Removal of impurities from metallurgical grade silicon by electron beam melting

Solar cells are currently fabricated from a variety of silicon-based materials.Now the major silicon material for solar cells is the scrap of electronic grade silicon(EG-Si).But in the current market it is difficult to secure a steady supply of this material.Therefore,alternative production processes are needed to increase the feedstock.In this paper,EBM is used to purify silicon.MG-Si particles after leaching with an initial purity of 99.88%in mass as starting materials were used.The final purity of the silicon disk obtained after EBM was above 99.995%in mass.This result demonstrates that EBM can effectively remove impurities from silicon.This paper mainly studies the impurity distribution in the silicon disk after EBM.     

镍、金掺杂n型硅材料的NTC和电学特性

Silicon materials compensated by deep level impurities such as nickel and gold have negative temperature coefficient （NTC） characteristics. In this work, n-type silicon wafers are smeared by nickel chloride ethanol solution and gold chloric acid ethanol solution, and subsequently put in the opening environment to heat. The electrical resistance and B-value of the thermistors made by this silicon material are measured and analyzed. When the silicon surface concentration of gold atoms is 2 × 10-6 mol/cm2, the uniformity of the single-crystal silicon material is optimal. When the diffusion temperature is between 900 and 1000 ℃, a material with high B-value and low electrical resistivity is obtained. The B-T and R-T change laws calculated by the theory of semiconductor deep level energy are basically consistent with the experimental results.     

Low-dimensional materials for photovoltaic application

The photovoltaic(PV)market is currently dominated by silicon based solar cells.However technological diversification is essential to promote competition,which is the driving force for technological growth.Historically,the choice of PV materials has been limited to the three-dimensional(3D)compounds with a high crystal symmetry and direct band gap.However,to meet the strict demands for sustainable PV applications,material space has been expanded beyond 3D compounds.In this perspective we discuss the potential of low-dimensional materials(2D,1D)for application in PVs.We present unique features of low-dimensional materials in context of their suitability in the solar cells.The band gap,absorption,carrier dynamics,mobility,defects,surface states and growth kinetics are discussed and compared to 3D counterparts,providing a comprehensive view of prospects of low-dimensional materials.Structural dimensionality leads to a highly anisotropic carrier transport,complex defect chemistry and peculiar growth dynamics.By providing fundamental insights into these challenges we aim to deepen the understanding of low-dimensional materials and expand the scope of their application.Finally,we discuss the current research status and development trend of solar cell devices made of low-dimensional materials.     

Prediction model for the diffusion length in silicon-based solar cells

A novel approach to compute diffusion lengths in solar cells is presented. Thus, a simulation is done; it aims to give computational support to the general development of a neural networks （NNs）, which is a very powerful predictive modelling technique used to predict the diffusion length in mono-crystalline silicon solar cells. Furthermore, the computation of the diffusion length and the comparison with measurement data, using the infrared injection method, are presented and discussed.     

Behavior modeling on the PIM performance in connectors using FEA method and circuit simulation

Coaxial connectors are generally regarded as a kind of potential passive non-linear source when magnetic materials are applied in the coating or under-plating, which may result in serious passive intermodulation (PIM) interference and degrade the communication quality. In this paper, the effect of connector coating materials on the PIM is theoretically studied using finite element analysis (FEA) and circuit simulations. Considering the material composition both in central and outer conductor, an FEA model of connector is proposed to identify the current density in magnetic material region. An equivalent circuit model expressing the nonlinearity in coating material is developed, coupled with the non-linear transfer model. The PIM product power of the connector with related material configuration is predicted by harmonic balance simulation. Intentionally design connector samples are used in PIM tests and the measurement results are consistent with the theoretical predictions. The PIM performance in coaxial connectors is demonstrated from the perspectives of both modeling analysis and experimental investigations.     

AN ALGORITHM OF K-LINE LOCATION FOR BBL IN LSI/VLSI

In this paper,a K-line location algorithm for building block cells in LSI/VLSI ispresented.When the relative positions of rectangular cells are given,there are 2 states accordingto the two orientations of a cell.It is proved that to find the optimum solution from the 2~N statescan be reduced to calculate the N states in K-line algorithm.So the algorithm is shown veryeffective and can be used with association for cluster method in BBL placement.Under certainconditions,this method can also be used to pesudo BBL placement directly.     

