Improved Nano-structures in hydrolysis-derived titanium dioxide particles for dye-sensitized solar cell applications

This thesis presents a study on the modification of titanium dioxide (TiO2) nanoparticle preparation through two hydrolysis routines: sol – gel and hydrothermal and the use of dopants for dye-sensitized solar cell (DSSC) application. Certain TiO2 characteristics, such as particle size, morphology, surface area, and phase structure, are crucial for obtaining superior power conversion efficiency in dye sensitized solar cells. Due to the agglomeration problem of the sol – gel process, this method has been found to make it difficult to control particle sizes with high surface area. In this work, we report a simple approach to improve the DSSC by controlling the degree of aggregation through zeta potential analysis. We found that different aqueous colloidal conditions, i.e., potential of hydrogen (pH), water/titanium alkoxide (titanium isopropoxide) ratio, and surface charge, obviously led to different particle sizes in the range of 10 to 500 nm. The stable sol solution was used for the blocking layer as well. Power conversion efficiency of 7.15% was obtained by using anatase TiO2 optimised to 10-20 nm in particle size with a compact blocking layer and a scattering layer. The scattering layer was made of particles with an average size of 100-200 nm, which were obtained through the sol – gel method by controlling the reaction parameters. Using the stable sol solution, one-dimensional (1D) TiO2 nanostructures were prepared using the electrospinning method to verify their potential for use as the photoelectrode of DSSCs. The achieved 1D mesoporous nanofibers were 100 nm in diameter and 10-20 μm in length, and were composed of aggregated anatase nanoparticles 20-30 nm in size. The employment of these novel 1D mesoporous nanofibers significantly improved the dye loading and light scattering of the DSSC photoanode, and resulted in conversion cell efficiency of 6.64%, corresponding to a ~65% enhancement over the Degussa P25 reference photoanode. Electrochemical impedance spectroscopy was used to investigate the electron transfer through the photoanode, and it showed improved charge transport and electron diffusion through the electrospun TiO2 nanofibers. The effects of phase structure on the photovoltaic performance were also investigated. To investigate the phase structure effects, it is very important to have the different phase structures in the same particle sizes. In this work, we controlled the phase transfer rate by controlling the synthesis parameters. The obtained nanoparticles were of the same size, but with different phase structures. The nanoparticles containing 75% anatase phase with 25% brookite phase showed the best photovoltaic performance, with power conversion efficiency of 6.8 % for operation with a TiO2 blocking layer. The hydrothermal synthesis method was used to synthesize oriented, single crystalline, one-dimensional TiO2 nanostructures. In this study, a precisely controlled procedure was used to synthesize 3D dendritic and 1D TiO2 nanostructures by tuning the hydrolysis rate of the titanium precursor. Based on this innovation, oriented 1D rutile TiO2 nanostructure arrays with continually adjustable morphologies, from nanorods (NRODs) to nanoribbons (NRIBs), and then nanowires (NWs), as well as transition state morphologies, were successfully synthesized. The photovoltaic performance tests showed that the photoanode constructed from the oriented NRIB arrays possessed not only a high surface area for sufficient dye loading and better light scattering in the visible light range than for the other morphologies, but also a wider band-gap and a higher conduction band edge, with more than 200% improvement in power conversion efficiency in dye-sensitized solar cells (DSSCs) compared with the NROD morphology. Titanium dioxide nanoparticles doped with nitrogen were prepared through the sol – gel method. Compared to undoped pure anatase titanium dioxide nanoparticles with the same particle size, the nitrogen doped titanium dioxide showed improved photovoltaic performance. Electrochemical impedance spectroscopy of cells with N doped TiO2 and pure TiO2 indicated that the charge transport of the photoelectrode was improved after doping with nitrogen. As a result, a photoelectric conversion efficiency of 6.8% was obtained for N doped TiO2 photoanode. In summary, the results show the systematic influence that the synthesis conditions have on the crystalline structure of titanium dioxide in such aspects as particle size, phase structure, surface area, and morphology. Greater attention to the synthesis of TiO2 for DSSCs showed how significantly the synthesis conditions can improve the photovoltaic performances

Polizeiliche Kriminalstatistik : (PKS)

Strategies for improving the photovoltaic performance of dye-sensitized solar cells (DSSCs) are proposed by modifying highly transparent and highly ordered multilayer mesoporous TiO 2 photoanodes through nitrogen-doping and top-coating with a light-scattering layer. The mesoporous TiO 2 photoanodes were fabricated by an evaporation-induced self-assembly method. In regard to the modification methods, the light-scattering layer as a top-coating was proved to be superior to nitrogen-doping in enhancing not only the power conversion efficiency but also the fill factor of DSSCs. The optimized bifunctional photoanode consisted of a 30-layer mesoporous TiO 2 thin film (4.15 μm) and a Degussa P25 light-scattering top-layer (4 μm), which gives rise to a ∼200% higher cell efficiency than for unmodified cells and a fill factor of 0.72. These advantages are attributed to its higher dye adsorption, better light scattering, and faster photon-electron transport. Such a photoanode configuration provides an efficient way to enhance the energy conversion efficiency of DSSCs

Structurally stabilized mesoporous TiO2 nanofibres for efficient dye-sensitized solar cells

One-dimensional (1D) TiO2 nanostructures are very desirable for providing fascinating properties and features, such as high electron mobility, quantum confinement effects, and high specific surface area. Herein, 1D mesoporous TiO2 nanofibres were prepared using the electrospinning method to verify their potential for use as the photoelectrode of dye-sensitized solar cells (DSSCs). The 1D mesoporous nanofibres, 300 nm in diameter and 10-20 μm in length, were aggregated from anatase nanoparticles 20-30 nm in size. The employment of these novel 1D mesoporous nanofibres significantly improved dye loading and light scattering of the DSSC photoanode, and resulted in conversion cell efficiency of 8.14%, corresponding to an ∼35% enhancement over the Degussa P25 reference photoanode

