Comparison of Photovoltaic Module Luminescence Imaging Techniques: Assessing the Influence of Lateral Currents in High-Efficiency Device Structures

We investigate the implications of using partial or patterned illumination for luminescence imaging of photovoltaic modules. Partial illumination induces local photovoltage variations that drive lateral current flow into non-illuminated cell regions, causing the average injection level to vary over the course of luminescence measurement. The extent of lateral current depends on material properties and is especially significant for high-efficiency devices such as the silicon heterojunction with intrinsic thin layer (HIT) modules that we investigate. We compare four module imaging methods including large-area illumination, scanning-laser illumination, contactless electroluminescence, and pattern-illuminated photoluminescence. By evaluating the statistical correlation among the different methods under varying laser fluences, we conclude that most methods converge to similar results at matched photon dose per cell (not laser power density) for the modules that we study, while pattern-illuminated photoluminescence yields distinct images from the other techniques regardless of the illumination power. Our combined analysis using multiple luminescence imaging techniques gives specific insight into the causes of HIT module degradation, including suspected degradation of the transparent conductive oxide and passivation layer.