Um dos grandes desafios do século XXI está na produção de energia limpa e renovável, já que a demanda mundial por energia continuará crescendo, assim como a necessidade de despoluir o planeta e de diminuir a emissão dos gases do efeito estufa. Nesse contexto, a conversão de energia solar em elétrica coloca-se como uma excelente alternativa, e com isso a dos dispositivos fotovoltaicos. A tecnologia fotovoltaica baseada no silício e em outros semicondutores orgânicos encontra-se em estágio relativamente avançado, porém o custo de produção e de manutenção a proíbe em uso de grande escala. Mais recentemente, iniciaram-se pesquisas com filmes de semicondutores orgânicos, e a rápida melhora na performance dessas células solares a coloca como promissora ao mercado fotovoltaico. Em nosso trabalho, realizamos estudos sobre a performance de dispositivos fotovoltaicos orgânicos baseados na estrutura de heterojunção, estudando a influência de vários parâmetros na performance dos dispositivos. Usamos como camada ativa para nossos dispositivos o poli(3-hexiltiofeno) (P3HT) regiorregular, que é um polímero condutor de gap eletrônico em torno de 1,8 eV misturado ao [6,6]-fenil-C61-ácido butírico-metil ester (PCBM). Essa mistura é apropriada à dissociação dos éxcitons gerados nas cadeias poliméricas pelos fótons absorvidos porque, sendo o PCBM muito eletronegativo, ele captura o elétron do éxciton antes do processo natural de recombinação. Como esse fenômeno ocorre em todo o volume da camada ativa, o dispositivo leva o nome de heterojunção de volume. A estrutura básica que usamos foi de ITO/P3HT-PCBM/Al, isto é, o ITO como eletrodo transparente e bom injetor de buracos e o alumínio como eletrodo injetor de elétrons. Outros dispositivos foram feitos adicionando uma camada transportadora de buracos entre o ITO e o polímero ativo, o Poli(3,4-etileno dióxido-tiofeno):poliestireno-sulfonado (PEDOT:PSS) e/ou cálcio (Ca) entre a camada de alumínio e o polímero. Verificamos que a performance do dispositivo fotovoltaico é bastante alterada quando mediante o contato utilizado, a espessura da camada ativa e a temperatura em que o tratamento térmico é realizado. Investigou-se também, os mecanismos de injeção, transporte e geração de portadores sob variação de temperatura, no intervalo de 90 à 330K. Foi mostrado que, mediante a variação da temperatura, a corrente de curto circuito (JSC), é governada principalmente pela mobilidade dos portadores. A eficiência dos dispositivos desenvolvidos neste trabalho é comparável aos principais valores obtidos na atualidade. Para obtenção destes resultados, foi necessária intensa pesquisa em processamento, principalmente mantendo todas as etapas de fabricação em atmosfera controlada. / One big challenge of the humanity along the 21st Century is to produce energy based on clean and renewable sources. The energy consumption certainly will increase, as well as the necessity in decreasing the emission of greenhouse gases. In this context, solar energy becomes an important alternative for the production of electric energy, in particular, that of photovoltaic devices. Photovoltaics made of silicon and of other inorganic semiconductors are already available, but due to the high cost is not an alternative to produce energy in a large scale. More recently, the organic photovoltaics, due to their quick progress, have becoming as promising technology for the solar energy market. In this work, we studied bulk heterojunction organic photovoltaics, varying several parameters and its influence on the device performance. We used regio-regular poli(3-hexylthiophene) (P3HT), that has an electronic gap close to 1.8 eV, mixed with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). PCBM acts in order to dissociate the photogenerated exciton because, being highly electronegative, it captures the electron form the exciton before the recombination process. We used as basic structure the ITO/P3HT-PCBM/Al. ITO as transparent electrode and injector of holes, and aluminum as the electrons injector electrode. In other devices we added a thin layer of Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), as hole transport layer and/or calcium (Ca) between the Al and the polymer. We verify that the device performance changes considerably with the insertion of such layers, and with the thickness of the active layer and the annealing treatment. We also investigated phenomena related to injection, generation and transport of charge carriers, in the 90-330 K temperature range. We showed that the temperature is the main factor that governs the short-circuit current (JSC). It is important to remark that our devices exhibited similar efficiency compared to that of the literature.

