Abstract: Due to their outstanding charge collecting and transport ability, carbon nanotubes (CNTs) can be used as electrode materials in solar cells. Here, we replaced the aluminum back electrode in a crystalline silicon (c-Si) solar cell with a CNT thin film to form a c-Si/CNT film solar cell. The c-Si/CNT solar cell had a high short-circuit current density of 35.5 mA cm^-2, which is higher than that using aluminum as the back electrode, indicating that CNTs can be used as hole transport materials in silicon-based solar cells. The power conversion efficiency of the c-Si/CNT solar cell was increased from 7.1% to 10.9% by dropping a few drops of a dilute HF solution (5 wt%) at the interface of the c-Si and the CNT film. This is because HF etches away the oxide layer on the c-Si surface, increases the interfacial contact area between the CNT film and the c-Si and decreases the recombination rate of electron/hole pairs.