High-Performance Barrier Layers for Flexible CIGS Thin-Film Solar Cells on Metal Foils
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High-Performance Barrier Layers for Flexible CIGS Thin-Film Solar Cells on Metal Foils Dirk Herrmann1, Friedrich Kessler1, Konrad Herz1, Michael Powalla1, Andreas Schulz2, Joachim Schneider2, Uwe Schumacher2 1 Zentrum fuer Sonnenenergie- und Wasserstoff-Forschung Baden-Wuerttemberg Industriestr. 6, D-70565 Stuttgart, Germany 2 Institut fuer Plasmaforschung, Universitaet Stuttgart, Pfaffenwaldring 31, D-70569 Stuttgart, Germany ABSTRACT Cu(In,Ga)Se2 (CIGS) solar cells have been fabricated on different metal foil substrates. Preferred foils with regard to costs and physical properties were stainless steel, titanium and Kovar® (Fe/Ni/Co alloy). SiOx films prepared by PECVD and sol/gel methods were deposited as barrier layers on these metal foil substrates. Both film types showed good suitability as diffusion barriers for substrate elements. In the SiOx:Na (sol/gel) barrier sodium could be incorporated to provide the Na-supply to the CIGS absorber. With relatively low sodium concentrations in these films (6% Na2O) large amounts of Na in the absorber have been found. Using a combination of SiOx (PECVD) and SiOx:Na (sol/gel) layers AM 1.5 cell efficiencies of 13.1% on titanium foil substrates have been achieved. With a newly implemented insulation test based on electrolysis measurements pinholes in the barrier layers could be localized and conclusions on their origin could be drawn. INTRODUCTION Commonly CIGS (Cu(In,Ga)Se2) based thin-film solar cells are deposited on sodium containing glass substrates at temperatures around 550°C. Highly efficient CIGS modules fabricated on thin and flexible non-glass substrates would on the other hand open entirely new application areas. Additionally, the option to produce CIGS thin-film solar cells in roll-to-roll processes has a large potential for cost reduction. Most suitable alternative materials with regard to costs, flexibility and conversion efficiency are metallic foils. Best results so far have been obtained on stainless steel substrates with efficiencies reaching 17.4%, compared to 18.8% on glass substrates [1]. While these results have been obtained with a sophisticated 3-stage batch process without additional barrier layer high reproducibility in a commercial in-line process can to our knowledge only be achieved with additional barrier layers. Their function is (1) to reduce the diffusion of substrate impurities into the solar cell and (2) to provide sufficient sodium supply to the CIGS absorber. Additionally, on the way towards a monolithically integrated high efficiency CIGS module on metal foils (3) electrical insulation between module and metal substrate is needed. These issues will be addressed in this paper. EXPERIMENTAL DETAILS In order to investigate the influences of the metal substrates 3 different commercially available metals were used: stainless steel, titanium and Kovar® (Fe/Ni/Co alloy). All these foils have thermal expansion coefficients close to those of the components of the CIGS solar cell with Kovar® showing the best match to Molybdenum whereas
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