Background and Originality Content
Organic
solar cells (OSCs) have gained attention for their light weight,
adjustable absorption range, flexibility, and solution
processability.[1-3]Recent
advancements in this field have been primarily driven by polymeric and
small-molecule donors/acceptors, with the power conversion efficiencies
(PCEs) surpassing 19% for single-junction
devices.[4-14] Among the high-performance acceptor
materials, the acceptor-donor-acceptor (A-D-A) and A-DA’D-A-structured
fused-ring electron acceptors (FREAs), exemplified by Y6 and ITIC
respectively, stand out.[15-20] With the aim of
exploring superior acceptors, considerable research has focused on
molecular engineering, such as π-conjugated extension, side-chain
engineering, end-group halogenation, and isomerization, to regulate the
light harvesting abilities, energy levels, and packing behaviors of
FREAs.[21-24] In addition, asymmetric strategy
serves as a key molecular design approach in enhancing the dipole moment
and dielectric constant while reducing the exciton binding energy,
thereby promoting exciton dissociation and charge
transport.[25, 26]
With the rapid development of FREAs, another type of nonfused-ring
electron acceptors (NFREAs), aimed at reducing product costs, has
emerged.[27-32] This type of acceptors typically
possess a partially or completely non-fused ring cores, greatly
simplifying molecular design and opening the opportunity for
constructing cost-effective acceptor materials.[33,
34] However, the additionally introduced rotatable σ -single
bonds can result in conformational isomers and twisted backbones. To
address this issue, large steric hindrance of side-chains and weak
intramolecular interactions (also known as noncovalently conformational
locks, NoCLs) are introduced in molecular design to lock-in the
conformation, thereby eliminating conformational isomers and achieving a
highly coplanar π -conjugated
backbones.[35-42] Recently, we reported a
PhO4T-series of symmetric NFREAs, demonstrating higher figure-of-merit
values than those of FREAs, due to their simple structures and concise
synthesis routes.[43] However, the PCEs of NFREAs
have still lagged far behind those of FREAs. Therefore, how to
rationally design NFREAs for low-cost and high-performance OSCs remains
challenging.
Herein, asymmetric engineering was employed to construct a new NFREA,
namely NoCA-19, with two distinct end-groups
(2-(5,6-dichloro-3-oxo-2,3-dihydro-1H -inden-1-ylidene)malononitrile,
IC-2Cl; and
2-(6,7-difluoro-3-oxo-2,3-dihydro-1H -cyclopenta[b ]naphthalen-1-ylidene)malononitrile,
NC-2F) on the basis of two symmetric NFREAs (one is previously reported
acceptor NoCA-17, and another one is newly synthesized acceptor NoCA-18)
(Figure 1a). Experimental and theoretical results show that asymmetric
acceptor NoCA-19 possesses broader light absorption range, more coplanarπ -conjugated backbone, and appropriate crystallinity, compared to
the two symmetric acceptors. When blending with the polymer donor J52,
the NoCA-19-based blend film afforded more balanced charge mobility,
suppressed recombination loss,
shorter charge extraction time, and more favorable morphology.
Therefore, the asymmetric NFREA NoCA-19-based OSCs achieved the highest
PCE of 12.26% among the three blend
systems. This work highlights the
importance of asymmetric end-group engineering in exploring low-cost and
high-performance NFREAs.
Results and Discussion
Synthesis and characterization
As depicted in Scheme S1, the
three NFREAs were synthesized in one-pot Knoevenagel condensation
reaction, and then separated individually by column chromatography. The
synthesis route is shown in Scheme S1, and the detailed synthesis
procedures and the 1H-/13C-NMR
spectra were given in Supporting Information. All the three acceptors
show good solubility in common solvents, such as chloroform, toluene,
and chlorobenzene.
The UV–vis absorption of these three acceptors in both dilute
chloroform solution and thin-film state are shown in Figures 1b and 1c,
and the corresponding optical parameters are summarized in Table 1. In
dilute chloroform solutions, the shapes of the normalized absorption
curves for three NFREAs are almost identical in the range of 550–850 nm
(Figure 1b). The maximum absorption peaks at 724 nm for NoCA-17 and 734
nm for NoCA-18 indicate a stronger electron-withdrawing ability of NC-2F