Impacts of PbI2 on high-efficiency perovskite solar cells: exploring intercalation orientations and defects

Abstract

Excess lead iodide (PbI₂) is commonly found in high-efficiency perovskite solar cells (PSCs). However, the impacts of the band alignments, passivation effects and defects caused by PbI₂ on the photovoltaic performance of PSCs remain undisclosed. Here, using first-principles calculations, we reveal that the intercalation orientation of the layered PbI₂ and interfacial terminations of PbI₂/perovskite dominate the functions of PbI₂ in perovskites. Among the PbI₂/CsPbI₃ heterointerfaces with various terminations, the PbI₂‖CsPbI₃–CsI, PbI₂‖CsPbI₃–Pb and PbI₂–Pb⊥CsPbI₃–I heterointerfaces are energetically favorable, which are formed by the layered PbI₂ parallelly (‖) interfaced with the CsI- and Pb-terminated (1 1 1) planes of CsPbI₃ and the Pb-terminated PbI₂ vertically (⊥) interfaced with the I-terminated (1 1 1) plane of CsPbI₃, respectively. The parallel and vertical intercalations of the layered PbI₂ in CsPbI₃ result in moderate type-I and pronounced type-II band alignments, respectively, explaining the discrepancies observed in experiments. The defect-free PbI₂ exhibits potential passivation effects in the perovskite, while the easily formed V_I and I_i defects in photoactive PbI₂ at the interface can result in deep levels. Nonadiabatic molecular dynamics simulations reveal that the deep-level at the PbI₂‖CsPbI₃–CsI heterointerface leads to severe carrier nonradiative recombination, while those at the PbI₂‖CsPbI₃–Pb and PbI₂–Pb⊥CsPbI₃–I heterointerfaces are not active recombination centers. To retain the beneficial effects of PbI₂ on CsPbI₃ solar cells by improving carrier lifetime and efficient carrier transfer, it is a promising approach to promote the parallel intercalation of thin-layered PbI₂ with the Pb-terminated (1 1 1) plane of the perovskite (PbI₂‖CsPbI₃–Pb). This study provides a comprehensive understanding of the influence of excess PbI₂ on perovskite and offers insights for optimizing its function in achieving high-efficiency PSCs.