Mechanism And Device Of Organic Pyridinium Salt ASPT

Organic light-emitting devices (OLEDs) have received great attention for their application in flat-panel displays because of their excellent properties such as high contrast, broad angle of view, high speed responses, etc. Search for small-molecular red-emissive materials for organic light-emitting devices applications has embarked on three different stages:(1) dopant guest luminescence materials, (2) assist dopant guest luminescence materials, (3) and the non-doped host luminescence materials. Doping could improve emission properties of devices effectively, but the concentration of guest dopants affects the device and makes the preparation process complicated. Meanwhile phase separation and crystallization take place. Therefore the development of ideal red luminescence material has become a hot topic in electroluminescence device. In this thesis, an organic pyridinium salt,trans-4-[p-[N-ethyl-N-(hydroxyethyl)-amino]styryl]-N-m-ethylpyridinium tetraphenylborate (ASPT) is used as the red luminescence material. The electroluminescent mechanism and device based on ASPT have been studied. Main work includes:Firstly, the experimental results indicate that with the increase in the polarity of the solvent, the absorption peak of ASPT blue-shifts while the fluorescence emission peak of ASPT red-shifts. When dissolved in chloroform, benzyl alcohol and dimethylformamide (DMF), ASPT exhibits a large Stokes shift. This Stokes shift increases with the increase in solvent polarity.Secondly, ASPT films are prepared via vacuum deposition. The ultraviolet-visible light absorption spectra show that the energy gap is 2.3 eV. The highest occupied molecular orbital (HOMO) of ASPT molecules is 5.1eV, and the lowest unoccupied molecular orbital (LUMO) is 2.8 eV, as determined by cyclic voltammetry (CV). The effect of various evaporation parameters on the surface features and compactness is analyzed using atomic force microscope (AFM).Thirdly, single-layer ASPT device is fabricated. The electroluminescence is orange-red in color and the EL spectrum shows a peak wavelength of 610 nm and a full width at half maximum (FWHM) of about 152 nm. Experiments show that the current is linearly dependent on voltage when the voltage is very low. As the external voltage increases, the current is proportional to (m+1)-th power of the voltage. The value of m changes with the variation in the thickness of the emissive layer.Fourthly, bi-layer and tri-layer ASPT devices are constructed. The introduction of electron-transport layer into ITO/ASPT/Alq3/Al bi-layer device improves its emission properties and decreases the turn-on voltage of the device. The EL peak wavelength of tri-layer ITO/TPD/ASPT/Alq3/Al device is 560 nm at low voltage, which shifts to 610 nm when the working voltage is a little bit high and then shifts to 530 nm as the voltage continues to increase. Therefore, a voltage-modulated color-varying EL device is realized.Lastly, by using hole-transfer poly(N-vinylcarbazole) (PVK) as the host material and dopant ASPT as the guest material, an ASPT:PVK-based single-layer organic light-emitting device is constructed. The experimental results confirm that there is an effective Forster Energy transfer between PVK and ASPT molecules in the blend system, meanwhile, guest molecules capture electric carriers and result in the red-emission. The EL peak wavelength red-shifts with the increase in the doping concentration of ASPT in the ASPT:PVK system.