Speaker: Dr. Russell Holmes, University of Minnesota

Speaker:  Dr. Russell Holmes, Department of Chemical Engineering and Materials Science, University of Minnesota

This is an MSE Joint ORaCEL Seminar

Title:  Engineering molecular orientation and polarization in organic semiconductor thin films  – Impact on optoelectronic device design and performance

Abstract: 

Organic semiconductors are conjugated molecular materials whose facile thin film processing and tunable optoelectronic properties have made them of interest for applications in light-emission, detection, and solar photoconversion. In addition, devices made from organic semiconductors can be integrated with mechanically flexible substrates, enabling novel form and functionality. Indeed, light-emitting devices (i.e. OLEDs) based on these materials have already been widely commercialized in mobile phones, with effort growing for their use in televisions, lighting, and solar energy conversion.

OLEDs in particular, are frequently constructed from glassy, molecular thin films. Despite their largely amorphous character, these films offer a rich tunability in molecular orientation that until recently, has been overlooked as a route for device engineering. Notable over the last decade have been advances in controlling emitter molecule transition dipole moment (TDM) orientation to enhance light extraction and device efficiency. This talk will focus on a related phenomenon that instead involves orientation of the molecular permanent dipole moment (PDM) leading to the formation of large, internal polarization fields. We find that this internal, spontaneous orientation polarization (SOP) is sufficiently large to modify device charge injection characteristics and induce significant exciton-polaron quenching, even at low-bias. The latter is surprising since prior work has assumed that bimolecular quenching only becomes severe under high injection. We demonstrate that SOP-induced quenching plays a significant role in determining peak device efficiency, while also potentially factoring into device stability. Finally, we demonstrate that SOP, and the associated quenching can be tuned with appropriate choice of thin film processing parameters. Since spontaneous polarization has been observed in a variety of organic semiconductors, we hypothesize that the associated quenching losses have gone largely unnoticed in OLEDs or attributed to other phenomena. These results will be discussed in the general context of maximum OLED efficiency and the refinement of materials selection and device design rules.

Bio:

Russell J. Holmes is a Professor in the Department of Chemical Engineering and Materials Science at the University of Minnesota, where he holds the Ronald L. and Janet A. Christenson Chair in Renewable Energy. He has also been named a Distinguished McKnight University Professor and Distinguished University Teaching Professor. He previously served as Director of Graduate Studies in Materials Science from 2013-2020, and currently leads the Flexible Electronics and Photovoltaics program of the Industrial Partnership for Research in Interfacial and Materials Engineering. From 2018-2019, he served as a Leverhulme Trust Visiting Professor at the University of Cambridge. Since 2017, Holmes has served as an Editor of Synthetic Metals, a journal dealing with organic electronic materials. He completed his B.Sc. (Honours) in Physics at the University of Manitoba in Canada, and his M.A. and Ph.D. in Electrical Engineering at Princeton University. His group carries out research in the optoelectronic characterization of organic and hybrid organic-inorganic thin film semiconductors, as well as their application in devices for light-emission and photoconversion.  Recent emphasis has been on elucidating structure-property relationships governing energy transport, engineering and understanding the role of interfaces, and developing new optoelectronic probes of exciton and charge behavior.