Zintl Phosphide CaZn₂P₂Thin Films as Candidate Semiconductors for Tandem Top Absorbers

The development of tandem photovoltaics and photoelectrochemical solar cells both calls for exploring new absorber materials with band gaps in the range of ~1.5–2.3 eV, for use in the top cell in conjunction with a suitable narrower-gap bottom cell. An outstanding challenge is to identify materials with suitable baseline optoelectronic and defect properties, good stability in operational environments, and the ability to be synthesized under conditions which will not cause damage to any underlying layers or withstand the synthesis of subsequent layers. Here, we demonstrate that the Zintl-phosphide compound CaZn₂P₂ represents a compelling candidate semiconductor for tandem solar cell applications as a top absorber. We prepare phase pure, ~500 nm-thick CaZn₂P₂ thin films using a scalable reactive sputter deposition process from Ca and Zn metallic targets in PH₃ gas. It is found that crystalline CaZn₂P₂ forms at growth temperatures as low as 100 °C, which is useful for monolithic device integration, but has remained elusive for many existing inorganic absorber materials. UV-vis spectroscopy shows that CaZn₂P₂ thin films exhibit a high optical absorption of ~10⁴ cm^-1 at the ~1.95 eV direct transition. Room temperature photoluminescence (PL) measurements show near band edge optical emission at 1.95 eV and time resolved microwave conductivity (TRMC) measurements indicate a long photoexcited carrier lifetime of up to 30 ns at a fluence of 2 10¹³ cm^-2, further confirming CaZn₂P₂ as an attractive top cell absorber. While carbonates are found to form on the surface of CaZn₂P₂ films exposed to the atmosphere, as expected from a Ca-containing material, the bulk is highly stable in both ambient conditions and moisture, as evidenced by the persistent PL and TRMC measurements. The observed optoelectronic properties of CaZn₂P₂ are supported by first-principles calculations, which show favorable defect properties, especially the absence of low-formation-energy, deep intrinsic defects. Furthermore, CaZn₂P₂ is but one of several AM₂P₂ materials, hinting at a new and exciting family of emerging Zintl phosphide semiconductors. Overall, our study should motivate future work integrating CaZn₂P₂, and similar potential top cell absorber materials into tandem solar cells.