Multicrystalline Wafers Research Articles

Novel imaging techniques for dislocation‐related D1‐photo‐luminescence of multicrystalline Si wafers – two different approaches

AbstractHere we report on two different approaches, which allow rapid photoluminescence imaging of dislocation‐related D1, and band‐to‐band radiation on multicrystalline solar cell wafers at room temperature. We compared a new light emitting diode based method with the recently introduced pulsed laser based method. With both techniques we obtained spatially resolved images from 15.6 × 15.6 cm2 wafers originating from different stages of the standard solar cell production process, showing that these techniques are highly interesting for inline quality control. Both methods provide homogeneous illumination over the whole sample area. With a resolution around 300 μm, and a total recording time of ∼45 s, full D1 images could be captured by the new diode technique. The most promising fact was that surface artifacts did not lead to any noticeable disturbance in the D1 image. For acid textured wafers, the pulsed laser based method was clearly in advantage over the continuous diode technique. Possible explanations are discussed in the text. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Mean Shift-Based Defect Detection in Multicrystalline Solar Wafer Surfaces

This paper presents an automated visual inspection scheme for multicrystalline solar wafers using the mean-shift technique. The surface quality of a solar wafer critically determines the conversion efficiency of the solar cell. A multicrystalline solar wafer contains random grain structures and results in a heterogeneous texture in the sensed image, which makes the defect detection task extremely difficult. Mean-shift technique that moves each data point to the mode of the data based on a kernel density estimator is applied for detecting subtle defects in a complicated background. Since the grain edges enclosed in a small spatial window in the solar wafer show more consistent edge directions and a defect region presents a high variation of edge directions, the entropy of gradient directions in a small neighborhood window is initially calculated to convert the gray-level image into an entropy image. The mean-shift smoothing procedure is then performed on the entropy image to remove noise and defect-free grain edges. The preserved edge points in the filtered image can then be easily identified as defective ones by a simple adaptive threshold. Experimental results have shown the proposed method performs effectively for detecting fingerprint and contamination defects in solar wafer surfaces.

Rapid dislocation‐related D1‐photoluminescence imaging of multicrystalline Si wafers at room temperature

AbstractHere we report on a novel method, which allows rapid photoluminescence imaging of band‐to‐band and dislocation‐related radiation, D1, on multicrystalline silicon wafers at room temperature. We demonstrate spatially resolved 5.0 × 5.0 cm2 D1‐images, with a resolution of ∼120 µm, within a total recording time of 550 ms. The method provides homogeneous illumination over the whole sample area. Comparison with results from a conventional photoluminescence mapping technique demonstrates the potential of this new method for application as an in‐line wafer characterization technique.

Using Design of Experiments Approach to Optimize Custom Emitter Clean Process used in PV Manufacturing

Abstract We describe a ‘Design of Experiment’ (DOE) technique used to optimize the process parameters of a new commercially available emitter clean solution aimed to enhance the output efficiency of the cell. Emitter etch solutions are known to substantially improve efficiency by reducing surface recombination of charge carriers and increasing the short circuit current of the cell. We have used a full factorial design of experiment with three variables kept at two levels. Appropriate randomization of the run order and three centre points were chosen in order to reduce experimental errors. Adjacent multicrystalline wafers chosen for this experiment were subjected to acid texturization, phosphorus doping, diffusion and phosphosilicate glass etching before the emitter clean step. We chose mean emitter sheet resistance (Rsheet) (average over 25 positions on each wafer) as the response of this experiment. The aim was to achieve an increment of 10% on Rsheet after emitter clean. Voc scan of the wafers were done to validate the Rsheet increments observed. The results showed that H2O2 concentration and temperature had statistically significant effects on change of Rsheet. Regression analysis was used to predict the optimized parameters that may achieve a 10% increase of Rsheet. A validation run was carried out with this recipe in an automated production line and solar cells fabricated from this run showed a 0.18% (absolute) increase in efficiency and 15% (relative) reduction of recombination current density (Jo2) compared to standard reference cells processed without emitter clean.

