I joined the theory group at the Max Planck Institute for Polymer Research as a master student from the University of Heidelberg in October 2011. I
am currently focusing on code development, in particular the implementation of electrostatics for site energy calculations.
During my studies I received a B.Sc. in Physics from the University of Heidelberg and an International Diploma (ICID) from Imperial College,
London. Before joining the group at MPI-P, I was affiliated with the Kirchhoff Institute for Physics at Heidelberg University and the
Spitzencluster Forum Organic Electronics, where I developed computational schemes to accurately determine molecular orientation at interfaces
based on infrared ellipsometry.
Impact of molecular quadrupole moments on the energy levels at organic heterojunctions
M. Schwarze, K. Schellhammer, Ch. Gaul, K. Ortstein, M. Lau, G. Cuniberti, C. Poelking, D. Andrienko, F. Ortmann, K. Leo
The functionality of organic semiconductor devices crucially depends on molecular energies namely the ionisation energy and the electron affinity. Ionisation energy and electron affinity values of thin films are however sensitive to film morphology and composition making their prediction challenging. In a combined experimental and simulation study on zinc-phthalocyanine and its fluorinated derivatives we show that changes in ionisation energy as a function of molecular orientation in neat films or mixing ratio in blends are proportional to the molecular quadrupole component along the π-π-stacking direction. We apply these findings to organic solar cells and demonstrate how the electrostatic interactions can be tuned to optimise the energy of the charge-transfer state at the donor−acceptor interface and the dissociation barrier for free charge carrier generation. The confirmation of the correlation between interfacial energies and quadrupole moments for other materials indicates its relevance for small molecules and polymers.
Influence of Orientation Mismatch on Charge Transport Across Grain Boundaries in Tri-isopropylsilylethynyl (TIPS) Pentacene Thin Films
F. Steiner, C. Poelking, D. Niedzialek, D. Andrienko, J. Nelson
Phys. Chem. Chem. Phys.,
We present a multi-scale model for charge transport across grain boundaries in molecular electronic materials that incorporates packing disorder electrostatic and polarisation effects. We choose quasi two-dimensional films of tri-isopropylsilylethynyl pentacene (TIPS-P) as a model system of technologically relevant crystalline organic semiconductors. We use atomistic molecular dynamics with a force-field specific for TIPS-P to generate and equilibrate polycrystalline two-dimensional thin films. The energy landscape is obtained by calculating contributions from electrostatic interactions and polarization. The variation in these contributions leads to energetic barriers between grains. Subsequently charge transport is simulated using a kinetic Monte-Carlo algorithm. Two-grain systems with varied mutual orientation are studied. We find relatively little effect of long grain boundaries due to the presence of low impedence pathways. However effects could be more pronounced for systems with limited inter-grain contact areas. Furthermore we present a lattice model to generalize the model for small molecular systems. In the general case depending on molecular architecture and packing grain boundaries can result in interfacial energy barriers traps or a combination of both with qualitatively different effects on charge transport.
Electrostatic phenomena in organic semiconductors: Fundamentals and implications for photovoltaics
G. D'Avino, L. Muccioli, F. Castet, C. Poelking, D. Andrienko, Z. Soos, J. Cornil, D. Beljonne
Journal of Physics: Condensed Matter,
This review summarizes the current understanding of electrostatic phenomena in ordered and disordered organic semiconductors outlines numerical schemes developed for quantitative evaluation of electrostatic and induction contributions to ionization potentials and electron affinities of organic molecules in a solid state and illustrates two applications of these techniques: interpretation of photoelectron spectroscopy of thin films and energetics of heterointerfaces in organic solar cells.
Band structure engineering in organic semiconductors
M. Schwarze, W. Tress, B. Beyer, F. Gao, R. Scholz, C. Poelking, K. Ortstein, A. A. Guenther, D. Kasemann, D. Andrienko, K. Leo
A key breakthrough in modern electronics was the introduction of band structure engineering the design of almost arbitrary electronic potential structures by alloying different semiconductors to continuously tune the band gap and band-edge energies. Implementation of this approach in organic semiconductors has been hindered by strong localization of the electronic states in these materials. We show that the influence of so far largely ignored long-range Coulomb interactions provides a workaround. Photoelectron spectroscopy confirms that the ionization energies of crystalline organic semiconductors can be continuously tuned over a wide range by blending them with their halogenated derivatives. Correspondingly the photovoltaic gap and open-circuit voltage of organic solar cells can be continuously tuned by the blending ratio of these donors.
