Multi-functional Phase-Changeable Salt for Inverted Perovskite Solar Cells.

Non-radiative recombination and uncontrolled crystallization at the buried perovskite interface remain key barriers to achieving highly efficient and stable perovskite solar cells (PSCs). Herein, we introduce a multi-functional phase-changeable salt, 2-(diphenylphosphino)ethanaminium tetrafluoroborate (DPPEABF₄), as a functional interfacial modulator. DPPEA⁺ interacts with perovskite precursors (FAI and PbI₂) through coordination and hydrogen bonding, suppressing excessive nucleation and reducing nucleation density. During annealing, DPPEABF₄ undergoes a reversible solid-to-liquid phase transition that buffers heat transfer, ensuring more uniform heating and slower, more controlled crystallization across the substrate. These effects collectively yield perovskite films with enlarged grains, reduced defect densities, and improved crystallinity. Concurrently, π-π stacking between DPPEA⁺ and Me-4PACz establishes an interfacial dipole that increases the work function and tunes the energy-level alignment at the buried interface, facilitating efficient hole transport. As a result, the optimized devices achieve a champion power conversion efficiency of 26.52% (certified 26.46%) and retain over 90% of their initial efficiency after more than 2000 h of storage under 30-40% relative humidity. This work provides a new paradigm for designing dynamic interfacial materials for high-performance optoelectronic devices.