Low-Temperature Synthesis of Stable CaZn$_2$P$_2$ Zintl Phosphide Thin Films as Candidate Top Absorbers

TL;DR

This paper reports the low-temperature synthesis of stable CaZn₂P₂ thin films with suitable optoelectronic properties, demonstrating their potential as top absorber materials for tandem photovoltaic applications.

Contribution

It introduces a scalable reactive sputter process to produce phase-pure CaZn₂P₂ films at low temperatures, with promising optoelectronic and stability characteristics.

Findings

CaZn₂P₂ films have a ~1.95 eV direct band gap.

Films exhibit high optical absorptivity (~10^4 cm^-1).

CaZn₂P₂ shows high stability in ambient and moisture conditions.

Abstract

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.