Keywords
Crystallographic orientation; Polycrystalline silicon; Grain boundary; Grain boundary engineering; Grain size distribution
1. Introduction
Multicrystalline silicon (mc-Si) is the dominating absorber material for solar cells [1]. It is produced in industry by directional solidification of a silicon melt contained in a Si3N4 coated SiO2 crucible. During the last years many processes were developed in order to influence the initial grain structure of the mc-Si ingots grown by directional solidification with the aim to reduce the crystal defect density. The first attempts were made to reduce the total grain boundary length of the wafer. The so-called dendritic growth method [2], [3] and [4] uses a high initial undercooling of the silicon melt to initiate the solidification process. The resulting grain structure near the bottom shows an increased grain size, as well as an increased fraction of ∑3 grain boundaries. The initial grains reveal a preferred {112} orientation parallel to the ingot solidification front. This special grain structure leads to a lower crystal defect density compared to the formerly used conventional mc-Si [5] and [6]. For establishing this grain structure a high undercooling is necessary at the beginning of the process. The development of such a process, with high repeatability to control the growth direction of the dendrites properly, is very challenging because highly dislocated regions will form when the dendrites grow away from the crucible bottom at wider angles [5]. Therefore, this technique is not used in industrial mass production.
Currently, the so-called high performance (hp) mc-Si [7] has entered the market. For this process a not back melted silicon nucleation layer is used on the crucible bottom. Such a nucleation technique results in a specific initial grain structure which is characterized by a uniform small grain size, a homogenous grain orientation distribution and a high fraction of random grain boundaries [7] and [8]. The obtained high fraction of random grain boundaries in combination with the small sized grain structure near the seeding position seems to be favourable to supress the formation of large dislocation clusters [7], [9] and [10] over the ingot height. Yang et al. [7] and Zhu et al. [11] showed clearly that the solar cell efficiency is increased by 0.5% absolute using these fine-grained wafers instead of conventional mc-Si material.
Until now only a few grain structure (grain size, grain orientation and grain boundary types) investigations were published. Most of them were performed on small regions (few cm2) of laboratory samples [8] and [9] without any clear quantification of the favourable grain size-, grain orientation-, and grain boundary type distribution due to the lack of statistics.
The aim of this work was a systematic and statistical relevant grain structure analysis for differently grown industrial mc-Si bricks. The grain structure results were correlated with the area fraction of electrical active defects and the solar cell efficiency of the investigated wafers, in order to quantify the most important grain structure features, which were relevant to obtain high quality mc-Si wafer material.
Keywords Crystallographic orientation; Polycrystalline silicon; Grain boundary; Grain boundary engineering; Grain size distribution1. IntroductionMulticrystalline silicon (mc-Si) is the dominating absorber material for solar cells [1]. It is produced in industry by directional solidification of a silicon melt contained in a Si3N4 coated SiO2 crucible. During the last years many processes were developed in order to influence the initial grain structure of the mc-Si ingots grown by directional solidification with the aim to reduce the crystal defect density. The first attempts were made to reduce the total grain boundary length of the wafer. The so-called dendritic growth method [2], [3] and [4] uses a high initial undercooling of the silicon melt to initiate the solidification process. The resulting grain structure near the bottom shows an increased grain size, as well as an increased fraction of ∑3 grain boundaries. The initial grains reveal a preferred {112} orientation parallel to the ingot solidification front. This special grain structure leads to a lower crystal defect density compared to the formerly used conventional mc-Si [5] and [6]. For establishing this grain structure a high undercooling is necessary at the beginning of the process. The development of such a process, with high repeatability to control the growth direction of the dendrites properly, is very challenging because highly dislocated regions will form when the dendrites grow away from the crucible bottom at wider angles [5]. Therefore, this technique is not used in industrial mass production.Currently, the so-called high performance (hp) mc-Si [7] has entered the market. For this process a not back melted silicon nucleation layer is used on the crucible bottom. Such a nucleation technique results in a specific initial grain structure which is characterized by a uniform small grain size, a homogenous grain orientation distribution and a high fraction of random grain boundaries [7] and [8]. The obtained high fraction of random grain boundaries in combination with the small sized grain structure near the seeding position seems to be favourable to supress the formation of large dislocation clusters [7], [9] and [10] over the ingot height. Yang et al. [7] and Zhu et al. [11] showed clearly that the solar cell efficiency is increased by 0.5% absolute using these fine-grained wafers instead of conventional mc-Si material.Until now only a few grain structure (grain size, grain orientation and grain boundary types) investigations were published. Most of them were performed on small regions (few cm2) of laboratory samples [8] and [9] without any clear quantification of the favourable grain size-, grain orientation-, and grain boundary type distribution due to the lack of statistics.The aim of this work was a systematic and statistical relevant grain structure analysis for differently grown industrial mc-Si bricks. The grain structure results were correlated with the area fraction of electrical active defects and the solar cell efficiency of the investigated wafers, in order to quantify the most important grain structure features, which were relevant to obtain high quality mc-Si wafer material.
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