Carbon nanotube-silicon (CNT-Si)-based heterojunction solar cells (HJSCs) are a promising photovoltaic (PV) system. Herein, few-layer black phosphorus (FL-BP) sheets are produced in N-methyl-2-pyrrolidone (NMP) using microwave-assisted liquid-phase exfoliation and introduced into the CNTs-Si-based HJSCs for the first time. The NMP-based FL-BP sheets remain stable after mixing with aqueous CNT dispersion for device fabrication. Due to their unique 2D structure and p-type dominated conduction, the FL-BP/NMP incorporated CNT-Si devices show an impressive improvement in the power conversion efficiency from 7.52% (control CNT-Si cell) to 9.37%. Our density-functional theory calculation reveals that lowest unoccupied molecular orbital (LUMO) of FL-BP is higher in energy than that of single-walled CNT. Therefore, we observed a reduction in the orbitals localized on FL-BP upon highest occupied molecular orbital to LUMO transition, which corresponds to an improved charge transport. This study opens a new avenue in utilizing 2D phosphorene nanosheets for next-generation PVs.