金刚石线锯电解磨削切割多晶硅片的试验研究

基于金刚石线锯切割系统,开展了金刚线电解磨削切割多晶硅片试验。结果表明:电解磨削复合加工方法在机械磨削的同时复合了阳极氧化和腐蚀,在硅片表面产生了机械损伤缺陷和电化学腐蚀缺陷。酸制绒时腐蚀反应在两种类型缺陷处顺利进行,形成均匀致密的绒面结构,有效降低硅片表面反射率,有利于后续电池片光电转换效率的提高。     

金属铜辅助化学刻蚀对金刚线切割多晶硅的制绒

针对金刚线切割多晶硅制绒后硅片反射率偏高且切割纹难以去除等问题,采用酸性湿法刻蚀预处理再结合低成本的金属铜辅助化学刻蚀成功的实现了金刚线切割多晶硅片表面制绒。研究结果表明,随着酸腐蚀时间的增加,金刚线切割多晶硅片表面切割纹、粗糙度得到有效改善。倒金字塔结构的引入能够有效地降低硅片表面的反射率。当酸洗预处理时间为5 min,金属铜辅助化学刻蚀时间为15 min时,样品表面倒金字塔结构最均匀,且在300~1 100 nm波长范围内,获得最低平均反射率3.32%。同时优越的减反效果和去除切割纹能力,使得制绒后金刚线切割多晶硅片有望实现高效率的太阳能电池。     

Low-reflective surface texturing for large area multicrystalline silicon using NaOH-NaClO solution

Multicrystalline silicon surface texturing using the mixed etching solution of the sodium hydroxide (NaOH) and of the sodium hypochlorite (NaClO) has been investigated. The reaction rate during the texturing process is easier to control due to the presence of NaOCl as an oxidizing agent in NaOH solution. The advantage of this etching is that the uniform mc-Si surface texturing with a low step height and less grain boundary delineation can be obtained. The Mc-Si surface after NaOH-NaOCl mixed etching with the 1: 4 ratio in the case of 20% NaOH has the optimum light trapping effect. In the case of the optimum etching condition, the average reflectivity for the textured surface of a large area (156 × 156 mm²) mc-Si can be reduced to less than 10%.     

Abrasive electrochemical multi-wire slicing of solar silicon ingots into wafers

To meet the growing demands of the global photovoltaic (PV) industry, preparing large scale and ultra-thin solar wafers becomes one of the key issues. This paper presents the preparatory investigations of slicing solar silicon ingot into wafers by an abrasive electrochemical method based on a multi-wire saw system. The anodic passivation on silicon can be controlled by applying an anodic potential during the mechanical slicing process, which improves the surface integrity and material removal rate remarkably. This new hybrid machining method has no influence on subsequent cleaning of wafers and preparing the solar cells, and the average photoelectric transformation efficiency is >17.5%.     

Influence of texture feature size on spherical silicon solar cells

The effects of surface texturing on spherical silicon solar cells were investigated. Surface texturing for spherical Si solar cells was prepared by immersing p-type spherical Si crystals in KOH solution with stirring. Two kinds of texture feature sizes (1 and 5μm pyramids) were prepared by changing stirring speed. After fabrication through our baseline processes, these cells were evaluated by solar cell performance and external quantum efficiency. The cell with 1 and 5μm pyramids shows the short circuit current density ( Jsc ) value of 31.9 and 33.2 mA·cm-2 , which is 9% and 13% relative increase compared to the cell without texturing. Furthermore, the cell with 5 μm pyramids has a higher open-circuit voltage (0.589 V) than the cell with 1 μm pyramids (0.577 V). As a result, the conversion efficiency was improved from 11.4% for the cell without texturing to 12.1% for the cell with 5 μm pyramids.     

