Dechun Zou

Carrier mobilities in organic electron transport materials determined from space charge limited current

We report the mobility in organic electron transport materials determined from current–voltage characteristics in the regime of space charge limited current. The electron mobilities obtained are highly dependent on applied electric field, similar to the cases using a conventional time-of-flight method. The electron mobilities of phenanthroline derivatives are high and comparable to hole mobilities of conventional hole transport materials. Two parameters, zero-field mobility and electric field activation parameter are discussed from the viewpoint of oxidation and reduction reactions. The electric field activation parameter increases gradually with the increase of ionization potential. These facts suggest that there is a good possibility for realizing low driving voltage in organic light-emitting diodes (OLEDs).

IMPROVEMENT OF CURRENT-VOLTAGE CHARACTERISTICS IN ORGANIC LIGHT EMITTING DIODES BY APPLICATION OF REVERSED-BIAS VOLTAGE

The effects of reversed-bias application on current-voltage and luminance-voltage characteristics of standard-type double-layer organic light emitting diodes [OLEDs], ITO/TPD(50 nm)/Alq3(50 nm)/Mg:Ag, were investigated. Both the magnitude of reversed-bias and the duration of reversed-bias application were systematically changed. Evident voltage shifts towards the lower voltage side in current-voltage and luminance-voltage characteristics were observed in the diodes which were treated under various reversed-bias condition. The longer the duration of reversed-bias application, the larger the voltage shift was. A maximum voltage shift of 0.7 V was observed in the diode treated under a -10 V bias for 3 h. Little change in luminance-current density relationships was observed for diodes which were treated under various reversed-bias conditions. The results were interpreted in terms of the movement of ionic impurities and orientational rearrangements of permanent dipoles in organic layers.

Spontaneous and reverse-bias induced recovery behavior in organic electroluminescent diodes

Double-layer organic electroluminescent diodes composed of a spin-coated polyvinylcarvbazole layer and a vacuum-sublimed tris-(8-hydroxyquinoline)aluminum layer were prepared. The diodes were driven at constant voltage and were also kept under short-circuit or reverse-bias conditions. Observations of luminance-current density-voltage relations at constant voltage driving were repeated. The decrease of both luminance and current density during constant voltage driving was observed. Both spontaneous and reverse-bias assisted recoveries of device performances were observed, and these degradation and recovery phenomena were discussed in terms of the movement of ionic impurities in organic layers.

Study on the degradation mechanism of organic light-emitting diodes (OLEDs)

Abstract The degradation mechanism occurring in organic light-emitting diodes (OLEDs), especially the recovery behaviour related to this degradation, was investigated. It was found that the recovery can to some extent occur spontaneously. However, recovery can also be achieved through the application of an external electric field. The recovery rate associated with electric-field induced recovery is considerably larger than that arising from spontaneous recovery. A degradation model which can explain well many of the phenomena related to the degradation of OLEDs is proposed.

Efficient blue emission from pyrazoline organic light emitting diodes

Abstract Two kinds of pyrazoline derivatives, 3-(4-chlorophenyl)-4,5-dihydro-1,5-diphenyl 1- H -Pyrazole, ( P 1 ) and 3-(4-methoxyphenyl)-4,5-dihydro-1,5-diphenyl 1- H -Pyrazole ( P 2 ) were synthesized. They showed efficient photoluminescence (PL) in both solid state and solution. The emission peak of P 1 was observed at 450 nm and that of P 2 was at 432 nm. Two types of organic electroluminescent devices (OELDs) using these materials as emitting layer were fabricated, one is a three-layered structure device fabricated by dry process and another is a single or double layer structure device fabricated by a wet technique. These materials were found to function as good blue light-emitting materials for OELDs. The device with a structure of ITO/TPD/ P 1 /Alq 3 /Mg:Ag showed a turn-on voltage of 3.5 V, while the device using P 2 instead of P 1 gave a turn-on voltage of 3 V, both of them give green light resulting from Alq 3 and the maximum light intensity was 5710 cd/m 2 under driving voltage of 8.5 V. The device of ITO/PVK+ P 1 /Alq 3 /Al emitted bright blue light resulting from P 1 . It exhibited a maximum luminance of 200 cd/m 2 at a driving voltage of 12 V, and the device of ITO/PVK+ P 2 /Al exhibited blue light of 200 cd/m 2 at a driving voltage of 11 V.

