Synthesis Of Nanoscale TiO₂/Organic Hybrid Semiconducting Materials And Their Application In Ultraviolet Photodetection

Ultraviolet (UV) photodetectors are becoming increasingly important for their applications in a number of areas, such as military use, UV communication, environmental monitoring, space research, and so on. Most of the UV photodetectors currently in use are made of wide-bandgap inorganic semiconductors. Inorganic/organic hybrid semiconductors are promising candidates for UV photodetection because they combine the advantages of two components, which exhibit high photosensitivity, fast response, good stability as well as easy processing, large-scale applications, low cost, and large field-of-view detection. In this dissertation, TiO₂ was chosen as the electron acceptor for its high mobility and high electron injection efficiency. Based on energy-band engineering theory, organic donors were selected to match the energy level between the donor and acceptor components and form the TiO₂/organic hybrid optoelectronic materials. The contact area between each component and the efficiencies of exciton separation and charge transportation were effectively improved by tailoring the microstructure and aggregate structure of the hybrids through some novel methods such as layer-by-layer assembly, in-situ synthesis, nanoscale synthesis and ordered construction. The photodetection properties of the hybrid materials were investigated in details.TiO₂/MTPD hybrid UV photoconductor was prepared via layer-by-layer deposition method. It was found that the hybrid material absorbed the lights below 420 nm and the maximum absorption position blue-shifted from 392 nm to 380 nm, which meant that its bandgap has been broadened. In our experiments, the photoconductive response velocity increased linearly with the increasing intensity of the incident light, and the photosensitivity increased by one time compared with single component. Meanwhile, the photosensivity also increased with the increased thickness of the TiO₂ film. The efficient light-induced charge transfer from MTPD to TiO₂ proved by SPS might be responsible for the improved photosensitivity. The study confirms that inorganic/organic hybrid materials might be used in the UV photodetection field. TiO₂ in-situ modified PVK was synthesized through nucleophilic reaction. The LUMO and HOMO energy levels of the modified PVK were found to be higher than those of pristine PVK. It also exhibited improved thermal stability according to the thermal analysis results. The UV photosensitivity increased by nearly one time compared with that of pure PVK when the TiO₂ content was 0.71%.Solution-processed TiO₂ nanocrystals/PFH hybrid semiconducting materials were synthesized at low temperature. Then the effects of TiO₂ content on the morphology and optical properties of the hybrid films were investigated systematically. The photodetector devices were made from the hybrid films and obvious photovoltaic effects were observed. The device characteristics, includingâ… -â…¤curves, spectral response, response time, and the bias dependence, were studied. High UV photo-to-dark current ratio of 103 was obtained for the hybrid photodetector under 3.2 mW/cm2 UV light or 0.7 mW/cm2 monochromatic light (365 nm) as well as fast response time less than 200 ms and high photoresponse of 54.6 mA/W at zero bias, which is comparable to some traditional GaN and SiC based UV photodetectors. Furthermore, the device showed changes in current under a wide range of UV irradiation with a linear relationship between them. The surfactant (Oleic acid) could dramatically improve the dispersion of TiO₂ nanocrystals in the polymer matrix and the compatability between them; however, the current was lowered accordingly. The thermal behavior of the device was also discussed in the end.Ordered inorganic/organic hybrid materials could provide large interface area, directional charge transport pathways, and reduced recombination ratio of the charge carriers. First, TiO₂ nanorod arrays were fabricated on the transparent FTO electrode. The formation mechanisms and the influences of the experimental parameters were discussed. Then PFH was spin-coated on the templates to form the ordered bulk heterojunction nanostructure which was applied in the fabrication of prototype UV photodetectors. The UV photo-to-dark current ratio approached three orders of magnitude under 3.2 mW/cm2UV light. Both the rise time and the decay time were shorter than 200 ms, which was the detecting limit of our equipments. Good reproducibility was also achieved.This dissertation is a first report for application of inorganic/organic hybrid semiconducting materials in the field of UV photodetection and concentrated on both theory and experiment aspects. Preparation of inorganic/organic hybrids provides theoretical and experimental evidence for further study of the hybrid semiconducting materials and corresponding devices used for photodetection, which is of great scientific significance and potential application value.