Radiometry in photovoltaics: calibration of reference solar cells and evaluation of reference values

The key quantity in the calibration of solar cells and photovoltaic modules is the short-circuit current of the device generated by a reference solar radiation with 1 kW m-2 total irradiance and with reference solar spectral irradiance distribution. Thus, (i) double spectral mismatch between test and standard device and between test and reference solar spectrum; and (ii) the calibration transfer from low to high irradiance level (non-linearity effects) may affect the calibration value. An overview of primary and secondary calibration procedures is given, focusing on the primary calibration methods applied by the four qualified laboratories establishing the World Photovoltaic Scale (WPVS) traceable to the International System of Units (SI). The differential spectral responsivity method of the Physikalisch-Technische Bundesanstalt (PTB) is described in more detail. Finally, different approaches are illustrated for the evaluation of a reference value, including the WPVS, based on the results for twenty transfer/travelling standards of the international comparison among eleven participants that was carried out to establish the WPVS.