Solar cell efficiency divergence due to operating spectrum variation

Abstract

A limitation in the performance rating of solar cells and modules is that they are tested using a single value for the solar spectrum. To map the performance expected under the varying spectra found in operating conditions, solar cell efficiencies have been evaluated using solar spectra generated by the National Solar Radiation Database, applied to confirmed record-efficiency cell parameters. Nine solar cell types (single-junction and multijunction) are evaluated using spectra at more than forty locations, spanning 76° of latitude and 150° of longitude, at hourly intervals over a year. Relative to the standard test efficiency, increases in annual operating efficiency are seen in cadmium telluride and single-junction perovskite designs, while efficiency decreases are observed in two-terminal multijunction structures. Though silicon exhibits the least variation, its -3% to +2% range is equivalent to 20° C of temperature variation. This divergence in operating efficiencies indicates that evaluation using a single spectrum is not a sufficient basis for comparison, or prediction of energy yield in operation. Application of an additional “operating spectrum,” to complement the standard test spectrum, is proposed.