Exciton Delocalization and Quantum Dynamics in Push-Pull Semiconductor Polymers

Conjugated polymers are a class of potential semiconductor materials that can be employed in stretchable microelectronics, electrophysiology and infrared photodetectors due to their intrinsic stretchability and highly-engineerable molecular levels. Their interesting photophysical behavior and electrical performance largely originates from the bound electron-hole pairs shared between monomer units upon excitation. Such quasiparticles were termed Frenkel excitons. Although the polymer chain backbone planarization enhances the -electron conjugation, polymers chains are disordered, with torsional modes breaking long-range exciton delocalization. This thesis work shows how tuning the polymer solutions concentrations will impact the photophysical responses in electron push-pull conjugated polymers, which are sensitively determined by the chain conformations. The well-characterized sample set allows us to further investigate the intriguing many-body phenomenon as well as their dynamics. By performing fluence-dependent transient absorption and excitation-correlation photoluminescence spectroscopy, we show that the temporal decays cannot be simply explained by a time-independent exciton-exciton annihilation model, which indicates that the exciton diffusion process in DPP-DTT is not isotropic. Instead, we have to employ the one-dimensional exciton diffusion model to describe dynamics, which results in a constant diffusion length of 9 nm. Combining this result and the exciton delocalization behavior we showed in the first work, we suggest that the exciton exciton interactions are most likely enhanced by short-range Coulomb interactions or wave function overlap. To further investigate the nature of the many-body interactions and multi-quantum correlations in this type of materials, we implemented the two-dimension coherent spectroscopy. Interestingly, we observed not only attractive biexcitons but also unbound biexciton pairs, manifested as the dominant spectral features along the diagonal axis. In addition, we ascribe the side peak to a heterogeneous vibronic state. By considering the complete set of Feynman pathways, we were able to explain the imbalanced side peak observed in the one-quantum spectra. In perspective, although solution concentrations are probably one of the most fundamental parameters to consider for polymer sample processing, their impact on the chain conformation and aggregation should be addressed carefully. The gentle alterations in macromolecular conformation in these materials can have a significant impact on both how well the device operates and the quantum phenomena occurring within the material. Our work shed new light on the many-body interactions and correlations in the Frenkel exciton systems and their implications could be considered as figures of merit in explaining different performance of novel optoelectronic devices.

Date

2024-04-08

Resource Type

Text

Resource Subtype

Dissertation

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