Reactive Epitaxial Formation of a Mg-P-Zn Ternary Semiconductor in Mg/Zn3P2 Solar Cells

Zinc phosphide (Zn3P2) has attracted considerable attention as an environmentally benign and earth abundant photoabsorber for thin-film photovoltaics. It is known that interdiffusion occurs at the Mg/Zn3P2 interface, which is a component of the record device, but the micro- and nanoscopic structures of the interface after interdiffusion have been controversial for over three decades. Here, we report on the formation of a Mg-P-Zn ternary semiconductor, Mg(MgxZn1-x)(2)P-2, at the Mg/Zn3P2 interface. Interestingly, Mg(MgxZn1-x)(2)P-2 is epitaxially grown on Zn3P2 with the orientation relationship of [2 (11) over bar0](0001)(Mg(MgxZn1-x))II[100](011)(Zn3P2) due to interdiffusion. The lattice mismatch of the Mg(MgxZn1-x)(2)P-2 layer on the Zn3P2 substrate is less than 0.5%, and this is favorable for carrier transport across the interface. Mg(MgxZn1-x)(2)P-2 is the material suggested as n-type Mg-doped Zn3P2 or a Mg-P-Zn alloy in the previous studies. Thus, only the optimization of Mg treatment as conducted in the previous studies is insufficient for the improvement of the cell performance. This work clarified that a suitable microstructure and band structure around Mg(MgxZn1-x)(2)P-2/Zn3P2 heterointerface should be established.