Time resolved photoluminescence and theoretical study of excitons in PTCDA [Elektronische Ressource] / vorgelegt von Andrei Yu. Kobitski

Kobitski

Sujets

• Physik
• Astronomie

Informations

• Publié par: technische_universitat_chemnitz
• Publié le: 01 janvier 2003
• Nombre de lectures: 34
• Langue: Deutsch
• Poids de l'ouvrage: 3 Mo

Extrait

Timeresolved Photoluminescence and Theoretical Study

of Excitons in PTCDA

von der Fakultät für Naturwissenschaften der Technischen Universität Chemnitz genehmigte Dissertation zur Erlangung des akademischen Grades doctor rerum naturalium (Dr. rer. nat.) vorgelegt von M. S.Andrei Yu. Kobitski geboren am 18.10.76 in Schwerin

eingereicht am ......................

Gutachter: Prof. Dr. D. R. T. Zahn  Prof. Dr. M. Hietschold  Prof. Dr. H. P. Wagner Tag der Verteidigung: 22.01.03 http://archiv.tuchemnitz.de/pub/……………………..

Bibliografische Beschreibung, Referat und Schlagwörter

Andrei Yu. Kobitski

Thema: Timeresolved Photoluminescence and Theoretical Study of Excitons in PTCDA

Einreichungsdatum: 18. Juli 2002

Im Rahmen dieser Arbeit wurden Exzitonen in molekularen Kristallen mittels zeitaufgelöster Photolumineszenz (PL) und theoretischer Berechnungen studiert. Zeitaufgelöste PL  Untersuchungen an einzelnen PTCDA  Kristallen und dünnen Filmen wurden im Zeitfenster bis zu 100 ns und in einem variablen Temperaturbereich von

10 bis 300 K durchgeführt. Die ermittelten PL  Spektren können durch überlappende Rekombinationkanäle mit zeitlich exponentiellem Abfall von beschrieben werde, die verschieden Typen von Exzitonen wie Frenkel  Exziton, Ladungstransfer  Exziton (CT), Excimer und Monomer entsprechen. Alle Exzitonen wurde quantitativ in bezug auf ihre Positionen, Linienformen und Zerfallszeiten charakterisiert. Durch Analyse der PL  Spektren bei verschiedenen Temperaturen wird der dominierende Beitrag des Frenkel  und des CT  Exzitons zur Rekombination bei niedrigen Temperaturen nachgewiesen, während der Excimer  Beitrag bei Raumtemperatur dominiert. Die gefundenen Lebenszeiten von Exzitonen sind viel langsamer als vorher für PTCDA berichtet. Ein schematisches Energie Diagramm von Exzitonzuständen, die die Prozesse der Absorption, Relaxation und Rekombination beschreiben, wird vorgeschlagen. Die entsprechendek Exzitonen zusammen mit denRaum  Dispersion der Frenkel externen Phononen und den drei tiefsten internen vibronischen Moden, wie in Resonanz  Ramanspektren beobachtet, sowie die Bildung von selbstgefangenen Exzitonen erklären die sehr hohe Stokes – Verschiebung, die zwischen Absorption und PL  Spektren beobachtet wird. Die quantenchemischen Berechnungen von Elektronenübergängen wurden für ein einzelnes PTCDA  Molekül und für verschiedene Typen von PTCDA  Dimeren

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durchgeführt. Die Berechnungen von vibronischen Moden eines einzelnen Moleküls und der Verformung im relaxierden angeregten Zustand zeigen gute Übereinstimmung mit der beobachteten Absorption von Molekülen in Lösung und dem unter Resonanzbedingung gemessenen Ramanspektren von dünnen Filmen. Mit den zeitabhängigen Dichtefunktionaltheorie  Berechnungen von vertikal gepackten und coplanaren Dimeren wurde die Bestimmung der intermolekularen Wechselwirkung und die Kalibrierung der Übergangsenergie durchgeführt. Weitere Berechnungen des verformten Dimers, z. B. Kation – Anion  Dimere und eines relaxieren angeregten Stapels, sagen Übergänge mit Energien von 1,80 1,63 und 1,75 eV, sehr nah den beobachteten Übergängen des Ladungstransfer  Exzitons und des Excimers voraus. Deshalb können diese Dimere als Modelle für selbstgefangene Exzitonen auf Molekularniveau betrachtet werden.

