STUDIES ON MAGNETORESISTANCE OF ORGANIC SEMICONDUCTORS UTILIZING FIELD-EFFECT TRANSISTOR STRUCTURES

of Thesis Organic semiconductors have been considered in the past few decades as promising candidates for future electronic devices. The organic materials research has been driven by their potential advantages such as low production cost with relative ease of processing, high electrical and electroluminescent efficiency, and structural flexibility of the devices. An intrinsic magnetic field dependent current observed in organic semiconductors, so-called, organic magnetoresistance (OMAR) is of great scientific interest because of its unique properties compared to other magnetoresistive effects. The OMAR effect can have a relatively large magnetoresistance value (up to 10–20%) at room temperature and small magnetic fields of 10 mT without using ferromagnetic contacts. Many models are suggested to explain this effect, but debate still exists about its nature. In this thesis, I have studied the OMAR utilizing field effect transistor structures, which delivers many advantages compared with transitional sandwich devices. The type of charge carrier, mobility, current density can be controlled and determined by the voltage applied to the gate electrode, which is very potent to elucidate the mechanism of OMAR. Firstly, the magnetoresistance effect of photocurrent on single-crystalline pentacene (PEN) and perfluoropentacene (PFP) field-effect-transistors has been demonstrated. General observation of MR can be obtained from highly disordered materials. Now, we can observe the OMAR on high order materials, pentacene and PFP single crystals. Our results indicate that it is required double carrier to cause the OMAR effect, hence, the bipolaron model is not suitable for explanation of OMAR. This statement has been confirmed by the gate voltage dependence of MR value in thin film of bilayer PEN/PFP FETs. Moreover, by analyzing the dependency of MR on gate voltage, drain source voltage, traps density and temperature, we suggested the possible mechanism based on the combination of electron-hole pair and exciton-charge interaction models. Secondly, the formation of electron-hole pair is essential to cause OMAR as shown in the study of bilayer thin film PEN/PFP FETs. The external magnetic field tents to increase triplet state density due to less mixing between singlet and triplet electron-hole states. The triplet electron-hole pairs can recombine into triplet excitons, which then interact with charges or traps causing different influences on current. The sign change showed in multilayer PEN/PFP comes from the switch in mechanism between triplet exciton detrapping and triplet exciton charge interaction process. The exciton-charge interaction becomes dominant when there is a large current density in device. Finally, I demonstrated a magneto-electroluminescence (MEL) effect of 1.3% for alpha sexithiophene (α-6T) light emitting FET (LEFET) under 100 mT at room temperature. The study confirms that the origin of magnetoconductance (MC) and MEL is different, and MEL is independent with MC. The magnetic-field-induced increase in the singlet exciton formation probability derives from intermolecular quantum correlated effect and the enhancement in irradiated recombination efficiency at the α-6T/Al interface. Therefore, by optimizing the magnetic field effect and device structures, the luminous efficiency of the LEFETs can be improved.