Optimal Filling of the Intermediate Band in Idealized Intermediate-Band Solar Cells

The optimal filling of the intermediate band (IB) of an IB solar cell is investigated. Using models based on detailed balance principles, it is shown that the optimal filling varies with the size of the subband gaps, the absorptivity of the cell, and the degree of the overlap between the absorption coefficients as well as the mutual sizes of the absorption cross sections for transitions over the subband gaps. The results of calculations that show how nonoptimal filling affects the cell efficiency are also presented. In several cases, a deviation from the optimal filling will only result in small changes in the efficiency. However, cases where the efficiency is reduced dramatically due to nonoptimal filling are also identified. For some cases, the negative impact of nonoptimal filling can be reduced by increasing the absorptivity of the cell or, when the effect of photofilling is significant, by increasing the light concentration.