Currently I am working on a joint project with BASF related to the "Spitzencluster"
Forum Organic Electronics, where I investigate charge transport in organic light emitting diodes (OLEDs).
I did my physics studies in Kaiserslautern, Mainz and Strasbourg and received my Diploma in November 2008 by
University of Frankfurt . In Frankfurt I investigated charge transport through single-molecule magnets using the
numerical renormalization group technique in the group of Prof Hofstetter.
2023
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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.
2022
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.
2021
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.
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.
2020
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.
2019
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.
2018
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.
2017
Simulations of organic light emitting diodes
P. Kordt, P. Bobbert, R. Coehoorn, F. May, C. Lennartz, D. Andrienko
1,
473-522,
2017,
[doi]
2015
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.
2012
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.
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.
2011
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.
