Experiments toward High-Performance Zintl Phosphide Solar Absorbers

Most photovoltaic absorber materials development is done on materials with known or slightly modified structural prototypes; for example, kesterite, chalcopyrite, and zincblende are all derived from mutations to the diamond crystal structure. It is rare and sometimes transformative—such as in halide perovskites—when new structural classes of absorber materials are discovered. The <i>AM</i>₂P₂ (<i>A</i> = alkaline earth cation such as Ba, Ca, Sr and <i>M</i> = 2+ cation such as Zn, Cd) compounds have previously been described as semiconducting zintl phases and are structurally distinct to widely studied PV materials. However, these have attracted recent interest from theory as materials with solar-spectrum matched band gaps and tolerance to intrinsic point defects. BaCd₂P₂ is particularly compelling with a direct 1.45 eV band gap and mostly comprising inexpensive elements already used in solar absorber applications, thus having high-grade feedstocks already available. Here, we report on the synthesis and properties of BaCd₂P₂ and related <i>AM</i>₂P₂ materials. BaCd₂P₂ can be synthesized in powder form from elemental precursors using conventional solid-state reaction methods. The material forms in a <i>P</i>-3<i>m</i>1 crystal structure (CaAl₂Si₂ prototype) and is stable in open air heating up to 300 °C as well as in acids, bases, and at even higher temperatures in inert environments. BaCd₂P₂ powder shows strong band-to-band photoluminescence that is stable at elevated temperature and time-resolved microwave conductivity shows long photoexcited carrier lifetimes of around 30 ns, which is already longer than observed in some much more mature absorber technologies. Since the direct-gap <i>AM</i>₂P₂ materials will be most compelling as thin film absorbers, we also discuss several ongoing efforts to realize these compounds in thin film format.