Elimination of Charge-Carrier Trapping by Molecular Design
O. Sachnik, X. Tan, C. Haese, N. Kinaret, K.-H. Lin, D. Andrienko, R. Graf, G.-J. A. H. Wetzelaer, J. J. Michels, Paul W. M. Blom
Nature Materials,
2023,
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[doi]
Molecular origin of balanced bipolar transport in neat layers of the emitter CzDBA
W. Liu, N. B. Kotadiya, P. W. M. Blom, G.-J. A. H. Wetzelaer, D. Andrienko
Adv. Mater. Technol.,
2000120,
2020,
[doi]
[abstract]
Recently an efficient single‐layer organic light‐emitting diode has been reported consisting of a neat layer of the diboron‐based thermally activated delayed fluorescence emitter 510‐bis(4‐(9H‐carbazol‐9‐yl)‐26‐dimethylphenyl)‐510‐dihydroboranthrene exhibiting remarkably balanced bipolar electron and hole transport. Here the donor–acceptor–donor architecture of the molecule is linked to the transport characteristics of its neat amorphous films. It is found that energetic disorder is larger for holes than for electrons explaining the experimentally observed difference in temperature activation of the mobility. Although a difference in energetic disorder would suggest unbalanced charge transport it is demonstrated that it is partly counteracted by larger coupling elements for holes.
A window to trap-free charge transport in organic semiconducting thin films
N. B. Kotadiya, A. Mondal, P. W. M. Blom, D. Andrienko, G.‐J. A. H. Wetzelaer
Nature Materials,
18,
1182-1186,
2019,
[doi]
[abstract]
Organic semiconductors which serve as the active component in devices such as solar cells light-emitting diodes and field-effect transistors often exhibit highly unipolar charge transport meaning that they predominantly conduct either electrons or holes. Here we identify an energy window inside which organic semiconductors do not experience charge trapping for device-relevant thicknesses in the range of 100 to 300 nm leading to trap-free charge transport of both carriers. When the ionization energy of a material surpasses 6 eV hole trapping will limit the hole transport whereas an electron affinity lower than 3.6 eV will give rise to trap-limited electron transport. When both energy levels are within this window trap-free bipolar charge transport occurs. Based on simulations water clusters are proposed to be the source of hole trapping. Organic semiconductors with energy levels situated within this energy window may lead to optoelectronic devices with enhanced performance. However for blue-emitting light-emitting diodes which require an energy gap of 3 eV removing or disabling charge traps will remain a challenge.
Direct and Energy-Transfer Mediated Charge Transfer State Formation and Recombination in Triangulene-Spacer-Perylenediimide Multichromophores: Lessons for Photovoltaic Applications
A. H. Balawi, S. Stappert, J. Gorenflot, C. Li, K. Muellen, D. Andrienko, F. Laquai
J. Phys. Chem. C,
123,
16602-16613,
2019,
[doi]
[abstract]
We study the dynamics of primary photoexcitations in three symmetric donor-spacer-acceptor-spacer-donor multichromophores with increasing oligophenylene spacer length following selective donor or acceptor excitation. Energy levels of the donor and acceptor moieties are tailored to facilitate splitting of the excited state into a lower-lying charge-transfer (CT) state mimicking the functionality of a donor–acceptor interface for charge generation thus resulting in long-lived charge separation. Ultrafast electronic energy transfer (ET) from the donor followed by fast hole (back)transfer from the acceptor populates the molecules’ CT states. However the CT efficiency is found to be close to unity independent of the donor or acceptor photoexcitation. The ratio of CT and recombination rates which reflects the population of CT states increases with the oligophenylene spacer length for both direct hole transfer and hole transfer following ET boosting the population of CT states under continuous excitation. We observe the population of high-lying “dark” excited states following ET from the donor to the acceptor. The “dark” states successively undergo CT and form CT states of higher energy with decreased recombination rates while maintaining the high charge generation efficiency. Changes in CT reaction rates are rationalized within the Marcus theory with driving forces and reorganization energies evaluated by density functional theory and polarizable continuum models. The present study demonstrates the importance of energetically higher-lying states which cannot be directly photoexcited yet are accessible through ET from local excited states. Similar processes are anticipated in other donor–acceptor systems which allow for both energy and CT processes such as bulk heterojunctions of the polymer and small-molecule donor/nonfullerene acceptor typically used in photovoltaic systems.
Polaron spin dynamics in high-mobility polymeric semiconductors
S. Schott, U. Chopra, V. Lemaur, A. Melnyk, Y. Olivier, R. DiPietro, R. Carey, X. Jiao, C. Jellett, M. Little, A. Marks, C. McNeill, I. McCulloch, E. McNellis, D. Andrienko, D. Beljonne, J. Sinova, H. Sirringhaus
Nature Physics,
15,
814-822,
2019,
[doi]
[abstract]
Polymeric semiconductors exhibit exceptionally long spin lifetimes and recently observed micrometer spin diffusion lengths in conjugated polymers demonstrate the potential for organic spintronics devices. Weak spin-orbit and hyperfine interactions lie at the origin of their long spin lifetimes but the coupling mechanism of a spin to its environment remains elusive. Here we present a systematic study of polaron spin lifetimes in field-effect transistors with high-mobility conjugated polymers as an active layer. We demonstrate how spin relaxation is governed by the charges' hopping motion at low temperatures while an Elliott-Yafet-like relaxation due to a transient localization of the carrier wavefunctions is responsible for spin relaxation at high temperatures. In this regime charge spin and structural dynamics are intimately related and depend sensitively on the local conformation of polymer backbones and the crystalline packing of the polymer chains.
Mechanistic study on the hydrolytic degradation of polyphosphates
K. Bauer, L. Liu, M. Wagner, D. Andrienko, F. Wurm
European Polymer Journal,
108,
286-294,
2018,
[doi]
[abstract]
Ring-opening polymerization of cyclic phosphates offers a fast access to well-defined water soluble and (bio)degradable polyphosphoesters (PPEs). In particular poly(alkyl ethylene phosphate)s have been used as building blocks for nanocarriers or hydrogels. The molecular mechanism of their degradation is however not well understood. Herein we study the hydrolytic degradation of two most frequently used PPEs poly(methyl ethylene phosphate) (PMEP) and poly(ethyl ethylene phosphate) (PEEP). The degradation process is analyzed by NMR spectroscopy which identifies and quantifies intermediates and degradation product(s). We prove that the major degradation pathway is backbiting leading to one dominating hydrolysis product ethyl or methyl ethylene phosphate (a diphosphate). Accelerated hydrolysis performed in basic and acidic conditions shows the high stability of PEEs under acidic conditions while they readily degrade under basic conditions. The backbiting mechanism is further supported by the reduction of the degradation kinetics if the terminal OH-group is blocked by a urethane. Our findings help to develop degradable nanodevices with adjustable hydrolysis kinetics.
Rigorous characterization and predictive modelling of hole transport in amorphous organic semiconductors
N. B. Kotadiya, A. Mondal, S. Xiong, P. W. M. Blom, D. Andrienko, G.-J. A. H. Wetzelaer
Adv. Electron. Mater.,
180366,
2018,
[doi]
[abstract]
Amorphous small-molecule hole-transporting materials are commonly used in organic light-emitting diodes and perovskite solar cells. Characterization of their main functionality hole transport has been complicated by the presence of large contact barriers. Using a recently developed technique to establish Ohmic hole contacts we investigate the bulk hole transport in a series of molecules with a broad range of ionization energies. The measured charge-carrier mobility dependence on charge concentration electric field and temperature is used to extract the energetic disorder and molecular site spacing. Excellent agreement of these parameters as well as ionization energies with simulation results paves the way to predictive charge-transport simulations from the molecular level.
Alcohol- and water-tolerant living anionic polymerization of aziridines
T. Gleede, E. Rieger, L. Liu, C. Bakkali-Hassani, M. Wagner, S. Carlotti, D. Taton, D. Andrienko, F. Wurm
Macromolecules,
51,
5713-5719,
2018,
[doi]
[abstract]
Living anionic polymerization gives access to well-defined polymers but it demands strict purification of reagents and solvents. This work presents the azaanionic polymerization (AAROP) of aziridines as a robust living polymerization technique with the ease of controlled radical polymerizations. AAROP does not require inert atmosphere and remains living in the presence of large amounts of water or alcohols. Mesyl- tosyl- or brosyl-activated aziridines were polymerized with up to 100-fold excess of a protic impurity with respect to the initiator and still being active for chain extension. This allowed the preparation of polyols by anionic polymerization without protective groups as only minor initiation occurred from the alcohols. The tolerance toward protic additives lies in the electron-withdrawing effect of the activating groups decreasing the basicity of the propagating species while maintaining a strong nucleophilic character. In this way competing alcohols and water are only slightly involved in the polymerization making living anionic polymerization an easy-to-conduct technique to well-defined polyamides and -amines.
Multiscale concepts in simulations of organic semiconductors
D. Andrienko
Handbook of Materials Modeling,
2018,
[doi]
[abstract]
We critically review simulation approaches developed to study organic solar cells light emitting diodes and field effect transistors. Special attention is paid to multiscale techniques. In particular we discuss how to parametrize coarse-grained models for morphology and charge transport simulations to account for finite-size effects and to treat long-range interactions in small systems.
Polymerizing phostones: A fast way to in-chain poly(phosphonate)s with adjustable hydrophilicity
K. Bauer, L. Liu, D. Andrienko, M. Wagner, E. Macdonald, M. Shaver, F. Wurm
Macromolecules,
51,
1272-1279,
2018,
[doi]
[abstract]
Phostones i.e. 2-alkoxy-2-oxo-13-oxaphospholanes are accessible in a one-pot reaction from commercially available 13-dibromopropane and alkyl phosphites. These 5-membered cyclic phosphonic acid esters are used for the preparation of linear poly(phosphonate)s via ring-opening polymerization resulting in polymers with a hydrolytically stable P-C bond in the polymer backbone. Phostones have the stable P-C bond within the cycle which leads to a dramatic increase of the monomer stability toward hydrolysis and long shelf-lives compared to other cyclic phosphoesters which hydrolyze immediately at contact with water. Two phostone monomers containing ethoxy or butoxy pendant chains were prepared in a single-step synthesis from inexpensive starting materials avoiding the usage of SOCl2 or POCl3. Polymers with ethoxy side chains are water-soluble without a lower critical solution temperature nontoxic to murine macrophages and hydrolytically degradable under basic conditions. The polymerization kinetics for different catalyst systems were evaluated for both monomers in order to identify optimal polymerization conditions resulting in polyphosphonates with molecular weights between 3000 and 25 100 g/mol with reasonable molecular weight dispersities (<1.6). Because of the ease of synthesis and distinct different hydrolysis kinetics compared to side-chain polyphosphonates we believe that these new polyphostones represent a valuable addition to water-soluble biopolymers for future biomedical applications.
Absoprtion
An ab initio method on large sized molecular aggregate system: Predicting absorption spectra of crystalline organic semiconducting films
W. Liu, D. Andrienko
J. Chem. Phys.,
158,
094108,
2023,
[doi]
[abstract]
Theoretical description of electronically excited states of molecular aggregates at an ab initio level is computationally demanding. To reduce the computational cost we propose a model Hamiltonian approach that approximates the electronically excited state wavefunction of the molecular aggregate. We benchmark our approach on a thiophene hexamer as well as calculate the absorption spectra of several crystalline non-fullerene acceptors including Y6 and ITIC which are known for their high power conversion efficiency in organic solar cells. The method qualitatively predicts the experimentally measured spectral shape which can be further linked to the molecular arrangement in the unit cell.
Acceptor-substituted oligothiophenes
Electronic excitations in push-pull oligomers and their complexes with fullerene from Many-Body Green's function theory with polarizable embedding
B. Baumeier, M. Rohlfing, D. Andrienko
J. Chem. Theory Comput.,
10,
3104-3110,
2014,
[doi]
Parametrization of extended Gaussian disorder models from microscopic charge transport simulations
P. Kordt, O. Stenzel, B. Baumeier, V. Schmidt, D. Andrienko
J. Chem. Theory Comput.,
10,
2508-2513,
2014,
[doi]
[abstract]
Simulations of organic semiconducting devices using drift-diffusion equations are vital for the understanding of their functionality as well as for the optimization of their performance. Input parameters for these equations are usually determined from experiments and do not provide a direct link to the chemical structures and material morphology. Here we demonstrate how such a parametrization can be performed by using atomic-scale (microscopic) simulations. To do this a stochastic network model parametrized on atomistic simulations is used to tabulate charge mobility in a wide density range. After accounting for finite-size effects at small charge densities the data is fitted to the uncorrelated and correlated extended Gaussian disorder models. Surprisingly the uncorrelated model reproduces the results of microscopic simulations better than the correlated one compensating for spatial correlations present in a microscopic system by a large lattice constant. The proposed method retains the link to the material morphology and the underlying chemistry and can be used to formulate structure-property relationships or optimize devices prior to compound synthesis.
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
Nature Materials,
14,
434-439,
2015,
[doi]
[abstract]
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 aperiodic excitations in a 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.
Charge transport in amorphous and smectic mesophases of dicyanovinyl-substituted oligothiophenes
M. Schrader, C. Koerner, C. Elschner, D. Andrienko
J. Mater. Chem.,
22,
22258-22264,
2012,
[doi]
[abstract]
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.
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
RSC Advances,
3,
12117-12123,
2013,
[doi]
[abstract]
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.
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.,
134,
6052-6056,
2012,
[doi]
[abstract]
By performing microscopic charge transport simulations for a set of crystalline dicyanovinyl-substituted oligothiophenes we find that the internal acceptor-donor-acceptor molecular architecture combined with thermal fluctuations of dihedral angles results in large variations of local electric fields substantial energetic disorder and pronounced Poole-Frenkel 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 pi-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.
Excited states of dicyanovinyl-substituted oligothiophenes from many-body Green's functions theory
B. Baumeier, D. Andrienko, Y. Ma, M. Rohlfing
J. Chem. Theory Comput.,
8,
997-1002,
2012,
[doi]
[abstract]
Excited states of dicyanovinyl-substituted oligothiophenes are studied using many-body Green's functions theory within the GW approximation and the Bethe-Salpeter equation. By varying the number of oligomer repeat units we investigate the effects of resonant-antiresonant transition coupling dynamical screening and molecular conformations on calculated excitations. We find that the full dynamically screened Bethe-Salpeter equation yields absorption and emission energies in good agreement with experimental data. The effect of resonant-antiresonant coupling on the first singlet excitation monotonically decreases with increasing size of the molecule while dynamical screening effects uniformly lower the excitation energies.
Biophysics
Twist-bend instability for toroidal DNA condensates
I. M. Kulic, D. Andrienko, M. Deserno
Europhys. Lett.,
67,
418-424,
2004,
[doi]
[abstract]
We propose that semiflexible polymers in poor solvent collapse in two stages. The first stage is the well-known formation of a dense toroidal aggregate. However if the solvent is sufficiently poor the condensate will undergo a second structural transition to a twisted entangled state in which individual filaments lower their bending energy by additionally orbiting around the mean path along which they wind. This topological ripening is consistent with known simulations and experimental results. It connects and rationalizes various experimental observations ranging from strong DNA entanglement in viral capsids to the unusually short pitch of the cholesteric phase of DNA in sperm heads. We propose that topological ripening of DNA toroids could improve the efficiency and stability of gene delivery.
Entropic torque
R. Roth, R. van Roij, D. Andrienko, K. R. Mecke, S. Dietrich
Phys. Rev. Lett.,
83,
088301,
2002,
[doi]
[abstract]
Quantitative predictions are presented of a depletion-induced torque and force acting on a single colloidal hard rod immersed in a solvent of hard spheres close to a planar hard wall. This torque and force which are entirely of entropic origin may play an important role for the key-lock principle according to which a biological macromolecule (the key) is functional only in a particular orientation with respect to a cavity (the lock).
Charge injection
Universal strategy for Ohmic hole injection into organic semiconductors with high ionization energies
N. B. Kotadiya, H. Lu, A. Mondal, Y. Ie, D. Andrienko, P. W. M. Blom, G.-J. A. H. Wetzelaer
Nature Materials,
17,
329-334,
2018,
[doi]
[abstract]
Barrier-free (Ohmic) contacts are a key requirement for efficient organic optoelectronic devices such as organic light-emitting diodes solar cells and field-effect transistors. Here we propose a simple and robust way of forming an Ohmic hole contact on organic semiconductors with a high ionization energy (IE). The injected hole current from high-work-function metal-oxide electrodes is improved by more than an order of magnitude by using an interlayer for which the sole requirement is that it has a higher IE than the organic semiconductor. Insertion of the interlayer results in electrostatic decoupling of the electrode from the semiconductor and realignment of the Fermi level with the IE of the organic semiconductor. The Ohmic-contact formation is illustrated for a number of material combinations and solves the problem of hole injection into organic semiconductors with a high IE of up to 6 eV.
