Photo: Gary Meek 12 member companies have joined the SEMI taskforce and are combining their efforts on the standard of quasimono wafers. Paving the way to acceptance Quasi-mono standards: The SEMI standards for quasi-monocrystalline silicon wafers can create a clear pathway for market acceptance of the technology and facilitate growth. The SEMI PV Group s Stephan Raithel and LDK Solar s Yuepeng Wan and Linyan Liu explore the potential of the technology and why standards could be so valuable. When perusing the different show floors at any major PV trade show, it becomes quickly apparent that nearly every leading PV manufacturer has quasi-mono (also known as mono-like) products on the market. The overall goal is simple: to be able to reach the same efficiency as monosilicon (mono-si) cells at the cost level of a multisilicon cell. In theory, of course, that sounds interesting. In reality however it is not that easy. Existing equipment can be used, but only after some modifications. Another challenge is to retain the required quality and purity during the crystallization process. The efficiency of the mono-like cells is definitely higher than multisilicon products approximately between 0.5 and 1%. So far the industry has not joined forces to work on pre-competitive issues and therein cut down the time to market. At the moment the answer to the question Hype or future commodity? needs to be answered with a pragmatic, maybe both. At its current status, mono-like technology will not replace mono-si, however it could be a new technology with a certain market share (the latest International Technology Roadmap for Photovoltaics predicts a market share of 40 percent by 2020, see www.itrpv.net). At the end, the costs per watt peak will be the deciding factor. SEMI s China Stan- 94
dards Committee has created a task force to determine specifications for mono-like wafers. All leading manufacturers are involved and therefore a promising result in a short time frame is expected. Background In today s PV market, the majority of PV modules are made with crystalline silicon materials. There are generally two types of crystalline silicon wafers, monocrystalline (also called single crystal) silicon wafers which are generally produced with the Czochralski (CZ) method and multicrystalline silicon wafers, which are commonly made with directional solidification methods (also called the casting method) with many variations of crystal grow systems. Multicrystalline silicon wafers have gained the highest market share (more than 40 percent) due to their low cost of crystallization. The CZ monocrystalline silicon wafer is still holding its market share, especially in rooftop applications, due to its higher cell conversion efficiency. The difference in the cell conversion efficiency between the mono and multisilicon modules is typically about 1.5%. Such a difference is a result of the surface property (about 0.5%) and the bulk crystal quality. The directional solidification method (or casting method) can also be used for growing single crystals. Examples can be found in the crystal growth of many different materials, such as sapphire. In fact, GT Solar s directional solidification system (DSS) furnace, which is widely used for multicrystalline silicon wafer production, originated from the heat exchange method (HEM) furnace for single crystals of sapphire. In the early stage of development of the HEM DSS furnace for silicon crystals, the focus was initially on the single crystal of silicon, and of course with a single crystal seed at the bottom of the melt. The results exposed that some parts of the crystal weren t single crystal (multicrystalline grains), and the cell conversion efficiency of the multi-section was close to that of the single section. The HEM furnace was eventually employed for producing multicrystalline silicon ingots, without putting seeds at the bottom of the crucible, simply and cost effectively, and it was further developed into the DSS furnace for multicrystalline silicon ingots. The growth of single crystal silicon with the directional solidification (or casting) method and the application of the resulting mono (or quasi-mono) silicon wafers for solar cells was also carried out by numerous researchers in the past 20 years. C.P. Khattak first studied the reduction of grain boundaries in DSS by large grains growth. It was regarded as the most straightforward approach to increase crystal quality and consequently solar cell efficiency back in 1987. Then, in 2000, the control of undercooling for preferred grain boundaries was studied extensively in Japan