Traceable spectral irradiance measurements in photovoltaics: Results of the PTB and JRC spectroradiometer comparison using different light sources

JRC ESTI and PTB performed a comparison of their respective reference standard spectroradiometers, aiming at improving knowledge on equivalence of spectral irradiance measurements and resulting uncertainty in calibration of photovoltaic devices. The comparison was conducted at the JRC premises using different light sources in order to test the reference instruments under various real-world operating conditions. The measured sources included four standard lamps (two calibrated by PTB and two calibrated by NPL), global natural sunlight under clear sky conditions, a steady-state xenon-halogen solar simulator for solar cell calibration (Wacom) and a steady-state xenon large area solar simulator (Apollo). The PTB system is composed of three compact array spectroradiometers (CAS) equipped with Si, InGaAs, and extended range InGaAs detectors, respectively, in order to cover the wavelength range from 250 to 2150 nm. The JRC-ESTI used a rotating grating spectroradiometer composed of a single stage monochromator and a Si-PbS sandwich detector in order to cover a wavelength range from 300 to 2500 nm, and, for the natural sunlight measurements, additionally a system composed of three compact array spectroradiometers covering the wavelength range from 300 to 1700 nm. Before the comparison all instruments were calibrated in house by the owning institution using their respective procedures and traceability chains. The measured spectra were compared and deviations were found to generally be within combined uncertainties. Finally, exemplary spectral mismatch calculations were made to evaluate the impact of the deviations in spectral measurements on PV devices calibration. The deviations were mostly less than 0.5%, but in some cases exceeded 1%.