BaZrS₃ Chalcogenide Perovskite Thin Films by H₂S Sulfurization of Oxide Precursors

Abstract

The earth-abundant ternary compound BaZrS3, which crystallizes in the perovskite-type structure, has come into view as a promising candidate for photovoltaic applications. We present the synthesis and characterization of polycrystalline perovskite-type BaZrS3 thin films. BaZrO3 precursor layers were deposited by pulsed laser deposition and sulfurized at various temperatures in an argon-diluted H2S atmosphere. We observe increasing incorporation of sulfur for higher annealing temperatures, accompanied by a red shift of the absorption edge, with a bandgap of E g = 1.99 eV and a large absorption strength >105 cm–1 obtained for sulfurization temperatures of 1000 °C. X-ray diffraction analysis and SEM indicate enhanced crystallization at the higher annealing temperatures, but no evidence for a crystalline solid solution between the BaZrO3 and BaZrS3 phases is found. The charge carrier sum mobility estimated from optical-pump–terahertz-probe spectroscopy indicates increasing mobilities with increasing sulfurization temperature, reaching maximum values of up to ∼2 cm2 V–1 s–1.

Chalcogenide Perovskites: Tantalizing Prospects, Challenging Materials

“…[ 42 ] Marquez et al confirmed that there is indeed no crystalline solid solution, but they also saw the gradual shift in optical absorption edge for partly sulfurized BaZrO 3 , see Figure a,b. [ 130 ] They reconciled these seemingly contradicting results by postulating the existence of an amorphous BaZr(O,S) 3 phase. Computationally, Vonrüti et al.…”

“…Reproduced with permission. [ 130 ] Copyright 2021, American Chemical Society. c) Ellipsometry data for Ba(Ti,Zr)S 3 and BaZr(S,Se) 3 alloys, extracted bandgaps are indicated by the solid circles.…”

“…These large absorption coefficients have been confirmed by experimental studies, which all agree that α exceeds 2 × 10 5 cm −1 at photon energies of 0.5 eV above the bandgap. [ 24,71,130,144 ] Possibly the most convincing data come from Nishigaki et al., who supplemented DFT calculations with ellipsometry measurements on pelletized powders of several chalcogenide perovskites, and were able to show extremely good agreement. [ 24 ] As a result of the excellent light absorption, an absorber thickness of around 500 nm would be sufficient for high efficiency solar cells, [ 24 ] less than half the material thickness used in commercial chalcogenide thin film solar cells.…”

“…converted PLD‐grown BaZrO 3 into increasingly sulfur‐rich films by heating in H 2 S from 700 to 1100 °C. [ 130 ] The use of oxide precursors carries the inherent problem noted by Clearfield, that complete conversion of an oxide precursor to a sulfide compound requires a combination of high temperatures and long times, e.g., >900 °C for several hours, due to the high stability of the oxides. Thus, routes in which oxide precursors are used, or in which any appreciable oxygen contamination is obtained, will invariably require high temperatures to generate the target compound.…”

“…[ 34 ] Furthermore, it seems difficult to remove all oxygen: for Márquez et al., the S/(O+S) ratio saturated at 0.85 for a 30 min treatment in H 2 S at 1100 °C. [ 130 ] For Wei et al., a strong oxygen peak was recorded by energy dispersive X‐ray spectroscopy (EDS), though not quantified, after a CS 2 sulfurization at 1050 °C for 4 h, see Figure a. [ 71 ]…”