Rough diamond

Ramped up production capacities, increased investment in thin-film modules for larger scale as well as building installations and falling prices are all helping thin-film technology become more mainstream in spite of the rapid price reductions in the crystalline silicon PV module market. Thin-film silicon falls between alternative thin-film PV technologies CIGS and CdTe, in terms of efficiencies and production costs, and it could emerge as the thin-film PV technology of choice, especially in China where investments in large-scale production are being made.
Driving the growth of thin-film silicon PV manufacturing and capacity increase – mainly amorphous silicon (a-Si) and microcrystalline silicon (µc-Si) – is the increased availability of large area deposition systems from several equipment suppliers. Device structures for single junction a-Si modules are more straightforward to fabricate compared to a-Si/µc-Si tandem structures. But while tandem cell thin-film silicon PVs are a newer development and more complex to produce they are able to achieve higher efficiencies than single junction. New thin-film silicon PV entrants will start out with capacity to produce a-Si-based modules, but will make the step change to manufacturing higher efficiency modules based on tandem and multi-junction cell structures in time.
Critical to the success and performance of a thin-film electronic device – and solar cells are no exception – is understanding how the layers in the thin-film sandwich interface with each other and to be able to adjust or tune these layers accordingly to improve device performance.
Like the crystalline PV module segment of the PV industry, the thin-film PV industry is focused on increased power conversion efficiencies in modules. In the thin-film silicon PV industry, depending on the market being supplied, efficiencies range between eight percent for a-Si and eleven percent for a-Si/µc-Si modules.
The transparent conductive oxide (TCO) components in thin-film silicon PV devices, and usually supplied as a TCO glass substrate to manufacturers is emerging as critical in terms of unlocking higher cell and module efficiencies.
This is becoming apparent as the number of sources of TCO coatings and glass flourish. Dominating the market for TCO glass in thin-film PV are Asahi Glass Company (AGC) and Nippon Sheet Glass (NSG) Group, respectively the world’s largest and second largest producers of flat glass. Both operate dozens of float glass lines – industrial equipment for producing uniform, flat glass for markets that include construction, automotive, displays and more recently solar energy. Both companies are introducing advances in TCO glass to enable increases in the power conversion efficiencies of silicon thin-film PV modules.
AGC is working to optimize both the advantages of its off-line (after float production) and online (during float production) TCO glass processing methods to produce TCO glass for amorphous silicon PVs, with high optical transmission, low electrical resistivity and high light scattering. A TCO glass substrate with these properties can enhance the efficiencies of thin-film silicon PV modules. AGC undertakes R&D on TCO developments in Japan, Europe and the U.S.
According to AGC’s customer evaluations, the use of the company’s recently introduced mobility enhanced TCO on low iron glass substrate, called ANS10 ME, increases module efficiency by five percent for single junction amorphous silicon, compared to the TCO on regular float glass.
Explains Jeroen Schotsaert, AGC Glass Europe’s Product Manager, Thin Film PV Solar Glass, “Some of our single junction customers have reached record efficiencies up to eight percent with the ANS10 ME.
The tandem junction customers were already using the low iron glass substrate but the use of the mobility enhanced TCO – ANS12 ME – allowed them to increase the module efficiency with two to three percent, taking efficiencies close to ten percent.” The ANS10 ME and ANS12 ME are produced online in Moustier, Belgium, to supply the European market. Schotsaert adds, “AGC aims to close the gap between the online and offline TCO within six months and to support customers in achieving module efficiencies to eleven percent and higher.” The company is one of the industrial partners in the EU Seventh Framework project HELATHIS (High Efficient Very Large Area Thin Film Silicon Photovoltaic Modules).
The project, which began in 2010 and is running for three years, is optimizing properties of TCO layers at the front contact and the back reflector, for large area deposition, to make thin-film silicon PV competitive with other electricity sources. Work to reduce reflection losses at the front glass is also being undertaken by the project partners. The project is developing a process for transfer into industrial facilities that will enable production of large scale modules – 5.7 square metres – with eight percent efficiencies for single junction and eleven percent for tandem modules with module cost well below one euro/watt.

