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PV innovations: Thin-film technology update

Joyce Laird

Joyce Laird examines the latest technology developments in CIGS thin-film solar cells, and picks out the key new players in the industry on both sides of the Atlantic.

The ongoing price war between solar thin-film and crystalline technology has been levelled by falling silicon prices, which has resulted in the toughest solar thin-film market for several years.

Despite this, companies such as First Solar, the largest manufacturer of thin-film solar panels in the world, are holding their own with technology trusted by organisations such as banks and other financial institutions. First Solar's CdTe (cadmium telluride) photovoltaic (PV) is the first and only solar thin-film PV technology to ever surpass crystalline silicon in terms of low cost for a significant part of the multi-kilowatt systems market.

Solar thin-films, including CdTe, are known to be less efficient than most wafer silicon modules. Typical wafer silicon modules are roughly 13% to 20% efficient, while the best CdTe modules were about 10.7% efficient; but recent modules produced at First Solar and measured by The National Renewable Energy Laboratory (NREL) have shown CdTe modules with efficiencies at 12.5% or greater.

Recent installations of large CdTe PV systems by First Solar confirm the competitiveness of CdTe PV with other forms of solar energy. Using First Solar technology – a 40 MW system being installed by juwi Group in Waldpolenz Solar Park, Germany was at the time of its announcement, both the largest planned and lowest cost PV system in the world. The price of €3.25/W translated at that time to US$4.2/W, lower than any other known system.

On the manufacturing side, First Solar's manufacturing cost per watt reached US$1.23 in 2007 and US$1.08 in 2008. On 24 February 2009, the cost/watt broke the US$1 barrier with US$0.98/W. In Q4 2009, their production cost had fallen to US$0.80/W. And First Solar is contractually bound to reduce price per watt by 6.5% per year, and says it plans to be competitive on an unsubsidised basis with retail electricity by 2010.

First Solar has indicated that its manufacturing costs had fallen in Q3 2009 to US$0.85/W, down 2% in three months and down 21% in a year. By 2014, it says it expects to drive down cost per watt to make solar modules at between US$0.52 and US$0.63. The biggest driver of the lower costs is better efficiency.

The chasing pack- getting closer

However, according to published reports, Sharp is hot on First Solar's tail with its own solar thin-film product that pairs amorphous silicon with a layer of its own microcrystalline silicon to achieve higher stability and better performance than previously thought possible.

This tandem-junction architecture captures more of the solar spectrum and converts more sunlight into electricity. It is produced with less than 1% of the silicon used in crystalline solar cells, creating a less material-intensive manufacturing process that contributes to a lower effective cost per kilowatt hour for large-scale applications. The company's experience in this area, combined with a strong brand name, gives Sharp's brand of solar thin-film a lot of credibility.

While the CIGS (copper indium gallium selenide) players may be having some difficulty getting funding, there have been quite a few rays of sunshine on the technical front and the news of higher efficiency on production lines is encouraging. And many believe that CIGS could be the best long term hope for solar thin-film PV, due to potentially higher efficiencies than other solar thin-film materials.

Avancis recently announced in Germany that it had an NREL certified 15.1% aperture area mini-module, which measures 30 cm by 30 cm. Achieving more than 15%, not just on a small cell but on any in-between-size module, is impressive and is the highest efficiency ever demonstrated of its kind.

There is also a lot going on with troubled German company Q-Cells, a diversified solar player that now wholly owns Solibro. Q-Cells reports high efficiency at the solar cell developer and has announced its intention to ramp up capacity to 135 MW this year.

Global Solar from Arizona recently announced that it has achieved 13.2% on a module with its roll-to-roll CIGS product – that makes it a world record for that type of CIGS production.

Finally, news reports confirm that Showa Shell Sekiyu, a major Japanese oil company, has reiterated its commitment to CIGS technology and plans to scale production up to 900 MW over the next few years. The data from around the world shows that even in this financially tight environment people are feeling confident about CIGS and other solar thin-film technologies.

Meet the manufacturers

Applied Quantum Technology (AQT) based in California manufactures solar cells that are drop-in replacements for conventional crystalline silicon cells. AQT's patented and proprietary technology is based on the production of CIGS-type solar thin-film PV cells using the reactive sputtering process.

Sputtering is the proven standard for producing other high-volume cost-sensitive high-tech products such as hard disks and optical discs. This approach includes single-step deposition, nano-engineering, device-enhancing source materials and a simplified cell interconnect design.

“The biggest thing is that we are commercialising now and we will take delivery of our first system in April 2010,” says Michael Bartholomeusz, CEO of AQT. “We will be in production and will be shipping to our first customer at the end of this year. So we are right on our timetable – that is a comfort. From both an expectation and technical aspect we are meeting our goals for full commercialisation this year. Our first project is an off-grid micro-utility.”

