NOVELTY - Preparing (M1) a chalcogen-containing solution, involves contacting at least one chalcogen element, a reducing agent and a liquid medium (L1) under conditions sufficient to produce a homogenous solution, where the reducing agent contains both boron and hydrogen, and is carbon free and metal free.    USE - For preparing chalcogen-containing solution, especially a metal-chalcogenide ink, which is used in forming kesterite film on a substrate, where the substrate is selected from metal foil substrate, glass substrate, ceramic substrate, aluminum foil coated with layer of molybdenum and polymer substrate; and used in the applications of photovoltaic device (all claimed), phase change memory and thin film transistors.    ADVANTAGE - The method is carried out without inert gas protection and in water-free environment; avoids the use of enhancing additives particularly organic polymers acting as binders, surfactants and/or extenders, which greatly reduces the manufacturing cost and complexity of the method; and avoids the use of highly toxic reactants such as hydrazine, hydrazine monohydrate or selenourea during the preparation of kesterite films. The reducing agent used in the method is free of carbon and metal.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) preparing (M2) a metal-chalcogenide ink, involving making the chalcogen-containing solution according to the method (M1), contacting at least one metal source (S1) with the chalcogen-containing solution under conditions sufficient to produce metal-chalcogenide nanoparticles, isolating the metal-chalcogenide nanoparticles, and dispersing the metal-chalcogenide nanoparticles in a liquid medium (L2) to form the metal-chalcogenide ink;    (2) preparing (M3) a kesterite film on a substrate, involving making the chalcogen-containing solution according to the method (M1), contacting at least one metal source (S2) with the chalcogen-containing solution under conditions sufficient to produce metal-chalcogenide nanoparticles containing Cu, Sn, Zn and at least one of S and Se, isolating the metal-chalcogenide nanoparticles, dispersing the metal-chalcogenide nanoparticles in a liquid medium (L2) to form an ink, depositing the ink on the substrate to form a metal-chalcogenide precursor layer on the substrate, and heat treating the metal-chalcogenide precursor layer to form the kesterite film on the substrate;    (3) a photovoltaic device comprising a substrate, a kesterite film absorber layer having formula Cu2-xZn1+ySn(S1-zSez)4+q, formed on the substrate by the method (M3), n-type semiconducting layer on the kesterite film, and a top electrode on the n-type semiconducting layer.    x and y=0-1 (preferably 0-0.5);    z=0-1;    q=(-1)-1 (preferably (-0.5)-0.5).