Charge transport
Finite-size scaling of charge carrier mobility in disordered organic semiconductors
P. Kordt, T. Speck, D. Andrienko
Phys. Rev. B,
94,
014208,
2016,
[doi]
[abstract]
Simulations of charge transport in amorphous semiconductors are often performed in microscopically sized systems. As a result charge carrier mobilities become system-size dependent. We propose a simple method for extrapolating a macroscopic nondispersive mobility from the system-size dependence of a microscopic one. The method is validated against a temperature-based extrapolation [A. Lukyanov and D. Andrienko Phys. Rev. B 82 193202 (2010)]. In addition we provide an analytic estimate of system sizes required to perform nondispersive charge transport simulations in systems with finite charge carrier density derived from a truncated Gaussian distribution. This estimate is not limited to lattice models or specific rate expressions.
Charge carrier photogeneration and extraction in polymer:fullerene bulk heterojunction organic solar cells
F. Laquai, D. Andrienko, C. Deibel, D. Neher
Adv. Polym. Sci.,
272,
267-291,
2016,
[doi]
[abstract]
In this chapter we review the basic principles of photocurrent generation in bulk heterojunction organic solar cells discuss the loss channels limiting their efficiency and present case studies of several polymer–fullerene blends. Using steady-state and transient optical and electrooptical techniques we create a precise picture of the fundamental processes that ultimately govern solar cell efficiency.
Molecular scale simulation of hole mobility and current densities in amorphous tridecane
M. Unge, Ch. Toernkvist, P. Kordt, D. Andrienko
IEEE Conference Publications,
14-18,
2015,
[doi]
[abstract]
The hole mobility of amorphous tridecane (a model of amorphous polyethylene) is simulated using a parameter-free approach which combines density functional theory molecular dynamics and kinetic Monte Carlo methods. We observe large variations of the current density in the samples typical to materials with large energetic disorder. The obtained mobility values are of the same order of magnitude as the highest experimentally reported values. By introducing carbonyl groups we assess the effect of material oxidation and find that the mobility is reduced by an order of magnitude already at moderate concentrations of these groups.
Coarse Graining
Benchmarking coarse-grained models of organic semiconductors via deep backmapping
M. Stieffenhofer, C. Scherer, F. May, T. Bereau, D. Andrienko
Frontiers in Chemistry,
10,
982757,
2022,
[doi]
[abstract]
The potential of mean force is an effective coarse-grained potential which is often approximated by pairwise potentials. While the approximated potential reproduces certain distributions of the reference all-atom model with remarkable accuracy important cross-correlations are typically not captured. In general the quality of coarse-grained models is evaluated at the coarse-grained resolution hindering the detection of important discrepancies between the all-atom and coarse-grained ensembles. In this work the quality of different coarse-grained models is assessed at the atomistic resolution deploying reverse-mapping strategies. In particular coarse-grained structures for Tris-Meta-Biphenyl-Triazine are reverse-mapped from two different sources: (1) All-atom configurations projected onto the coarse-grained resolution and (2) snapshots obtained by molecular dynamics simulations based on the coarse-grained force fields. To assess the quality of the coarse-grained models reverse-mapped structures of both sources are compared revealing significant discrepancies between the all-atom and the coarse-grained ensembles. Specifically the reintroduced details enable force computations based on the all-atom force field that yield a clear ranking for the quality of the different coarse-grained models.
Coarse-graining
Ultra-Coarse-Graining of Homopolymers in Inhomogeneous Systems
F. Berressem, C. Scherer, D. Andrienko, A. Nikoubashman
J. Phys. Condens. Matter,
33,
254002,
2021,
[doi]
[abstract]
We develop coarse-grained (CG) models for simulating homopolymers in inhomogeneous systems focusing on polymer films and droplets. If the CG polymers interact solely through two-body potentials then the films and droplets either dissolve or collapse into small aggregates depending on whether the effective polymer–polymer interactions have been determined from reference simulations in the bulk or at infinite dilution. To address this shortcoming we include higher order interactions either through an additional three-body potential or a local density-dependent potential (LDP). We parameterize the two- and three-body potentials via force matching and the LDP through relative entropy minimization. While the CG models with three-body interactions fail at reproducing stable polymer films and droplets CG simulations with an LDP are able to do so. Minor quantitative differences between the reference and the CG simulations namely a slight broadening of interfaces accompanied by a smaller surface tension in the CG simulations can be attributed to the deformation of polymers near the interfaces which cannot be resolved in the CG representation where the polymers are mapped to spherical beads.
Kernel-based machine learning for efficient simulations of molecular liquids
C. Scherer, R. Scheid, D. Andrienko, T. Bereau
J. Chem. Theory Comput.,
16,
3194-3204,
2020,
[doi]
[abstract]
Current machine learning (ML) models aimed at learning force fields are plagued by their high computational cost at every integration time step. We describe a number of practical and computationally-efficient strategies to parametrize traditional force fields for molecular liquids from ML: the particle decomposition ansatz to two- and three-body force fields the use of kernel-based ML models that incorporate physical symmetries the incorporation of switching functions close to the cutoff and the use of covariant meshing to boost the training set size. Results are presented for model molecular liquids: pairwise Lennard-Jones three-body Stillinger-Weber and bottom-up coarse-graining of water. Here covariant meshing proves to be an efficient strategy to learn canonically averaged instantaneous forces. We show that molecular dynamics simulations with tabulated two- and three-body ML potentials are computationally efficient and recover two- and three-body distribution functions. Many-body representations decomposition and kernel regression schemes are all implemented in the open-source software package VOTCA.
Understanding three-body contributions to coarse-grained force-fields
C. Scherer, D. Andrienko
Phys. Chem. Chem. Phys.,
20,
22387-22394,
2018,
[doi]
[abstract]
Coarse-graining (CG) is a systematic reduction of the number of degrees of freedom (DOF) used to describe a system of interest. CG can be thought of as a projection on coarse-grained DOF and is therefore dependent on the functions used to represent the CG force field. In this work we show that naive extensions of the coarse-grained force-field can result in unphysical parametrizations of the CG potential energy surface (PES). This issue can be elevated by coarse-graining the two- and three-body forces separately which also helps to evaluate the importance of many-body interactions for a given system. The approach is illustrated on liquid water where three-body interactions are essential to reproduce the structural properties and liquid methanol where two-body interactions are sufficient to reproduce the main features of the atomistic system.
Comparison of systematic coarse-graining strategies for soluble conjugated polymers
C. Scherer, D. Andrienko
Eur. Phys. J. Spec. Top.,
225,
1441-1461,
2016,
[doi]
[abstract]
We assess several systematic coarse-graining approaches by coarse-graining poly(3-hexylthiophene-25-diyl) (P3HT) a polymer showing $\pi$-stacking of the thiophene rings and lamellar ordering of the $\pi$-stacked structures. All coarse-grained force fields are ranked according to their ability of preserving the experimentally known crystalline molecular arrangement of P3HT. The coarse-grained force fields parametrized in the amorphous melt turned out to accurately reproduce the structural quantities of the melt as well as to preserve the lamellar ordering of the P3HT oligomers in $\pi$-stacks. However the exact crystal structure is not reproduced. The combination of Boltzmann inversion for bonded and iterative Boltzmann inversion with pressure correction for nonbonded degrees of freedom gives the best coarse-grained model.
Structure-based coarse-graining in liquid slabs
M. Jochum, D. Andrienko, K. Kremer, C. Peter
J. Chem. Phys.,
137,
064102,
2012,
[doi]
[abstract]
Structure-based coarse-graining relies on matching the pair correlation functions of a reference (atomistic) and a coarse-grained system. As such it is designed for systems with uniform density distributions. Here we demonstrate how it can be generalized for inhomogeneous systems by coarse-graining slabs of liquid water and methanol in vacuum as well as a single benzene molecule at the water-vacuum interface. Our conclusion is that coarse-graining performed in inhomogeneous systems improves thermodynamic properties and the structure of interfaces without significant alterations to the local structure of the bulk liquid.
Versatile Object-oriented Toolkit for Coarse-graining Applications
V. Ruehle, C. Junghans, A. Lukyanov, K. Kremer, D. Andrienko
J. Chem. Theory Comput.,
5,
3211-3223,
2009,
[doi]
Coarse-grained modelling of polypyrrole morphologies
V. Ruehle, J. Kirkpatrick, K. Kremer, D. Andrienko
Phys. Stat. Sol. B,
245,
844,
2008,
[doi]
[abstract]
A multiscale model to simulate large scale morphologies and study charge transport in polypyrrole is developed. First \em ab-initio methods are used to derive an atomistic force field. Coarse graining of this atomistic model is then performed. At a final stage the analysis of simulated morphologies allows to split polymer chains into conjugated segments which can further be used to simulate both inter- and intrachain charge dynamics.
Coarse-grained interaction potentials for polyaromatic hydrocarbons
O. Anatole von Lilienfeld, Denis Andrienko
J. Chem. Phys.,
124,
054307,
2006,
[doi]
[abstract]
Using Kohn-Sham (KS) density-functional theory we have studied the interaction between various polyaromatic hydrocarbon molecules. The systems range from monocyclic benzene up to hexabenzocoronene (hbc). For several conventional exchange-correlation functionals total potential-energy curves of interaction of the pi-pi stacking hbc dimer are reported. It is found that all pure local density or generalized gradient approximated functionals yield qualitatively incorrect predictions regarding structure and interaction. Inclusion of a nonlocal atom-centered correction to the KS Hamiltonian enables quantitative predictions. The computed potential-energy surfaces of interaction yield parameters for a coarse-grained potential which can be employed to study discotic liquid-crystalline mesophases of derived polyaromatic macromolecules.
Conjugated polymers
Electron trapping in conjugated polymers
D. Abbaszadeh, A. Kunz, N. B. Kotadiya, A. Mondal, D. Andrienko, J. J. Michels, G.‐J. A. H. Wetzelaer, P. W. M. Blom
Chem. Mater.,
31,
6380-6386,
2019,
[doi]
[abstract]
Electron trapping is a well-recognized issue in organic semiconductors in particular in conjugated polymers leading to a significant electron mobility reduction in materials with electron affinities smaller than 4 eV. Space-charge limited current measurements in diodes indicate that these traps have similar molecular origin while calculations show that hydrated molecular oxygen is a plausible molecular candidate with the tail of the solid-state electron affinity distribution reaching values as high as 4 eV. By decreasing the trap density by mixing conjugated polymers with an insulating polymer matrix one can fill the traps with charges and hence eliminate their effect on electron mobility. Trap-dilution does not only improve transport but also reduces trap-assisted recombination boosting the efficiency of polymer light emitting diodes.
Generic model for lamellar self-assembly in conjugated polymers: linking mesoscopic morphology and charge transport in P3HT
C. Greco, A. Melnyk, K. Kremer, D. Andrienko, K. Daoulas
Macromolecules,
52,
968-981,
2019,
[doi]
[abstract]
We develop a generic coarse-grained model of soluble conjugated polymers capable of describing their self-assembly into a lamellar mesophase. Polymer chains are described by a hindered-rotation model where interaction centers represent entire repeat units including side chains. We introduce soft anisotropic nonbonded interactions to mimic the potential of mean force between atomistic repeat units. The functional form of this potential reflects the symmetry of the molecular order in a lamellar mesophase. The model can generate both nematic and lamellar (sanidic smectic) molecular arrangements. We parametrize this model for a soluble conjugated polymer poly(3-hexylthiophene) (P3HT) and demonstrate that the simulated lamellar mesophase matches morphologies of low molecular weight P3HT experimentally observed at elevated temperatures. A qualitative charge-transport model allows us to link local chain conformations and mesoscale order to charge transport. In particular it shows how coarsening of lamellar domains and chain extension increase the charge carrier mobility. By modeling large systems and long chains we can capture transport between lamellar layers which is due to rare but thermodynamically allowed backbone bridges between neighboring layers.
Self-organization and charge transport properties of selenium and tellurium analogues of polythiophene
Sh. Ye, L. Janasz, W. Zajaczkowski, J. G. Manion, A. Mondal, T. Marszalek, D. Andrienko, K. Muellen, W. Pisula, D. S. Seferos
Macromol. Rapid Commun.,
1800596,
2019,
[doi]
[abstract]
A series of conjugated polymers comprising polythiophene polyselenophene and polytellurophene with branched 37‐dimethyloctyl side chains well‐matched molecular weight dispersity and regioregularity is synthesized. The ionization potential is found to vary from 5.14 to 5.32 eV with polytellurophene having the lowest potential. Field‐effect transistors based on these materials exhibit distinct hole transport mobility that varies by nearly three orders of magnitude with polytellurophene having the highest mobility (2.5 × 10−2 cm² V−1 s−1). The large difference in mobility demonstrates the significant impact of heteroatom substitution. Although the series of polymers are very similar in structure their solid‐state properties are different. While the thin film microstructure of polythiophene and polyselenophene is identical polytellurophene reveals globular features in the film topography. Polytellurophenes also appear to be the least crystalline even though their charge transport properties are superior to other samples. The torsional barrier and degree of planarity between repeat units increase as one moves down group‐16 elements. These studies show how a single atom in a polymer chain can have a substantial influence on the bulk properties of a material and that heavy group‐16 atoms have a positive influence on charge transport properties when all other variables are kept unchanged.
Macroscopic Structural Compositions of π-Conjugated Polymers: Combined Insights from Solid-State NMR and Molecular Dynamics Simulations
A. Melnyk, M. Junk, M. McGehee, B. Chmelka, M. R. Hansen, D. Andrienko
J. Phys. Chem. Lett.,
2017,
[doi]
[abstract]
Molecular dynamics simulations are combined with solid-state NMR measurements to gain insight into the macroscopic structural composition of the π-conjugated polymer poly(25-bis(3-tetradecyl-thiophen-2-yl)thieno[32-b]thiophene) (PBTTT). The structural and dynamical properties as established by the NMR analyses were used to test the local structure of three constitutient mesophases with: (i) crystalline backbones and side chains (ii) lamellar backbones and disordered side chains or (iii) amorphous backbones and side chains. The relative compositions of these mesophases were then determined from the deconvolution of the 1H and 13C solid-state NMR spectra and dynamic order parameters. Surprisingly based on molecular dynamics simulations the powder composition consisted of only 28% of the completely crystalline mesophase while 23% was lamellar with disordered sidechains and 49% amorphous. The protocol presented in this work is a general approach and can be used for elucidating the relative compositions of mesophases in π-conjugated polymers.
The PCPDTBT Family: Correlations between Chemical Structure Polymorphism and Device Performance
F. Fischer, G. Schulz, D. Trefz, A. Melnyk, M. Brinkmann, D. Andrienko, S. Ludwigs
Macromolecules,
50,
1402-1414,
2017,
[doi]
[abstract]
We highlight the influence of processing conditions on polymorphism and structure formation on the mesoscale for the family of PCPDTBT polymers with branched alkyl side chains. Direct correlations of morphology to the chemical structure and to transistor device performance are established. We found that up to four different packing motifs could be realized depending on the polymer derivative and the processing conditions: amorphous π-stacked cross-hatched and dimer-containing polymorphs. While C- and F-PCPDTBT display similar packing behavior organizing in π-stacked and dimer-like structures Si-PCPDTBT gives rise to cross-hatched structures upon simple deposition from solution. The observed differences in chain packing for C-/F-PCPDTBT versus Si-PCPDTBT are attributed to differences in backbone conformations and aggregation behavior in solution. The effect of polymorphism on charge transport is probed using field-effect transistors in which both π-stacked and cross-hatched polymer chain arrangements yield the highest hole mobilities. Mesoscopic morphology and mobility simulations rationalize our experimental findings by relating mobility to distributions of electronic coupling elements between the chains.
Morphology and charge transport in P3HT: A theorist's perspective
C. Poelking, K. Daoulas, A. Troisi, D. Andrienko
Adv. Polym. Sci.,
265,
139-180,
2014,
[doi]
[abstract]
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 donorâÃÂÃÂacceptor-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
Macromolecules,
46,
5762-5774,
2013,
[doi]
[abstract]
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
Macromolecules,
46,
8941-8956,
2013,
[doi]
[abstract]
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,
117,
1633-1640,
2013,
[doi]
[abstract]
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.
