Provided herein are methods of incorporating additives into thin-film solar cell substrates and back contacts. In certain embodiments, sodium is incorporated into a substrate or a back contact of a thin-film photovoltaic stack where it can diffuse into a CIGS or other absorber layer to improve efficiency and/or growth of the layer. The methods involve laser treating the substrate or back contact in the presence of a sodium (or sodium-containing) solid or vapor to thereby incorporate sodium into the surface of the substrate or back contact. In certain embodiments, the surface is simultaneously smoothed.