Published online 17 February 2000 | Nature | doi:10.1038/news000217-11


Let there be more light

Light bulbs and television screens made from organic materials should start to appear in the coming decade. A new trick might help to make them brighter, says Philip Ball.

The stage is set for light-emitting diodes to displace the electric bulb and the cathode-ray tube as the lights in our lives. A report in Nature1 now shows how a class of potentially cheap LEDs can be made four times more energy-efficient than before. This should help them to become still more commercially attractive.

Thomas Edison's light bulb was a good idea at the time -- but that time has passed. The idea of making light by heating a twist of wire until it glows white-hot is starting to look decidedly archaic. Conventional bulbs don't last for long -- typically about 1500 hours. And much of the electrical power is wasted as heat, not light.

In light-emitting diodes, electricity is converted directly to light. This means that they can generate as much light as a bulb for a fraction of the power. LEDs are now being introduced into traffic lights, where they will last much longer and consume less energy than bulbs. If they can be made cheaply enough, LEDs will become the main light source of the twenty-first century.

Commercial LEDs use inorganic semiconducting materials that emit light when a current flows through them. Until recently, the only known such materials that had sufficient brightness and longevity for use in commercial devices were those that emit red and yellow light. In 1993 a blue-light LED was produced, making it possible to deliver the entire visible spectrum -- or to combine it in white-light LEDs suitable for indoor lighting.

Yet the light sources of the future might not be made from these hard, brittle materials, but from plastics and other organic compounds. Plastics that conduct electricity and emit light emerged in the early 1990s from research on polymers that behave like metals. The first polymer LED glowed a feeble yellow.

Now polymer LEDs straddle the rainbow, and are bright and long-lived enough to be close to commercial application. Cambridge Display Technologies, a company that emerged from the group at the University of Cambridge who made the first polymer LED, is now collaborating with Seiko-Epson to produce the world's first full-colour all-plastic television, "in the first quarter of 2000".

Stephen Forrest and co-workers at Princeton University have explored a different kind of organic LED, based on small organic molecules rather than the long polymer chains of the materials used for plastic devices. But, they say, nearly all organic materials used so far in LEDs waste about three quarters of the energy used to excite the material to a light-emitting state.

LEDs work by injecting electrons into, or extracting them from, the emitting semiconductor. The injected charges boost molecules into 'excited states', which then lose their excess energy by emitting a photon of light. Typically, however, only one in four of the excited states (called the 'singlet state') is of the sort that decays through light emission. The other three in four, called triplet states, can't get rid of their energy so easily, and they slowly fritter it away in other forms, mostly heat.

Now Forrest and colleagues report a way of extracting this hitherto squandered energy, which in principle boosts the energy-conversion efficiency of the LED by a factor of four. The trick is to channel all the excitation energy into the one in four states of the emitter that generates photons efficiently. Forrest and colleagues achieve this by mixing three molecular components in their LED material.

The first is a conducting organic material which provides a 'host' matrix for the other two, and which is excited by the flow of current. Dispersed amongst this host material is an organic dye that emits red light. Energy is shovelled from the excited host to the dye via a 'sensitizer' molecule, which is chosen for its ability to put energy from either its singlet or its triplet state into just the singlet state of the dye. The dye and sensitizer molecules were incorporated into separate thin layers of the host because, if they are too close together, it becomes easier for the sensitizer to feed energy into the 'wasteful' triplet states of the dye.

Forrest and colleagues say that this approach could be used to increase the efficiency of any organic LED, whether made from polymers or small molecules. 

  • References

    1. Baldo,M.A., Thompson, M.E. & Forrest, S.R. High-efficiency fluorescent organic light-emitting devices using a phosphorescent sensitizer. Nature 403, 750 2000. | Article | PubMed | ISI | ChemPort |