Cation Volatility Governs the Functionality of Thiocyanate Additives in FAPbI ₃ Perovskite Solar Cells

The performance of formamidinium lead iodide (FAPbI 3 ) 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 FAPbI 3 perovskites. Two representative additives are investigated: fully volatile NH 4 SCN and PEASCN, which contain a retainable, interface-active PEA + component. In both systems, SCN − transiently coordinates with Pb 2+, restructures precursor chemistry, and promotes phase-pure crystallization with enlarged grains and improved film quality. Their cationic components, however, follow distinct pathways during annealing. NH 4 + 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, NH 4 SCN- 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.