Ab initio triplet-triplet annihilation rates for phosphorescent OLED emitters

Spin-orbit coupling (SOC) in phosphorescent emitter molecules containing heavy elements such as iridium and platinum enables radiative decay of triplet excitons, allowing nearly 100% charge-to-photon conversion in organic light emitting diodes (OLEDs) using these emitters as guests in host-guest emission layers. However, SOC also makes long-range Förster-type transfer of the triplet exciton energy from an emitter molecule to another emitter carrying a triplet exciton possible, leading to triplet loss. This triplet-triplet annihilation (TTA) decreases the efficiency and operational lifetime of phosphorescent OLEDs. TTA can be quantified by a Förster radius that can be calculated from the overlap between the emission spectrum and the triplet absorption spectrum of the emitter. Using advanced ab initio quantum-chemical methods that include solvation and vibrational effects, we calculate the emission and triplet absorption spectra of 16 phosphorescent emitters emitting in the visible and the infrared. As a general rule, we find that the calculated TTA Förster radius, and thus the TTA rate, decreases with increasing emission energy, in agreement with experimental results. However, emitter-specific exceptionally large or small Förster radii occur because of coincidentally large or small overlaps between peaks in the emission and absorption spectra. This emphasizes the importance of accurate ab initio calculations of these spectra in the search for emitters with low TTA. We find that stereoisomers can have distinct TTA Förster radii, highlighting the importance of isomeric effects. In contrast to previous studies of triplet-polaron quenching for the same 16 emitters embedded in various charge transporting hosts, we find that quadrupolar contributions to TTA are negligible.