Bilayer order in a polycarbazole conjugated polymer
X. Lu, H. Hlaing, D. S. Germack, J. Peet, W. H. Jo, D. Andrienko, K. Kremer, B. M. Ocko
Nature Communications,
3,
795,
2012,
[doi]
[abstract]
One of the best performing semiconducting polymers used in bulk heterojunction devices is PCDTBT a polycarbazole derivative with solar-conversion efficiencies as high as 7.2\%. Here we report the formation of bilayer ordering in PCDTBT and postulate that this structural motif is a direct consequence of the polymer's molecular design. This bilayer motif is composed of a pair of backbones arranged side-to-side where the alkyl tails are on the outer side. This is in stark contrast to the monolayer ordering found in other conjugated polymers. The crystalline bilayer phase forms at elevated temperatures and persists after cooling to room temperature. The existence of bilayer ordering along with its high-packing fraction of conjugated moieties may guide the synthesis of new materials with improved optoelectronic properties.
DSSC
Two Channels of Charge Generation in Perylene Monoimide Solid-State Dye-Sensitized Solar Cells
I. Howard, M. Meister, B. Baumeier, H. Wonneberger, N. Pschirer, R. Sens, I. Bruder, L. Chen, K. Muellen, D. Andrienko, F. Laquai
Adv. Energy Mater.,
4,
1300640,
2014,
[doi]
Observing Charge Dynamics in Surface Reactions by Time-Resolved Stark Effects
M. Meister, B. Baumeier, N. Pschirer, R. Sens, I. Bruder, F. Laquai, D. Andrienko, I. Howard
J. Phys. Chem. C,
117,
9171-9177,
2013,
[doi]
[abstract]
Surfaces facilitate chemical reactions occurring in biological and synthetic systems with wide-ranging applications from energy conversion to catalysis and sensing. Microscopic understanding of the structure and dynamics that underpin these reactions is keenly pursued with novel experimental techniques such as sum frequency generation and laser-assisted photoemission spectroscopy. Herein we demonstrate a method for interpreting the time-resolved observation of the Stark effect to provide an in situ optical probe of the charge dynamics during an interfacial reaction. The analysis holds broad potential for investigating charge migration in surface-bound catalysts and sensors as well as photocenter and retinal proteins even when the Stark parameters of the material are unknown. We demonstrate the analysis with respect to the energy conversion reaction in solid-state dye-sensitized solar cells.
Discotics
Supramolecular Structure of Perylene Tetracarboxdiimides
V. Marcon, J. Kirkpatrick, W. Pisula, D. Andrienko
Phys. Stat. Sol. B,
245,
820,
2008,
[doi]
[abstract]
Using atomistic molecular dynamics (MD) simulations we study the columnar phases of perylene tetracarboxdiimides (PDI) and establish correlations between the molecular structure packing and dynamical properties of these materials. Combining electronic structure calculations MD and kinetic Monte Carlo simulations a correlation is then established between molecular structure and charge mobility of columnar mesophases of perylene tetracarboxdiimide derivatives.
Structure-Charge Mobility Relation for Hexabenzocoronene Derivatives
D. Andrienko, J. Kirkpatrick, V. Marcon, J. Nelson, K. Kremer
Phys. Stat. Sol. B,
245,
830,
2008,
[doi]
[abstract]
Charge mobilities of several derivatives of discotic liquid crystals have been determined by combining three methods into one scheme: (i) quantum chemical methods for the calculation of molecular electronic structures and reorganization energies (ii) molecular dynamics for simulation of the relative positions and orientations of molecules in a columnar mesophase and (iii) kinetic Monte Carlo simulations and Master Equation approach to simulate charge transport. Applying this scheme to differently substituted hexabenzocoronene derivatives we reproduce the trends and magnitudes of mobilities as measured by pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) and connect mobility directly to the microscopic morphology of the columns. Our study also shows that it is possible to understand and reproduce experimental charge transport parameters and in some cases accurately predict them.
Charge mobility of discotic mesophases: a multiscale quantum/classical study
J. Kirkpatrick, V. Marcon, J. Nelson, K. Kremer, D. Andrienko
Phys. Rev. Lett.,
98,
227402,
2007,
[doi]
[abstract]
A correlation is established between the molecular structure and charge mobility of discotic mesophases of hexabenzocoronene derivatives by combining electronic structure calculations molecular dynamics and kinetic Monte Carlo simulations. It is demonstrated that this multiscale approach can provide an accurate ab initio description of charge transport in organic materials.
Atomistic simulation of structure and dynamics of columnar phases of hexabenzocoronene derivatives
D. Andrienko, V. Marcon, K. Kremer
J. Chem. Phys.,
125,
124902,
2006,
[doi]
[abstract]
Using atomistic molecular dynamics simulations we study solid and liquid crystalline columnar discotic phases formed by alkyl-substituted hexabenzocoronene mesogens. Correlations between the molecular structure packing and dynamical properties of these materials are established.
Dye-doped nematics
Light-induced surface sliding of the nematic director in liquid crystals
O. Francescangeli, F. Simoni, S. Slussarenko, D. Andrienko, V. Reshetnyak, Yu. Reznikov
Phys. Rev. Lett.,
82,
1855-1858,
1999,
[doi]
[abstract]
We report the effect of light-induced sliding of the nematic director over an isotropic boundary surface in an azo-dye doped liquid-crystal cell. We show that illumination of the cell with polarized laser light induces transient dynamic sliding followed by permanent reorientation of the director. The two effects are in competition and tend to orient the director along mutually orthogonal directions. The sliding can be controlled and even completely quenched by the amount of induced anchoring energy. A physical model is proposed which accounts for the experimental results.
Light-induced alignment and reorientation effects in liquid crystals doped with azo-dyes
D. Andrienko, D. Fedorenko, E. Ouskova, V. Reshetnyak, Yu. Reznikov, S. Slussarenko, D. Voloshchenko, O. Lavrentovich
Ukr. J. Phys.,
44,
149-157,
1999,
Photoalignment effect induced by angular momentum of light in dye-doped liquid crystals
D. Andrienko, D. Fedorenko, Yu. Reznikov, S. Slussarenko, O. Francescangeli, F. Simoni
Mol. Cryst. Liq. Cryst.,
329,
613-622,
1999,
[doi]
[abstract]
We studied the effective interaction of the angular momentum of light with a liquid crystal (LC) doped with azo-dye. The experiments were carried out in a combined cell with one substrate providing strong unidirectional planar alignment of LC and the other covered by isotropic polymer layer providing a degenerated planar alignment. The effect manifests itself as azimuth reorientation of the liquid crystal director under the action of circularly polarized light in the absorption band of ate-dye. The new orientation of LC was captured by the command surface because of a light-induced alignment memory effect. To describe the experimental dependencies we solved the self-consistent problem of the propagation of a circularly polarized light in the absorbing nonlinear media using the geometric optics approximation. Comparing the experimental data with numerical evaluation we obtain the basic parameters of the system: the orientation nonlinearly coefficient and the value of anchoring of the director with the polymer surface.
Light-induced spatially modulated orientation of liquid crystal with photosensitive dopant
D. Andrienko, Yu. Reznikov, O. Uskova, D. Fedorenko
Ukr. J. Phys.,
43,
459-462,
1998,
Laser beam modulation freezing on a liquid crystal surface
D. Andrienko, O. Francesangeli, E. Ouskova, F. Simoni, S. Slussarenko, Yu. Reznikov
Mol. Cryst. Liq. Cryst.,
321,
69-76,
1998,
[abstract]
We report the formation of permanent light-induced patterns in a planar cell filled with dye-doped liquid crystal. The sample is placed between one rubbed surface giving unidirectional planar alignment and one isotropic surface providing degenerated planar alignment. The patterns appear under the irradiation of the isotropic surface through the liquid crystal layer. The experimental results are explained in terms of spatial modulation of the light polarization over the isotropic surface due to conformation nonlinearity of the mixture. The light-induced adsorption of azo-dye on the initially isotropic surface leads to freezing of this modulation and the appearance of a twisted director structure.
Dye-sensitized Solar Cells
Efficiency-limiting Processes in Cyclopentadithiophene-bridged Donor-Acceptor-type Dyes for Solid-State Dye-sensitized Solar Cells
F. Hinkel, Y. Kim, Y. Zagraniarsky, F. Schluetter, D. Andrienko, K. Muellen, F. Laquai
J. Chem. Phys.,
148,
044703,
2018,
[doi]
[abstract]
The charge generation and recombination processes in three novel push-pull photosensitizers for dye-sensitized solar cells (DSSCs) are studied by ps–μs transient absorption (TA) and quasi-steady-state photoinduced absorption (PIA) spectroscopy. The three cyclopentadithiophene-based photosensitizer dye molecules exhibit comparably low power conversion efficiencies ranging from 0.8% to 1.7% in solid-state DSSCs. We find that the photocurrents increase in the presence of Li-salt additives. Both TA and PIA measurements observe long-lived dye cations created by electron injection from the dyes’ excited state for two dyes from the series. However the third dye shows significantly lower performance as a consequence of the less efficient electron injection even after the addition of Li-salts and faster electron-hole recombination on the ns-μs time scale. In essence the prerequisites for this class of donor-π bridge-acceptor photosensitizers to reach higher charge generation efficiencies are a combination of strong dipole moments and fine tuning of the electronic landscape at the titania-dye interface by Li-salt addition.
Electret
Solid-State Electron Affinity Analysis of Amorphous Fluorinated Polymer Electret
S. Kim, A. Melnyk, D. Andrienko, Y. Suzuki
J. Phys. Chem. B,
124,
10507-10513,
2020,
[doi]
[abstract]
In this study electron trapping phenomena in amorphous polymer electrets were studied using a solid-state electron affinity analysis by means of molecular dynamics simulations parametrized with ab initio calculations. Negatively charged amorphous systems of a cyclic transparent optical polymer (CYTOP) with different end groups were reproduced by molecular dynamics simulations parametrized by density functional theory analysis. Quantum mechanical calculations were performed for electron trapping sites to determine the electron affinity of an isolated molecule. The influence of the amorphous system surrounding the trapping site was considered by accounting for electrostatic interactions with surrounding molecules and multipole induction. A series of analyses were carried out to mimic the conformational diversity of the amorphous system. The calculated solid-state electron affinities were found to adopt a Gaussian-type distribution and were in good accordance with the experimental data for surface potential and thermally stimulated discharge spectra indicating the reliability of the present analysis for predicting the charging performance of amorphous polymer electrets.
Excited and charge transfer states
Frenkel and charge-transfer excitations in donor-acceptor complexes from many-body Green's functions theory
B. Baumeier, D. Andrienko, M. Rohlfing
J. Chem. Theory Comput.,
8,
2790-2795,
2012,
[doi]
[abstract]
Excited states of donor-acceptor dimers are studied using many-body Green's functions theory within the GW approximation and the BetheâÃÂÃÂSalpeter equation. For a series of prototypical small-molecule based pairs this method predicts energies of local Frenkel and intermolecular charge-transfer excitations with the accuracy of tens of meV. Application to larger systems is possible and allowed us to analyze energy levels and binding energies of excitons in representative dimers of dicyanovinyl-substituted quarterthiophene and fullerene a donor-acceptor pair used in state of the art organic solar cells. In these dimers the transition from Frenkel to charge transfer excitons is endothermic and the binding energy of charge transfer excitons is still of the order of 1.5 eV. Hence even such an accurate dimer-based description does not yield internal energetics favorable for the generation of free charges either by thermal energy or an external electric field. These results confirm that for qualitative predictions of solar cell functionality accounting for the explicit molecular environment is as important as the accurate knowledge of internal dimer energies.
Force fields
Transferable atomic multipole machine learning models for small organic molecules
T. Bereau, D. Andrienko, A. O. von Lilienfeld
J. Chem. Theory Comput.,
11,
3225-3233,
2015,
[doi]
[abstract]
Accurate representation of the molecular electrostatic potential which is often expanded in distributed multipole moments is crucial for an efficient evaluation of intermolecular interactions. Here we introduce a machine learning model for multipole coefficients of atom types H C O N S F and Cl in any molecular conformation. The model is trained on quantum-chemical results for atoms in varying chemical environments drawn from thousands of organic molecules. Multipoles in systems with neutral cationic and anionic molecular charge states are treated with individual models. The modelsâÃÂàpredictive accuracy and applicability are illustrated by evaluating intermolecular interaction energies of nearly 1000 dimers and the cohesive energy of the benzene crystal.
Kinetic Monte Carlo
Efficient simulation of Markov chains using segmentation
T. Brereton, O. Stenzel, B. Baumeier, D. Andrienko, V. Schmidt, D. Kroese
Methodol. Comput. Appl.,
16,
465-484,
2014,
[doi]
[abstract]
A methodology is proposed that is suitable for efficient simulation of continuous-time Markov chains that are nearly-completely decomposable. For such Markov chains the effort to adequately explore the state space via Crude Monte Carlo (CMC) simulation can be extremely large. The purpose of this paper is to provide a fast alternative to the standard CMC algorithm which we call Aggregate Monte Carlo (AMC). The idea of the AMC algorithm is to reduce the jumping back and forth of the Markov chain in small subregions of the state space. We accomplish this by aggregating such problem regions into single states. We discuss two methods to identify collections of states where the Markov chain may become trapped: the stochastic watershed segmentation from image analysis and a graph-theoretic decomposition method. As a motivating application we consider the problem of estimating the charge carrier mobility of disordered organic semiconductors which contain low-energy regions in which the charge carrier can quickly become stuck. It is shown that the AMC estimator for the charge carrier mobility reduces computational costs by several orders of magnitude compared to the CMC estimator.
Liquid crystals
Introduction to Liquid Crystals
D. Andrienko
J. Mol. Liq.,
267,
520-541,
2018,
[doi]
[abstract]
This pedagogical overview of liquid crystals is based on lectures for postgraduate students given at the International Max Planck Research School “Modeling of Soft Matter”. I am delighted to dedicate it to my scientific advisor Prof. Yuriy Reznikov thus acknowledging his valuable contribution to my life.
A personal recollection: a tribute to Yuriy Reznikov
D. Andrienko
J. Mol. Liq.,
accepted,
2017,
[doi]
[abstract]
Meeting the right people at the right time is as important as hard work persistence and talent. Yuriy Reznikov was one of the key people who shaped my life and had an important impact on my scientific and personal development. His motivation diplomacy openness and engagement taught me how to appreciate the work of other people to collaborate with peers and even to face the world outside academia. As a tribute to Yuriy I would like to share with you this brief personal recollection of time spent in the Department of Physics of Crystals at the Institute of Physics in Ukraine first as a master and then a PhD student.
Theory and simulation of the nematic zenithal anchoring coefficient
Denis Andrienko, Michael P. Allen
Phys. Rev. E,
65,
021704,
2002,
[doi]
[abstract]
Combining molecular simulation Onsager theory and the elastic description of nematic liquid crystals we study the dependence of the nematic liquid crystal elastic constants and the zenithal surface anchoring coefficient on the value of the bulk order parameter.
Molecular simulation and theory of a liquid crystalline disclination core
Denis Andrienko, Michael P. Allen
Phys. Rev. E,
61,
504-510,
2000,
[doi]
[abstract]
Molecular simulations of a nematic liquid crystal confined in cylinder geometry with homeotropic anchoring have been carried out. The core structure of a disclination line defect of strength +1 has been examined and comparison made with various theoretical treatments which are presented in a unified way. It is found that excellent fits to the cylindrically symmetrized order tensor profiles may be obtained with appropriate parameter choices; notwithstanding this on the time scales of the simulation the cylindrical symmetry of the core is broken and two defects of strength +1 / 2 may be resolved.
Liquid crystal director fluctuations and surface anchoring by molecular simulation
Denis Andrienko, Guido Germano, Michael P. Allen
Phys. Rev. E,
62,
6688-6693,
2000,
[doi]
[abstract]
We propose a simple and reliable method to measure the liquid crystal surface anchoring strength by molecular simulation. The method is based on the measurement of the long-range fluctuation modes of the director in confined geometry. As an example molecular simulations of a liquid crystal in slab geometry between parallel walls with homeotropic anchoring have been carried out using the Monte Carlo technique. By studying different slab thicknesses we are able to calculate separately the position of the elastic boundary condition and the extrapolation length.
