Theses and Dissertations from UMD
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Item Self-Assembled InAs/GaAs Quantum Dot Solar Cells(2015) Li, Tian; Dagenais, Mario; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Our work focuses on experimental and theoretical studies aimed at establishing a fundamental understanding of the principal electrical and optical processes governing the operation of quantum dot solar cells (QDSC) and their feasibility for the realization of intermediate band solar cell (IBSC). Uniform performance QD solar cells with high conversion efficiency have been fabricated using carefully calibrated process recipes as the basis of all reliable experimental characterization. The origin for the enhancement of the short circuit current density (Jsc) in QD solar cells was carefully investigated. External quantum efficiency (EQE) measurements were performed as a measure of the below bandgap distribution of transition states. In this work, we found that the incorporation of self-assembled quantum dots (QDs) interrupts the lattice periodicity and introduce a greatly broadened tailing density of states extending from the bandedge towards mid-gap. A below-bandgap density of states (DOS) model with an extended Urbach tail has been developed. In particular, the below-bandgap photocurrent generation has been attributed to transitions via confined energy states and background continuum tailing states. Photoluminescence measurement is used to measure the energy level of the lowest available state and the coupling effect between QD states and background tailing states because it results from a non-equilibrium process. A basic I-V measurement reveals a degradation of the open circuit voltage (Voc) of QD solar cells, which is related to a one sub-bandgap photon absorption process followed by a direct collection of the generated carriers by the external circuit. We have proposed a modified Shockley-Queisser (SQ) model that predicts the degradation of Voc compared with a reference bulk device. Whenever an energy state within the forbidden gap can facilitate additional absorption, it can facilitate recombination as well. If the recombination is non-radiative, it is detrimental to solar cell performance. We have also investigated the QD trapping effects as deep level energy states. Without an efficient carrier extraction pathway, the QDs can indeed function as mobile carriers traps. Since hole energy levels are mostly connected with hole collection under room temperature, the trapping effect is more severe for electrons. We have tried to electron-dope the QDs to exert a repulsive Coulomb force to help improve the carrier collection efficiency. We have experimentally observed a 30% improvement of Jsc for 4e/dot devices compared with 0e/dot devices. Electron-doping helps with better carrier collection efficiency, however, we have also measured a smaller transition probability from valance band to QD states as a direct manifestation of the Pauli Exclusion Principle. The non-linear performance is of particular interest. With the availability of laser with on-resonance and off-resonance excitation energy, we have explored the photocurrent enhancement by a sequential two-photon absorption (2PA) process via the intermediate states. For the first time, we are able to distinguish the nonlinearity effect by 1PA and 2PA process. The observed 2PA current under off-resonant and on-resonant excitation comes from a two-step transition via the tailing states instead of the QD states. However, given the existence of an extended Urbach tail and the small number of photons available for the intermediate states to conduction band transition, the experimental results suggest that with the current material system, the intensity requirement for an observable enhancement of photocurrent via a 2PA process is much higher than what is available from concentrated sun light. In order to realize the IBSC model, a matching transition strength needs to be achieved between valance band to QD states and QD states to conduction band. However, we have experimentally shown that only a negligible amount of signal can be observed at cryogenic temperature via the transition from QD states to conduction band under a broadband IR source excitation. Based on the understanding we have achieved, we found that the existence of the extended tailing density of states together with the large mismatch of the transition strength from VB to QD and from QD to CB, has systematically put into question the feasibility of the IBSC model with QDs.Item Synthesis of Novel Alkaline Polymer Electrolyte for Alkaline Fuel Cell Applicaitons(2012) Luo, Yanting; Wang, Chunsheng; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Development of the intrinsically OH- conductive polymeric electrolyte (alkaline polymer electrolyte, APE) is the critical component to enable the wide application of alkaline fuel cell (AFC) technology. Alkaline polymer electrolyte fuel cell (APEFC) based on AFC technology has been revived recently for applications in transportation and portable electronic devices due to its advantages of using non-noble metal catalysts, faster oxygen reduction in alkaline medium, and compact design. The research described in this dissertation aims to synthesize a novel APE, with controlled ionic conductivity and mechanical strength to achieve high fuel cell power density and long durability. Most APEs synthesized up to now use a modification of existing engineering polymer backbones, which are very difficult to balance its mechanical properties with its ionic conductivities. In this research, we copolymerized APE precursor polymers, namely poly (methyl methacrylate-co-butyl acrylate-co vinylbenzyl chloride) (PMBV) from three functional monomers, methyl methacrylate (MMA), butyl acrylate (BA) and vinylbenzyl chloride (VBC), where VBC was the functional group that was attached with trimethylamine (TMA) and was the OH- carrier after ion-exchanging. MMA was used for mechanical support and BA was used to alleviate the brittleness coming from MMA and VBC. We synthesized alkaline polymer electrolytes from bottom-up polymerization of these selected functional monomers using free radical solution and miniemulsion copolymerization techniques. By miniemulsion copolymerization, the properties of the obtained APEs could be precisely controlled by tuning the (1) monomer ratio, (2) glass transition temperature (Tg), (3) molecular weight (MW), and (4) crosslinking the copolymer. The increase in Tg was realized by eliminating BA from monomers, which was a low Tg component. MW was optimized through investigating binary copolymerization kinetics factors (initiator and surfactant). For crosslinking, the newly obtained poly (methyl methacrylate-co-vinylbenzyl chloride) (PMV) was crosslinked as a semi-interpenetrating network (s-IPN) to reduce water uptake and thus enhanced the mechanical strength in a humidified environment for APEFCs. After the optimization, our best quaternized PMBV (QPMBV) series APE membranes could reach a maximum power density of 180 mW/cm2 and the crosslinked QPMV APE could last 420 hours on APEFCs, which was among the best overall performance in APE technologies. In the future, we propose to use fluorinated polymer monomers to redesign the polymer backbone. Another direction in the design of APEs is to reselect the possible functional OH- carrier groups to make APEs more chemically and mechanically stable in a high pH environment. And last but not least, atomic force spectroscopy (AFM) is proposed to observe the APE nanostructure, the ionic conductive path, and the local mechanical strength by applying a small voltage between the tip and stage.