Organic Light-emitting Diode Carrier Injection And Transport Mechanism Of The Study

Organic light-emitting devices (OLEDs) are now attracting more and more attentions because of their possible applications in the new-generation flat panel display technology. This thesis mainly considers aspects of carrier injection and transport occurring in OLEDs. After a brief review of some fundamental knowledge of carrier injection and transport in OLEDs, experimental details, such as fabrication and measurement processes, are described for possible repeat of our experiments by other people. The main work in this thesis consists of four parts, including theoretical and experimental investigations of the tunneling model of buffer layer, the role of hole in improving performance of OLEDs by buffer layer, the electron blocking effect of NPB and the temperature dependence of OLED properties.1. Based on the WKB approximation of the tunneling model, we calculate the J-V characteristics of OLEDs having Duffer layers of different thickness. The results show how the insertion of a buffer layer with proper thickness lowers the OLED turn-on voltage. Further calculation suggests some parameters, such as the resistivity ratio and the position of the conduction band minimum of the buffer layer relative to the lowest unoccupied molecular orbital of the organic layer, are important in selecting a buffer material. A quantitative estimation of the optimal buffer layer thickness is also presented to serve as a guide to device design. The model is validated by comparison of its predictions to experimental results.2. Mechanisms of carrier injection enhancement by inserting insulating buffer layer in organic light-emitting devices (OLEDs) have been investigated. According to the model of tunneling barrier reduction by raising the cathode Fermi energy level, the electric field in the buffer layer should be more intensive than that in the organic layer. Because there is no free intrinsic carrier in OLEDs, to obtain higher electric field intensity in the buffer layer, holes must be injected and accumulated at the buffer-organic interface. By varying the hole injection and transport ability of the OLEDs and investigating the effect of Al2O3 layer insertion on the current-voltage characteristics, we have found the improvementof electron injection is positively correlated to the amount of hole reaching the organic-buffer interface, which is in good agreement with the analysis. The improvement of luminescence efficiency and the abnormal variation of the turn-on voltage with buffer layer thickness, i.e., initially increasing, then decreasing and then increasing once more, are also consistently explained by the model.3. Current and luminance characteristics of OLEDs with NPB of various thicknesses as the hole transport layer have been investigated. It is found that for conventional structures of ITO/NPB/Alq3(60 nm)/LiF(0.5 nm)/Al the optimal hole injection and luminescence efficiencies appear at NPB thicknesses of 5 and 20 nm, respectively. The large difference between the two optimal thicknesses suggests that the effective block of the NPB layer against electrons from across the Alqa/NPB interface is essential for high-efficiency operation of the OLEDs. The electron blocking effect of NPB is further confirmed by the electroluminescence (EL) behavior of devices with the structure of ITO/NPB(5 nm)/Alq3:DCM(30 nm)/NPB/Alq3(60 nm)/LiF(0.5 nm)/Al. The proportion of DCM's EL to the whole EL decreases with increasing NPB thickness. This suggests that the NPB blocks electron transport to the Alq3:DCM layer. The photo-induced energy transfer to the DCM molecules is ruled out by the EL behavior observed after quenching excitons in the Alq3 layer. The origin of the difference in the optimal TPD thicknesses reported by other two different groups is also discussed.4. By investigating the temperature dependence of OLED's efficiency, we find the dependence is closely related to the NPB thickness. When no NPB layer is inserted in OLED, the electroluminescence efficiency of Alq3 decreases with temperature monotonously. When the NPB layers of...