Field-induced orientational transitions in a nematic with chiral dopand
D. Andrienko, Yu. Reznikov, V. Reshetnyak
Mol. Cryst. Liq. Cryst.,
331,
2333-2342,
1999,
Director gratings and light diffraction in a nematic cell with spatially modulated easy axis
D. Andrienko, I. Pinkevich
Mol. Cryst. Liq. Cryst.,
309,
143-156,
1998,
[doi]
Light-induced Frederiks transition in the nematic liquid crystal with chiral dopant
D. Andrienko, I. Pinkevich, V. Reshetnyak
Liq. Cryst.,
25,
95-100,
1998,
[doi]
[abstract]
The influence of phototransformed molecules with chiralproperties changing on the absorption of light field on the light-induced Freedericksz transition threshold in a homeotropically oriented nematic cell is considered. It is shown that the appearance of the light-induced chiral molecules can decrease or increase the Freedericksz threshold value depending on the chirality sign of the phototransformed molecules and of the initial chiral dopant. Expressions for the threshold are obtained for circular and linear polarization of the incident light. The dependence of the threshold on the periodicity of the spatially modulated light intensity is estimated for large periods of modulation. The dependence of dopant threshold chirality on the director anchoring energy has been found.
Orientational transitions in a nematic cell with reverse director distributions
D. Andrienko, Yu. Reznikov
Mol. Cryst. Liq. Cryst.,
321,
291-297,
1998,
[doi]
[abstract]
We predict orientational transitions in a nematic cell with co-directed and reversed orientations of easy axes on the aligning surfaces. Aside from a forward director distribution there can exist the reverse distributions with zero and pi/2 polar director angle at some point in the cell bulk. In the Rapini approach for the surface free energy these distributions can be unstable and transition two the forward distribution can occur. Dimensionless parameters controlling the director distribution are the easy axis angle and an anchoring strength parameter WL/K.
Diffraction gratings in a nematic cell due to spatial variation of surface order parameter
D. Andrienko, I. Pinkevich, Yu. Reznikov
Mol. Cryst. Liq. Cryst.,
321,
283-289,
1998,
[doi]
[abstract]
The influence of a periodical spatial modulation of a surface order parameter on the distribution of the order parameter in a nematic liquid crystal bulk is studied. The problem is treated in the framework of the Landau - de Gennes theory with an anchoring energy in the form F ~ Tr(Q-S)^2 where S is the NLC order parameter and Q is the surface order parameter. It is shown that the order parameter grating which damps exponentially with the distance to the cell plates appears in the cell bulk. The damping coefficient is inversely proportional to the coherence length ksi of the nematic-isotropic phase transition. Diffraction efficiency of this grating is proportional to delta Q^2 where delta Q is an amplitude of the order parameter modulation and is sufficient for experimental observations.
Dynamic holographic gratings in nematic cell with periodic boundary conditions
D. Andrienko, I. Pinkevich, M. Lednei
Mol. Cryst. Liq. Cryst.,
304,
95-100,
1997,
[doi]
Long-range interactions
Long-range embedding of molecular ions and excitations in a polarizable molecular environment
C. Poelking, D. Andrienko
J. Chem. Theory Comput.,
12,
4516-4523,
2016,
[doi]
[abstract]
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.
Materials design
Computational materials discovery in soft matter
T. Bereau, D. Andrienko, K. Kremer
APL Mat.,
4,
053101,
2016,
[doi]
[abstract]
Soft matter embodies a wide range of materials which all share the common characteristics of weak interaction energies determining their supramolecular structure. This complicates structure-property predictions and hampers the direct application of data-driven approaches to their modeling. We present several aspects in which these methods play a role in designing soft-matter materials: drug design as well as information-driven computer simulations e.g. histogram reweighting. We also discuss recent examples of rational design of soft-matter materials fostered by physical insight and assisted by data-driven approaches. We foresee the combination of data-driven and physical approaches a promising strategy to move the field forward.
Molecular Electronics
Tuning single-molecule conductance by controlled electric field-induced trans-to-cis isomerisation
C.S. Quintans, D. Andrienko, K. F. Domke, D. Aravena, S. Koo, I. Diez- Perez, A. C. Aragones
Applied Sciences,
11,
3317,
2021,
[doi]
[abstract]
External electric fields (EEFs) have proven to be very efficient in catalysing chemical reactions even those inaccessible via wet-chemical synthesis. At the single-molecule level oriented EEFs have been successfully used to promote in-situ single-molecule reactions in the absence of chemical catalysts. Here we elucidate the effect of an EEFs on the structure and conductance of a molecular junction. Employing scanning tunnelling microscopy break junction (STM-BJ) experiments we form and electrically characterize single-molecule junctions of two tetramethyl carotene isomers. Two discrete conductance signatures show up more prominently at low and high applied voltages which are univocally ascribed to the trans and cis isomers of the carotenoid respectively. The difference in conductance between both cis-/trans- isomers is in concordance with previous predictions considering π-quantum interference due to the presence of a single gauche defect in the trans isomer. Electronic structure calculations evidence the conductance switching originates in the EEF-induced excited state mixing that enhances the electron transport contribution of states involved in the isomerisation dynamics promoting the cis-isomerisation. Our work opens new routes for the in-situ control of isomerisation reactions in single-molecule contacts.
Nanographene
Tuning Interfacial Charge Transfer in Atomically Precise Nanographene-Graphene Heterostructures by Engineering van der Waals Interactions
X. Yu, Sh. Fu, M. Mandal, X. Yao, Zh. Liu, W. Zheng, P. Samorì, A. Narita, K. Muellen, D. Andrienko, M. Bonn, H. Wang
J. Chem. Phys.,
156,
2022,
[doi]
[abstract]
Combining strong light absorption and outstanding electrical conductivity hybrid nanographene-graphene (N-Gr) van der Waals heterostructures (vdWHs) represent an emerging material platform for versatile optoelectronic devices. Interfacial charge transfer (CT) a fundamental process whose full control remains limited plays a paramount role in determining the final device performance. Here we demonstrate that the interlayer vdW interactions can be engineered by tuning the sizes of bottom-up synthesized NGs to control the interfacial electronic coupling strength and thus the CT process in NG-Gr vdWHs. By increasing the dimensions of NGs from 42 to 96 sp2 carbon atoms in the polyaromatic core to enhance the interfacial coupling strength we find that the CT efficiency and rate in NG-Gr vdWHs display a drastic increase of one order of magnitude despite the fact that the interfacial energy driving the CT process is unfavorably reduced. Our results shed light on the CT mechanism and provide an effective knob to tune the electronic coupling at NG-Gr interfaces by controlling the size-dependent vdW interactions.
Nematic alignment
Control of the anchoring energy of rubbed polyimide layers by irradiation with depolarized UV-light
D. Andrienko, Yu. Kurioz, M. Nishikawa, Yu. Reznikov, J. L. West
Jap. J. Appl. Phys,
39,
1217-1220,
2000,
[doi]
[abstract]
Exposure of rubbing polyimide (PI) film to depolarized ultraviolet (UV) light suppressed the effective anchoring energy of liquid crystal (LC) with aligning surface. Polarized light changed the orientational distribution of PI molecules obtained by rubbing by changing both the anchoring energy and easy axis direction. These results show that ultraviolet exposure can be effectively used to control anchoring parameters.
Mechanically and light induced anchoring of liquid crystal on polyimide film
M. Nishikawa, J. L. West, D. Andrienko, Yu. Kurioz, Yu. Reznikov, D. S. Kang, J. H. Kim, C. Rosenblatt
Mol. Cryst. Liq. Cryst.,
329,
623-630,
1999,
Photoalignment of pentyl-cyanobiphenyl on the fluorinated polyvinil-cinnamates induced by UV and visible light
D. Andrienko, A. Dyadyusha, Yu. Kurioz, Yu. Reznikov, F. Barbet, D. Bormann, M. Warenghem, B. Khelifa
Mol. Cryst. Liq. Cryst.,
329,
831-838,
1999,
[doi]
Tilted photoalighnment of a nematic liquid crystal induced by a magnetic field
D. Andrienko, Yu. Kurioz, Yu. Reznikov, Ch. Rozenblatt, R. G. Petschek, O. D. Lavrentovich, D. Subacius
J. Appl. Phys.,
83,
50-55,
1998,
[doi]
[abstract]
Nematic liquid crystal cells with polyvinyl cinnamate coated substrates were subjected to ultraviolet light. When this was done in the presence of an oblique magnetic field the photoalignment was found to be temporally and thermally robust with a large pretilt angle and weak polar anchoring. Moreover two easy axes with equal and opposite pretilt angle were obtained such that a magnetic field could switch the director from one easy axis to the other.
Measurements of controllable azimuthal anchoring energy of liquid crystal on photoaligning surface
D. Andrienko, A. Iljin, A. Dyadyusha, Yu. Kurioz, Yu. Reznikov
Mol. Cryst. Liq. Cryst.,
321,
271-281,
1998,
[abstract]
A method to determine the surface azimuthal anchoring energy of a nematic liquid crystal is proposed. The technique implies the measurement of the director deviation on the cell substrate as a function of strength and direction of the applied magnetic field. As an example the dependence of the azimuthal anchoring coefficient on the exposure time is measured at the interface between the nematic K15 and polyvinylcinnamate film exposed by UV light. The analogous measurements performed in a wedge cell show that the method with magnetic field is more precise.
Light-induced anchoring transitions and bistable nematic alignment on a polysiloxane aligning surface
D. Andrienko, A. Dyadyusha, Yu. Kurioz, V. Reshetnyak, Yu. Reznikov
Mol. Cryst. Liq. Cryst.,
321,
299-307,
1998,
[abstract]
We report a novel azimuthal anchoring transition of nematic liquid crystal on a polysiloxane-cinnamate treated surface under the UV irradiation. It is found that the direction of the light-induced easy axis strongly depends on the irradiation time. Under short exposure times the orienting surface provides an orientation of LC parallel to the direction of the UV light polarization. Longer times give the orthogonal to the UV polarization orientation of LC. There is a region of exposure times that give a bistable anchoring conditions and domains with different directions of the director orientation. We propose a phenomenological model of this anchoring transition and surface bistability based on the competition between trans-cis isomerisation of the side fragments of the polysiloxane-cinnamate material and cross-linking photoreaction of the trans-form of side fragments resulting in orthogonal molecular easy axes.
Surface driven transition in a nematic liquid crystal cell
D. Andrienko, Yu. Kurioz, Yu. Reznikov, V. Reshetnyak
JETP,
85,
2045-2055,
1997,
[doi]
[abstract]
Surface driven reorientation effects in a nematic liquid crystal cell caused by light-induced changes of the anchoring parameters were studied. Theoretical consideration of one-dimensional flat distributions of the director has shown that the director can undergo threshold reorientation between hybrid homeotropic and planar alignments as the anchoring energy varies continuously. The threshold reorientation takes place when the reference and light-induced easy axes are perpendicular. In the one-elastic-constant approximation the light-induced transition was found to be of second order as shown by a critical increase of the director thermal fluctuations in the vicinity of the transition point. These effects were experimentally studied in the cells containing 5CB liquid crystal aligned by the photosensitive azo-containing polymer layer.
Nematic colloids
Colloidal particles in liquid crystal films and at interfaces
Mykola Tasinkevych, Denis Andrienko
Cond. Mat. Phys.,
13,
33603,
2010,
[doi]
Effective triplet interactions in nematic colloids
M. Tasinkevych, D. Andrienko
Eur. Phys. J. E,
21,
277-282,
2006,
[doi]
[abstract]
Three-body effective interactions emerging between parallel cylindrical rods immersed in a nematic liquid crystals are calculated within the Landau-de~Gennes free energy description. Collinear equilateral and midplane configurations of the three colloidal particles are considered. In the last two cases the effective triplet interaction is of the same magnitude and range as the pair one.
Colloidal particles at a nematic-isotropic interface: effects of confinement
J. L. West, K. Zhang, A. Glushchenko, D. Andrienko, M. Tasinkevych, Y. Reznikov
Eur. Phys. J. E,
20,
237-242,
2006,
[doi]
[abstract]
When captured by a flat nematic-isotropic interface colloidal particles can be dragged by it. As a result spatially periodic structures may appear with the period depending on a particle mass size and interface velocity [1]. If liquid crystal is sandwiched between two substrates the interface takes a wedge-like shape accommodating the interface-substrate contact angle and minimizing the director distortions on its nematic side. Correspondingly particles move along complex trajectories: they are first captured by the interface and then glide towards its vertex point. Our experiments quantify this scenario; simple estimates and numerical minimization of the Landau-de Gennes free energy allow for a qualitative description of the interfacial structure and the drag force.
Effective pair interactions between colloidal particles at a nematic-isotropic interface
D. Andrienko, M. Tasinkevych, S. Dietrich
Europhys. Lett.,
70,
95-101,
2005,
[doi]
[abstract]
The Landau-de Gennes free energy is used to study theoretically the interaction of parallel cylindrical colloidal particles trapped at a nematic-isotropic interface. We find that the effective interaction potential is non-monotonic. The corresponding force-distance curves exhibit jumps and hysteresis upon approach/separation due to the creation/annihilation of topological defects. Minimization results suggest a simple empirical pair potential for the effective colloid-colloid interaction at the interface. We propose that the interface-mediated interaction can play an important role in self-organization and clustering of colloidal particles at such interfaces.
Drag of micro-particles by an extended nematic-isotropic interface
J. L. West, Ke Zhang, A. V. Glushchenko, Y. Reznikov, D. Andrienko
Mol. Cryst. Liq. Cryst.,
422,
73-82,
2004,
[doi]
[abstract]
We studied the behaviour of polymer particles in a moving interface between the nematic and isotropic phases of a nematic liquid crystal. We showed that theNI-interface is extended(E)and has a layeredN-I-Nstructure in the vertical cross-section of the sample; the wedge of the isotropic phase is bounded by the nematic phase which is limited by the cell substrates. The minimum of the cell free energy defines the position of particles in the interface region. We find that the preferable position of the particle is at the vertex of the wedge formed by the isotropic phase. The particles are captured by the vertex line and follow the interface when it moves.
Mechanism of formation of three dimensional structures of particles in a liquid crystal
J. L. West, Ke Zhang, G. Liao, A. V. Glushchenko, Y. Reznikov, D. Andrienko, M. P. Allen
Mol. Cryst. Liq. Cryst.,
410,
83-93,
2004,
[doi]
[abstract]
In this work we report methods of formation of three-dimensional structures of particles in a liquid crystal host. We found that under the appropriate conditions the particles are captured and dragged by the moving isotropic/nematic front during the phase transition process. This movement of the particles can be enhanced significantly or suppressed drastically with the influence of an electric field and/or with changing the conditions of the phase transition such as the rate of cooling. As a result a wide variety of particle structure can be obtained ranging from a fine-grained cellular structure to stripes of varying periods to a course-grained ~root~ structure. Changing the properties of the materials such as the size and density of the particles and the surface anchoring of the liquid crystal at the particle surface can also be used to control the morphology of the three-dimensional particle network and adjust the physical properties of the resulting dispersions. These particle structures may be used to affect the performance of LCD's much as polymers have been used in the past.
Capillary bridging and long-range attractive forces in a mean-field approach
D. Andrienko, P. Patricio, O. I. Vinogradova
J. Chem. Phys.,
121,
4414,
2004,
[doi]
[abstract]
When a mixture is confined one of the phases can condense out. This condensate which is otherwise metastable in the bulk is stabilized by the presence of surfaces. In a sphere-plane geometry routinely used in atomic force microscope and surface force apparatus it can form a bridge connecting the surfaces. The pressure drop in the bridge gives rise to additional long-range attractive forces between them. By minimizing the free energy of a binary mixture we obtain the force-distance curves as well as the structural phase diagram of the configuration with the bridge. Numerical results predict a discontinuous transition between the states with and without the bridge and linear force-distance curves with hysteresis. We also show that similar phenomenon can be observed in a number of different systems e.g. liquid crystals and polymer mixtures.
Interaction of colloids with a nematic-isotropic interface
D. Andrienko, M. Tasinkevych, P. Patricio, M. M. Telo da Gama
Phys. Rev. E,
69,
021706,
2004,
[doi]
[abstract]
The Landau-de Gennes free energy is used to calculate the interaction between long cylindrical colloids and the nematic-isotropic interface. This interaction has two contributions: one is specific of liquid crystals and results from the deformation of the director field close to the particles or to the interface while the other is generic and results from wetting and surface tension effects. Deep in the nematic phase the director field of long cylindrical colloids with strong homeotropic anchoring exhibits two half-integer defect lines. As the colloid moves towards the interface the director configuration changes through a series of discontinuous transitions where one or two of the defects are annihilated. In addition the NI interface bends towards the colloid in order to minimize the elastic free energy in the nematic. In the isotropic phase the colloid is surrounded by a thin nematic layer that reduces the surface free energy under favorable wetting conditions. The interaction has a well-defined minimum near the interface. In this region the director and interfacial structures are complex and cannot be described analytically. Using the numerical results for the Landaude Gennes free energy in the harmonic region we obtained simple scaling laws for the linear force on the colloid.
