This class of materials has energetically close singlet and triplet excited states, in which ambient thermal energy upconverts the triplet states into the singlet states through reverse intersystem crossing (RISC). The other uses organic molecules that exhibit thermally activated delayed fluorescence (TADF) 5, 6, 7. The first relies on organometallic complexes with transition metals, such as iridium and platinum, which induce a large spin–orbit coupling to allow triplet states to emit photons as phosphorescence 2, 3, 4. To overcome this issue, two strategies for harvesting the ‘dark’ triplet excited states as photons have been established. This spin statistics limits the internal quantum efficiency of OLEDs and leads to the energy loss owing to the spin-forbidden nature of triplet excited states to emit photons. In the case of OLEDs, recombination of charge carriers leads to the formation of singlet and triplet excited states in a 1:3 ratio. The spin multiplicity of molecular excited states plays a crucial role in organic optoelectronic devices.
