Study On Red Organic Electroluminescent Material And Device

Organic electroluminescent device (OELD) have extensive application prospect in the field of display and illumination because of its high efficiency and energy saving, wide viewing angle, rapid response and shock-resistance. Referring to the full color display of organic electroluminescence, the efficiency and color purity of red OELD are low. The development and research of high-performance red OELD is of significance to achieve full color display.The OELD with the structure of ITO/2TNATA (25nm)/NPB:F4-TCNQ (xwt.%)(80nm)/EML (20nm)/BCP (10nm)/Alq3(40nm)/Al (100nm) were fabricated. F4-TCNQ was doped into NPB transport layer. The influences of different F4-TCNQ doping concentration on the device performances were discussed. The experimental results indicate doping F4-TCNQ enhance the hole concentration and increase the conductivity of the transport layer through local charge transfer. When the doping concentration of F4-TCNQ is2wt.%, the device exhibits an optimal performance. A turn-on voltage of8V and a maximum brightness of25860cd/m2under the forward bias of25V are obtained. And the maximum efficiency is increased from281m/w to361m/w, comparing with the undoped device on the same condition.In this thesis, four types of sample were prepared at first by using the vacuum vapor thermal evaporation method, which are ITO/2TNATA, ITO/NPB, ITO/2TNATA/NPB and ITO/Rubrene, respectively. Their surface morphologies were inspected by atomic force microscope (AFM). The results show that the2TNATA film is smooth, which can increases the adhesive force of ITO and NPB. In addition, the Highest Occupied Molecular Orbital of2TNATA locate between the work function of ITO and the HOMO of NPB, which is beneficial to low the injection barrier of hole and enhance the efficiency of hole. The AFM results manifest that the Rubrene film deposited on ITO is great uniformity. The interface electronic state of ITO/Rubrene is investigated by using X-ray photoelectron spectroscopy (XPS). The results indicate that an inter-diffusion system is formed under the interaction of Rubrene film and ITO at the interface. The combination of Rubrene film and ITO is actually a combination of C and O in the form of covalent bond. The combination at the interface induces a type of dipole far away from the surface of ITO, improving the injection efficiency of hole and the device performance. Subsequently, in order to investigate the luminescence mechanism of the devices, fluorescent DCJTB and Rubrene were doped into the Alq3emitting layer. The impacts of DCJTB and Rubrene on the electroluminescence properties of the devices were also studied. The Absorption spectra and photoluminescence spectra of DCJTB and Rubrene and the electroluminescent spectra of the doped devices were measured. The results show that there are both the Forster energy transfer mechanism and the direct carrier trapping mechanism working in the doped devices. In the devices, Rubrene can serve as a bridge of energy transfer. As the carrier concentration is high, Rubrene and DCJTB can capture the carriers directly from the host materials and then emit light immediately.At last, the fabricated devices were encapsulated by using epoxy resin. It is found that the emitting light area is decrease after a period of time, which is ascribed to an untight encapsulation. Following, the failure mechanism of the OELDs is analyzed by using microscopy techniques in detail. It is observed that the main cause of the failure is the existing of water and oxygen, originating from the surrounding. Of which, water results in the generation of bubbles at the electrode/organic interface, which reduces injection of electron and produces some nonluminous dark-spots. Oxygen oxidizes the Al cathode. In addition, an excessive high working current leads to crystallization of materials in the organic layer and the fusion of the cathode. Due to the non-conformal coverage of thin film, the device usually fails from the edge.