Dye-sensitized solar cells based on perylene derivatives

The oil price rises more and more, and the world energy consumption is projected to expand by 50 percent from 2005 to 2030. Nowadays intensive research is focused on the development of alternative energies. Among them, there are dye-sensitized nanocrystalline solar cells (DSSCs) “the third generation solar cells”. The latter have gained attention during the last decade and are currently subject of intense research in the framework of renewable energies as a low-cost photovoltaic. At present DSSCs with ruthenium based dyes exhibit highest efficiencies (ca 11%). The objective of the present work is to fabricate, characterize and improve the performance of DSSCs based on metal free dyes as sensitizers, especially on perylene derivatives. The work begins by a general introduction to the photovoltaics and dye-sensitized solar cells, such as the operating principles and the characteristics of the DSSCs. Chapter 2 and 3 discuss the state of the art of sensitizers used in DSSCs, present the compounds used as sensitizer in the present work and illustrate practical issues of experimental techniques and device preparation. A comparative study of electrolyte-DSSCs based on P1, P4, P7, P8, P9, and P10 are presented in chapter 4. Experimental results show that the dye structure plays a crucial role in the performance of the devices. The dye based on the spiro-concept (bipolar spiro compound) exhibited a higher efficiency than the non-spiro compounds. The presence of tert-butylpyridine as additive in the electrolyte was found to increase the open circuit voltage and simultaneously decrease the efficiency. The presence of lithium ions in the electrolyte increases both output current and the efficiency. The sensitivity of the dye to cations contained in the electrolyte was investigated in the chapter 5. FT-IR and UV-Vis were used to investigate the in-situ coordination of the cation to the adsorbed dye in the working devices. The open-circuit voltage was found to depend on the number of coordination sites in the dye. P1 with most coordination sites has shown the lowest potential drop, opposite to P7, which is less sensitive to cations in the working cells. A strategy to improve the dye adsorption onto the TiO2 surface, and thus the light harvesting efficiency of the photoanode by UV treatment, is presented in chapter 6. The treatment of the TiO2 film with UV light generates hydroxyl groups and renders the TiO2 surface more and more hydrophilic. The treated TiO2 surface reacts readily with the acid anhydride group of the dye that acts as an anchoring group and improves the dye adsorption. The short-circuit current density and the efficiency of the electrolyte-based dye cells was considerably improved by the UV treatment of the TiO2 film. Solid-state dye-sensitized solar cells (SSDs) based on spiro-MeOTAD (used as hole transport material) are studied in chapter 7. The efficiency of SSDs was globally found to be lower than that of electrolyte-based solar cells. That was due to poor pore filling of the dye-loaded TiO2 film by the spin-coated spiro-MeOTAD and to the significantly slower charge transport in the spiro-MeOTAD compared to the electrolyte redox mediator. However, the presence of the donor moieties in P1 that are structurally similar to spiro-MeOTAD was found to improve the wettability of the P1-loaded TiO2 film. As a consequence the performance of the P1-based solid-state cells is better compared to the cells based on non-spiro compounds.

Role of Rab5 in synaptic vesicle recycling

During synaptic transmission, NT-filled synaptic vesicles are released by Ca2+-triggered exocytosis at the active zone. Following exocytosis, SV membrane is immediately re-internalized and synaptic vesicles (SVs) are regenerated by a local recycling mechanism within the presynaptic terminal. It is debated whether an endosomal compartment is involved in this recycling process. In contrast, it is well known from cultured mammalian cells, that endocytic vesicles fuse to the early sorting endosome. The early endosome is a major sorting station of the cell where cargo is send into the degradative pathway to late endosome and lysosome or towards recycling. Each trafficking step is mediated by a certain protein of the Rab family. Rab proteins are small GTPases belonging to the Ras superfamily. They accumulate at their target compartments and have thereby been used as markers for the different endocytic organelles in cultured mammalian cells. Rab5 controls trafficking from the PM to the early endosome and has thereby been used as marker for this compartment. A second marker is based on the specific binding of the FYVE zinc finger protein domain to the lipid PI(3)P that is specifically generated at the early endosomal membrane. This study used the Drosophila NMJ as a model system to investigate the SV recycling process. In particular, three questions were addressed: First, is an endosomal compartment present at the synapse? Second, do SVs recycle through an endosome? Third, is Rab5 involved in SV recycling? We used GFP fusions of Rab5 and 2xFYVE to visualize endosomal compartments at the presynaptic terminal of Drosophila third instar larval NMJs. Furthermore, the endosomes are located within the pool of recycling SVs, labeled with the styryl-dye FM5-95. Using the temperature-sensitive mutation in Dynamin, shibirets, we showed that SV recycling involves trafficking through an intermediate endosomal compartment. In cultured mammalian cells, interfering with Rab5 function by expressing the dominant negative version, Rab5SN causes the fragmentation of the endosome and the accumulation of endocytic vesicles. In contrast, when Rab5 is overexpressed enlarged endosomal compartments were observed. In Drosophila, the endosomal compartment was disrupted when loss of function and dominant negative mutants of Rab5 were expressed. In addition, at the ultrastructural we observed an accumulation of endocytic vesicles in Rab5S43N expressing terminals and enlarged endosomes when Rab5 was overexpressed. Furthermore, interfering with Rab5 function using the dominant negative Rab5S43N caused a decrease in the SV recycling kinetics as shown by FM1-43 experiments. In contrast, overexpression of Rab5 or GFP-Rab5 caused an increase in the FM1-43 internalization rate. Finally, standard electrophysiological techniques were used to measure synaptic function. We found that the Rab5-mediated endosomal SV recycling pathway generates vesicles with a higher fusion efficacy during Ca2+-triggered release, compared to SVs recycled when Rab5 function was impaired. We therefore suggest a model in which the endosome serves as organelle to control the SV fusion efficacy and thereby the synaptic strength. Since changes in the synaptic strength are occuring during learning and memory processes, controlling endosomal SV recycling might be a new molecular mechanism involved in learning and memory.

