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)2P2, at the Mg/Zn3P2 interface. Interestingly, Mg(MgxZn1–x)2P2 is epitaxially grown on Zn3P2 with the orientation relationship of [21̅1̅0](0001)Mg(MgxZn1–x)||[100](011)Zn3P2 due to interdiffusion. The lattice mismatch of the Mg(MgxZn1–x)2P2 layer on the Zn3P2 substrate is less than 0.5%, and this is favorable for carrier transport across the interface. Mg(MgxZn1–x)2P2 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)2P2/Zn3P2 heterointerface should be established.