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Title: Nanostructured titania films in inorganic/organic composite pholtovoltaic devices
Author: Xie, Zhibin
ISNI:       0000 0004 2721 6522
Awarding Body: Oxford University
Current Institution: University of Oxford
Date of Award: 2011
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Composite solar cells (CSCs) composed of a titania layer 'and a conjugated polymer thin film sandwiched between anode and cathode have attracted considerable interest as cost- effective alternatives to silicon-based photovoltaic (PV) devices. The titania layer not only affects the dissociation of excitons photogenerated in the conjugated polymer but also plays a crucial role in collecting injected electrons at the anode. As a result, the morphology and electron transport properties of the titania thin films have a great impact on the PV performance of the CSCs. In this project, I first investigated the microstructure of titania thin films fabricated . by doctor blading a colloidal paste (DB-HT), spin coating sol-gel solutions (SC-SG or SC-PP) and glancing angle deposition (GLAD). Random nanocrystalline Ti02 thin film were obtained by the DB-HT, SC-SG and SC-PP techniques while the pore size, porosity and surface roughness all decrease in this order. The power efficiencies of CSCs based on DB- HT Ti02 were reproducibly greater than 0.5% at 80mW/cm2, outperforming similar devices by other groups. Under the same illumination conditions, CSCs based on SC-SG and SC-PP Ti02 gave efficiencies of only 0.10% and 0.007%. These studies suggested that the titania surface roughness was a critical factor controlling Jsc in bi-Iayer CSCs. For the SC-PP titania layer, observations include that anatase nanocrystals with size 5~ 1 Onm exist in as-deposited layers, and that these can work as a blocking layer and have no detrimental effect on their PV performance. Titania layers with controllable columnar structures were fabricated also by glancing angle deposition. The effects of post-deposition annealing and deposition parameters on the microstructures and optical properties of these textured titania thin films were investigated. After preliminary optimization of annealing conditions and deposition angle, CSCs based on GLAD titania achieved an efficiency of 0.13%. A limiting factor for the inferior performance could be limited polymer penetration into the GLAD titania layer. In the second part of the project, photoconductivity (PC) in nanocrystalline Ti02 thin films was studied to understand the electron transport. It was found that PC evolution is heavily influenced by ambient atmosphere, sample temperature, and illumination conditions. These phenomena were interpreted in terms of electron generation, electron trapping and detrapping, hole trapping and detrapping and electron scavenging. I discovered that the 1 initial normalized decay rate increased with reduced light intensity. The experimental data was fitted using a model developed by a colleague to estimate electron mobility and concentration. Finally, it was found that electron transport in the Ti02 thin films at least in the initial stage of illumination controls the performance of Ti02-based CSCs, and that the quasi steady-state PC in Ti02 film appears to be a good indicator of Jsc value in the devices. !.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available