| In decade, Zinc Oxide (ZnO) nanomaterials have caught a great deal of attention due to their extensive applications in light emitting diodes, photocatalysts, photodetectors and solar cells, etc. Meanwhile, ZnO nanomaterials have also exhibited the tremendous potentials to deal with the global energy and environmental crises. As a kind of traditional metal oxide semiconductor, the optical and electronic characteristics of ZnO have been studied from the 1940s. Though ZnO has presented the outstanding performances in various optoelectronic applications, the behind mechanisms are still not understood. Since the 1970s, many researchers have proposed different theoretical models and approaches to describe the photoelectric response and relaxation processes in ZnO, in order to reveal the photogenerated carrier dynamics in ZnO. Actually, the analyses on the photogenerated carrier dynamics are the theoretical basis for further improving the photoelectric properties of ZnO. So far, the descriptions on the photoelectric response and relaxation processes in ZnO are mainly relied on either the depletion layer model or the photogenerated carrier rate function, but both of them are qualitative. Besides, it also should be noted that both models own their deficiencies, and furthermore there are also some discrepancies between them. Therefore, this thesis will discuss the above models in detail, and then imitate their advantages to propose an innovative analytical approach and a new theoretical model to reveal the photoelectric response and relaxation processes in ZnO. Experimentally, through systematically tuning the extrinsic and the intrinsic influence factors on the photoelectric response and relaxation processes in ZnO, the time-resolved photoconductance spectra and the related photoelectric characteristics of ZnO will be carried out and studied in detail. Based on the experimental results, the complex photogenerated carrier dynamics in ZnO will be resolved into the different extrinsic/intrinsic carrier dynamic sub-processes, and each carrier dynamic sub-process will be discussed and described by a corresponding rate function, respectively. According to the simultaneous rate functions of all carrier dynamic sub-processes, a thorough-process photogenerated carrier dynamic (rate function) model will be established. On the basis of the rate functions of the thorough-process photogenerated carrier dynamic model, the corresponding analytical equations will be solved to quantitatively decipher the time-resolved photoconductance spectra of ZnO. Combined with the experimental data, the full time-resolved photoconductance spectra of ZnO will be thoroughly fitted by the analytical equations, so that the photogenerated carrier dynamic parameters can be extracted, and then the photoelectric response and relaxation mechanisms of ZnO will be revealed. According to the establishing procedures of the thorough-process photogenerated carrier dynamic model and the analytical equations of photoconductance spectra, this thesis will present a new concept and an innovative methodology to study and evaluate the photoelectric characteristics of ZnO.According to a literature search, this thesis performed the studies based on the ZnO nanoparticle film and the ZnO nanorod array, because both of them were the most widely used ZnO nanostructures in various practical applications. Through presenting the time-resolved photoconductance spectra, the photoelectric response and relaxation characteristics of the ZnO nanoparticle film and the ZnO nanorod array were studied. Compared to the ZnO nanoparticle film, the ZnO nanorod array performed the outstanding persistent photoconductance (PPC) behavior. Depending on the differences in morphology, the carrier transporting models of the ZnO nanoparticle film and the ZnO nanorod array were established, respectively. It suggested that the oxygen vacancies owned the different charge states in the different depletion and non-depletion regions in ZnO nanocrystals. Because the oxygen vacancies with different charge states could act as the electron traps or the carrier recombination centers, they gave the significant influences on the photogenerated carrier lifetimes. In the ZnO nanorod array, the non-depletion regions in the nanorods connected with each other, and then formed a "conductivity network" with few recombination centers for the photogenerated carrier transporting in the array; therefore the photogenerated carrier recombination rate in the array could be reduced, and the photogenerated carrier lifetimes could be prolonged. Due to the long photogenerated carrier lifetimes, the ZnO nanorod array performed the capacity to storage the photogenerated carrier for a long period of time, resulting in the significant PPC effect.Afterwards, through establishing the thorough-process photogenerated carrier dynamic model and the analytical equations of time-resolved photoconductance spectra, this thesis quantitatively described the photoelectric response and relaxation processes in the ZnO nanoparticle film and the ZnO nanorod array, respectively. By changing the oxygen partial pressure in the atmosphere and meanwhile tuning the wavelength of photoexcitation source, the different extrinsic/intrinsic