Improved Photovoltaic Performance through Ionically Regulated Space Charge Zones in α-FAPbI₃ Perovskite Solar Cells

The alpha-phase formamidinium lead triiodide (α-FAPbI₃) is considered the most promising photoactive layer material due to its narrow bandgap and superior thermal stability for perovskite solar cells (PSCs). However, interfacial lattice defects created on SnO₂ electron transport layer (ETL) due to its oxygen vacancies (V_O) and hydroxide ions (OH^-) during the fabrication of α-FAPbI₃ thin films limit power conversion efficiency (PCE) and increase material degradation rates. Herein, the applied interfacial engineering and the regulation of space charge distribution at the FAPbI₃/SnO₂ interface are discussed, both of which are of paramount significance for solar cells. The interfaces were modified using pre-extracted NH₄⁺ ions [1], which exhibited a low migration barrier energy of 0.21 eV, as determined from density functional theory (DFT) simulations. Due to the low migration barrier energy of NH₄⁺, the FAPbI₃ perovskite top surface is p-doped, effectively aligning with hole transport layer interfaces. The redistribution of space charge zones at the FAPbI₃/SnO₂ interface is accomplished through the regulation of V_O and OH^-, combined with the application of octyl-ammonium iodide (OAI) perovskite top passivation materials. To substantiate the impact of space charge redistribution on the photovoltaic characteristics at the FAPbI₃/SnO₂ interface, parallel conductance measurements were carried out. These measurements entailed the manipulation of FAPbI₃ thin film thickness under both dark and illuminated conditions, with the outcomes assessed through D.C and A.C polarization measurements. These results demonstrate that the regulation of space charge zones effectively enhances the modification of bifacial interfaces, leading to improved crystallinity and charge carrier extraction. Ultimately, this approach results in increased PCE up to 24.38% and enhanced operational stability for 1600 hours of PSCs.<br/><br/><br/>[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)