Bulk Lifetimes up to 20 ms Measured on Unpassivated Silicon Discs Using Photoluminescence Imaging

Abstract

With high-efficiency silicon solar cells approaching 25% efficiency in mass production, the requirements on the bulk lifetime and its uniformity across the wafer and the ingot increase dramatically. Since some cell architectures require these high lifetimes on starting material, the need arises for characterization methods to measure very high bulk lifetimes that are spatially resolved at an early stage before cell processing. A method based on the spectral ratio of two photoluminescence images is applied here on two unpassivated silicon discs from different positions within a Czochralski-grown phosphorous-doped n-type silicon ingot. The method allows the determination of spatially resolved bulk lifetime images on samples with adequate thickness and can be done within seconds and without the need to passivate surfaces. As-grown bulk lifetimes up to 20 ms are measured on the ingot's central disc, indicating recent improvements in crystallization technology, but are strongly reduced closer to the crown. Evidence suggesting the impact of thermal donors on the lifetime and effective doping concentration near the crown is found from combining spectral photoluminescence and infrared spectroscopy analyses. The technique could find applications in research and development activities, particularly in the optimization of Czochralski silicon crystal growth conditions.