Functional metallophosphors for effective charge carrier injection/transport: New robust OLED materials with emerging applications

Organic light-emitting diodes (OLEDs) show great promise of revolutionizing display technologies in the scientific community. One successful approach for improved device efficiency has been to maximize the electron-hole recombination using dopants that emit from the triplet excited state. In this context, heavy transition metal complexes have recently gained tremendous academic and industrial research interest for fabricating highly efficient phosphorescent OLEDs by taking advantage of the 1:3 exciton singlet/triplet ratio predicted by simple spin statistics. Traditional room-temperature phosphorescent dyes are monofunctional materials working only as light-emitting centres but other key issues including charge generation and transport remain to be addressed in the electroluminescence. This Feature Article highlights recent and current advances in developing new synthetic strategies for multifunctional organometallic phosphors, which integrate both luminescent and charge carrier injection/transport functions into the same molecules so that they perform most, if not all, of the necessary functional roles (viz. photoexcitation, charge injection and transport as well as recombination) for achieving high-efficiency devices. Considerable focus is placed on the design concepts towards the tuning capability of charge-transport characteristics and phosphorescence emission colour of this prominent class of metallophosphors. In particular, the latest research endeavor in accomplishing novel triplet emitters with enhanced charge injection/charge transport of both hole and electron carriers is criticially discussed, which can provide good implications regarding their possible routes for future research development in the field.