Synthesis and characterization of new asymmetric thieno[3,4-b]pyrazine-based D?π?A?A type small molecular donors with near-infrared absorption and their photovoltaic applications

Article abstract of DOI:10.1016/j.orgel.2019.02.013

Two new asymmetric D?π?A1?A2 configured small molecules named PTP and TTP bearing thieno[3,4-b]pyrazine (TP) unit as the A1 group have been designed and synthesized for solution-processed bulk-heterojunction organic solar cells (OSCs). These new dipolar molecular donors not only exhibit strong near-IR absorption but pack into antiparallel dimers, in which PTP shows a coexistence of coplanar and cofacial antiparallel dimers configuration, while TTP reveals a coplanar centrosymmetric dimer structure. Solution-processed OSCs for PTP blended with pristine C₇₀ achieves a promising power conversion efficiency (PCE) of 3.40% along with an open-circuit voltage (V_OC) of 0.64 V, a short-circuit current density (J_SC) of 9.83 mA cm^?2 and a fill factor (FF) of 54.0%, which is the highest efficiency to date among the TP-based materials. Conversely, the optimized TTP:C₇₀ solar cell delivers a V_OC of 0.48 V, a J_SC of 6.02 mA cm^?2 and a FF of 43.3%, corresponding to a moderate PCE of 1.25%.

Full text of DOI:10.1016/j.orgel.2019.02.013

Accepted Manuscript
Synthesis and characterization of new asymmetric thieno[3,4-b]pyrazine-based D−π
−A−A type small molecular donors with near-infrared absorption and their photovoltaic
applications
Shu-Hua Chou, Hsieh-Chih Chen, Chun-Kai Wang, Chin-Lung Chung, Chieh-Ming
Hung, Jung-Chun Hsu, Ken-Tsung Wong
PII:
S1566-1199(19)30076-X
DOI:
https://doi.org/10.1016/j.orgel.2019.02.013
ORGELE 5119
Reference:
To appear in: Organic Electronics
Received Date: 17 November 2018
Revised Date: 3 February 2019
Accepted Date: 8 February 2019
Please cite this article as: S.-H. Chou, H.-C. Chen, C.-K. Wang, C.-L. Chung, C.-M. Hung, J.-C. Hsu,
K.-T. Wong, Synthesis and characterization of new asymmetric thieno[3,4-b]pyrazine-based D−π−A−A
type small molecular donors with near-infrared absorption and their photovoltaic applications, Organic
Electronics (2019), doi: https://doi.org/10.1016/j.orgel.2019.02.013.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Synthesis and characterization of new asymmetric thieno[3,4-b]pyrazine-
based D−π−A−A type small molecular donors with near-infrared absorption
and their photovoltaic applications
Shu-Hua Choua, Hsieh-Chih Chenb,*, Chun-Kai Wanga, Chin-Lung Chunga, Chieh-Ming Hungb,
Jung-Chun Hsub, Ken-Tsung Wonga,c,*
a. Department of Chemistry, National Taiwan University, Taipei 106, Taiwan. Email:
kenwong@ntu.edu.tw
b. Department of Fiber and Composite Materials, Feng Chia University, Taichung 407, Taiwan.
Email: hchihchen@fcu.edu.tw
c. Institute of Atomic and Molecular Science Academia Sinica, Taipei 106, Taiwan.
Two new asymmetric D−π−A1−A2 configured small molecules named PTP and TTP bearing
thieno[3,4-b]pyrazine (TP) unit as the A1 group have been designed and synthesized for
solution-processed bulk-heterojunction organic solar cells (OSCs). These new dipolar molecular
donors not only exhibit strong near-IR absorption but pack into antiparallel dimers, in which
PTP shows a coexistence of coplanar and cofacial antiparallel dimers configuration, while TTP
reveals a coplanar centrosymmetric dimer structure. Solution-processed OSCs for PTP blended
with pristine C70 achieves a promising power conversion efficiency (PCE) of 3.40% along with
an open-circuit voltage (VOC) of 0.64 V, a short-circuit current density (JSC) of 9.83 mA cm-2 and
a fill factor (FF) of 54.0%, which is the highest efficiency to date among the TP-based materials.
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Conversely, the optimized TTP:C70 solar cell delivers a VOC of 0.48 V, a JSC of 6.02 mA cm-2
and a FF of 43.3%, corresponding to a moderate PCE of 1.25%.
Introduction
Bulk-heterojunction (BHJ) organic solar cells (OSCs) offer a promising option for next
generation photovoltaics with the merits of light weight, mechanical flexibility, color-tunable
feature, solution-processability and low-cost fabrication [1-10]. Small molecules in solution-
processed BHJ OSCs in comparison with the polymeric counterparts have some superior
advantages including easy modification of the molecular skeletons for fine-tuning of absorption
range and molecular energy levels, a well-defined molecular structure easy for purification
together with good batch reproducibility, that render highly reproducible device characteristics
[11-14]. For solution processability and light-harvesting capability considerations, the p-type
small molecule donors are typically equipped with relatively long π−conjugated cores and
decorated with long alkyl side chains. To date, p-type small molecules with power conversion
efficiencies (PCEs) in excess of 10% have been successfully developed in OSCs [15-17].
However, the tedious synthetic efforts render the cost-effective production of OSCs still
challenging. Small molecules with strong dipolar feature are also feasible for long wavelength
absorption. But, the PCEs of solution-processed OSCs employing small molecules with strong
dipolar property remain low, probably owing to the poor film quality, low exciton dissociation
probability and relatively high non-radiative recombination [18,19]. Therefore, substantial efforts
should be dedicated to design promising dipolar small molecules as well as develop both the
processing strategies and device architectures to boost the device efficiencies.
To create strong molecular dipole, molecule consisting of electron-rich donor (D) and
electron-withdrawing acceptor (A) moieties as the conjugated constituents is essential. This D−A
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hybridization strategy benefits to lower optical energy gap [1-10]. Particularly, dipolar molecules
with push-pull D−A configuration together with high quinoidal mesomeric structures over the
conjugated skeleton are substantially active for spectral response towards longer wavelength
[20]. Recently, the electron-deficient bicyclic heteroarene of thieno[3,4-b]pyrazine (TP) has
emerged as an attractive building block for organic field effect transistors (OFETs) [21] and
OSCs [22]. TP can perform a high propensity of quinoidal character along the conjugated
backbone with the aid of resonance stabilization from a six-membered aromatic sextet
(pyrazine). This feature is beneficial for enhancing the intramolecular D−A electronic coupling
and eventually reducing the optical energy gap of the corresponding conjugated materials [23].
