title: Exciton Dynamics in Perfluoropentacene Single Crystals
title: Exziton-Dynamik in Perfluoropentacen Einkristallen
Creators: name: Kolata, Kolja
Publisher: name: Philipps-Universität Marburg
Year: 2015
Subjects: Halbleiter
Anrege-Abfrage-Spektroskopie
Spektroskopie
pump-probe
Ladungsträger
Physik
Dynamik
Photoeffekt
Singlet-Aufspaltung
excitons
Triplet-Zustände
Ultrakurzzeit-Dynamik
singlet-fission
Exzitonen
Acene

description: In dieser Arbeit wird anhand des
      Modelsystems von Perfluoropentacen (PFP) Einkristallen,die
      Singlet-Exziton-Aufspaltung, zum ersten Mal, entlang aller
      Kristallachsen mit Hilfe der Anrege-Abfrage-Spektroskopie
      vermessen. Dies wird ermöglicht durch das besondere
      Kristallwachstum von PFP, welches auf Naf(100) eine stehende
      Geometrie einnimmt und auf KCl(100) eine Liegende. Dadurch
      sind alle Achsen in normaler Transmissions-Geometrie
      adressierbar. Die Messungen ergeben, dass die intermolekulare
      Kopplung primär entlang der versetzten pi-Stapelung, bzw.
      entlang der b-Achse erfolgt. Diese Beobachtung entspricht der
      Vorhersage der Theorie, welche in diesem Aspekt bestätigt
      wird. Zusätzlich wird der kohärente Zustand zwischen
      Singlet-Exziton und Triplet-Exzitonen, das sogenannte
      Triplet-Paar (1TT), als direkt nach der Anregung induzierte
      Fano-Resonanz entlang der a-Achse beobachtet. Die
      asymmetrische Resonanz resultiert aus der Interferenz der
      Übergangsamplituden der neun einzelnen, nicht-entarteten
      Sublevels des Triplet-Paars. Während der Aufspaltung,
      relaxiert das Triplet-Paar in einen Excimer-artigen Zustand,
      dessen Übergänge sich in Form von einer breitbandig
      induzierten Absorption entlang der b-Achse manifestieren. Ein
      Großteil der Triplet-Paare relaxiert strahlend zurück in den
      Grundzustand, sichtbar als Photolumineszenz 300 meV unterhalb
      der untersten Exziton-Resonanz. Die Rekombination der
      Triplet-Exzitonen, welche nach Dephasierung des
      Triplet-Paares als individuelle Exzitonen agieren, erfolgt
      auf Nanosekunden-Skala aufgrund des nötigen
      Spin-Umklapp-Prozesses. Außerdem werden
      polarisationsabhängige Absorptionsmessungen der PFP
      Einkristalle in der Arbeit präsentiert. Diese weisen eine
      schwache Davydov-Aufspaltung von 25 meV auf. Die
      Davydov-Aufspaltung ist,in erster Näherung, ein Resultat der
      Dipolkopplung zwischen den nicht-Translationsinvarienten
      Basis-Molekülen. Da diese im PFP Kristall fast einen rechten
      Winkel aufspannen, ist die Kopplung klein. Allerdings durch
      die starke Kopplung entlang der b-Achse, wird eine H- und
      J-Aggregate-artige Absorption bei höheren Energien
      beobachtet. Interessanterweise kann durch angeregte
      Ladungsträger diese Kopplung gehemmt werden, was sich in den
      Anrege-Abfrage Messungen zeigt. Abschließend wird mit Hilfe
      von kohärenten Oszillationen die Dephasierungszeit von
      exzitonischen Resonanzen in Ge/SiGe Quantenfilmen ermittelt.
      Kohärente Oszillationen treten bei Anrege-Abfrage-Messungen
      kurz vor dem Zeitüberlapp von Anrege- und Abfrage-Impuls auf.
      Aus der Transiente dieses kohärenten Effekts, lässt sich die
      Dephasierung der entsprechenden Resonanz extrahieren.
      Dementsprechend, kann bei der Ladungsträger-Analyse, im
      Anrege-Abfrage-Experiment, die Dephasierungszeit der
      adressierten Übergänge mitgemessen werden. Die Methode wird
      auf eine Reihe von Ge/SiGe Quantenfilm Proben angewandt,
      welche nominell die selben Wachstumsparameter besitzen. Es
      zeigt sich, dass der Dephasierungsprozess durch den
      intrinsischen Streukanal von Gamma- zu L-Tal dominiert ist
      und somit lediglich eine maximale Dephasierungszeit von 300
      fs erreicht werden kann. Die beste Probe zeigt eine
      Dephasierungszeit von 250 fs und somit eine homogene
      Verbreiterung über den gesamten, untersuchten
      Temperaturbereich.
description: The realization of the first
      bipolar junction transistor in the year 1948 by Bardeen,
      Brattain and Shockley [1] sparked off the semiconductor
      industry, which gradually revolutionized the way we live.
