Cu<sub>2</sub>ZnSn(S, Se)<sub>4</sub> (CZTSSe) is a promising alternative absorber material for thin-film photovoltaic applications because of its earth-abundant constituents, tunable band gap, and high optical absorption coefficient. Using binary and ternary chalcogenide nanoparticles as precursors we have developed a chemical route to produce high efficiency CZTSSe photovoltaic (PV) devices via solution based methods. The printed CZTSSe films show an interesting microstructure consisting of an upper micrometer-sized polycrystalline layer (large-grain layer) and a bottom fine-grain layer. In this paper, we present our results on characterization of the layers including composition, electronic and optical properties. Based on the observed properties we develop a numerical model for the CZTSSe PV device and present the simulation results. We anticipate that the combination of detailed characterization and device model will help us better understand the limitations of our current devices and indicate potential improvement paths.