Uniform, high crystalline, (100) crystal orientated perovskite films without PbI2 residue by controlling the nanostructure of PbI2

Abstract

The photovoltaic performance and stability of perovskite solar cells (PVSCs) is deeply dependent on the morphology of the perovskite film and the amount of lead iodide (PbI2) residue. However, the morphology of perovskite film fabricated through two-step sequential deposition method usually suffers from poor film coverage and low crystallinity due to the ungovernable crystallization kinetics. Moreover, owing to the compact stacking of PbI2 crystals, the conversion of PbI2 is incomplete as well. In this work, we propose for the first time to control the nanostructure of PbI2 to get high crystallinity, identical (100) crystal orientation and PbI2 residue-free film. Porous PbI2 with plate structure was obtained by selected solvent annealing (SSA). The reaction rate between PbI2 and methylammonium iodide (MAI) was accelerated because the contact area between PbI2 and MAI was enlarged due to the porous nanostructure of PbI2. Thus, uniform, high crystalline perovskite film without PbI2 residue was obtained. Furthermore, (100) crystal orientation of perovskite was obtained due to the PbI2 shape changing from grain to plate, which originated from the changing growth frontier of PbI2 crystals. As a result, a promising power conversion efficiency of 16.61% is achieved in planar-heterojunction PVSCs due to improved photoabsorption and carrier extraction efficiency. This work for the first time establishes the relationship between PbI2 structure and the corresponding structure of perovskite in detail, and shows a reasonable design of PbI2 nanostructure is important to improve the perovskite morphology.