Fabrication of Functional Nanostructures Based on Polymeric Nanomaterials and Their Nanotechnology Applications

Abstract

Controlling or optimizing the functional nanostructures based on polymeric nanomaterials and their device applications have been intensively studied. Because nanostructured thin films of block copolymers (BCPs) and conjugated polyelectrolytes (CPEs) act as building block and interlayer material, respectively, they have been widely studied as promising candidates for nanoscale device applications. Furthermore, nanostructures of TiO2 from self-assembled BCP have unique optoelectronic properties, they are useful for optoelectronic applications, including solar cell, photocatalysts, and sensors. Diblock copolymers (BCPs), composed of two different polymers connected to each other with a covalent bonds, form nanosize structures such as spheres, cylinders, and lamellae 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. The CPEs as polymeric nanomaterials, which are conjugated backbones with side-chains bearing ionic functional groups, are widely used by nanostructured thin film based devices such as light emitting devices, thin-film transistors, and solar cells as well as biological and chemical sensors. The chemical structures of CPEs can be modified by synthetic chemistry, offering good opportunities for tuning the chemical, electrical and optical properties of the materials. In this thesis, the fabrication and their applications of vertically aligned TiO2 nanostructures from BCP nanotemplates and the application of a polymeric film based on 1,4-bis(4-sulfonatobutoxy)benzene and thiophene moieties (PhNa-1T) are discussed. The controllability over the dimension and shape of BCP nanoporous templates allowed for the adjustment of TiO2 nanostructures for various aims. Since the reduced graphene oxide (rGO) film provided effective transferring, superhydrophobic TiO2 nanorods from BCP cylindrical nanotemplate were conveyed onto a flexible polymer film and a metal substrate. Furthermore, due to the effective building block for deposition of perovskite absorbers and anatase crystallinity of perpendicularly oriented TiO2 nanostructures from BCP nanotemplates, it is applied to electron transporting layer for perovskite solar cells. In addition, because of the polymeric film based on PhNa-1T with excellent charge transporting properties and extremely smooth surface, it is beneficial for hole transporting layer between the electrode and perovskite layers. In Chapter I, the usefulness and significance of polymeric nanomaterials are discussed and the theoretical background of BCPs and conjugated polymers (CPs) are briefly introduced. Lastly, methods which are used to fabricate functional nanostructures using BCP nanostructures are discussed as well. In addition, the importance of CPEs is discussed and the several unique advantages of CPEs are shortly introduced. Also, the functional nanostructures of BCPs and CPEs from polymeric nanomaterials are applied to nanotechnology such as superhydrophobic surfaces and photovoltages. In Chapter II, we report transferrable superhydrophobicity which was enabled by fabricating TiO2 nanorods on a reduced graphene oxide film. Superhydrophobic TiO2 nanorods were synthesized from a nanoporous template of BCPs. The controllability over the dimension and shape of nanopores of the BCP template allowed for the adjustment of TiO2 nanostructures for superhydrophobicity. Since the reduced graphene oxide film provided effective transferring, superhydrophobic TiO2 nanorods were conveyed onto a flexible polymer film and a metal substrate. Thus, the surface of the designated substrate was successfully changed to superhydrophobic surface without alteration of its inherent characteristics. In Chapter III, we fabricated perovskite solar cells (PSCs) with enhanced device efficiency based on vertically oriented TiO2 nanostructures using BCP nanoporous templates. Structured dimension and shape controllability of the nanopores of the BCP template allowed for the synthesis of TiO2 nanorods and TiO2 nanowalls as an electron transport layer (ETL). The TiO2 nanorods-based PSCs showed more efficient charge separation and lower charge recombination, leading to better performance compared to TiO2 nanowalls based solar cells. The champion solar cells with TiO2 nanorods showed an efficiency of 15.5% with VOC = 1.02 V, JSC = 20.0 mA/cm2 and fill factor = 76.1%. Thus, perpendicularly oriented TiO2 nanostructures fabricated from BCP nanotemplates could be applied to the preparation of electron transport layers for improving the efficiency of PSCs. In Chapter IV, we introduce a novel polymeric hole-transport material based on conjugated polyelectrolytes (PhNa-1T) and its application as a hole-transport layer (HTL) material of high-performance PSCs on flexible substrate. Compared with the conventionally used PEDOT:PSS, the incorporation of PhNa-1T into HTL of the PSC device was demonstrated to be more effective for improving charge extraction from the perovskite absorber to the HTL and suppressing charge recombination in the bulk perovskite and HTL/perovskite interface. As a result, the flexible PSC using PhNa-1T achieved high photovoltaic performances with an impressive power conversion efficiency of 14.7%. Moreover, the neutral characteristic of PhNa-1T-based flexible PSCs showed much improved stability under an ambient condition than the acidic characteristic of PEDOT:PSS-based PSCs. Thus, flexible PSCs with high efficiency and good air stability can be fabricated by using the polymeric HTL (PhNa-1T).