Polaron Dynamics Properties With Magnetic Impurity In Conjugated Polymers

As the application of organic materials in spintronics, organic functional materials crossed with Chemistry area and spintronics crossed with Physics area combined, giving birth to some novel phenomena and forming a new branch of learning—"Organic spintronics". The exploration of new application of organic materials in the field of spintronics has important value of basic research and potential application background.Organic spintronics research is at the beginning, many key issues are still under debate, for instance, the mechanism of organic magnetoresistance, spin relaxation of carriers and spin polarization evolution in conjugated polymers. Compared with conventional inorganic materials, organic materials have their own characters, such as the structure of low dimensional and "soft". Due to the strong electron-lattice coupling, carriers in conjugated polymers are self-trapped states such as soliton, polaron or bipolaron rather than ordinary electrons or holes. These local elementary excitations have complicate charge-spin relationship, which make organic electronics have abundant properties. Consequently, in order to design operable spintronic devices, we need to study fundamental mechanism of spin injection and transport in organic materials.The manipulation of spin during the transport process is one of the basic issues for the application of spintronic devices. In the traditional organic spin devices, spin-polarized current is usually achieved by spin injection. However, as a result of the great difference of the conductivity of ferromagnetic metals and organic semiconductors, the spin injection efficiency is very low, and the separation of spin and charge signals can not be achieved while the whole injection process needs a strong magnetic field to be completed.In addtion, impurity effect will also have essential effect on the transport process of the system. In the previous studies, impurities only reflect charge properties which affect the charge transport of carriers. With the introduction of magnetic impurity, due to the introduction of spin degree of freedom, the effect caused by magnetic impurity on the spin transport process are studied. Researches about the micro-process of the spin transport in organic polymers in the presence of magnetic impurity and the mechanism are not deep, therefore, in this paper, we use nonadiabatic dynamics method to study the polaron dynamics properties especially the spin evolution in polymer chain in the presence of magnetic impurity, possbile phenomenons such as spin-filter and spin polarization are discussed. The detailed research and main results are given below:1. Spin transport process of polarons in conjugated polymers in the presence of magnetic impurityCurrently, the investigation of a quantum-impurity or quantum dot in a correlated system is an interesting topic both for fundamental and for technological reasons, such as charging effects, Coulomb blockade, spin blockade, voltage-controlled spin polarization, tunneling magnetoresistance, quantum tunneling of magnetization and Kondo effect, etc. There have been a number of experimental and theoretical studies of magnetic impurity that it can make the spin of polaron in conjugated polymers flipped or make bipolaron transformed into two polarons. In Chapter three, we will study the spin transport process of polarons in conjugated polymers in the presence of magnetic impurity. We found that, it can generate a spin polarization current. Different with inorganic semiconductors, threshold values of the magnetic impurity when spin-filter effect happens.2. Application of magnetic impurities in organic spin devicesCurrently in organic light-emitting diodes (OLEDs) study, people usually use ferromagnetic metal electrodes instead of normal metal electrodes to achieve spin-polarized injection through adjusting magnetic moment direction of both sides. According to the result of Chapter three, we found that spin polarization are caused by the magnetic impurity, therefore, we use this important property of magnetic impurity to design a spin-OLEDs model. In our model, ferromagnetic metal electrodes are replaced by two magnetic impurities, the spin of polarons polarized at both sides while polarons with different charge and antiparallel spin recombine in the middle region of impurities. It can generate the collision process of polaron pairs with antiparallel spin. This research can enrich people’s understanding of organic spintronic and provide important theoretical basis for the development of organic spin devices.