2.5 percent efficiency gain

NSG Group, another leading global suppliers of TCO glass for the thin-film PV industry, is also advancing its TCO glass for silicon PV. Tim McKittrick, who is Photovoltaic Sector Manager at Pilkington Group in the UK, NSG’s flat glass business, gave a presentation during EU PVSEC on his company’s prototype advanced TCO glass, termed ‘Mashu,’ for online TCO glass manufacturing. Mashu TCO gives typically an additional 2.5 percent efficiency gain over standard TCO products achieving a greater angle of light scatter, and reduced absorption in the TCO layer, thereby increasing the amount of light that enters the PV junction. The results have been verified by several commercial manufacturers of thin-film silicon modules. The Mashu TCO prototype is the result of improving morphology of the grains in the TCO layer. In simple terms, better light scattering, which in turn enables more photons to enter the active PV cell layer, is achieved when the grains of the TCO are larger and more uniform. Gaining better control over grain morphology is key, as grains that are too small fail to enable adequate light scattering and grains that are too big can reduce cell voltage. Compared with NSG’s most widely supplied TCO glass for thin-film silicon PV, known as TEC A9X, Mashu is able to produce better overall grain uniformity.
To help measure and collect data to assess light scattering, NSG Group has used a technology developed by the University of Southampton. Wavelength Angle Resolved Scattering (WARS) is where a light source is directed onto the back of the substrate, scattered light intensity is measured with a photo-detector mounted at different angles and light is measured over a range of wavelengths at each angle.
The plan is to roll out the prototype Mashu TCO technology across NSG’s production lines, says McKittrick. ‘This will start to happen in 2012 and ultimately all lines should have such capability.’ Currently NSG’s plant in China, Changsu, is being converted over to TCO glass production, which will start to produce its first samples in autumn this year. The investment in Changsu will prepare NSG for the anticipated capacity expansion in the region for producing silicon thin-film PV modules.
TCO glass and coating is increasingly a concern for more players along the PV value chain besides the global TCO glass manufacturers. Equipment supplier Oerlikon Solar has generated much interest with its turnkey thin-film silicon PV module line ThinFab, since its launch in September 2010. The company announced in June the first order of a complete 120 megawatt ThinFab production line, based on Micromorph technology, by a customer in Asia. Micromorph is an amorphous-microcrystalline cell design, with roadmap target efficiencies of 11.9 percent. Oerlikon Solar’s fully automated low-pressure chemical vapor deposition (LPCVD) TCO system forms part of the ThinFab line. The system is able to produce TCO layers with tunable surface morphology. The ability to adjust light scattering achieves maximum light efficiency while allowing the manufacturer to reduce the thickness of the thin-film silicon absorber layers that if too thick, result in a reduction in the cell’s power output, but if too thin also impact light absorption.
As thin-film PV manufacturers increasingly look to tandem and other types of multi-junction thin-film silicon PV cell configurations to achieve higher efficiency modules, control of the morphology and size distribution of the deposited µc–Si, becomes increasingly important. Changes to the balance between microcrystalline and amorphous composition will affect the properties of the film and impact the cell’s efficiency.
While installing entire turnkey production lines such as ThinFab is one, albeit expensive, route for PV manufacturers looking to produce high efficiency thin-film silicon PV modules, demand for either TCO tools or suppliers of TCO glass that are equipped to enable tunable surface morphology is growing. One such glass producer that has invested in a process able to produce TCO glass based on needs of the manufacturer is Euroglas. The firm supplies high quality flat glass, with operations in Germany and central Europe. Euroglas was set up in 1993 by a consortium of five mid-sized glass processing companies. The parent company is Glas Trösch in Switzerland. Euroglas promotes itself as an alternative source of flat glass independent of the major corporations and currently operates four float lines. In 2009, Euroglas set up its solar business making glass at its plant in Haldensleben on state of the art production lines. The company works closely with customers in the PV industry and processes substrates according to client specifications. Earlier this year, around the time of Intersolar in May, the firm launched its TCO glass.

Aluminium zinc oxide

Instead of using tin oxide, which is applied by pyrolytic methods – where the coating is deposited at high temperatures within the float glass process – Euroglas uses aluminium zinc oxide for the TCO coating. Processing is done off-line so that optical and electrical specifications can be adjusted variably according to the produced module type. The process is able to achieve high flexibility, sheet resistance and achieve high optical transparency, electrical conductivity and high light scattering. So far the company’s offering has garnered positive feedback, says Euroglas’ Rüdiger Schulz. “Much of the TCO glass supplied to the thin-film PV industry is a “take it or leave it” product. We adjust our TCO to the customer’s absorber, not the other way round. By working with Euroglas Solar, clients can deposit thinner layers of silicon within their existing facility and can therefore enhance their output.” While it’s difficult to quantify the cost of TCO glass as part of the cost of producing a thin-film silicon PV module, around 15 to 20 percent is a good benchmark. If manufacturers can enhance the performance of TCO glass, by reducing the amount of material required for the PV absorber layers, and enhance module efficiencies, this suggests there may be significant scope in future for greater diversity in TCO glass and coating offerings in the thin-film silicon PV industry.
One of the newest kids on the block is the Finnish start–up Beneq. The company is commercializing two types of advanced deposition technology within thin film and other microelectronics industries, including PVs and flat panel displays. To target applications for its nAero technology in thin-film PV, Beneq has collaborated with Finnish supplier of glass processing machinery and software tools Glaston. Glaston’s range of solar glass process tools are mainly for flat tempering, flat laminating, bending, drilling and edging. The company’s newest line, the TFC2000, targets the TCO glass industry and can be used by both conventional glass manufacturers planning on supplying TCO glass and also thin-film PV manufacturers. The first order for TFC2000, worth about 14 million euros, is being shipped to a thin-film silicon PV manufacturer in China. Beneq’s CEO, Sampo Ahonen, observes, “China is aggressively ramping up thin-film silicon production and we have an offering that’s different. The nAero process we are commercializing is more cost effective because there is less waste.” Compared with some TCO glass and coating processes TFC2000 can reduce costs by 20 or even 30 percent.
The nAERO process combines advantages of spray pyrolysis and CVD, without their drawbacks, and can be applied both on-line and off-line, claims Beneq. The process is based on the vaporisation of well-controlled droplets in the coating chamber. The primary droplet size and size distribution is of critical importance for the resulting thin film. The glass substrate is brought to the coating process at such a temperature so that the thermal energy of the glass substrate vaporizes the droplets close to the surface of the substrate to be coated, before coming into contact. As well as TCO coatings for PV, low -emissivity glass and self-cleaning that meet industrial requirements have been produced with the process. The company is also working with partners and potential clients in the thin-film silicon PV industry to gather more performance data on the TCO glass application for thin-film PV.
All of these various approaches by suppliers of TCO glass, thin-film equipment and more novel deposition tools suggest that thin-film silicon PV will play a key part in a thin-film future, combining higher efficiencies with the economics of large area substrate manufacturing.