Teaching

Chapter 1 – Basic Principles of Electronic Transitions: This chapter introduces basic principles of light matter interaction, electronic transitions, and fundamental equations allowing the calculation of parameters related to electronic transitions.

Chapter 2 – Basic Measurement Principles: In this chapter, we discuss basic steady-state and time-resolved spectroscopic techniques and measurement principles, specifically UV-Vis spectrophotometry, photoluminescence spectroscopy, fluorescence excitation spectroscopy, quantum yield measurements, as well as time-resolved fluorescence measurement techniques and basic data analysis methods.

Chapter 3 – Spectra of Molecules I: In this chapter, we discuss the origin of electronic transitions in organic molecules by investigating their orbital electronic structure and probability of electron transition between different types of molecular orbitals. Furthermore, we discuss fundamental principles of electronic transitions including Fermi´s Golden Rule, the Born-Oppenheimer approximation, and the Franck-Condon principle. Lastly, we derive the four selection rules of electronic transitions.

Chapter 3 – Spectra of Molecules II: In this part of Chapter 3, we will cover fundamentals of electronic transitions in organic molecules, spectral shifts induced by the environment (for instance, solvents of different polarity), and radiation-less transitions leading to dissipation of energy and non-radiative recombination of excited states.

Chapter 4 – High-resolution optical spectroscopy: In this chapter, we discuss the intrinsic absorption and emission lineshape of individual molecules observed by high-resolution fluorescence spectroscopy under cryogenic temperatures and we compare the emission to that observed from an ensemble of molecules, where emission from multiple emitters / molecules is observed.

Chapter 5 – Intermolecular Interactions: In this chapter we discuss the photophysical properties of H-type and J-type aggregates created by intermolecular interaction of two or more molecules, more precisely electronic coupling of their transition dipole moments. Furthermore, we discuss charge transfer states observed in dimers of donor and acceptor molecules, as well as excimers and exciplexes, which are molecular dimers existing in the excited state only.

Chapter 6 – Energy Transfer Processes: In this chapter, we discuss the fundamentals of excited state energy transfer in organic molecules. After a general introduction into the prerequisites of energy transfer processes, the two most common energy transfer mechanisms, Förster resonance energy transfer (FRET) and Dexter energy transfer are introduced and the difference to trivial energy transfer is discussed.

Chapter 7 – Excitonic States: In this chapter, we discuss the concept of excitons in the framework of quasiparticles and then introduce and discuss the origin and physics of different types of excitons including Frenkel-type, Mott-Wannier-type, and Charge-Transfer-type excitons. The physics of exciton motion in disordered and ordered solid states are briefly discussed and experimental evidence of the existence of excitons in matter is shown.

Chapter 8 – Photoexcitations in Conjugated Polymers: In this chapter, we discuss the concept of delocalization of pi-electrons in a conjugated molecular system, including oligomers and polymers. Different models to describe the dependence of the energy gap on the oligomer / polymer length are introduced. Furthermore, we discuss elementary excitations and quasi-particles in conjugated materials and electronic transitions between excited states.

Chapter 9 – Specific Examples of Conjugated Polymers: In this chapter, we discuss some specific examples of conjugated polymers, their photophysical properties, as well as routes to make conjugated polymers.

Chapter 10 – Photophysics and Devices: In the first part of this chapter, we discuss the photophysical properties of conjugated polymers, including disorder causing, for instance, spectral relaxation, as well as charge carrier transport in thin films. In the second part, we discuss applications of conjugated polymers in thin film optoelectronic devices, specifically organic light emitting diodes (OLEDs) and organic solar cells (OSCs).

Chapter 11 – Organic Photovoltaics: This chapter presents a glance into the fascinating topic of organic photovoltaics, which uses organic semiconductors to convert light into electricity. Here, we briefly discuss the general layout, the available donor and acceptor materials and the impact of the thin film morphology on device performance. Next, we discuss the physics of charge generation and recombination and what controls the device voltage and fill factor, in other words the figures of merit determining the power conversion efficiency.

Gruppentheorie 1: In diesem Teil der Vorlesung werden zunächst Symmetrieoperationen beschrieben und dann die Punktgruppen und deren Symmetrieelemente eingeführt. Es wird ein Schema zur Bestimmung der Punktgruppe von Molekülen vorgestellt.