Colloidal disks in nematic liquid crystals
N. M. Silvestre, P. Patricio, M. Tasinkevych, D. Andrienko, M. M. Telo da Gama
J. Phys. Condens. Matter,
16,
S1921-S1930,
2004,
[doi]
[abstract]
We use adaptive finite elements methods to investigate a variety of structures in inverted nematic emulsions numerically. In particular we study dipolar and quadrupolar interactions between colloidal discs in two-dimensional nematics. The behaviour of colloidal particles near a substrate and at a nematic-isotropic interface are also considered.
Forces between elongated particles in a nematic colloid
D. Andrienko, M. Tasinkevych, P. Patricio, M. P. Allen, M. M. Telo da Gama
Phys. Rev. E,
68,
051702,
2003,
[doi]
[abstract]
Using molecular dynamics simulations we study the interactions between elongated colloidal particles (length to breath ratio >1) in a nematic host. The simulation results are compared to the results of a Landau-de Gennes elastic free energy. We find that depletion forces dominate for the sizes of the colloidal particles studied. The tangential component of the force however allows us to resolve the elastic contribution to the total interaction. We find that this contribution differs from the quadrupolar interaction predicted at large separations. The difference is due to the presence of nonlinear effects namely the change in the positions and structure of the defects and their annihilation at small separations.
Drag on particles in a nematic suspension by a moving nematic-isotropic interface
J. L. West, A. Glushchenko, G. Liao, Y. Reznikov, D. Andrienko, M. P. Allen
Phys. Rev. E,
66,
012702,
2002,
[doi]
[abstract]
We report a clear demonstration of drag on colloidal particles by a moving nematic-isotropic interface. The balance of forces explains our observation of periodic striplike structures that are produced by the movement of these particles.
Defect structures and torque on an elongated colloidal particle immersed in a liquid crystal host
D. Andrienko, M. P. Allen, G. Skacej, S. Zumer
Phys. Rev. E,
65,
041702,
2002,
[doi]
[abstract]
Combining molecular dynamics and Monte Carlo simulation we study defect structures around an elongated colloidal particle embedded in a nematic liquid crystal host. By studying nematic ordering near the particle and the disclination core region we are able to examine the defect core structure and the difference between two simulation techniques. In addition we also study the torque on a particle tilted with respect to the director and modification of this torque when the particle is close to the cell wall.
Computer simulation of topological defects around a colloidal particle or droplet dispersed in a nematic host
Denis Andrienko, Guido Germano, Michael P. Allen
Phys. Rev. E,
63,
041701,
2001,
[doi]
[abstract]
We use molecular dynamics to study the ordering of a nematic liquid crystal around a spherical particle or droplet. Homeotropic boundary conditions and strong anchoring create a hedgehog (radial point defect) director configuration on the particle surface and in its vicinity; this topological defect is canceled by nearby defect structures in the surrounding liquid crystal so as to give a uniform director field at large distances. We observe three defect structures for different particle sizes: a quadrupolar one with a ring defect surrounding the particle in the equatorial plane; a dipolar one with a satellite defect at the north or south pole; and a transitional nonequatorial ring defect. These observations are broadly consistent with the predictions of the simplest elastic theory. By studying density and order-parameter maps we are able to examine behavior near the particle surface and in the disclination core region where the elastic theory is inapplicable. Despite the relatively small scale of the inhomogeneities in our systems the simple theory gives reasonably accurate predictions of the variation of defect position with particle size.
Non-fullerene acceptors
Electronic coarse-graining of long conjugated molecules: Case study of non-fullerene acceptors
A. Zhugayevych, K.-H. Lin, D. Andrienko
J. Chem. Phys.,
159,
024107,
2023,
[doi]
[abstract]
By considering only one electronic state per molecule charge transport models of molecular solids neglect intramolecular charge transfer. This approximation excludes materials with quasi-degenerate spatially separated frontier orbitals such as non-fullerene acceptors (NFAs) and symmetric thermally activated delayed fluorescence emitters. By analyzing the electronic structure of room-temperature molecular conformers of a prototypical NFA ITIC-4F we conclude that the electron is localized on one of the two acceptor blocks with the mean intramolecular transfer integral of 120 meV which is comparable with intermolecular couplings. Therefore the minimal basis for acceptor–donor–acceptor (A–D–A) molecules consists of two molecular orbitals localized on the acceptor blocks. This basis is robust even with respect to geometry distortions in an amorphous solid in contrast to the basis of two lowest unoccupied canonical molecular orbitals withstanding only thermal fluctuations in a crystal. The charge carrier mobility can be underestimated by a factor of two when using single site approximation for A–D–A molecules in their typical crystalline packings.
Semitransparent Organic Photovoltaics Utilizing Intrinsic Charge Generation in Non-Fullerene Acceptors
A. Sharma, N. Gasparini, A. Markina, S. Karuthedath, J. Gorenflot, J. Han, A. Balawi, W. Liu, D. Bryant, J. Bertrandie, J. Troughton, S. H. K. Paleti, H. Bristow, F. Laquai, D. Andrienko, D. Baran
Advanced Materials,
accepted,
2023,
abstract not found
Ambipolar charge transport in a non-fullerene acceptor
F. H. Hasenburg, K.-H. Lin, B. van der Zee, P. W. M. Blom, D. Andrienko, G.-J. A.H. Wetzelaer
APL Materials,
11,
021105,
2023,
[doi]
[abstract]
Charge transport is one of the key factors in the operation of organic solar cells. Here we investigate the electron and hole transport in the non-fullerene acceptor (NFA) IT-4F by a combination of space-charge-limited current measurements and multiscale molecular simulations. The electron and hole mobilities are fairly balanced amounting to 2.9 x 10-4 cm2 V-1 s-1 for electrons and 2.0 x 10-5 cm2 V-1 s-1 for holes. Orientational ordering and electronic couplings facilitate a better charge-percolating network for electrons than for holes while ambipolarity itself is due to sufficiently high electron affinity and low ionization energy typical for narrow-gap NFAs. Our findings provide a molecular-level understanding of balanced hole and electron transport in an archetypical NFA which may play a key role in exciton diffusion and photogenerated hole transfer in organic solar cells.
Reduced bimolecular charge recombination in efficient organic solar cells comprising non-fullerene acceptors
Y. Wu, Y. Li, B. van der Zee, W. Liu, A. Markina, H. Fan, H. Yang, C. Cui, Y. Li, P. W. M. Blom, D. Andrienko, G.-J. A. H. Wetzelaer
Scientific Reports,
13,
4717,
2023,
[doi]
Reduced Intrinsic Non-Radiative Losses Allow Room-Temperature Triplet Emission from Purely Organic Emitters
Y. Li, L. Jiang, W. Liu, S. Xu, T.-Y. Li, F. Fries, O. Zeika, Y. Zou, C. Ramanan, S. Lenk, R. Scholz, D. Andrienko, X. Feng, K. Leo, S. Reineke
Advanced Materials,
2101844,
2021,
[doi]
[abstract]
Persistent luminescence from triplet excitons in organic molecules is rare as fast non-radiative deactivation typically dominates over radiative transitions. This work demonstrates that the substitution of a hydrogen atom in a derivative of phenanthroimidazole with an N-phenyl ring can substantially stabilize the excited state. This stabilization converts an organic material without phosphorescence emission into a molecular system exhibiting efficient and ultralong afterglow phosphorescence at room temperature. Results from systematic photophysical investigations kinetic modeling excited-state dynamic modeling and single-crystal structure analysis identify that the long-lived triplets originate from a reduction of intrinsic non-radiative molecular relaxations. Further modification of the N-phenyl ring with halogen atoms affects the afterglow lifetime and quantum yield. As a proof-of-concept an anticounterfeiting device is demonstrated with a time-dependent Morse code feature for data encryption based on these emitters. A fundamental design principle is outlined to achieve long-lived and emissive triplet states by suppressing intrinsic non-radiative relaxations in the form of molecular vibrations or rotations.
Chemical design rules for non-fullerene acceptors in organic solar cells
A. Markina, K.-H. Lin, W. Liu, C. Poelking, Y. Firdaus, D. R. Villalva, J. I. Khan, S. H. K. Paleti, G. T. Harrison, J. Gorenflot, W. Zhang, S. De Wolf, I. McCulloch, T. D. Anthopoulos, D. Baran, F. Laquai, D. Andrienko
Adv. Energy Mater.,
2102363,
2-11,
2021,
[doi]
[abstract]
Efficiencies of organic solar cells have practically doubled since the development of non-fullerene acceptors (NFAs). However generic chemical design rules for donor-NFA combinations are still needed. Such rules are proposed by analyzing inhomogeneous electrostatic fields at the donor–acceptor interface. It is shown that an acceptor–donor–acceptor molecular architecture and molecular alignment parallel to the interface results in energy level bending that destabilizes the charge transfer state thus promoting its dissociation into free charges. By analyzing a series of PCE10:NFA solar cells with NFAs including Y6 IEICO and ITIC as well as their halogenated derivatives it is suggested that the molecular quadrupole moment of 75 Debye A balances the losses in the open circuit voltage and gains in charge generation efficiency.
Impact of Acceptor Quadrupole Moment on Charge Generation and Recombination in Blends of IDT-Based Non-Fullerene Acceptors with PCE10 as Donor Polymer
J. I. Khan, M. A. Alamoudi, N. Chaturvedi, R. S. Ashraf, M. N. Nabi, A. Markina, W. Liu, T. A. Dela Pena, W. Zhang, W. Alsufyani, D. Andrienko, I. McCulloch, F. Laquai
Adv. Energy Mater.,
2100839,
2021,
[doi]
[abstract]
Advancing non-fullerene acceptor (NFA) organic photovoltaics requires the mitigation of the efficiency-limiting processes. Acceptor end-group and side-chain engineering are two handles to tune properties and a better understanding of their specific impact on the photophysics could facilitate a more guided acceptor design. Here the device performance energetic landscape and photophysics of rhodanine and dicyanovinyl end-capped IDT-based NFAs namely O-IDTBR and O-IDTBCN in PCE10-based solar cells are compared by transient optical and electro-optical spectroscopy techniques and density functional theory calculations. It is revealed how the acceptors’ quadrupole moments affect the interfacial energetic landscape in turn causing differences in exciton quenching charge dissociation efficiencies and geminate versus non-geminate recombination losses. More precisely it is found that the open circuit voltage (VOC) is controlled by the acceptors’ electron affinity (EA) while geminate and non-geminate recombination and the field dependence of charge generation rely on the acceptors’ quadrupole moments. The kinetic parameters and yields of all processes are determined and it is demonstrated that they can reproduce the performance differences of the devices’ current–voltage characteristics in carrier drift-diffusion simulations. The results provide insight into the impact of the energetic landscape specifically the role of the quadrupole moment of the acceptor beyond trivial considerations of the donor–acceptor energy offsets.
Charge Photogeneration in Non-Fullerene Organic Solar Cells: Influence of Excess Energy and Electrostatic Interactions
M. Saladina, P. S. Marques, A. Markina, S. Karuthedath, C. Woepke, C. Goehler, Y. Chen, M. Allain, P. Blanchard, C. Cabanetos, D. Andrienko, F. Laquai, J. Gorenflot, C. Deibel
Adv. Funct. Mater.,
31,
2007479,
2020,
[doi]
[abstract]
In organic solar cells photogenerated singlet excitons form charge transfer (CT) complexes which subsequently split into free charge carriers. Here we consider the contributions of excess energy and molecular quadrupole moments to the charge separation process. We investigate charge photogeneration in two separate bulk heterojunction systems consisting of the polymer donor PTB7-Th and two non-fullerene acceptors ITIC and h-ITIC. CT state dissociation in these donor--acceptor systems is monitored by charge density decay dynamics obtained from transient absorption experiments. We study the electric field dependence of charge carrier generation at different excitation energies by time delayed collection field (TDCF) and sensitive steady-state photocurrent measurements. Upon excitation below the optical gap free charge carrier generation becomes less field dependent with increasing photon energy which challenges the view of charge photogeneration proceeding through energetically lowest CT states. We determine the average distance between electron--hole pairs at the donor--acceptor interface from empirical fits to the TDCF data. The delocalisation of CT states is larger in PTB7-Th:ITIC the system with larger molecular quadrupole moment indicating the sizeable effect of the electrostatic potential at the donor--acceptor interface on the dissociation of CT complexes.
Long-range exciton diffusion in molecular non-fullerene acceptors
Y. Firdaus, V. M. Le Corre, S. Karuthedath, W. Liu, A. Markina, W. Huang, S. Chattopadhyay, M. M. Nahid, M. I. Nugraha, Y. Lin, A. Seitkhan, A. Basu, W. Zhang, I. McCulloch, H. Ade, J. Labram, F. Laquai, D. Andrienko, L. J. A. Koster, T. D. Anthopoulos
Nature Communications,
11,
5220,
2020,
[doi]
[abstract]
The short exciton diffusion length associated with most classical organic semiconductors used in organic photovoltaics (5-20 nm) imposes severe limits on the maximum size of the donor and acceptor domains within the photoactive layer of the cell. Identifying materials that are able to transport excitons over longer distances can help advancing our understanding and lead to solar cells with higher efficiency. Here we measure the exciton diffusion length in a wide range of nonfullerene acceptor molecules using two different experimental techniques based on photocurrent and ultrafast spectroscopy measurements. The acceptors exhibit balanced ambipolar charge transport and surprisingly long exciton diffusion lengths in the range of 20 to 47 nm. With the aid of quantum-chemical calculations we are able to rationalize the exciton dynamics and draw basic chemical design rules particularly on the importance of the end-group substituent on the crystal packing of nonfullerene acceptors.
Barrierless Free Charge Generation in the High-Performance PM6:Y6 Bulk Heterojunction Non-Fullerene Solar Cell
L. Perdigon-Toro, H. Zhang, A. Markina, J. Yuan, C. M. Wolff, M. Stolterfoht, Y. Zou, F. Gao, D. Andrienko, S. Shoaee, D. Neher
Advanced Materials,
32,
1906763,
2020,
[doi]
[abstract]
Organic solar cells (OSCs) are currently experiencing a second golden age thanks to the development of novel non-fullerene acceptors (NFAs). Surprisingly some of these blends exhibit high efficiencies despite a low energy offset at the heterojunction. Herein we thoroughly investigate free charge generation in the high-performance blend of the donor polymer PM6 with the NFA Y6 as a function of internal field temperature and excitation energy. Results show that generation is essentially barrierless with near-unity efficiency regardless of excitation energy. Efficient generation is maintained over a wide temperature range down to 100 K despite the small driving force for charge generation. Studies on a blend with a low concentration of the NFA measurements of the energetic disorder and theoretical modelling suggest that charge generation is assisted by the electrostatic interfacial field which for Y6 is large enough to compensate the Coulomb dissociation barrier. This field also repels charges from donor-acceptor interfaces reducing non-geminate recombination.
Intrinsic efficiency limits in low-bandgap non-fullerene acceptor organic solar cells
S. Karuthedath, J. Gorenflot, Y. Firdaus N. Chaturvedi, C. S. P. De Castro, G. T. Harrison, J. I. Khan, A. Markina, A. H. Balawi, T. A. D. Pena, W. Liu, R.-Z. Liang, A. Sharma, S. H. K. Paleti, W. Zhang, Y. Lin, E. Alarousu, D. H. Anjum, P. M. Beaujuge, S. De Wolf, I. McCulloch, T. D. Anthopoulos, D. Baran, D. Andrienko, F. Laquai
Nature Materials,
20,
378-384,
2021,
[doi]
[abstract]
In bulk heterojunction (BHJ) organic solar cells (OSCs) both the electron affinity (EA) and ionization energy (IE) offsets at the donor–acceptor interface should equally control exciton dissociation. Here we demonstrate that in low-bandgap non-fullerene acceptor (NFA) BHJs ultrafast donor-to-acceptor energy transfer precedes hole transfer from the acceptor to the donor and thus renders the EA offset virtually unimportant. Moreover sizeable bulk IE offsets of about 0.5 eV are needed for efficient charge transfer and high internal quantum efficiencies since energy level bending at the donor–NFA interface caused by the acceptors’ quadrupole moments prevents efficient exciton-to-charge-transfer state conversion at low IE offsets. The same bending however is the origin of the barrier-less charge transfer state to free charge conversion. Our results provide a comprehensive picture of the photophysics of NFA-based blends and show that sizeable bulk IE offsets are essential to design efficient BHJ OSCs based on low-bandgap NFAs.
