion spectroscopy (Figure 3b). NITI shows a maximum
absorption at 715 nm with a large absorption coefficient (
ε
)
of 1.90 × 105 m−1 cm−1. In thin film, the absorption spectrum
becomes broadened with the maximum absorption peak con-
siderably red-shifted to 762 nm, suggesting the strong intermo-
lecular
π
–
π
interactions. The optical bandgap is determined to
be 1.49 eV. As seen in Figure 3b, the large-bandgap PBDB-T
donor forms a perfect complementary absorption with NITI in
thin films. Cyclic voltammetry measurement was then applied
to evaluate the frontier orbital energy levels of NITI in thin
film (Figure S1, Supporting Information). The potentials were
calibrated with ferrocene/ferrocenium (Fc/Fc+) redox couple
(4.8 eV below the vacuum level). According to the oxidation
and reduction onsets, the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO)
Adv. Mater. 2017, 1704510
Figure 1. a) Molecular structure and b) optimized molecular geometry of
NITI. (2-Ethylhexyl groups were simplified to methyl ones).
Figure 2. Synthesis of compound NITI. Reagents and conditions:
a) Pd(PPh3)4, aqueous Na2CO3 (2.0 m) dioxane; b) 2-(5,6-difluoro-3-oxo-
2,3-dihydro-1H-inden-1-ylidene)malononitrile, pyridine, CHCl3.

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Figure 3. a) The molecular structures of PBDB-T and NITI. b) UV–vis–NIR absorption spectra of NITI in dichloromethane and in thin film and PBDT-T
in thin film. c) Energy diagram of materials used in OPVs.
Figure 4. a) J–V curves and b) EQE curves for OPV devices under AM 1.5 G irradiation (100 mW cm−2). c) Jsc dependence on light intensity. d) Transient
photocurrent measurements of OPV devices with and without 1-CN.

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energy levels were determined to be −5.68 and −3.84 eV,
respectively. The HOMO and LUMO offset between NITI and
PBDB-T is 0.35 and 0.92 eV, which can provide enough driving
force for efficient charge separation.
To evaluate the photovoltaic performance of NITI, we fabri-
cated OPV devices with a conventional structure of indium tin
oxide(ITO)/ poly(3,4-ethylenedioxythiophene):poly (styrenesul-
fonate) (PEDOT:PSS)/active layer/PNDIT-F3N/Al, where the
active layer consisted of PBDB-T, NITI, and PNDIT-F3N devel-
oped by Huang and co-workers was selected as the cathode
buffer layer.[17] The optimized active layer was spin-casted from
chlorobenzene solution with an optimized donor/acceptor
(D/A) weight ratio of 1:1 followed by utilizing 1-chloronaph-
thalene (1-CN) as the processing additive to improve the inter-
mixing of the electron donor and acceptor phases. As shown
in Figure 4a and summarized in Table S3 in the Supporting
Information, the as-cast devices exhibit significantly high PCE
of 9.49% with a Voc of 0.84 V, a short-circuit current (Jsc) of
17.80 mA cm−2, and a fill factor (FF) of 0.63. After the addi-
tion of 1% (v/v) 1-CN, the PCE was further increased to 12.74%
with a Voc of 0.86 V, a Jsc of 20.67 mA cm−2, and a FF of 0.71%.
External quantum efficiency (EQE) curves were shown in
Figure 4b. The optimized device has a broad photon response
from 300 to 850 nm with the maximum value reaching 81.5% at
730 nm, indicating the efficient photon harvesting and charge
collection. The integrated photocurrents from EQE spectra are
20.10 mA cm−2, which agrees well with the J–V measurements
(≈3% error). The bulk charge transport property of PBDB-
T:NITI blend films was investigated by the space-charge limited
current method using hole only (ITO/PEDOT:PSS/active layer/
Au) and electron only (ITO/ZnO/active layer/Al) devices. The
hole and electron mobilities are determined to be 0.79 × 10−4
and 0.21 × 10−4 cm2 v−1 s−1 for the as-cast film and 1.34 × 10−4
and 1.19 × 10−4 cm2 v−1 s−1 for the optimized film. The high and
balanced mobilities can suppress the charge recombination and
thus ensure the improvement of the device performance.
To investigate the relationship between charge recombina-
tion and transport in the photoactive layer, we measured the
light intensity (P) dependence of Jsc. If the slope (s) of the Jsc
versus P curve approaches 1, the bimolecular recombination
is weak in an OPV device. In Figure 4c, the recombination
parameters s are 0.91 and 0.96 for the devices with as-cast and
optimized blends, respectively, suggesting that bimolecular
recombination is effectively suppressed during operating the
optimized device. We further utilize transient photocurrent
measurement to study the competition between charge carrier
sweep-out by the internal field and recombination in the OPV
devices. As shown in Figure 4d, the transient photocurrent
of the OPVs was measured at 0 bias. Apparently, the charge
extraction time was estimated to be 1.30 µs for the device based
on the as-cast blend film, which was further reduced to 0.82 µs
after optimization. Considering that the charge recombination
is efficiently suppressed in the optimized device, we think the
enhanced charge mobility is responsible for the reduced charge
extraction time.
The structure order of pure and BHJ thin films were studied
by grazing incidence X-ray diffraction. The diffraction pat-
terns and the line-cuts of neat PBDB-T, NITI, and their blends
without/with 1-CN additive are shown in Figure 5. PBDB-T
shows a broad (100) diffraction peak at 0.31 Å−1 with wide
azimuthal angle spreading and an in-plane crystal coherence
length (CCL) of 5.3 nm. A (010) diffraction peak in the out-of-
plane (OOP) direction at 1.69 Å−1 is seen, corresponding to a
π
–
π
stacking distance of 3.72 Å. The neat NITI film exhibits a
Adv. Mater. 2017, 1704510
Figure 5. 2D GIXS patterns for a) PBDB-T and b) NITI pristine film, c) as-cast PBDB-T:NITI blend film, and d) PBDB-T:NITI blend film processed with
1-CN additive. e) Corresponding line-cuts of GIXS patterns. (Solid line: out of plane, Dashed line: in-plane).

