Physicists at the University of Utah say that the efficiency of organic light-emitting diodes is unlikely to exceed 25%. Or at least in the case of one particular organic polymer used to fabricate devices that are finding their way into computer displays, and which in future could provide low-cost, flat-panel lighting solutions.
Organic LEDs (OLEDs) work by injecting positive and negative charge carriers, each of which carries a magnetic dipole, or spin. When the charges meet there are four possible combinations of total spin that can arise, but generally speaking only the singlet will produce light. It is possible to use so-called triplet emitters made from organic phosphors, but the emission from these materials is slow, and the resulting photochemistry can have a negative impact on device lifetimes.
“Our results provide the most compelling evidence so far that a spin polarisation in an organic semiconductor is remarkably persistent,” says John Lupton, who with his colleague Christoph Boehme led the research effort. “If the spins did flip-flop around, then we could get more than 25% in the singlet channel, which would be good news. But this does not seem to be so.”
The results apply to one particular polymer – MEH-PPV – but could also be relevant to others. Doping of organic semiconductors may improve matters, but even if this doesn’t work out, OLEDs could still find wide application owing to their very low cost.
What we will not see, says Lupton, is OLEDs being used in high-power applications such as spotlights. Large area illumination such as flat-panel lighting, on the other hand, should not be adversely affected by the reported efficiency limitation.
Further reading: “Spin Rabi flopping in the photocurrent of a polymer light-emitting diode”, McCamey et al., Nature 7, 723 (2008).
This is a revised version of an article first published in Nanomaterials World.