Thesis advisor: Krzysztof Kempa / The resonant modes of a sequence of periodically spaced microstrip resonators is studied. The system is analyzed as transmission line with periodic capacitive gaps, as a waveguide with apertures via normal mode expansion, and through a derivation of the static fields in the gap between two microstrip resonators via conformal mapping. FDTD simulations are also performed to numerically calculate the resonant modes of the sequence and also its absorption spectrum when it contains a lossy dielectric. It is found, as expected, that when the gap size is large, the microstrip resonators are uncoupled and there resonant modes are unperturbed. As the gap size narrows, the resonators become strongly coupled, and changing boundary conditions perturb the resonant modes upwards in frequency. Moreover, an additional resonant mode is observed that does not correspond to any uncoupled mode. / Thesis (BS) — Boston College, 2011. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: College Honors Program. / Discipline: Physics Honors Program. / Discipline: Physics.

Thesis advisor: Krzysztof Kempa / In my work presented in this dissertation, I have focused on simulation studies of light interaction with nanostructures made of metals and dielectrics. Of particular interest have been plasmonic effects. The structures included the wire and coaxial nanowaveguides, as well as periodic arrays of planar quasi-triangles, and periodic arrays of nanoholes in thin metallic films. In the nanowaveguides I focused on plasmon polariton modes which resemble the TEM modes propagating in the corresponding conventional radio transmission lines. This collaborative research, involving an experimental effort, showed how the nanoscopic plasmon polariton modes reduce in the retarded limit to the TEM modes, and in the non-retarded limit to the corresponding surface plasmon modes. My simulations explained details of recent experimental results involving plasmonic waveguiding in metallic nanowires. Similar results have been obtained for nanocoaxial waveguides. My simulations of the optical absorption in the arrays of nano quasi-triangles, recently observed experimentally, helped identify those as due to Mie plasmonic resonances in these nanoparticles. They also explained the peak shifts in terms of the 2D surface plasmon dispersion, and the plasmon momentum quantization. In the study of the arrays evolution from holes to quasi-triangles, my simulations provided the clue to the critical behavior of the peak position for structures approaching the percolation threshold (the transitional structure in the series, for which film resistance diverges), and allowed to identify the series of structures as an analog of the percolation threshold problem. Finally, I have simulated optical performance of nanorod arrays (or multi-core nanocoax), which have been employed as platform for novel solar cells. My simulations have been employed to predict and optimize these cells. My work resulted in 5 publications and 2 manuscripts in preparation. / Thesis (PhD) — Boston College, 2011. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Les semiconducteurs organiques (SO) ont attiré une grande attention ces dernières années en raison de leur facilité de fabrication, de leurs modifications des propriétés optiques et électriques et de leur rentabilité. Ils forment la classe de matériaux les plus adaptés à l'électronique flexible et à la bioélectronique, en particulier en association avec des matériaux inorganiques / hybrides solubles en solution. Cependant, la mobilité des charges dans ces matériaux est fortement affectés par leur désordre structurel et énergétique introduit par les défauts qui "piègent" les transporteurs de charge. Selon l'emplacement physique des pièges et leur distribution en énergie, ils pourraient affecter de manière significative le transport de charge dans un dispositif. Le présent travail s'efforce de sonder l'interface et les états défectueux en masse dans des diodes à base de polymère. Au lieu de cela, une partie de l'étude implique de caractériser le système avec et sans encapsulation, en utilisant des techniques pour enregistrer le comportement de courant-tension à l'état stationnaire (IV), les transitoires d'extraction de charge par la tension augmentant linéairement (CELIV) et les courants transitoires d'injection en obscurité (DiTC), ainsi que la photoluminescence (PL) et l'électroluminescence (EL) des systèmes. Les mêmes caractéristiques ont été effectuées pour observer l'effet de pénétration de la lumière ultraviolet (UV) sur les systèmes. Tous les tests ont été effectués sur trois polymères différents, à savoir P3HT, MDMO:PPV et PCDTBT. La comparaison des dispositifs encapsulés et non encapsulés donne un aperçu des différences caractéristiques des mesurables lors de l'exposition à l'air et humidité. Les tests de pénétration lumineuse indiquent la modification de la fonction de travail de la cathode après une désorption d'oxygène assistée par UV sur l'interface polymère/cathode. Un effort simultané s'est traduit par une étude in situ de la dynamique de transport des charges dans les semi-conducteurs organiques sur une large gamme de temps à une échelle microscopique. / Organic semiconductors (OSs) have garnered a great attention in the recent years due to their ease of processibility, optical and electrical property-tunability, and to their cost-effectiveness. They form the class of materials most suitable for flexible electronics and bioelectronics, especially in association with solutionprocessable inorganic/hybrid materials. However, the charge mobility in these materials is strongly affected by their structural and energetic disorder introduced by the defects that ‘trap’ the charge carriers. Depending upon the physical location of the traps and their distribution in energy, they could significantly affect the charge transport in a device. The present work strives to probe the interface and bulk defect states in polymer-based diodes. In lieu of that, a part of the study involved characterizing the device with and without encapsulation, using techniques to record steady-state current-voltage (IV)behaviour, transients of charge extraction by linearly increasing voltage (CELIV) and dark-injection transient currents (DiTC), as well as photoluminescence (PL) and electroluminescence (EL) off the devices. The same characteristics have been carried out to observe the effect of ultra-violet (UV) lightsoak on the devices. All the tests were performed on three different polymers, namely P3HT, MDMO:PPV and PCDTBT. The comparison of the encapsulated versus unencapsulated devices gives an insight into characteristic differences in the measurables upon exposure to air and moisture. The light-soak tests indicate the modification of the cathode work function after a UV-assisted oxygen desorption off the polymer/cathode interface. A simultaneous effort went into an in-situ investigation of charge transport dynamics in organic semiconductors over wide time range at a microscopic scale.

