Intermediate band solar cell provides novel alternative to multi-junction solar cell, but its efficiency is significantly degraded when spectral overlap exists between different absorption bands. Here, a scheme using non-uniform sub-bandgap state filling together with intermediate band transport is proposed to resolve the spectral overlap issue. On the basis of detailed balance calculation, spectrally decoupled devices using low–high state filling is shown to achieve 52.8% conversion efficiency when 4 eV spectral overlap is present between absorption coefficients of different bands, compared with baseline efficiency equal to 35.1% for conventional half-filled intermediate band devices. If a base material without intermediate band is added to the two section low–high state filling devices, the efficiency is further increased to 61.5%, which approaches efficiency of 63.2% for intermediate band devices with no spectral overlap and 63.8% for unconstrained triple-junction tandem cells. The junction thermalization loss associated with proposed new structures is shown to be equal to conventional half-filled intermediate band devices. Copyright © 2013 John Wiley & Sons, Ltd.