Organic light-emitting diodes (OLEDs) have emerged as an important alternative to traditional light-emitting devices such as LCDs. Amongst their advantages, OLEDs are self-emissive and addressable on the scale of single pixels, and so do not require a backlighting panel. As a result they are energy efficient, and can produce pure black with high contrast, while also are able to be fabricated on a multitude of diverse substrates opening the door to very thin, foldable/rollable and even semi-transparent displays. The first-generation of OLEDs used fluorescent emitters and thus were limited to a maximum internal quantum efficiency (IQE) of 25%, as only the singlet excitons could produce light. The two main classes of compounds used to raise this efficiency ceiling to 100% IQE are organometallic phosphorescent (Ph) complexes (used in most current commercialized OLEDs) and more recent thermally activated delayed fluorescence (TADF) emitters. In PhOLEDs singlet excitons are converted to triplet excitons, all of which phosphoresce with the assistance of the heavy metal ion, which promotes strong spin-orbit coupling (SOC) facilitating both intersystem crossing (ISC) and phosphorescence emission. In TADF OLEDs, triplet excitons are up-converted to singlet excitons by reverse intersystem crossing (RISC), all of which may then fluoresce. TADF is possible when there is both a sufficiently small singlettriplet energy gap (ΔEST) and non-zero SOC between the relevant singlet and triplet states of different orbital type.