Models for electron transport and back reaction in dye-sensitized nanocrystalline solar cells were investigated by developing novel measurement techniques and the results were used to test two complementary models; diffusive electron transport within the TiO2 medium and the quasi-static approximation to deal with non steady-state conditions where trapping plays a role. These will be shown to be partly correct and the shortfalls highlighted and discussed. In the end it was found that more knowledge of the parameters governing the behaviour of electrons is required to further test and develop the models. The incorporation of a secondary sensing electrode allowed the internal quasi-Fermi level (QFL) within the TiO2 to be probed. The behaviour of the voltage measured by the secondary sensing electrode was in accordance with diffusive electron transport in the TiO2. This was confirmed by measuring the QFL along the current-voltage curve of the cell, and by the temperature dependence of the measured QFL. Discrepancies concerning the behaviour of the ideality of the open-circuit voltage (and hence the electron lifetime) between experiment and modelling are highlighted and discussed throughout. Assuming an Arrhenius relationship simple expressions for the temperature dependence of the open-circuit voltage were derived and experimentally tested. The trapped electron density was measured along the current-voltage curve. With the inclusion of the secondary sensing electrode and measuring the trap distribution, the way the trapped charge varied could be modelled and compared to experiment. This provided an important link between the free and trapped electron density profiles but again highlighted shortcomings of the applied models. The quasi-static approximation was tested against a full numerical solution (continuum model) to determine the phase space in which it is applicable. Knowing this, an almost ideally behaving cell was used to test the quasi-static approximation. Having shown that it was valid for the given cell, the quasi-static approximation was used to determine how the conduction band electron lifetime varied with temperature, resulting in an Arrhenius dependence of the back reaction rate of electrons. A strong temperature dependence of the electron lifetime, and hence a strong temperature dependence of the electron diffusion length was demonstrated.

