Diffusion-Free Intramolecular Triplet–Triplet Annihilation Contributes to the Enhanced Exciton Utilization in OLEDs

Triplet–triplet annihilation (TTA), or triplet fusion, is a biexcitonic process in which two triplet-excited molecules can combine their energy to promote one into an excited singlet state. To alleviate the dependence of the TTA rate and yield on triplet diffusion in both solid and solution environments, intramolecular TTA (intra-TTA) has been recently proposed in conjugated molecular systems able to hold multiple triplet excitons simultaneously. Developing from the previous demonstration of TTA performance enhancement in sensitized upconversion solutions, here similar improvements in triplet harvesting in solid-state films are reported under electrical excitation in organic light emitting diodes (OLEDs). At low dye concentration and low current densities, the intra-TTA active OLED shows a +40% improved external quantum efficiency with respect to the reference device, and a TTA spin-statistical factor f ^4DPA of 0.4, close to that determined in fluid solution for the individual chromophore (0.45). These results therefore indicate the utility of this molecular design strategy across a wider range of TTA applications, and with particular utility in the further development of low-power TTA-enhanced OLEDs.