Polymorphism and Strong Light-Matter Coupling in Organic Semiconductors

The extensive structural variability of organic semiconductors enables wide-ranging modulation of their optical, electronic, and mechanical properties. However, the number of organic semiconductors suitable for long-term stable organic transistors, solar cells, LEDs, and photodetectors is comparatively small. Two promising strategies for targeted property modulation – without altering chemical structure – are controlling the crystal structures of organic semiconductors (i.e., their polymorphism) and utilizing strong light-matter coupling in optical cavities (polaritonics). Despite decades of research, the targeted deposition and stabilization of metastable polymorphs in organic thin films remain a significant challenge. Using the example of a newly synthesized p-type semiconductor, a 5,10-dihydroindolo[3,2-b]indole derivative, this dissertation demonstrates how long-term retention of a metastable polymorph and its electronic and optical properties can be achieved through control of the initial film morphology. Whereas rough and polycrystalline evaporated films exhibit rapid crystal structure transformation, defect-free, smooth zone-cast films are stable for several months. Annealing these films does not accelerate conversion; instead, it leads to thermodynamic stabilization of the metastable polymorph and thus even a reversal of aged films. These approaches to polymorph stabilization may be extended to other organic polymorphic systems. Furthermore, this dissertation demonstrates the potential of zone-casting for controlling morphology and polymorphism of thin films, using the well-known n-type semiconductor PDIF-CN₂ as an example. Highly aligned films of the standard single-crystal polymorph and of a previously unknown thin film polymorph with unique polarization-sensitive optical properties and robust charge transport are selectively deposited through optimization of the deposition parameters. The molecular packing structure of the new thin film polymorph, a twisted molecular “sandwich” structure, exhibits both H- and J-aggregate properties depending on light polarization making it an interesting candidate for polarization-sensitive photodetectors. Anisotropic organic thin-film layers with strongly polarization-dependent optical properties are particularly relevant for the field of molecular polaritonics and its applications in lasers and photodetectors. However, the few studies on anisotropic polaritonic systems report contrasting interpretations of their observed optical properties. This dissertation investigates the polarization-dependent strong light-matter coupling of the newly discovered birefringent PDIF-CN₂ polymorph within Fabry-Pérot microcavities. Angle-resolved reflectivity and photoluminescence measurements reveal two orthogonally polarized systems, each with distinct coupling strengths that remain nearly invariant under variations in light polarization and propagation direction. This analysis supports earlier theoretical predictions about the influence of birefringence on the formation of exciton polaritons. It can serve as a foundation for the potential use of PDIF-CN2 and similar systems in polarization-selective polaritonic devices.