Polymer photovoltaic systems whose photoactive layer is a blend of a semiconducting polymer with a fullerene derivative in a bulk heterojunction configuration are amongst the most successful organic photovoltaic devices nowadays. The three-dimensional organization in these layers (the morphology) plays a crucial role in the performance of the devices. Detailed characterization of this organization at the nanoscale would provide valuable information for improving future material and architectural design and for device optimization. In this thesis, the results of morphology studies of blends of several polyfluorene copolymers (APFOs) blended with a fullerene derivative are presented. Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy was combined with dynamic Secondary Ion Mass Spectrometry (dSIMS) for surface and in-depth characterization of the blend films. NEXAFS was performed using two different electron detection methods, partial (PEY) and total (TEY) electron yield, which provide information from different depth regimes. Quantitative compositional information was obtained by fitting the spectra of the blend films with a linear combination of the spectra of films of the pure components. In blends of APFO3 with PCBM in two different blend ratios (1:1 and 1:4 of polymer:fullerene) NEXAFS data show the existence of compositional gradients in the vertical direction for both blend ratios, with clear polymer enrichment of the free surface. A series of APFOs with systematic changes in the side-chains was studied and it was shown that those small modifications can affect polymer:fullerene interaction and induce vertical phase separation. Polymer-enrichment of the free surface was clearly identified, in accordance with surface energy minimization mechanisms, and a compositional gradient was revealed already in the first few nanometers of the surface of the blend films. dSIMS showed that this vertical phase separation propagates throughout the film. It was possible to determine that as the polar character of the polymer increases, and thus the polymer:fullerene miscibility is improved, the tendency for vertical phase separation becomes stronger. / <p>Paper II was not published at the time of the licentiate defence and had the title: NEXAFS spectroscopy study of the surface composition in APFO3:PCBM blend films</p>

M.Sc. / CuInSe2/CdS/ZnO heterojunction solar cells are currently one of the most promising technologies for the production of economically viable energy in the form of electricity. The key component of this thin film solar cell device is the chalcopyrite absorber film. CuInSe2 and its related alloys such as Cu(In,Ga)Se2 have been deposited by a number of techniques, including methods which have been demonstrated to be scalable to mass production volumes. In this study attention was focused on (i) developing a relatively simple deposition technology for the production of chalcopyrite absorber films, (ii) detailed characterization of the semiconductor thin films in terms of the experimental parameters and (iii) fabrication of completed CuInSe2/CdS/ZnO solar cell devices. Metallic precursors comprising of copper and indium were deposited with electron-beam evaporation. The number of elemental layers in the precursor stack as well as the substrate temperature was optimized in order to produce metallic