Fig. 1: Electroluminescence spectrum at 100 cd/m2 and the molecular structure of the Ir(III)-containing true-blue phosphor. Note that the luminescent profile has a good coverage of the true-blue region of visible spectrum.

Organic light-emitting diodes (OLEDs) are promising for the development of efficient, low-cost, flat-panel displays such as computer monitors and TV screens. Conventional fluorescent OLEDs have low efficiencies and research is moving towards the use of phosphorescent OLEDS, which in principle could be three times more efficient, thus displaying enhanced performance and greater energy efficiency.

Ideally, red–green–blue-emitting pixels could be used for producing the full-colour spectrum. Although red and green emitters are available, blue-emitting phosphors have proved much more difficult to realize because of their large emission energy gap—requiring a high excitation energy—and producing emission that encompasses many quenching processes, which reduce the efficiency of the phosphorescence. Thus, most phosphors have poor device performance—relative brightness, lifespan and chromaticity toward true-blue color—and there are few reports on these elusive materials.

Yun Chi from the National Tsing Hua University in Taiwan and colleagues1 have now successfully prepared true-blue phosphorescent OLEDs with much improved efficiency. They used benzyl-substituted carbene—a compound in which the pi-electrons between the carbene and the phenyl ring benzyl group are not conjugated — as ancillary chelating ligands for an Ir(III)-containing phosphorescent material. The carbenes have a strong Ir–ligand bond and quenching processes centered around the metal are avoided, thus greatly improving the efficiency of blue phosphorescence.

Still, these OLEDs are not as efficient as would have been expected, because some energy is transferred back from the phosphor to the host matrix materials, particularly under the operating conditions expected in a device. Nevertheless, this work opens up a route for further development of efficient true-blue phosphors. The next step is to investigate other host materials that may further enhance device performance and to improve the thermal and electrochemical stabilities of their phosphors.