Controlled Passivation by Lead Halides of Interfacial Defects of Perovskite Films for Emerging Electronic Applications

In mixed organic–inorganic lead halide perovskites APbX₃ (A = methylammonium (MA) or formamidinium (FA); X is Cl, Br or I), the formation of secondary-phase excess lead iodide (PbI₂) has some positive effects as a grain-boundary passivation on interstitial defects but can be detrimental to device stability and lead to large hysteresis effects in voltage sweeps. We converted PbI₂ into an inactive (NH₄⁺)-PbI₂ compound via pre-extracted NH₄+/SnO₂ by NH₄Cl passivator [1], which effectively stabilizes the perovskite crystalline phase. These effects were discussed with density functional theory (DFT) and shown to grain voltage sweep measurement by conductive atomic force microscopy (C-AFM) and surface photo-voltage by illuminated Kelvin probe force microscopy. We compared the current and voltage properties for insulating domains of PbI2, (Cl)-PbI₂, and (NH₄⁺)-PbI₂ in-situ perovskite films. The ferro-electric properties were discovered on PbI₂ and (Cl)-PbI₂, on the other hand (NH₄⁺)-PbI₂ show no ion migration. This means that inactive (NH₄⁺)-PbI₂ effects can be stabilized the indiscriminate ion movements on the perovskite-based optoelectronics. As a result, the perovskite solar cells (PSCs) achieve an average efficiency of 21.57 ± 0.45 % and best efficiency of 22.25 % [2] due to the dramatically enhanced open circuit voltage by 1.180 V, which is the main effect of efficiency improvement, and increase the operational device stability over 50 days in ambient conditions with ~50 % of relative humidity. It is very curious that the results also show passivated interface is an effective way to modify bifacial defects and electronic structures improving charge configuration with high resistant / low resistant states and its transfer and extraction, and eventually it is leading to new emergent electronic devices.<br/>[1] J. H. Kim, Y. S. Kim, H. R. Jung, and W. Jo, “Chlorine-passivation of the ozone-treated SnO₂ thin films: occurrence of oxygen vacancies for manipulation of conducting states and bipolarities in resistive switching”, Applied Surface Science 555, 149625 (2021).<br/>[2] J. H. Kim, J. H. Park, Y. H. Kim, and W. Jo, “Improvement of Open-Circuit Voltage Deficit via Pre-treated NH₄⁺ Ion Modification of Interface between SnO₂ and Perovskite Solar cells”, Small, 2204173 (2022).