LEDs made from long-chain polymers should emit more light than those based on short molecules, according to physicists in the US and the Netherlands. Valy Vardeny of the University of Utah and colleagues say they were amazed to find that the production of light-emitting ‘excitons’ in these materials depends on the length of the polymer chain. The discovery could lead to the development of ultra-efficient polymer LEDs (M Wohlgenannt et al 2002 Phys. Rev. Lett. 88 197401).
Many organic polymers emit light when a voltage is applied to them. This effect arises from the ‘conjugated’ structure of these polymers – that is, the alternate single and double bonds that link the carbon atoms in their ‘backbones’. In these compounds, electrons are ‘delocalized’ from their parent atoms and form two ‘molecular orbitals’ of different energies, which act as a valence band and a conduction band.
When a voltage is applied to such a material, electrons enter the conduction band and positive holes enter the valence band. An electron and a hole from these bands can bind together to form a neutral – but excited – entity known as an exciton. The exciton falls to its ground state when the electron and the hole recombine, and light can be emitted.
But light is only emitted by ‘singlet’ excitons, formed when the spins of the electron and the hole add up to zero. ‘Triplet’ excitons – formed when the spins add up to one – emit no light. This means that a polymer LED will emit more light if more singlet excitons are produced. But physicists long thought that the rules of quantum mechanics allowed just one singlet exciton to be produced for every three triplet excitons, limiting the quantum efficiency of polymer LEDs to 25%. But recent experiments have reached efficiencies of up to 63%.
In order to understand these high efficiencies, 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 team used techniques known as photo-induced absorption (PA) and PA-detected magnetic resonance to study films of the materials.
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. Earlier studies had found some evidence for such a link, but team member René Janssen says that he was still very surprised by the discovery. “In particular, the fact that polymers of different chemical natures appear on the same curve was very unexpected,” he told PhysicsWeb.
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,” says Janssen.