Interfacial Modification of Perovskite Solar Cells Using an Ultrathin MAI Layer Leads to Enhanced Energy Level Alignment, Efficiencies, and Reproducibility.

For the first time, we intentionally deposit an ultrathin layer of excess methylammonium iodide (MAI) on top of a methylammonium lead iodide (MAPI) perovskite film. Using photoelectron spectroscopy, we investigate the role of excess MAI at the interface between perovskite and spiro-MeOTAD hole-transport layer in standard structure perovskite solar cells (PSCs). We found that interfacial, favorable, energy-level tuning of the MAPI film can be achieved by controlling the amount of excess MAI on top of the MAPI film. Our XPS results reveal that MAI dissociates at low thicknesses (<16 nm) when deposited on MAPbI. It is not the MAI layer but the dissociated species that leads to the interfacial energy-level tuning. Optimized interface energetics were verified by solar cell device testing, leading to both an increase of 19% in average steady-state power conversion efficiency (PCE) and significantly improved reproducibility, which is represented by a much lower PCE standard deviation (from 15 ± 2% to 17.2 ± 0.4%).