Annealing Kinetics of Amorphous Silicon Alloy Solar Cells Made at Various Deposition Rates
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Annealing Kinetics of Amorphous Silicon Alloy Solar Cells Made at Various Deposition Rates Baojie Yan, Jeffrey Yang, Kenneth Lord, and Subhendu Guha United Solar Systems Corp., 1100 West Maple Road, Troy, Michigan 48084 ABSTRACT A systematic study has been made of the annealing kinetics of amorphous silicon (a-Si) alloy solar cells. The cells were deposited at various rates using H2 dilution with radio frequency (RF) and modified very high frequency (MVHF) glow discharge. In order to minimize the effect of annealing during light soaking, the solar cells were degraded under 30 suns at room temperature to quickly reach their saturated states. The samples were then annealed at an elevated temperature. The J-V characteristics were recorded as a function of annealing time. The correlation of solar cell performance and defect density in the intrinsic layer was obtained by computer simulation. Finally, the annealing activation energy distribution (Ea) was deduced by fitting the experimental data to a theoretical model. The results show that the RF low rate solar cell with high H2 dilution has the lowest Ea and the narrowest distribution, while the RF cell with no H2 dilution has the highest Ea and the broadest distribution. The MVHF cell made at 8 Å/s with high H2 dilution shows a lower Ea and a narrower distribution than the RF cell made at 3 Å/s, despite the higher rate. We conclude that different annealing kinetics plays an important role in determining the stabilized performance of a-Si alloy solar cells. INTRODUCTION It is well known that the performance and stability of amorphous silicon (a-Si) alloy solar cells depend on the deposition conditions. The best solar cells are made at a low deposition rate with high hydrogen dilution just before the onset of microcrystalline formation [1,2]. Materials deposited at this so-called “near-the-edge” condition exhibit various unique properties such as linear-like microstructure [2] and a low temperature peak in their hydrogen evolution spectra [3]. The most important feature is that materials with high H2 dilution have better stability than materials with no H2 dilution. Generally, the initial efficiency and stability of solar cells decrease with the increase of deposition rate. Materials deposited at high rates contain a high density of microvoids [4]. However, the high rate MVHF deposited a-Si alloy solar cells show initial performance comparable to the low rate RF deposited cells [5-7]. The light-induced degradation of the solar cells is normally believed to be due to the creation of silicon dangling bonds during light soaking. The stabilized efficiency is reached by the balance of thermal annealing and light-induced defect generation. A remaining question is whether the difference in stability for high and low deposition rates is due to different annealing kinetics or different saturated defect density independent of annealing. We previously reported [8] that the density of microvoids affects not only the cell performance but also the kinetics of light soaking and annealin
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