Amorphous Silicon And Photovoltaics

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Amorphous Silicon and Photovolatics  This article is based in part on research from Materials for Thin-Film Silicon Photovoltaics      Page | 1  Thin-film photovoltaics (PV) are gaining ground in the currently expanding PV landscape. Currently at 23 percent of the worldwide production, these technologies are set to represent close to 48 percent of the market by 2015, with crystalline silicon (c-Si) making up the rest. Of the thin-film PV technologies, amorphous silicon (and other silicon-based TF materials) represents the majority of the market, followed by cadmium telluride (CdTe) with copper indium gallium diselenide (CIGS) and others just starting to penetrate the market. Thin film PV materials possess several attractive advantages over c-Si that are allowing them to gain a prominent role in the overall PV market. Lower overall materials cost, less expensiv e manufacturing techniques, lower final module weight, lower-cost installation options, and the ability to use flexible substrates are some of the advantages that make thin film PV attractiv e for many applications going forward. And amorphous silicon (a-Si) in particular has several advantages when compared to other thin film technologies. One key advantage is that a-Si is an extremely well studied material with 20 years of experience both in research facilities and as a product in the market place. Multiple manufacturers are making a profit manufacturing a-Si today, and a-Si should become more profitable as new cell designs increase efficiency, manufacturing costs decrease with more advanced manufacturing techniques and demand increases as fossil fuels become more expensiv e in the long term. In addition, it is well supported by equipment vendors with several turnkey manufacturing solutions now available. Because of these factors, it has the highest probability of success in the thin film PV market with the least risk in the next 8 years. Amorphous Silicon's Opportunity in the PV Landscape Thin-film silicon, in particular a-Si, is the most well understood TFPV solution currently available. And from its current position in the market it is well positioned to become the low cost solution of choice for many applications that currently use crystalline silicon and for new applications where

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current c-Si PVs are not feasible either from a cost perspective or application perspective such as flexible substrate applications. The initial a-Si PV was patented in 1977 by D. E. Carlson. Although the efficiency of the initial single junction cells was low, the low overall module cost made them suitable for small lowerpower, cost-sensitive applications such as solar calculators. Over time, improved module geometries such as double and triple junction cells, improved understanding of the importance of interface engineering between deposited layers and cost reductions from manufacturing improvements have made a-Si (and other thin-film silicon materials) the low-cost PV solution of choice for low power applications, as well as competitive with c-Si in power generation such that it now enjoys around 5 percent of the overall world wide PV market. Also, with the temperature coefficient being such that a-Si cells get more efficient as they warm combined with the superior performance of a-Si in light conditions, a-Si is very competitive in overall power output. The two major perceiv ed disadvantages of a-Si versus other PV technologies (both c-Si and other TF technologies) are its lower initial module efficiency and some further efficiency degradation during its initial exposure to sunlight. However, new module geometries, and a better understanding of the effect of particle size distributions of Si (a-Si vs. nanocrystalline silicon vs. microcrystalline silicon) have improved efficiency over earlier devices. At least one vendor of a turnkey a-Si PV manufacturing solution is now offering assurances of terminal efficiency of its products of more than 8 percent. While this is certainly lower than c-Si, CdTe, or CIGS, the key metric to profitability is overall cost-per-watt not module efficiency and in this realm a-Si is very competitive. The overall efficiency of a-Si may lag the potential of more recent entrants into the thin film PV market such as CIGS, a-Si is the only low-cost thin film PV material that is available in volume now to meet today's demand. In addition, a-Si is the closest to taking advantage of the economies of scale that result from a transition to large-scale manufacturing, which will drive down the price even further and meet the projected demands of the next 5-8 years. The second most important thin film PV in terms of installed generating capacity in the field is CdTe, which currently has about half the market penetration of a-Si thin film PV. While CdTe is reliable and economical, growth of CdTe PV technology may be hampered by nagging

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environmental concerns and associated increased costs. While a proactive recycling program by manufacturers may minimize impaired growth of CdTe due to environmental concerns, the recycling programs adds additional costs to CdTe PV that are not associated with a-Si. While other technologies such as CIGS are now just beginning production ramps or on the cusp of feasibility at many of that industry’s start up companies, a-Si has been well known for years and is now rapidly becoming available as a turnkey solution from several large equipment vendors, including Applied Materials, Oerlikon, and Ulvac. Applied Materials now offers an automated 5.7- m2 glass to module production line. The key advantage to these turnkey solutions is that they are supported by large multinational corporations with established track records of success in the semiconductor and equipment manufacturing industries. They have worldwide support teams and experience supply ing turnkey manufacturing solutions. Compared to other thin film PV technologies, the commitment of major industry equipment manufacturers to a-Si increases the likelihood of market penetration for a-Si in the low cost thin film PV marketplace and an industry commit ment to develop a cost effective, reliable product. An additional advantage of available turnkey solutions is that they can be tailored to the global customer base. Customers with generous funding but little expertise in the field can purchase everything (equipment, process flow, factory engineering consultants, test and reliability engineering, etc.) from a single vendor with guarantees of factory costs and product yields. Other customers with more expertise in the field can pick and choose from an à la carte menu of equipment, support, and available IP options to make purchases based on their individual business model

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With this commitment to turnkey, high-volume manufacturing capability, suppliers up and down the value chain are increasing available supply of substrates, encapsulation materials, transparent conducting oxides (TCO) such that all of the pieces are in place to aggressively drive down a-Si PV manufacturing costs. Turnkey solution vendors are quoting low double-digit returns based on current market (fall 2008) and regulatory conditions (especially in Europe with current feed-in tariffs). The demand in Europe will be somewhat lower than earlier estimates for 2009 based on recent reductions of feed-in tariffs and 500 MW cap recently announced in Spain.

