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Why OLEDs degrade over time

OLEDs are widely used in modern displays due to their excellent colour quality and efficiency. In phosphorescent OLEDs, heavy metal atoms enable nearly 100% conversion of electricity into light.

However, over time users often notice dimming brightness and colour shifts. These effects originate from chemical degradation inside the OLED, driven by interactions between energy carriers known as excitons and electrical charges.

Excitons live for microseconds, long enough to collide with other excitons or charges. During such interactions, energy can be lost instead of emitted as light, and this excess energy may damage the molecular structure of the material. This issue is especially severe in blue OLEDs, which operate at higher energies.

Beyond the standard theory of exciton interactions

Traditionally, these energy-loss processes are described as F枚rster-type interactions, where energy is transferred through dipolar coupling, similar to nanoscale antennas exchanging electromagnetic waves.

Van Hoesel showed that this widely used approximation is not always sufficient. In a detailed case study, he discovered that quadrupolar interactions, a higher-order, more complex type of coupling, can dominate the interaction between excitons and charges. This finding resolves long-standing discrepancies between experimental observations and theoretical predictions.

A new framework for energy transfer in OLEDs

Building on this discovery, Van Hoesel developed a general theoretical framework that includes higher-order multipole effects in excitonic interactions. This framework allows researchers to calculate interaction rates using measurable optical properties, extending the predictive power of existing models.

Applying this approach to a wide range of material combinations revealed that while many materials behave similarly, some exhibit atypical absorption properties that lead to significantly slower energy-loss processes, particularly important for blue emitters.

Understanding exciton鈥揺xciton interactions

In addition to interactions with charges, excitons can also interact with each other through triplet鈥搕riplet annihilation (TTA). Van Hoesel showed that for these processes, the traditional dipole description remains sufficient and that theoretical predictions align closely with experimental data.

Interestingly, the interaction rate decreases with emission energy, which suggests that higher-energy (blue) emitters may benefit from certain interaction properties, offering new perspectives for improving their stability.

Toward longer-lasting OLED displays

By combining advanced theoretical modeling with practical material analysis, this research provides design rules to suppress energy-loss processes, particularly triplet鈥損olaron quenching. These insights enable more accurate prediction of degradation mechanisms and help guide the development of more durable OLED materials.

Van Hoesel鈥檚 work shows that understanding the microscopic physics of exciton interactions is key to improving both efficiency and lifetime, bringing us closer to OLED displays that remain bright and colour-accurate for many years.