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.

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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.

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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.

This paper presents the results of an international intercomparison of photovoltaic (PV) performance measurernents and calibrations. The intercomparison, which was organized and operated by a group of experts representing national laboratories from across the globe (i.e., the authors of this paper), was accomplished by circulating two sample sets. One set consisted of twenty silicon reference cells that would, hopefully, form the basis of an international PV reference scale. PI qualification procedure applied to the calibration results gave average calibration numbers with an overall standard deviation of less than 2% for the entire set. The second set was assembled from a wide range of newer technologies that present unique problems for PV measurements. As might be expected, these results showed much larger differences among laboratories. Methods were then identified that should be used to measure such devices, along with problems to avoid.

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More quality assurance activities are needed to minimize uncertainties and improve equivalence of the UV radiation data obtained at different sites worldwide in order to monitor trends and to determine changes due to the depletion of the stratospheric ozone layer. Aspects of traceability based on a calibration chain from a cryogenic radiometer to the spectroradiometers within a UV monitoring network, as well as equivalence and conformity based on intercomparisons of standard lamps and instruments, are discussed. Some recent developments to be used for quality assurance/control in UV monitoring networks are presented, covering portable monitor radiators, monitor systems containing photodiodes and/or LEDs as well as the calibration transfer from the horizontal to the vertical optical axis.