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 mT) and high MFE ( 10 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...[ Read more ]

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 mT) and high MFE (> 10 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 (T_XPI) is responsible for the high MFE. The trion model has been used to explain the underlying physical origin for T_XPI 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. Polaron trapping and de-trapping processes based on T_X-free polaron interaction and T_X-trapped polaron interaction have been identified in NPB based single layer devices. The different characteristic magnetic fields of T_XPI are determined by zero field splitting of T_X and the different electronic coupling between T_X and free/trapped polarons. The magnetic field suppressed energy transfer based on T_xPI has been observed. The existence of doublet trion has been proved with its life time determined to be ~ 120 ps at room temperature in NPB doped by DCJTB device. The T_XPI based energy transfer plays the decisive role in EL generation in doped OLEDs. The evidence of the electronic coupling between T_x and polaron has been proved by the resonance effect in MFE on EL in DCJTB doped organic light emitting diodes. The positive effect of the T_xPI based energy transfer processes can enhance external quantum efficiency of fluorescent doped organic light emitting diodes.