The most common emissive pathways are fluorescence and phosphorescence. Fluorescence20 occurs through the recombination of a singlet exciton, while phosphorescence occurs through the recombination of triplet excitons. Radiative recombination of triplet excitons is spin forbidden; however, in phosphorescent materials, spin–orbit–exciton–photon interaction, typically due to the presence of a heavy metal atom, facilitates the spin flip necessary for a triplet state exciton to decay into a singlet ground state.20 Strong spin-orbital coupling also enables intersystem crossing, which is the conversion of singlet excitons into the higher spin multiplicity triplet energy state. Generally, all singlet excitons formed on phosphorescent materials are transferred to the triplet state, where they eventually decay resulting in the emission of a photon. Since the transition from a triplet state into the ground state is kinetically unfavorable, phosphorescence occurs over much longer timescales than fluorescence, typically in the microsecond domain. This allows more time for triplet excitons to undergo undesirable nonemissive pathways. However, the ability to harvest both singlet and triplet excitons allows phosphorescent materials to achieve much higher efficiencies than fluorescent emitters. As mentioned previously, in electroluminescence, singlet and triplet states are created in a 1 to 3 ratio. As only singlet excitons can decay emissively on fluorescence materials, the efficiency of fluorophores is limited to 25%. Phosphorescent OLEDs, however, can theoretically reach a quantum efficiency of 100%. Figure 4 shows a schematic diagram of fluorescent versus phosphorescent emission.