In situ X-ray radiography and diffraction imaging of the growth of silicon for photovoltaic applications

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Crystalline silicon based photovoltaics (PV) is one of the key technologies on the way into the age of renewable energy. Increasing the energy efficiency of solar cells and at the same time reducing the production costs is the major challenge of the PV industry. One way to achieve this is to improve the crystalline quality of the silicon wafers. The presence of grain boundaries, dislocations and impurities significantly lowers the PV performance. Such defects act as recombination sites for electrons and holes and therefore reduce the solar-cell efficiency. Impurities segregate at higher order grain boundaries during the solidification process and can modify the solid-liquid interface creating structural defects. Consequently, controlling the density and nature of defects during silicon growth will result in better solar-cell performances.To achieve this, a fundamental understanding of the mechanisms governing defect generation is essential. We will use a unique combination of in-situ X-ray radiography, diffraction imaging and different post-mortem techniques, to reveal the generation of different defects during silicon solidification and to study their impact on PV properties. Synchrotron X-ray radiography enables us to observe the solid-liquid interface during crystal growth. With this technique the generation of new grains at grain boundary grooves can be studied. Diffraction imaging gives information on dislocations and strain fields in the growing crystals. The implementation of a new camera system of high spatial and temporal resolution will allow us to track a selected diffraction spot in greater detail so that the propagation of dislocations and their interaction with different grain boundary types (random angle, twin) can be studied. This will help us to understand the influence of different grain boundary types on the emission and also on the blocking of dislocations which is crucial when aiming at improving the product quality. Additionally, the impact of impurities on the PV properties will be analyzed in more detail by studying samples that contain different amounts of light impurities. As impurities are usually introduced during the fabrication process it is essential to investigate their interaction with dislocations and different grain boundary types.In general, this study aims at deepening the knowledge about defect generation in directionally solidified silicon and its influence on the PV properties. The new information might indicate potential improvements for the fabrication of crystalline silicon solar cells.