Impact of “Nonfullerene” Acceptor Core Structure on the Photophysics and Efficiency of Polymer Solar Cells
M. Alamoudi, J. Khan, Y. Firdaus, K. Wang, D. Andrienko, P. Beaujuge, F. Laquai
ACS Energy Letters,
3,
802-811,
2018,
[doi]
[abstract]
Small-molecule (SM) ‘nonfullerene’ acceptors are promising alternatives to fullerene (PC61/71BM) derivatives often used in bulk heterojunction (BHJ) organic solar cells; yet the efficiency-limiting processes and their dependence on the acceptor structure are not clearly understood. Here we investigate the impact of the acceptor core structure (cyclopenta-[21-b:34-b′]dithiophene (CDT) vs. indacenodithiophene (IDTT)) of malononitrile (BM)-terminated acceptors namely CDTBM and IDTTBM on the photophysical characteristics of BHJ solar cells. Using PCE10 as donor polymer the IDTT-based acceptor achieves higher power conversion efficiencies (8.4%) than the CDT-based acceptor (5.6%) due to a concurrent increase in short-circuit current and open circuit voltage. Using (ultra)fast transient spectroscopy we demonstrate that reduced geminate recombination in PCE10:IDTTBM blends is the reason for the difference in short-circuit currents. External quantum efficiency measurements indicate that the higher energy of interfacial charge transfer states observed for the IDTT-based acceptor blends is the origin of the higher open-circuit voltage.
OFETs
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.,
19,
10854-10862,
2017,
[doi]
[abstract]
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.
OLED
Monitoring the charge-carrier occupied density-of-states in disordered organic semiconductors under non-equilibrium conditions using thermally stimulated luminescence spectroscopy
A. Stankevych, R. Saxena, A. Vakhnin, F. May, C. Pflumm, N. Kinaret, D. Andrienko, J. Genoe, H. Baessler, A. Koehler, A. Kadashchuk
Phys. Rev. Appl.,
19,
054007,
2023,
[doi]
[abstract]
The dynamics of charge carriers in disordered organic semiconductors is inherently difficult to probe by spectroscopic methods. Thermally stimulated luminescence (TSL) is an approach that detects the luminescence resulting from the recombination of spatially-well-separated geminate charge pairs usually at low temperature. In this way the density of states (DOS) for charges can be determined. In this study we demonstrate that TSL can also be used for probing an occupied density of states formed by a low-temperature energetic relaxation of photogenerated charges. Another approach used to gain an insight into the charge-relaxation process is kinetic Monte Carlo (KMC) simulations. Here we use both techniques to determine the energetic distribution of charges at low temperatures. We find that the charge dynamics is frustrated yet this frustration can be overcome in TSL by using an infrared (IR) push pulse and in KMC simulations by a long simulation time that allows for long-range tunneling. Applying the IR-push TSL to pristine amorphous films of 18 commonly used low-molecular-weight organic light-emitting diode materials we find that the width of the occupied DOS amounts to about 2/3 of the available DOS. The same result is obtained in KMC simulations that consider spatial correlations between site energies. Without the explicit consideration of energetic correlations the experimental values cannot be reproduced which testifies to the importance of spatial correlations.
Glass transition temperature prediction of disordered molecular solids
K.-H. Lin, L. Paterson, F. May, D. Andrienko
npj Computational Materials,
7,
2021,
[doi]
[abstract]
Glass transition temperature Tg is the key quantity for assessing morphological stability and molecular ordering of films of organic semiconductors. A reliable prediction of Tg from the chemical structure is however challenging as it is sensitive to both molecular interactions and analysis of the heating or cooling process. By combining a new fitting protocol with an automated determination of interaction parameters we predict Tg with a mean absolute error of ca 20 deg C for a set of organic compounds with Tg in the 50-230 deg C range. Our study establishes a reliable and automated pre-screening procedure for design of amorphous organic semiconductors essential for the optimization and development of organic light emitting diodes.
N-Doping improves charge transport and morphology in the organic non-fullerene acceptor O-IDTBR
A. F. Paterson, R. Li, A. Markina, L. Tsetseris, S. MacPhee, H. Faber, A.-H. Emwas, J. Panidi, H. Bristow, A. Wadsworth, D. Baran, D. Andrienko, M. Heeney, I. McCulloch, T. D. Anthopoulos
J. Mater. Chem. C,
9,
4486,
2021,
[doi]
[abstract]
Doping improves critical performances metrics in electronic devices. Research into n-doped organic small molecules is comparatively limited because of a historical combined lack of available n-dopants and unfavourable microstructural effects. This prevents further advancement of existing materials such as non-fullerene acceptors (NFAs) that have already shown great promise for (opto)electronic devices: from record-efficiency organic photovoltaics to promising semiconductors for n channel organic thin-film transistors. Here we show that several molecular n dopants namely [12-b:2'1'-d]benzo[i][2.5]benzodiazocine potassium triflate adduct (DMBI-BDZC) tetra-n-butylammonium fluoride (TBAF) and 4-(23-dihydro-13-dimethyl-1H-benzimidazol-2-yl)-NN-dimethylbenzenamine (N-DMBI) improve charge transport properties in the NFA O-IDTBR and increase the electron charge carrier mobility to over 1 cm2/Vs in thin-film transistors. By combining complementary experimental techniques with computer simulations of doping and charge dynamics we show that improved charge transport arises from synergistic effects of n-type doping and morphological changes. Specifically a new previously unreported dopant-induced packing orientation results in one of the highest electron mobility values reported to-date for an NFA molecule. Overall this work underpins an important mechanism on dopant-interactions and their impact on morphology showing that dopant induced packing orientation morphology and preferential phase is a key part of the charge transport enhancement process in doped organic systems – rather than a hinderance.
Molecular Library of OLED Host Materials - Evaluating the Multiscale Simulation Workflow
A. Mondal, L. Paterson, J. Cho, K.-H. Lin, B. van der Zee, G.-J. A. H. Wetzelaer, A. Stankevych, A. Vakhnin, J.-J. Kim, A. Kadashchuk, P. W. M. Blom, F. May, D. Andrienko
Chem. Phys. Rev.,
2,
031304,
2021,
[doi]
[abstract]
Amorphous small-molecule organic materials are utilized in organic light emitting diodes (OLEDs) with device performance relying on appropriate chemical design. Due to the vast number of contending materials a symbiotic experimental and simulation approach would be greatly beneficial in linking chemical structure to macroscopic material properties. We review simulation approaches proposed for predicting macroscopic properties. We then present a library of OLED hosts containing input files results of simulations and experimentally measured references of quantities relevant to OLED materials. We find that there is a linear proportionality between simulated and measured glass transition temperatures despite a quantitative disagreement. Computed ionization energies are in excellent agreement with the ultraviolet photoelectron and photoemission spectroscopy in air measurements. We also observe a linear correlation between calculated electron affinities and ionization energies and cyclic voltammetry measurements. Computed energetic disorder correlates well with thermally stimulated luminescence measurements and charge mobilities agree remarkably well with space charge–limited current measurements. For the studied host materials we find that the energetic disorder has the greatest impact on the charge carrier mobility. Our library helps to swiftly evaluate properties of new OLED materials by providing well-defined structural building blocks. The library is public and open for improvements. We envision the library expanding and the workflow providing guidance for future OLED material design.
Density of states of OLED hosts from thermally stimulated luminescence
A. Stankevych, A. Vakhnin, D. Andrienko, L. Paterson, J. Genoe, I. I. Fishchuk, H. Baessler, A. Koehler, A. Kadashchuk
Phys. Rev. Appl.,
15,
044050,
2021,
[doi]
[abstract]
The electronic density-of-states (DOS) plays a central role in controlling the charge-carrier transport in amorphous organic semiconductors while its accurate determination is still a challenging task. We have applied the low-temperature fractional thermally stimulated luminescence (TSL) technique to determine the DOS of pristine amorphous films of OLED host materials. The DOS width is determined for two series of hosts namely (i) carbazole-biphenyl (CBP) derivatives: CBP mCBP and mCBP-CN and (ii) carbazole-phenyl (CP) derivatives: mCP and mCP-CN. TSL originates from radiative recombination of charge-carriers thermally released from the lower energy part of the intrinsic DOS that causes charge trapping at very low temperatures. We find that the intrinsic DOS can be approximated by a Gaussian distribution with a deep exponential tail accompanying this distribution in CBP and mCBP films. The DOS profile broadens with increasing molecular dipole moments varying from 0 to 6 Debye in a similar manner within each series in line with the dipolar disorder model. The same molecular dipole moment however leads to a broader DOS of CP compared to CBP derivatives. Using computer simulations we attribute the difference between the series to a smaller polarizability of cations in CP-derivatives leading to weaker screening of the electrostatic disorder by induction. These results demonstrate that the low-temperature TSL technique can be used as an efficient experimental tool for probing the DOS in small-molecule OLED materials.
Computer aided design of stable and efficient OLEDs
L. Paterson, F. May, D. Andrienko
J. Appl. Phys.,
128,
160901,
2020,
[doi]
[abstract]
Organic light emitting diodes (OLEDs) offer a unique alternative to traditional display technologies. Tailored device architecture can offer properties such as flexibility and transparency presenting unparalleled application possibilities. The commercial advancement of OLEDs is highly anticipated and continued research is vital for improving device efficiency and lifetime. The performance of an OLED relies on an intricate balance between stability efficiency operational driving voltage and colour coordinate with the aim of optimising these parameters by employing appropriate material design. Multiscale simulation techniques can aid with the rational design of these materials in order to overcome existing shortcomings. For example extensive research has focused on the emissive layer and the obstacles surrounding blue OLEDs in particular; namely the trade-off between stability and efficiency while preserving blue emission. More generally due to the vast number of contending organic materials and with experimental pre-screening being notoriously time-consuming a complementary in silico approach can be considerably beneficial. The ultimate goal of simulations is the prediction of microscopic device properties from chemical composition prior to synthesis. However various challenges must be overcome to bring this to a realisation some of which are discussed in this perspective. Computer aided design is becoming an essential component to future OLED developments and with the field shifting towards machine-learning-based approaches in silico pre-screening is the future of material design.
Perspectives of Unicoloured Phosphor-sensitised Fluorescence (UPSF)
L. Paterson, A. Mondal, P. Heimel, R. Lovrincic, F. May, C. Lennartz, D. Andrienko
Adv. Electron. Mater.,
5,
1900646,
2019,
[doi]
[abstract]
Unicoloured phosphor-sensitised fluorescence (UPSF) is a dual emitting concept proposed for improving efficiencies and operational lifetimes of blue organic light emitting diodes (OLEDs). To overcome the limitations of the individual emitters it uses a phosphorescent donor to sensitise a fluorescent acceptor. To quantify the potential of the concept we develop a multiscale model of a UPSF OLED. We start from atomistic morphologies parameterise the rates of all processes on the available experimental data and solve the respective master equation with the help of the kinetic Monte Carlo algorithm. Our simulations show that the energy transfer between donor molecules is essential to reproduce the results of the time-resolved photoluminescence experiment. We then expand the scope of experiment by studying the effect of the acceptor concentration as well as Förster and (parasitic) Dexter energy transfer from the donor to acceptor on the characteristics of the UPSF OLED. Our study shows that an appropriate material design can further improve efficiency by more than 30\% and at the same time achieve radiative decay times below 0.02 µs thus significantly extending OLED operational lifetime.
Unicolored phosphor-sensitized fluorescence for efficient and stable blue OLEDs
P. Heimel, A. Mondal, F. May, W. Kowalsky, C. Lennartz, D. Andrienko, R. Lovrincic
Nature Communications,
9,
4990,
2018,
[doi]
[abstract]
Improving lifetimes and efficiencies of blue organic light-emitting diodes is clearly a scientific challenge. Towards solving this challenge we propose a unicolored phosphor-sensitized fluorescence approach with phosphorescent and fluorescent emitters tailored to preserve the initial color of phosphorescence. Using this approach we design an efficient sky-blue light-emitting diode with radiative decay times in the submicrosecond regime. By changing the concentration of fluorescent emitter we show that the lifetime is proportional to the reduction of the radiative decay time and tune the operational stability to lifetimes of up to 320 h (80 % decay initial luminance of 1000 cd/m2). Unicolored phosphor-sensitized fluorescence provides a clear path towards efficient and stable blue light-emitting diodes helping to overcome the limitations of thermally activated delayed fluorescence.
Simulations of organic light emitting diodes
P. Kordt, P. Bobbert, R. Coehoorn, F. May, C. Lennartz, D. Andrienko
1,
473-522,
2017,
[doi]
Design rules for charge-transport efficient host materials for phosphorescent OLEDs
F. May, M. Al-Helwi, B. Baumeier, W. Kowalsky, E. Fuchs, C. Lennartz, D. Andrienko
J. Am. Chem. Soc.,
134,
13818-13822,
2012,
[doi]
[abstract]
The use of blue phosphorescent emitters in organic light-emitting diodes (OLEDs) imposes demanding requirements on a host material. Among these are large triplet energies the alignment of levels with respect to the emitter the ability to form and sustain amorphous order material processability and an adequate charge carrier mobility. A possible design strategy is to choose a pi-conjugated core with a high triplet level and to fulfill the other requirements by using suitable substituents. Bulky substituents however induce large spatial separations between conjugated cores can substantially reduce intermolecular electronic couplings and decrease the charge mobility of the host. In this work we analyze charge transport in amorphous 28-bis(triphenylsilyl)dibenzofuran an electron-transporting material synthesized to serve as a host in deep-blue OLEDs. We show that mesomeric effects delocalize the frontier orbitals over the substituents recovering strong electronic couplings and lowering reorganization energies especially for electrons while keeping energetic disorder small. Admittance spectroscopy measurements reveal that the material has indeed a high electron mobility and a small Poole-Frenkel slope supporting our conclusions. By linking electronic structure molecular packing and mobility we provide a pathway to the rational design of hosts with high charge mobilities.
Can lattice models predict density of states of amorphous organic semiconductors?
F. May, B. Baumeier, C. Lennartz, D. Andrienko
Phys. Rev. Lett.,
109,
136401,
2012,
[doi]
[abstract]
We extend existing lattice models of amorphous semiconductors by accounting for changes in molecular polarizability upon charging/excitation. A compact expression of this contribution to the density of states is provided. Although the lattice model and the description based on a microscopic morphology both qualitatively predict an additional broadening shift and an exponential tail (traps) of the density of states a quantitative agreement between the two cannot be achieved.
Organic Interfaces
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
Nature Communications,
10,
2466,
2019,
[doi]
[abstract]
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.
Organic Semiconductors
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
J. Phys. Condens. Matter,
28,
433002,
2016,
[doi]
[abstract]
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
Science,
352,
1446-1449,
2016,
[doi]
[abstract]
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.
Organic electronics
Simulations of morphology and charge transport in supramolecular organic materials
D. Andrienko
RSC Smart Materials Series,
book chapter,
2014,
[doi]
[abstract]
In this chapter we describe the current state of the art of microscopic charge transport simulations in partially ordered and disordered organic semiconductors including simulations of atomistic morphologies evaluation of electronic couplings driving forces charge transfer rates and charge carrier mobilities. Special attention is paid to finite-size effects long-range interactions and charge localization.
Challenges for in silico design of organic semiconductors
B. Baumeier, F. May, C. Lennartz, D. Andrienko
J. Mater. Chem.,
22,
10971-10976,
2012,
[doi]
[abstract]
We outline the objectives of microscopic simulations of charge and energy transport processes in amorphous organic semiconductors describe the current status of techniques used to achieve them and list the challenges such methods face when aiming at quantitative predictions.
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.,
7,
3335-3345,
2011,
[doi]
[abstract]
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.
Relationship between supramolecular assembly and charge-carrier mobility in discotics: The impact of side chains
F. May, V. Marcon, M. R. Hansen, F. Grozema, D. Andrienko
J. Mater. Chem.,
21,
9538-9545,
2011,
[doi]
[abstract]
Discotic mesophases are known for their ability to self-assemble into columnar structures which serve as semiconducting molecular wires. Charge-carrier mobility along these wires strongly depends on molecular packing which is controlled by intermolecular interactions. Using solid-state NMR and molecular dynamics simulations we relate how conformations of alkyl and glycol side chains affect helical pitch and angular distribution of molecules within the columnar structures of perylenediimide derivatives. Using the high-temperature limit of Marcus theory we then establish a link between the secondary structure and charge-carrier mobility. Simulation results are compared to pulse-radiolysis time-resolved microwave conductivity measurements. We conclude that for achieving high charge-carrier mobilities in discotics side chains with specific interactions are required in order to minimize the translational and orientational molecular disorder in the columns.
