Fluorescence spectroscopy was examined as a potential technique for identifying aerosol particles like bacterial spores. This technique was used for laboratory measurements on some common biological agent simulants. We have measured the intrinsic steady-state fluorescence emission spectra as a function of the excitation wavelength for several bacterial spores (washed and unwashed) in dry and aqueous suspensions at room temperature using excitation wavelengths from 200 to 600 nm. These measurements were compared to those of common, naturally occurring biological components like fungal spores and pollen and non spore samples like ovalbumin. The spectra of samples were combined into fluorescence profiles or fluorescence fingerprints. Different substrates were used for collection and detection of spores. Each bacterium produces a unique in vitro fluorescence profile when measured in dried and aqueous suspension and exhibits a strong maximum in its fluorescence emission spectrum near 330-340 nm. The fluorescence profiles were reproducible. The complexity of microorganisms made the interpretation of their spectral signature a difficult task. Principal components analysis (PCA) and cluster analysis were done as a data reduction technique for detection and identification from different backgrounds. PCA illustrates that linear combination of detected fluorescence intensities, which are present in different ratios in each of samples studied, can be used to discriminate biological agent simulants from other biological samples. The hydration effects, washing effects and the role of tryptophan on spore fluorescence were also investigated. The emission spectra of the dried spores showed a maximum near 330 nm, suggesting a hydrophobic environment for its tryptophan residues. The aqueous solution of tryptophan showed fluorescence shifted to 360 nm and in ethanol solution the maximum was shifted to 340 nm, suggesting a rather more polar average location of the tryptophan. To find the limit of detection we measured the quantum efficiency (QE) of a few samples. We concluded that spectroscopy techniques coupled with effective interpretation models are applicable to biological simulants agents. Index Heading: Bacteria; Spores; Identification; Fluorescence; Fluorescence Quantum Efficiency; Principal Components Analysis; Cluster Analysis.

Ce travail porte sur la croissance par épitaxie sous jets moléculaires (EJM) et sur les propriétés structurales et optiques de boîtes quantiques (BQs) AlyGa1-yN insérées dans une matrice AlxGa1-xN (0001). L’objectif principal est d’étudier le potentiel des BQs en tant que nouvelle voie pour la réalisation d’émetteurs ultraviolets (UV) efficaces.Tout d'abord, nous avons étudié la croissance des BQs GaN en utilisant soit une source plasma (N2, appelée PAMBE) soit une source ammoniac (NH3, appelée NH3-MBE) afin de choisir la meilleure approche pour former les BQs les plus efficaces. Il a été montré que le procédé de croissance est mieux contrôlé en utilisant l’approche PAMBE, conduisant à la croissance de BQs GaN avec des densités plus élevées, une meilleure uniformité en taille et des intensités de photoluminescence (PL) jusqu’à trois fois plus élevées. En outre, l'influence de la contrainte épitaxiale sur le processus d'auto-assemblage des BQs a été étudiée en fabriquant des BQs GaN sur différentes couche tremplins d’AlxGa1-xN (avec 0,5 ≤ x ≤ 0,7). Nous avons montré que des BQs avec des densités plus élevées et des hauteurs plus faibles sont formées en augmentant le désaccord de paramètre de maille (c.à.d en utilisant des tremplins avec xAl plus élevé). Cependant, les mesures de photoluminescence (PL) indiquent un fort décalage de l'énergie d'émission vers le rouge lorsque xAl augmente, en raison de l'augmentation de la discontinuité du champ électrique interne de 3 à 5,3 MV/cm.Ensuite, des études approfondies sur les conditions de croissance et les propriétés optiques des BQs Al0,1Ga0,9N / Al0,5Ga0,5N ont été présentées, montrant les différents défis pour fabriquer des BQs efficaces. L’optimisation de la procédure de croissance, notamment l’étape de recuit post-croissance, a montré une modification de la forme des BQs. Plus précisément, un changement d’une forme allongée (pour un recuit à 740 °C), à une forme symétrique (pour un recuit à une température proche de ou supérieure à 800°C) a été observé. En plus, une bande d’émission supplémentaire vers les plus grandes longueurs d’onde a également été observée pour les BQs formées avec un recuit à 740°C. Cette bande a été attribuée à une fluctuation de composition des BQs, induisant la formation d’une famille additionnelle de BQs avec des hauteurs plus grandes et une compostions en Al inférieure à 10 %, estimée proche de l’alliage binaire GaN. Enfin, il a été démontré qu’en faisant un recuit à plus haute température (≥ 800°C), l’émission de PL de cette famille supplémentaire de BQs (BQs riche en Ga ou (Al)GaN) diminue très fortement. De plus, cette étape de recuit impacte fortement la forme des BQs et a conduit à une amélioration de leur efficacité radiative d’un facteur 3. Ensuite, la variation de la composition en Al des BQs AlyGa1-yN (0,1 ≤ y ≤ 0,4), ainsi que la quantité de matière déposée ont permis d’évaluer la gamme de longueurs d’onde d’émission accessibles. En ajustant les conditions de croissance, l’émission des BQs a été déplacée de l’UVA vers l’UVC, atteignant une émission autour de 270 - 275 nm (pour les applications de purification de l’eau et de l’air) avec des rendements radiatifs élevés. Les mesures de photoluminescence résolue en temps (TRPL), combinées avec les mesures de PL en fonction de la température, nous ont permis de déterminer les efficacités quantiques internes (IQE) des BQs GaN / AlxGa1-xN (0001). Des valeurs d’IQE comprises entre 50 % et 66 % ont été obtenues à basse température, avec la possibilité d’atteindre un rapport d’intensité intégré de PL, entre 300 K et 9 K, allant jusqu’à 75 % pour les BQs GaN et 46 % pour les BQs AlyGa1-yN (contre 0,5 % pour des structures équivalents à base de puits quantiques).Enfin, nous avons montré la possibilité de fabriquer des DELs à base de BQs (Al,Ga)N couvrant une grande gamme de longueurs d’onde allant du bleu-violet jusqu’à l’UVB (de 415 nm à 305 nm). / This PhD deals with the epitaxial growth, structural and optical properties of AlyGa1-yN quantum dots (QDs) grown on AlxGa1-xN (0001) by molecular beam epitaxy (MBE), with the aim to study their potential as a novel route for efficient ultraviolet (UV) emitters.First, we have studied the growth of GaN QDs using either plasma MBE (PAMBE) or ammonia MBE (NH3-MBE) to find the most adapted nitrogen source for the fabrication of UV emitting QDs. It was shown that the growth process is better controlled using PAMBE, leading to the growth of GaN QDs with higher densities, better size uniformity and up to three times higher photoluminescence (PL) intensities. Also, the influence of the epitaxial strain on the QD self-assembling process was studied by fabricating GaN QDs on different AlxGa1-xN surfaces (with 0.5 ≤ x ≤ 0.7). We showed that QDs with higher densities and smaller sizes (heights) are formed by using a larger lattice-mismatch (i.e. a higher xAl composition). However, photoluminescence (PL) measurements indicated a strong redshift in the emission energy as the Al content of the AlxGa1-xN template increases due to the increase of the internal electric field discontinuity from 3 to 5.3 MV/cm.Next, in-depth investigations of the growth conditions and optical properties of Al0.1Ga0.9N QDs / Al0.5Ga0.5N were done presenting the different challenges to be solved to grow efficient QDs. Changing the growth procedure, especially the post-growth annealing step, has shown a modification of the QD shape from elongated QDs, formed with an annealing at 740°C, to symmetric QDs, formed with an annealing at a temperature around or above 800°C. An additional band emission at lower energies was also observed for QDs grown with a lower annealing temperature (740°C). This additional band emission was attributed to the formation of QDs with higher heights and a reduced Al composition less than the nominal one of 10 % (i.e. forming Ga-rich QDs). The influence of the annealing step performed at higher temperature has been shown to strongly decrease the PL emission from this additional QD family. In addition, this annealing step strongly impacted the QD shape and led to an improvement of the QD radiative efficiency by a factor 3. Then, the AlxGa1-xN