Fused Cyclopentadithienothiophene Acceptor Enables Ultrahigh Short-Circuit Current and High Efficiency >11% in As-Cast Organic Solar Cells

Article abstract of DOI:10.1002/adfm.201904956

A new method to synthesize an electron-rich building block cyclopentadithienothiophene (9H-thieno-[3,2-b]thieno[2″,3″:4′,5′]thieno[2′,3′:3,4]cyclopenta[1,2-d]thiophene, CDTT) via a facile aromatic extension strategy is reported. By combining CDTT with 1,1-dicyanomethylene-3-indanone endgroups, a promising nonfullerene small molecule acceptor (CDTTIC) is prepared. As-cast, single-junction nonfullerene organic solar cells based on PFBDB-T: CDTTIC blends exhibit very high short-circuit currents up to 26.2 mA cm^?2 in combination with power conversion efficiencies over 11% without any additional processing treatments. The high photocurrent results from the near-infrared absorption of the CDTTIC acceptor and the well-intermixed blend morphology of polymer donor PFBDB-T and CDTTIC. This work demonstrates a useful fused ring extension strategy and promising solar cell results, indicating the great potential of the CDTT derivatives as electron-rich building blocks for constructing high-performance small molecule acceptors in organic solar cells.

Full text of DOI:10.1002/adfm.201904956

FULL PAPER
Organic Solar Cells
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Fused Cyclopentadithienothiophene Acceptor Enables
Ultrahigh Short-Circuit Current and High Efficiency >11%
in As-Cast Organic Solar Cells
Qiao He, Munazza Shahid, Jiaying Wu, Xuechen Jiao, Flurin D. Eisner,
Thomas Hodsden, Zhuping Fei, Thomas D. Anthopoulos, Christopher R. McNeill,
James R. Durrant, and Martin Heeney*
tunability.[1–3] So far, a variety of building
A new method to synthesize an electron-rich building block cyclopenta-
dithienothiophene (9H-thieno-[3,2-b]thieno[2″,3″:4′,5′]thieno[2′,3′:3,4]
cyclopenta[1,2-d]thiophene, CDTT) via a facile aromatic extension strategy
is reported. By combining CDTT with 1,1-dicyanomethylene-3-indanone
endgroups, a promising nonfullerene small molecule acceptor (CDTTIC)
is prepared. As-cast, single-junction nonfullerene organic solar cells based
on PFBDB-T: CDTTIC blends exhibit very high short-circuit currents up to
26.2 mA cm−2 in combination with power conversion efficiencies over 11%
without any additional processing treatments. The high photocurrent results
from the near-infrared absorption of the CDTTIC acceptor and the well-inter-
mixed blend morphology of polymer donor PFBDB-T and CDTTIC. This work
demonstrates a useful fused ring extension strategy and promising solar cell
results, indicating the great potential of the CDTT derivatives as electron-rich
building blocks for constructing high-performance small molecule acceptors
in organic solar cells.