点杂多晶太阳能硅片电解磨削多线切割试验研究

提出了一种电解磨削多线切割点杂多晶太阳能硅片的新方法。电源的正极接硅锭,负极接切割线网,电解过程中硅锭发生微区钝化反应,形成硬度较小的钝化膜,使点杂多晶太阳能硅片更易切割。试验结果表明:该技术具有切割效率高、切片合格率高等优点。进一步检测发现,硅片宏观表面线痕浅、隐裂少,微观表面平整性好。该技术的应用降低了硅片加工的成本,提高了硅材料的利用率,也为太阳能电池的运用拓宽了空间。     

Experimental and simulation study for ultrathin (∼100 μm) mono crystalline silicon solar cell with 156×156 mm2 area

A reduction in silicon material consumption in the photovoltaic industry is required for cost reduction. Using crystalline silicon wafers of less than 120 microns of thickness is a promising way for cost and material reduction in the solar cell production. The standard thickness of crystalline silicon solar cells is currently around 180 microns. If the wafers are thinner than 100 microns in the silicon solar cells, the amount of silicon will be reduced by almost half, which should result in prominent cost reduction. With this aim, many groups have worked with thin crystalline silicon wafers. However, most of them have studied with small size substrates. In this paper, we present the electrical characteristics for thin single crystalline silicon solar cells of 100 and 115 μm thickness and 156×156 mm² area manufactured through a conventional process. We have achieved 17.2% conversion efficiency with a 115 μm silicon substrate and 16.8% with a 100 μm substrate. This enables the commercialization of the thin crystalline silicon solar cells with high conversion efficiency. We also suggest issues to be solved in thin crystalline silicon solar cell manufacturing.     

Effects of rapid thermal process on the junction properties of aluminum rear emitter solar cells

N-type silicon with aluminum emitters for rear junctions was studied; aluminum back surface fields were replaced with n-type silicon wafers. Aluminum rear emitters for n-type silicon solar cells were studied with various rapid thermal processing conditions. With fast ramping-up and fast cooling, an aluminum rear junction was formed uniformly with low emitter recombination current. The effects of junction quality on solar cell efficiency were investigated.     

Laser induced quasi-periodical microstructures with external field modulation for efficiency gain in photovoltaics

The overall reflectivity of silicon is decreased by 10% altering the surface topology by ultra-short pulsed laser ablation and resulting in an efficiency increase of solar cells. The size of quasi-periodical μm-structures on the surface can be defined by the applied laser parameters. The topology is additionally adapted in size and distance of the microstructures at constant laser parameters with a specifically applied external electrical field leading to a cone-like microstructure with an adjustable light-trapping geometry. On large scale multicrystalline silicon solar wafers with a laser generated μm-scale surface topology were processed into cells with an absolute efficiency gain.     

基于时频域混合分析的太阳能硅片缺陷检测方法

针对太阳能硅片的水印缺陷,提出一种基于时频域混合分析的检测方法。在此基础上,设计太阳能硅片表面缺陷检测硬件系统。对采集的硅片图像,首先在时域进行图像预处理,然后利用实数快速傅里叶变换得到频率谱图像;在频域里,设计高斯滤波器滤波,再通过傅里叶逆变换获得重构图像,转换到时域,进行差分得到差分图像;最后通过阈值化和形态学操作获得缺陷区域。实验结果表明:此方法对包含水印缺陷的太阳能硅片样本进行缺陷检测的准确率高,具有较高的实用价值。     

Applications of porous silicon thin films in solar cells and biosensors

An overview of the applications of porous silicon (PS) thin films, as antireflection coatings (ARC) in silicon solar cells and transducers in biosensors, is presented. The reflectance spectra of PS films have been compared with other conventional ARCs (such as SiN_x TiO₂/MgF₂ and ZnS), and optimal PS ARC with minimum reflectance has been obtained. The implementation of PS into an industrially compatible screen-printed (SP) solar cell by both the electrochemical etching (ECE) and chemical etching (CE) methods are reviewed. Porous silicon films, formed via ECE for short anodization times, on textured n⁺ emitter ofc-Si solar cell having SP front and back contacts, lead to improvements in the performance of solar cells and demonstrate their viability in industrial applications.     

