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Modelling stability of organic phosphorescent light-emitting diodes

Periodic Reporting for period 2 - MOSTOPHOS (Modelling stability of organic phosphorescent light-emitting diodes)

Reporting period: 2016-12-01 to 2018-05-31

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. MOSTOPHOS addresses the problem of stability of blue emitting organic LEDs based on phosphorescent dyes in order to achieve all-organic white sources for lighting. The challenge of the project is to provide a theoretical understanding of the dominant degradation mechanisms, that are at the basis of possible improvements. The impact of MOSTOPHOS is not limited to potential innovation in materials. Being primarily a theoretical effort, the project aims at considerably deepening the insights into the microscopic processes at the basis of OLEDs functioning, from a more fundamental level, strongly pushing forward the state-of-art of analysis tools available to the scientific community and to the industry.
During the course of the project, we have developed and validated microscopic (atomistic) models for charge and exciton transport in amorphous small-scale morphologies of phosphorescent and TADF OLEDs. We have introduced a model for the evaluation of injection rates from oxides into an organic layer. We have provided the model for the degradation of a phosphorescent OLED and a conceptual solution capable of extending the blue OLED lifetime. We have performed extensive kinetic Monte Carlo simulation of devices using both atomistic and lattice codes and modelled operational devices, including exciton/exciton and charge/exciton interactions. We have linked continuous and lattice-based models and performed drift-diffusion simulations of OLEDs with light outcoupling and heat transport. We have transferred the knowledge from BASF and CYNORA and completely characterized their OLED stacks, identifying the crucial for OLED functionality and degradation parameters. We have used the state-of-the art concepts (ultrastable glasses) to improve the efficiency and lifetime of blue OLEDs. We have developed a GUI for the integrated workflow of OLED simulations, on multiple scales, from morphology simulations and electronic couplings to master and drift-diffusion equation solvers.

We have organized an international workshop in Grenoble in 2017 ( The workshop was one of the major network and training events in the field. Moreover, a summer school was held in Mittelwihr in 2017 ( an international training activity for PhD students.

We have actively communicated with industrial partners, also outside the consortium: Schroedinger, Philips, sim4tec, OSRAM-OLED, BASF, FLUXIM, MERCK and CYNORA. Several collaborative spin-offs will continue even after the end of the project.
Several large European industries related to materials synthesis are impacted by the results of the project, such as MERCK, SAES Getters, CYNORA or Novaled. The developed within MOSTOPHOS software is now not limited to phosphorescent OLEDs but can also be used to analyze other OLED technologies based on small molecules, for instance Thermally Activated Delayed Fluorescence (TADF).

A clear set of results has been achieved:

1. We have developed and validated the models for the degradation process of OLEDs and the so-called efficiency roll-off, which is the reduction in efficiency of an OLED as the voltage is increased.

2. We have developed a developed software suite, including a streamlined GUI, which could help companies identify the right materials for OLED applications. Software package is designed to obtain OLED characteristics from scratch, starting from the chemical structures.

3. We have established the contacts with major European industrial players in the field (MERCK, CYNORA, OSRAM-OLED).

4. We have streamlined and aligned our codes, including interfaces and documentation of the data output.

5. We have proposed a new concept for overcoming limiting life-times of a blue pixel (Unicolor Phosphor Sensitized Fluorescence), proved it experimentally and identified directions for its further improvements.

Overall, this was a very dynamic and complex project. We have seen how the extremely competitive field of OLEDs forced a relocation of a large research flagship project from one company to another. Because of this, the project goals had to be re-oriented from a pure phosphorescent to a TADF OLED right in the middle of the action.
In spite of this, the project clearly fulfilled its initial goals. Moreover, because of this unforeseen changes in the consortium, we had to practically double our efforts, which of eventually resulted in an exceptionally high output: more than 60 publications and proceedings, 3 book chapters, and more than 80 conference presentations. In just three years we have co-organized an international workshop with more than 100 participants, and organized 8 well-attended software workshops.

Most importantly, the project has proposed a practical solution to the challenging problem of blue OLED stability, the new Unicolor Phosphor Sensitized Fluorescence (UPSF) concept. It has also contributed to the issue of reducing the injection barrier from a transparent electrode (oxide) into an organic layer by a novel way of injecting charge through a thin organic interlayer, thus helping to improve OLED efficiency by decreasing the contact resistance. Thus results will have an impact on both academic and industrial OLED designs.