Initial theoretical study of solar cells with an intermediate band of non-zero width and a thermalized electron population

Abstract

A model based on detailed balance principles is developed to study how the thermalized nature of the electrons in the intermediate band (IB) affects the efficiency of intermediate band solar cells. Published work on intermediate band solar cells with finite IB width has focused on the fundamental case when the absorptivity is assumed to be high for all photon energies above the smallest band gap. In this work, an attempt is made to incorporate variations in the absorptivity due to the thermal distribution of the IB electrons. In a wide IB with a thermalized electron population, there will be a low density of electrons close to the upper band edge. The density of unoccupied electron states close to the lower band edge will also be low. As a consequence, the absorption coefficients for photon energies where the only energetically allowed transitions involve exciting electrons from or to, respectively, such states can be expected to be low. The presented model incorporates the effect of the thermalized electron population in an idealized way. In some cases, the calculated efficiency is well above the limit for single band gap cells, whereas in other cases it is not. It is concluded that absorption coefficients rising rapidly from very low values to higher values are advantageous, that overlap between the absorption coefficients can be beneficial when the IB becomes sufficiently wide, and finally, that a case-by-case study probably is required to evaluate whether a particular IB material can give cells with high efficiency. Copyright © 2011 John Wiley & Sons, Ltd.