  The novel concept of non-compensated n-p codoping has made it possible to
create tunable intermediate bands in the intrinsic band gap of TiO2, making the
codoped TiO2 a promising material for developing intermediate band solar cells
(IBSCs). Here we investigate the quantum efficiency of such IBSCs within two
scenarios - with and without current extracted from the extended intermediate
band. Using the ideal equivalent circuit model, we find that the maximum
efficiency of 57% in the first scenario and 53% in the second are both much
higher than the Shockley-Queisser limit from single gap solar cells. We also
obtain various key quantities of the circuits, a useful step in realistic
development of TiO2 based solar cells invoking device integration. These
equivalent circuit results are also compared with the efficiencies obtained
directly from consideration of electron transition between the energy bands,
and both approaches reveal the intriguing existence of double peaks in the
maximum quantum efficiency as a function of the relative location of IBs.
