Nanoimprint Lithography of Light Trapping Structures in Sol-gel Coatings for Thin Film Silicon Solar Cells
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Nanoimprint Lithography of Light Trapping Structures in Sol-gel Coatings for Thin Film Silicon Solar Cells Maurits Heijna1, Jochen Loffler1, Bas Van Aken1, Wim Soppe1, Herman Borg2, and Patrick Peeters2 1 ECN Solar Energy, P.O. Box 1, Petten, 1755 ZG, Netherlands 2 OM&T B.V., Glaslaan 2, Eindhoven, 5616 LW, Netherlands ABSTRACT For thin-film silicon solar cells, light trapping schemes are of uppermost importance to harvest all available sunlight. Typically, superstrates with randomly textured TCO front layers are used to scatter the light diffusively in pin-cells on glass. Here, we investigate methods to texture opaque substrates with both random and periodic textures, for use in nip-cells on metal foil. We applied an electrically insulating SiOx-polymer coating on a stainless steel substrate, and textured this barrier layer by nanoimprint. On this barrier layer the back contact is deposited for further use in the solar cell stack. Replication of masters with various random and periodic sub-micron patterns was tested, and, using scanning electron microscopy, replicas were found to compare well with the originals. The embossing of the barrier layer does not diminish its electrically isolating properties, and thus adds extra functionality to this layer. Masters with Ugrooves of various sub-micrometer widths have been used to investigate the optimal dimensions of regular patterns for light trapping in the silicon layers. Angular reflection distributions were measured to evaluate the light scattering properties of both periodic and random patterns. These periodic patterns, comprising diffraction gratings, show promising results in scattering the light to specific angles, enhancing the total internal reflection in the solar cell. INTRODUCTION Thin film microcrystalline silicon solar cells are an emerging PV technology as a cheaper alternative to the more traditional wafer-based crystalline silicon solar cells since they can be produced using low-cost manufacturing methods [1]. However, the absorber layer in thin film cells is typically 1 µm versus 200 µm for wafer-based cells, and is too thin for the incident light to be absorbed in one pass. As a result, thin film microcrystalline Si solar cells need light trapping schemes to enlarge the pathway and thus to enhance the absorption [2], especially in the red and near infrared part of the spectrum [3]. To enhance light trapping in thin film silicon solar cells, usually nanotextured back reflectors and/or nanotextured transparent conducting oxide (TCO) front electrodes are used. For pin “superstrate” devices, glass with randomly textured front TCO electrodes is widely used, which can be made for instance by sputtering combined with texture etching [3]. An alternative to random structures to enhance light trapping in solar cells is the use of periodic structures like diffraction gratings. These gratings are quite appealing in that they diffract light into the solar cell under specific angles, which may be larger than the angle of total internal reflection, thus
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