Photovoltaics

From Amorph to Micromorph:
Production Solutions for Thin Film Solar Modules

Photovoltaic production is rapidly growing to meet global demand. Detlev Koch- Ospelt, Head of Oerlikon Solar discusses recent success in enabling a quick start for manufacturing.

Since 2003, Oerlikon Solar, headquartered in Switzerland, has supplied turnkey production solutions for large-area thin film solar modules. It now has a global market presence as a provider of end-to-end production solutions for amorphous and, recently, micromorph tandem modules. Oerlikon Solar operates in a volatile market place with doubledigit growth rates and is well positioned to handle future growth in the solar industry as it supports its customer’s ever-changing needs.

Just a year after breaking ground in July 2006, Ersol Thin Film, a customer of Oerlikon Solar, has started production at its 6,000-square-meter thin film solar module fab. By August, mass production at the industrial-scale manufacturing facility for thin film solar modules based on amorphous silicon was already underway.

“This is another milestone for the participating companies, ersol Thin Film and Oerlikon Solar, and also for the photovoltaic sector,” said Dr. Uwe Krüger, CEO of Oerlikon, headquartered in Pfäffikon.

Proprietary Technologies Fill a Need
Oerlikon Solar delivered the required turnkey processing equipment for the range of thin film applications and installed it on schedule in close cooperation with Ersol Thin Film. Key elements for the amorphous modules are the Type KAI 1200 coating systems, which deposit the photovoltaic layer, the TCO 1200 systems (Transparent Conductive Oxide) for applying the transparent zinc oxide, and the LSS 1200 lasers for structuring the layers. Another advantage for customers of the company: the production systems of the entire thin film front-end and the corresponding proprietary process technology have all been optimally coordinated in the development.

By the end of 2007, Ersol Thin Film plans to boost its annual capacity to 40 MWp (megawatt peak). With the entire first year’s production already sold, planning of further expansion has started. According to Ersol Thin Film, 90 percent of the orders will be used for large-scale open-area solar power plants.

The modules, which have an area of 1.4 square meters, are produced through a fully automated process in the clean room, where all the work is normally done by robots. In the first step, a transparent, conductive TCO film is deposited on a 3-millimeter float glass substrate. Next the amorphous silicon film – thinner by a factor of 200 as compared with wafer-based (crystalline and polycrystalline) silicon cells – is applied in the KAI 1200. This method also makes a significant contribution to saving scarce semiconductor resources. The cells are then coated with a second conductive film and provided with a glass cover to protect the thin layer stack. Ersol Thin Film has invested over 80 million Euros in the production facility and created more than 160 new jobs. In the long term, experts expect thin film technology to gain a similar market share to that of waferbased technology.

That said, thin film silicon modules have the potential to achieve efficiencies of 10 percent and higher at significantly lower cost per watt peak than with wafer-based technologies. Amorphous silicon has advantages over competing thin film technologies in large-scale manufacturing, including the availability of raw materials and the environmental friendliness of the raw materials used. In this sense, the Ersol Thin Film solar technology fab represents the dawn of a new era of regenerative energy conversion.

Although the costs have already decreased by over 80 percent since 1980, additional drastic cuts will be needed if solar energy is to become more widespread. Apart from improvements to the productivity of manufacturing lines and reductions in material costs, cost-cutting efforts are focused on the development of innovative cell concepts and improved efficiency of the cells themselves. It is exactly these two aspects that are addressed by the micromorph tandem cell, the second-generation thin film solar cell developed by Oerlikon Solar. The company is now ready to supply customers with production equipment for micromorph modules. The market is already showing strong interest, and the first contracts for the new production lines have already been signed.

Micromorph Modules Are Wave of Future
Helping to strengthen Oerlikon Solar’s demand as a partner is Dr. Johannes Meier, the inventor of the micromorph tandem cell, who is in charge of the company’s research and development activities.

“In the early 1990s, the world was still unaware of the photovoltaic properties of microcrystalline silicon. It was believed to have too many flaws, rendering it unsuitable for these purposes,” explained Meier, now the chief technology officer of Oerlikon Solar. “We were able to disprove that notion.”

