Improved interface between front TCO and microcrystalline silicon p-i-n solar cells.
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Improved interface between front TCO and microcrystalline silicon p-i-n solar cells . E. Vallat-Sauvain, S. Faÿ, S. Dubail, J. Meier, J. Bailat, , U. Kroll, A. Shah, University of Neuchâtel, Institute of Microtechnology, rue A.L. Breguet 2, 2000 Neuchâtel, SWITZERLAND ABSTRACT The microstructure of microcrystalline silicon p-layers on zinc oxyde (ZnO) has been studied by Transmission Electron Microscopy (TEM). ZnO has been characterised by X-Rays diffraction and by Scanning Electron Microscopy (SEM). These reveal that the ZnO surface topography consists of regular pyramids with a (110) preferential crystallographic growth axis. High Resolution TEM observations indicate that the p- layer growth starts at the top of the ZnO pyramids. This results, for fully microcrystalline p-layers, in a decreased thickness at the bottom of the ZnO pyramids. When the p-layer is fully microcrystalline, no amorphous incubation layer could be observed between ZnO and p-silicon layer. Moreover, the high crystallinity of the players in microcrystalline p-i-n devices is accompanied by an increased fill-factor in the I-V characteristics. INTRODUCTION Hydrogenated microcrystalline silicon (µc-Si:H) is a promising new photovoltaic (PV) absorber material [1-3]. Thin films of this material have been used to fabricate single-junction µc-Si:H and micromorph (a-Si:H/ µc-Si:H) tandem solar cells, providing thereby encouraging results. While detailed characteristics of microcrystalline absorber material in terms of absorption behavior, transport properties, device performance and stability are available, relatively little is known about the internal structure of this material. One major observation relates to the surprisingly complex microstructure of layers deposited on glass at various silane dilutions [4-6]. Recently, we have reported in a Transmission Electron Microscopy (TEM) study of fully µcSi:H cells [7], that microstructural variations of the material (i.e. at the micrometer scale) can be observed for small changes in the deposition parameters. On the other hand, it is known that the µc-Si:H microstructure is critically substrate-dependent, at least during the early stage of growth [9]. At the early stage of growth, the surface topology of the substrate seems to be the determining parameter for both crystallite orientation and grain size in µc-Si:H layers [7,9,10]. In case of entirely microcrystalline p-i-n devices deposited on a Transparent Conductive Oxide (TCO), not only the microstucture but also the quality of the electrical contact is essential for obtaining satisfactory solar cell performance. Especially the initial crystalline growth of the doped µc-Si:H interface on the TCO is critical for obtaining the desired device properties. An improved interface between the TCO and the p-layer will avoid creating unintentional barriers that result in additionnal series resistance, and therefore can noticeably lead to a decrease of the fill factor and Voc of the whole p-i-n solar cell. The present work addresses these problems, based a study f
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