Influence of Frenkel excitons and charge transfer states on the spectroscopic properties of organic molecular crystals [Elektronische Ressource] / Linus Gisslén

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• Publié par: technische_universitat_munchen
• Publié le: 01 janvier 2010
• Nombre de lectures: 31
• Langue: English
• Poids de l'ouvrage: 11 Mo

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Walter Schottky Institut
Zentralinstitut fu¨r physikalische Grundlagen der Halbleiterelektronik
Fakult¨at fu¨r Physik der Technischen Universit¨at Mu¨nchen
Influence of Frenkel Excitons and Charge Transfer
States on the Spectroscopic Properties of Organic
Molecular Crystals
Linus Gissl´en
Vollst¨andiger Abdruck der von der Fakult¨at fu¨r Physik der Technischen Universit¨at
Mu¨nchen zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften
(Dr. rer. nat.)
genehmigten Dissertation.
Vorsitzender: Univ.-Prof. Dr. Johannes Barth
Pru¨fer der Dissertation 1. Priv.-Doz. Dr. Reinhard Scholz
2. Univ.-Prof. Dr. Philipp Scherer
Die Dissertation wurde am 10.12.2009 bei der Technischen Universit¨at Mu¨nchen
eingereicht und durch die Fakult¨at fu¨r Physik am 26.01.2010 angenommen.Abstract
Organic semiconductors represent a large group of materials consisting of small molecules
or longer polymer chains. In the condensed phase, these polyaromatic molecules are held
together by relatively weak interactions between their electric quadropoles and by van
der Waals forces. Perylene and its derivatives have attracted significant interest as active
layers for light harvesting, photovoltaics, and photoinduced charge and energy transfer
processes. This thesis focuses mainly on crystals of perylene-based molecules because
these substances areamongthe bestcharacterized organicmaterials. Perylene derivatives
are robust organic dyes absorbing and emitting light in the visible range and in the near
infrared. Theydisplayastrongtendencytoself-assembleintomolecularaggregates,liquid
crystals, orevencrystals. Inordertoincreasetheirefficiency andstability, thesematerials
have been studied quite intensively. In particular, their possible application in solar-cells
needs insight into optical excitation and charge transport processes. The vision is that
in the near future organic electronics will successfully compete with inorganic electronics
for applications that require mechanical flexibility, high area coverage, and low cost mass
production.
The performance of these materials as charge or energy transport materials does not
arise exclusively fromthe electronic properties ofthe individual molecules, but it depends
as well on favorable intermolecular interactions, such as π stacking. In fact it can be
shown that many perylene derivatives display very similar optical spectra as dissolved
monomers, whereas in their crystalline phase, the interactions between the π orbitals of
adjacent molecules result in quite diversified optical properties. The interactions between
these π orbitals depend strongly on the side wings attached to the perylene core since
different side groups result in different stacking geometries. This difference in geometric
overlap governs the level of interactions between neutral excitons and charge transfer
states, so that it becomes the starting point for understanding the microscopic processes
involved.
Afteracarefulinvestigationofelectronicexcitationsinasinglemolecule,thisthesisde-
velops theoretical tools allowing to quantify intermolecular interactions and their impact
onto the optical properties of molecular crystals. As the deformation of a relaxed excited
molecule defines the vibronic progressions observed inabsorption andphotoluminescence,
the relation between electronic excitations, deformation patterns and the elongation of
molecular vibrations are studied for a monomer. The deformation of positively or nega-
tively charged molecular ions with respect to the neutral ground state is calculated with
density functional theory (DFT), and the geometry in the optically excited state is de-
iduced from constrained DFT and time-dependent DFT. These deformations are then
projected onto the vibrational eigenvectors, allowing in turn to compare calculated ab-
sorption, photoluminescence, and resonant Raman spectra to experimental observations.
For later use in an exciton model addressing molecular crystals, all of these deformations
are reinterpreted in terms of the elongation of an effective internal vibration.
In the crystalline phase, neutral molecular excitations and charge transfer between
adjacent molecules are coupled via electron and hole transfer, two quantities relating
directly to the width of the conduction and valence band. Based on the crystal structure
determined by X-ray diffraction, DFT and Hartree-Fock are used for the calculation of
the electronic states of a dimer of stacked molecules. The resulting transfer parameters
for electron and hole are inserted into the exciton model for the coupling between Frenkel
excitons and charge transfer states.
