Thin film photovoltaic cells and methods of manufacturing such cells that include one or more diffusion barrier layers configured to provide a relatively smooth growth surface for subsequent deposition of a p-type semiconductor layer. Diffusion barrier layers according to the present teachings may be amorphous, microcrystalline or nanocrystalline layers of materials including molybdenum, conductive oxides, conductive nitrides, conductive carbides, or mixtures thereof. In some cases a diffusion barrier layer may be configured to have surface roughness less than a predetermined threshold value.