Light Emitting Diodes (LEDs) are an essential component of flat panel display systems and, demands for low cost-systems have led to many researchers to consider device fabrication using efficient organic phosphorescent materials. A significant benefit over LCD displays is that they do not require backlighting, and hence consume less power. As with other color display systems, a colour image is generated from a mixture of red, blue and green light. While highly efficient green organic LEDs (OLEDs) are known, blue OLEDs have until now been somewhat less efficient.

Fig. 1: Blue LEDs

Now, Junji Kido and co-workers from Yamagata University and the Optoelectronic Industry and Technology Development Association in Japan1 have developed a highly efficient blue OLED. The efficiency of OLEDs is assessed by measuring the ratio of photons produced per second to the number of electrons supplied per second — known as the external quantum efficiency (EQE). Green OLEDs have been produced with EQEs up to 30%, but until now, blue OLEDs have not approached this level.

A typical OLED consists of two organic layers — a conductive layer and an emissive layer sandwiched between two electrodes. When a voltage is applied across the OLED, electrons flow from the cathode to the anode. The emissive layer gains electrons from the cathode, and the conductive layer transports positive charge (electron-holes) from the anode towards the emissive layer. When the charges recombine the energy is emitted in the form of a photon of visible light. The low efficiency of blue OLEDs has been partly attributed to the low mobility of electrons in the conductive layer.

Kido and co-workers synthesized a new electron transport material for use as the conductive layer. The compound was based on a substituted benzene ring bearing two dipheylpyridine groups. When the new material was used in an OLED combined with standard emissive and electrode materials, they obtained an EQE of up to 21%.