A method is provided for forming a solution-processed metal and mixed-metal selenide semiconductor using selenium (Se) nanoparticles (NPs). The method forms a first solution including SeNPs dispersed in a solvent. Added to the first solution is a second solution including a first material set of metal salts, metal complexes, or combinations thereof, which are dissolved in a solvent, forming a third solution. The third solution is deposited on a conductive substrate, forming a first intermediate film comprising metal precursors, from corresponding members of the first material set, and embedded SeNPs. As a result of thermally annealing, the metal precursors are transformed and the first intermediate film is selenized, forming a first metal selenide-containing semiconductor. In one aspect, the first solution further comprises ligands for the stabilization of SeNPs, which are liberated during thermal annealing. In another aspect, the metal selenide-containing semiconductor comprises copper, indium, gallium diselenide (CIGS).