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 π–π-interactions. The photoemission intensity from the highest occupied molecular orbitals (HOMO and HOMO − 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 p -quinquephenyl pyridine (5P-Py) adsorbed on ZnO(1 0   −1 0). The parent anchoring group, pyridine, significantly lowers the work function by up to 2.9 eV 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 2.1 eV 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 2 eV 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 Grenzflächen zwischen organischen Molekülen und anorganischen Elektroden und der damit korrelierten Anordnung der Energieniveaus an diesen Grenzflächen. Anhand von vier Modellsystemen, welche unterschiedliche Teile einer Solarzelle oder LED darstellen, werden diese Prozesse mittels zeitaufgelöster Photoelektronenspektroskopie, welche die Messung besetzter und unbesetzter Zustände, sowie die Erfassung ultraschneller Prozesse ermöglicht, untersucht. ZnO ist ein vielversprechendes Material für transparente Elektroden und als aktives LED-Medium, weshalb die Prozesse in optisch angeregtem ZnO von großem Interesse sind. Die Untersuchungen zeigen, dass die elektronische Struktur der O-terminierten ZnO-Oberfläche stark durch Adsorption von Wasserstoff beeinflusst wird und dass die Exzitonenentstehung mit größerer Elektronendichte an der Oberfläche langsamer wird, da die effziente Elektron- Phonon-Kopplung abgeschirmt wird. Die SP6/ZnO-Grenzfläche dient einerseits als Modellsystem für Ladungstransfer, in dicken Filmen jedoch stellt SP6 ein potentielles LED-Medium dar. Zusätzlich zu den bereits bekannten Relaxationsprozessen, die mit zeitaufgelöster optischer Spektroskopie beobachtet wurden, zeigt die Photoeletronenspektroskopie eine weitere ultraschnelle Komponente. Die Wechselwirkung langlebiger Tripletzustände führt in diesem System zur Emission von Elektronen. Außerdem erlaubt die Photoelektronenspektroskopie Rückschlüsse auf die absoluten Energien der angeregten Zustände. Die Anordnung molekularer Energieniveaus an der Grenzfläche zwischen einer Metallelektrode und pi-konjugierten Molekülen wird an dicyanovinyl-substituierten Oligothiophenen auf Gold untersucht. Die Goldoberfläche hat nicht nur Einfluss auf die elektronische Struktur der Monolage, sie beeinflusst auch erheblich die Lebensdauer der angeregten Zustände, welche mit dem Abstand zur Metalloberfläche zunimmt. Als Modellsystem für einen polymeren Halbleiter wurde P3HT eingesetzt. Die Untersuchung zweier Filme mit unterschiedlich großen kristallinen Anteilen zeigt, dass die Relaxationsdynamik in dem Film schneller abläuft, der die größere Kristallinität und somit die besseren Transporteigenschaften aufweist. Diese Ergebnisse geben einen Einblick in die komplexen zusammenhängenden Relaxationsprozesse optisch angeregter Zustände. Das detaillierte Verständnis 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 γ-aminobutyric acid (GABA) measurements using J-difference editing, before and after spectral realignment—a 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: “Independent alignment” (aligning each subspectrum independently) and “Pairwise alignment” (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