Synthesis Of ZnO Hierarchical Structures And Their Application In Dye-sensitized Solar Cells

Currently, the world is facing the increasingly severe problems of energyexhaustion and environmental contamination. Exploitation and utilization of newenergy especially for the solar energy, which is abundant, safe and green, havegradually become the focus. Dye-sensitized solar cell (DSSC) has been theconcerntration since its first appearance in the world, simulating the principle ofphotosynthesis in green plants. As a neoteric solar energy conversion device, DSSChas drawn much attention for researchers since its simple assembly, easy productionand low cost. As a key component of the cell, the photoanode plays a key role incharge collection and transmission during the photoelectric process. The use of TiO2as the photoanode material shows the best performance in DSSC. Among thesemiconductor oxides with wide band gap, the band gap of ZnO is the most similarto TiO2and the electron mobility and the electronic diffusion coefficient of ZnO aremuch higher than TiO2, which is regarded as its great potential to be used in DSSC.In recent years, some progress has been received for the DSSC application of ZnO.Considering the weaknesses in light scattering and relatively low surface area forzero-dimensional, one-dimensional or two-dimensional structures of ZnO, this worktries to manufacture three-dimensional ZnO hierarchical structures to achieve largersurface area and increase the efficiency of capturing and scattering for the incidentlight in order to improve the performance of DSSCs. The main research contents inthis article are as follows.(1) A hydrothermal route was used to produce hierarchical ZnO structures basedon one-dimensional nanorods. The effects of the preparation conditions such as thetemperature, pH value of the solution and hydrothermal time on the morphology andphotoelectric properties of the products had been investigated in this work. Theresults showed the product prepared with pH value of10at120°C for18h presentedthe best performance in DSSC application with a short-circuit current density of7.29mA·cm-2and a power conversion efficiency of2.42%since its structural superiority for dye adsorption, electronic transmission and light trapping.(2) A hydrothermal method was utilized to produce hierarchical ZnO structuresbased on two-dimensional nanosheets and the impacts of the molar ratios of thereactants on the morphologies of products had also been surveyed. Experimentalresults showed that the nanosheets-based ZnO hierarchical microspheres (ZnO-MS)displayed better photoelectrochemical performance for their larger specific surfacearea, pore size and pore volume for dye loading, more remarkable light reflectionability for scattering and relatively lower rate for charge recombination. After thepost-treament by tetrabutyl titanate alcoholic solution at90°C, the performance ofZnO-MS-DSSC received a great increase with a short-circuit current density of6.65mA·cm-2, an open-circuit voltage of0.676V and a power conversion effiency of2.94%, which were respectively44.9%,12.5%and67.0%higher than the bare one.(3) A hydrothermal way was employed to prepare hierarchical ZnO structuresbased on zero-dimensional nanoparticles with a large scale of production, highcrystallinity and ordered morphology. Results showed that the abilities of thenanoparticles-based ZnO hierarchical structures (ZnO-HS) for light diffusion anddye adsorption were better than the commercial ZnO nanoparticles (ZnO-NP). TheZnO-HS products were used as the light-blocking layer materials to apply in theDSSCs based on ZnO-NP and TiO2nanoparticles (TiO2-NP) thin films. After addingthe light reflective layer, the capacity for light diffusion of the photoanode wassignificantly enhanced. The short-circuit current density and the power conversioneffiency of the ZnO-NP-DSSC had been increased from5.47mA·cm-2and1.68%to12.6mA·cm-2and3.40%of the ZnO-NP/ZnO-HS-DSSC respectively. However, anopposite result was performed in the TiO2-NP/ZnO-HS-DSSC, which was related tothe internal charge recombination of the cell and confirmed by the results inelectrochemical impedance spectra measurements.