Latest Trends in Development and Manufacturing of Industrial, Crystalline Silicon Solar-Cells

Abstract Reliability, performance and cost are the key parameters for all PV products. The technology dominating the PV market until today with a market share of ∼80% is the very robust and versatile double-side contacted silicon solar-cell. During previous years the development and manufacturing of this standard cell architecture has been very successful with respect to the continuous reduction of unit-cost and improvement in efficiency, maintaining its dominance in the PV arena despite a huge number of competing thick- and thin-film concepts and technologies. In this paper we present an overview about the ongoing development and manufacturing activities at Q-Cells, which show, that also for the next years to come, the continuous improvement of the standard cell architecture still has an enormous potential with respect to cost and efficiency and hence to €/Wp and finally €/kWh reduction. The main levers are the optimization of optical and electrical front side properties and the successful transfer of low cost backside passivation and metallization concepts from the R&D lab into pilot- and finally high volume production. We applied these well known concepts to our 6” p-type standard monoand multi-crystalline wafers including umg-Si. The results, achieved in our R&D Pilot Production facility at Thalheim, show stable median efficiencies exceeding 19% for our mono- and 18% for our multi-crystalline solar cells including umg-Si. These cells feature a lowly doped emitter, a fineline-printed Ag grid and a dielectric passivated rear with point contacts. Narrow efficiency distributions are achieved with help of laser marked single wafer tracking, enabling sophisticated equipment and process control. By applying advanced module manufacturing technologies to these high-efficient multi-crystalline cells we achieve module power output of > 260 Wp for a standard 60 cell module.

Spatially resolved carrier lifetime calibrated via quasi-steadystate photoluminescence

Abstract The estimation of solar cell efficiency from minority carrier lifetime measurements requires precise and robust lifetime techniques. For multicrystalline wafers and solar cells this brings about the necessity of adequately averaged spatially resolved lifetime measurements. Materials such as e.g. multicrystalline upgraded metallurgical grade silicon frequently feature relatively low lifetimes, high trap densities, and several material parameters (charge carrier mobilities and net dopant concentration) that are not straightforwardly predictable or measurable. As this may substantially compromise conventional lifetime measurements, a lifetime technique which is unaffected by such restrictions is of general interest. We present a lifetime imaging technique based on a calibration of photoluminescence images via quasi-steady-state photoluminescence (QSSPL), which requires no a priori information about material parameters such as carrier mobilities and net dopant concentration. It is virtually unaffected by effects related to reabsorption of luminescence, by optical effects related to a sample's surface morphology, and by trapping. Injection dependence of lifetime is properly taken into account. We further accomplished a substantial upgrade of the hitherto existing sensitivity limit of QSSPL in terms of lifetime, to where average lifetimes down to one microsecond are now reliably measurable – yielding a sensitivity of spatially resolved lifetime well below one microsecond.

Imaging of chromium point defects in p-type silicon

In this work a method for the spatially resolved detection of the Cr point defect distribution in p-type silicon based on photoluminescence imaging was developed and successfully applied on monocrystalline and multicrystalline wafers. Chromium was identified by its metastable defect configuration and the association time during the formation of chromium–boron pairs. Further defects which could influence the Cr concentration determination have been studied and their influence was suppressed. For a quantitative evaluation of the chromium concentration, the knowledge of exact values of defect parameters such as energy level and capture cross sections for electrons and holes for both Cr states is essential. From the doping dependence of the lifetime, published defect parameters have been analyzed and adjusted slightly in order to explain experimental results. With these parameters reasonable averaged Cr point defect concentrations have been determined for different monocrystalline and multicrystalline samples. Furthermore, space-resolved images of the Cr point defect concentration could be acquired, permitting study of the distribution within a multicrystalline sample with respect to grains and grain boundaries or dislocation clusters.

