Highly efficient organic light-emitting diodes with completely oriented delayed fluorescent emitters

Thermally activated delayed fluorescent (TADF) materials are those that have a small energy gap between the lowest singlet and triplet excited states, enabling singlet excitons through thermal upconversion of triplet excitons even at room temperature. Thus, they can achieve external electroluminescence (EL) quantum efficiency (EQE) equal to that of phosphorescence based organic light-emitting diodes (OLEDs) [1]. Considering fluorescence and phosphorescence based OLEDs as the first and second generations, respectively, TADF materials can be positioned as the third generation realizing highly efficient OLEDs. Recently, the best EQE of 31.2% in green TADF based OLEDs was demonstrated [2]. Further, EQE can be greatly improved by transition dipoles horizontally oriented to a substrate surface; such a preferential dipole orientation is achievable by controlling molecular orientation.