Harald Ade (NCSU): "Experimental Considerations and Prospects for Tuning the Exciton Binding Energy in Organic Polymers"

A graduate of Stony Brook University, H. Ade has been a faculty member at NCSU since Nov. 1992, rising through the ranks to Full Professor by 2001, and been named Distinguished Professor of Physics in 2014 and Goodnight Innovation Distinguished Professor in 2017. He has had an active and continually funded research program  and served as Director of Graduate Program in Physics from 2006-2013. Recognitions include R&D100 Award, NSF Young Investigator Award, APS Fellow, AAAS Fellow, MRS Fellow, Alumni Outstanding Research Award (twice, NCSU), Holladay Medal (NCSU), K. F. J. Heinrich Award, and Shirley Price for Outstanding Science and Halbach Award of Innovative Instrumentation (both at the Advanced Light Source). He is a Clarivate Analytics Highly Cited Researcher in the field of Materials Science since 2017, based in large part of his development of soft-ray resonant scattering methods and it use in organic electronics. Some of his external engagements include serving on the Scientific Advisory Committee of the Advanced Light Source (2011- 2019) and the BESSY-II Synchrotron Facility in Berlin, Germany (2006-2009), as well as the Scientific Advisory Council of the Helmholz Zentrum Berlin, Germany (2009 – 2012). He has conceived and coordinates interdisciplinary efforts at NCSU across the Carbon and Organic Electronics Laboratories (ORaCEL) and the Carbon Electronics Cluster as part of the NCSU Chancellor Faculty Excellence Program. He was Chair of the 15th International Symposium on Functional π-electron Systems (Fπ -15).

Abstract: Discrepancies in reported values of exciton binding energy (Eb) for organic semiconductors (OSCs) indicate methodological or systematic inconsistencies that necessitate a comprehensive study. Traditionally, the Eb of Frenkel excitons is defined as the difference between the transport gap (Et) and the optical gap (Eopt). Here, we determined the Eb values for various PBnDT-TAZ polymer variants using two commonly employed methods to measure Et: a combination of ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy (UPS-IPES) and solid-state cyclic voltammetry (CV). Uniform experimental protocols were employed to investigate the potential relationship between the chemical structure and Eb. The Eb values obtained by UPS-IPES show no correlation with the polymer structure and range from 0 to 600 meV. In contrast, CV reveals an apparent Eb-molecular structure correlation, with values ranging from 200 meV to 1eV, depending on electron-donating or electron-withdrawing substituents on the polymer backbone/sidechain and side-chain polarity. We discuss the discrepancy the measurement methods by examining the limitations of the traditional definition of Eb, which overlooks the temporal and spatial dependence of Eb. Additionally, we address the limitations of each method in accurately measuring the transport gap. This highlights the context-dependent nature of method selection and its significance in drawing reliable conclusions from such experiments. Notably, the observed Eb trend derived from CV may be linked to electrolyte swelling and associated increase in dielectric constant, suggesting that high-efficiency single-material organic photovoltaics with high dielectric materials is conceptually valid. Additional studies are needed to further elucidate the observed trends.