Quadrupole moments of radium isotopes from the 7p^2P_3/2 hyperfine structure in Ra II

The hyperfine structure and isotope shift of ^{221- 226}Ra and ^{212, 214}Ra have been measured in the ionic (Ra 11) transition 7s^2 S_{1/2} - 7p ^2 P_{3/2} (\lamda = 381.4 nm). The method of on-line collinear fast-beam laser spectroscopy has been applied using frequency-doubling of cw dye laser radiation in an external ring cavity. The magnetic hyperfine fields are compared with semi-empirical and ab initio calculations. The analysis of the quadrupole splitting by the same method yields the following, improved values of spectroscopic quadrupole moments: Q_s(^221 Ra)= 1.978(7)b, Q_s (^223 Ra)= 1.254(3)b and the reanalyzed values Q_s(^209 Ra) = 0.40(2)b, Q_s(^211 Ra) = 0.48(2)b, Q_s(^227 Ra)= 1.58(3)b, Q_s (^229 Ra) = 3.09(4)b with an additional scaling uncertainty of ±5%. Furthermore, the J-dependence of the isotope shift is analyzed in both Ra II transitions connecting the 7s^2 S_{1/2} ground state with the first excited doublet 7p^ P_{1/2} and 7p^ P_{3/2}.

Femtosecond time-resolved wave packet motion in molecular multiphoton ionization and fragmentation

The dynamics of molecular multiphoton ionization and fragmentation of a diatomic molecule (Na_2) have been studied in molecular beam experiments. Femtosecond laser pulses from an amplified colliding-pulse mode-locked (CPM) ring dye laser are employed to induce and probe the molecular transitions. The final continuum states are analyzed by photoelectron spectroscopy, by ion mass spectrometry and by measuring the kinetic energy of the formed ionic fragments. Pump-probe spectra employing 70-fs laser pulses have been measured to study the time dependence of molecular multiphoton ionization and fragmentation. The oscillatory structure of the transient spectra showing the dynamics on the femtosecond time scale can best be understood in terms of the motion of wave packets in bound molecular potentials. The transient Na_2^+ ionization and the transient Na^+ fragmentation spectra show that contributions from direct photoionization of a singly excited electronic state and from excitation and autoionization of a bound doubly excited molecular state determine the time evolution of molecular multiphoton ionization.

Molecular photochemistry with femtosecond laser pulses

Femtosecond laser pulses generated from an amplified coiliding pulse modelocked ring dye laser have been employed in molecular beam experiments to study the dynamics and the pathways of multiphoton induced ionization, autoionization and fragmentation of Na2 . Energy distributions of photoelectrons arising from these processes and the mass and released kinetic energy of the corresponding fragment ions are measured by time-of-flight spectroscopy.

Biological Photonic Crystals: Diatoms

The aim of this work was to produce a variety of fluorescent diatom cell wall material as a basis for spectroscopic investigations of the influence of the photonic structure on the emission of an incorporated laser dye. This goal was achieved by the method of in vivo-fluorochromation, in which the fluorescence dyes are incorporated by the diatom cells during cell wall formation. Several fluorescent dyes (mostly rhodamines) known as strong laser dyes, were tested for a possible application within this method. The results of this work show that half of the tested rhodamines can be applied for an in vivo-fluorochromation of diatom cells. For a successful incorporation into the diatom cell wall, a relatively low toxicity to diatom cells is necessary. Replacement of the carbon acid function at the carboxyphenyl ring of the rhodamine by a methyl or ethylester function showed to convert a rhodamine of relatively low toxicity to a rhodamine leading to severe lethal effects within the cells. In contrast to their carbon acid forms, which posses a net neutral charge of the molecule, rhodamine esters exhibit a net positive charge. The enhanced toxicological effects seem to be due to an increased accumulation of positive charged rhodamines within the mitochondria, an increased hydrophobicity due to the attachment of an alkyl substituent, an increased retention time of the dyes within the mitochondria and a therefore stronger negative effect on the mitochondrial membrane bound energy processes of the diatom cell. Therefore rhodamines with a positive net charge deriving from a methyl or ethylester function at the carboxy phenyl ring instead of a carbon acid substituent showed not to be suitable for long-term investigations/ biomineralization studies of diatoms. Investigations performed on diatom species of different orders showed that rhodamine 19, rhodamine B, and rhodamine 101 can presumably be successfully applied for in vivo-fluorochromation to all diatom species. The results obtained here can help to find further laser dyes for an in vivo-fluorochromation of diatom cells and therefore for the production of fluorescent nanostructural elements for a detailed optical investigation of the diatom cell wall. First optical measurements performed on in vivo-fluorochromated cell walls did not give any hints concerning the photonic structure of the diatom cell. Cell wall parts with different nanostructural elements were investigated and by comparison of the obtained fluorescence emission spectra, no special features that might derive from photonic structural effects could be observed. Results concerning the concentration dependent shifts within the emission spectra, as well as the decrease of fluorescence intensity of the stained cell wall structures with increasing dye concentration, depict that several effects occurring by interaction of the molecules within the cell wall can have an impact on the technical application of fluorescent cell walls. It can be assumed that the investigation of the photonic crystal behaviour and the possibility to achieve laser action within the diatom cell wall can be hampered by molecular interactions. The results give hints to prevent such obstacles. Comparison of the recent findings and state of the art of in vivo-fluorochromation of diatom cell wall material, make clear that the here presented results are of importance and can offer a considerable contribution to the development and establishment of new biosilification markers, for diatoms as well as for other biosilifying organisms.

