| Due to the unique one-dimensional nanostructure, excellent electrical, optical and mechanical properties, carbon nanotubes (CNTs) have attracted a wide variety of attentions in various fields including Chemistry, Physics, Materials Science and Engineering, and Biology since discovery in1991. Nature further commented that CNT would be one of the most important nanomaterials and could be widely uesd in the fields of optoelectronic devices and energy in this century at the first issue in2011. It was also pointed out that the traditional idea for CNTs as nanoscaled optoelectronic devices became almost impossible as the structure and property of CNTs could not be accurately controlled. A possible solution is to assemble CNTs into macroscopic structures such as fiber. As CNTs are highly aligned, the resulting fiber maintains the excellent physical properties of CNTs such as a high electrical conductivity and mechanical strength. In addition, the CNT fiber can be controlled and repeated as required, because the minor difference among CNTs in the structrue does not obviously affect their properties and applications. In fact, CNT fibers have been already investigated for the use in the high-performance structure material and supercapacitor, however, their application in the optoelectronic field is rare. In this dissertation, the preparation, structure, and property of CNT fiber and composite fiber were carefully investigated. A series of dye-sensitized solar cells and polymer solar cells with novel structrues and high-perfornances had been then developed by using the CNT fiber as a new electrode material. The mechanism and rule for the raplid charge seperation and tranport in the aligned CNTs were explored at nanoscale. It is detailed as below.Preparation and properties of aligned CNT fiber and composite fiber. High-quality and spinnable CNT arrays were first synthesized by a typical chemical vapor deposition, from which a CNT fiber can be continuously drawn out by a rotating micro-probe. Nitrogen-doped CNT composite fibers and silver/carbon nanotube composite fibers were further prepared based on the pure CNT fiber. The electrical, mechanical, and electrocatalytic property of CNT fiber and composite fiber had been investigated. It was found that the electrical conductivity and tensile strength of CNT fiber achieved400S cm-1and600MPa, respectively. At the same time, these CNT fibers and composite fibers exhibit excellent electrocatalytic activity with a large surface area. Due to the combined remarakble properties, CNT fibers are very promising as a new family of electrodes in optoelectronic devices, e.g., photovoltaics. Planar dye-sensitized solar cells with CNT fiber as a working electrode. Based on the high electrical conductivity and large specific surface area of CNT fiber, a planar dye-sensitized solar cell was fabricated by using CNT fiber as working electrode, cis-diisothiocyanato-bis(2,2’-bipyridyl-4,4’-dicarboxylato) ruthenium(II) bis(tetrabutylammonium) as photosensitive dye, platinum-coated conductive glass as counter electrode, and LiI/I2/dimethyl-3-n-propyl-imidazolium iodide/4-tert butyl-pyridine in dehydrated acetonitrile as electrolyte. The open-circuit voltage, short-circuit current density, and fill factor could achieve0.47V,10.3mA cm-2, and0.45, respectively. The power conversion efficiency is calculated to be2.6%. The incident photon to electron conversion efficiency of this solar cell exceeded90%. It was also found that the power conversion efficiencies decreased with the increasing diameter of CNT fiber as the effective contact area did not increase obviously.All CNT fiber based wire-shaped dye-sensitized solar cells. A planar solar cell did not fully take advantages of the CNTs in both structrue and property. Recently, wire-shaped solar cells have attracted increasing attentions due to their unique flexible and weavable properties. Generally, metal wires or conventional polymer fibers coated with conductive layer (e.g., indium tin oxide) were used as conductive fiber substrates. However, metal wires may be corrupted by the electrolyte or air, while the conductive layer on polymer fibers tend to crack or break during the use, which has largely lowered the efficiency and stability of solar cell. Due to the high flexibility, high mechanical strength, and high thermal stability, CNT fibers can overcome the above challenges. In this dissertation, a novel wire-shaped solar cell was fabricated in which a CNT fiber coated with a layer of titanium dioxide nanoparticles was used as working electrode while another bare CNT fiber was used as counter electrode. The energy conversion efficiency achieved3%. In addition, the wire-shaped solar cell exhibited a good flexibility and high mechanical stability, and both output voltage and current can be easily tuned by connection in series and in parallel, respectively.Wire-shaped dye-sensitized solar cell based on titanium dioxide nanotube and CNT fiber. To further improve the photovoltaic performance, another wire-shaped solar cell was fabricated by using a titanium wire on which aligned titanium dioxide nanotubes were perpendicularly grown as a working electrode and a CNT fiber as the counter electrode. Compared with the widely used titanium dioxide nanoparticle network, the titanium dioxide nanotube effectively decreased the transport distance of electron, which decreased the recombination of photogenerated charges. Therefore, it showed a higher power conversion efficiency than the all CNT fiber based wire cell. The highest efficiency was4%.Wire-shaped polymer solar cell based on CNT fiber. Although the dye-sensitized solar cell has achieved relatively high power conversion efficiency, it also meets some critical problems, e.g., leakage or evaporation of the electrolyte which largely decreases the lifetime of solar cell. Compared with a dye-sensitized solar cell, polymer solar cell can be more easily fabricated by a solution process such as dip-coating without the use of any liquid electrolyte, which also favors a stable photovoltaic performance. Therefore, a wire-shaped polymer solar cell was developed by using the CNT fiber and titanium wire with perpendicularly grown titanium dioxide nanotubes on the surface as two electrodes. The energy conversion efficiency of this wire-shaped polymer solar cells was about0.15%, which can be greatly further improved if the lengths of titanium dioxide nanotubes could be decreased to100-200nm. Right now their lengths were on the level of micrometers.In conclusion, the fabrication, structure, and property of CNT fibers and CNT composite fibers have been systematically investigated in this dissertation. A series of dye-sensitized solar cells and polymer solar cells based on CNT fiber as electrode have been developed, and the effect of related experimental parameters on the photovoltaic performance has been carefully explored. This dissertation shows that CNT fiber represents a new and effective electrode to fabricate high-performance organic solar cells, especially for the wire-shaped solar cells. In summary, this dissertation expands the application scope of CNT fiber and indicates a new direction to improve the photovoltaic performance based on aligned one-dimensional nanomaterials. |
PDF Full Text Request