The temperature (T) dependence of the yellow-red photoluminescence band is studied in the range from 300 K to 10 K. From the simultaneous intensity and lifetime measurements it is possible to separate unambiguously the radiative and non-radiative components of the decay process. From the analysis of the non-radiative component a model is suggested in which potential barriers can confine excitons, limiting in this way the possible paths for non-radiative recombination.