Material Quality Requirements for Intermediate Band Solar Cells

Intermediate band (IB) solar cells hold the promise of efficiency as high as triple-junction solar cells with much simpler cell design, containing only two semiconductor material interfaces. Although several IB materials have been demonstrated, no cells have shown promising efficiencies. Many of the fundamental required properties of IB materials are well known; they need strong subgap absorption, long carrier lifetimes, and good carrier mobilities. The tradeoffs between these properties, however, are not well understood. We present the first results using a new coupled Poisson/drift-diffusion model designed for IB materials, called Simudo. We compare the results from Simudo to a simpler semianalytic model for IB device performance, highlighting where they agree well. Using both of these models, we perform a systematic study of a figure of merit for IB materials ν. We consider the standard p-IB-n architecture with a high-density IB, in which two depletion junctions are formed. Considering materials with identical electron and hole properties, we show that ν is well correlated with device efficiency. We show for the first time a threshold behavior, where the efficiency of the IB device exceeds that of the standard p-n-junction only for sufficiently high-quality material. We show that ν is similarly predictive of device performance as an equivalent figure of merit is for standard solar cells. These results both give guidance for experiments regarding required IB material properties and demonstrate how detailed device modeling can aid in the design of IB devices, for example, by choosing layer thicknesses optimally.