      Nowadays, semiconductors are the fundamental building blocks
      of every high-tech electronic device, most notably the
      computer which has become an inescapable part of our daily
      lives. Besides voltage and current control capabilities,
      semiconductors exhibit intriguing opto-electronic properties;
      the best known and commercially most successful applications
      are light emitting diodes (LEDs), laser diodes, charged
      coupled devices (CCD) and solar cells.[2, 3, 4] Due to the
      broad variety of material systems, they cover virtually the
      complete optical spectrum while simultaneously being
      cost-efficient and easy to miniaturize. Until the late 90ies,
      commercially available devices were exclusively based on
      inorganic semiconductors, primarily on Silicon. However, over
      the last decade, the class of organic semiconductors has
      gained an increasing amount of interest, e.g., now one of the
      most popular smartphone’s display1 is based on
      OLED2-technology. Flexibility upon stress and deeper color
      contrasts are typically named as their main advantages over
      conventional liquid crystal displays (LCD). While organic
      semiconductor devices are already well established as light
      emitters, they are still in research state as light
      harvesters. In general, organic solar cells offer high photon
      cross sections in combination with similar flexibility as
      OLED displays. Additionally, they exhibit the potential for
      low-cost mass-production, including innovative and versatile
      procedures such as ink-jet printing.[6, 7] However, two major
      challenges still exist which need to be addressed before
      organic solar cells become compatible: the long-term
      stability and the quantum efficiency.[8] The fast degradation
      of organic solar cells is caused by oxidation, reduction and
      thermal instabilities. Research in this field focuses on the
      synthesis of new organic molecules, thus, it can be assigned
      to the organic chemistry sector. Quantum efficiencies are
      determined by the microscopic photon to carrier conversion,
      i.e., the photovoltaic effect, therefore, it is predominantly
      a research topic of solid state physics. This thesis focuses
      mainly on aspects of the quantum efficiency in the polyacene
      Perfluoropentacene (PFP) and its underlying decay processes,
      namely the electronic relaxation dynamics after optical
      excitation. In particular, the process of singlet exciton
      fission is analyzed which promises to double the quantum
      efficiencies, as it converts one singlet exciton into two
      triplet excitons.[9] Excitons are correlated electron and
      hole pairs: neutral excitations of the crystal after
      absorption of a photon. Singlet exciton fission was first
      proposed in 1968 in order to explain the drastic
      photoluminescence quench of Tetracene crystals compared to
      Anthracene crystals.[10] It has gained renewed attention
      lately, due to its potential application in the growing field
      of organic solar cells. However, the microscopic
      understanding is still in its infancy which hampers essential
      progress in this field; for instance, the influence of the
      geometrical order of the molecules within the crystal on
      singlet exciton fission has only been analyzed theoretically.
      The reason is the lack of single crystal samples allowing for
      the correlation of molecular packing and electronic
      dynamics.[9] This issue is resolved in Chapter 5 for the
      model system of PFP single crystals, where for the first time
      the singlet exciton fission dynamics are observed along the
      three crystal axes by polarization-resolved pump-probe
      spectroscopy. Moreover, the efficient coupling direction is
      identified as well as the preceding electronic species of the
      two triplet excitons. Although spectroscopic analysis on
      polyacenes date back to the 40ies [11], lack of computational
      power and interest lead to the sad state that even
      interpretations of the linear absorption are still debated
      today. However, basic knowledge of the linear absorption is
      essential in order to interpret the non-linear dynamics.
      Therefore, Chapter 4 serves as a precursor, where the linear
      absorption of the PFP samples is interpreted using
      phenomenological models. Here, first indications are given
      for a dominant coupling direction within the PFP crystal
      which are then confirmed in Chapter 5. Furthermore, the
      amount of exciton splitting in PFP is determined, also known
      as the Davydov-splitting.[12] It is induced by dipole
      coupling between the two basis molecules of the crystal
      lattice during excitation. In Chapter 6 the focus is shifted
      to inorganic semiconductors. The chapter introduces a fast
      and convenient method to determine dephasing times of induced
      coherent exciton polarizations with more precision than a
      common lineshape analysis of the absorption spectrum. In
      pump-probe spectroscopy, the transients of the coherent
      oscillations are exploited to serve as phase indicators for
      the several excitonic transitions. These transients are
      observed during the coherent regime before pump and probe
      pulses perfectly overlap in time.[13] As a proof of
      principle, the methodology is applied to a set of Germanium
      quantum well samples and evaluated in respect to their
      optical quality. In addition, the main dephasing mechanism in
      Germanium quantum wells is identified. These three chapters
      capture the results of the thesis and are preceded by
      introductory chapters covering basic light-matter
      interactions and experimental details; they are succeeded by
      a conclusion chapter summarizing the essential
      findings.