Spectral mismatch in calibration of photovoltaic reference devices by global sunlight method

The state of the art for calibration of photovoltaic reference devices has been reviewed recently. In the global sunlight method the total irradiance is measured by a pyrheliometer and a shaded pyranometer while the device is held at 25DGC. The method is supposed to have high repeatability even if the spectral irradiance is not measured, provided that the atmospheric conditions are selected carefully. Here we investigate seasonal variation of this method by calculating the spectral mismatch for crystalline silicon reference solar cells for spectra measured at air mass of 1.5 and the defined reference spectral irradiance. A seasonal variation is found with negligible mismatch for certain periods but significant deviations for other periods. Based on this a modified global sunlight method using global natural sunlight for the calibration of photovoltaic reference devices is proposed, using measured spectral irradiance and applying mismatch correction.