78.00.00 Optical properties, condensed-matter spectroscopy and other interactions of radiation and particles with condensed matter
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78.20.-e Optical properties of bulk materials and thin films (for optical properties related to materials treatment, see 81.40.Tv; for optical materials, see 42.70-a; for optical properties of superconductors, see 74.25.Gz; for optical properties of rocks and mine
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78.30.-j Infrared and Raman spectra (for vibrational states in crystals and disordered systems, see 63.20.-e and 63.50.-x respectively)
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- 78.35.+c Brillouin and Rayleigh scattering; other light scattering (for Raman scattering, see 78.30.-j)
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78.40.-q Absorption and reflection spectra: visible and ultraviolet (for infrared spectra, see 78.30.-j)
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- 78.45.+h Stimulated emission (see also 42.55.-f Lasers)
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78.47.-p Spectroscopy of solid state dynamics (see also 42.65.-k Nonlinear optics; 42.50.-p Quantum optics)
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78.55.-m Photoluminescence, properties and materials
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78.60.-b Other luminescence and radiative recombination
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78.66.-w Optical properties of specific thin films (for optical properties of low-dimensional, mesoscopic, and nanoscale materials, see 78.67.-n; for optical properties of surfaces, see 78.68.+m)
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78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
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78.68.+m Optical properties of surfaces
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78.70.-g Interactions of particles and radiation with matter
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78.90.+t Other topics in optical properties, condensed matter spectroscopy and other interactions of particles and radiation with condensed matter (restricted to new topics in section 78)
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- Doctoral Thesis (3)
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This thesis addresses the identification and characterization of spin states in optoelectronic materials and devices using multiple spin-sensitive techniques. For this purpose, a systematic study focussing on triplet states as well as associated loss pathways and excited state kinetics was carried out. The research was based on comparing a range of donor:acceptor systems, reaching from organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) to organic photovoltaics (OPV) employing fullerene and multiple non-fullerene acceptors (NFAs). By developing new strategies, e.g., appropriate modeling, new magnetic resonance techniques and experimental frameworks, the influence of spin states in the fundamental processes of organic semiconductors has been investigated. Thereby, the combination of techniques based on the principle of electron paramagnetic resonance (EPR), in particular transient EPR (trEPR) and optically detected magnetic resonance (ODMR), with all-optical methods, such as transient electroluminescence (trEL) and transient absorption (TA), has been employed. As a result, excited spin states, especially molecular and charge transfer (CT) states, were investigated in terms of kinetic behavior and associated pathways, which revealed a significant impact of triplet states on efficiency-limiting processes in both optoelectronic applications.
This thesis describes novel concepts for the measurement of the static and dynamic properties of the electronic structure of molecules and nanocrystals in the liquid phase by means of coherent fluorescence-detected spectroscopy in two and three frequency dimensions. These concepts are based on the systematic variation ("phase cycling") of a sequence of multiple time-delayed femtosecond excitation pulses in order to decode a multitude of novel nonlinear signals from the resulting phase-dependent fluorescence signal. These signals represent any permutation of correlations between zero-, one-, two-, and three-quantum coherences. To this end, two new phase-cycling schemes have been developed which can simultaneously resolve and discriminate several nonlinear signals of sixth order, including those of the fourth order of nonlinearity.
By means of the sixth-order signals recorded in this work, static properties of highly excited electronic states in molecules such as their energies, transition dipole moments, and relative displacement of electronic potential surfaces, as well as dynamic properties in terms of their relaxation kinetics, can be ascertained. Furthermore, it was shown that these signals are suitable for the characterization of exciton-exciton correlations in colloidal quantum dots and for the measurement of ultrafast exciton-exciton annihilation in molecular aggregates.
The experiments performed in this thesis mark an important step towards the complete characterization of the nonlinear response of quantum systems. In view of this, the concept of fluorescence-detected multiple-quantum coherence multidimensional spectroscopy introduced here offers a unified, systematic approach.
In virtue of the technical advantages such as the use of a single excitation beam and the absence of nonresonant contributions, the measurement protocols developed here can be directly transferred to other incoherent observables and to sample systems in other states of matter. Furthermore, the approaches presented here can be systematically extended to higher frequency dimensions and higher orders of nonlinearity.
Der Fokus dieser Arbeit liegt in der Untersuchung des exzitonischen Transports, sowie der
Dynamik exzitonischer Zustände in organischen Halbleitern. Als fundamentale Fragestellung werden die inhärenten, materialspezifischen Parameter untersucht, welche Einfluss auf die Exzitonen-Diffusionslänge besitzen. Sowohl der Einfluss der strukturellen Ordnung als auch die fundamentalen exzitonischen Transporteigenschaften in molekularen Schichten werden anhand der archetypischen, morphologisch unterschiedlichen organischen Halbleiter Diindenoperylen (DIP), sowie dessen Derivaten, α-6T und C60 studiert. Die resultierende Filmbeschaffenheit wird mittels Röntgendiffraktometrie (XRD) und Rasterkraftmikroskopie (AFM) analysiert, welche Informationen über die Morphologie, die strukturelle Ordnung und die Mikrostruktur der jeweiligen molekularen Schichten auf verschiedenen Längenskalen liefern.
Um Informationen über die Exzitonen-Diffusion und die damit einhergehende Exzitonen-
Diffusionslänge LD zu erhalten, wurde die Methode des Photolumineszenz (PL)-Quenchings
gewählt. Um umfassende Informationen zur Exzitonen-Bewegung in molekularen Dünnschichten zu erhalten, wurde mit Hilfe der Femtosekunden-Transienten-Absorptionsspektroskopie (TAS) und der zeitkorrelierten Einzelphotonenzählung (TCSPC) die Dynamik angeregter Energiezustände und deren jeweiliger Lebensdauer untersucht. Beide Messverfahren gewähren Einblicke in den zeitabhängigen Exzitonen-Transport und ermöglichen eine Bestimmung des Ursprungs möglicher Zerfallskanäle.
Die zentralen Ergebnisse dieser Arbeit zeigen zum einen eine Korrelation zwischen LD und der strukturellen Ordnung der Schichtmorphologie, zum anderen weist die temperaturunabhängige
Exzitonen-Bewegung in hochgeordneten polykristallinen DIP-Filmen auf die Möglichkeit
der Existenz eines kohärenten Exzitonen-Transports bei tiefen Temperaturen unterhalb von 80 K hin. Zeitaufgelöste spektroskopische Untersuchungen lassen zudem auf ein breites Absorptionsband höherer angeregter Zustände schließen und weisen eine höhere Exzitonen-
Zustandsdichte in polykristallinen DIP-Schichten im Vergleich zu ungeordneten Filmen auf.