This dissertation presents the magnetic field effects (MFEs) in conjugated polymer/ organic molecules-based diodes, including the phenyl-substituted poly(p-phenylene vinylene) copolymer (super yellow, SY-PPV)-based polymer light-emitting diodes (PLEDs), tetracene- and pentacene-based diodes. For the first topics in the first part, the effect of device architecture (current injection through the diodes) and operating condition (the external applied bias) on magnetoconductance (MC) response was investigated and analyzed by the fitting analysis in SY-PPV-based PLEDs. Using the mathematical analysis to fit the curves with two empirical equations of a non-Lorentzian and a Lorentzian function, we are able to extract the hidden negative MC component from the positive MC resposes in charge-unbalance SY-PPV-based PLEDs. We attribute the negative MC component to the triplet excitons-charge reaction. The negative MC component can be further increase by increasing the concentration of free hole carriers in hole-blocking SY-PPV-based PLED. Thus, the negative MC component corresponds for the line shape broadening of MC curves. In the next part, we investigate the triplet-triplet annihilation (TTA) process in the charge-balanced SY-PPV-based PLEDs. We found that the temperature and current density may induce the TTA process in SY-PPV-based PLEDs. The TTA process may harvest the energy from triplet to singlet excitons in SY-PPY active layer and in part contribute the emission to fluorescence in PLEDs especially in the high current density regime.
In the second topics, we study the magnetic field effect in tetracene-based diodes. We found the singlet fission (SF) reaction occurs in tetracene-based diodes based on the magnetophotocurrent (MPC) and magnetophotoluminescence (MPL) characterization. By depositing the fullerene (C60) on the tetracene active layer to yield a planar heterojunction device, we found that the MPC response show the sign-change and the PL spectra of tetracene/C60 PHJ-based diode show almost completely quenched. It indicates that the charge separation by charge transfer (CT) complex states is more effective than the SF reaction. Consequently, the singlet fission reaction is suppressed by this charge separation of the opposite charge carriers at the donor/acceptor interfaces.
Finally, at the third topics, we investigate the MPC response of pentacene-based diodes in different magnetic fields orientation either perpendicular (90°) or parallel (0°) orientation. We found that the MPC magnitude is magnetic field-orientation dependent. We attribute the change of MPC magnitude under magnetic field-orientation to the interaction of polaron pair’s spins (dipole-dipole or exchange interaction). Depositing C60 on pentacene layer in addition to the change of MPC magnitude, it also narrows the MPC line shape. Due to the weaker exchange interaction in CT complex states from pentacene/C60 interface, the external applied magnetic field can modulate this interaction result in the modulation in MPC line shape. The MPC line shape narrowing is observed at low magnetic field regime (B < 300 Oe) in pentacene/C60-based diode by changing the magnetic field orientation from 90° to 0°. We contribute this MPC line shape narrowing to the suppression of hyperfine interaction to induce the intersystem crossing. Our experimental results strengthen the previous studies that the dipole-dipole, exchange or hyperfine interactions between polaron pairs or CT complex states are responsible for the magnetic field orientation dependence of organic magnetoresistance.

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