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Microcracks in silicon wafers II: Implications on solar cell characteristics, statistics and physical origin

 
: Demant, M.; Welschehold, T.; Kluska, S.; Rein, S.

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Postprint urn:nbn:de:0011-n-3791024 (1.4 MByte PDF)
MD5 Fingerprint: a86167f49e25b0266cb17459cc4e3cb0
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Created on: 7.7.2016


IEEE Journal of Photovoltaics 6 (2016), No.1, pp.136-144
ISSN: 2156-3381
English
Journal Article, Electronic Publication
Fraunhofer ISE ()
PV Produktionstechnologie und Qualitätssicherung; Silicium-Photovoltaik; Charakterisierung von Prozess- und Silicium-Materialien; Messtechnik und Produktionskontrolle; micro-crack; electroluminescence; photoluminescence; shunt; cell

Abstract
Microcracks that are induced in early processing stages, especially before emitter diffusion, strongly influence the current-voltage (I-V) characteristics of the solar cell. We focus on the impact of crack morphology measured by photoluminescence imaging in the as-cut stage on the electrical solar cell parameters. To provide a sufficient statistical base, microcracks are intentionally induced in a well-defined way in multi- (mc-Si) and mono- (Cz-Si) crystalline silicon wafers in the as-cut stage, the damaged wafers being processed to solar cells afterwards. From the dataset, a sorting criterion for microcracks concerning their electrical impact is derived, which depends on wafer thickness and material type. It is shown that cracks above 4 mm2 lead with high probability to severe shunts and, thus, need to be sorted out. Investigations by means of scanning electron microscopy (SEM) and electron-beam induced current (EBIC) measurements reveal that shunts with very low parallel resistance in Cz-Si solar cells can be attributed to metal-to-metal contacts between front and rear sides of the solar cell. Moreover, it is shown that the reduced robustness of Cz-Si compared with mc-Si concerning the formation of shunts at microcracks originates from a widening of the crack channels above 10 μm in alkaline texturing, which facilitates the formation of metal-to-metal contacts.

: http://publica.fraunhofer.de/documents/N-379102.html