Single junction crystalline silicon (c-Si) solar cells are reaching their practical efficiency limit while perovskite/c-Si tandem solar cells have achieved efficiencies above the theoretical limit of single junction c-Si solar cells. Next to low-thermal budget silicon heterojunction architecture, high-thermal budget carrier-selective passivating contacts (CSPCs) based on polycrystalline-SiO _x (poly-SiO _x) also constitute a promising architecture for high efficiency perovskite/c-Si tandem solar cells. In this work, we present the development of c-Si bottom cells based on high-temperature poly-SiO _x CSPCs and demonstrate novel high-efficiency four-terminal (4T) and two-terminal (2T) perovskite/c-Si tandem solar cells. First, we tuned the ultra-thin, thermally grown SiO _x. Then we optimized the passivation properties of p-type and n-type doped poly-SiO _x CSPCs. Here, we have optimized the p-type doped poly-SiO _x CSPC on textured interfaces via a two-step annealing process. Finally, we integrated such bottom solar cells in both 4T and 2T tandems, achieving 28.1% and 23.2% conversion efficiency, respectively.