For photovoltaics, cast multi-crystalline silicon (mc-Si) is often used as a cheaper alternative to the single-crystal silicon used for integrated circuits. The photoluminescence image on the right shows a mc-Si wafer. Dark areas have low carrier lifetime ("bad"); bright areas have long carrier lifetime ("good"). Mc-Si is packed with extended defects (dislocations, grain boundaries and precipitates) and transition metal impurity point defects. These transition metals reduce efficiency by acting as recombination centres for electrons and holes generated by the incoming sunlight. Processes that effectively remove metallic impurities from single-crystal silicon do not work fully in mc-Si. This is because the metals interact with extended defects in the material. Our research aims to understand the interaction between transition metals and extended defects (particularly precipitates and dislocations), and to design new “gettering” processes to overcome this interaction.
Mohammad Al Amin started his PhD in this area in September 2013.