Extraction and microscopic analysis of partial shading-induced defects in a commercial CIGS PV module

The ever-increasing instalment capacity of Cu (In, Ga)(Se, S)2-based photovoltaicscalls for a better understanding and control of their reliability. In this paper, we showhow using a coring-based method, small samples can be extracted from full size com-mercial modules, and prepared for lab-scale analysis. The method is applied to a Cu(In, Ga)(Se, S)2(CIGS) module where a non-reversible, propagating (‘wormlike’) defecthas been created in a controlled partial shading experiment. Through current–voltage,photoluminescence and illuminatedlock-in thermography analyses on anundamaged part of the module, the method used is shown to yield fully functional,undamaged active cells, with a photovoltaic conversion efficiency above the fullmodule efficiency. Where the wormlike defects were present, a typical strongshunting behaviour is observed, as well as an increased sulphur content near theedge of the wormtrails. Furthermore, thewormlike defect propagation is shown tobe strongly influenced by the present of specific features near the interconnects,which could be the result of manufacturing. These results demonstrate the poten-tial of coring to analyse module failure with all the laboratory tools available. Theyalso shed some light on how wormlike defects, which are a rare but serious hazardfor CIGS modules reliability, can form and propagate in commercial, monolithicallyinterconnected modules.KEYWORDSCIGS, coring, illuminated lock-in thermography (ILIT), partial shading, photoluminescence,reliability, wormlike defects1|INTRODUCTIONCIGS is a well-established PV technology, with an estimated 1.9GWp produced in 20171and plans for larger production in thefuture.2,3Its range of applications includes not only PV field instal-lations but also cladding for the built-in environment,4where it hasthe advantage over crystalline silicon that it can be made suffi-ciently flexible to fit on surfaces of any shapes (e.g., buildingfacades and roofs).5,6However, one crucial requirement for thebankability of any PV technology is reliability: CIGS must withstandthe various stresses present in outdoor operation, at least within itsdesigned operational lifetime. In this respect, high stability in out-door conditions7as well as in accelerated ageing tests8and fieldtests9has been demonstrated for some CIGS modules. However, asthe base CIGS material and full device stack evolve over the yearsto achieve higher efficiencies, the reliability needs to be continu-ously monitored. The cause behind the degradation of a PV deviceis usually studied in detail at the laboratory level, on dedicatedsamples (cells or small modules) which can differ significantly fromcommercial devices. On the other hand, the characterisation offield-degraded, full-size modules is limited to the techniques thatcan be used at this scale (e.g., I-V and electroluminescence).