Transient optical studies of photoinduced charge transfer in semiconductor quantum dot solar cells

Semiconductor quantum dots (also referred to as 'nanocrystals‘) are well suited as light-harvesting agents in solar cells because they are robust, have tuneable effective band gaps, and are easy to process. The research presented in this thesis is targeted towards the study of excitonic solar cells employing semiconductor nanocrystals as a light harvesting component. Gaining control of the interfacial charge transfer processes in operation in these devices forms a crucial part of any attempt to optimise their performance. In particular, the use of transient spectroscopic techniques reveals how efficient and long-lived charge separation can be achieved in these solar cell architectures. The primary focus of this research is to investigate the parameters influencing charge transfer in dye-sensitised solar cells (DSSCs) using colloidal quantum dots as light-absorbers. One aim is to study the impact of varying the thermodynamic driving forces provided for interfacial electron transfer on the yield of both the electron injection and hole regeneration reactions occurring within the DSSC; this can be achieved by varying the energetics of each component of the system (metal oxide, quantum dot and hole conductor) in turn. In addition, the interfacial morphology can be modulated by changing the passivating ligands present at the QD surface, and by modifying the structure of the redox mediator (or hole conductor). In doing so, we also attempt to improve our understanding of how charge carrier trapping in quantum dots impacts upon solar cell performance. Furthermore, new strategies towards solar cell design are presented, which show great potential as a result of their favourable photophysical properties. One of these approaches (presented in the final chapter) is to effect the in situ growth of CdS nanocrystals in a conducting polymer, a method which circumvents many of the processing issues associated with the use of nanocrystals in polymer blend solar cell architectures. It is hoped that the work presented in this thesis is used to develop design rules for the construction of semiconductor nanocrystal-based excitonic solar cells. By identifying which key parameters control the rates and yields of electron transfer at the nanocrystal interface, improvements in device efficiency can be realised. It is believed that these studies fill an important gap in our current understanding, and highlight some of the potential benefits and shortcomings of using semiconductor nanocrystals in cheap, solution-processed solar cells

The 2008 Federal Intervention to Stabilize Fannie Mae and Freddie Mac

Fannie Mae and Freddie Mac are government-sponsored enterprises that play a central role in U.S. residential mortgage markets. In recent years, policymakers became increasingly concerned about the size and risk-taking incentives of these two institutions. In September 2008, the federal government intervened to stabilize Fannie Mae and Freddie Mac in an effort to ensure the reliability of residential mortgage finance in the wake of the subprime mortgage crisis. This paper describes the sources of financial distress at Fannie Mae and Freddie Mac, outlines the measures taken by the federal government, and presents some evidence about the effectiveness of these actions. Looking ahead, policymakers will need to consider the future of Fannie Mae and Freddie Mac as well as the appropriate scope of public sector activities in primary and secondary mortgage markets

Transient optical studies of photoinduced charge transfer in semiconductor quantum dot solar cells

Semiconductor quantum dots (also referred to as 'nanocrystals‘) are well suited as light-harvesting agents in solar cells because they are robust, have tuneable effective band gaps, and are easy to process. The research presented in this thesis is targeted towards the study of excitonic solar cells employing semiconductor nanocrystals as a light harvesting component. Gaining control of the interfacial charge transfer processes in operation in these devices forms a crucial part of any attempt to optimise their performance. In particular, the use of transient spectroscopic techniques reveals how efficient and long-lived charge separation can be achieved in these solar cell architectures. The primary focus of this research is to investigate the parameters influencing charge transfer in dye-sensitised solar cells (DSSCs) using colloidal quantum dots as light-absorbers. One aim is to study the impact of varying the thermodynamic driving forces provided for interfacial electron transfer on the yield of both the electron injection and hole regeneration reactions occurring within the DSSC; this can be achieved by varying the energetics of each component of the system (metal oxide, quantum dot and hole conductor) in turn. In addition, the interfacial morphology can be modulated by changing the passivating ligands present at the QD surface, and by modifying the structure of the redox mediator (or hole conductor). In doing so, we also attempt to improve our understanding of how charge carrier trapping in quantum dots impacts upon solar cell performance. Furthermore, new strategies towards solar cell design are presented, which show great potential as a result of their favourable photophysical properties. One of these approaches (presented in the final chapter) is to effect the in situ growth of CdS nanocrystals in a conducting polymer, a method which circumvents many of the processing issues associated with the use of nanocrystals in polymer blend solar cell architectures. It is hoped that the work presented in this thesis is used to develop design rules for the construction of semiconductor nanocrystal-based excitonic solar cells. By identifying which key parameters control the rates and yields of electron transfer at the nanocrystal interface, improvements in device efficiency can be realised. It is believed that these studies fill an important gap in our current understanding, and highlight some of the potential benefits and shortcomings of using semiconductor nanocrystals in cheap, solution-processed solar cells.EThOS - Electronic Theses Online ServiceEPSRCGBUnited Kingdo

