NOVELTY - Fabricating a kesterite film (302) involves providing substrate (202); forming bulk precursor layer on substrate, where precursor layer comprises copper (Cu), zinc (Zn), tin (Sn) and one of sulfur (S) and selenium (Se); forming capping layer on precursor layer, where capping layer comprises one of tin (Sn), sulfur (S) and selenium (Se); and annealing precursor layer and capping layer to produce film.    USE - For fabricating a kesterite film of formula Cu2-xZn1+ySn(S1-zSez)4+q used in photovoltaic device (claimed) and in solar cells.    ADVANTAGE - The method improves properties of inorganic film having copper, zinc, tin and at least one of sulfur and selenium. The film provides high efficiency photovoltaic devices using abundant, environmentally friendly materials at reduced cost. The method controls the concentration and gradient of volatile elements Sn, S and Se within the bulk CZTSSe film to target improved device performance. The use of a solid capping layer, rather than a vapor-phase addition process, provides at least temporarily molten Se-rich compositions during the high-temperature treatment that may assist in recrystallizing the chalcogenide-based thin film (leading to improved grain structure) and act as a temporary capping layer to reduce the probability of elemental loss during the high temperature treatment (leading to better phase purity in the film). The capping layer serves to compensate for the elemental loss due to evaporation (especially critical for the metal ratio, i.e. the Sn content) during the final anneal. The capping layer provides additional elemental chalcogen at the surface of the film that promotes crystallization without forming voids in the bulk of the layer, in contrary to excess chalcogen added in the bulk layer(s).    DETAILED DESCRIPTION - A method of fabricating a kesterite film (302) of formula Cu2-xZn1+ySn(S1-zSez)4+q, involves providing a substrate (202); forming a bulk precursor layer on the substrate, where the bulk precursor layer comprises copper (Cu), zinc (Zn), tin (Sn) and at least one of sulfur (S) and selenium (Se); forming a capping layer on the bulk precursor layer, where the capping layer comprises at least one of tin (Sn), sulfur (S) and selenium (Se); and annealing the bulk precursor layer and the capping layer to produce the kesterite film having values of x, y, z and q for any given part of the film that deviate from average values of x, y, z and q throughout the film by less than 20%.    x=0-1 (preferably 0-0.5);    y=0-1 (preferably 0-0.5);    z=0-1;    q=-1 to 1 (preferably -0.5 to 0.5).    INDEPENDENT CLAIMS are included for the following:    (1) a kesterite film of formula Cu2-xZn1+ySn(S1-zSez)4+q formed by the fabrication method such that values of x, y, z and q for any given part of the film deviate from average values of x, y, z and q throughout the film by less than 20%; and    (2) a photovoltaic device comprising: a substrate; kesterite film absorber layer of formula Cu2-xZn1+ySn(S1-zSez)4+q formed on the substrate by the fabrication method such that values of x, y, z and q for any given part of the film deviate from average values of x, y, z and q throughout the film by less than 20%; an n-type semiconducting layer (402) on the kesterite film; and a top electrode (404) on the n-type semiconducting layer. The photovoltaic device further comprises an electrically conductive layer (204) on the substrate.    DESCRIPTION OF DRAWING(S) - The figure shows cross-sectional diagram illustrating an n-type semiconducting layer having been formed on the kesterite film and a top electrode having been formed on the n-type semiconducting layer.    Substrate (202)    Layer of conductive material (204)    Kesterite film (302)    N-type semiconducting layer (402)    Top electrode (404)