Dow Solar Solutions (DSS) from Michigan is the breakthrough company to watch. Its groundbreaking technology, the Dow Powerhouse Solar Shingle, integrates low-cost, solar thin-film CIGS PV cells provided by Global Solar into a proprietary roofing shingle design. The product design reduces installation costs because the conventional roofing shingles and solar generating shingles are installed simultaneously by roofing contractors. DSS expects an enthusiastic response from roofing contractors since no specialised skills or knowledge of solar array installations are required.

“Our objective was to introduce a game changing BIPV [Building Integrated Photovoltaics] product,” says David Parrillo, the company's senior R&D director. “We think we've done that simply by attacking the balance of system cost, the installation associated with BIPV. Our product is a functional building product. Like any other shingle, its primary purpose is to keep water out of your home, only this shingle produces power too. Any standard roofer can install the product easily in a short amount of time.

“We wanted an efficient solar technology and we selected CIGS. We wanted our product to be able to be nailed into a roof with galvanised roofing nails just as a roofer would use on any roof. This product does do that. We designed proprietary connectors that would allow a roofer to nail down a shingle and place the one next to it and connect it very easily, seamlessly, and then nail down the next shingle.

“Think of each shingle as an independent solar generator; it goes in as fast as you can nail down three galvanised roofing nails so it all happens very quickly. There are no intricate interconnection schemes. That end-to-end connection of the shingles is proprietary to Dow. After the installation is complete, an electrician is called and it is connected to an inverter and then finally to the grid. Our end goal is to capitalise on this technology and to translate it into other products,” he adds.

Since 85% of all rooftops are asphalt, it will be very interesting to see where this technology leads when the products roll out in 2011.

High efficiency

With two manufacturing plants in Arizona and Berlin, Global Solar was one of the first companies to start putting CIGS down on flexible foil substrates.

“We have a lightweight, flexible, high-efficiency product and these are all things that work well in that market because of the many ways they can be integrated into a building in a cost-effective and very aesthetically pleasing way,” says Jeffrey Britt, president and CEO of Global Solar.

“We have a couple of large projects in place as well as a number of smaller things in the last two years and we have got partnerships with Dow Building Solutions for integrating our flexible solar CIGS product into its roof shingle product. We are proud to be its first choice partner as far as providing the PV portion of its product.”

In the latest example of increasing CIGS conversion efficiencies, the Department of Energy's NREL has confirmed that Global Solar has achieved 13.2% aperture-area efficiency on a solar thin-film PV module integrated with flexible cell-strings from its CIGS production line in Tucson, Arizona. Global Solar produces glass modules for utility in which its strings can be laminated into glass for the utility market, but its prime focus is BIPV.

“We plan on keeping a strong partnership with Dow and also really to come out with our own unique products for BIPV that are flexible and have an array of beneficial features,” says Britt. “We are in development to discover new cell connection technologies. We think we can reduce the cost of interconnection materials by about half by using a new technology. The interconnection portion is about half of total materials and production cost so we are looking at very serious cost reduction when we finally integrate these new technologies into our product ion lines.”

Intermolecular from California has started working on clean energy projects such as improving alkaline texturing of monocrystalline silicon wafers, and developing a more durable and cheaper anti-reflective coating for PV glass. The company is just starting work on a new development programme focused on making high efficiency CIGS easier to manufacture. This programme will be heavily leveraging Intermolecular's sputtering R&D platform, the Tempus P-30, which allows it to rapidly screen a wide variety of co-sputtered film compositions using multiple source targets simultaneously. This capability permits combinatorial fabrication of 100-200 unique CIGS solar cells every week. These in turn are fully characterised in an automated solar cell tester with the resulting data captured and analysed using advanced software to help researchers quickly sift through the results.

To help characterise the materials, processes and devices more comprehensively than has previously been possible, Intermolecular is taking the R&D platforms developed for the semiconductor industry and bringing them to the solar and clean energy arena.

“In the case of thin-film PV, and particularly in the case of CIGS, our combinatorial capabilities allow you to very comprehensively explore the phase space of many different materials that you want to put into a stack,” says Craig Hunter, vice-president and general manager of the company's solar and energy technologies unit.

“We can run a very comprehensive design of experiments where we characterise the composition of the thin-film cell and correlate the performance of that cell against its composition.”

Matt Roberts, director of media and communication at Washington-based independent consulting company Mac Strategies, believes he has a good perspective of what is happening in PV, in both regular silicon and solar thin-film technologies:

“Thin-film has done great things in the area of cost per watt,” he says. “They are competitive in that area but they have a way to go to get the efficiency to be competitive in the area of large-scale installs or home owners,” says Roberts.

