In-line Thermography for Reliable Hot Spot Detection and Process Control

We discuss and demonstrate the usage of in-line thermography as a versatile tool for reliable detection of solar cells prone to cause hot-spots in modules under shaded operation conditions and for optimization of production processes to avoid producing such cells. Thermography images of reverse biased solar cells are recorded during a time interval of 60 ms, along with the measurement of the reverse IV-characteristics of the solar cell, avoiding the necessity of a second contacting unit in cell testers. The obtained images are corrected for thermal background radiation and evaluated in two ways – i) predicting the cells temperature in module operation when a solar cell is reverse biased due to partial shading of the module. Using the predicted cell temperature to reject cells that are prone to hot-spot failure is a superior metric to the current methodology of rejecting such cells. ii) Analyzing possible causes for hot-spots by a shape detection to find the most important error classes. This information is important to give a quick response about the origin of reverse currents caused by production deviations.