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(Mg xZn1- x)2P2, at the Mg/Zn3P2 interface. Interestingly, Mg(Mg xZn1- x)2P2 is epitaxially grown on Zn3P2 with the orientation relationship of [21̅1̅0](0001)Mg(Mg xZn1- x)||[100](011)Zn3P2 due to interdiffusion. The lattice mismatch of the Mg(Mg xZn1- x)2P2 layer on the Zn3P2 substrate is less than 0.5%, and this is favorable for carrier transport across the interface. Mg(Mg xZn1- 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(Mg xZn1- x)2P2/Zn3P2 heterointerface should be established.