| Due to the novel electrical, optical, and mechanical properties, carbon nanotubes (CNTs) exhibit promising applications in a wide variety of fields, such as electrical and optical devices, energy, and biomedicine. However, in many cases, CNTs are required to be further assembled into bulk materials including array, film, and fiber. Among them, the CNT film may represent the most studied system. Unfortunately, as CNTs tend to aggregate during the conventional solution or melt process, it still remains challenging to control and improve the structure and property in the film, which has largely hindered the application in many fields. For instance, CNT films have been widely proposed as electrodes in organic solar cells such as dye-sensitized solar cells (DSSCs). DSSCs have been considered as the third generation solar cells and extensively investigated due to a low cost, easy fabrication, and high energy conversion efficiency. The development of new electrode materials represents a promising direction to improve the cell efficiency with target for a practical application. Therefore, CNT films have been widely explored for the electrode in DSSCs, particularly, as counter electrode to replace the conventional expensive and unstable platinum. However, the aggregated structure of CNTs has greatly lowered the efficiency of the resulting cell. To further improve the structure and property of CNT film, it is critically important to realize a high alignment of CNTs. Herein, two new methods have been developed for the preparation of aligned CNT films and composite films, and the use of them as counter electrodes to fabricate highly efficient DSSCs was carefully studied in this thesis. The main work is summarized as bellow.Synthesis of ultrahigh CNT array. To prepare aligned CNT films and aligned CNT composite films with remarkable electrical and mechanical properties, a simple and effective method is to use highly aligned CNT arrays as building materials, so the quality of CNT arrays is very important at this point. Herein, CNT arrays with a high quality were synthesized by chemical vapor deposition. The resulting CNT show a multi-walled structure with diameters of7-20nm. The effect of growth parameters including catalyst, gas flow rate, temperature, and time has been carefully explored in the synthesis of CNT arrays. The optimal conditions for the synthesis of ultrahigh CNT arrays were discovered.Preparation of ultralong aligned CNT films and their application in DSSCs. Currently, aligned CNT films are generally dry-spun from CNT arrays. However, the synthesis of spinnable CNT array remains challenging. In addition, the spinnable array mainly shows a height of300-400μm (typically less than1mm), the resistance of CNT film is very high. It has been well known that the resistance of a CNT film is dominated by the contact resistances among CNTs as the resistances of individual CNTs are very low. Shorter CNTs produce more contact points with higher contact resistances. Therefore, the increase of CNT lengths is critical to improve the application of CNT film in electrode. Herein, a new approach called "adhesion and transfer method" has been developed to produce ultralong aligned CNT films. It represents a general approach in the preparation of CNT films, particularly appropriate for ultrahigh CNT arrays. The resulting ultralong and aligned CNT film indicated a conductivity of1150S/cm at room temperature, compared with～500S/cm for an aligned CNT film by a dry-spinning method. The aligned ultralong CNT film was further used as counter electrode to fabricate DSSCs. The energy conversion efficiency has achieved9.00%, much higher than4.18%for the cell based on the aligned CNT film synthesized by the dry-spinning process and8.05%based on the conventional platinum.Preparation of horizontally aligned CNT/epoxy composite films and their application in DSSCs. CNTs exhibit high mechanical strength and high electrical conductivity, while polymers could be easily processed with broad source and low cost. The above advantages may be combined by producing a CNT/polymer composite material. Herein, the conventional slice technology has been developed to prepare aligned CNT/epoxy composite film in which CNTs are aligned along the slicing direction. The thickness of the composite film can be accurately controlled from50nm to50μm. The resulting composite film could be also highly flexible, transparent, and conductive, which enable a promising application in electrode. Herein, the composite film has been used as counter electrode to fabricate DSSCs.Preparation and application of perpendicularly aligned CNT/olefin composite films. Polymer materials such as epoxy, polystyrene, and poly (methy1methacrylate) have been generally used as a second phase in the preparation of CNT composite materials, and solution or burning process has been mainly applied to remove the second phase in case that pure CNTs are required. However, the above post-treatments shares a big and common problem, i.e., difficulty in preparing high-purity CNTs as some impurities are often attached on CNTs. Herein, olefin has been mainly used to prepare vertically aligned CNT composite films also by using a slicing technique. The thickness of the composite film may be controlled from50nm to50μm. Olefin can be then easily removed by evaporation at high temperature at which CNTs would not be affected in both structure and property. Because CNTs are perpendicularly penetrated through the composite film, the lengths of CNTs are accordingly tuned from50nm to50μm. The CNTs were further unzipped into graphene nanoribbons after heating and ultrasonic treatments.In summary, two new methods have been developed for the preparation of aligned CNT films and composite films with high efficiency. The aligned CNT films showed excellent optical, electrical, and mechanical properties, and exhibited promising applications in various fields, particularly, as a new family of electrode materials. The use of them as counter electrodes in DSSCs have been studied as a demonstration. |
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