Towards Microcrystalline Silicon n-i-p Solar Cells with 10% Conversion Efficiency
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Towards microcrystalline silicon n-i-p solar cells with 10% conversion efficiency L. Feitknecht, C. Droz, J. Bailat, X. Niquille, J. Guillet, A. Shah Institut de Microtechnique, Université de Neuchâtel, Breguet 2, CH-2000 Neuchâtel, Switzerland. ABSTRACT High-performance microcrystalline and amorphous silicon solar cells are the key elements for a successful combination to form the "micromorph" tandem cell [1,2]. A microcrystalline silicon (mc-Si:H) solar cell in the n-i-p configuration was fabricated by the VHF PE-CVD deposition process. The cell has a conversion efficiency exceeding 9% (VOC=520 mV, FF=73%, JSC=24.2 mA/cm2). This result was achieved by a successful combination of the following elements: first a fine-tuning of the silane concentration (SC) in hydrogen feedstock gas used for deposition of the intrinsic absorber layer, second, the incorporation of an optimised back-reflecting substrate into the cell; and, third, the ideal combination of each of these keycomponents. Compared to earlier results with n-i-p-type mc-Si:H solar cells, a substantial increase in VOC was now obtained, while maintaining reasonable JSC-values. Earlier investigations on the role of the i-layer material had revealed a trade-off between cells with high JSC but low VOC or cells of low JSC and high VOC. In the present contribution the authors now show the successful combination of a cell with an acceptable VOC and good JSC generation in the long-wavelength region (above 700 nm). This is mainly because of suitable light-diffusing back-reflectors which perform well with respect to both, optical and electrical aspects. INTRODUCTION The fabrication of substrate-nip solar cells is substantially different from that of superstrate-pin solar cells, because of the change in the deposition sequence of the layers and its technological consequences (like initial n-type or p-type nucleation layer and optically transparent and doped window-layers). Whereas a relatively large number of papers have dealt with the design and fabrication of superstrate-pin cells, only a relatively small number of papers have looked at substrate-nip cells. The present work is a further contribution to latter topic and will deal with a specific problem in this context: The interface of the back-TCO (transparent conductive oxide) to the n-type silicon film (we call it here the 'back-interface') takes on a very important role for the deposition of nip-type cells and this interface has an influence on the structure of the whole cell and its performance: Indeed, nucleation of silicon grains, electronic and optical properties are key issues which depend on the nature of the underlying back-TCO material [3,4]. Investigations on the interface of the back-TCO / n-layer of the n-i-p solar cell are not straight-forward since the interesting part is hidden by the solar cell and thus not directly accessible. Since most of the light is absorbed within the first 2 µm of the absorber, only the long wavelength part of the light spectrum reaches the back-interface under white-l
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