alloys with optimum structural properties. These precursors were subsequently reacted in vacuum to elemental Se vapour or to H2Se/Ar at atmospheric pressure in a separate diffusion reactor. In order to investigate the growth kinetics of the respective processes, the precursors were reacted to the Se in the temperature range between 350°C and 450°C. The structural features (morphology, presence of crystalline phases and bulk compositional properties) of the respective films were compared and correlated against the growth parameters. From this systematic study, optimum growth parameters were determined for the production of completed solar cell devices. / Professor V. Alberts

本論文示範了兩款過渡金屬硫化物量子點 (二硫化銀銦量子點與硫化錳量子點) 在量子點敏化太陽能電池上作為光敏化劑的應用，這也是它們在量子點敏化太陽能電池上的初次應用。 / 二硫化銀銦量子點的合成採用了一鍋合成法，合成的量子點隨後透過3-巰基丙酸連接到二氧化鈦的表面。研究發現，當量子點溶液的濃度處於較低水平的時候，量子點在二氧化鈦的吸附量會較高。另外，從不同研究小組在量子點吸附行為的報告中，觀察到量子點的吸附行為決定於實驗條件，如量子點的大小和表面活劑，納米二氧化鈦多孔膜的孔隙度和量子點的溶劑。實驗中，最高性能的二硫化銀銦量子點敏化太陽能電池的短路電流為0.49 mA/cm²，開路電壓為0.245 V，填充因子為38.26 %，光電轉換效率為0.046 %。 / 透過連續離子層沉積反應法，硫化錳量子點生長並組裝到二氧化鈦的表面上。能譜測量顯示，錳跟硫的比率在不是1:1。這現象懷疑是源於錳(2+)的小離子半徑，對錳(2+)和硫(2-)之間的化學反應產生了不良的影響，導致吸附了的錳(2+)沒有反應過來。通過優化連續離子層沉積反應法週期的數量，最高性能的硫化錳量子點敏化太陽能電池的短路電流為0.65 mA/cm²，開路電壓為0.30 V，填充因子為48.21 %，光電轉換效率為0.095 %。 / QD-SSCs sensitized with novel transition metal sulfides have been demonstrated. Both AgInS₂ QD-SSC and MnS QD-SSC presented in this thesis are new and are the first demonstrated works in the research field. / AgInS₂ QDs was synthesized by one-pot hot colloidal synthesis approach. The as-synthesized QDs were attached to the TiO₂ surface through 3-mercaptopropionic acid. Optimization process on QDs adsorption was done, and it has been observed that the amount of QDs adsorbed is higher when the concentration of the QDs solution is at low level. The variations in the behaviors in QDs adsorption between works from different research groups are considered to originate from experimental conditions such as the sizes and surfactants of QDs, porosities in the TiO₂ matrix, and the solvent for QDs dispersion. The optimized AgInS₂ QD-SSC attained a short-circuit current of 0.49 mA/cm², an open-circuit voltage of 0.245 V, a fill factor of 38.26 % and a power conversion efficiency of 0.046 %. IPCE measurements confirm the successful sensitization from AgInS₂ QDs, indicating the energetically favourable electron injection from AgInS₂ QDs to TiO₂. / By adopting the SILAR technique, MnS QDs was in-situ grown and deposited on the TiO₂ surface. EDX measurements indicated that the Mn/S ratio in the TiO₂/MnS film is not 1:1. The reason is suspected to originate from the small ionic radius of Mn²⁺ that promoted an adverse effect on the reaction between Mn²⁺ and S²₋. It is proposed that a portion of the adsorbed Mn²⁺ did not react with the S²₋., resulting an excess concentration of Mn²⁺ in the film. By optimizing the number of SILAR cycles, MnS QD-SSC was optimized to exhibit a short-circuit current of 0.65 mA/cm², an open-circuit volatge of 0.30 V, a fill factor of 48.21 % and a power conversion efficiency of 0.095 %. IPCE measurements confirm the sensitization is originated from MnS QDs, which consequently reveal an energetically favourable electron injection from the MnS QDs to TiO₂. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Cheng, Kai Chun. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2012. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts also in Chinese. / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Background --- p.1 / Chapter 1.2 --- Solar cells --- p.2 / Chapter 1.2.1 --- Developments --- p.2 / Chapter 1.2.2 --- Nanostructured solar cells --- p.4 / Chapter 1.2.2.1 --- Bilayer organic solar cells --- p.4 / Chapter 1.2.2.2 --- Bulk heterojunction organic solar cells --- p.5 / Chapter 1.2.2.3 --- Organic-inorganic hybrid solar cells --- p.7 / Chapter 1.2.2.4 --- Dye-sensitized solar cells --- p.8 / Chapter 1.2.2.5 --- Quantum dot-sensitized solar cells --- p.11 / Chapter 1.2.3 --- Characterization of solar cells --- p.11 / References --- p.14 / Chapter Chapter 2 --- Quantum dot-sensitized solar cells --- p.18 / Chapter 2.1 --- Quantum dots --- p.18 / Chapter 2.1.1 --- Quantum confinement --- p.18 / Chapter 2.1.2 --- Multiple exciton generation --- p.20 / Chapter 2.2 --- Quantum dot-sensitized solar cell --- p.22 / Chapter 2.2.1 --- Principles --- p.22 / Chapter 2.2.2 --- Assembly of oxide/quantum dot film --- p.25 / Chapter 2.2.3 --- Light harvesting and electron injection --- p.29 / Chapter 2.2.4 --- Titanium dioxide as electron acceptor --- p.33 / Chapter 2.2.5 --- Redox process of electrolyte --- p.37 / Chapter 2.2.6 --- Counter electrode materials --- p.39 / References --- p.41 / Chapter Chapter 3 --- Experimental Details --- p.45 / Chapter 3.1 --- Materials --- p.45 / Chapter 3.2 --- Preparation of the TiO₂ mesoporous film --- p.46 / Chapter 3.3 --- Synthesis of AgInS₂ quantum dots --- p.47 / Chapter 3.4 --- Preparation of the TiO₂/QDs film --- p.47 / Chapter 3.5 --- Configuration of the QD-sensitized solar cell --- p.49 / Chapter 3.6 --- Characterization and Photoelectrochemical Measurements --- p.51 / References --- p.52 / Chapter Chapter 4 --- Experimental Results --- p.53 / Chapter 4.1 --- AgInS₂ QD-sensitized solar cell --- p.54 / Chapter 4.1.1 --- Characterization of AgInS₂ QDs --- p.54 / Chapter 4.1.2 --- Adsorption of AgInS₂ QDs on the TiO₂ surface --- p.56 / Chapter 4.1.3 --- Photoelectrochemical measurements of the AgInS₂ QD-SSC --- p.60 / Chapter 4.2 --- MnS QD-sensitized solar cell --- p.64 / Chapter 4.2.1 --- Characterization of MnS QDs --- p.64 / Chapter 4.2.2 --- Photoelectrochemical measurements of the MnS QD-SSC --- p.69 / References --- p.74 / Chapter Chapter 5 --- Discussions and Conclusions --- p.75 / Chapter 5.1 --- Discussions --- p.76 / Chapter 5.1.1 --- AgInS₂ QD-SSC --- p.76 / Chapter 5.1.1.1 --- Adsorption of AgInS₂ QDs on the TiO₂ surface --- p.76 / Chapter 5.1.1.2 --- Electron injection --- p.80 / Chapter 5.1.1.3 --- Problems encountered and future directions --- p.83 / Chapter 5.1.2 --- MnS QD-SSC --- p.84 / Chapter 5.1.2.1 --- Growth of MnS QDs on the TiO₂ surface --- p.85 / Chapter 5.1.2.2 --- Effects of SILAR cycles on MnS QD-SSC --- p.86 / Chapter 5.1.2.3 --- Problems encountered and future directions --- p.88 / Chapter 5.1.3 --- AgInS₂ QD-SSC versus MnS QD-SSC --- p.89 / Chapter 5.2 --- Conclusions --- p.91 / References --- p.94

Ma, Xuhang = 柔性襯底銅銦鎵硒太陽能電池製備工藝 / 馬續航. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2013. / Includes bibliographical references (leaves 88-91). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts also in Chinese. / Ma, Xuhang = Rou xing chen di tong yin jia xi tai yang neng dian chi zhi bei gong yi / Ma Xuhang.