barrier composition (0.5 ≤ x ≤ 0.7), the AlyGa1-yN QD composition (0.1 ≤ y ≤ 0.4) as well as the deposited amount were varied in order to assess the range of accessible emission energies. Also, the influence of varying the AlxGa1-xN barrier composition on the QD formation was studied. By varying these growth conditions, the QD wavelength emission was shifted from the UVA down to the UVC range, reaching a minimum wavelength emission of 270 - 275 nm (for water and air purification applications) with a high radiative efficiency. Time resolved photoluminescence (TRPL) combined with temperature dependent PL measurements enabled us to determine the internal quantum efficiencies (IQE) of AlyGa1-yN QDs / AlxGa1-xN (0001). IQE values between 50 % and 66 % were found at low temperature, combined with the ability to reach a PL integrated intensity ratio, between 300 K and 9 K, up to 75 % for GaN QDs and 46 % for AlyGa1-yN QDs (versus 0.5 % in a similar quantum well structure emitting in the UVC range).Finally, the demonstration of AlyGa1-yN QD-based light emitting diode prototypes, emitting in the whole UVA range, using GaN and Al0.1Ga0.9N QDs, and in the UVB range down to 305 nm with Al0.2Ga0.8N QDs active regions, was shown.

Resumo: A cana-de-açúcar (Saccharum spp.) é uma das principais culturas das regiões tropicais, cuja produtividade agrícola pode ser afetada pelo déficit hídrico. Para investigar o processo de tolerância e sensibilidade ao déficit hídrico, diferentes parâmetros fisiológicos foram avaliados em cultivares de cana-de-açúcar tolerantes (SP83-2847 e CTC15) e sensível (SP86-155) ao déficit hídrico. A deficiência hídrica afetou todo o aparato fotossintético das plantas de forma diferenciada dentro e entre as cultivares. A taxa fotossintética e condutância estomática diminuíram significativamente para todas as cultivares submetidas ao estresse. Nas plantas controle das cultivares (cv) tolerantes SP83-2847 e CTC15 observou-se que a taxa fotossintética apresentou valores mais altos em comparação a cultivar sensível SP86-155. Resultados do teor relativo de água mostraram que a cultivar CTC15 apresentou melhor condição hídrica durante o período de déficit hídrico. A eficiência quântica do fotossistema II da cultivar SP83-2847 mostrou maior estabilidade nos últimos dias do tratamento experimental, sugerindo que o decréscimo do teor relativo de água estimulou o ajuste da capacidade fotossintética frente às alterações da disponibilidade hídrica. De modo geral, as cultivares SP83-2847 e CTC15, consideradas tolerantes, sob déficit hídrico exibiram melhor desempenho em relação a cultivar sensível SP86-155. Os dados permitem sugerir que tais parâmetros fisiológicos podem ser empregados na avaliação e distinção de genótipos de cana-de-açúcar tolerantes e sensíveis ao déficit hídrico. / Abstract: The sugarcane (Saccharum spp.) is one of the main crops cultivated in tropical areas, whose agricultural productivity can be affected by drought. To investigate the tolerance and sensitivity process to water deficit, various physiological parameters were evaluated in sugarcane cultivars considered tolerant (SP83-2847 and CTC15) and sensitive (SP86-155) to drought. The water deficit affected the entire photosynthetic apparatus of all plants in different manners, inside and among cultivars. The photosynthetic rate and stomatal conductance decreased significantly for all cultivars, submitted to water stress. In control plants of the tolerant cultivars SP83-2847 and CTC15, it was observed that the photosynthetic rate showed better values in comparison to sensitive cultivar SP86-155. According to relative water content results of the cultivar CTC15 showed better condition water performance during the drought. The quantum efficiency photosystem II of the cultivar SP83-2847 showed greater stability in recent days of the experimental treatment, suggesting that the decline in the relative water content stimulated the adjustment of photosynthetic capacity to face the changes in water availability. Thus, cultivars SP83-2847 and CTC15, considered tolerant under water deficit, showed better performance in comparison to sensitive cultivar SP86-155. The data suggest that these physiological parameters can be used in the evaluation and distinction of drought tolerant and sensitive sugarcane genotypes. / Orientador: Sonia Marli Zingaretti / Coorientador: Clara Beatriz Hoffmann-Campo / Banca: Rinaldo Cesar de Paula / Banca: José Renato Bouças Farias / Mestre

