Xu Shao-hong

The effect of rubrene as a dopant on the efficiency and stability of organic thin film electroluminescent devices

Rubrene was doped into the hole transport layer of an organic thin film electroluminescent (OTFEL) device with a double-layered structure. It was found that the dopant has a profound influence on the EL characteristics - it changed the region of light emission, increased the luminescence efficiency by more than 50% and improved the device stability tenfold. The reasons for these effects are discussed based on injection theory and the energy level diagram of the device.

Energy transfer and white emitting organic thin film electroluminescence

Abstract Energy transfer between in N , N ′-bis-(1-naphthyl)- N , N ′-diphenyl-1,1′-biphenyl-4-4′-diamine (NPB) and rubrene was investigated. The device ITO/CuPc/NPB:rubrene/blocking layer /Alq/MgAg, in which copper phthalocyanine (CuPc) is used as buffer layer, NPB as the hole transporting layer (HTL), trimer of N -arylbenzimidazoles (TPBi) [or 2–(4–biphenylyl)–5–(4–tertbutylphenyl)–1,2,3–oxadiazole(PBD) or 1,2,4–triazolederivative(TAZ)] as the blocking layer, Tris(8-quinolinolato)aluminum complex (Alq) as electron transporting layers (ETL), can not give white emitting light. White emitting light can be realized in a new device with the structure ITO/CuPc/NPB/blocking layer:rubrene/Alq/MgAg, in which rubrene is doped in blocking layer instead of in NPB. The emission spectrum of this device covers a wide range of visible region and can be adjusted by the concentration of rubrene. The white emitting devices with CIE coordinates x =0.31, y =0.32, maximum luminance 8635 cd/m 2 and luminous efficiency 1.39 Lm/w, were obtained with the blocking layer TPBi doped with 1.5% rubrene.

Improving Stability of Organic Electroluminescent Diode by Inserting Copper Phthalocyanine Between the Anode and Hole Transport Layer

Copper phthalocyanine(CuPc) thin film layer was inserted between the anode indium tin oxide and the hole transport layer TPD of an organic thin film electroluminescent device with a double-layered structure. It is found that the CuPc layer greatly improves the device stability. The durability of the device with CuPc layer increases about 8 times. The fact that at a constant current density the driving voltage remains unchanged with operation time for the device with CuPc layer means that the barriers of the carriers injection are stable due to the inserted CuPc layer.

Effects of microcavities on the spontaneous emission of organic light-emitting diodes with ZnO:Al as the anode

Organic light-emitting diodes (LED) with a microcavity structure and an aluminium-doped zinc oxide ZnO:Al (AZO) anode have been fabricated. Effects of microcavities on the spontaneous emission of the organic LED, such as spectral narrowing, intensity enhancement and angle dependence of the emission, have been observed. Different emission colours have been obtained by changing the thickness of the AZO layer and that of a filler layer. The wavelengths of the cavity modes can be explained on the basis of the calculated total optical thickness of the individual cavities.

Red organic light-emitting diodes based on energy levels matching of dopant with the host materials

By doping red dye 4-dicyanomethylene-2-(tert-butyl)-6-methyl-4H-pyran(DCJTB) in the tris-(8-hydroxyquinolinato) metal Mq3(where M=Al, Ga, In) chelate complexes, a series of red-dopant organic light-emitting diodes with different doping concentrations have been fabricated. The electroluminescence efficiency of these red diodes with a DCJTB doped Mq3 emitting layer is found to be decreased markedly with the increasing of doping concentration. Electroluminescence characteristics of these devices are studied in terms of energy levels matching of red-dopant with the host materials and carrier transporting layers.

A stable blue organic electroluminescent material

In order to compare two kinds of blue electroluminescent materials, we have investigated two kinds of blue OLEDs with the similar structrue ITO/CuPc/NPB/JBEM: perylene/Alq/Mg:Ag [device(J)] and ITO/CuPc/NPB/DPVBi: perylene/Alq/Mg:Ag [device (D)]. The difference of luminance and efficiency was not obvious for the two devices. However, there was remarkable difference for their lifetime. The device(J) achieved longer half lifetime of 1035 h at initial luminance of 100 cd/m2, and that of device(D) was only 255 h. According to their energy level diagrams, the difference of their stability may originate from different host materials in the two devices. It may be attributed to the better thermal stability of JBEM molecules than that of DPVBi. It is shown that JBEM may be a promising blue organic electroluminescent material with great stability.

Organic Thin Film Electroluminescent Passive Matrix Display

Long life green-emitting matrix display based on organic light-emitting diode is reported. The pixel number is 96×60, equivalent pixel size 0.4×0.4 mm2, and the pixel gap 0.1 mm. An image with no crosstalk between pixels is obtained. The average luminance of these pixels at duty cycle of 1/64 is 100 cd/m2, and the power consumption is 0.6 W. The dark room contrast of 1:100 is achieved without using a polarization filter.

Threshold Lowering and by Intensity and Efficiency Enhancement by Dopants in Polymer Emitting Diodes

A new method to increase the luminance and quantum efficiency of polymer light emitting diodes with a lower threshold voltage has been reported. The threshold voltage, luminance and quantum efficiency have been significantly improved by doping certain dopants with a lower highest occupied molecular orbital (HOMO) level into the hole transporting layer. A high performance device has been achieved by addition of the perylene and triphenylamine as a dopant into poly(N-vinylcarbazole). The luminance and quantum efficiency increase by 2-3 times in comparison with the undoped device, reaching 10000 cd/m2 in luminance and 0.58% in quantum efficiency, while threshold voltage is reduced to one half value. The energy diagram has been obtained by measuring the HOMO levels and band gap values. Based on this, the carriers injection and balance between electrons and holes as well as the action of dopant are discussed.