Study On Organic Light Emitting Materials And Diodes

Organic Light-Emitting Diodes (OLEDs) have been attracting considerable attention due to their low drive voltage, high brightness, wide viewing angle and fast response. So they have been called"Third generation display"or"Magical display". Usually, they are the first choice for people to choose as displays and light sources. Nowadays, the low efficiency, short life, poor stability properties restraint them abroad applications. Although the small size displays have been commercialized, the large size displays are still a difficult problem.In this paper, we introduced the recent progress, the diodes structures and luminescence principles of the organic light emitting diodes (OLEDs). Presentation of the materials: electrode materials, luminescent materials, hole transport materials, electron transport materials and buffer materials which are used in OLEDs. Synthesis and characterization a new yellow light-emitting material Zn(ODZ)q, Fabricated three red phosphorescent materials diodes and optimized the structures of these red phosphorescent diodes by added Alq₃ as an electron transport layer to improve electron and hole injection in device for balance, thereby improved device performances. Also we use one of these red Ir complexes to preparation of white OLEDs.There are some conclusions are obtained as follows.1. A new yellow material Zn(ODZ)q was synthesized by 8-ydroxyquinoline, 2-(2-ydroxyphenyl)-5-benzo-oxadiazole and zinc acetate. The material's structure was confirmed by HNMR and Infrared spectrum. The OLEDs with structure of ITO (120 nm) / NPB (40 nm) / Zn (ODZ)q (60 nm) / Al (150 nm) were fabricated and tested. The maximum brightness of these diodes were 1290 cd/m² at the bias of 14 V, while the maximum current efficiency of these diodes were 1.49 cd/A, at the current density of 21.8 mA/cm². The EL peak of these diodes was 588 nm.2. The three newly synthesized Ir complexes ((Piq)₂Ir(Ph6Pic), (Piq)₂Ir(TPAPh6Pic) and (Piq)₂Ir(OXD6TPAPh6Pic)) were used as dopants in the emissive layer, in which PVK was used as host material and PBD as electron-transporting material (the weight ratio of PVK and PBD was 7:3). Two series of device A and B were prepared which the structure of devices A are ITO / PEDOT: PSS (40 nm) / Sample (80 nm) / Ca (10 nm) / Al (150 nm) and that of devices B are ITO / PEDOT: PSS (40 nm) / Sample (80 nm) / Alq₃ (10 nm) / Al (150 nm). The data shown that the performances of B diodes are better than that of A diodes. One of the greatest change is Sample 3, the biggest brightness of the deivce A is 6 times than that of device B and the highest current efficiency is 7 times than that of device B. The reason is that Alq₃ was introduced as electron transporting material in device B. Alq₃ can help the electron injection, resulting in improved-balance of hole and electron and increasing the probability of excitons formation. So the optical and electrical properties of the deivce B were improved. The entire EL peak of the different diodes are the same, that is because the Eg of the three compounds are equally and the Alq₃ in the devices do not change the EL peak. From these experiments we can know that introduce an electron transporting layer or a hole blocking layer in devices can enhance the performance of PLEDs.3. Researches show that it is impossible to obtain white light in PLEDs in which the (Piq)₂Ir(Ph6Pic) doped in PVK simply, because there is almost no energy transfer between PVK and (Piq)₂Ir(Ph6Pic). Directly captured the carriers is the main reason of the (Piq)₂Ir(Ph6Pic) emitting light. In order to obtain white light, the green fluorescent material Alq₃ was introduced. Changed the thickness of Alq₃ and the concentration of the doping red Ir compounds, obtained a white light. In the experiment 1 the CIE coordinate was (0.32, 0.33), that are matching very well with the standard white light with the CIE coordinates of (0.33, 0.33). In the experiment 2, the coordinates of the CIE coordinates were the same as the standard white light. In addition, from experimental 1 and 2, we know that experimental 2 are better than the experimental 1, the reason were that the thickened Alq₃ in the diodes, not only play a emitting material but also play a electron transport layer. So the maxium brightness of the experiment 2 is up to 5320 cd/m², while that of experimental 1 is only 2740 cd/m². The highest current efficiency of experiment 1 is 0.96 cd / A, while that of experiment 2 is about 1.5 times high than that of experiment 1.