blocks have been developed and incor-
porated into SMAs in order to optimize
charge transport, energy levels and optical
absorption.[1,4] Fused ladder-type aromatics
have been widely applied to construct high
mobility organic semiconductors because
the extended conjugation in fused rings is
beneficial to facilitate π-electron delocali-
zation, enlarging interchain π–π overlap
and hence enhancing the intermolecular
charge carrier mobility.[5–8] The incorpora-
tion of electron-deficient endgroups onto
the periphery of fused ladder-type aro-
matics is an effective strategy to lower and
stabilize the lowest unoccupied molecular
orbital (LUMO) energy levels, thus pro-
moting electron accepting behavior and
reducing the optical bandgap.[9,10] SMAs
based on such fused ladder-type aromatics
have demonstrated great promise with
several reports of high power conversion efficiencies (PCEs)
over 13%.[11–15] Very recently a SMA with a bent, electron-defi-
cient core has exhibited an efficiency of 15.7% with a high JSC
of 25.3 mA cm−2.[16]
For current high performance SMA based BHJ OSCs with
efficiencies around 13%, the typical photocurrent (JSC) densi-
ties are around 19–23 mA cm−2.[11–14,17,18] Given the positive
1. Introduction
For high performance bulk heterojunction (BHJ) organic solar
cells (OSCs), nonfullerene small molecule acceptors (SMAs)
have been shown to be highly promising alternatives to resolve
the inadequacies of conventional fullerene derivatives, such as
poor absorption in the visible region and limited energy level
Q. He, Dr. M. Shahid, J. Wu, T. Hodsden, Prof. J. R. Durrant,
Prof. M. Heeney
Department of Chemistry and Centre for Plastic Electronics
Imperial College London
Dr. Z. Fei
Institute of Molecular Plus
Tianjin Key Laboratory of Molecular Optoelectronic Science
Tianjin University
London W12 0BZ, UK
Tianjin 300072, P. R. China
E-mail: m.heeney@imperial.ac.uk
Prof. T. D. Anthopoulos
Dr. X. Jiao, Prof. C. R. McNeill
Department of Materials Science and Engineering
Monash University
King Abdullah University of Science and Technology (KAUST)
KAUST Solar Centre
Thuwal 23955-6900, Saudi Arabia
Victoria 3800, Australia
Prof. J. R. Durrant
SPECIFIC IKC
College of Engineering
Swansea University
Swansea SA2 7AX, UK
Dr. F. D. Eisner, Prof. T. D. Anthopoulos
Department of Physics and Centre for Plastic Electronics
Imperial College London
London SW7 2AZ, UK
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201904956.
DOI: 10.1002/adfm.201904956
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Scheme 1. Synthetic route to CDTTIC.
correlation between JSC and the PCE in OSCs, there have been
several approaches toward achieving higher JSC. One major
strategy, for instance, is to design near-infrared (NIR) absorbing
SMAs with a narrow bandgap, which can form a complemen-
tary absorption profile with wide bandgap donor materials.
Other approaches have focused on processing techniques such
as hot spin coating, thermal annealing, and solvent additive pro-
cessing in order improve the blended film morphology. Such
approaches can enhance charge transport and reduce recombi-
nation.[19] Recently high JSC values over 25 mA cm−2 have been
achieved with solvent additive processing (1,8-diiodooctane,
DIO or 1-chloronaphthalene, CN) and thermal annealing treat-
ment.[16,20,21] However, these additional treatments can increase
processing complexity and are less desirable for the practical
fabrication of large area BHJ OSCs. Therefore, developing new
SMAs which can achieve high JSC and PCE under simple pro-
cessing techniques remains of interest.
Cyclopenta[2,1-b;3,4-b]dithiophene (CDT) is a fused aromatic
bithiophene derivative, which has shown versatile applica-
tions[22] in organic field-effect transistors (OFETs),[23,24] organic
solar cells,[25,26] dye-sensitized solar cells (DSSC),[27] and perov-
skite solar cells (PSCs),[28] due to its high co-planarity with
strong intermolecular π–π interactions and electron donating
capability. Recently, CDT based SMAs have been reported
with PCEs ranging from 3.53% to 10.14%, but with relatively
low photocurrents peaking at 16 mA cm−2.[29–31] These results
suggest that CDT could be a promising donor if it could be
modified to reduce the bandgap of the resulting SMA. An
attractive approach for such a modification is the extension of
the ring system by the fusion of additional thiophene groups
onto the periphery of CDT to afford cyclopentadithienothio-
increase the conjugation length, decrease bandgap and addi-
tionally facilitate intramolecular interactions. Surprisingly given
the interest in CDT, there is, to the best of our knowledge, only
one report of the synthesis of CDTT in which it was utilized
in the development of conjugated polymers for thin-film tran-
sistor applications.[32] However, the reported synthesis of CDTT
was low yielding and required the use of the pyrophoric reagent
tert-butyllithium together with cryogenic temperatures.