<Schlagwörter> Organische Moleküle, PTCDA, Molekülkristall, Zeitaufgelöste Photolumineszenz, Zeitabhängige Dichtefunktionaltheorie, FrenkelExzitonen, LadungstransferExzitonen, Exzimer, Monomer, Dimer

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Contents List of Tables vii List of Figures ix List of Abbreviations xv Chapter 1. Introduction 1 Chapter 2. Theoretical background 5 2.1 Electronic structure of organic solids 5 2.2 Single molecule absorption and recombination 7 2.3 Excitons in molecular crystals 9 Chapter 3. Calculation of optical properties of a single PTCDA molecule 13 3.1 Selecting an appropriate method of calculation 13 3.2 Geometry and molecular orbitals 15 3.3 Vibrational modes 17 3.4 Excited states 22 3.5 Relaxed geometry of the first excited state 24 3.6 Parameters of the absorption and emission spectra of a single molecule 26 Chapter 4. Timeresolved photoluminescence measurement technique and samples description 29 4.1 Timeresolved photoluminescence (TRPL) setup 29 4.2 PTCDA crystal structure and sample preparation 30 Chapter 5. Absorption and photoluminescence of crystalline PTCDA: The Stokes shift. 33 Chapter 6. Low  temperature TRPL measurements of PTCDA single crystals 37

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Chapter 7. TRPL measurements of PTCDA single crystals at variable temperature 47 7.1 Frenkel exciton PL band 48 7.2 PL bands resulting from localized selftrapped excitons 50 7.3 Exciton kinetics as a function of temperature 53 Chapter 8. Quantum chemical analysis of excited states in PTCDA dimers 57 8.1 Regular dimer 58 8.2 Vertically stacked dimers 61 8.2.1 Anion  Cation dimers  61 8.2.2 Relaxed excited dimers  65 8.3 Dimers with nonequivalent molecules 66 Chapter 9. TRPL characterisation of thin PTCDA films 69 9.1 PTCDA films with different thicknesses 69 9.2 PTCDA films grown at different substrate temperatures 71 9.3 TRPL measurement at variable temperature 72 Summary 78 Bibliography 86 Erklärung I Curriculum Vitae III List of Publications VII Acknowledgements XVII

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List of Tables

Table 3.1 Table 3.2 Table 3.3 Table 3.4 Table 6.1

Calculated frequencies of A g modes are shown together with vibrational experimental assignments, including observed Davydov splittings and experimental uncertainties. B3LYP/321G calculations of Raman active vibrational modes of neutral molecule, anion and cation. E 00 energies for the first excited state in absorption of transition naphthalene, anthracene, pyrene and perylene in solution and gas l phase are shown (taken from [Clair52, Joblin99, Pino99]). E 00 + energies are calculated as centres of mass of the experimental spectra using Eqs. 2.3  2.5. The two last columns show the calculated transition energies for the first excited state using the 321G and 631G(d) basis sets. A g modes of PTCDA in the electronic ground state, as breathing calculated with the B3LYP/321 method: frequencies, dimensionless 2 2 a a reorganisation energies and reorganisation energies. The and reorganisation energies calculated with the densityfunctional tight binding method are taken from [Scholz00]. Gaussian contributions to the model function eq. (6.1) for the time integrated PL spectra, where each of the prefactorsA j can be expressed 1 × as an integral A j dt a j (t)  over contributionsthe delaydependent ò

a j (t). The full width at half maximum (FWHM) can be related to the s1hs broadening parametersin eq. (6.1) byFWHM j j 8 ln 2. The sum

over the timeintegrated coefficientA j is normalized to 100%. Table 7.1 Parameters of the model including Frenkel component and three STE components at T = 0 K. In the first column the “centreofmass” peak positions for the first and second vibronic bands of excitons are shown. In the second column the area ratio between vibronic bands are given. The last column represents the broadenings of Gaussian functions used

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Table 7.2

to model the lineshape of vibronic bands. Slash symbols separate the quantities related to different vibronic bands. Parameters of the model curves for the Frenkel and selftrapped excitons from the fit of temperature dependences of the decay times and PL intensities according to eqs. (7.4) – (7.6).

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List of Figures

Fig. 2.1 Fig 2.2

Fig. 3.1

Fig. 3.2

Fig. 3.3 Fig. 3.4

Fig 3.5

Fig. 3.6

Fig. 3.7

Fig. 3.8

Electronic structure of an organic solid. Optical cycle including the elongation q 0  of an internal vibration of w energy h between the geometry in the electronic ground state and the relaxed excited state. Calculated geometric parameters for PTCDA by means of theB3LYP/6 31G, B3LYP/321G andDFTBmethods. Energy levels of several occupied and unoccupied MO’s are shown with respect to the vacuum level. Electron wavefunctions of the PTCDA molecular orbitals. Resonant Raman spectrum of 40 nm PTCDA film on Si(100) and ¯ calculated A g for neutral ( frequencies r) PTCDA, and negatively ( )  and positively ( ) charged molecules. Oscillator strengthes of the calculated transitions (B3LYP/321G) are shown together with the absorption spectra of dissolved PTCDA [Gomez97] and a thin PTCDA film [Paraian02]. Absorption and fluorescence spectrum of PTCDA dissolved in DMSO [Hoffmann00a]. Configuration coordinate diagrams for the relaxation along the 1 elongation of the vibrational mode at 1346 cm (a) and relaxation of the molecule summed over all calculated vibrational modes (b). Calculated values are shown as points and solid curves show a parabolic Y approximation. The curvature for the ground state potential curve and Y excited state potential curve * are assumed to be equal. The arrows in Fig.3.7(b) show the absorption, relaxation and recombination processes, respectively. Linear absorption of PTCDA dissolved in dimethylsulfoxide. Circles: experimental spectrum [Hoffmann00a], discrete vertical lines: multi Poisson distribution of elongated A g  modes, solid line: absorption spectrum derived from the discrete transitions and a Gaussian