Long-range embedding of molecular ions and excitations in a polarizable molecular environment
C. Poelking, D. Andrienko
J. Chem. Theory Comput.,
We present a perturbative treatment of localized aperiodic excitations (charge charge transfer and excitonic states) interacting with a periodic molecular environment. The method rigorously accounts for the long-ranged interaction of charges with a net-quadrupolar background with the conditional convergence of the interaction sum removed by bulk or thin-film shape corrections. We illustrate how long-range interactions qualitatively and quantitatively impact the densities of states and level profiles in heterostructures of organic semiconductors investigating the role of molecular architecture packing and orientation. In accounting for mesoscale fields we obtain the energetics of charge carriers in both crystalline and mesoscopically amorphous systems with high accuracy.
Design Rules for Organic Donor-Acceptor Heterojunctions: Pathway for Charge Splitting and Detrapping
C. Poelking, D. Andrienko
J. Am. Chem. Soc.,
Organic solar cells rely on the conversion of a Frenkel exciton into free charges via a charge transfer state formed on a molecular donor-acceptor pair. These charge transfer states are strongly bound by Coulomb interactions and yet efficiently converted into charge-separated states. A microscopic understanding of this process though crucial to the functionality of any solar cell has not yet been achieved. Here we show how long-range molecular order and interfacial mixing generate homogeneous electrostatic forces that can drive charge separation and prevent minority-carrier trapping across a donor-acceptor interphase. Comparing a variety of small-molecule donor-fullerene combinations we illustrate how tuning of molecular orientation and interfacial mixing leads to a tradeoff between photovoltaic gap and charge-splitting and detrapping forces with consequences for the design of efficient photovoltaic devices.
Effect of mesoscale ordering on the density of states of polymeric semiconductors
P. Gemuenden, C. Poelking, K. Kremer, K. Daoulas, D. Andrienko
Macromol. Rapid Commun.,
Impact of Mesoscale Order on Open-Circuit Voltage in Organic Solar Cells
C. Poelking, M. Tietze, C. Elschner, S. Olthof, D. Hertel, B. Baumeier, F. Wuerthner, K. Meerholz, K. Leo, D. Andrienko
Structural order in organic solar cells is paramount: It reduces energetic disorder boosts charge and exciton mobilities and assists exciton splitting. Due to spatial localization of electronic states microscopic descriptions of photovoltaic processes tend to overlook the influence of structural features at a mesoscale. Long-range electrostatic interactions nevertheless probe this ordering making local properties depend on the mesoscopic order. To account for this a technique that addresses spatially \em aperiodic excitations in a \em periodic polarizable environment is developed. We show that structural order can reverse the role of donor and acceptor as conditioned by gas-phase energy levels. This finding resolves the controversy between experimental and theoretical results for the band shape and level alignment in efficient photovoltaic systems. Furthermore we rationalize the acceptor-donor-acceptor paradigm for molecular design of the successful DCVnT series of dyes which makes optimal use of these long-range effects. Comparing atomistic simulations to UPS experiments we provide an alternative interpretation for the empirical link between molecular energy levels and open-circuit voltage.
Morphology and charge transport in P3HT: A theorist's perspective
C. Poelking, K. Daoulas, A. Troisi, D. Andrienko
Adv. Polym. Sci.,
Poly(3-hexylthiophene) (P3HT) is the fruit fly among polymeric organic semiconductors. It has complex self-assembling and electronic properties and yet lacks the synthetic challenges that characterize advanced donorÃÂ¢ÃÂÃÂacceptor-type polymers. P3HT can be used both in solar cells and in field-effect transistors. Its morphological conductive and optical properties have been characterized in detail using virtually any and every experimental technique available whereas the contributions of theory and simulation to a rationalization of these properties have so far been modest. The purpose of this review is to take a snapshot of these results and more importantly outline directions that still require substantial method development.
