Integration of Expanding Thermal Plasma deposited Hydrogenated Amorphous Silicon in Solar Cells
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Integration of Expanding Thermal Plasma deposited Hydrogenated Amorphous Silicon in Solar Cells B.A. Korevaar1,2, C. Smit1,2, A.M.H.N. Petit1, R.A.C.M.M. van Swaaij1, and *M.C.M. van de Sanden2 1 Delft University of Technology, DIMES, Feldmannweg 17, 2628 CT DELFT, The Netherlands 2 Eindhoven University of Technology, Department of Applied Physics, P.O. Box 513, 5600 MB EINDHOVEN, The Netherlands *[email protected]; ABSTRACT A cascaded arc expanding thermal plasma is used to deposit intrinsic hydrogenated amorphous silicon at growth rates between 0.2 and 3 nm/s. Incorporation into a single junction p-i-n solar cell resulted in an initial efficiency of 6.7%, whereas all the optical and initial electrical properties of the individual layers are comparable with RF-PECVD deposited films. In this cell the intrinsic layer was deposited at 0.85 nm/s and at a deposition temperature of 250ºC, which is the temperature limit for growing the p-i-n sequence. The cell efficiency is limited by the fill factor and using a buffer layer at the p-i interface deposited with RF-PECVD at low growth rate can increase this. The increase in fill factor is a result of a lower initial defect density near the p-i interface then obtained with the expanding thermal plasma, resulting in better charge carrier collection. To use larger growth rates, while maintaining the material properties, higher deposition temperatures are required. Higher deposition temperatures result in a smaller optical bandgap for the intrinsic layer and deterioration of the p-type layer, resulting in a lower opencircuit voltage. First results on applying a buffer layer will also be presented.
INTRODUCTION Both hydrogenated amorphous silicon (a-Si:H) [1-6] and hydrogenated microcrystalline silicon (µc-Si:H) [7] can be deposited with a cascaded arc expanding thermal plasma (ETP). Compared with solar grade RF-PECVD [8] the material properties of ETP a-Si:H are similar at low growth rates (< 1 nm/s) and low deposition temperatures (≤ 250ºC). At large growth rates (> 1 nm/s) and high deposition temperatures (> 250ºC) the material properties of ETP a-Si:H are somewhat different to RF-PECVD a-Si:H and are similar to the properties obtained with HW-CVD [9]. Especially at deposition temperatures of 400ºC and rates > 2 nm/s the ETP a-Si:H is better structured reflected by a larger hole drift mobility and refractive index than obtained for RFPECVD a-Si:H [10-12]. A consequence of the high deposition temperature is the lower hydrogen content (~6 at.%) for ETP a-Si:H and therefore a smaller bandgap. A smaller bandgap implies more absorption close to the p-i interface and consequently a smaller total intrinsic layer thickness is required when applied in solar cells. Simulations are performed to obtain the ideal thickness of a solar cell as a function of the bandgap of the intrinsic layer. Furthermore, the high deposition temperature required to obtain good material properties at those large growth rates [12] will deteriorate the p-type layer by enhanced hydrogen out-diffusion fro
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