58 research outputs found
a trimolecular composition
Photochemical upconversion based on tripletâtriplet annihilation (TTA-UC) is
employed to enhance the short-circuit currents generated by two varieties of
thin-film solar cells, a hydrogenated amorphous silicon (a-Si:H) solar cell
and a dye-sensitized solar cell (DSC). TTA-UC is exploited to harvest
transmitted sub-bandgap photons, combine their energies and re-radiate
upconverted photons back towards the solar cells. In the present study we
employ a dual-emitter TTA-UC system which allows for significantly improved UC
quantum yields as compared to the previously used single-emitter TTA systems.
In doing so we achieve record photo-current enhancement values for both the
a-Si:H device and the DSC, surpassing 10â3 mA cmâ2 sunâ2 for the first time
for a TTA-UC system and marking a record for upconversion-enhanced solar cells
in general. We discuss pertinent challenges of the TTA-UC technology which
need to be addressed in order to achieve its viable device application
Increased upconversion performance for thin film solar cells: A trimolecular composition
Photochemical upconversion based on triplet-triplet annihilation (TTA-UC) is employed to enhance the short-circuit currents generated by two varieties of thin-film solar cells, a hydrogenated amorphous silicon (a-Si:H) solar cell and a dye-sensitized solar cell (DSC). TTA-UC is exploited to harvest transmitted sub-bandgap photons, combine their energies and re-radiate upconverted photons back towards the solar cells. In the present study we employ a dual-emitter TTA-UC system which allows for significantly improved UC quantum yields as compared to the previously used single-emitter TTA systems. In doing so we achieve record photo-current enhancement values for both the a-Si:H device and the DSC, surpassing 10-3 mA cm-2 sun-2 for the first time for a TTA-UC system and marking a record for upconversion-enhanced solar cells in general. We discuss pertinent challenges of the TTA-UC technology which need to be addressed in order to achieve its viable device application
Improving the light-harvesting of amorphous silicon solar cells with photochemical upconversion
Single-threshold solar cells are fundamentally limited by their ability to
harvest only those photons above a certain energy. Harvesting below-threshold
photons and re-radiating this energy at a shorter wavelength would thus boost
the efficiency of such devices. We report an increase in light harvesting
efficiency of a hydrogenated amorphous silicon (a-Si:H) thin-film solar cell
due to a rear upconvertor based on sensitized tripletâtriplet-annihilation in
organic molecules. Low energy light in the range 600â750 nm is converted to
550â600 nm light due to the incoherent photochemical process. A peak
efficiency enhancement of (1.0 ± 0.2)% at 720 nm is measured under irradiation
equivalent to (48 ± 3) suns (AM1.5). We discuss the pathways to be explored in
adapting photochemical UC for application in various single threshold devices
Improving the light-harvesting of second generation solar cells with photochemical upconversion
Photovoltaics (PV) offer a solution for the development of sustainable energy
sources, relying on the sheer abundance of sunlight: More sunlight falls on
the Earthâs surface in one hour than is required by its inhabitants in a year.
However, it is imperative to manage the wide distribution of photon energies
available in order to generate more cost efficient PV devices because single
threshold PV devices are fundamentally limited to a maximum conversion
efficiency, the Shockley-Queisser (SQ) limit. Recent progress has enabled the
production of c-Si cells with efficiencies as high as 25%,1 close to the
limiting efficiency of âŒ30%. But these cells are rather expensive, and
ultimately the cost of energy is determined by the ratio of system cost and
efficiency of the PV device. A strategy to radically decrease this ratio is to
circumvent the SQ limit in cheaper, second generation PV devices. One
promising approach is the use of hydrogenated amorphous silicon (a-Si:H),
where film thicknesses on the order of several 100nm are sufficient.
Unfortunately, the optical threshold of a-Si:H is rather high (1.7-1.8 eV) and
the material suffers from light-induced degradation. Thinner absorber layers
in a-Si:H devices are generally more stable than thicker films due to the
better charge carrier extraction, but at the expense of reduced conversion
efficiencies, especially in the red part of the solar spectrum (absorption
losses). Hence for higher bandgap materials, which includes a-Si as well as
organic and dye-sensitized cells, the major loss mechanism is the inability to
harvest low energy photons
Effect of a back reflector
Photochemical upconversion is applied to a hydrogenated amorphous silicon
solar cell in the presence of a back-scattering layer. A custom-synthesized
porphyrin was utilized as the sensitizer species, with rubrene as the emitter.