Development of hybrid UV VCSEL with organic active material and dielectric DBR mirrors for medical, sensoric and data storage applications

Lasers play an important role for medical, sensoric and data storage devices. This thesis is focused on design, technology development, fabrication and characterization of hybrid ultraviolet Vertical-Cavity Surface-Emitting Lasers (UV VCSEL) with organic laser-active material and inorganic distributed Bragg reflectors (DBR). Multilayer structures with different layer thicknesses, refractive indices and absorption coefficients of the inorganic materials were studied using theoretical model calculations. During the simulations the structure parameters such as materials and thicknesses have been varied. This procedure was repeated several times during the design optimization process including also the feedback from technology and characterization. Two types of VCSEL devices were investigated. The first is an index coupled structure consisting of bottom and top DBR dielectric mirrors. In the space in between them is the cavity, which includes active region and defines the spectral gain profile. In this configuration the maximum electrical field is concentrated in the cavity and can destroy the chemical structure of the active material. The second type of laser is a so called complex coupled VCSEL. In this structure the active material is placed not only in the cavity but also in parts of the DBR structure. The simulations show that such a distribution of the active material reduces the required pumping power for reaching lasing threshold. High efficiency is achieved by substituting the dielectric material with high refractive index for the periods closer to the cavity. The inorganic materials for the DBR mirrors have been deposited by Plasma- Enhanced Chemical Vapor Deposition (PECVD) and Dual Ion Beam Sputtering (DIBS) machines. Extended optimizations of the technological processes have been performed. All the processes are carried out in a clean room Class 1 and Class 10000. The optical properties and the thicknesses of the layers are measured in-situ by spectroscopic ellipsometry and spectroscopic reflectometry. The surface roughness is analyzed by atomic force microscopy (AFM) and images of the devices are taken with scanning electron microscope (SEM). The silicon dioxide (SiO2) and silicon nitride (Si3N4) layers deposited by the PECVD machine show defects of the material structure and have higher absorption in the ultra violet range compared to ion beam deposition (IBD). This results in low reflectivity of the DBR mirrors and also reduces the optical properties of the VCSEL devices. However PECVD has the advantage that the stress in the layers can be tuned and compensated, in contrast to IBD at the moment. A sputtering machine Ionsys 1000 produced by Roth&Rau company, is used for the deposition of silicon dioxide (SiO2), silicon nitride (Si3N4), aluminum oxide (Al2O3) and zirconium dioxide (ZrO2). The chamber is equipped with main (sputter) and assisted ion sources. The dielectric materials were optimized by introducing additional oxygen and nitrogen into the chamber. DBR mirrors with different material combinations were deposited. The measured optical properties of the fabricated multilayer structures show an excellent agreement with the results of theoretical model calculations. The layers deposited by puttering show high compressive stress. As an active region a novel organic material with spiro-linked molecules is used. Two different materials have been evaporated by utilizing a dye evaporation machine in the clean room of the department Makromolekulare Chemie und Molekulare Materialien (mmCmm). The Spiro-Octopus-1 organic material has a maximum emission at the wavelength λemission = 395 nm and the Spiro-Pphenal has a maximum emission at the wavelength λemission = 418 nm. Both of them have high refractive index and can be combined with low refractive index materials like silicon dioxide (SiO2). The sputtering method shows excellent optical quality of the deposited materials and high reflection of the multilayer structures. The bottom DBR mirrors for all VCSEL devices were deposited by the DIBS machine, whereas the top DBR mirror deposited either by PECVD or by combination of PECVD and DIBS. The fabricated VCSEL structures were optically pumped by nitrogen laser at wavelength λpumping = 337 nm. The emission was measured by spectrometer. A radiation of the VCSEL structure at wavelength 392 nm and 420 nm is observed.