With suitable doping and molecular configuration, the characteristics and capacities of plastics such as polymeric poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT PSS) can be expanded and enable them to be used as electrical conductors and semiconductors. They can act as elements and system components in the developing world of “organic” and “printed” microelectronics.
“Organic” circuitry structures, including transistors, sensors and LEDs, are based on carbon derivatives, rather than silicon or gallium arsenide. Circuit patterns measuring just a few micrometres deep can be printed onto light, flexible and transparent substrates, by conventional mass printing processes.
Integration in objects
Printing and vapour deposition yield versatile, electronically or photonically functionalised films or coatings that can be applied in any desired curvatures to a wide range of objects, and even to textiles. They form capacitive touch sensors and large-area luminous fields in the form of organic light-emitting diodes (OLEDs) as well as complete sensors and detectors for environmentally or medically important data, such as temperature and humidity. They can be used as lightweight, flexible organic solar cells or as flat, printed batteries for powering miniaturised devices. This means that electronics and data technology can be seamlessly integrated beyond the conventional applications such as PCs, tablets or mobile phones, and opens up opportunities for exploitation of “smart” objects and the expansion of networking and independently operating data systems in the “Internet of Things”.
Organic and printed electronics is still a research-intensive sector. The latest (fifth) edition of the Roadmap of the Organic and Printed Electronics Association (OE-A) on the applications and technologies of organic electronics, illustrates the state of progress and trends for the coming ten-year period. With over 220 members worldwide, the OEA coordinates research and development projects and standardisation under the supervision of the International Electrotechnical Commission (IEC) TC119 and other organisations.
The first plastics-based microelectronics products are already available and the technology is regarded as a platform for a future industry that will unite printing technology, electronics and materials research. A number of developments were on show at the Printed Electronics Products and Solutions Pavilion at K 2013, including printing technologies and functionalised surfaces such as RFID solutions, flexible displays and OLEDs.
OLED screens and displays, the first mass market
The small OLED displays in mobile phones and smartphones have become a highly successful mass market for organic electronics. British market researcher Smithers Pira is forecasting growth of the sector as a whole into a global annual market of US $200bn by 2025 – equivalent to today’s market for silicon chips. Larger, colour-intensive and high-contrast 55″ OLED televisions screens have been announced or are already available, for instance, Samsung, LG, albeit at very high prices. The “electronic paper” e-readers from Amazon and Sony use energy-efficient, bistable electrophoretic displays.
The next step is the development of lighter, more flexible, possibly roll-up e-readers and tablets that will be produced without cover glass. British company Plastic Logic is already producing a backplane loaded with organic thin film transistors (OTFTs). The latest milestone development is a thin, flexible e-paper display with a 10.7″ diagonal that, with a resolution of 150dpi, makes up a matrix of 1,280 x 960 TFTs; 1.2 million pixels in total. Plastic Logic, in cooperation with Isorg in France, recently unveiled a 4 x 4cm image sensor with 8,930 pixels on a thin plastic substrate.
Sealed against water vapour
Flexible organic photovoltaics and display technology still need hermetic encapsulation for protection from atmospheric water vapour. So far, this has only been possible with rigid cover glass. The appropriate barrier solution for flexible displays is laminated films, for which transparent layers of amorphous silicon dioxide appear to be well suited. These are being collaboratively researched and developed at various locations, including the Fraunhofer Polymer Surface Alliance (Polo) and at the Japanese National Institute of Advanced Sciences (AIST).
Application driving forces
The OE-A Roadmap says that four major industries are driving application development: automotive, pharmaceutical, consumer electronics and food packaging. “Smart” packaging of consumer items with printed RFID tags can make merchandise management and large-scale logistics more efficient. Printed, dynamically updated display fields can communicate the best-before date to the consumer, monitor cooling for sensitive goods or guarantee the authenticity of high-grade articles with data links to traceable supply chains. German company PolyIC has developed RFID tags and printed antennas with conductive transparent organic films.
Some premium class cars are already fitted with printed antennas as well as printed sensors for seat occupancy integrated in the seat covers, which trigger airbags as necessary. OLED displays can be used for instrument illumination; barely-visible printed window de-icers; and linked to reverse-view cameras to replace traditional mirrors. OLED reversing lights are in the pipeline at Audi and organic displays and touch sensors have been used as replacements for mechanical indicators and switches.
OLEDs offer dynamically colour-controllable light emitted over a large area and so they can be attached to the surfaces of various objects in the home, turning them into active light sources. OLED lights are already available in design studies and premium products, including examples from Osram and Philips.
Organic photovoltaics and batteries
Researchers in organic photovoltaics (OPV) are developing hybrids made of titanium oxide and dye-sensitised solar cells in parallel with purely organic, polymer-based cells. Because of their relatively low efficiency, they are marketed as off-grid local supply sources for charging the batteries of mobile data, consumer devices and measuring stations. The long-term outlook of the OE-A Roadmap envisages applications in the envelopes of vehicles and buildings by 2021.
System components of organic electronics are available in printed data memories – for example, in the ferroelectric, non-volatile memory films of Thinfilm, the Norwegian manufacturer. Thinfilm combines its memories with a printed, first transistor logic board produced by Parc in California, to yield a software-addressable memory module. It can be extended with a printed thermistor as a temperature sensor, a display field from Sweden’s ICT Acreo research institute and a printed battery to create a compact measuring system.
Printed batteries are also a focus of development. Energy-rich supercapacitors can provide temporary power supplies and can be integrated with display and luminous fields, touch sensors and solar cells in packages, textiles and other consumer items.