ABSTRACT
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25% of formed excitons are singlet (total spin 0) and 75% are triplet (total spin 1) excitons (Baldo et al., 1999b; Segal et al., 2003). Fluorescent dopants emit from singlet states, so only singlet excitons are utilized and triplet excitons are lost to nonradiative decay processes. As a result, uorescent OLEDs are limited to 25% internal quantum eciency, which is only ≈5% external quantum eciency (EQE; see Chapter 12 for a discussion of optical losses which reduce the external quantum eciency). Phosphorescent dopants can eciently harvest both singlet and triplet excitons, thus 100% internal quantum eciency can be achieved (20%–25% external eciency). erefore, phosphorescent emitters are crucial for high eciency OLEDs.
