Ultra-thin defect tolerant high efficiency III–V tandems for development of low-cost photovoltaics

Here we theoretically and experimentally demonstrate that through a careful design optimization, and despite very high dislocation densities (>10⁸cm^-2), ultra-thin (~1 micron) dual junction solar cells with practical efficiencies in excess of 23% could be achieved. The study has also attempted to avoid the use of conventional indium-bearing III-V alloys and has focused on III-V semiconductor alloys made with more earth-abundant elements (i.e. Al, Sb, N...). We have evaluated as a function of dislocation densities, the design parameters for devices that comprise a 1.7 eV top AlGaAs solar cell and a 1.25 eV bottom GaAsSb/GaAs(N)Sb cells. The experimental validation of modeling data was undertaken by the fabrication of proposed thin film subcells on intentionally dislocated buffers. As shown here, and in-line with our modeling results, even for defect densities in excess of 10⁸cm^-2, we were able to fabricate sets of top and bottom cells with combined open circuits voltages in excess of 1.7 volts. A direct extrapolation of these preliminary experimental results already indicate the potential for fabricating thin-film III-V devices on metal foils with 1 sun efficiencies in excess of 20%.