Thin-film tandem cells with thin CdTe

Models using realistic parameters indicate that two-terminal, double-junction tandem solar cells based on CdTe or CuInGaSe2 and related alloys should be able to reach 25% efficiencies. 1 Key to achieving this is high transparency for below-band-gap photons. Cells with transparent back contacts (TBC) and CdTe thickness between 2.4 μm and 0.3 μm show over 65% average transmission for photons below bandgap. The short circuit current is not compromised as a 10.6% cell with only 0.5 μm CdTe is demonstrated with a Jsc of 23.1mA/cm². For the bottom cell we have focused on optimizing Hg1-xCdxTe, where a single junction efficiency of 6.7% was obtained. Calculations based on the transmission of cells and the quantum efficiency of the best HgCdTe cell indicate that efficiency gains can be made by making the CdTe layer thinner. Finally, a four terminal cell with 7.6% efficiency is discussed.