Studies on Dielectric Properties of Silicon Nitride at High Temperature

In this paper, the dielectric properties of silicon nitride are studied using the dielectric polarization theories. According to the developed dielectric models, the temperature dependence of dielectric constant and loss of silicon nitride is mainly analyzed. In addition, the impact of Li^＋, K^＋, Ca^2＋, Al^3＋ and Mg^2＋ doping on the dielectric properties of silicon nitride are also estimated.     

Preface to the Special Issue on the Celebration of the 60th Anniversary of Dedicating to Scientific Research of Prof.Zhanguo Wang

Prof.Zhanguo Wang,a world-famous semiconductor materials physicist,was born on December 29,1938,in Zhenping County,Henan Province,China.After graduating from the Department of Physics,Nankai University in 1962,he joined the Institute of Semiconductors,Chinese Academy of Sciences,until now.Prof.Wang has made outstanding achievements in the field of semiconductor materials and material physics.He has engaged in the study of the irradiation effect of silicon solar cells used in artificial satellites and the devices/modules in nuclear transient irradiation in his early career,which significantly contributed to the realization of atomic/hydrogen bombs and artificial satellites in China.Prof.Wang joined the Department of Solid State Physics,the University of Lund,from 1980 to 1983,where he worked on deep energy level physics and photoluminescence studies of semiconductors.He and collaborators developed a new method to identify whether the two-deep levels within a bandgap are coupled,thus solving the longexisting argument for the nature of gold-related donors and acceptors in silicon and A and B deep levels in liquid phase epitaxy grown GaAs.     

Feasibility of BCB as an interlevel dielectric in integrated circuits

This paper investigates the feasibility of using an organic polymer based on benzocyclobutene as an interlevel dielectric material in very large scale integrated (VLSI) circuits. The material is a thermoset resin with attractive electrical and mechanical properties for application as an interlevel dielectric in VLSI circuits. It has a low relative dielectric constant of 2.7. The single coating planarization achieved by spin coating the material is superior to currently used materials and makes it a very attractive material for the fabrication of multilevel metal systems. The planarization properties of this material are presented and compared with those of polyimide. The patterning and dry etching of BCB to define 1 μm vias is described. As the material has limited thermal stability at temperatures greater than 350°C, compatible materials for low via resistivity have been investigated using a double level metal structure. The effect of post metal anneals on via resistivity of various via structures is presented. It is found that a low via resistivity of 3 × 10^-9 gW-cm² without any post metal anneal is obtained by using an AlCu/Pd-AlCu metallurgy.     

背表面形貌对硅太阳电池效率的影响

Different processes are used on the back surface of silicon wafers to form cells falling into three groups：textured, planar, and sawed-off pyramid back surface.The characteristic parameters of the cells, ISC, VOC, FF, Pm, and Eff, are measured.All these parameters of the planar back surface cells are the best.The FF, Pm, and Eff of sawed-off pyramid back surface cells are superior to textured back surface cells, although ISC and VOC are lower.The parasitic resistance is analyzed to explain the higher FF of the sawed-off pyramid back surface cells.The cross-section scanning electron microscopy（SEM） pictures show the uniformity of the aluminum-silicon alloy, which has an important effect on the back surface recombination velocity and the ohmic contact.The measured value of the aluminum back surface field thickness in the SEM picture is in good agreement with the theoretical value deduced from the Al-Si phase diagram.It is shown in an external quantum efficiency（EQE） diagram that the planar back surface has the best response to a wavelength between 440 and 1000 nm and the sawed-off back surface has a better long wavelength response.     

废弃多晶硅太阳电池回收高纯硅片工艺研究

论述分析了国内外晶体硅太阳电池回收技术现状,研究了太阳电池的结构及制备工艺,提出了废弃多晶硅太阳电池回收高纯硅片的工艺.依次去除铝背场/铝硅合金层/背银、氮化硅减反膜/正银、磷扩散层及金属杂质,得到高纯硅片.硅原料的回收率高达76.4％,回收的高纯硅片经检验检测,其电阻率、间隙氧浓度、代位碳含量和少子寿命均符合GB/T 29055-2012中规定的性能参数.该回收工艺路线简单,回收率高,成本低,适于产业化推广.废弃太阳电池的回收再利用不仅可以在一定程度上缓解硅原料短缺的问题,还可以减少废弃的太阳电池给环境造成负担.     