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sharp diffraction at 0.44 Å–1 in OOP direction, an alkyl stacking
packing at 0.36 Å–1 in in-plane (IP) direction, and a broad
π
–
π
stacking peak at 1.83 Å–1 in OOP direction. Thus, the interla-
mellae spacing is 14.2 Å and a
π
–
π
stacking distance is 3.43 Å.
The lamellae CCL in IP are estimated to be 17.4 nm, and thus
NITI has good structure order in neat films. When blended,
both PBDBT-T and NITI contents showed reduced crystalline
feature as seen from much weaker scattering profiles. Thus,
the presence of the other molecule retards material crystalliza-
tion due to good mixing. Diffraction features both PBDB-T and
NITI can be seen. The low q region is dominated by PBDB-T
and NITI alkyl-to-alkyl peaks, and the high q region is domi-
nated by NITI
π
–
π
stacking peaks. Compared to the as-cast
blend, the 1-CN processed blend film shows sharper alkyl–alkyl
packing and
π
–
π
stacking peaks. The PBDB-T (100) intensity
shows obvious improvement comparing to NITI alkyl packing
peak, and the
π
–
π
stacking peak intensified and centered at
≈1.82 Å−1, closing to NITI structure feature. Thus, materials
show improved crystallinity upon additive processing, which
corresponds well with mobility measurements.
The surface and bulk morphology of blend films were
studied by the atomic force microscopy (AFM) and transmis-
sion electron microscopy (TEM). As shown in Figure S6 in the
Supporting Information, the as-cast film exhibits large phase
aggregation, shown as bumps and dark areas in AFM and
TEM. Additive processed BHJ film showed reduced material
aggregations, both in size and extend. And the TEM image
becomes quite smooth. These aggregations should come
from agglomeration of NITI, and when additive is used, it
disperses more uniformly inside polymer matrix and orders
more readily. Such a feature improves the transport and pho-
tovoltaic function. Transmission resonant soft X-ray scattering
(RSoXS) at 294.8 eV was used to investigate the BHJ thin film
phase separation. As seen from Figure S7 in the Supporting
Information, the as-cast blend film shows higher scattering
intensity in low q region, shown as a broad hump, giving a
distance of ≈200–300 nm; and when additive is used, RSoXS
intensity reduces largely, with a scattering hump at similar
length scales, thus the NITI aggregation reduces largely. The
improved crystalline feature from PBDB-T polymers would
form fibrils (seen from TEM images), which forms a network
to boost photovoltaic function.
In summary, we have designed and synthesized a new
TbT-based small-molecule electron acceptor, NITI, featuring
a bulky and planar indenoindene core. Despite of its twisted
molecular geometry, NITI shows a low optical bandgap of
1.49 eV in thin film with a high molar extinction coefficient
of 1.90 × 105 m−1 cm−1 in solution. By matching NITI with the
large-bandgap polymer donor, an extraordinary PCE of 12.74%
was achieved after device optimizations, attributing to the
improved charge transport property and proper phase separa-
tion. To the best of our knowledge, this is the first example that
fullerene-free OPVs achieve a high PCE over 12% without using
nonfullerene acceptors with an indacenodithienothiophene
core,[18] which is inspiring for the design of new electron accep-
tors. Through further efforts such as molecular optimization of
NITI, design and/or selection of suitable polymer donors with
deeper HOMO energy level to reduce energy loss,[4,19] we may
further promote the PCE even up to 15%.[20]
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
S.X. and Z.Z. contributed equally to this work. The authors thank
the National Basic Research Program of China (973 Program) (No.
2014CB643502) for financial support, the Strategic Priority Research
Program of the Chinese Academy of Sciences (XDB12010200), and the
National Natural Science Foundation of China (91333113, 21572234).
Conflict of Interest
The authors declare no conflict of interest.
Keywords
indenoindene, nonfullerene acceptors, organic solar cells, power
conversion efficiency, thieno[3,4-b]thiophene
Received: August 9, 2017
Revised: August 30, 2017
Published online:
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