Investigation of Magnetic Field Dependent Electroluminescence and Charge Injection in Organic Light Emitting Diodes

After 20 years of development, conjugated polymers have been extensively applied in organic light emitting diodes (OLED), solar cells, transistors, and chemical or bio-sensors. Recently it is discovered that magnetic field can tune the electroluminescence intensity and conductivity in OLEDs, leading to the development of organic magneto-optoelectronics. However, the underlying mechanisms are still unclear.

In this dissertation, we investigated a wide range of conjugated polymers and low molecular weight molecules and proposed that the magnetic field effect on electroluminescence and magnetoresistance arise from the magnetic field enhanced polaron pair dissociation and reduced triplet-charge reaction. The final magnetic field effects are determined by the sum of the two contributions.

The magnetic field effect on polaron pair dissociation can be tuned by varying the spin-orbital coupling of the organic semiconductor. Stronger spin-orbital coupling leads to the reduction of magnetic field effect on both electroluminescence and magnetoresistance. Phosphorescent dye doping can also tune the magnetic field effects through energy transfer process and intermolecular interaction.

Triplet-charge reaction can be largely controllable by manipulating the bipolar injection. It has found that unbalanced bipolar injection enhance the triplet-charge injection, leading to more positive magnetoresistance and more negative magnetic field effect on electroluminescence. Balanced bipolar injection reduces triplet charge reaction, resulting in more negative magnetoresistance and more positive magnetic filed effect on electroluminescence. The triplet-charge reaction can also be morphologically tuned. In poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) based OLEDs, low energy crystalline domains can be induced in PFO amorphous matrix by either high boiling point solvent or annealing treatments. The low energy domains can both spatially confine both excitons and charges to enhance the triplet-charge reaction. Consequently the enhanced triplet-charge reaction reduces the magnitude of magnetic field effects

Our study successfully built a bridge between the magnetic field effects and the spin dependent excitonic processes in OLEDs. Scientifically, the excitonic processes, e.g. intersystem crossing, triplet-charge reaction, can be investigated by simply measuring the magnetic responses. Technically, this tunable magnetic field effects have the potential to be used to in new generation smart screens, magnetic sensors.