Materials

China has become the largest manufacturing base for crystalline silicon modules in the world, and is becoming increasingly reliant on a domestic supply base. This article discusses the emergence of local supply chains and the strategic responses of global suppliers to this domestic competition. It proceeds to review a set of conclusions from four case studies of formulated material supply within China that can apply to supply chain participants in the PV industry, concluding with some strategic considerations for suppliers on the cusp of entering the Chinese market.

The PV industry is undergoing dramatic changes. Like a carnival ride gone dreadfully wrong, exhilaration has been supplanted by dread; joy has been replaced by fear. Just look around you – provided you are able to turn your head to defy the g-forces acting upon you as we bank and turn wildly along. You will see PV companies closing their doors for good. You will see extraordinarily talented people throughout the supply chain, shifting positions everywhere and looking for safe-haven jobs. And you will also see once-leading PV companies burning cash and losing their status as ‘bankable’. Everywhere we turn, we see companies in the supply chain shuttering production as if to balance markets.

The purpose of this paper is to give an overview of the use and potential of diamond wire for the silicon-shaping process in the PV industry. The current market and future prospects for helping to meet the goals of 2020’s roadmap of thinner wafers and reduced $/W are described.

Most high-efficiency solar cells are fabricated from monocrystalline Czochralski silicon (Cz-Si) wafers because the material quality is higher than multicrystalline silicon (mc-Si) wafers. However, the material study presented in this paper reveals strong variations in the material quality of commercially available Cz-Si wafers, leading to a loss in solar cell efficiency of 4% absolute. The reason for this is the presence of defects, which appear as dark rings in photoluminescence (PL) images of the finished solar cells. It is shown that these efficiency-limiting defects originate from oxygen precipitation during emitter diffusion. It is demonstrated that an incoming inspection in the as-cut state is difficult, as strong ring structures in as-cut wafers turn out to originate most often from thermal donors. These are dissolved during high-temperature treatments and are therefore harmless, whereas moderate ring structures in the as-cut state may become severe. That is why critical wafers can be identified and sorted out reliably only after emitter diffusion, by using QSSPC-based lifetime measurements or PL imaging. The two-year statistics gathered from the research line at Fraunhofer ISE on the occurrence of ring defects in Cz-Si wafers indicate that ring defects are highly relevant in terms of material yield.

Silver paste is a key component of the design of nearly all silicon wafer solar cells manufactured in 2011. The high cost of the precious metal in the paste formulation means that silver paste is also the second-highest component of the total cost of materials. This article reviews the silver paste supply chain and the challenges in silver paste formulation and manufacture, and discusses some of the approaches for reducing or removing entirely the use of silver in crystalline silicon cell manufacture.

The reliable analysis of trace elements in silicon is of fundamental importance for the understanding of material properties and quality control of solar cells. This paper presents a demonstration of the power of two analytical techniques for the determination of trace elements in solar silicon: inductively coupled plasma mass spectrometry (ICP-MS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). These techniques are among the few that achieve sufficiently low detection limits and they may complement each other because of their specific performance. Examples are given of the quantitative chemical analysis of boron, phosphorus and iron in different types of solar silicon, as well as of the enrichment of metals and alkali metals in Si3N4 precipitates.

Competition in the premium sector is becoming more and more fierce. This is forcing PV module manufacturers to differentiate themselves through product benefits and better performance in terms of efficiency. While attention has previously been focused on cell technology, it is likely that, in the future, all module components will become part of this competition – a competition in which premium front glasses present an especially promising element. Antireflective coating (ARC) is only the beginning of this evolution. Not only do deeply textured front glasses promise significant increases in output – up to 7% – but their specific product characteristics also make them suitable for niche applications, such as airplane entry lanes and airport buildings. EuPD Research has issued a white paper devoted to solar glass, of which a synopsis is presented here.

The last several years have seen a significant number of publications on wire saw data in regard to process optimization theory applied to solar wafering. The methods vary, but fundamentals concern the mechanical dynamics of the wire sawing process, where measurements of the wire forces in the silicon slot using free abrasive are studied; however, these data are not yet fully correlated to a complete thermodynamic analysis of the problem. The objectives of the empirical development of the process theory are also widely varied, but there is industry agreement that it is being faced with the fundamental limits of cutting rates in processes that use free abrasive slurries and a single wire. The limit arises from intrinsic thermodynamic limits of the delivery of work energy to the silicon slot. Similarly, these same principles prevent us from increasing the wafer load to overcome the limitation as work energy transfer rates are countered by higher entropic losses that occur as power and wafer load are increased. The effect results in the problem that the wafer load may not be increased without proportionately reducing table speed. The fundamental nature of these limits suggests that they involve theoretically calculable energy quantities of thermodynamic limiting functions, which restrict the ‘useful’ work that we can extract from the system, where the work energy of interest is the abrasion of the silicon in forming the wafers. The present work reviews the theoretical issues of determining process efficiency optimums that could be used to achieve throughput gains.

t is well known that the cost of silicon materials is the major cost factor in crystalline silicon PV module production. Polysilicon price accounts for about 30% of total module production costs. While the PV industry has set a polysilicon price target of US$40/kg by 2015, this goal will not be reached if demand continues to exceed supply and if new plants cannot reduce operating costs below US$25/kg. Given a continued 30% annual growth in demand for PV modules, new polysilicon plants and expansions are needed to avoid shortages of high-purity, cost-effective polysilicon. This paper discusses the major factors in polysilicon production costs, the important elements of polysilicon plant design for reducing operating costs, the key cost elements of polysilicon plant operations, and how the design of polysilicon products can reduce crystal growth costs.

Upgraded metallurgical-grade silicon (UMG-Si), once looked on as a cost-effective and energy-efficient alternative to Si produced via the Siemens route, has experienced a severe regression of late. This has been caused both by the market conditions and by specific physical properties of these materials. Meanwhile, the qualities and the rated influence of negative physical effects have changed partially. Hopes are again rising that these materials, which have to be compensated to meet the desired net doping specifications, might achieve an economical breakthrough instead of long-dreaded low breakdown voltages. In the following paper, we summarize a few of our results on multicrystalline UMG silicon as well as results published by other research groups in the last few years.