Improved treatment of the strongly varying slope in fitting solar cell I-V curves

Straightforward least squares fitting of I-V curves leads to nonoptimal fits: residuals around and above the open-circuit voltage dominate the fit, leading to a bad fit at the maximum power point and lower voltage values. To deal with this problem the authors have resorted to using weighting functions or to minimizing the area between data and fit instead of the least squares procedure. Both approaches lack a sound statistical basis. Voltage noise has a big influence on fitting due to the steep slope of an I-V curve for higher voltage values. For this reason we have used orthogonal distance regression (ODR), which is a mathematical method for fitting measurements with errors in both voltage- and current measurements. It allows for computing both the I-V curve parameters and their uncertainties.