on a laboratory scale. The first large-scale (pilot) application of quasi-mono wafers could be considered that which was implemented by BP Solar. As early as 2006, BP Solar branded its quasi-mono wafers and solar cells as Mono2 TM. However, the quasi-mono wafers didn t receive much attention until 2011. As the market shifted increasingly towards higher efficiency solar modules, and solar cell technology advanced to a much higher level, the requirements for Advertisement As one of the world s leading technology providers in the photovoltaic industry with production and R&D facilities in Europe and Asia, Manz offers a full range of single equipment or totally integrated production lines. They combine the unbeatable competitive advantages of German high-tech engineering with highest quality standards and local production, sales and service in Asia. That means for our customers: lowest cost of ownership, highest efficiency and yield, and thus the most profitable solution ever. YOU HAVE CHALLENGES? WE OFFER SOLUTIONS! DISCOVER OUR EFFICIENCY-BOOSTING PRODUCTION TECHNOLOGY EQUIPMENT FOR CSI AND TFS EU PVSEC, Frankfurt Live Demonstration: Hall 3.0 / Booth D11a Manz AG Steigaeckerstrasse 5 72768 Reutlingen Germany info@manz.com www.manz.com
Graphics: Solarpraxis AG/ Harald Schütt Crystalline silicon wafer technology by efficiency 24 % Cell efficiency 23 % 22 % 21 % 19 % 18 % 17 % 16 % 2010 mono Si, n-type mono Si, p-type mono-like Si p-type multi Si, p-type Crystalline silicon market share by technology 100 % Shares of materials within c-si market 90 % 80 % 70 % 60 % 50 % 40 % 30 % 10 % 0 % 2010 2012 2014 2016 high quality wafers became more pressing. Companies such as JA Solar, LDK Solar and ReneSola announced their quasi-mono products, and modules based on quasi-mono materials also found acceptance in the market. Since then, quasimono wafers have slowly gained market share. As a new product, modules made of quasi-mono wafers are facing several challenges to gain market acceptance. The color difference on the grains with different orientation after alkali texturing causes aesthetic concerns in residential applications. The endurance and attenuation of the electrical properties also remain causes for concern among some cautious customers. A consensus on the quality of the quasi-mono wafers is also still missing. There is even no standard name for this new type of wafer. 2012 2014 2016 2018 Source: ITRPV 2020 24 % 23 % 22 % 21 % 19 % 18 % 17 % 16 % Therefore, there appears to be an urgent need for the standardization of this new type of wafer. Based on the market needs and under the guidance of the newly formed SEMI China Photovoltaic Standards Committee, LDK Solar initiated a new Standards New Activity Report Form (SNARF) for the standard of quasi-mono wafers. The PV Silicon Wafer Taskforce was established with the leadership of LDK Solar, JA Solar and Suntech. 12 member companies have joined the taskforce and are combining their efforts to produce industry standards for quasi-mono wafers. mono Si mono-like Si multi Si 2018 Source: ITRPV 100 % 90 % 80 % 70 % 60 % 50 % 40 % 30 % 10 % 0 % 2020 Properties of quasi-mono wafers Quasi-monocrystalline silicon ingots can be made with current crystal growth furnaces for multicrystalline silicon ingots. In comparison to the growth process of multicrystalline silicon ingots, the major difference in the process is that a layer of monocrystalline silicon seeds are employed on the surface of the bottom side of the crucible. The seeds are usually made with the CZ process. Silicon feedstock and dopant are then loaded on top of the seed. In the process of melting, the silicon feedstock starts to melt from the top. The process is controlled so that the seeds will not be completely melted. When the solidification process begins, the remaining seeds act as the nucleation surface and the grown crystal will follow the orientation of the seeds and gradually form a large ingot consisting of crystals with some sections having multiple grains, especially in the areas close to the crucible and between the seeds. Due to this special growth process, the appearance and properties of a quasi-mono-wafer is different from that of a monocrystalline silicon wafer or a multicrystalline silicon wafer. Firstly, due to the seeding control the growth of quasi-mono is from the middle to the outside. The crystalline silicon grown in the region close to the crucible surface is mostly multicrystalline silicon, while in other regions it forms mono-like crystals, in the case of good growth. As a result, wafers with very different characteristics