Electron Transport In The Nanostructure Film Electrodes And Application For Dye-sensitized Solar Cells

The utilization of the solar energy is a permanent subject. Dye-sensitized solar cells (DSSCs) are considered attractive for energy conversion application because of their relatively low cost and high efficiency. In 2003, the Gr?tzel group in EPFL (Switzerland) reported 11% light-to-electrical energy conversion efficiency. This arose more interests in the world for the research of this kind of solar cell. Dye-sensitized nanocrystal TiO2 porous film electrode is the key part of this kind of solar cell. The film electrodes consist of nanosize TiO2 colloids that are sintered on a transparent conducting substrate, which results in a porous geometry and a very large surface area. The film electrode is mesoscopic network structure formed by the interconnected nano-sized TiO2 particles. The interconnection of the particles can allow for the electronic conduction to take place. Electron transport through the nanoparticle network occurs by trap-mediated diffusion, a slow mechanism (with electron escape times of 1 to 10 ms for about 10μm-thick TiO2 film) that is nonetheless efficient for TiO2 cells that use the traditional radox couple in a liquid electrolyte. Due to such a slow transport, back transfer of electrons into the electrolyte can occur, which is believed to be one of the recombination mechanisms. So, improvement of electron transport and suppression of recombination are the important projects for the fundamental studies of the dye-sensitized nanocrystalline solar cell. In our dissertation, firstly, we have fabricated successfully the model of the dye-sensitized nanocrystalline TiO2 solar cell. And then, we study electron transport properties and suppression of recombination in dye-sensitized solar cells (DSSCs), which will provide a certain degree of experimental and theoretical basis for the preparation of high-efficiency solar cells. The main contents are following: 1. Decrease the recombination of photo-generated charge in the thin-film electrode through constructing the interface barrier or blocking layer.By the glass rod or blade Guachu prepared about 10 um thick porous nano-crystalline TiO2 thin film electrode to dye - bipyridine ruthenium (II)-sensitization agent to assemble photoelectrochemical cell prototype device, and its performance were tested. The energy conversion efficiency of 4.0 to 7.0 % is obtained by adjusting the light intensity, the thickness of film electrode and the effective lighting area. The two routes for recombination via the nanocrystalline TiO2 and via the conducting fluorine doped tin oxide (FTO) substrate were presented usually. In order to prevent the loss of photoinjected electrons by back reaction, Electron transfer via both routes needs to be minimized. In the last years, it has been demonstrated that nanocrystalline film was coated by a thin overcoat of a different metal oxide with a higher conduction band edge, such as Al2O3, Nb2O5 etc. This overcoat is intended to increase the physical separation of injected electrons and oxidized dye/red-ox couple and thereby retarding the recombination reactions. To the second route of recombination, the FTO substrate may be partly exposed to the electrolyte through the porous TiO2 film, which could result in recombination via electrons in the FTO reducing I3?.(1) Design and Constructing three different kinds of TiO2 surface barrier. The annealed TiO2 nanoparticles electrode was used as the electrode for fabricating DSSCs after soaking in TiCl4, ZnAc and SnCl4 aqueous solutions, respectively. Electrochemical measurements reveal that the TiO2/ZnO electrode posses the highest open circuit photovoltage and minimum dark current. Surface work function results indicate that the treating with three different types of aqueous solutions induce the different surface energy band structures at TiO2 nanoparticles surface. The downward bend of TiO2/ZnO interface energy band causes the formation of electron traps in ZnO, which should effectively hinder the recombination of electrons and holes at the surface states of TiO2, while the higher barrier at ZnO and electrolyte reduce the dark current. By comparing the TPV results of four film electrode, it indicates that the existence of energy band structure could affect the separation process of photo-generated electron-hole pairs. The design of the interface barrier should be beneficial for decreasing the dark current and increasing the open circuit photovoltage.