| Energy and environmental issues are very important for the sustainable human development in this century. One of the ways to solve these problems should be the utilization of solar energy. Development of photoelectric functional materials and application of photoelectric conversion systems will be the important methods for the conversion from solar energy to electrical energy. In recent years, the developments of semiconductor nanomaterials, sub-micro- and micro-materials make people invent new photoelectric conversion systems, for example, dye-sensitized solar cells, photocatalysis, and photoelectric gas sensors. In general, these photoelectric conversion systems usually use TiO2, ZnO and other wide bandgap semiconductors as the basic components. However, these wide bandgap semiconductors do not absorb visible light which occupy a large proportion of the solar spectrum. Therefore, construction of composite structures with narrow bandgap semiconductors or organic molecules would be an important method to enhance their photoelectric activity. Studying the properties of photogenerated charge carriers in photoelectric functional materials can make us understand the essence of their photovoltaic differences, establish new evaluation tools and theoretical basis for photoelectric conversion systems. Then we can design and synthesis photoelectric functional materials with high photoelectric activity to construct photovoltaic devices. The properties of photogenerated charges in micro/nano composite materials are affected by many factors, for example, the built-in field at the interface of heterogeneous structure, the doping to a material to change its energy band structure and charge transporting properties, the functional groups in organic molecules.The study of the properties of photogenerated charges requires appropriate research methods. The separation of photogenerated charges in space will be accompanied by the photovoltaic effect. The surface photovoltage technique is a non-destructive measurement which can directly show the information of separation of photogenerated charge carriers caused by surface states, drift under built-field and diffusion under concentration gradient etc. Therefore, surface photovolage techniques based Kelvin probe (KP) and lock-in amplifier and transient photovoltage measurement were used to study the properties of photogenerated charges in these micro/nano composite materials. The main results of this doctoral thesis are illustrated as follows:1. Highly ordered TiO2 nanowire arrays growing on the F-doped SnO2 (FTO) glass were prepared by solvothermal methods. The morphology and structure of the sample were characterized by scanning electronic microscopy (SEM) and X-ray diffraction (XRD). The photovoltaic properties were studied by surface photovolage techniques based KP and lock-in amplifier. Transient photovoltage measurement revealed the dynamic information of the separation of photogenerated charges. These results show that charge separation is controlled by drift in space charge region, diffusion under concentration gradient and surface trapping. According to the surface work functions of FTO and TiO2 nanowire arrays, the schematic band diagram of the TiO2/FTO interface is given, which is closely related to the transient photovoltage. All the experiments results could be a guide to the practical application of this nanostructure.2. ZnO nanowire arrays coated with CdS quantum dots were successfully fabricated by a chemical bath deposition method. SEM, transmission electron microscopy (TEM) and XRD have been utilized to characterize the samples. We have studied the processes of separation and recombination of the photo-generated charges in the heterogeneous films in the visible region by surface photovoltage and transient photovoltage measurements. By controlling the amount of attached CdS QDs, we found that the photovoltaic characteristics change significantly. Then, we fabricated quantum dot sensitized nanowire solar cells (QDSSCs) with a liquid electrolyte as the hole transport medium. The QDSSCs exhibited short-circuit currents ranging from 0.8 to 2.6 mA cm-2 and open-circuit voltages of 0.35-0.44 V when illuminated with light intensity 100 mW cm-2. The best power conversion efficiency of these QDSSCs was 0.34%. The relation between the performance of QDSSCs and their photovoltage characterization was also discussed.3. The photoelectric properties of ZnO microspheres, ZnO/Ag2S heterogeneous microspheres and Ag2S hollow microspheres were investigated systematically by surface photovoltage, transient photovoltage and surface photocurrent techniques. The ZnO/Ag2S heterostructure shows superior photoelectric properties in visible-light region compared with pure Ag2S. Transient photovoltage results reveal the separation processes of photo-generated charge carriers in the samples. Due to the excellent photoelectric properties in visible light region a photoelectric gas sensor was fabricated. The photoelectric ethanol sensing property induced by visible light for the ZnO/Ag2S heterostructure is obvious, which should be valuable for the practical application of semiconductor gas sensors at room temperature.4. Zn-doped TiO2 microspheres have been synthesized by introducing a trace amount of zinc nitrate hexahydrate to the reaction system. SEM, Field-emission scanning electron microscope (FESEM), XRD, and X-ray photoelectron spectroscopy (XPS) have been utilized to characterize the samples. Both surface photovoltage spectroscopy technique based on lock-in amplifier and transient photovoltage measurement reveal that the slight doping of Zn can promote the separation of photo-generated charges as well as restrain the recombination due to the strong interface built-in electric field and the decreasing of surface trap states. The photovoltaic parameters of dye-sensitized solar cells (DSSCs) based on Zn-doped TiO2 are significantly better, compared to that of a cell based on undoped TiO2. When Zn/Ti molar ratio was 0.5%, near-monodisperse microspheres are obtained, and the DSSC based on this sample had the best power conversion efficiency. The relation between the performance of DSSCs and their photovoltaic properties is also discussed.5. Surface photovoltage techniques based on Kelvin probe and lock-in amplifier, and transient photovoltage technique were all employed to study the properties of photo-generated charges in a series of novel dendritic oligocarbazole attached [60]fullerenes. It was shown that the conductivity types of these samples change from n-type to p-type with the increasing generation of the carbazole groups by all the three kinds of photovoltaic measurement. The C6o core in the molecular plays a role of chromophore. By analyzing the data gained from transient photovoltage and contact potential difference (CPD) measurements, we consider the n-type characteristic should originate from the fullerene-related photogenerated charges separation, while the p-type behavior of the dendritic molecule may arise from the transport of photogenerated holes (diffusion) in the hole-transporting carbazole dendrimers.
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