Modelling stability of organic phosphorescent light-emitting diodes (MOSTOPHOS)
European UnionStatus: ongoing project
The lifetime, reliability, and efficiency of organic light emitting diodes (OLED) are critical factors precluding a number of novel devices from entering the market. Yet, these stability issues of OLEDs are poorly understood due to their notorious complexity, since multiple degradation and failure channels are possible at different length- and timescales. Current experimental and theoretical models of OLED stability are, to a
large extent, empirical. They do not include information about the molecular and meso-scales, which prevents their integration into the workflow of the industrial R&D compound design. It is the idea of this project to integrate various levels of theoretical materials characterization into a single software package,to streamline the research workflows in order for the calculations to be truly usable by materials engineers,
complementary to experimental measurements. Towards this goal, this project brings together the academic and industrial expertise of the leading experimental and theoretical groups in the field of organic semiconductors.
The primary objectives of the project are
• To develop simulation methods needed to unveil stability-limiting mechanisms in phosphorescent
organic light emitting diodes.
• To develop a software suite for prescreening of host/guest materials for OLED applications.
• To design optimal OLED stack configurations with improved lifetimes, reliabilities, and efficiencies
set by the roadmaps of the participating industrial partners (BASF, Philips).
In particular, we plan
1. To develop efficient schemes for modeling of the vacuum deposition process of host-guest layers in OLED stacks (MPIP). This will include (i) automatic parameterization of accurate polarizable force-fields (ii) development of coarse-grained models capable of addressing time- and length-scales typical for surface/orientational diffusion of deposited compounds; (iii) simulations (molecular dynamics) of the deposition process using these models and (iv) re-introduction of atomistic details into the coarsegrained
2. To compute the rates for charge/exciton transfer/recombination/injection. This includes polaronpolaron annihilation and triplet-polaron quenching, which are often responsible for the efficiency rolloff and fast OLED degradation. Special focus here will be on the quantitative descriptions (based on the realistic morphologies), appropriate choice of diabatic states, environmental effects (BASF, UPV, MPIP),and quantitative models for charge injection rates at metal organic-interfaces (CNR). Mechanisms not yet explored in the context of OLED simulations (e.g. charge transfer via the superexchange mechanism) will also be studied. In combination with the realistic morphology this will allow to
formulate a master equation for a relatively small (tenths of thousands of atoms)microscopic system.
The role of degradation will be introduced by adding events such as electrical and optical trap formation, creation of voids, and modification of the density of states. At this level QM/MM calculations will be performed in order to understand how chemical degradation can be prevented. The experimental backup at this stage would be UPS, XPS (level alignment), MALDI-TOF (mass spectroscopy for the reactants), and the emission spectrum measurements (TUD).
3. To parameterize stochastic off-lattice models based on statistical analysis of microscopic domains and to bring the information about underlying chemical composition to the mesoscopic scale (MPIP). The stochastic model will be employed to perform kinetic Monte Carlo (KMC) simulations of charge and exciton dynamics in the OLED stack (TUE). This will allow weighting the probabilities of degradation events by the occupation probabilities (obtained from the excitonic and polaronic density profiles) and hence account for the effect of spatial inhomogeneities on degradation.
4. To analyze KMC simulations and to develop a way of parameterizing the macroscopic drift-diffusion model, which would then complement the micro- and mesoscopic models with electro-thermal (heat dissipation) as well as electromagnetic (light in- and out-coupling)processes (UTV, S4T). The purpose of this work package is two-fold: first, it provides a fast way of analyzing the I-V-L curves with a potential feedback about degradation mechanisms at the mesoscopic level; second, it allows accounting for the role of heat dissipation and optical effects on degradation.
5. To validate the models on a test system suggested by the industrial partners (BASF). BASF will synthesize the compounds present in the test stack. Device manufacturing and characterization will be performed both at TUD and BASF.
6. To incorporate the developed methods (via including source codes, providing separate modules or interfaces) into a single software suite (SCH) that will streamline calculation workflows and enable efficient reporting and analysis of the results via an intuitive graphical user interface. The usability of the package will be tested by the industrial end-users (BASF, PHIL) for prescreening their candidate compounds. The software suite developed in this project will be marketed (SHR) to the OLED companies enabling the industry to accelerate development of novel products
Consiglio Nazionale delle Ricerche- CNR- Italy- Alessandro Pecchia
BASF SE BASF- Germany- Christian Lennartz
Max Planck Institute for Polymer Research-MPIP- Germany- Denis Andrienko
Universidad del País Vasco- Donostia UPV -Spain -Angel Rubio
University Rome “Tor Vergata”- UTV- Italy- Aldo Di Carlo
Technische Universiteit Eindhoven- TUE- Netherlands- Peter Bobbert
Philips Eindhoven -PHIL -Netherlands -Reinder Coehoorn
Technische Unversität Dresden- TUD -German- Karl Leo
sim4tec -S4T -Germany -Robert Nitsche
COSMOLOGIC GMBH & COKG-CL-Germany