Control of Doping in Cu2SnS3 Through Defects and Alloying

As the world's demand for energy grows, the search for cost competitive and earth abundant thin film photovoltaic absorbers is becoming increasingly important. A promising approach to tackle this challenge is through thin film photovoltaics made of elements that are abundant in the Earth's crust. In this work, we focus on Cu₂SnS₃, a promising earth abundant absorber material. Recent publications have presented 3% and 6% device efficiencies using Cu₂SnS₃-based absorber materials and alloys, respectively. However, little is understood about the fundamental defect and doping physics of this material, which is needed for further improvements in device performance. Here, we identify the origins of the changes in doping in sputtered cubic Cu₂SnS₃ thin films using combinatorial experiments and first-principles theory. Experimentally, we find that the cubic Cu₂SnS₃ has a large phase width and that the electrical conductivity increases with increasing Cu and S content in the films, which cannot be fully explained by the theoretical point defect model. Instead, theoretical calcuations suggest that under Cu-rich conditions alloying with an isostructural metallic Cu₃SnS₄ phase occurs, causing high levels of p-type doping; this theory is consistent with experimental Raman and NEXAFS spectroscopy data. These experimental and theoretical works lead to the conclusion that Cu₂SnS₃ films must be grown both S-poor and Cu-poor in order to achieve moderate hole concentrations. These new insights enable the design of growth processes that target the desired carrier concentrations for solar cell fabrication. Using the strategies described above, we have been able to tune the carrier concentration over >3 orders of magnitude and achieve films with p-type doping of 10¹⁸ cm^-3, facilitating future device integration of these films.