Search

Organic light emitting bistable memory devices (OLEBDs) with a dual function of organic light emitting diodes and organic memory devices were developed by using 0.5 nm thick MoO3 as an interlayer between hole injection layer and hole transport layer. The hole transport unit with MoO3 interlayer played a role of a memory unit as well as a hole transport unit. High on/off ratio over 1000 was obtained at a reading voltage of 1 V and driving voltage was lowered by MoO3. In addition, two different luminances were obtained at the same driving voltage by changing writing voltage of OLEBDs.

Transparent organic bistable memory devices (OBDs) were developed by employing indium tin oxide (ITO) as an anode and a cathode for OBD. A cathode structure of aluminum (Al)/ITO was used and bistability could be realized with pure polyphenylenevilylene based polymer active material without any metal nanoparticle. Transmittance of over 50% could be obtained in Al/ITO based OBD at an Al thickness of 10nm, and an average on/off ratio around 100 was observed.

Origin of efficiency roll-off in phosphorescent organic light-emitting diodes was investigated with triplet mixed host devices and stable devices with little efficiency roll-off was developed. Efficiency roll-off was significant in the device with narrow recombination zone (RZ) and charge leakage out of emitting layer at high luminance was critical to efficiency roll-off. Efficiency roll-off could be reduced in triplet mixed host device with broad RZ and little charge leakage at high driving voltage. Triplet mixed host devices with an exciton blocking layer showed a quantum efficiency over 90% of maximum quantum efficiency at a luminance of 20000cd∕m2.

Color stability of white organic light-emitting diodes was improved by using a charge confining device structure, which can confine charges at the center of the emitting layer. A red phosphorescent emitting layer was sandwiched between blue phosphorescent emitting layers and there was little change of color coordinate from 100to10000cd∕m2.

Device performances of green devices with cathode structure of lithium quinolate (Liq)∕Ca∕Al were investigated and electron injection mechanism was studied using ultraviolet photoelectron spectroscopy. Power efficiency could be improved by 70% by using Liq∕Ca∕Al cathode structure due to efficient electron injection, and interfacial energy barrier lowering by Liq∕Ca∕Al metal cathode was observed.

Green phosphorescent organic light-emitting diodes (PHOLEDs) with a triplet mixed host emitting layer were developed and device performances were studied by changing host materials in light-emitting layer. Power efficiency of green PHOLEDs could be improved from 12.7to29.1lm∕W by using triplet mixed host emitting layer. Combination of hole-transport-type host with good hole injection properties and electron-transport-type host with good electron injection properties was effective to get high efficiency in triplet mixed host devices.

Device performances of green phosphorescent organic light-emitting diodes using (4,4′-N,N′-dicarbazole)biphenyl (CBP) and N,N′-dicarbazolyl-3,5-benzene (mCP) as an exciton blocking layer were investigated. CBP and mCP were introduced between hole transport layer and emitting layer to block triplet exciton quenching and efficient hole transport to emitting layer. The efficiency of green devices could be improved by more than three times by using mCP exciton blocking layer.

Light emission in phosphorescent quantum well structure which can confine excitons within an emitting layer was investigated. A multilayer quantum well structure which has a narrow triplet band gap host material sandwiched between wide triplet band gap host materials was designed, and device performances were studied. The multilayer emitting structure gave high efficiency of 47cd∕A compared with 11cd∕A of standard green devices.

The effect of indium tin oxide (ITO) surface treatment on hole injection in organic light-emitting diode with C60 as a buffer layer on ITO was studied. Double surface dipole layer was induced on oxygen plasma treated ITO surface, while no dipole formation was observed on ITO without surface treatment. Interfacial energy barrier between ITO and hole transport layer was reduced by 0.4eV by C60 modification on oxygen plasma treated ITO surface, while there was no change of interfacial energy barrier by C60 on ITO without surface treatment.