A method for preparing CIGS absorber layers using CIGS nanoparticles on a substrate comprises one or more annealing steps that involve heating the CIGS nanoparticle film(s) to dry the film and possibly to fuse the CIGS nanoparticles together to form CIGS crystals. Generally, at least the final annealing step will induce particle fusion to form CIGS crystals. Reactive gas annealing has been found to facilitate the growth of larger grains in the resulting CIGS absorber layers and lead to improved photovoltaic performance of those layers. It is suspected that the presence of carbon in CIGS nanoparticle films hinders grain growth and limits the size of crystals which can be obtained in CIGS films upon annealing. It has been discovered that exposing the CIGS nanoparticle films to a reactive atmosphere containing sulfur can decrease the amount of carbon in the film, resulting in the growth of larger CIGS crystals upon annealing.