BaZrS$_3$ perovskite thin films by H$_2$S sulfurization of solution-processed precursors

Inorganic BaZrS$_3$ perovskite is a potential non-toxic chalcogenide material for photovoltaic applications due to its bandgap tunability, thermodynamic stability and the abundance of constituent elements. This study demonstrates the preparation of BaZrS$_3$ thin films from solution-processed precursors, with which the formation of secondary phases could be avoided. Metal-organic precursors were spin-coated and subsequently sulfurized at various temperatures, ranging from 600 ◦C to 1100 ◦C, using Ar-diluted H$_2$S gas. The atomic ratio of the films,
determined by X-ray fluorescence spectroscopy, reveals an equimolar ratio of Ba and Zr both before and after the sulfurization process. No oxide secondary phases are detected in the sulfurized films. The Le Bail analysis of the X-ray diffractograms reveals the formation of BaZrS$_3$ in the orthorhombic perovskite-type crystal structure (space group Pnma, #62). We observe an increased crystallinity of the films with increasing preparation temperature, accompanied by a decrease of the bandgap from 2.16 ± 0.04 eV at 900 ◦C to 2.02 ± 0.01 eV at 1000 ◦ C. For
sulfurization temperatures of 1100 ◦ C, a secondary Ruddlesden-Popper phase, Ba$_4$Zr$_3$S$_{10}$, is detected. ... mehrKelvin probe measurements of the films prepared at 1000 ◦C indicate n-type conductivity with a work function of ~4.26 eV. Photoelectron yield spectroscopy reveals an ionization energy of ~5.38 eV which defines the position of the valence band maximum. The experimental density of state spectra are found in good agreement with the theoretical data. The sum charge carrier mobility of ~2.4 cm$^2$V $^1$s $^1$ is determined from optical-pump–terahertz-probe spectroscopy measurements.