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1245-A02-03

OXYGENATED PROTOCRYSTALLINE SILICON THIN FILMS FOR WIDE BANDGAP SOLAR CELLS R.E.I. Schropp, J.A. Schüttauf, C.H.M. van der Werf Utrecht University, Faculty of Science, Debye Institute for Nanomaterials Science, Department of Physics and Astronomy, Section Nanophotonics – Physics of Devices, P.O. Box 80.000, 3508 TA Utrecht, The Netherlands 1 Present address: Eindhoven University of Technology, Eindhoven, The Netherlands ABSTRACT Protocrystalline silicon, which is a material that has enhanced medium range order (MRO), can be prepared by using high hydrogen dilution in PECVD, or, alternatively, using high atomic H production from pure silane in HWCVD. We show that this material can accommodate percentage-level concentrations of oxygen without deleterious effects. The advantage of protocrystalline SiO:H for application in multijunction solar cells is not only that it has an increased band gap, providing a better match with the solar spectrum, but also that the solar cells incorporating this material have a reduced temperature coefficient. Further, protocrystalline materials have a reduced susceptibility to light-induced defect creation. We present the unique result in the PV field that these oxygenated protocrystalline silicon solar cells have an efficiency temperature coefficient (TCE) that is virtually zero (TCE is between -0.08%/°C and 0.0/°C). It is thus beneficial to make this cell the current limiting cell in multibandgap cells, which will lead to improved annual energy yield. INTRODUCTION Oxygen and carbon are well known as bandgap widening constituents in hydrogenated amorphous silicon (a-Si:H). Both additions have been used primarily in p-type a-Si:H layers to make them more transparent for blue light and thus achieve higher photocurrent generation in the intrinsic layer of p-i-n type solar cells. The use of oxygenated p-layers was for instance reported by Fuji Electric, Co. [1]. The addition of oxygen to a-Si:H i-layers has been studied before [2], but the use of oxygenated i-layers in practical solar cells has not been investigated much [3,4]. Oxygen is commonly considered to lead to donor centers in silicon, accompanied with mid-gap defects particularly in amorphous silicon. In this paper, we show that the situation is completely different for protocrystalline silicon (proto-Si:H), a material much like amorphous silicon except for an enhanced medium range order (MRO). A fingerprint for enhanced MRO is the reduced width (FWHM < 5.5o [5]) of the first scattering peak in X-ray scattering measurements. In this FWHM the partial pair distribution functions based on next-nearest neighbor separations play an important role. A higher correlation length obtained from the narrowed FWHM then means that the bonds are more ordered. Enhanced MRO is usually achieved in Plasma Enhanced Chemical Vapor Deposition (PECVD) by strong dilution of the silane (SiH4) with H2. A disadvantage is that the deposition rate is only 1-3 Å/s. Alternatively, it can be obtained from pure SiH4 at 10 Å/s using Hot Wire CVD [6]. The