The $Cu_2ZnSnSe_4$ (CZTS) thin film solar cell has drawn significant attention due to its earth-abundant and non-toxic constituents as a solar cell absorber. CZTS, however, possesses intrinsic drawbacks including phase insta-bility, secondary phases, and defects that harm the device performance. Considering the ease of processing con-ditions to achieve CZTS with a pure-phase, CZTS solar cells were prepared by thermal co-evaporation. The an-nealing conditions including temperature and Se vapor were adjusted in such a way to suppress the Sn-loss and to reduce the thickness of $MoSe_2$. Also, CdS thickness was controlled to improve the incident light collection. Afterwards, we focused on passivating the interface of CZTS - both a front CZTS surface and a back CZTS/Mo interface - to reduce the interfacial recombination accounting for the large open-circuit deficit in most of CZTS solar cells. Structural and electrical properties were studied using scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), x-ray diffraction technique (XRD), J-V and external quantum efficiency measurement (EQE), and low-temperature photoluminescence (PL) analysis. The effect of interface passivation in correlation with the device performance were studied as well.