DFT and AMPS-1D simulation analysis of all-perovskite solar cells based on CsPbI₃/FAPbI₃ bilayer structure

Very recently, perovskite solar cells (PSCs) have been proposed with both new device structure and materials to replace the conventional Methyl Ammonium-based structures in order to develop cells with higher operational stability. All-perovskite solar cells are the most emerging class of organic-inorganic PSCs which combine the photogeneration and absorption capability of two different perovskite materials to ensure both photo conversion performance and device operational stability. This simulation work aims to develop a systematic investigation of an all-perovskite solar cell structure made of CsPbI₃/FAPbI₃ heterojunctions using wxAMPS-1D platform supported with Density Functional Theory (DFT) simulations to analyze the band structure of both CsPbI₃ and FAPbI₃ materials. The quantum efficiency, device characteristics, and degradation trend have been optimized against the thickness of perovskite layers. The optimum thicknesses are obtained to be 200 nm for CsPbI₃ layer at an assumed thickness range of∼300 nm for FAPbI₃ as stated in different literature, 30 nm for TiO₂ electron transporting layer, and 80 nm for Spiro-OMeTAD hole transporting layer. The optimized CsPbI₃/FAPbI₃ bilayer all-perovskite solar cell results in 19.94% efficiency (PCE), short-circuit current density (J_sc = 24.5 mA/cm²), and open circuit voltage (V_oc = 1.1 V), and a fill factor (FF) = 74%. The bilayer structure (CsPbI₃/FAPbI₃) shows 20% improvement in external quantum efficiency (in visible range), compared to the cell made of single CsPbI₃ or FAPbI₃ layer. Moreover, the degradation analysis shows that increasing the mid-gap defect density (in both layers) up to 10¹⁵ cm⁻³ was detrimental to device performance.