Numerical Simulation of the Effect of Mg-Doped TiO₂ Electron Transport Layer on the Performance of Tetragonal MAPbI₃ Perovskite Solar Cells

A systematic SCAPS-1D simulation was performed to evaluate the effects of charge-transport-layer selection, MAPbI₃ crystal phase, TiO₂ modification, and key device parameters on the photovoltaic performance of MAPbI₃-based perovskite solar cells. Twenty-five combinations of five electron transport layers (ETLs) and five hole transport layers were first screened using tetragonal MAPbI₃ as the absorber. The TiO₂/P₃HT combination exhibited the highest power conversion efficiency (PCE) of 23.653% and was selected for subsequent analysis. Comparison of cubic, orthorhombic, and tetragonal MAPbI₃ showed that the tetragonal phase delivered the highest PCE, primarily owing to its higher short-circuit current density despite its relatively lower open-circuit voltage. Li-, Co-, Mg-, and Al-doped TiO₂ ETLs were then evaluated, among which Mg:TiO₂ produced the largest and most consistent performance improvement, increasing the PCE of the tetragonal device to 23.951%, mainly through an enhancement in fill factor. The selected Glass/FTO/Mg:TiO₂/tetragonal-MAPbI₃/P₃HT/Au device was further optimized by varying the absorber thickness, Mg:TiO₂ donor concentration, MAPbI₃ bulk defect density, interface defect density, and back-contact work function. Under the stepwise-selected low-defect conditions, the device achieved a simulated open-circuit voltage of 1.166 V, short-circuit current density of 27.600 mA cm⁻², fill factor of 87.580%, and PCE of approximately 28.18%. The results highlight the importance of jointly optimizing absorber properties, electron-transport characteristics, defect density, and contact energetics, and they provide a theoretical basis for narrowing the experimental design space of high-performance MAPbI₃ perovskite solar cells.