Under a bias of 24 suns, a peak external quantum efficiency (EQE) enhancement
of ~2 % was observed at a wavelength of 720 nm. Without the scattering layer,
the EQE enhancement was half this value, indicating that the effect of the
back-scatterer is to double the efficacy of the upconverting device. The
results represent an upconversion figure of merit of 3.5 Ă 10â4 mA cmâ2 sunâ2,
which is the highest reported to date
Measuring non-radiative relaxation time of fluorophores with biomedical applications by intensity-modulated laser-induced photoacoustic effect
Modulated tone-burst light was employed to measure non-radiative relaxation time of fluorophores with biomedical importance through photoacoustic effect. Non-radiative relaxation time was estimated through the frequency dependence of photoacoustic signal amplitude. Experiments were performed on solutions of new indocyanine green (IR-820), which is a near infrared dye and has biomedical applications, in two different solvents (water and dimethyl sulfoxide (DMSO)). A 1.5 times slower non-radiative relaxation for the solution of dye in DMSO was observed comparing with the aqueous solution. This result agrees well with general finding that non-radiative relaxation of molecules in triplet state depends on viscosity of solvents in which they are dissolved. Measurements of the non-radiative relaxation time can be used as a new source of contrast mechanism in photoacoustic imaging technique. The proposed method has potential applications such as imaging tissue oxygenation and mapping of other chemophysical differences in microenvironment of exogenous biomarkers
EinzelmolekĂŒlspektroskopische und quantenchemische Untersuchungen zum elektronischen Energietransfer
In der vorliegenden Arbeit wurden Untersuchungen zum Mechanismus, der Dynamik und der Kontrolle des elektronischen Energietransfers in multichromophoren Modellsystemen durchgefĂŒhrt. Als Untersuchungsmethoden wurden hauptsĂ€chlich die konfokale EinzelmolekĂŒlspektroskopie und die Quantenchemie eingesetzt. Der Aufbau des EinzelmolekĂŒlmikroskops wurde bezĂŒglich der Anregungs- und Detektionskomponenten variiert, um die unterschiedlichen Experimente durchzufĂŒhren. Die quantenchemischen Rechnungen wurden auf Dichtefunktional- und Coupled-Cluster-Niveau durchgefĂŒhrt. Die aus den Rechnungen erhaltenen zusĂ€tzlichen
Informationen ĂŒber experimentell zum Teil schwer zugĂ€ngliche Eigenschaften der Farbstoffe unterstĂŒtzten die Interpretation der experimentellen Befunde. rnIn frĂŒheren Untersuchungen der AG BaschĂ© wurden die Energietransfer-Raten von Donor-Akzeptor-Systemen gemessen, die erhebliche Abweichungen von nach der Förster-Theorie berechneten Raten zeigten. Daher war ein Ziel der vorliegenden Arbeit, diese Abweichungen zu erklĂ€ren. Zu diesem Zweck wurde die Geometrie der Diaden experimentell untersucht, sowie die elektronische Kopplung zwischen den Chromophoren quantenchemisch berechnet. Die relative Orientierung der Chromophore in den Diaden wurde in einem EinzelmolekĂŒl-Experiment mit rotierender Anregungspolarisation abgefragt. Die erhaltenen Winkelverteilungen konnten schlieĂlich eindeutig auf die FlexibilitĂ€t der die Chromophore verbrĂŒckenden Oligophenyl-Einheiten zurĂŒckgefĂŒhrt werden. Die Unterschiede der gemessenen Energietransfer-Raten zu den nach der Förster-Theorie ermittelten Werten konnten jedoch nicht ĂŒber
die molekulare FlexibilitĂ€t der Systeme erklĂ€rt werden. AufklĂ€rung ĂŒber die Diskrepanzen zur Förster-Theorie ergaben die quantenchemischen Rechnungen. In Rahmen dieser Arbeit wurde zum ersten Mal die Coupled-Cluster-Theorie zur Berechnung der elektronischen Kopplung eingesetzt. Die Betrachtung der isolierten Chromophore reichte aber nicht aus, um die gemessenen Abweichungen von der Förster-Theorie zu erklĂ€ren. Erst ĂŒber die BerĂŒcksichtigung der molekularen BrĂŒcke konnten die gefunden Abweichungen erklĂ€rt werden. Die deutliche VerstĂ€rkung der elektronischen Kopplung ist auf die Polarisierbarkeit der BrĂŒcke zurĂŒckzufĂŒhren.rnNach diesen Betrachtungen stand die Kontrolle des Energietransfers im Fokus der weiteren Untersuchungen. In den durchgefĂŒhrten EinzelmolekĂŒlexperimenten wurden die Chromophore der Donor-Akzeptor-Systeme selektiv mit zwei Laserpulsen unterschiedlicher WellenlĂ€nge angeregt. Beim gleichzeitigen Anregen beider Chromophore wurde Singulett-Singulett-Annihilation (SSA) induziert, ein
