Solar cells that combine single-crystalline silicon (Si) with graphene (G) have been widely researched in order to develop next-generation photovoltaic devices. However, the power conversion efficiency (PCE) of G/Si solar cell without chemical doping is commonly low due to the relatively high resistance of graphene. In this work, through combining graphene with carbon nanotube (CNT) networks, we fabricated three kinds of hybrid nanocarbon film/Si heterojunction solar cells in order to increase the PCE of the graphene based Si solar cell. We investigated the characteristics of different nanocarbon film/Si solar cells and found that their performance depends on the heterojunctions. Specifically, a doping-free G-CNT/Si solar cell demonstrated a high PCE of 7.9％, which is nearly equal to the combined value of two individuals (G/Si and CNT/Si). This high efficiency is attributed to the synergistic effect of graphene and CNTs, and can be further increased to 9.1％after applying a PMMA antireflection coating. This study provides a potential way to further improve the Si based heterojunction solar cells.