Improving the view - coating applications for photovoltaics

Materials World magazine
1 Mar 2009

Greg Gibson, Chief Technology Officer at digital fluid dispensing and coating company NexTechFAS, USA, describes developments for electronic applications.

The manufacture of electronic materials involves sophisticated processes requiring precise control, which means the technologies need constant redesigns to remain competitive. Current and next generation electronic materials are being used for a variety of products.

While companies face issues relating to material deposition, many are incorporating a time-tested solution – extrusion deposition (ED) coating. This has been used to apply wet chemistry to semiconductor wafers and glass panels for almost two decades. It is now being explored for thin film photovoltaic solar cells.

Sunny disposition

Solar power is expected to deliver increasing amounts of eco-friendly electricity in the future. However, solar is not expected to reach grid-parity (the point where it may equal to or cheaper than electrical grid power) without subsidies until 2015. Lowering manufacturing costs is key to achieving parity. Equipment is needed for vacuum deposition, such as chemical vapour deposition or physical vapour deposition. The cost of this equipment can be millions of US dollars and can take up a substantial area in the solar factory. Material consistency and repeatability of the process are also crucial.

High-performance ED equipment to apply a wet chemistry process to cells would eliminate vacuum- based equipment for certain processes. Extrusion deposition equipment has the potential to decrease the total average cycle time, therefore increasing the throughput rate. It can also reduce the cost of manufacturing equipment, and has the ability to generate highly uniform thin film coatings. This allows for well-defined coating edges for solar cells with diverse chemistries.

Picture perfect

Flexible active-matrix displays are considered a milestone in the advance of appliance and consumer electronics industries. Engineers and designers will be far less restricted when integrating them into products, and the range using the displays will broaden significantly.

Plastic Logic, headquartered in the USA, built the first commercial plastic electronics manufacturing factory in Dresden, Germany, in September 2008. The company solves critical issues in manufacturing high resolution transistor arrays on flexible plastic substrates by using a low temperature process without mask alignment. This uses a mix of production equipment from display manufacturing, including ED.

A variety of organic and inorganic materials can be used in the displays. Deposition onto flexible substrates, however, is challenging. It requires planarity of the surface, substrate handling and fixturing (single sheet or roll-to-roll), and control of electrostatic charge, thermal processing and the effect of associated dimensional variances. Like solar, consistency and repeatability is essential, and traditional deposition processes cannot always ensure this.

Extruded deposition can handle both rigid and flexible materials, and accurately deposits inorganic or organic solution-based substances to thicknesses of 20nm (dry film thickness), with uniformity better than ±3%.

Natural view

Organic light emitting diodes (OLED) can provide brighter, clearer displays on electronic devices with faster response times, wider viewing angles and higher quality colour reproduction, while using less power than conventional LEDs or liquid crystal displays (LCDs). The OLEDs can also be used in computer displays, mobile phones, personal digital assistants, advertising displays and for general lighting and large-area light-emitting elements. They do not require a backlight function, draw less power, and can be thinner than traditional LCD panels.

In order for OLED displays to become more competitive and replace some entrenched technologies such as LCD and plasma, ongoing development of the materials and identifying effective manufacturing processes will be critical.

Small molecular OLEDs use inorganic material with small molecular structures. Like solar, layers can be built using sophisticated vacuum vapour deposition.

Polymer OLEDs (Poly-OLEDs) use organic polymers consisting of larger molecular structures, where traditional photolithography techniques are being researched because of advances in chemical-resistant polymers. Poly-OLEDs are used as a thin film for full-spectrum colours. Hole injection layers and hole transport layers (HIL and HTL) are most commonly deposited during the development of OLED. Emissive layer and electron transport materials (EL and ETM) can also be used together, as DuPont is in its OLED materials set.

There is a lot of expectation surrounding ink-jet technologies for material deposition because of their selective application and quick output design. However, ink-jet still has some challenges with resolution, repeatability and layer smoothness. For patterned materials, manufacturers are looking towards ink-jet, but ED equipment is more suitable for un-patterned layers and large areas. This can apply organic materials in the very thin layers required for uniformity and well defined coated areas, and in certain cases it eliminates the need for the edge bead removal process.

In addition, the low level of waste is important due to the high cost of these materials. The coating process of HIL, HTL, EML and ETL can all be conducted using ED equipment.

Highly flexible poly-OLED display panels are now being fabricated onto plastic substrates, as ED has the ability to handle both rigid and flexible materials, making it ideal for flexible OLED displays.

Reaching out

While touch screens have been a part of our daily lives for a long time, optical touch technology is developing, with promising new multi-touch screens that can support existing equipment while giving additional benefits for new applications such as Microsoft Windows 7. This will allow consumers to control computers using their fingers rather than a mouse or stylus.

The technology uses multiple optical sensors that track the movement of any object close to the surface by detecting interruption of an infrared (IR) light source. Some manufacturers are expanding on the concept of basic IR optical-touch systems with optical waveguides that deliver lower component costs and higher resolution due to digitisation of the receiving optical signal into separate optical waveguide channels.

In addition to the optical touch application, ED equipment can be used to apply thin conductive layers and films, which may have the potential to replace sputtered indium-tin-oxides in traditional touch screens that typically use capacitive and resistive technologies.

Further information: NexTechFAS