Degradation of Individual Cells in a Module Measured with Differential IV Analysis

A methodology is developed for the extraction of cell-level properties from the analysis of differential IV response in a solar module with series connected cells. Through a combination of simulation and experimental verification we show that the shunt resistance and short circuit current of individual cells can be determined from a peak in the module differential resistance with cells that are partially shaded. The magnitude of the peak is equal to the shunt resistance of the cell for small values of shunt resistance. The current at which the peak occurs is proportional to the product of the short circuit current and the shading factor of the particular cell. With this methodology, we are able to measure degradation of 72 individual cells in a single commercial module after a high temperature/high humidity/high voltage stress test. Therefore, the statistics of degradation in this test were improved 72-fold.