I have joined the theory group at MPIP in May 2009 as a Ph.D student.
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.
Computer aided design of stable and efficient OLEDs
L. Paterson, F. May, D. Andrienko
J. Appl. Phys.,
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.,
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
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
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.,
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.,
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.,
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.,
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.
Microscopic simulations of charge transport in disordered organic semiconductors
V. Ruehle, A. Lukyanov, F. May, M. Schrader, T. Vehoff, J. Kirkpatrick, B. Baumeier, D. Andrienko
J. Chem. Theory Comput.,
Charge carrier dynamics in an organic semiconductor can often be described in terms of charge hopping between localized states. The hopping rates depend on electronic coupling elements reorganization energies and driving forces which vary as a function of position and orientation of the molecules. The exact evaluation of these contributions in a molecular assembly is computationally prohibitive. Various often semiempirical approximations are employed instead. In this work we review some of these approaches and introduce a software toolkit which implements them. The purpose of the toolkit is to simplify the workflow for charge transport simulations provide a uniform error control for the methods and a flexible platform for their development and eventually allow in silico prescreening of organic semiconductors for specific applications. All implemented methods are illustrated by studying charge transport in amorphous films of tris-(8-hydroxyquinoline)aluminum a common organic semiconductor.
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.,
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.