Organic radical emitters: nature of doublet excitons in emissive layers

Organic radical emitters have received significant attention as a new route to efficient organic light-emitting diodes (OLEDs). The electronic structure of radical emitters allows bypassing the triplet harvesting issue in current OLED devices. However, the nature of doublet excited states remains elusive due to the complex nature of emissive layers. To date, the computational efforts have treated radical carrying materials as isolated entities in the gas phase. However, OLED materials are applied as thin solid films where intermolecular interactions significantly impact optoelectronic properties of the devices. Here, we combine molecular dynamics simulations and quantum chemical calculations to evaluate the effect of emitter-host interactions on the performance of radical-based emissive layers. Results demonstrate that intermolecular interactions remarkably modulate the electronic properties of the radicals in the thin solid films. The doublet excitons of isolated emitters demonstrate a hybrid character of charge-transfer (CT) and local-excitation (LE), while the emitter-host clusters present a significant CT character. Further, the impact of static and dynamic disorders on the hole-electron recombination is studied. Although the host-emitter interactions simultaneously decrease radiative rates and increase non-radiative rates, the latter rates are 100 times smaller than the former rates, allowing internal quantum efficiency to reach 100% for the doublet-based emission process. The results of this study highlight the significant impact of host-emitter interactions on radiative and non-radiative recombination processes and offer guidelines to tune these interactions for advancing radical-based OLEDs.