The effect of dust on photovoltaic modules is investigated with respect to concentration and spectral transmittance. Samples were collected in the form of raw dust as well as accumulated dust on exposed sheets of glass at different tilt angles. Spectral transmittance of the samples was determined. Transmittance variation between top, middle and bottom was identified for samples collected at different inclinations, where the worst case was seen at a tilt angle of 30 o with a nonuniformity of 4.4% in comparison with 0.2% for the 90 tilt. The measured data showed a decrease in transmittance at wavelengths <570 nm. Integrating this with measured spectral responses of different technologies demonstrates that wide band-gap thin-film technologies are affected more than, for example crystalline silicon technologies. The worst case is amorphous silicon, where a 33% reduction in photocurrent is predicted for a dust concentration of 4.25 mg/cm 2 . Similarly, crystalline silicon and CIGS technologies are predicted to be less affected, with 28.6% and 28.5% reductions in photocurrent, respectively. The same procedure was repeated with varying Air Mass (AM), tilt angle and dust concentration values to produce a soiling ratio table for different technologies under different AM, tilt angle and dust concentration values. Copyright

The optical properties of ␣-Al 2 O 3 have been determined by two independent methods, vacuum ultraviolet (VUV) spectroscopy and electron energy-loss spectroscopy (EELS) over the energy range from 6 to 142 eV. For each experimental method two sets of high-quality data have been measured and analyzed from ␣-Al 2 O 3 . The evolution of the differences between the data for each experimental method and its associated analytical method indicate the reliability of the results, and the comparison between the methods emphasizes the relative advantages of each. VUV spectroscopy offers a higher energy resolution, resolving sharper features in the spectra, whereas EELS significantly extends the energy range, which has been exploited here to 140 eV but can be extended even further. Good overall consistency is found, but there are pronounced differences in the valence region from 10 to 30 eV, where strong absorptions are present, increasing the relative variability of the analysis. This demonstrates that accuracy estimates of the optical properties are meaningful only when supplied as a function of energy, because both strong intrinsic absorptions and weak experimental signals make accurate determinations difficult. Moreover, variations in the optical properties are observed most easily in the interband transition strength, J cv , and are less evident when the optical properties are represented as the complex index of refraction. Because the optical properties can change with specimen origin and preparation and because of the details of data acquisition and numerical analysis procedures, assessing the contributions of each of these methods to our accurate knowledge of the optical constants is essential.

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The strong growth of the PV market is accompanied by an increasing number of ''new'' PV technologies and concepts now mature for commercialization. A correct calibration of these devices is in some cases very difficult, because indoor and outdoor performance measurements often lead to different results. In this paper we compare the indoor and outdoor performance measurements of a set of recent commercially available PV modules (conventional and high-efficiency c-Si, single-, double-, and triple-junction thin film (TF) technologies) and we observe that the maximum power P max of some devices measured indoors using our large area pulsed solar simulator is usually lower than the power measured outdoors under natural sunlight. The major effects which lead to these discrepancies are identified, as follows: (a) spectral mismatch errors, very significant for CdTe, and all a-Si TF technologies; (b) measurement-related sweep-time effects, which seem to strongly influence the performance of high efficiency c-Si devices and to a lesser extend of all a-Si TF technologies; and (c) short-time light-soaking effects, which influence the performance of CIS and to a lesser extent CdTe.