Yong Lei

Low temperature magnetic field effects in Alq3-based organic light emitting diodes

The magnetic field effects on injection current and electroluminescence have been investigated for aluminum tris(8-hydroxyquinoline) (Alq3)-based organic light emitting diodes at the temperature of 12 K. The experimental traces of electroluminescence exhibit a rapid rising at low magnetic field, followed by a decrease at high field strength, whereas the injection current increases continuously. The drive dependence of the high field effect of the quantum efficiency matches that which is expected for the triplet-triplet annihilation process, indicating that the delayed fluorescence from the triplets’ annihilation significantly contributes to the field dependent light emission in our devices.

Magnetoelectroluminescence in tris (8-hydroxyquinolato) aluminum-based organic light-emitting diodes doped with fluorescent dyes

The influences of fluorescent dye doping on the magnetoelectroluminescence in tris (8-hydroxyquinolato) aluminum (Alq3)-based organic light-emitting diodes have been investigated systematically by varying the dopant concentrations and its energy band gap. Our results show that the decrease in electroluminescence intensity at high magnetic field, which survives only at low temperatures for pure Alq3-based devices, persists in dye-doped devices even at room temperature. This is explained here as the result of magnetic field dependent triplet-triplet annihilation process, in which the triplet excitons trapped on the dye molecules play the most important role.

Positive and negative components of magnetoconductance in hole transport limited organic light-emitting diodes

Two kinds of devices using N,N′-Di(naphthalen-1-yl)-N,N′-diphenyl-benzidine (NPB) and dye-doped NPB as emitting layer were fabricated to study their magnetoconductance (MC) response. The MC of the NPB devices contains a positive low-field (0 40 mT) component at low temperatures. Similar MC is presented in the dye-doped NPB devices even at room temperature. Magnetoelectroluminescence results and energy-level diagram indicate that long lifetime triplet excitons and excessive holes are in these devices. All these observations suggest that triplet exciton-hole reaction is responsible for the negative MC while positive MC is assigned to hyperfine mixing of electron-hole pairs.

Control of magnetoconductance through modifying the amount of dissociated excited states in tris-(8-hydroxyquinoline) aluminum-based organic light-emitting diodes

Magnetoconductance (MC) is generally believed to be controlled by the ratio of singlet to triplet excited states. In this study, it is found that the MC magnitude of tris-(8-hydroxyquinoline) aluminum-based organic light-emitting diodes decreases substantially upon the introduction of narrow band gap fluorescent dopants. Since singlet to triplet ratio of excited states keeps unchanged in doped devices, this large reduction in MC means that other underlying mechanism affects the MC. The charge carrier trapping effect is proposed here to vary the magnitude of MC. By using this trapping effect, the controlling of the total amount of dissociated electron-hole pairs and consequently the magnitude of MC are realized by changing the dopant’s concentration or band gaps.