Recoverable degradation phenomena of quantum efficiency in organic EL devices

Abstract Double-layer organic electroluminescent (EL) devices composed of vacuum-sublimed TPD and Alq 3 layers were fabricated. The double-layer device was driven under constant-current condition and allowed to stand under short-circuit or reverse-bias conditions. Luminance–voltage–current density relations at a constant current driving were repeatedly measured. The decrease in luminance at constant current driving conditions, in other words, the decrease in quantum efficiency in addition to the increase of drive voltage was observed. The decreased quantum efficiency showed spontaneous and reverse-bias induced recovery. One of the two phenomena, the increase in driving voltage can be well explained using the internal electrical field model, [D. Zou, M. Yahiro, T. Tsutsui, Synth. Met. 91 (1997) 191; T. Tsutsui, M. Yahiro, D. Zou, Mat. Res. Soc. Symp. Proc. 488 (1998) 611] however, the decrease in quantum efficiency cannot be explained.

Red electroluminescence from a novel europium β-diketone complex with acylpyrazolone ligand

Abstract A novel poly-ligand europium β-diketone complex was prepared, which contains ligand of 1-phenyl-3-methyl-4(4-butylbenzoyl)-5-pyrazolone (HPMBBP). Its structure is Eu(TTA) 2 (PMBBP)Phen. We investigated its PL and EL properties. When it was used as emitting material, the new Eu-complex exhibited intense photoluminescence (PL) and electroluminescence (EL) at 613 nm in a fabricated three-layer electroluminescent device: ITO/TPD/Eu(TTA) 2 (PMBBP)Phen/Alq 3 /Mg:Ag. The maximum luminance of 16 cd/m 2 from the device was obtained at 12.5 V.

Recoverable degradation and internal field forming process accompanied by the orientation of dipoles in organic light emitting diodes

Recoverable degradation of a polyurethane (PU) derivative with large dipoles on the side chain was investigated. Spontaneous recovery and reverse bias-assisted recovery were observed. The observed spontaneous recovery and reverse bias-assisted recovery are explained by the formation of an internal electronic field due to the re-orientation of dipoles. The re-orientation of dipoles was evaluated by second harmonic generation measurement. The relatively large estimated value of x zyy (2) , approximately 7 X 10 -7 esu (2.93 X 10 -10 m/V) at 10 V, indicates a high degree of orientation of dipoles, which generates a relatively large internal electronic field.

Novel binuclear europium β-diketone chelate used as red emitter in organic electroluminescent device

A novel binuclear europium and yttrium b -diketone complex EuY(TTA)6Phen2 was synthesized. The electroluminescent device using it as an emitter was fabricated. When the structure of device is ITO/TPD/EuY(TTA)6Phen2/Alq3/Mg:Ag, the emission peak of 613 nm and the luminance of 99 cd/m 2 at 11 V from the device have been achieved. The results showed that the new b -diketone chelate possessed better PL and EL properties, and film formation than those of Eu(TTA)3Phen. q 2000 Elsevier Science S.A. All rights reserved.

Short-term degradation behaviors of light emitting diodes made of polyurethane derivative with large permanent dipoles on the side chain

Abstract The effect of permanent dipole on the performance of organic electroluminescent devices (OELDs) was investigated by using a model polyurethane derivative (PU) with large dipoles on its side chain. The used OLEDs emits green color with an emission maximum at 540 nm. Decreases in luminance and current density during a constant voltage driving were observed. Evident spontaneous recovery and reverse bias assisted recovery of the degradation were confirmed. The observed degradation and recovery phenomena were discussed in terms of internal electronic field formation due to the re-orientation of dipole in organic layer.