Verifying the Structure-Property-Performance Relationship of Y6-Based Small Molecule Acceptors by Alkoxy-Side Chain Isomerization and Conjugated Skeleton Asymmetry

Non-fullerene small molecule acceptor (NF-SMAs) such as Y6 derivatives were the current working horse for the top performing polymer solar cells (PSCs). For this reason, numerous chemical modifications have been explored for establishing structural-property-performance relationship for Y6-based NF-SMAs. In this work, we explored the isomerization on asymmetrical Y6 derivatives by varying the position of alkoxy side chain on the thiophene linkage between the core and one of the end groups of Y6. The isomers with different conformational restriction raised by noncovalent interactions as well as steric hindrance revealed drastically different behavior in crystallization and polymorphism, which led to significant contrast in PSC performances. The X-TO1 molecule with alkoxy side chain facing the end group (outward) displayed preferred molecular configuration and polymorph, delivering power conversion efficiency (PCE) of 15.63%, while its isomer X-TO2 showed coexisted two polymorphs which significantly increased the charge recombination in the PSC devices led to a low PCE around 3%. This structure-property-performance relationship not only highlighted the elegancy of isomerization in tuning the photovoltaic performances but also identified polymorphism as one of the causes for impairing photovoltaic performances of PSCs