Research efforts in various multitudes have been demonstrated to stabilize methylammonium (MA)- and bromide (Br)-free formamidinium lead triiodide (FAPI) perovskite thin films. Despite these commendable efforts, pure FAPI perovskite thin film is prone to critical phase-transition issues due to its thermodynamically stable non-perovskite phase (2H). Here, in this work, we propose a rational additivization strategy to overcome this challenge. Our multifunctional ammonium salt containing a sulfur heteroatom shifts the thermodynamic stability from the 2H phase to an intermediate phase closer to the cubic phase. Along with the high crystallinity, micron-sized grains with preferred (00h) facet orientation stem the Pb …S interaction to offer exceptional stability against high relative humidity, direct water incursion, and shelf-life aging. Our findings through experimental and theoretical studies substantiate the role of Pb …S interaction in stabilizing the perovskite cubic phase and the stoichiometric distribution of elemental components.
Haris et al. implement a multifunctional additivization strategy where the sulfur heteroatom induced strong interactions; this prevents water and damp invasion to the formamidinium lead triiodide perovskite layer. The strategy also demonstrates the shifting of thermodynamic stability of FAPbI 3 from an unfavorable 2H phase to a favorable intermediate phase.