We demonstrate the use of embedded CdS nanowires in improving spectral transmission loss and the low mechanical and electrical robustness of planar CdS window layer and thus enhancing the quantum efficiency and the reliability of the CdS-CdTe solar cells. CdS nanowire window layer enables light transmission gain at 300nm-550nm. A nearly ideal spectral response of quantum efficiency at a wide spectrum range provides an evidence for improving light transmission in the window layer and enhancing absorption and carrier generation in absorber. Nanowire CdS/CdTe solar cells with Cu/graphite/silver paste as back contacts, on SnO2/ITO-soda lime glass substrates, yield the highest efficiency of 12% in nanostructured CdS-CdTe solar cells. Reliability is improved by approximately 3 times over the cells with the traditional planar CdS counterpart. Junction transport mechanisms are delineated for advancing the basic understanding of device physics at the interface. Our results prove the efficacy of this nanowire approach for enhancing the quantum efficiency and the reliability in window-absorber type solar cells (CdS-CdTe, CdS-CIGS and CdS-CZTSSe etc) and other optoelectronic devices. We further introduce MoO3-x as a transparent, low barrier back contact. We design nanowire CdS-CdTe solar cells on flexible foils of metals in a superstrate device structure, which makes low-cost roll-to-roll manufacturing process feasible and greatly reduces the complexity of fabrication. The MoO3 layer reduces the valence band offset relative to the CdTe, and creates improved cell performance. Annealing as-deposited MoO3 in N2 reduces series resistance from 9.98 Ω/cm2 to 7.72 Ω/cm2, and hence efficiency of the nanowire solar cell is improved from 9.9% to 11%, which efficiency comparable to efficiency of planar counterparts. When the nanowire solar cell is illuminated from MoO3-x /Au side, it yields an efficiency of 8.7%. This reduction in efficiency is attributed to decrease in Jsc from 25.5mA/cm2 to 21mA/cm2 due to light transmission loss in the MoO3-x /Au electrode. Even though these nanowire solar cells, when illuminated from back side exhibit better performance than that of nanopillar CdS-CdTe solar cells, further development of transparent back contacts of CdTe could enable a low-cost roll-to-roll fabrication process for the superstrate structure-nanowire solar cells on Al foil substrate.

This thesis describes an investigation into the suitability of complementary metal oxide semiconductor (CMOS) active pixel sensor (APS) devices for scientific imaging applications. CMOS APS offer a number of advantages over the established charge-coupled device (CCD) technology, primarily in the areas of low power consumption, high-speed parallel readout and random (X-Y) addressing, increased system integration and improved radiation hardness. The investigation used a range of newly designed Test Structures in conjunction with a range of custom developed test equipment to characterise device performance. Initial experimental work highlighted the significant non-linearity in the charge conversion gain (responsivity) and found the read noise to be limited by the kTC component due to resetting of the pixel capacitance. The major experimental study investigated the contribution to dark signal due to hot-carrier injection effects from the in-pixel transistors during read-out and highlighted the importance of the contribution at low signal levels. The quantum efficiency (QE) and cross-talk were also investigated and found to be limited by the pixel fill factor and shallow depletion depth of the photodiode. The work has highlighted the need to design devices to explore the effects of individual components rather than stand-alone imaging devices and indicated further developments are required for APS technology to compete with the CCD for high-end scientific imaging applications. The main areas requiring development are in achieving backside illuminated, deep depletion devices with low dark signal and low noise sampling techniques.

