In this work, we study type-II GaSb/GaAs quantum-dot intermediate band solar cells (IBSCs) by means of quantum efficiency (QE) measurements. We are able, for the first time, to measure an absolute QE which clearly reveals the three characteristic bandgaps of an IBSC; EG, EH, and EL, for which we found the values 1.52, 1.02, and 0.49 eV, respectively, at 9 K. Under monochromatic illumination, QE at the energies EH and EL is 10–4 and 10–8, respectively. These low values are explained by the lack of efficient mechanisms of completing the second sub-bandgap transition when only monochromatic illumination is used. The addition of a secondary light source (E = 1.32 eV) during the measurements produces an increase in the measured QE at EL of almost three orders of magnitude.