Na induction effects for J–V properties of Cu₂SnS₃ (CTS) solar cells and fabrication of a CTS solar cell over-5.2% efficiency

The effects of Na on Cu₂SnS₃ (CTS) solar cells were investigated by focusing on the electrical properties obtained using electrochemical impedance spectroscopy and Hall measurements to clarify which factors and regions are strongly affected by sulfurization with a NaF layer. The increase in the depletion layer width (x_dep) suggested that the number of non-radiative recombination centers decreased at the p–n interface of the CTS solar cell owing to the sulfurization process with the NaF layer. Thus, the increase in Jsc indicated that the carrier collection width widened as x_dep increased. Furthermore, Voc increased because the CTS lifetime was lengthened. The performance of the CTS solar cells was improved by these factors. Moreover, as a result of an investigation of the dark and light J–V properties, suppression of the crossover behavior was accomplished by sulfurization with the NaF layer. This technique improved the CTS/Mo and CdS/CTS interfaces owing to the Na content, leading to increases in all desirable solar cell properties. Therefore, the induction of Na from a soda-lime glass substrate as well as sulfurization with a NaF layer is required to obtain highly efficient CTS solar cells. As a result of this study, a CTS solar cell with an efficiency higher than 5.24% was achieved.