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Light Emitting Polymers leading to Plastic Electronics

Author: S.S.Newaz. Polyorganix, Inc. (www.Polyorganix.com)

Use of Liquid Crystal Display or LCD is ubiquitous in electronic consumer goods. The technology has become better and cheaper over the years for a varieties of applications. But LCD technology has insurmountable limitations like: high power consumption, bulkier weight, lack of robustness, limited contrast, limited viewing angle, just to name a few. Large LCD-run displays are expensive and difficult to produce.. Most of the LCD-run technologies also need an auxiliary backlight for their performance. Other alternative innovations also suffer from similar physical and electronic technological constrains -- especially when applied in high electronic information displays. Increase in this technical demand needs a breakthrough to overcome these barriers.

Light emitting polymers (LEP's) for the first time will offer a single technology that will combine ease of device fabrication with enormous flexibility for the choice of the substrate and high energy efficiency. This technology is on its way to revolutionize the electronic display industry. We could see flexible TV screen rolled on like a magazine or a high-powered nano computer on our palm.

What are the LEP's and how do they work?

The Nobel prize in chemistry in 2000 was awarded to three scientists: Prof. Shirakawa of Japan, Prof. Heeger and Prof. MacDiermid of USA. Their work led to the development of so called conductive plastics, with semiconducting properties like silicon. Polymers like common rubbers and plastics are long chain compounds, which do not conduct electricity. Conductive polymers, therefore, are a very special type of materials, which do conduct electricity. The structural requirement for these polymers are based on the fact that these polymers have a cloud of "movable electrons" which can flow, just like in metals -- and thereby become conductive materials. They are also known as conjugated polymers or electroactive polymers. Some of these polymers when subjected to an electric field, do not only conduct electricity, but also emit light of different colors -- red, green or blue or even beyond the visible light range in the UV or near infra red region. This special class of conductive polymers is nown as Light Emitting Polymers (LEP's).

In the nature, organic pigments absorb sunlight and then absorb, emit or fluoresce different part of the visible light -- which we then see as a particular color. Similarly, LEPs emit light in response to applied electric current. The wavelength or the color of the light produced depend on the precise chemical composition of the polymer used. In terms of wavelength availability, efficiency and usefulness, LEP's are equivalent to traditional LCDs.

One other parameter for LEPs to be versatile and simple in its applications is that the polymer be soluble in common solvents or even water. Research chemists are using the knowledge of synthetic organic chemistry to develop and improve a host of these polymers for optimum performance. Set up of a LEP device consists of simple steps like:

  • Use a transparent plastic or glass substrate.
  • Cover it with columns of thin electrode (anode).
  • Cover the electrode columns with layers of LEP solutions.
  • Finally cover the top with rows of electrode (cathode).

When electric current is applied, light emits at the junctions of columns and rows (anodes and cathodes). These can be the pixels for display. The LCD technology requires many more layers of components and only inflexible inorganic substrates can be used, making the choices limited and initial investment high for sophisticated equipment needed. LEP technology on the other hand provides simpler technology requiring a substrate, an anode, LEP solutions, and a cathode. Use of LEP solutions allows one to use a varieties of substrate including thin flexible plastics. This would make the Plastics electroactive and light emitting. A Fantastic product!

Potential Use:

  • Plastic TV: Flexible flat TV screens for television or computer are one of the most common application anticipated. Use of the solutions of LEPs, would allow much higher pixel density , which would be extremely useful for high density video display.
  • Ink jet printer ink: Use of the solutions of LEPs are being developed for use in ink cartridge of color printers. Since all different types of patterns of various shapes and sizes are possible with the LEPs, patterning becomes much simpler.
  • New industry of Plastic Electronics: will develop using conducting and Light emitting polymers. For example, these polymers would replace components of transistors. Instead of rigid silicone surface, flexible plastic electronics would be printed more like colored pages of a magazine and they would be rolled like newspaper rolled in machines.
  • Solar cells: Conductive polymers can also be used in solar cells. These polymers can provide ways to fabricate cheap solar cells and provide readily accessible solar energy.

Benefit of LEPs:

Unlike LCDs, LEPs actually emit light. The advantage of these organic polymeric materials over metallic or liquid crystal materials is based on the fact that the application for fabricating the electric devices is relatively simpler. Their solubility in common solvents make it easier to apply. There are also advantages of using lower amount of electrical energy making them more energy efficient. Simpler technology would allow lower investment requirement. It would also have less environmental impact.

Summarizing the overall benefits of LEPs over LCDs in different areas:

  • Good contrast and viewing angles; no complex back light is required; complex light emission pattern possible; very high resolution possible for any shape or size.
  • Highly patternable; suitable for ink jet and for flexible substrates leading to light weight solid-state construction with portability and ruggedness.
  • Fast switching speed for video displays.
  • Highly energy efficient: Lower power consumption leading to greatly increased battery life.
  • Printable onto thin films for continuous roll-to-roll coating leading to innovative product concepts.
  • Low barrier of entry to technology of Plastic Electronics for new semiconductor markets.
  • Less environmental impact:
    • Fewer raw materials required.
    • Huge energy saving.
    • No toxic material -- like mercury used in the backlight required in LCD display.

There are various different types of LEP's -- based on their chemical structures. They may show different colors before or after electrical conductance: may have different physical characteristics in terms of shelf life or solubility in common solvents or may have different energy efficiencies. Some of the major classes of the LEP's and conductive polymers along with their potential use are named below:

  • Poly phenylenevinylene (PPV) -- Phone display application, flat screen TV, color printers cartridge, nanotube composites.
  • Polyflourene (PF and PFE) -- Color screen for TV and Video
  • Polythiophene (PT) -- Antistatic photographic film.
  • Polyaniline and Polyphenylene (PANI & PP)-- Computer screens.

Within each class of these polymers, there are tremendous variations in chemical structure, molecular chain length (molecular weight) and chain structures. These variations may impart improved ease of application (solubility, for example), color intensity, photoluminescence or electroluminescense efficiencies. Overall synthetic procedure would differ as well. Because of this tremendous diversity of structures and relatively new chemical procedures required, intense R&D works are currently underway for both to make these materials efficiently (chemical synthetic procedures) as well as for their specific applications. This all adds up for a host of new innovative products for a bright colorful future.

At Polyorgaix, we have the expertise and basic understanding of the application requirements and the synthetic routes to produce these exciting new polymers, the LEP's and other conductive polymers. We cordially invite any request to undertake projects in this field.

 

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