In recent years, quasi-one-dimensional materials have attracted a lot of research attention due to their remarkable properties, and their potential as building blocks for nanoscale electronic and optoelectronic devices. A modified chemical vapor deposition (CVD) method has been used to synthesize ZnO nanowires. Electron microscopy and other characterization techniques show that nanowires having distinct morphologies when grown under different conditions. The effects of reaction parameters including reaction time, temperature, carrier gas flow rate, substrates and catalyst material upon the size, shape, and density of ZnO nanowire arrays have been investigated. Excitonic solar cells —including Gratzel-type cells, organic and hybrid organic/inorganic solar cells—are promising devices for inexpensive, large-scale solar energy conversion. Hybrid organic/inorganic solar cells are made from composites of conjugated polymers with nanostructure metal oxides, in which the polymer component serves the function of both light absorber and hole conductor, and the ZnO nanowire arrays act as the electron conductors. Organic solar cells have been fabricated from environmentally friendly water-soluble polymers and ZnO nanowire arrays.

Approved for public release; distribution is unlimited. / Thermophotovoltaic cells are a good candidate for use in high efficiency radioisotope thermoelectric generator (RTG) power devices for deep space missions. This thesis examines the use of Silvaco Virtual Wafer Fabrication Software as a tool for designing and optimizing TPV cells for different possible spectra. It gives results for GaSb and InGaAs cells optimized to the AM0 spectrum which closely match published data as well as hypothetical cells optimized to the spectrum of a 1300K blackbody. / Ensign, United States Navy

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M.Sc. / Current solar cell research programmes in general aim to develop a high conversion efficiency photovoltaic (PV) module from high quality thin films. In this study, Cu (In,Ga)Se2 (CIGS) thin films were grown and characterized. These films were grown by selenization of Cu-In-Ga precursors. These precursors were prepared by co-sputtering In and (Cu, Ga). All the precursors were grown on Mo coated soda lime glass substrates. The selenization was conducted under different conditions in Ar/H2Se atmosphere, i.e. taking different values of flow rate of H2Se (5.00, 1.00, 0.25 mol%) in Ar, temperature (350, 450, 550 ºC) and time (10, 20, 30, 40, 50, 60 min). At each selenization condition, two samples were placed at different positions in the chamber. The structural properties of the produced films were analyzed by the techniques of X-ray Diffraction (XRD) for phases, Scanning Electron Microscopy (SEM) for morphology and Energy Dispersive Spectroscopy (EDS) for the bulk composition. The effect of temperature variation, the effect of flow rate variation and the effect of time variation were analyzed by comparing the structural properties as analyzed by the techniques mentioned. All in all this specific study delivers important information about the sensitivity of Cu(In,Ga)Se2 (CIGS) thin films to the temperature, gas flow rate and exposure time of the selenization step. / Doctor C.A. Engelbrecht Professor Vivian Alberts

Organic semiconducting materials have been attracted considerable attention as a promising technology for the next generation flexible electronic devices, such as solar cells and field-effect transistors because of their advantages of low-cost, structural versatility and flexibility. Many organic semiconducting materials have been developed in recent years. In this thesis, four pi-conjugated building blocks based on benzodithiophene and quinoxalinedithienothiophene were applied to develop novel photovoltaic materials, including donor-acceptor alternating copolymers as a donor material for polymer solar cells, photosensitizers for dye sensitized solar cells, small molecule hole transporting materials for perovskite solar cells and small molecule acceptors for organic solar cells. A comprehensive review of current development of organic photovoltaic materials was presented in Chapter 1. In Chapter 2, a series of D-A copolymers (PBB-n) based on 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole and 4,5-bis((2-ethylhexyl)oxy)benzo[2,1-b:3,4-b'] dithiophene attached with different solubilizing side-chains were