Excited state dynamics in novel energy conversion materials

Biewald, Alexander (2024): Excited state dynamics in novel energy conversion materials. Dissertation, LMU München: Fakultät für Chemie und Pharmazie

Abstract

The photophysical properties of light-absorbing semiconducting materials are of significant interest, as these materials are essential for light-induced energy conversion processes such as those in solar cells. This work investigates photo-induced excited state dynamics in semiconductors using confocal microscopic measurement techniques, focusing specifically on the radiative photoluminescent decay of both static and diffusing excited states. The thesis begins with an introduction to the physical description of excited states in materials, comparing their characterization in semiconductors to that in molecules. Additionally, the excited state can represent a mobile state, driven by a chemical potential. A subsequent chapter explains the dynamics of diffusion combined with photophysical decay, linking measured data with diffusion functionality through various numerical and analytical approaches. Chapter three is introducing the confocal microscope and the implemented advanced time-resolved and spectroscopy techniques. Furthermore, a detailed description of the signal generation is given determining the measured photo luminescence. The final section explains the numerical analysis of the measured data. These foundational chapters serve to introduce the main projects discussed in this work. In methyl ammonium lead iodide (MAPbI 3 ) thin films, the diffusive transport of excited ambipolar charge carriers is hindered by optical phonons and lattice fluctuations. Additionally, the disorder induced by phase transitions in thin films was found to halt this transport. In ceasium formamidium lead triiodide (CsFAPbI 3 ) quantum dot films, restructuring of the clustering improved photophysical properties and consequently enhanced power conversion efficiency. For lead-free perovskite systems, the inferior solar cell performance was attributed to strong localization of excited states due to coupling to lattice phonons and immobile defect states. The novel approach using wurster-benzodithiophene-dialdehyde covalent-organic-framework (WBDT COF) highly crystaline thin films, a tunable material composed solely of organic molecules, exhibited semiconductor-like behavior, with disorder identified as a critical factor influencing excited state diffusive transport. Finally, measurements on rare-earth-ion metallic-organic-frameworks (REI MOFs) revealed a non-parametric downconversion process in ytterbium 2,5-dihydroxy-1,4-benzoquinone metallic-organic-frameworks (DHBQ MOFs), demonstrated through second-order time correlation function analysis of emitted photons.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: Perovskite, COF, covalent organic framework, Exciton, Polaron, Microscopy, Nanooptics
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 540 Chemie
• Fakultäten: Fakultät für Chemie und Pharmazie
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 17. Oktober 2024
• 1. Bericht­erstatter:in: Hartschuh, Achim