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Title: Nanostructured transparent conducting oxides via blockcopolymer patterning
Author: Kim, Joung Youn Ellie
ISNI:       0000 0004 5364 5427
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2014
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The aim of this thesis was to develop a new class of TCO material in thin film form, one featuring a mesoporous structure with a 20-40nm length scale. The microphaseseparation of block copolymer was exploited for patterning the TCO materials into this nanostructure. As portable electronics become more widespread and integrated into daily life, and impending energy shortages drive progress towards more efficient and economical photovoltaic technologies, the development of fabrication routes to nanostructured TCO has become a subject of study for many. This thesis concentrates on the development of zinc oxide (ZnO) based materials. One of the more common TCOs, ZnO has many potential applications in piezoelectric devices, organic (and dye-sensitised) photovoltaics, gas sensors, and so on. Unfortunately, nanostructured ZnO has been particularly difficult to realise, due to its rapid crystallisation into wurtzite crystals. Therefore, much of this works focus lies in bypassing the widely recognised problems of creating such a nanostructured material, and its processing into a thin film form useful for many applications. The first chapter of this work reviews the main principles of transparent conducting oxides, trends in past research, and current directions of inquiry. Chapter 2 presents the basic underlying principles of block copolymer self-assembly, and discusses its application to the creation of mesoporous metal oxides. Broadly speaking, there are two different ways to do so either as a sacrificial template, or as a structure guiding agent. Both uses of BCP are demonstrated throughout this work. Chapter 3 describes the experimental methods used in this study, including approaches to film processing as well as characterization techniques. In Chapter 4, a reliable and controllable route to preparing 3D bicontinuous polymeric template is presented, which employs phase separation of block copolymer. Using the templates developed in Chapter 4, two fabrication routes to ZnO based materials are presented in Chapter 5 and 6. In Chapter 5, atomic layer deposition was utilised to realise 3D structures of ZnO with 20-40 nm length scale, exploiting its ability to conformally deposit one layer at a time. These structures include an extremely periodic gyroid structure, as well as a bicontinuous wormlike morphology. This mesoporous ZnO was processed into thin film form, and its use was demonstrated in an inverted P3HT-ZnO hybrid solar cell. In Chapter 6, a solution impregnation approach was explored, using sol gel of a recently developed amorphous TCO material, In-Ga-ZnO. Various thin film processing techniques were explored to arrive at the optimum fabrication route to 3D mesoporous In-Ga-ZnO. Upon electrical and optical characterisation, this material revealed excellent transparency and electrical conductivity even in mesoporous form, exhibiting great potential for use as a transparent electrode. Chapter 7 presents a different approach to the previous chapters, where block copolymer was used not as a template, but as a structure guiding agent to fabricate 3D nanostructured ZnO. Due to its simplicity and elegance, this bottom-up coassembly method has been utilised and well-established for other metal oxide systems, but hardly applied to zinc oxide successfully. The issues of rapid crystallisation were bypassed by first synthesising ZnO nano crystals, and incorporating them into the BCP nano-architecture. Finally, Chapter 8, concludes this work by summarising the re- sults and insights gained, and by presenting some preliminary results for possible future work.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral