Control Of The Magnetic Field Effects In Organic Optoelectronic Devices

Recently, there has been a significant driving force in studying the magnetic field effects (MFEs) from nonmagnetic organic materials and organic devices because of two potential impacts. First, organic MFEs provide a useful tool for revealing the device physic in organic semiconductors. Second, organic MFEs can lead to the development of multifunctional organic semiconductor devices with integrated electronic, optical and magnetic properties. Although plenty of magnetic field-sensitive injection current change (magneto-conductance, MC) and electroluminescence intensity change (magneto-electroluminescence, MEL) were readily observed in various organic semiconductors, these MFEs were usually reported singly and discontinuously due to the debatable factors governing the organic MFEs. Therefore, people still have difficulties in obtaining the desirable MFEs to meet the actual requirements, which forms a bottle-neck of the industrialization. One solution of this bottle-neck is developing the technics of controlling MFEs in organic semiconductors.In our previous work, organic MFEs (including the MC and the MEL) were systematically investigated in the organic light-emitting diodes (OLEDs) and the high magnetic field effect of MEL was demonstrated to be an effective "probe" for the triplet-triplet annihilation (TTA) process in the organic optoelectronic devices. In this M.S. Dissertation, by using this probe, we try to control the organic MFEs by means of doping guest materials into the host. The underlying mechanism related to the tunable MFEs in OLEDs was the modification of spin formation and spin evolvement of the interfacial "e-h pair". According to this result, we developed the technics related to controllable MFEs in organic semiconductors and invented a new type of organic magnetic sensor with double parameters and high sensitivity.Our efforts focus on four parts:1. The fluorescent dye doped OLEDs with structure of ITO/NPB/Alq3:dopant/Alq3/LiF/Al were fabricated by using co-evaporation technics. From 15 K to room temperature, the magnetic field dependent electroluminescence of devices, i.e., MEL was investigated. Results show that the decrease in electroluminescence at high magnetic field, which survives only at low temperatures for pure Alq3-based devices, persists in dye-doped devices even at room temperature. This is explained as the result of magnetic field dependent TTA process, in which the triplet excitons trapped on the dye molecules play the most important role.2. The influences of fluorescent dye doping on the magneto-conductance (MC) in Alq3-based OLEDs have been systematically investigated. MC is generally believed to be controlled by the ratio of singlet to triplet excited states in organic semiconductors. However, we found that the MC magnitude decreases substantially upon the introduction of narrow band gap fluorescent dopants. Since singlet to triplet ratio of excited states keeps unchanged in doped devices, this large reduction of MC means that other underlying mechanism affects the MC. The charge carrier trapping effect is proposed here to vary the magnitude of MC. By using this trapping effect, the controlling of the total amount of dissociated electron-hole pairs and consequently the magnitude of MC are realized by changing the dopant's concentration or band gaps.3. The MEL of Alq3-based OLEDs is studied by mixing hole transport material NPB into Alq3 emission layer (EML). We found the MEL decay at low temperature (15 K), unlike the remarkable decay trend of MEL in OLEDs with pristine Alq3 EML, is greatly suppressed in OLEDs with NPB-mixed EML, and nearly vanishes when NPB concentration reaches to～50 wt%. The rate model shows the MEL and triplet exciton density consistently vary with NPB concentration in the mixed EML, suggesting a strong correlation between the MEL and triplet exciton density. Our findings may present a possible way to tune the MEL through adjusting the triplet exciton population in OLEDs.4. Based on the discussions in first part and second part, we public a patent application:"a new type of organic magnetic sensor with double parameters and high sensitivity" (Application No.201010162269 and Publication No.CN101858961), in order to introduce an actual application of organic MFEs.