High‐efficiency solar cells on phosphorus gettered multicrystalline silicon substrates

Measurements of the dislocation density are compared with locally resolved measurements of carrier lifetime for p‐type multicrystalline silicon. A correlation between dislocation density and carrier recombination was found: high carrier lifetimes (>100 µs) were only measured in areas with low dislocation density (<10⁵ cm⁻²), in areas of high dislocation density (>10⁶ cm⁻²) relatively low lifetimes (<20 µs) were observed. In order to remove mobile impurities from the silicon, a phosphorus diffusion gettering process was applied. An increase of the carrier lifetime by about a factor of three was observed in lowly dislocated regions whereas in highly dislocated areas no gettering efficiency was observed. To test the effectiveness of the gettering in a solar cell manufacturing process, five different multicrystalline silicon materials from four manufacturers were phosphorus gettered. Base resistivity varied between 0·5 and 5 Ω cm for the boron‐ and gallium‐doped p‐type wafers which were used in this study. The high‐efficiency solar cell structure, which has led to the highest conversion efficiencies of multicrystalline silicon solar cells to date, was used to fabricate numerous solar cells with aperture areas of 1 and 4 cm². Efficiencies in the 20% range were achieved for all materials with an average value of 18%. Best efficiencies for 1 cm² (20·3%) and 4 cm² (19·8%) cells were achieved on 0·6 and 1·5 Ω cm, respectively. This proves that multicrystalline silicon of very different material specification can yield very high efficiencies if an appropriate cell process is applied. Copyright © 2006 John Wiley & Sons, Ltd.     

Investigation of bulk and solar cell properties of ingots cast from compensated solar grade silicon

In this work, the effect of the concurrent presence of B and P on bulk and solar cell properties of directionally solidified multicrystalline ingots from commercially compensated solar grade silicon (SoG‐Si) feedstock produced by Elkem Solar was investigated. The initial B and P content prior to the directional solidification experiment was 1260 and 762 ppba, respectively. Two reference ingots have been solidified in a silica crucible from 100% electronic grade silicon (EG‐Si) feedstock, with 332 ppba of boron added. All ingots have been cast under similar process parameters. The resistivity measurements by Four Point Probe (FPP) are in good agreement with the net dopant content, i.e., N_A − N_D for p‐type material, measured by Glow Discharge Mass Spectrometer (GDMS). Bulk lifetime measurements show a decrease in the values compared to the EG reference. Lifetime distributions show the highest values of 13 and 19 µs at approximately half ingot height, compared to 30 and 44 µs in the reference ingots. This decrease can be due to the concurrent effect of compensation and of other impurities present in the ingot. However, the content of several transition metals measured by GDMS at half ingot height was not significantly higher than that of the reference ingots. Oxygen content as measured by Fourier Transform Infra‐Red (FTIR) spectroscopy shows no significant difference compared to the references. Solar cells made from the compensated ingots and processed under standard process conditions show efficiency values up to 15.5% and fill factor values up to 78%, comparable to conventional multicrystalline silicon cells. Copyright © 2010 John Wiley & Sons, Ltd.     

Ohmic contacts on high purity P-type silicon

A new method of forming electrical contacts on high purity p-type silicon has been developed. The resultant contacts are ohmic from 18 to 300K. The electrical sparking technique permits localized doping in the contact region of the device while avoiding the over-all high annealing temperatures required by diffusion. Indium solder is used for metallization and attachment of wires to the sparked regions. The process is simple and easily applied to samples for measuring electrical transport properties or to devices, such as silicon solar cells.     

超快激光微构造硅的研究与应用

在特定的气体氛围下,用一定能量密度的超短脉冲激光连续照射单晶硅片表面,或者离子注入在硅中引入硫族元素等方法,可在硅表面得到具有奇特光电性质的微米量级尖锥结构,该微锥结构被称为黑硅。这一新材料有奇特的光电性质,如对0.25～17μm波长的光有强烈的吸收,具有良好的场致发射特性等,为硅提供许多新的功能。Mazur教授预言这种新材料相当于60年前的半导体,在探测器、传感器、显示技术及微电子等领域都有重要的潜在应用价值,尤其在高效太阳能电池领域具有其他材料无可比拟的优越性。本文介绍了超快激光微构造硅的形成机理,研究进展、光电特性以及应用前景。     

Impact of phosphorus gettering parameters and initial iron level on silicon solar cell properties

