DEVELOPING HIGH-EFFICIENCY LOW-COST SOLAR CELL TECHNOLOGY IS ESSENTIAL TO BOTH ENABLING A WIDER VARIETY OF SPACE MISSIONS AND MAKING SOLAR ENERGY COMPETITIVE WITH FOSSIL FUELS. ONE PROMISING CELL DESIGN IS TO INTEGRATE A III-V TOP CELL ONTO AN INEXPENSIVE SI BOTTOM CELL TO FORM A TANDEM (DUAL JUNCTION). IN PARTICULAR A GAASP/SI TANDEM CELL HAS AN OPTIMAL MATERIAL COMBINATION TO ACHIEVE THEORETICAL EFFICIENCIES OF 37-44% WELL ABOVE THE 26.7% OF RECORD SI CELLS. UNFORTUNATELY GAASP CELLS GROWN ON SI SUFFER FROM THE FORMATION OF PERFORMANCE-ROBBING THREADING DISLOCATIONS DUE TO THE LARGE LATTICE CONSTANT MISMATCH BETWEEN GAASP AND SI (~3.2%). THESE DISLOCATIONS LEAD TO POOR DIFFUSION LENGTHS OF LIGHT-GENERATED ELECTRONS AND HOLES REDUCING THE SHORT-CIRCUIT CURRENT DENSITY (JSC). TO INCREASE THE JSC AND OVERALL EFFICIENCY OF THE GAASP CELL I PROPOSE TO GROW A THIN GAASP SOLAR CELL ON TOP OF A DISTRIBUTED BRAGG REFLECTOR (DBR). THE DBR REFLECTS PHOTONS THAT WOULD OTHERWISE TRANSMIT THROUGH THE GAASP CELL GIVING A SECOND CHANCE FOR THE PHOTONS TO BE ABSORBED AND GENERATE ADDITIONAL ELECTRON-HOLE PAIRS. SINCE THE DBR EFFECTIVELY DOUBLES THE OPTICAL THICKNESS THE GAASP CELL CAN ALSO BE THINNER MITIGATING THE ISSUES ASSOCIATED WITH LOW DIFFUSION LENGTH DUE TO DEFECTS. EARLY SIMULATION WORK PREDICTS A ~1.5MA/CM2 INCREASE IN JSC AND 1.5% HIGHER CELL EFFICIENCY WITH A DBR. I WILL CARRY OUT SIGNIFICANT EXPERIMENTAL WORK TO GROW THE DBR S REFINE SIMULATION MODELS AND IMPROVE DBR DESIGN. I WILL ALSO ADDRESS POTENTIAL CHALLENGES OF DISLOCATION FORMATION IN THE DBR AND DEVISE STRATEGIES TO REDUCE THE COST OF THE ADDITIONAL DBR GROWTH. ULTIMATELY I AIM TO INCREASE THE GAASP CELL EFFICIENCY FROM THE CURRENT RECORD OF 16.5% TO>20%. THIS WILL ENABLE THE DEVELOPMENT OF LOW-COST GAASP/SI TANDEM CELLS WITH>30% EFFICIENCY AND WIDESPREAD APPLICATIONS FOR BOTH SPACE AND TERRESTRIAL USE.