Electroluminescence and Dark Lock-In Thermography for the Quality Assessment of Metal-Wrap-Through Solar Devices

Imaging techniques, like electroluminescence and dark lock-in thermography, are valuable quality control tools as they yield quantitative and spatially resolved information about the device. In this paper, we isolated some of the conductive foil-cell interconnections of back-contact solar cells to study the appearance of these intentional failures in electroluminescence, dark lock-in thermography, and series resistance images. It has been found that isolated emitter-to-foil contacts are clearly visible in the three imaging techniques, as they show characteristic features that deviate from the features typical of functioning emitter-to-foil dots. Isolated base-to-foil contacts are instead invisible in the images obtained by electroluminescence and only hardly visible in the images obtained by the other two techniques. Only after a large amount of contacts are isolated, a local current redistribution or drastic series resistance increase is noticeable. Two graphical methods for the automatic identification of isolated emitter-to-foil contacts in electroluminescence, dark lock-in thermography, and series resistance images were also designed, showing a success rate of 97% in the investigated cells. Such techniques could represent useful tools for implementation in inline quality control processes. Moreover, the techniques and conclusions drawn in this paper can be extended to a large number of other conventional and emerging photovoltaic technologies.