Optical Properties of Organic-Inorganic Hybrid (TMS)₃Cu₂I₅ Thin Films Formed by Mist Deposition

In recent years, Cu-based metal halides such as Cs₃Cu₂I₅[1] have garnered considedrable attention as lead-free metal halides due to their excellent optical properties. These materials exhibit blue emission with a high photoluminescence quantum yield (PLQY) under UV irradiation, high ambient stability, and non-toxicity. These properties make them promising candidates for optical applications, including LEDs,[2] photodetectors[3], and scintillators[4]. Additionally, similar to conventional Pb-based perovskites, Cu-based metal halides can also tune their emission wavelength by adjusting the halogen composition. However, the tunable emission is limited to the blue and green regions, and the green-emissive Cs₃Cu₂Cl₅ suffer from low ambient stability, degrading under air conditions. To address these challenges, we explored organic-inorganic hybrid materials as potential candidates for long-wavelength emissive materials with high PLQYs. Among them, (TMS)₃Cu₂I₅[5] (TMS: trimethyl sulfonium) is a novel material that exhibits yellow emission with a PLQY of 26% in single crystals and high ambient stability. Despite these promising properties, (TMS)₃Cu₂I₅ has been minimally studied, with research focused on single crystal growth and not extending to thin film deposition, which is crucial for device applications. In this study, we successfully deposited (TMS)₃Cu₂I₅ thin films using mist deposition with various precursor composition ratios and investigated their optical properties.
We prepared the precursor solution by dissolving various ratios of (TMS)I and CuI in DMF and DMSO. The precursor mist, generated by ultrasonic transducers, was transferred to heated substrates using carrier gas to form thin films. We analyzed the composition of the deposited (TMS)₃Cu₂I₅ thin films using energy dispersive X-ray spectroscopy (EDX), confirming of the presence of S, Cu, and I, with sulfur originating from TMS. The film compositions were Cu-rich compared to the precursor solution ratios. When the precursor solutions ratio of (TMS)I / CuI was 2.25, the concentration ratio of S:Cu:I reached approximately 3:2:5. The thin films exhibited bright yellow emission with a peak at 550 nm under UV irradiation. Photoluminescence (PL) measurements revealed broad emission spectra with a full width at half maximum (FWHM) of 0.42 eV. Photoluminescence excitation (PLE) spectra showed two distinct peaks at 287 nm and 313 nm. The Stokes shift of (TMS)₃Cu₂I₅ was calculated 1.72 eV. This large Stokes shift is advantageous for LEDs application by reducing self-absorption. These PL/PLE results align with previous reports on the single crystals. We further evaluated the PLQY of (TMS)₃Cu₂I₅ thin films. The PLQY increased as the film composition approached stoichiometry, reaching 60% in the thin films. This enhancement is attributed to the suppression of non-radiative recombination centers caused by impurities as the composition of the thin films reached near stoichiometry.
In the symposium, we will discuss the more detailed crystal structure, composition in the thin films, and optical properties of (TMS)₃Cu₂I₅ thin films deposited by mist deposition.

Reference
[1] T. Jun et al., Adv. Mater., 30, 1804547 (2018)
[2] L. Wang et al., Nano. Lett., 20, 3568-3576 (2020)
[3] j. Ma et al., ACS Appl. Mater. Interfaces, 13, 15409-15419 (2021)
[4] S. Cheng et al., Phys. Status Solidi RRL, 14, 2000374 (2020)
[5] D. Banerjee et al., ACS Appl. Mater. Interfaces, 15, 30455-30468 (2023)