In addition, various alkyl and/or aryl groups can be introduced onto 2 and 3 positions of TP to
further fine tune the physical and electronic properties. Previously, BHJ OSCs employing TP-
based polymer has achieved a PCE of 2.37% [24], whereas vacuum-processed OSC with TP-
based small molecule only delivered a PCE of 1.3% [25]. Therefore, it still has room to uplift the
PCE of OSCs utilizing TP-containing small molecules. From reported literatures, we found the
majority of TP-based conjugated systems for OSCs are symmetric which were derived from the
double cross-couplings at 5 and 7 positions of TP moiety. In this work, we report herein a rarely
explored asymmetric TP-embedded bipolar system, which was inspired from our previous works
of highly polar small molecule donors with electron deficient heteroarenes like pyrimidine or
benzothiadiazole as the conjugated bridge to link electron-donating triarylamine and electron-
accepting dicyanovinylene. This molecular architecture not only can rationally modulate the
energy levels but also dictate panchromatic light-harvesting ability, leading to high performing
OSCs [26-29].
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In this study, we have synthesized and fully characterized two newly bipolar small molecules,
namely PTP and TTP, configure with D−π−A1−A2 architecture, in which the ditolylamine (D)
and dicyanovinylene (A2) are bridged by 5-phenyl-2,3-dimethylthienopyrazine or 5- thiophenyl-
2,3-dimethylthienopyrazine (π−A1), respectively (Scheme 1). Furthermore, methyl groups are
introduced as the protecting groups of TP moiety to supress the close molecular packing in the
solid-state and also improve the photostability. The electron deficiency together with quinoidal
propensity of TP unit allow the resulting molecules performing strong intramolecular charge
transfer (ICT) character, and thus imparting long wavelenght absorption. Additionally, the
inherent bipolar nature induces both PTP and TTP molecules to self-assemble into
centrosymmetric dimers as explicated by X-ray analysis, that can effectively cancel molecular
dipole moments in the solid state, and thus elude the carrier traps which has been considered to
be detrimental to the charge carrier transport. PTP and TTP blended with pristine C70 delivered
conspicuous PCEs of 3.40% and 1.25%, respectively, in solution-processed small-molecule
OSCs.
Results and discussion
The synthetic routes for PTP and TTP are depicted in Scheme 1. It is noteworthy that
incorporation of trimethylsilyl groups on 1a and 1b can stabilize the TP-derivatives, preventing
the decomposition and/or self-coupling during the follow-up aryl-aryl coupling reactions. On
ther other hand, 1a instead of 1b was used for phenyl-bridged Stille cross-coupling to inhibit
self-coupling of TP moiety owing to the relatively low yield for preparing stanylated benzene
and lower reactivity of stanylated benzene as compared to thiophene. The freshly prepared
stannylated-TP intermediate 1a underwent a Stille cross-coupling with bromotriarylamine and
followed by the work-up with potassium fluoride aqueous solutions and then chromatography
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purification on silica gel afforded the desilylated compound 2a. Stille cross-coupling of freshly
prepared N,N-di-p-tolyl-5-(tributylstannyl)thiophen-2-amine and 1b, and subsequently
desilylation under the basic condition (K2CO3) in a mixed solution of THF and methanol gave
compound 2b. Deprotonation conducted on 2a-b at -100 °C with n-BuLi and followed by
quenching with ethyl formate gave the carbaldehyde compounds 3a-b. The lower temperature
for formylation were conducted compared to common conditions at -78 °C to prevent side
reactions due to the high reactivity of anions of TP moieties with electron-donating group. Final
Knoevenagel condensation of the resulting carbaldehydes 3a-b with malononitrile in the
presence of triethylamine furnished targets PTP and TTP in good yields of 85% and 87%,
respectively.
Scheme 1. Syntheses of PTP and TTP. Reagents and conditions: (I) (1) Pd(PPh3)2Cl2, copper
iodide, THF, 70 °C, 48 h; (2) KF(aq), r.t., 12 h; (II) (1) Pd(PPh3)2Cl2, copper iodide, THF, 70 °C,
16 h; (2) K2CO3, methanol, THF, r.t., 12 h; (III) (1) n-BuLi, THF, -100 °C, 0.5 h; (2) ethyl
formate, -100 °C, 0.5 h; (3) HCl, r.t., 2 h; (IV) malononitrile, triethylamine, CH2Cl2, r.t., 1 h.
The thermal stability of these small molecules was investigated by thermal gravimetric
analysis (TGA) under a nitrogen atmosphere at a heating rate of 10 °C min-1. The decomposition
temperatures (Td) of PTP and TTP at 5% weight loss are 284 °C and 260 °C, respectively (Fig.
1a), which indicates that the thermal stability of the study small-molecules is adequate for
application in optoelectronic devices. Cyclic voltammetry (CV) of PTP and TTP was recorded
in THF and CH2Cl2 solution to determine the reduction (Ered) and oxidation (Eox) potentials,
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respectively (Fig. 1b). The electrochemical data are summarized in Table S1, Supporting
Information. Two one-electron reduction processes were found in PTP (-1.18 and -1.73 V) and
TTP (-1.11 and -1.70 V), which are assigned to the reduction occuring at TP moiety and
dicyanovinylene, respectively. In addition, both TP-based materials display a reversible one-
electron oxidation potential because of the electron-rich triarylamines. TTP exhibits a lower
(∆Eox = 220 mV) oxidation potetial at 0.21 V (i.e. EHOMO, sol = -5.01 eV) as compared to that of
PTP at 0.43 V (i.e. EHOMO, sol = -5.23 eV), which can be rationalized as the fortified electronic
coupling of the HOMO energy level between the TP and electron-rich thiophene spacer. This
result qualitatively matchs the HOMO energy levels observed in thin films (EHOMO, film) by
ultraviolet photoelectron spectroscopy (UPS), in which a lower-lying HOMO energy level was
obtained for PTP (EHOMO, film = -5.32 eV) as compared to that of TTP (EHOMO, film = -5.08 eV).
Fig. 1. (a) TGA traces of PTP and TTP small molecules. (b) Cyclic voltammograms of PTP and
TTP in solution at a scan rate of 100 mV s-1.
The UV–vis absorption spectra of PTP and TTP in dichloromethane solution and thin films
are shown in Fig. 2a. The short-wavelength absorption bands from 310 to 450 nm are ascribed to
the π–π* electronic transitions, while the long-wavelength absorption bands in the range of 500
to 1000 nm are assigned to the ICT bands due to interactions between D and A unit of the
conjugated backbone. In comparison with phenyl-bridged PTP (λmax, sol = 612 nm, ε = 41600 M-1
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cm-1), the thiophene-bridged TTP (λmax, sol = 700 nm, ε = 62600 M-1 cm-1) exhibits a strong red-
shiftλmax together with the higher extinction coefficient (ε), stemming from the increased ICT
character imparted by the quinoidal feature of thiophene. PTP shows a significantly red-shifted
absorption in the solid state (λmax, film = 637 nm) compared to that in solution (λmax, sol = 612 nm),
suggesting that PTP molecules exhibit significant intermolecular interactions in thin film. In
contrast, the optical-shift is less pronounced for TTP film, in which a vibronic shoulder of the
broadening band emerged near ~810 nm is likely ascribed to the higher structural organization in
the solid state [30].
