List of Abstracts 2016: PLENARY SESSION

The working principle of opto-electronic applications such as photovoltaic devices, light-emitting diodes or field-effect transistors relies on a combination of elementary electronic and optical processes, including photoinduced exciton generation and dissociation, charge transport and recombination. While it is in principle possible to assess these processes using experimental investigation techniques, they often show a strong dependence on the detailed microstructure in the bulk or at interfaces, which are by far more challenging to explore. Here, numerical simulations prove to be useful, if combining the best of two worlds: classical molecular dynamics simulations of structural organization in the solid and quantum-mechanical calculations of (molecular) electronic/excitonic effects. With the information provided by such all-atom simulations at hands, model Hamiltonians encompassing the main physical effects can be built and solved for samples of physical sizes and over timescales that are relevant for applications.
Here, I will review modeling work we have done in our Laboratory that mostly covers the use of organic conjugated molecules and polymers in the field of organic electronics, namely: - Charge transport models: Crossover from hopping to band regime in molecular crystals [1] and conjugated polymers showing resilience to conformational disorder [2].; - Exciton dissociation at organic interfaces, with focus on the role of electrostatic, delocalization and hybridization effects: How can one fight Coulomb law? [3, 4].; Mechanisms for singlet exciton fission in conjugated molecules to boost photovoltaic conversion efficiency beyond Shockley-Queisser limit: Towards structure-property relationships [5, 6].; Light emission from thermally-activated delayed fluorescent (TADF) molecular materials: How does it work?
If time permits, I will then move on to discuss two types of materials that have attracted lots of attention in the last few years, namely 2D materials and mixed organic-inorganic lead-halide perovskites, highlighting similarity and differences with respect to conjugated molecular materials; REFERENCES: [1] “Mixed Quantum-Classical Dynamics for Charge Transport in Organics”. L. Wang, O.V. Prezdhdo, and D. Beljonne. Physical Chemistry Chemical Physics 17 (2015) 12395-12406.; [2] “Approaching Disorder-Free Transport in High Mobility Conjugated Polymers”. D. Venkateshvaran, M. Nikolta, A. Sadhanala, V. Lemaur, M. Zelazny, M. Kepa, M. Hurhangee, A.J. Kronemeijer, V. Pecunia, I. Nasrallah, I. Romanov, K. Broch, I. McCulloch, D. Emin, Y. Olivier, J. Cornil, D. Beljonne, and H. Sirringhaus. Nature 515 (2014) 384-388.; [3] “Charge Separation and Recombination at Polymer-Fullerene Heterojunctions: Delocalization and Hybridization Effects”. G. D’Avino, L. Muccioli, Y. Olivier, and D. Beljonne. Journal of Physical Chemistry Letters 7 (2016) 536-540.; [4] “Charge Separation Energetics at Organic Heterojunctions: On the Role of Structural and Electrostatic Disorder”. F. Castet, G. D’Avino, L. Muccioli, J. Cornil, and D. Beljonne. Physical Chemistry Chemical Physics 16 (2014) 20279-20290.; [5] “Maximizing Singlet Fission by Intermolecular Packing”. L. Wang, Y. Olivier, O.V. Prezhdo, and D. Beljonne. Journal of Physical Chemistry Letters 5 (2014) 3345-3353.; [6] “First-Principles Quantum Dynamics of Singlet Fission: Coherent versus Thermally Activated Mechanisms Governed by Molecular π Stacking”.H. Tamura, M. Huix-Rotllant, I. Burghardt, Y. Olivier, and D. Beljonne. Physical Review Letters 115 (2015) 107401.

2D-layers, like graphene or a monolayer of hexagonal boron nitride (h-BN), enable the creation of new materials, unforeseen reaction pathways or striking confinement effects. Three examples for this statement are given. (i) Room temperature deposition on a 2D-layer moiré with an Ir(111) substrate results in the formation of a regular array of clusters with a narrow size distribution and high thermal stability. Such arrays may be of great use for applications in nanocatalysis and nanomagentism. Here we will specifically discuss the formation of a novel carbon cluster / graphene compound. (ii) The inertness of 2D-layers together with the confinement of the diffusion for the supplied reactands to two dimensions enables new reaction pathways in organo-metallic chemistry. As an example, it is shown how graphene and h-BN enable the growth of Europium- cyclooctatetraene nanowires of micrometer length through supply of atomic Eu and cyclooctatetraene molecules under well-defined ultrahigh vacuum conditions. (iii) Using ion implantion and thermal processing highly pressurized precipitates of the implanted species can be created in the space between a 2D-layer and its substrate. Thereby new high temperature and high pressure reactions might come into reach. As an example for the high pressure in these precipitates, we demonstrate crystalline Xe underneath h-BN on Ir(111). ACKNOWLEDGEMENTS: Contributions to this work by H. Åhlgren, M.A. Arman, N. Atodiresei, S. Blügel, C. Busse, V. Caciuc, F. Craes, T. Gerber, E. Granas, C. Herbig, F. Huttmann, W. Jolie, J. Knudsen, T. Knispel, J. Kotakoski, A.K. Krasheninnikov, A.J. Martinez-Galera, A. N’Diaye, N. Schleheck, U.A Schröder, S. Schumacher, S. Simon, and P. Valerius are gratefully acknowledged.