In this work we report a new mild and effective synthetic
route to prepare the CDTT core through aromatic ring exten-
sion of a commercially available CDT monomer. We subse-
quently incorporate the CDTT core into a new nonfullerene
acceptor (CDTTIC) (Scheme 1). CDTTIC possesses a strong
absorption in the 500–860 nm solar region with a high molar
−1
absorptivity in solution (2.4 × 105
m
cm−1), a bandgap of
1.44 eV in the solid state and a high electron mobility (4.2 ×
10−3 cm2 V−1 s−1) in thin-film diode devices. Density functional
theory (DFT) calculations suggest that the backbone of CDTTIC
is highly planar with good overlap between the HOMO and
LUMO frontier molecular orbitals. As-cast single-junction BHJ
OSCs based on CDTTIC without additional treatments yielded
high PCEs of up to 11.5% with ultrahigh currents observed (JSC
of up to 26.2 mA cm−2). These results suggest that CDTT is
a promising electron-rich building block in constructing NIR
nonfullerene acceptors to achieve excellent JSC and PCE.
2. Results and Discussion
The synthesis route toward the compound CDTTIC is shown
in Scheme 1. Alkylation and bromination of the commercially
available monomer 4H-cyclopenta[2,1-b:3,4-b′]dithiophene
with n-octyl bromide and NBS, followed by base-catalyzed
phene
(9H-thieno-[3,2-b]thieno[2″,3″:4′,5′]thieno[2′,3′:3,4]-
cyclopenta[1,2-d]thiophene, CDTT). Such ring fusion should
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rearrangement[33,34] afforded CDT-dicarbaldehyde (4). Subse-
quent treatment with ethyl mercaptoacetate under basic con-
ditions afforded CDTT-dicarboxylate (5). Conversion of the
dicarboxylate (5) to dialdehyde (7) was readily achieved by a
two-step reduction/oxidation route using LiAlH4, followed by
the Dess–Martin periodinane. The final acceptor CDTTIC was
synthesized by the Knoevenagel condensation reaction between
7 and 1,1-dicyanomethylene-3-indanone (IC).
This is further supported by the relatively high electron
mobility of CDTTIC (4.2 × 10−3 cm2 V−1 s−1), which is extracted
from space charge limited current measurements in electron-
only devices (Figure S2, Supporting Information).
The electrochemical properties of CDTTIC were investigated
by cyclic voltammetry (CV) using films prepared on a plat-
inum rod working electrode (Figure S3, Supporting Informa-
tion). The LUMO and HOMO energy levels of CDTTIC were
estimated to be −3.98 and −5.62 eV respectively based on the
onset of the reduction and oxidation potentials (Eox = 1.24 V,
Ered = −0.4 V) and assuming the ferrocene/ferrocenium couple
to 4.8 eV below vacuum (Efe = 0.42 V vs Ag/AgCl). These energy
levels are in reasonable agreement with those predicted by
density functional theory at the B3LYP/6-31G* level (Figure S1,
Supporting Information). These energy levels are well matched
to typical high-performance polymer donors such as our pre-
viously reported polymer donor PFBDB-T, which also has a
complementary absorption profile (Figure 1 and Figure S4,
Supporting Information).[14]
To demonstrate the potential of CDTTIC as an electron
acceptor in OSCs, we blended it with PFBDB-T[14] and fabri-
cated BHJ OSCs with an inverted device structure of glass/ITO/
ZnO/PFBDB-T:CDTTIC/MoO3/Ag. The chemical structure,
optical absorption, and energy levels of PFBDB-T are shown in
Figure 1. PFBDB-T and CDTTIC have complementary absorp-
tion (Figure 1b and Figure S4, Supporting Information) and
matched energy levels (Figure 1c, calculated from CV method).
Table 1 summarizes the open-circuit voltage (VOC), short-cir-
cuit current density (JSC), fill factor (FF), and PCE of OSCs at
different donor/acceptor (D/A) weight ratios. When the D/A
ratio was 1:1, the as-cast OSCs demonstrated the best overall
CDTTIC possesses good solubility in common organic sol-
vents such as chloroform, chlorobenzene and o-dichloroben-
zene. The chloroform solution (10−6 m) of CDTTIC exhibits
strong absorption in the 500–750 nm region with a maximum
extinction coefficient of 2.4 × 105 −1 cm−1 at 707 nm. Compared
m
to its solution, CDTTIC exhibits a boarder absorption profile in
the thin film, with a more pronounced shoulder and a signifi-
cant redshift of the maximum absorption peak to 778 nm, with
an optical bandgap of 1.44 eV, as estimated from the absorption
edge (861 nm).