broadening of FWHM = 90 meV.

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Fig. 4.1 Fig. 4.2a

Fig. 4.2b

Fig. 4.2c Fig 5.1

Fig. 6.1

Fig. 6.2

Fig. 6.3

Fig. 6.4

Fig. 6.5

Fig. 6.6

Fig. 6.7

Timeresolved photoluminescence setup. ’ PTCDA crystal structure (top view). Thebandcaxes crystallographic ’ are shown, whereccorresponds to the (c2a) direction of the published crystal basis vectors [Lovinger84]. PTCDA crystal structure: Side view, showing the spacing between consecutive monolayers. PTCDA crystal structure: Side view, showing the crystallographic axisa. Absorption and PL spectra of epitaxial films and single crystals of PTCDA. Evolution of the PTCDA PL spectrum as a function of delay after the excitation pulse. The inset shows the decay of the PL intensity integrated over the whole spectral range on a logarithmic scale. Timeintegrated PL spectrum of a PTCDA crystal (circles) and the fit (solid curve) based on eq. 6.1. All 6 contributions are shown together w with their positionsh j. Decay of the PL intensity integrated over the whole spectral range (circles) and the fit based on eq. 6.2 (solid line) on a logarithmic scale. Timedependent prefactors of the 6 different contributions to the PL spectra, and fitting curves based on a biexponential decay model (solid lines). The model curve for the PL feature at the highest energy contains t components decaying on the fast and slow time scales fast = 3 ns and t slow = 33.5 ns, respectively, while all other PL bands are based on the t t two decay times medi =12.7 ns and slow = 33.5 ns, cp. eqs. (6.3, 6.4). PL spectra for selected delays (circles) together with the calculated curves based on the model for the timedependent PL (solid lines). For clarity, 5 consecutive spectral data points in Fig. 6.1 are summed up. Timeintegrated PL spectrum (circles) together with contributions of the three different decay channels (fast: blue solid line, intermediate: red short dashed line, slow: green long dashed line). Absorption and low temperature PL scenario calculated in the model based on the transfer of Frenkel excitons [Vragovic02b].

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Fig 7.1

Fig 7.2

Fig 7.3

Fig 7.4

Fig 7.5

Fig. 7.6

Timeintegrated PL spectra of a PTCDA single crystal measured at different temperatures in the range 10  300 K. Timedependence of the spectrallyintegrated normalized PL spectra at different temperatures, on a logarithmic scale. Temperature dependence of the Frenkel exciton broadening experimental (circles) and calculated curves according to eq. (7.2), with contributions of the lowest internal mode (dasheddotted), the effective external mode (dashed), and total (solid). For the total broadenings, temperature  independent contributions of 40, 65, 77, 92, and 120 meV are added for the lines at 1.85, 1.80, 1.70, 1.63, and 1.53 eV, respectively. Left: Evolution of the PTCDA PL spectra measured at 40 and 280 K as a function of delay after the excitation pulse (circles). Each trace for a given delay t corresponds to a timeintegration over an interval of 2.5 ns for a representative sample of delay times t = 5, 10, 20, 30, and 45 ns. The solid lines represents the calculated curves based on our model for the timedependent PL. Right: Timeintegrated PL spectra (circles) are shown at 40 and 280 K together with the Frenkel component (dashed olive), Excimer (dotted orange), CT (dash – dotted dark cyan and red) and the sum of all components including two high energy bands (solid, black). Decay times (left) and intensities (right) of the Frenkel exciton (squares), excimer (circles) and CT excitons (triangles) are shown as a function of temperature. The model curves obtained using eqs (7.4)  (7.6) are plotted as solid lines. Schematic potential energy diagram of the Frenkel exciton, the high and lowenergy CT excitons, and the excimer state, showing the formation barriers between the state of optical absorption and the selftrapped exciton states. The direction towards the r.h.s. shows the dispersion of the Frenkel exciton in kspace, and the configuration coordinate towards l.h.s. corresponds to the different deformation patterns for each of the

localized selftrapped exciton states visualized.

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