Impact of sheet resistance and contact shading on ´Suns-V/sub OC/´ measurements on thin-film solar cells

The measurement of the open-circuit voltage (V/sub OC/) as a function of the illumination intensity (Suns-V/sub OC/) is a useful tool for characterising solar cells, giving a characteristic curve with virtually no influence from series resistance. In particular, Suns-V/sub OC/ measurements allow the extraction of the diode properties without a complete contacting scheme. In this paper, using resistive network calculations, we show that the combination of contact shading and high sheet resistance can cause severe deviations of the measured Suns-V/sub OC/ curve from that measured without contact shading and/or negligible sheet resistance. These deviations bear the danger of erroneous assessment of the fundamental diode properties. For sheet resistances typical for thin layers of doped hydrogenated amorphous Si (a-Si:H), even the shadow of the tip of a needle-shaped contacting probe (O/spl sime/5 /spl mu/m) is sufficient to cause a severely distorted Suns-V/sub OC/ curve. Results of experiments performed on a microcrystalline p-i-n Si thin-film solar cell with an a-Si:H n/sup +/ layer are presented and explained within the framework of the resistive network model.