Efficiency of Ge solar cells designed for GeGaAs bicolor systems

Abstract

This paper presents the results of a detailed theoretical analysis of Ge solar cells designed for the two-cell GeGaAs system. Spectrum splitting and tandem stacked concepts are considered. The maximum intrinsic limit efficiency for the two concepts are 38 and 33%, respectively (300 K, 300 AM 1.5 suns). The analysis of the Ge cell, taking into account published data of minority carrier lifetimes versus doping level is made as a function of temperature, solar concentration ratio, series resistance and air mass number. The intrinsic parameters of the device, x j (junction depth) and surface recombination velocity are not critical if x j < 1 μ m . Efficiencies of 13.5% for the Ge solar cell and 5.8% for the Ge cell with solar spectrum limited to 1.4 eV are obtained at 300 K for 300 AM 1.5 suns. With the state-of-art GaAs solar cell, a realistic efficiency of 30% can be obtained for the two-cell system.