The electronic structure and exciton dynamics of the molecules and polymers that form the active layer in organic electronic devices can change dramatically during solution deposition. As solvent vaporizes, molecules aggregate and become electronically coupled, sometimes dramatically changing the exciton dynamics and thus the suitability of the material for electronic devices. The exciton dynamics of molecules in solution and in films of aggregates can be measured using transient absorption spectroscopy. However, the progression of exciton dynamics during film formation is unknown since measurements typically cannot be performed quickly enough to collect accurate transient absorption spectra of these species. The exciton dynamics of evolving material systems can be measured by increasing the speed of data collection. A novel implementation of transient absorption spectroscopy is introduced that can measure transient spectra with up to a 60 ps pump-probe time delay in one shot. The exciton dynamics of intermediate aggregation states are revealed during the formation of an organic film. We use a model to simulate absorbance spectra and fit the simulated spectra to those collected experimentally. The resulting fits provide insight into the structure of the nascent aggregates during the film formation process. Notably, we find that the measured spectra likely represent a mixture of aggregates of varying size and structure. Together, the presented experiments and modelling provide a detailed picture of the progress of molecular aggregation during thin film formation.

In situ transient absorption spectroscopy during the formation of organic thin films

ACS Fall 2020 Virtual Meeting

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