Effect Of Devices' Materials And Structures On Their Properties Of Blue Electroluminescence

As the one of three primary colors to realize full-color electroluminescence display, it has practical significance to improve properties of blue electroluminescence device. Considering structures and materials of devices, the pathes were researched which to improve properties of bule inorganic and organic electroluminescence in this dissertation.Firstly, it was studied that effect of structures and materials on properties of blue inorganic electroluminescence device. ZnSe electroluminescence device with layered optimization structure was deposited, in which SiO₂ is as acceleration layer. The electroluminescence of ZnSe was observed which could not be found in device with traditional double-insulator layer structure. There are two electroluminescence peaks, one is 469nm corresponding to band-band transition of ZnSe and the other is 544nm corresponding to defects. The upper energy levels of these two electrons transition processes are the same. The mechanism of luminescence was discussed. Original electrons accelerated in SiO₂ and got high energy. These hot electrons have enough energy to impact ZnSe and then the electrons in ZnSe valence band are excited to conduction band. When the electrons in conduction band transit to valence band or defects energy level and recombine with holes, the luminescence is given. The luminescence mechanism of the device is solid state cathodoluminescence (SSCL) because electrons are accelerated in solid instead of in vacuum. This phenomenon extended SSCL from organic materials to inorganic materials. It can provide a new way to realize blue inorganic electroluminescence.Based on layered optimization structure device of ZnSe, serial layered optimization structure device was obtained by inserting SiO₂ into the central part of the luminescence layer. The luminescence intensity is higher than that of layered optimization structure device. The causes are as follows: 1) SiO₂ inserting layer can make electrons accelerated and multiplied. This results SSCL. The distance of SSCL is relatively short, so the brightness increases. The average energy of hot electrons increases. 2) SiO₂ conduction band is higher 0.5 eV than ZnSe's and this makes hot electrons get 0.5eV energy during they come from SiO₂ to ZnSe. 3) SiO₂ inserting layer has two interface with ZnSe. So the source of original electrons increases and the quantity of original electrons and hot electrons increase.According to that blue inorganic electroluminescence matrix should has wide bandgap, (ZnS)_x(MgO)_1-x were prepared. The energy band of samples increases with the thickness of MgO thin film. This change can avoid ionization of electrons on excited state of luminescence centers. This affords opportunity to obtain high efficiency blue inorganic electroluminescence.Secondly, it was studied that effect of structures and materials on properties of blue organic electroluminescence device. In the trilayer structure organic electroluminescence device, organic materials Alq₃ and BCP were inserted to the interface between ITO and luminescence layer NPB. Emission of NPB was enhanced. The effect of the thickness of inserting layer on properties of devices was studied. The properties of devices are enhanced when the inserting layer has suitable thickness. The thickness of inserting layer BCP is thinner than that of Alq₃ because the block-hole ability of BCP is stronger than that of Alq₃.The chosen of blue organic materials is important besides device structure. The effect of usual blue materials TPB and DPVBi on properties of devices was researched. They were doped into PVK with same doped concentration. Their spectra were studied and it is believed that the energy transfer efficiency between PVK and DPVBi is higher than that between PVK and TPB according to the deduction formula. Taking Rubrene as probe, DPVBi blue electroluminescence mechanism of devices with different structures on was discussed to prove the energy transfer between PVK and DPVBi further. The experiment results show the vary extent of DPVBi relative to the luminescence intensity of Rubrene of doped device is more obviors than that of device which takes DPVBi as single emission layer. According to this result, luminescence of DPVBi is mainly due to energy transfer when DPVBi is doped into PVK. So this result can be a direct proof to obtain high performance blue electroluminescence used energy transfer from PVK to DPVBi. In addition, 15wt% is the optimal dope concentration of DPVBi.Lastly, it was attempted that improving properties of blue electroluminescence device through organic-inorganic recombine structure. The causes of improvement were researched. There are three kinds of stuctrue: inorganic material ZnS with high carrier mobility was as electron transport layer, ZnS nano particles were doped into hole transport layer PVK and LiF was inserted into different interfaces. All of these can improve properties of blue electroluminescence devices through controlling the quantity of electrons and holes. In Tandem structure blue electroluminescence device, two luminescence units were connected by Au as charge generating layer. Its current efficiency was higher than that of two luminescence units at same driving voltage. The dipole layers on Au/organic layer interfaces induce the difference of luminescence intensity between two units.Summarized the above, it is found blue electroluminescence property can be improved through appropriate materials and structures of device. Inorganic blue electroluminescence is still weaker. Organic blue electroluminescence devices have worse stability. In contrast to them, high blue emission properties of organic/inorganic recombination blue electroluminescence device can be obtained used the characteristics of organic materials and inorganic materials. It is a kind of promising electroluminescence device. Of course, every type devices still have many aspects need to research, such as interface, stability and lifetime.