Impact of Heterointerfaces in Solar Cells Using ZnSnP₂ Bulk Crystals

We report on the optimization of interface structure in ZnSnP₂ solar cells. The effects of back electrode materials and related interface on photovoltaic performance were investigated. It was clarified that a conventional structure Mo/ZnSnP₂ showed a Schottky-behavior, while an ohmic-behavior was observed in the Cu/ZnSnP₂ structure annealed at 300 °C. STEM-EDX analysis suggested that Cu-Sn-P ternary compound was formed at the interface. This compound is considered to play an important role to obtain the ohmic contact between ZnSnP₂ and Cu. In addition, it was clarified that the aqua regia etching of ZnSnP₂ bulk crystals before chemical bath deposition process for the preparation of buffer layer was effective to remove the layer including lattice defects introduced by mechanical-polishing, which was supported by TEM observations and photoluminescence measurements. This means that the carrier transport across the interface was improved because of the reduced defect at the interface. Consequently, the conversion efficiency of approximately 2% was achieved with the structure of Al/ZnO;Al/ZnO/CdS/ZnSnP₂/Cu, where the values of short circuit current density, J_SC, open circuit voltage, V_OC, and fill factor, FF, were 8.2 mA cm^-2, 0.452 V, and 0.533, respectively. However, the value of V_OC was largely low considering the bandgap value of ZnSnP₂. To improve the conversion efficiency, the optimization of buffer layer material is considered to be essential in the viewpoint of band alignment.