26th European Photovoltaic Solar Energy Conference and Exhibition, 5-9 September 2011, Hamburg, Germany INLINE DEPOSITED THIN-FILM SILICON SOLAR CELLS ON IMPRINTED FOIL B.B. Van Aken, M.C.R. Heijna, J. Löffler and W.J. Soppe ECN Solar Energy, P.O. Box 1, NL-1755 ZG Petten, the Netherlands Phone: +31 224 564905; Fax: +31 224 568214; email: vanaken@ecn.nl ABSTRACT: ECN is developing n-i-p solar cells based on a-Si and μc-Si absorber layers deposited with inline PECVD, using linear plasma sources, on an imprint-textured UV curable coating layer on foil. We show that solar cells deposited on foil with random texture can achieve good light trapping (J sc ~ 15-16 mA/cm 2 for a-Si cells). Furthermore, we show that a-Si nip cells on foil, processed in dynamic mode in an industrial pilot roll-to-roll system for 30 cm wide foils, can achieve efficiencies of over 7%. Future work will focus on developing and implementing optimised periodic nanotextures for μc-Si and micromorph tandem. Keywords: roll-to-roll deposition, thin film Si solar cells, flexible substrate 1 INTRODUCTION Roll-to-roll production of thin film Si solar cells has several advantages over batch-type reactor systems, for instance high-throughput fabrication and the application of cheap foil substrates. Flexible, lightweight PV modules gear up to building integrated PV: the most important market for PV in densely populated, developed countries [1,2]. Our novel concept for roll-to-roll production of high efficiency nip solar cells is based on amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon thin films on steel foil coated with an insulating barrier layer and sputtered back contact and reflection layer. Furthermore, the barrier layer can be imprinted with any periodic or random texture to increase the light trapping. Light trapping is most important for microcrystalline Si solar cells and μc-Si bottom cells in micromorph tandems, due to the lower absorption strength of μc-Si. To absorb the longer wavelengths (700 nm – 1100 nm), textures with larger periods and heights are applied. In the framework of the EU project Silicon-Light [3], we are investigating and will demonstrate the fabrication of ideal periodic structures for light scattering in microcrystalline silicon solar cells and micromorph tandem cells. Our concept for roll-to-roll fabrication of thin film silicon solar cells contains a few unique features [4], which offer a great potential for high efficiencies and low cost fabrication. Two of these features are presented and discussed in this paper, i) the suitability of UV curable coating as substrate for thin film Si solar cells and ii) the application of linear plasma sources for the inline deposition of silicon layers. The standard way to improve the light management of thin film solar cells is to apply a light scattering structure, either on the front window or at the back reflector. Typically, the growth conditions of the TCO layers are adjusted to get appropriate surface roughness. In contrast, imprinting the UV curable coating layer allows full control of the applied, random or periodic, texture to fully optimise the light trapping. We have combined in our roll-to-roll PECVD system the previously reported linear symmetric RF sources [5,6], which are excellently suited for deposition of amorphous and microcrystalline doped silicon layers, with a commercially available linear VHF source for the high rate deposition of intrinsic Si absorber layers. The main advantages of linear plasma sources are that only uniformity in a single direction is required and the ease of upscaling the plasma sources to enable deposition on foil substrates of one metre width or more. 2 EXPERIMENTAL DETAILS A UV-curable barrier layer (C-Coatings B.V.) is applied by doctor blading. The texture is imprinted by a PDMS shim in the wet layer and the substrate/layer/shim stack is put through a UV belt oven to harden the lacquer and fixate the texture. Back contacts are sputtered in an AJA lab scale sputter tool, and typically consist of ~250 nm Ag and 80 nm ZnO. The silicon layer deposition is carried out either in a cluster tool or an inline PECVD system. The cluster tool has three separate UHV chambers for n, i and p layer deposition. All chambers have identical flat-plate RF PECVD sources and are closed during deposition to prevent contamination of the transfer chamber and the other chambers. Samples are transported on standard sample holders via a transport arm. Typically, four substrates of 10×2.5 cm 2 are co-deposited. The inline PECVD system is an industrial pilot roll- to-roll system for foils of width up to 300 mm. The Flexicoat300 has three inline deposition vacuum chambers. Two chambers are equipped with the previously reported linear symmetric RF (13.56 MHz) sources [5,6], which are excellently suited for deposition of amorphous and microcrystalline doped silicon layers. The intrinsic Si absorber layers are deposited with a linear VHF plasma source (70 MHz). Samples (typically several 10×2.5cm 2 substrates) are fixed to a custom-made sample holder, which is placed in the steel foil that is used as conveyor belt. The vacuum chambers are separated by independently pumped gas sluices to prevent contamination by dopant gasses. The main advantages of linear plasma sources are that deposition uniformity is only required in one direction, perpendicular to the motion of the substrate(s) and the ease of upscaling the plasma sources to enable deposition on foil substrates of one metre width or more. The solar cells are defined by the area of the ITO front contact: 4×4 mm 2 and 10×10 mm 2 . For contacting purposes a silver contact pad is e-beamed on the ITO front contact. IV measurements are done on a WACOM sun simulator. The area of the silver contact pad is excluded for the determination of the current density.