Rational design of 3D dendritic TiO2 nanostructures with favorable architectures

Controlling the morphology and size of titanium dioxide (TiO2) nanostructures is crucial to obtain superior photocatalytic, photovoltaic, and electrochemical properties. However, the synthetic techniques for preparing such structures, especially those with complex configurations, still remain a challenge because of the rapid hydrolysis of Ti-containing polymer precursors in aqueous solution. Herein, we report a completely novel approach-three- dimensional (3D) TiO2 nanostructures with favorable dendritic architectures-through a simple hydrothermal synthesis. The size of the 3D TiO2 dendrites and the morphology of the constituent nano-units, in the form of nanorods, nanoribbons, and nanowires, are controlled by adjusting the precursor hydrolysis rate and the surfactant aggregation. These novel configurations of TiO2 nanostructures possess higher surface area and superior electrochemical properties compared to nanoparticles with smooth surfaces. Our findings provide an effective solution for the synthesis of complex TiO2 nano-architectures, which can pave the way to further improve the energy storage and energy conversion efficiency of TiO 2-based devices

Aqueous colloidal stability evaluated by zeta potential measurement and resultant TiO2 for superior photovoltaic performance

Controlling the morphological structure of titanium dioxide (TiO 2) is crucial for obtaining superior power conversion efficiency for dye-sensitized solar cells. Although the sol-gel-based process has been developed for this purpose, there has been limited success in resisting the aggregation of nanostructured TiO2, which could act as an obstacle for mass production. Herein, we report a simple approach to improve the efficiency of dye-sensitized solar cells (DSSC) by controlling the degree of aggregation and particle surface charge through zeta potential analysis. We found that different aqueous colloidal conditions, i.e., potential of hydrogen (pH), water/titanium alkoxide (titanium isopropoxide) ratio, and surface charge, obviously led to different particle sizes in the range of 10-500 nm. We have also shown that particles prepared under acidic conditions are more effective for DSSC application regarding the modification of surface charges to improve dye loading and electron injection rate properties. Power conversion efficiency of 6.54%, open-circuit voltage of 0.73 V, short-circuit current density of 15.32 mA/cm2, and fill factor of 0.73 were obtained using anatase TiO 2 optimized to 10-20 nm in size, as well as by the use of a compact TiO2 blocking layer

Mesoporous anatase single crystals for efficient Co(2+/3+)-based dye-sensitized solar cells

Highly crystalline mesoporous architectures are very promising for a number of applications due to their unique properties arising from their excellent electronic connectivity, structural coherence, mass-transport-efficient channels, and large surface area/pore volume. Herein, we report, for the first time, a facile synthesis approach to mesoporous anatase single crystals (MASCs) with special polyhedral pores (similar to 7 nm). This architecture is employed to construct MK-2-sensitized solar cells using a cobalt redox shuttle, with a maximum efficiency of 8.7% achieved, which is significantly higher than for analogous devices based on commercial Dyesol TiO2 (6.3%). It is worth emphasizing that not only applications in solar cells, but also in sensing, drug delivery, and photocatalysis may benefit from these innovative MASCs. Crown Copyright (C) 2014 Published by Elsevier Ltd. All rights reserved

Mesoporous anatase single crystals for efficient Co(2+/3+)-based dye-sensitized solar cells

Highly crystalline mesoporous architectures are very promising for a number of applications due to their unique properties arising from their excellent electronic connectivity, structural coherence, mass-transport-efficient channels, and large surface area/pore volume. Herein, we report, for the first time, a facile synthesis approach to mesoporous anatase single crystals (MASCs) with special polyhedral pores (~7nm). This architecture is employed to construct MK-2-sensitized solar cells using a cobalt redox shuttle, with a maximum efficiency of 8.7% achieved, which is significantly higher than for analogous devices based on commercial Dyesol TiO2 (6.3%). It is worth emphasizing that not only applications in solar cells, but also in sensing, drug delivery, and photocatalysis may benefit from these innovative MASCs

Continually adjustable oriented 1D TiO2 nanostructure arrays with controlled growth of morphology and their application in dye-sensitized solar cells

Oriented, single-crystalline, one-dimensional (1D) TiO nanostructures would be most desirable for providing fascinating properties and features, such as high electron mobility or quantum confinement effects, high specific surface area, and even high mechanical strength, but achieving these structures has been limited by the availability of synthetic techniques. In this study, a concept for precisely controlling the morphology of 1D TiO nanostructures by tuning the hydrolysis rate of titanium precursors is proposed. Based on this innovation, oriented 1D rutile TiO nanostructure arrays with continually adjustable morphologies, from nanorods (NRODs) to nanoribbons (NRIBs), and then nanowires (NWs), as well as the transient state morphologies, were successfully synthesized. The proposed method is a significant finding in terms of controlling the morphology of the 1D TiO nano-architectures, which leads to significant changes in their band structures. It is worth noting that the synthesized rutile NRIBs and NWs have a comparable bandgap and conduction band edge height to those of the anatase phase, which in turn enhances their photochemical activity. In photovoltaic performance tests, the photoanode constructed from the oriented NRIB arrays possesses not only a high surface area for sufficient dye loading and better light scattering in the visible light range than for the other morphologies, but also a wider bandgap and higher conduction band edge, with more than 200% improvement in power conversion efficiency in dye-sensitized solar cells (DSCs) compared with NROD morphology