Low-Cost Passivated Al Front Contacts for III-V/Ge Multijunction Solar Cells

Improving the performances and reducing costs of III-V multijunction solar cells are crucial in aerospatial energy systems and in terrestrial concentrator modules. We attempted to achieve both objectives by implementing non-ohmic metal/semiconductor interface contacts on the front surface of III-V/Ge triple-junction solar cells. We demonstrate the feasibility of this concept for this type of solar cell by a simple evaporation of Al only either on the GaAs contact layer or the AlInP window. The best results were obtained when sulfur passivation by (NH4)2Sx was conducted on the GaAs contact layer. This allowed for a reduction in reverse saturation dark current density by one order of magnitude and a slight increase in Voc of almost 20 mV under 1 sun illumination relative to a reference device with Pd/Ge/Ti/Pd ohmic contacts. However, poor performances were observed at first under concentrated sunlight. Further annealing the solar cells with Al front metallization resulted in the reduction of Voc to the same level as the reference solar cell but allowed for good performances under high illumination. Indeed, an efficiency over 34% was observed at 500 suns light intensity both for Al and Pd/Ge/Ti/Pd contacted solar cells.