photogenerated carrier dynamic sub-processes in the ZnO nanoparticle film were investigated in detail, and then a donor photoionization model (DPM) were proposed. In the DPM model, a series of rate functions were employed to describe the photogenerated carrier dynamic sub-processes occurring in ZnO, including the band-to-band transitions, the donor photoionization, the surface oxygen adsorption/desorption, and the photogenerated electron recapture by defects. Correspondingly, a series of analytical equations of the DPM model were further solved to thoroughly fit the full time-resolved photoconductance spectra of the ZnO nanoparticle film. Subsequently, the photogenerated carrier dynamic parameters (such as the electron yield from donor photoionization, the electron capture rates by defects, and the oxygen adsorption reaction rates on ZnO surface, etc.) were quantitatively extracted to decipher the photogenerated carrier dynamics in the photoelectric response and relaxation processes of the ZnO nanoparticle film. It suggested that the photoconductance gain and the PPC effect mainly depended on the donor photoionization in ZnO; whereas the adsorbed oxygen molecules on ZnO surface acted as the electron acceptors, which captured the photogenerated electrons and thus reduced the photoconductance gain and the PPC effect.Furthermore, this thesis carried out the studies focusing on the remarkable PPC effect and the photogenerated carrier storage properties of the ZnO nanorod arrays. Through controlling the ZnO nanorod diameter, both the PPC effect and the photogenerated carrier storage of the ZnO nanorod arrays could be tuned. According to the dependencies of the photoelectric characteristics of the ZnO nanorod arrays on the nanorod diameter, it suggested that the Debye length and the photon penetration depth in ZnO could spatially partition a standalone nanorod into the different photoelectric functional regions (PFRs). The different extrinsic/intrinsic photogenerated carrier dynamic sub-processes occurred in the different PFRs, which synergistically determined the photoelectric properties of the ZnO nanorod arrays. Accordingly, a series of rate functions were employed to quantitatively describe the various photogenerated carrier dynamic sub-processes occurring in the different PFRs, and then a thorough-process photoelectric response and relaxation dynamic model (TPRDM) was established. Furthermore, on the basis of the corresponding analytical equations, the TPRDM model was employed to fit the full time-resolved photoconductance spectra of the ZnO nanorod arrays. After the photogenerated carrier dynamic parameters had been quantitatively extracted from the fitting processes, it indicated that the oxygen vacancy photoionization occurring in the non-depletion region of the ZnO nanorods was the root cause for the significant PPC effect and the photogenerated carrier storage properties of the ZnO nanorod arrays. Besides, the TPRDM model fitting processes on the time-resolved photoconductance spectra not only deciphered the photogenerated carrier dynamics in the ZnO nanorods, but also provided a numerical-analytical method to quantitatively evaluate the photoelectric properties of the ZnO nanorod array-based devices. Moreover, the TPRDM model revealed the different functional roles of the PFRs in different optoelectronic applications, and thus it also illuminated a practical approach to design the ZnO nanorod array-based devices via optimizing the modularized spatial configuration of the PFRs.Finally, the ZnO nanorod array was annealed in a hydrogen atmosphere, in order to study the influences of the oxygen vacancies with high concentration on the photoelectric response and relaxation processes of the ZnO nanorod array. The high concentration of oxygen vacancies in ZnO remarkably increased the free electron concentration in the conduction band, resulting in the degeneration of the conduction band. Meanwhile, the high concentration of oxygen vacancies also led the defect levels to extending into the defect band in the band gap of ZnO, and the extended defect band from the shallow oxygen vacancy levels in ZnO were overlapped with the degenerated conduction band. The degenerated ZnO nanorod array performed the less photoelectric responses to the ultraviolet, the blue, and the green photoexcitations; however, it exhibited a remarkable photoelectric response to the near-infrared photoexcitation. Besides, the degenerated ZnO nanorod array presented the selective enhanced photoluminescence in both the ultraviolet and near-infrared regions. According to the photoluminescence characteristics and the time-resolved photocurrent spectra, the influences of both the oxygen vacancy and the Moss-Burstein shift on the photogenerated carrier dynamics in the degenerated ZnO nanorod array were further discussed. In the degenerated ZnO nanorod array, the photogenerated carriers owned a much shorter lifetime, and the photogenerated carriers easyly experienced the radiative recombination. Therefore, the degenerated ZnO nanorod array exhibited the remarkably enhanced photoluminescence properties, but both the PPC effect and the photogenerated carrier storage were abolished. |
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