Amorphous Silicon Solar Cells Techniques for Reactive Conditions
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ABSTRACT The preparation of amorphous silicon films and solar cells using SiH 2Cl2 source gas and electron cyclotron resonance assisted chemical vapor deposition (ECR-CVD) was investigated. By using buffer layers to protect previously deposited layers improved a-Si:H(CI) solar cells were prepared and studied. The high quality a-Si:H(CI) films used in this study exhibited low defect densities (-10 15cm•3) and high stability under illumination even when the deposition rate was increased to -15A/s. The solar cells were deposited in the n-i-p sequence. These solar cells achieved V, values of - 0.89V and - 3.9% efficiency on Ga doped ZnO (GZO) coated specular substrate. The a-Si:H(C1) electron and hole g-r products were -10-8cm 2/V. INTRODUCTION Amorphous silicon solar cells continues to be one of the most promising thin film photovoltaic technologies due to its high performance and its low cost. Guha [1] et al. have reported amorphous silicon based solar cells with stabilized efficiencies of greater than 13%. Even so, further cost reductions and performance increases must be implemented in order to achieve more widespread commercialization. For example, cost reductions can be obtained through increased film deposition rates which increases the cost effective use of production equipment and by using less expensive source materials. The light induced degradation of amorphous silicon materials is still one of the greatest remaining performance issue. Previously Azuma et al. [2,3] reported that a-Si:H(Cl) films prepared using less expensive SiH12 Cl. source gas could be deposited at high growth rates. These a-Si:H(Cl) films also showed improved stability under illumination compared to that of standard a-Si:H prepared using SiH4[4]. Also, the defect density of these a-Si:H(C1) films was 5 X 1015cm 3 in the annealed state, and 2 X 10 16 cm-3 in the degraded state. The time to reach the saturation degraded state was also short. These results are an indication that a particularly well relaxed, stable network was formed by the ECR-hydrogen plasma technique used in these reports. This paper focuses on the study and optimization of SiH 2 Cl 2 based n-i-p solar cells. The effect of band gap tuning and of the a-Si:H(Cl) i-layers properties on the solar cell performance was systematically examined. Capacitance measurement and the dependence of illumination direction on collection efficiency were used to probe electric transport. Also, the i-layer space charge regions were examined by the analysis of the light intensity dependence of photovoltaic parameters. Solar cell stability was also measured. 791 Mat. Res. Soc. Symp. Proc. Vol. 557 ©1999 Materials Research Society
EXPERIMENT
Amorphous silicon thin films and solar cells were fabricated in a load locked three chamber system. The a-Si:H(C1) intrinsic layers were deposited in a ECR-CVD chamber using SiH2Cl1 and 142 gas mixtures. The band gap of the a-Si:H(Cl) i-layers were relatively wide at -1.85eV. The doped layers and buffer layers used in this work were prepared using convent
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