Darkfield Transient Absorption Spectroscopy of Evolving Domain Boundaries in Organic Films

With support from the Chemical Structure and Dynamics (CSD) program in the Division of Chemistry, Professor Cathy Wong of the University of Oregon-Eugene is developing a new darkfield transient absorption spectroscopy that uses spatial frequency filtering strategies to investigate the intermolecular arrangement and photophysics at domain boundaries in films of fused ring electron acceptors (FREAs). The performance of a film of molecular aggregates is determined by how domain boundaries form, and how their electronic structure, excited state dynamics, and intermolecular arrangement emerge during molecular aggregation.

However, the challenge in a typical bulk measurement is that signal from a boundary is usually overwhelmed by the larger signal from adjacent domains. Professor Wong and her students will develop an in situ darkfield transient absorption spectrometer for measuring domain boundaries in films of electron donors and FREAs and will determine the measured molecular arrangements by simulating the measured spectra.

Their studies could provide better understanding of molecular aggregation at interfaces and could develop darkfield transient absorption as a widely adopted technique in photophysics. The central concept of this research is brought into the undergraduate classroom with a complementary physical chemistry laboratory focused on darkfield microscopy.

Single-shot transient absorption spectroscopy, developed in Professor Wong’s laboratory, can measure the excited state dynamics and electronic structure of materials systems in a few seconds. This spectroscopy enables the measurement of materials while they change, for example during their formation. Professor Wong and her team will combine this technique with spatial frequency filtering to isolate the signal from domain boundaries as they form in thin films of FREAs.

They will isolate the signal from domain boundaries with particular spatial orientations, and measure rates of charge transfer and exciton recombination as a function of pump polarization. In this way, the rates can be determined as a function of molecular orientation relative to a domain boundary. By performing these measurements while varying film deposition parameters, they can determine how domain boundaries that result in higher charge transfer rates can be produced.

FREAs have been shown to improve the efficiency of organic photovoltaics, and these studies may elucidate how functionality can be further improved. The broader impacts of this work include the ability to control domain boundaries in organic films that are used in electronics and photovoltaics, and the development of a new spectroscopy that can isolate signal from tiny boundaries and defects.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.