Imaging Defect Dynamics in Organic Semiconductor Films

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Technical Description: The goal of this research project is to apply novel scanning probe microscopy tools to understand how local structure and disorder influence photochemical stability and charge transfer state formation in organic semiconductor films. To accomplish this goal, the project uses new experimental capabilities such as time-resolved electrostatic force microscopy to make direct nanoscale correlations between local structural disorder and local variations in photochemical trap formation, doping, and interfacial charge transfer state energies. These observations can add a unique perspective to our understanding of how structure and processing affect the photochemistry and photophysics of organic semiconductors with technological implications for photodiodes and photovoltaics. The project tackles three challenges related to the overall goal: (1) using sub-microsecond time-resolved electrostatic force microscopy methods to study the role of heterogeneity and local disorder on the photochemical degradation of state-of-the art high-efficiency organic bulk heterojunctions; (2) understanding the role of local structure and disorder on local carrier transport, photochemical doping, and the evolution of these photochemical processes in time using non-contact energy dissipation (quality factor) measurements; and (3) understanding the role of morphology and structural heterogeneity on the energetic distribution of charge transfer states in a spatially and spectrally resolved fashion.Non-Technical Description: Organic semiconductors are currently used in electronic displays, and are of interest for use as light-weight, low-cost solar cells (for instance to power and recharge consumer electronics, or eventually harvest solar energy for the grid). Long term stability is a major challenge for these applications, and this project seeks to better understand the details of how imperfections and defects form in these materials at very fine length scales of tens of nanometers through the application of newly developed scanning-probe microscopy tools. The possible applications of organic semiconductors in lighting and solar energy not only have significant direct economic and societal impact, but can be valuable hooks for engaging students and the broader public in science education. The outreach component of the project develops new digital and visual media for large scale public engagements reaching K-12 students and the local community. In addition to training graduate students at the interface of chemistry, physics, and materials science, the project directly supports undergraduate research and seeks to address science education pipeline issues by beginning a new partnership with the Rainier Scholars program.