Improved Efficiency of Single Junction Microcrystalline Silicon n-i-p Solar Cells with an i-Layer Made by Hot-Wire CVD
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0989-A18-03
Improved Efficiency of Single Junction Microcrystalline Silicon n-i-p Solar Cells with an iLayer Made by Hot-Wire CVD Hongbo Li, Ronald H.J. Franken, Robert L. Stolk, C. H.M. van der Werf, Jan-Willem A. Schuttauf, Jatin K. Rath, and Ruud E.I. Schropp Surfaces, Interfaces and Devices, Faculty of Science, Utrecht University, P.O.Box 80.000, Utrecht, 3508TA, Netherlands ABSTRACT The influence of the surface roughness of Ag/ZnO coated substrates on the AM1.5 J-V characteristics of microcrystalline silicon (µc-Si:H) solar cells with an i-layer made by the hotwire chemical vapour deposition (HWCVD) technique is discussed. Cells deposited on substrates with an intermediate rms roughness show the highest efficiency. When using reverse hydrogen profiling during i-layer deposition, an efficiency of 8.5% was reached for single junction µc-Si:H n-i-p cells, which is by far the highest for µc-Si:H n-i-p cells with a hot-wire i-layer.
INTRODUCTION Intrinsic hydrogenated microcrystalline silicon (µc-Si:H) have been proven to be suitable for use in thin film silicon cells. For single junction µc-Si:H solar cells, independently confirmed efficiencies over 10% have been reported [1, 2], whereas multi-junction cells with a µc-Si absorber in the bottom cell have yielded an efficiency of around 12% [1,3]. Although most of the groups use plasma enhanced chemical vapour deposition (PECVD) for the deposition of intrinsic µc-Si:H [4], it is of interest to study the hot-wire CVD technique [5], because of its potential to be faster and more cost-effective than conventional PECVD, while scaling-up for industrial production is expected to be straightforward. Almost 10 years ago, we initially developed highly crystalline silicon material at a high substrate temperature (~500 ∫C). Samples made on Corning glass showed a compact, singly oriented poly-Si structure with purely intrinsic electrical transport properties [6, 7]. Single junction solar cells made on bare stainless steel foil with 1.2 µm of this material as the absorber layer showed a remarkably high short-circuit current density (Jsc) of over 19 mA/cm2 [8], considering no textured back reflector was used at that time. However, the cell open circuit voltages (Voc) and fill factors (FF) obtained at that time are low compared to those of µc-Si:H cells developed nowadays. These differences can be attributed to the high density of electrical defects present in the i-layer [6]. Though this poly-Si material did not yet result in high efficiency solar cells, crucial knowledge of silicon growth with the hot-wire technique was obtained [8 -11]. Lowering the substrate temperature, the J¸lich group obtained significantly higher solar cell efficiencies with hot-wire deposited µc-Si:H i-layers: an initial efficiency of 7.5% was reported for an n-i-p cell with an i-layer deposited at a substrate temperature around 260 ∫C [12]. Successive study further improved the efficiency of p-i-n cells to 9.4% [13], which is the highest published efficiency for single junction hot-wire µc-Si:H thin fil
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