Tunability of electronic and optical properties of the Ba-Zr-S system via dimensional reduction

Transition metal sulfide perovskites offer lower band gaps and greater tunability than oxides, along with other desirable properties for applications. Here we explore dimensional reduction as a tuning strategy using the Ruddlesden-Popper phases in the Ba-Zr-S system as a model. The three dimensional perovskite BaZrS$_3$ is a direct band semiconductor, with a band gap of 1.5 eV suitable for solar photovoltaic application. However, the three known members of the Ruddlesden-Popper series, are all indirect gap materials, and additionally have lower fundamental band gaps. This is accompanied in the case of by a band structure that is more favorable for carrier transport for oriented samples. The layered Ruddlesden-Popper compounds, show significantly anisotropic optical properties, as may be expected. The optical spectra show tails at low energy, which may complicate experimental characterization of these materials.