| Person in charge of the project: |
Advanced Characterization of Novel Solar Grade Materials for Solar Cells.|
Mc-Si (multi-crystalline silicon) is the primary material used in the production of industrial bulk Si solar cells. The presence of grain boundaries, of clean or impurity-decorated crystal defects perturbs the ideal band structure of the semiconductor. If this perturbation leads to the formation of energy levels within the band-gap, SRH (Shockley-Read-Hall) recombination is activated and the solar cell performance consequently reduced. Therefore, effort must be put in reducing/limiting the electrical activity of the material defects during the solar cell process and on the other side, in recognizing and determining the constraints the starting material should comply with in order to yield sufficient conversion efficiency.|
Phosphorus gettering and hydrogenation from a-SiNx:H layers are the two processes investigated in this work. In analogy with computer-science concepts, the characterization of defect-impurity structures in solar cells can be unfold over different abstraction layers, the device level being the top one and the structural-compositional level the bottom one. This work, rather than focusing on the effect of gettering and hydrogenation on one specific layer, is devoted to study the reciprocal correlation among the various layers. A crucial layer is represented by recombination lifetime, given its sensitivity to electrically active defects. Consequently, part of the work has been devoted to the development and enhancement of advanced tools and methodologies for recombination lifetime determination, both spatially and injection level resolved. Corollary to recombination lifetime, another important phenomenon referred to as “carrier trapping” is of interest due to its correlation with material quality. The underlying physical phenomenon and the effect at device level being still not completely clear (it can be modeled as a temporary capture of excess charge carriers at defect sites), it is a promising source of information on relevant defect-impurity structures and needs therefore accurate characterization.
Duration of the project:
14.05.2007 - 14.05.2011