In-situ photoisomerization of azobenzene to inhibit ion-migration for stable high-efficiency perovskite solar cells

Ion migration is a notorious problem in perovskite solar cells (PSCs) that severely mutilates device performance. Herein, a strategy to inhibit ion migration in situ is developed by using photoisomerization of azobenzene (AZO) to immobilize cations in the lattice. During the nucleation process, the photoisomerized cis-AZO reacts with FA+, MA+ and Pb2+ cations in the perovskite precursor by synergistic cation-π interaction and Lewis base coordination, leading to heterogeneous nucleation to produce uniform perovskite film. Meanwhile, it accelerates conversion of intermediate yellow δ-phase to desired black α-phase of FAPbI3 for improved crystallinity with well-passivated grain surface. Consequently, defect density is effectively reduced for the perovskite film to demonstrate suppressed carrier recombination and enhanced carrier extraction. Subsequently, the solar cell efficiency is elevated from 21.29% to 23.58% with negligible J-V hysteresis. Long-term stability is also improved, with the bare device without any encapsulation retaining 84% of its initial efficiency after aging 744 hours in ambient.