The future development of Cu2ZnSnS4 (CZTS) as an alternative absorber for low cost and high efficiency thin film photovoltaic modules depends strongly on the understanding and control of the formation of secondary phases in this system. With this aim, a detailed chemical-physical characterization of Zn-rich CZTS films prepared by sulfurization of metallic stacks is presented, combining x-ray diffraction, Raman, and Auger spectroscopy. Under such conditions a reaction pathway for the formation of CZTS by a solid-liquid-gas reaction with binary compounds as intermediates is proposed. This is supported by the detailed depth-resolved analysis of the films annealed at different temperatures and times. The experimental data support a formation mechanism of CZTS that proceeds rapidly when a eutectic Cu2S+SnS mixture melts above 480 degrees C, acting as a flux to react the remaining solid binary phases and sulfur gas present in the annealing atmosphere. During reaction Cu and Sn are completely consumed, while excess Zn segregates towards the back contact regions as shown by a combined Raman-Auger depth-resolved analysis. Using the presented thermal treatment process a device with 5.5% efficiency and a maximum external quantum efficiency of 79% is obtained. (C) 2013 Elsevier B.V. All rights reserved.