David Tweddle (35Min)

University of Oxford

Abstract

The performance of multicrystalline silicon solar cells, which currently represent around 60% of the world’s photovoltaic module production, is strongly limited by their impurities. These impurities, primarily transition metals, are present in the raw material and can also be introduced from the production environment. Transition metals are known to segregate to crystallographic defects, such as dislocations and grain boundaries, which then act as strong recombination centres. Gettering, notably phosphorus diffusion gettering, is a common industry technique used to minimise the effect of such impurities. However, this process is not completely effective and the reasons for this are not well understood. This is as a result of the very low concentrations of impurities associated to specific isolated microstructural features that can dramatically influence the electrical properties of the material. Hence, this represents a significant microscopy challenge to characterise the atomic scale distribution of these trace elements and correlate this with material performance.

This study develops a complementary Atom Probe Tomography (APT), Electron Beam Induced Current (EBIC) and Photoluminescence (PL) approach to characterise and compare the electrical and chemical properties of specific microstructural defects in multicrystalline silicon pre and post gettering treatment. To this end, a novel Focused Ion Beam (FIB) lift-out method which enables correlative Transmission Electron Microscopy (TEM) and atomic scale APT analysis of large targeted sections of grain boundaries is presented.

A second commonly used, and perhaps least understood, industrial technique is hydrogen passivation. In this talk, an experimental protocol is presented, involving a combination of isotopic charging and APT, which enables the unambiguous 3D characterisation of hydrogen atoms at individual defects within multicrystalline silicon. To this end, our results demonstrate the first direct observation of the location of hydrogen at the atomic scale to crystallographic defects in silicon. In addition, the method presented allows for quantitative comparisons to be drawn regarding the difference in hydrogen content at different types of grain boundaries and also between individual dislocations.

Brief Bio

David is currently a second year DPhil Student in the Department of Materials, University of Oxford, using Atom Probe Tomography to analyse specific microstructural defects in multicrystalline silicon solar cells. Previously achieved a Masters in Engineering at the University of Sheffield in Materials Science (1st class), completing a thesis investigating hole transport materials for perovskite solar cells.