Role of Tosylate Anions in Small Molecule Electrolytes Enabling Defect Passivation by Regulating Zinc Coordination of Zinc Oxide Hybrid Films for Organic Solar Cells

Electron transport layers (ETLs) with efficient electron extraction are essential for high-performance organic solar cells (OSCs). Sol-gel-derived zinc oxide (ZnO) is widely used as an ETL because of its high electron mobility and suitable work function; however, intrinsic defects in ZnO often limit the power conversion efficiency (PCE) of the device. To overcome this limitation, inverted OSCs employing ZnO doped with small molecule electrolytes (SMEs) as the ETL are developed. Rylene diimide-based SMEs containing tosylate anions, PDIN-OTs and NDIN-OTs, are synthesized and incorporated into ZnO for non-fullerene OSCs. The resulting ZnO-SME hybrid films significantly enhance device performance, yielding a PCE of up to 18.3%. Devices modified with PDIN-OTs exhibit higher short-circuit current density (J_sc), while those using NDIN-OTs show improved fill factor (FF). These enhancements arise from the effective passivation of ZnO trap states through interactions between tosylate anions and Zn ions in the ZnO lattice, consistent with a trap-filling mechanism. This interaction facilitates electron transport, suppresses charge recombination, and increases ZnO conductivity. In addition, reduced work function, Urbach energy, and trap density further promote efficient charge transport and collection. Overall, this study demonstrates that organic SMEs are effective ZnO modifiers and offer a promising strategy for improving OSC performance.