Hybrid lead halide perovskites, AMX 3 compounds in which A = CH 3 NH 3 (MA), CH(NH 2 ) 2(FA), Cs; M = Pb,Sn; X = I, Br, Cl, display remarkable performance in solution-processed optoelectronic devices, including > 22% efficient thin film photovoltaic cells. These compounds represent the first class of materials discovered to exhibit properties associated with high performance compound semiconductors, while being formed at or near room temperature using simple solution chemistry techniques. This thesis is focused on the synthesis, structural characterisation and phase behaviour of MAPbI 3 , FAPbI 3 , A-site solid solutions and novel organic metal halide framework materials. The complete atomic structure and phase behaviour of methylammonium lead iodide is elucidated for the first time, including hydrogen positions, using high flux, constant wave-length neutron powder diffraction. At 100 K an orthorhombic phase, space group Pnma, is observed, with the methylammonium cations ordered as the C–N bond direction alternates in adjacent inorganic cages. Above 165 K a first order phase transition to tetragonal, I4/mcm, occurs with the unlocking of cation rotation, which is disordered primarily in the ab plane. Above 327 K a cubic phase, space group Pm3m, is formed, with the cations isotropically disordered on the timescale of the crystallographic experiment. The high temperature phase of formamidinium lead iodide, α-FAPbI 3 is shown for the first time to be cubic, (Pm3m), at room temperature using time-of-flight, high resolution neutron powder diffraction. Polymorphism and the low temperature phase behaviour of FAPbI 3 have been further investigated using reactor and spallation neutron sources with high resolution in temperature. A tetragonal phase, P4/mbm, is confirmed in the temperature range 140-285 K.The composition, structural and optical parameters of ’A’ site solid solutions (MA/FA)PbI 3 have been investigated by single crystal X-ray diffraction, UV-vis spectroscopy and 1 H solution NMR. A composition-dependent transition in the crystal class from tetragonal to cubic(or pseudo-cubic) at room temperature is identified and correlated to trends in the optical absorption. Novel hybrid materials with inorganic frameworks of varying dimensionality from 0D to 2D, including imidazolium lead iodide and piperazinium lead iodide, have been synthesised using various templating organic cations and their atomic structures solved by single crystal X-ray diffraction.

CoordenaÃÃo de AperfeÃoamento de Pessoal de NÃvel Superior / This research describes the process of obtaining a photovoltaic cell, since getting electrical conductor glasses used for the flow of electrons coming from the photovoltaic effect until the deposition of thin films of semiconductor titanium dioxide (TiO2) and cadmium sulfide (CdS) at each of these glasses. The use of natural or synthetic dyes deposited on titanium dioxide layer has the objective to increase the absorption spectrum of the TiO2, since sunlight emits most of its energy in the frequency range of visible light. After joining the two glasses with thin films deposited over TiO2 plus dye and CdS, it was used a potassium triiodide electrolyte for regeneration and consequently the activation of photovoltaic solar cell. After mounting the cell concerned, tests of photoactivity have been performed by exposing the cells to sunlight collected for specified periods and the values of voltage and photocurrent generated. Theoretical studies have been conducted to mathematical modeling of the behavior of the solar cell mounted, and then we have analyzed the efficiency of converting solar energy into electrical energy. The constituents of the cell have been characterized by the techniques of X-ray diffraction (XRD) and scanning electron microscopy (SEM) for analyzing the porosity, uniformity and other physical parameters of thin films. / O presente trabalho descreve o processo de obtenÃÃo de uma cÃlula fotovoltaica, desde a obtenÃÃo de vidros condutores elÃtricos utilizados para o fluxo dos elÃtrons oriundos do efeito fotovoltaico, atà a deposiÃÃo dos filmes finos dos semicondutores diÃxido de titÃnio (TiO2) e sulfeto de cÃdmio (CdS) em cada um dos vidros. O uso de corantes naturais ou sintÃticos na camada depositada de diÃxido de titÃnio possuiu como objetivo aumentar o espectro de absorÃÃo do mesmo, uma vez que a luz solar emite uma grande parte de sua energia na faixa de frequÃncia da luz visÃvel. Depois de unir os dois vidros com os filmes finos depositados de TiO2 mais corante e o CdS, utilizou-se o eletrÃlito de tri-iodeto de potÃssio para a regeneraÃÃo e consequentemente a ativaÃÃo da cÃlula solar fotovoltaica. ApÃs a montagem da cÃlula em questÃo, foram realizados testes de fotoatividade, expondo as cÃlulas ao sol por perÃodos determinados e coletados os valores da fotocorrente gerada e a tensÃo, alÃm disso, foram realizados estudos teÃricos para modelagem matemÃtica do comportamento da cÃlula solar montada e em seguida analisou-se a eficiÃncia de conversÃo de energia solar em energia elÃtrica. Os constituintes da cÃlula foram caracterizados pelas tÃcnicas de difraÃÃo de raios-X (DRX) e microscopia eletrÃnica de varredura (MEV) para analisar a porosidade, uniformidade e outros parÃmetros fÃsicos dos filmes finos.

