| dc.description.abstract | This thesis investigates the multilayer organic field effect transistors fabricated with high-mobility P-type (DNTT) and N-type (DFH-4T) small molecular semiconductors. We characterize the devices including ambipolar field effect transistors and trilayer light-emitting transistors, and correlate their optoelectronic performance with the morphology, microstructures, and the energy levels of organic films.
For ambipolar field effect transistors, we perform a series of study on different deposition order of organic films and different top contact electrodes to optimize the mobilities and injection efficiencies of electrons and holes. We observe that N-type organic semiconductor material (DFH-4T) as deposited on dielectric material (PMMA) would form a unique morphology with randomly packed large-sized crystals. Such a unique morphology of DFH-4T has dual effects on the sequential deposition of DNTT and top contact metals: (1) The film quality of P-type materials (DNTT) remains good and hence a high mobility of holes; (2) Evaporated metals will easily penetrate into the bulk of organic films, resulting in an increased contact area between metals and organic films. Therefore, electron injection can be still efficient even using high work-function metals. With DFH-4T/ DNTT deposition order and high work function metal (Ag), we demonstrate the ambipolar organic field effect transistors with electron mobility and hole mobility exceeding 1 cm2/Vs.
Next, we insert a light-emitting layer into the DNTT and DFH-4T, to provide a space for electrons and holes to recombine and emit light. In consideration of the energy level of DNTT and DFH-4T, we choose the red phosphorescent material (Ir(piq)3) with a narrow energy bandgap as the guest material, and dope it into the fluorescence material (Bebq2) as the host material to form the light-emitting layer. We perform a series of experiments to understand the effects of the doping concentration of phosphorescent materials in the emitting layer, deposition order of organic semiconductors, and the thickness of the bottom layer (DNTT) on the EQE of organic lighting-emitting transistor. With the doping concentration of 5-10 wt %, deposition order of DNTT/Bebq2:Ir(piq)3/DFH-4T , and 10 nm DNTT as bottom layer, we achieve the highest EQE up to 0.2 %. This light-emitting transistor has significant features of the broad recombination zone approaching to 100 mm and the high emissive intensity, which is promising for display applications in the future.
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