Efficiency limiting bulk recombination in multicrystalline silicon solar cells

Abstract

In this paper we measure, spatially resolved, the efficiency potential of multicrystalline material. The excess carrier lifetime in the bulk affects the open-circuit voltage, the short-circuit current and the fill factor of a solar cell differently as different injection conditions apply. Therefore, we measure the bulk lifetime injection-dependent and analyze each IV-parameter separately. Furthermore, multicrystalline wafers with laterally varying bulk recombination need a spatially resolved analysis to explore the impact of large grains or dislocation clusters on the efficiency. The crucial information we need is the spatially resolved injection-dependent bulk lifetime. For this purpose, we use lifetime calibrated photoluminescence imaging with a varying generation rate. Measuring a surface passivated wafer after the important high temperature steps of a solar cell process ensures that we attain the bulk recombination that is relevant for the solar cell. Combining this information with a cell simulation, we deduce the maximum efficiency for every image point. We also discuss how to obtain global values from these images and investigate the effect of charge currents flowing laterally through the emitter using Sentaurus Device and Spice network simulations. A comparison of these simulated global values to measured IV-parameters on finished solar cells shows good agreement between the two. A detailed analysis of two mc-wafers from different feedstock shows limiting IV-parameters in different regions, efficiency reductions due to dislocations and explains differences in the fill factor of two equally processed solar cells. The analysis shows that evaluating the material related efficiency limitation of silicon wafers has to be done at the efficiency level, as the lifetime at one generation level alone is not sufficient.