Natural sunlight calibration of silicon solar cells

Abstract

The light source is very important when calibrating solar cells. Commonly used light sources – solar simulators – are expensive and frequently inaccurate. This work shows that testing solar cells under natural sunlight is simpler, cheaper, and more accurate than all but the most careful simulator measurements. Solar spectra generated with the model SMARTS2 show that the direct-beam solar spectrum, under clear sky and low air mass conditions, is an excellent match to the AM1.5G standard. Millions of simulations of a broad range of silicon cells (efficiencies 6-25%) under the modelled direct-beam spectra show that measurement uncertainty of less than 5% is achievable. This is comparable to the reproducibility of results achieved by standards laboratories. Climate data shows that the required atmospheric conditions occur commonly in summer for all but polar latitudes. Finally, it is shown that the important atmospheric conditions can be measured without expensive equipment.