| dc.description.abstracteng | Multicrystalline silicon, which is among the most common materials for solar cells [3], contains extended defects like grain boundaries and dislocations and a high amount of metal impurities potentially reducing the solar cell efficiency [1,7].
In order to reduce the detrimental effect of metal impurities on the efficiency of multicrystalline solar cells, the metal impurities can be redistributed
into electrical inactive areas of the solar cell (gettering) [12] or at few large accumulation sites, e.g. at grain boundaries [16].
In this work the interaction of metal impurities with extended defects is investigated for the purpose of a better understanding of the underlying physical mechanisms, which is necessary to effectively redistribute the metal impurities.
Experimental investigations on the atomistic scale with a combination of in-situ EBIC/FIB and TEM are preformed on multicrystalline silicon samples intentionally contaminated with copper and iron.
The combination method was developed in this work to investigate the distribution, the atomic structure and the chemical nature of selected extended recombination active defects at high resolution, but low necessary defect density [43,71]. Accumulation of copper at light elements like nitrogen and oxygen located at grain boundaries, and dislocation networks decorated with copper are identified to be the origin of recombination-active defects.
Simulations on the wafer scale of the redistribution of the iron concentration during temperature treatments and gettering processes in presence of grain boundaries are performed.
Simulations of gettering processes show that kinetics of phosphorus diffusion gettering and aluminium gettering are limited by the dissolution of precipitates at a low temperature regime. For high gettering temperatures aluminum gettering has the advantage of being only limited by the thermodynamic conditions, while phosphorus diffusion gettering is limited by the phosphorus indiffusion.
A comparison of simulations of the redistribution of iron during temperature treatments in presence of a grain boundary with experimental LBIC and PL measurements and simulations investigating the influence of segregation and precipitation as mechanisms of impurity accumulation at grain boundaries show that quantitative modeling with precipitation as mechanism of impurity accumulation at grain boundaries is possible, while segregation has only minor effects.
The simulations illustrate that grain boundaries can serve as sinks or sources for metal impurities depending on the temperature treatments.
A variation of grain size in the simulations imply that there is an optimal grain size in the range of two times the diffusional range to achieve
optimal diffusion lengths in solar cells.
The simulations show that for some temperature treatments, it is important to take the history of temperature treatments of the sample into account.
The comparison of simulations and LBIC/ PL measurements hint that especially for low iron concentrations and in the close vicinity of grain boundaries, interstitial iron is not the limiting factor for the diffusion length. As a result, other recombination processes, like the recombination activity of the grain boundary itself or recombination due to precipitates, have to be taken into account when calculating the interstitial iron concentration from LBIC/ PL measurements. | de |