All-Optical Mid-Infrared Photodetectors Based on Two-Dimensional Metal Halide Perovskites

Lead halide perovskites have received significant attention in solid-state lighting (SSL) due to their high photoluminescence quantum yields, tunable emissions, and ease of solution processing. However, the presence of lead in these materials raises substantial environmental and health concerns. Environmentally friendly copper halides, which exhibit broadband emissions from self-trapped excitons (STEs), are emerging as promising alternatives for energy downconverting phosphors in multifunctional light-emitting applications. In this study, we successfully synthesized three new lead-free organic-inorganic hybrid copper halides: (HA)₂CuI₃ (HA⁺ = C₆H₁₃NH₃⁺, n-hexylammonium cation) and (OA)₄CuX₅ (X = Br, I; OA⁺ = C₈H₁₇NH₃⁺, n-octylammonium cation). (HA)₂CuI₃ crystallizes in the monoclinic space group C2/c, featuring 0D CuI₄ dimers, while (OA)₄CuX₅ (X = Br, I) crystallizes in the triclinic space group P, possessing 0D CuX₄ clusters. These different emission centers lead to distinct optical properties for the materials. Under 257 nm excitation, (OA)₄CuBr₅ and (HA)₂CuI₃ emit broad yellow light with PL peaks at 587 and 573 nm, respectively. Notably, OA₄CuI₅ exhibits white-light emission with two PL peaks at 493 and 616 nm, CIE chromaticity coordinates of (0.38, 0.38), and a high CRI value of 91. Time-resolved PL (TRPL) measurements reveal that their lifetimes extend into the microsecond range. Additionally, we observed that the two excitonic states in (OA)₄CuI₅ have different lifetimes, which we attribute to free excitons and STEs, respectively. Given their broad emission spectra, large Stokes shifts, and long lifetimes, their emissions likely originate from STEs. This work stimulates the exploration of single materials with white-light emissions for potential applications in lighting and display technologies.