Developing Spatially-Offset Femtosecond Stimulated Raman Spectroscopy to Investigate Charge Transport Through a Vibrational Lens

Abstract

Current solar cell technology based on a single p-n junction has a maximum efficiency dictated by the Shockley-Queisser limit of 33%. Singlet fission, which occurs in some organic semiconductors, has the potential to push that limit to 44% because it results in the formation of two separate triplets from the input of one photon. In order for organic semiconductor solar cells based on singlet fission to be a viable option, the excitons need be capable of traveling a sizable distance (up to microns) to reach the electrodes. Thus, understanding what facilitates or hinders exciton transport is crucial in the optimization of charge transport efficiency. However, exciton transport is notoriously difficult to study because they are short-lived, have short diffusion lengths, and easily recombine. In this project, I focused on developing a brand new ultrafast Raman imaging technique called spatially-offset femtosecond stimulated Raman spectroscopy (SO-FSRS) that has both the spatial and temporal resolution to track structural changes in molecular systems during exciton transport. Any structural changes can alter the frequenciesor intensities of vibrational modes which are reflected in the Raman spectra. In the experimental setup, the photoexcitation pulse is displaced from the Raman probe and pump pulses such that excitons are generated at a known distance from the probing region. The photoexcitation pulse is then raster-scanned to generate a Raman map of exciton transport. The details on how SO-FSRS are developed are documented in this thesis. After the successful development of SO-FSRS, its utility was first demonstrated with 6,13-bis(triisopropylsilylethynl) pentacene. I showed that the fast exciton and free charge carrier transport axes are identical, but the exciton transport is less anisotropic by a factor of ~3. SO-FSRS is the first technique that can directly track molecular structural evolution during exciton transport, which can provide us with chemical insights on how to tailor-make molecules for specific electronic devices.