A review and comparison of different methods to determine the series resistance of solar cells

This work presents a review of five different methods to determine the lumped series resistance RS of solar cells and an experimental investigation of these to find the most reliable and robust method(s) for cell characterization under operating conditions. The methods under consideration are: fitting of the two-diode equation function to a dark IV-curve, comparison of a one-sun with a dark IV-curve, comparison of a Suns-VOC with a one-sun IV-curve, comparison of two or more IV-curves measured at different illumination intensities, and computation of the area under a one-sun IV-curve. Firstly, for a quantitative evaluation, all series resistance values were plotted against the fill factor FF of the corresponding cell. The accuracy of the methods is quantified using a wide range of solar cells. Secondly, the robustness of the methods in the presence of other FF-limitations such as shunts is also explored. The results and the interpretation of a first analysis of small 2×2 cm2 solar cells of the integration method led to a successful improvement of this method, which was proven by a second measurement. All the conducted investigations led us to the conclusion that of these five methods under consideration, the illumination intensity variation, the comparison of a Suns-VOC with a one-sun IV-curve, and the modified comparison of a one-sun IV-curve with a dark IV-curve method are the most reliable and robust ways to determine the series resistance under operating conditions.