We further investigated the influence of the π-bridge on the electronic transitions of PTP and
TTP by theoretical TD-DFT calculations B3LYP/6-31G(d,p) level in dichloromethane. As
depicted in Fig. 2b,c, the LUMO energy levels of PTP and TTP are mainly localized at the
electron-deficient thienopyrazine-dicyanovinylene units. Meanwhile, the HOMO energy level of
PTP exhibits a greater population on the electron-donating ditolylamine group, whereas the
HOMO energy level of TTP is delocalized along the molecular backbone with a substantial
population on the TP moiety. This result agrees well with the observed data in CV and UV–vis
absorption experiments. It is clearly demonstrated that the replacement of the phenylene spacer
by the thiophene bridge modulated not only the HOMO energy level but also the LUMO energy
level, resulting in the decrease of the optical energy gap due to the strong and intrinsic ICT
character between TP and thiophene π-spacer. It has been reported that parent TP moiety
underwent rapid photodegradations not only in the solution but also in the solid-state when
exposed to the air and sunlight at the same time [25,31]. We tried to attach the methyl
substituents as protecting groups to the 2 and 3 positions of TP moiety to enhance the
photostability which was proven by the same UV-vis absorption spectra of PTP and TTP in
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dichloromethane solution with various exposed times under ambient atmosphere (see Fig. S1,
Supporting Information). The conformation and intermolecular packing of PTP and TTP were
determined via single crystal structures. PTP forms a coplanar conformation along the
conjugated backbone with dihedral angles of 6.6° and 7.4° between the benzene and thienyl ring
of TP moiety (Fig. 2d), whereas TTP exhibits a slightly twisted conformation along the
conjugated backbone with dihedral angles of 6.2° and 11.2° (Fig. 2e). In addition, both of PTP
and TTP stack into antiparallel dimers owing to the highly polar nature of them. It is noteworthy
that the cofacial and coplanar antiparallel dimers are coexisted in the PTP packing motif, while
there are only coplanar antiparallel dimers in the TTP packing motif, demonstrating that PTP
possesses superior intermolecular interactions in the solid state and has potential for better
charge carrier transport in OSCs.
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Fig. 2. (a) Absorption spectra of PTP and TTP in CH2Cl2 and as thin films. Isodensity surface
plots of the lowest energy transitions for (b) PTP and (c) TTP energy levels. Single crystal X-
ray structure analysis for (d) PTP and (e) TTP packing motif.
Fig. S2a depicts a sketch of the energy levels of the OPV device architecture consisting of
indium tin oxide (ITO)/CuSCN/small molecule:C70/C70/Al. in which CuSCN was employed as a
hole-transporting layer due to its high optical transparency, high work function, high
conductivity and ease of solution processability [2]. A thin C70 layer was evaporated to form
ohmic contact, which is desired for efficient charge carrier extraction and transport, and thereby
reduce the recombination loss caused by the accumulation of charge in the interface [28,32]. The
HOMO and LUMO energy levels for both small molecule and C70 are in positions such that hole
injection is favorable toward CuSCN while electron transport is directed toward the Al electrode.
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Various conditions for device fabrication were examined, and the device performances are
summarized in Table 1. The active layers of PTP and TTP were spin-casted from o-DCB
solutions with 1 vol% 1-chloronaphthalene (CN) additives to improve the device performances.
In the optimized device with PTP:C70 (1:1) as the active layer and thermal annealing at 120 °C
for 10 min, the VOC achieves to 0.64 V with a JSC of 9.83 mA cm-2 and a FF of 54.0%, delivering
a PCE of 3.40% (Fig. 3a). In contrast, device based on the optimized TTP:C70 (1:2) blend
exhibits only an inferior PCE of 1.25%, with a VOC of 0.48 V, a JSC of 6.02 mA cm-2 and a FF of
43.3% (Fig. 3b). Surprisingly, TTP with a thiophene spacer exhibited distinct red-shifted
absorption spectra, and yet, offered not only a lower VOC but also an inferior JSC. The hole
mobility of PTP and TTP measured using the space-charge-limited current (SCLC) model is 8.1
× 10-6 and 1.7 × 10-6 cm2 V-1 s-1, respectively (Fig. S2b, Supporting Information). This fivefold
of magnitude increase in the hole mobility of PTP improves the charge carrier collection
efficiency and partly accounts for the higher FF and JSC values for PTP devices. The
corresponding IPCE spectra of PTP- and TTP-based devices (Fig. 3c,d) indicate the responses
in the visible region can be attributed to the intrinsic absorptions of small molecules and C70.
Regardless of the red-shifted absorption of the TTP-based devices, the highest IPCE does not
surpass 30% and thus results in a poor JSC. Conversely, PTP-based blend displays a rather
panchromatic spectrum over the entire excitation spectral range. The integrated JSC values from
the IPCE spectra are 9.40 and 5.76 mA cm-2 for the champion 1:1 PTP:C70 and 1:2 TTP:C70
devices, respectively, which are within a 5% error, according well with those obtained from the
corresponding J−V measurements and supporting the reliability of the photovoltaic
measurement.
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Fig. 3. Current density versus voltage (J–V) characteristics of optimized devices made from
various blend ratios of a) PTP:C70 and b) TTP:C70 with 1 vol% 1-chloronaphthalene (CN)
adopted as an additive under AM1.5 white light illumination at 100 mW cm-2. Corresponding
IPCE spectra of c) PTP:C70 and d) TTP:C70 devices.
Table 1 Photovoltaic parameters of optimized OSCs.
a
dye
dye:C70
VOC (V)
JSC (mA cm-2)
FF
PCE (%)b
1:1
1:1.5
1:2
1:1
1:1.5
1:2
0.64 ± 0.01
0.62 ± 0.01
0.49 ± 0.01
0.40 ± 0.01
0.45 ± 0.01
0.48 ± 0.01
9.83 ± 0.12
9.17 ± 0.20
6.77 ± 0.20
3.74 ± 0.10
4.47 ± 0.10
6.02 ± 0.06
0.540 ± 0.002
0.495 ± 0.005
0.452 ± 0.005
0.322 ± 0.006
0.396 ± 0.005
0.433 ± 0.003
3.40 ± 0.10
2.84 ± 0.18
1.49 ± 0.23
0.48 ± 0.15
0.80 ± 0.15
1.25 ± 0.13
PTP
TTP
aDevice configuration of ITO/CuSCN/dye:C70/C70/Al. bAverage PCE of 20 optimized devices.