A new route to a well-defined block copolymer with alternating PEO-solubilising groups and fluorinated distyrylbenzene units was established. The Horner Wittig reaction was used as the polycondensation reaction. The non-fluorinated analogue of this block copolymer was prepared via the Wittig reaction. Both polymers were soluble in chloroform and free-standing films could be cast from solution. The position of the HOMO and LUMO energy levels of these two materials were determined by a combination of cyclic voltammetry, UV photoelectron spectroscopy and UV/Vis absorption spectroscopy. The presence of fluorine substituents on the distyrylbenzene unit had no influence on the HOMO-LUMO band-gap (3.0 eV). However, the position of these two energy levels relative to the vacuum level was shifted to higher energies (0.85 eV shift) in the case of the fluorinated block copolymer. The photoluminescence quantum efficiency of the fluorinated block copolymer was 17%, that of the non-fluorinated block copolymer was 34%. The former was used as the electron conducting layer in a light emitting diode with poly(p-phenylene vinylene) as the emissive layer. The latter was used as the emissive layer in light emitting diodes. Luminances over 2000 cd/m(^2) were observed for devices based on the non-fluorinated block copolymer using indium tin oxide as the anode and aluminium as the cathode. The luminescence efficiency of such devices was as high as 0.5 cd/A, corresponding to an internal quantum efficiency of 1.1%.Furthermore, an oligo(p-phenylene vinylene) was synthesised that contained two terminal fluorinated benzene rings and two central non-fluorinated benzene rings, all connected by vinylene bridges. This material aggregated in a 'brickwall' motif, where each molecule overlaps with two halves of molecules in the row above and below. The structure of this J aggregate is due to aryl-fluoroaryl-interactions and was demonstrated by X-ray crystal structure analysis.

Fluorescence spectroscopy was examined as a potential technique for identifying aerosol particles like bacterial spores. This technique was used for laboratory measurements on some common biological agent simulants. We have measured the intrinsic steady-state fluorescence emission spectra as a function of the excitation wavelength for several bacterial spores (washed and unwashed) in dry and aqueous suspensions at room temperature using excitation wavelengths from 200 to 600 nm. These measurements were compared to those of common, naturally occurring biological components like fungal spores and pollen and non spore samples like ovalbumin. The spectra of samples were combined into fluorescence profiles or fluorescence fingerprints. Different substrates were used for collection and detection of spores. Each bacterium produces a unique in vitro fluorescence profile when measured in dried and aqueous suspension and exhibits a strong maximum in its fluorescence emission spectrum near 330-340 nm. The fluorescence profiles were reproducible. The complexity of microorganisms made the interpretation of their spectral signature a difficult task. Principal components analysis (PCA) and cluster analysis were done as a data reduction technique for detection and identification from different backgrounds. PCA illustrates that linear combination of detected fluorescence intensities, which are present in different ratios in each of samples studied, can be used to discriminate biological agent simulants from other biological samples. The hydration effects, washing effects and the role of tryptophan on spore fluorescence were also investigated. The emission spectra of the dried spores showed a maximum near 330 nm, suggesting a hydrophobic environment for its tryptophan residues. The aqueous solution of tryptophan showed fluorescence shifted to 360 nm and in ethanol solution the maximum was shifted to 340 nm, suggesting a rather more polar average location of the tryptophan. To find the limit of detection we measured the quantum efficiency (QE) of a few samples. We concluded that spectroscopy techniques coupled with effective interpretation models are applicable to biological simulants agents. Index Heading: Bacteria; Spores; Identification; Fluorescence; Fluorescence Quantum Efficiency; Principal Components Analysis; Cluster Analysis.

