Explanation of high solar cell diode factors by nonuniform contact resistance

The current density–voltage (J–V) curve that characterises the performance of a solar cell is often extra 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 nonuniform 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 with 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 nonuniform contact resistance.