Analysis of tandem solar cell efficiencies under AM1.5G spectrum using a rapid flux calculation method

We report the use of a rapid flux calculation method using incomplete Riemann zeta functions as a replacement for the Bose-Einstein integral in detailed balance calculations to study the efficiency of tandem solar cell stacks under the terrestrial AM1.5G spectrum and under maximum concentration. The maximum limiting efficiency for unconstrained and constrained tandem stacks of up to eight solar cells, under the AM1.5G spectrum and maximum concentration, are presented. The results found agree well with previously published results with one exception highlighting the precautions necessary when calculating for devices under the AM1.5G spectrum. The band gap sensitivities of two tandem solar cell stack arrangements of current interest were also assessed. In the case of a three solar cell tandem stack the results show a large design space and illustrate that the constrained case is more sensitive to band gap variations. Finally, the effect of a non-optimum uppermost band gap in a series constrainedfive solar cell tandem stack was investigated. The results indicate that a significant re-design is only required when the uppermost band gap is greater than the optimum value with a relatively small effect on the limiting efficiency. It is concluded that this rapid flux calculation method is a powerful tool for the analysis of tandem solar cells and is particularly useful for the design of devices where optimum band gaps may not be available. Copyright (C) 2007 John Wiley & Sons, Ltd.