| From the eighties of last century, organic light-emitting diodes and organic photovoltaic cells are constructed. Until2002, spin of carrier is introduced into organic materials, and then different types of organic materials and devices are constructed. Because organic materials contain very light elements (such as C, H, O, N), the spin-orbit coupling and hyperfine interaction are very weak. Therefore, spin diffusion length is long in organic materials. With further investigating the spin and charge of organic materials, more and more new devices will emerge in our life.Up to now, organic device can be divided into three types. The first type device adopts magnetic molecule as interlayer to realize spin dependent functions. Organic magnet is the combination of organic material and magnet, which has been investigated in the past decade. Up to now, several organic magnet have been synthesized, such as organic ferromagnet poly-BIPO, which substitutes part of H atoms in polyethylene with magnetic side radicals to achieve magnetism. In the following, some theorist did research on the origin of magnetism in organic ferromagnet poly-BIPO. They got the property of spin density wave (SDW) as well as the effect of electron-electron interaction and boundary condition on SDW. The second type device contains two different Ferromagnetic electrodes and organic layer. With the effect of magnetic field, the electrodes present spin parallel or spin anti-parallel, which induces the resistant of device changing with magnetic field. The focus of this device is how to increase the spin polarized injection. The third type device does not contain any magnetic element, the electrodes and organic layer are all non-magnetic. In this type device, conductance (or electroluminescence) changes as a function of external magnetic field (B) and is observed in relatively low magnetic fields (lower than100mT) at room temperature. To understand the physical mechanism of OMFE, a large number of bipolar and unipolar organic devices have been fabricated and investigated in experiments. Up to now, a unified theory to explain this magnetic field related phenomenon is lack. Now, we focus on the second and the third devices to investigate magnetic field related phenomenon. Many experiment measurements have been given, but the explanations are lack. In this thesis, we build the transition equations among carriers, and take into account the effects of Zeeman effect, hyperfine interaction and exchange interaction on the transitions. We try to give the explanations on magnetic field related phenomenon.1. Organic spin valveTemperature can change the effect of magnetoresistance in organic spin valve. Two factors can be responsible for this temperature related phenomenon:one is the interfacial polarization of ferromagnetic electrodes, the other is temperature related spin relaxation time. We employ a drift-diffusion equation and take into account the temperature influence on mobility and spin relaxation to investigate spin polarization and magnetoresistance of organic semiconductor device. And the change of magnetoresistance with temperature is mainly dependent on spin relaxation time of organic layer.2. Organic unipolar device2.1For unipolar device, bipolaron model is popular. Bobbert el al. proposed steady-state rate equation for polarons and spinless bipolarons. By considering a two-site scheme, they obtained the magnetoconductance (MC) and showed that MC is caused by blocking of the current by certain sites at which bipolaron formation can take place. We try to establish a group of dynamic equations for polarons, and bipolarons. These equations include the transition rate between polarons and bipolarons, which is closely related with both the B and the hyperfine interactions of the hydrogen nuclei. By considering the mobility or velocity of a polaron different from that of a bipolaron, we calculate the MC in a unipolar organic device and compare the results with possible experimental data.2.2Voltage can effectively change MC effect. We study the voltage effect on MC in an organic semiconductor device based on voltage-related carrier density and mobility. With the effects of magnetic field and hyperfine interaction, we present the transition between polaron and bipolarons, and provide the dependence of MC on total carrier density and mobility. The MC effect will become weak with increasing voltage in a unipolar polymer device and the sign of MC will change at a critical voltage. In a small molecular device, the situation is opposite:MC increases with increasing voltage.2.3We have mentioned that MC can be obtained at room temperature. At room temperature, not only singlet bipolaron can be emerged in organic materials, but also triplet bipolaron will be emerged. Because of thermal activation, singlet bipolaron will change into triplet bipolaron. By considering the mobility of a polaron different from that of singlet bipolaron and triplet bipolaron, we calculate the OMC in a unipolar organic device, and MC can be positive and negative. Besides, ultra small magnetic field effect in organic device can be obtained with the effect of exchange interaction.3. Organic bipolar deviceOrganic bipolar devices are widely used around us (such as organic light-emitting diodes and organic photovoltaic cells). It is very interesting for us to investigate the magnetic field related current and electroluminescence in these devices. By constructing dynamic equations including electrons, holes and their pairs, we calculate the MC and the magnetoelectroluminescence (MEL) separately. It is indicated that MC and MEL may result from different response on the applied magnetic field. MC is from the scattering of polarons by magnetic field related triplet excitons, while MEL is mainly from magnetic field related conversion between singlet and triplet electron-hole pairs. Furthermore, we discuss the relation between MC and MEL. |
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