Elucidating the impacts of structural heterogeneity on excited state dynamics in solution-processed materials

Abstract

Solution-processed inorganic and organic semiconductors hold enormous promises due to their low manufacturing cost, scalability, and compatibility with flexible substrates. However, solution processing techniques do not require control over crystal growth, which can lead to structural defects within the crystal structure. The defects within solution-processed semiconductors can create significant challenges in optimizing device functionality; therefore, it is crucial to understand the impact of structural defects on photophysical properties. Traditional ensemble measurement techniques can conceal the effects of microscale structural defects on functional properties in the structure-averaged observation of solution-processed materials. The work presented in this dissertation employs time-resolved and spectrally resolved microscopy techniques to investigate the influence of structural heterogeneity on the photophysical properties of microscale solution-processed materials. Measurements collected across multiple discrete and highly crystalline domains of multiple classes of solution-processed materials have helped establish a relationship between the functionality and the local structure of these materials. Initially, the focus was on elucidating anisotropic carrier transport in lead halide perovskites by investigating lattice strain and energetic distribution in microcrystals. Later, the focus shifted towards characterizing and understanding the impact of structural defects on the excited state dynamics in another class of solution-processed material called metal-organic frameworks (MOFs). PCN-222 exhibited rapid exciton transport with time-averaged diffusion coefficients ranging from 0.27 to 1.0 cm2/s and subdiffusive behavior, showing transport slowing on the tens of ps time scale. Subdiffusivity indicated that excited states were rapidly transported through the porphyrin network of PCN-222 before being trapped. Moreover, the first transport measurements and transient absorption microscopic measurements in PCN-222 are reported here. Photoluminescence quenching and heterogeneous relaxation pathways were noted in regions with higher structural heterogeneity. Furthermore, the spectral evolution of porphyrinic PCN-222 MOF was investigated, which revealed excitation-dependent chromophore coupling in the MOF structure. Soret band excitation with enhanced coupling can create more mobile excited states, whereas Q band excitation with reduced coupling will generate fewer mobile excited states. Excitation-dependent chromophore coupling strongly dictates the transport and relaxation properties in MOF microstructures that also illustrate the impact of structural defects on the excited state transport and relaxation dynamics. A significant spectral shift has also been observed in microrods stemming from structural heterogeneity. These findings contribute to a deeper understanding of the impact of structural defects on the photophysical properties of solution-processed materials, facilitating the development of optimized semiconductor devices for various applications. The results reported in this dissertation will not only continue to aid in the characterization of MOFs but will also advance our understanding of excited state dynamics in a variety of solution-processed materials.