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

Determining Utility System Value of Demand Flexibility From Grid-interactive Efficient Buildings

This report focuses on ways current methods and practices that establish the value to electric utility systems of distributed energy resource (DER) investments can be enhanced to determine the value of demand flexibility in grid-interactive efficient buildings that can provide grid services. The report introduces key valuation concepts that are applicable to demand flexibility that these buildings can provide and links to other documents that describe these concepts and their implementation in more detail.The scope of this report is limited to the valuation of economic benefits to the utility system. These are the foundational values on which other benefits (and costs) can be built. Establishing the economic value to the grid of demand flexibility provides the information needed to design programs, market rules, and rates that align the economic interest of utility customers with building owners and occupants. By nature, DERs directly impact customers and provide societal benefits external to the utility system. Jurisdictions can use utility system benefits and costs as the foundation of their economic analysis but align their primary cost-effectiveness metric with all applicable policy objectives, which may include customer and societal (non-utility system) impacts.This report suggests enhancements to current methods and practices that state and local policymakers, public utility commissions, state energy offices, utilities, state utility consumer representatives, and other stakeholders might support. These enhancements can improve the consistency and robustness of economic valuation of demand flexibility for grid services. The report concludes with a discussion of considerations for prioritizing implementation of these improvements

Adhesion Enhancement of Polymeric Films on Glass Surfaces by a Silane Derivative of Azobisisobutyronitrile (AIBN).

Adhesion of polymeric films on surfaces can be due to a combination of van der Waals, electrostatic or covalent interactions between the two materials. The interfacial adhesion between a polymer film and glass or metal can be improved by using a broad class of silane coupling agents. Typically, silane coupling compounds used for adhesion improvement have structure similar to (R´O)3-Si-R, where R´O- is an alkoxy group and -R is an organofunctional group. Under appropriate reaction conditions alkoxy groups condense with hydroxyl groups available on the surface, resulting in surfaces decorated with organofunctional -R groups, which promote formation of covalent bonding of the coupling agent with polymeric networks. Alkoxy silanes with amino and vinyl organofunctional groups are common silane coupling agents and their adhesion-promoting abilities with polymeric films have been well-documented. In analogy to our previous work of forming conformal polymer coatings on three dimensional assemblies of silica nanoparticles (aerogels) via surface initiated polymerization (SIP), here we expand the scope of that work demonstrating the application of a new bidentate free radical initiator (Si-AIBN) as coupling agent that enhances adhesion of polystyrene (PS) and polymethylmethacrylate (PMMA). Si-AIBN was synthesized via a condensation reaction between 3-aminopropyltriethoxysilane (APTES) and azobiscyanovaleric acid. Si-AIBN is attached to the surface of glass by hydrolysis of the ethoxy groups and reaction with the hydroxyl groups of the surface. On supply of thermal energy those glass surfaces act like a macro initiator generating surface-bound radicals. In the presence of olefin monomers, surface-bound initiator starts formation of polymeric chains in analogy to a “grafting from” approach. Since each polymer chain is terminated by a molecule of the initiator, which is surface-bound, the adhesion of the resulting polymeric films on the substrate is promoted not only by mechanical interlocking of the polymeric chains but also by covalent bonding with the glass surface

Preparation of Macroporous Conductive Carbon Cerogels from Pyrolysis of Isocyanate-Crosslinked Resorcinol Formaldehyde Aerogels

Carbon aerogels combine typical aerogal properties such as high surface area and low density with electrical conductivity. They are prepared by pyrolysis under N2 or Ar at 600-2100 degrees C of resorcinol formaldehyde (RF) aerogels, which in turn are prepared via aqueous sol-gel chemistry. Carbon aerogels are considered for numerous applications such as separation media in HPLC, as supercapacitors, as materials for hydrogen storage, as non-reflective materials, and as anodes in lithium-ion batteries. Meanwhile, porous carbons with high hydrophobic surface areas and large pore volumes are used as industrial adsorbents. In addition, macropores enhance mass transport for applicatoins in energy storage and lithium intercalation batteries. It is well-established with silica that monodispersed polystryrene beads can be used to introduce ordered mesoporosity or macroporosity. In the same approach, plystyrene beads have been also incorportated as templates in RF sol-gel matrices and have been removed later by dissolving in toluene. Here, we report that RF gels crosslinked with isocyanates yield macroporous, electrically conducting carbon aerogels without need for templating

