Television screen

Scientists in Italy have developed organic light-emitting transistors (OLETs) that are said to be 10 times more efficient than those previously reported and twice as efficient as organic light-emitting diodes (OLEDs).

The technology could be used to fabricate monitors with improved resolution and may enable the development of electrically pumped organic laser devices using transistors with no loss of light. These could be used in fibre optics for the telecommunications industry.

‘Our OLET devices display a micrometre-sized light source with light generation kept separate from the electrical current, which helps maintain a brightness and efficiency,’ says Professor Gianluca Generali from the Institute of Nanostructured Materials in Bologna. ‘This has led to an external quantum efficiency of five per cent, compared to 2.2% for optmised OLEDs using the same structural materials.’

Existing OLEDs have intrinsic efficiency limitations due to their structure.

‘Unfortunately, the close spatial proximity of the electrical contacts and the light-generation region [in OLEDs] cause some emitted photons to be absorbed, resulting in photon loss’, explains Generali. ‘Similarly, the largest quenching effect in OLEDs, called exciton-charge quenching, reduces the number of excitons, and occurs due to a spatial overlap of excitons and charges.’

While OLETs can reduce this quenching effect, they have not been optimised until now, achieving an efficiency of just 0.6%.

The ‘trilayer field-effect’ OLET structure, developed by the team in Bologna, uses a three-layer organic field effect transistor structure, consisting of a hole-transporting (p-type) layer, a light emitting middle layer, and an electron-transporting (n-type) layer on a polymethyl methacrylate dielectric and an indium tin oxide gate layer deposited on a glass substrate. By providing better control over the electrical fields, this structure enables a double order of magnitude increase in efficiency compared with using the same organic-electro luminescent materials in
an OLED configuration.

‘This “trilayer” architecture enables the light-formation and light-emitting regions to be located far enough away from the electrodes so that photon losses and exciton-metal quenching are prevented,’ says Generali. ‘Also, the light-emitting region is
physically separated from the charge flows, which prevents exciton-charge quenching.’

These structures are made by thermal evaporation, with shadow masks and patterning, and can be applied to a range of substrates, such as silicon, glass and plastic.

The team are now looking to further improve the efficiency of the trilayer OLETs by decreasing the operating voltage and carefully tuning each part of the structure.

Dr Geoff Williams of the OLED group at Thorn Lighting in Spennymoor, UK, says that the work shows potential for future OLET devices. However, he argues that, while the structures could be used for small area niche applications, the cost of mass-production may prove to be a barrier for general lighting applications.

Generali acknowledges that this is an area that will need further investigation.