  To absorb the photons below the band-gap energy effectively, we proposed a
quantum dot (QD) photocell modeled by multi-level system for the quantum yields
and photo-to-charge efficiency limits. The theoretical results show the quantum
yields are enhanced as compared to the single band-gap solar cell, and the
photo-to-charge efficiencies are larger than Shockley and Queisser efficiency
in the same absorbed spectrum. What's more, at the room temperature the
efficiency limits are well beyond 63% achieved by Luque and Marti
(Ref\cite{26}) due to absorbing the low-energy photons via two sub-bands in
this proposed photocell system. The achievements may reveal a novel theoretical
approach to enhance the QD photocell performance modeled a multi-level
absorbing photons system.
