Bifacial Growth InGaP/GaAs/InGaAs Concentrator Solar Cells

A three-junction concentrator photovoltaic (CPV) cell with 1.9-eV InGaP top and 1.42-eV GaAs middle cells on one side of an infrared-transparent N-GaAs wafer and a 0.94-eV InGaAs bottom cell on the opposite wafer side (backside) is described. This architecture isolates the upper lattice-matched subcells from threading dislocations generated during the growth of the lattice-mismatched bottom subcell. The cell uses a unique epitaxial bifacial growth technique with only a simple water rinse and spin dry between growths on opposite wafer sides. The best independently verified efficiency for a 1-cm $^2$ cell is 42.3% at 406-suns AM1.5D at 25 °C (V $_{\rm oc}$ 3.452 V, 87.1% FF, and 1-sun J $_{\rm sc}$ of 14.07 mA/cm $^2$ ). We give data on single-junction subcells and tunnel junctions in the tandem, quantum efficiency, and temperature coefficient data, discuss use of a thin pseudomorphic layer at the back of the GaAs middle subcell to extend the wavelength response, and discuss the benefit of graded doping layers in increasing subcell 1-sun J $_{\rm sc}$ at the top and bottom subcells.