Papers by Keyword: Multicrystalline Silicon

Abstract: Large temperature gradient was introduced to improve the removal rate of metal impurity in silicon ingot during direction solidification. The concentration of metal impurities in the silicon ingot with a large temperature gradient is 0.96 ppmw. The solidification time is reduced by 20% due to the fast speed of crystal growth improved; meanwhile the purity is increased by 64%.

Abstract: In this work, directional solidification was performed for multicrystalline silicon (mc-Si) ingot casting. The initial nucleation at the bottom of the silicon melt could be controlled by changing the cooling rate from 9 to 20μm/s. Metallographic microscope, X-Ray Diffraction (XRD), Microwave photoconductivity decay meter (μ-PCD) and four-point probe resistivity tester were used to investigate the microstructure, crystal orientation and electrical properties of the mc-Si ingots. The obtained results showed that cooling rate at 17μm/s is the optimum condition for the mc-Si ingots casting, under which the prepared ingot has lower dislocation density of 6×10^-3 cm^-2, better electrical properties, more uniformer resistivity distribution with an average value of 0.68 Ω×cm and higher minority carrier lifetime with a maximum value of 1.8 μs than that of in the other cooling rate conditions.

Abstract: The multicrystalline silicon wafers purified by directional solidification route were used to introduce copper impurities. The resistivity and minority carrier lifetime of multicrystalline silicon wafers were investigated by four-point probe resistivity tester and μ-PCD, respectively. Annealing temperature, atmosphere and cooling rate were researched. It was found that copper contaminants have a greater impact on the electrical properties of multicrystalline silicon. Research results showed that copper impurities tend to exist at defect sites at high temperature, and high annealing temperature, argon atmosphere and slow cooling conditions make more impact on the electrical properties of multicrystalline silicon than low annealing temperature, air atmosphere and fast cooling.

Abstract: The finite element model of double etching pits was established, optical performance of multicrystalline silicon wafer before and after etching was simulated by RF MODULE of COMSOL Multiphysics version 3.5a. Optical characteristic of unetching wafer and acidic textured were compared. It is indicates that acidic textured (double etching pits) has low reflectivity, high power flow Y component , the better light trapping. When the wavelength is 600nm, the maximum and minimum value of surface electric field Z component of acidic textured are 1.9 times and 1.4 times respectively than that of unetching wafer, and two extremum value of surface magnetic field Z component are 2.1 times and 1.9 times respectively than that of unetching wafer. Numerical simulation results of Multi hole model are closely with experimental values, which can guide the practical production.

Abstract: Small angle grain boundaries have been grown in a small Bridgman furnace, using seeded growth method, at three different pulling rates i.e. 3 μm/s, 13 μm/s and 40 μm/s. In order to assess segregation mechanisms of impurities towards the central grain boundary, melt has been polluted by 50ppma of either copper or indium. Secondary ion mass spectrometry (SIMS) local analyses have been performed to investigate the impact of solid state diffusion and limited rejection of solute at the grain boundary for each growth rate. The results are discussed in connection with an atomistic model built on Vienna Ab-initio Simulation Package (VASP).

Abstract: Cast mc-Si ingots are widely used in photovoltaic manufacturing. The utilization rate of industrial polycrystalline silicon ingot only about 70%, most of them are less than 70%, the main influence factors are casting process, raw and auxiliary materials as well as the crucible material. The growth process and the overall characteristics of 420 kg polycrystalline silicon ingot are analyzed and researched in detail. This paper focuses on the distribution characteristics and causes of the casting defect which are analyzed in detail and discussed, at the same time suggestions are given to improve the utilization rate of the ingot.

Abstract: A comparative study has been made to analyze the impact of interstitial iron in minority carrier lifetime of multicrystalline silicon (mc-Si). It is shown that iron plays a negative role and is considered very detrimental for minority carrier recombination lifetime. The analytical results of this study are aligned with the spatially resolved imaging analysis of iron rich mc-Si.

Abstract: The temperature distribution has important influence on the position and shape of solid-liquid interface during directional solidification process. So the calculation of temperature field is fairly significant for both structural analysis and temperature control. In this paper, the finite element method is applied to establish the 2D axisymmetric model for modeling the temperature distribution and the solid-liquid interface shape of multicrystalline silicon in semi-industrial directional solidification furnace. The numerical results show that the temperature field and solid-liquid interface shape can be controlled by adjusting the pulling rate in directional solidification process, and an optimized pulling rate of this system was obtained for large diameter silicon crystals with low defect density and uniform dopant distribution.

Abstract: In multicrystalline silicon for photovoltaic applications, high concentrations of iron are usually found, which deteriorate material performance. Due to the limited solubility of iron in silicon, only a small fraction of the total iron concentration is present as interstitial solute atoms while the vast majority is present as iron silicide precipates. The concentration of iron interstitials can be effectively reduced during phosphorus diffusion gettering (PDG), but this strongly depends on the size and density of iron precipitates, which partly dissolve during high-temperature processing. The distribution of precipitated iron varies along the height of a mc-Si ingot and is not significantly reduced during standard PDG steps. However, the removal of both iron interstitials and precipitates can be enhanced by controlling their kinetics through carefully engineered time-temperature profiles, guided by simulations.

Abstract: Light microscopy, electron backscatter diffraction and transmission electron microscopy is employed to investigate dislocation structure and impurity precipitation in commonly occurring dislocation clusters as observed on defect-etched directionally solidified multicrystalline silicon wafers. The investigation shows that poligonised structures consist of parallel mostly similar, straight, well-ordered dislocations, with minimal contact-interaction and no evidence of precipitate decoration. On the other hand, disordered structures consist of various dislocation types, with interactions being common. Decoration of dislocations by second phase particles is observed in some cases. Enhanced recombination activity of dislocations may therefore be a result of dislocation interaction forming tangles, microscopic kinks and jogs, which can serve as heterogeneous nucleation sites that enhance metallic decoration.