Fabrication and Optimization of CdSe Solar Cells for Possible Top Cell of Silicon-Based Tandem Devices

Silicon-based tandem solar cells are regarded as one of the most feasible ways to break the single-junction Shockley–Queisser limit efficiency and further reduce the cost of solar electricity. Recently, wide-bandgap ( ≈ 1.7 eV) perovskite solar cells have drawn intense research interest as the top cell for Si-based tandem devices. Despite significant progress in device efficiency, the unsatisfactory stability of perovskites is still a huge concern. Besides halide perovskites, there are many inorganic semiconductors worthy of investigation. It is believed that cadmium selenide, a binary compound enjoying outstanding optoelectronic properties, high stability, and low cost, is very promising as the top cell for Si-based tandem devices. Herein, the CdSe thin-film solar cells that have been neglected for 3 decades are revisited. Using rapid thermal evaporation, high-quality CdSe thin films with large grain size, high-photoluminescence, small Stokes shift, and intrinsic n-type conductivity are obtained. Furthermore, CdSe solar cells are redesigned and a champion efficiency of 6.00% is achieved. Through in-depth analysis, it is identified that bulk and interface defects limit the open-circuit voltage and hence device performance. This work highlights the great potential of CdSe solar cells as top cells for Si-based tandem devices.