Oxygen distribution on a multicrystalline silicon ingot grown from upgraded metallurgical silicon (original) (raw)
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Oxygen Distribution in Directionally Solidified Multicrystalline Silicon
Oxygen is one of the major unintentional-doped impurities in the multicrystalline silicon grown by directional solidification method. There are several sources of oxygen during casting of multicrystalline silicon, e.g. quartz crucible and coating layer as well as the feedstock itself. Oxygen is known to form defects and thermal donors, CO and BO complexes. The existence of oxygen in multicrystalline silicon will therefore affect the quality of the final solar cells. A model for distribution of oxygen in multicrystalline silicon is proposed, where both the evaporation and micro-segregation has been considered. Comparing with the FTIR measurements, the new model is able to describe the oxygen distribution in the final ingots. The influence of oxygen dissolution from the crucible has also been discussed in the paper.
Oxygen precipitation in polycrystalline ingot and ribbon solar silicon
1997
Multicrystalline silicon grown by directional solidification and polycrystalline RGS silicon are low cost materials for photovoltaic applications. The properties of solar cells made from these materials are mainly determined by dislocations, grain boundaries and intragrain defects such as impurities, small clusters of atoms or precipitates. Because of the use of quartz and graphite parts in both processes oxygen and carbon are the main impurities but are introduced in different quantities. The concentrations are generally higher in ribbon material and are close to the solubility limits. Oxygen is known to affect the conversion efficiency of solar cells. Both for Cz and multi-crystalline silicon an improvement of the solar cell performance as well as a degradation has been reported [1, 2]. Oxygen can form a variety of defects that affect the electrical behavior differently. Clusters of a few oxygen atoms and various sizes and crystal structures are observed for larger SiO2 precipitat...
Thermal oxidation processes for high-efficiency multicrystalline silicon solar cells
2004
For solar cells with highest efficiencies, thermally grown oxides are used for surface passivation. In this paper we compare wet oxidation in pyrogenic steam ambience at temperatures in the range from 800 °C to 900 °C with dry oxidation at 1050 °C. The wet oxidation process is suitable for mono-and multicrystalline silicon and does not degrade minority carrier lifetime. At the same time it maintains the high quality of surface passivation. The application of the wet oxide in a simplified process scheme with laser-fired rear contacts leads to conversion efficiencies under standard testing conditions of 20.3 % for multicrystalline and 21.2 % for monocrystalline silicon solar cells on small device areas (1 cm 2 ).
The effect of oxide precipitates on minority carrier lifetime in n-type silicon
Journal of Applied Physics, 2015
Supersaturated levels of interstitial oxygen in Czochralski silicon can lead to the formation of oxide precipitates. Although beneficial from an internal gettering perspective, oxygen-related extended defects give rise to recombination which reduces minority carrier lifetime. The highest efficiency silicon solar cells are made from n-type substrates in which oxide precipitates can have a detrimental impact on cell efficiency. In order to quantify and to understand the mechanism of recombination in such materials, we correlate injection level-dependent minority carrier lifetime data measured with silicon nitride surface passivation with interstitial oxygen loss and precipitate concentration measurements in samples processed under substantially different conditions. We account for surface recombination, doping level, and precipitate morphology to present a generalised parameterisation of lifetime. The lifetime data are analysed in terms of recombination activity which is dependent on precipitate density or on the surface area of different morphologies of precipitates. Correlation of the lifetime data with interstitial oxygen loss data shows that the recombination activity is likely to be dependent on the precipitate surface area. We generalise our findings to estimate the impact of oxide precipitates with a given surface area on lifetime in both n-type and p-type silicon. V
2020
Abstract. In this study we try to indentify relation between carrier lifetime, resistivity and two mains impurities concentration in a p-type upgrade metallurgical multicrystalline (UMG) silicon ingot. Thanks to this relation, we could prevent the Light Induced Degradation (LID) phenomenon and the SiC particles formation which are, respectively, at the origin of Voc losses and shunts in solar cells. So these 2 parameters are important for photovoltaic panels' efficiency. Lifetime measurements are achieved by means of the Microwave Photoconductivity Decay "μw-PCD" technique, and concentration measurements are determined by FTIR. We demonstrate that resistivity variations depend on oxygen's concentration but carbon analyses must be continued.
