Cu2ZnSnS4 (CZTS) is a low cost, non-toxic material composed of earth-abundant elements with a large optical absorption coefficient. However, in CZTS solar cells, CdS buffer layers are widely used, which contain toxic elements and lead to lattice mismatch with CZTS thin films. In this paper, we have systematically investigated a I-II-IV-VI semiconductor material to replace the CdS buffer layer with a non-toxic material that offers good lattice matching with the CZTS film. For the first time, Cd-free Cu2ZnSnS4-Ag2ZnSnS4 heterojunction photovoltaic device is fabricated, where Cu2ZnSnS4 and Ag2ZnSnS4 layers are obtained by co-sputtering method. The cell exhibits a 4.51% efficiency which is higher than those of CZTS solar cells with ZnS and In2S3 buffer layers. This work offers the possibility to realize environmentally benign, scalable, low-cost, and high-efficiency solar cells.