The Research On Interfacial Properties And Exciton Recombination Characteristics In Organic Electroluminescent Devices

Organic electroluminescence device (OLED) is one of the spotlights in the modern flat panel displays and it will be served as a kind of green illuminating sources in the 21st century. But its shortcomings of not long lifetime and unstable properties need solving urgently for use. Nowadays, the interfacial properties and exciton recombination luminous characteristics are the key problems. Thus a systematic and comprehensive study of the physics involved the formation of the interfacial states is very important. The investigation on interfacial problems can be served as a guide to devices and electroluminescent materials design, which is beneficial to making all relative science mutual progress.The main theory models and their research results of this dissertation are as following: 1. Both metal/small organic material interface (MOI) and metal/polymer interface (MPI) have been researched in detail.(1) A model for calculation the built-in potential (V_bi) in OLED was presented. The voltage at which the current under illumilation in OLED is equal to the dark current, namely the net photocurrent is zero, which we term the compensation voltage (V₀). The relation between the V_bi and V₀ was maintained. It is only dependent of material parameters and the temperature. At low temperature, V_bi is equal to V₀. However, because of diffusion of thermally injected charges at room temperature, V₀ is lower than V_bi.(2) The evolution of the core XPS intensity of C1s, N1s, and O1s for Alq3/Mg, and Alq3/Al have shown that the core banding energy is lower than that of bare Alq3, which means that there are a lot of charge carriers transfer at the interface. The new shoulder of A12p confirms that the Al have passed through the organic material, reacted with C, O, finally formed C-Al and C-O-Al bonds. The width of dipole layer for Alq3/Al is about 5nm. The dipole potential is equal to the difference of the change of metal function and the core energy level. The charge transfer at the interface results in a lower electron injection barrier, thus forming a good electron injection interface.(3) Based on the density function theory and the self-consistent field iterations, we obtained the relation between organic energy change and metal work function. For a large charge transfer, the chemical dipole potential D_chem can be larger than ΔD_met in absolute value, the resulting interface dipole is characterized by an increase in metal work function, which is against electron injection. While for a small charge transfer, D_chem<ΔD_met, and the resulting interface dipole leads to a net work function decrease, which is beneficial to electron injection. Those factors include (a) the image potential including polarization of the organic material;(b) formation of the interface state;(c) the electric field resulting in alignment of the permanent dipole of the organic material.(4) Inserting 1.5nm LiF into four kinds of devices: ITO/Alq3/Al, ITO/Alq3/LiF(1nm):Al,ITO/Alq3/LiF(1.5nm): Al, and ITO/Alq3/LiF(2nm):Al, it will improve properties of the devices. The carrier tunneling theory model of the metal buffer/organic interface has been built, which is approach to the performance of real devices. In this model, we have considered firstly both the effect of leakage current on the injection current density and of the interfacial energy levels realignment, which is coming from a dipole layer owing to charge carrier transfer at the interface. By using of WKB approximation method, we have also found out that the buffer layer thickness, the conductivity ratio of the organic to the buffer, the difference between the position of the conductivity band of buffer layers and the lowest unoccupied molecular orbital (LUMO) are the main parameters affecting properties of device, which is very agreement with our experimental results.(5) According to Monte-Carlo model, a generalized Monte-Carlo dynamics model for charge-carrier injection from a metal electrode to polymer materials was presented in detail. The injection efficiency is determined by trapped charge density, the structure of spatial and energy disorder of polymer. The injection barrier is not only determined by the materials wok function, but also by palors ionizable potential and image potential.2. A disorder hopping model for carriers recombination at organic/organic interface (OOI) in double layer organic light emitting diodes (OLEDs) was presented. According to the structure of an OOI as well as spatial and energetic disorder of hopping states, it was more reasonable to use the disorder hopping model than to use the Fowler-Nordheim formalism. It was shown that the carriers hopping distance, the effective barrier height and the electric field contribution had heavy effects on recombination efficiency. Firstly, when the applied voltage was less than 19.5F(in double layer OLEDs ITO/oc-NPD/Alq3/Al), recombination efficiency increased with the increase of hopping distance, while the applied voltage was lager than 19.5F, it decreased with the increase of hopping distance;secondly, it also increased with the increase of effective barrier height at OOI;Finally, it increased with the increase of differences of electric field at OOI, while it decreased when the value of differences of electric field attached 24*lO5V/cm.3. A model for carriers injection, transport and recombination in single layer organic light emitting diodes was presented. The electric field contribution, the relationship between current density and the applied voltage, and the recombination efficiency were obtained by solving the nonlinear Painleve equation. When the majority carriers had lower mobility, the carriers were easy to injection, transport, and the device efficiency could be improved. If the holes mobility was larger than the electron mobility, the recombination zone moved to cathod, and vice versa. The theory model of recombination was presented and the effect of temperature and applied voltage on the recombination efficiency was investigated in double layer organic light-emitting diodes: ITO/PPV/PBD/Ga. At lower applied voltage, two peaks have been observed in the recombination efficiency with temperature. With increasing voltage,the two peaks shifted toward each other, and at voltage around 9V the two peaks converged. These phenomena were attributed to the excited deep and shallow trap levels and the change of recombination zone: In the Frenkel exciton model, the temperature dependence of the quantum efficiency depended on the carriers mobilities and carriers densities. The carriers mobilities increased with decreasing temperature, while the carriers densities decreased with decreasing temperature. Therefore, a peak in the quantum efficiency with temperature was expected in the model.The high-temperature peak originated due to the recombination of the deep trap levels, but the low-temperature peak due to shallow ones. On the other hand, as the voltage increased, the recombination zone would be changed, which had some effects on the recombination efficiency.4. Considering the formation and disassociation of polaron-excitons, an analytical model to calculate the width of recombination zone and the external quantum efficiency in single layer OLED was presented. The influences of applied bias and the thickness of the device on the width of recombination zone and the external quantum efficiency were thoroughly studied. Based on the bilayer organic light-emitting diode with ohmic anode and injection limited cathode, we conclude that the width of recombination zone is approximately equal to that of OOI. While one barrier is larger than the other's, the width will be determined by minority. The exciton recombination theory model was built for doped organic thin films. The position of recombination zone will shift in the doped OLED. The width of recombination zone is dependent of trapped charge density, dopant concentration, electric field, temperature, etc.The features of this dissertation embody its prominent utility and its profound mechanism investigation. The main features are as following:(1) The interfacial theory models combine with highlights and heading fields of several sciences such as materials, physics, chemistry, etc. The investigation of the effect of interfacial properties on luminous characteristics of OLED is very important. Density functional theory and Monte-Carlo model are good for systematic and comprehensive understanding of the various luminous dynamics processes.(2) A disorder hopping theory model has been modified by us, and it is suitable for interface theory, which has the characteristics of transient analytical formation and accurate calculation. While the other models such as the tunneling model are difficult to do it.(3) It was first built by us that the exciton recombination theory model for doped multilayer OLED. The change of the positions of recombination zone and its width vs dopant's concentration was analyzed.