designed, synthesised and characterized. In general, PBB-n polymers showed good absorption in the region of visible light and UV region, indicating such polymers are a promising light harvester. Also, PBB-n exhibited suitable energy levels, suggesting that they could be applied as the donor materials in polymer solar cells. PBB-n also exhibited various extent of aggregation behaviour. Chapter 3 described syntheses and the fluorination effect of two series of fluoro-substituted PBB-n copolymers, namely PfBB-n and PffBB-n on optical, electrochemical, and optoelectronic properties. Among them, PfBB-n series was characterized with photovoltaic performance. The champion devices fabricated from PfBB-12 showed a PCE as high as 9.7%, with a Voc of 0.92 V, a Jsc of 16.60 mA/cm-2 and a FF of 63.49%. Cells fabricated from other PfBB-n copolymers also exhibited good PV performance with PCE ranging from 7.4 - 8.5%. For PffBB-n polymers, temperature-dependent aggregation behaviour was exploited by modulating the coating temperature during device fabrication. PSC devices based on PffBB-n exhibited good PV performance with PCE ranging from 7.4% to 9.9%. Among which, PffBB-14 provided the most promising PV performance with PCE of 9.9%, a Voc of 0.92 V, a Jsc of 16.8 mA/cm-2 and a FF of 64.36%. Electron deficient conjugated structure was seldom used as the π-bridge in metal-free photosensitizers. In Chapter 4, four novel organic photosensitizers, namely QC5-m and PC5-n were designed with an electron deficient π-bridge. Typical sandwich-structured DSSCs based on the newly developed photosensitizers exhibited promising photovoltaic performance with PCE ranging from 5.23 - 7.77 %, with a maximum Jsc as high as 15.63 mA cm-2. These results suggest that the use of electron deficient π-bridge provides alternative approach to construct efficient organic photosensitizers. Chapter 5 and Chapter 6 described the design, synthesis and investigation of novel hole-transporting materials and electron acceptor materials based on benzo[2,1-b:3,4-b']dithiophene-4,5-dione derived building blocks as potential organic photovoltaic materials for solar cell applications. Keywords: organic photovoltaic materials, photosensitizers, polymer solar cell, electron acceptor, hole-transporting materials.

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. Johannesburg, 2015. / Organic Photovoltaic (OPVs) is a promising alternative technology to provide clean and inexhaustible energy due to their excellent optoelectronic properties of the active polymer blends. The organic polymers have low weight, tunable electrical and optical properties besides being relatively insensitive to film imperfections which in the long run enable low-cost high-throughput roll-to-roll processing. However, their photo-conversion efficiency (PCE) and instability to air remains their greatest drawback as these preclude their commercialization. Indeed the highest power-conversion efficiency reported in literature is between 10-12 % compared to their inorganic counterparts (40 %). Therefore there is great need for improvement to make them competitive with grid parity. In this thesis, the major factors limiting the efficiency of organic solar cells such as light absorption, exciton diffusion and dissociation as well as charge collection are investigated and discussed. Despite the high thickness dependent absorption coefficients (>105 cm-1) within the visible spectrum the materials exhibit short exciton diffusion lengths (10-20 nm) which limit the optimal active layer thickness to a few nanometers. Improving optical absorption within this thickness forms the basis of this project. We report the use of surface Plasmons synthesized by both thermal evaporation and Radio Frequency (RF) magnetron sputtering system to tune and enhance optical absorption and scattering using the surface Plasmon resonance effect. The NPs were annealed at various temperatures and for different times to reconstruct and modify their shapes, sizes as well as the inter-particle distance (coverage). Stability is of paramount importance in organic semiconductor devices. Serious degradation in air constrains their applications potential. The study further investigates the mechanisms that determine the stability of organic photovoltaic devices. Our results depict the degradation mechanisms and their circumvention through the use of high mobility pentacene to improve stability.

