Healing the Buried Cavities and Defects in Quasi-2D Perovskite Films by Self-Generated Methylamine Gas

Perovskites with grain size comparable to film thickness are intensively pursued for high-efficiency solar cells. Geometrically, large grains with high crystallinity tend to form polyhedral shapes that have difficulty forming compact and smooth films. When quasi-two-dimensional RP perovskite films adopt a downward growth mode, defective contacts tend to form at their bottom interfaces with many nanocavities. This is attributed to the angular growing fronts of RP perovskite grains adopting [111] (or/and [101]) growth directions. Self-generated methylamine gas, by a replacement reaction in solution, is introduced to in situ heal these irregular nanocavities that are deeply buried in perovskite films during crystallization processes. The amount of self-generated methylamine gas should be adequately controlled to avoid the homogeneous nucleation of perovskites from a liquid perovskite-amine intermediate phase, which is a key to avoid ruining the grain size and film composition. This in situ healing strategy offers significantly enhanced charge collection efficiency and device working stability.