Photocatalytic reactions mediated by semiconductors such as ZnS, TiO2, ZnO, etc. can harvest solar energy into chemical bonds, a process with important prebiotic and environmental chemistry applications. The recycling of CO2 into organic molecules (e.g., formate, methane, and methanol) facilitated by irradiated semiconductors such as colloidal ZnS nanoparticles has been demonstrated. ZnS can also drive prebiotic reactions from the reductive tricarboxylic acid (rTCA) cycle such as the reduction of fumarate to succinate. However, the mechanism of photoreduction by ZnS of the previous reaction has not been understood. Thus, this thesis reports the mechanisms for heterogeneous photocatalytic reductions on ZnS for two model reactions in water with sulfide hole scavenger. First the reduction of CO2 is carried out under variable wavelength of irradiation and proposed to proceed thorough five steps resulting in the exclusive formation of formate. Second the reduction of the double bond of fumaric acid to succinic acid is reported in detail and compared to the previous conversion of CO2 to formic acid. Both reactions are carried out under variable wavelength of irradiation and proposed to proceed thorough one electron transfer at a time. In addition, a new method to measure the bandgap of colloidal ZnS suspended in water is established. Furthermore, the time scales of electron transfer and oxidizing hole loss during irradiation of ZnS for both reactions are reported and interpreted in terms of the Butler-Volmer equation. The sunlight promoted production of succinate introduced above, provides a connection of this prebiotic chemistry work to explore if central metabolites of the rTCA cycle can catalyze the synthesis of clay minerals. Clay minerals are strong adsorbents that can retain water and polar organic molecules, which facilitate the polymerization of biomolecules and conversion of fatty acid micelles into vesicles under prebiotic conditions relevant to the early Earth. While typical clay formation requires high temperatures and pressures, this process is hypothesized herein to be accelerated by central metabolites. A series of synthesis are designed to last only 20 hours to study the crystallization of sauconite, an Al- and Zn-rich model clay, at low temperature and ambient pressure in the presence of succinate as a catalyst. Succinate promotes the formation of the trioctahedral 2:1 layer silicate at ≥ 75 °C, 6.5 ≤ pH ≤ 14, [succinate] ≥ 0.01 M. Cryogenic and conventional transmission electron microscopies, X-ray diffraction, diffuse reflectance Fourier transformed infrared spectroscopy, and measurements of total surface area and cation exchange capacity are used to study the time evolution during the synthesis of sauconite. While the studies with ZnS presented above advanced the fundamental understanding of photocatalysis with single semiconductors, the environmental applications of this material appear limited. A common limitation to photocatalysis with single semiconductors is the rapid recombination of photogenerated electron-hole pairs, which reduces significantly the efficiency of the process that in the case of ZnS also suffers from photocorrosion in the presence of air. In order to overcome the fast charge recombination and the limited visible-light absorption of semiconductor photocatalysts, an effective strategy is developed in this work by combining two semiconductors into a nanocomposite. This nanocomposite is solvothermally synthesized creating octahedral cuprous oxide covered with titanium dioxide nanoparticles (Cu2O/TiO2). The nanocomposite exhibits unique surface modifications that provide a heterojunction with a direct Z-scheme for optimal CO2 reduction. The band structure of the nanocomposite is characterized by diffused reflectance UV-visible spectroscopy, X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy. The photoreduction of CO2(g) to CO(g) on the nanocomposite is investigated in the presence water vapor as the hole scavenger that generates the quantifiable hydroxyl radical (). The quantum efficiency of CO production under irradiation at λ ≥ 305 nm with the nanocomposite is 2-times larger than for pure Cu2O. The detection of and XPS analysis contrasting the stability of Cu2O/TiO2 vs Cu2O during irradiation prove that TiO2 prevents the photocorrosion of Cu2O. Overall, the studies of photocatalytic reductions on single component semiconductors reveal new knowledge needed for developing future photocatalytic application for fuel production, wastewater treatment, reducing air pollution, and driving important prebiotic chemistry reactions. Furthermore, the design of a photocatalyst operating under a Z-scheme mechanism provides a new proof of concept for the design of systems that mimic photosynthesis. Finally, this work also demonstrates how molecules obtained by mineral mediated photochemistry can catalyze clay formation; highlighting the important role that photochemistry may have played for the origin of life on the early Earth and other rocky planets.