Characterization of Silicon Crystals Grown from Melt in a Granulate Crucible

The growth of silicon crystals from a melt contained in a granulate crucible significantly differs from the classical growth techniques because of the granulate feedstock and the continuous growth process. We performed a systematic study of impurities and structural defects in several such crystals with diameters up to 60 mm. The possible origin of various defects is discussed and attributed to feedstock (concentration of transition metals), growth setup (carbon concentration), or growth process (dislocation density), showing the potential for further optimization. A distinct correlation between crystal defects and bulk carrier lifetime is observed. A bulk carrier lifetime with values up to 600 μs on passivated surfaces of dislocation-free parts of the crystal is currently achieved

Health and safety in quarries A hundred years of law

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Microstructural investigation of LID sensitive mc-PERC solar cells

Light induced degradation can lead to a severe efficiency loss in multi-crystalline PERC solar cells depending on bulk Si material properties and solar cell processing parameters, known as mc-LID or LeTID. Various defect models have been suggested which indicate a clear distinction to BO- and FeB-LID mechanisms. Cu is known to cause light induced degradation and therefore is one of the possible causes of mc-LID in PERC cells. However, until now, a direct microstructural proof of its presence is still missing. In this contribution, we investigate mc-LID sensitive PERC cells which show the typical lateral appearance of mc-LID where structural defects, such as grain boundaries, show a reduced degradation. Investigations of grain boundaries from front and rear side with respect to the recombination activities (LBIC) in correlation to the crystalline structure (XRD Laue mapping) indicate that gettering at grain boundaries reduces degradation. Furthermore, enhanced rear recom bination at grain boundaries and scattered local spots of μm size is detected. At these regions with damaged rear passivation, Cu-containing microscopic particles are unveiled by microstructural investigations (SEM) and elemental micro-analysis (EDX). Target preparation (TEM) shows a Cu-filled channel that connects the Cu-containing microscopic particles and the silicon bulk. These observations indicate that the presence, diffusion, and precipitation of Cu might play a role in the mc-LID defect formation

Evaluation of improvement strategies of grain structure properties in high performance multi-crystalline silicon ingots

High performance multi-crystalline silicon (HPM-Si) for the use in photovoltaics is characterized by a very fine grain structure and a high content of random grain boundaries, finally resulting in a low dislocation density and consequently in a high material quality. Typically, the grain size increases and the fraction of random grain boundaries decreases over ingot height due to annihilation mechanisms, especially in the first 150 mm. One approach for further material improvement is to further increase the initial random grain boundary fraction and to maintain it as high as possible over the complete ingot height. In this work, several theoretical approaches to achieve these points were evaluated by experiments in G1 scale. Firstly, the influence of the silicon seeding material on the initial grain structure was investigated regarding the effect of extremely fine Si particles in the µm to nm range and the bulk density of the particle layer. Secondly, the effect of the initial geometrical grain boundary arrangement in the seed layer was evaluated. For that purpose, special seed alignments similar to the Quasimono approach were tested. Finally, the process parameter growth rate was varied in a wide range to investigate its influence on the evolution of the grain boundaries during growth. The results show that the optimum for the initial random grain boundary fraction is already reached by the existing methods/commonly used seed materials. Concerning the decrease of the random grain boundary fraction over ingot height, some technical aspects were identified which are able to keep the amount of random grain boundaries at a high level. However, the practical realization within an industrial setup might be difficult

Impact of different SiO2 diffusion barrier layers on lifetime distribution in multi-crystalline silicon ingots

Three different SiO2-based barrier layers, used in industry for prevention of metallic impurity diffusion into directionally solidified silicon ingots, were investigated in detail by G1 experiments in order to find out the most relevant barrier properties responsible for the blocking behaviour. Minority carrier lifetime and interstitial iron measurements on the grown silicon ingots show significant differences between the barriers regarding the red zone extension as well as the maximum lifetime/minimum iron content in the ingot centre. Structural analysis of the barriers by optical microscopy and Raman spectroscopy reveal a clear correlation to the porosity and thickness of the barrier layers. Further, indications were found that the barriers become effective only during the process and that this should occur as early as possible to obtain an optimal impact of the barrier. Only one of the tested barriers shows an overall positive impact on the lifetime, coming close to using a high purity quartz glass SiO2 crucible instead of the standard ceramic SiO2 crucible

Light induced degradation: Kinetic model and grain boundary impact on sponge-LID

High-performance multi-crystalline silicon material (HPmc-Si) dominates the market for casted p-type silicon. Solar cells made from HPmc-Si material might suffer from light induced degradation due to the so called sponge-LID mechanism. In this work, we present a kinetic sponge-LID model showing that the degradation follows a pairing reaction involving two reactants. This implies that sponge-LID is based on a different reaction scheme compared to the well-known models for boron-oxygen- or iron-boron-degradation. Based on our model, degradation rates are investigated reading the influence of temperature and illumination on the degradation. Finally, we present statistical results implying that Sigma-3 grain boundaries are less affected of the degradation than other grain boundary types. Using a detailed spatially resolved analysis of the effective carrier diffusion length, the different behaviour of grain boundaries and intra-grain regions is quantified

Electronic nose performance optimization for continuous odour monitoring in ambient air

Industrial plants with odour emissions affect the quality of air and are often cause of public complaints by the people living surrounding the plant. For this reason, the control of odour represent a key issue. The starting point for an effective odour control it‟s their objective quantification. The electronic nose represent the odour measurement technique with probably the greatest potential, but currently there is not a universally recognized procedure of their application for the continuous monitoring of environmental odours.The aim of this paper is to present and describe a novel procedure to training electronic noses in order to maximize their capability of operating a qualitative classification and estimating the odour concentration of ambient air. This novel approach will reduce the uncertainty and increase the reliability of the continuous odour measures. The research is carried out through a real case study application in a big liquid waste treatment plant (LWTP). The seedOA system, patented by the SEED group of the University of Salerno, was used as e.nose device. The characterization of the odour concentrations from the different treatment units and the identification of the principal odour sources is discussed

Investigation of gas bubble growth in fused silica crucibles for silicon Czochralski crystal growth

Gas bubbles in crucibles for Czochralski (Cz) silicon growth are both necessary and detrimental: In the outer, bubble-containing (BC) layer of the crucible, they are required for mechanical stability, while in the inner, bubble-free (BF) layer, bubbles can cause the release of particles from the crucible into the melt which may disrupt the single-crystalline growth. In this work, a vacuum bake-out test (VBT) procedure was set up for unused crucible parts and a microscopic characterization routine was developed to systematically investigate bubble formation and growth. Longer process time, higher temperature, and lower atmospheric pressure lead to an increased bubble growth in both, the BC and BF layer. During the VBT, no new bubbles form in the BF layer, while existing bubbles grow. The comparison to experimental data from crucibles used in an industrial Cz process indicates that VBTs can simulate this process. This allows the prediction of the gas-bubble formation in Cz crucibles using a cost-effective and less time-consuming analyzation method