Energietransferprozess, bei dem die Anregungsenergie vom vorigen Akzeptor zum vorigen Donor ĂŒbertragen wird. Da ĂŒber SSA Fluoreszenzphotonen gelöscht wurden, konnte ĂŒber den Abstand der Laserpulse die FluoreszenzintensitĂ€t des einzelnen MolekĂŒls moduliert werden. Konzeptionell verwandte EinzelmolekĂŒlexperimente wurden an einem weiteren molekularen System durchgefĂŒhrt, das aus einem Kern und einer Peripherie bestand. Fluoreszenzauszeiten des Gesamtsystems bei selektiver Anregung des Kerns wurden auf die Population eines Triplett-Zustandes zurĂŒckgefĂŒhrt, der die Fluoreszenz der Peripherie löschte. rnAbschlieĂend wurde der SSA-Prozess zwischen zwei gleichartigen Chromophoren untersucht. Es wurde eine Methode entwickelt, die es zum ersten Mal erlaubte, die SSA-Zeitkonstante individueller MolekĂŒle zu bestimmen. HierfĂŒr wurden die Daten der gemessenen Photonen-Koinzidenzhistogramme mittels eines im Rahmen dieser Arbeit hergeleiteten analytischen Zusammenhangs ausgewertet, der ĂŒber Monte-Carlo-Simulationen
bestÀtigt wurde.The aim of the present work was the investigation of the mechanism, the dynamics and the control of electronic energy transfer processes in multichromophoric model systems. To reach these goals, confocal single molecule spectroscopy at room temperature as well as quantum chemical calculations were utilized. The excitation and detection pathways of the confocal microscope were varied to carry out the different experiments. For the quantum chemical calculations coupled-cluster and density functional theory were used. The results of the calculations supported and complemented the experimental findings. rnExperiments on single donor-acceptor dyads preceding this work showed that the measured energy transfer rates
deviate substantially from rates calculated via Försterâs theory of energy transfer. Thus one goal of the present dissertation was to investigate these deviations. For this purpose the molecular geometry and the electronic coupling in the dyads were investigated. The relative orientation of the chromophores was studied by single molecule experiments with rotating excitation polarisation. Eventually the resulting distributions of angles unambiguously could be ascribed to the flexibility of the oligophenylene bridges connecting the chromophores. The deviations of the measured energy transfer rates from Förster theory, however, could not be explained with the molecular flexibility. The quantum chemical calculations clarified this point. Here, for the first time coupled-cluster theory was used to calculate the electronic coupling between two chromophores. Taking into account the isolated chromophores only, however, could not explain the measured deviations from Förster theory. Consideration of the influence of
the molecular bridge on the electronic coupling strength explained the discrepancies of measured rates and Förster theory. The drastic enhancement of the electronic coupling is attributed to the polarizability of the molecular bridge.rnThe next step was the control of the energy transfer process. The chromophores of the investigated individual dyad molecules were selectively excited by two laser pulses of different wavelengths. Simultaneous excitation of both chromophores induced singlet-singlet annihilation (SSA), an electronic energy transfer process whereby the energy was transferred from the former acceptor to the former donor chromophore. Since the SSA process quenched fluorescence, the fluorescence intensity of the single molecule could be modulated by changing the time delay between the two pulses. Related experiments were carried out on a molecular system that consisted of a core and a periphery. Fluorescence off-times of the molecule upon selective excitation of the core were ascribed to formation
of a core triplet state which quenches the fluorescence of the periphery.rnThe last experiment investigated the SSA process between two chemically identical chromophores. A method was derived to quantify the SSA time constants of individual molecules. The recorded photon coincidence histograms were evaluated with an analytical expression. The method was validated via Monte Carlo simulations
WÀhlen im rÀumlichen Kontext : eine empirische Untersuchung zum Einfluss der sozialen Komposition des Wahlkreises auf die individuelle Wahlentscheidung bei der Bundestagswahl 2009
