Improved Outdoor Measurements for Very High Efficiency Solar Cell Sub-Modules

Very High Efficiency Solar Cell (VHESC) program is developing integrated optical/photovoltaic modules for portable applications that operate at 50 percent efficiency. Test sub-modules incorporating four p-n junctions and corresponding optics have been realized and are predicted to realize efficiency greater than 40%. Phased implementation requires corresponding measurement to inspect accomplished work and provide improvement direction for the next step. The comparison between the real performance of the four-junction test sub-module and the theoretical prediction of its efficiency is a significant indication of the realizability of the final VHESC module including six junctions which is designed to achieve 50% efficiency. For the sub-module measurement, a test bed was set up for outdoor test. Previous outdoor measurements of the VHESC test sub-modules resulted in a preliminary sub-module efficiency of 36.2% [1]. As solar cells with better performance were fabricated, the measurement methodology was refined and corresponding improvements were made to the initial test bed. Three test sub-modules containing new solar cells were measured with the new test setup for three different concentration levels at University of Delaware (UD). One test sub-module demonstrated efficiency as high as 39.5%, coupled with 44.3% efficient solar cells and 89.1% efficient optics, at 30.48X concentration. The measurements were taken when the direct light intensity was over 860W/m² and the I_sc was not calibrated to 1000W/m². Another two test sub-modules including solar cells in the same batch as the ones tested at UD were taken to National Renewable Energy Laboratory (NREL). The I_sc data of the two test sub-modules were recorded outdoors at NREL when the direct light intensity was over 970 W/m ². In addition, the I_sc was calibrated to the standard spectrum condition using ASTM G173 direct data. Comparison of the results shows the difference between the test sub-module efficiency measured at UD and NREL is less than 4%.