Materials World Network: Growth, Kinetics, and Morphology of Multi-Layered Organic Thin Films via Low-Energy Secondary Ion Mass Spectrometry

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

This project is based on a partnership between three research groups, one at the Science and Analysis of Materials (SAM) Department at the G. Lippmann Research Center in Luxembourg and two at the Materials Science Department of the University of Michigan (UM). This partnership merges resources that are not available to each participant individually. The purpose of this research is to investigate the shape, compositional definition, and energetics of interfaces in vapor-deposited multi-layer organic semiconductor thin films and devices fabricated by one of the UM groups. The increasing sophistication of optoelectronic devices requires molecular-level dimensional control in the fabrication of multi-layered structures with specifically engineered interfaces. However, the effectiveness of growth and doping strategies devised to achieve the desired device structures oftentimes remains unverified due to the lack adequate characterization techniques. This is particularly true for devices based on conjugated organic compounds, which find increasing use in energy applications (e.g. organic light-emitting diodes and organic photovoltaic cells, etc.). The buried interfaces are simply inaccessible or suffer damage when using conventional characterization techniques. Low-energy secondary ion mass spectrometry (LE-SIMS), a specialty of the SAM group, provides a promising avenue for the analysis of organic-based thin-film layered structures, because sub-keV impact energies of the primary ions result in reduced fragmentation of molecular species at the specimen surface. The physics of the collision cascades and the processes that lead to the ejection of secondary ions at low energies is still poorly understood, and a unified formalism for the identification of ejected species does not yet exist. Pursuing this knowledge, the other UM group combines large-scale molecular dynamics (MD) simulations with first-principles density functional theory (DFT) calculations to study the detailed atomic trajectories in collision cascades and predict the nature of ejected molecular fragments. This computational framework serves to interpret experimental data obtained from LE-SIMS, thereby improving the depth resolution of the technique and its ability to reliably identify organic molecular species, thus further establishing LE-SIMS as a technique for depth-profiling organic thin film materials. The goal of this project is to establish the relationship between growth conditions, structure, and properties of multi-layer thin film organic semiconductors with unprecedented precision. Fundamental insights for the advancement of organic electronic device design and fabrication techniques are anticipated. The project serves as the basis for three Ph.D. theses. Students benefit from a diverse educational experience through exchange visits to partner institutions, remote interactions between researchers, sharing of data, and the use of cyber infrastructure for the dissemination of findings through. Undergraduate students are involved directly at an academic level, and K-12 students through new outreach initiatives at UM.