NOVELTY - Formation of semiconductor thin film involves use of precursor material comprising group IB-chalcogenide and/or group IIIA-chalcogenide particles. The amount of group IB or IIIA element and amount of chalcogen in the particles are selected to be at a desired stoichiometric ratio for the group IB or IIIA chalcogenide that provides a melting temperature less than a highest melting temperature found on a phase diagram for any stoichiometric ratio of elements for the group IB or IIIA chalcogenide.    USE - For forming semiconductor thin film (claimed) useful for fabricating solar cells and solar modules; as absorber layer in photovoltaic cells; and as an ink, paste, or paint in solution-deposited absorber layers for solar cells.    ADVANTAGE - The chalcogen has low melting points and forms core-shell nanoparticles having core that is an elemental or binary nanoparticle and the shell is chalcogen coating. The chalcogen melts and quickly reacts with the material of the core nanoparticles. The method provided low-cost, highly tunable, reproducible, and rapid synthesis of a nanoparticulate chalcogenide and chalcogen material for use as an ink, paste, or paint in solution-deposited absorber layers for solar cells. Coating the nanoparticles allows precisely tuned stoichiometry, phase, size, orientation, and/or shape of the chalcogenide crystals in the chalcogenide film e.g., for copper-indium-gallium-sulfo-di-selenide (CIGS) polycrystalline film. The method provides an absorber layer with several desirable properties, including high density, high uniformity, low porosity, and minimal phase segregation.    DETAILED DESCRIPTION - Formation of semiconductor thin film involves forming a precursor material comprising group IB-chalcogenide and/or group IIIA-chalcogenide particles; disposing the particle precursor material over a surface of a substrate; and heating the particle precursor material to react the particles to form a film of a group IB-IIIA-chalcogenide compound. The amount of the group IB or IIIA element and amount of chalcogen in the particles are selected to be at a desired stoichiometric ratio for the group IB or IIIA chalcogenide that provides a melting temperature less than a highest melting temperature found on a phase diagram for any stoichiometric ratio of elements for the group IB or IIIA chalcogenide. The melting temperature is at a eutectic temperature. The group IB or IIIA chalcogenide has a stoichiometric ratio that results in the group IB or IIIA chalcogenide being less thermodynamically stable than the group IB-IIIA-chalcogenide compound. The film is formed from a precursor layer of the particles and ink containing a sodium compound with an organic counter-ion or a sodium compound with an inorganic counter-ion. An INDEPENDENT CLAIM is included for precursor material comprising group IB-chalcogenide particles containing an oxygen-free chalcogenide material in the form of an alloy of a chalcogen with an element of group IB; and/or group IIIA-chalcogenide particles containing an oxygen-free chalcogenide material in the form of an alloy of a chalcogen with elements of group IIIA.    DESCRIPTION OF DRAWING(S) - The figure shows a flow diagram of the fabrication of the chalcogenide layer using inks formed from nanoparticles.