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Why excitons matter in OLEDs

OLEDs are widely used in smartphones, televisions, and virtual and augmented reality devices because of their high efficiency, flexibility, and excellent image quality. Yet even today, important physical processes inside OLEDs are not fully understood.

A key role is played by excitons, bound electron-hole pairs that store the energy eventually released as light. Some of these excitons are lost before they can emit photons, reducing device efficiency, especially at high brightness levels. Understanding these loss mechanisms is therefore essential for designing better OLEDs.

Measuring how strongly excitons are bound

The first part of Tomita鈥檚 research focuses on field-induced dissociation (FID), a process in which an external electric field breaks apart excitons. By combining field-dependent photoluminescence measurements with three-dimensional kinetic Monte Carlo (3D-KMC) simulations, he was able to determine the binding energies of excitons in a range of OLED materials.

The study revealed exciton binding energies between 1.0 and 1.3 electronvolts, depending on the material system. These values are significantly larger than often assumed in OLED research and show that exciton binding energy cannot be neglected when determining the energy-level structure of OLED materials.

Understanding energy-loss mechanisms

The second part of the thesis investigates triplet-polaron quenching (TPQ), one of the main efficiency-loss mechanisms in phosphorescent OLEDs. TPQ occurs when an exciton interacts with an electrical charge carrier, causing the exciton鈥檚 energy to be lost instead of being converted into light.

Using a combination of device experiments, transport measurements, and simulations, Tomita quantified the interaction strengths of various TPQ processes. His work reveals how factors such as charge transport, trap states, and material combinations influence quenching behavior and contribute to efficiency losses.

Combining experiments and simulations

A key achievement of the thesis is the development of an integrated framework that connects optical measurements, electrical characterization, quantum-chemical calculations, and device-scale simulations. This approach makes it possible to interpret experimental observations at the molecular level and extract material properties that are otherwise difficult to determine directly.

By linking experiments and simulations, the framework provides a powerful tool for understanding the complex excitonic interactions that govern OLED performance.

Towards more efficient OLED displays

Tomita鈥檚 findings show that accurate knowledge of exciton binding energies and quenching processes is crucial for optimizing OLED device design. Since the alignment of energy levels strongly affects OLED efficiency, the improved understanding gained in this research can help engineers develop devices with reduced efficiency losses, improved brightness, and better overall performance.

Ultimately, this work contributes to a deeper understanding of the fundamental physics of OLEDs and provides new tools for creating the next generation of high-performance display technologies.