Experimental and Simulation Based Approach (CTDyn

3.2

Dynamics of Charge Transfer States in Organic Semiconductor Devices: A Combined Experimental and Simulation-Based Approach (CTDyn)

This new Swiss-German collaboration investigates charge transfer states (CT excitons) that play a crucial role in the understanding and further improvement of the efficiency of organic electronic devices such as OLEDs and organic solar cells. We choose an approach that combines optical and electronic measurements with numerical simulations, and will improve the underlying physical models and numerical methods.

Contributors: Partner(s): Funding: Duration: M. Regnat, C. Kirsch, S. Züfle, B. Ruhstaller Prof. W. Brütting, Universität Augsburg SNF/DFG 2020–2023

Organic light-emitting devices (OLEDs) are a disruptive flat panel display technology rivalling with established LCD technology. OLEDs have reached a very high maturity level for applications in mobile devices and large TV panels. Hereby a major increase of the luminance yield has been realized by an improved understanding and control of the exciton physics in organic materials in the last decades. This involved moving from fluorescent to phosphorescent dyes, to host-guest systems and recently to thermally activated delayed fluorescence (TADF) systems where 100% internal quantum efficiency can be realized. Especially in TADF devices important material parameters are determined using temperature-dependent (mostly optical) measurements. Furthermore, with increased driving currents additional excitonic processes occur that can lead to (reversible or irreversible) performance loss. While these processes are well-known today, a complete model including all relevant physics for charges and excitons and describing a full set of experiments is still a big challenge. The project CTDyn studies the dynamics of excitons and interplay between different exciton species as well as between excitons and charge carriers, both in the bulk of individual layers (intra-molecular) and at interfaces of multilayer organic semiconductor devices (inter-molecular). We will extend our established 1D drift-diffusion approach and combine it with a novel 3D Master equation model as well as compare it with 0D analytical formulas. This requires the preparation of clean model systems to isolate the underlying processes by dedicated experiments on the one hand; on the other hand the model needs to be extended to capture the enhanced complexity of these systems. The physics of exciton processes can be studied in both OLED and solar cell configurations. In fact, due to basic thermodynamics and reciprocity many of the findings from OLEDs may be transferred to solar cells and vice versa. For example, we plan to measure the open-circuit voltage of OLEDs (suns-Voc experiment) or the luminance of solar cells. With regards to simulation we aim for a comprehensive and universal model covering all processes and which can be employed for both OLEDs and solar cells.

Figure 1: General exciton and charge transfer processes. FC denotes the free carriers, S1 the singlet and T1 the triplet exciton, and S0 the ground state. The TADF process is linked to the krISC rate.