A future for photovoltaics

Some areas of vacuum web coating have progressed in the past decade, others have lagged behind. A hot topic 20 years ago was transparent barrier coatings. Silica and alumina coatings had been produced and evaluated for barrier performance, and it was predicted that an enormous amount of vacuum-deposited transparent barrier would be required.

Following this prediction, many research programs were started and completed with many different vacuum deposition processes and materials evaluated. A decade later the same predictions were heard, but the explosion of use of transparent vacuum-deposited barrier coatings still had not arrived.

A few machines were sold for the electron beam deposition or plasma-enhanced chemical vapor deposition of transparent barrier coatings, but the cost of the coatings was deemed too high by potential users, and the market growth never took place. In Japan, there was some production using induction-heated sources to deposit silica but only on a limited scale.

Hidden from view ten years ago, one company followed up a research paper presented at one of the International Web Coating Conferences organized by Bob Bakish and developed a method of oxidizing the depositing aluminum coating in a standard metallizer. This technique of using a modified resistance-heated aluminum evaporation metallizer to deposit transparent aluminum oxide coatings has become the new hope for producing transparent barrier coatings.

It is a technology that looks to offer transparent barrier coatings at a cost much closer to that of opaque aluminum metallized coatings. So again we hear predictions for a huge growth in transparent barrier coatings.

High barriers needed

Ten years ago the requirement for ultra-high-barrier coatings for organic displays had been identified, and it was established that a barrier performance six orders of magnitude better than for food packaging was required. This barrier performance target has proven difficult to achieve.

The reasons why the deposited metal, glass-like, or ceramic coatings fail to meet the bulk barrier properties are now much better understood. The effect of substrate quality on the barrier performance leads to the development of higher-performance substrates that have been heat stabilized, cleaned, and planarized. It has been shown that if you produce a clean, smooth, flat, high-surface-energy surface, a single-layer dense barrier coating can achieve ultra-barrier performance.

When the surface is not ideal, it is common for a polymer layer to be deposited in the vacuum system before the inorganic layer is deposited in order to produce a new smooth and clean surface. This polymer deposition process also can be used after the inorganic layer has been deposited, so it can protect the freshly deposited layer as well as fill in any defects in the inorganic coating.

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So in the last ten years, we have moved from research laboratory exemplars to pilot production. However, the promises of product so far have been exaggerated; large quantities of material can still be difficult to achieve; and the costs are significantly higher than the display industry would like.

Although this in-vacuum polymer deposition process has been available for about 20 years, it has been massively underused. I suspect the patent position and commercial considerations actually prevented this technology from being developed and used much more widely. Over the next ten years, I would expect that as the original patents run out, there will be an increased use of this technology, not only to enable higher-performance barrier production but as a way of developing multilayer optical and electronic materials.

This need for ultra-barrier transparent material has become even more urgent as the organic display market for flexible photovoltaics has appeared. The flexible photovoltaics (solar cells) are being scaled up, and as the process changes from research to pilot production and full production, there is an urgent need to produce modules and arrays that have a long life. It has been established that these materials also need to be encapsulated — and with a barrier material that is at the ultra-barrier type of performance.

These flexible photovoltaics include the copper indium diselenide type cells, but eventually these barrier materials will be needed for both organic and inorganic flexible photovoltaics.

It does not take a genius to see that one of the potentially large growth markets for vacuum-deposited coatings is in the area of barrier coatings. The rate of growth for transparent barrier food packaging coatings is good, with predictions for retortable transparent barrier materials expected to be in excess of 20%. However, the growth for both the organic display and the photovoltaic markets will dwarf this figure and will continue for much longer. The challenges also will be much greater as the performance must be so much better, and the price likely will be reduced considerably over the next few years.

With the markets so large, the competition will be considerable, and this too will help drive prices down.

Machinery widens

Metallizers from previous generations still are recognizable, but while there have not been any radical step changes in the technology, there has been significant progress. Over the past ten years, metallizers have continued to get larger; the widest one now produced stands at 4.45 m wide and is able to metallize at speeds of 1,250 mpm. This width is approximately half the mill roll size for some film production lines. It will be interesting to see if anyone will take the leap to try to produce a metallizer that can use a full-width mill roll and gain the benefits of the film symmetry, reduced slitting, and cleaner film.

The use of printing technology to enable pattern metallization has continued development throughout the past ten years, with the latest improvement being the ability to produce metallized patterns in register with earlier embossing or printing. This simple use of in-register pattern metallizing is expected to open up opportunities for electronic applications. This technology works well for aluminum metallizing but is expected to need further development to enable the pattern deposition of thicker electronic coatings that may be deposited at slower speeds.

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Seeking greater efficiency

Another area that attracts some interest has been in reducing power requirements. This has included different options for the boat shape that can reduce the volume of material and surface area, thus reducing the radiant heat loss and achieving the same temperature at lower power.

