Reflectance Improvement by Thermal Annealing of Sputtered Ag/ZnO Back Reflectors in a-Si:H Thin Film Silicon Solar Cells
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Reflectance Improvement by Thermal Annealing of Sputtered Ag/ZnO Back Reflectors in a-Si:H Thin Film Silicon Solar Cells Karin Söderström1, Franz-Josef Haug1, Céline Pahud1, Rémi Biron1, Jordi Escarré1, Martial Duchamp2, Rafal Dunin-Borkowski3-2 and Christophe Ballif1 1 Ecole Polytechnique Fédérale de Lausanne, Institute of Microengineering, Photovoltaics and Thin Film Electronics Laboratory, 2000 Neuchâtel, Switzerland 2 Center for Electron Nanoscopy, Technical University of Denmark, 2800 Kongens Lyngby, Denmark 3 Institute for Microstructure Research, Forschungszentrum Jülich, D-52425 Jülich, Germany ABSTRACT Silver can be used as the back contact and reflector in thin film silicon solar cells. When deposited on textured substrates, silver films often exhibit reduced reflectance due to absorption losses by the excitation of surface plasmon resonances. We show that thermal annealing of the silver back reflector increases its reflectance drastically. The process is performed at low temperature (150°C) to allow the use of plastic sheets such as polyethylene naphthalate and increases the efficiency of single junction amorphous solar cells dramatically. We present the best result obtained on a flexible substrate: a cell with 9.9% initial efficiency and 15.82 mA/cm2 in short circuit current is realized in n-i-p configuration. INTRODUCTION Interest in thin film silicon solar cells has increased strongly in the last few years. Their development promises to help to produce abundant, low cost electricity at a time when mankind faces the challenge to move to alternative energy production processes. The low carrier lifetime in amorphous and microcrystalline silicon materials results in the need to reduce the thickness of the solar cells’ active layer to well below the absorption length of light in the red part of the spectrum. Textured interfaces are used to scatter light within the solar cells to lengthen the light path and increase the generated photocurrent [1, 2, 3]. The n-i-p process is interesting because it can be used to produce light-weight, flexible and unbreakable modules, since the cell layers can be deposited on opaque substrates such as steel or on plastic sheets. The back contact and reflector in the n-i-p configuration is often realized by using metals having a high reflectivity like silver. Unfortunately, as the substrate itself needs to be rough to increase the light trapping, the silver layer has to be deposited onto a nano textured surface. In contrast to a silver layer deposited onto a flat surface, for which low parasitic absorptions are observed, silver deposited onto a rough substrate leads to parasitic absorption losses that take place within the metallic layer by the excitation of surface plasmon polaritons (SPPs) [4]. SPPs are known to be stronger when the metal quality is low [5]. In this contribution it will be shown that a proper thermal annealing of the silver layer leads to a large improvement of the film reflectance. The annealing is carried out at a moderate temperature, which allows the use
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