While single-junction solar cells may be capable of attaining AM1.5 theoretical efficiency of 33.16%, infinite multijunction (MJ, Tandem) solar cells will have a limiting efficiency of 86.8%. Tandem solar cells based on crystalline silicon (c-Si) bottom cells are therefore attracting great interest. An interesting candidate for the top cell absorber is represented by copper zinc tin sulfide Cu 2 ZnSnS 4 (CZTS). In this work, the CZTS/Si tandem solar cell is optimized by using indium oxide / Cadmium sulfide (In 2 O 3 /CdS) hybrid buffer. This present work reports CZTS/Si solar cell with an open-circuit voltage V OC of over 0.942 V and a J SC of 34.7 mA cm − ² by adding In 2 O 3 layer in the CZTS top cell. The added In 2 O 3 layer has a thickness of 0.022 µm and is n-doped with a concentration of 1e20 cm −3 . Compared with a CZTS/Si in which the hybrid buffer is of CdS, the efficiency is increased from 13.5% to 28.4%. These hybrid In 2 O 3 /CdS buffers provide a promising way to reduce the V OC deficit and further boost the efficiency of CZTS/Si solar cells.