Zhu Wen-qing

Organic thin film transistors with PMMA modified insulator

A double insulation layer structure organic thin films transistor (OTFT) was investigated for improving the performance of the SiO2 gate insulator. A 50 nm PMMA layer was coated on top of the SiO2 gate insulator as the organic insulator layer. The results demonstrated that using inorganic/organic compound insulator as the gate dielectric layer is an effective method to fabricate OTFTs with improved electric characteristics and decreased leakage current. Electrical parameters of carrier mobility and on/off ratio were calculated. OTFT based on Si substrate with a field-effect mobility of 4.0 × 10−3 cm2/Vs and on/off ratio of 104 was obtained.

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.

Life Prediction for Vacuum Fluorescent Display Based on Curve Fitting of Brightness Decay

In order to estimate the life-time of vacuum fluorescent display (VFD) in a short time, a constant stress accelerated life test was performed by increasing its cathode temperature. The fitting of the brightness decay curve for VFD was achieved by applying the Weibull function to describe the brightness decay and the Least Square Method (LSM) to estimate the distribution parameters. Furthermore, the brightness decay formula in the normal stress, which was employed to design the initial brightness for the VFD according to life-time requirements of the users, was determined by the acceleration parameter. The numerical results show that the predicted life-time has a high accuracy, and that the brightness decay formula can provide some significant guideline to manufacturers and technicians for life-time estimation and initial brightness design.

Blue emission from exciplex in electroluminescent devices made from vinyl polymer and oxadiazole derivative

Two types of molecular and polymer devices employing (2-(4′-biphenyl)-5-(4″-tert-butylphenyl)-l,3,4-oxadiazole) (PBD) and poly(N-vinylcarbazole)(PVK) as electron transport and hole transport materials, respectively, have been fabricated. The structures of these two devices are ITO/PVK(70 nm)/PBD(60 nm)/Al and ITO/FVK: PBD(1:1 by mass, 70 nm)/Al. The formation of exciplex is evident by comparing the electroluminescence (EL) of a bi-layer device and the photoluminescence (PL) of a PVK and PBD mixed film. The maximum emission energy of exciplex evaluated is consistent with the peak emission of EL. Blue emissions from both devices are obtained. The type of exicplex is interpreted based on the energy level diagram of the bi-layer device.

Blue-violet organic electroluminescent devices based on exciton-confined structure

Three types of blue-violet light-emitting devices based on an exciton-confined structure have been prepared, in which different materials were used as emitting layers and hole-transporting layers. They had structures of ITO/CuPc/NPB/CPB/TPBi/Alqs/LiF/Al (DNC), ITO/CuPc/J03/CBP/TPBi/Alqs/LiF/Al (DJC) and ITO/CuPc/J03/FNPD/TPBi/Alqs/LiF/Al (Djf). Here copper phthalocyanine (CuPc) acted as hole-injecting layer(HIL), N,N-bis-(1-naphthyl)-N,N-diphenyl-1.1biphenyl-4-4-diamine(NPB) and J03 hole-transporting layers (HTLs), 4,4′-dicarbazolyl-1,1′-biphenyl (CBP) and FNPD as emitting layers(EMLs), N,arylbenzimidazoles (TPBi) as hole-blocking layer(HBL), and tris(8-quinolinolato)aluminium complex(Alqs) as electron-transporting layer(ETL). TPBi applied here is a good confinement to both charges and excitons, which make the devices emit blue-violet light originating from the emitter, CBP and FNPD. Their characteristics have also been investigated. The result shows that the device DNC based on NPB/CBP has the best performance among the three devices. The excellence of DNC is attributed to the better hole-transporting ability of NPB as compared with J03, and the better emitting ability of CBP as compared with FNPD, although the best matching of energy levels is found in the hole-transporting layer and emitting layer of the device DPJF.