Thin film solar cells based on CZTS (Cu2ZnSnS4) have record efficiency in 2013 around 12.6%. Its materials are low cost, non-toxic and earth abundant and this makes it suitable low cost solar cells. Enhancing its efficiency over this limit could be achieved using nanophotonic techniques for guiding light. Our approach here is designing optimum back contact plasmonic nanostructure for increasing light trapping based on Mie theory. Using Mie theory parameters, we could identify position of electric and magnetic dipoles, absorption and scattering efficiency of different nanostructures, and select optimum nanostructure for highest back scattering light and light trapping. Our design methodology for selecting optimum back contact nanostructure includes reducing absorption efficiency, increasing scattering efficiency, and increasing back scattered light. Our simulation results indicate that, light trapping is totally depending on dimension and pitch of closed plasmonic nanostructure, a good enhancement in light trapping achieved using plasmonics structures, after using dielectric coating we found enhancement in generated photocurrent over planar back contact. In addition, our results give clearer picture about how absorption and scattering efficiency of each proposed nanostructure would change back scattered light ratio. For more accurate results, we have done all our simulations using three-dimensional models based on finite element method tool. For verifying our results, we started comparing our results of plasmonic nanostructure over substrate with previous reported work, and results matched accurately. Fabricating optimum gold nanostructure with dielectric coating over back contact of CZTS would result in increasing light trapping and its overall efficiency.