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10.1002/chem.202102252
Chemistry - A European Journal
Introduction
In recent years, bulk heterojunction (BHJ) polymer solar cells (PSCs) are steadily and rapidly developing
with power conversion efficiencies (PCEs) over 18%[1-10] due to the continuous optimization of the
molecular structure of materials,[11-13] the configuration and fabrication process of photovoltaic devices.[14-16]
This outstanding achievement is mainly ascribed to the fact that fullerene-based acceptors were substituted
by the springing up of new nonfullerene small-molecule acceptors (SMAs) thanking to their superior optical
absorption properties, easily tunable molecular energy levels, and potential for relatively low-cost
production processes.[17-20] The first example of efficient SMAs was the well-known ITIC, which derived
from dithienoindacenodithiophene (IDTT) and reported by Zhan group.[21] In 2019, a new high-performance
SMA, named Y6 which features a ladder-type fused ring of dithienothiophen[3.2-b]-pyrrolobenzothiadiazole
(TPBT), was pioneered by Zou and the co-workers.[22] Thereafter, PCEs of PSCs based this kind of SMAs,
so-called Y-series SMAs, boomed rapidly and exceeded 18%.[1-10]
Including ITIC and Y6, most of high-performance SMAs possess an acceptor-donor-acceptor (A-D-A)
type structural skeleton, where a ladder-type fused ring (like IDTT, TPBT) was used as the central donor (D)
unit and an electron-withdrawing unit is employed as the end-capping acceptor (A) group.[21-25] Here, it
should be noted that the ladder-type central unit is fused by an electron-deficient core (e.g. benzothiadiazole)
with electron-rich unit (typically dithienothiophen[3.2-b]pyrrole) in Y-series SMAs. To well tune the
molecular structure, absorption spectrum, and electronic energy levels (HOMO/LUMO) of these SMAs,
various molecular design strategies are concentrated on the backbone manipulation of D unit, the substituent
modification of A group, and the conjugated π-bridge engineering between D and A units.[26-31] In addition, it
is demonstrated that side-chain engineering upon the central D unit is also crucial in tailoring solubility,
molecular structure, intermolecular packing and film morphology of SMA materials. Alkylphenyl is
generally used as the side chains of ITIC-like SMAs in view of the relatively simple synthetic routine as
compared with alkyl side chains.[32,33] These above-mentioned properties can be greatly tuned when the
alkylphenyl side chain is replaced by an alkylthienyl or alkyl.[34-36] In contrast, for Y-series SMAs, long
chain alkyls are usually employed as the effective side chains for high device performance.[37,38] Generally,
the alkylphenyl (alkylthienyl) side chain is linked to the backbone of D unit by the sp3-hybridized carbon
atom of the cyclopentadiene moiety. Therefore, the alkylphenyl (alkylthienyl) groups are just
non-conjugated side chains like alkyl side chains. These non-conjugated side chains have limit effect on
absorption properties and HOMO/LUMO energy levels, which are closely related with short-circuit current
density (Jsc), open-circuit voltage (Voc) and ultimate PCE of a PSC device. On the contrary, the introduction
of the conjugated side chains onto the central D unit can efficiently improve absorption properties and
optimize HOMO/LUMO energy levels apart from controlling solubility, structure, intermolecular packing
and film morphology of SMAs as same as non-conjugated side chains (alkylphenyl, alkyl).
In summary, there are three approaches to introduce the conjugated side chains onto the polycyclic fused
central unit of SMAs. First, the conjugated side chains are introduced into the molecular backbone through
sp2-hybridized C atom into aromatic ring (e.g. benzene ring).[39-44] Zhan group reported the first example of
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