First-principles study of La and Mg doped/co-doped TiO2 for tailoring trap states in the electron transport layer of perovskite solar cells

Doping TiO₂ with La and Mg shows promise in enhancing its performance as an electron transport layer (ETL) in perovskite solar cells. Yet, a comprehensive understanding of the specific contributions of these dopants to TiO₂ properties remains elusive. In the present study, we employ density functional theory calculations with the Hubbard U correction (DFT + U) to explore the structural and electronic characteristics of La-doped TiO₂ and two structural variants co-doped with La and Mg. In one of the two co-doped materials, La and Mg atoms replace O and Ti, respectively (TiO₂–La₍₁₎-Mg₍₁₎), while in the other, La and Mg substitute for two Ti atoms (TiO₂–La₍₂₎-Mg₍₁₎). The singularly La-doped material, TiO₂–La₍₃₎, is formed by replacing one Ti atom with an La atom. Our comprehensive analysis, formation energy and Koopman's calculations, aimed to determine structural stability. The results indicate that under O-rich or Ti-rich conditions, TiO₂–La₍₂₎-Mg₍₁₎ shows the most promising characteristics. Assessment of the electronic structure reveals that TiO₂–La₍₂₎-Mg₍₁₎ and TiO₂–La₍₃₎ have a lower density of trap states compared to TiO₂–La₍₁₎-Mg₍₁₎. Analysis of the optical spectrum indicates that TiO₂–La₍₂₎-Mg₍₁₎ exhibits the highest absorption ability in visible range, closely followed by TiO₂–La₍₁₎-Mg₍₁₎ and TiO₂–La₍₃₎, respectively. In conclusion TiO₂–La₍₂₎-Mg₍₁₎ and TiO₂–La₍₃₎ are identified as the most promising structures, emphasizing the importance of striking a balance between trap states and a narrowed band gap to achieve optimal performance. Furthermore, our findings suggest the potential for efficient TiO₂ co-doped structures under specific growth conditions, highlighting a novel avenue for exploration in both theoretical and experimental domains.