Background and Originality Content
In recent years, the development
of organic solar cells (OSCs) has largely benefited from the revolution
of non-fullerene acceptors, especially fused-ring electron acceptors
(FREAs) with the acceptor-donor-acceptor (A-D-A) architecture, which
were pioneered by Zhan et al. in 2015.[1-3] Later
in early 2019, Zou et al. reported a new generation of A-DA′D-A
structured FREAs (represented by Y6), containing electron-deficient
units in the fused-ring cores.[4, 5] Due to the
unique structural feature, FREAs possess superior absorption property
and tight intermolecular π–π stacking in
common.[6] With the continuous efforts on
molecular tailoring and device engineering, the power conversion
efficiencies (PCEs) of the state-of-the-art single junction OSCs have
exceeded 19%.[7-14] However, due to the
remarkable structural feature of highly fused conjugated backbones,
FREAs generally require complicated synthetic approaches and suffer from
low yields and high product costs, which may severely hinder their
large-scale applications.
To address the disadvantage of
FREAs, an alternative strategy for low-cost electron acceptors was
proposed, namely nonfused-ring electron acceptors
(NFREAs).[15] In general, this novel type of
acceptors shares an unfused central core (a simply fused or even
completely unfused π-conjugated core), thus making the synthesis route
more concise.[16-21] More importantly, the
noncovalently conformational locks (NoCLs) strategy is generally
introduced in the molecular design of NFREAs, which can effectively
restrict the rotation of single bonds and reduce conformational isomers
raised from the unfused-ring cores, thus improving the molecular
planarity and rigidity.[22-26] Therefore, NFREAs
possess a simple-structured yet co-planar conjugated backbone, opening
the opportunity for constructing cost-effective acceptor
materials.[27-29] Regarded as tailoring from
A-DA′D-A type FREAs, A-D-A′-D-A structured NFREAs have received much
attention recently. Due to the key role of the A′ unit in regulating
light-harvest ability, energy levels, intramolecular conformation, and
intermolecular packing behaviors, a lot of work has been devoted to the
selection of A′ units, including
benzo[c ][1,2,5]thiadiazole,[30, 31]2H -benzo[d ][1,2,3]triazole,[32,
33] thieno[3,4-c ]pyrrole-4,6-dione,[34,
35] and
benzo-[1,2-c :4,5-c′ ]dithiophene-4,8-dione.[36,
37] However, it is still challenging to rationally choose an
electron-withdrawing motif as the A′ unit.
Thiazole is a widely-used electron-deficient heteroaromatic ring due to
its electron-withdrawing nitrogen of imine
(C=N),[38] which has been employed in constructing
organic semiconductors for optoelectronics, such as
OSCs,[39] organic field-effect transistors
(OFETs),[40] and organic light-emitting diodes
(OLEDs).[41] Compared with thiazole,
4-chlorothiazole should be a more electron-deficient unit, which is
expected to build organic conjugated molecules with deep-lying highest
occupied molecular orbital (HOMO) energy levels. Particularly, the N and
Cl atoms in 4-chlorothiazole may be able to form S···N and S···Cl NoCLs
with neighboring thiophene units to enhance the molecular planarity.
Therefore, 4-chlorothiazole is an excellent candidate as the A′ unit to
construct high-planarity and low-cost NFREAs.
Herein, thiazole and
4-chlorothiazole were utilized as the A′ units to construct two
A-D-A′-D-A type NFREAs, namely Tz-H and Tz-Cl ,
respectively (Figure 1a). Compared with Tz-H , Tz-Clpossessed a facile and concise synthesis, due to the unexpectedly C-H
direct arylation of 2,5-dibromo-4-chlorothiazole. Both density function
theory (DFT) calculations and single-crystal X-ray diffraction (SC-XRD)
data revealed that the simultaneous incorporation of S···N and S···Cl
NoCLs can provide a more planar conjugated backbones, and thus a more
compact and ordered packing than the sole introduction of S···N NoCLs.
When blending with the polymer donor PM6, the Tz-Cl -based blend
film afforded higher exciton dissociation efficiency, more balanced
charge mobility, lower recombination loss, shorter charge extraction
time, and more favorable morphology. Therefore, a PCE of 11.10% was
achieved for Tz-Cl -based devices, much higher than that of theTz-H counterpart (6.41%).
This work demonstrated the
potential of NoCLs to construct low-cost NFREAs.
Results and Discussion
Synthesis and characterization
The synthesis routes of the two NFREAs were shown in Scheme S1, and the
detailed synthesis procedures,1H-/13C-NMR spectrometry, and mass
spectrometry were given in Supporting Information. Initially, we tried
to obtain the dialdehyde intermediate 4 from
2,5-dibromothiazole (1 ) and compound 6 by
Pd(0)-catalyzed C-H activation, but failed. Thus, we utilized compound1 with 2 equivalents of mono-organotin compound (2 ) to
afford a nonfused-type conjugated core (3 ), followed by the
Vilsmeier-Haack reaction to convert dialdehyde intermediate 4 .
Different from the chloro-free thiazole derivation,
2,5-dibromo-4-chlorothiazole
(5 ) can unexpectedly undergo a two-fold
C-H activation with compound6 to directly afford dialdehyde intermediate 7 .
Finally, Tz-H and Tz-Cl was obtained through
Knoevenagel condensation between the dialdehyde intermediate (4or 7 ) and
2-(5,6-dichloro-3-oxo-2,3-dihydro-1H -inden-1-ylidene)malononitrile
(IC-2Cl). Obviously, the synthetic route for Tz-Cl is more
concise, which may be beneficial to reducing the production cost. BothTz-H and Tz-Cl showed good solubility in common
organic solvents, such as chloroform, tetrahedron, and chlorobenzene.