Charge-induced Spin Polarization In Organic Molecule

Organic semiconductor, acting as a new functional material, not only has the characteristics of traditional inorganic semiconductors, but also consist of some particular advantages, such as low production costs, large-scale production and easy synthesize. Particularly, it has novel electromagnetic and photophysical characteristics, which leads to its widely application in the organic light emitting diode (OLED), organic photovoltaic cells (OPV), organic magnetic field effect (OMFE) transistors et.al. Currently, Japan, Korea and most countries in Europe has achieved industrialization of organic light-emitting diodes.In recent years, a significant progress of organic semiconductors take place in spintronics. It contains the organic magnetic field effect, the spin characteristics (spin generation, transport, storage, and detection) and other new physical phenomena in functional materials and its relevant devices. Organic Spintronic is concerned with two areas:organic functional materials related to chemistry and spintronic of physics. On the one hand, it is practical for the application and significant for the basic research to seek the new organic solid materials with more obvious physics characteristics. Which become one of the important research topics soft materials science; on the other hand, spintronics make a great progress in electronics in recent several decades. It not only caused the emergence of high-density memory such as a major application of the device, but also led to the revolution of some basic physics, such as the spin current, self-spinning, spin Hall Effect, spin rectification and some other new physics concepts or phenomena. The combination of them forms a new discipline, organic spintronics, which is to explore the possible potential applications of organic functional materials.The object of study on organic spintronics currently focus on two types of structure device. One is the organic devices containing magnetic electrodes, such as La1-xSrxMnO3 /T9/La1-xSrxMnO3 and Co/Alq3/La1-xSrxMnO3, in which the main research is spin polarized electron (hole) injection, transport and detection, reflected by devices magnetoresistance or spin valve effect. The other device does not contain any magnetic element organic devices, such as ITO/PEDOT/polyfluorene/Ca and ITO/PEDOT /T6/Ca/Al. In 2004, Francis et.al found that the magnetic resistance can reach more than 10% in an organic device ITO/PEDOT/polyfluorene/Ca at room temperature by applying a weak magnetic field (100mT). The magnitude and sign of magnetoresistance are related to the bias voltage and the thickness of organic layer. Since this phenomenon is rare in inorganic devices, organic magnetoresistance (OMR) quickly attracted widespread attention in physics, chemistry, materials and electronic academia. By preliminary research in recent years, OMR has rich content and complex mechanism and thereby has a significant potential applications. Organic semiconductor (device) have many advantages over inorganic semiconductor, such as variable magnetoresistance. Otherwise, photoluminescence (PL), electroluminescent (EL) and photocurrent (PC) can response to the external magnetic field to some extent. Thereby, organic magnetic field effect (OMFE) become an important focus of organic functional materials and device research and its strong response mechanism to weak magnetic field attracts the interest of physics workers.Due to the natural features of organic materials:weak spin-orbit coupling and the hyperfine interaction, the carriers in organic devices have a relatively long spin relaxation time. Therefore, it becomes the ideal material for spin-polarized transport. On the other hand, the charge carriers in organic semiconductors differ from that in inorganic for the strong electron-lattice coupling. Organic material carriers are some self-trapped quasiparticles, such as solitons, polarons, having more complex relations between spin and charge. It is these characteristics of the organic semiconductor that makes spintronics richer. Small molecule organic semiconductor material is characterized by the good molecular symmetry and rigidity. The whole molecule is usually a conjugated system. They are relatively easy to form an ordered film, facilitating the carrier transmission. As a result, small molecule materials are used as the intermediate layer in organic field-effect transistor.Based on the TO model of small organic molecules, this paper investigates the effect of the spin reversal (caused by impurities, temperature and some other factors) reversal strength on the carrier spin-polarization, and the impact of electron-electron interaction on the carrier spin polarization. The research content and results are as follows:1. When an electrons is injected into molecular crystal materials, it will form a localized polaron. Regardless of the spin-flip effect, it is completely spin polarized, i.e. magnetic moment of one. But in the case of considering the electron spin reversal on its site, the greater is the intensity of the spin reversal, the smaller is the polaron magnetic moment. In addition, there exist a crucial value of spin-flip strength, to which the polaron magnetic moment will suddenly drop to zero. When two electrons with opposite spin values are injected, it will form a bipolaron. In this case, however, spin-flip strength has no impact on the total spin for bipolaron contain two opposite spin electrons and their magnetic moment is always zero.2. spin-flip effect connect the magnetic moment with the injected electron number and its localization. Meanwhile, we take the electron-electron interaction on nearest neighbor sites into consideration. It is found that the critical value making the electron spin magnetic moment disappear turns to decrease with the increasing e-e strength. But if the electrons are spin-polarized, the electron-electron interaction between nearest neighbor sites has little effect on the net magnetic moment.