Importance of Defect Density near the p-i Interface for a-Si:H Solar Cell Performance
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Importance of Defect Density near the p-i Interface for a-Si:H Solar Cell Performance *B.A. Korevaar1,2, C. Smit1,2, R.A.C.M.M. van Swaaij1, D.C. Schram2, 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]; [email protected];
ABSTRACT A cascaded arc expanding thermal plasma is used to deposit intrinsic hydrogenated amorphous silicon at growth rates larger than 2 Å/s. Implementation into a single junction p-i-n solar cell resulted in initial efficiencies of ~7%, although all the optical and initial electrical properties of the individual layers are comparable with RF-PECVD deposited films. The somewhat lower efficiency is due to a smaller fill factor. Spectral response measurements, illuminated J,Vmeasurements, and simulations indicate that a higher local defect density in the region near the p-i interface might be responsible for the smaller fill factor in comparison with conventional lowrate RF-PECVD. The higher defect density is most likely caused by the initial growth in the first 10 to 50 nm. Therefore, controlled initial growth of the intrinsic layer is suggested for good solar cell performance.
INTRODUCTION A cascaded arc expanding thermal plasma (ETP) is a relatively new deposition technique that allows growth rates of solar grade1 a-Si:H of up to 100 Å/s. However, single junction solar cells of which the intrinsic layer has been grown with ETP have a 3% lower initial efficiency than cells grown with RF-PECVD. The lower efficiency is mainly due to a lower fill factor (FF). The advantage of the ETP is the higher growth rate, which can result in a cost reduction of a-Si:H based solar cells. In 1998 a p-i-n grown solar cell was presented with an initial efficiency of 4.1% with the intrinsic layer deposited at 100 Å/s [1]. To obtain good intrinsic material at those growth rates deposition temperatures of ~400ºC are required, which is not compatible with the deposition temperature of the p-type window layer. This high temperature and the vacuum breaks in between the doped layers and the intrinsic layer are the main reason for the obtained efficiency. In order to be able to deposit p-i-n structures, without influencing the p-layer properties, the growth rate has been reduced to rates smaller than 10 Å/s, which results in solar grade material at 250ºC [2,3] and a cell efficiency of 6.7% grown at 8.5 Å/s. For a better understanding, a comparison has been made with a cell grown with RF-PECVD by measuring the spectral response before and after light-induced degradation. Further, solar cells are simulated with the ASA program [4] in order to understand the differences found between both cells. 1
The individual intrinsic layer can be prepared with the same material properties as with RF-PECVD [7].
A24.4.1
EXPERIMENTAL SET-UP Intrinsic a-Si:H is grown using an ETP chamber that is connected via a load-
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