Cation Volatility Governs the Functionality of Thiocyanate Additives in FAPbI3 Perovskite Solar Cells

The performance of formamidinium lead iodide (FAPbI3) perovskite solar cells is limited by insufficient bulk crystallinity and defect-rich grain boundaries, which drive nonradiative recombination and poor operational stability. Herein, we establish a comparative thiocyanate additive model to reveal how cation volatility governs additive functionality in FAPbI3 perovskites. Two representative additives are investigated: fully volatile NH4SCN and PEASCN, which contain a retainable, interface-active PEA+ component. In both systems, SCN− transiently coordinates with Pb2+, restructures precursor chemistry, and promotes phase-pure crystallization with enlarged grains and improved film quality. Their cationic components, however, follow distinct pathways during annealing. NH4+ is completely removed and therefore mainly enhances bulk crystallization, whereas PEA+ remains preferentially at grain boundaries and surfaces, where it introduces additional interfacial passivation and hydrophobic protection. As a result, NH4SCN- and PEASCN-based devices achieve champion efficiencies of 23.3 and 22.9%, respectively, together with markedly improved stability. This work identifies cation volatility as a key descriptor of thiocyanate additive function and provides a volatility-informed design principle for high-efficiency and durable perovskite solar cells.