(1) EQE is the product of four terms (eq. 1) 3 , the most important of which for emitter design is the exciton utilization efficiency (nST), the ratio of generated excitons that can undergo allowed radiative transitions to the ground state. This is because the charge carrier balance (γ) and photoluminescence quantum yield (PLQY, ΦPL) can both equal unity in an optimally functioning device, while the outcoupling efficiency (nout) is typically limited to only 20% as the transition dipole moments of the emitter molecules distributed isotropically. Meanwhile, nST depends on the mechanism of emission, because under electrical excitation excitons are generated through hole and electron recombination in a 75:25 ratio of triplets to singlets. Thus if a material is only capable of productively harvesting singlet excitons to produce light, as is the case for purely fluorescent emitters, its internal quantum efficiency (IQE; the product of γ, nST and ΦPL) will be limited to 25%, and its maximum EQE will be limited to around 5%. By contrast, phosphorescent emitters can harvest both singlet and triplet excitons, owing to strong spin-orbit coupling between the excited states which permits singlet to triplet interconversion via intersystem crossing (ISC) and reverse intersystem crossing (RISC). These triplet excitons can then radiatively decay via phosphorescence, thereby allowing 100% of the generated excitons to emit light. High spin-orbit coupling is normally induced by the inclusion of a heavy metal atom (or atoms) in the emitter, as is the case with organometallic complexes. However, the heavy metals, most commonly Pt and Ir, that these so-called 2 nd -generation emitters rely upon have a number of inherent drawbacks, including: uneven distribution across the Earth's crust, low natural abundance compared to the atoms of all-organic emitters and, to a degree, relatively high cost. As such, there is still significant demand for materials that emit via alternative triplet harvesting mechanisms, and of these the most promising is that of thermally activated delayed fluorescence (TADF). In this mechanism the singlet-triplet splitting energy (ΔEST) is sufficiently small to permit endothermic upconversion of triplet excitons into singlets via RISC. If the gap is sufficiently small, this can be achieved even with the modest spin-orbit coupling present in all-organic emitters. These upconverted triplet excitons can then emit from the singlet excited state at the same emission wavelength as the initially generated singlet excitons, but with a longer overall excited state lifetime. This mechanism likewise allows 100% of generated excitons to emit light but without necessitating the inclusion of a heavy metal.