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