At least one system manufacturer offers an economy operating mode in which various parts of the system are switched off for periods of time to save energy. This is done without affecting the process speed or coating quality. As the aluminum deposition process is not particularly material-efficient — with some machines operating at material efficiency as low as 35% and the best around 60% — there has been some interest in a technology that has deposited aluminum onto metal foil at greater than 99% material efficiency.

This process uses a magnetically levitated molten pool of aluminum and a single exit from the source across the width of the substrate. This type of slot source or jet vapor source is being actively developed by several groups for a variety of different materials, but all have the common aim of producing highly uniform coatings with a high material efficiency.

The green factor grows

The ecological aspect of vacuum coating was not much of an issue ten years ago, but with sustainability and the move toward “green” materials of major importance today, this aspect is expected to grow in the foreseeable future.

There is confusion in this area, with some fighting for biodegradable films as being the best way to achieve environmental goals, while others want the use of biodegradable films restricted as it is believed that this encourages disposal rather than recycling. Biodegradable films are being metallized by a number of companies, and this quantity is expected to increase while the debate continues.

What is clear is the expectation for packaging to continue to be reduced by either elimination of material layers or reduction in thickness. However, the performance of the packaging — whether mechanical or barrier — is expected to remain the same.

The sputtering sector

Sputter roll coating as a general business appears to be stable but has never quite achieved expectations. Ten years ago, there was a desire for more transparent conducting coated material, but there was a worldwide overcapacity for machines that could produce these coatings. The gap between the words and the reality was the cost and quality of the coatings.

In reactively depositing the indium tin oxide (ITO) coatings, there always was the possibility of arcing disrupting the conductivity of the coating, but as the coating was transparent irrespective of the conductivity, the ability to map the good parts and bad parts of any roll became a quality requirement. The difficulty of meeting some of the specifications and the slow rate of deposition meant that many of the coatings were too expensive, and so many potential users stayed with thin glass substrates. The advantage of converting to roll-to-roll transparent conducting substrates never was deemed compelling enough. Over the past ten years, this market continues to be lower than predicted. In recent years, there was a large price increase for indium targets, and the concern over diminishing resources has caused some companies to change strategy.

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There is currently a great effort to find a low-cost transparent conducting coating to replace ITO. The most widely used alternative is probably aluminum zinc oxide (AZO). Both the aluminum and zinc are cheaper and more abundant than the indium.

Over the next decade, I would expect there will be a gradual move away from ITO toward AZO and other transparent conducting coatings. However, unless there is another large spike in the price of indium, I would expect this to take time since ITO generally offers superior performance, meaning compromises must be made to change to an alternative.

Powering up

Power supplies for sputtering have continued development to help address deposition problems associated with reactive deposition. Various arc suppression or management techniques have been developed that help production sources work arc-free for longer periods of time.

The more exciting development, however, has been the high-power impulse magnetron sputtering (HIPIMS), in which a very high power is applied to the magnetron source for a very short time. This enables a very high ionization of the depositing material, and in turn, this enables denser coatings with more equiaxed crystal structures to be deposited.

The speed of pulsing and the change in the plasma characteristics can help improve the sputtering rate and uniformity as well as the adhesion and density of the deposited coatings. As this process becomes better understood and more widely available, there will be more roll-to-roll coating systems that will use the process.

Two other technologies that also will become more widely used for roll-to-roll coating are atomic layer deposition (ALD) and atmospheric plasma deposition. The first is being developed in two different ways, and both look as if they will result in production equipment within the next ten years.

Atmospheric plasma has been around for longer than ALD for roll coating machines and has the attraction of not requiring a vacuum system. Thus it was expected to produce lower-cost coatings. However, the cost of helium for the process and the limited deposition rate mean it has not yet delivered coatings to challenge those produced by vacuum deposition.

Ten years ago, roll-to-roll vacuum system manufacturing was dominated by the production of aluminum metallizers. This is no longer the case. With the explosion of the photovoltaic market, there have been a large number of startup companies requiring pilot and production machines to produce photovoltaics by roll-to-roll vacuum deposition.

Many of the photovoltaic materials require several vacuum coating machines, since some of the layers are sputtered and others evaporated. Also, between the layers there may be a scribing step.

With the economic crisis, the initial surge in requirement for these photovoltaic deposition machines has waned, and as the market becomes more cost-conscious and some companies fail, there may be a thriving second-hand machine market for some time. But this is only a short-term change; the market has not disappeared but has only been delayed.

It is expected that the dominant market for vacuum coating machines will be for photovoltaics and the barrier materials required to encapsulate them, as well as for organic displays.

To sum up, despite the general economic world gloom and the fact that metallizing will remain a challenging market with relatively low margins, I would say the opportunities for the vacuum coating market as a whole have never been brighter.

Dr. Charles A Bishop, C. Eng., is principal of C.A. Bishop Consulting Ltd., Shepshed, Leices, UK. He is a member of AIMCAL's Technical Advisory Panel and has published more than 50 technical papers and holds five patents. He can be reached at cabishopconsulting@cabuk1.co.uk.


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