Thin-film amorphous Silicon (a-Si) technology has been up and coming in recent years in order to achieve an efficiency comparable to crystalline Silicon (c-Si). Although the technology has existed for quite some time, it has not been researched as much as its crystalline counterpart, especially not in the hotspot reliability sector. The hotspot phenomenon is noticed in field-aged modules commonly due to partial shading or soiling losses. When one of the cells in a string gets partially or fully shaded, the shaded cells are forced to be in reverse bias and dissipate power in the form of heat, causing the formation of hotspots. The hotspots can reach temperatures of beyond 100C. These can lead to the melting of module encapsulants and faster degradation of the module, as well as being a fire hazard because of the elevated temperatures. This thesis was done in collaboration with HyET Solar BV, a company that produces a-Si thin film solar foils. The modules used in this thesis are from HyET Solar BV and we have selected highly degraded modules for the experiments.

This report aims to achieve a couple of things: first, get insight of hotspot formation in a-Si thin-film modules in accordance with the latest International Electrotechnical Comission (IEC) 61215 Certification norms.

Secondly, we have utilized electroluminescence (EL) imaging to classify various defects present in the modules and to evaluate their endurance to hotspot formation. Based on those results, we created a map that shows the probability of hotspot formation from different defects in the degraded modules.

And lastly, we classified the different kinds of hotspots based on factors like shape and location. Also, the possibilities of how these defects originated are explained on the basis of a schematic overview of the interconnection between the cells.

After testing and experimenting, it became apparent that the highly degraded a-Si foils are susceptible to hotspot formation. After light soaking two foils according to the IEC 61215 norms, a couple of hotspots were created in the shaded area. Some of these hotspots could be seen on EL images by defects before they became visible during the visual inspection step. EL imaging is used to characterize different defects that could lead to hotspot formation. These defects are white dots, current crowding and dark regions. The probability map for these defects is as follows: 46.6% of the white dots have turned into hotspots, 16.7% for current crowding and 32% for dark regions. It is suspected that these white dots are a manufacturing process problem; they could be areas of high-impurity contaminants or a sharp point. Current crowding and dark regions are always paired together and depending on which side of the cell they are on, it is suspected that it is a fault in either the P2 or P3 scribes of a cell in the module.