The nanoscale microstructures of the active layers were characterized by tapping-mode atomic
force microscopy (TM-AFM). Fig. 4 shows the topographic and phase images of the best
performing PTP:C70 (1:1) and TTP:C70 (1:2) blends. PTP:C70 blend exhibits a coarser and well
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intermixed morphology with a root mean square (RMS) roughness of 2.36 nm (Fig. 4a), whereas
TTP:C70 blend lacks well-defined structural features with RMS~1.12 nm) (Fig. 4b). The better
phase separation and bicontinuous interpenetrating networks could improve the exciton
dissociation probability and charge transport efficiency. These results are in good agreement
with the higher JSC and FF for the PTP:C70-based OSCs. Therefore, the diverse domain sizes and
morphological features suggest the nature of π-bridge would give rise to the different miscibility
and crystallinity.
Fig. 4. Morphology characterization of TM-AFM topography images (upper row) and phase
images (lower row) of 1:1 PTP:C70 blend film (panels a and c) and 1:2 TTP:C70 blend film
(panels b and d) with 1% CN. The imaging size is 1 µm × 1 µm for each panel.
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Fig. 5. 2D-GIWAXS patterns of a) pristine PTP, b) pristine TTP, c) 1:1 PTP:C70 blend and d)
1:2 TTP:C70 blend films. e) 1D line-cuts of GIWAXS patterns for neat and blend films.
The crystallinity and nanostructural order within the pristine and blend films were conducted
through grazing incidence wide angle X-ray scattering (GIWAXS, Fig. 5). Both the neat films
exhibit similar preferable face-on molecular packing orientations with (100) lamellar stacking
peaks in the in-plane direction and (010) π–π stacking peaks in the out-of-plane direction at q =
1.17 Å-1 and 1.94 Å-1 (d100 spacing = 5.37 Å and d010 spacing = 3.24 Å) for PTP molecule and q
= 1.16 Å-1 and 1.94 Å-1 (d100 spacing = 5.42 Å and d010 spacing = 3.24 Å) for TTP molecule. All
of the peaks and the corresponding d spacing values are listed in Table 2. Additionally, the (100)
lamellar peaks and (010) π–π stacking peaks in the blend films are weaker than those of the neat
films, indicating the addition of C70 disrupts the crystallization of PTP and TTP molecules.
Nevertheless, the PTP blend possesses a stronger in-plane inter-lamellar scattering peak and a
more intense (010) π–π stacking peak in the out-of-plane direction as compared with the TTP
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blend, which is beneficial for improving charge transport and has often correlated with high
performing OSCs.
Table 2 Summary of packing parameters derived from GIWAXS measurements.
Lamellar stacking, d100
π–π stacking, d010
Materials
Scattering vector (q) d-spacing Scattering vector (q) d-spacing
(Å-1)
1.17
1.16
1.16
1.15
(Å)
5.37
5.42
5.42
5.46
(Å-1)
1.94
1.94
1.93
1.92
(Å)
3.24
3.24
3.26
3.27
Pristine PTP
Pristine TTP
1:1 PTP:C70
1:2 TTP:C70
Conclusions
−A1−A2 configuration,
In conclusion, we report two new dipolar donors PTP and TTP with D−π
in which thieno[3,4-b]pyrazine (TP) was introduced as the A1 moiety. The physical
characterizations together with theoretical analysis indicate that the HOMO energy level is
higher as phenylene was replaced with thiophene as π spacer. Nevertheless, the high quinoidal
character of TP allows them to have panchromatic light-harvesting abilities. The highly polar
features lead these molecules to pack into antiparallel dimers, in which the coexistence of
coplanar and cofacial antiparallel dimers were disclosed for PTP, and the coplanar
centrosymmetric dimer was found for TTP. The solution-processed OSCs made from PTP and
TTP blended with pristine C70 exhibited promising PCEs of 3.40% and 1.25%, respectively.
Moreover, the superiority of PTP in the OSC performance is in line with the higher hole
mobility, extensive panchromatic spectrum response, better bicontinuous interpenetrating
morphology and stronger (010) π–π stacking peak in the out-of-plane direction. We believe that
the established structure−property-function relationship based on TP-based D−π−A1−A2
structures can trigger new ideas for designing of the small molecule donors for future
optoelectronics.
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Experimental section
Materials
All reactants were reagent grades and purchased commercially without further purifications.
Solvents for syntheses were freshly distilled according to standard procedures before used. All
reactions were carried out under an atmosphere of nitrogen or argon. C70 was purchased from
Nano-C and was purified by temperature- gradient sublimation before use in this study.
Synthetic procedures
4-(2,3-Dimethylthieno[3,4-b]pyrazin-5-yl)-N,N-di-p-tolylaniline (2a)
n-BuLi (1.6 M, 7 mL, 11.2 mmol) was added dropwise to a solution of 2,3-dimethyl-5-
(trimethyl-silyl)thieno[3,4-b]pyrazine (2.5 g, 10.6 mmol) in anhydrous THF (55 mL) cooled to -
78 °C under Ar atmosphere. The reaction was stirred at this temperature for 0.5 h before the
addition of tributyltin chloride (3.1 mL, 10 mmol) and then slowly warm back to the room
temperature. The resulting mixture was kept stirring at room temperature for 9 h, then quenched
with H2O and extracted with diethyl ether. The combined organic phase was washed with H2O
and brine, dried with anhydrous MgSO4, and evaporated under reduced pressure to obtain freshly
prepared tin reagent 2,3-dimethyl-5-(tributylstannyl)-7-(trimethylsilyl)thieno[3,4-b]pyrazine
(1a), which was directly used in the following Stille reaction. A mixture of stannyl derivative 1a
(10.6 mmol), 4-bromo-N,N-di-p-tolylaniline (4.1 g, 11.6 mmol), CuI (162 mg, 0.85 mmol) and
Pd(PPh3)2Cl2 (600 mg, 0.85 mmol) in anhydrous THF (70 mL) was heated to reflux under Ar
atmosphere for 48 h. After cooling, the resulting mixture was dried in vacuo, diluted with
CH2Cl2, mixed with concentrated KF aqueous solution and stirred overnight at room
temperature. Water was poured into the flask and the mixture was extracted with CH2Cl2. The
organic phase was dried over MgSO4 and the solvent was removed by rotary evaporation. The
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residue was purified by column chromatography on silica gel (gradient eluent: CH2Cl2/hexane =
1/6 to 1/2) to afford 2a as a deep-orange solid (2.2 g, 48%). M. p. 172–173 °C; IR (KBr) ν 3026,
1
2998, 2918, 2855, 1598, 1507, 1322, 1276, 814, 750; H NMR (Acetone-d6, 400 MHz) δ 8.09
(dd, J = 6.8, 2.0 Hz, 2H), 7.73 (s, 1H), 7.15 (m, 4H), 7.02 (m, 6H), 2.62 (s, 3H), 2.58 (s, 3H),
2.32 (s, 6H); 13C NMR (CD2Cl2, 100 MHz) δ 153.8, 152.8, 148.1, 145.2, 143.7, 137.5, 133.5,
130.3, 128.8, 127.0, 125.3, 122.2, 112.0, 112.0, 24.2, 24.0, 21.2; HRMS (m/z, FAB+) calcd. for
C28H25N3S 435.1769, found 435.1767.
