MW plasma enhanced CVD of intrinsic Si for thin film solar cells
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1066-A03-05
MW plasma enhanced CVD of intrinsic Si for thin film solar cells Bas B. Van Aken, Hans Leegwater, Maarten Dorenkamper, Camile Devilee, Jochen Loffler, Maurits C.R. Heijna, and Wim J. Soppe ECN - Solar Energy, P.O. box 1, Petten, 1755 ZG, Netherlands ABSTRACT The aim of the thin film silicon PV research program at ECN is the development of highthroughput production technology for high efficiency, microcrystalline and amorphous thin film silicon photovoltaics (PV) on flexible substrates. For this purpose, a roll-to-roll system has been designed and constructed, consisting of three deposition chambers for the continuous deposition of n-type, intrinsic and p-type Si layer. In this paper, we will present optical and electrical characterisation of device quality intrinsic Si layers, deposited with Microwave (MW) plasma enhanced chemical vapour deposition (PECVD), with a special focus on UV-reflection spectroscopy (UVRS). UVRS can be used to determine the crystallinity in very thin silicon layer and is interesting as a possible inline tool for layer quality assessment and crystallinity control. INTRODUCTION Roll-to-roll (R2R) production of thin film Si solar cells combines the advantages of flexible substrates and high-throughput fabrication. Advantages include reduced handling costs, clean (closed) environment processing and the absence of pump-down cycles leading to potential significant cost reduction compared to batch-type processing using glass superstrates. However, in contrast to equipment adapted from display technology for the fabrication of thin Si layers on glass, equipment for roll-to-roll production is not commercially available. Therefore, ECN is developing high-throughput, roll-to-roll production technology for high efficiency thin film silicon solar cells. In our concept, steel foil, coated with an insulating barrier layer, is used as substrate. A sputtered back-contact and –reflector is applied, before the active Si layers are deposited by PECVD. On top of the Si layers, a transparent conductive oxide is sputtered. In order to achieve monolithical series connection, cells are defined and isolated by laser scribing followed by printing of insulating and conducting lines. In principle, the same concept can be used for plastic substrates. For the continuous deposition of amorphous and microcrystalline silicon n-i-p layers and thin film solar cells by PECVD, ECN and Roth&Rau AG developed a roll-to-roll system, the FLEXICOAT300. In this contribution, we will report on intrinsic Si layers grown with MW-PECVD. We will pay particular attention to UVRS as characterization tool for determination of the crystalline fraction in thin silicon layers. The standard method for analysis of crystallinity of silicon layers is Raman spectroscopy, which, however, is less suitable for analysis of very thin layers (on top of other silicon layers), unless a UV laser is used for excitation. Further, Raman spectroscopy is rather difficult to implement as an inline monitoring tool in a PECVD system. UVRS is suited for me
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