Polaron dynamics in thin polythiophene films studied with time-resolved photoemission

Femtosecond time-resolved two-photon photoemission spectroscopy is employed to study the dynamics of an excited state in a thin regioregular poly(3-hexylthiophene) (RR-P3HT) film deposited on a conducting polymer poly(3,4-ethylene-dioxythiophene): poly-(styrenesulfonate) (PEDT:PSS) electrode following optical excitation at 2.1 eV. We found that the biexponential decay of this excited state has a fast component (2.6 ps) assigned to bound polaron pairs which recombine quickly or separate to be added to the slow component (7.6 ps). The latter is attributed to polarons generated via charge transfer between adjacent polymer chains

Dynamics of optically excited electrons in the conducting polymer PEDT:PSS

Femtosecond time-resolved two-photon photoemission spectroscopy is employed to study the dynamics of the non-equilibrium electron distribution in the conducting polymer poly(3,4-ethylene-dioxythiophene): poly-(styrenesulfonate) (PEDT:PSS) film following optical excitation at 2.1 eV. We found that the electron thermalization occurs on a ultrafast timescale of around 60 fs analogous to the relaxation times of optically excited electrons in Au(111)

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

Exziton- und Polarondynamik in duennen Oligo- und Polithiophenfilmen

In the framework of this thesis the exciton and polaron dynamics in thin oligo- and polythiophene films have been investigated. Two-photon photoemission (2PPE) has been employed to study the electronic structure as well as the exciton formation, relaxation and decay dynamics in the oligothiophene films and at their interface with Au(111). Performing coverage, photon-energy-dependent and time-resolved experiments on sexithiophene (6T)/Au(111), we found that a photoinduced electron transition between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is essential in order to observe the population of a Frenkel exciton that takes place within 100 femtoseconds (fs). In thin 6T films the exciton exhibits a lifetime of 650 fs. On a timescale of 400 fs an energetic stabilization is observed resulting in the formation of a polaron or electron trapping at defect states. The lifetime of this state is 6 picoseconds (ps). Moreover, beside the intramolecular relaxation channel of the exciton and the polaron/trap, a substratemediated relaxation channel exists. In order to investigate the influence of the adsorption geometry on the excited state dynamics, we have studied an alkyl-substituted 6T, namely alpha, omega- dihexylsexithiophene (DH6T) on Au(111). The exciton and polaron/trap dynamics is 5-6 times faster in DH6T compared to 6T. We believe that the wall-brick- like adsorption geometry of DH6T is responsible for this behavior. Indeed, the alkyl chains in DH6T act as spacers between the 6T cores of neighboring molecules, thus reducing stabilization effects due to intermolecular interactions. Furthermore, by means of scanning tunneling microscopy (STM) and surface vibrational spectroscopy we have demonstrated a change in the adsorption geometry of alpha-octithiophene (8T) on Au(111), from flat-lying in the submonolayer to a tilted geometry with the long axis parallel to the substrate in the monolayer. This tilted configuration leads to a strong electronic decoupling between the Frenkel exciton and the metal substrate, leading to the suppression of the substrate-mediated relaxation channel in 8T/Au(111). Finally we have applied time-resolved 2PPE to study the hot electron dynamics in poly(3,4-ethylene-dioxythiophene): poly-(styrenesulfonate) (PEDT:PSS) and the excited state dynamics in regioregular poly(3-hexylthiophene) (RR-P3HT). The latter shows a biexponential decay, with the fast component (2.6 ps) assigned to bound polaron pairs which recombine quickly or separate to be added to the slow component (7.6 ps) that is attributed to polarons generated via charge transfer between adjacent polymer chains. The present study shows that 2PPE is the ideal tool to study the dynamics of excited states in small organic molecules as well as in polymers. Therefore this method promises to provide