Photoluminescence study of polycrystalline photovoltaic CdS thin film layers grown by close-spaced sublimation and chemical bath deposition

Photoluminescence PL measurements were used to study the effect of postdeposition treatments by annealing and CdCl2 activation on polycrystalline CdS layer grown by close-spaced sublimation CSS and chemical bath deposition CBD. CdS films were either annealed in a temperature range of 200–600 °C or CdCl2 treated between 300–550 °C. The development of “red, ” “intermediate orange, ” “yellow, ” and “green ” luminescence bands is discussed in comparison with PL assignments found in literature. PL spectra from CdS layer grown by CSS are dominated by the yellow band with transitions at 2.08 and 1.96 eV involving Cdi-A, VS-A complex states where A represents an acceptor. Green luminescence bands are observed at 2.429 and 2.393 eV at higher annealing temperature of 500–600 °C or CdCl2 treatment above 450 °C, and these peaks are associated with zero and a longitudinal optical phonon replica of “free-to-bound ” transitions. As grown CBD-CdS films show a prominent red band with four main peaks located at 1.43, 1.54, 1.65, and 1.77 eV, believed to be phonon replicas coupled with local vibrational modes. This remains following postdeposition treatment. The red luminescence is associated with VS surface states and in the case of CdCl2 treatment with VCd-ClS centers. Postdeposition treatments of CBD and CdS promote the evolution of an intermediate orange band at 2.00 eV, most likely a donor-acceptor pair, and a yello