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

基于晶体硅的光伏技术是进入可再生能源时代的关键技术之一。提高太阳能电池的能源效率，同时降低生产成本是光伏产业面临的主要挑战。实现这一目标的一种方法是提高硅片的晶体质量。晶界、位错和杂质的存在显著降低了PV性能。这些缺陷作为电子和空穴的重组位点，因此降低了太阳能电池的效率。在凝固过程中，杂质在高阶晶界处偏析，并能改变固液界面，产生结构缺陷。因此，在硅生长过程中控制缺陷的密度和性质将导致更好的太阳能电池性能。要实现这一点，对控制缺陷产生的机制的基本理解是必不可少的。我们将使用原位x射线照相、衍射成像和不同的尸检技术的独特组合，揭示硅凝固过程中不同缺陷的产生，并研究它们对PV性能的影响。同步加速器x射线照相使我们能够观察晶体生长过程中的固液界面。利用该技术可以研究晶界凹槽处新晶粒的生成。衍射成像提供了生长晶体中位错和应变场的信息。高空间和时间分辨率的新相机系统的实现将使我们能够更详细地跟踪选定的衍射点，以便研究位错的传播及其与不同晶界类型（随机角度，孪晶界）的相互作用。这将有助于我们了解不同晶界类型对发射的影响，以及对位错阻塞的影响，这对于提高产品质量至关重要。此外，杂质对PV性能的影响将通过研究含有不同量轻杂质的样品来更详细地分析。由于杂质通常是在制造过程中引入的，因此有必要研究它们与位错和不同晶界类型的相互作用。总的来说，本研究旨在加深对定向固化硅缺陷产生及其对PV性能影响的认识。新的信息可能表明晶体硅太阳能电池制造的潜在改进。

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.