5-(2,3-Dimethyl-thieno-[3,4-b]pyrazin-5-yl)-N,N-di-p-tolyl-thiophen-2-amine (2b)
n-BuLi (1.6 M, 5.9 mL, 9.4 mmol) was added dropwise to a solution of N,N-di-p-tolylthiophen-
2-amine (2.43 g, 8.7 mmol) in anhydrous THF (40 mL) cooled to -78 °C under Ar atmosphere.
The reaction was stirred at this temperature for 0.5 h before the addition of tributyltin chloride
(2.8 mL, 10 mmol) and the cooling bath was removed. The resulting mixture was kept stirring at
room temperature for 3 h, then quenched with H2O and extracted with diethyl ether. The
combined organic phase was washed with H2O and brine, dried with anhydrous MgSO4, and
evaporated under reduced pressure to obtain freshly prepared N,N-di-p-tolyl-5-(tributylstannyl)-
thiophen-2-amine which was directly used in the following Stille reaction. A mixture of stannyl
derivative (8.7 mmol), 1b (2.5 g, 7.9 mmol), CuI (120 mg, 0.63 mmol) and Pd(PPh3)2Cl2 (445
mg, 0.63 mmol) in anhydrous THF (90 mL) was heated to reflux under Ar atmosphere for 16 h.
After cooling, a mixed solution of K2CO3 (4.36 g, 31.6 mmol) in MeOH (40 mL) and THF
(40mL) was added to the reaction mixture, and stirred overnight. The resulting mixture was
extracted with ethyl acetate, saturated potassium fluoride aqueous solution and water. The
organic phase was dried over MgSO4 and the solvent was removed by rotary evaporation. The
residue was purified by column chromatography on silica gel (gradient eluent: pure hexane to
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ethyl acetate/hexane = 1/5) to afford 2b as a light red solid (2.8 g, 80%). M. p. 177–178 °C; IR
1
(KBr) ν 3107, 3022, 2947, 2917, 1543, 1506, 1466, 1434, 1274, 1209, 929, 817, 750; H NMR
(CDCl3, 400 MHz) δ 7.45 (s, 1H), 7.42 (d, J = 4.0 Hz, 1H), 7.10 (m, 8H), 6.56 (d, J = 4.0 Hz,
1H), 2.61 (s, 3H), 2.59 (s, 3H), 2.33 (s, 6H); 13C NMR (CDCl3, 100 MHz) δ 153.3, 153.0, 152.0,
145.1, 142.3, 136.5, 132.7, 129.6, 128.5, 127.2, 123.6, 122.9, 118.8, 110.5, 24.4, 24.1, 21.4;
HRMS (m/z, FAB+) calcd. for C26H23N3S2 441.1333, found 441.1339.
7-(4-(Di-p-tolylamino)phenyl)-2,3-dimethylthieno[3,4-b]pyrazine-5-carbaldehyde(3a)
To a stirred solution of 2a (2.2 g, 5.05 mmol) in anhydrous THF (200 mL) at -100 °C was added
dropwise a solution of n-BuLi (1.6 M, 3.3 mL, 5.3 mmol) under Ar atmosphere, and stirred for
another 0.5 h. Anhydrous ethyl formate (4.1 mL, 50.8 mmol) was added to the reaction mixture
and then stirred for 0.5 h at this temperature. The resulting mixture was quenched with 1.5 M
HCl in THF solution (4.5 mL). The cooling bath was removed and the reaction mixture was
stirred for 2 h at room temperature. After addition of water, the mixture was extracted with ethyl
acetate and the combined organic phase was washed with brine and dried over MgSO4. After
removal of solvent under reduced pressure, the crude was purified by silica chromatography
(gradient eluent: CH2Cl2/hexane = 1/1 to 2/1) to yield 3a as a brown solid (1.2 g, 51%). M. p.
193–194 °C; IR (KBr) ν 3021, 2957, 2917, 2840, 1638, 1593, 1508, 1440, 1321, 1267, 1236,
1
1200, 1148, 814, 750; H NMR (CD2Cl2, 400 MHz) δ 10.51 (s, 1H), 8.14 (d, J = 8.8 Hz, 2H),
7.15 (d, J = 8.0 Hz, 4H), 7.06 (d, J = 8.4 Hz, 4H), 7.00 (d, J = 8.8 Hz, 2H), 2.66 (s, 3H), 2.65 (s,
3H), 2.35 (s, 6H); 13C NMR (CD2Cl2, 100 MHz) δ 180.2, 156.7, 153.5, 149.8, 146.8, 145.8,
144.0, 138.1, 134.2, 130.1, 129.4, 125.8, 124.5, 120.1, 24.4, 24.2, 21.4; HRMS (m/z, FAB+)
calcd. for C29H25N3OS 463.1718, found 463.1716.
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7-(5-(Di-p-tolylamino)thiophen-2-yl)-2,3-dimethylthieno[3,4-b]pyrazine-5-carbal-dehyde
(3b)
To a stirred solution of 2b (1.45 g, 3.28 mmol) in anhydrous THF (130 mL) at -100 °C was
added dropwise a solution of n-BuLi (1.6 M, 2.1 mL, 3.36 mmol) under Ar atmosphere, and
stirred for another 0.5 h. Anhydrous ethyl formate (2.7 mL, 33.4 mmol) was added to the
reaction mixture and then stirred for 0.5 h at this temperature. The resulting mixture was
quenched with 1.5 M HCl in THF solution (3 mL). The cooling bath was removed and the
reaction mixture was stirred for 2 h at room temperature. After addition of water, the mixture
was extracted with ethyl acetate and the combined organic phase was washed with brine and
dried over MgSO4. After removal of solvent under reduced pressure, the crude was purified by
silica chromatography (gradient eluent: CH2Cl2/hexane = 2/1 to ethyl acetate/CH2Cl2 = 1/10) to
yield 3b as a deep-blue solid (850 mg, 55%). M. p. 182–183 °C; IR (KBr) ν 3050, 3017, 2914,
1
2834, 1639, 1508, 1433, 1378, 1316, 1265, 1224, 1141, 1055, 816, 768; H NMR (CD2Cl2, 400
MHz) δ 10.41 (s, 1H), 7.75 (d, J = 4.0 Hz, 1H), 7.14 (m, 8H), 6.43 (d, J = 4.4 Hz, 1H), 2.63 (s,
3H), 2.60 (s, 3H), 2.35 (s, 6H); 13C NMR (CD2Cl2, 100 MHz) δ 180.1, 159.4, 157.8, 153.5,
146.9, 144.8, 141.5, 137.6, 135.3, 130.5, 129.3, 124.9, 123.3, 122.5, 115.2, 24.2, 24.0, 21.3;
HRMS (m/z, FAB+) calcd. for C27H23N3OS2 469.1283, found 469.1287.
