Remarkable new materials that combine novel semiconducting electronic properties with the processing characteristics of polymers, offer the prospect of flexible displays and broadband optical amplifiers.
These flexible, low-cost polymers under development are made up of snowflake-shaped molecules called dendrimers, which emit light when electricity is passed through them. During tests, light emitting diodes using the new dendrimer proved the most efficient ever recorded in the world for a device manufactured in a similar way. “The advantage is that it is flat but flexible”, said Samuel. “We can design materials with desirable properties, such as the ability to emit the three primary colours enabling us to make any colour on the spectrum. Dendrimers have the potential to make long-term improvements in colour quality, power efficiency, lifetime and processing costs of displays.”
Applications of Organic Semiconductors
These organic semiconductors, synthesised by Dr Paul Burn at Oxford University and Dr Oleg Salata at Opsys Limited, can be used in a variety of display applications, from mobile phone displays to food packaging. In the future it may be possible to use the material in light emitting wallpaper in a variety of colours as an alternative to traditional overhead lighting. The material could potentially be applied to clothing, from school uniforms to sports gear. Other potential uses for curved display screens could be for demonstrating animated cooking instructions on food tins or downloading football results on to beer cans.
Figure 1. Organic semiconductors glowing under ultraviolet light.
Semiconducting Polymers and Optical Amplifiers
Semiconducting polymers is also one strand of research of the UK-wide Ultra fast Photonics Collaboration (UPC), with its core at St Andrews University. Optical amplifiers are one of the key components of a fibre optic system and it was their invention that heralded the large-scale introduction of optical fibre into transmission systems. The bandwidth of existing amplifiers is limited, however and UPC is looking at novel types of amplifiers based on polymers or quantum dots, to extend the useful bandwidth range.
Understanding and Controlling the Properties of Organic Semiconductors
“We are looking at the fascinating interaction between light and matter from a variety of angles” explains Prof Samuel, based at St Andrews University, the core of the consortium. “The light emitted by a material provides a window through which we can study the underlying physics. By changing the electromagnetic environment of a light emitter, e.g. in a photonic crystal, a micro cavity or even a simple grating, its radiation can be dramatically altered and tailored to a specific application. Engineering the size of the material itself gives us another handle on radiation properties, e.g. by reducing the emitters down to clusters of 10's or 100's of atoms (‘quantum dots’ or ‘nanocrystals’), we can dramatically change the emission wavelength, even create white light. The new materials thus created are very exciting, both from a fundamental physics point-of-view and for display applications. We are also looking at the spin of an electron, how it interacts with light and how these phenomena can be applied.”
Performance of Optical Amplifiers
In their aim to achieve operating speeds in excess of 100Tb/s, UPC has recently demonstrated that dilute polymer solutions can act as broadband optical amplifiers by increasing the intensity of weak light pulses 200,000-fold.