07 Jan 2003
Researchers produce an organic LED that is 25 times more efficient than the best quantum-dot LEDs to date.
Researchers have produced an organic light-emitting diode (LED) device that is about 25 times more efficient than the best quantum-dot LEDs to date. The structure contains a single layer of cadmium-selenium quantum dots sandwiched between two organic thin films. Seth Coe and colleagues at the Massachusetts Institute of Technology believe that their approach could be used to fabricate other hybrid organic-inorganic devices (Nature 420 800).
The advantages of organic LEDs are that they can be easily processed and have a high performance. When coupled with the excellent light-emitting properties of inorganic nanocrystals they can be used to produce real devices. They also have good colour tunability so they can be used to build full colour displays of red-green-blue emitters.
For an LED to be efficient electrons and holes must be brought together in a small region so that they can recombine effectively and emit photons -- without escaping or dissipating. Ideally, the structure of an LED is made up of three layers: a thin emissive layer that is held between a hole-transport layer and an electron-transport layer. The emissive layer should be thin to allow direct transfer of electrons and holes from the surface of the two transporting layers.
Seth Coe and colleagues, at the Massachusetts Institute of Technology, made a quantum-dot LED, in which the emissive layer of cadmium-selenium is only a few nanometres thick. This emissive layer is made up of single nanocrystals - each about 3nm in diameter - that are equally distributed in the array.
Coe and co-workers suggest that the electrons and holes in their structure are captured directly at the surfaces of the cadmium-selenium nanocrystals - into discrete electronic levels of the quantum dots - and emit light by recombination. The spread, or bandwidth, of the emission spectrum from these devices has a gaussian profile that is determined by the uniformity of the size of the quantum-dots -- which can be controlled during processing. This is important as the luminescent spectrum could be 'tuned' to particular wavelengths by varying the size of the quantum dots.
Another advantage of this type of LED, say the team, is that every electron-hole recombination can produce a photon -- leading to a possible 100% quantum efficiency at any visible wavelength. Compared to many other materials -- including the polymers used in conventional LEDs -- fewer than half of electron-hole recombinations can result in light emission because of statistical mechanics restrictions.
Belle Dumé is Science Writer at PhysicsWeb