Current Status of Earth-Abundant Cu₂SnS₃-Based Materials for Next-Generation Solar Cells

Recently, low-cost and nontoxic compounds have garnered considerable attention for applications in sustainable energy devices such as solar cells, photocatalysts, and thermoelectric materials for securing resources and protecting the global environment. In compound solar cells, earth-abundant compounds are needed to develop highly efficient sustainable solar cells that can replace the existing CuInxGa1-xSe2 (CIGS) and CdTe. In particular, these proposed semiconductors consist of earth-abundant elements such as Cu2 ZnSnS4, Cu2O, CuSbS2, Sb2S3, Tin sulfide, and FeS. Among them, Cu2SnS3 (CTS) is expected to have a high absorption coefficient and suitable bandgap (E_g) for solar cells that can absorb visible light.1) In addition, CTS thin films are used as the bottom layer of tandem solar cells because they have a relatively narrow gap (approximately 0.9 eV).1) The history of CTS solar cells began with the report of a Schottky diode based on a CTS with a photovoltaic power conversion efficiency (PCE) of 0.11% by Kuku and Fakolujo in 1987.2) Araki group optimized the atomic composition ratios of CTS thin films and achieved a high PCE of 4.29%.3) Subsequently, several researchers have reported that adding Na to CTS films improved the PCE.4, 5) Moreover, our group achieved an efficiency of 5.24% by 2021.6) Various elements such as Na, Ag, Ge, Sb, and Ni have recently been added to CTS solar cells to improve their PCE. Among these, Cu1-xAgxSnS3 (CATS) and Cu2Sn1-y GeyS3 (CTGS) are expected to be applied to the band-graded structures used in highly CIGS solar cells 7), as shown in Fig. 1. As shown in Fig. 1 (a), the excited photogenerated carriers cause interface recombination near the light-absorption layer and the n-type semiconductor, thereby preventing efficiency improvement. As shown in Fig. 1 (b), high-efficiency CIGS solar cells achieve high efficiency by preventing interface recombination through band swelling near the interface with the p-n junction. In CATS and CTGS, the valence band maximum of CATS and the conduction band minimum of the CTGS solid solution can be continuously controlled by changing the Ag/(Ag + Cu) x and Ge/(Ge + Sn) ratios y, respectively. Indeed, PCE of 4.07%8) for CATS solar cells and 6.0%9) for CTGS solar cells have been reported, and CTGS solar cells with a band-grading structure and PCE of 6.73% were reported in 2017.10) However, the PCE of CATS and CTGS solar cells are still low. Our group has researched CATS and CTGS solar cells to develop high-efficiency band-graded CTS-related solar cells. In particular, it has been reported that the low x region, such as x \quad =0.03, is suitable for solar cells because of the correlation between the carrier lifetime measured by time-resolved photoluminescence (PL) and the column size measured by X-ray diffraction.11) However, the defect properties against the composition ratio of x have not been explained. In this study, to evaluate the formation of Ag-related defects, low-temperature (LT) PL was investigated, focusing on CATS solar cells with x \quad =0.10 a relatively high PCE was obtained.