Study On The Magnetoresistance Of Two-terminal Organic Thin Film Devices

As a new class function materials, organic semiconductors have been attracting much interest because of the excellent optical, electric and magnetic properties occurring in these materials. As we know, apart from charge, an electron also has a spin. However electrons are treated only as the carrier of the charges with their spins being usually neglected in the traditional micro-electronic devices. The discovery of the effect of giant magneto-resistance and tunneling magneto-resistance in metallic spin valves caused the revolution in the field of magnetic memory and magnetic recording. Organic semiconductor material is the most promising candidate to realize spin polarized transport because of its week spin-orbit coupling and hyperfine interaction which lead to longer spin diffusion lengths. It is significant to study the magnetic resistance in organic devices for further understand of the physical properties and to explore the special functions for use scientific proven in spintronics.In this paper, we mainly investigated the magnetic resistance of two terminal organic thin film devices with a structure of anode/organic semiconductor materials/cathode. Following concludes are obtained:(1) It is demonstrated that the organic magnetic resistance (OMAR) is negative and its absolute value increases with the external field. When the external field increased to 250mT, the organic magnetic resistance reached to-57.5%. while in devices which structure of ITO/CuPc/Al, we found that the OMAR is positive and increases with the external magnetic field. It reaches a value of 30% for an external magnetic field of 140mT. The origin of the OMAR is as follows:in an external magnetic field, the triplet excitons translate into singlet excions through intersystem crossing, which leads to the number of singlet exciton increase and decrease of that of triplet excitons which lowers the effect of the capture and scattering between carriers. Thus the field effect mobility of carriers increases which means that the current increases and the absolute value of the organic magnetic resistance 100x[p(H)-p(0)]/p(0) increased.(2) It was found experimentally that the current-voltage (I-V) curve shifts towards small voltage direction with the increases of the external magnetic field. The reason may be that the magnetic field approaching that of spin-orbital interaction and hyperfine interaction field, the triplet-triplet exciton annihilation rate increases which results in an increase of singlet exciton. The reduction of triplet excition and increase of singlet exciton increases the carrier mobility, which results in a left-shift of the I-V curves.