Influence of Defect Post-deposition Treatments on poly-Si Thin-film Solar Cells on Glass grown by ECRCVD
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0910-A25-03
Influence of Defect Post-deposition Treatments on poly-Si Thin-film Solar Cells on Glass grown by ECRCVD Björn Rau1, Jens Schneider1,2, Erhard Conrad1, and Stefan Gall1 1 Silicon Photovoltaics, Hahn-Meitner-Institut Berlin, Kekuléstr. 5, Berlin, D-12489, Germany 2 now at: CSG Solar AG, Sonnenalle 15, Thalheim, D-06766, Germany
ABSTRACT The epitaxial thickening of a thin polycrystalline Si (poly-Si) film (seed layer) is a promising approach to realize an absorber layer of a poly-Si thin-film solar cell on glass. Such cell concept combines the benefits of crystalline Si and the high potential for cost reduction of a thin-film technology. Here, we discuss the influence of post-deposition treatments on the properties of absorber layers grown by electron-cyclotron resonance chemical vapor deposition (ECRCVD) and the solar cell performance, respectively. Defect annealing was used to improve the structural quality of the absorber layers and to increase the doping efficiency. For this, we used rapid thermal annealing (RTA) processes. Annealing times (up to 400 s) were applied at temperatures of up to 950 °C. Defect passivation treatments were carried out at temperatures of about 350 °C to passivate the remaining defects in the films by hydrogen. The impact of both treatments on the solar cell parameter will be discussed focusing on RTA. Excellent VOC’s of up to 361 mV were achieved without hydrogenation showing the high potential of ECRCVD-grown absorbers. Applying both treatments resulted so far in an increase of VOC of about 400 mV. Because of the fact, that both post-treatments (particularly hydrogenation) are still not yet optimized, further improvements can be expected. INTRODUCTION An attractive low-temperature route to a poly-Si thin-film solar cell on a low-cost substrate like glass bases on the seed layer concept. In such a cell concept, we use a thin large-grained poly-Si seed layer on glass formed by aluminium-induced crystallisation (AIC). The absorber layer is grown on this seed layer in a subsequent epitaxial deposition process. The epitaxial growth process is limited by the glass to temperatures of about 600°C. At these low temperatures it becomes necessary to apply deposition techniques which provide additional non-thermal energy to the surface of the growing film in order to obtain epitaxial growth. ECRCVD, as an ion-assisted deposition technique, is suitable for epitaxial growth of Si in this temperature regime [1,2]. But in contrast to conventional CVD at high temperatures (~1100 °C), at low temperatures the structural quality of epitaxial growth is mainly determined by the seed layer properties (e.g. crystal orientation). This results typically in a higher density of crystal defects in the grown films leading to a decreased carrier lifetime and therefore enhanced recombination losses in solar cells. Another feature of low-temperature Si epitaxy is a very low doping efficiency [3].
The presence of crystal defects influencing the performance of an electrical device can be reduced by post-de
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