Molecular mixtures as a pathway to enhance the electronic and mechanical properties of organic films

Significance 

The pursuit to increase the efficiency of low cost solar-to-electric energy conversion has led to the development of organic photovoltaics. Such advances are hoped to enable fabrication of larger and flexible devices in comparison with convectional inorganic photovoltaics. Such auspicious developments have however encountered multiple challenges in organic photovoltaics regarding their efficiency, scalability, cost and flexibility in order to effectively compete with inorganic and hybrid photovoltaics. Consequently, development of a large number of novel materials in the fields of organic photovoltaics and organic electronics has led to an optimization challenge for the selection process. Presently, fullerenes and fullerene derivatives, have shown to have very good mechanical characteristics while preserving the required electronic properties. Currently, comprehension of the transport and thermo-mechanical properties in pure phases has rapidly evolved, nonetheless, there is a need for evaluating at a fundamental level how having mixtures of molecules based on similar conjugated moieties, such as those of fullerenes, affects the macroscopic properties.

Recently, Dr. Naga Rajesh Tummala, Dr. Veaceslav Coropceanu and Professor Jean-Luc Bredas from the School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics Georgia Institute of Technology and Professor Saadullah Aziz from King Abdulaziz University assessed and described how mixing affects the molecular packing, mechanical properties, and electronic parameters of interest for solar-cell applications. They also quantified the electronic and mechanical characteristics of mixtures of fullerenes by using a combination of molecular dynamics simulations and density functional theory calculations, following established procedures for pure phases of fullerenes. Their research work is published in Journal of Materials Chemistry C.

The research method employed the GROMACS software to perform molecular dynamics simulations of the various selected fullerenes. These simulations were based on the all-atom optimized potentials for liquid simulations force field to represent the Lennard-Jones and point-charge parameters for the carbon, oxygen, and hydrogen atoms. The morphologies derived from molecular dynamics simulations were then used as input for electronic-structure calculations performed at the density functional theory level with an optimally tuned range-separated hybrid functional.

The authors observed that at less than 10% mole fraction, C70 fullerenes could not form a percolation network in the mixtures of C60:C70, however, the formation of small domains was observed. They also found out that the mixtures of fullerenes show increased fracture energy and similar tensile modulus and fracture toughness, indicating that the mechanical properties of the parent system were not compromised.

In summary, the study by Jean-Luc Bredas and colleagues presented an in-depth quantification of the packing, mechanical properties, and electronic disorder and partial density of states of mixtures of fullerenes and fullerene derivatives. Generally, they observed that the doping of PC61BM with PC71BM leads to the formation of shallower trap states than in the case of doping C60 with C70 and C84. Altogether, the results obtained by the Georgia Institute of Technology and King Abdulaziz University scientists can serve as a guide for future investigations of electronic disorder in various mixtures of small organic molecules exploited in organic electronics.

Molecular mixtures as a pathway to enhance the electronic and mechanical properties of organic films - Advanced Engineering

About the author

Jean-Luc Bredas received his Ph.D. from the University of Namur, Belgium, in 1979. 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 before moving in 2003 to the Georgia Institute of Technology. In July 2014, he took a 2-½ leave of absence to King Abdullah University of Science and Technology (KAUST) where he served as Distinguished Professor of Materials Science and Engineering and Director of the KAUST Solar and Photovoltaics Research and Engineering Center. At Georgia Tech, where he resumed his activities in January 2017, he is Regents’ Professor of Chemistry and Biochemistry and holds the Vasser-Woolley and Georgia Research Alliance Chair in Molecular Design. He is a Georgia Research Alliance Eminent Scholar since 2005.

Jean-Luc Bredas is a 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 Charles H. Stone Award of the American Chemical Society, the 2013 David Adler Award in Materials Physics of the American Physical Society, and the 2016 Award in the Chemistry of Materials of the American Chemical Society. He is a Fellow of the American Chemical Society (Inaugural Class of 2009), American Physical Society, Optical Society of America, Royal Society of Chemistry, and Materials Research Society (Inaugural Class of 2008), and an Honorary Professor of the Institute of Chemistry of the Chinese Academy of Sciences. He holds honorary degrees from the University of Linköping, Sweden, and the Free University of Brussels.

He has published over 1,100 refereed articles (that have garnered over 75,000 citations on the Web of Science, leading to a current Web of Science h-index of 123) and given over 500 invited presentations. Since 2008, he has served as Editor for “Chemistry of Materials”, published by the American Chemical Society.

His research interests focus on the computational characterization and design of novel organic materials for organic electronics and photonics.

About the author

Naga Rajesh Tummala obtained his PhD in 2010 from the School of Chemical, Biological, and Materials Engineering at the University of Oklahoma under the supervision of Dr. Alberto Striolo (now at University College London). His PhD work focused on understanding the structure of water at interfaces and gaining molecular level insights into the aggregation of surfactants on hydrophilic and hydrophobic surfaces. His work provided mechanistic details of surfactant-assisted carbon nanotube dispersion and guidelines for the molecular design of efficient dispersants. After working on continuum modeling of flow in porous media for enhanced oil recovery applications, he moved to the Georgia Institute of Technology in 2011 to work with Dr. Jean-Luc Bredas. Initially as a post-doctoral fellow and then as a research scientist, he was involved in understanding and evaluating the microscopic and materials properties of organic molecules used in organic electronics and photovoltaics using multi-scale molecular simulation approaches and in providing guidelines for the design of next-generation materials.

About the author

Dr. Veaceslav Coropceanu 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.

He is currently a Principal Research Scientist in the School of Chemistry and Biochemistry at the Georgia Institute of Technology. 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

Saadullah G. Aziz received his Master’s Degree in theoretical chemistry from King Abdulaziz University in 1984, under the supervision of Prof. Rifaat Hilal, and his Ph.D. in Molecular Spectroscopy from Sussex University, UK, in 1988, under the supervision of the late Sir H. W. Kroto. After his Ph.D., he joined King Abdulaziz University, first as assistant professor, then as associate professor until he was appointed as professor of physical chemistry in 1998. He has held visiting professorships in a number of universities: Case Western Reserve University in 1996, University of La Plata, Argentina, in 1997, and University of Malaya, Malaysia, in 2002.

He was a member of several international cooperation research groups. He enjoyed working with Prof. Jean-Luc Bredas, Georgia Tech, in a project on organic electronics and its application to solar cells. He has interest with Prof. Oliver Kuhn, University of Rostock, Germany, in a project related to theoretical X-ray Spectroscopy and Ultrafast Electron Dynamics. He has also collaborations with Prof. Mario Barbatti, Aix-Marseille University in Marseille, France, in the area of Nonadiabatic Dynamics based on TD-DFTB Surface Hopping.

Reference

Naga Rajesh Tummala, Saadullah G. Aziz, Veaceslav Coropceanu, Jean-Luc Bredas. Characterization of the structural, mechanical, and electronic properties of fullerene mixtures a molecular simulations description. Journal of Materials Chemistry C, 2018, volume 6, page 3642.

Go To Journal of Materials Chemistry C

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