Interpreting module EL images for quality control

A strong supply-side photovoltaic market coupled with increasing customer awareness of the importance of quality module manufacturing is resulting in customers increasingly dictating supply terms with manufacturers. With customers focusing on quality metrics, this in general becomes a positive development which is of benefit for high quality manufacturers. In Australia, with Australian Consumer Law leaving ultimate warranty responsibility in the hands of the local service providers, the need for these service providers to have confidence in the quality of the manufactured product is higher still. A quality control technique that is becoming increasingly important is electroluminescence (EL) testing. EL testing is useful for identifying variations in wafer quality, wafer impurities and dislocations, microcracks, front grid finger breaks, poor solder joints and shunted cells. There is a wealth of scientific literature dealing with most of these issues. Some of this information is useful for manufacturing process control and some of it is useful to customers. For example, features such as microcracks can impact a module’s durability in the field and this information is important. But other features of the EL image – such as the uniformity of EL response from the cells – has no reliable relationship to module performance. By reviewing the theory and application of module scale EL testing, interpretation of the images can be improved and areas where further work is required can be highlighted.