Influence of non-uniform illumination on spectral response and efficiency measurements of large area solar cells

In most set-ups used to measure the spectral response of large area solar cells, it is not very easy to get uniform monochromatic and bias illumination over the total area of the solar cell, so usually only a part of the cell is illuminated or the uniformity of the irradiation is not guaranteed. This leads to an error due to the different weighting of the local spectral response of the cell with the local irradiation. An estimation of the uncertainty due to non-uniform or partial illumination for different types of solar cells is given and the necessity to guarantee the uniformity of the irradiation for precision measurements and calibration is shown. In addition, the uncertainty in the mismatch factor is calculated as an example of the influence of spectral response uncertainty to integral values. The effects resulting from inhomogeneity of spectral response under non-uniform monochromatic irradiation are determined by measuring the spatial distribution of the irradiation and the spat ially resolved spectral response. In addition a model will be used to extract the influence of extreme cases of non- uniformity on different kinds of solar cells. We will show the errors arising for series-connected mini-modules and for multicrystalline Si-cells, which can be treated as cells with slightly different spectral responses connected in parallel.