Fabrication of functional nanostructures based on self-assembled block copolymers and their application to optoelectronic devices

Abstract

The fabrication of nanostructured functional materials from block copolymer self-assembled nanostructures and their application to optoelectronic devices are presented. Because nanostructures of ZnO and TiO2 have unique optoelectronic properties, they have been widely studied as promising candidates for optoelectronic applications, including photocatalysts, sensors, and solar cells. In addition, polymer nanocomposites with well-dispersed functional nanomaterials are useful for their nature of enhancing efficiency of a given property. Diblock copolymers (BCPs), composed of two different polymers connected to each other with a covalent bond, form nanosize structures such as spheres, cylinders, lamellae, and micelles by microphase separation. The morphology, size, and spacing of the nanostructures can be easily controlled by varying the volume ratios between the blocks and molecular weights. BCP nanostructures are useful to design nanostructured materials by using the BCP nanostructures as patterning masks for etching or as templates for the deposition or growth of materials. Moreover, micelles formed by BCPs can solubilize otherwise insoluble substances and can therefore be used to mix incompatible substances microscopically. They can also stabilize colloidal particles or form microemulsions. In this thesis, the fabrication of ZnO and TiO2 nanostructures with BCP nanostructures and the fabrication of polystyrene nanocomposites with well-dispersed fluorophores with BCP micelles are discussed. Because the ZnO nanostructures are prepared via a hydrothermal process, a process which is conducted at a low temperature, the method is applied to a flexible conductive substrate. Due to the good electron mobility of the ZnO and TiO2 nanostructures, they are applied to solar cells. The semiconducting properties of the ZnO nanostructures are also characterized. In addition, the colors of the light emitted by light-emitting diodes are controlled by polystyrene nanocomposites, immiscible with polar substances, with well-dispersed micelle-encapsulated polar fluorophores in them. In Chapter I, the usefulness and significance of nanomaterials are discussed and the theoretical background of BCPs is briefly introduced. Lastly, methods which are used to fabricate functional nanostructures using BCP nanostructures are discussed as well. In Chapter II, we discuss the fabrication of ZnO nanorods and nanowalls with a method which combines BCP nanotemplates and a hydrothermal growth technique. ZnO nanostructures were controlled by means of the size and shape of the BCP templates. Two types of ZnO nanorods were fabricated with BCP cylindrical templates with different molecular weights, and ZnO nanowalls were fabricated with BCP lamellar templates. We showed that the method used is not only applicable to an amorphous ZnO seed layers but can also be applied to a ZnO single crystalline substrate. ZnO nanostructures were also fabricated on a conductive substrate, indium tin oxide, to apply them to organic solar cells as electrodes. We characterized the photovoltaic properties of the devices and confirmed that the ZnO nanostructures were effective as electron transporting materials. In Chapter III, we demonstrate the fabrication of ZnO nanorods and nanowalls directly on flexible substrates by combining a hydrothermal growth technique with nanoporous templates obtained from block copolymers. First, templates with cylindrical nanopores in two sizes and a template with nanogrooves were fabricated on flexible substrates by employing block copolymers with different molecular weights. From these nanotemplates, we synthesized vertically oriented ZnO nanorods with controlled diameters and ZnO nanostructures in a wall shape. Because the ZnO nanostructures were produced without an electrically insulating epitaxial layer, the semiconducting properties of the ZnO nanorods were characterized as synthesized. Thus, this combined method of hydrothermal growth and block copolymer templates for ZnO nanostructures can be directly applied to flexible electronic devices without alteration of the substrate. In Chapter IV, we demonstrate the fabrication and the application of TiO2 nanorods and nanowall networks by combining a sol-gel method with BCP templates. As templates, perpendicularly oriented templates with cylindrical nanopores and nanowall networks were fabricated by the self-assembly of BCPs. From these nanotemplates, TiO2 nanostructures were successfully fabricated by the dip-coating of a TiO2 precursor solution followed by CF4 etching and sintering processes, sequentially. The morphologies and the crystallinity of the TiO2 nanostructures were characterized with a microscopic technique and an X-ray analysis. To confirm the electronic properties of the TiO2 nanostructures, we synthesized perovskite materials and applied the nanostructures to perovskite solar cells as electrodes. From the photovoltaic properties, we confirmed that the synthesized nanostructures and perovskite materials were effective as electrodes and active layers, respectively. In Chapter V, we demonstrated that fluorescent dyes could be nanoscopically dispersed in a polymer matrix that was immiscible with the dyes

the dyes were encapsulated in micelles. Using a model polymer composite, we also showed that the color of light emitted by light-emitting diodes (LEDs) could be controlled by coating fluorescent polymer composites onto the LEDs. For this purpose, fluorophores that were insoluble in toluene were solubilized into a solution of block copolymer micelles in toluene by the selective incorporation of fluorescent dyes into micellar cores. Because the micelles could be dispersed well in the polymer matrix without the formation of aggregates, fluorescent dyes encapsulated in the micelles were also effectively dispersed in the polymer matrix without macroscopic separation. The polymer composite can be evenly coated onto most substrates, regardless of their surface characteristics. Thus, light-emitting devices with well-controlled emission wavelengths and emission intensities can be fabricated by coating the polymer composite onto the surface of the device.