Microstructures of Microcrystalline Silicon Solar Cells Prepared by Very High Frequency Glow-Discharge
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Microstructures of Microcrystalline Silicon Solar Cells Prepared by Very High Frequency Glow-Discharge J. Dubail, E. Vallat-Sauvain, J. Meier, S. Dubail and A. Shah Institut de Microtechnique (IMT), A.L. Breguet 2, Université de Neuchâtel, CH-2000 Neuchâtel, Switzerland. ABSTRACT The microstructure of two different μc-Si:H solar cells was studied by Transmission Electron Microcopy (TEM). At the micrometer scale, a difference in the grain structure is observed close to the ZnO substrate: cracks are found in the cell deposited with the higher silane concentration. Surprisingly, the cell with the cracks close to the transparent conductive oxide shows the largest VOC (530 mV) and FF (68%). These first studies reveal that microstructures of fully microcrystalline silicon devices may show a quite large variation with corresponding effects in solar cell performance. INTRODUCTION Hydrogenated microcrystalline silicon (μc-Si:H) is a promising new photovoltaic (PV) absorber material. Thin films of this material have been grown to fabricate single-junction μcSi:H and micromorph (a-Si:H/μc-Si:H) tandem solar cells, providing thereby encouraging results [1-3]. While detailed characteristics of microcrystalline absorber material in terms of absorption behavior, transport properties, device performance and stability are available, only very little is known about the internal structure of this material. Recently, we have reported, in a transmission electron microscopy (TEM) study of undoped µc-Si:H films deposited on glass substrates that µc-Si:H shows a surprisingly large variation of internal microstructures [4, 5]. In this former study we found that the morphology is strongly dependent on the hydrogen to silane dilution ratio used during deposition. On the other hand, it is known that the microstructure is also critically substrate-dependent, at least during the early stage of growth [6-8]. 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 µc-Si:H p-i interface on the TCO is critical for a good working device. Sofar, only very little has been reported on the microstructure of entire µcSi:H cells. This present work is a first attempt to focus on the structure of microcrystalline p-i-n devices by using TEM investigations. We show results of TEM cross-sections of two µc-Si:H cells with conversion efficiencies of 6 and 7 %, respectively. These two devices were deposited under slightly different deposition conditions, using in both cases the Very High Frequency Glow Discharge technique (VHF-GD) [9]. EXPERIMENTAL Cell deposition
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This study is based on two microcrystalline single-junction p-i-n solar cells named Cell A and Cell B. A fully μc-Si:H p-i-n structure was grown by VHF-GD, on a rough ZnO (TCO transparent conductive oxide) layer deposited by us on a glass substrate. The doped layer
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