Submitted by Emanuel Varela Cardoso (emanuel.varela@ufcg.edu.br) on 2018-09-24T18:34:57Z No. of bitstreams: 1 ÉRICA CALDAS SILVA DE OLIVEIRA – TESE (PPGMet) 2011.pdf: 3050030 bytes, checksum: 38d32142dec7d2ec8b6853ae9d180ca6 (MD5) / Made available in DSpace on 2018-09-24T18:34:57Z (GMT). No. of bitstreams: 1 ÉRICA CALDAS SILVA DE OLIVEIRA – TESE (PPGMet) 2011.pdf: 3050030 bytes, checksum: 38d32142dec7d2ec8b6853ae9d180ca6 (MD5) Previous issue date: 2011-06 / A busca por novas alternativas de uso dos recursos naturais emerge da necessidade da demanda por novas matérias-prima que fomentem o desenvolvimento. Com relação ao bioma Caatinga, a produção de conhecimento nas mais diversas áreas do saber, reveste-se de uma importância fundamental para a geração de políticas públicas, que viabilizem estratégias de gestão ambiental e priorize, conservação e preservação da biodiversidade da caatinga, riqueza, endemismos, potencial econômico e social, valor ecológico. Assim entendendo, realizou-se uma pesquisa sobre a espécie Cnidoscolus quercifolius Pohl (faveleira), em áreas do Seridó Ocidental do Estado da Paraíba, precisamente nos municípios de Santa Luzia e São Mamede, no período de maio de 2009 a fevereiro de 2011, abordando aspectos da biologia desta espécie, que guarda um valor sócio-ambiental importante nas áreas em que ocorre. Através de coletas mensais de dados, a pesquisa abordou aspectos etnobotânicos da faveleira, naquelas comunidades trabalhadas, aspectos da ecofisiologia da espécie, em que se evidenciou características anatômicas e citogenéticas, avaliação da eficiência quântica fotoquímica do fotossistema II, avaliação do potencial hídrico e estudo de área foliar, biomassa de folhas e frutos, fitossociologia do estrato arbustivo-arbóreo em áreas de caatinga no Seridó Ocidental paraibano e fenologia de populações de faveleira, considerando períodos estacionais chuvosos e secos. A espécie C. quercifolius apresentou, de acordo com os resultados obtidos, uma dinâmica biológica que se ajusta bem aos fatores ambientais em que a mesma se desenvolve, apresentando uma sazonalidade acentuada para os aspectos ecofisológicos estudados. / The search for new alternatives for use of natural resources emerges the necessity of demand for new raw materials to facilitate development. Regarding the Caatinga biome, the production of knowledge in several areas of knowledge, is of fundamental importance for the generation of public policies that allow environmental management strategies and prioritize, conservation and preservation of biodiversity of the savanna, wealth, endemics, economic and social potential, ecological value. Thus understood, there was a survey on the species Cnidoscolus quercifolius Pohl (faveleira) in areas of the West Seridó of Paraiba State, precisely in Santa Luzia and São Mamede, from march 2009 to february 2011, addressing aspects of the biology of this species, which houses a major socio-environmental value in areas where it occurs. Were monthly data, the study addressed aspects of ethnobotanical faveleira, worked in those communities, aspects of the ecophysiology of the species, which showed anatomical and cytogenetic characteristics, evaluation of photochemical quantum efficiency of photosystem II, evaluation of water potential and study leaf area, biomass of leaves and fruits, phytosociology the woody layer in the savanna areas of West Seridó Paraiba and phenology of populations faveleira considering seasonal rainy and dry periods. The species C. quercifolius presented, according to the results, a biological dynamics that fits well with environmental factors in which it unfolds, showing a marked seasonality for aspects ecofisológicos studied.

