Long polymers emit more light

17 Jun 2002

The amount of light emitted by polymer LEDs depends on the polymer chain length, say researchers.

From PhysicsWeb

For polymer LEDs, the longer the polymer chain, the more light is emitted - this is the conclusion of work carried out by researchers in the US and the Netherlands.

Valy Vardeny of the University of Utah and colleagues say they have discovered a direct link between the polymer chain length and the number of light-emitting excited states, or excitons, produced (Phys. Rev. Lett. 88 197401).

Last year, the group made polymer LEDs that converted between 41% and 63% of incoming energy into light (see related story).

Until now, conventional theory has suggested that spin-triplet exciton formation within polymer LEDs restricts efficiency to 25% as only one light-emitting singlet exciton is produced for every three non-light-emitting triplet excitons.

In order to understand the high efficiencies produced in their previous work, Vardeny and colleagues compared the proportion of singlet and triplet excitons produced in polymers ranging in length from just a few polymer units to hundreds of polymer units.

The researchers found that the proportion of singlet excitons produced was much larger in longer polymer chains, irrespective of the shape of the polymer molecule.

Vardeny and colleagues speculate that this effect could arise from the wavefunctions of the singlet and triplet excitons. They suggest that the wavefunction of a singlet exciton is spread over the whole length of the polymer, while that of the triplet exciton is localized.

This would mean that the singlet and triplet wavefunctions are similar in small molecules but very different in longer molecules. But the researchers say they need to do more work to understand how the wavefunctions affect the production of singlet and triplet excitons.

"Our discovery is good news for LED research - especially for polymer LEDs - because it means that efficiencies may not have reached their theoretical limits," said team member René Janssen.