Lead-sulfur interaction induced damp and water stability in pure formamidinium lead triiodide.

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.