Studies On The Magnetic Field Effects In Alq₃-based Organic Light-emitting Devices

This thesis mainly includes the studies on the following three subjects:the magnetic field effects (MFE) on the electroluminescence of organic light-emitting diodes (OLEDs); tuning the magnetic-field effect in OLEDs using LiF interlayer; the investigation on the magnetoconductance (the variation of injection current vs. applied magnetic field). It was demonstrated that an applied magnetic field can apparently affect the electroluminescence and injection current. And the variation of these MFEs depends on the temperature.The first part of our experimental work is the research on the MFEs in conventional OLEDs. Organic light emitting diodes with structure of ITO/NPB/Alq3/LiF/Al was fabricated, the magnetic field effects (MFEs) on its injection currents and electroluminescence at different temperatures (12 K, 150 K,200 K and room temperature) were measured. It was found that the applied magnetic field can obviously modulate the injection current and electroluminescence in devices. At low temperature (12 K～150 K), the MFE on EL is strongly dependent on the applied bias (namely injection current). The EL first rapidly increases with the magnetic field and then saturate at about 40 mT. However, at high bias, the MFE on EL decreases after the saturation at 40 mT. The larger the bias is, the stronger the decrease shown. From 200 K to R.T. the EL doesn't decrease after the saturation at 40mT. At any temperature, the current first increase with the magnetic field and then saturate after 40 mT. It is suggested that the physical mechanism of the phenomena may be due to the magnetic field induced suppression of intersystem crossing between singlet and triplet polaron-pair and field modulated triplet-triplet annihilation process.To further explore the influence of triplet excitons'concentration on the MFEs in OLEDs, and to realize the effective tuning on the MFE of electroluminescence, the OLEDs composing inserted LiF interlayer were fabricated. The device structure is ITO/NPB/LiF/Alq3/LiF/Al. By changing the thickness of LiF deposited between Alq3 and NPB, the optoelectrical properties and the magnetic field effect on electroluminescence were studied at different temperatures. The measurement results show that using LiF layer is able to affect the transport of carrier and the formation of the excited states inside the devices. A thick LiF layer could block the transport of holes, lowering the efficiency of the devices. However, using LiF layer could effectively tune the magnetic field effect of electroluminescence:when compared with conventional devices, the magnetic field effect of devices with inserted LiF layer were strengthened within small field rang (B<40mT) at different temperatures; at low temperatures, inserting LiF layer significantly weakened the magnitude of high field (B≥40mT) decrease of electroluminescence, the thicker the LiF layer is, the smaller the magnitude of decrease exhibited. These observations indicate that the concentrations of the triplet excitons can influent the high magnetic field decrease of electroluminescence at low temperatures.Finally, OLED with structure of ITO/CuPc/NPB/Alq3/LiF/Al was fabricated, and the MFE on the injection current (magnetoconductance or MC effect) at 300 K,260 K,220 K, and 180 K were measured with constant voltage bias. During the transition of injection current from bipolar current to unipolar current, the MC of the device increased firstly and fell with the decreasing current. The MC became smaller at the lower temperatures. However, under any measurement conditions, the values of the MC were always positive. The inversion of MC form positive to negative as reported in literatures was not observed. The experimental results demonstrate that the±MC effects in OLED not only depend on the unipolar or bipolar current. It is also related with the organic materials and device's structure. Using the magnetic field modulated "electron-hole pair" mechanism and "bipolaron" model; the positive MC effects in bipolar and unipolar injection current are interpreted respectively.