Formation of thin-film crystalline silicon on glass observed by in-situ XRD

Thin-film poly-crystalline silicon (poly c-Si) on glass obtained by crystallization of an amorphous silicon (a-Si) film is a promising material for low cost, high efficiency solar cells. Our approach to obtain this material is to crystallize a-Si films on glass by solid phase crystallization (SPC). As the grain size of SPC poly c-Si films will be smaller than that of multi-crystalline wafers, lower solar cell efficiencies are expected for this technology. Despite the smaller grain size, a 2-micron-thick polycrystalline silicon solar cell with light trapping was shown to have a conversion efficiency of more than 10% [1]. Obtainable efficiencies up to 15% are expected for solar cells made using SPC of a-Si:H films. Expanding thermal plasma chemical vapor deposition (ETP-CVD) was used to prepare hydrogenated a-Si films; this technique is chosen because the deposition rates are much higher than with plasma enhanced CVD. A-Si:H films with different hydrogen contents were annealed using temperatures ranging from 500 °C to 700 °C. The evaluation of the films after annealing treatments revealed that the hydrogen content and bonding configuration did not influence the structural properties of the crystallized films significantly. The average crystallite size in the fully crystallized films was between 100 and 150 nm. Full crystallization of 1 micrometer thick films was achieved within 20 minutes for annealing at 625 °C and 650 °C. During annealing at 600 °C crystallization is much slower, and no crystallization is observed at 500 °C. The relation between the annealing temperature and the rate with which the films are fully crystallized is of great importance to develop a solar cell technology, to limit the thermal budget and processing time.

LBIC and Reflectance Mapping of Multicrystalline Si Solar Cells

Multicrystalline silicon (mc-Si) is increasingly used in the photovoltaic industry. However, this material is characterized by intrinsic structural heterogeneities (dislocations, grain boundaries, etc.), which are detrimental to the performance of the cells. The minority-carrier diffusion length is sensitive to these defects, and gives an indication of the material quality and its suitability for solar cell use. The laser beam induced current (LBIC) technique makes it possible to estimate the local minority-carrier diffusion length from photocurrent contrast data. The purpose of this work is to show an advanced homemade LBIC system that highlights the importance of controlling the laser power excitation and the reflected light in inhomogeneous mc-Si samples. This control demonstrates that the estimated minority-carrier diffusion length (LDiff) in texturized multicrystalline wafers strongly depends on the collecting conditions of the reflected light.

New modes of FFT impedance spectroscopy applied to semiconductor pore etching and materials characterization

AbstractA systematic approach for the application of fast Fourier transform (FFT) impedance spectroscopy to semiconductor photo‐electrochemistry is given. In particular, photo‐impedance spectroscopy in two novel modes is used in conjunction with conventional current–voltage impedance spectroscopy, and applied to some aspects of pore etching in InP and Si as well as to the characterization of multicrystalline Si or other solar cell material. The technique, applicable in situ with high time resolution, employs optimized hardware and software, extensive mathematical modeling of carrier transport, and the numerical implementation of fast parameter extraction algorithms. The complete system with a fully integrated impedance spectrometer delivers a wealth of new data including key parameters of pore etching. Examples given include the in situ tracking of the fast switch‐over between pore growth modes in InP as well as following a pore etching process over long time scales in Si. FFT impedance spectroscopy in the novel back side illumination mode, in combination with the conventional mode, allows not only some active control of macropore etching in n‐type Si by supplying crucial real‐time data like the valence of the dissolution process and the pore depth, but also provides new fundamental insights into the electrochemical processes occurring at the (porous) electrode. FFT photo‐impedance spectroscopy in the front side illumination mode used in conjunction with a scanned laser beam allows fast local measurements of all semiconductor parameters relevant for solar cell applications, like doping concentration, minority carrier lifetime, diffusion constant, or surface recombination velocity with high spatial resolution; this is demonstrated for large multicrystalline Si wafers. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultra high-speed characterization of multicrystalline Si wafers by photoluminescence imaging with HF immersion

Ultra high-speed characterization of crystalline quality in non-passivated multicrystalline Si wafers and ingots was demonstrated by photoluminescence (PL) imaging with the samples immersed in a HF solution. We confirmed that the present technique has about ten times higher spatial resolution and ten thousand times faster rapidity than those of conventional electrical characterization methods, and that strong correlations of the results between the present and conventional techniques were obtained. These results show clearly that the present technique is a powerful tool for characterizing the accurate bulk property of non-passivated wafers and ingots.

