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

摘要

Photoluminescence (PL) measurements were used to study the effect of postdeposition treatments by annealing and CdCl_2 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 ℃ or CdCl_2 treated between 300-550 ℃. 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 (Cd_I-A), (V_s-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 ℃ or CdCl_2 treatment above 450 ℃, 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 V_s surface states and in the case of CdCl_2 treatment with (V_(Cd)-Cl_s) 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 yellow band at 2.12 eV correlated with (Cd_I-V_(Cd)) centers. The green luminescence bands observed at 2.25 and 2.34 eV are associated with transitions from deep donor states (e.g., Cd_I) to the valence band. These states form due to crystallinity enhancement and lattice conversion during annealing or CdCl_2 activation. Observed changes in PL bands provide detailed information about changes in radiative recombination centers in CdS layer, which are suggested to occur during device processing of CdTe/CdS thin film solar cells.