Application and analysis of silicon nitride films for surface passivation of high efficiency silicon solar cells

Two solar cell types are discussed in this thesis. Firstly, the Metal Wrap-Through cell, where the emitter-contact metallization of the front side is wrapped through holes in the wafer to the cell back. Optimization of several cell processing steps led to an increase of more than 2% absolute in cell efficiency. By integrating these optimized process steps a cell efficiency of 17.9% was reached. Using these multicrystalline cells in modules, a highest module efficiency of 17.0% on aperture area was obtained, which was the world record at the end of 2009 and the beginning of 2010.
Secondly, the Interdigitated Back Contact cell, where both the emitter and base region and their contacts are positioned at the cell back. This thesis describes the fabrication of this cell based on all-screen-printed patterning and metallization. By optimizing the front surface passivation, a short circuit current of 41.5 mA/cm² and a cell efficiency of 19.1% was obtained.
To further improve cell efficiency, surface passivation by a-SiN_x:H is studied.
By applying a NH₃ plasma prior to a-SiN_x:H layer deposition, it was found that the amount of K-centers and thereby the surface passivation could be varied without changing the actual a-SiN_x:H deposition process and thus the a-SiN_x:H bulk layer properties.
Detailed characterization showed a gradual change in the composition of a-SiN_x:H in the first 2 nm. By changing the temperature of the NH₃ plasma, the amount and penetration depth of N in the first few nm's in the Si substrate were altered.