2-((7-(4-(Di-p-tolylamino)phenyl)-2,3-dimethylthieno[3,4-b]pyrazin-5-yl)-methy-
lene)malononitrile (PTP)
A mixture of aldehyde derivative 3a (400 mg, 0.86 mmol), malononitrile (85 mg, 1.29 mmol)
and few drops of triethylamine in CH2Cl2 (55 mL) was stirred at room temperature for 1 h under
N2 atmosphere. The solvent was evaporated to dryness and purified by silica chromatography
(eluent: CH2Cl2/hexane = 2/1). The solid was reprecipitated twice with hexane/CH2Cl2 and
18

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filtered to ensure the purity for device fabrication. PTP (374 mg, 85%) was obtained as a brown
solid after dried under vacuum overnight. M. p. 212–213 °C; IR (KBr) ν 3029, 2833, 2210, 1592,
1
1542, 1508, 1477, 1438, 1418, 1331, 1297, 1258, 1196, 945, 828, 750; H NMR (CD2Cl2, 400
MHz) δ 8.71 (s, 1H), 8.25 (d, J = 8.8 Hz, 2H), 7.18 (m, 4H), 7.08 (m, 4H), 6.99 (d, J = 9.2 Hz,
2H), 2.68 (s, 3H), 2.67 (s, 3H), 2.36 (s, 6H); 13C NMR (CD2Cl2, 125 MHz) δ 157.7, 155.3, 151.5,
149.0, 148.7, 146.1, 144.2, 138.7, 135.5, 130.8, 130.6, 126.7, 124.3, 120.0, 116.3, 116.0, 70.5,
24.2, 23.9, 21.2; HRMS (ESI, m/z, [M+H]+) calcd. for C32H26N5S 512.1909, found 512.1918;
Anal. calcd. for C32H26N5S: C 75.12; H 4.93; N 13.69, found C 74.77; H 4.76; N 13.55.
2-((7-(5-(Di-p-tolylamino)-thiophen-2-yl)-2,3-dimethylthieno-[3,4-b]-pyrazin-5-
yl)methylene)malononitrile (TTP)
To a mixture of aldehyde derivative 3b (0.65 g, 1.38 mmol) and malononitrile (137 mg, 2.07
mmol) in CH2Cl2 (80 mL) was added few drops of triethylamine and stirred at room temperature
for 1 h under N2 atmosphere. The solvent was evaporated to dryness and purified by silica
chromatography (gradient eluent: CH2Cl2/hexane = 2/1 to pure CH2Cl2). The solid was
reprecipitated twice with hexane/CH2Cl2 and filtered to ensure the purity for device fabrication.
TTP (620 mg, 87%) was obtained as a dark-green solid after dried under vacuum overnight. M.
p. 193–194 °C; IR (KBr) ν 3097, 2927, 2855, 2208, 1555, 1507, 1406, 1381, 1350, 1308, 1258,
1
1180, 1155, 1075, 967, 750; H NMR (CD2Cl2, 400 MHz) δ 8.48 (s, 1H), 7.97 (d, J = 4.4 Hz,
13
1H), 7.21 (m, 8H), 6.40 (d, J = 4.4 Hz, 1H), 2.64 (s, 3H), 2.61 (s, 3H), 2.38 (s, 6H); C NMR
(CDCl3, 100 MHz) δ 163.2, 157.1, 153.4, 148.0, 144.1, 143.7, 143.6, 137.2, 136.3, 133.1, 128.7,
125.4, 121.4, 116.8, 116.5, 113.7, 65.8, 24.0, 23.8, 21.4; HRMS (ESI, m/z, [M+H]+) calcd. for
C30H24N5S2 518.1473, found 518.1475; Anal. calcd. for C30H24N5S2: C 69.60; H 4.48; N 13.53,
found C 69.63; H 4.51; N 13.55.
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Characterization
Nuclear magnetic resonance (NMR) spectroscopy was recorded in deuterochloroform (or other
1
given solvents) as an internal reference using Varian Mercury Plus-400 spectrometer for H
1
NMR, and Bruker AVIII-500 or Varian Mercury Plus-400 spectrometer for 13C NMR. The H
and 13C chemical shifts (δ) were quoted relative to the internal standard, tetramethylsilane. High-
resolution mass spectrometry data were acquired by a JMS-700 spectrometer in FAB mode or a
FINNIGAN LCQ spectrometer for electrospray ionization mass spectrometry (ESI-MS). IR
spectra were obtained on Varian 640-IR. Melting points were determined on a Fargo MP-1D
melting point apparatus. Combustion elemental analyses were analyzed on a Elementar Vario
EL-Cube analyzer. Thermogravimetric analysis (TGA) was taken with a TA instrument 5100
thermal analysis system. The electrochemical properties were performed on a CH Instruments
model CHI619B in a one-compartment three-electrode electrochemical cell equipped with a
glassy carbon working electrode, a platinum wire counter electrode, and an Ag/AgCl reference
electrode. CV experiments were performed at a scan rate of 100 mV s-1 in anhydrous CH2Cl2
(1.0 mM) with 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6) as a supporting
electrolyte for oxidation potentials, and in anhydrous THF (1.0 mM) with 0.1 M tetra-n-
butylammonium perchlorate (TBAP) as a supporting electrolyte for reduction potentials. All
potentials reported were calibrated with the ferrocene/ferrocenium redox couple (Fc/Fc+) as
internal standard. For each molecule, its Eox and Ered was obtained by halving the sum of two
reversible peaks and the HOMO/LUMO energy levels can be estimated according to the formula
EHOMO/LUMO, sol = -e (Eox/red + 4.8) (eV) for the CV measurement. Ultraviolet photoelectron
spectroscopy (UPS) measurements were conducted to further estimate the ionization potentials
of materials in thin films. The thickness of the active films was measured with a depth-profile
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meter (Veeco Dektak 150, USA). Five lines on a film of 1 cm × 1 cm were made by carefully
scratching with a plastic tip and the average height between the hills and valleys is used to
represent the film thickness. UV−vis absorption spectra in anhydrous CH2Cl2 solutions were
acquired with a Jasco V-670 spectrophotometer. Pristine thin-films were prepared by spin-
coating onto pre-cleaned ITO substrates and the absorption spectra were recorded on a
PerkinElmer spectrophotometer. The morphology of polymer film surface was obtained with a
Nanoscope 3D Controller atomic force micrographs (AFM, Digital Instruments) operated in the
tapping mode at room temperature.
Computational methodology
All the molecular conformations and photopysical properties of molecules were performed based
on the density functional theory (DFT) at the B3LYP level with the 6-31G(d,p) basis set using
the Gaussian 09 program package. The S0 geometries of all molecules were fully optimized
without symmetric constraints with solvent effects included by the polarizable continuum model
(PCM) using the dielectric constant of dichloromethane. The time-dependent DFT (TD-DFT)
calculation shows the vertical lowest singlet excitation wavelengths, oscillator strengths and
molecular orbital compositions in terms of corresponding contributions.
