Ionic Self-Diffusion Approaches: Enhancing Stability and Efficiency in Perovskite Solar Cells through Ion Migration Suppression and Fermi Level Tuning

Lead halide perovskites have garnered attention as potential energy-harvesting materials for optoelectronic devices due to their remarkable photovoltaic properties. Among these, the FA cation demonstrates the most promising performance and attributes to the formamidinium lead triiodide (FAPbI₃) perovskite, which has a narrow bandgap (E_g) and great thermal stability. Despite FAPbI₃ perovskite displaying great stability as an absorber layer for perovskite solar cells, issues of device instability persist due to absorbed Pb²⁺ or I^{<span style="font-size:10.8333px">-</span>}at the oxide surface which induced the ion migration of iodine vacancies. This phenomenon generates the hysteresis loop and accelerates degradation rate in photovoltaic performance. In this study, we customized the charge-selective interfaces, revealing the induced adjustment of the Fermi level at the FAPI₃/SnO₂ junction by ionic diffusion into the bulk. This adjustment enhances charge transport and mitigates ion migration. These findings were confirmed using computational simulations and ultraviolet photoelectron spectroscopy. To understand ion migration mechanism at the FAPbI₃/SnO₂ interface, we analyze the normalized ionic and electronic conductance as functions of film thickness through D.C polarization measurements. The space charge zone of cation and anion-treated SnO₂ (NH₄⁺-SnO₂ and Cl^--SnO₂) [1], [2] was found to exhibit reduced zones under induced light. Based on our findings, it is evident that interfaces modified with cationic and anionic treatments lead to a decrease in Pb²⁺ and I^{<span style="font-size:10.8333px">-</span>} absorption at the oxide surface. Additionally, these modifications enhance charge transport while significantly minimizing the occurrence of interfacial defects. As a result of this approach, the power conversion efficiency (PCE) increased to 24.38%, and the operational stability of perovskite solar cells (PSCs) was extended to 1600 hours.

[1] 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)

[2] 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)