銅銦鎵硒薄膜太陽能電池因為其高效率及相對低廉的成本，商業應用已經開始陸續出現。我們自主研發的集成式銅銦鎵硒薄膜電池生產系統可以全程製作襯底大小為10cm x 10cm 的電池及剃型組件。本研究工作主要分為兩個方向:第一個方向是研究及測試生長高效率太陽能電池及組件的具體條件。通過儀器改進及電池每層鍍膜的條件優化，能夠重複的生長高效率電池及組件; 第二個方向是通過減少銅銦鎵硒吸光習的厚度從而達到降低電池生產成本的目的。 / 銅銦鎵硒採用三步共蒸法製備吸收層。第一步先蒸發銦、鎵、硒三種元素形成n型硒化銦(鎵)薄膜;第二步蒸發銅、硒形成銦鎵硒半導體薄膜; 第三步蒸發一層額外的型硒化銦(鎵)薄膜保證整體電池是p型半導體。三步期間的襯底溫度經過小心調試，以使得合適的鎵梯度能夠在吸收層裹形成。通過每一層的條件優化我們能夠生長出高光電轉換效率的太陽能電池(17%)及組件(12%)。 / 太陽能電池的變溫測試及弱光測試對瞭解其應用潛能存在非常重要的作用。通過多組對比實驗發現銅銦鎵硒電池的溫度係數可以通過增加鎵在吸收層的組分而得到改善。同時，電池的弱光表現可以通過減少銅的量得到很大的提高。STM 的研究發現弱光表現得到改善是因為吸收層顆粒介面電阻的增加而導致的。 / 減少吸收層的厚度有利於進一步減少太陽能電池的材料成本。當電池的吸收層厚度小於一微米時，開路電壓跟短路電流都明顯有所減少，從而導致太陽能電池效率降低。更薄電池效率的提高可以從兩個方面來實現:氧化鋅表面的陷光結構及更加合適的鎵含量的使用。通過這兩艇改進方法，電池效率被提高到14%以上，使得超薄電池有更好的應用前景。 / Cu(In,Ga)Se₂ (CIGS)-based thin film solar cell has been commercialized recently due to its high energy conversion efficiency. We have designed an integrated satellite deposition system for producing CIGS solar cell with substrate size of 10cm x 10cm. This work mainly contains two parts with first part focusing on growing and characterizing high quality baseline solar cells and solar modules and second part concentrating on further reducing the material costs by growing thinner absorber layer with high efficiency. / The most difficult part in growing high quality CIGS solar cells originate from the absorber layers which contain p-type chalcopyrite structures with four different elements: Cu, In, Ga and Se. The widely used three-stage process is employed to co-evaporate In, Ga and Se first, then Cu and Se are evaporated to form the chalcopyrite CIGS structure and additional In, Ga and Se are deposited in the end to ensure an overall Cu deficiency, which is important for getting p-type semiconductors. The substrate temperatures during these three stages are carefully adjusted to introduce proper gallium gradients which is important for collecting electrons efficiently. Together with optimizing other layers we are able to get cell efficiency (area around 0.5 cm²) over 17%. To produce CIGS mini-modules, laser scribing as well as mechanical scribing are employed for series interconnection of individual cells using monolithic integration. The power and speed of laser together with the condition of mechanical scriber are carefully adjusted to ensure a minimum dead area in the module. Module (area around 80 cm²) with efficiency over 12% is produced. / Solar cells were fabricated and tested under varied temperature and weak light conditions. Temperature coefficient is compared between CIGS solar cells and other types of solar cells. Temperature coefficient is improved a lot with higher gallium content in the absorber layer. Weak light performance is shown to be increased a lot when copper percentage is lowered down. In order to examine the origin of beneficial effects from Cu-poor absorber, solar cells are grown with comparable grain sizes using our technique and I-V performances are examined under STM in grain/atomic scale. Leakage current is found to be mainly originates from boundary area. CIGS solar cells with Cu-poor absorber benefit from the reduced leakage from boundary area. / CIGS solar cells with thinner absorber thickness are studied and compared with conventional CIGS solar cells. We have found that high conversion efficiency solar cells can be grown for absorber thickness as thin as 1.5μm. Further reduction in absorber thickness deteriorates solar cell performances in both V∝ and Jsc resulting in conversion efficiency as low as 11%. / Two major approaches are performed to improve solar cell performances. Light trapping by etching AZO top contact for creating pyramid-structures to enhance light scattering. Efficiency is increased by more than 1.5% for solar cells with etched AZO surfaces. Solar cells with efficiency larger than 13% can be grown by using AZO etching. Another approach is by using suitable Ga content in absorber layer. Solar cells with efficiency as high as 14.17% are grown which makes thinner CIGS solar cells very competitive. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Yang, Shihang = 高效率、低成本銅銦鎵硒薄膜太陽能電池的製造 / 楊世航. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2012. / Includes bibliographical references (leaves 101-109). / Abstract also in Chinese. / Yang, Shihang = Gao xiao lu, di cheng ben tong yin jia xi bo mo tai yang neng dian chi de zhi zao / Yang Shihang. / Chapter 1 --- Introduction to Photovoltaics --- p.1 / Chapter 1.1 --- Energy crisis --- p.1 / Chapter 1.2 --- Physics of solar cells --- p.4 / Chapter 1.2.1 --- Light Absorption --- p.4 / Chapter 1.2.2 --- Charge Carrier Separation --- p.8 / Chapter 1.2.3 --- Solar Cell I-V Characteristics --- p.9 / Chapter 1.3 --- Classifications of Solar Cells --- p.11 / Chapter 1.3.1 --- Crystalline silicon solar cell --- p.11 / Chapter 1.3.2 --- Thin film solar cells --- p.12 / Chapter 1.3.3 --- Organic and polymer solar cells --- p.13 / Chapter 1.4 --- Cu(In,Ga)Se₂ (CIGS) based Solar Cells --- p.13 / Chapter 1.4.1 --- State of the art --- p.13 / Chapter 1.4.2 --- Material properties and structures --- p.14 / Chapter 1.4.3 --- CIGS advantages --- p.17 / Chapter 2 --- Integrated CIGS deposition system and fabrication process optimization --- p.21 / Chapter 2.1 --- Introduction to vacuum deposition system --- p.21 / Chapter 2.1.1 --- Integrated CIGS solar cell deposition system --- p.21 / Chapter 2.1.2 --- Ni-Al top grid evaporation system --- p.23 / Chapter 2.2 --- Fabrication processes --- p.23 / Chapter 2.2.1 --- Substrate treatment --- p.23 / Chapter 2.2.2 --- Molybdenum back contact deposition --- p.24 / Chapter 2.2.3 --- CIGS absorber layer formation --- p.26 / Chapter 2.2.4 --- Hetero-junction formation --- p.31 / Chapter 2.2.5 --- Window layer optimization --- p.32 / Chapter 2.2.6 --- Laser and mechanical scribing for mini-modules fabrication --- p.37 / Chapter 2.3 --- Equipment improvements --- p.42 / Chapter 2.3.1 --- Heating uniformity of substrate --- p.42 / Chapter 2.3.2 --- Use of pyrometer for improved control of absorber thickness/composition --- p.43 / Chapter 2.3.3 --- Se cracking unit --- p.45 / Chapter 2.4 --- Characterization of CIGS solar cells --- p.47 / Chapter 2.4.1 --- Morphology, composition and crystallinity --- p.47 / Chapter 2.4.2 --- Depth profile of CIGS --- p.49 / Chapter 2.4.3 --- Electrical property measurements --- p.51 / Chapter 2.5 --- Conclusion --- p.54 / Chapter 3 --- Performance of CIGS solar cells under non-standard test conditions --- p.56 / Chapter 3.1 --- Temperature coefficient measurement of CIGS --- p.57 / Chapter 3.1.1 --- Equipment set-up --- p.57 / Chapter 3.1.2 --- Temperature coefficients for different types of solar cells . --- p.60 / Chapter 3.1.3 --- CIGS solar cells with varied Ga/III composition --- p.65 / Chapter 3.2 --- Weak Light Performance of CIGS --- p.69 / Chapter 3.2.1 --- Introduction --- p.69 / Chapter 3.2.2 --- Experiment --- p.72 / Chapter 3.2.3 --- Results and discussion --- p.73 / Chapter 3.3 --- Conclusion --- p.81 / Chapter 4 --- CIGS solar cells with lower fabrication cost --- p.83 / Chapter 4.1 --- Fabrication cost analysis for commercial CIGS solar cells --- p.83 / Chapter 4.2 --- Thinner CIGS absorber layer --- p.84 / Chapter 4.2.1 --- Solar cell performances with different absorber thicknesses --- p.84 / Chapter 4.2.2 --- Performance improvement for thinner solar cell --- p.87 / Chapter 4.3 --- Conclusion --- p.96 / Chapter 5 --- Conclusion --- p.98 / Chapter 5.1 --- Summary of previous researches --- p.98 / Chapter 5.2 --- Future work --- p.99 / Bibliography --- p.101