The structural and electronic properties of CDTTIC were
investigated by density functional theory at the B3LYP/6-31G*
level (Figure S1, Supporting Information). The calculations
predict that CDTTIC has a co-planar core, with effective delo-
calization of the HOMO and LUMO frontier molecular orbitals
over the backbone. CDTTIC is also predicted to have a bent or
banana like shape, due to the preferential orientation of the

C O bond of the indanone toward the sulfur of the CDTT core.
We were unable to grow a suitable single crystal of CDTTIC to
confirm this prediction but note that similar endgroups have
been shown to orientate in this manner, due to stabilizing non-
[35,36]

covalent C O…S interactions.
Thin film diffraction meas-
urements, vide infra, suggest that CDTTIC is highly crystalline.
Figure 1. a) Chemical structures, b) absorption (sol.: in chloroform solution (10−6 m); f.: in neat thin film), and c) energy levels of PFBDB-T and CDTTIC.
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Table 1. Device data of OSCs based on as-cast PFBDB-T: CDTTIC blends.
indicative of an efficient sweep-out of carriers and negligible
bimolecular recombination losses under 1 sun conditions, in
agreement with the high photocurrent observed,[37,38] but devi-
ates from linearity at higher light intensities (Figure 3a, inset).
To better resolve this dependency, the derivative lnJSC/lnΦ is
shown in Figure 3a, main plot. This derivative plot clearly dem-
onstrates that at intensities greater than 2 sun, JSC increases
sub-linearly with light intensity, indicative of increased bimo-
lecular recombination attributed to the high charge carrier den-
sities photogenerated at these high light fluxes. At maximum
power point, the lower internal electric field will also result in
higher charge densities even at one sun irradiation, suggesting
that bimolecular losses are likely to explain, at least in part, the
low fill factors observed for these PFBDB-T: CDTTIC based
OSCs.
D:A
[W/W]
JSC
[mA cm−2 a)
VOC
[V]a)
FFa)
PCE
[%]a)
]
1:2
23.9 0.4(24.3) 0.69 0.03(0.71) 0.56 0.01(0.56) 9.2 0.4(9.7)
26.1 0.2(26.2) 0.72 0.02(0.73) 0.55 0.01(0.55) 10.3 0.3(10.5)
24.9 1.0(25.8) 0.78 0.01(0.78) 0.56 0.02(0.57) 11.0 0.5(11.5)
23.9 0.6(24.5) 0.79 0.01(0.79) 0.51 0.01(0.52) 9.6 0.3(9.9)
1:1.5
1:1
1:0.8
a)Average values with standard deviation from over 15 devices; the values of the
best devices in parentheses.
performance (Figure 2a) with an average VOC of 0.78 V and a
remarkably high average JSC of 24.9 mA cm−2. The overall PCE
was 11.0% (average values over 15 devices), mainly as a result
of a rather low FF of 0.56. In an effort in increase the latter,
we investigated the device performance under various pre/post-
treatments, such as mixed solvents, annealing and additives
(Table S1, Supporting Information). However no improvement
in FF or PCE was observed, with all treatments resulting in
small decreases in PCE. To further confirm the high JSC values,
we performed external quantum efficiency (EQE) measure-
ments. As shown in Figure 2b, the as-cast device, exhibited a
broad response from 350 to 840 nm, with EQE values of over
70% over the 445–805 nm range. The calculated JSC obtained
from the integration of the EQE spectrum with the AM 1.5G
reference spectrum was 23.8 mA cm−2, which is <5% mismatch
with that from J–V measurement (24.9 mA cm−2). The broad
EQE spectrum from 350 to 840 nm from 350 to 840 nm sug-
gests that both polymer and acceptor make considerable contri-
The charge generation and extraction properties were also
studied by corrected photocurrent data, as plotted in Figure 3b.
The corrected photocurrent density Jph was determined from
the difference between the current density measured in the
light and the dark using pulsed irradiation to avoid sample
heating: Jph = Jlight − Jdark.
