Trash into Treasure: δ-FAPbI Polymorph Stabilized MAPbI Perovskite with Power Conversion Efficiency beyond 21.

Effective passivation and stabilization of both the inside and interface of a perovskite layer are crucial for perovskite solar cells (PSCs), in terms of efficiency, reproducibility, and stability. Here, the first formamidinium lead iodide (δ-FAPbI ) polymorph passivated and stabilized MAPbI PSCs are reported. This novel MAPbI /δ-FAPbI structure is realized via treating a mixed organic cation MA FA PbI perovskite film with methylamine (MA) gas. In addition to the morphology healing, MA gas can also induce the formation of δ-FAPbI phase within the perovskite film. The in situ formed 1D δ-FAPbI polymorph behaves like an organic scaffold that can passivate the trap state, tunnel contact, and restrict organic-cation diffusion. As a result, the device efficiency is easily boosted to 21%. Furthermore, the stability of the MAPbI /δ-FAPbI film is also obviously improved. This δ-FAPbI phase passivation strategy opens up a new direction of perovskite structure modification for further improving stability without sacrificing efficiency.