Vapor-assisted deposition of highly efficient, stable black-phase FAPbI3 perovskite solar cells

Moving a perovskite into the blackThe bandgap of the black α-phase FAPbI3 (where FA is formamidinium) is nearly ideal for solar cells, but it is unstable with respect to the photoinactive yellow δ-phase. Lu et al. found that a film of the yellow phase was converted to a highly crystalline black phase by vapor exposure to methylammonium thiocyanate at 100°C, and it retained this structure after 500 hours at 85°C. Solar cells fabricated with this material had a power conversion efficiency of more than 23%. After 500 hours under maximum power tracking and a period of dark recovery, 94% of the original efficiency was retained.Science, this issue p. eabb8985INTRODUCTIONMetal halide perovskite solar cells (PSCs) have reached a power-conversion efficiency (PCE) of 25.2%, thus exceeding other thin-film solar cells. FAPbI3 (where FA is formamidinium) has been shown to be an ideal candidate for efficient, stable PSCs. Obtaining highly crystalline, stable, and pure α-phase FAPbI3 films has been of vital importance. However, FAPbI3 undergoes a phase transition from the black α-phase to the photoinactive δ-phase below 150°C. Previous approaches to overcoming this problem include mixing it with MA, Cs or Br ions. Here, we report a deposition method using methylammonium thiocyanate (MASCN) vapor treatment to convert δ-FAPbI3 to the desired pure α-phase below the thermodynamic phase-transition temperature. Molecular dynamics (MD) simulations show that the SCN– anions promote the formation and stabilization of α-FAPbI3. These vapor-treated FAPbI3 PSCs exhibit outstanding photovoltaic and electroluminescent performance.RATIONALEAlthough the phase transition from δ- to α-phase FAPbI3 requires a high temperature, the treatment of δ-phase FAPbI3 films with MASCN vapor allows the conversion to occur at temperatures below 150°C. MD simulations show that SCN– ions preferentially adsorb on the surface of δ-FAPbI3 to replace iodide ions that are bound to Pb2+. This process disintegrates the top layer of face-sharing octahedra and induces the transition to the corner-sharing architecture of α-FAPbI3. Once the corner-sharing α-form is formed on the top surface, this layer templates the progression of the phase transition from δ- to α-FAPbI3 toward the bulk. Once the pure α-FAPbI3 is formed, its back conversion to the δ-phase is prevented by a high energy barrier.RESULTSWe show a complete conversion from δ- to α-FAPbI3 at 100°C using the MASCN vapor treatment method. This phase transi