Crosslinking 3D Assemblies of Silica Nanoparticles (Aerogels) by Surface-Initiated Free Radical Polymerization of Styrene and Methylmethacrylate

Quasi-stable, ultra-low density, three-dimensional assemblies of nanoparticles are referred to as aerogels. Aerogels are open-cell foams derived from supercritical fluid (SCF) drying of wet gels. Their large internal void space is responsible for low dielectric constants, low thermal conductivities and high acoustic impedance. At the same time though those materials are fragile and impractical for high load applications. The fragility problem has been addressed by casting a thin conformal polymer coating over the entire internal porous surface of the nanostructure.1 That process is referred to as crosslinking. The coating connects chemically skeletal nanoparticles and renders interparticle necks wider. Thus, the internal void space is not compromised significantly, while the flexural strength of a typical monolith is increased by 300Ã for a nominal increase in density only by a factor of three. A major issue, however, is the fact that current preparation procedures for crosslinked aerogels involve several solvent exchange steps, which are expensive and must be eliminated before crosslinked aerogels become commercially viable. In order to eliminate solvent exchange steps, the reagents for the crosslinking process should be included in the sol, which in turn means that the crosslinking chemistry should be deconvoluted from the gelation chemistry (an ionic process). In this context, there are several chemistries involving nanoparticle surface modification for making core-shell structures. These methods span the entire range from layer-by-layer electrostatic assembly of oppositely charged materials,2-6 to atom transfer radical polymerization,7 and direct free radical polymerization of olefins from surface-bound initiators such as peroxides,8 and AIBN.9,10 by comparison, free-radical initiators have received the least attention as silica surface modifiers, while all literature examples concern asymmetric peroxide and AIBN derivatives, which are attached on silica only from one side. Such monodentate free-radical initiators would not work for our purposes, because upon homolytic cleavage they would produce one surface-bound radical, which is the desirable outcome, but they would also release a second radical in the mesopores. The polymer formed in the solution filling the mesopores will have to be removed, and that introduces more solvent exchange steps. For our purposes, we need bidentate free-radical initiators that will attach themselves on silica from both sides. In that regard, we report here the synthesis of AIBN analogue as well as its incorporation into silica, and the surface initiated polymerization of styrene and methylmethacrylate to yield conformal polymer coatings on the mesoporous surfaces of typical base-catalyzed aerogels

Acid-catalyzed Time-efficient Synthesis of Resorcinol-Formaldehyde Aerogels and Crosslinking with Isocyanates

Aerogels are open-cell foams derived from supercritical fluid (SCF) drying of wet gels. Their large internal void space is responsible for low thermal conductivity, high surface area and high acoustic impedance. Most aerogels are based on inorganic metal or semimetal oxide frameworks. Pekala and co-workers synthesized resorcinol-formaldehyde (RF) organic aerogels by poly-condensation of resorcinol with formaldehyde in the presence of Na2CO3 as base catalyst, followed by drying with SCF CO2. Low-density RF aerogels prepared by this method exhibit high porosities (\u3e80%), high surface areas (400-900 m2g-1), ultrafine cell-size (\u3c500 \u3e Å) and densities as low as 0.03 g cm-3. The major drawback though, has been the length of the preparation procedure that typically spans several days. Looking at the mechanism of the process, the RF gel formation has been associated with two major reactions: (1) formation of hydroxymethyl derivatives of resorcinol; and, (2) condensation of those derivatives to methylene (-CH2-) and methylene ether (-CH2-O-CH2-) bridges. The effect of the resorcinol to catalyst (R/C) ratio on the final aerogel structure has been studied extensively. That ratio was typically varied in the range between 50- 300. Formation of particles connected with large necks was reported for low and for very high (~1500) R/C ratios. The final pore structure and the gelation time depend strongly on the sol pH; at low pHs, precipitation rather than gelation was reported. The extensive literature on base-catalyzed RF aerogels has obscured attempts towards acid-catalyzed processing. Recently, Brandt and Fricke reported an aqueous acetic acid catalyzed route for RF gel synthesis, where they still allowed a two-day period for gelation and aging. Reasoning that not only hydroxy methylation of resorcinol, but also subsequent condensation to methylene and methylene ether bridges should be all acid-catalyzed processes, we undertook a systematic look at the reaction of resorcinol with formaldehyde in CH3CN, developing a time-efficient method that yields within a few hours (as opposed to weeks) gels indistinguishable from those reported in the literature. The -OH groups of resorcinol in the resulting gels are reactive with di- and tri-isocyanate crosslinkers in analogy to silica, leading to isocyanate-derived polymer crosslinked RF aerogels, which are more robust, and more resistive to shrinkage than their native (noncrosslinked) counterparts