Acomparisonbetweenthecalculateddielectrictensorandtheobservedopticalspectra
allowstodeducetherelativeenergeticpositionsofFrenkelexcitonsandthecharge-transfer
state involving stack neighbors, a key parameter for various electronic and optoelectronic
device application. Irrespective ofthe energetic orderingofthese two types ofexcitations,
the exciton modelprovides anew sum rule forthe second moment ofthe opticalresponse,
giving a direct measure of the impact of electron and hole transfer onto the observed
absorption spectra of molecular crystals.
For six out of seven perylene pigments studied, the exciton model results in excellent
agreement between calculated and observed optical properties, and for the seventh com-
pound, theagreement was stillwithin acceptablerange. Moreover, the modelcalculations
described in this thesis have revealed that the published dielectric tensor of one of these
molecular crystals (PTCDA) has resulted from an erroneous evaluation of ellipsometry
data, but a refined analysis of these spectra gives experimental line shapes that can in-
deed be reproduced by the exciton model. Among the materials studied, PTCDA is the
only compound where the charge transfer state along the stacking direction has an en-
ergy far below the neutral molecular excitation. Therefore, photoluminescence excitation
spectroscopy with photon energy below the main absoption features allows the selective
excitation of photoluminescence from charge transfer states. The dispersion branches
arising from the exciton model corroborate previous interpretations of the radiative re-
combination mechanisms, and they allow to assign the respective excitation resonances
to specific charge transfer states.
In conclusion, we have demonstrated a successful realization of a theoretical approach
describingthefundamentalinteractionsinfluencingonexcitontransferincrystallinepery-
lenepigments. Furthermore,themicroscopic parametersetobtainedhasallowedtocalcu-
lateanopticalresponsecongruentwithexperimental spectra, quantifyingseveral intrinsic
variables of the molecular crystals for the first time.
iiZusammenfassung
OrganischeHalbleitersindeinesehrvielfaltigeKlassevonMaterialien,diesichauskleinen¨
Moleku¨len undlangenPolymerketten zusammensetzen k¨onnen. Dieaus polyaromatischen
Molekulen aufgebauten Festkorper entstehen durch relativ schwache attraktive Krafte¨ ¨ ¨
zwischen den Moleku¨len, die auf ihren elektrischen Quadrupolmomenten und der van der
Waals Wechselwirkung beruhen. Insbesondere Perylen und seine Derivate finden Anwen-
dungenalsaktiveMaterialienfu¨rSolarzellen,Lichtsensorenundlichtinduzierten Ladungs-
und Energietransport. Die vorliegende Dissertation besch¨aftigt sich haupts¨achlich mit
kristallinen Perylen-Pigmenten, weil diese Substanzen zu den am besten charakterisierten
organischen Materialien geh¨oren.
Derivate von Perylen sind besonders stabile organische Farbstoffe, die im Sichtbaren
und Nahinfrarot Licht emittieren und absorbieren. Sie tendieren dazu, Moleku¨laggregate,
flussigkristalline Phasen und sogar kristalline Festkorpern zu bilden. Diese Materialien¨ ¨
wurden bereits intensiv untersucht, um ihren Wirkungsgrad und ihre Best¨andigkeit zu
verbessern. In Bereichen, die hohe mechanische Flexibilitat und niedrige Produktionskos-¨
ten auf großen Fl¨achen voraussetzen, werden organische Halbleiter in absehbarer Zukunft
erfolgreich mit anorganischen Halbleitern konkurrieren. Allerdings erfordern insbesondere
AnwendungenalsSolarzelleneintieferestheoretischesVersta¨ndnisderoptischenAnregun-
gen, der Bildung von Elektron-Loch-Paaren, sowie des Transports von Ladungstr¨agern.
Der Ladungs- und Energietransport in diesen Materialien h¨angt nicht nur von den
elektronischenEigenschafteneinzelnerMolekuleab,sondernvielmehrauchvondenWech-¨
selwirkungen zwischen benachbarten Moleku¨len (z.B. π stacking). Tats¨achlich kann man
zeigen,dassvielePerylenderivate inFormgel¨osterMonomeresehr¨ahnlicheoptischeSpek-
trenzeigen. ImGegensatzdazubewirken dieWechselwirkungen zwischen denπ Orbitalen
benachbarter Moleku¨le deutlich verschiedene optischen Eigenschaften.
DieWechselwirkungen zwischen diesenπ Orbitalenhangenstarkdavonab,welcheSei-¨
tengruppenamPerylenmoleku¨langebundensind,daverschiedene Seitengruppenverschie-
dene Kristallgeometrien nach sich ziehen. Deren unterschiedliche Geometrie verandert¨
sowohl die Wechselwirkungen zwischen neut