Single crystal structures
The needlelike crystal of PTP suitable for X-ray single crystallography analysis was grown from
slow interface mixing of CH2Cl2/Hexane. PTP crystallite had a monoclinic crystal system with a
space group of P21/c. The fibrous tan crystal of TTP suitable for X-ray single crystallography
analysis was grown from slow interface mixing of CH2Cl2/Hexane. TTP crystallite had a
triclinic crystal system with a space group of P-1. Single crystal X-ray data were collected on
Agilent Xcalibur (Atlas, Gemini ultra) diffractometer using graphite-monochromatic radiation
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ACCEPTED MANUSCRIPT
for PTP (Cu-Kα, 1.54178 Å) and TTP (Mo-Kα, 0.71073 Å) at 200 (2) K and 295 (2) K,
respectively. Data and ω-scans were collected with CrysAlis Pro (Agilent) software for all
observed reflections. Cell refinement and data reduction were performed with CrysAlis Pro
(Agilent) software. SHELXS97 (Sheldrick, 2008) was applied for solving structures by direct
methods, and refining structures by full-matrix least-squares techniques on F2. The
crystallographic data for PTP (CCDC: 1045238) and TTP (CCDC: 1045237) can be obtained
from the Cambridge Crystallographic Data Centre.
OSC device fabrication and characterization
The small molecule solar cells were fabricated with the device structure of ITO/CuSCN (40
nm)/dye:C70 (70-100 nm)/C70 (6 nm)/Al (100 nm). Prior to use, patterned ITO-coated glass
substrates (10 Ω/square) were successively cleaned by ultrasonication in 1% neutral detergent in
water, then deionized water, followed by acetone and finally isopropanol for 10 min each and
subsequently dried under a stream of dry nitrogen. The substrates were then underwent UV-
ozone treatment for 10 min. CuSCN (Aldrich) was dissolved in dipropyl sulfide with a
concentration of 20 mg mL-1. The solution was stirred 12 h at room temperature and then
centrifuged to remove undissolved material. Afterwards, the CuSCN solution in an inert N2-filled
glove box (< 0.1 ppm O2 and H2O) was spin-coated on ITO-coated glass substrates at 2000 rpm
for 60 s and heated at 100 °C for 10 min. Blend solutions were prepared by dissolving the target
small molecule (PTP or TTP) and C70 in o-DCB solution in the blend ratios of 1:1, 1:1.5 and 1:2
with a small molecule concentration of 30 mg mL-1 and were heated to 100 °C and stirred 8 h for
complete dissolution. Then the blend solution was spin-coated after adding 1% (v/v) 1-
chloronaphthalene (CN) as additives. The wet film was slowly dried in a covered Petri dish for 3
h in the glove box and then annealed at 120 °C for 10 min. Afterwards, methanol was dropped
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onto the active layer with a spin-coating rate at 2500 rpm for 60 s, and subsequently the coated
substrates were transferred to a thermal evaporator and evacuated to ≤ 5 × 10-6 Torr. C70 was
thermally deposited on top of the active layer with an evaporation rate of 1 Å s-1. Ultimately, the
coated substrates were then transferred to a thermal evaporator and evacuated again to ≤1 × 10−6
Torr, followed by a 100-nm aluminum electrode layer was deposited through a shade mask. The
J–V curves were measured using a Keithley 4200 source-measuring unit. A calibrated solar
simulator (Oriel) with a 100 mW cm-2 power density was used as the light source. An IPCE (QE-
R3011) characterization platform supplied by Enlitech Inc. was used for data acquirement. All
IPCE spectra were recorded using a lock-in technique at a chopping frequency of 100 Hz.
Acknowledgements
This work was financially supported by the Ministry of Science and Technology, Taiwan
(MOST 104-2113-M-002-006-MY3 and MOST 105-2113-M-035-003-MY2).
References
1 L. Lu, T. Zheng, Q. Wu, A. M. Schneider, D. Zhao, L. Yu, Recent advances in bulk
heterojunction polymer solar cells, Chem. Rev. 115 (2015) 12666.
2 P. Pattanasattayavong, N. Yaacobi-Gross, K. Zhao, G. O. Ndjawa, J. Li, F. Yan, B. C.
O'Regan, A. Amassian, T. D. Anthopoulos, Hole transporting transistors and circuits
based on the transparent inorganic semiconductor copper(I) thiocyanate (CuSCN)
processed from solution at room temperature, Adv. Mater. 25 (2013) 1504.
3 C. Lu, H.-C. Chen, W.-T. Chuang, Y.-H. Hsu, W.-C. Chen, P.-T. Chou, Interplay of
molecular orientation, film formation, and optoelectronic properties on isoindigo- and
23

ACCEPTED MANUSCRIPT
thienoisoindigo-based copolymers for organic field effect transistor and organic
photovoltaic applications, Chem. Mater. 27 (2015) 6837.
4 C. Sun, Z. Wu, Z. Hu, J. Xiao, W. Zhao, H.-W. Li, Q.-Y. Li, S.-W. Tsang, Y.-X. Xu, K.
Zhang, H.-L. Yip, J. Hou, F. Huang, Y. Cao, Interface design for high-efficiency non-
fullerene polymer solar cells, Energy Environ. Sci. 10 (2017) 1784.
5 H. Zhou, Y. Zhang, C.-K. Mai, S. D. Collins, G. C. Bazan, T.-Q. Nguyen, A. J. Heeger,
Polymer homo tandem solar cells with best efficiency of 11.3%, Adv. Mater. 27 (2015)
1767.
6 I. Osaka, T. Kakara, N. Takemura, T. Koganezawa, K. Takimiya, Naphthodithiophene–
naphthobisthiadiazole copolymers for solar cells: alkylation drives the polymer backbone
flat and promotes efficiency, J. Am. Chem. Soc. 135 (2013) 8834.
7 Z. Zhang, F. Lin, H.-C. Chen, H.-C. Wu, C.-L. Chung, C. Lu, S.-H. Liu, S.-H. Tung, W.-
C. Chen, K.-T. Wong, P.-T. Chou, A silole copolymer containing a ladder-type
heptacylic arene and naphthobisoxadiazole moieties for highly efficient polymer solar
cells, Energy Environ. Sci. 8 (2015) 552.
8 C. E. Small, S. Chen, J. Subbiah, C. M. Amb, S.-W. Tsang, T.-H. Lai, J. R. Reynolds, F.
So, High-efficiency inverted dithienogermole–thienopyrrolodione-based polymer solar
cells, Nat. Photonics
6 (2012) 115.
9 J. J Intemann, K. Yao, Y. X. Li, H. L. Yip, Y. X. Xu, P. W. Liang, C. C. Chueh,
F. Z. Ding, X. Yang, X. Li, Y. Chen, A. K Y Jen, Highly efficient inverted organic
solar cells through material and interfacial engineering of indacenodithieno[3,2-
b]thiophene-based polymers and devices, Adv. Funct. Mater. 24 (2014) 1465.
