Electronic Processes in Organic Semiconductors

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.