Rapid and Accurate Determination of Series Resistance and Fill Factor Losses in Industrial Silicon Solar Cells

Abstract

Lower than ideal fill factors (FF) are caused by parasitic series (R(s)) and shunt (R(shunt)) resistances, and non-ideal diode properties. The challenge is to quantify the FF losses quickly, simply and without ambiguity. Extracting the parameters by fitting the illuminated or dark measured data with the double diode equation is inaccurate since the externally apparent R(s) is not constant; it varies with illumination level and electrical load. It is shown that the variations in R(s) are not a second order effect only noticeable in laboratory cells, but that the variations are even more important in industrial solar cells and many methods underestimate R(s). It is also common to estimate the cause of FF loss by visual inspection of the IV curve, but this also leads to a misinterpretation of loss mechanisms. A very high R(s) affecting 10% of the cell causes a slope at short circuit current that is very similar in appearance to a cell with low R(shunt), and that a high R(s) affecting 50% of the cell appears similar to high second diode saturation current. A superior method to measure R(s) at the maximum power point is to shade the cell to 0.1 suns and measure open circuit voltage and short circuit current. Using this extra data with standard one sun measurements also measures the average diode ideality factor, R(shunt), and reveals non-ohmic contacts.