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Renewable energy sources are becoming increasingly more vital as the supply of fossil fuels is depleted, and awareness of the impact of non-sustainable fuel sources on the environment is increasing. A solar cell is an example of one such renewable energy source, which converts sunlight into electricity; providing an eternal, clean energy supply. The concept of converting solar energy into electricity has been around since the late 1800's [1]; but it wasn't until well after Einstein's landmark paper on a completely new quantum theory of light in 1905 [2] that a modern single-junction solar cell was made using a semiconducting p-n junction, as is used today [3]. This type of solar cell, although used almost exclusively in the world today, has a fun- damental efficiency limit of 31% [4]; so despite falling costs of materials and manufacturing, efficiencies have remained relatively low. Intermediate band solar cells, the focus of this project, potentially increase this limit to 47% [5] by attempting to absorb a greater portion of the solar spectrum, without loss of voltage over an equivalent single-junction cell. Examples have been made using quantum dots, however poor efficiencies are reported due to the low absorption cross section of quantum dots and short lived intermediate states, which prevents efficient absorption of low energy photons. This project however, aims to experimentally demonstrate a modification to the inter- mediate band cell, called a "Quantum Ratchet" intermediate band solar cell. This uses the coupled subbands of a quantum well superlattice as the "Quantum Ratchet" state, to increase the absorption cross section, and spatially separate excited electrons from their cor- responding holes to extend the lifetime of the carriers in this state, allowing more efficient absorption of a second photon. The goal of the project is therefore to experimentally demonstrate a photocurrent due to a sequential absorption of two below-bandgap photons, and show an extended lifetime of carriers in the "Quantum Ratchet" state. Presented here will be the theory behind intermediate band solar cells, along with experimental results from a specifically designed "Quantum Ratchet" device which have been submitted for publishing. Also presented are the experimental results from an 'upconverting' sample - a process which shows promise as a 'natural ratchet', the results of which have been published in Physical Review B [60].

This dissertation examines the relationship between space and identity in the city of Urumqi, the capital of Xinjiang Uyghur Autonomous Region in the People's Republic of China. The work explores the shifting interface between various types of spaces within the city, focusing on three major areas: dominant state space, Han Chinese space and Turkic Muslim space. Central to my argument is that that the dominant form of spatial identity arises through the process ofrepetition. From the city's establishment in the Qing dynasty up until present, there has been a sustained tendency to self-consciously reproduce an urban spatial template deployed throughout cities within inner China. At the same, time each new regime, be it imperial or modernizing, deploys a different series of repetition to the one which precedes it. The current work traces the trajectory of some of these forms as they evolve over space and time. In doing so, it describes how the political incorporation of Xinjiang into China functions at the level of the city over an extended period of time. Urumqi has, from its inception, attracted migrants in large numbers from various parts of inner China and Xinjiang itself, and thus the city's neighbourhoods have simultaneously produced a number of spatial and territorial differences within this larger framework of repetition. With a focus on Han and Turkic Uyghur communities, the dissertation also describes the parallel trajectory of these spatial differences in relation to those of the state. I portray a general situation in which Han communities have become more incorporated into the dominant spatial mode and Uyghurs less so. This observation is fleshed out in greater detail in the chapters which deal with contemporary city. Despite a multiplicity of urban forms, I argue that the difference inherent in Uyghur spatial practices is increasingly designated and reified by the state, on its behalf. Through a detailed description of this "inauthentic" and coercive expression of difference, I trace how the Uyghur neighbourhoods have become increasingly subject to what Carl Schmitt ([1922] 2004) refers to as "the state of exception". This shift in the spatial dynamics of the city serves as significant pre-cursor to the Urumiq Riots of 2009. The dissertation concludes with a discussion of ethnographic methodologies and their relation to time. The influence of the past upon the present, coupled with the highly transient nature of present spatial organization within Chinese cities, necessitates a type of ethnography which is sensitive to the processual nature of space and place. Drawing on Bergson's concept of duration ([1896] 1911 ), I argue for an ethnographic methodology which takes into account the persistence of the past in descriptions of the present.

Decision-making in uncertain environments poses a conflict between the goals of exploiting past knowledge in order to maximize rewards and exploring less-known options in order to gather information. The descriptive modeling framework utilized in previous studies of exploratory choice behavior characterizes exploration as the result of a noisy decision process, rather than a process reflecting beliefs and/or uncertainty about the environment. It stands to reason that people do not merely negotiate the exploration-exploitation dilemma by stochastically making choices, but rather, fully utilize their knowledge of the environment structure and integrate their trial-by-trial observations of choice in order to direct exploratory choice. The work presented in this dissertation evaluates this hypothesis. As the previous used tasks structures and descriptive models obfuscate this more sophisticated form of belief-directed exploration, I describe a novel exploration-exploitation task that affords disentanglement of reflective belief-directed exploration strategy from a reflexive and naïve exploration strategy. The former strategy is distinguished from latter by its ability to update its belief states in the absence of direct observations of choice payoff changes. Accordingly, we specify cognitive models instantiating these two choice strategies and in the first experiment, we find evidence that behavior is by and large better characterized by a reflective strategy, and further, that choice latencies appear to index value computations carried out in implementing such a strategy. In a second experiment, I reveal how physiological arousal (measured by Skin Conductance Responses) appears to index a form of value computation similar to what is prescribed this reflective model, and further, how individual differences in physiological response to these value signals bear on choice behavior. In a third experiment, I demonstrate how this sophisticated form of choice behavior carries cognitive costs, and following the contemporary model-based/model-free reinforcement learning framework, I show how placing concurrent decision-makers under cognitive load diminishes the contribution of the more sophisticated reflective exploration strategy, fostering reliance on stochastic, reflexive form of exploratory choice behavior.