(2) Three different metal oxides are used as blocking layers, TiO2, ZnO and SnO2 respectively. The experimental results show that ZnO is the best. From the results of work function and the open circuit voltage and (Voc-Jsc) curves measurements, work function of ZnO is greater than that of conductive glass substrate and TiO2 films. ZnO as a blocking layer can not only inhibit charge recombination, but also be propitious to charge transport from TiO2 film to conductive glass substrate.2. Improving the charge transfer properties in the thin film electrode by using one-dimensional nanostructures.(1). To solve the problem of slow electron flow, several vertical TiO2 nanotubular or ZnO nanorod-like architectures are used for DSSCs, which offer the potential for the improvement of electron transport. But the preparation processes are commonly time-consuming, complicated, and demanding. ZnO nanorod has a large surface area and a long conduction pathway that lead straight to the electrode for efficient and fast electron transport. It was reported that electron transport is tens to hundreds of times faster in nanorod array electrodes than in nanocrystalline particulate electrodes in dye sensitized ZnO solar cells. However, ZnO solar cells show significantly lower conversion efficiencies compared to the most efficient naoctrystalline TiO2 photoanodes. The high efficiency of DSSCs is achieved only when the nanoporous TiO2 electrodes are introduced. In this works, The TiO2 based dye-sensitized solar cells doped with different sizes of ZnO nanorods were fabricated and studied by photoelectrochemical measurements. The results show that the solar conversion efficiency of the dye-sensitized solar cells after the addition of ZnO nanorods (1 wt %) was increased by about 15 % compared to that without ZnO nanorods. The effect of different sizes of ZnO nanorods on the charge carrier transport properties has been studied in the composite semiconductor film by means of transient photovoltage technique. The result indicates that the carrier diffuse rate in N3-sensitized TiO2/ZnO film electrode was about 1 to 3 order of magnitude faster than that in TiO2 electrode. The cyclic voltammograms suggest that the conduction band edge shifts toward the vacuum level after the addition of ZnO nanorods in working electrodes, which may be the main cause of the enhancement of the open-circuit photovoltage (Voc). The experimental results indicate that the addition of ZnO nanorods can improve charge carrier transport, decrease recombination, enhance Voc, and increase efficiency of energy conversion.(2). The vertical nano-architectures are used for DSSCs. The electron transport properties were studied in TiO2 nanotube arrays electrodes and ZnO nanorod arrays electrodes by means of transient photovoltage technique. Part I: TiO2 nanotube arrays were grown from a starting titanium sheet by potentiostatic anodization. When different light intensity of laser excitation (355 nm in wavelength), Two diametrically opposite-voltage transient phenomenon in the 10-7 s are showed. Under poor light, the small amounts of freedom carriers are in nanotube arrays, the carriers can be fast captured by surface states, which results in a negative-voltage transient phenomenon. And under glare light, the relatively more freedom carriers are in nanotube arrays. Due to the increment of the light penetration depth, the Schottky barrier between Ti metal and TiO2 has played a decisive direction factors on the charge transmission, which lead to the positive voltage transient phenomenon. In the chapter, the electron transport properties are also studied in the dye-sensitized TiO2 nanotubes arrays and nanocrystalline TiO2 films (532 nm in wavelength). The experimental results show that the electron diffusion coefficient in the TiO2 nanotube arrays electrode is about 3 orders of magnitude larger compared with that in TiO2 electrode. Efficient and fast transport in TiO2 nanotubes indicates that the traped/detraped electrons are decreased greatly. The superior transport properties may allow for the quasi-Fermi level for electrons is closer to the conduction band minimum, which results in a large value of the open-circuit photovoltage. The vertical growth ZnO nanorod arrays of the different scales on conductive glass are in synthesis of the water bath method, The carrier transport mechanism have been studied. The energy conversion efficiency of the ZnO-based on devices is far less that of traditional ZnO nanoparticle film solar cells. This phenomenon has also been discussed.