Gruppentheorie 2: In diesem Teil der Vorlesung diskutieren wir verschiedene Darstellungen, die es uns mit Hilfe von Darstellungsmatrizen und Basen erlauben, Symmetrieoperationen mathematisch zu beschreiben. Sie lernen Charaktertafeln für Punktgruppen zu erstellen und anhand der Charaktertafeln Moleküleigenschaften wie z.B. das Vorhandensein von Schwingungsbanden in der IR-Spektroskopie vorauszusagen. Dieser Teil der VL leitet am Ende in den zweiten Teil der VL über, wo wir uns mit Molekülspektroskopie beschäftigen werden.

Molekülarspektroskopie 3: In diesem Kapitel der Vorlesung besprechen wir die Natur des Lichts, einerseits elektromagnetische Welle, andererseits Teilchen (Welle-Teilchen-Dualismus), weiterhin die Wechselwirkung von Licht mit Materie und die damit verbundenen Übergänge zwischen Grundzustand und angeregten Zuständen in Atomen und Molekülen. Im weiteren Verlauf betrachten wir aus quantenmechanischer Perspektive Übergänge und formulieren Fermis Goldene Regel sowie eine fundamentale Auswahlregel, die Laporte oder Symmetrieregel.

Molekülarspektroskopie 3b: In diesem Teil der Vorlesung besprechen wir verschiedene Licht-Materie-Wechselwirkungen wie Absorption und Emission und diskutieren die Charakteristika von Spektren und Banden wie z.B. die homogene und inhomogene Linienverbreiterung. Danach werden die wichtigsten Lichtquellen, Detektoren, Monochromatoren und Polychromatoren besprochen, die bei spektroskopischen Experimenten zum Einsatz kommen. Dann erfolgt eine Einführung in das Prinzip des Interferometers und dessen Anwendung in der Fouriertransform-Spektroskopie.

Molekülarspektroskopie 4: In diesem Teil der Vorlesung betrachten wir die Grundlagen von schwingungsangeregten Zuständen in organischen Molekülen und beschreiben diese klassisch und quantenmechanisch. Dann schauen wir uns Besetzungswahrscheinlichkeiten von Schwingungszuständen an und diskutieren deren Wechselwirkung mit Licht. Im weiteren Verlauf besprechen wir zunächst die Grundlagen der Schwingungs-Ramanstreuung und dann die Schwingungsbewegung mehratomiger Moleküle. Abschließend bestimmen wir die Normalschwingungen einfacher Moleküle mit Hilfe der Gruppentheorie.

Molekülarspektroskopie 5: In diesem Teil der Vorlesung geht es um Energieniveauschema organischer Moleküle sowie deren elektronische Zustände. Es werden optische Übergänge besprochen sowie Spektroskopiemethoden wie die (zeitaufgelöste) Absorptions- und Fluoreszenzspektroskopie.

Climate Change and Energy Economy: This part of chapter 1 discusses the world´s energy reserves and annual energy consumption and introduces photovoltaics as an interim solution to the world´s growing demand for electrical energy.

Quantum Statistics: I derive distribution functions for integer spin particles (bosons) and half-integer spin particles (fermions) and discuss their properties with regard to the already known Maxwell-Boltzmann statistics of particles. Then, I introduce Fermi-Dirac (FD) quantum statistics and its application to the physical description of the valence electron gas in metals.

Semiconductors and pn-junctions: We discuss the intrinsic properties of semiconductors, specifically their energy levels (band structure), the density of states and carrier distribution functions, as well as intrinsic carrier concentrations. Then, we discuss the effect of doping on the intrinsic carrier concentration and Fermi levels in semiconductors. Finally, what happens when a semiconductor is exposed to light and how the generation of excess charge carriers in a solar cell leads to the so-called quasi-Fermi level splitting, resulting in work performed by the solar cell.

Radiative recombination and charge transport: We discuss radiative carrier recombination in the framework of the detailed balance principle and the continuity equation, as well as charge carrier transport by drift and diffusion.

Non-radiative recombination: We discuss non-radiative (dark, radiation-less) carrier recombination processes including Shockley-Read-Hall (SRH), surface recombination (SR), and Auger-Meitner recombination (AMR) processes and their impact on photovoltaic device performance.

pn junctions: What happens when you bring together in physical contact an n-type and a p-type semiconductor to create what is called a pn-junction, which is the core of any photovoltaic device.

The efficiency of ideal solar cells: In this final chapter, we discuss the theoretical efficiency limit of the ideal solar cell and compare it with the efficiency of real solar cells.