[39] In most OPV devices, Jph saturates
at far reverse bias, where the internal electric field is very strong,
and all field dependent losses pathways can be turned off. As
can be seen from Figure 3b, Jph for this device does not reach
a saturation point even when a relatively strong reverse bias of
−8 V was applied, indicative of field dependent generation in
these blends, which most likely results from field dependent
geminate recombination of interfacial charge transfer states
(as discussed above, bimolecular losses are not significant at
one sun irradiation at short circuit, and therefore can not be the
origin of the strong increase in Jph at reverse bias). This field
dependent charge generation will also contribute to the rela-
tively low FF observed for these blends.[40]
The transport properties of the blends were also assessed by
hole and electron only devices using the space charge-limited
current (SCLC) method. The charge carrier mobilities were
calculated by fitting the J–V characteristics of the respective
diodes (Figure S5, Supporting Information). The blends dem-
onstrate relatively good and balanced charge transport with a
hole mobility of 2.6 × 10−4 cm2 V−1 s−1 and an electron mobility
of 3.5 × 10−4 cm2 V−1 s−1.
bution to EQE and JSC
.
Charge recombination in the devices was studied by meas-
uring JSC as a function of irradiation intensity (Φ) as shown in
Figure 3a (device thickness ≈110 nm). The relationship between
JSC and Φ can be described by JSC ∝ Φα. If all free carriers are
swept out and collected at the electrodes prior to recombina-
tion, α should be equal to 1, while α < 1 indicates relatively
strong bimolecular recombination driven loss of photocurrent.
The current density shows an almost linear dependence of
JSC upon Φ with a slope (α) of 0.98 (≈1) for light intensities
in the range of 1% to 200% of 1 sun equivalent illumination,
(a)
(b) 100
0
80
60
V
J
= 0.78 V
oc
sc
-5
-10
-15
-20
-25
= 25.8 mA cm-2
FF = 0.57
PCE = 11.5%
MoO3/Ag
40
PFBDB-T:CDTTIC
20
ZnO/ITO
0
0.0
0.4
0.8
400 500 600 700 800 900
Voltage (V)
Wavelength (nm)
Figure 2. a) J–V curve and b) EQE spectrum of as-cast devices with the structure ITO/ZnO/PFBDB-T: CDTTIC (1:1, w/w)/MoO3/Ag without any
additional treatments.
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Figure 3. Photocurrent analysis of as-cast PFBDB-T: CDTTIC devices with the active layer thickness d of 110 nm. a) Analysis of JSC as a function of
light intensity Φ, plotted on a log–log scale (Inset), and as the derivative lnJSC/lnΦ versus light intensity (main plot). b) Corrected photocurrent Jph as
a function of applied bias Vapp measured under ≈1 sun irradiation.
The surface morphology of the PFBDB-T: CDTTIC blended
active layer was investigated by atomic force microscopy (AFM)
3. Conclusion
in the tapping mode. The PFBDBT-T: CDTTIC blended film
shows a smooth and uniform surface (Figure S6, Supporting
Information) with a small root-mean-square roughness of
1.81–2.27 nm, which is beneficial to the contact between active
layer and the top electrode. The molecular packing in as-cast
PFBDB-T and CDTTIC neat films as well as PFBDB-T: CDTTIC
blend films was examined through grazing incidence wide-
angle X-ray scattering (GIWAXS) measurements.[41] The 2D
GIWAXS patterns of neat CDTTIC and its blend with PFBDB-
T are shown in Figure 4a–c. For the neat CDTTIC sample,
CDTTIC did not form a film but rather small 3D crystallites
that produce a powder-like diffraction pattern. This suggests
that CDTTIC is quite crystalline, preferring to form 3D crystals
rather than a thin film. The sample annealed at 100 °C is largely
unchanged. Upon blending with the PFBDB-T the pattern is
significantly different, with the main diffraction peaks corre-
sponding to PFBDB-T[14] with perhaps a few diffraction spots
from CDTTIC crystallites. This suggests that CDTTIC is highly
miscible with PFBDB-T; at the very least blending CDTTIC
with PFBDB-T suppresses the crystallization of CDTTIC.
