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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 2D polymers (2DPs) are promising as structurally well‐defined, permanently porous, organic semiconductors. However, 2DPs are nearly always isolated as closed shell organic species with limited charge carriers, which leads to low bulk conductivities. Here, the bulk conductivity of two naphthalene diimide (NDI)‐containing 2DP semiconductors is enhanced by controllably n‐doping the NDI units using cobaltocene (CoCp₂). Optical and transient microwave spectroscopy reveal that both as‐prepared NDI‐containing 2DPs are semiconducting with sub‐2 eV optical bandgaps and photoexcited charge‐carrier lifetimes of tens of nanoseconds. Following reduction with CoCp₂, both 2DPs largely retain their periodic structures and exhibit optical and electron‐spin resonance spectroscopic features consistent with the presence of NDI‐radical anions. While the native NDI‐based 2DPs are electronically insulating, maximum bulk conductivities of >10⁻⁴ S cm⁻¹are achieved by substoichiometric levels of n‐doping. Density functional theory calculations show that the strongest electronic couplings in these 2DPs exist in the out‐of‐plane (π‐stacking) crystallographic directions, which indicates that cross‐plane electronic transport through NDI stacks is primarily responsible for the observed electronic conductivity. Taken together, the controlled molecular doping is a useful approach to access structurally well‐defined, paramagnetic, 2DP n‐type semiconductors with measurable bulk electronic conductivities of interest for electronic or spintronic devices.