Optical Properties of Pentacene and Perfluoropentacene Thin Films

The optical properties of pentacene (PEN) and perfluoropentacene(PFP) thin films on various SiO2 substrates were studied using variable angle spectroscopic ellipsometry. Structural characterization was performed using X-ray reflectivity and atomic force microscopy. A uniaxial model with the optic axis normal to the sample surface was used to analyze the ellipsometry data. A Strong optical anisotropy was observed and enabled the direction of the transition dipole of the absorption bands to be determined. Furthermore, comparison of the optical constants of PEN and PFP thin films with the absorption spectra of the monomers in solution shows significant changes due to the crystalline environment. Relative to the monomer spectrum the HOMO-LUMO transition observed in PEN (PFP) thin film is reduced by 210 meV (280 meV). Surprisingly, a second absorption band in the PFP thin film shows a slight blueshift (40 meV) compared to the spectrum of the monomer with its transition dipole perpendicular to that of the first absorption band.Comment: 6 pages, 6 figures, submitted to J. Chem. Phy

Optische Eigenschaften organischer dünner Filme: Statische Spektren und Echtzeit-Untersuchungen während des Wachstums

The aim of this work was to establish the anisotropic dielectric function of organic thin films on silicon covered with native oxide and to study their optical properties during film growth. While the work focuses mainly on the optical properties of Diindenoperylene (DIP) films, also the optical response of Pentacene (PEN) films during growth is studied for comparison. Both molecules are polycyclic aromats that form highly ordered thin films with high charge carrier mobilities. This makes them interesting for applications in organic electronics, such as for organic field effect transistors, OLED´s or organic photovoltaics. Apart from electrical and structural properties also the intimately related optical properties are of strong interest. The experimental findings concerning homogeneous single films can be viewed as a basis for investigations of more complex systems like films grown on nanostructured substrates or multilayer systems that are of high relevance for technical applications. Spectroscopic ellipsometry and differential reflectance spectroscopy are used to determine the dielectric function of the films ex-situ and in-situ, i.e. in air and in ultrahigh vacuum. Both methods are capable to follow the film growth in real-time for detecting possible functional changes already during growth. Additionally, Raman- and fluorescence spectroscopy is utilized to characterize the DIP films serving also as a basis for spatially resolved optical measurements beyond the diffraction limit. Furthermore, X-ray reflectometry and atomic force microscopy are used to determine important structural and morphological film properties. The absorption spectrum of DIP in solution serves as a monomer reference. The observed vibronic progression of the HOMO-LUMO transition allows the determination of the Huang-Rhys parameter experimentally, which is a measure of the electronic vibrational coupling. The corresponding breathing modes are measured by Raman spectroscopy. The optical properties of DIP films on native oxide show significant differences compared to the monomer spectrum due to intermolecular interactions. First of all, the thin film spectra are highly anisotropic due to the structural order of the films. The type of anisotropy measured by spectroscopic ellipsometry is consistent with structural studies on DIP thin films on Si substrates, showing that the molecule is standing nearly upright. Furthemore, based on theoretical investigations done in cooperation with R. Scholz, the Frenkel exciton transfer is studied and the energy difference between Frenkel and charge transfer excitons is determined. Real-time measurements reveal optical differences between interfacial or surface molecules and bulk molecules that play an important role for device applications. They are not only performed for DIP films but also for PEN films. While for DIP films on glass the appearance of a new mode is visible, the spectra of PEN show a pronounced energy red-shift during growth. It is shown how the thickness dependence gives insight into possible mechanisms of these effects. Additional spatially resolved optical measurements are performed for studying in cooperation with D. Zhang et al. the microscopic optical properties of DIP films in detail, utilizing a tip-enhanced near field optical microscope. It is possible to optically detect domain boundaries far beyond the diffraction limit. This opens new perspectives for analyzing spatially resolved spectroscopic properties of molecular films together with the local morphology.In dieser Arbeit werden umfassende Untersuchungen zu den optischen Eigenschaften von Diindenoperylene (DIP)-Filmen auf oxidierten Siliziumsubstraten und Quarzglas angestellt und mit Pentacene (PEN)-Filmen verglichen. DIP und PEN sind polyzyklische Aromaten, die in dünnen Filmen wohlgeordnete Kristallstrukturen formen und vergleichsweise hohe Ladungsträgermobilitäten aufweisen. Diese Eigenschaften machen die Moleküle für Anwendungen in der organischen Elektronik interessant, wie z.B. für OLED´s, organische Feldeffekttransistoren oder organische Solarzellen. Neben der elektrischen und strukturellen Charakterisierung sind auch die damit eng verbundenen optischen Eigenschaften für mögliche Anwendungen von großer Bedeutung. Die Untersuchung homogener Einzelschichten stellt dabei eine wichtige Grundlage für das Verständnis komplexer Systeme wie Mischkristalle oder nanostrukturierter Proben dar. Spektroskopische Ellipsometrie und differentielle Reflexionsspektroskopie werden zur Bestimmung der dielektrischen Funktion dünner Filme ex-situ und in-situ, also an Luft und in Ultrahhochvakuum verwendet. Beide Methoden machen auch Echtzeitmessungen möglich, so dass die optischen Eigenschaften unmittelbar während des Filmwachstums gemessen werden können. Zur weiteren Charakterisierung werden Raman- und Fluoreszenzmessungen an DIP Filmen durchgeführt, die als Grundlage für ortsaufgelöste Messungen dienen. Außerdem kommen Röntgenreflektometrie und Rasterkraftmikroskopiemessungen zum Einsatz, um wichtige strukturelle Eigenschaften zu bestimmen. Der Vergleich zwischen Dünnfilm und Monomer-Spektren, die in Lösung mit UV-vis Spektroskopie gemessen wurden, zeigt charakteristische Unterschiede, die auf intermolekulare Wechselwirkungen im Kristall zurückzuführen sind und auch von der Präparation der Filme abhängen, wie z.B. von der Substrattemperatur oder von der Wahl des Substrates. Für DIP-Filme wird eine ausgeprägte Anisotropie der dielektrischen Funktion gemessen, die Rückschlüsse auf die Kippwinkel der Moleküle zulässt und sehr gute Übereinstimmung zu Ergebnissen in der Literatur darstellt, die auf anderen Methoden beruhen. Außerdem kann mit Unterstützung theoretischer Modelle auch der Transfer von Frenkel- und Ladungstransfer-Exzitonen untersucht werden. Ebenso wird die Energiedifferenz zwischen beiden Exzitonen aus den optischen Spektren bestimmt. Die beobachtete Dickenabhängigkeit der Spektren während des Wachstums deckt Unterschiede zwischen Molekülen an der Oberfläche bzw. einer Grenzfläche und im Volumen auf, was von großer Bedeutung für Bauelemente in elektronischen Anwendungen ist. Dies kann nicht nur bei DIP-Filmen sondern auch bei PEN-Filmen beobachtet werden. Während bei DIP-Filmen auf Glas das Entstehen einer neuen Mode beobachtet wird, ist im Fall von Pentacene eine Energieverschiebung ins Rote zu sehen. Die Schichtdickenabhängigkeit gibt Auskunft über den möglichen Mechanismus des beobachteten Effektes. Zusätzliche ortsaufgelöste Messungen in Kooperation mit D. Zhang et al., die auf einem Spitzen verstärkten Nahfeldmikroskop beruhen, zeigen zudem, dass es möglich ist die optischen Filmeigenschaften auf der Nanometer-Skala zu untersuchen. So können Domänenränder optisch sichtbar gemacht werden, die eine wichtige Rolle für mögliche Anwendungen spielen und neue Perspektiven für die Zukunft eröffnen

