Theoretical Investigation On Magnetic Field Effect In Organic Devices With Asymmetrical Molecules

In2003Kalinowski et al. found that the current in the device changed obviously when the magnetic field is applied. Since then, many people spend much spirit and energy to investigate organic magnetic field effect (OMFE). Experiments showed that OMFE has following surprising universal features.(1) The OMFE appears in vast different organic semiconductors without any magnetic elements at room temperature although the possible energy level shifts due to the presence of a magnetic field are orders magnitude smaller than the thermal energy and other energy scales. For instance, T. D. Nguyen obtained the magetoconductance (MC) as large as-23%in room temperature.(2) The electroluminescence, photocurrent, photoluminescence, and electrical-injection current are very sensitive to a weak magnetic field with both positive and negative OMFE though positive OMFE at very weak field is typically observed.(3) The observed MC seems to have different behaviors from small molecules, oligomers to polymers, and from weak to strong magnetic field. It may be fitted with the empirical Lorentzian B2/(B2+B02) or non-Lorentzian [B/(B+B0)]2or their combination. It may be also fitted with power law B", f1/B2+f2/B4or d1B2+d2B4.(4) It is not very clear the relationship between OMFE and bias or temperature.(5) There are two kinds of OMFE, positive OMFE and negative OMFE.With these interesting and abundant experimental phenomena, many theoretical physicists would like to take much time to investigate the origin of OMFE. In the present, a large number of people believe spin is the reason of the OMFE. And in all of the theories based on spin, the hyperfine field is regarded as a key factor. Three mechanisms, including bipolaron model, e-h pair model, and exciton-charge interaction model, which are based on the correlation of the spins of interacting carriers and its dependence on the magnetic field, have come on. So far, people have make specific calculations of the three mechanisms with hopping theory and obtain many conclusions, which can be used to explain the Lorentzian’s and non-Lorentzian’s behaviors, the peculiar behavior in super small magnetic field and the sign of OMFE. However, we are not clear about the relationship between bias, temperature, device’s size and OMFE according to the three mechanisms. Beside spin, some people believe the electronic orbit’s variation should be the reason of OMFE. In2010Wang and Xie suggested to understand OMFE from charge-Lorentz effect. They predicated that a non-symmetrical electronic orbit may result in an apparent change when a magnetic field is applied. Just recently, Alexandrov et al. put forward hopping MC via nonzero orbital momentum. They found that a weak magnetic field could shrink/expand electronic p orbit.In this paper, we consider the influence of the magnetic field on the transfer integral. By using a nonadiabatic dynamic method, we have simulated the dynamic process of the collision between a positive polaron and a negative polaron. Combined with experiments, we explain the significant effect of Lorentz effect on OMC and emphasis the importance of the asymmetrical structure and the electron-phonon (e-ph) coupling of the organic material. The detailed research and main results are given below:1. With SSH model and nonadiabatic dynamic method, the relationship between polaron’s velocity and the external magnetic field has been investigated. It’s found that an external magnetic field will increase transfer integral which adds polaron’s velocity. However the increment is small, when the magnetic field is300mT, the variation is only0.01%, which is too small to be regarded as an error in experiments. So the variation of the polaron’s velocity is not the essential reason of the MC.2. The current through a device is dependent upon not only the carrier velocity, but also the carrier density. Let us consider a bipolar device where electrons and holes are injected separately from the cathode and anode. A negative polaron and a positive one may meet each other to annihilate into an exciton. When an external magnetic field is applied, it will act on a moving polarons and change its phase, which is possible to change the collision of polaron pairs or the yield of excitons. So the appearance of magnetic field effect (MFE) is expected. With our calculation, it is found that the yield of excitons decreases linearly with increasing the magnetic field, which leads to a negative MC. 3. The organic magnetic field effect is only occurred in organic materials, and people have not found it in inorganic materials. Organic semiconductors have a few distinct properties that their inorganic counterparts do not have. Firstly, unlike an atom that is sphere-like, an organic molecule is highly irregular or asymmetrical. As a result, an organic molecular orbit has no obvious symmetry. For example, the itinerant π-electron in the hybrid sp orbital. With our calculation, it is found that a large asymmetrical structure contributes to an apparent MC. For a symmetric molecule, there is not any magnetic field effect. Secondly, molecules in organic semiconductors are bonded by the Van der Waals force so that their bands are very narrow in comparison with an order of10eV bandwidth for their inorganic counterparts, which means that the charge carrier is strongly localized. Therefore, one will expect an appreciable value for the magnetic correction in organic semiconductors for localized polaron state. In our model, the electron-phonon coupling decides the local level of the polaron. Increasing electron-phonon coupling, which makes a polaron more localized, leads to larger MC.