Development of a New Class of Perovskite Quantum Dots for Diverse Applications

Chalcogenide perovskites have recently attracted much attention as a new class of optoelectronic materials^[1]. Typical chalcogenide perovskites consist of the composition IIA-IVB-VIA₃, and the prototypical material is BaZrS₃. They are characterized by optoelectronic properties intermediate between oxide and halide perovskites and exhibit high stability due to the presence of divalent anions, similar to oxides. In particular, chalcogenide perovskite has a band gap that can respond to visible light, a high optical absorption coefficient, and high stability enough to withstand aqueous environments, making it a candidate for next-generation perovskite absorber material to replace halides in photovoltaics. The high synthesis temperature of chalcogenide perovskites (which typically requires temperatures well above 600°C) makes their application to the thin-film solar cells a challenging task. On the other hand, it has been reported that the band-edge absorption in BaZrS₃ is extraordinary strong^[2], indicating that it can also exhibit excellent luminescent properties^[3]. Against this background, we are developing chalcogenide perovskite nanocrystals with multiple objectives. One is to develop highly durable and luminescent quantum dots as materials for color conversion and emission layers in displays and lighting, and the other is to develop nanocrystals as precursors for low-temperature deposition of the absorber layer in thin-film solar cells. Colloidal nanoparticles of BaZrS₃ were synthesized using dithiocarbamate complexes as precursors, using our independently developed recipe (similar to that of Ref [4]). While the X-ray diffraction patterns of the synthesized nanoparticles were consistent with the crystal peaks of BaZrS3, they exhibited a wide particle size distribution, irregular shape, and poor orange emission properties with FWHM &gt; 100 nm and PLQY &lt; 10%. Major improvement of synthesis technique is needed as a luminescent material, since monodispersion and narrow emission peak are required in practical applications. On the other hand, the optical absorption of the dispersed solution for blue light was demonstrated to be several to ten times higher than that of commercially available InP quantum dots. Detailed analysis of XRD peaks and compositions indicates that the synthesized BaZrS₃ is sulfur deficient, a possible origin of the low luminescence properties. However, theoretical calculation predicted that sulfur vacancies do not create deep defects. Therefore surface defects may dominate the nonradiative recombination. We are currently working with universities to improve the performance. Osaka University and Tohoku University are working to improve luminescence performance by understanding the synthesis mechanism and improving synthesis techniques including core-shell formation. Single-particle spectroscopy by Prof. Vacha at Tokyo Institute of Technology has shown that BaZrS₃ exhibited a very narrow luminescence peak as a single quantum dot. We are also collaborating with groups at Tokyo Institute of Technology and Ritsumeikan University to develop chalcogenide perovskite thin-film solar cells using BaZrS₃ nanoparticles or their precursors and have so far achieved in depositing BaZrS₃ thin-film. Meanwhile, we are exploring potential collaborators for diverse applications, from light emitting components and devices to photovoltaics, sensors and catalysts, and medical applications.<br/>[1] A. Swarnkar et al., Chem. Mater. 2019, 31, 565-575.<br/>[2] Y. Nishigaki et al., Sol. RRL 2020, 1900555.<br/>[3] K. Hanzawa et al., J. Am. Chem. Soc. 2019, 141, 5343-5349.<br/>[4] R. Yang et al., J. Am. Chem. Soc. 2022, 144, 35, 15928-15931.