We studied the photovoltaic properties of single-walled carbon nanotube/Si (SWNT/Si) heterojunction cells. We observed an optimal thickness of the SWNT network film that maximizes the photovoltaic conversion efficiency. The spectra of incident photon to charge carrier efficiency indicate that the production of carriers in the Si layer mainly contributes to the photovoltaic conversion. The experimental results and loss analysis based on the equivalent circuit model suggest that the fabrication of a high-density semiconducting SWNT network at the interface of Si is the key to improving the conversion efficiency of the SWNT/n-Si heterojunction solar cell. (C) 2012 The Japan Society of Applied Physics