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第16862号 / 工博第3583号 / 新制||工||1541(附属図書館) / 29537 / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 木本 恒暢, 教授 藤田 静雄, 准教授 浅野 卓 / 学位規則第4条第1項該当

Dans cette thèse, deux semi-conducteurs en séléniure ayant d'excellentes propriétés ont été étudiés afin de développer des matériaux performants pour des applications photoélectriques. Tout d'abord, les nanorodes de Sb2Se3 ont été synthétisés en utilisant une méthode d'injection à chaud, et le plus grand défi associé à la faible conductivité de Sb2Se3 a été relevé en formant des hétérojonctions et/ou par un dopage. Les nanorodes de Sb2Se3 à conductivité électrique nettement améliorée ont été utilisés pour fabriquer des photo-détecteurs prototypes, qui présentent un grand potentiel d'application grâce à leur grande efficacité. Le Sb2Se3 dopés au Sn a été préparé en utilisant un procédé de fusion à haute température. Avec l'augmentation de la concentration en Sn, les cristaux (SnxSb1-x)2Se3 présentent également une grande amélioration de la conductivité et des propriétés photoconductrices. Quatre cibles à base de Sb2Se3 avec la composition chimique de Sb2Se3, Sb2Se3.3, (Sn0.1Sb0.9)2Se3 et Sb2(Se0.9I0.1)3 ont été préparées et les couches minces ont été déposées en utilisant la pulvérisation cathodique. Une étude systématique de la cristallinité, de la morphologie de surface, des propriétés optiques, du type de conduction (p ou n) et des performances photo-électro-chimique des couches minces a été réalisée. Une nouvelle cellule solaire à couches minces de Sb2Se3 avec une quasi-homojonction a été fabriquée pour la première fois et le rendement de conversion atteint déjà un taux très intéressant de 2,65%. Une méthode efficace d'injection à chaud a également été développée pour la synthèse de nano-fleurs uniformes de γ-In2Se3. Une photodiode à hétérojonction formée en déposant une couche mince de nanoflower γ-In2Se3, du type p, sur un substrat en Si de type n, a été fabriquée pour la première fois. Il a été démontré que ce photo-détecteur peut être auto-alimenté avec d'excellentes performances, notamment une réponse rapide et une sensibilité à large bande. / In this dissertation, two different selenide semiconductors with excellent properties have been studied in order to develop high performance materials and devices for photoelectric applications. Firstly, Sb2Se3 nanorods were synthesized via hot-injection method, and the biggest challenge of low conductivity of Sb2Se3 nanorods has been overcome successfully by forming heterojunction and/or doping. The Sb2Se3 nanorods with enhanced electrical conductivity were used for fabricating prototype photodetectors, which show great application potential as highly efficient photodetectors. The Sn-doped Sb2Se3 crystals were successfully prepared by using high-temperature melting process. With increasing Sn doping concentration, the (SnxSb1-x)2Se3 crystals also exhibit a great improvement of conductivity and photoconductive properties. Four Sb2Se3-based targets with the chemical composition of Sb2Se3, Sb2Se3.3, (Sn0.1Sb0.9)2Se3 and Sb2(Se0.9I0.1)3 have been successfully prepared by using high-temperature melting technique. Then thin films have been deposited by using RF magnetron-assisted sputtering. A systematic investigation of the crystallinity, surface morphology, optical properties, p/n type and photo-electro-chemical performance of the thin films has been performed. A novel quasi-homojunction Sb2Se3 thin film solar cells was fabricated for the first time and the highest conversion efficiency obtained in our work reaches already a highly interesting 2.65%. An effective hot-injection method has also been developed for synthesizing uniform γ-In2Se3 nanoflowers. An efficient heterojunction photodiode formed by n-type Si substrate and p-type γ-In2Se3 nanoflower film was fabricated for the first time. It has been demonstrated that this photodetector can be self-powered with excellent performance including fast response and broadband sensibility.

