Carrier Recombination in Multicrystalline Silicon: A Study using Photoluminescence Imaging

Abstract

This thesis applies photoluminescence imaging technique to study various carrier recombination mechanisms in multicrystalline silicon materials. One emphasis of the work has been recombination at grain boundaries, which is one of the limiting factors for the performance of multicrystalline silicon solar cells. An approach for quantifying the recombination activities of a grain boundary in terms of its effective surface recombination velocity, based on the photoluminescence intensity profile across the grain boundary, is developed. The approach is applied to compare the recombination properties of a large number of grain boundaries in wafers from different parts of a p-type boron doped directionally solidified multicrystalline silicon ingot. The results show that varying impurity levels along the ingot significantly impact the electrical properties of as-grown grain boundaries, and also their response to phosphorus gettering and hydrogenation. The work is then extended to various types of multicrystalline silicon materials. The electrical properties of conventionally solidified p-type, n-type and also recently developed high performance p-type multicrystalline silicon wafers were directly compared in terms of their electronic behaviours in the intra-grain regions, the grain boundaries and the dislocation networks. All studied samples reveal reasonably high diffusion lengths among the intra-grain regions after gettering and hydrogenation, suggesting that the main performance limiting factors are likely to be recombination at crystal defects. Overall, grain boundaries in the conventional p-type samples are found to be more recombination active than those in the high performance p-type and conventional n-type samples. As-grown grain boundaries and dislocations in the high performance p-type samples are not recombination active and only become active after thermal processes. In contrast, grain boundaries in the n-type samples are already recombination active in the as-grown state, but show a dramatic reduction in their recombination strength after gettering and hydrogenation. Apart from recombination through crystal defects within the bulk, recombination at surfaces acts as another significant loss mechanism in solar cells. This thesis also demonstrates the use of the photoluminescence imaging technique to study surface recombination in silicon wafers, and provides some examples of such applications.