Analysis of the dependence of the spectral factor of some PV technologies on the solar spectrum distribution

The outdoor performance of thin film technologies is not so satisfactorily understood as that of crystalline silicon. Despite all PV materials are influenced by the spectral irradiance, thin film PV modules are especially sensitive to it. Thus, the spectral factor (SF) is a useful index to quantify spectral gains or losses a PV module may experience under the actual spectrum when compared to the standard AM 1.5-G. This work is mainly aimed at looking into the dependence of the reciprocal of the spectral factor (SF−1) of four modules representative of four technologies – amorphous silicon (a-Si), cadmium telluride (CdTe), copper indium diselenide (CIS) and monocrystalline silicon (m-Si) – on the average photon energy (APE), an index that characterizes the spectrum shape. Results show that in general SF−1 increases as APE does. PV modules made of m-Si, CIS, CdTe and a-Si are ordered from lowest to highest dependence of the former parameter on the latter. Thus, figures of APE ranging from 1.79 to 1.92 eV cause deviations on SF−1 – from the ideal ‘one’ of the AM 1.5-G – of some −25/15% for a-Si, but only of some −5/5% for m-Si. Correlations between figures of APE obtained in cloudy and fine weather imply that spectral gains – highly noticeable in a-Si – take place in overcast conditions, while clear days with high irradiance profiles exert a limited influence on the spectral behavior of the four considered technologies, with the exception of a-Si. Last, low levels of irradiance in clear days imply spectral losses in all the cases.