Density Functional Theory Study on Alkaline Earth-Based Titanate Perovskite Oxides: Unraveling Their Significance for Solar Cell Application

Charge transport layers (CTLs) and transparent conductive electrodes (TCEs) are important constituents of polymer solar cells (PSCs) and perovskite solar cells (Per-SCs), affecting the efficiency and stability of these devices. We employed density functional theory to study the structural, optoelectronic, thermal, and elastic properties of alkaline earth-based titanate perovskite oxides to determine the appropriate compounds for PSCs and Per-SCs. Based on the calculations, CaTiO₃ exhibits a direct band gap of 3.535 eV, while BeTiO₃, MgTiO₃, SrTiO₃, and BaTiO₃ displayed indirect band gap energies of 3.618, 4.852, 3.193, and 2.960 eV, respectively. Considering the calculated valence and conduction band edges and energy band diagram alignment of the perovskite oxide structures with widely used photoactive layers, SrTiO₃, BaTiO₃, and CaTiO₃ emerge as promising materials to be applied as electron transporting layer (ETL) in the structure of the PSCs and Per-SCs. The findings also reveal that SrTiO₃ and CaTiO₃ exhibit the greatest electron mobility, making them more appropriate candidates for ETL. The minimal exciton binding energy found in SrTiO₃ signifies its high separability and enhances its suitability for efficient carrier generation as the most effective ETL. The results obtained from optical parameters confirmed that the investigated compounds are appropriate candidates for TCE and CTL as they demonstrate low optical conductivity and absorptivity, minimal refractive index, and reflectivity in the solar range of the light spectrum (1–4 eV). The calculated elastic parameters verified that SrTiO₃ and CaTiO₃ are mechanically and thermally stable, which further supports their potential function in solar cells.