Extracting nondispersive charge carrier mobilities of organic emiconductors from simulations of small systems
A. Lukyanov, D. Andrienko
Phys. Rev. B,
82,
193202,
2010,
[doi]
[abstract]
Predictions of charge-carrier mobilities in amorphous semiconductors often rely on charge transport simulations in microscopically sized systems where transport is dispersive and mobilities are system-size dependent. We propose a method for extrapolating a macroscopic nondispersive mobility from the temperature dependence of a microscopic one. The method is tested on an amorphous phase of tris(8-hydroxyquinoline) aluminum for which the temperature dependence of a microscopic hole mobility is obtained by combining molecular-dynamics simulations for generating material morphologies electronic-structure calculations for determining charge hopping rates and kinetic Monte Carlo simulations for studying charge dynamics. The extracted value of the nondispersive mobility and its electric field dependence agree well with the results of time-of-flight experiments.
Charge transport in columnar mesophases of carbazole macrocycles
T. Vehoff, B. Baumeier, D. Andrienko
J. Chem. Phys.,
133,
134901,
2010,
[doi]
Solvated poly-(phenylene vinylene) derivatives: conformational structure and aggregation behavior
A. Lukyanov, A. Malafeev, V. Ivanov, H.-L. Chen, K. Kremer, D. Andrienko
J. Mater. Chem.,
20,
10475-10485,
2010,
[doi]
Charge transport in organic crystals: role of disorder and topological connectivity
T. Vehoff, B. Baumeier, A. Troisi, D. Andrienko
J. Am. Chem. Soc.,
132,
11702-11708,
2010,
[doi]
Density-functional based determination of intermolecular charge transfer properties for large-scale morphologies
B. Baumeier, J. Kirkpatrick, D. Andrienko
Phys. Chem. Chem. Phys.,
12,
11103-11113,
2010,
[doi]
Charge transport in self-assembled semiconducting organic layers: role of dynamic and static disorder
T. Vehoff, Y. S. Chung, K. Johnston, A. Troisi, D. Y. Yoon, D. Andrienko
J. Phys. Chem. C,
114,
10592-10597,
2010,
[doi]
A multiscale description of charge transport in conjugated oligomers
V. Ruehle, James Kirkpatrick, Denis Andrienko
J. Chem. Phys.,
132,
134103,
2010,
[doi]
Graphitic nanoribbons with dibenzo[el]pyrene repeat units: synthesis and self-assembly
Yulia Fogel, Linjie Zhi, Ali Rouhanipour, Denis Andrienko, Hans Joachim Raeder, Klaus Muellen
Macromolecules,
42,
6878-6884,
2009,
[doi]
Amorphous films of tris-8(hydroxyquinoline aluminium): force-field morphology and charge transport
A. Lukyanov, C. Lennartz, D. Andrienko
Phys. Stat. Sol. A,
206,
2737-2742,
2009,
[doi]
Understanding structure-mobility relations for perylene tetracarboxdiimide derivatives
V. Marcon, W. Pisula, J. Dahl, D. W. Breiby, J. Kirkpatrick, S. Patwardhan, F. Grozema, D. Andrienko
J. Am. Chem. Soc.,
131,
11426-11432,
2009,
[doi]
Charge transport in semiconductors with multiscale conformational dynamics
A. Troisi, D. L. Cheung, D. Andrienko
Phys. Rev. Lett.,
102,
116602,
2009,
[doi]
Towards high charge-carrier mobilities by rational design of the shape and periphery of discotics
X. Feng, V. Marcon, W. Pisula, M. R. Hansen, J. Kirkpatrick, F. Grozema, D. Andrienko, K. Kremer, K. Mullen
Nature Materials,
8,
421-426,
2009,
[doi]
Columnar mesophases of hexabenzocoronene derivatives. I. Phase transitions
V. Marcon, T. Vehoff, J. Kirkpatrick, Ch. Jeong, Do. Y. Yoon, K. Kremer, D. Andrienko
J. Chem. Phys.,
129,
094505,
2008,
[doi]
Columnar mesophases of hexabenzocoronene derivatives: II. Charge carrier mobility
J. Kirkpatrick, V. Marcon, K. Kremer, J. Nelson, D. Andrienko
J. Chem. Phys.,
129,
094506,
2008,
[doi]
Charge transport parameters of HBC at different temperatures
J. Kirkpatrick, V. Marcon, J. Nelson, D. Andrienko
Phys. Stat. Sol. B,
245,
835,
2008,
[doi]
Atomistic force field and electronic properties of carbazole: from monomer to macrocycle
T. Vehoff, J. Kirkpatrick, K. Kremer, D. Andrienko
Phys. Stat. Sol. B,
245,
839,
2008,
[doi]
Electron-Deficient N-Heteroaromatic Linkers for the Elaboration of Large Soluble Polycyclic Aromatic Hydrocarbons and Their Use in the Synthesis of Some Very Large Transition Metal Complexes
Yulia Fogel, Marcel Kastler, Zhaohui Wang, Denis Andrienko, Graham J. Bodwell, Klaus Muellen
J. Am. Chem. Soc.,
129,
11743-11749,
2007,
[doi]
[abstract]
The selective oxidation of the perimeter of an extended polycyclic aromatic hydrocarbon (PAH) namely a six-fold tert-butylated tetrabenzo[bcefhiuv]ovalene led to the formation of an -diketone. The newly installed carbonyl centers allowed this building block to be converted into the largest known heteroatom-containing polycyclic aromatic hydrocarbons (up to 224 atom in the aromatic core) by way of the quinoxaline ring condensation reaction. The tert-butyl substituents caused a distortion of the usually planar aromatic frameworks which hampered the aggregation tendency of the extended aromatic -systems and led to extraordinarily high solubilities. All of the systems described here even giant phthalocyanine could thus be purified using standard chromatographic techniques and characterized using typical spectroscopic methods. For the first time fully resolved 1H NMR spectra of soluble diamagnetic 98 and 104 atom-containing aromatic systems are presented. The computed and experimental UV/vis spectra emphasize the dependence of the characteristic p and bands upon the size of the PAHs. It was also possible to obtain the largest known ligand to yet be complexed around a ruthenium center. A quadrupolar solvatochromic effect was observed when two donating PAH moieties were fused to an accepting quinoxaline center in which case the photoluminescence spanned a range of about 80nm. Electrochemical properties of the new nanographenes were investigated using cyclic voltammetry (CV) and this showed quasi-reversible reductions.
Organic light emitting diodes
Modeling of organic light emitting diodes: from molecular to device properties
P. Kordt, J. M. van der Holst, M. Al Helwi, W. Kowalsky, F. May, A. Badinski, C. Lennartz, D. Andrienko
Adv. Funct. Mater.,
25,
1955-1971,
2015,
[doi]
[abstract]
In this chapter we describe the current state of the art of microscopic charge transport simulations in partially ordered and disordered organic semiconductors including simulations of atomistic morphologies evaluation of electronic couplings driving forces charge transfer rates and charge carrier mobilities. Special attention is paid to finite-size effects long-range interactions and charge localization.
Parameter-free continuous drift-diffusion models of amorphous organic semiconductors
P. Kordt, S. Stodtmann, A. Badinski, M. Al Helwi, C. Lennartz, D. Andrienko
Phys. Chem. Chem. Phys.,
17,
22778-22783,
2015,
[doi]
Organic photovoltaics
Charge Carrier Dynamics in P3HT:PCBM Photovoltaic Blends
F. Laquai, R. Mauer, D. Andrienko, P. Blom
Macromol. Rapid Commun.,
36,
1001-1025,
2015,
[doi]
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.,
36,
1047,
2015,
[doi]
Sub-ns Triplet State Formation by Non-Geminate Recombination in PSBTBT:PC70BM and PCPDTBT:PC60BM Organic Solar Cells
F. Etzold, I. A. Howard, N. Forler, A. Melnyk, D. Andrienko, M. R. Hansen, F. Laquai
Energy and Environmental Science,
8,
1511-1522,
2015,
[doi]
[abstract]
The solid-state morphology and photo-generated charge carrier dynamics in low-bandgap polymer:fullerene bulk heterojunction photovoltaic blends using the donor-acceptor type copolymers PCPDTBT or its silicon-substituted analogue PSBTBT as donors are compared by two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) and femto-to microsecond broadband Vis-NIR transient absorption (TA) pump-probe spectroscopy. The 2D solid-state NMR experiments demonstrate that the film morphology of PCPDTBT:PC60BM blends processed with additives such as octanedithiol (ODT) are similar to those of PSBTBT:PC60BM blends in terms of crystallinity phase segregation and interfacial contacts. The TA experiments and analysis of the TA data by multivariate curve resolution (MCR) reveal that after exciton dissociation and free charge formation fast sub-nanosecond non-geminate recombination occurs which leads to a substantial population of the polymer's triplet state. The extent to which triplet states are formed depends on the initial concentration of free charges which itself is controlled by the microstructure of the blend especially in case of PCPDTBT:PC60BM. Interestingly PSBTBT:PC70BM blends show a higher charge generation efficiency but less triplet state formation at similar free charge carrier concentrations. This indicates that the solid-state morphology and interfacial structures of PSBTBT:PC70BM blends reduces non-geminate recombination leading to superior device performance compared to optimized PCPDTBT:PC60BM blends.
Organic semiconductors
Charge transport in highly ordered organic nanofibrils: lessons from modelling
G. Gryn'ova, A. Nicolai, A. Prlj, P. Ollitrault, D. Andrienko, C. Corminboeuf
J. Mater. Chem. C,
5,
350-361,
2017,
[doi]
[abstract]
H-Aggregates featuring tight π-stacks of the conjugated heterocyclic cores represent ideal morphologies for 1D organic semiconductors. Such nanofibrils have larger electronic couplings between the adjacent cores compared to the herringbone crystal or amorphous assemblies. In this work we show that for a set of seven structurally and electronically distinct cores including quaterthiophene and oligothienoacenes the co-planar dimer model captures the impact of the monomer's electronic structure on charge transport but more advanced multiscale modelling featuring molecular dynamics and kinetic Monte-Carlo simulations is needed to account for the packing and disorder effects. The differences in the results between these two computational approaches arise from the sensitivity of the electronic coupling strength to the relative alignment of adjacent cores in particular the long-axis shift between them imposed by the oligopeptide side chains. Our results demonstrate the dependence of the performance of H-aggregates on the chemical nature of the cores and the presence of the side chains as well as the limitations in using the simple dimer model for a rapid computational pre-screening of the conjugated cores.
Modeling of spatially correlated energetic disorder in organic semiconductors
P. Kordt, D. Andrienko
J. Chem. Theory Comput.,
12,
36-40,
2016,
[doi]
Organic solar cells
Open-circuit voltage of organic solar cells: morphology makes the difference
C. Poelking, J. Benduhn, D. Spoltore, M. Schwarze, K. Leo, S. Roland, F. Piersimoni, D. Neher, K. Vandewal, D. Andrienko
Communications Physics,
5,
2022,
[doi]
[abstract]
Organic photovoltaics (PV) is an energy-harvesting technology that offers many advantages such as flexibility low weight and cost as well as environmentally benign materials and manufacturing techniques. Despite a growth of power conversion efficiencies to around 19 % in the last years organic PVs still lag behind inorganic PV technologies mainly due to high losses in open-circuit voltage. Understanding and improving open circuit voltage in organic solar cells is challenging as it is controlled by the properties of a donor-acceptor interface where the optical excitations are separated into charge carriers. Here we provide an electrostatic model of a rough donor-acceptor interface and test it experimentally on small molecule PV materials systems. The model provides concise relationships between the open-circuit voltage photovoltaic gap charge-transfer state energy and interfacial morphology. In particular we show that the electrostatic bias generated across the interface reduces the photovoltaic gap. This negative influence on open-circuit voltage can however be circumvented by adjusting the morphology of the donor-acceptor interface.
Organic-inorganic interfaces
Stark Effect of Hybrid Charge Transfer States at Planar ZnO/Organic Interfaces
U. Hoermann, S. Zeiske, F. Piersimoni, L. Hoffmann, R. Schlesinger, N. Koch, T. Riedl, D. Andrienko, D. Neher
Phys. Rev. B,
98,
155312,
2018,
[doi]
[abstract]
We investigate the bias dependence of the hybrid charge transfer state emission at planar heterojunctions between the metal oxide acceptor ZnO and three donor molecules. The electroluminescence peak energy linearly increases with the applied bias saturating at high fields. Variation of the organic layer thickness and deliberate change of the ZnO conductivity through controlled photodoping allow us to confirm that this bias-induced spectral shift relates to the internal electric field in the organic layer rather than the filling of states at the hybrid interface. We show that existing continuum models overestimate the hole delocalization and propose a simple electrostatic model in which the linear and quadratic Stark effects are explained by the electrostatic interaction of a strongly polarizable molecular cation with its mirror image.
Other
Computing Inelastic Neutron Scattering Spectra from Molecular Dynamics Trajectories
T. Harrelson, M. Dettmann, C. Scherer, D. Andrienko, A. Moule, R. Faller
Scientific Reports,
11,
7938,
2021,
[doi]
[abstract]
Inelastic neutron scattering (INS) provides a weighted density of phonon modes. Currently INS spectra can only be interpreted for perfectly crystalline materials because of high computational cost for electronic simulations. INS has the potential to provide detailed morphological information if sufficiently large volumes and appropriate structural variety are simulated. Here we propose a method that allows direct comparison between INS data with molecular dynamics simulations a simulation method that is frequently used to simulate semicrystalline/amorphous materials. We illustrate the technique by analyzing spectra of a well-studied conjugated polymer poly(3-hexylthiophene-25-diyl) (P3HT) and conclude that our technique provides improved volume and structural variety but that the classical force field requires improvement before the morphology can be accurately interpreted.
Perovskite nanocrystals
Efficient and stable perovskite-silicon tandem solar cells through contact displacement by MgF
J. Liu, M. De Bastiani, E. Aydin, G. T. Harrison, Y. Gao, R. R. Pradhan, M. K. Eswaran, M. Mandal, W. Yan, A. Seitkhan, M. Babics, A. S. Subbiah, E. Ugur, F. Xu, L. Xu, M. Wang, A. ur Rehman, A. Razzaq, J. Kang, R. Azmi, A. A. Said, F. H. Isikgor, T. G. Allen, D. Andrienko, U. Schwingenschloegl, F. Laquai, S. De Wolf
Science,
377,
302-306,
2022,
[doi]
[abstract]
The performance of perovskite solar cells with inverted polarity (p-i-n) is still limited by recombination at their electron extraction interface which also lowers the power conversion efficiency (PCE) of p-i-n perovskite-silicon tandem solar cells. A 1 nm thick MgFx interlayer at the perovskite/C60 interface through thermal evaporation favorably adjusts the surface energy of the perovskite layer facilitating efficient electron extraction and displaces C60 from the perovskite surface to mitigate nonradiative recombination. These effects enable a champion Voc of 1.92 volts an improved fill factor of 80.7 % and an independently certified stabilized PCE of 29.3 % for a ~1 cm2 monolithic perovskite-silicon tandem solar cell. The tandem retained ~95 % of its initial performance following damp-heat testing (85 Celsius at 85 % relative humidity) for 1000 hours.
Quantum Efficiency Enhancement of Lead-Halide Perovskite Nanocrystal LEDs by Organic Lithium Salt Treatment
T. Naujoks, R. Jayabalan, Ch. Kirsch, F. Zu, M. Mandal, J. Wahl, M. Waibel, A. Opitz, N. Koch, D. Andrienko, M. Scheele, W. Bruetting
ACS Appl. Mater. Interfaces,
14,
28985-28996,
2022,
[doi]
[abstract]
Surface-defect passivation is key to achieving a high photoluminescence quantum yield in lead halide perovskite nanocrystals. However in perovskite light-emitting diodes these surface ligands also have to enable balanced charge injection into the nanocrystals to yield high efficiency and operational lifetime. In this respect alkaline halides have been reported to passivate surface trap states and increase the overall stability of perovskite light emitters. On the one side the incorporation of alkaline ions into the lead halide perovskite crystal structure is considered to counterbalance cation vacancies whereas on the other side the excess halides are believed to stabilize the colloids. Here we report an organic lithium salt viz. LiTFSI as a halide-free surface passivation on perovskite nanocrystals. We show that treatment with LiTFSI has multiple beneficial effects on lead halide perovskite nanocrystals and LEDs derived from them. We obtain a higher photoluminescence quantum yield and a longer exciton lifetime and a radiation pattern that is more favorable for light outcoupling. The ligand-induced dipoles on the nanocrystal surface shift their energy levels toward a lower hole-injection barrier. Overall these effects add up to a 4- to 7-fold boost of the external quantum efficiency in proof-of-concept LED structures depending on the color of the used lead halide perovskite nanocrystal emitters.
