Influence of built-in voltage in optimized extremely thin absorber solar cells

Extremely thin absorber (ETA) solar cells based on CdTe 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 due to the high electrical fields in the absorber. Nevertheless, 15% efficient CdTe 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 as a function of the minority carrier diffusion length and diffusion constant. The calculations predict that with moderate to high diffusion lengths above 10 nm, the built-in voltage enhances the conversion efficiency, but in case of having poor diffusion lengths, low built-in voltages are necessary to restrict tunneling recombination and avoid major efficiency losses.