Bulk Lifetime Limiting Defects in Czochralski Silicon and Graphene Oxide as a Surface Passivation Material

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Abstract

Crystalline silicon continues to be the backbone of solar cell technologies. Enor- mous efforts towards process optimisation, cell architecture and defect identification, amongst others, thus continue to be made in order to push the limits of achievable ef- ficiencies. Consequently, this thesis aims to provide an improved understanding of a variety of defects related to light element impurities commonly found in silicon mate- rials used for cell fabrication, and their effect on the minority carrier lifetime. This thesis begins with a chapter which includes the motivation and outline of the present document. There, a summary of the most important aspects of the presented investigations and the relevance of the carried out work is described. In the subse- quent chapter, a review on diverse silicon materials used for solar cell fabrication and the characterization techniques applied by the author for the shown studies is con- ferred. The most important chapters then are those which follow Chapter 2, where all the contributions from this thesis are presented in the form of published, submit- ted or submission-pending papers. Lastly, a concluding chapter is presented with final remarks from the contributions from the thesis and further work. Amongst the most important contributions from this work stand out the following: 1. The identification of a lifetime-limiting complex which includes atomic H on its composition in phosphorous-doped Cz-Si crystals containing high levels of O and C. These findings thus include the rather controversial remark of having the ubiquitous H as a detrimental impurity for solar cell fabrication. 2. The identification of two previously unreported spectroscopic signals related to two thermal-donor species present in P-doped Cz-Si which have recently been suggested to limit the minority carrier of the material in literature. By means of various junction capacitance techniques evidence disproving the validity of such suggestion is provided. 3. The demonstration of a previously undetected spectroscopic signal attributed to a BO-complex in B-doped Cz-Si. Complex that appears to be related to the responsible defect of the decades-long problem of light-induced degradation oc- curring in solar cells made from this type of materials during the first few days of their operation. 4. The demonstration of graphene oxide as a potential cheap alternative for silicon surface passivation, and the identification of its primary passivation mechanism as a field-effect coming from the GO̢۪s negative surface charge.

Keyword(s)

Czochralski silicon; Minority carrier transient spectroscopy; Silicon defects; carbon defects; deep level transient spectroscopy; graphene oxide passivation; hydrogen defects; light induced degradation; minority carrier lifetime; oxygen defects; recombination; solar cells

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