Sub-bandgap photoluminescence and defect-controlled transport in layered Cu3In5S9 single crystals

High-quality Cu 3 In 5 S 9 single crystals were grown by the Bridgman method and systematically investigated to correlate structural order, defect-related vibrational features, charge transport, and sub-bandgap optical response. X-ray diffraction revealed a highly crystalline monoclinic structure with well-defined reflections, while energy-dispersive X-ray spectroscopy verified the presence of Cu, In, and S as the constituent elements of the crystal. Raman spectroscopy revealed a dominant first-order phonon mode at 288.9 cm -1 , accompanied by Cu–S and In–S vibrational features in the 200–350 cm -1 range, while FTIR spectra exhibited characteristic metal–sulfur stretching bands in the 400–600 cm -1 region. Near-infrared (NIR) absorption measurements revealed a pronounced absorption edge in this spectral range, and Tauc analysis confirmed a direct optical band gap of approximately 1.43 eV. A weak sub-bandgap absorption tail indicates the presence of defect-related localized states, which are likely to influence the charge-transport behavior. Temperature-dependent Hall measurements revealed two distinct mobility regimes: the mobility increases with temperature (μ h ∝ T +3/2 ) in the 150–250 K range, consistent with ionized-impurity scattering, whereas it decreases with increasing temperature in the 270–370 K range, indicating a crossover to phonon-limited transport. The crystals exhibit pronounced photoluminescence under sub-bandgap excitation at 1064 nm, providing clear evidence of a nonlinear excitation pathway consistent with two-photon absorption. The two-photon absorption coefficient is estimated to be approximately 6 × 10 -2 cm -1 . These results establish Cu 3 In 5 S 9 as a structurally ordered layered semiconductor exhibiting coupled defect-mediated transport and nonlinear optical response, highlighting its potential for near-infrared optoelectronic and multiphoton applications.