Modeling extremely thin absorber solar cells for optimized design

Extremely thin absorber (ETA) solar cells based on inorganic semiconductors are theoretically analyzed by a model that considers the absorber as being a pin junction, with tunneling-assisted defect recombination. Tunneling recombination turns out to be very important in ETA solar cells, owing to the high electrical fields in the absorber, which establishes a minimum thickness for the absorber layer, which is calculated to be around 15 and 20 nm for CdTe and CuInS2, respectively. Nevertheless, 15% efficient CdTe and CuInS2 ETA solar cells are possible, even at low diffusion lengths down to 10 nm. Additionally, the modeling provides optimum values for the thickness and number of absorber layers for CdTe and CuInS2 ETA solar cells, as a function of the minority-carrier diffusion length and diffusion constant, for cells with and without light trapping. The calculations predict that light trapping serves two purposes: to enhance the cell efficiency by up to 5% absolute, and to use a simpler structure compared with the situation without light-trapping. Copyright (C) 2004 John Wiley Sons, Ltd.