Impact of excess lead on the stability and photo-induced degradation of lead halide perovskite solar cells

The stability of perovskite solar cells exposed to prolonged solar irradiation is a major concern that has not been thoroughly investigated in the past. In this investigation, devices fabricated with the architecture, glass/ITO/PEDOT:PSS/MAPbI₃/PCBM/Ag using Pb acetate as a source material, were found to operate with an efficiency of about 13%. Adding excess amounts of Pb to the precursor resulted in a presence of Pb in the perovskite (PVS) active layer. In addition, the impact of photo-induced degradation on the device efficiency was investigated. X-ray Diffraction and Time-resolved Photoluminescence analyses were used to evaluate the impact of excess Pb before and after illumination. The results indicate that changes in the crystallinity occur without any significant decrease in charge carrier lifetimes, and this is attributed to the formation of the degradation product, PbI_2. This by-product has been shown to have beneficial effects due to its influence in passivating grain boundaries and in altering the band structure at the interface between the active layer and the electron transport layer. This work demonstrates that 5 mol% excess Pb is the optimal concentration with respect to efficiency and stability of these devices. The devices can retain more than 50% of their initial efficiency after 1 h of simulated solar exposure, when compared to 0 mol% and 10 mol% excess Pb samples.