My doctoral study at the Max Planck Institute for Polymer Research focuses on charge transport in organic solar cells. With the aim of improving photovoltaic efficiencies, macroscopic properties of charge transport are traced back to the nanoscopic structures of organic semiconductors. These structure-property relationships are devised by combining results of experimental material characterization and methods of computational modeling.
The work involves close collaborations with groups at the University of Dresden (Prof. Dr. Karl Leo), the University of Ulm (Prof. Dr. Peter Baeuerle), and Seoul National Universty (Prof. Dr. Do Yeung Yoon). The project started in May 2009 and is supported by the German Research Foundation (IRTG 1404) and the German National Academic Foundation.
Water-free proton conduction in hexakis(p-phosphonatophenyl)benzene nano-channels
C. Wehmeyer, M. Schrader, D. Andrienko, D. Sebastiani
J. Phys. Chem. C,
We elucidate the proton conduction mechanism in self-assembling stacks of phosphonic-acid-functionalized molecules (hexakis(p-phosphonatophenyl)benzene) at different temperatures (400-600K) and at zero humidity conditions. We employ first-principles molecular dynamics simulations in combination with large-scale force-field simulations forming a specific arrangement of the molecules in the columnar stacks. This arrangement leaves space for quasi-one-dimensional hydrogen bond nano-wires along which protons are transported. We observe spontaneous autodissociation of the phosphonic acid groups leading to proton displacements of up to 10A along the nano-wires. Our simulations show that there is a fast (200fs) and a slow (3-12ps) component in the dynamics of the hydrogen bond network corresponding to orientation fluctuations of the hydrogen bonds and persistent long-range proton transport respectively. Our results support the hypothesis that significant proton conduction is possible in this compound at fully dehydrated conditions and at high temperatures. In such circumstances the material may outperform the common NafionÃÂÃÂ® polymer as membrane materials for proton exchange fuel cells.
Molecular ordering and charge transport in a dicyanovinyl-substituted quaterthiophene thin film
C. Elschner, M. Schrader, R. Fitzner, A. A. Levin, P. Baeuerle, D. Andrienko, K. Leo, M. Riede
By combining computer simulations grazing incidence and powder X-ray-diffraction measurements we reconstruct the crystal structure of a thin film of terminally dicyanovinyl-substituted quaterthiophene (DCV4T). The crystal structure differs from the known single crystal structure of the same compound but resembles the molecular packing of a methylated DCV4T. Charge transport simulations on the molecular level show that the 2 dimensional thin-film charge-transport network is well suited for hole transport in solar cells.
Charge transport in amorphous and smectic mesophases of dicyanovinyl-substituted oligothiophenes
M. Schrader, C. Koerner, C. Elschner, D. Andrienko
J. Mater. Chem.,
By analyzing electrostatic and polarization effects in amorphous dicyanovinyl-substituted oligothiophenes we conclude that local molecular dipole moments result in a large spatially correlated energetic disorder. This disorder increases with the number of thiophene units in the oligomer and leads to an unexpected reduction of charge carrier mobility in a more ordered (smectic) mesophase observed for the longest of the studied oligomers (hexamer). This reduction in mobilities contradicts the common belief that more ordered phases of organic semiconductors have a better charge carrier mobility. In this particular case the amorphousness leads to a better-connected charge percolating network helping to bypass deep energetic traps. By comparing mobilities of amorphous and crystalline mesophases we conclude that vacuum deposited thin organic films have well ordered polycrystalline morphologies.
Comparative study of microscopic charge dynamics in crystalline acceptor-substituted oligothiophenes
M. Schrader, R. Fitzner, M. Hein, C. Elschner, B. Baumeier, M. Riede, K. Leo, P. Baeuerle, D. Andrienko
J. Am. Chem. Soc.,
By performing microscopic charge transport simulations for a set of crystalline dicyanovinyl-substituted oligothiophenes we find that the internal acceptorÃÂ¢ÃÂÃÂdonorÃÂ¢ÃÂÃÂacceptor molecular architecture combined with thermal fluctuations of dihedral angles results in large variations of local electric fields substantial energetic disorder and pronounced PooleÃÂ¢ÃÂÃÂFrenkel behavior which is unexpected for crystalline compounds. We show that the presence of static molecular dipoles causes large energetic disorder which is mostly reduced not by compensation of dipole moments in a unit cell but by molecular polarizabilities. In addition the presence of a well-defined ÃÂÃÂ-stacking direction with strong electronic couplings and short intermolecular distances turns out to be disadvantageous for efficient charge transport since it inhibits other transport directions and is prone to charge trapping.
Microscopic simulations of charge transport in disordered organic semiconductors
V. Ruehle, A. Lukyanov, F. May, M. Schrader, T. Vehoff, J. Kirkpatrick, B. Baumeier, D. Andrienko
J. Chem. Theory Comput.,
Charge carrier dynamics in an organic semiconductor can often be described in terms of charge hopping between localized states. The hopping rates depend on electronic coupling elements reorganization energies and driving forces which vary as a function of position and orientation of the molecules. The exact evaluation of these contributions in a molecular assembly is computationally prohibitive. Various often semiempirical approximations are employed instead. In this work we review some of these approaches and introduce a software toolkit which implements them. The purpose of the toolkit is to simplify the workflow for charge transport simulations provide a uniform error control for the methods and a flexible platform for their development and eventually allow in silico prescreening of organic semiconductors for specific applications. All implemented methods are illustrated by studying charge transport in amorphous films of tris-(8-hydroxyquinoline)aluminum a common organic semiconductor.