We have developed a ring extension strategy to synthesize
9H-thieno-[3,2-b]thieno[2″,3″:4′,5′]thieno[2′,3′:3,4]cyclopenta[1,2-d]
thiophene (CDTT) in good yields. By incorporation of electron
accepting endgroups a novel nonfullerene acceptor, CDTTIC,
was synthesised. CDTTIC is a low bandgap materials with
a broad absorption profile and a high absorption coefficient.
Photovoltaic devices prepared from blends of CDTTIC with the
donor polymer PFBDB-T exhibited ultrahigh short-circuit cur-
rents up to 26.2 mA cm−2 with efficiencies up to 11.5% for as-
cast films without any treatments, such as processing additives
or thermal annealing. These results indicate that cyclopenta-
dithienothiophene derivatives are promising candidates for the
construction of near-infrared nonfullerene acceptors in OSCs,
and that the ring extension strategy is a useful approach to the
development of low bandgap acceptors.
4. Experimental Section
Synthesis: Unless stated otherwise, solvents and chemicals
were obtained commercially and were used without further purification.
Figure 4. a–c) 2D GIWAXS patterns for CDTTIC, annealed CDTTIC and PFBDB-T: CDTTIC (1:1, w/w) blends.
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ZnO-nanoparticles were purchased from Avantama AG, Switzerland.
PFBDB-T and INCN was synthesized according to the literature
procedure.[14,42]
mixture was cooled to 0 °C, and then saturated NH4Cl solution (5 mL)
and KOH (2 m, 5 mL) were added carefully. The mixture was extracted
with DCM (3 × 30 mL). The combined organics were dried by MgSO4,
filtered and concentrated under reduced pressure. The residue was
purified by silica gel chromatography (eluent: DCM) to afford a yellow
4,4-Dioctyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (2)
To a suspension of 4H-cyclopenta[2,1-b:3,4-b′]dithiophene (1, 868 mg,
4.87 mmol) in anhydrous DMSO (30 mL) was added sodium tert-
butoxide (1.4 g, 14.6 mmol) in parts. The reaction mixture was heated
at 60 °C for 1 h, followed by the addition of 1-bromooctane (2.82 g,
14.6 mmol) dropwise. After complete addition, the resultant mixture
was heated at 60 °C for 5 h, then poured into ice-water. The precipitate
was collected by filtration and washed with water. This was purified by
column chromatography on silica, eluting with petroleum ether, to give
a pale-yellow solid (1.87 g, 95%). 1H NMR (400 MHz, CDCl3, δ): 7.18
(d, J = 8.0 Hz, 2H), 6.99 (d, J = 8.0 Hz, 2H), 1.91–1.87 (m, 4H),
1.42–0.98 (m, 24H), 0.96–0.88 (m, 6H). 13C NMR (100 MHz, CDCl3, δ):
158.25, 136.67, 124.56, 121.75, 53.43, 37.97, 32.02, 30.24, 29.55, 29.44,
24.72, 22.82, 14.27. MS (GC-MS) m/z: 402.3 (M+).
1
compound (310 mg, yield: 82.6%). H NMR (400 MHz, CDCl3, δ): 7.18
(s, 2H), 4.85 (s, 4H), 2.09–2.05 (m, 4H), 2.02 (s, 2H), 1.33–0.63 (m,
30H). 13C NMR (100 MHz, CDCl3, δ): 147.45, 144.02, 139.36, 138.78,
133.56, 118.92, 60.88, 54.11, 36.88, 31.82, 29.90, 29.27, 29.24, 24.48,
22.67, 14.14. MS (MALDI-TOF) m/z: [M + H]+ calculated for C31H42O2S4,
574.92; found, 574.9.
