The photovoltaic properties of carbon nanotube/Si heterojunction solar cells were investigated using network films of high-quality single-walled carbon nanotubes (SWNTs) grown by atmospheric-pressure floating-catalyst chemical vapor deposition. Because of the optimization of the device window size and the utilization of SWNT thin films with both low resistivity and high transparency, a high photovoltaic conversion efficiency of greater than 12% was achieved for SWNT/Si heterojunction solar cells without any postprocessing, such as carrier doping treatment. In addition, the high reproducibility of the photovoltaic performance of these devices in air was demonstrated.