Hopping Transport Of Carriers And Its Magnetic Effects In Organic Semiconductors

Organic semiconductors are organic functional materials with semiconductor characteristics. They are mainly organic small molecules and polymers with π-conjugated structure. Compared with inorganic semiconductors, they have light weight, good flexibility, wide source, controllable structure and so on, thus are becoming a material of choice for preparation of large area and low cost electronic devices. They can be widely used in electroluminescent devices, solar cells, field effect transistors, spintronic devices, sensors, and many other fields.Organic semiconductors are usually nonmagnetic materials, so in our traditional concept, the current and luminescence properties in organic light-emitting devices (OLEDs) will not be affected by magnetic field. Until2003, J. Kalinowski’s team firstly reported magnetic effects in OLEDs. When a magnetic field was applied parallel to the light-emitting layer, the luminous intensity, current and the quantum efficiency in the device changed in different degrees. Since then, the magnetic effects in organic semiconductor materials and devices have gradually aroused people’s concern. The strength of the magnetic effect is commonly showed by organic magnetic resistance (OMAR), i.e. the relative change of the resistance or current due to the magnetic field.So far, the experimental research on the OMAR effects has many new progresses. For most of the researched organic semiconductors, although their molecular characteristics, device structures and working condition are not identical, but the OMAR shows some common characteristics. Based on these experimental phenomena and characteristics, theoretical researchers have presented a lot of models for illustrating the mechanism of OMAR, but so far, it is far from reaching a unified understanding. The OMAR is related to carriers’injection, migration, recombination and decay processes, thus it is a complex integrated effect. There is no doubt that the effect is closely related to the carriers’ hopping transport and spin orientation.Based on the OLEDs in this thesis, we focus mainly on carriers’hopping transport process and spin orientation change, and present some theoretical analysis on the OMAR effect. Our analysis is mainly based on two aspects:the carriers’ migration is a hopping process from one molecular site to another, i.e. the two-site model, and the changing spin state of electrons is obtained by solving the Schrodinger equation under action of spin Hamiltonian. The full contents are arranged as follows:In the first chapter, we introduce the background information related to this thesis, including the basic properties of organic semiconductor materials, typical elementary excitation in the organic semiconductor, basic structure and the luminescent principle of OLEDs, and the experiment and theoretical research progress of OLEDs’ magnetic effect, etc. Finally, we introduce the arrangement of the full content.In the second chapter, we introduce the typical theories of carriers’ migration under an electric field, including the macroscopic drift-diffusion theory, the microscopic theory of Miler-Abrahams hopping theory and the Marcus theory, as well as the space-charge-limited current formed in the OLED, etc.In the third chapter, we consider the magnetic field effect on electron state. We first give a comprehensive spin electronic Hamiltonian including applied magnetic field, hyperfine interaction and spin-orbit coupling effect. Then set out from the Hamiltonian, we derive the electron spin angular momentum’s change with time according to the basic principles of quantum mechanics, and analyze the effect of magnetic field on the spin under individual effect of the hyperfine interaction and spin-orbit coupling. Describing the organic molecular vibration by using a harmonic vibration potential, we discuss the magnetic field effect on the electron energy level and wave function local length.In the fourth chapter, we consider the effect of spin flip on electric current. Viewing from electron’s hopping between two molecular sites, we first get the spin angular momentum of the’electron pair’. Then extending the result to recombination process of the ’electron-hole pair’, we calculate the OMAR under the framework of space-charge-limited current, and analyze the effects of hyperfine interaction and spin-orbit coupling.In the last chapter, we summarize our work, and outline the trend of the further research and development.