Roll to roll nanotexturization of layers on steel foil substrates for NIP silicon solar cells

Abstract

ROLL-TO-ROLL NANOTEXTURISATION OF LAYERS ON STEEL FOIL SUBSTRATES FOR NIP SILICON SOLAR CELLSM.C.R. Heijna*, M.J.A.A. Goris, W.J. SoppeECN, Solar Energy, P.O. Box 1, 1755 ZG Petten, theNetherlands.*Corresponding author, phone +31 (224) 56 4953, e-mail: heijna@ecn.nlW. Schipper, R. WildeNanoptics GmbH, Innungsstrasse, 21244 Buchholz, GermanyABSTRACT: Light management is of utmost importance for thin film silicon solar cells. For nip cells, light management can be provided by textured back reflectors. In this paper, we demonstrate the fabrication of textured back contact foil on an industry compatible scale. A UV-curing lacquer on a stretch of 80 m of 30 cm wide steel foil was nanotexturedinroll-to-roll mode, using the contact imprint method. Lab-scale samples were textured statically to test the effect on the growth of siliconby PECVD and cell efficiencies. The light trapping effect increases with decreasing lateral feature size. High aspect ratio structures result in strong domain boundariesdefined by the period of the gratingsand in non-conformal growth of the active layers of the cell. In its turn, this non-conformal growth, with thinner coverage of the steep slopes of the texture can lead to shunting problems in the solar cells.So, abalanceneeds to be found between optimum light scattering of the texturedback contact and optimum layer growth on that back contact.Keywords:Light trapping, thin film solar cell, substrates1 INTRODUCTIONRoll-to-roll production allows for flexible PV modules and a significant decrease in production cost of thin-film solar cells compared to batch-type reactor systems.Flexible and lightweight PV modules gear up to building integrated PV: the most important market for PV in densely populated, developed countries [1,2]. ECN is developing a pilot line for roll-to-roll production of high efficiency n-i-p solar cells based on amorphous and microcrystalline silicon thin films on steel foil coated with an insulating layer and sputtered back contact and reflectionlayer. The main purpose of the insulatinglayer on the steel foil is to enable monolithic series interconnection of cells in a later stage of the fabrication process, after deposition of all layers. The cell and module design of the ECN concept is depicted in Figure 1.Figure 1: ECN’s thin film Si solar cell and module concept.For thin-film silicon solar cells, light trapping schemes are of uppermost importance to harvest all available sunlight. These schemes typically comprise the use of textured surfaces with lateral and vertical dimensions in the submicron range. For pin silicon solar cells on glass, naturally texturedor etched TCOs are used to provide the light-scattering texture [3,4]. Whereas etching or natural growth processes for acquiring textures limit the possibilities for size and geometry of the texture, nanoimprint allows for any master structure to be replicated; the nanotexture appliedcan be, within limits, any kind of random or periodic texture.Roll-to-roll nano-replication is usually performed onplastic substrates, allowing for illumination through the substrate. Here, we demonstrate replication ofsubmicrontextures in UV-curing lacquerson steel foil.The aim of this work is to demonstrate a roll-to-roll nanoimprint process on coated steel foil substrates that is feasible to be applied on an industrial scaleto produce large-scale light trapping textured substrates for thin film PV.2 ROLL-TO-ROLL NANOTEXTURISATION2.1Industrial production with contact imprintIn order to produce structured materials with significantly higher speed while maintaining quality and producing under industrial conditions, at Nanoptics we follow the contact imprint technology as a new and innovative machine conceptdedicated for UV and e-beam embossing of opaque substrates.Figure 2:Schematicshowing the principle of the contact imprint technology.Unlike embossing with a structured cylinder, on this system a freshly coatedbase film will be laminated by a so calledmaster film. After a long period in contact with the lacquer it will be cured by UV-and / or electron beam radiation(see Figure 2). It is technicallyfeasible on metal film which has been demonstrated successfully. In addition it allows a huge number of other base films to be