Computer modeling of a two-junction, monolithic cascade solar cell

Abstract

The theory and design criteria for monolithic, two-junction cascade solar cells are described. These developments provide materials selection criteria for cascade structures and the basis on which the AlGaAs-GalnAs combination was selected. The departure from the conventional solar cell analytical method and the reasons for using the integral form of the continuity equations are briefly discussed. The results of design optimization are presented. The energy conversion efficiency that is predicted for the optimized structure is greater than 30 percent at 300 K, AMO, and one sun. The analytical method predicts device performance characteristics as a function of temperature. In this paper, the range is restricted to 300 to 600 K. The characteristics include the family of solar cell V-I curves, conversion efficiency, voltage at the maximum power point, dark current of top and bottom cells, fill factor, and spectral response. Where unexpected or unusual properties are identified, the pertinent phenomena are analyzed using the continuity equation solution and are presented and discussed in greater detail. While the analysis is capable of determining most of the physical processes occurring in each of the individual layers, only the more significant device performance characteristics are presented.