An electronic ratchet is required in nanostructured intermediate-band solar cells

We investigate in this paper the intrinsic properties that have limited the efficiency of nanostructured intermediate-band solar cells. Those devices take advantage of intraband transitions, which occur on narrow energy width, and present low radiative recombination efficiency. We show that the combined effect of these two nonidealities strongly impacts the power conversion efficiency of intermediate-band solar cells. Minimum requirements in terms of those two characteristics to achieve efficiencies in excess of the Shockley-Queisser limit are computed, which show that compatible nanostructures are challenging to obtain. Especially, we evidence that currently experimentally considered materials cannot overcome the best single-junction cells. In order to solve those issues, we consider devices including an electronic ratchet mechanism. First, such devices are shown to be much less sensitive on the limitations of the nanostructures characteristics so that requirements for high efficiencies can be met. Second, we show that quantum well devices present advantages over their quantum dot counterparts, although they have attracted much less interest so far.