Synthesis of alkaline-earth Zintl phosphides M Zn 2 P 2 ( M = Ca, Sr, Ba) from Sn solutions

Exploration of suitable partner materials (so-called buffer layer or n-type emitter) for each light-absorbing material is essential to practicalize various emerging photovoltaic devices. Motivated by our recent discovery of a partner material, Mg(Mg x Zn 1− x ) 2 P 2 , in Mg/Zn 3 P 2 solar cells, the related series of materials M Zn 2 P 2 ( M = Ca, Sr, Ba) is of interest to the application in pnictide-based solar cells. In this study, we synthesize these materials to evaluate the optoelectronic properties concerning photovoltaic applications. To deal with the difficulties of the high vapor pressure and reactivity of the constituent elements, we utilized Sn as a solvent to reduce their activities during heat treatments. Powders that are mainly composed of M Zn 2 P 2 were obtained by crushing the samples after solution growth, although single-phase crystals of M Zn 2 P 2 could not be obtained in this study. The optical bandgap and the ionization potential of each M Zn 2 P 2 were evaluated through the diffuse reflectance and the photoelectron yield spectroscopy measurements of the powder. As a result, we found that CaZn 2 P 2 would be a promising partner material in photovoltaics based on Zn 3 P 2 and ZnSnP 2 .