a link between optical and electron spectroscopy.Im Rahmen dieser Arbeit wurden die Exziton- und Polarondynamik in duennen Oligo- und Polythiophenfilmen untersucht. Dabei wurde mithilfe der Zwei- Photonen Photoemission (2PPE) sowohl die elektronische Struktur, als auch die Bildungs-, Relaxation-, und Zerfallsdynamik von Exzitonen in duennen Oligothiophenfilmen und an der Grenzoberflaeche mit der Au(111)-Oberflaeche untersucht. Bedeckungs- und photonenenergieabhaengige Messungen, sowie zeitaufgeloeste Experimente an alpha-Sexithiophen (6T)/Au(111) haben gezeigt, dass ein Uebergang vom hoechsten besetzen Molekuelorbital (HOMO) zum niedrigsten unbesetzten Molekuelorbital (LUMO) fuer die Population eines Frenkel-Exzitons unerlaesslich ist. Die Bildung dieses Exzitonzustandes erfolgt auf einer Zeitskala von 100 Femtosekunden (fs), und weist in duennen Filmen eine Lebensdauer von 600 fs auf. Des Weiteren wurde eine energetische Stabilisierung dieses angeregten Zustandes innerhalb von 400 fs beobachtet, welche von der Bildung eines Polarons oder eines an Defektstellen gebundenen Elektrons herruehrt. Dieses Polaron/gebundenes Elektron verfuegt ueber eine Lebensdauer von 6 Picosekunden (ps). Es existiert ein intramolekularer und ein substrat-induzierter Zerfallskanal. Um den Einfluss der Adsorptionsgeometrie auf die Dynamik der angeregten Zustaende zu untersuchen, wurden Experimente mit dem alkylsubsituierten 6T, naemlich alpha,omega-Dihexylsexithiophen (DH6T) auf Au(111) durchgefuehrt. Im Vergleich zu 6T zerfallen das Exziton und das Polaron/gebundenes Elektron 5-6 mal schneller. Wir gehen davon aus, dass die Adsorptionsgeometrie von DH6T fuer diese schnellere Zerfallsdynamik verantwortlich ist. In der Tat vergroessern die Dihexylketten den Abstand zwischen den Thiopheneinheiten benachbarter Molekuele und reduzieren dadurch die stabilisierenden intermolekularen Wechselwirkungen. Darueberhinaus haben wir im Falle von alpha-Octithiophene (8T)/Au(111) mittels Rastertunnel Mikroskopie (STM) sowie schwingungsspektroskopisch eine bedeckungsabhaengige Aenderung der Adsorptionsgeometrie beobachtet. Waehrend die 8T Molekuele im Submonolagenbereich flach auf Au(111) adsorbieren, nehmen sie ab Bedeckungen von einer Monolage eine Adsorptionsgeometrie ein, in der das Molekuelrueckgrat parallel zur Oberflaeche verkippt ist. Diese Konfiguration fuehrt zu einer starken elektronischen Entkopplung des Frenkel-Exzitons von der Metalloberflaeche, was zur Unterdrueckung des substrat-induzierten Zerfallkanals fuehrt. Schliesslich haben wir mittels zeitaufgeloester 2PPE die Dynamik heisser Elektronen in Poly(3,4-ethylen-dioxythiophen): Poly-(styrensulfonat) und die Dynamik angeregter Zustaende in regioregularen Poly(3-hexylthiophen) (RR-P3HT) untersucht. Letztere weist einen biexponentiellen Zerfall auf. Die schnelle Komponente (2.6 ps) wird gebundenen Polaronpaaren zugeordnet, welche entweder schnell rekombinieren oder sich separieren. Die langsame Komponente (7.6 ps) wird Polaronen zugeordnet, welche durch einen Ladungstransfer zwischen benachbarten Polymerketten entstehen

Optically Induced Inter- and Intrafacial Electron Transfer Probed by Two-Photon Photoemission: Electronic States of Sexithiophene on Au(111)

Using two-photon photoemission spectroscopy, we investigated the electronic structure of the organic semiconductor α-sexithiophene (6T) adsorbed on Au(111). Beside the quantitative determination of the energetic position of electronic states originating from the highest occupied molecular orbitals (HOMO and HOMO-1) and the lowest unoccupied molecular orbitals (LUMO and LUMO+1), a localized exciton state that possesses a binding energy of 0.9 eV has been identified. Whereas the creation of the exciton is the result of an intramolecular excitation involving a HOMO−LUMO transition, the transient population of the LUMO and LUMO+1 follow from an optically induced charge transfer from the metallic substrate to the molecule. The present study provides important parameters such as the energetic position of the transport level and the exciton binding energy, which are needed to understand the physics in organic-molecules-based optoelectronic devices