Studying the change in organic light-emitting diode performance at various vacuum-deposition rates of hole and electron transport layers

Abstract

The electroluminescence (EL) of classic and thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) at various vacuum-deposition rates of hole and electron transport layer (HTL and ETL) has been studied. The external quantum efficiency (EQE) measurements showed that the best performance devices were those with a high charge carrier balance inside the emitting layer, which was engineered using hole and electron current manipulation as a result of vacuum-deposition rate control. Changing the vacuum-deposition rate of HTL and ETL leads to a change in the maximum EQE ( $$ {\text{EQE}}_{ {\mathrm{max}} } $$ ) of the classic and TADF OLEDs without obvious changes in EQE roll-off ratio at high current density. We used a simple analytical model to clarify that the enhanced hole current in HTL at high deposition rates is dominated by high hole mobility attributed to the increased hole hopping rate due to the reduction of the intermolecular separation between horizontally oriented N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) molecules. The increase in the electron current of tris-(8-hydroxyquinoline) aluminium (Alq3) ETL at low deposition rate was ascribed to high electron injection from cathode into ETL by the fabrication and comparison of J–V characteristic of two electron-only devices with a difference at deposition rate of ETL near cathode interface. Finally, we introduced an OLED with novel gradient and barrier structures for the emitting layer in which high injected charge carriers recombined inside added recombination zone to raise radiative recombination and efficiency of the device. Our results demonstrated that EL efficiency of an OLED can be changed by controlling the vacuum-deposition rate of organic layers.