can be obtained from the same ingot (see the images at the top of page 98). Secondly, as previously outlined, a layer of seed crystals of the same crystallographic directions are placed at the bottom of the crucible for quasi-mono casting. The crystals will merge together and become a large grain during the growth, and result in small angle sub-grain boundaries originating from the gap of the seeds. The surface of the mono-like crystal looks uniform, but in some cases a large number of sub-grain boundaries, which may cause a dislocation cluster, can be seen from an oblique direction. Besides, the oxygen content of quasi-mono is less than that of mono-crystalline silicon, which can in turn reduce the effect of light induced degradation (LID) of quasi-mono solar cells. A quasi-mono wafer is a multicrystalline wafer with a large monocrystallinelike grain. To make full use of the surface of the mono-grain, the alkali texturing process can be applied so that the light reflection on the cell surface can be greatly reduced. In general, for different percentages of large single grains of the quasi- 96
mono wafer, different texturing processes should be selected, to minimize the surface reflectivity and hence maximize the cell efficiency. For the high percentage of large single grains of quasi-mono wafer, alkali texturing can form an inverted pyramid texture surface and improve the efficiency of the solar cell. But due to the anisotropic etching of alkaline, the pyramids that form on the other grains, with an orientation different from that of the large grain, will have a different structure and orientation. This area will have a different reflectivity for the incoming light and exhibit different colors from that of the large grain as shown in the image at the bottom of page 98. And for a low percentage of large single grains of quasi-mono wafer, acid texturing can be applied. However the dislocation density, the concentration of oxygen and carbon, and the impurity made the efficiency obviously lower than that of a high percentage of large single grains. Typically if the efficiency of a monocrystalline silicon cell is 18.5%, the efficiency of a quasimono cell with a high large single grain percentage can reach about 17.5 to 18.2%. But for low single grain percentage quasimono cells, the efficiency can be about 16.6 to 17.0%. Advertisement Thin Film Metrology for Quality and Production Control Silicon Solar Cells SE 400adv PV Laser ellipsometer and spectroscopic ellipsometer for the measurement of thickness and refractive index of AR coatings on textured multi- and monocrystalline silicon wafers Quasi-mono wafer standards At present, manufactures have their own internal standards for quasi-mono wafers, which can present a problem relating to various interfaces and terminology, inhibiting communication between suppliers and their customers. Standardized specifications for quasi-mono wafers are expected to reduce these interface problems and to enable a common understanding of material properties and terms. This has the potential to allow the reliable manufacturing of solar cells, benefiting all parties. This standard will cover quasi-mono wafers produced by the directional solidification (casting) method. The comprehensive specification will define the appearance (the percentage of the largest single grain), geometrical properties, physical properties, and electrical properties along with the test methods suitable for determining their magnitudes, which give a reference for customers, manufactures and suppliers. During the formulation of standards for quasi-mono wafers, many discussions are devoted to the definition of some basic terminologies. Also, there are issues that require a large amount of data for verification. The following are some examples: The name of quasi-mono wafers: there are various names for quasi-mono wafers such as casting monocrystalline silicon wafers, or seeded casting mono-wafers, according to the method of growth. They have also been called mono-like silicon wafers, which was due to their appearance. And the name SEMI International Standards Over 4,000 volunteers, 23 global technical committees, and more than 200 task forces have worked tirelessly to create the library of over 800 SEMI Standards and Safety Guidelines. The industry supports the standards development process because it provides a forum for free exchange of technical expertise and is protected by anti-trust regulations. Companies that participate actively in standards work have a head start on their competitors in adapting to market demands and new technologies. In addition, a recent study has shown that research risks and development costs are reduced for companies contributing to the standardization process. Thin Film Solar Cells SE 800 PV SenSol Haze Horizontal computer controlled mapping system with multiple sensor platform to measure: - Film thickness - Haze - R&T - Sheet resistance SENTECH Instruments GmbH Schwarzschildstrasse 2, 12489 Berlin, Germany Phone: +49 30 / 63 92 55 20, Fax: +49 30 / 63 92 55 22 E-mail: marketing@sentech.de, Web: www.sentech.de
