Extraction of Saturation Current and Ideality Factor from Measuring Voc and Isc of Photovoltaic Modules

Abstract

Saturation current (I0) and ideality factor (n) of a p-n junction solar cell are an indication of the quality of the cell. These two parameters are usually estimated from dark current-voltage measurements. In this study, a quick and easy method to determine these two parameters by measuring open-circuit, Voc, and short-circuit current, Isc, is presented. Solar cell designers can use this method as a grading or diagnostic tool to evaluate degradation in photovoltaic (PV) modules. In order to verify the Voc-Isc method, a series of experiments have been conducted on a single cell and a 36-cell module. Good agreement between our Voc-Isc method and dark I-V measurements was obtained. An application of the method on the performance degradation of a single-junction a-Si:H module revealed that the module’s I0 increased by more than three orders of magnitude and n increased by 65% after an outdoor exposure of 130 kWh/m2. This increase in n indicates that after exposure, the recombination current in the cells’ space charge region increased due to the light-induced formation of metastable defects. The method is also used to assess the quality of five PV module technologies and proved to be reliable despite defective cells in a module.