Solid-state reaction process for high-quality organometallic halide perovskite thin film

Recently, a hybrid perovskite material, ABX₃ (A=Cs, CH₃NH₃, NH₂CHNH₂; B=Pb, Sn; X=Cl, Br, I), has received much attention as an active layer in new-generation solar cells. This material is usually fabricated with either a one-step or a two step process in solution. However, the surface morphology, nucleation rate and grain growth rate of the CH₃NH₃PbI₃ perovskite light absorber prepared by the solution reaction process (SRP) are hard to control. Here, we show a fast solid-solid reaction process (SSRP), to fabricate ultraflat (roughness of approximately 12 nm) CH₃NH₃PbI₃ perovskite thin films with large grain sizes (∼947 nm). The SSRP simply involves directly contacting a lead iodide thin film (PbI₂) with methylammonium iodide powder (CH₃NH₃I) without any chemical reagents at 120°C under a normal atmospheric environment. The SSRP reaction dynamics is investigated by an in situ heating scanning electron microscope (SEM) system. This innovative SSRP is an easy approach for the large-scale fabrication of planar heterojunction perovskite solar cells and allows us to demonstrate a power conversion efficiency of approximately 15.27% (active area of 0.16 cm²).