Computational Study of SnO2 Polymorphs as Electron Transport Layer for Pb-free Cs2TiBr6-Based Double Perovskite Solar Cells

Abstract

In this study, a two-stage computational approach combining ab initio density functional theory (DFT) and SCAPS-1D device simulation was employed to investigate the Effect of SnO2 polymorphs as electron transport layers (ETLs) in Cs2TiBr6-based double perovskite solar cells (PSCs). Bandgap underestimation was corrected by the DFT+U adjustment, which produced band gaps of 3.80, 3.44, and 3.58 eV for cubic, tetragonal, and orthorhombic SnO2 structures, respectively. The optical absorption coefficients of 10⁴–10⁶ cm⁻¹ confirm their suitability as ETLs. The cubic phase exhibits the highest electron mobility of 192.13 cm² V⁻¹ s⁻¹. Device simulations of FTO/SnO2/Cs2TiBr6/CuAlO2/C structures demonstrate that the tetragonal SnO₂-based device achieved the highest performance (PCE = 9.93%). Further optimization revealed that interfacial trap density significantly increases recombination, reducing VOC and overall PCE. The optimized device shows the maximum efficiency of 11.6%. This work emphasizes the significant influence of SnO2 polymorphic structures and interfacial properties on improving the photovoltaic efficiency of lead-free Cs2TiBr6 double perovskite solar cells.