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Two major factors have periodically accelerated and retarded the materials research and productization of PV over the past 35 years. These two factors are global energy commodity prices and levels of government intervention (subsidies for PV, increased taxes for fossil fuels). For most of this time the subsidies were driven by the price of oil and balance of trade issues. In the past 10-15 years increasing environmental concerns over the predicted effects of greenhouse gasses emitted from fossil fuel consumption have driven much of the government subsidies and mandated targets of production for zero green house emissions PV solutions. Like all PV materials, historical a-Si materials development and commercial adoption curve has mirrored that of the entire alternative energy solutions sector. In the PV area, with the exception of earlier development for specialty applications such as space and some remote off grid applications, the capital available for PV development and commercialization follows global energy commodity prices. The initial development of commercial PV was a reaction of the initial energy crisis in 1973 and the spike in prices in the late 70's early 80's. The increase in oil prices and the extremely high cost of available c-Si technologies was a real driver for the initial a-Si PV developed and brought to market in the pre 1985 timeframe. The subsequent crash in energy prices in 1985 severely curtailed research in improved PV materials and associated further expansion of commercial applications and installed PV base. From the economic perspectiv e, the crash in energy prices in the mid 80's effectively limited PV applications to space, remote locations and low power applications (calculators/etc). As time went on, cost reductions for a-Si and other PV technologies and the relentless increasing march of energy prices starting in 2001 and cascading upwards since (until very recently) have resulted in intense interest in improved low cost PV solutions. Amorphous silicon is in a unique position to meet this need. Traditional c-Si by its nature does not have the cost reduction path available to it that the thin film technologies have, and a-Si's lead in ramp to large volumes among thin film solutions gives it the lead in being the probable large volume solution of choice for the foreseeable future. Because a-Si volume manufacturing solutions are available now at a time of high demand creates a unique opportunity for it to be the leading thin film PV solution for the foreseeable future and make major gains in the overall PV landscape. There is not enough c-Si available for it to meet the current market demand. CdTe is a known technology but the environmental concerns real or

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perceived may limit its growth. CIGS does not have the backing of multiple major equipment vendors with turnkey solutions. And, the majority of the CIGS manufacturers are by comparison small and in the prototype/early production phase with no guarantee that they can produce the quantity needed to meet current demand in the volumes that a-Si can provide. Organic PV’s are even farther from commercialization. Amorphous silicon is positioned as the right solution at the right time. The subsequent drop in energy prices in the fall of 2008 actually may benefit a-Si in the long run as the investment in a-Si production has been made while investment on the same scale for other thin film technologies to get them from prototyping/initial production to high volume manufacturing in most cases has not been committed. On the other side of the current global economic slump, the a-Si capacity will be there as the aggressive growth of the past several years and associated increases in commodity energy prices return. Cost Sensitive Application Advantages of Thin-Film Silicon PV Any PV application where cost is the key is an area where a-Si will be attractive in the future. The first application area is low-cost PV building materials. As a-Si PV prices drop, a-Si based PV roofing tiles will reach a point where they become a viable options compared to traditional composite roofing; this will open a huge market for a-Si PV's. Another area in building materials is inexpensiv e a-Si based facades. These are already available, but the current cost limits penetration to either remote off grid applications where there is no alternative, or applications where the high cost is not an object (7-10 years to see payoff). A final application that could create huge demand is the use of flexible a-Si PV for mobile and wearable applications. As the number of mobile devices carried by the average indiv idual in developed nations increases and the necessity of remaining in constant contact becomes pervasiv e, the power requirements of current mobile devices will outstrip the ability of present battery systems to supply this demand for a full day. Low cost wearable (hats, bags and/or foldable-collapsible a-Si based PV solutions) could offer a viable bridge for power users to grab some extra power during the day without plugging into the grid. A similar argument applies in developing regions of the world where the communications network is often more reliable than the power grid. Small portable flexible a-Si based chargers for mobile

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devices would represent a large market opportunity in these developing regions. Several manufacturers are starting to address this need for military and outdoor remote location needs, but it has not been expanded to exploit some of these larger general consumer market opportunities. Future Advantages of a-Si Along with the cost reductions from the transition to volume manufacturing, a-Si also offers a path to higher efficiency through optimization of the thickness, morphology and size distribution of the deposited silicon film whether it be purely amorphous, nanocrystalline or microcrystalline. A second area of investigation is more robust passivation of dangling bonds within the a-Si film by reactiv e species other than hydrogen to reduce/eliminate the Staebler-Wronski Effect. A typical cell contains upper and lower electrodes, 2p, 2I and 2N regions and associated interfaces, and each needs to be optimized for composition, particle size distribution and interface characteristics to achieve optimum performance and reliability. This multi variable problem space is an ideal area to apply combinatorial chemistry techniques to optimize the system. An early attempt was made in 2003 that showed it was possible to vary particle size of amorphous to microcrystalline silicon, doping concentration, and deposition thickness using combinatorial techniques. If this technique was expanded to fully characterize the system, it would be possible to efficiently find an empirical solution that maximizes efficiency and minimizes Staebler-Wronski Effect (SWE). Such techniques have been successfully applied to advanced LEDs, advanced phosphors and several cataly sts used in the plastics industry.  

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