Improvement of Photophysical Properties of CsPbBr3 and Mn2+:CsPb(BrCl)3 Perovskite Nanocrystals by Sr2+ Doping for White Light-Emitting Diodes
H. Yuce, M. Mandal, Y. Yalcinkaya, D. Andrienko, M. Demir
J. Phys. Chem. C,
126,
11277-11284,
2022,
[doi]
[abstract]
All-inorganic metal halide perovskite nanocrystals (NCs) having the general formula ABX3 where A is a monovalent cation for example Cs+ B is a divalent cation typically Pb2+ and X is Cl- Br- I- or their binary mixture show potential in optoelectronic devices. In this work we explore the effect of B-site doping on the optoelectronic properties of CsPbX3 NCs. First the Pb2+ ions in the pristine CsPbBr3 NC are partially substituted by Mn2+ ions. The alkaline earth metal strontium is then doped on both pristine and the Mn2+-substituted NCs. We found that a small percentage of Sr2+ doping remarkably improves the photoluminescence quantum yield of CsPbBr3 and Mn2+-state emission in Mn2+:CsPb(BrCl)3 NCs. Perovskite NC film/poly(methyl methacrylate) composites with all four NC variants were used in a white light-emitting diode (WLED) where Sr2+ doping increased the luminous efficiency of the WLED by ~4.7%. We attribute this performance enhancement to a reduced defect density and an attenuated microstrain in the local NC structure.
Perovskites
Grain Engineering for Improved Charge Carrier Transport in Two-Dimensional Lead-Free Perovskite Field-Effect Transistors
S. Wang, S. Frisch, H. Zhang, O. Yildiz, M. Mandal, N. Ugur, B. Jeong, C. Ramanan, D. Andrienko, H. Wang, M. Bonn, P. W. M. Blom, M. Kivala, W. Pisula, T. Marszalek
Materials Horizons,
9,
2633,
2022,
[doi]
Spatially resolved fluorescence of caesium lead halide perovskite supercrystals reveals quasi-atomic behavior of nanocrystals
D. Lapkin, C. Kirsch, J. Hiller, D. Andrienko, D. Assalauova, K. Braun, J. Carnis, Y. Y. Kim, M. Mandal, A. Maier, A. J. Meixner, N. Mukharamova, M. Scheele, F. Schreiber, M. Sprung, J. Wahl, S. Westendorf, I. A. Zaluzhnyy, I. A. Vartanyants
Nature Communications,
13,
2022,
[doi]
Porphyrin-functionalization of CsPbBrI2/SiO2 core-shell nanocrystals enhances the stability and efficiency in electroluminescent devices
J. Wahl, M. Engelmayer, M. Mandal, T. Naujoks, P. Haizmann, A. Maier, H. Peisert, D. Andrienko, W. Bruetting, M. Scheele
Adv. Optical Mater.,
2101945,
2021,
[doi]
[abstract]
Surface ligand exchange on all-inorganic perovskite nanocrystals of composition CsPbBrI2 reveals improved optoelectronic properties due to strong interactions of the nanocrystal with mono-functionalized porphyrin derivatives. The interaction is verified experimentally with an array of spectroscopic measurements as well as computationally by exploiting density functional theory calculations. The enhanced current efficiency is attributed to a lowering of the charging energy by a factor of 2-3 which is determined by combining electronic and optical measurements on a selection of ligands. The coupled organic–inorganic nanostructures are successfully deployed in a light-emitting device with higher current efficacy and improved charge carrier balance magnifying the efficiency almost fivefold compared to the native ligand.
Polycarbonates
Flow boundary conditions for chain-end adsorbing polymer blends
X. Zhou, D. Andrienko, L. Delle Site, K. Kremer
J. Chem. Phys.,
123,
104904,
2005,
[doi]
[abstract]
Using the phenol-terminated polycarbonate blend as an example we demonstrate that the hydrodynamic boundary conditions for a flow of an adsorbing polymer melt are extremely sensitive to the structure of the epitaxial layer. Under shear the adsorbed parts (chain ends) of the polymer melt move along the equipotential lines of the surface potential whereas the adsorbed additives serve as the surface defects. In response to the increase of the number of the adsorbed additives the surface layer becomes thinner and solidifies. This results in a gradual transition from the slip to the no-slip boundary condition for the melt flow with a non-monotonic dependence of the slip length on the surface concentration of the adsorbed ends.
Adhesion of polycarbonate blends on a nickel surface
D. Andrienko, S. Leon, L. Delle Site, K. Kremer
Macromolecules,
38,
5810 -5816,
2005,
[doi]
[abstract]
We demonstrate that the adhesive behavior of a phenol terminated bisphenol-\em A polycarbonate melt to a (111) nickel surface changes significantly if a small amount of short chains is present i.~e. in polydispersed melts or self-blends. Attractive interaction of the chain ends with the surface results in an adsorbed layer made of single- and two-end attached chains. Short chains however diffuse from the bulk and occupy the adsorption sites much faster than the long ones. Interplay between the surface concentration of short chains their molecular conformation and excluded volume results in a non-monotonic dependence of the surface coverage by long chains on the molecular weight of the additive. The smallest polycarbonate coverage is achieved for diphenyl carbonate due to its high mobility and relatively large excluded volume. We propose that self-blending can be used to modify in a controlled fashion the friction coefficient of a melt sheared past nickel surface.
Dynamic surface decoupling in a sheared polymer melt
X. Zhou, D. Andrienko, L. Delle Site, K. Kremer
Europhys. Lett.,
70,
264-270,
2005,
[doi]
[abstract]
We propose that several mechanisms contribute to friction in a polymer melt adsorbed at a structured surface. The first one is the well-known disentanglement of bulk polymer chains from the surface layer. However if the surface is ideal at the atomic scale the adsorbed parts of polymer chains can move along the equipotential lines of the surface potential. This gives rise to a strong slippage of the melt. For high shear rates chains partially desorb. However the friction force on adsorbed chains increases resulting in quasi-stick boundary conditions. We propose that the adsorbed layers can be efficiently used to adjust the friction force between the polymer melt and the surface.
Polyelectrolite microcapsules
Elasticity of polyelectrolyte multilayer microcapsules
V. V. Lulevich, D. Andrienko, O. I. Vinogradova
J. Chem. Phys.,
120,
3822-3826,
2004,
[doi]
[abstract]
We present a novel approach to probe elastic properties of polyelectrolyte multilayer microcapsules. The method is based on measurements of the capsule load-deformation curves with the atomic force microscope. The experiment suggests that at low applied load deformations of the capsule shell are elastic. Using elastic theory of membranes we relate force deformation elastic moduli and characteristic sizes of the capsule. Fitting to the prediction of the model yields the lower limit for Young's modulus of the polyelectrolyte multilayers of the order of 1-100 MPa depending on the template and solvent used for its dissolution. These values correspond to Young's modulus of an elastomer.
Young's Modulus of Polyelectrolyte Multilayers from Microcapsule Swelling
O. I. Vinogradova, D. Andrienko, V. V. Lulevich, S. Nordschild, G. B. Sukhorukov
Macromolecules,
37,
1113-1117,
2004,
[doi]
[abstract]
We measure Young's modulus of a free polyelectrolyte multilayer film by studying osmotically induced swelling of polyelectrolyte multilayer microcapsules filled with the polyelectrolyte solution. Different filling techniques and core templates were used for the capsule preparation. Varying the concentration of the polyelectrolyte inside the capsule its radius and the shell thickness yielded an estimate of Young's modulus of the order of 100 MPa. This corresponds to an elastomer and reflects strong interactions between polyanions and polycations in the multilayer.
Polymers
Simulations. In book Macromolecular Engineering: Structure-properties correlations using modern characterization techniques
D. Andrienko, K. Kremer
3,
1431-1470,
2007,
[doi]
[abstract]
This chapter will give a short introduction into computer simulations of polymers or soft matter in general. It should serve as a guide to the literature as well as provide a first impression of the opportunities but also of the current limitations.
Proton conduction
Water-free proton conduction in hexakis(p-phosphonatophenyl)benzene nano-channels
C. Wehmeyer, M. Schrader, D. Andrienko, D. Sebastiani
J. Phys. Chem. C,
117,
12366-12372,
2013,
[doi]
[abstract]
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 NafionÃÂî polymer as membrane materials for proton exchange fuel cells.
Raman spectroscopy
Electrochemical TERS Elucidates Potential-Induced Molecular Reorientation of Adenine/Au(111)
N. M. Sabanes; T. Ohto; D. Andrienko; Y. Nagata; K. F. Domke
Angew. Chem. Int. Ed.,
56,
9796-9801,
2017,
[doi]
[abstract]
Electrochemical surface activity arises from the interaction and geometric arrangement of molecules at electrified interfaces. We present a novel electrochemical tip-enhanced Raman spectroscope that can access the vibrational fingerprint of less than 100 small non-resonant molecules adsorbed at atomically flat Au electrodes to study their adsorption geometry and chemical reactivity as a function of applied potential. Combining experimental and simulation data for showcase adenine/Au(111) we conclude that protonated physisorbed adenine adapts a tilted orientation at low potentials while it is vertically adsorbed around the potential of zero charge. Further potential increase induces adenine deprotonation and reorientation to a planar configuration. The extension of EC-TERS to the study of adsorbate reorientation significantly broadens the applicability of this advanced spectroelectrochemical tool for the nanoscale characterization of a full range of electrochemical interfaces.
Rational compound design
Toward quantitative structure-property relationships for charge transfer rates of polycyclic aromatic hydrocarbons
M. Misra, D. Andrienko, B. Baumeier, J.-L. Faulon, O. A. von Lilienfeld
J. Chem. Theory Comput.,
7,
2549-2555,
2011,
[doi]
[abstract]
Quantitative structure-property relationships (QSPRs) have been developed and assessed for predicting the reorganization energy of polycyclic aromatic hydrocarbons (PAHs). Preliminary QSPR models based on a combination of molecular signature and electronic eigenvalue difference descriptors have been trained using more than 200 PAHs. Monte Carlo cross-validation systematically improves the performance of the models through progressive reduction of the training set and selection of best performing training subsets. The final biased QSPR model yields correlation coefficients q2 and r2 of 0.7 and 0.8 respectively and an estimated error in predicting reorganization energy of ±0.014 eV.
Tuning electronic eigenvalues of benzene via doping
V. Marcon, O. A. von Lilienfeld, D. Andrienko
J. Chem. Phys.,
127,
064305,
2007,
[doi]
Reviews
Virtual screening for organic solar cells and light emitting diodes
N. C. Forero-Martinez, K.-H. Lin, K. Kremer, D. Andrienko
Advanced Science,
2200825,
2022,
[doi]
[abstract]
The field of organic semiconductors is multifaceted and the potentially suitable molecular compounds are very diverse. Representative examples include discotic liquid crystals dye-sensitized solar cells conjugated polymers and graphene-based low-dimensional materials. This huge variety not only represents enormous challenges for synthesis but also for theory which aims at a comprehensive understanding and structuring of the plethora of possible compounds. Eventually computational methods should point to new better materials which have not yet been synthesized. In this perspective it is shown that the answer to this question rests upon the delicate balance between computational efficiency and accuracy of the methods used in the virtual screening. To illustrate the fundamentals of virtual screening chemical design of non-fullerene acceptors thermally activated delayed fluorescence emitters and nanographenes are discussed.
Slippage
Boundary slip as a result of a prewetting transition
D. Andrienko, B. Duenweg, O. I. Vinogradova
J. Chem. Phys.,
119,
13106,
2003,
[doi]
[abstract]
Some fluids exhibit anomalously low friction when flowing against a certain solid wall. To recover the viscosity of a bulk fluid slip at the wall is usually postulated. On a macroscopic level a large slip length can be explained as a formation of a film of gas or phase-separated lubricant with lower viscosity between the fluid and the solid wall. Here we justify such an assumption in terms of a prewetting transition. In our model the thin-thick film transition together with the viscosity contrast gives rise to a large boundary slip. The calculated value of the slip length has a jump at the prewetting transition temperature which depends on the strength of the fluid-surface interaction contact angle. Furthermore the temperature dependence of the slip length is nonmonotonous.
Nematic director slippage: Role of the angular momentum of light
D. Andrienko, V. Reshetnyak, Yu. Reznikov, T. J. Sluckin
Phys. Rev. E,
63,
11701-11708,
2001,
[doi]
[abstract]
We propose a theoretical model of the light-induced director slippage effect. In this effect the bulk director reorientation contributes to the surface director reorientation. It is found that the director and ellipticity profiles obtained in the geometric optics approximation are dependent on the ellipticity of the incident light wave. The director distribution is spatially modulated in linearly polarized light but grows monotonically in circularly polarized light. The surface director deviation has been examined and comparison made with existing experimental data which then permits the magnitude of the orientational nonlinearity coefficient to be calculated.
Electrically controlled director slippage over photosensitive aligning surface; in-plane sliding mode
D. Andrienko, F. Barbet, D. Bormann, Yu. Kurioz, S.-B. Kwon, Yu. Reznikov, M. Warenghem
Liq. Cryst.,
27,
365-370,
2000,
[doi]
[abstract]
We have studied the electro-optical characteristics of a homogeneously aligned nematic liquid crystal (LC) with weak planar anchoring of the director at the bounding substrates. By using the in-plane switching (IPS) of the LC which is achieved by an in-plane electric field the driving voltage was confirmed to be far less than that of the conventional IPS mode in which both substrates possess strong anchoring characteristics. Moreover because of the absence of strong subsurface director deformations the cell could operate optically in the Mauguin regime. Using these features we propose a new type of LC switching mode - in-plane sliding (IPSL) mode. We have realized this mode in a LC cell comprising one reference substrate with strong director anchoring and one substrate covered with photoaligning material with weak anchoring. In order to clarify the switching process we derived a simplified expression for the threshold voltage on the assumption of uniformity of the in-plane electric field. For the dynamical response of the LC to the in-plane electric field the switching on and off relaxation times of the IPSL mode were found to be longer than for the traditional IPS mode. However we have proposed an optimized cell geometry for the IPSL mode with a response time comparable to that of the IPS mode.
Solar cells
Design Rules for Organic Donor-Acceptor Heterojunctions: Pathway for Charge Splitting and Detrapping
C. Poelking, D. Andrienko
J. Am. Chem. Soc.,
137,
6320-6326,
2015,
[doi]
[abstract]
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.
Stochastic modelling
A general framework for consistent estimation of charge transport properties via random walks in random environments
O. Stenzel, C. Hirsch, V. Schmidt, T. Brereton, D. Kroese, B. Baumeier, D. Andrienko
Multiscale Model. Simul.,
12,
1108-1134,
2014,
[doi]
Stochastic networks
Stochastic modeling of molecular charge transport networks
B. Baumeier, O. Stenzel, C. Poelking, D. Andrienko, V. Schmidt
Phys. Rev. B,
86,
184202,
2012,
[doi]
[abstract]
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.
TADF
Virtual screening of TADF emitters for single-layer OLEDs
K.-H. Lin, G.-J. A. H. Wetzelaer, P. W. M. Blom, D. Andrienko
Frontiers in Chemistry,
9,
2021,
[doi]
[abstract]
Thermally-activated delayed fluorescence (TADF) is a concept which helps to harvest triplet excitations boosting the efficiency of an organic light-emitting diode. TADF can be observed in molecules with spatially separated donor and acceptor groups with a reduced triplet-singlet energy level splitting. TADF materials with balanced electron and hole transport are attractive for realizing efficient single-layer organic light emitting diodes greatly simplifying their manufacturing and improving their stability. Our goal here is to computationally screen such materials and provide a comprehensive database of compounds with a range of emission wavelengths ionization energies and electron affinities.
Trapping
The influence of impurities on the charge carrier mobility of small molecule organic semiconductors
P. Friederich, A. Fediai, J. Li, A. Mondal, N. B. Kotadiya, F. Symalla, G.-J. A. H. Wetzelaer, D. Andrienko, X. Blase, D. Beljonne, P. W. M. Blom, J.-L. Bredas, W. Wenzel
submitted,
2023,