Influence of the CdCl₂ thermal annealing on the luminescent properties of CdS-CSVT thin films

In this work we present results about the influence of a postdeposition heat treatment with CdCl₂ on CdS films grown by the Close Spaced Vapour Transport (CSVT) technique. Such heat treatment has been used as a standard procedure to improve the overall performance of CdTe/CdS based solar cells. The films were studied by analyzing the luminescent properties in the range 10-300 K. The room temperature PL-spectrum of the as-grown CdS films showed a very broad band centered at 2.26 eV and a shoulder in the low energy side at 1.80 eV approximately. After the CdCl₂ thermal annealing, the spectrum, also at 300 K, showed better PL characteristics: a strong band in the low energy side at 1.67 eV and a band in the high energy side at 2.47 eV. The analysis at lower temperatures allows us to observe the evolution of these bands. The high energy band becomes more intense and shifts to higher energies reaching an energy close to the energy band-gap at 10 K. The analysis of the PL intensity as a function of temperature allows us also to apply a theoretical model for the quenching of the PL-intensity.