Novel-structure dye-sensitized solar cells based on highly ordered titania nanotube arrays

Abstract

The nanocrystalline dye-sensitized solar cell (DSSC), initially developed by Gratzel and his research group in 1991, has drawn much attention due to its low cost, facile fabrication procedures and relatively high conversion efficiency. In general, a classically configured DSSC is built up as a flat sandwich structure which uses two pieces of transparent conductive oxide (TCO) glasses as the substrates. One of the TCO glass is coated with a thin layer mesoporous nanocrystalline titania film attached with dye molecules as the photoanode. Another TCO glass is platinized as the counter electrode. An liquid electrolyte layer is between the two substrates, which contains a redox couple such as iodide/triiodide to transfer electrons between the electrodes. Although the record power conversion efficiency of over 11% has been achieved so far, there are still several problems preventing this kind of solar cell towards industrialization: (i) the highest conversion efficiency of 11.5% is obtained only on a very tiny scale DSSC (0.158 cm²). For large-scale DSSCs, as they employ TCO glass as the substrates, the energy conversion efficiency is low in virtue of the high sheet resistance of TCO glass. (ii) Currently the titania nanoparticle film is coated onto the surface of the TCO glass through the screen printing method. Due to the affinity of interface between the TCO glass and titania film is not firm enough, sometimes the oxide coating would peel off after a period of time. (iii) As the most extensively studied DSSCs adopt nanoparticle film as the active layer of photoanode and due to the multiple trapping/detrapping events occurring within the grain boundaries between the 3D networks of interconnected nanoparticles, the electron transportation rate in the nanoparticle film is very low, which will lead to high interface recombination reactions and finally lower the efficiency. Recently the highly ordered, self-organized titania nanotube arrays obtained through electrochemical anodization of an Ti foil have been shown to offer direct electrical pathways for rapid collection of electrons. However, due to the opacity of the Ti foil, the highly ordered titania nanotube arrays can be only applied on backside illuminated DSSCs. In the backside illumination configuration, the incident light will be attenuated by the platinized counter electrode and the electrolyte, which lowers the sunlight utilization and restricts the efficiency. Therefore, it is necessary to develop new structures and components to overcome the shortages and strive for a breakthrough to get better achievements in the development and commercialization of the inexpensive and high efficiency DSSCs. The objective of this thesis is to improve the photovoltaic performance of the DSSCs by introducing new-structure, all-Ti-substrates based DSSCs combined with highly ordered titania nanotube arrays. According to the bionic idea, a series of DSSCs with novel structures are developed and systematically studied. By using the principle of the capillary network in a lung, a mesh-like DSSC is designed, which contains no TCO glass. The photoanode is prepared by using a Ti mesh as the substrate and then through the anodization method to directly synthesize highly ordered titania nanotube arrays as the active layer, where the Ti metal mesh serves as both the substrate and the source material. As a result, the affinity of the titania layer with the Ti mesh is better than its structure with a piece of TCO glass. The counter electrode is prepared by using a Ti sheet as the substrate and electrodeposited with Pt thin film as the catalyst layer. In the mesh-like configuration, the electrolyte could diffuse through the holes on the mesh freely to transfer the redox couple. With the increase of mesh number, the surface area of the substrate is also increased, which means more titania nanotube arrays could be synthesized to enhance the dye loading capacity. Different lengths of titania nanotube arrays are also investigated to find their influence on the photovoltaic performances. The results indicate that a proper length of the nanotube array layer is a key factor to achieve high conversion efficiency. Compared with the backside illuminated DSSC, the mesh-like DSSC could allow sunlight irradiating through the photoanode side to avoid light loss. Besides, due to the low sheet resistance of Ti substrate, the size of the cell has only a little impact on the conversion efficiency. Through optimization, for a single unit cell with 4 cm² area, the energy conversion efficiency could achieve 5.0% under standard AM 1.5 sunlight, which makes it possible to produce this type of DSSC in large scale with relatively high efficiency. The flexible mesh-like DSSCs are also fabricated by using transparent thermoplastic film as the sealing materials. The bendability is investigated and shows good mechanical and photovoltaic stability. A DNA-like DSSC is developed by using an anodized Ti wire as photoanode and platinized Ti wire as counter electrode. These two wire-like electrodes are twisted together to make a double-helix structure just like a DNA molecule. The DNA-like DSSC is based on a 3-dimentional (3D) structure which shows superiority of tracking sunlight due to its symmetrical double-helix structure. Different thickness of the nanotube arrays are investigated to find their influence on the photovoltaic parameters and the cell with a 15.3 micron layer exhibited the highest conversion power, about 0.49 mW. To further increase the photovoltaic performance, a two-step formation of the titania nanowire-covered nanotube bilayer film technique is developed and applied in DNA-like DSSCs. The bilayer film is prepared by the electrochemical anodization first to grow the lower nanotube layer and then through the hydrothermal method to grow the upper nanowire layer. From the reflectivity spectrum and scanning electron microscopy it is observed that the nanowire layer on the top could not only decrease the reflectivity of the film, but also play a role to modify the film cracks. Compared with the DSSC based on a single layer electrode, the cell with bilayer film showed higher photovoltaic parameters and lower dark current, which is due to its higher light harvesting efficiency and lower charge recombination process between the electrolyte and the substrates. The seriesparallel connection characteristics of the DNA-like DSSCs reveal that the total voltage and the total short current equalled the sum of each cell's in series and in parallel, respectively. It is anticipated that the DNA-like structured DSSCs have great potential for the application in larger modules using integrated circuit techniques. By combining the merits of the mesh-like DSSCs and DNA-like DSSCs, a new type of 3D double deck mesh-like DSSCs is further developed. One of the Ti mesh is anodized to in-situ synthesize highly ordered titania nanotue arrays. Another Ti mesh is platinized through electrodeposition as the counter electrode. The effect of mesh number on the 3D DSSCs is investigated through dye loading measurement, cyclic voltammetry and electrochemical impedance analysis. The results show that with the increase of mesh number, the dye loadings on the photoanode and the active surface area of Pt on the counter electrode are increased, while the diffusion of the electrolyte becomes more difficult due to the reduced diameter of the openings in the mesh. It has also been demonstrated that the performance of this 3D DSSC is capable of tracking sunlight just like the DNA-like DSSC due to its axial symmetrical structure. In the I-V measurement, the 3D DSSC based on the 90-mesh photoanode and the 120-mesh counter electrode shows the highest conversion efficiency of 5.5% under standard AM 1.5 sunlight.