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Orientador: Francisco Carlos Lavarda / Banca: Julio Ricardo Sambrano / Banca: Fernando Sato / O Programa de Pós-Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividades de pesquisa em materiais de diversos campi da Unesp / Resumo: Hoje existe uma necessidade de novos polímeros com propriedades ajustadas para utilização em camadas ativas de céluas solares, uma vez que os materiais mais utilizados ainda não são eficientes, nem suficientemente estáveis, de modo que possam substituir os dispositivos de silício. O poli(3-hexiltiofeno) (P3HT), é um polímero amplamente utilizado em dispositivos fotovoltaicos, e muito importante em células solares. Tem sido relatado modificações na posição 4 das suas unidades monoméricas que levaram a novos polímeros com propriedades diferentes daquele sem modificações. O trabalho que será apresentado visou estudar as propriedades estruturais e ópticas do P3HT e possíveis alterações ocorridas devido às modificações realizadas na posição 4 de suas unidades monoméricas. Tal estudo utilizou ferramentas de cálculo de estrutura eletrônica de materiais, com os métodos PM6 e DFT. Concluímos que o P3HT em solução e no estado sólido não é plantar. Com o nosso modelo para o P3HT, através da conjunção dos métodos ZINDO-S/CIS, obtivemos um pico de absorção óptica teórico em clorofórmio de aproximadamente 385 nm, sendo este em boa concordância com o valor experimental. Em relação às substituições químicas, estudamos teoricamente 15 derivados de P3HT e os cálculos indicaram que é possível obter compostos com uma diminuiçao significativa do gap com estabilidade e solubilidade similares à do P3HT, principalmente com a ligação do átomos de oxigênio ou grupos orgânicos conjugados ao anel tiofênico / Abstract: Today there is a need new polymers with properties adjusted for use in active layers of solar cells since the most employed materials still are not efficient nor stable enough so that they can replace the silicon devides. The poly(3-hexylthioplene) (P3HT), is a polymer widely used in photovoltaic devices, and very important in solar cells. It has been reported modifications at position 4 of its monomeric units that led to new polymers with different properties than the pristine one. The work that will be presented aimed to study the structural and optical properties of P3HT and possible changes due to chemical substitutions made in position 4 of the monomer units. This study used tools calculating the electronic structure materials, with PM6 and DFT methods. We conclude that the P3HT, in solution and in the solid state, is not planar. With our model for the P3HT, through the combinations of the ZINDO-S/CIS methods, we obtained a theoretical optical absorption peak in chloroform of about 385 nm, in good agreement with the experimental value. In relation to the chemical substitutions, we studied 15 derivatives of P3HT and the calculations indicated that it is possible to obtain compunds with a significant decrease in the gap with stability and solubility similar to that of P3HT, especially with the binding of oxygen atoms or conjugated organic groups to the thiophenic ring / Mestre