EFFICIENCY LIMITS FOR AN IDEAL SOLAR CELL WITH COMBINED UP AND DOWN CONVERSION OR FOR TWO LEVEL UP CONVERSION USING BLACKBODY MODELLING AND AM1.5G DATA

摘要

Up [1] or down [2] conversion of transmitted or incident photons respectively has been theoretically shown by one of the current authors (Green) to increase the conversion efficiency of sunlight to electrical power. The current work extends the analysis to investigate a combination of up and down conversion; to model an up converter with two levels; to compare analyses using real air mass data with that modelling the sun as a blackbody (as has been carried out for up-conversion in [3]). Analysis has been carried out both with the band gap of the cell as an optimized parameter and at a fixed value of 1.1eV. Both up and down conversion are shown to improve efficiencies for both spectra. Combined up and down conversion improves the efficiency further for the 1.1eV band gap but give a lower efficiency for the optimised band gap. The explanation for this unexpected result is presented based on the small coupling losses that result from absorption/re-emission in the down converter not being offset by useful down conversion at the high band gaps at which the optimum occurs. However, further work on tailoring the absorptance of the down converter to prevent this loss is expected to give an increase in efficiency for combined up and down conversion.