Lattice mismatched dual junction tandem cells

High performance solar cells with capabilities covering a broad range of mission parameters are of great interest to the space photovoltaic community. Current areas of interest include improving efficiency of multi-junction cells by adjusting bandgaps to more optimum values, adding junctions to existing structures and investigating the effects of various substrate materials. The goal is to merge the highest efficiency multijunction solar cell with a low cost, lightweight substrate. This paper focuses on developing a multijunction solar cell with optimum bandgaps by relaxing the constraint for lattice matching between the substrate and the epitaxial cell structure. A III-V lattice mismatched dual junction solar cell composed of a 1.6 eV InGaP top cell and a 1.1 eV InGaAs bottom cell has been grown with an Air Mass Zero (AM0) efficiency of 16.4% without an antireflective coating (ARC). An AM0 efficiency of 23% is anticipated when a dual layer antireflective coating is applied. Both sub-cells are lattice matched to each other but mismatched to the GaAs substrate. Accommodation of the lattice strain was accomplished via an InGaAs buffer structure. Extension of the lattice mismatched approach to three junction devices holds the promise to demonstrate AM0 efficiencies in excess of 30%.