Magnetic Field Dependent Electroluminescence and Charge Transport in Organic Semiconductors

It has been found that a small magnetic field (<300 mT) can substantial change the electroluminescence, photoluminescence, photocurrent, electrical injection current in nonmagnetic organic semiconductors. It is generally believed that these magnetic field effects (MFE) are related to the spin dependent processes in organic semiconductor. However, the origin of MFE is still not well understood. In this dissertation, we investigate the underlying mechanism for magnetic field effects on electroluminescence (MFE_EL) and magnetoresistance (MR) and demonstrate the complete tuning of MFE_EL and MR based on our theoretical understanding.

We consider MFE arising from magnetic field sensitive intersystem crossing (ISC) and triplet charge reaction. Magnetic field can increase the singlet ratios through ISC, accounting for positive MFE_EL. Magnetic field modulated ISC strongly depends on the electron-hole pair separation distance. MFE can be enhanced by increasing the electron hole pair distance through material mixing and interplaying the electric dipole-dipole interaction. Meanwhile, two possible mechanisms corresponding for negative MFE_EL: triplet-triplet annihilation and triplet charge reaction are also discussed. The negative MFE_EL is achieved through adjusting triplet density charge confinement and exciton/charge ratio, which indicates that triplet charge reaction is a dominate process accountable for negative MFE_EL.

Significant MR and MFE_EL are observed in strong spin orbital coupling iridium complex based OLED device after introducing the non-magnetic insulating blocking PVA layer. A possible mechanism for this new interface induced MR and MFE_EL is proposed based on magnetic field perturbed spin-spin interaction at short capture distance of inter-charge carriers. The comparative study of two strong spin orbital coupling materials Ir(ppy)₃ and Ir(ppy)₂(acac) with different electrical dipole moments indicate the electric dipole-dipole interaction can change MR and MFE_EL from short distance capture based regime to long distance intersystem-crossing regime.

At last, we demonstrate the fully tuning sign of magnetic field effect on the fluorescence (MFE_FEL) and phosphorescence (MFE_PEL) by using the ISC, energy transfer and spin-spin interaction. In addition, we demonstrate a giant MFE_EL (400%) in electrochemical cells and attribute this giant MFE_EL to Lorentz force driven ion transport and Lorentz force dependent diffusion layer thickness through convection.