Performance-Limiting Oxygen-Related Defects in Silicon Solar Cells
ECS Transactions, 2006
The energy conversion efficiency of solar cells fabricated from oxygen-containing crystalline silicon wafers with boron as p-type dopant is ultimately limited by boron-oxygen-related recombination centers which form under illumination or forward-biasinduced electron injection into the p-type base of the cell. This paper reviews the recent progress in understanding the physics of this degradation effect. It is shown that two different types of boronoxygen centers are simultaneously formed at very different formation rates. Electronic defect properties, formation mechanisms and the impact on device properties are discussed.
Oxygen and lattice distortions in multicrystalline silicon
Solar Energy Materials and Solar Cells, 2002
Oxygen is one of the main impurities in multicrystalline silicon for photovoltaic applications. Precipitation of oxygen occurs during crystal growth and solar cell processing. It is shown that dislocations enhance the oxygen precipitation. Depending on the thermal conditions and the initial oxygen content various types of SiO 2 +precipitates and oxygen related defects are observed and investigated by fourier transform infrared (FTIR) spectroscopy and transmission electron microscopy. The large area distribution of oxygen decorated dislocations is studied by scanning infrared microscopy (SIRM). Both inhomogeneous distributions of dislocations and oxygen precipitates occur and can lead to internal stresses. The internal stresses of multicrystalline-silicon wafers are investigated by an optical method using polarized infrared light. The results are compared with the dislocation microstructure and the oxygen distribution in wafers produced by different growth techniques.
Materials Science and Engineering: B, 2009
Nowadays the photovoltaic (PV) market suffers the severe shortage of silicon. One possible solution is to produce SoG-Si via a direct metallurgical route, followed by a final casting step. The use of such lower quality materials in solar cell production depends on the possibility of improving the electrical quality during the cell processing and requires a deep understanding of the interaction between defects. The aim of this work is to study the electrical properties and the minority charge carrier recombination behaviour of extended defects in a mc-Si ingot grown from metallurgical Si produced directly by carbothermic reduction of very pure quartz and carbon. The combined application of photoluminescence, infrared spectroscopy, electron beam induced current technique and transmission electron microscopy succeeded in identifying oxygen precipitates, decorated grain boundaries and dislocations as the defects which limit the quality of the metallurgical mc-Si and, therefore, the efficiency of the related solar cells.
Solar Energy Materials and Solar Cells, 2011
Solar grade, p-type multicrystalline silicon wafers with large grains from different parts of silicon ingots produced by the metallurgical route (SoG-Si) at ELKEM Solar were studied using a number of complementary methods such as microwave photoconductivity decay, deep level transient spectroscopy, transmission and scanning electron microscopy, X-ray fluorescence, and secondary ion mass spectroscopy. Wafers from the top of the ingots have uniform spatial distributions of both minority carrier lifetime (average lifetime t¼3.2 ms) and concentrations of illumination-sensitive recombination centers (N rc ¼ 3 Â 10 10 À 2 Â 10 11 cm À 3) over the whole wafers. Wafers from the bottom of the ingots have regions of very low lifetimes (t ¼ 0.3 ms) and high concentrations of illumination-sensitive recombination centers (N rc ¼2 Â 10 12 cm À 3). In the top part of the ingots the observed DLTS peaks can be attributed to copper-related extended defects, and the DLTS results from grains and grain boundaries are not significantly different. The main factors limiting the lifetime in the high lifetime regions are concluded to be illumination-sensitive recombination centers such as Fe-B pairs, B-O complexes, and Cu-related extended defects. The low lifetimes in the bottom part of the ingots are explained by a combination of several factors including high concentrations of illumination-sensitive recombination centers and of some deleterious elements (S, Na and Al), and a large amount of structural defects.
Journal of Crystal Growth, 2012
Silicon nitride is an alternative material to the widely used silica crucibles for directional solidification of mc-Si ingots, its main advantages being the reusability in successive castings and elimination for a source for oxygen contamination of the ingot. In this work, several ingots were cast in these crucibles and compared to reference ingots cast in silica crucibles. The thermal properties of the Si 3 N 4 crucible differ from those of the SiO 2 crucible and lead to a different thermal history during melting and casting. The oxygen contamination of the ingot was observed to depend mainly on the melting and holding temperature, rather than on the crucible material. The lowest oxygen concentration was observed in the ingots with the lowest melting temperature. However, the thermal properties of the Si 3 N 4 crucible influence the oxygen profile along ingot height, with a faster decrease in the concentration with increasing ingot height. This is believed to be due to a different mechanism for oxygen transport compared to that of the silica crucibles. The concentration of dopants in the ingots showed that contamination from the Si 3 N 4 crucible occurred, probably due to diffusion of B-and P-oxides into the Si melt.