Heteroleptic platinum(II) NHC complexes with a C^C* cyclometalated ligand – synthesis, structure and photophysics

Platinum(II) complexes [(NHC)Pt(L)] with various β-diketonate based auxiliary ligands (L: 3-meacac = 3-methylacetylacetonato, dpm = dipivaloylmethanato, dbm = dibenzoylmethanato, mesacac = dimesitoylmethanato, duratron = bis(2,3,5,6-tetramethylbenzoyl)methanato) and a C^C* cyclometalated N-heterocyclic carbene ligand (NHC: dpbic = 1,3-iphenylbenzo[d]imidazol-2-ylidene, dpnac = 1,3-diphenylnaphtho[2,3-d]imidazol-2-ylidene or bnbic = 1-phenyl-3-benzylbenzo[d]imidazol-2-ylidene) were found to show different aggregation and photophysical properties depending on the auxiliary ligand. Eight complexes were prepared from a silver(I)–NHC intermediate by transmetalation, cyclometalation and subsequent treatment with potassium-tert-butanolate and β-diketone. They were fully characterized by standard techniques including ¹⁹⁵Pt NMR. Five complexes were additionally characterized by 2D NMR spectroscopy (COSY, HSQC, HMBC and NOESY). Solid-state structures of five complexes could be obtained and show the tendency of the square-planar compounds to form pairs with different Pt–Pt distances depending on the bulkiness of the substituents at the auxiliary ligand. The result of the photophysical measurements in amorphous PMMA films reveals quantum yields of up to 85% with an emission maximum in the blue region and comparatively short decay lifetimes (3.6 µs). Density functional theory (DFT/TD-DFT) calculations were performed to elucidate the emission process and revealed a predominant ³ILCT/³MLCT character. Organic light-emitting devices (OLEDs) comprising one of the complexes achieved 12.6% EQE, 11.9 lm W⁻¹ luminous efficacy and 25.2 cd A⁻¹ current efficiency with a blue emission maximum at 300 cd m⁻². The influence of an additional hole-transporter in the emissive layer was investigated and found to improve the device lifetime by a factor of seven