21 February 2011 Change of electrical properties of CIGS thin-film solar cells after structuring with ultrashort laser pulses
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Low-damage laser scribing of thin films to perform series interconnection (external and integrated) of thin-film CIGS solar cells for module fabrication is still a challenge. In consequence, the influence of laser scribing parameters on the electrical characteristics of thin-film CIGS solar cells must be studied in addition to standard analytical techniques for imaging and spectroscopy. Hence, CIGS solar cells were scribed with ultrashort Ti:Sapphire laser pulses with a wavelength of 775 nm and a pulse length of 150 fs. The I-V curves with the open circuit voltage, parallel, and series resistance were measured directly after the laser-scribing process and were compared with initial cell parameters. Apart from studying the influence of laser fluence etc. also various laser-scribing geometries were examined. The most significant effect of the laser-scribing procedure can be found for the parallel resistance. Laser ablation and laser-induced material modifications during scribing results in (i) alterations of the material properties of the films, e.g. the CIGS, and (ii) material modifications outside of the laser scribe, where the interfaces, e.g. p-n junction, primarily are effected; both effects are leading to the sudden decrease in parallel resistance. Morphology, topography, geometry and material modifications of the laser-scribed areas were analyzed by scanning electron microscopy (SEM) in combination with energy-dispersive X-ray spectroscopy (EDX) and focused ion beam (FIB) cross sectioning. The results of the laserscribing induced alterations are discussed in relation to the applied scribing parameters. A model is introduced to improve the understanding of the physical reasons of the measured solar cell degradation while scribing.
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A. Wehrmann, A. Wehrmann, H. Schulte-Huxel, H. Schulte-Huxel, M. Ehrhardt, M. Ehrhardt, D. Ruthe, D. Ruthe, K. Zimmer, K. Zimmer, A. Braun, A. Braun, S. Ragnow, S. Ragnow, "Change of electrical properties of CIGS thin-film solar cells after structuring with ultrashort laser pulses", Proc. SPIE 7921, Laser-based Micro- and Nanopackaging and Assembly V, 79210T (21 February 2011); doi: 10.1117/12.874999; https://doi.org/10.1117/12.874999

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