Limiting efficiencies for intermediate band solar cells with partial absorptivity: the case for a quantum ratchet

The intermediate band solar cell (IBSC) concept aims to improve upon the Shockley-Queisser limit for single bandgap solar cells by also making use of below bandgap photons through sequential absorption processes via an intermediate band (IB). Current proposals for IBSCs suffer from low absorptivity values for transitions into and out of the IB. We therefore devise and evaluate a general, implementation-independent thermodynamic model for an absorptivityconstrained limiting efficiency of an IBSC to study the impact of absorptivity limitations on IBSCs. We find that, due to radiative recombination via the IB, conventional IBSCs cannot surpass the Shockley-Queisser limit at an illumination of one Sun unless the absorptivity from the valence band (VB) to the IB and the IB to the conduction band (CB) exceeds ≈ 36%. In contrast, the introduction of a quantum ratchet into the IBSC to suppress radiative recombination can enhance the efficiency of an IBSC beyond the Shockley-Queisser limit for any value of the IB absorptivity. Thus, the quantum ratchet could be the vital next step to engineer IBSCs that are more efficient than conventional single gap solar cells.