24

ACCEPTED MANUSCRIPT
10H.-C. Chen, Y.-H. Chen, C.-C. Liu, Y.-C. Chien, S.-W. Chou, P.-T. Chou, Prominent
short-circuit currents of fluorinated quinoxaline-based copolymer solar cells with a power
conversion efficiency of 8.0%, Chem. Mater. 24 (2012) 4766.
11I. Ata, D. Popovic, M. Linden, A. Mishra, P. Bauerle, The influence of the central
acceptor unit on the optoelectronic properties and photovoltaic performance of A–D–A–
D–A-type co-oligomers, Org. Chem. Front. 4 (2017) 755.
12B. Walker, C. Kim, T.-Q. Nguyen, Small molecule solution-processed bulk
heterojunction solar cells, Chem. Mater. 23 (2011) 470.
13A. Mishra, P. Bauerle, Small molecule organic semiconductors on the move: promises for
future solar energy technology, Angew. Chem. Int. Ed. 51 (2012) 2020.
14A. A. Raheem, S. Kamaraj, V. Sannasi, C. Praveen, New D–π–A push–pull
chromophores as low band gap molecular semiconductors for organic small molecule
solar cell applications, Org. Chem. Front.
5 (2018) 777.
15B. Kan, Q. Zhang, M. Li, X. Wan, W. Ni, G. Long, Y. Wang, X. Yang, H. Feng, Y.
Chen, Solution-processed organic solar cells based on dialkylthiol-substituted
benzodithiophene unit with efficiency near 10%, J. Am. Chem. Soc. 136 (2014) 15529.
16D. Deng, Y. Zhang, J. Zhang, Z. Wang, L. Zhu, J. Fang, B. Xia, Z. Wang, K. Lu, W. Ma,
Z. Wei, Fluorination-enabled optimal morphology leads to over 11% efficiency for
inverted small-molecule organic solar cells, Nat. Commun.
7 (2016) 13740.
17B. Kan, M. Li, Q. Zhang, F. Liu, X. Wan, Y. Wang, W. Ni, G. Long, X. Yang, H. Feng,
Y. Zuo, M. Zhang, F. Huang, Y. Cao, T. P. Russell, Y. Chen, A series of simple
oligomer-like small molecules based on oligothiophenes for solution-processed solar cells
with high efficiency, J. Am. Chem. Soc. 137 (2015) 3886.
25

ACCEPTED MANUSCRIPT
18D. Demeter, V. Jeux, P. Leriche, P. Blanchard, Y. Olivier, J. Cornil, R. Po, J. Roncali,
Tuning of the photovoltaic parameters of molecular donors by covalent bridging, Adv.
Funct. Mater. 23 (2013) 4854.
19A. Tang, C. Zhan, J. Yao, Series of quinoidal methyl-dioxocyano-pyridine based π-
extended narrow-bandgap oligomers for solution-processed small-molecule organic solar
cells, Chem. Mater. 27 (2015) 4719.
20C.-L. Chung, H.-C. Chen, Y.-S. Yang, W.-Y. Tung, J.-W. Chen, W.-C. Chen, C.-G. Wu,
K.-T. Wong, S,N-heteroacene-based copolymers for highly efficient organic field effect
transistors and organic solar cells: critical impact of aromatic subunits in the ladder π-
system, ACS Appl. Mater. Interfaces 10 (2018) 6471.
21R. D. Champion, K.-F. Cheng, C.-L. Pai, W.-C. Chen, S. A. Jenekhe, Electronic
properties and field effect transistors of thiophene based donor–acceptor conjugated
copolymers, Macromol. Rapid Commun. 26 (2005) 1835.
22S. C. Rasmussen, R. L. Schwiderski, M. E. Mulholland, Thieno[3,4-b]pyrazines and their
applications to low band gap organic materials, Chem. Commun. 47 (2011) 11394.
23R. Mondal, S. Ko, Z. Bao, Fused aromatic thienopyrazines: structure, properties and
function, J. Mater. Chem. 20 (2010) 10568.
24R. S. Ashraf, M. Shahid, E. Klemm, M. Al-Ibrahim, S. Sensfuss, Thienopyrazine based
low bandgap poly(heteroaryleneethynylene)s for photovoltaic devices, Macromol.
Rapid Commun. 27 (2006) 1454.
25S. Steinberger, A. Mishra, E. Reinold, E. Mena-Osteritz, H. Müller, C. Uhrich, M.
Pfeiffer, P. Bäuerle, Synthesis and characterizations of red/near-IR absorbing A–D–A–
26

ACCEPTED MANUSCRIPT
D–A-type oligothiophenes containing thienothiadiazole and thienopyrazine central units,
J. Mater. Chem. 22 (2012) 2701.
26H.-C. Ting, Y.-H. Chen, L.-Y. Lin, S.-H. Chou, Y.-H. Liu, H.-W. Lin, K.-T. Wong,
Benzochalcogenodiazole based donor–acceptor–acceptor molecular donors for organic
solar cells, ChemSusChem 7 (2014) 457.
27S.-W. Chiu, L.-Y. Lin, H.-W. Lin, Y.-H. Chen, Z.-Y. Huang, Y.-T. Lin, F. Lin, Y.-H.
Liu, K.-T. Wong, A donor–acceptor–acceptor molecule for vacuum-processed organic
solar cells with a power conversion efficiency of 6.4%, Chem. Commun. 48 (2012) 1857.
28H.-I. Lu, C.-W. Lu, Y.-C. Lee, H.-W. Lin, L.-Y. Lin, F. Lin, J.-H. Chang, C.-I. Wu, K.-T.
Wong, New molecular donors with dithienopyrrole as the electron-donating group for
efficient small-molecule organic solar cells, Chem. Mater. 26 (2014) 4361.
29X. Che, C.-L. Chung, C.-C. Hsu, F. Liu, K.-T. Wong, S. R. Forrest, Donor–acceptor–
acceptor's molecules for vacuum deposited organic photovoltaics with efficiency
exceeding 9%, Adv. Energy Mater.
8 (2018) 1703603.
30R. C. Coffin, J. Peet, J. Rogers, G. C. Bazan, Streamlined microwave-assisted preparation
of narrow-bandgap conjugated polymers for high-performance bulk heterojunction solar
cells, Nat. Chem.
1 (2009) 657.
31A. P. Kulkarni, Y. Zhu, S. A. Jenekhe, Photodegradation of emissive conjugated
copolymers and oligomers containing thienopyrazine, Macromolecules 41 (2008) 339.
32H. Ma, H.-L. Yip, F. Huang, A. K.-Y. Jen, Interface engineering for organic electronics,
Adv. Funct. Mater. 20 (2010) 1371.
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ToC:
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