Stable blue emission and sub-gap mediated UV photodetection in lead-free 1D CsCu₂Br₃ single crystals

In this work, we report a comprehensive study of the structural, optical, and photoelectrical properties of one-dimensional (1D) CsCu₂Br₃ single crystals. Powder X-ray diffraction (XRD) confirms the orthorhombic Cmcm phase with excellent crystallinity, while thermogravimetric analysis (TGA) demonstrates thermal stability up to ~ 300 °C, with only a minor (~ 6%) extrinsic mass loss near 287 °C. Optical characterization reveals a direct bandgap of 4.09 eV, a low Urbach energy (0.104 eV), and strong blue photoluminescence (451.6 nm) with a large Stokes shift and unusually broad full width at half maximum (FWHM) (~ 250 nm), indicative of pronounced electron-phonon coupling and self-trapped excitons (STEs). Time-resolved PL yields two recombination channels (τ₁ = 10.07 ns, τ₂ = 2.27 ns), consistent with free/shallow carriers and stabilized STEs. Electrical measurements show symmetric Current-voltage (I-V) behavior and UV-enhanced conduction dominated at high fields by a trap-limited space-charge-limited current (SCLC) mechanism (m ≈ 2.45, V_TFL ≈ 13.2 V). Under 365 nm excitation (photon energy below E_g), the device exhibits a reproducible but modest photoconductive response, mediated by sub-gap absorption pathways-namely Urbach-tail states, defect-related channels, and STE manifolds-rather than direct interband excitation. Accordingly, the responsivity at 365 nm is modest (R ≈ 4.38 × 10^-5 A W^-1), requires a higher bias (+ 20 V), and shows trap-limited SCLC signatures, fully consistent with a weak sub-gap absorption regime. Collectively, the structural robustness, stable excitonic blue emission, and sub-gap mediated UV photoresponse position CsCu₂Br₃ as a durable lead-free platform for UV optoelectronics, with maximum efficiency expected under deep-UV excitation (≈ 280-320 nm), to be mapped in future calibrated spectral responsivity studies.