The term micromorph combines the names of the two different silicon materials contained in a tandem cell: microcrystalline and amorph. Compared with the simple amorphous cell, the new cell has an additional microcrystalline silicon film. This structure makes optimal use of the sun’s light spectrum because the two silicon layers – the top and bottom cells – convert the entire light spectrum into electrical power. The visible radiation, with wavelengths between 400 and 700 nanometres, and a portion of the infrared spectrum (700 to 1,100 nanometres) are used for power generation. As a result, the new cell has an efficiency level approximately 50 percent higher than the amorphous cell. “This gives our micromorph tandem cell the potential for efficiencies of 10 percent and more,” explained Meier.

Furthermore, the use of other material technologies may yield further optimisations of the new cell. It is also conceivable that the absorption of the cell could be adjusted for location as well as the type and duration of exposure to the sun. In addition, laboratory development work has already demonstrated triple cells with special reflective coatings that achieve an efficiency of up to 15 percent. For this reason in particular, many experts are convinced that the micromorph tandem modules will take its place as the strongest rival to wafer-based cells.

Micromorph Advantages Stack Up
Micromorph tandem modules advantages are not limited to the fact that it is made of environmentally friendly, non-toxic and readily available materials. It also uses only about half as much embodied energy as compared with crystalline cells. Consequently, thin film modules take just about two years to deliver the energy consumed in producing them – about half as long as wafer-based cells. A major hurdle was the transfer of laboratory results to wide-area solar modules and the ability to uniformly deposit high quality films across an area of more than one square meter. To achieve this, Oerlikon used synergies from the development of reactors to deposit amorphous silicon in the manufacturing of large flat-panel displays.

This advanced technology has a more complex structure than the conventional amorphous cell. The initial production steps, such as the TCO coating of the glass and creation of the amorphous layer of the top cell (approximately 0.3 micrometers), begin in the usual way. A bottom cell composed of approximately 1.5 micrometers of microcrystalline silicon is then deposited on this substrate. The back contact, lamination and protective glass cover are then identical to the first generation production process. This production sequence reveals a major advantage of the tandem cell: the existing FAB 1200 production line concept for amorphous solar modules could easily be upgraded by just adding the systems depositing the additional microcrystalline layer. Another benefit is that customers of Oerlikon Solar could start immediately with a production line for amorphous modules and later retrofit their facilities to micromorph tandem.

TCO Films Vital to Production
Regardless of whether an application is for amorph or micromorph; in thin film silicon technology, TCO films are indispensable for the direct conversion of sunlight into electrical power. Playing a major role, therefore, in the advance towards market-ready micromorph tandem modules was the development of the new TCO 1200, which meets the special efficiency requirements of the new technology through the special surface morphology of the deposited layers. This tool is an integral part of Oerlikon Solar’s FAB 1200 production line concept.

Oerlikon Solar’s TCO 1200 coating system, TCO stands for, transparent and conductive layers applied to one or both sides of the light-converting, semiconducting material. The characteristics of silicon thin film solar modules are defined by several key parameters: In addition to good conductivity and low optical absorption of the TCO, these include the so-called light trapping potential imparted to the photoactive silicon layer by the surface morphology of the TCO. Ultimately, all of these factors determine the performance of the solar modules and directly reduce the cost per watt peak. The new system by Oerlikon Solar is based on a low pressure chemical vapour deposition (LPCVD) process, which uses low-cost, environmentally friendly zinc oxide instead of the commonly used flouride-doped tin oxide. Low deposition temperatures of approximately 200°C impose less stress on the substrate and also permit more substrate options. In the deposition process, diethyl zinc and steam react at subatmospheric pressure to form zinc oxide. Under suitable process conditions, films can be formed with an excellent surface texture with no need for post-processing. Oerlikon Solar conducts direct testing of this quality during the production process by discharging and checking the TCO-coated glasses.

Book to bill
With the modular FAB 1200 concept, Oerlikon Solar covers both types of thin film production lines and is ideally positioned for the future solar market. At the end of July 2007 the company announced another major new order: CMC, a company based in Taiwan, has awarded Oerlikon the contract to deliver the entire production lines to manufacture solar modules with a total annual output of 40 MWp. It will be a fully automated production line with integrated testing systems. Moreover, the contract includes a mutual option not only to expand the capacity but also to convert the facility to micromorph tandem technology. Oerlikon Solar currently has orders on its books with a total value exceeding US$400 million. The new thin film solar modules are taking the economic utilisation of solar energy to the next level.