Deciphering the role of anionic groups in zwitterionic electron transport layers for organic solar cells

Abstract

The intrinsic advantages of zwitterions have driven their widespread use as electron transport layers (ETLs) in organic solar cells (OSCs). However, the role of anionic groups in governing interfacial behavior and device performance remains largely unexplored. Herein, we propose two zwitterions, PDIM-DC and PDIM-z, which share an identical imidazolium cation but feature distinct anions: ethoxydicyanoethenolate and sulfonate, respectively. Comprehensive characterization studies reveal that the ethoxydicyanoethenolate anion endows PDIM-DC with stronger self-doping, enhanced crystallinity, and reduced interfacial energy, leading to markedly superior work function tunability, higher electron conductivity, and better active layer compatibility than PDIM-z. When deployed as the ETL in PM6:Y6-based OSCs, PDIM-DC achieves an impressive power conversion efficiency (PCE) of 17.64%, outperforming its PDIM-z counterpart (15.65%) due to more efficient charge collection and suppressed recombination. More strikingly, PDIM-DC exhibits superior thickness tolerance, retaining a respectable PCE of 16.52% at 32 nm, whereas PDIM-z-based devices almost fail under identical conditions. The versatility of PDIM-DC is further validated and a PCE of 19.19% is achieved in the D18:L8-BO system. Collectively, our findings establish rational anion engineering as a powerful yet previously overlooked strategy to unlock the full potential of zwitterionic ETLs, opening a new avenue for designing high-performance interfacial layers for OSCs.