23 research outputs found
Localization-dependent charge separation efficiency at an organic/inorganic hybrid interface
By combining complementary optical techniques, photoluminescence and
time-resolved excited state absorption, we achieve a comprehensive picture of
the relaxation processes in the organic/inorganic hybrid system SP6/ZnO. We
identify two long-lived excited states of the organic molecules of which only
the lowest energy one, localized on the sexiphenyl backbone of the molecule, is
found to efficiently charge separate to the ZnO conduction band or radiatively
recombine. The other state, most likely localized on the spiro-linked biphenyl,
relaxes only by intersystem crossing to a long-lived, probably triplet state,
thus acting as a sink of the excitation and limiting the charge separation
efficiency.Comment: 6 pages, 5 figure
Ultrafast Exciton Population, Relaxation, and Decay Dynamics in Thin Oligothiophene Films
Femtosecond time-resolved two-photon photoemission spectroscopy is utilized to
determine the electronically excited states dynamics at the α-sexithiophene
(6T)/Au(111) interface and within the 6T film. We found that a photoinduced
transition between the highest occupied molecular orbital and lowest
unoccupied molecular orbital is essential in order to observe exciton
population, which occurs within 100 fs. In thin 6T films, the exciton exhibits
a lifetime of 650 fs. On a time scale of 400 fs, an energetic stabilization is
observed leading to the formation of a polaron or electron trapping at defect
states. The lifetime of this state is 6.3 ps. Coverage-dependent measurements
show that apart from the excited state decay within the film, a substrate-
mediated relaxation channel is operative. The present study demonstrates that
two-photon photoemission spectroscopy is a powerful tool to investigate the
whole life cycle from creation to decay of excitons in an organic
semiconductor
Coverage-dependent adsorption geometry of octithiophene on Au(111)
The adsorption behavior of α-octithiophene (8T) on the Au(111) surface as a
function of 8T coverage has been studied with low-temperature scanning
tunneling microscopy, high resolution electron energy loss spectroscopy as
well as with angle-resolved two-photon photoemission and ultraviolet
photoemission spectroscopy. In the sub-monolayer regime 8T adopts a flat-lying
adsorption geometry. Upon reaching the monolayer coverage the orientation of
8T molecules changes towards a tilted configuration, with the long molecular
axis parallel to the surface plane, facilitating attractive intermolecular
ÏâÏ-interactions. The photoemission intensity from the highest occupied
molecular orbitals (HOMO and HOMO â 1) possesses a strong dependence on the
adsorption geometry due to the direction of the involved transition dipole
moment for the respective photoemission process. The change in molecular
orientation as a function of coverage in the first molecular layer mirrors the
delicate balance between intermolecular and molecule/substrate interactions.
Fine tuning of these interactions opens up the possibility to control the
molecular structure and accordingly the desirable functionality
Uncovering the (un-)occupied electronic structure of a buried hybrid interface
The energy level alignment at organic/inorganic (o/i) semiconductor interfaces is crucial for any light-emitting or -harvesting functionality. Essential is the access to both occupied and unoccupied electronic states directly at the interface, which is often deeply buried underneath thick organic films and challenging to characterize. We use several complementary experimental techniques to determine the electronic structure of pâ-quinquephenyl pyridine (5P-Py) adsorbed on ZnO(1â0ââââ1â0). The parent anchoring group, pyridine, significantly lowers the work function by up to 2.9âeV and causes an occupied in-gap state (IGS) directly below the Fermi level EF. Adsorption of upright-standing 5P-Py also leads to a strong work function reduction of up to 2.1âeV and to a similar IGS. The latter is then used as an initial state for the transient population of three normally unoccupied molecular levels through optical excitation and, due to its localization right at the o/i interface, provides interfacial sensitivity, even for thick 5P-Py films. We observe two final states above the vacuum level and one bound state at around 2âeV above EF, which we attribute to the 5P-Py LUMO. By the separate study of anchoring group and organic dye combined with the exploitation of the occupied IGS for selective interfacial photoexcitation, this work provides a new pathway for characterizing the electronic structure at buried o/i interfaces.Deutsche Forschungsgemeinschafthttps://doi.org/10.13039/501100001659Peer Reviewe
Ultraschnelle Relaxationsprozesse nach optischer Anregung von anorganisch/organischen Hybridsystemen
This work focuses on fundamental processes which influence the efficiencies of
organic solar cells and LEDs, for instance the formation and decay dynamics of
excitons, their diffusion, the charge transfer at interfaces between organic
materials and inorganic electrodes and the correlated energy level alignment
at these interfaces. These processes are investigated on the basis of four
model systems, which represent different parts of a solar cell or an LED, by
means of time-resolved photoelectron spectroscopy which facilitates the
measurement of occupied and unoccupied states as well as the acquisition of
ultrafast processes. ZnO is a promising material for transparent electrodes
and as an active LED medium, therefore the processes in optically excited ZnO
are of great interest. The investigations show that the electronic structure
of the O-terminated ZnO surface is strongly influenced by the adsorption of
hydrogen and that the exciton formation slows down at higher electron
densities at the surface as the electron phonon coupling is screened. The
SP6/ZnO interface can serve on the one hand as a model system for charge
transfer processes, however SP6 in thick films represents a potential LED
medium. In addition to the already known relaxation processes which have been
observed before by time-resolved optical spectroscopy, photoelectron
spectroscopy reveals another ultrafast component. The interaction of long-
lived triplet states results in this system in the emission of electrons.