In this thesis, I will present and discuss research performed on InGaN/GaN multiple quantum well (QW) structures. The results of which were taken using photoluminescence (PL) spectroscopy and PL time decay spectroscopy. In the first two experimental chapters, I report on the effects of QW growth methodology on the optical properties of c-plane InGaN/GaN QWs. I compare structures grown using the single temperature (1T), quasi-two temperature (Q2T), temperature bounced (T-bounced) and two temperature (2T) QW growth methodologies. The T-bounced and 2T structures are observed to have gross well width fluctuations (GWWF), where the QW width varies from 0 to 100 % created when the QWs are exposed to a temperature ramp. Whereas, the 1T and Q2T structures have continuous QWs with only one or two monolayer well width fluctuations. The structures with GWWFs are observed to have a larger room temperature internal quantum efficiency (RT-IQE) at low excitation conditions i.e. below efficiency droop compared to those without. The larger RT-IQE is ascribed to several factors which include an increased radiative recombination rate, increased thermal activation energy of non-radiative recombination and reduced defect density of the QWs. The effect of barrier growth temperature is also investigated. No clear trend is observed between barrier growth temperature and RT-IQE.In the last experimental chapter I report on studies of carrier localisation in InGaN/GaN QWs using resonant PL spectroscopy. The effect of carrier localisation on the independently localised electrons and holes are investigated and the resonant PL spectrum is studied in detail. The InGaN/GaN QW structure is observed to exhibit an effective mobility edge at 12 K where delocalised carriers are created above a particular excitation energy. The emission from the resonantly excited localised states which are accompanied by the emission of a longitudinal optical phonon (resonant LO feature) is investigated as a function of temperature and excitation energy. The integrated PL intensity of the resonant LO feature is observed to quench rapidly with temperature up to around 45 K, independent of excitation energy. The integrated PL intensity of the resonant LO feature is fitted to an Arrhenius model and a thermal activation energy of ∼ 1(±1) meV is extracted. This activation energy is speculated to be consistent with the localisation energy of electrons.

This thesis focuses on the analysis of the external quantum efficiency of quantum dot-enhanced multi-junction solar cells. Divided in four major parts, it uses the experimental methodology developed in the SUNLAB. At first, a model is introduced to calculate the external quantum efficiency of single and multi-junction solar cells. This model takes into account the semiconductor physics governing the electrical property of the solar cell. It furthermore takes into account the optical transmission and reflection in the semiconductor structure using a transfer matrix method. The calculated curve fits a single junction GaAs solar cell's external quantum efficiency to a high degree of precision. Finally, an InGaP/GaAs/Ge solar cell's external quantum efficiency is calculated and it reproduces accurately the behavior of a measured cell. Second, the reflectivity of a solar cell is studied. An analysis technique involving using the fast Fourier transform of the oscillation in the reflectivity is introduced. This technique extracts the thicknesses of the top and middle subcells. The reflectivity is subsequently calculated using the transfer matrix method and it reproduces the behavior of the measured samples. Third, the effect of the addition of quantum dots in the middle subcell is studied. It is demonstrated that they extend the absorption range of the middle subcell. This is completed by first modeling the quantum mechanical behavior of the electrons and holes in the nanostructure. Their emission and absorption properties are derived. Those derived properties are verified by experimentally measured photoluminescence and electroluminescence of the nanostructures. The resulting model is then compared to experimentally measured external quantum efficiencies of single junction and multi-junction quantum dot-enhanced solar cells. Finally, a study of the bottom subcell artifact is completed. Using the fill-factor bias experiment, each of the contribution of the light coupling and the internal voltage biasing is decoupled. For the measured sample, an optimal voltage of 2.1 V is found to minimize the artifact. At this point, the internal voltage biasing creates an artifact of 1 % and the light coupling artifact is 8 %.