Low-Energy Defects and Oxidative Stability in Sn and Sn/Ge Perovskites from First-Principles Calculations

Motivated by the low power conversion efficiency of Sn-based perovskite solar cells, primarily attributed to oxidative processes (Sn(II) → Sn(IV)) and the formation of defect levels due to tin loss, we used first-principles density functional theory calculations to analyze the defect structures in Sn single perovskites (FASnI3 and MASnI3) and Sn/Ge double perovskites (FA2SnGeI6 and MA2SnGeI6). We focused on the optical absorption and defect levels, which are closely related to the photoelectric conversion efficiency. The photocarrier effective masses were calculated using the same approach, with the interesting outcome that alloying with germanium seems to balance the transport properties of electrons and holes, possibly enhancing the final device performance. Regarding defect structures, in all investigated systems, the defect formation energies of iodine-on-tin antisite defects (ISn) and tin vacancies (VSn) were low compared to other types of defects, indicating a material’s tendency to lose tin. The formation of tin vacancies, in particular, is a fingerprint of the enhanced oxidation of tin, as the Sn(IV) environment is reproduced. Additionally, in single perovskites, defect levels appear within the bandgap, whereas in double perovskites, these defect levels move inside the band edges. The emergence of stable defect levels was found to depend strongly on the chemical potentials of the constituent elements of the perovskite. The defect level formation in both Sn single perovskites and Sn/Ge double perovskites can be better controlled under Sn-rich and I-poor conditions.