Adverse Effects of Excess Residual PbI₂ on Photovoltaic Performance, Charge Separation, and Trap-State Properties in Mesoporous Structured Perovskite Solar Cells

Organic-inorganic halide perovskite solar cells have rapidly come to prominence in the photovoltaic field. In this context, CH₃ NH₃ PbI₃ , as the most widely adopted active layer, has been attracting great attention. Generally, in a CH₃ NH₃ PbI₃ layer, unreacted PbI₂ inevitably coexists with the perovskite crystals, especially following a two-step fabrication process. There appears to be a consensus that an appropriate amount of unreacted PbI₂ is beneficial to the overall photovoltaic performance of a device, the only disadvantageous aspect of excess residual PbI₂ being viewed as its insulating nature. However, the further development of such perovskite-based devices requires a deeper understanding of the role of residual PbI₂ . In this work, PbI₂ -enriched and PbI₂ -controlled perovskite films, as two extreme cases, have been prepared by modulating the crystallinity of a pre-deposited PbI₂ film. The effects of excess residual PbI₂ have been elucidated on the basis of spectroscopic and optoelectronic studies. The initial charge separation, the trap-state density, and the trap-state distribution have all been found to be adversely affected in PbI₂ -enriched devices, to the detriment of photovoltaic performance. This leads to a biphasic recombination process and accelerates the charge carrier recombination dynamics.