Furthermore, photoelectron spectroscopy allows to draw conclusions on the
absolute energies of the excited states. The energy level alignment at the
interface between a metal electrode and pi- conjugated molecules is
investigated on dicyanovinyl-substituted oligothiophenes on gold. The gold
surface influences not only the electronic structure of the monolayer, it also
significantly affects the lifetimes of excited states that increase with the
distance to the metal surface. As a model system for a polymer semiconductor
P3HT was established. The investigation of two films with different amounts of
crystalline fractions shows that the relaxation dynamics proceeds faster in
the film that features higher crystallinity and therefore superior transport
properties. These results give an insight into the complex interrelated
relaxation processes of optically excited states. The detailed comprehension
of these processes promises their targeted utilization to optimize the
efficiency of organic solar cells and LEDs.Diese Arbeit befasst sich mit fundamentalen Prozessen, welche die Effzienz
organischer Solarzellen und LEDs beeinussen, beispielsweise die Entstehungs-
und Zerfallsdynamik von Exzitonen, deren Diffusion, der Ladungstransfer an
GrenzflĂ€chen zwischen organischen MolekĂŒlen und anorganischen Elektroden und
der damit korrelierten Anordnung der Energieniveaus an diesen GrenzflÀchen.
Anhand von vier Modellsystemen, welche unterschiedliche Teile einer Solarzelle
oder LED darstellen, werden diese Prozesse mittels zeitaufgelöster
Photoelektronenspektroskopie, welche die Messung besetzter und unbesetzter
ZustÀnde, sowie die Erfassung ultraschneller Prozesse ermöglicht, untersucht.
ZnO ist ein vielversprechendes Material fĂŒr transparente Elektroden und als
aktives LED-Medium, weshalb die Prozesse in optisch angeregtem ZnO von groĂem
Interesse sind. Die Untersuchungen zeigen, dass die elektronische Struktur der
O-terminierten ZnO-OberflÀche stark durch Adsorption von Wasserstoff
beeinflusst wird und dass die Exzitonenentstehung mit gröĂerer
Elektronendichte an der OberflÀche langsamer wird, da die effziente Elektron-
Phonon-Kopplung abgeschirmt wird. Die SP6/ZnO-GrenzflÀche dient einerseits als
Modellsystem fĂŒr Ladungstransfer, in dicken Filmen jedoch stellt SP6 ein
potentielles LED-Medium dar. ZusÀtzlich zu den bereits bekannten
Relaxationsprozessen, die mit zeitaufgelöster optischer Spektroskopie
beobachtet wurden, zeigt die Photoeletronenspektroskopie eine weitere
ultraschnelle Komponente. Die Wechselwirkung langlebiger TripletzustĂ€nde fĂŒhrt
in diesem System zur Emission von Elektronen. AuĂerdem erlaubt die
Photoelektronenspektroskopie RĂŒckschlĂŒsse auf die absoluten Energien der
angeregten ZustÀnde. Die Anordnung molekularer Energieniveaus an der
GrenzflĂ€che zwischen einer Metallelektrode und pi-konjugierten MolekĂŒlen wird
an dicyanovinyl-substituierten Oligothiophenen auf Gold untersucht. Die
GoldoberflÀche hat nicht nur Einfluss auf die elektronische Struktur der
Monolage, sie beeinflusst auch erheblich die Lebensdauer der angeregten
ZustÀnde, welche mit dem Abstand zur MetalloberflÀche zunimmt. Als
Modellsystem fĂŒr einen polymeren Halbleiter wurde P3HT eingesetzt. Die
Untersuchung zweier Filme mit unterschiedlich groĂen kristallinen Anteilen
zeigt, dass die Relaxationsdynamik in dem Film schneller ablÀuft, der die
gröĂere KristallinitĂ€t und somit die besseren Transporteigenschaften aufweist.
Diese Ergebnisse geben einen Einblick in die komplexen zusammenhÀngenden
Relaxationsprozesse optisch angeregter ZustÀnde. Das detaillierte VerstÀndnis
dieser Prozesse verspricht deren gezielte Ausnutzung, um die Effzienz von
organischen Solarzellen und LEDs zu optimieren
Subtraction artifacts and frequency (Mis-)alignment inJ-difference GABA editing
Purpose:
To compare the repeatability of Îł-aminobutyric acid (GABA) measurements using J-difference editing, before and after spectral realignmentâa technique which has previously been demonstrated to improve the quality of J-difference GABA spectra.
Materials and Methods:
We performed in vivo measurements in three brain regions (occipital, sensorimotor, and dorsolateral prefrontal cortex [DLPFC]), and analyzed these using alternative alignment approaches to evaluate the impact of alignment on repeatability: âIndependent alignmentâ (aligning each subspectrum independently) and âPairwise alignmentâ (aligning each on and off subspectrum as a pair) were compared.
Results:
Pairwise alignment improved the group mean coefficient of variation in all regions; 0.4% in occipital, 1.1% in sensorimotor, and 1.1% in DLPFC. Independent alignment resulted in subtraction artifacts in the majority of cases, and increased the coefficient of variation in the DLPFC by 9.4%. Simulations demonstrate that the GABA quantification error in datasets with high B0 drift, is 4.5% without alignment, but <1% with optimal alignment.
Conclusion:
Pairwise alignment improves the repeatability of GABA spectroscopy data. However, independently aligning all on and off subspectra can lead to artifacts and worse repeatability when compared with nonaligned data. J. Magn. Reson. Imaging 2013;. © 2013 Wiley Periodicals, Inc