The development of mesoporous titania (meso-TiO2) films is a considerable research goal in the field of mesoporous material development due to their proven applicability in solar cells and phtocatalysts. In this work, the meso-TiO2 films were fabricated through different methods and these home-made titania structures were applied in DSSCs and photocatalysts. Meso-TiO2 powders were first prepared from ethanol/water or ethanol solvent. The meso-TiO2 made from the ethanol/water solvent did not have an ordered mesostructure, but that made from ethanol solvent had 2D-hexagonal mesostructure. Films were prepared by adding ordered meso-TiO2 particles into paste formulations of P25 nanoparticles with weight proportion ranging from 0 to 100%. These were used to form films by doctor blading, and the influence of paste composition on film structure, morphology, porosity, optical properties and cell performance were investigated. Secondly, ordered meso-TiO2 films were fabricated by dip coating from aqueous or ethanol solvent. Both films had cubic mesostructures, but the film coated from aqueous solvent was not uniform. The film formed from ethanol solvent was doped with sulphur. The effects of doping on the mesostructure, morphology, structure, optical properties and photocatalytic activity were studied. The thickness of films was increased by repeated coating. The number of layers had an influence on the mesostructure, morphology, optical properties and cell performance when these films were applied in DSSCs Finally, a novel method was adopted to prepared meso-TiO2 films. Molecular titania precursors or titania colloidal seeds were used as the titania source. Both of them can be used to prepare free-standing hybrid films at air-water interface by a self-assembly method, however the one synthesised from the molecular titania precursor did not contain very much titania and became a powder after calcination. In contrast, after calcination, the films formed from the colloidal titania solution remained intact, and were composed of mixtures of TiO2 nanoparticles and nanowires with mesopores arising from interparticle porosity. These films were applied in DSSCs. This interfacial method was also successfully extended to prepare free-standing ZnO films from a molecular precursor. After calcination, the free-standing ZnO films were found to be composed of rough spheres formed by flocculation of smaller nanoparticles.

Perovskite photovoltaics is one of the fastest growing opto-electronic technologies with device efficiencies currently exceeding 23%. The opportunity to deposit these abundant materials with large area solution processing techniques could make perovskites viable for low-cost production. However, since perovskite materials are prone to degradation, their lifetime needs to be improved to that of silicon solar cells before these devices can be commercialized. Moreover, unlike most semiconductors, trap densities in polycrystalline perovskite films in high-performing devices have been determined to be relatively large, suggesting a remarkable defect tolerance in perovskite films that needs to be understood in the context of the nature of the trap states and any residual non- radiative losses. These non-radiative losses are observed as photoluminescence heterogeneity within perovskite films, even for high-performing perovskite systems. In this work, we explore the degradation kinetics of perovskite devices under stress conditions and find that further stability improvements should focus on the mitigation of trap generation during ageing. Furthermore, we fabricate perovskite solar cells with a novel back-contact structure, in which electron- and hole-selective electrodes are co-positioned on the back side of the cell and spaced by 100 μm. By utilising grazing-incidence X-ray diffraction, we show that even in the earliest stages of conversion of precursors to perovskite we achieve remarkably high open-circuit voltages, suggesting that the defect tolerance of perovskites appears at an early stage in the conversion process. Moreover, we employ scanning X-ray diffraction with nanofocused beam and obtain detailed information, revealing overlapping grains located at different depths within perovskite films. We find that the critical grain size is the longer-range structural super-grains rather than the grains viewed with conventional microscopy techniques. These findings further highlight the presence of structural defects in perovskite materials and provide important insights towards improving the optoelectronic behaviour of these materials.