冶金法制备太阳能级硅的原理及研究进展

随着光伏市场需求不断增加,满足光伏电池技术经济指标要求的硅材料出现严重短缺,低成本提纯冶金硅至太阳能级硅工艺技术越来越受到广泛重视,成为研究开发热点.本文分析了全球光伏产业的发展现状和趋势,对目前获得太阳能级多晶硅的化学路径和冶金路径进行了对比分析;重点介绍了冶金法制备太阳能级硅的工艺原理,以及目前常用的提纯技术;同时,简单介绍低成本生产太阳能级多晶硅的新工艺,并指出了冶金法可能是今后提纯多晶硅的主要研究方向.     

Preparation of pyramid-SiNWs binary structure with Ag nanoparticles-assisted chemical etching

It is an important way to improve the efficiency of solar cells by using the special microstructures of surface. In this work, a pyramid-silicon nanowires(pyramidSiNWs) binary structure was prepared on the silicon surfaces with the metal-assisted chemical etching(MACE)method. Scanning electron microscope(SEM) was used to observe the micromorphology of the pyramid-SiNWs binary structure. The formation mechanism of the binary structure was discussed. The role of Ag nanoparticles in MACE is considered to be the template and the catalyzer.The optical reflectivity of the silicon surfaces was studied with ultraviolet-visible(UV-Vis) spectrophotometer.Compared with the flat silicon surface and the simple pyramidal structure, the silicon surfaces with the pyramidSiNWs binary structure achieve a much lower reflectance in a wide range of wavelength. The effect of etching time as a parameter on the reflectivity was also discussed.     

直流辉光氧等离子体刻蚀金刚石膜的研究

在直流辉光放电等离子体装置上，利用不同直流功充和工作气压下产生的氧等离子体对CVD金刚石厚膜的表面进行了刻蚀。利用扫描电子显微镜、Raman光谱和电子微量分析天平，分别对刻蚀前后金刚石膜表面的形貌、结构和刻蚀速率进行了观测。结果发现：在工作气压一定时，刻蚀速率随着直流功率的增加而增大，并且刻蚀由各向同性转变为各向异性。但过高的直流功率会导致金刚石膜表面沉积出无定形碳。基于实验研究结果和相关基本理论建立了刻蚀模型，并根据模型得到了影响刻蚀的主要原因在于等离子体中的电子温度和金刚石膜的悬浮鞘电位。     

EDM texturing of multicrystalline silicon wafer and EFG ribbon for solar cell application

This paper presents a novel electrical discharge machining (EDM) texturing method for roughening mc-Si wafers and EFG ribbons for solar cell application. Experiments were carried out on an EDM die-sinker using a specially designed conductive and soft magnetic brush to texture the workpiece. The textured substrates were investigated and analysed using scanning electron microscope, and solar cells were made on textured samples to evaluate the effect of this method. Preliminary experimental results show that the throughput of this method can be over 1000 mm² minute with a brush of 100 mm diameter. Solar cells made on textured substrates give reasonable output.     

High efficiency slicing of low resistance silicon ingot by wire electrolytic-spark hybrid machining

As the semiconductor industry requires the cutting of silicon ingots into wafers, the slicing of large, ultra thin wafers is one of the main technologies to prevent wastage. Recently, apart from conventional inner diameter (ID) blade and multi-wire saw methods, wire electrical discharge machining (WEDM), which has no cutting force, has been introduced to this area and low resistance silicon may be sliced by WEDM. In this paper, a novel approach, based on wire electrolytic-spark slicing strategy using hybrid oil/aqueous electrolyte, combining electric discharge and anodic etching into a single process, is investigated experimentally. Some improvements, such as a new wire winding system, hybrid electrolyte and high efficiency pulse generator, have been adopted in a kind of high speed (HS)-WEDM machine. Experiments have been conducted to evaluate the machining rate, surface quality and wafer thickness of low resistance (0.5–3 Ω cm) mono-crystalline silicon. It has been demonstrated that with properly selected electrical parameters and electrolyte, a maximum machining rate of 600 mm²/min can be obtained and with a wafer thickness less than 120 μm. Furthermore, in comparison with WEDM, heat affected zone and harmful metal residues are considerably diminished, which provides significant theoretical and experimental support for future applications.     

