Uniform, high crystalline, (100) crystal orientated perovskite films without PbI_2 residue by controlling the nanostructure of PbI_2

摘要

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 (PbI_2) 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 PbI_2 crystals, the conversion of PbI_2 is incomplete as well. In this work, we propose for the first time to control the nanostructure of PbI_2 to get high crystallinity, identical (100) crystal orientation and PbI_2 residue-free film. Porous PbI_2 with plate structure was obtained by selected solvent annealing (SSA). The reaction rate between PbI_2 and methylammonium iodide (MAI) was accelerated because the contact area between PbI_2 and MAI was enlarged due to the porous nanostructure of PbI_2. Thus, uniform, high crystalline perovskite film without PbI_2 residue was obtained. Furthermore, (100) crystal orientation of perovskite was obtained due to the PbI_2 shape changing from grain to plate, which originated from the changing growth frontier of PbI_2 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 PbI_2 structure and the corresponding structure of perovskite in detail, and shows a reasonable design of PbI_2 nanostructure is important to improve the perovskite morphology.