Orientador: Francisco Carlos Lavarda / Banca: Luiz Carlos da Silva Filho / Banca: Fernando Sato / O Programa de Pós Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atvidades de pesquisa de diversos campi / Resumo: Atualmente existe uma intensa busca por novos materiais com propriedades ajustadas para utilização em células solares orgânicas de modo de obter um aumento em sua eficiência de conversão para que possa substituir os dispositivos de silício. O politieno [3,4-b]-tiofeno-co-benzoditiofeno (PTB7) é um polímero recentemente proposto na literatura e com propriedades muito interessantes em células solares orgânicas, o que o coloca como uma possível alternativa ao amplamente utilizado poli(3-hexiltiofeno) (P3HT). Tem sido relatadas modificações diferentes posições da unidade monomérica deste copolímero, tanto na etrutura benzoditiofeno (BDT), quanto no estrutura tienotiofeno (TT), que o compõe. Estas modificações levaram a novos polímeros com propriedades diferentes e por vezes mais interessantes que aquelas do PTB7 sem substituições. O trabalho que será apresentado visou estudar as propriedades estruturais, eletrônicas e ópticas do PTB7 e possíveis alterações ocorridas devida às modificações químicas realizadas na estrutura do BDT de suas unidades monoméricas. Tal estudo utilizou ferramentas de otimização de estruturas como Mecânica Molecular, Dinâmica Molecular e o método semi-empírico Parametric Method 6 (PM6), assim como de cálculo de estrutura eletrônica de materiais, como a Teoria do Funcional da densidade (DFT) e de cálculos de propriedades ópticas como a Teoria do Funcional da Densidade Dependente do Tempo (TD-DFT). Concluímos que o PTB7 no estado sólido pode ser considerado planar. Com o nosso modelo para o PTB7, obtivemos uma diferença de energia entre o Último Orbital Molecular Ocupado HOMO (do inglês Highest Occupied Molecular Orbital) e o Primeiro Orbital Molecular Desocupado LUMO (do inglês Lowest Unoccupied Molecular Orbital) de aproximadamente 1,84 eV, sendo que este valor está em boa concordância com o valor experimental. Em relação às substituições químicas, estudamos... / Abstract: Currently there is an intensive search for new materials with turned properties for use in organic solar cells to obtain an increase in its conversion efficiency and replace silicon devices. The polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7) is a polymer recently proposed in the literature and with very interesting properties in organic solar cells, which places it a a possible alternative to the widely used poli(3-hexilthiophene) (P3HT). It has been reported changes in different positions of the monomeric unit of this copolymer, both in benzodithiphene (BDT) structure, as in the thienothiphene (TT) structure that compose it. These modifications led to new polymer with different properties and sometimes more interesting than those of PTB7 without substituions. The work to be presented aimed to study the structural, electronic and optical properties of PTB7 and possible changes due to chemical changes made in the BDT structure of its monomeric units. This study employed optimization tools like Molecular Mechanics, Molecular Dynamics and Parametric Method 6 (PM6), as well as calculations of the electronic structures with the Density Functional Theory (DFT) method, and optical properties such as the Time Dependent Density Functional Theory (TD-DFT) calculations. We conclude that the PTB7 chains in the solid state can be considered planar. With our model for PTB7, we obtained a difference between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) of approximately 1.84 eV, and this value is in good agreement with the experimental value. Regarding chemical substantions, we studied theoretically 8 derivatives of PTB7 and the results showed that it is possible to obtain compounds with a significant decrease in and that it is possible to obtain compounds with HOMO and LUMO energy values more adjusted to the widely employed acceptor material phenyl-C61-butyric acid methyl ester (PCBM) / Mestre

Orientador: Maria Aparecida Zaghete Bertochi / Banca: Talita Mazon / Banca: Fenelon Martinho Lima Pontes / Banca: Luiz Vicente de Andrade Scalvi / Banca: Claudia longo / O Programa de Pós Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividades de pesquisa em materais de diversos campi / Resumo: O desenvolvimento das células solares de perovskita foi acompanhado por uma revolução no campo dos dispositivos fotovoltaicos. Células solares de perovskita atingiram eficiências de conversão de energia maiores que 21% em apenas 5 anos após sua descoberta, colocando-as em competição com as células solares comerciais de silício. Apesar de promissores, os dispositivos de perovskita enfrentam desafios que impedem sua comercialização, sendo o maior deles o problema de estabilidade. Nesse