Highly efficient hybrid warm white organic light-emitting diodes using a blue thermally activated delayed fluorescence emitter: exploiting the external heavy-atom effect

To attain high efficiencies in hybrid white organic light-emitting diodes (WOLEDs), mutual quenching of the fluorophors and phosphors should be minimized. Efforts have been devoted to reducing the triplet quenching of phosphors; however, the quenching of fluorophors by the external heavy-atom effect (EHA) introduced by the phosphors is often ignored. Here, we observed that conventional fluorophors and fluorophors with thermally activated delayed fluorescence (TADF) behave differently in the presence of EHA perturbers. The efficiencies of the conventional fluorophors suffer greatly from the EHA, whereas the TADF fluorophors exhibit negligible changes, which makes TADF materials ideal fluorophors for hybrid devices. WOLEDs using a blue TADF fluorophor and an orange phosphor achieve a maximum forward viewing external quantum efficiency of 19.6% and a maximum forward viewing power efficiency of 50.2 lm W−1, among the best values for hybrid WOLEDs. This report is the first time that the EHA effect has been considered in hybrid WOLEDs and that a general strategy toward highly efficient hybrid WOLEDs with simple structures is proposed. Warm white organic light-emitting diodes with a power efficiency of up to 85.3 lm W–1 are reported by scientists in China. The emitters fabricated by Dongdong Zhang and co-workers from Tsinghua University in Beijing are based on the use of a yellow phosphor (PO-01) and a blue fluorophore (2CzPN), which exhibits thermally activated delayed fluorescence (TADF). The device operates with a total external quantum efficiency of 33.3%. The team says that such high efficiency is made possible by the minimizing the fluorescence quenching processes that can occur between the phosphor and the fluorophore. In particular, their TADF fluorophore is shown to be highly robust against quenching resulting from external heavy atom effects introduced by metal phosphors. The approach could lead to a simpler design of white emitters.

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