Optoelectronic properties of candidate photovoltaic Cu2PbSiS4, Ag2PbGeS4 and KAg2SbS4 semiconductors

Abstract

High temperature synthesis and optical band gaps are reported for three candidate photovoltaic earth-abundant Cu2PbSiS4, Ag2PbGeS4 and KAg2SbS4 semiconductors. The reported synthesis method is found to be more advantageous for KAg2SbS4 compared to the literature reported synthesis utilizing supercritical ammonia as a reaction medium, which produces a mixture of chalcogenide products. Based on optical diffuse reflectance data, Cu2PbSiS4, Ag2PbGeS4 and KAg2SbS4 have band gaps in the 1.6–1.8 eV range, and are shown to be stable in ambient air for a period of 6 weeks, making them attractive candidates for solar cell applications. Density functional theory (DFT) calculations indicate indirect band gaps for Cu2PbSiS4 and KAg2SbS4, and a nearly direct band gap for Ag2PbGeS4 with the calculated difference between indirect and direct gaps of only 30 meV. The p-type semiconducting behavior of Cu2PbSiS4, Ag2PbGeS4 is also verified by the transport measurments. The 3D connectivity of the polyanionic networks in these compounds results in dispersive valence and conduction bands, which is especially noticeable for KAg2SbS4. This fact is in part attributed to the presence of formally pentavalent SbV in this compound leading to empty Sb 5s orbitals in the conduction band. We discuss the potential of Cu2PbSiS4, Ag2PbGeS4 and KAg2SbS4 for photovoltaic applications based on synthesis, stability, band gap and electronic structure considerations.