Die meisten Studien der empirischen Wahlforschung fĂŒhren das WĂ€hlverhalten bei deutschen Bundestagswahlen gemÀà den bewĂ€hrten ErklĂ€rungsansĂ€tzen (Columbia School, Cleavage-Theorie, Michigan School, âŠ) auf Faktoren der Individualebene zurĂŒck. Nur wenige analysieren darĂŒber hinaus den Einfluss rĂ€umlicher Kontextmerkmale. Diese BeitrĂ€ge gelangen zudem zu widersprĂŒchlichen Befunden, z.B. darĂŒber, welcher Anteil der Gesamtvarianz ĂŒberhaupt durch Kontextfaktoren erklĂ€rt werden kann. Daher will die vorliegende Arbeit klĂ€ren, inwiefern die soziale Komposition des rĂ€umlichen Kontexts ĂŒber
individuelle Merkmale der WĂ€hler hinaus ihre individuelle Wahlentscheidung bei der Bundestagswahl 2009 beeinflusst hat. Dazu wird zunĂ€chst ein rĂ€umliches Mehrebenen-Modell des individuellen Wahlverhaltens entwickelt, das den Einfluss von Kontextmerkmalen u.a. auf soziale Interaktionsmechanismen innerhalb der Kontexteinheiten zurĂŒckfĂŒhrt. Zudem werden die zentralen individuellen ErklĂ€rungsfaktoren der oben genannten Theorien (Parteiidentifikation, Kandidaten-, Sachfragen-Orientierung, soziale Gruppenzugehörigkeit) in das Modell integriert. Auf Grundlage von Daten der German Longitudinal Election Study werden anschlieĂend logistische Mehrebenen-Modelle fĂŒr die alten und erstmals auch fĂŒr die neuen BundeslĂ€nder und Deutschland geschĂ€tzt. Erstmals werden zudem Wahlkreise als relevante Kontexteinheiten untersucht. Es zeigt sich, dass ein kleiner Teil der Varianz der individuellen Wahlentscheidung allein auf Merkmale des Wahlkreises zurĂŒckgefĂŒhrt werden kann. Es treten sowohl direkte Kontexteffekte als auch
Mehrebenen-Interaktionseffekte auf, die sich jedoch in ihrer Wirkung zwischen den Regionen und auch zwischen den Parteien erheblich unterscheiden.Most empirical studies of voting behavior explain vote choice at German federal elections using individual level variables referring on established theories of voting behavior (Columbia School, Cleavage Theory, Michigan School, ...). Only a few analyze in addition to them specific effects of the geographical context that surrounds the voters. On top of that these studies produce contradictory results, e.g. about how much of the total variance can be explained by contextual factors. That is why this study seeks to analyze if the social composition of the geographical
context of voters had a particular effect on their vote choice in addition to their most important individual characteristics at the German federal election 2009. For that purpose first of all a geographical multilevel-model of vote choice is generated that explains contextual effects among other mechanisms by social interactions among the voters inside the contextual units controlling for the most important individual level factors mentioned by the theories above (party identification, candidate-, issue-orientation, social group membership). Using data of the German Longitudinal Election Study logistic multilevel-models for West- and for the first time for East-Germany and Germany are estimated. Equally for the first time electoral districts are analyzed as the relevant units of the context level. The results show that a small part of the variance of individual vote choice can be explained particularly by characteristics of the electoral districts. Not only direct context effects but also cross-level-
interactions can be found. But their strength of influence differs as well between the analyzed regions as between the analyzed parties
The spectroscopy and thermochemistry of phenylallyl radical chromophores
The resonant two-color two-photon ionization and laser induced fluorescence excitation spectra of the 1-phenylallyl (cinnamyl) and inden-2-ylmethyl radicals are reported. The 1-phenylallyl radical is found to fluoresce with low yield, permitting only a coarse dispersed fluorescence spectrum, while the inden-2-ylmethyl radical yields sufficient fluorescence to obtain ground-state vibrational frequencies and two-dimensional fluorescence spectra. Computed ionization energies and thermochemical properties including radical stabilization energies are reported for a range of resonance-stabilized radicals, including the phenylpropargyl, vinylpropargyl and phenylallyl radicals
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