Intermediate band solar cells (IBSCs) are third-generation photovoltaic (PV) devices that can harvest sub-bandgap photons normally not absorbed in a single-junction solar cell. Despite the large increase in total solar energy conversion efficiency predicted for IBSC devices, substantial challenges remain to realizing these efficiency gains in practical devices. We evaluate carrier escape mechanisms in an InAs/GaAs quantum dot intermediate band p-i-n junction PV device using photocurrent measurements under sub-bandgap illumination. We show that sub-bandgap photons generate photocurrent through a two-photon absorption process, but that carrier trapping and retrapping limit the overall photocurrent. The results identify a key obstacle that must be overcome in order to realize intermediate band devices that outperform single junction photovoltaic cells.