Effects of sheet resistance and contact shading on the characterization of solar cells by open-circuit voltage measurements

The measurement of the open-circuit voltage (Voc) as a function of the illumination intensity (Suns–Voc) is a useful tool for characterizing solar cells, giving a characteristic curve with virtually no influence from series resistance. In particular, Suns–Voc measurements allow the extraction of the diode properties without a complete contacting scheme, such as for test structures in research or for quality control between processing steps during production. In this article, we show by means of resistive network calculations, that the combination of contact shading and high sheet resistance can cause severe deviations of the measured Suns–Voc curve from that measured without contact shading or with only negligible sheet resistance. These deviations bear the danger of an erroneous assessment of the fundamental diode properties. For sheet resistances typical for thin layers of doped hydrogenated amorphous Si even the shadow of the tip of a needle-shaped contacting probe can be sufficient to cause a distorte...