电镀金刚石线锯切割光伏多晶硅的表面特性与锯丝磨损分析

电镀金刚石线锯切割的光伏多晶硅切片表面特性,影响其断裂强度和后续的制绒工艺;为探究线锯锯切工艺参数对多晶硅切片表面特性的影响规律,揭示电镀金刚石锯丝的磨损机理,开展了光伏多晶硅的电镀金刚石线锯切片试验。研究结果表明:锯切的多晶硅表面存在由金刚石磨粒的塑性剪切、微切削去除形成的塑性浅划痕与较深的沟槽,及材料脆性去除留下的表面破碎微凹坑;切片表面材料的塑性去除和脆性去除相对比例随工艺参数组合变化而变化,增大晶片进给速度,降低走丝速度,切片表面粗糙度增大,表面形貌逐渐由塑性沟槽为主转变为以破碎微凹坑为主;使用表面镀镍(金属化)的金刚石颗粒制备的电镀金刚石锯丝的磨损形态在稳定阶段主要是磨粒磨平,使用后期主要是磨粒脱落和镀层磨损。      

Effect of porous silicon layer on the performance of Si/oxide photovoltaic and photoelectrochemical cells

Photovoltaic and photoelectrochemical systems were prepared by the formation of a thin porous film on silicon. The porous silicon layer was formed on the top of a clean oxide free silicon wafer surface by anodic etching in HF/H₂O/C₂H₅OH mixture (2:1:1). The silicon was then covered by an oxide film (tin oxide, ITO or titanium oxide). The oxide films were prepared by the spray/pyrolysis technique which enables doping of the oxide film by different atoms like In, Ru or Sb during the spray process. Doping of SnO₂ or TiO₂ films with Ru atoms improves the surface characteristics of the oxide film which improves the solar conversion efficiency.The prepared solar cells are stable against environmental attack due to the presence of the stable oxide film. It gives relatively high short circuit currents (I_sc), due to the presence of the porous silicon layer, which leads to the recorded high conversion efficiency. Although the open-circuit potential (V_oc) and fill factor (FF) were not affected by the thickness of the porous silicon film, the short circuit current was found to be sensitive to this thickness. An optimum thickness of the porous film and also the oxide layer is required to optimize the solar cell efficiency. The results represent a promising system for the application of porous silicon layers in solar energy converters. The use of porous silicon instead of silicon single crystals in solar cell fabrication and the optimization of the solar conversion efficiency will lead to the reduction of the cost as an important factor and also the increase of the solar cell efficiency making use of the large area of the porous structures.     

Fine grinding of silicon wafers

Silicon wafers are used for the production of most microchips. Various processes are needed to transfer a silicon crystal ingot into wafers. As one of such processes, surface grinding of silicon wafers has attracted attention among various investigators and a limited number of articles can be found in the literature. However, no published articles are available regarding fine grinding of silicon wafers. In this paper, the uniqueness and the special requirements of the silicon wafer fine grinding process are introduced first. Then some experimental results on the fine grinding of silicon wafers are presented and discussed. Tests on different grinding wheels demonstrate the importance of choosing the correct wheel and an illustration of the proper selection of process parameters is included. Also discussed are the effects of the nozzle position and the flow rate of the grinding coolant.     

Fabrication of concave micro lens array using laser patterning and isotropic etching

Micro lens arrays are widely used in optical communication and laser-fiber coupling applications. In this paper, a technique to fabricate concave micro lens arrays on glass substrate using a third harmonic Nd:YAG laser direct patterning and followed by chemical wet isotropic etching is presented. The patterning process was done on gold film, which was coated on a glass substrate by using a NC controlled laser ablation tool paths. The glass substrate is then etched by using hydrofluoric acid (HF) solutions whereby the exposed area will be dissolved away by chemical reaction with HF. The type of etching process is an isotropic etching which the etching rate is equal at all direction thus produce hemispherical concave profile on glass. The optimum laser patterning parameters is obtained and the effect of different types of HF solutions on etching efficiency is studied. The surface morphology, 2D and 3D profiles are also measured. Various micro lens diameters are fabricated with different values of lens sag.