Intermediate band solar cells (IBSCs) promise high efficiencies while maintaining a low device structural complexity. A high efficiency can be obtained by harvesting below-band-gap photons, thus increasing the current, while at the same time preserving a high voltage. Here, we provide experimental proof that below-band-gap photons can be used to produce nonzero electrical work in an IBSC without compromising the voltage. For this, we manufacture a GaSb/GaAs quantum-dot IBSC. We use light biasing and make our cell operate at the maximum power point at 9 K. We measure the photocurrent response to absorption of photons with an energy of less than 1.15 eV while the cell is operating at 1.15 V. We also show that this result implies the existence of three quasi-Fermi levels linked to the three electronic bands in our device, as demanded by the IBSC theory to preserve the output voltage of the cell.