Annealing Effects of Microstructure in Thin-film Silicon Solar Cell Materials Measured by Effusion of Implanted Rare Gas
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Annealing Effects of Microstructure in Thin-film Silicon Solar Cell Materials Measured by Effusion of Implanted Rare Gas Atoms W. Beyer1,2, D. Lennartz2, P. Prunici1, H. Stiebig1 1 Malibu GmbH & Co. KG, Böttcherstr. 7, D-33609 Bielefeld, Germany 2 IEK5-Photovoltaik, Forschungszentrum Jülich, D-52425 Jülich, Germany ABSTRACT In thin film silicon solar cell technology, annealing (heat treatment) effects are of interest since (i) annealing of underlying films often cannot be avoided during deposition and (ii) heat treatment (e.g. by laser) may be actively used for improvement of as-deposited material. Changes in the microstructure of several thin film silicon solar cell materials like hydrogenated amorphous silicon, microcrystalline silicon and zinc oxide by heat treatment were investigated by effusion measurements of hydrogen and implanted helium. Densification is observed for all materials studied, i.e. interconnected voids disappear or are transformed to isolated voids. We attribute the observed annealing effects primarily to an incomplete polymerization during growth. Important for solar cell processing is the result that the annealing effects involving structural changes set in at temperatures close to the temperature of deposition. INTRODUCTION Knowledge on temperature treatment (annealing) processes in thin film silicon solar cell materials is desirable by various reasons. On one hand, during processing of thin film silicon related devices, annealing of underlying material often cannot be avoided. Annealing processes, on the other hand, may be of interest to change and improve individual layers during processing of devices like solar cells. In particular, the application of large area laser annealing and laser crystallization may serve as a tool for improvement of thin film silicon related devices. Various annealing processes are conceivable, like (i) diffusion of hydrogen, (ii) structural changes of amorphous material and (iii) partial and complete crystallization of the amorphous material. Improvement of individual thin film layers applied in silicon solar cell technology by annealing have been reported, like the reduction of light-induced defect generation for hydrogenated amorphous silicon (a-Si:H) films [1], enhanced passivation properties of thin film silicon materials for crystalline silicon processing [2] and the improvement of zinc oxide (ZnO) films used as transparent contacts [3]. Here, we focus on microstructural changes in thin film materials caused by annealing. For characterization of microstructure, we apply effusion measurements of hydrogen and in particular of implanted helium. Since the presence of interconnected voids or of compact material causes quite different hydrogen effusion processes, hydrogen effusion is sensitive to microstructure [4-6]. Effusion of rare gas atoms like helium is highly sensitive to material density and voids as these gas atoms do not bind to the host material [6,7]. EXPERIMENT Thin films of 1 – 2.5 μm thickness deposited on c-Si wafers were investigated. The aSi:H fi
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