Molecular engineering of N,S-heteroarene-based small-molecule acceptors: effects of side chains, backbone extension and end groups on structure, optoelectronic properties and solar cell performance

Abstract

Studying the structure–property correlation of small-molecule acceptors (SMAs) is beneficial in optimizing their optoelectronic properties and enhancing photovoltaic performance. Here, we designed and synthesized six N,S-heteroarene (axa-indacenodithiophene, NIT)-based SMAs through molecular engineering (side chain, backbone extension and end group), which are defined as the pentacyclic central core series (NIT68, NIT810, and NIT810-F) and the backbone extension series (T-NIT68, NIT810-T, and NIT810-T-F), respectively. The results reveal that end group engineering (fluorination vs. non-fluorination) has a greater effect on the absorption properties and molecular energy levels as compared with side chain (2-hexyldecyl vs. 2-octyldodecyl) and backbone extension engineering. Theoretical calculation suggests that molecular engineering has a tiny impact on the backbone planarity of these NIT-based SMAs, but has a certain effect on the geometric configuration and electronic structure of molecules. Organic solar cells (OSCs) based on these SMAs were fabricated and device performance was optimized. OSCs based on the backbone extension series showed a higher open-circuit voltage (V_oc) than those of the pentacyclic central core series, because of upshifted LUMOs. PBDB-T:NIT810-based OSCs exhibited the best power conversion efficiency (PCE) of 7.25% among these six SMAs. However, when PBDB-T was replaced by fluorinated PM6, the fluorinated NIT810-F and NIT810-T-F-based OSCs exhibited an improved PCE of up to 7.78% and 9.51%, respectively, due to the obviously increased fill factor (FF) and short-circuit current density (J_sc). The improved FF and J_sc should be primarily attributed to the increased exciton dissociation and charge transport properties, suppressed bimolecular recombination, and optimized phase separation. Our work focusing on the effect of molecular engineering on molecular structure, optoelectronic and photovoltaic properties will certainly provide some significant guidance for the molecular design of high-performance SMAs.