We have studied experimentally the effect of different initial iron contamination levels on the electrical device properties of p‐type Czochralski‐silicon solar cells. By systematically varying phosphorus diffusion gettering (PDG) parameters, we demonstrate a strong correlation between the open‐circuit voltage (V_oc) and the gettering efficiency. Similar correlation is also obtained for the short‐circuit current (J_sc), but phosphorus dependency somewhat complicates the interpretation: the higher the phosphorus content not only the better the gettering efficiency but also the stronger the emitter recombination. With initial bulk iron concentration as high as 2 × 10¹⁴ cm⁻³, conversion efficiencies comparable with non‐contaminated cells were obtained, which demonstrates the enormous potential of PDG. The results also clearly reveal the importance of well‐designed PDG: to achieve best results, the gettering parameters used for high purity silicon should be chosen differently as compared with for a material with high impurity content. Finally we discuss the possibility of achieving efficient gettering without deteriorating the emitter performance by combining a selective emitter with a PDG treatment. Copyright © 2012 John Wiley & Sons, Ltd.     

Improved micromorph solar cells by means of mixed‐phase n‐doped silicon oxide layers

A good light trapping scheme is necessary to improve the performance of amorphous/microcrystalline silicon tandem cells. This is generally achieved by using a highly reflective transparent conducting oxide/metal back contact plus an intermediate reflector between the component cells. In this work, the use of doped silicon oxide as alternative n‐layer in micromorph solar cells is proposed as a means to obtain high current values using a simple Ag back contact and no extra reflector between the component cells n‐doped silicon oxide layers with a wide range of optical and electrical properties have been prepared. The influence of different deposition regimes on the material properties has been studied. The main findings are the following: (i) when carbon dioxide is added to the gas mixture, sufficiently high hydrogen dilution is necessary to widen the transition region from highly conductive microcrystalline‐like films to amorphous material characterized by low electrical conductivity; (ii) lower refractive index values are found with lower deposition pressure. Optimal n‐doped silicon oxide layers have been used in both component cells of micromorph devices, adopting a simple Ag back contact. Higher current values for both cells are obtained in comparison with the values obtained using standard n‐doped microcrystalline silicon, whereas similar values of fill factor and open circuit voltage are measured. The current enhancement is particularly evident for the bottom cell, as revealed by the increased spectral response in the red/infrared region. The results prove the high potential of n‐doped silicon oxide as ideal reflector for thin‐film silicon solar cells. Copyright © 2011 John Wiley & Sons, Ltd.     

A New View of Microcrystalline Silicon: The Role of Plasma Processing in Achieving a Dense and Stable Absorber Material for Photovoltaic Applications

To further lower production costs and increase conversion efficiency of thin‐film silicon solar modules, challenges are the deposition of high‐quality microcrystalline silicon (μc‐Si:H) at an increased rate and on textured substrates that guarantee efficient light trapping. A qualitative model that explains how plasma processes act on the properties of μc‐Si:H and on the related solar cell performance is presented, evidencing the growth of two different material phases. The first phase, which gives signature for bulk defect density, can be obtained at high quality over a wide range of plasma process parameters and dominates cell performance on flat substrates. The second phase, which consists of nanoporous 2D regions, typically appears when the material is grown on substrates with inappropriate roughness, and alters or even dominates the electrical performance of the device. The formation of this second material phase is shown to be highly sensitive to deposition conditions and substrate geometry, especially at high deposition rates. This porous material phase is more prone to the incorporation of contaminants present in the plasma during film deposition and is reported to lead to solar cells with instabilities with respect to humidity exposure and post‐deposition oxidation. It is demonstrated how defective zones influence can be mitigated by the choice of suitable plasma processes and silicon sub‐oxide doped layers, for reaching high efficiency stable thin film silicon solar cells.     

Efficiency-limiting defects in silicon solar cell material

The precipitation rate of intentionally introduced iron during low-temperature heating is studied among a variety of single-crystal and polycrystalline silicon solar cell materials. A correlation exists between the iron precipitation rate and the carrier recombination rate in dislocation-free as-grown material, suggesting that diffusion-length-limiting defects in as-grown material are structural defects which accelerate iron precipitation. Phosphorous diffusion gettering was found to be particularly ineffective at improving diffusion length after intentional iron contamination in materials with high iron precipitation rates. We propose that intragranular structural defects in solar cell silicon greatly enhance transition metal precipitation during cooling from the melt and become highly recombination-active when decorated with these impurities. The defects then greatly impair diffusion length improvement during phosphorus gettering and limit carrier lifetimes in as-grown material.