archives@tulane.edu / This dissertation describes several key developments in semiconductor devices and technologies designed for solar power conversion and other applications. The first development is of two new growth techniques for producing large-area two-dimensional molybdenum disulfide (MoS2). Such two-dimensional materials have the potential to miniaturize photovoltaic volume and mass by orders of magnitude without sacrificing performance. While large-scale 2D-material-based photovoltaics have not yet been realized, large-area growths such as those described in this dissertation provide meaningful progress toward that goal. The described techniques enable 2D MoS2 thickness control on the order of angstroms and increase 2D MoS2 growth speed by two orders of magnitude relative to the current state of the art. Furthermore, the grown materials are developed into preliminary optoelectronic devices, with performance characterization, as a step toward more advanced photovoltaic devices. The second development presented in this dissertation is the design, fabrication, test, and analysis of a kW-scale hybrid spectrum-splitting photovoltaic module. The module is designed to be transmissive to incident infrared radiation, allowing for infrared light to be separately collected by a thermal receiver, while simultaneously collecting high-energy visible and ultraviolet light via photovoltaics. A system is built and tested on an outdoor testbed and shows 75% total power conversion efficiency (thermal and electric) of the incident solar spectrum, surpassing the capability of conventional photovoltaics. This high efficiency and combination of electrical and thermal power accelerates solar energy penetration into new applications requiring multiple power streams. Across these varied length scales, this dissertation gives glimpses into new innovations throughout the photovoltaic and semiconductor fields and aims to share this knowledge and outlook with the next generation of researchers. / 1 / John Robertson

archives@tulane.edu / 1 / Meghan Bush

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This work primarily concerns the chemistry involved in the preparation of zinc oxide semiconducting films. Chapter One is ah outline of the background to this project. It also indicates the reasons for studying the areas described in the subsequent chapters. General details of experimental techniques are described in Chapter Two. The third chapter covers the synthetic work carried out during this project. This includes the preparation and characterisation of Zn(acac)(_2-)H(_2)O, related zinc compounds and abetylacetonate complexes of group ill metals (Al, Ga, In). It also includes details of the reaction between alcoholic solutions of Zn(acac)(_2-)H(_2)O and water. Chapter Four describes the study, by differential scanning calorimetry, of Zn(acac)(_2-)H(_2)O and Zn(acac)(_2-) This includes d.s.c. traces carried out for a variety of conditions (including heating rate and particle size). Particular attention is paid to enthalpies and peak-maxima temperatures. The experiments described in Chapter Five were designed to gain further insight into the complex thermal behaviour of Zn(acac)(_2-)H(_2)O. Infra-red spectroscopy, proton n.m.r. , elemental analysis and X-ray powder photography were used to identify the compounds involved. The sixth chapter Outlines the synthesis of zinc oxide films from aqueous solutions of zinc formate dihydrate using spray equipment designed and assembled at the University of Durham. X-ray photography, and thickness and transparency measurements were carried out on the films produced.

Concentrator photovoltaics (CPV) is one of the most promising renewable technologies owing to its high efficiency, scalability, low operating expense, and small environmental impact. However, there is much research and advancements to be made before CPV is established as a cost competitive energy technology. To this end, Morgan Solar has developed the Sun Simba, an innovative light weight and low cost CPV module. Under the “Advancing Photonics for Economical Concentration Systems” (APECS) project, outdoor CPV test and measurement systems were designed and constructed at the University of Ottawa and at Little Rock, CA. The performance and reliability of development stage Sun Simba modules installed at the University of Ottawa is assessed. The Little Rock test system was constructed for purposes of future comparison and assessment. To properly assess the performance, instrumentation and data acquisition systems to measure meterological parameters and the associated electrical performance are described and the long-term performance of Sun Simba modules installed at the University of Ottawa is summarized. A finite element model of a cell-on-carrier assembly was constructed to explore the parameter space of the carrier and suggest improvements in carrier design. The effect of carrier geometry, material choices, and convective boundary conditions and their influence on the cell efficiency is determined. The modelling results connected to the measured data is used to estimate the heat sinking capability of the second generation Sun Simba modules.