Nanowire solar cells have great potential as candidates for high efficiency, next-generation solar cell devices. To realize their potential, accurate and efficient modeling techniques en- compassing both optical and electrical phenomena must be developed. In this work, a coupled optical and electronic model of GaAs nanowire solar cells was developed, with the goal of building a platform for automated, algorithmic device optimization. Significant work was done on the optical portion of model, with the goal of reducing run- times and improving the level of automation. Enhancements were made to an open-source implementation of the Rigorous Coupled Wave Analysis method for solving Maxwell’s equations, to make it more accurate for modeling nanowire solar cells. Its accuracy and efficiency were thoroughly investigated, and with the enhancements presented here it was shown to be an effective technique for rapid optical modeling of nanowire devices. Purely optical optimizations of a sample AlInP-passivated GaAs nanowire on a GaAs substrate were performed to demonstrate the efficacy of the technique using a Nelder-Mead simplex optimization of device geometry. The optical model was then coupled into a finite volume method based electrical model implemented in TCAD Sentaurus, to compute device efficiencies and ultimately optimize electrical device performance. As a first step, an algorithmic optimization of a p-i-n nanowire solar cell consisting of an AlInP-passivated GaAs nanowire on a Si substrate was performed using the generation rates computed by the enhanced RCWA implementation. The overall geometry was fixed to the result of the optical optimization, and only internal electrical parameters were optimized. The results showed that significant performance improvements can be obtained with the right choice of doping levels and doping region configurations, even without optimizing the global device geometry.

This thesis undertakes the development, characterization and optimization of boron diffusion for silicon solar cells. Heavy diffusions (sheet resistance < 40 Ohm/square) to form a back surface field, and light diffusions (sheet resistance > 100 Ohm/square) to form oxide-passivated emitters were developed. Test structures and solar cells were fabricated to assess uniformity, lifetime and recombination effects due to the light and heavy boron diffusions. It was found that the growth of a thin ~200 ??, thermal oxide, during stabilization ??? immediately prior to the boron diffusion - was required to maintain high lifetime and reduce surface recombination (reducing the emitter saturation current density) for all boron diffusions. The heavy boron diffusion process was incorporated into the single side buried contact solar cell processing sequence. The solar cells fabricated had both boron diffused and Al/Si alloyed P+ regions for comparison. This research conclusively showed that boron diffused solar cells had significantly higher open circuit voltage compared to Al/Si alloyed devices. Fabrication of n-type solar cells, and their subsequent characterization by overlayed secondary electron image and the electron beam induced current map showed that the Al/Si alloy varied in depth from 5 to 25 micrometers deep. Methodology and characterization for light, oxide-passivated boron diffusions are also presented. This study yielded boron diffused emitters (sheet resistance > 100 Ohm/square) with low emitter saturation current. It was observed that this was possible only when the thermal oxidation after the boron diffusion was minimal, less than 1,000 ??. This was due to the segregation effect of boron with oxide, decreasing the surface concentration that in turn decreased the electric field repulsion of electrons from the surface. Device modelling of n-type solar cells is presented where the parameters of the modelling include the results of the light, oxide-passivated boron diffusions. This modelling shows n-type-base material with light oxide-passivated boron diffusion has higher potential conversion efficiency than forming a solar cell from phosphorous diffused p-type material.

<p>Patterning of multi crystalline silicon Solar cell is prepared with photolithography etching. Electroless plating is used to get metallization of Nickel contacts. SEM analysis of Nickel deposition and measurement of contact resistance for series and shunt resistance is done. To increase the fill factor, the screen printed electrodes are subjected to different firing temperatures there by increasing the efficiency of solar cell. Nickel-silicide formation at the interface between the Silicon and Nickel enhances stability and reduces the contact resistance, resulting in higher energy conversion efficiency.</p><p> </p>

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