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

Pure-phase α-FAPbI₃ 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 FAPbI₃ -QD film surface, in which a MAPbI₃ 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 FAPbI₃ QD solar cell, and it retains over 90% of the initial PCE after being stored in ambient environment for 1000 h.