353 research outputs found
X-ray photoelectron spectroscopy for understanding molecular and hybrid solar cells
No full textX-ray photoelectron spectroscopy is a powerful tool for the characterization of molecular and hybrid solar cells. This technique allows for atomic-level characterization of their components as well as for the determination of the electronic structure that governs the key conversion processes. In this chapter, we introduce the basic concepts of electronic structure in molecules and semiconducting materials followed by a description of the concepts of photoelectron spectroscopy and how they relate to electronic structure. Finally, we give examples of the application of photoelectron spectroscopy to different types of molecular and hybrid solar cell materials demonstrating the type of information that can be obtained, to gain fundamental understanding and to further develop such devices
Defect-Induced Water Bilayer Growth on Anatase TiO2(101)
No full textPreparing an anatase TiO2(101) surface with a high density of oxygen vacancies and associated reduced Ti species in the near-surface region results in drastic changes in the water adsorption chemistry compared to adsorption on a highly stoichiometric surface. Using synchrotron radiation excited photoelectron spectroscopy, we observe a change in the water growth mode, from layer-by-layer growth on the highly stoichiometric surface to bilayer growth on the reduced surface. Furthermore, we have been able to observe Ti3+ enrichment at the surface upon water adsorption. The Ti3+ enrichment occurs concomitant with effective water dissociation into hydroxyls with a very high thermal stability. The water bilayer on the reduced surface is thermally more stable than that on the stoichiometric surface, and it is more efficient in promoting further water dissociation upon heating. The results thus show how the presence of subsurface defects can alter the wetting mechanism of an oxide surface
Probing the dye–semiconductor interface in dye-sensitized NiO solar cells
No full textThe development of p-type dye-sensitized solar cells (p-DSSCs) offers an opportunity to assemble tandem photoelectrochemical solar cells with higher efficiencies than TiO2-based photoanodes, pioneered by O'Regan and Gratzel [Nature 353, 737-740 (1991)]. This paper describes an investigation into the behavior at the interfaces in p-DSSCs, using a series of BODIPY dyes, BOD1-3. The three dyes have different structural and electronic properties, which lead to different performances in p-DSSCs. We have applied photoelectron spectroscopy and transient absorption spectroscopy to rationalize these differences. The results show that the electronic orbitals of the dyes are appropriately aligned with the valence band of the NiO semiconductor to promote light-induced charge transfer, but charge-recombination is too fast for efficient dye regeneration by the electrolyte. We attribute this fast recombination, which limits the efficiency of the solar cells, to the electronic structure of the dye and the presence of Ni3+ recombination sites at the NiO surface
Light-driven transformation processes of anisotropic silver nanoparticles
No full textThe photoinduced formation of silver nanoprisms from smaller silver seed particles in the presence of citrate anions is a classic example of a photomorphic reaction. In this case, light is used as a convenient tool to dynamically manipulate the shape of metal nanoparticles. To date, very little is known about the prevailing reaction mechanism of this type of photoreaction. Here we provide a detailed study of the shape transformation dynamics as a function of a range of different process parameters, such as photon energy and photon flux. For the first time, we provide direct evidence that the photochemical synthesis of silver nanoprisms from spherical seed nanoparticles proceeds via a light-activated two-dimensional coalescence mechanism. On the other hand, we could show that Ostwald ripening becomes the dominant reaction mechanism when larger silver nanoprisms are grown from photochemically synthesized smaller nanoprisms. This two-step reaction proceeds significantly faster and yields more uniform, sharper nanoprisms than the classical one-step photodevelopment process from seeds. The ability to dynamically control nanoparticle shapes and properties with light opens up novel synthesis avenues but also, more importantly, allows one to conceive new applications that exploit the nonstatic character of these nanoparticles and the ability to control and adjust their properties at will in a highly dynamic fashion.George P. Lee, Yichao Shi, Ellen Lavoie, Torben Daeneke, Philipp Reineck, Ute B. Cappel, David M. Huang and Udo Bac
Partially Reversible Photoinduced Chemical Changes in a Mixed Ion Perovskite Material for Solar Cells
No full textMetal halide perovskites have emerged as materials of high interest for solar energy-to-electricity conversion, and in particular, the use of mixed-ion structures has led to high power conversion efficiencies and improved stability. For this reason, it is important to develop means to obtain atomic level understanding of the photoinduced behavior of these materials including processes such as photoinduced phase separation and ion migration. In this paper, we implement a new methodology combining visible laser illumination of a mixed-ion perovskite ((FAP-bI(3))(0.85)(MAPbBr(3))(0.15)) with the element specificity and chemical sensitivity of core-level photoelectron spectroscopy. By carrying out measurements at a synchrotron beamline optimized for low X-ray fluxes, we are able to avoid sample changes due to X-ray illumination and are therefore able to monitor what sample changes are induced by visible illumination only. We find that laser illumination causes partially reversible chemistry in the surface region, including enrichment of bromide at the surface, which could be related to a phase separation into bromide- and iodide-rich phases. We also observe a partially reversible formation of metallic lead in the perovskite structure. These processes occur on the time scale of minutes during illumination. The presented methodology has a large potential for understanding light-induced chemistry in photoactive materials and could specifically be extended to systematically study the impact of morphology and composition on the photostability of metal halide perovskites
First layer water phases on anatase TiO2(101)
