Atom Probe Characterisation of Individual Dislocations in Multi-Crystalline Silicon

The production of low-cost and reliable electricity with minimal environmental impact is one of the biggest challenges for the future of humanity. Solar power is the cleanest and most abundant source of energy, and as such photovoltaic devices that convert light to electricity will likely be a major part of overcoming that challenge. Multicrystalline silicon (mc-Si) solar cells, which currently represent around 60% of the world market for solar energy production, have limited performance due to the impurities in the raw material. Their efficiency is heavily dependent on control of the impurity distribution (gettering) and effective hydrogen passivation of defects present in the material. However, the gettering and passivation processes are not completely effective and the reasons for this are not well understood.

This project which is based both in the Atom Probe Group and Semiconductor and Silicon Photovoltaics Group will used advanced FIB (focused ion beam) and APT (atom probe tomography) techniques to perform "atom by atom" analyses on individual dislocations whose electrical properties have been well characterised. The techniques to be used have only been very recently developed and are unique to the Materials Department at Oxford. The result of this project will be a better understanding of the problems that limit the efficiency of mc-Si solar cells and potentially it will result in improved performance devices. In particular, the correlation between defect type, impurity atoms present, and electrical activity after commercial cell gettering and passivation processes will be elucidated. Measurements of the electrical activity of defects will be made at different stages in cell processing so that key parameters in the processing with regard to dislocation activity can be identified. The project is in collaboration with the mc-Si manufacturer CrystaloxPV and Oxford Instruments who are supplying a dedicated Electron Beam Induced Current (EBIC) system for use with the project to characterise electrical activity of specific microstructural features. The project will be also used to assess the efficacy of novel hydrogen passivation and gettering techniques being developed in the Oxford Semiconductor Group. The research will also contribute to the EPSRC-funded multi-institutional SuperSilicon PV: extending the limits of material performance project. It contributes directly to some of EPSRCs strategic research areas in particular the Energy theme and the Renewable Energy sub-theme.