Study On Spin Polarization Of Charge Carriers In Organic Semiconductors

Compare with inorganic semiconductors and full carbon materials (such as graphene), organic semiconductors have many unique properties. Most of the or-ganic semiconductor materials have highly disordered morphology, low chemical purity and unique electronic structures. Over the past thirty years, as important electromagnetic optical functional materials, the field of organic semiconductors has made great progress. Compared with inorganic devices, organic semicon-ductor devices are easy preparation, flexibility and have lower price, which have broad prospects in application. Many organic electronic devices, such as organic light-emitting diodes, organic solar cells, organic field effect transistors, organic sensors and organic lasers have been developed. The great success of OLED lighting and display in commercial applications has arouse large-scale research in organic spintronics. Many magnetic field-sensitive device, such as magnetic field sensors, spin valves and spin organic light emitting diodes, have been pro-posed. Device-oriented research have make great advances, while organic spin-tronics contains not only spin-dependent technology, but also the study of phys-ical mechanism. At present, the literature have been proposed a large number of microscopic physical models and assumptions.The charge carriers in organic semiconductors are polarons and bipolarons. Charge transport takes place mostly by hopping of injected carriers between localized states, which results in low mobility in carrier density. Because of the high disorder in configuration, the carrier density may be different from sample to sample which made by the same organic material. Organic Spintronics contains not only spin exchange of information carriers, but also the magnetic field sensitive carrier transport, these phenomena are all related with the spin relaxation. In organic spin valve devices, the spin-polarized current inject into the organic active layer from the electrodes with different coercive force. The magnetization direction of the two electrodes can be varied independently. This device can be made of carrier spin filter. Conversely, the device is sensitive to the external magnetic fields, it also can be used as magnetic field sensor. Organic spin valve devices require high carrier mobility and long spin relaxation time. The spin-orbit coupling in organic semiconductor is weak, which is favorable to spin polarization. But the low mobility, hyperfine field, high disorder and high localization of the carriers in organic semiconductors are not conducive to spin polarization. These features result in abundant physical properties, while it also lead to the difficult in experimental study. Due to the stray field from the electrode, the conventional magneto-optical Kerr effect no longer applicable. So far, the most promising experimental technique that could track the spin polarization in microcosmic level is the μ spin rotation spectroscopy. According to the experiment, the spin diffusion length in organic semiconductor is of only several nanometers. Another experiment is two-photon photoelectron emission spectroscopy, which offers compelling evidence for interfacial spin transfer, but cannot probe the transition of spin-polarized carriers.The phenomena that the charge transport in organic light emitting diode devices could be affected by external magnetic field further complicate the prob-lem. It is called organic magnetoresistance (OMAR). The OMAR effect needs only a weak external magnetic field (tens of mT), and have a resistivity rate 1-10% changes. In recent years, the OMAR effect have been reported in OLEDs with different organic materials, especially the devices prepared by means of so-lution and vacuum deposition method. Magnitude, sign and bias dependence were also studied. OMAR in Several mT and magnetic electroluminescent effect also have been reported. Also it was found that the OMAR may be change the sign with the external magnetic field changes. Another interesting point of view is, OMAR effect could possibly reveal the physical mechanism in the magnetic field sensitive birds, and it may also involve in the phenomenon of macroscopic quantum coherence at room temperature.There are three main theoretical models on OMAR effect. That are the electron-hole pair model (it is also called polaron pair model), bipolaron model and exciton-polaron collision model. In the electron-hole pair model, the con-version between singlet and triplet electron-hole pair are controlled by the spin relaxation process, and the concentration of singlet and triplet electron-hole pairs determines the conductivity of the device. The external magnetic field can change the effect on spin relaxation from the hyperfine field, which lead to the OMAR effect. Generally, this model can only describe the positive, or negative OMAR. It can not describe both. In exciton-polaron collision model, the collision between excitons and polarons will decreased the mobility. The external magnetic field can change the concentration of triplet excitons, thereby changing the number of collisions, And then change the mobility and generate OMAR effect. Both models require devices exist positive charge carriers, but OMAR can appear in a unipolar device. Bobbert et al proposed a bipolaron model may well explain the OMAR effect in unipolar devices. This model assumes that the external magnetic field could regulate polaron and bipolarons ratio. Due to the different mobilities of polarons and bipolarons, the conductivity will change with the magnetic field. It is worth noting that there was found up to 2000% of OMAR in a unipolar device, and can be well expressed by bipolaron model. The magnetic interaction in OLED devices have been studied, such as the hyperfine interaction and spin-orbit interaction. Xie et al suggest a model based on the Lorentz interaction, the magnetic field could changing transition integral between two neighboring molecules, thus changing the current. Now the biggest challenge is to directly observe and examine the validity of these theories in experiments. Currently, there are spectroscopic techniques to carry out this related research.The OMAR effect promote the depth study of the spin characteristics of charge carriers in organic semiconductor materials. In 2010, Tarafder et al studied spin polarization in a Alq3 molecule by using first-principle simulation method. They found