No full textThe anatase TiO2(101) surface and its interaction with water is an important topic in oxide surface chemistry. Firstly, it benchmarks the properties of the majority facet of TiO2 nanoparticles and, secondly, there is a controversy as to whether the water molecule adsorbs intact or deprotonates. We have addressed the adsorption of water on anatase TiO2(101) by synchrotron radiation photoelectron spectroscopy. Three two-dimensional water structures are found during growth at different temperatures: at 100 K, a metastable structure forms with no hydrogen bonding between the water molecules. In accord with prior literature, we assign this phase to chains of disordered molecules. Growth 160 K results in a metastable structure with expressed hydrogen bonding between the water molecules. At 190 K, the water molecules become disordered as the thermal energy is too high and hence the hydrogen bonds break. The result is a structure with isolated monomers. Partial dissociation is observed for all three growths, with the molecular state only slightly favored in energy (20–40 meV) over the dissociated state. Heating of a thick film leads to more dissociation compared to a bilayer, when formed at 100 K. Thus, extending the water network facilitates proton transport and hence dissociation. The results reconcile apparent conflicting experimental results previously obtained by scanning tunneling microscopy (STM) and core level photoelectron spectroscopy
Oxygen-Induced Doping of Spiro-MeOTAD in Solid-State Dye-Sensitized Solar Cells and Its Impact on Device Performance
No full textSolid state dye-sensitized solar cells (sDSCs) employing the hole conductor 2,2'7,7'-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene (spiro-MeOTAD) require the presence of oxygen during fabrication and storage. In this paper, we determine the concentrations of oxidized spiro-MeOTAD within devices under different operating and storage conditions by UV-vis spectroscopy. Relative concentrations of spiro-MeOTAD(+) were found to be greater than 10% after illumination for standard sDSCs, where no chemical dopant had been used in the solar cell fabrication but oxygen and lithium ions were present. We suggest that oxidized spiro-MeOTAD is created as a byproduct of oxygen reduction at the TiO(2) surface during cell illumination. Furthermore, we studied the effect of light soaking under different conditions and associated changes in spiro-MeOTAD(+) concentration on the solar cell measurements. Our findings give insights to photochemical reactions occurring within sDSCs and provide guidelines for which doping levels should be used in device fabrication in absence of oxygen.</p
A speech understanding and dialog system with a homogeneous linguistic knowledge base
No full textMast M, Kummert F, Ehrlich U, et al. A speech understanding and dialog system with a homogeneous linguistic knowledge base. IEEE transactions on pattern analysis and machine intelligence. 1994;16(2):179-194.This article presents the speech understanding and dialog system EVAR. All levels of linguistic knowledge are used both to control the analysis process and for the interpretation of an utterance. All kinds of knowledge are integrated in a homogeneous knowledge base. The control algorithm used for the analysis is defined within the representation scheme and does not depend on the application. One of the aims of EVAR is to develop a system structure where linguistic and non-linguistic expectations could be used not only for the interpretation but also as predictions for the recognition process
Cutting'aesthetic teeth' : Flannery O'Connor's habit of art
No full textDissertação (mestrado) - Universidade Federal de Santa Catarina, Centro de Comunicação e ExpressãoEste trabalho foi sugerido pela afirmação de Flannery O'Connor que sua "dedicação estética" nasceu através do contato com Art and Scholasticism de Jacques Maritain. O propósito foi chegar a uma interpretação do sentido da frase. Uma investigação detalhada foi feita do conteúdo de Art and Scholasticism, posteriormente contrastada com os resultados de uma pesquisa feita em seus ensaios e suas cartas, o que revelou numerosos ecos de diversos trechos constando no texto de Maritain. Três pontos principais foram escolhidos como critérios na análise do hábito artístico de O'Connor: 1) a prática de arte implica uma luta; 2) a arte somente pode ser percebida pelos sentidos; e 3) a prática de arte exige do artista a dedicação indivisa à obra nascente. O estudo conclui que, para O'Connor, o brotar da dentição estética, através da leitura de Art and Scholasticism, significou que, ao perceber na análise da natureza da arte algo com que podia concordar, ela reconheceu tanto sua própria capacidade de tornar-se uma artista literária, quanto sua vontade de assumir a tarefa de desenvolver em sua pessoa o hábito de arte
Effect of different halide-based ligands on the passivation and charge carrier dynamics in AgBiS2 nanocrystal solar cells
No full textAgBiS2 nanocrystals have been shown to be a promising material for solar cell applications due to their high absorption coefficient, solution-processability and stability. However, detailed and systematic insight into how different surface passivation agents affect the overall material properties and corresponding device performance is still limited. Herein, a study about AgBiS2 nanocrystals treated with five different halide-based compounds - TBAI, TMAI, TBABr, TMABr and TMACl - is presented, with the nanocrystals themselves being synthesised via a newly adapted route under atmospheric conditions. For the differently passivated samples, variation in the ligand uptake, as well as shifts in the position of the valence and conduction bands could be observed. Incorporating these ligand-treated thin films into solar cell devices allowed for further investigation of their overall performance as well as into their respective charge carrier dynamics. Markedly longer charge carrier lifetimes were observed for the bromide- and chloride-passivated samples through transient photovoltage and photocurrent measurements as well as impedance spectroscopy. The effect of the surface modification on the charge carrier transport behaviour, on the other hand, was found to be less pronounced. Overall, this work demonstrates the importance of better understanding how different ligands affect nanocrystal properties, showcasing how it influences a wide variety of parameters controlling final device performance
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