High efficiency InGaP/InGaAs tandem solar cells on Ge substrates

Over 30% AM1.5G efficiency was achieved by adding a small quantity of Indium into a GaAs bottom cell in the conventional tandem cell on a Ge substrate. Characteristics of InGaAs cells on Ge were investigated by varying In-composition. The maximum efficiency was obtained for the cell with 0.01 In-composition, which was lattice-matched to Ge and produced no misfit-dislocations. Relatively high efficiencies were obtained for the cells with In-compositions less than 0.1, which did not produce cracks but misfit-dislocations. InGaP/InxGa1-x As tandem cells with In-composition x between 0.01 and 0.07 demonstrated higher efficiency than the conventional InGaP/GaAs cells, that was attributed to an increase in photo-currents both in the top and bottom cells. Remarkably, an In0.49Ga0.51P/In0.01Ga0.99As tandem cell lattice-matched to Ge showed an improvement in Voc which was attributed to an elimination of misfit-dislocations in thick GaAs layers. Also, those InGaP/InxGa1-xAs cells with low In-compositions were found to be favorable for improving efficiency of triple junction cells using Ge cells. Over 31% AM1.5G efficiency was demonstrated for the InGaP/InxGa1-xAs/Ge triple-junction cells with In-composition x of 0.01 and 0.06, at present.