A method for synthesizing a thin film of CZTS such as for use as an absorber in a photovoltaic device. The method includes providing a substrate in a chamber, and, then, depositing a film of CZTS material on the substrate, the CZTS material comprising copper, zinc, tin, and at least on chalcogen species. The depositing includes tuning an optical bandgap of the film with heterovalent alloying. The depositing is performed at low temperatures with the substrate provided in the chamber free of direct/active heating. For example, the substrate may be maintained at a temperature below about 150.degree. C. during the depositing of the film. The heterovalent alloying involves controlling deposition rates for the copper and the zinc to define a copper to zinc ratio set the optical bandgap such as a value between about 1.0 eV and about 2.75 eV.
 The United States Government has rights in this invention under Contract No. DE-AC36-08G028308 between the United States Department of Energy and the Alliance for Sustainable Energy, LLC, the Manager and Operator of the National Renewable Energy Laboratory.