Controllable Synthesis Of Novel Ⅱ-Ⅵ Group Semiconductor Hierarchical Nanoarrays And Their Photovoltaic Devices

In the past decades, II-VI group nanomaterials have attracted much attention in lightof their unique properties distinct from their bulk counterparts and thus great potentialin a wide range of applications. Specially, arrays of one-dimensional (1-D) II-VI groupnanowires are of particular interest to researchers due to their highly-ordered geometrystructure that enables them to be building blocks in the construction of nanodevices. Inthis dissertation, the work was focused on controllable synthesis and assembly ofhighly-ordered nanowire arrays of II-VI group materials, including CdS and ZnO,through the construction of hierarchical nanostructures. From the perspective ofapplication, we systematically investigated the construction of PbS quantum dot baseddepleted bulk heterojunction solar cells in light of those requirements includingenergy-band match and light absorption. We first developed a novel synthetic system forPbS quamtum dots. Following the synthesis work, in view of practical applications, adip-coating process was successfully developed to address the large-scale fabrication ofquantum dot film. At the same time, bulk heterojunction solar cells were constructedbased on the controllable synthesis of ZnO and TiO2nanowire arrays, from which thechoice of the materials and the related working principles were investigated, offeringgood reference for such kind of solar cells.In details, we present our work as follows.1. Novel CdS hierarchical nanoarrays were successfully developed through a two-stepprocess in which preformed Cd microplatelet arrays (MPs) were prepared to inducesubsequent in situ growth of nanowire arrays on the Cd MPs in a solvothermalprocess. The morphologies, structure, composition, and phase of the resultantnanostructures were analyzed. The nucleation and growth behavior of CdS nanowirearrays were investigated by a combination of manipulation of the syntheticconditions and analysis of the product. Based on the electrodeposition system, aH2-bubble assisted growth mechanism was proposed for the formation of patternedCd MPs, thus providing guidance for the synthesis of other II-VI groupsemiconductors with such structure.2. Based on the synthetic roadmap of CdS hierarchical nanoarrays, ZnO hierarchicalnanoarrays were successfully achieved. The morphologies, structure, compositionand phase were analyzed whereas the formation mechanism of such ZnOhierarchical nanoarrays was thoroughly investigated. The growth of Zn MPs also follows the H2-bubble assisted growth mechanism as proposed for Cd MPs byanalyzing the electrodeposition system. The controllable fabrication of ZnOhierarchical nanoarrays were achieved by manipulating the structure parameters ofthe ZnO MPs.3. In light of those factors limiting the facile synthesis of PbS quantum dots, we havedeveloped a double-sulfur-source strategy to address the reactivity of sulfurprecursor, by which highly monodisperse PbS quantum dots were successfullyobtained employing a low-cost and environmentally-friendly synthetic system. Themorphology, size and phase were analyzed while the corresponding growth kineticswas thoroughly investigated. The work offers new thought to regulate the precursorreactivity in the field of colloidal synthetic chemistry.4. A dip-coating process was developed for the preparation of high-quality quantumdot film, featuring the advantages of improved material utilization and large-scalefabrication. Those factors that affect the quality of the resultant quantum dot filmwere investigated, including the concentration of the quantum dots, the pullingspeed, the humidity and the temperature. Based on the dip-coating approach,depleted heterojunction solar cells were constructed to evaluate the photovoltaicperformance of the dip-coated quantum dots. Meanwhile, we demonstrated theuniversality of this dip-coating process for conformal coating of highly-coarsesubstrate employing ZnO nanowire arrays with high aspect ratio. Furthermore,based on the conformal coating of ZnO nanowire arrays, novel radial-junctionquantum dot solar cells were constructed. The photovoltaic performance of suchkind of solar cells was investigated and those factors limiting the photovoltaicperformance were put forward, thus providing reference for the construction ofhigh-performance radial junction solar cells.5. Depleted bulk heterojunction solar cells based on PbS quantum dots wereconstructed. Highly-ordered ZnO nanowire arrays were prepared via a hydrothermalprocess, in which controlled growth was achieved by manipulating the growthconditions. PbS quantum-dot-based depleted bulk heterojunction (DBH) solar cellsthat employ ZnO nanowire arrays as the electron-acceptor were built, by which theeffect of the thickness of the PbS quantum dots on the resultant photovoltaicperformance was studied. TiO2nanowire network-based structured electrode wasdeveloped employing the ZnO nanowire arrays as the template. Based on theas-developed TiO2structured electrode, DBH solar cells were constructed in conjunction with PbS quantum dots. The choice of the n-type structured electrodeand their related structure parameters were investigated in term of the photovoltaicperformance of the resultant solar cells. Finally, the origin of the performanceenhancement as involved in the depleted bulk heterojunction structure was studied.