Three kesterite thin-film solar cells, Cu2ZnSnSe4 (CZTSe), Cu2ZnSn(S,Se)4 (CZTSSe), and Cu2ZnSnS4 (CZTS), and based on low light intensity measurements, examined the possibility of using kesterite devices for indoor applications.

The use of flexible and highly conducting molybdenum (Mo) foil as a substrate offers several advantages such as a high thermal stability, smooth surface, and chemical inertness for the fabrication of high-efficiency thin film solar cells (TFSCs) by lowering the manufacturing costs. Here, we report a record preliminary efficiency of ∼8% for sputtered-grown Cu 2 ZnSn(S,Se) 4 (CZTSSe) TFSCs on flexible and lightweight Mo foils. Careful studies were focused on identifying the role of preparative parameters such as annealing temperature, absorber composition, and post-preparative optimization to bridge the obtained record efficiency of ∼8% to a previous record efficiency of 7.04% for Naincorporated CZTSSe sputter-based TFSCs. Interestingly, the preliminary record efficiency of ∼8% for our CZTSSe device grown via a scalable sputtering method was achieved by optimizing the absorber quality and post-preparative device optimization. While our preliminary results with a record efficiency demonstrate the potential of sputtering method, there is much scope for further improvement in the device efficiency by thoroughly understanding alkali element doping in the absorber layer. KEYWORDS: Cu 2 ZnSn(S,Se) 4 (CZTSSe), kesterite, thin film solar cells, flexible substrate, Mo foil

Recent efficiency advancements in kesterites have reinforced the use of Cu2ZnSn(S,Se)4 (CZTSSe) in indoor photovoltaic applications. However, the performance of kesterites under low light intensity conditions is mainly hindered by deep‐level defects. In this study, a strategic approach of silver (Ag) and germanium (Ge) cation substitution to cure these defects are employed. The Ag‐doped CZTSSe (CZTSSe:Ag) and Ge‐doped (CZTSSe:Ge) samples experimentally demonstrated a significant improvement in kesterite device performance under all intensities of LED and white fluorescent lamp conditions are prepared. Interestingly, the CZTSSe:Ag device exhibited the highest performance levels, i.e., 1.2–1.5 and 2.5–3 times better than those of Ge‐doped CZTSSe:Ge and undoped CZTSSe, respectively. This improved device performance is mainly attributed to the reduced energy level of deep‐level defects in CZTSSe:Ag. Moreover, these defects assisted in the generation of a larger potential difference between the grain boundary and grain interior in the CZTSSe:Ag sample, attracting minority carriers near the grain boundary. Consequently, the improved carrier separation process reduced the carrier recombination losses and enhanced the power output under low light intensity conditions. This Ag and Ge cation substitution in kesterite is found to be an effective approach to improve the device performance under low light intensity conditions.