Suppression of efficiency roll-off characteristics in thermally activated delayed fluorescence based organic light-emitting diodes using randomly oriented host molecules

The efficiency roll-off characteristics in highly efficient thermally activated delayed fluorescence (TADF) based organic light-emitting diodes (OLEDs) were effectively suppressed by controlling the molecular orientation of a 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host matrix. The hole mobility in the light emitting layer was found to govern the magnitude of this suppression. Three-dimensional finite-difference time-domain calculations and photoluminescence quantum yield measurements revealed that the optical characteristics of the fabricated devices and photophysical properties of the TADF emitter did not affect efficiency roll-off. CBP molecules adopted random orientations when films were fabricated at high temperature (350 K), resulting in low hole mobility, and shifting the recombination zone away from the interface of the emitting layer with the electron transporting layer. When CBP was randomly orientated, efficiency roll-off was suppressed by 30% at a current density of 100 mA cm^-2. This result indicates that control of the molecular orientation of the host can allow us to indirectly tune the carrier balance in OLEDs.