Nanchang University researchers recently reported a molecular doping strategy that addresses a central bottleneck in formamidinium–cesium (FA1−xCsxPbI3) perovskites: stabilizing the photoactive α-phase while maintaining high device efficiency and long-term operational stability.
Metal halide perovskites' large-scale deployment has been limited by phase instability and performance degradation. In FA–Cs systems, achieving the desired α-phase is particularly challenging because of limited incorporation of Cs+ ions and an incomplete understanding of the phase transition pathway. While two-step fabrication offers improved control over crystallization compared to one-step processing, it still struggles to deliver uniform cation distribution and stable phase formation. To address this, the team introduced a tailored additive, cesium 4-(diphenylphosphino)benzoate, designed to regulate Cs+ incorporation during film formation.
As the authors explain: "We design cesium 4-(diphenylphosphino)benzoate to enable efficient Cs+ doping and to homogenize cation distribution, obtaining high-quality perovskite films with improved phase stability". The molecule facilitates more uniform dispersion of Cs+ across the lattice, which in turn stabilizes the α-phase and suppresses unwanted phase transitions.
Mechanistically, the additive improves both the kinetics and thermodynamics of crystallization. By coordinating with precursor species during the two-step deposition, it promotes controlled nucleation and growth while ensuring that Cs+ ions are effectively incorporated into the perovskite lattice rather than segregating. This homogeneous cation distribution reduces local strain and stabilizes the crystal structure, providing a clearer pathway toward the α-phase. The study further sheds light on the transition-state structure and phase transition pathway of FA0.9Cs0.1PbI3, clarifying how Cs-driven lattice stabilization operates at the atomic level.
Devices fabricated using this approach show strong performance metrics. "The solar cells fabricated via the two-step process achieve an efficiency of 26.91% (certified 26.61%)," wrote the authors. In addition to efficiency gains, the devices demonstrate notable thermal and operational stability. "The devices incorporating a thermally stable charge-transport layer retain 95% of their initial efficiency (23.76%) after continuous operation under 1-sun illumination at the maximum power point tracking and 85°C (ISOS-L-2 protocol) for 1,500 hours."
Overall, the work highlights how targeted molecular design can resolve long-standing issues in perovskite processing, linking cation distribution control with phase stability and device durability. The insights into Cs+-driven lattice stabilization and phase transition mechanisms could guide the development of broader doping strategies, potentially enabling more reliable and scalable perovskite solar technologies.
