Magnetic Field Effect in Small Molecule Organic Semiconductors

Abstract

The magnetic field effect (MFE) exerted on charge transport and energy transfer processes in fluorescent small molecule organic semiconductors has been investigated in this thesis. By varying both internal factors such as device structures and external factors such as bias applied, various MFEs have been observed and identified in conductance, photocurrent, photoluminescence (PL), and electroluminescence (EL) measurements at room temperature. The observed MFEs generally consist of so-called low MFE ($< 10$ $\text{mT}$) and high MFE ($> 10$ $\text{mT}$) components and can exhibit either positive or negative field dependence. The low MFE is attributed to magnetic field suppression of electron-hole pairs spin mixing via hyperfine interaction between electron spin and nuclear spin. But the triplet-exciton-polaron interaction (TXPI) is responsible for the high MFE. The trion model has been used to explain the underlying physical origin for TXPI mechanism with the hypothesis of the doublet and quartet trions as the intermediate states. The magnetic field suppression of spin mixing between doublet and quartet trions results in the various high magnetic field effects. The characteristic magnetic field of TXPI is determined by zero field splitting of TX and the additional electronic coupling between TX and polaron if the polaron wave function is extended. The positive effect of the TXPI based energy transfer processes can enhance the external quantum efficiency of fluorescent doped organic light emitting diodes.