abstract: The maximum theoretical efficiency of a terrestrial non-concentrated silicon solar cell is 29.4%, as obtained from detailed balance analysis. Over 90% of the current silicon photovoltaics market is based on solar cells with diffused junctions (Al-BSF, PERC, PERL, etc.), which are limited in performance by increased non-radiative recombination in the doped regions. This limitation can be overcome through the use of passivating contacts, which prevent recombination at the absorber interfaces while providing the selectivity to efficiently separate the charge carriers generated in the absorber. This thesis aims at developing an understanding of how the material properties of the contact affect device performance through simulations.The partial specific contact resistance framework developed by Onno et al. aims to link material behavior to device performance specifically at open circuit. In this thesis, the framework is expanded to other operating points of a device, leading to a model for calculating the partial contact resistances at any current flow. The error in calculating these resistances is irrelevant to device performance resulting in an error in calculating fill factor from resistances below 0.1% when the fill factors of the cell are above 70%, i.e., for cells with good passivation and selectivity. Further, silicon heterojunction (SHJ) and tunnel-oxide based solar cells are simulated in 1D finite-difference modeling package AFORS-HET. The effects of material property changes on device performance are investigated using novel contact materials like Al0.8Ga0.2As (hole contact for SHJ) and ITO (electron contact for tunnel-oxide cells). While changing the bandgap and electron affinity of the contact affect the height of the Schottky barrier and hence contact resistivity, increasing the doping of the contact will increase its selectivity. In the case of ITO, the contact needs to have a work function below 4.2 eV to be electron selective, which suggests that other low work function TCOs (like AZO) will be more applicable as alternative dopant-free electron contacts. The AFORS-HET model also shows that buried doped regions arising from boron diffusion in the absorber can damage passivation and decrease the open circuit voltage of the device. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2020

In today’s electronics, transistors are the main building blocks of the vast majority of electronic devices and integrated circuits. Types of transistors vary depending on the device structure and operation principle. Metal-oxide-based thin film transistors (MO TFTs), in particular, are an emerging technology that has a promising future in many applications, such as large-area display and wearable electronics. It exhibits unique features that make it superior to the existing Si-based technology, such as optical transparency and mechanical flexibility. However, some technical challenges in MO TFTs limit their emplyoment in today’s applications, such as low carrier mobility and high processing temperature. Solution-processed MO TFT based on spray pyrolysis combined with a carefully engineered TFT structure offers a dramatically enhance carrier mobility at low processing temperature. In this work, we are utilizing spray pyrolysis to grow In2O3 and ZnO based TFTs at low processing temperature. The structural effects of the channel layer on the electrical performance is investigated in two parts. The first part highlights the impact of thickness of the channel layer on the device performance of both In2O3 and ZnO, while the second part explores In2O3/ZnO heterojunction-based active layer. The results showed that increasing the channel thickness of both In2O3 and ZnO based TFTs enhanced the carrier mobility due to a reduced surface-roughness scattering effect. In addition, evidence showed that the electron transport mechanism in In2O3/ZnO heterojunction transitioned from trap-limited conduction (TLC) to percolation conduction (PC) process. Thanks to the existence of a 2D-confined electron sheet at the atomically sharp In2O3/ZnO heterointerface, the electron mobility was dramatically enhanced.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22072号 / 工博第4653号 / 新制||工||1725(附属図書館) / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 木本 恒暢, 教授 川上 養一, 准教授 杉山 和彦 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

