p-i interface engineering and i-layer control of hot-wire a-Si:H based p-i-n solar cells using in-situ ellipsometry

In this paper we report on the effect of monitoring the i-layer region near the p-i interface with the help of in-situ kinetic and spectroscopic ellipsometry on the performance of hot-wire deposited hydrogenated amorphous silicon p-i-n solar cells. It is very clearly observed that the microstructure at the p-i interface region in terms of the Si-Si bond packing density and surface roughness significantly affects the cell performance. The filament temperature, T-Fil, Was the main parameter varied to control the above mentioned two properties near the p-i interface as well as in the bulk i-layer. In order to achieve significant enhancement in the cell performance we extended the idea of the ''soft start'', earlier employed for the glow discharge deposited solar cells, to the hot-wire deposited i-layer. We were able to control the i-layer properties at the p-i interface and in the bulk independently and correlate these to the cell performance. It is shown that a major increase in cell performance can be achieved by improving the microstructure of the growing film directly at the p-i interface. Most interestingly, no significant deterioration in cell efficiency has been observed if only the p-i interface was properly controlled but the i-layer was of lower quality. These results are also shown to be consistent with model calculations of a numerical simulation. Our results therefore provide a clue to prepare hot-wire a-Si:H based solar cells with high efficiency and in the whole at high growth rates, which is needed for a more economic a-Si:H solar cell production.