Efficient and Stable Organic Solar Cells Enabled by Backbone Engineering of Nonconjugated Polymer Zwitterion Interlayers

Improving the efficiency and stability of fused-ring electron acceptor (FREA)-based organic solar cells (OSCs) by interface engineering is presently an emerging topic in the photovoltaic research field. Herein, we propose the design and efficient synthesis of four nonconjugated self-doped polymer zwitterions composed of the same electron-rich dopant but varied electron-deficient host fragments (perylene diimide and naphthalene diimide) and linkages (imidazolium and ammonium). Our results reveal that both their electrical properties and interfacial compatibility with active layers can be fine-tuned by structural modification, therefore impacting the power conversion efficiencies (PCEs) of the OSCs. The zwitterion combining perylene diimide and ammonium exhibits a more suitable energy level, higher conductivity, and more favorable film-forming ability with respect to others, which markedly modify the electrode/active layer interface, promote efficient charge extraction, and diminish charge recombination. This results in improved efficiency and stability of both binary and ternary FREA-based OSCs over a wide range of interlayer thickness with a maximum PCE value of 18.67% and high operational stability with T₈₀ > 800 h (the time scale for solar cell efficiency reaching 80% of the initial value). Our work provides an ingenious way to systematically optimize the molecular structures of nonconjugated polymer zwitterions toward more efficient and robust OSCs.