We present a photoluminescence study of CdTe/CdS thin film photovoltaic devices. Post growth treatment is known to increase the efficiency of the devices from a few percent to over 10%. Using a bevel etch technique, we are able to study the photoluminescence emission originating in the CdTe layer as a function of depth from the CdTe/CdS interface to the surface. By studying the intensity variation of the observed luminescence emission bands, we are able to correlate the changes in the impurity distribution deduced from the photoluminescence data with the changes in the photovoltaic efficiency. The air anneal does not significantly alter the impurity distribution whereas the anneal following CdCl2 deposition produces a redistribution of impurities. We clearly observe the diffusion of sulphur from the CdS into the CdTe layer affecting the impurity distribution. There is evidence for incorporation of oxygen into the lattice as a substitutional impurity with a greater concentration at the CdTe/CdS interface rather than the surface. A change in the concentration of cadmium vacancy-chlorine defect complexes may be related to the change in the photovoltaic efficiency.