Decoupling spectral overlap of intermediate band solar cells using low-high state filling

The high efficiency predicted for solar cells based on multi-photon processes relies on the assumption of optical transitions with non-overlapping spectral bands. Spectral overlap reduces the conversion efficiency due to energy loss associated with improper usage of higher energy photons for lower energy optical transitions. In this study, device structures with non-uniform occupation of intermediate electronic states are proposed to reduce the dependence of conversion efficiency on spectral overlap. Solar cell conversion efficiencies are calculated for structures where absorption bands are spatially decoupled due to defined occupation of intermediate states. Conversion efficiencies for a two section and three section device with spectral overlap of 4 eV are determined to be 52.8 % and 61.5 %, respectively, and are significantly larger than the calculated efficiency of 35.1% for the case of a standard multi-photon cell with uniform half-filled intermediate states The spectrally-decoupled device provides a means to achieve high theoretical efficiency independent of spectral overlap that approaches the detailed balance efficiency limit of 63.2 % for intermediate state devices without spectral overlap and 63.8 % for unconstrained triple-junction tandem cells.