âmbito, a presente tese teve como principal foco o desenvolvimento de células solares de perovskita baseadas em filmes nanoestruturados de Nb2O5 e TiO2, visando melhor compreensão do funcionamento desses dispositivos afim de se obter a solução dos problemas hoje enfrentados. Os resultados obtidos mostram eficiências maiores que 13% para o sistema: filme compacto de Nb2O5/ filme mesoporoso de TiO2/ CH3NH3PbI3; e eficiências tão elevadas quanto 15% para sistema usando filme compacto e mesoporoso de TiO2/ CH3NH3PbI3. As melhores células solares montadas com filmes compactos de Nb2O5 apresentaram correntes de curto circuito de 19 mA/cm2, tensão de circuito aberto de 960 mV, fator de preenchimento de 75% e eficiências de 13%. Para as células formadas com filmes de compactos de TiO2 foram obtidas correntes de curto circuito de 20 mA/cm2, tensão de circuito aberto de 1V, fator de preenchimento de 70% e eficiências de 15%. A estabilidade dos dispositivos e a presença de histerese nas curvas de tensão-cor... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The development of perovskite solar cells was accompanied by a revolution in the photovoltaics field. Perovskite solar cells have reached higher energy conversion efficiencies of 21% in just 5 years after its discovery, putting them in competition with commercial silicon solar cells. Although promising, the perovskite devices face some challenges which delay their commercialization, and one of most important is the stability. In this context, the present thesis intended the development of perovskite solar cells based on nanostructured films of Nb2O5 and TiO2, in order to better understand the functioning of these devices. Efficiencies up to 13% were obtained for the system composed of: compact Nb2O5 / mesoporous TiO2/ CH3NH3PbI3 and efficiencies as high as 15% for compact system using compact TiO2/ mesoporous TiO2/ CH3NH3PbI3. The best solar cells prepared using compact Nb2O5 films showed a short circuit current of 19 mA/cm 2, open circuit voltage of 900 mV, fill factor of 75% and 13% of efficiency. Devices prepared using compact TiO2 films reached short circuit current of 20 mA/cm2, open circuit voltage of 1V, fill factor of 70% and 15% of efficiency. The stability of the devices and the presence of current-voltage hysteresis were studied by changing parameters such as the composition and the thickness of the compact layer (TiO2 vs. Nb2O5), as well as the synthesis method used to prepare the perovskite films (sequential deposition method vs solvent-engineering method). Ove... (Complete abstract click electronic access below) / Doutor

Orientador: Carlos Frederico e Oliveira Graeff / Banca: André Sarto Polo / Banca: Fenelon Martinho Lima Pontes / Banca: Francisco das Chagas Marques / Banca: José Humberto Dias da Silva / Resumo: Este projeto consiste na produção e estudo das propriedades de novos materiais para a otimização de células solares sensibilizadas por corante (DSSC) baseadas em filmes de TiO2 nanoestruturado e corantes da família do N3. A pesquisa foi focada no estudo e na otimização de novos contra elétrodos baseados em CuS e CoS nanoestruturados, os quais apresentam grande interesse no setor de pesquisa e desenvolvimento industrial, pelas suas promissoras características eletrocatalíticas e pelos seus baixos custos. Além da obtenção das nanopartículas de CuS e CoS pela técnica hidrotermal assistida por micro-ondas, um grande esforço foi feito a fim de obter tintas precursoras ideais para a deposição dos filmes de CuS e CoS pela técnica de screen-printing, compatível com as modernas linhas de produção industriais. Portanto, foram desenvolvidos e otimizados métodos simples, rápidos e de baixo custo para obter filmes finos dos materiais estudados. Em particular tivemos um grande sucesso utilizando a estratégia do precursor químico dissolvido em solventes. Na primeira aplicação deste método, um precursor químico (Co(II) dietilditiocarbamato) foi utilizado em tintas à base de CH2Cl2. Foram obtidos elétrodos de CoS nanocristalinos e de alta eficiência. Todavia o uso do solvente orgânico clorado não é muito apropriado para grandes superfícies. Portanto, desenvolvemos um método baseado num precursor químico do CoS hidrossolúvel e de baixa toxidade. Com este método aprimorado foram obtidos bons re... