  The development of tandem photovoltaics and photoelectrochemical solar cells
requires new absorber materials with band gaps in the range of ~1.5-2.3 eV, for
use in the top cell paired with a narrower-gap bottom cell. An outstanding
challenge is finding materials with suitable optoelectronic and defect
properties, good operational stability, and synthesis conditions that preserve
underlying device layers. This study demonstrates the Zintl phosphide compound
CaZn$_2$P$_2$ as a compelling candidate semiconductor for these applications.
We prepare phase pure, 500 nm-thick CaZn$_2$P$_2$ thin films using a scalable
reactive sputter deposition process at growth temperatures as low as 100
{\deg}C, which is desirable for device integration. UV-vis spectroscopy shows
that CaZn$_2$P$_2$ films exhibit an optical absorptivity of ~10$^4$ cm$^-$$^1$
at ~1.95 eV direct band gap. Room-temperature photoluminescence (PL)
measurements show near-band-edge optical emission, and time-resolved microwave
conductivity (TRMC) measurements indicate a photoexcited carrier lifetime of
~30 ns. CaZn$_2$P$_2$ is highly stable in both ambient conditions and moisture,
as evidenced by PL and TRMC measurements. Experimental data are supported by
first-principles calculations, which indicate the absence of
low-formation-energy, deep intrinsic defects. Overall, our study should
motivate future work integrating this potential top cell absorber material into
tandem solar cells.
