Solution-Processed Semitransparent CZTS Thin-Film Solar Cells via Cation Substitution and Rapid Thermal Annealing

Semitransparent solar cells are able to capitalize on land scarcity in urban environments by co-opting windows and glass structures as power generators, thereby expanding the capacity of photovoltaics to meet energy needs. To be successful, devices must be efficient, possess good visual transparency, long-term stability, and low cost. Copper zinc tin sulfide is a promising thin-film material that consists of earth-abundant elements. For optical transparency, the usual molybdenum back contact is replaced with a transparent conducting oxide (TCO). However, due to subsequent high-temperature annealing, the TCO degrades, losing conductivity, or forms a poor interface with CZTS. Lower temperatures mitigate this issue but hinder grain growth in CZTS films. Herein, cadmium substitution and silver and sodium doping are used to aid grain growth and improve film quality at lower annealing temperatures. Thin molybdenum is sputtered on TCO to help improve the interface transition postannealing by conversion to MoS₂. Rapid thermal processing is used to minimize high-temperature exposure time to preserve the TCO. With these methods, a semitransparent device with a front illumination efficiency of 2.96% is demonstrated.