SIMS Study of C, O and N Impurity Contamination for Multi-Crystalline Si Solar Cells

AbstractThis paper is a case study of using SIMS to quantitatively measure C, O and N impurity contamination at two sequential commercial process steps: (1) Si feedstock: 7N (modified Siemens) and 5N feedstock (UMG-Si); and (2) multi-crystalline (mc-Si) solar wafers: cut and etched, from directional solidification bricks grown from 7N and 7N/5N (80:20) feedstock. The conclusion of this study is twofold: (a) the primary opportunity to reduce C, O and N contamination in mc-Si solar cells is at the directional solidification process, and (b) the costly specification of highly pure Si feedstock is unnecessary from a C, O and N perspective if a directional solidification process is used.

Study of Gettering Mechanisms in Silicon: Competitive Gettering between Phosphorus Diffusion Gettering and Other Gettering Sites

The effectiveness of phosphorus diffusion gettering (PDG) and related segregation coefficients for different metal impurities were measured applying thermal treatments in the temperature range 800-950 °C for different times. We used multi-crystalline and mono-crystalline CZ p-type wafers with different boron concentrations and different levels of dislocations and bulk micro-defects (BMD). In all sample types, for Cu and Ni we found complete gettering in the temperature range investigated. In the case of Fe, the segregation coefficient increases with both increase in temperature and extension of time. The increase is qualitatively changing when going above 900 °C. At 950 °C the segregation coefficient increases faster at shorter diffusion time but at extended diffusion time it increases slower as compared to diffusion at 900 °C. At the same temperature and time of phosphorus diffusion the segregation coefficient is found to be independent of the metal impurity concentration in the range of 1012-1015 cm-3 investigated. We have shown that the presence of BMD and dislocations in bulk silicon does not impede the ability of PDG to completely remove Fe, Ni and Cu metal impurities from the bulk. Further analysis suggests that the PDG has the same gettering efficiency for mono-crystalline silicon and multi-crystalline silicon. We conclude that if any bulk precipitation of Fe, Ni and Cu impurities is present in multi-crystalline silicon it cannot seriously compete with PDG. However we found that increasing the boron concentration in the samples reduces the segregation coefficient of Fe, and this reduction is more severe at lower temperatures. Finally, by applying a post anneal ramp down from 900 °C to 700 °C after phosphorus diffusion, we found that the Fe segregation coefficient increases by a factor of 36 for lightly B doped samples, from 53 to 1919, leading to a significant reduction of Fe in the bulk after 2 hours ramp down anneal.

Impact of Iron and Molybdenum in Mono and Multicrystalline Float-Zone Silicon Solar Cells

This paper investigates the impact of iron (Fe) and molybdenum (Mo) when they are introduced in the feedstock for mono- and multicrystalline Float-Zone (FZ) silicon (Si) growth. Neutron Activation Analysis shows that the segregation coefficient is in agreement with literature values. Lifetime maps on monocrystalline wafers show a uniform lifetime which decreases with the increase of contamination levels. Multicrystalline wafers show low lifetime areas, corresponding to grain boundaries and highly dislocated areas, which are independent from the contamination levels. Intra grain areas have a higher lifetime which changes with the contamination levels. The solar cells show a reduced diffusion length in multicrystalline uncontaminated cells compare to the monocrystalline uncontaminated. In multicrystalline cells the lowest level of Fe introduced (1012 atm/cm3) has hardly any influence, whereas in the Mo-contaminated cells the impact is visible from the lowest level (1011 atm/cm3). In monocrystalline cells the diffusion length is reduced already at the lowest contamination level of Fe.

