Improving the Minority Carrier Lifetime of Bulk n-Type GaAs Enables Epitaxy-Free, Thin-Film GaAs Solar Cells with Voc Greater than 900 mV

Photovoltaics for future applications in space solar power and large satellite constellations will require large-scale space power systems comprised of ultralight, radiation-resilient, high specific power, and very low cost (approximately terrestrial Si PV cost) solar cells. We have developed an approach to address scalable fabrication of epitaxy-free, thin film, GaAs solar cells with Voc’s over 900 mV and a method for improving the minority carrier lifetime of bulk n-type GaAs. We fabricate non-epitaxial cells by using low-cost zinc diffusion into a bulk wafer to form a p/n junction, electroplating nickel, and then mechanically exfoliating this structure via spalling in order to make a thin-film solar cell, and switching which side the film is handled in order to make an upright-junction solar cell. Under AM 1.5G illumination, without an anti-reflection coating, the cell achieves a Voc of 903 mV and FF of 75%. Including the GaAs, metal, and tape handle, we achieve a specific power of approximately 500 W/kg at an areal mass density of 94 g/m^2. Preliminary technoeconomic analysis indicates this approach can yield scalable cell fabrication with $/Watt and LCOE costs comparable to commercial terrestrial Si photovoltaics.

We also report a process for improving the minority carrier properties of melt-grown GaAs by annealing at high-temperature in an arsenic poor-ambient. The annealing apparatus we have developed requires neither sealed ampoules nor high vacuum. Small area solar cells fabricated on GaAs annealed at 850-1000C with p/n junctions formed by zinc doping have performance comparable to similarly prepared devices that use vapor-phase-epitaxy grown GaAs layers. We find that thermal processing at 1000C can enhance the hole diffusion length of commercially available GaAs from 0.6 um to 7.6 um. We further aim to combine these two advances to fabricate thin-film, epitaxy-free GaAs solar cells with efficiencies over 10% and specific powers of 1 kW/kg.