Radically Reimagining III-V Compound Semiconductor Photovoltaics: Epitaxy-Free Approach to Scalable Synthesis of Flexible Low-Cost Thin Film Solar Cells

For III-V compound semiconductor solar cells to achieve scalability for very large-scale photovoltaics, we need to radically reimagine how they are fabricated. We outline an approach for scalable synthesis of GaAs thin film solar cells that bypasses epitaxial growth and eliminates or minimizes vacuum processing. Starting with thin film absorbers that are formed by mechanical exfoliation from GaAs bulk ingots, we demonstrate that high efficiency cells are possible using methods adopted from silicon and perovskite thin film photovoltaics, such as use of diffused junctions formed at ambient pressures, and solution processing of ohmic contacts and passivation layers using earth-abundant materials. We demonstrate diffused junction GaAs solar cells with 1-Sun AM1.5G efficiencies as high as 23.5%.<br/><br/>Thin-film GaAs photovoltaic pn junction structures were synthesized from bulk GaAs crystals by diffusion doping and spalling to mechanically exfoliate 3-micron thick films of GaAs. The p/n junction is fabricated with a simple zinc-diffusion doping technique, which has yielded open circuit voltages &gt; 960 mV. The films are spalled onto electroplated nickel stressor layers that also serve as the mechanical support for the film. Using 110 oriented GaAs crystals results in large-area mirror-smooth spalled films with 0.1 nm RMS roughness. Preliminary fabrication of thin film diodes yielded implied V_oc values over 830 mV, with ideality factor of two, and an absence of parasitic shunts. The diodes are supported on a Kapton tape, demonstrating their potential application for lightweight photovoltaic manufacturing and deployment. These diodes represent the first steps toward high-efficiency low-cost, epitaxy-free, III-V photovoltaics. Finally, we also performed a technoeconomic analysis indicating a scalable pathway for epitaxy/vacuum-free III-V photovoltaics to achieve LCOE parity with future silicon photovoltaics, with a capital cost of 8cent/Watt for module manufacturing.