9,9-Dioctyl-9H-thieno[3,2-b]thieno[2″,3″:4′,5′]thieno[2′,3′:3,4]-
cyclopenta[1,2-d]thiophene-2,7-dicarbaldehyde (7)
A solution of compound 6 (310 mg, 0.54 mmol) in dry DCM (10 mL)
was added dropwise into the solution of Dess–Martin periodinane
(687 mg, 1.62 mmol) in dry DCM (20 mL) at 0 °C under argon. After
10 min, the ice bath was removed and the reaction mixture was left
stirring at room temperature for 2 h. Saturated Na2SO3 solution (30 mL)
was added, and the mixture was extracted with DCM (3 × 30 mL). The
combined organics were dried by MgSO4, filtered and evaporated under
reduced pressure. The resulting residue was purified by chromatography
on silica gel (eluent: chloroform/petroleum ether = 1.5: 1) to give
2,6-Dibromo-4,4-dioctyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (3)
To a solution of compound 2 (1.6 g, 3.98 mmol) in chloroform
was added N-bromosuccinimide (1.56 g, 8.75 mmol) at room
temperature. The mixture was stirred for 12 h in the absence of light
at room temperature, then poured into water. The organics were
extracted with chloroform (3 × 30 mL) and the combined solution was
concentrated under vacuum. The pure compound was obtained by
column chromatography over silica eluting with petroleum ether (1.83 g,
1
yellow-orange solid (246 g, yield: 80%). H NMR (400 MHz, CDCl3, δ):
9.97 (s, 2H), 8.00 (s, 2H), 2.12–2.07 (m, 4H), 1.23–0.72 (m, 30H). 13C
NMR (100 MHz, CDCl3, δ): 182.93, 149.64, 145.04, 144.39, 141.68,
139.58, 130.15, 54.85, 36.74, 31.84, 29.76, 29.24, 24.52, 22.74, 14.21. MS
(MALDI-TOF) m/z: [M + H]+ calculated for C31H38O2S4, 570.88; found,
570.8.
1
82%). H NMR (400 MHz, CDCl3, δ): 6.95 (s, 2H), 1.81–1.77 (m, 4H),
1.42–0.94 (m, 24H), 0.92–0.87 (m, 6H). 13C NMR (100 MHz, CDCl3, δ):
156.03, 136.50, 124.69, 111.35, 55.17, 37.76, 32.02, 30.17, 29.52, 29.48,
24.64, 22.85, 14.32. MS (GC-MS) m/z: 560.1 (M+).
2,2′-[(9,9-Dioctyl-9H-thieno[3,2-b]thieno[2″,3″:4′,5′]thieno[2′,3′:3,4]
cyclopenta[1,2-d]thiene-2,7-diyl)bis{(Z)methanylylidene[(2Z)-3-oxo-1H-
indene-2,1(3H)-diylidene]}]dipropanedinitrile (CDTTIC)
3,5-Dibromo-4,4-dioctyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-
dicarbaldehyde (4)
To a solution of compound 3 (1.36 g, 2.43 mmol) in THF (25 mL) at
−78 °C was added a solution of lithium diisopropylamide (LDA) (9.7 mL
of a 1 m solution in THF/heptanes/ethylbenzene, 9.7 mmol) dropwise.
The mixture was stirred for 30 min at this temperature, and then
warmed to room temperature for 2 h. After the mixture was cooled to
0 °C, anhydrous DMF (1.2 mL) was added and the reaction stirred at this
temperature for 30 min. Then the mixture was allowed to warm to room
temperature for 1 h and water (30 mL) was added. The organics were
extracted (3 × 30 mL DCM), dried by MgSO4, filtered and concentrated
under reduced pressure. The residue was purified by silica gel
chromatography (eluent: DCM/petroleum ether = 1/1) to afford a yellow
To a mixture of 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile
(340 mg, 1.75 mmol) and compound 7 (200 mg, 0.35 mmol) in CHCl3
(25 mL) under argon was added pyridine (1 mL). After addition, the
mixture was heated to reflux for 12 h. After cooling to room temperature,
the mixture was poured into water (30 mL) and extracted with DCM
(3 × 30 mL). The combined organics were dried by MgSO4, filtered
and concentrated under reduced pressure. The residue was purified by
silica gel chromatography (eluent: DCM/petroleum ether = 2/1) giving
a purple dark solid (252 mg, yield: 78%). 1H NMR (400 MHz, CDCl3, δ):
8.95 (s, 2H), 8.71 (d, J = 8.0 Hz, 2H), 8.17 (s, 2H), 7.97 (d, J = 8.0 Hz,
2H), 7.82–7.72 (m, 4H), 2.38–2.22 (m, 4H), 1.39–0.66 (m, 30H). 13C
NMR (100 MHz, CDCl3, δ): 160.32, 151.57, 148.32, 146.10, 144.02,
140.33, 140.04, 138.17, 137.09, 135.54, 134.78, 125.58, 124.05, 123.32,
114.70, 55.18, 37.07, 31.88, 29.89, 29.74, 29.31, 29.23, 24.59, 22.73,
14.20. MS (MALDI-TOF) m/z: [M + H]+ calculated for C55H46NO2S4,
923.94; found, 923.9. Anal. calculated for C55H46N4O2S4: C, 71.55;
H, 5.02; N, 6.07; found: C, 71.47; H, 4.96; N, 6.13.
