Protocrystalline Silicon for Micromorph Tandem Cells on Gen. 5 Size
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Protocrystalline Silicon for Micromorph Tandem Cells on Gen. 5 Size Gijs van Elzakker1, Daniel Sixtensson1, Niklas Papathanasiou1, Klaus Neubeck1 and Roland Sillmann1 1 Inventux Technologies AG, Wolfener Str. 23, 12681 Berlin, Germany ABSTRACT Inventux Technologies AG is a high volume producer of Micromorph (a-Si:H/µc-Si:H) tandem modules. The light-induced degradation of hydrogenated amorphous silicon (a-Si:H), called Staebler-Wronski effect (SWE), limits the stabilized efficiency of a-Si:H-based solar cells. Several laboratories have reported on the development of a-Si:H with increased resistance against light-soaking. This so-called ‘protocrystalline’ silicon can be grown with plasmaenhanced chemical vapor deposition (PECVD) by diluting the silane source gas with hydrogen. The aim of the work presented in this paper was to scale-up the laboratory results on protocrystalline silicon to a size of 1.43 m2 (Gen. 5) using a process that is suitable for high volume production. We demonstrate that the strict boundary conditions regarding uniformity and growth rate, which are necessary for a production process, can be met. The reduced light-induced degradation of protocrystalline solar cells fabricated with the newly developed process is confirmed by a light-soaking experiment. As an outlook towards future work, we discuss issues related to the implementation of a protocrystalline top cell in the Micromorph tandem configuration. The challenge of choosing the right top-cell thickness is illustrated by experimental results on two tandem cells. The top cells of these tandems contain protocrystalline i-layers of different thicknesses. INTRODUCTION To date, the most promising silicon-based thin film solar-cell configuration is the socalled Micromorph tandem cell. A Micromorph tandem cell consists of a microcrystalline silicon (μc-Si:H) bottom cell and an amorphous silicon (a-Si:H) top cell. Since the a-Si:H top cell of the a-Si:H/µc-Si:H tandem can be made thinner, and because µc-Si:H is hardly affected by lightsoaking, the light-induced degradation in such a cell structure is significantly reduced in comparison to a single-junction a-Si:H cell. This is one of the reasons why Inventux focuses exclusively on the production of Micromorph modules. Despite the success of the Micromorph concept, the influence of the light-induced degradation in the top cell, which generates approximately 2/3 of the power, on the overall performance is still significant. Therefore, stability against light-soaking of the top cell material itself should be addressed. Several research laboratories have shown that hydrogen dilution of silane in the PECVD deposition of a-Si:H can lead to an enhanced stability against SWE [1]. The a-Si:H deposited under these conditions is called protocrystalline silicon. It has been shown that by using protcrystalline silicon the light-induced degradation can be reduced to 10% for a single-junction cell with a 300-nm thick absorber [2]. The enhanced stability is due to an improved structural order, which is
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