Cadmium sulfide continues to be the most successful and widely used n-type heterojunction partner in thin film CdTe solar cells. In most cases solar cell performance is enhanced if the CdS films are heat treated prior to the deposition of the CdTe. This paper discusses the effect ofH2annealing on the resistivity of CSS-CdS films and the use of a mobility activation model to explain the observed changes in resistivity. Photoluminescence measurements of CSS CdS films heat-treated in He and inCdCl2vapor have also been carried out. In all cases the heat treatments lead to an increase in the intensity of a photoluminescence band believed to be associated with sulfur vacancies. ©1999 American Institute of Physics.