Gallium phosphide as the high bandgap material in high-efficiency multi-junction solar photovoltaics

Abstract

In the endeavor to meet the ever increasing energy demands of the world, two methodologies of converting solar energy to electrical energy have dominated the solar photovoltaic field. The first method utilizes a material with high conversion efficiencies over a very large area, balancing material efficiency with material cost. The second method utilizes relatively expensive materials with very high conversion efficiencies but uses optical concentration systems to greatly reduce the quantity of material required. The increase in efficiency of this method is accomplished by incorporating multiple materials in the photovoltaic device. State of the art laboratory results have achieved efficiencies close to 43%.

In an effort to push this efficiency higher, this research focuses on a weak spot in the multi-junction photovoltaic field; the conversion of high-energy photons (green-blue to UV). The material chosen for this is Gallium Phosphide. Results for a baseline p-n junction device are presented, as well as improvements on the baseline device yielding an efficiency of 2:6% of AM1:5-global. A V_oc of 1.59V is achieved at 12x concentration. An internal quantum efficiency of ≈ 100% is achieved at 440nm. Methods for simulating photovoltaic devices to identify key performance limiters are presented. Schottky devices are analyzed. Temperature effects are investigated. Suggestions for future improvements on a GaP photovoltaic device are presented.