In organic photovoltaics (OPVs) the key processes relevant to device operation such as exciton dissociation and free carriers recombination occur at the donor-acceptor (D-A) interface. OPV devices require the bulk heterojunction (BHJ) architecture to function efficiently. In these BHJs, D-A interfaces are arranged in three dimensions, which makes molecular arrangements at these interfaces ill defined and hard to characterize. In addition, molecular materials used in OPVs are inherently disordered and may exhibit variable degrees of structural order in the same BHJ. Yet, D-A molecular arrangements and structure are crucial because they shape the energy landscape and photovoltaic (PV) performance in OPVs. Studies that use well-defined model systems to look in details at the interfacial molecular structure in OPVs and link it to interfacial energy landscape and device operation are critically lacking. We have used in situ photoelectron spectroscopy and ex situ x-ray scattering to study D-A interfaces in tailored bilayers and BHJs based on small molecule donors. We show preferential miscibility at the D-A interface depending on molecular conformation in zinc phthalocyanine (ZnPc)/ C60 bilayers and we derive implications for exciton dissociation. Using sexithiophene (6T), a crystalline donor, we show that the energy landscape at the D-A interface varies markedly depending on the molecular composition of the BHJ. Both the ionization energies of sexithiophene and C60 shift by over ~0.4 eV while the energy of the charge transfer state shifts by ~0.5 eV depending on composition. Such shifts create a downward energy landscape that helps interfacial excitons to overcome their binding energies. Finally, we demonstrate that when both disordered and ordered phases of D coexist at the interface, low-lying energy states form in ordered phases and significantly limit the Voc in devices. Overall our work underlines the importance of the aggregation and conformation states of molecular materials at and near the D-A interface in determining the operation and performance of OPV devices. This work shows that the role of D-A interfaces in complex BHJ devices can be unraveled through careful experimental design and by in depth characterization of planar heterojunction bilayer devices recreating model interfaces.

Thin-film solar cells based on an absorber layer of chalcogenide compounds (CIGS, CdTe) are today among the most promising photovoltaic technologies due to their long-term ability to gain a foothold in mass commercial production as an alternative to conventional Si solar cells. Despite this success, the physical origin of the defects present in the thin films are still insufficiently elucidated, especially in the compounds of the chalcopyrite family Cu(In_{1x},Ga_{x})(S_{y},Se_{1y})_{2}. The research focuses on the identification and analysis of microstructural defects responsible for the electrical instability of chalcopyrite-based thin-film solar cells with a typical heterostructure arrangement ZnO:Al/i-ZnO/CdS/Cu(In,Ga)Se_{2}/Mo. The non-uniform polycrystalline nature of semiconductor materials in this complex multilayer structure requires a comprehensive analysis of electro-optical, structural and compositional properties associated with the actual morphology at the macroscopic, microscopic or even nanoscopic level. The observed predominant ohmic or non-ohmic current conduction in the dark transport characteristics was also reflected in the slope deviations of the excessive noise fluctuations, which were in the spectral domain exclusively in the form of flicker noise with dependency S_{i} ~ f^{1}. Spatially resolved electroluminescence based on stimulated photon emission by charge carriers injecting into the depletion region, not only showed a significantly inhomogeneous distribution of intensity in planar heterojunction under forward bias, but also revealed light emitting local spots in reverse bias due to a trap-assisted radiative recombination through the high density of defect states. Microscopic examination of the defect-related light emitting spots revealed rather extensive defective complexes with many interruptions through the layers, especially at the heterojunction CdS/Cu(In,Ga)Se_{2} interface. Besides, the high leakage current via these defective complexes subsequently led to a considerable local overheating, which caused a clearly observable structural and morphological changes, such as deviations in absorber layer stoichiometry due to Cu–In–Ga–Se segregation, Cu-rich and Ga-rich grains formation with an occurrence of Se-poor or Cu_{x}Se_{y} secondary phases regions, material redeposition accompanied by evaporation of ZnO:Al/i-ZnO/CdS layers together with the formation of Se structures on the surface around the defects. Within the research, analytical modelling of transport characteristics was implemented with parameters extraction of individual transport mechanisms to understand the non-ohmic shunt behaviour due to leakage current. In addition to the proper current path along the main heterojunction, the proposed model contains parasitic current pathways as a consequence of recombination-dominated charge transport or current conduction facilitated by multi-step tunnelling via high density of mid-gap defect states in the depletion region, ohmic leakage current caused by pinholes or low-resistance paths along grain boundaries in Cu(In,Ga)Se_{2}, or space-charge limited current due to metals diffusion from the ZnO:Al layer and grid Ag contacts through disruptions in i-ZnO/CdS layers.

No description available.