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Morphology and composition of wormlike defects, occurring in CIGSe solar cells under reverse bias conditions
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
2018 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

Thin film solar cells consist in a family of solar cells having an absorber layer thickness in the order of microns, offering both a decrease in material usage and the possibility of using flexible substrates. CIGSe solar cells is one of the main thin film solar cells on the market, with record efficiencies reaching over 22%. However, the knowledge about the long-term reliability and stability of CIGSe is still limited. One major problem is its stability under partial shading, forcing the solar cell to operate under reverse bias conditions. This does not only induce temporary loss in power output but can also lead to permanent damage in the form of worm like defects, extending several mm over the active area of the cell. The origin of these defects is a reverse bias break down, giving rise to the formation of a hotspot, which then propagates through the material, creating a worm like defect.

The objective of this thesis is to study the morphology and composition of these wormlike defects to gain a better understanding of its propagation. 11 cells were studied, having the same stacking sequences but varying processing specifications, and different thicknesses of the AZO layer. The samples were provided by Institute de Jean Rouxel, Nantes and partially processed at Solliance Solar Research, Eindhoven, Netherlands. For the study of the morphology of the worm, optical microscopy, confocal microscopy and SEM was used. The characterization methods used in the study of the material composition and morphology include Raman spectroscopy, photoluminescence spectroscopy and EDX. All characterization was carried out at Solliance.

The result showed that worms were formed in all samples but two. Moreover, worms were formed in all different types of samples. There was an indication that there is a correlation between a low shunt resistance and a low break down voltage. Furthermore, in the samples having a thicker AZO layer, localized worms were formed, limited to area around their origin. In the samples with a thinner AZO the worms propagated several mm across the active area. For the propagating worms, a few sizes and propagation patterns were observed. Moreover, the worms were established to be quite big in relation to the thickness of the cell, measuring over 20microns in width, destroying the material in its path of propagation. Furthermore, large voids were observed being formed towards the front contact. Regarding the composition of the worm it could be established that there was a segregation of elements in the worm, where Cu-agglomerations were formed and an increase in Ga content was observe in certain areas. Moreover, in the area next to the worm an increase in defect levels as well as the formation of CIS, the remnants of CdS, as well as the diffusion of Cd into the CIGSe layer was established. These effects were observed up to 20 microns from the worm. Both the study of the morphology of the worm as well as the change in material composition next to it indicates high temperatures during worm formation, and a suggestion that the changes in the area around the worm are due to heat.

The conclusion drawn from these results is that the propagation of the hotspot occurs due to the increase in resistivity in the worm related to the voids formed towards the front contact, forcing the current to seek a path of less resistance. This new path provided by the increase in defect levels in the CIGSe layer next to the worm, as well as the diffusion of Cd. The elevated temperature around the hotspot also increase the probability of defect propagation.

Place, publisher, year, edition, pages
2018.
Keywords [en]
CIGS, reverse bias, wormlike defect
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-71852OAI: oai:DiVA.org:ltu-71852DiVA, id: diva2:1267353
External cooperation
Solliance Solar Research
Subject / course
Student thesis, at least 30 credits
Educational program
Materials Engineering, master's level
Examiners
Available from: 2018-12-13 Created: 2018-12-01 Last updated: 2018-12-13Bibliographically approved

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