Modelling of quantum dot intermediate band solar cells: effect of intermediate and linewidth broadening

A detailed balanced approach is used to model a quantum-dot intermediate band solar cell (QD-IBSC). A model assuming identical QDs with no carrier losses has peak efficiency of 63.24% for a bandgap of 1.95 eV with the intermediate
band (IB) positioned either 1.24 eV above the valence band or below the conduction band. The effect of inhomogeneities in the QDs is introduced by incorporating an inhomogeneous broadening of the IB, and the homogeneous broadening describing life-time broadening effects is also introduced. A Gaussian inhomogeneous broadening of the ground state is considered initially and then other inhomogeneous broadening are considered, including a ground plus excited state distribution. The inhomogeneous broadening removes state density from the ideal IB energy position and thereby reduces efficiency. However, the homogeneous broadening of the QD-IB levels allows more off-resonance QD states to be able to contribute to absorption of the optimum IB photon. This broadening also allows photons near the optimum IB photon energy to be absorbed as well. Both these effects lead to an increase in efficiency. The inter-play between these inhomogeneous and homogeneous
broadening allows the efficiency in realistic inhomogeneous arrays of QDs to remain close to the ideal values.