1
solid (0.98 g, yield: 65%). H NMR (400 MHz, CDCl3, δ): 10.02 (s, 2H),
2.41–2.37 (m, 4H), 1.39–1.01 (m, 24H), 0.98–0.74 (m, 6H). 13C NMR
(100 MHz, CDCl3, δ): 183.00, 157.33, 144.61, 139.68, 115.19, 58.81,
34.16, 31.93, 29.61, 29.27, 29.22, 24.07, 22.77, 14.24. MS (MALDI-TOF)
m/z: [M + H]+ calculated for C27H36Br2O2S2, 616.51; found, 616.5.
Diethyl 9,9-dioctyl-9H-thieno[3,2-b]thieno[2″,3″:4′,5′]thieno[2′,3′:3,4]
cyclopenta[1,2-d]thiophene-2,7-dicarboxylate (5)
Ethyl mercaptoacetate (0.45 mL, 3.96 mmol) was added dropwise to
a mixture of compound 4 (610 mg, 0.99 mmol) and K2CO3 (820 mg,
5.94 mmol) in DMF (25 mL) at 50 °C under argon. After addition, the
mixture was stirred at this temperature for 24 h, and then poured into
water (30 mL) and extracted with DCM (3 × 30 mL). The combined
organics were dried by MgSO4, filtered and concentrated under reduced
pressure. The residue was purified by silica gel chromatography (eluent:
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
1
DCM/hexane = 2/3) to afford a yellow solid (430 mg, yield: 65.9%). H
Acknowledgements
NMR (400 MHz, CDCl3, δ): 8.05 (s, 2H), 4.43–4.38 (m, 4H), 2.09–2.05
(m, 4H), 1.47–1.36 (m, 6H), 1.35–0.71 (m, 30H). 13C NMR (100 MHz,
CDCl3, δ): 162.68, 148.73, 143.13, 140.72, 137.98, 133.51, 126.91, 61.59,
54.56, 41.55, 36.89, 31.90, 29.83, 29.27, 27.87, 24.45, 22.81, 22.77, 20.64,
18.96, 14.61, 14.51, 14.22, 11.62. MS (MALDI-TOF) m/z: [M + H]+
calculated for C35H46OS4, 658.99; found, 658.9.
The authors thank the China Scholarship Council (CSC) via the CSC
Imperial Scholarship and the Royal Society and the Wolfson Foundation
(for Royal Society Wolfson Fellowship). This work was performed in part
at the SAXS/WAXS beamline,[41] part of ANSTO.
(9,9-Dioctyl-9H-thieno[3,2-b]thieno[2″,3″:4′,5′]thieno[2′,3′:3,4]
cyclopenta[1,2-d]thiene-2,7-diyl)dimethanol (6)
A solution of LiAiH4 (3.9 mL of a 1 m solution in THF, 3.9 mmol) was
added dropwise into compound 5 (430 mg, 0.65 mmol) in THF (20 mL)
at room temperature. After stirring at this temperature for 5 h, the
Conflict of Interest
The authors declare no conflict of interest.
©
Adv. Funct. Mater. 2019, 1904956
1904956 (6 of 7)
2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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[21] X. Song, N. Gasparini, L. Ye, H. Yao, J. Hou, H. Ade, D. Baran,
ACS Energy Lett. 2018, 3, 669.
Keywords
aromatic extension, conjugated polymers, cyclopentadithienothiophene,
organic solar cells, small molecule acceptor
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©
Adv. Funct. Mater. 2019, 1904956
1904956 (7 of 7)
2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim