Among the perovskite solar cells that are attracting attention as next-generation solar cells, tin perovskite is expected to be a lead-free perovskite material owing to its lack of toxic lead content and exemplary light absorption properties. However, compared to lead perovskite, tin perovskite exhibits inferior photoelectric conversion efficiency and atmospheric stability, necessitating performance enhancement. The observed deficiencies in the photoelectric conversion efficiency and atmospheric stability are attributed to the oxidation of Sn(II) to Sn(IV) and the occurrence of self-p doping. Both are mainly caused by defects in the crystal owing to crystal growth and exposure to air. In other words, (i) the presence of defects in the crystal causes the appearance of deep levels in the band gap, and (ii) these defect levels trap carriers and promote recombination, thereby preventing smooth charge transfer and lowering the the conversion efficiency [1].

We have previously reported that the “Sn-rich and I-poor condition” is beneficial for controlling the formation of defect levels in the tin single perovskites FASnI₃ and MASnI₃ [2]. In recent years, double perovskites have attracted attention owing to the ease of tuning material properties, such as bandgap and light absorption characteristics, compared to single perovskites. In particular, in Sn/Ge double perovskites, the defect formation energies are high, and it has been reported that defects such as antisites (e.g., Sn_I or Ge_I) and vacancies (V_I) have relatively less severe effects [3]. In addition, when tin perovskites are used as tandem solar cells, the bandgap is too small. However, it has been reported that the band gap can be increased by replacing some or all of the iodine at the x site with bromine [4]. Therefore, in this study, the double perovskite MA₂SnGeI₆, in which germanium is substituted for tin single perovskite, and MA₂SnGeI₄Br₂, in which a part of the x-site of the structure is substituted with bromine, were selected as the target structures. We analyzed the defect structures by calculating the defect levels and defect formation energies using first-principles calculations and searched for clues to improve the conversion efficiency.

Comparing tin single perovskite with Sn/Ge double perovskite, the analysis of effective mass analysis shows that Ge alloying leads to a better balance between electrons and holes, and thus Sn/Ge double perovskite is expected to have improved device performance. From the point of view of the defect structure, it was found that in all structures, the anti-site defect I_Sn, where tin is replaced by iodine, and the tin vacancy defect V_Sn have low defect formation energies, indicating a tendency for tin to be easily displaced. In addition, defect levels that appeared within the band gap in the single perovskite were transferred to the valence band and conductor, which are outside the band, by the double perovskite. The appearance of stable defect levels was found to be strongly dependent on the chemical potential of the perovskite constituent elements. Similar to tin single perovskites, it was found that defect level formation in Sn/Ge double perovskites can be controlled by adopting “Sn-rich and I-poor conditions.” The results for MA₂SnGeI₄Br₂ are presented in our poster.