Model Predicts How Hybrid Light-Matter State Increases OLED Efficiency

Lightweight, flexible, and eco-friendly, OLEDs are reshaping the lighting industry with innovative illumination solutions and high-definition displays. However, they continue to be slow at converting electric current into light. Only 25% of the electronic states of organic molecules can emit light upon electrical excitation, which limits the overall efficiency of OLEDs. With only a 25% probability of emitting photons efficiently and rapidly, OLEDs tend to be dimmer than other lighting technologies.

To map out a potential solution to OLED inefficiency, researchers at the University of Turku and Cornell University developed a theoretical model predicting that the brightness of OLEDs can be increased by fine-tuning polaritons.

Polaritons are hybrid states of light and matter created through strong light-matter coupling. The coupling is achieved by using mirrors to confine light within the OLEDs. When sandwiched between two semi-transparent mirrors, the organic emitters couple with the confined light, creating polaritons.

Polaritons have the potential to activate the remaining 75% of the electronic triplet states in OLEDs.

Thermally activated delayed fluorescence (TADF) emitters are a class of OLED emitters with high internal quantum efficiency (IQE). In TADF emitters, triplet excitations are efficiently converted into singlets by reverse inter-system crossing (RISC).

A standard blue OLED with a width of 15 mm and an emitting pixel width of 2 mm. A polariton OLED could be obtained by replacing the thin films above and below with a semi-transparent material with a thickness of 10 to 100 nm. Courtesy of Mikael Nyberg and Manish Kumar.

Microcavity polaritons have the potential to achieve high RISC rates without compromising the emitters' ability to emit photons. In the context of OLEDs, this means that by using straightforward cavity designs, the emitters inside a cavity can exhibit high RISC rates and high IQE, resulting in optoelectronic devices that combine simple architectures and superior performance.

However, existing theoretical models for this approach are rudimentary, and provide a limited view of how polaritons interact with these molecular processes. The researchers saw the lack of an effective model as a bottleneck hindering efficient triplet harvesting in actual OLED devices.

They introduced a theoretical model for polaritonic OLED processes that was not restricted to the single-excitation subspace. The model allowed the team to explore RISC together with triplet-triplet annihilation and singlet-singlet annihilation.

Using the Marcus theory of electron transfer and Fermi's golden rule, the researchers derived rates for both polaritonic RISC and triplet-triplet annihilation. By scanning the parameter space of the model, they constructed "enhancement maps” for both RISC and triplet-triplet annihilation. They also analyzed the effect of the number of molecules and studied how singlet-singlet annihilation could be reduced with strong coupling. They applied the model to six molecules previously studied under strong coupling.

“While the general idea of using polaritons in OLED technology is not entirely original, a theory that examines the boundaries of performance gains has been missing,” professor Konstantinos Daskalakis said. “In this work, we carefully examined where the polariton sweet spot lies in different scenarios. We found that the strength of the polaritonic effect in OLEDs’ performance depends on the number of coupled molecules. The fewer, the better.”

By tuning the polaritonic states, it is possible to find where the 75% dark states start to become bright polaritons.

“With the molecules we studied and a single coupled molecule, the efficiency improved significantly,” researcher Olli Siltanen said. “The dark-to-bright conversion rate increased by a whopping factor of 10 million at best.”

The team further found that the polaritonic effect was negligible in many of the molecules, which could indicate that the dark-to-bright conversion rate of existing OLEDs cannot be enhanced simply by equipping them with mirrors.

“The next challenge is to develop feasible architectures facilitating single-molecule strong coupling or invent new molecules tailored for polariton OLEDs,” Daskalakis said. “Both approaches are challenging, but as a result, the efficiency and brightness of OLED displays could be significantly improved.”

Although OLEDs offer several advantages over traditional lighting solutions, their widespread adoption has been hindered by limitations in efficiency and brightness. The results of this study could provide a path forward, laying the foundation for OLEDs that are not only more efficient but also capable of achieving performance levels previously thought impossible, and opening the way for more advanced hybrid light-matter technologies.

The research was published in Advanced Optical Materials (www.doi.org/10.1002/adom.202403046).