Failure root cause investigation in corrugated flexible thin film modules

Cu(In,Ga)(Se,S)2 (CIGS) - based devices, which, in rigid modules, already have a solid track-record and demonstrated high reliability, are also very promising for the emerging field of curved vehicle- and building-integrated photovoltaics (VIPV and BIPV). However, such new applications imply additional stresses compared to flat, rigid modules, that can be induced for example by bending or coefficient of thermal expansion mismatch. Prior to the present study, we laminated flexible CIGS modules onto corrugated rooftiles and installed them outdoor for power generation for three years. We then unpackaged the modules using an in-house procedure and found two characteristic defects. The focus of this study was on the replication of these defects in the lab, and the search for a realistic root cause. Both defects were found to arise from the sample flexibility, and should therefore not be specific to CIGS. The first defect, which was relatively mild and due to very local front grid delamination, could be replicated in thermal cycling tests. The second one, which severely impacted the performance by causing kinks in IV curves, consisted in local delamination between the Mo back contact and the CIGS absorber layer. We investigated mechanical, thermal and environmental stresses to replicate this last defect. Although thermal cycling and sequential testing failed to replicate it, tensile and hail testing revealed that the Mo/CIGS interface, in the present experiment, was indeed the weakest in the stack and could delaminate when high strain were applied locally and combined with damp-heat conditions.