In one aspect, a method for forming CIGSSe-based thin films includes depositing at least two layers of particles on a substrate. At least one layer includes a CIGSSe particle having a chemical composition denoted by Cu(InI-xGax)(S1-ySey)2 where 0≦x ≦1 and 0≦y≦1. The particle layers are annealed individually or in combination to form a CIGSSe thin film having a composition profile along the depth of the film In addition, one or more of the particle layers may be also deposited on a pre-existing absorber and annealed to form a film having a composition profile along the depth of the film After depositing thin film precursor layers containing CIGSSe nanoparticles (and/or any other particles) on a suitable substrate in accordance with a desired concentration profile, a subsequent treatment under an Se and/or S containing atmosphere at elevated temperature may be used to convert the precursor layers into a CIGSSe absorber film In a further aspect, a method for forming multinary metal chalcogenide semiconductor layers directly on a substrate from a solution of precursors, includes depositing a plurality of metal chalcogenide particles onto a substrate to form a precursor film A species containing a metal, chalcogen, or combination thereof is dissolved in a solution containing one or more solvents to form a liquid chalcogen medium. The precursor film is contacted with the liquid chalcogen medium at a temperature of at least 50 C to form a multinary metal chalcogenide thin film