Effect of matrix parameters on mesoporous matrix based quantum computation

We present a solid state implementation of quantum computation, which improves previously proposed optically driven schemes. Our proposal is based on vertical arrays of quantum dots embedded in a mesoporous material which can be fabricated with present technology. We study the feasibility of performing quantum computation with different mesoporous matrices. We analyse which matrix materials ensure that each individual stack of quantum dots can be considered isolated from the rest of the ensemble-a key requirement of our scheme. This requirement is satisfied for all matrix materials for feasible structure parameters and GaN/AlN based quantum dots. We also show that one dimensional ensembles substantially improve performances, even of CdSe/CdS based quantum dots

Novel Porous Polymer Compositions for the Synthesis of Monolithic Bimodal Microporous/Macroporous Carbon Compositions Useful for Selective CO₂ Sequestration

The present invention discloses novel porous polymeric compositions comprising random copolymers of amides, imides, ureas, and carbamic-anhydrides, useful for the synthesis of monolithic bimodal microporous/macroporous carbon aerogels. It also discloses methods for producing said microporous/macroporous carbon aerogels by the reaction of a polyisocyanate compound and a polycarboxylic acid compound, followed by pyrolytic carbonization, and by reactive etching with CO2 at elevated temperatures. Also disclosed are methods for using the microporous/macroporous carbon aerogels in the selective capture and sequestration of carbon dioxide

Understanding the Fundamental Properties of Conjugated Materials for Organic Electronics

The effects of heteroatomic substitution, conjugation length and aggregation on the absorptive and emissive properties of conjugated organic materials was explored. Heteroatomic substitution was first investigated with the synthesis of two nitrogencontaining polymeric derivatives of benzothiadiazole (BT): a pyridylthiadiazole (PyT)-based polymer, PTTPy; and the novel pyridazine thiadiazole (PzT)-based polymer, PTTPz. It was discovered that PzT was a much stronger acceptor in comparison to the more commonly used BT and PyT moieties, and that per electronegative nitrogen atom that is substituted into the acceptor, an effective HOMO stabilisation energy of ~0.1 eV can be achieved. The second part of this thesis introduces the renowned chromophore diketopyrrolopyrrole (DPP) and was used to study how chain length, polydispersity and defects effect the absorption ability of conjugated materials. A series of welldefined DPP-based conjugated oligomers were synthesised (from n=1-6 repeat units long) via iterative Suzuki-Miyaura cross-coupling, as well as their polymeric counterpart. The extinction coefficients were measured however their unusual trends were difficult to rationalise. Significant jumps in the red-shift from pentamer to hexamer were tentatively credited to the oligomers self-aggregating, however the exact nature of which (inter- or intra-molecularly) was inconclusive. Thus, to understand the unusual red-shift, aggregation had to be controlled. Encapsulating alkyl chains that prevent polymer backbones from !–! stacking were therefore incorporated onto a phenyl-DPP derivative and gave rise to extremely high fluorescence quantum yields in both solution (>70%) and thin film (>20%). Their absorption and emission spectra showed clearer, more defined features compared to their naked counterparts, demonstrating the suppression of both inter and intramolecular aggregation. The macrocyclic shield resulted in a dramatically increased backbone co-linearity and gave rise to structurally ordered, defect-free polymer domains, as evidenced by STM, allowing DPP to be used, for the first time, as a emissive material in the solid state. Finally, using the optimised reaction conditions for phenyl-DPP, the synthesis of the corresponding encapsulated polymer based on thienyl-DPP was also attempted