In actual thin film PV device based on chalcopyrite CIGS absorber, zinc blend ZnS has proven to replace successfully toxic and expensive CdS buffer layer, with high efficiency of about 18 %. In the other class of materials based on chalcogenide absorber where costly and scarce Indium and Gallium are replaced by Zinc and Tin, ZnS mix rather badly with CZTS due to small lattice misfit of 1.38% and large valence band offsets of 0.9 eV. Among possible alternatives to ZnS mixed with CZTS, the pyrite FeS2 is a promising candidate who can address economic and environmentally issues. To do so, we investigate in a comparative study the structural, electronic and optical properties of ZnS and new introduced pyrite FeS2 as a buffer layer in kesterite CZTS based solar cells. First-principles calculations are performed using FP-LAPW method implemented in Wien2k code within the framework of Perdew-Burke-Ernzerhof-GGA approximation. The addition of the Hubbard term U in LDA+U/ (GGA+U) approximation was necessary to correct the energy gaps. The calculated band structure gives reliable results close to the experimental values of 3.78 and 1.29 eV which are direct gaps located at point in BZ for ZnS and CZTS, respectively. For FeS2, the modified Becke-Johnson potential is used to calculate its band gap and it gives a value of 1.093 eV. It is found that FeS2 can be used as an efficient buffer in CZTS heterostructure based solar cells.