Spin Injection And Transport Properties In Organic Device

In the past few decades, spintronics as a very potential research area of condensed physics has attracted a lot of interests. Different from the classical electronics, spintronics involves both the electronic and spin characters of an electron. Fundamental studies of spintronics include investigations of spin injection, transport and detection in electronic materials, as well as spin manipulation. Its goal is to understand the interaction between the spin of electrons and crystalline environments and to make useful devices. Spintronics based on electronics, in the past decades, has resulted in not only a lot of applied device, such as high-density memory, but also some foundational renovation of physics, for example the emergence of some new physics concepts or phenomena such as spin current, spin valve, spin hall effect and so on. Spintronics device includes the conformations of magnetic metals or magnetic semiconductors with insulator, semiconductor, conductor, or superconductor, and the mixture of them, also includes molecule or nanometer device. Recently, several model devices have been schemed out. For example, Datta and Das designed the first spin transistor. However, the low efficiency of spin injection into semiconductor blocks the practical applications of such devices. The spin polarization of injected current is closely related to the ratio between resistances of the two layers. Therefore, more attentions should be paid to the influence of interfacial effects on the spin injection and spin evolution during the transport process.Compared with conventional semiconductors, soft organic semiconductors (OSCs) have an opportunity to form a good interface with ferromagnetic metal (FM) or half-metal contacts, reducing the probability of spin scattering at the interface. The spin relaxation time is much longer than that of conventional semiconductors due to the weak spin-orbital and hyperfine interactions. Different from the traditional semiconductors, in which the carriers are electrons or holes, the carriers in OSCs are some "quasi-particles" such as polarons, bipolarons, and solitons. They have more complex spin-charge relation, which will result in more abundant properties of an organic spin device. The electric, magnetic and optical properties are unique in both the organic molecules and conjugated polymers. Up to now, the electric properties of quasi-one dimensional conducting polymers have been well understood. However, there is a lack of full understanding about the spin property of OSCs, since it is a new field called organic spintronics.Organic spintronics is a subject that mainly study some physics mechanisms or phenomenon such as the creation, annihilate, transfer or storage of spin in the organic materials or device. It is an interdisciplinary subject, including two regions: organic materials in chemistry and spintronics in physics. Discussing the application of organic material in the spintronics apparently has significant value of basic research and potential foreground of applications. Therefore, it is an aspect that a lot of international research groups are interested in.In 2002, Dediu's group firstly report the spin injection and transport in organic material, they use semi-metal material La_xSr_1-xMnO₃ (LSMO) as the source of electrons and sexithenyl (T₆) as the organic layer. It has been found that spin injected in the organic layer, and the current is spin polarized. In recent years, a lot of experiments have confirmed the spin injection and transport in organic materials, for example, in 2004, Xiong et al. have also observed spin injection and transport in a LSMO/Alq3/Co organic spin valve. The measured magnetoresistance can be as high as 40% at low temperature. Majumdar et al. have observed as much as 80% magnetoresistance (MR) at 5 K and 1.5% MR at room temperature in the structure of LSMO/polymer/Co organic spin valve. They also found that there is a thin spin-selective tunneling interface between LSMO and the polymer, which improves the spin injection.Quantum theory used by Xie et al. and spin diffusion theory used by Smith, and Z. G. Yu et al. have been demonstrated successfully for studying the spin polarized injection and transport in OSCs. The quantum theory can describe the microcosmic mechanism of spin transport, while the macroscopical theory can obtain some results comparing to the experiments. In recent years, although much effort has been devoted to the study of organic spintronics, many questions are still under debate or indistinct. For example, effect of the transition between polarons and bipolarons on the spin injection and transport; the density of bipolarons depends on which factors; effect of the permeation of magnetic atoms at the FM/OSC interface on the spin injection and transport, and so on. In this paper, based on the classic drift-diffusion theory, we will investigate the above questions. The detailed research contents include:1. Effect of polaron and bipolaron on the spin polarized injection and transportDue to strong electron-lattice interaction in OSCs, injected electrons can induce the distortion of the lattice and result in charged self-trapped states called spin polarons or spinless bipolarons. A polaron has 1/2 spin, while a bipolaron has no spin. It should be pointed out that, due to the effect of temperature, pressure or external electric field, bipolarons and polarons in OSCs are not necessarily independent excitations. Two spin polarons may annihilate into one spinless bipolaron, while a bipolaron may also dissociate into two spin polarons. Street et al. has suggested a spin-irrelevant polarons interaction model to investigate the transition between polarons and bipolarons. Neglecting that transition, people have understood the preliminary effect of polarons and bipolarons. However, effect of the transition between polarons and bipolarons on the spin injection and transport is indistinct. Based on the Street's model, we suggested a spin related polaron-polaron model to describe the transition between polarons and bipolarons and investigated the spin injection and transport in an organic spin device. The evolutions of spin polarons and spinless bipolarons are calculated from the drift-diffusion equations, in which both the polaron-bipolaron transition and the spin flipping of a spin polaron are included. Then the spin polarized current is obtained. It is found that the polarons are responsible for the spin polarized transport in an OSC. Different from the case in a normal inorganic semiconductor, spinless bipolarons will affect the spin polarization of the OSC device. Finally, effects of the spin-flip time and the mobility of the carriers on the spin polarization in an organic device are discussed. It is found that large spin-flip time or mobility is helpful to the spin transport in OSCs. 2. The factors affecting the density of bipolaron in an OSC spin deviceThrough considering the transition between polarons and bipolarons, we found that bipolaron will apparent affect the spin polarized transport in the chapter III. Otherwise, Bobbert et al. considered that the influence of magnetic field on the density of bipolaron resulted in the organic magnetoresistance (OMAR). It can be seen that bipolaron as one of carriers play an important role in the spin transport, its density will decide the property of organic spin device to some extent. Considering the transition between polarons and bipolarons, we calculated the density distribution of carriers based on the drift-diffusion equations, and discussed the effect of the spin-flip time, mobility on the density of bipolarons. It was found that the value of the spin-flip time does not influence the saturation density of bipolarons, but the splitting of the spin-flip time is not helpful to the creation of bipolarons, which indicate a spin non-degenerate state in the OSC does not make for the creation of bipolarons. It also was found that the value of the mobility does not affect the saturation density of bipolarons. However, the saturation density of bipolarons will increase with the decrease of the proportion of mobility between polarons and bipolarons.3. Effect of Co permeation on spin polarized transport in a Co/OSC structureCo atoms will permeate into the soft organic material to form a magnetic permeated sublayer (MPS) during the fabrication of an organic spin device, such as Co/OSC/LSMO. We considered the OSC as a two-sublayer structure of MPS and pristine OSC, and then established a dynamic spin-diffusion equation to study the effect of MPS on the spin current polarization and the magnetoresistance of the device. It was found that the MPS will change the spin transport due to its different spin-flip time and mobility from that in the pristine OSC. The splitting of spin-flip times will be favorable to the spin polarization transport. Mobility of spin polarons in the MPS will be reduced due to the scattering of the Co atoms, which will weaken the spin polarization. For a given device, effect of the thickness of the MPS on the spin polarization is discussed. Also we calculated the magnetoresistance of a Co/OSC/LSMO device by the Julliere formula. A theoretical result which is consistent with the experimental data was obtained. Finally, effects of magnetic permeation at interface on spin injection are discussed. It can be found that the interface with magnetic permeation is spin selective, which will be helpful to spin injection. If the magnetization of permeation layer is large, the selectivity of interface will be strong.