Benzyl-imidazolium fluoroborate showing synergistic anion-cation passivation and surface encapsulation for efficient and stable perovskite solar cells

While the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has seen remarkable progress, interfacial defects continue to pose a fundamental challenge by incurring non-radiative losses and undermining operational stability. Conventional passivation strategies often lack the multifunctionality required to concurrently address diverse defect types and environmental degradation pathways. Here, we introduce a multifunctional ionic compound, 1-benzyl-3-methylimidazolium tetrafluoroborate (BzMIMBF₄), for effective interface regulation in PSCs. The imidazolium cation of [BzMIM]⁺ acts as a Lewis base to coordinate with undercoordinated Pb²⁺, while [BF₄]⁻ electrostatically passivates iodine vacancies. Moreover, the hydrophobic benzyl groups self-assemble into a compact barrier that shields the perovskite from moisture and oxygen ingress. Consequently, the optimized device achieves a champion PCE of 25.53% with negligible hysteresis. Impressively, the unencapsulated device retains over 90% of its initial efficiency after 1000 h under 30–40% relative humidity and demonstrates over 905 h of stability under continuous one-sun illumination. This work illustrates a synergistic molecular design strategy for simultaneously mitigating interfacial defects and enhancing stability in high-performance perovskite photovoltaics.