Radiative and Nonradiative Recombinations in Organic Radical Emitters

The Effect of Guest–Host Interactions

Significance 

Organic Light Emitting Diodes (OLEDs) are solid-state devices composed of thin films of organic molecules that create light with the application of electricity. Their recent penetration into the high-end market of commercial electronic devices has been triggered by their outstanding properties: i.e. light weight, panel flexibility, low power consumption, high brightness, and high contrast. In purely organic emitters with a closed-shell electronic structure, according to spin statistics, the recombination of the electrons and holes injected at their respective electrodes generally produces singlet and triplet excitons in a ratio of 1:3. Since the lowest triplet electronic state (T1) is normally located below the lowest singlet state (S1) in fluorescent emitters, only 25% of generated excitons can be harvested. Thus, to avoid losing 75% of the input power, several strategies have been explored to harvest the triplet excitons. The most notable involve the use of heavy metal-based phosphors (which are at the heart of the light-emitting materials currently used in commercial OLEDs) or, over the past decade, of purely organic thermally activated delayed fluorescence (TADF) emitters. Very recently, in parallel to these efforts, a novel design paradigm has emerged based on neutral organic radical-carrying emitters with an open-shell electronic structure.

In general, despite the extensive efforts conducted to improve the luminescence efficiency of radical-based optoelectronic devices, the details of how the excited states form remain elusive, a consequence of the complexity of these systems. Also, there is a lack of in-depth investigations to unveil the effect that the host matrix can have on the radical excited-state radiative and nonradiative decay properties. Since these processes are critical in device operation, it appears highly desirable to gain a much greater understanding of their characteristics in order to guide the development of radical-based materials for OLEDs and other applications. On this account, scientists at the University of Arizona: Dr. Hadi Abroshan, Professor Veaceslav Coropceanu and Professor Jean-Luc Brédas used multi-scale simulations to describe the factors determining the optoelectronic properties of radical emitters in realistic solid-state morphologies. Their work is currently published in the research journal, Advanced Functional Materials.

In essence, the research team proposed to address the aforementioned challenges of emitter–host interactions from a theoretical perspective. Generally, by combining molecular dynamics simulations and density functional theory calculations, the impact of the host matrix on the optoelectronic performance of radical emitters was evaluated, taking as a representative example the (4-ncarbazolyl-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)-methyl (TTM-3NCz) radical emitter dispersed in a 4,4-bis(carbazol-9-yl)biphenyl (CBP) host. Ideally, the team chose to rely on a multi-tiered computational approach to study the morphology and electronic properties of emissive layers containing radical emitters.

The morphological analysis showed that steric effects around the radical centers, carried by the TTM electron-poor moieties of the emitters, disfavored π–π interactions with the host molecules, which lead to random intermolecular orientations around the TTM moieties. Moreover, the authors reported that the 3NCz electron-rich moieties of the emitters, however, had much lesser spatial hindrance for intermolecular π–π stacking, which modulates the structural and electronic properties of the emitters in the host matrix. The results also underlined that the host–emitter interactions taking place in the solid state can reduce the dynamic disorder in the excited states of the radical emitters, a feature that can be used to tune the optoelectronic properties of radical-based OLEDs.

In a statement to Advances in Engineering, the authors highlighted that their work provided vital inputs in this field and future studies will determine the best ways to tune these interactions via a selection of the host materials that can promote stronger electroluminescence of the emissive layers. Importantly, applications of radical-based organic materials are not limited to OLEDs but can potentially extend to fields including spintronics, imaging, and quantum information technologies.

Radiative and Nonradiative Recombinations in Organic Radical Emitters: The Effect of Guest–Host Interactions - Advances in Engineering

About the author

Hadi Abroshan is currently a Senior Scientist at Schrödinger, Inc. He received a Ph.D. in Chemistry from Carnegie Mellon University, working with Prof. Hyung J. Kim and Prof. Rongchao Jin. As a Postdoctoral Researcher, he worked with Prof. Jens K. Nørskov in the Department of Chemical Engineering at Stanford University and Prof. Jean-Luc Brédas in the Department of Chemistry and Biochemistry at the Georgia Institute of Technology and the University of Arizona.

With an extensive research background in computational materials science, Hadi has performed multiscale simulations and led successful projects to design cost-effective multifunctional materials for optoelectronics, catalysis, and nanotechnology. He has developed computational strategies and simulation protocols to tackle fundamental challenges in materials modeling in synergistic collaboration with multiple research institutes. Furthermore, he has conducted projects to extract accurate information on the time- and size-evolution of functional devices. His efforts have led to the discovery of novel environmentally friendly materials as well as processes with superior efficiencies.

About the author

Dr. Veaceslav Coropceanu is currently a Research Professor in the Department of Chemistry and Biochemistry at the University of Arizona. He received his Ph.D. in Theoretical and Mathematical Physics from the State University of Moldova in 1985. In 1994 he was appointed as Associate Professor at the same university. After research stays at the University of Sussex, United Kingdom, on a NATO/Royal Society Fellowship and at the Medical University of Lübeck, Germany, on an Alexander von Humboldt Fellowship, he joined the Brédas research group in 2000 at the University of Arizona. In 2003 he moved to the Georgia Institute of Technology where he became a Principal Research Scientist in the School of Chemistry and Biochemistry.

He moved back to the University of Arizona in 2020. His research interests revolve around theoretical studies of the electronic and optical properties of organic and inorganic systems, including energy-transfer and electron-transfer phenomena.

About the author

Jean-Luc Bredas received his B.Sc. (1976) and Ph.D. (1979) degrees from the University of Namur, Belgium. In 1988, he was appointed Professor at the University of Mons, Belgium, where he established the Laboratory for Chemistry of Novel Materials. While keeping an “Extraordinary Professorship” appointment in Mons, he joined the University of Arizona in 1999. In 2003, he moved to the Georgia Institute of Technology where he became Regents’ Professor of Chemistry and Biochemistry and held the Vasser-Woolley and Georgia Research Alliance Chair in Molecular Design. Between 2014 and 2016, he joined King Abdullah University of Science and Technology (KAUST) as a Distinguished Professor and served as Director of the KAUST Solar & Photovoltaics Engineering Research Center. He returned to Georgia Tech in 2017 before moving back to the University of Arizona in 2020.

Jean-Luc Bredas is an elected Member of the International Academy of Quantum Molecular Science, the Royal Academy of Belgium, and the European Academy of Sciences. He is the recipient of the 1997 Francqui Prize, the 2000 Quinquennial Prize of the Belgian National Science Foundation, the 2001 Italgas Prize, the 2003 Descartes Prize of the European Union, the 2010 ACS Charles Stone Award, the 2013 APS David Adler Award in Materials Physics, the 2016 ACS Award in the Chemistry of Materials, the 2019 Alexander von Humboldt Research Award, and the 2020 MRS Materials Theory Award. He is Honorary Professor of the Institute of Chemistry of the Chinese Academy of Sciences and Adjunct Professor of Chemistry at the Georgia Institute of Technology and King Abdulaziz University. He has served as editor for Chemistry of Materials since 2008. He has published over 1,100 refereed publications, which have garnered over 113,000 citations; his current Google Scholar h-index is 154.

Reference

Hadi Abroshan, Veaceslav Coropceanu, Jean-Luc Brédas. Radiative and Nonradiative Recombinations in Organic Radical Emitters: The Effect of Guest–Host Interactions. Advanced Functional Materials 2020, volume 30, 2002916.

Go To Advanced Functional Materials

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