Study of the Physical and Chemical Origin of Features Observed in Luminescence and Thermography Images of Cu(In,Ga)Se2

Luminescence and thermography have proven to be effective tools for the detection of failure modes and defects in Cu(In,Ga)Se2 (CIGS)-based photovoltaic (PV) devices. However, the chemical and physical origin of many of the features observed with these techniques are still unclear. This makes it difficult to asses their impact on device performance and lifetime. Here, features were identified in CIGS cells using spatial photoluminescence (PL), electroluminescence (EL), illuminated lock-in thermography (ILIT) and dark lock-in thermography (DLIT). Localized features were studied using optical microscopy, scanning electron microscopy, electron dispersive X-ray spectroscopy and confocal microscopy. The most commonly observed features could be associated with three different origins. Firstly, TCO sheet resistance resulting in a gradient in the direction of current flow was visible in each sample. For samples with high TCO sheet resistance this compromised detection of other features in EL. Secondly, about half of the detected features corresponded to areas of exposed molybdenum resulting in dark spots in PL and EL. These occurred in shunted and non-shunted form, with only the former causing hotspots in thermography. Thirdly, ohmic shunts induced by current-injection (current-induced shunts) were found to form large hotspots in ILIT and DLIT and a significant drop in luminescence intensity in EL and PL. Localized features in luminescence were only observed for the largest current-induced shunts as clear bright spots in PL and clear dark spots in EL. The results from this study contribute to the distinction of different defect types in modules using exclusively luminescence and thermography imaging techniques.