Nematic ordering conjugation and density of states of soluble polymeric semiconductors
P. Gemuenden, C. Poelking, K. Kremer, D. Andrienko, K. Daoulas
We develop a generic coarse-grained model for describing liquid crystalline ordering of polymeric semiconductors on mesoscopic scales using poly(3-hexylthiophene) (P3HT) as a test system. The bonded interactions are obtained by Boltzmann-inverting the distributions of coarse-grained degrees of freedom resulting from a canonical sampling of an atomistic chain in $\theta$-solvent conditions. The non-bonded interactions are given by soft anisotropic potentials representing the combined effects of anisotropic $\pi-\pi$ interactions and entropic repulsion of side chains. We demonstrate that the model can describe uniaxial and biaxial nematic mesophases reproduces the experimentally observed effect of molecular weight on phase behavior and predicts Frank elastic constants typical for polymeric liquid crystals. We investigate charge transport properties of the biaxial nematic phase by analyzing the length distribution of conjugated segments and internal energetic landscape for hole transport. Results show how conjugation defects propagate from the terminal chain monomers and how long-range orientational correlations lead to a spatially correlated non-Gaussian density of states.
Effect of polymorphism regioregularity and paracrystallinity on charge transport in poly(3-hexyl-thiophene) [P3HT] nanofibers
C. Poelking, D. Andrienko
We investigate the relationship between molecular order and charge-transport parameters of a crystalline conjugated polymer poly(3-hexyl-thiophene) (P3HT) with a particular emphasis on its different polymorphic structures and regioregularity. To this end atomistic molecular dynamics is employed to study an irreversible transition of the metastable (form I') to the stable (form I) P3HT polymorph caused by side-chain melting at around 350~K. The predicted side-chain and backbone-backbone arrangements in unit cells of these polymorphs are compared to the existing structural models based on X-ray electron diffraction and solid-state NMR measurements. Molecular ordering is further characterized by the paracrystalline dynamic and static nematic order parameters. The temperature-induced changes of these parameters are linked to the dynamics and distributions of electronic coupling elements and site energies. We demonstrate that a small concentration of defects in side-chain attachment (90\% regioregular P3HT) leads to a significant (factor of ten) decrease in charge-carrier mobility. This reduction is due to an increase of the intermolecular part of the energetic disorder and can be traced back to the amplified fluctuations in backbone-backbone distances i.e. paracrystallinity. The simulated hole mobilities are in excellent agreement with experimental values obtained for P3HT nanofibers.
Characterization of charge-carrier transport in semicrystalline polymers: Electronic couplings site energies and charge-carrier dynamics in poly(bithiophene-alt-thienothiophene) [PBTTT]
C. Poelking, E. Cho, A. Malafeev, V. Ivanov, K. Kremer, C. Risko, J.-L. Bredas, D. Andrienko
J. Phys. Chem. C,
We establish a link between the microscopic ordering and the charge-transport parameters for a highly crystalline polymeric organic semiconductor poly(25-bis(3-tetradecylthiophen-2-yl)thieno[32-b]thiophene) (PBTTT). We find that the nematic and dynamic order parameters of the conjugated backbones as well as their separation evolve linearly with temperature while the side-chain dynamic order parameter and backbone paracrystallinity change abruptly upon the (also experimentally observed) melting of the side chains around 400 K. The distribution of site energies follows the behavior of the backbone paracrystallinity and can be treated as static on the time scale of a single-charge transfer reaction. On the contrary the electronic couplings between adjacent backbones are insensitive to side-chain melting and vary on a much faster time scale. The hole mobility calculated after time-averaging of the electronic couplings reproduces well the value measured in a short-channel thin-film transistor. The results underline that to secure efficient charge transport in lamellar arrangements of conjugated polymers: (i) the electronic couplings should present high average values and fast dynamics and (ii) the energetic disorder (paracrystallinity) should be small.
Stochastic modeling of molecular charge transport networks
B. Baumeier, O. Stenzel, C. Poelking, D. Andrienko, V. Schmidt
Phys. Rev. B,
We develop a stochastic network model for charge transport simulations in amorphous organic semiconductors which generalizes the correlated Gaussian disorder model to realistic morphologies charge transfer rates and site energies. The network model includes an iterative dominance-competition model for positioning vertices (hopping sites) in space distance-dependent distributions for the vertex connectivity and electronic coupling elements and a moving-average procedure for assigning spatially correlated site energies. The field dependence of the hole mobility of the amorphous organic semiconductor tris-(8-hydroxyquinoline)aluminum which was calculated using the stochastic network model showed good quantitative agreement with the prediction based on a microscopic approach.