Radiative defects in chloride-activated CdSe thin films

Sub-gap recombination—not the intrinsic band structure—limits wide-gap Se-based chalcogenide devices, yet how chloride activation rewires radiative pathways has remained unclear. Here we show that a 30 min CdCl₂ anneal transforms evaporated CdSe from porous nanograins into dense micrometer-scale polycrystals and sharpens the optical band edge, reducing the Urbach energy from 85 to 17 meV at 300 K. Combining temperature- and fluence-dependent photoluminescence (PL), time-resolved PL and hyperspectral mapping with hybrid-DFT, we resolve three emissive channels and quantify their mechanisms. A near-edge band is excitonic at low temperature and becomes free-carrier emission above ∼120 K, with linewidth set by a ∼25 meV polar phonon. A sub-gap band at Eg–0.45 eV requires above-gap carriers and quenches with 0.16 eV. A broad ∼1.05 eV band is driven by both above- and below-gap photons, blue-shifts by ∼20 meV from 100–300 K and retains microsecond lifetimes at room temperature. Mapping links infrared emission to edge-rich microstructure where band-edge PL is dimmer, blue-shifted and broadened. Calculations are consistent with chlorine donors, a selenium-vacancy pathway for the sub-gap band, and a cadmium-vacancy–chlorine complex for the infrared band, pointing to concrete routes to suppress sub-gap loss in chalcogenide wide-gap photovoltaics and detectors.