A Processing Route to Chalcogenide Perovskites Alloys with Tunable Band Gap via Anion Exchange

Chalcogenides perovskites are of growing interest for photovoltaics (PV), for reasons including their chemical stability, Earth-abundant elements, and tunable direct band gap [1,2]. Unlike their halide cousins, chalcogenide perovskites are challenging to make, requiring high temperature and aggressive sulfurizing environments. This challenge is being met by a growing number of researchers; in recent years there have appeared reports of synthesis of powders, crystals, nanocrystals, and thin films, and strategies to lower the synthesis temperature. Almost without exception, these reports focus on the most widely-studied compound, BaZrS₃. BaZrS₃is highly stable in the perovskite phase, but its direct band gap of 1.9 eV limits its usefulness for single- or dual-junction PV. Last year, we demonstrated that the band gap of BaZr(S,Se)₃ alloys can be tuned over the range of 1.4 – 1.9 eV, entirely within the perovskite phase [3]. This synthesis included co-delivery of sulfur and selenium during epitaxial thin film growth, which is an uncommon set of capabilities.<br/>Here we demonstrate an alternative and more-accessible route to making BaZr(S,Se)₃ alloys with tunable band gap: selenization-after-sulfurization [4]. We first grow BaZrS₃ thin films, and then convert to BaZr(S,Se)₃ alloys by selenium exposure. At sufficiently high temperature, the process of sulfur-selenium anion exchange proceeds rapidly, without disrupting the phase or even the crystal microstructure of the as-growth film. The alloy films resulting from this process have lower defect concentration than the alloys made by direct growth (using co-delivery of sulfur and selenium), and superior electronic transport properties, measured by photoconductive response. The selenization-after-sulfurization process can be carried out immediately after film growth (without breaking vacuum), or after an intervening air exposure. The thin, self-limiting native oxide that forms on BaZrS₃ after air exposure does not impede the subsequent selenization process. We also find that the perovskite structure is stable in high-selenium-content thin films with and without epitaxy. For instance, we demonstrate that a textured, polycrystalline BaZrS₃ thin film grown on a 2” sapphire wafer can be completely converted to an BaZr(S,Se)₃ alloy with band gap of 1.5 eV. These samples have excellent uniformity and no apparent change to the film morphology, microstructure, or crystal structure, besides an increase in unit cell size to accommodate the larger selenium atoms.<br/>The selenization-after-sulfurization process is similar to processing steps developed and established at industrial scales for CIGS manufacturing. It may enable other researchers, having developed innovative ways to synthesis BaZrS₃, to expand their work into alloys with tunable band gap, and move the field closer to relevance for single- and dual-junction PV.<br/><br/>1. R. Jaramillo and J. Ravichandran, "In praise and in search of highly-polarizable semiconductors: Technological promise and discovery strategies," APL Mater. <b>7</b>(10), 100902 (2019).<br/>2. S. Niu, J. Milam-Guerrero, Y. Zhou, K. Ye, B. Zhao, B. C. Melot, and J. Ravichandran, "Thermal stability study of transition metal perovskite sulfides," J. Mater. Res. <b>33</b>(24), 4135–4143 (2018).<br/>3. I. Sadeghi, J. Van Sambeek, T. Simonian, M. Xu, K. Ye, T. Cai, V. Nicolosi, J. M. LeBeau, and R. Jaramillo, "Expanding the Perovskite Periodic Table to Include Chalcogenide Alloys with Tunable Band Gap Spanning 1.5–1.9 eV," Adv. Funct. Mater. <b>33</b>(41), 2304575 (2023).<br/>4. K. Ye, I. Sadeghi, M. Xu, J. Van Sambeek, T. Cai, J. Dong, R. Kothari, J. M. LeBeau, and R. Jaramillo, "A Processing Route to Chalcogenide Perovskites Alloys with Tunable Band Gap via Anion Exchange," Adv. Funct. Mater. 2405135 (2024).