Application of Distributed Bragg Reflector for Improved Radiation Tolerance of IMM Solar Cells

MicroLink Devices (MLD) has developed an advanced lightweight, high-efficiency solar cell that is sold as the enabling power generating technology for high-altitude, long endurance unmanned platforms such as the Airbus Zephyr and the Prismatic PHASA-35, as well as smaller, battery-powered unmanned aerial vehicles (UAV). These unique cells are fabricated by an epitaxial lift-off (ELO) process where ultrathin and flexible solar cells are grown on GaAs wafers as an inverted metamorphic (IMM) triple junction solar cell. These peeled off semiconductor crystal layers are fractions of the thickness of human hair.  Through earlier work, MicroLink has improved the conversion efficiency of the ELO solar cells to >30% under the AM0 1-sun solar spectra and increased the specific power to the highest of any solar cell technology at >2000 W/kg. These cells are sold by MLD to primarily non-Governmental customers, with total sales in 2019 for greater than 25kW of solar cells for over $5M, and a gross profit of over $1.7M. Satellites are a key defense technology enabling persistent intelligence, surveillance, and reconnaissance (ISR). With key modifications to the current device structure, MicroLink’s IMM cells will provide unique properties for power generation of satellites purchased by the Air Force, and at a reduced cost. Improving the end of life (EOL)/beginning of life (BOL) efficiency ratio for IMM solar cells is critical to bring them into this market. A key limitation in MicroLink’s IMM solar cells for satellites is the degradation of the GaAs middle cell under radiation. A thinner GaAs subcell can be grown, absorbing less radiation, if some unabsorbed light is reflected back into it by a distributed Bragg reflector (DBR) optimized by MicroLink. During this two year program, a DBR layer will be installed below the GaAs junction to reflect back unabsorbed near IR light, but not light below the GaAs bandgap (>875nm). The DBR structure will achieve >90% reflectance with an 80nm bandwidth, allowing the GaAs layer to be substantially thinned from >3µm to nearly 1µm, improving its radiation tolerance. After optimization and along with improvements to other areas of the solar cell, the goal will be a solar cell with 30% efficiency and an EOL/BOL efficiency ratio of 85% under 1MeV electron irradiance with 1e15 cm-2 fluence. Coupons of these cells will undergo significant reliability testing internally and with the Air Force. By the end of the program, a production process will be scaled up so that cells can be manufactured and sold to Air Force defense satellite manufacturers including Lockheed Martin and Airbus. These companies have provided letters of support for this program. They will install IMM cells on defense satellites, improving the power generation of Air Force satellites. Compared to conventional cells of similar efficiency, these cells will have only 20% of the mass, and operate at higher efficiency (~1.8%), all while saving costs.