Preparation Of Macroscopically Oriented Mesoporous Films And Their Primary Application In Solar Cells

As advanced nanostructured materials, ordered mesoporous films have become one of the hot inter-disciplinar areas due to their relatively large surface area, highly order microstructures and widely applications in separation, optics, electron and magnetics, etc. According to the definition of international unit of pure and applied chemistry (IUPAC), mesoporous films are defined as solid porous films with pore size of 2~50 nm. Macroscopically oriented mesoporous films are specific mesoporous films for the mesochannels are arrayed as 2D-hexagonal (such as SBA-3,15 or MCM-41 type) and in one direction at large scale.The preparation of macroscopically oriented mesoporous films has been mainly focused on controlling the arrangement of SBA-3 type mesochannels (pore size is 2-3 nm) in 2D field in earlier researches. The control of mesochannels arrangement in 3D is hardly reported to date. Importantly, small pore size restricts the introduction of functional guests (such as organic molecular or biologic proteins with high molecular weight) into the mesochannels. This limitation largely restricts the research of macroscopically oriented mesoporous films incorporated with advanced functional guests. This paper provides two simple and facile methods to prepare macroscopically oriented mesoporous films with large pore size (5-7 nm) based on the knowledge and technique from continuous researches in orienting liquid crystal molecules in our groups. These two methods simplify earlier technology of film preparation and advance the control of mesochannels arrangement from 2D to 3D field. Additionally, this work also employed these specific films in solar cells by introducing functional guests into their mesochannels. These techniques provided new approaches on fabrication macroscopically oriented mesoporous films, and have potential applications in optical or electronic industry. The content of this dissertation could be summarized as follows:1. Laser induced periodic microgrooves were prepared on polyimide films employing UV laser with wavelengths of 266, 355 and 532 nm. SBA-15 type macroscopically oriented mesostructured films were prepared on such specific microstructured surface by dip-coating or spin-coating methods. The possible deposition mechanism of arrayed mesochannels based on experimental results can be deduced as follows: With the rapid evaporation of ethanol solvent in sol precursor solution, surfactant molecular self-assembled into rod-like micelles with silica around. Such rod-like micelles were inclined to arraying parallel to the microgrooves direction in order to decrease their surface free energy based on the confined effect of laser induced periodic microgrooves. Micelles, which were close to microgrooves, were first regularly arrayed and continued arraying from bottom to top for confined effect. At last, homogenous macroscopically oriented mesostructured films were successfully prepared. By investigating the effect of depth or widen of microgrooves and preparing technology, the depth (large than 20 nm) was very important to obtain great oriented films while widen (between 260 and 550 nm) showed no obvious influence to the orientation of films. Spin-coating method obtained greater result to prepare oriented mesostructured films than dip-coating method on the same microgrooved samples. Besides preparing SBA-15 type oriented films, SBA-3 type oriented films were also obtained on such specific surface and showed widely adaptability of such microgrooved surfaces.2. Patterned laser induced periodic microgrooves were prepared on polyimide films employing UV laser with wavelengths of 355 nm through a photomask. Patterned SBA-15 type macroscopically oriented mesostructured films were prepared on such specific microstructured surface by dip-coating method. The chemical structure and property of polyimide films before/after laser irradiation was characterized by SEM, EDS, AFM, XPS, and contact angle measurements. The hydrophilicity of irradiated regions was increased greatly compared with that in unirradiated regions. Therefore, sol precursor solution was selectively deposited on irradiated regions. In the meanwhile, laser induced periodic microgrooves have the ability to arrange mesochannels in one direction. In this work, preferable patterned mesostructured silica films can be obtained when dip speed was slow and environment relative humidity was high. What's more, the growth of patterned films does not depend on the relationship between LIPS direction and dip coating direction. Furthermore, self-luminescent patterned mesostructured films incorporated with CdTe quantum dots were also prepared.3. A simple and facile method employing hot air as driving force to prepare SBA-15 type macroscopically oriented mesoporous films was provided and it showed wide adaptability for its free of specific substrate. The microstructure of mesochannels was detected by HR-TEM and in-plane XRD measurements and it showed wonderful arrangement and controllable. The possible deposition mechanism based on experimental results can be deduced as follows: With the rapid evaporation of ethanol solvent in sol precursor solution by hot air flow, surfactant molecular self-assembled into rod-like micelles with silica around. Such rod-like micelles were inclined to arraying parallel to the flow direction under the great uniaxial shearing force in sol solution and homogenous macroscopically oriented mesostructured films were successfully prepared. For its free of specific substrates, double- even multi-layered oriented mesostructured films can be obtained by this method and it advance the control of mesochannels arrangement from 2D to 3D field. Besides preparing SBA-15 type oriented films, SBA-3 type oriented films were also obtained by this method.4. Cadmium sulfide (CdS) quantum dots were introduced into the mesochannels of SBA-15 type macroscopically oriented mesoporous films and the whole film was then served as working electrode for solar cells measurement. The distribution and morphology of CdS quantum dots were detected by HR-TEM, EDS, XRD, N2 absorption-desorption measurements and it showed successful inclusion of functional guests into the mesochannels. CdS quantum dots harvested photons and inverted their energy into electrons, then, million of electrons were moved in one direction in the uniaxial mesochannels and the energy conversion process was completed as one effect cycle. The photo-responsivity of such films was quiet sensitive but the conversion efficiency of the whole device was limited requiring continuous modification and advancement. Furthermore, a complex architecture based on introducing various quantum dots with different absorption wavelengths (for example, CdS or PbS quantum dots) into the mesochannels occupying in different layers of multi-layered oriented mesoporous films was also fabricated and such kind of design can have the ability to absorb more solar energy and increase convert efficiency.