Development of lattice-mismatched GaInAsP for radiation hardness
Ryan M. France1, Pilar Espinet-Gonzalez2, Harry A. Atwater2, Don Walker3
1National Renewable Energy Laboratory, Golden, CO, United States
/2California Institute of Technology, Pasadena, CA, United States
/3The Aerospace Corp., El Segundo, CA, United States

We develop lattice-mismatched GaInAsP as an alternative alloy to pure As-based alloys currently used in III-V multijunction solar cells. Increasing the alloy phosphorous content while maintaining an optimal bandgap may allow high efficiency multijunction devices with increased radiation hardness. Here, 1.0-eV GaInAsP is developed and implemented into single and multijunction solar cell devices. The lattice-mismatched GaInAsP must be grown strain-free, which involves understanding the nonlinear group V incorporation efficiency, and the subcell thickness must be maintained below the thickness where surface-driven phase separation occurs. We show single junction 1.0-eV Ga0.5In0.5As0.7P0.3 with excellent carrier collection and a bandgap-voltage offset of 0.40 V. This material quality approaches that of 1.0-eV Ga0.7In0.3As used in inverted metamorphic multijunction devices, but has increased phosphorus content and consequently is expected to have a higher radiation resistance. We incorporate the 1.0 eV GaInAsP subcell into a 3-junction inverted metamorphic solar cell, which, although not optimized, has an efficiency over 20% under the AM0 spectrum without an anti-reflection coating. These preliminary results indicate that lattice-mismatched GaInAsP can be effectively used in multijunction solar cells to replace radiation-soft materials.