Contents I. Introduction
Organic light-emitting diodes (OLEDs) are a promising technology for displays and lighting. They enable vivid, bright and high contrast displays to be made. They are now widely used in mobile phone displays and also used in some televisions. Advances in OLEDs are closely related to materials developments. In particular, efficient OLEDs need to harvest both singlets and triplets. One strategy for doing this is to use phosphorescent light-emitting materials in which a heavy metal causes strong spin-orbit coupling that enables the triplet to emit efficiently. An alternative strategy is thermally assisted delayed fluorescence in which a small energy difference between singlet and triplet energy levels means that triplets can emit via the singlet state by reverse intersystem crossing. Whilst both approaches have been very effective for green and red emitters, making blue emitters that fully satisfy the requirements of displays has proved very difficult.
