We fabricated Si-based solar cell with stacked Ge islands grown via the Stranski–Krastanov growth mode in the intrinsic layer of pin diodes. The onset of the external quantum efficiency in the near infrared regime was extended up to approximately 1.4 μm for the solar cells with stacked Ge islands. The quantum efficiency was found to increase with increasing number of stacking, showing that a part of electron–hole pairs generated within Ge islands was separated by the internal electric field and contributed to the photocurrent. Increase of the processing temperature for the impurity diffusion was found to bring blue-shift of the band gap through the intermixing of Si and Ge as well as the deformation of Ge islands. Therefore, low-temperature process was suggested to be necessary for further enhancement of quantum efficiency in the near infrared regime.