Recently, the monolithic spin-coating perovskite/planar silicon heterojunction tandem solar cells with high performance have attracted attention mainly due to simple fabrication, low preparation cost and high efficiency, especially compared with fully textured multi-junction perovskite/silicon tandem device. As is well known, the excellent passivation of a-Si:H/c-Si interface is the key to achieving a high-efficiency planar silicon heterojunction solar cell, which further improves the performance of the corresponding tandem cell. Therefore, we investigate the elements affecting a-Si:H/c-Si interface passivation, including the c-Si surface treatment technique, a-Si:H passivation layer and P-type emitter layer and so on. In these experiments, we adjust the immersed time of diluent hydrofluoric acid and pre-deposited hydrogen plasma with different gas mixture flows. Also, the suitable deposition parameters of intrinsic a-Si:H passivation layer are regulated by varying hydrogen dilution and time, and variously slight silane content is embedded into i-a-Si:H /P-type (I/P) emitter interface by hydrogen-rich plasma treating which is for acquiring optimal experimental processing conditions to promote the chemical passivation. In addition, the p-a-Si:H and p-nc-Si:H are comparatively studied as buffer layers to further improve the I/P interface passivation by varying the hydrogen dilution in the gas mixture during deposition. It can be found that p-nc-Si:H buffer layer with high conductivity and wide bandgap can not only reduce the defect density at the I/P interface, but also increase the conductivity of P-type emitter, which further improves the field passivation effect. By the above- mentioned optimization, the highest minority carrier lifetime and implied open-circuit voltage (iV_oc) of the structure of P-type emitter/a-Si:H(i)/c-Si/a-Si:H(i)/N-type layer (inip) sample can respectively reach 2855 μs and 709 mV, which demonstrates authentically outstanding passivation performance. An efficiency of 18.76% can be obtained for the planar a-Si/c-Si heterojunction solar cell with a V_oc of 681.5 mV, which is 34.3 mV higher than that of the reference device. Regarding the optimized planar a-Si:H/c-Si heterojunction solar cell as the bottom cell, we also obtain an efficiency of 21.24% for perovskite/silicon heterojunction tandem solar cell with an open-circuit voltage of 1780 mV, which proves that the above strategies are very effective for improving the passivation optimization and performance of bottom cell in the tandem device.