abstract: Photovoltaics (PV) is an environmentally promising technology to meet climate goals and transition away from greenhouse-gas (GHG) intensive sources of electricity. The dominant approach to improve the environmental gains from PV is increasing the module efficiency and, thereby, the renewable electricity generated during use. While increasing the use-phase environmental benefits, this approach doesn’t address environmentally intensive PV manufacturing and recycling processes. Lifecycle assessment (LCA), the preferred framework to identify and address environmental hotspots in PV manufacturing and recycling, doesn’t account for time-sensitive climate impact of PV manufacturing GHG emissions and underestimates the climate benefit of manufacturing improvements. Furthermore, LCA is inherently retrospective by relying on inventory data collected from commercial-scale processes that have matured over time and this approach cannot evaluate environmentally promising pilot-scale alternatives based on lab-scale data. Also, prospective-LCAs that rely on hotspot analysis to guide future environmental improvements, (1) don’t account for stake-holder inputs to guide environmental choices in a specific decision context, and (2) may fail in a comparative context where the mutual differences in the environmental impacts of the alternatives and not the environmental hotspots of a particular alternative determine the environmentally preferable alternative This thesis addresses the aforementioned problematic aspects by (1)using the time-sensitive radiative-forcing metric to identify PV manufacturing improvements with the highest climate benefit, (2)identifying the environmental hotspots in the incumbent CdTe-PV recycling process, and (3)applying the anticipatory-LCA framework to identify the most environmentally favorable alternative to address the recycling hotspot and significant stakeholder inputs that can impact the choice of the preferred recycling alternative. The results show that using low-carbon electricity is the most significant PV manufacturing improvement and is equivalent to increasing the mono-Si and multi-Si module efficiency from a baseline of 17% to 21.7% and 16% to 18.7%, respectively. The elimination of the ethylene-vinyl acetate encapsulant through mechanical and chemical processes is the most significant environmental hotspot for CdTe PV recycling. Thermal delamination is the most promising environmental alternative to address this hotspot. The most significant stake-holder input to influence the choice of the environmentally preferable recycling alternative is the weight assigned to the different environmental impact categories. / Dissertation/Thesis / Doctoral Dissertation Civil and Environmental Engineering 2016

This work is focused on developing novel techniques for characterizing organic bulk heterojunction (BHJ) materials for organic photovoltaic (OPV) applications. Polymer:fullerene BHJs are a promising class of photovoltaic materials, but an improved understanding of the charge transport processes and materials science of BHJs is required for them to effectively compete with other photovoltaic systems. Key parameters of BHJ systems that need to be evaluated include both electron and hole mobilities, the carrier concentrations, the recombination mechanism and the recombination coefficient. For these studies, poly(3-hexylthiophene) (P3HT):(6,6)-phenyl C₆₁-butyric acid methyl ester (PCBM) have been characterized due to its wide use among researchers. Traditional characterization techniques have focused on transient measurements in a vertical device configuration, but we demonstrate the use of lateral BHJ devices as materials diagnostic platforms. Lateral devices allow for direct access to the active layer for spatially resolved and environmental effect measurements. The devices are also measured under steady state operation, similar to a working OPV cell. Under these conditions, lateral BHJ devices exhibit space charge limited transport behavior. A detailed charge transport model is presented to describe the potential, electric field, and carrier concentration profiles of lateral BHJ devices, as well as the current versus voltage characteristics of different regions of the device. We are able to calculate the slower carrier mobility from photocurrent measurements of lateral devices and the carrier mobility ratio from the device potential profile, even in ambipolar BHJ systems. In situ potentiometry is used to construct detailed potential profiles of the device channel and calculate both carrier mobilities. The carrier concentration and recombination coefficient are calculated from lateral conductivity measurements, and we show that bimolecular recombination is the dominant mechanism in bulk P3HT:PCBM. A simplified in situ potentiometry and photocurrent measurement technique is presented to measure the time evolution of organic BHJ performance. Due to the open geometry of the lateral BHJ device, we are also able to monitor the change in key charge transport parameters, including the recombination mechanism, in response to environmental degradation, analyte exposure, and ambient temperature. We show increased geminate recombination in P3HT:PC₇₁BM after prolonged light exposure. Lateral BHJ device measurements offer a useful complement to measurements on vertical photovoltaic structures and provide a more complete and detailed picture of OPV materials. / text

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