Explanation of high solar cell diode factors by non-uniform contact resistance

The current density-voltage (J-V) curve that characterises the performance of a solar cell is often rounded, resulting in reduced efficiency. When fitting to the standard one dimensional models, it is often found that the rounding cannot be fitted by the series resistance only. In these cases, the diode factor m or the depletion region saturation current density J0DR (depending on the model used) is increased. This behaviour could not be explained so far; this paper discusses if a non-uniform contact resistance of the front side metallisation leads to an increase of m or J0DR . The theoretical part of the investigation is the simulation of the curve for a cell with two regions with different contact resistance. It was found indeed that m or J0DR is increased, while the series resistance is not increased as much as expected. The experimental part was the calculation of the J-V curve of a high m solar cell using local contact resistances measured with the so-called Corescan and the cell’s resistanceless J-V curve as measured with the so-called Suns-Voc method. The calculated curve approximated the actual curve quite well, demonstrating in practice that high diode factors can be explained by non-uniform contact resistance.