Synthesis Of Semiconductor Nanocrystals And Their Application In Electroluminescence Devices

Due to quantum confinement effect, semiconductor nanocrystals which are smaller than the bulk exciton diameter exhibit special physical and chemical properties. Different emission color, absorption peaks, electron affinities and ionization potentials can be obtained from the same material just by size variation. Recently, some reports on the use of semiconductor nanocrystals in combination with polymer materials application in the light emitting diodes (LEDs) have attracted a wide attention. In this thesis, semiconductor nanocrystals were synthesized in aqueous solution, and electroluminescence devices using nanocrystals/polymer composite thin film as the emitting layer and hole transporting layer were investigated. Results are summarized as follows:Water-dispersible ZnSe nanocrystals were synthesized by a green and simple route. Growth of the nanocrystals was performed under water-bath temperature, using selenium powder and zinc acetate as precursors, upon addition of mercaptoacetic acid as a stabilizer. Products were characterized by X-ray photoelectron spectra (XPS), X-ray diffraction patterns (XRD), transmission electron microscopy (TEM). Photoluminescence properties of ZnSe nanocrystals in evolution are discussed. Photoluminescence is the contribution of both excitonic and defect emission. The excitonic emission peak can be tuned by changing the refluxing time and pH value. Mercaptoacetic acid is available in controlling the ratio of excitonic emission to defect emission. ZnSe/ZnS core/shell nanocrystals were synsthesised by using ZnSe nanocrystals as a core material. The photoluminescence spectra and absorption spectra were employed to study the luminescence and absorption properties, and the results indicated that the luminescence intensities of core/shell ZnSe/ZnS nanocrystals were enhanced greatly in comparison with the uncoated ZnSe nanocrystals, and the surface of nanocrystal was well passivated.ZnSe and ZnSe/ZnS nanocrystals were transferred from aqueous solution to organic solution by using surfactant placing on the surfaces of nanocrystals, thus the well-dispersed nanocrystals could mix with the polymer material MEH-PPV, and electroluminescence nanocomposite devices were fabricated. We studied the effect of the different mass ratio of ZnSe to MEH-PPV and the different applied voltages on the electroluminescence properties. The recombination mechanism of the nanocomposite film under photo excitation and electric injection was discussed respectively. We investigated the photo- and electroluminescence properties of the device, and found the electroluminescence intensity of nanocrystals increased with the applied voltages. I-V characteristic of this device is similar to that of a classic diode.ZnSe nanocrystals were doped into PVK using as hole transporting layer in polymer light emitting devices. We investigated the effect of doped mass concentration on photo- and electroluminescence properties of the devices. After doping with ZnSe nanocrystals, carriers mobility of polymer increased and injection barrier for carriers became lower. As a result, the nanocrystal doped devices had higher luminance and lower onset voltage.