Theoretical Study On Low Field Magnetic Effects In Organic Light-emitting Materials

Organic semiconductor materials with light weight, good flexibility, range, and controllable structure and easy to large area of film forming, become the preferred material for low cost electronic devices and obtain research and application. Organic light emitting device is made up of non-magnetic materials and organic semiconductor are non-magnetic materials, and can make through the current and luminous intensity of device change and produce organic field effect. The strength of this effect can be showed through the relative change rate of device resistance before and after applying magnetic field, Known as the organic magnetic resistance.Studies have shown that, organic magnetoresistance tend to be a certain saturation values under the high magnetic field, as well as present a certain maximum or minimum value at low magnetic fields. Researchers have presented a lot of models for illustrating the mechanism of OMR, but so far, it is far from reaching a unified understanding. The OMR is related to carriers’injection, migration, recombination and decay processes, thus it is a complex integrated effect. There is no doubt that the effect is closely related to the carriers’hopping transport and spin orientation.In this paper, organic electroluminescent devices is studied as the research background. the magnetic field effect on the carrier transport that based on the hopping migration of carriers and the changes of their spin orientation in organic semiconductor light-emitting materials is simulated and establish a calculation model of organic magnetic resistance. This paper calculates and analyzes the characteristics of organic magnetic resistance and its forming reason In the range of 0～1000 mt field. Research content or result are as follows:(1) Based on the two molecular lattice model, M-A equation was used to describe the electron jump process. Hamiltonian including orbital motion of the electron, spin motion and molecular vibration and local super fine interaction describes electronic energy levels and spin state. Assume electronic spin orientation invariant while before and after transition, the expression of organic magnetic resistance is analyzed by the relative change of the hopping frequency, which can be obtained by study the possible jump processes and hopping frequencies of electrons between the two molecules.(2) In consideration of the influence of magnetic field on frequency of molecular vibration, when we transform the Schrodinger equation to the confluent hypergeometric equation in the plane polar coordinates with the vertical magnetic field, we can obtain the exact solution of electron energy level and wave function. After applying magnetic field, the multiple energy level degeneracy was lifted. Energy and energy level splitting is greater with the stronger magnetic field. Energy overlap occurred between adjacent energy levels when the magnetic field meets certain conditions. Due to the calculation of the wave function is too complicated, the formula for estimating the local length is given when the complex calculation of the wave function replaced by the average value of the square of the distance. We can obtain that the molecular vibration frequency increases with the increasing of magnetic field, and the degree of overlap of the wave function reduces when the local length of the wave function decreasing.(3) To the composite spin system that including with an nuclear spin, local nuclear spin is taken as 1/2, and obtain spin splitting energy level and state distribution when we take into account the influence of the magnetic field and the local field. The electron spin state do not change if electrons spin and local spin are homodromous before having a magnetic-field action. A spin state of electron changes with a time and spin energy level becomes splitting if the electrons spin and the local spin are retrorse. The distribution probability of electron spin state at each split level could expressed by three probability density:Pi, P2 and P3, and be the Pi+P2+2P3=1.(4) Regard the electron spin and local spin as a composite system. Regard the process of electron hopping molecular lattice as a process of back entanglement. In the non entangled state, we define a disentanglement process parameter using disentanglement process initial electron spin density average value. The size of η represents the strength of entanglement in the process of characterizing the entanglement. It is modulated by the external magnetic field and ultra fine effect, its mean value appears min in the low field region and saturation in high field regions. The change of η is gentler with the increase of the magnetic saturation process and the magnetic field Bmin is greater when the hyperfine effect is stronger.(5) Thinking about molecular vibration, hyperfine interaction, spin splitting, spin reversal, and electron-local spin entanglement that have an impact on M-A hopping frequency, we establish the calculation model of organic magnetic resistance. Calculation results show that The effect of super fine strength influence on low field magnetoresistance, and the high field magnetoresistance is influenced by the molecular vibration frequency through the local area length. There is a maximum value of organic magnetic resistance in the low field region, the corresponding Bmax. decreases with the increase of the hyperfine interaction effect. Bmax is also related to the spin inversion and entanglement, and the spin reversal has great influence on organic magnetoresistance in low field region. The effect of entanglement influencing on organic magnetoresistance is reflected in the whole magnetic field.In the low field region, the spin entanglement is the fundamental reason for the low field of organic magnetoresistance, while the spin entanglement makes the organic magnetic resistance changed from positive to negative in the high field region.