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Abstract The development of tandem photovoltaics and photoelectrochemical solar cells requires new absorber materials with bandgaps 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 CaZn₂P₂as a compelling candidate semiconductor for these applications. Phase‐pure, ≈500 nm‐thick CaZn₂P₂thin films are prepared using a scalable reactive sputter deposition process at growth temperatures as low as 100 °C, which is desirable for device integration. Ultraviolet‐visible  spectroscopy shows that CaZn₂P₂films exhibit an optical absorptivity of ≈10⁴ cm⁻¹at ≈1.95 eV direct bandgap. 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. CaZn₂P₂is 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, this study shall motivate future work integrating this potential top cell absorber material into tandem solar cells. 

ABSTRACT A key bottleneck to solar fuels is the absence of stable and strongly absorbing photoelectrode materials for the oxygen evolution reaction (OER). Modern approaches generally trade off between stable but weakly absorbing materials, such as wide bandgap oxides, or strongly absorbing materials that rely on encapsulation for stability and are weakly catalytic, such as the III‐V family of semiconductors. Of interest are materials like transition metal phosphides, such as FeP₂, that are known to undergo beneficial in situ surface transformations in the oxidative environment of OER, though stability has remained a primary hurdle. Here, we report on CaCd₂P₂, a Zintl phase visible‐light absorber with favorable 1.6 eV bandgap, that we identified using high‐throughput computational screening. Using a combination of photoelectrochemical measurements, microscopy, and spectroscopy, we show that CaCd₂P₂undergoes a light‐stabilized surface transformation that renders it stable under alkaline OER conditions. We also show that the well known OER catalyst CoPi can act as a stable co‐catalyst in synergy with the modified in situ CaCd₂P₂surface. The light‐induced stabilizing transformation that CaCd₂P₂undergoes is in sharp contrast to the photocorrosion commonly observed in visible light‐absorbing photoelectrodes. The broader AM₂P₂family of Zintl phases offers a significant opportunity to explore stabilizing interface chemistry and re‐design the manner in which low‐bandgap semiconductors are used for photoelectrochemical energy conversion.