Fabrication And Electrical Characterization Of Several Organic Composite Films Embedded With Nanoparticles

Over the past decades, organic nonvolatile memory devices have been intensively investigated owing to their advantages of flexibility, low fabrication cost, light-weight and large-area fabrication. Among various types of nonvolatile memory devices, organic bistable devices (OBDs) are a promising alternative to the conventional semiconductor-based nonvolatile memory devices. However, the devices with single organic layer generally have low current On/Off ratio and short rentention time which can hardke meet the application requirements of the memory devices.. OBDs with an organic layer embedded with metal and metal oxide nanoparticles (NPs) are particularly attractive because of their unique electrical properties and easily controllable processing factors. The embedded NPs types consist of metal NPs (Au, Ag, Cu), metal-oxide NPs (ZnO, CdSe, TiO2), organic dyes and bioactive molecules.The hole transport material PMMA and electron material Alq3 were selected as the molecular matrix in this paper, and the embedded NPs were Au, NiO, TiO2, and NiTiO. The effects of process conditions including the NPsl concentration, film thickness, annealing temperatures, and composite structure etc. on morphology, and photoelectric characteristic of the thin films were studied by Scanning electron microscope, Transmission electron microscopy, UV-Vis photometer, Electrochemical workstation etc. The specific contents of the dissertation are as follows:First, the properties of devices with Au NPs embedded in PMMA, Alq3, and PMMA:Alq3 were investigated. The results show that the Au NPs were uniformly distributed in the organic molecules and the films had good morphology. The composite devices showed bistable electrical switching behavior and the On/Off ratio waa increased by two orders of magnititude compared with that of the pure Au NPs. When the film was deposited for three times, the volume ratio of the PMMA and Au NPs solution was 1:1 and the annealing temperature was 110?, the GaIn/Au-PMMA/ITO device showed a better electrical behavior. The properties of the Galn/Au-Alq3/ITO device was slightly influenced by the composite structure, and the On/Off ratio was close to that of the Au NPs. For the GaIn/Au-PMMA:Alq3/ITO devices with different composite structure, the On/OFF ratio is bigger when the organic layer contacts with the electodes than those with the Au NPs linked with the electrodes. The rentention time of the devices is longer than half a month. The first principle calculation was conducted on the Au-PMMA and Au-Alq3 systems which showed that compared with the pristine organic molecules, new energy levels emerged and the band gap decresed in the composite systems.Then, the properties of devices with NiO and TiO2 NPs embedded in PMMA, Alq3, and PMMA:Alq3 were studied. The GaIn/NiO-PMMA/ITO and GaIn/NiO-Alq3/ITO device showed a better On/Off ratio at the mass to volume ratio of 2.5 mg/3ml and 10 mg/3ml, respectively, which was two orders of magnititude bigger than that of the NiO NPs. The organic molecule embedding TiO2 NPs exhibited a bisable electrical characteristic and a relatively small On/Off ratio. No threshold voltage was observed. The conductance of the films increased with the increase of the NPs concentration.Finally, the electrical performance of devices with NiTiO NPs embedded in PMMA, Alq3, and PMMA:Alq3 as the function of the composite ratio of Ni and Ti were studied. The results showed that the GaInTiO-PMMA/ITO and GaIn/NiTiO-Alq3/ITO device had the biggest On/Off ratio and smaller threshold voltge when the composite Ni and Ti ratio was 7:1, while the GaIn/NiTiO-PMMA:Alq3/ITO device had the biggest On/Off ratio and smallest threshold voltge at the Ni:Ti ratio of 3:1. The GaIn/NiTiO-PMMA:Alq3/ITO device was unipolar swept, which showed that the device remain the On state after the transition from the Off to On state. That is to say the reverse bias was essential to the reproducibility of the device switching behavior.Conclusively, the memory performance of the organic composite thin films depends significantly on the matching of the energy levels of the organic molecule and the NPs. The proposed switching mechanism of the compsite device is charge trapping/detrapping.