Zintl Phosphide Thin Films as Emerging Materials for Solar Absorption

The discovery of an altogether new structural class of semiconductor is a rare occurrence. For example, in the case of solar absorption, nearly all relevant semiconductors can broadly be described as materials derived from the tetrahedrally-coordinated diamond structure (e.g., Si, III-Vs, II-VIs, chalcopyrites, kesterites). This is part of the reason that new high-performing materials, such as perovskites, which are made up of octahedral bonding motifs, garner so much interest and help generate new materials design concepts. Using high-throughput computational workflows, we recently discovered that several AM₂P₂ (A = Ca, Ba, Sr and M = Cd, Zn) compounds possess the requisite intrinsic materials properties for high optoelectronic performance. The calculations predict solar spectrum matched band gaps, strong optical absorption, and benign intrinsic defects, leading to long photoexcited carrier lifetimes. This family of compounds, which exhibits a mixed octahedral + tetrahedral bonding motif, has been known for several decades but the optoelectronic properties of these materials are almost entirely unexplored experimentally. Thus, AM₂P₂ phosphides might become a new structural class of functional thin film solar absorbers.

We have prepared AM₂P₂ compounds such as BaCd₂P₂, CaZn₂P₂, CaCd₂P₂, and SrZn₂P₂ as bulk powders, thin films, and nanocrystals. Despite the various sample formats, we consistently measure optical properties consistent with computational predictions, including photoexcited carrier lifetimes up to 30 ns at low fluence. This talk will focus on thin films of CaZn₂P₂ and SrZn₂P₂, prepared using a hybrid PVD/CVD reactor. Taking the example of CaZn₂P₂, it is found that crystalline films form at growth temperatures as low as 100 °C. These films exhibit a high optical absorption of ~10⁴ cm^-1 at the ~1.95 eV direct transition and room temperature photoluminescence (PL) measurements show near band edge optical emission. While carbonates are found to form on the surface of CaZn₂P₂ films exposed to atmosphere, the material’s bulk is highly stable in both ambient conditions and moisture, as evidenced by persistent PL and microwave conductivity measurements. While work on AM₂P₂ semiconductors is just beginning, our early results suggest they might become the first candidates in a promising new structural class of optoelectronic materials.