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PGI-1 Talk: Prof. Dr. Noa Marom

Effect of Crystal Packing on the Electronic Properties of Molecular Crystals

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25 Jul 2016 11:30
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PGI Lecture Hall

Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh PA 15213

Abstract:

Molecular crystals are typically bound by weak dispersion (van der Waals) interactions, which give rise to shallow potential energy landscapes with many local minima. As a result, molecular crystals often have several polymorphs that are very close in energy. Commonly used approximations for the exchange-correlation functional of density functional theory (DFT) lack a proper description of dispersion interactions. While pairwise dispersion corrections provide a significant improvement, the many-body dispersion (MBD) method reaches the desired meV accuracy by accounting for long-range electrostatic screening and for many-body dispersion interactions.
Crystal structure prediction is of paramount importance, particularly for the pharmaceutical industry, as most drugs are marketed in the form of molecular crystals. The massively parallel genetic algorithm (GA), GAtor, relies on the evolutionary principle of survival of the fittest to find the most stable crystal structure of a given rigid molecule. Dispersion-inclusive DFT is used for structural relaxation and energy evaluations. The structure generation package, Genarris, performs fast screening of randomly generated structures with a Harris approximation, whereby the molecular crystal density is constructed by replicating the single molecule density, which is calculated only once. GAtor and Genarris have been used within the 6th crystal structure prediction blind test for tricyano-1,4-dithiino[c]-isothiazole (TCS3).
Crystal packing may significantly alter the electronic and optical properties of molecular crystals, which are essential for applications in organic electronics and photovoltaics. Many-body perturbation theory within the GW approximation and the Bethe-Salpeter equation (BSE) is employed to describe properties derived from charged and neutral excitations. Layered structures of TCS3 are expected to exhibit smaller gaps, greater band dispersion, and broader optical absorption than other packing motifs. Singlet fission (SF), the conversion of a high-energy singlet exciton into two triplet excitons, may potentially lead to a twofold increase in solar cell efficiency. The monoclinic form of rubrene is expected to exhibit more efficient SF than the more common orthorhombic form.

Contact

Prof. Dr. Stefan Blügel
Phone: +49 2461 61-4249
Fax: +49 2461 61-2850
email: s.bluegel@fz-juelich.de