Application of hydrogenated amorphous silicon (a-Si:H) films on solar cells is limited due to a significant quality decrease over light exposure. Previous studies indicated that the incorporation of clusters into a-Si:H films are related to light induced degradation, though the mechanisms which leads to light induced degradation are still unclear [1].
To study the effects of clusters on the light induced degradation and control their deposition into films, we have developed a multi-hollow plasma CVD method by which the incorporation of clusters is reduced in the upstream region using the gas flow that drives clusters formed in discharges toward the downstream region of the reactor. Thus, we can simultaneously deposit films in which the volume fraction of clusters incorporated into films varies by changing the position of the substrate in the reactor, since the transport of species varies with their size. The deposition rate decreases exponentially with the distance from the discharge region, which agrees with our simple model of spatial density of SiH3, the main precursor of a-Si:H film deposition. Moreover, films deposited far from the discharges in the downstream region have a higher deposition rate compared to those deposited in the upstream region; which indicates that clusters are driven by gas flow towards the downstream region where they deposit. We evaluated the stability of the films against light exposure by measuring their defect density with electron spin resonance spectroscopy, and we found out that the stability of the films increases with the distance from the electrode. In addition, the films deposited far from the discharges in the upstream region were highly stable. In short, a-Si:H films with a lower volume fraction of clusters incorporated into the films tend to show better stability against light exposure.
[1] Y. Watanabe, A. Harikai, K. Koga, and M. Shiratani, Pure Appl. Chem., (2002) 74 483
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