Close to the crucible Close to the crucible Different categories of quasi-mono wafer in the same ingot quasi-mono-crystalline silicon wafer is more technical and usually used in academic documents. In the standard, we are reaching a consensus on a name that can express the nature, appearance and principle of such a wafer. After discussion, the name quasi-monocrystalline silicon wafer has been chosen for the standard, according to the principle of crystal growth. Besides, a monocrystalline silicon wafer has only one single crystal with no large-angle boundaries or twin boundaries, while a multicrystalline silicon wafer consists of many crystal grains and contains large-angle boundaries or twin boundaries. The obvious difference makes the monocrystalline silicon and Photo: LDK Solar Photo: LDK Solar Cell with grains with different colors. 98
Photo: SEMI/Kempinski A presentation of SEMI s International Roadmap for Photovoltaics (ITRPV) will be held during this year s EU PVSEC trade show in Frankfurt between September 24 and 28. multicrystalline silicon easy to define and distinguish. But as mentioned above, there are several types of wafers that can be found in the same quasi-mono ingot. One type is the same as monocrystalline silicon, and another is the same as multicrystalline silicon, as well as some others in between. Thus, the definition of quasimono is particularly important. It shall express the nature of quasi-mono and the easy-to-distinguish three types. Next, maybe the hardest item in the standard is the new concept mentioned above: the percentage of large single grains, which will influence the process, appearance and efficiency of subsequent quasi-mono cells. The name, the definition, the classification and the method of this item have no standard or document to reference. For the classification, after several internal discussions in the task force, three categories are temporarily employed to classify the quasimono wafer. Category 1: 100% large single grain, where the appearance and quality is comparable with that of monocrystalline silicon wafers. Category 2: The wafer contains large single crystal grains, but also contains a twinned or multicrystalline silicon region. Category 3: There are small single crystal grains (few tens of micrometers to few tens of millimeters) with different orientations, which is the same as multicrystalline silicon wafers (see the picture at the top of page 98). The criterion (i.e., the percentage of large single grains, for the classifications of Category 2 and 3) needs to be verified by a significant amount of experimental data and will be done by the task force. In the appendix of this standard, a test method of measuring the percentage of large single grains on quasi-mono wafers is briefly described, to provide a reference of the measuring method. Based on the principle of digital image processing, this method referred to an LDK patent that can analyze and calculate the value of the percentage. As a competitor to multicrystalline silicon wafers and monocrystalline wafers, quasi-mono wafers are finding their market position, with the potential advantage of lower costs than mono wafers and quality higher than multi wafers. The standardization of quasi-mono wafers will help manufacturers communicate better with customers, and focus on the development of higher performance products. The standard will pave the way for quasimono wafers to gain market share. S Stephan Raithel, Yuepeng Wan and Linyan Liu. Participate in the SEMI standards PV silicon wafer task force Participating companies: LDK, JA Solar, Suntech, Rena solar, GCL, JYT, Canadian Solar, RIETECH, Tianwei, Gdsolar, 48th institute, Schott Solar, Q-Cells. Task Force leaders: Wan Yuepeng, LDK / Wang Xiaoyong, JA Solar / Lu Jinggang, Suntech Please get in contact with Stephan Raithel to learn more about the International Technology Roadmap for PV (ITRPV) and the SEMI Standards program. More information is also available under www.itrpv.net and www.pvgroup.org/standards. Stephan Raithel Director PV Europe SEMI PV Group sraithel@semi.org +49.30.303080770 99