In Situ Surface Reconstruction toward Planar Heterojunction for Efficient and Stable FAPbI3 Quantum Dot Solar Cells

Abstract Pure‐phase α‐FAPbI 3 quantum dots (QDs) are the focus of an increasing interest in photovoltaics due to their superior ambient stability, large absorption coefficient, and long charge‐carrier lifetime. However, the trap states induced by the ligand‐exchange process limit the photovoltaic performances. Here, a simple post treatment using methylamine thiocyanate is developed to reconstruct the FAPbI 3 ‐QD film surface, in which a MAPbI 3 capping layer with a thickness of 6.2 nm is formed on the film top. This planar perovskite heterojunction leads to a reduced density of trap‐states, a decreased band gap, and a facilitated charge carrier transport. As a result, a record high power conversion efficiency (PCE) of 16.23% with negligible hysteresis is achieved for the FAPbI 3 QD solar cell, and it retains over 90% of the initial PCE after being stored in ambient environment for 1000 h.