Novel intermediate reflector layer for optical and morphological tuning in the Micromorph thin film tandem cell
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Novel intermediate reflector layer for optical and morphological tuning in the Micromorph thin film tandem cell J.-B. Orhan1, E. Vallat-Sauvain1, M. Marmello1, U. Kroll1, J. Meier1, E. Laux2, D. Grange2, S. Farine Brunner2, H. Keppner2 1 TEL Solar-Lab S.A., rue du Puits-Godet, 12a, 2000 Neuchâtel, Switzerland 2 Haute Ecole Arc Ingénierie, Eplatures-Grise 17, 2300 La Chaux-de-Fonds, Switzerland ABSTRACT PECVD growth of the microcrystalline silicon junction on a highly textured amorphous top cell often leads to defective absorber layers and finally to low quality bottom cell. This paper reports on the current status of using an innovative smoothening/reflective layer (SRL) as alternative intermediate reflector between top and bottom cell of a Micromorph tandem device deposited on as-grown highly textured LPCVD ZnO layer. Manufacturing of the SRL layer is realized by “liquid phase” deposition technologies. Optical and electrical properties, smoothening effect and photoelectrical results of Micromorph tandem devices are discussed. The implementation of our novel SRL results in the growth of a crack-free bottom cell and to an efficient current transfer from the bottom to the top cell. INTRODUCTION TEL Solar is active in the field of production equipment for silicon based thin film PV modules, providing turn-key thin film production lines for a-Si and, a-Si/μc-Si tandem technologies [1-2]. To achieve the goal of high efficiency, TEL Solar uses a proprietary transparent conductive oxide (TCO) for the coating of the glass which is based on zinc oxide (ZnO) deposited by low pressure chemical vapor deposition (LPCVD). Its advantage lies in the fact that it is rough as grown and, therefore, it acts as an excellent light diffuser. As a result, the absorption of the light in the silicon layers increases, generating high current density of the solar cell. If even thicker and rougher TCO layers could be used, the current density would increased further [3]. Such layers would however lead to devices with low Voc and FF. The formation in the bottom microcrystalline cell of low-density defects [4], so-called cracks, probably acting as shunts and recombination centers, is believed to be the origin of the photoelectrical properties degradation. We report on our first results in the development of a novel interlayer to be inserted between the top and bottom cells of a Micromorph tandem device. This layer presents two main functionalities: on one hand it levels the surface by lowering its roughness; on the other hand it acts, thanks to its low refractive index, as an optical reflector. Levelling allows the growth of a better quality μc-Si bottom cell. The reflection of light improves the current in the top amorphous cell. Therefore, this interlayer is expected to allow the use of highly texture LPCVD ZnO as front contact, and hence to reach high currents and high efficiencies. The manufacturing of this layer is proposed using inventive “liquid phase” deposition technologies. Furthermore, “liquid phase” deposition equipments and consumabl
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