Longer lives for organic LEDs

The performance of organic light-emitting diodes can be improved by doping them with carbon-60 according to scientists at Samsung in South Korea.

The carbon-60 molecules can also extend the lifetime of the devices by a factor of two. Organic LEDs are potentially attractive for applications because they are easy to process and can emit over the full visible spectrum.

Light emission from organic materials relies on electrons and 'holes' combining to form excited states called 'excitons' that subsequently emit photons when they decay. A typical LED contains a thin light-emitting layer sandwiched between layers that transport the holes and the electrons. One way of improving the performance of organic LEDs is to increase the mobility of the holes in the hole-transport layer by adding a dopant. This should lead to more holes combining with electrons in the device.

Jun Yeob Lee and Jang Hyuk Kwon at Samsung's Corporate R&D Centre in Yong-In City studied the effect of carbon-60 doping in phosphorescent devices that rely on an organic material called 'TDAPB' as the hole-transport layer.

Lee and Kwon varied the concentration of carbon-60 in the TDAPB from 0 to 3% while measuring the properties of the device with a spectrophotometer. They found that the mobility of holes in devices doped with 3% carbon-60 was five times higher than that of pure TDAPB. The current density also increased by a factor of three, and there was a 30% increase in the luminance of the LED.

Doping with 3% carbon-60 also increased the lifetime from 700 h for the undoped device to 1400 h.

Carbon-60 is an electron acceptor that protects the TDAPB from being 'attacked' by excess electrons ejected from the light-emitting layer.