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

Zinc phosphide (ZnP) 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/ZnP 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 Zn)P, at the Mg/ZnP interface. Interestingly, Mg(Mg Zn)P is epitaxially grown on ZnP with the orientation relationship of 21̅1̅0||100 due to interdiffusion. The lattice mismatch of the Mg(Mg Zn)P layer on the ZnP substrate is less than 0.5%, and this is favorable for carrier transport across the interface. Mg(Mg Zn)P is the material suggested as "n-type Mg-doped ZnP" 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 Zn)P/ZnP heterointerface should be established.