Panchromatic response composed of hybrid visible-light absorbing polymers and near IR absorbing dyes for nanocrystalline TiO₂-based solid-state solar cells

In pursuit of panchromatic sensitizers for mesoporous TiO2-based solid-state solar cells, a near-IR absorbing zinc phthalocyanine dye (coded TT1) was firstly adsorbed over relatively thin (similar to 1 mu m) TiO2 mesoporous films and then a visible-light absorbing polymer [regioregular poly(3-hexylthiophene). P3HT] was incorporated into the mesopores as both a second sensitizer and a solid hole conductor. After optimizing some experimental parameters, these hybrid solid-state cells exhibited a clear panchromatic response, and an overall conversion efficiency of around 1% at full sun intensity. (C) 2010 Elsevier B.V. All rights reserved

Cysteine methyl ester modified glassy carbon spheres for removal of toxic heavy metals from aqueous media

Glassy C spherical powder (10-20 µm diam.) modified with cysteine Me ester is an inexpensive, novel material for the rapid removal of large quantities of toxic heavy metals such as Cd(II), Cu(II) and As(III) from water, with wide ranging potential applications such as 3rd world drinking water filtration or environmental cleanup. [on SciFinder(R)

Panchromatic response composed of hybrid visible-light absorbing polymers and near-IR absorbing dyes for nanocrystalline TiO2-based solid-state solar cells

In pursuit of panchromatic sensitizers for mesoporous TiO2-based solid-state solar cells, a near-IR absorbing zinc phthalocyanine dye (coded TT1) was firstly adsorbed over relatively thin (similar to 1 mu m) TiO2 mesoporous films and then a visible-light absorbing polymer [regioregular poly(3-hexylthiophene). P3HT] was incorporated into the mesopores as both a second sensitizer and a solid hole conductor. After optimizing some experimental parameters, these hybrid solid-state cells exhibited a clear panchromatic response, and an overall conversion efficiency of around 1% at full sun intensity. (C) 2010 Elsevier B.V. All rights reserved

4-Nitrobenzylamine Partially Intercalated into Graphite Powder and Multiwalled Carbon Nanotubes: Characterization Using X-ray Photoelectron Spectroscopy and in Situ Atomic Force Microscopy

We report the characterization of partial intercalation of 4-nitrobenzylamine (4-NBA) into edge-plane or edge-plane-like defect sites on the surface of both graphite powder and "bamboo-like" multiwalled carbon nanotubes (MWCNTs) using XPS. By comparing the XPS spectra of 4-NBA derivatized graphite powder and MWCNTs with that of graphite powder treated with benzylamine in a similar fashion, we conclude that benzylamine itself does not undergo partial intercalation. Using in situ at. force microscopy, we are able to observe the partial intercalation of 4-NBA into an edge-plane-like "step" defect on the surface of a highly ordered pyrolytic graphite crystal in real time. Together these observations provide further evidence for the partial intercalation of 4-NBA and lead us to propose a new hypothesis to explain this phenomenon. [on SciFinder(R)

AP-1 and clathrin are essential for secretory granule biogenesis in Drosophila.

International audienceRegulated secretion of hormones, digestive enzymes, and other biologically active molecules requires the formation of secretory granules. Clathrin and the clathrin adaptor protein complex 1 (AP-1) are necessary for maturation of exocrine, endocrine, and neuroendocrine secretory granules. However, the initial steps of secretory granule biogenesis are only minimally understood. Powerful genetic approaches available in the fruit fly Drosophila melanogaster were used to investigate the molecular pathway for biogenesis of the mucin-containing "glue granules" that form within epithelial cells of the third-instar larval salivary gland. Clathrin and AP-1 colocalize at the trans-Golgi network (TGN) and clathrin recruitment requires AP-1. Furthermore, clathrin and AP-1 colocalize with secretory cargo at the TGN and on immature granules. Finally, loss of clathrin or AP-1 leads to a profound block in secretory granule formation. These findings establish a novel role for AP-1- and clathrin-dependent trafficking in the biogenesis of mucin-containing secretory granules