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: This project consists of the production and study of new materials for the optimization of dye-sensitized solar cells (DSSC) based on nanostructured TiO2. The research was focused on the design and optimization of new counter electrodes based on CuS and CoS nanoparticles, which are very interesting in the research and development of DSSC, due to their promising electrochemical characteristics and for their lower costs. Along with the hydrothermal technique assisted by microwave, a great effort was made to obtain optimum precursor inks for deposition of CuS and CoS films by screen-printing technique, compatible with modern industrial production lines. Hereby facile, inexpensive and rapid methods for thin films have been developed and optimized for the deposition of CuS and CoS layers. In particular, we developed a chemical precursor process using solvent-based inks. In the first application of this approach, a chemical precursor CoS (Co (II) diethyldithiocarbamate) was used in CH2Cl2. Nanocrystalline CoS electrodes and high efficiency have been obtained, however the use of chlorinated organic solvent is not very suitable for large surfaces. Therefore, we developed a method based on a water-soluble (WS) chemical precursor. Good results have been obtained, in terms of stability and efficiency, with the WS precursor. The counter electrode developed with our methods were tested with alternative redox pairs such as Co2+/Co3+ and ferrocene. Such an application of CuS and CoS electro... (Complete abstract click electronic access below) / Mestre

In 2017, 98 gigawatts of solar capacity were added globally, outpacing new contributions from coal, gas and nuclear plants combined, based on 161 billion dollars of investment. Solar is the leading contributor to the clean energy revolution and continues to grow in market share and drop in price every year as economy of scale advances the technology. Within this market, silicon and cadmium telluride solar cells dominate nearly all of market share, converting roughly 20% of incident solar power into electricity. It is worth noting that the gains from a 1% increase in power conversion efficiency of the typical 20% solar cell to 21% would be measured, annually, in billions of dollars. If the solar cells installed last year had 1% more power conversion efficiency and the power displaced coal power generation, this enhancement in efficiency would now save roughly 8,000,000 pounds of carbon dioxide emission per hour every hour for the ~220,000-hour (~25 year) lifetime of the solar cells. Within this context, enhancing the power conversion efficiency of solar cells is crucial economically and environmentally. Because sunlight is incident on the earth as a broad spectrum of different colors, the energy of the photons spans a wide range. Unfortunately, the spectral range limits power conversion efficiency. For example, solar cells are transparent to photons with insufficient energy, while photons with excess energy relax to the band edge of the solar material, losing the excess energy as heat. This thesis focuses on improving the utilization of high energy photons currently lost to this thermalization process. In Chapter 1, we introduce the photophysical process of singlet exciton fission and give an overview of the field, with a focus on its potential for incorporation into photovoltaic devices. In Chapter 2-8, we discuss our results realizing singlet exciton fission in molecular systems, specifically bipentacenes. This chapter includes the synthesis of these materials, theoretical calculations predicting and rationalizing their photophysical behavior, and the spectroscopic characterization used to demonstrate the singlet fission process. In Chapter 3, we detail a modular synthetic approach to oligomers and even the first polymer of pentacene. We also discuss some basic properties of these materials using techniques such as linear absorption, cyclic voltammetry, and grazing incidence wide angle X-ray scattering spectroscopy. In Chapter 4, we investigate the photophysics of these materials. Photoluminescence upconversion spectroscopy reveals the decay of the singlet exciton on ultrafast timescales, while transient absorption spectroscopy is used to assign the singlet fission timescale, as well as to characterize the triplet absorption spectra. Chapter 5 discusses the synthesis and photophysics of homoconjugated and non-conjugated pentacene dimers, where singlet fission occurs through sigma bonds. Again, transient absorption spectroscopy is crucial to the assignment of the photophysics at play, but continuous wave time resolved electron spin resonance measurements yield additional insights into interaction between the resulting triplet pair excitons. Chapter 6 provides further detail into the formation of strongly exchange coupled triplet pair states. Continuous wave time resolved electron spin resonance spectroscopy is used to determine the quintet character of these states, and pulsed electron spin resonance measurements nutate the spin of these states to confirm this assignment. Chapter 7 provides the first demonstration that singlet exciton fission is also possible in heterodimer systems. Finally, Chapter 8 delves more deeply into the exciton correlations in these materials with a special focus on the pentacene-tetracene dimer system.