Roberts sees solar shingles as having great potential because many home owners' associations are trying to block solar – which they deem unsightly – from being used in their neighbourhoods. Furthermore, solar panels are simply not allowed in areas that are considered historical.

Odersun, which has its headquarters in Frankfurt, has been making huge strides in spreading its unique solar thin-film technology into both main and niche markets in the BIPV area. The company has achieved certification from the International Electrotechnical Commission (IEC) and is ramping up for full production in Europe. It is planning a new plant in Beijing within the next three months.

Odersun's silicon-free solar thin-film solar cells and modules are made of copper indium disulphide semiconductor materials on long reels of copper tape (CISCuT). This copper tape is just 1 cm wide and 0.1 mm thick. It is used to form the CIS semiconductor and also acts as the substrate and carrier material for the solar cell. Using an internationally patented ‘reel-to-reel’ production process, the 0.001 mm thin active cell layer is created in just proprietary three stages.

“We have been working with our existing processes and have done some experiments to increase performance and that we have achieved,” says Hein Van der Zeeuw, CEO of Odersun. “With certification we are ready to go and as for efficiency we are at 6-7% and we aim to be at 8% by the end of the year.”

Partnerships with China

One of the biggest moves for Odersun has been its partnership with Advanced Technology & Materials (AT&M) in China. The companies have joined forces to address the fast-growing solar market in China, where they will start producing and selling solar cells and modules based on Odersun's solar thin-film technology.

“We expect to be in full production in Beijing in the second half of 2011,” says Van der Zeeuw. “So we will be in production in Germany as Odersun and in China in partnership with AT&M.”

Oerlikon, by contrast, is focusing on solar thin-film silicon and specifically on its ‘micromorph’ technology. This is the high-efficiency version of amorphous solar thin-film silicon. Since Oerlikon is a prime supplier of manufacturing lines for this technology, the financial crunch facing its customers has definitely been an issue.

But Christopher O'Brien, head of market development in North America, says the company is making very good headway on the performance of its equipment.

“A year ago we were talking about production costs of around US$1.50/W. Today the speed of the tools and efficiency of the production has brought this down to a dollar and by the end of this year we estimate this to drop to US$0.70/W. This will make us very highly competitive against the other solar technologies. The market is improving and financing is opening up again so companies like Oerlikon will have a bankable advantage.

“We are supplying the best core manufacturing for this technology. Our ‘factory in a box’ concept is a highly automated facility. If you go to see a line you will see the glass go in at one end and come out finished at the other end. It's automated and robots handle the movement along with other automated conveyors from step to step.

“We have greatly improved both the speed and yield of the line. We have rigorous operation protocols. But overall what we provide that is unique is that we allow customers to produce this thin-film silicon technology with a very low cost and with a predicted lower cost in the future. This is an investment pathway.”

John Drexinger, COO of Pro-Tech Energy Solutions based in New Jersey, says that the common factor when deciding on any solar thin-film technology is cost.

“Like any emerging technology, thin-film will have its place but it has been stalled a bit because of the slow market. The markets are coming back in China and Australia particularly – it will level itself out again,” he says.

“We use mostly crystalline but we have used some First Solar thin-film and like any technology it has a place in the market, particularly in the South West because of its thermal advantages.”

Veeco, also based New Jersey, focuses on producing the best deposition systems possible for flexible substrate or the ‘roll-to-roll’ metal or poly CIGS technology manufacturers. Veeco is now the only equipment company offering integrated production-scale solutions for CIGS manufacturing on glass using thermal evaporation sources for the CIGS/absorber layer.

“Thermal evaporation, as compared to other deposition methods, produces the highest efficiency thin-film solar cells and has the lowest materials costs with high material utilisation driving down the manufacturing cost per watt,” says John Patrin, senior director of product marketing and business development at Veeco.

“Veeco's FastLine systems can handle up to thirty 1.1 m by 1.4 m glass panels an hour. The modular architecture of the system allows customers to scale their output according to their needs. Today with our equipment we say that the solar manufacturer can come very close to a dollar a watt manufacturing and even below that with CIGS.”

A tight technology race

Overall, seeing some CIGS companies develop efficiencies up to 13% – which is very close to silicon at around 15% – it becomes apparent that this is becoming a tight race. Although each can be used for some of the same applications, there is room for both technologies if they focus on the strengths each brings to specific markets; for example BIPV for solar thin-film and residential for crystalline. From there, depending on cost per watt and the geography of the installation, the utility scale can divide up to use either technology depending on what fits the location.

About the author:

Based in California, Joyce Laird has been writing for a wide range of industrial magazines for over a decade. Her extensive background in the semiconductor industry created a perfect transition to covering developments in photovoltaics.

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