다결정 실리콘 태양전지의 표면 텍스쳐링 및 반사방지막의 영향

The effects of texturing and anti-reflection coating on the reflection properties of multi-crystalline silicon solar cell have been investigated. The chemical solutions of alkaline and acidic etching solutions were used for texturing at the surface of multi-crystalline Si wafer. Experiments were performed with various temperature and time conditions in order to determine the optimized etching condition. Alkaline etching solution was found inadequate to the texturing of multi-crystalline Si due to its high reflectance of about 25%. The reflectance of Si wafer texturing with acidic etching solution showed a very low reflectance about 10%, which was attributed to the formation of homogeneous. Also, deposition of ITO anti-reflection coating reduced the reflectance of multi-crystalline si etched with acidic solution(<TEX>$HF+HNO_3$</TEX>) to 2.6%.

실리콘 태양전지

태양광발전이란 태양에너지를 직접 전기 에너지로 변환시키는 것이다. 지난 5년 동안 태양광발전은 세계적으로 높은 성장률을 보여 왔다. 특히 2006년에는 30%, 이상의 성장을 가져왔으며 앞으로 20년 동안 평균 생산 성장률은 매년 27%-34%가 될 것으로 예상하고 있다. 현재까지는 태양광발전을 이용해 생산된 전력의 가격은 기존 전력발전의 가격보다 높지만 태양광 기술의 발전과 효율의 향상으로 점점 그 가격이 떨어지고 있다. 뿐만 아니라 태양전지용의 실리콘 기판의 대량생산은 점점 더 태양전지의 가격 저하를 가져오고 있다. 태양전지의 변화효율의 한계는 30%이다. 현재에는 결정질 실리콘 태양전지가 주를 이루고 있지만 미래에는 박막 실리콘 태양전지가 주도를 이룰 것이다. 2030년에는 박막 태양전지가 90%이상을 이루고 결정질 태양전지는 10% 이하로 떨어질 것을 예상하고 있다. 성균관대학교에서는 결정질 실리콘 태양전지의 저가화와 고효율화를 주 연구로 수행하고 있다. 현재 성균관대학교에서는 스크린 프린트를 이용해서 16% 이상의 다결정 실리콘 태양전지와 17% 이상의 단결정 실리콘 태양전지를 성공적으로 제작하였다. 제 1세대에서 다음 세대의 태양전지 발전의 과정은 새로운 접근법으로 확대되지만 여전히 실리콘이 지금까지 주된 재료로 쓰이고 있다. 2010년까지 이러한 기술들에 대한 격차는 여전히 있지만 태양광발전을 통한 전력생산의 가격은 60 cent/watt 정도로 예상하고 있다. 태양광발전은 청정에너지로서 재생불가능 하고 고갈되어가고 환경오염을 일으키는 다른 에너지와 비교하여 점점 대체에너지로서의 자리를 확립해 가고 있다. Photovoltaic (PV) technology permits the transformation of solar light directly into electricity. For the last five years, the photovoltaic sector has experienced one of the highest growth rates worldwide (over 30% in 2006) and for the next 20 years, the average production growth rate is estimated to be between 27% and 34% annually. Currently the cost of electricity produced using photovoltaic technology is above that for traditional energy sources, but this is expected to fall with technological progress and more efficient production processes. A large scale production of solar grade silicon material of high purity could supply the world demand at a reasonably lower cost. A shift from crystalline silicon to thin film is expected in the future. The technical limit for the conversion efficiency is about 30%. It is assumed that in 2030 thin films will have a major market share (90%) and the share of crystalline cells will have decreased to 10%. Our research at Sungkyunkwan University of South Korea is confined to crystalline silicon solar cell technology. We aim to develop a technology for low cost production of high efficiency silicon solar cell. We have successfully fabricated silicon solar cells of efficiency more than 16% starting with multicrystalline wafers and that of efficiency more than 17% on single crystalline wafers with screen printing metallization. The process of transformation from the first generation to second generation solar cell should be geared up with the entry of new approaches but still silicon seems to remain as the major material for solar cells for many years to come. Local barriers to the implementation of this technology may also keep continuing up to year 2010 and by that time the cost of the solar cell generated power is expected to be 60 cent per watt. Photovoltaic source could establish itself as a clean and sustainable energy alternate to the ever depleting and polluting non-renewable energy resource.

