Theoretical Investigation On Organic Magnetoresistance

Organic solid material is a new chiasmal area, which consists of both chemistry-dependent organic materials synthetizing and processing and the research of the physics-dependent electrical, magnetic and optical characteristic and the development and application of organic functional devices. Organic solid materials were for long time only associated with electrical insulators and could not get much attention. Until the1970s, MacDiarmid et al found that the electric conductivity of the doped polyacetylene had a much increase, and then open the times of the organic materials. Organic light emitting diodes (OLED), organic field effect transistors (OFET) and organic photovoltaic cells (OPVC) et al new organic functional devices have entered the people's lives. For the groundbreaking contributions in the research and application of organic materials, MacDiarmid et al got the Nobel Prize in Chemistry in2000.In the new century, a significant breakthrough in organic materials is the application in Spintronics. Due to the weak spin-orbital coupling and hyperfine interaction, the carriers in organic materials have a relative long spin relaxation time, so it is a good candidate for spin injection and transport. Different from the carriers in traditional semiconductors, the carriers in organic materials is soliton, polaron and bipolarons, which have a more complicate charge-spin relation, and make organic electronics has more abundant properties. Combined the organic electronics with the Spintronics, exploring the spin-dependent applications in organic materials, then forming a new subject-Organic Spintronics. In recent years, the generation, transport, storage and detection of spin et al., new physical phenomenons and underlying mechanisms in organic functional materials and devices are the main contents in Organic Spintronics. There is important basic research value and potential application in the application of organic material in spintronics, which is also a hot subject among some international research groups.People have gotten a deep cognition for organic materials and the excited states through a thorough research for decades. On the other hand, people have developed several organic functional devices, such as organic field effect transistors, organic solar cells and organic light emitting diodes. In2002, Dediu et al. realized the spin polarized injection into organic materials, a ferromagnetic electrode was adopted as the source. Then, the performance of the device is improved to achieve the spin polarized injection at room temperature. In2004, Francis et al found that with the application of a weak magnetic field (≤100mT), the magnetoresistance of the device ITO/PEDOT/polyfluorene/Ca can be up to10%at room temperature, the magnitude and the sign is also dependent on the thickness and the bias. The electrode adopted in the device is normal metal without any spin polarized carriers, the corresponding magnetoresistance is the intrinsic property of organic materials, which is dubbed Organic magnetoresistance (OMAR). OMAR has been found in many organic materials containing conjugated polymers and small molecules. However, the organic magnetoresistance is complex, and the dependence on temperature and bias is complicated. As this phenomenon is very unusual in inorganic device, organic magnetoresistance (OMAR) obtained much attention from physics, chemistry, material and electronics. From the investigation in the last few years, there are many significant applications in OMAR, and it is rich in content and is very complicated. There are not only magnetoresistance, but also large magnetic field effect in photoluminescence (PL), electroluminescence (EL) and photocurrent (PC) in organic semiconductor (device), we can call all of them organic magnetic field effect (Organic magnetic field effect, OMFE). This is a hot issue in the current research on organic functional materials and devices, even more the exploration of its mechanism attracted the interest of the physics workers. The investigation on organic magnetic field effect is helpful for the understanding of microscopic process in organic materials and the development of organic functional devices.UP to now, although people have carried a deep research on organic magnetic field effect experimentally, the mechanism governing OMFE is highly debated and the origin has not been achieved with one voice. There are mainly three models to explore the origin of the OMFE:the electron-hole model, the triple exciton-polaron quenching model and the bipolaron model. All these models are based on an assumption that the carriers are spin sensitive to the external magnetic field and the internal hyperfine field of the hydrogen nucleus. Recently, based on the Lorentz force caused by magnetic field, Wang et al. put forward the "magnetic field caused hopping" mechanism, which believes that the electron hopping between molecules is dependent on the magnetic field. However, the peculiar properties of organic materials are not reflected, it is hard to explain why the corresponding magnetic field effect can not be found in its inorganic counterparts. In addition, the diversity of the organic materials and the dependence on temperature and bias are also not reflected. In this paper, we take the calculation of organic magnetoresistance for example, emphasized the peculiar electron-phonon interaction, to illustrate the mechanism of organic magnetic field effect. Based on the Zeeman interaction and Lorentz interaction separately, we give the quantitative calculation on the OMAR. Additionally, the dependence of organic magnetoresistance on electron transfer integral and bias are also investigated. The detailed research and main results are given below:1. Calculation on OMAR based on Zeeman interactionConsidering the crystalline structure of organic small molecule, the band transport mechanism is applicable. Under this mechanism, the conductance of the material is mainly dependent on the density of states near the Fermi level, i.e. σ∞ρ(EF). In the third chapter of this paper, we calculate the change of the density of the states near the Fermi level caused by the magnetic field based on the Zeeman interaction. And then OMAR is obtained.1.1The magnetoresistance in organic materials is much dependent on the density of the carriers. With the decrease of density, the magnitude of magnetoresistance increases. This is consistent with the dependence on the bias that obtained in experiments.1.2For a fixed density of carriers, with the decrease of electron integral, the magnitude of magnetoresistance also increases. The organic molecules are usually bonded by weak Van der Waals force, the corresponding electron transfer integral is in the scale of mT, and this is the prerequisite for the obvious magnetic field effect.1.3The magnetoresistance in organic materials is much dependent on the electron-phonon interaction constant. For a stronger electron-phonon interaction, the magnetoresistance is more obvious. There is no electron-phonon interaction in inorganic materials, so one can not observe any magnetic field effect.2. Calculation on OMAR based on Zeeman interactionThe previous researches are all based on the change of energy caused by magnetic field, but this change is much less than the thermal perturbation, it is difficult to explain this effect at room temperature. As we all know, apart from the Zeeman interaction, there is also Lorentz interaction between the charge and the magnetic field, and this effect can also influence the performance of the device. Compared to the energy, the change of momentum is easier and less dependent on the temperature. In the fourth chapter of this paper, based on the magnetic field-induced hopping mechanism, we investigate the collision process between two oppositely charged polarons under the magnetic field.2.1An imaginary part of electron transfer integral will be induced by the magnetic field. The series Wannier states that compose the electronic state will have a phase difference which is dependent on the imaginary part of electron transfer integral.2.2The mobility of the carrier is related with the electron transfer integral. With the application of magnetic field, the electron transfer integral increases. However, only under a very strong magnetic field, the increase of the velocity of polaron is obvious, and that is not consistent to the observation in experiments.2.3In bipolar device, the combination process between two oppositely charged polarons is inevitable, and this will change the effective density of the carrier. The phase difference caused by the magnetic field make the collision process which is related with superposition between two electron states is magnetic field dependent. With the increase of the magnetic field, the combination probability decreases, and then the effective density of the carrier increases, a negative magnetoresistance is naturally obtained. In addition, it is shown that the magnetoresistance is also dependent on t electron-phonon interaction.