| As a novel ?at panel display technology with the advantages of self-lighting, solid,high brightness, and quick response rate, organic light-emitting diodes (OLED) has beennoticed widely and studied extensively. However, their electroluminescence e?ciency,lifetime, operating voltage, and design are still need to be delved to make advancement.Through studying and assimilating predecessors' research fruit, this dissertation studiedhow to improve the device performance and meanwhile discussed the optimal design ofOLED. (1) The charge injection and transport mechanism of OLED are investigated bystudying the current–voltage data. Except for the transition from the injection limitedcurrent to space charge limited current, there are other two transitions. One is thetransition from single injection in the low applied voltage area to the dual injectionin the high applied voltage area. The other is the transition for the major injectioncurrent mechanism from Richardson-Schotty thermionic emission to Fowler-Nordheimtunnelling injection current induced by high electrical ?eld in high applied voltage area.In addition, by comparing the experimental data with combination theory, it was foundthat F2/J (F is the electricity ?eld intensity and J is current density) could re?ect thechange tendency of quantum e?ciency with current density directly. (2) Integrating the theory and the experiments, the optimal design of OLED wasdiscussed from device structure, energy level matching, and ?lm thickness three perspec-tives. The heterojunction is formed in the multi-layer structure OLED, which makes therecombination area far away from anode and cathode and decreases the possibility of theself-quenching of carriers at electrodes. The energy level matching degree is in?uencedby not only the carrier injection barriers at the electrode interface but also the barriersat the organic heterojunction interface. (3) The location of dopants in organic ?lms a?ects the device performance greatly.The in?uence of doping area on the carrier mobility could be depressed when only therecombination area were doped. Similar to doping, the device lifetime can be prolongedif the carrier transporting layer and luminescence layer were mixed, which could avoidthe high electricity ?eld intensity at the organic interface. Moreover, the carrier injection IIIcould be promoted when LiF were used as the anodic bu?er layer, which is similar to thatof LiF as the cathodic bu?er layer. So, it is reasonable to use the tunnelling injectionmechanism to explain the enhanced current injection for inorganic bu?er layer. (4) The morphology and electrical properties of indium-tin-oxide (ITO) ?lms withoxygen plasma treatment was studied from microscopic view by atomic force microscopy.It is found that ITO surface roughness decreased after treatment, which can improve itswetting performance and consequently improve the ?lm performance. On the other hand,oxygen plasma treatment makes ITO ?lm oxidized further, which decreases the numberof oxygen vacancy and Sn4+. This could increase the work function of ITO, whichwould decrease the device threshold voltage and increase the luminescence e?ciencyconsequently. (5) The in?uence of spin method and solvent vapor posttreatment on the morphol-ogy of polymer ?lms was studied. Results show that the ?lm morphology and dopantdistribution in the polymer ?lms cast with the conventional spin method are depen-dent on the positions of polymer ?lms because of the di?erence of centrifugal force. Toavoid this negative e?ect, a new spin method was put forward by locating the speci-men at an appropriate distance from the center of the spin plate during spin casting.It is found that polymer ?lms prepared with the new method are more uniform andtheir electroluminescence performances are independent of the positions. Meanwhile,a promising approach called solvent vapor post treatment, which can control...
|
PDF Full Text Request