Papers by Keyword: Multicrystalline Silicon

Abstract: The evolution of Fe-related defects is simulated for di erent P di usion gettering (PDG) processes which are applied during silicon solar cell processing. It is shown that the introduction of an extended PDG is bene cial for some as-grown Si materials but not essential for all of them. For mc-Si wafers with an as-grown Fe concentration 14 cm³, a good reduction of the Fe_i concentration and increase of the electron lifetime is achieved during standard PDG. For mc-Si wafers with a higher as-grown Fe concentration the introduction of defect engineering tools into the solar cell process seems to be advantageous. From comparison of standard PDG with extended PDG it is concluded that the latter leads to a stronger reduction of highly recombination active Fe_i atoms due to an enhanced segregation gettering e ect. For an as-grown Fe concentration between 10¹⁴ cm³ and 10¹⁵ cm³, this enhanced Fe_i reduction results in an appreciable increase in the electron lifetime. However, for an as-grown Fe concentration >10¹⁵ cm³, the PDG process needs to be optimized in order to reduce the total Fe concentration within the wafer as the electron lifetime after extended PDG keeps being limited by recombination at precipitated Fe.

Abstract: Elaborated characterization tools play an important role for the further improvement of solar material and the development of solar cells. Besides the huge variety of highly advanced methods mainly based on optical and electrical measurements, the direct measurement of surface currents by the detection of their induced magnetic fields has gained less attention. The novel method current-analysis-by-inductive-coils (CAIC) based on an inductive coil detector and is reviewed and compared with already established methods, which are light-beam-induced-current (LBIC) and dark-lock-in-thermography (LIT). The detector reveals complementary information at high resolution. The LIT measurements depicted shunting defects in forward and reverse current. Because of the high spatial resolution of the CAIC measurement technique it was determinable that in some cases the positions of current sinks in the CAIC maps are not corresponding with the microstructure. The analyses of the superpositions reveals macroscopic precipitates like SiC and Si₃N₄ filaments and clusters as an origin of some of the shunts.

Abstract: Lifetime distribution of a multicrystalline silicon ingot of 250 mm diameter and 100 mm height, grown by unidirectional solidification has been modeled. The model computes the combined effect of interstitial iron and dislocation distribution on minority carrier lifetime of the ingot based on Shockley Read Hall (SRH) recombination model for iron point defects and Donolato’s model for recombination on dislocations. The iron distribution model was based on the solid state diffusion of iron from the crucible and coating to the ingot during its solidification and cooling, taking into account segregation of iron to the melt and back diffusion after the end of solidification. Dislocation density distribution is determined from experimental data obtained by PVScan analysis from a vertical cross section slice. Calculated lifetime is fitted to the measured one by fitting parameters relating the recombination strength and the local concentration of iron

Abstract: Silicon solar cell is well known as one of the cleanest and most potential renewable resources. As the major photovoltaic (PV) material in PV industry, multi-crystalline silicon (mc-Si) grown by directional solidification has recently attracted increasing attention because of its low production cost, low pollution and high throughput. Deeper understanding of the physic and optic properties, and preparation methods of the materials will lead to improved device design. This paper briefly presents basic directional solidification theory of multi-crystalline silicon, and reviews recent development of solar-grade multi-crystalline silicon. The directional solidification preparation techniques of high-quality solar-grade multi-crystalline silicon are detailed introduced and summarized. Furthermore, the existing problems and further development direction of directionally solidified multi-crystalline silicon for solar cell are discussed.

Abstract: Surface texturing of silicon can reduce the reflectance of incident light and hence increase the conversion efficiency of solar cells. Many approaches have been present in texturing silicon solar cell. As a practical method, chemical wet etching has been widely used in monocrystalline silicon and multicrystalline silicon (mc-Si), but the cost and high reflectance hamper its widely used. In this paper, a new approach is present by using ionized bubble to texture multicrystalline solar cell. The electric field and ultrasonic wave is introduced into traditional chemical wet etching. After mixed solution ionized by electrolysis and gasified by the ultrasonic wave, the ionized bubble is formed. The movement of ionized bubble is observed to study the impact on the surface of mc-Si solar cell. At last, a spectrophotometer was used to measure mc-Si surface reflectance to test the efficiency of two methods. The result express that the reflectance of mc-Si surface textured by ionized bubble is much low than that of chemical mixed acid etching.

Abstract: The Si3N4 protective coating has an important impact on avoiding melting silicon from contacting with the crucible wall directly. A mixed Si/Si3N4 layer was formed on the interface of silicon and Si3N4 coating, and the declination of N content was observed in this mixed layer. With the ingots condition of 1500oC for 2 h, the large Si3N4 and SiC particles appeared in the mixed layer and the formation mechanism was discussed. The Si3N4 coating had significantly increased the lifetime of minority carriers by decreasing impurity content.

Abstract: The distribution of resistivity, impurity and polarity in multicrystalline silicon ingot prepared by directional solidification method was detected. The effect of impurity distribution on resistivity was also researched. The results show that the shapes of equivalence line of resistivity in the cross section and vertical section of the silicon ingot depend on the solid-liquid interface. The resistivity in the vertical section increases with the increasing of solidified height at the beginning of solidification and reaches to maximum at the polarity transition point, then decreases rapidly with the increasing of solidified height and tends to zero on the top of the ingot because of the high impurity concentration. Study proves that the variation of resistivity in the vertical section is mainly relevant to the concentration distribution of the impurities such as Al, B and P in the growth direction.

Abstract: The electromagnetic continuous pulling is a newly growth technology as a promising process for silicon preparation, start-up heating is necessary for the semiconductivity of silicon at room temperature. Investigations were carried out to study this process in a square crucible with the frequency of 50kHz, details of the experimental procedure were given, the affecting factors, electrical performance and heat explosion problems during the process were measured and discussed. The results indicated that the best conditions for the preheating were the central position of the base in the coil, the higher power and the proper primary mass of silicon setting. The electrical performance indirectly showed the pool conditions and so it can be effectively used to control the melting operation.

Abstract: An integrated cell and module technology based on metal-wrap-through (MWT) cells has been developed and demonstrated. 243 cm2 large and 160 µm thin multicrystalline silicon MWT cells were made with a best cell efficiency of 17.9%. From 36 cells with an average efficiency of 17.8% a full-size module was made with an efficiency of 17.0% (aperture area). The module was made using a conductive rear-side foil with conductive adhesive for the interconnection. The module was constructed using a dedicated module manufacturing line that is designed to be able to work with extremely thin cells and provide a high through-put of one 60 cell module per minute.

Abstract: We have characterized optical property of small-angle (SA) grain-boundaries (GBs) in high-pure multicrystalline Si by using cathodoluminescence (CL). Prior to CL measurement, the electrical activity of GBs were evaluated by using electron-beam-induced current (EBIC). The SA-GBs are categorized into two groups with room temperature (RT-) EBIC contrast. The SA-GBs with misorientation angle about 1º give weak RT-EBIC contrast and yield D3 and D4. The SA-GBs with 2.5º show strong EBIC contrast and yield D1 and D2. These correspondences reflect the dislocation density at the SA-GBs. We also found the curious distribution of D1 emission in some special GBs, which is now difficult to explain. It is noticed that large-angle GBs do not show any D-line emissions at all.