Absolute measurements demonstrate internal quantum efficiencies in silicon solar cells to exceed unity for photon energies above the first direct band gap and to show distinct spectral features that correspond to specific points in the Brillouin zone. Ultraviolet radiation can generate hot carriers with sufficient energy to cause impact ionization which results in two electron hole pairs per incident photon.
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.
Differential spectroradiometry is based on the determination and analysis of the differential spectral response of photoelectric devices in the presence of bias irradiance that can be varied with wavelength and over many orders of magnitude. The principle is described. Differential spectroradiometry is particularly suitable for the following applications that are reviewed or illustrated by some examples: determination of spectral and weighted responsivities allowing the transfer of the spectral responsivity scale to high levels of irradiance; determination of nonlinearities in the photocurrent-irradiance characteristic; non-destructive characterization of photoinduced effects.
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