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Since 2010, the ultraviolet fluorescence (UVF) method is used to identify defects in wafer-based crystalline silicon photovoltaic (PV) modules. We summarize all known applications of fluorescence imaging methods on PV modules to identify defects and characteristics. The aim of this review is to present the basic principles for the interpretation of UVF images. The method allows for detection of cell cracks in a chronological order of occurrence, visualizing hot parts in a PV module, and identifying deviating bill of materials of PV modules. The effects of various material combinations on the UVF are reproduced in the lab and explained for the first time. Seasonal effects on the UVF are presented for the first time. In addition, some not yet understood features in the images are shown and discussed. Furthermore, the application of UVF imaging for manual, hood-based, and drone-based inspection is presented. The analysis speed of the three methods has been measured under real conditions. For the manual inspection, we found an evaluation speed of 250 modules/h, for a hood-based system 200 modules/h and the drone-based method allows an imaging speed of up to 720 modules/h.

We introduce an approach to determine the operating voltage of individual solar cells in photovoltaic (PV) modules by electroluminescence (EL) imaging. The highest EL signal of each solar cell is proportional to its operating voltage. Moreover the sum of all operating voltages equals the externally applied module voltage. Thus the operating voltage of individual solar cells is determined from the measured EL signal. The reliability of this relation is verified by measurements on specially prepared PV modules allowing us to measure the individual operating cell voltage. The experimentally measured cell voltages are deduced with an uncertainty of ±1% from an EL image. Moreover, the operating cell voltages determined from the EL image are used to calculate the module series resistance. Comparing experimentally determined values from the operating cell voltage and the total current flowing supplied to the module with calculated module series resistances using tabulated material and typical solar cell parameters, a very good correspondence is found. Copyright © 2010 John Wiley & Sons, Ltd.

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