Silicon for photovoltaic applications

Silicon is used in photovoltaics (PV) as the starting material for monocrystalline and multicrystalline wafers as well as for thin film silicon modules. More than 90% of the annual solar cell production is based on crystalline silicon wafers. Therefore, silicon is the most important material for PV today. The challenge which the PV-industry is currently facing is to decrease the manufacturing costs per Wp annually by 5%. Since approximately 70% of the costs for solar cells are caused by wafer costs, there are two main avenues to achieve the cost reduction. One is the development of cheap solar grade Si feedstock material, the other is the development of a cheap ingot manufacturing process for multicrystalline silicon wafers. Therefore, one aim of the PV-industry is to produce sufficiently pure solar grade silicon at low manufacturing costs. Three different routes for the production of solar grade Si are currently considered. Processes like decomposition of trichlorosilane by means of a fluidized bed reactor or decomposition and melting by means of a tube reactor are under development. Monosilane decomposition by means of a free space reactor is also in progress. Every process should be able to achieve the criteria for solar grade Si. Especially, the decomposition of monosilane by means of a free space reactor will be able to meet this challenge as will be explained in more detail. Within the last 10 years multicrystalline (mc) silicon ingots for PV with weights of 150, 240 and 300 kg have been developed and are produced today in the standard production process. The state of the art growth rate of these ingots is 0.5–1.5 cm/h. The two main targets of the PV-industry today are first to increase the ingot weight and second to accelerate the growth of multicrystalline silicon ingots. First, results of the preparation of very large ingots with an ingot weight of 400 kg or ingots grown with growth rates higher than 2 cm/h will be presented.

Minority-carrier trapping in Ga-doped multicrystalline Si wafers

Abstract B- and Ga-doped multicrystalline-silicon (mc-Si) wafers with different resistivity and different positions of grown ingots have been used to evaluate the stability, quality improvement and thermal annealing effects on carrier lifetimes. The effective carrier lifetimes are improved and high lifetimes as high as 200 μs are realized after P-diffusion and SiN x coating. Ga-doped wafers show a certain stability after thermal annealing up to 250 °C which insures the possibilities of eliminating the light-induced degradation effects generated in p-type mc-Si wafers.

Antireflective porous layer formation on multicrystalline silicon by metal particle enhanced HF etching

Antireflection of silicon (Si) surface is one key technology for the manufacture of efficient solar cells. Metal particle enhanced HF etching is applied to produce uniform antireflecting porous layer on multicrystalline Si wafers that cannot be uniformly texturized by anisotropic etching with an alkaline solution. Fine platinum (Pt) particles are deposited on multicrystalline n-Si wafers by electroless displacement reaction in a hexachloroplatinic acid solution containing HF. Both macroporous and luminescent microporous layers are uniformly formed by immersing the Pt-particle-deposited multicrystalline Si wafers in a HF solution. The reflectance of the wafers is reduced from 30% to 6% by the formation of porous layer. The photocurrent density of photoelectrochemical solar cells using porous multicrystalline n-Si has a 25% higher value than non-porous Si cells.

Texturization of multicrystalline silicon wafers for solar cells by chemical treatment using metallic catalyst

A new etching method for texturing multicrystalline p-type Si wafers for solar cells was developed. In this method, we used platinum or silver particles as the catalysts, which were loaded on the wafers by means of the electroless-plating technique. After deposition of the catalysts, the wafers were etched and textured in HF solution, to which in some cases chemical oxidants were added. The solar cells (4 cm2) manufactured from the textured wafers showed efficiency as high as 16.6%, which was about 1% (absolute) higher than that of the cells made from the wafers treated by the conventional alkaline method.