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Diffractive backside structures via nanoimprint lithography

: Hauser, H.; Mellor, A.; Guttowski, A.; Wellens, C.; Benick, J.; Müller, C.; Hermle, M.; Bläsi, B.

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Energy Procedia 27 (2012), pp.337-342
ISSN: 1876-6102
International Conference on Crystalline Silicon Photovoltaics (SiliconPV) <2, 2012, Leuven>
Journal Article, Conference Paper, Electronic Publication
Fraunhofer ISE ()
Solarthermie und Optik; Angewandte Optik und funktionale Oberflächen; Silicium-Photovoltaik; Mikrostrukturierte Oberflächen

For decreasing thicknesses of wafer based silicon solar cells, photon management structures to maintain high quantum efficiencies will gain importance. Diffractive gratings on the wafer back side can be designed to achieve very high path length enhancements, especially for weakly absorbed infrared radiation. This technologically demanding concept has to be realised using processes with upscaling potential. Therefore, we present a fabrication process for producing photonic structures in silicon based on interference lithography and nanoimprint lithography (NIL). We realised linear as well as crossed gratings of different depths, which were etched into the wafer back side. Polarisation dependent reflection measurements were made to get information about potential absorption enhancement as well as the occurrence of parasitic absorption in the metal reflector. This is conducted for a PECVD silicon oxide buffer layer between grating and reflector as well as a spin coated silicon oxide layer. Besides these optical characterisations, we further investigated the electrical properties of the back surface, where we applied a concept in which electrical and optical properties are decoupled. This is realised by a layer stack on the wafer back side, consisting of a thin Al2O3 passivation and a doped amorphous silicon layer.