Photoluminescence and electroluminescence mechanisms at polycrystalline CdS in air and in contact with aqueous electrolytes

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Abstract
Room-temperature photoluminescence (PL) and electrolyte electroluminescence (EL) spectra of polycrystalline CdS samples, in air and in contact with aqueous electrolytes, have been studied as a function of the atmosphere and temperature of annealing, light excitation intensity and wavelength, applied bias, electrolyte composition, and photoetching treatments. Both luminescence spectra are characterized by two broad bands centered at about 770 and 920 nm, associated with a red (R) and an infrared (IR) emission, respectively. According to our experimental results, the R band can be attributed to radiative recombination between electrons trapped at sulfur vacancies, localized at about 0.79 eV below the bottom of the conduction band, and valence-band free holes. The IR emission, on the other hand, is associated with a radiative recombination process involving electrons trapped at sulfur vacancies and holes trapped at cadmium vacancies located at about 0.26 eV above the top of the valence band. The luminescence spectrum shape is shown to depend on the rate of hole injection into the semiconductor and on the concentration of cadmium and sulfur vacancies. From the analysis of the R and IR band intensities, in both PL and EL spectra, valuable information about the generation and spatial distribution of vacancies under different annealing treatments can be obtained. Such information is of interest for the fabrication of high-performance liquid-junction solar cells based on CdS polycrystalline electrodes.