that the injected charges have a lower energy in a spin polarized state than a non-polarized one. The polarization or magnetic moment increases nearly linearly with the injected charge quantity. They analyzed that, when the asymmetric Alq3 molecule is doped with electrons, the three Al-N bonds will change their lengths which causes spin splitting and finally magnetic moment induced by ferromagnetic coupling appears in the molecule. But Alq3 contains a metal atom which make it difficult to elucidate the reason for the charge in-duced magnetism. Then in 2013, Hou et al. studied spin polarization of a pure organic material. They calculated a charged thiophene oligomer by using density functional theory method. They also found that the charged oligomer is spin polarized. However the polarization characteristic in the polymerized oligomer is different from that in small molecule Alq3. The emergence and variation of the net magnetic moment is related to both the amount of charge injected and the polymerization of the oligomer.The OMAR effect and spontaneous spin polarization in semiconductors are both indicate that the charge and spin characteristics in organic semiconductors are unique and rich. Recently, the room temperature multiferroics in organic semiconductors has aroused great interest. Multiferroic material refers to the materials that simultaneously show two or more ferroic orders, such as ferroelec-tric, ferromagnetic and ferroelasticity. Over the past decade, because of the rich physical properties and potential applications in the field of spintronics, opto-electronics and thermal sensors, multiferroic materials quickly became a hotspot in material science. The discovery and study of the room temperature organic multiferroics offer a new to the application. In 2009, Giovannetti et al used first-principles calculation combining model method, predicted TTF-CA charge-transfer salts could occur in multiferroics for the first time. After this, several organic magnetoelectric materials have been created in experiments. In 2012, Ren et al reported photo excitation ferromagnetic in nw-P3HT/C60 composite. They found that in P3HT nanowire single crystal doped with C60, light will cause the device show significant ferromagnetism. Further experiments found that the external electric field and stress could also affect the magnetization. In inorganic multiferroic materials, the Curie temperature is usually very low. while in or-ganic charge-transfer composites not unlimited, so it may have potential in room temperature multiferroic devices.In summary, charge carriers in organic semiconductor materials are rich in spin characteristics. There have been found a lot phenomena in experiments, but the physical mechanism behind them still need to be carefully studied. This will help us to understand the charge carriers in organic semiconductor better. In this paper, we use the classical theory of transition and one-dimensional tight binding (SSH) model, and study organic magnetoresistance phenomenon, charge induced spontaneous spin polarization in organic semiconductors, and photoex- citation ferromagnetics in charge-transfer blend devices. The following are the brief introductions and conclusions.1. OMAR effect in OLED device. There have been several theoretical models on organic magnetic field effect. Here we use the classical Marcus transition model. The magnetic field causes Zeeman splitting of the energy level, and changes the intermolecular charge transition integral. We also take into account the hyperfine interaction from the hydrgen atom, and the Lorentz force from the external magnetic field on the carriers of the Lorentz force. We found that, at low temperature, magnetic field induced charge transition integral through Zeeman splitting may be one reason that cause OMAR. However, at high temperature, the thermal disturbance energy is higher than Zeeman splitting energy. In these case, the Lorentz force maybe the main role on the OMAR effect.2. Charge induced spontaneous spin polarization in polythiophene molecules. By using one-dimensional SSH model, including electron-electron interaction, spin-flip effect and spin-orbit coupling, we study the spin polarization phenomenon in polythiophene molecules. Compared with the first-principles calculation, the model approach can easily calculate a long molecule chain, inject more charges into the molecule and more convenient for parametric analysis. Our calcula-tions shows that, with electron-phonon coupling constant increases, the injected charges will emerge from the extended state to localized state. Along with the emergence of the localized electronic states, the spin polarization appears. Thus a strong electron-phonon interaction is the prerequisite for the emergence of spin polarization. With the same amount of charge injection, the spin polarization is different with different polymerization, which is related with the localization of the electronic state. For small molecule, the injected charge is always localized in the molecule, so it can easily appear spin polarization. In a polymer, in the case of small amount of charge injection, charge injected first form an extended state, the charge injection amount is increased to a certain degree of localized states appear, and the accompanying spin polarization generated. For the spin-orbit coupling, because organic materials spin-orbit coupling strength is small. It is found that the spin-orbit coupling has no effect on the spin polarization.3. Photoexcitation ferromagnetics in organic charge-transfer complexes, we use the extended tight-binding model including electron-electron coupling and spin-flip effect, and study the spin polarization of excitons and charge transfer states in organic charge-transfer complexes which were found by Ren et al in 2012. It is generally believed that with the product of a single excitation light between excitons, triplet exciton is transition forbidden.because their number is negligible. But after considering the relevant spin interaction, the spin is no longer a good quantum number, all of the excitons will become singlet and triplet excitons superposition state. It calculated that the charge transfer state, the electron donor and acceptor on the spin polarization is not the same, so no matter how excited charge transfer state is always displayed to the outside net magnetic moment. In this part of the work,we also proposed a model to explain ferromagnetism magnetic excitation light source.