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

Abstract

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.

The Method of Calculating the Spectral Sensitivity of Solar Cells

“…In work [1] is proposed the method of determining the two most important characteristics of solar cells: the short-circuit current of solar exposure -the spectral sensitivity under the same conditions. Since the solar cell is not a linear element (depend on level of radiation) measurement scheme is created in which the solar cell is irradiated simultaneously by two streams of radiation:…”

“…The result is an array of these differential sensitivity-, , depending on the wavelength -λ and the shortcircuit cell -. The true method of calculating short circuit current cell for standard conditionsis created in work [1].…”

“…The paper [1] contains also a description of the method of computation of the spectral sensitivity of the solar cells (SC) measured by their differential sensitivity to various levels of radiation. Formula for the calculation of spectral sensitivityat the selected wavelength λ in standard irradiation conditions…”

“…Moreover, the spectral sensitivity becomes zero in the case when the magnitude of differential sensitivity on any level is set to zero (with any sensitivity at other levels). In figure 1 (taken on [1]) the gray color indicates the area of the wavelengths above 1100 nm. In this range of spectral sensitivity (and hence differential) is equal to a zero.…”

“…However, the above formula involves integration from zero values of current and the calculation algorithm becomes uncertain. There is no reason for a reasonable choice of the lower limit of integration in form (1). Additionally, as shown above, process of measurements should maintain the constancy of the spectral composition of the background radiation.…”