A team of researchers from the University of St Andrews, in collaboration with the University of Cologne, has developed a way to enhance the brightness and color of TVs and smart screen displays while maintaining device efficiency. The researchers used the scientific principle of strong coupling of light and matter to modify the emission spectra of organic light-emitting diodes (OLEDs) without the viewing angle dependence typically associated with micro-cavity designs. Professor Malte Gather, the lead investigator, explained that placing the OLED stack between thin mirrors made from metallic materials widely used in the display industry significantly enhances the coupling between light and the organic material. However, this coupling typically reduces device efficiency.
Researchers investigated the development of polariton-based organic light-emitting diodes (POLEDs) by inserting a strong coupling layer into a high-quality micro-cavity (MC) OLED with a doped emissive layer (EML). The study demonstrated efficient electrically driven polariton generation and light emission in both red- and green-emitting POLEDs, with the devices retaining over 70% of the external quantum efficiency achieved by weakly coupled reference OLEDs. The emission characteristics of the devices were dominated by polariton formation, with emission exclusively emanating from the lower polariton branch (LPB).
The use of organic semiconductors offers advantages over inorganic materials for the realization of POLEDs due to their large exciton binding energies and high oscillator strengths. However, previous studies using strongly absorbing bulk materials as the emissive layer to achieve strong coupling have led to low external quantum efficiency and low brightness. In this study, researchers used a doped emissive layer with high luminescence quantum yield and an assisting strong coupling layer to improve device efficiency.
The researchers’ POLEDs offer a strategy to realize polariton emission and enable the development of OLED displays with viewing-angle-independent emission color and improved color saturation. By tuning the emission characteristics, the devices can enter the ultra-strong coupling (USC) regime without major losses in performance. The hybridized light-matter polaritons in the POLEDs exhibit flat, exciton-like angular dispersion, which eliminates any noticeable color change with angle and maintains the high color purity and efficiency of an MC OLED. The devices offer desirable properties for displays and other emerging applications requiring narrowband and angle-independent light sources.
A Significant Increase in EQE
By adding a strong coupling layer in the second field maximum of a second-order cavity device, efficient Ir-based emitters combined with either Cl6SubPc in the red or C545T in the green yielded highly tunable polariton emission, with EQEs of 10% and luminance levels over 20,000 cd/m2 at 5 V forward bias. This new approach represents a significant increase in EQE of at least an order of magnitude and an increase in luminance and luminous efficacy of at least two orders of magnitude when compared with previously demonstrated POLEDs. The assistant strong coupling layer ensures that the overall device characteristics remain dominated by the strongly coupled cavity, even reaching the ultrastrong coupling regime without major losses in device efficiency.
By decoupling light emission and polariton formation, researchers avoided catastrophic losses in device efficiency and gained access to phosphorescent emitters for the design. This approach unlocks the entire visible spectrum and parts of the near-infrared spectrum for efficient polariton emission. The POLEDs developed here could also provide a promising monolithic pathway to electrically pumped polariton lasing in organic systems, for example, by combining metallic contacts with high-optical-quality distributed Bragg reflectors.
These POLEDs offer attractive features for display applications by enabling the emission of more saturated colors than possible with current OLED technology without introducing a change in perceived color when viewed from different angles. Furthermore, they offer the potential for improving the robustness and reliability of future flexible displays, particularly for top-emitting OLED architectures. The rearrangement of exciton energies to polariton states that occurs in a POLED may also enhance device efficiency through the improved recycling of triplet states.
Overall, this strategy offers numerous possibilities for reducing the remaining gap in EQE between POLEDs and conventional OLEDs by employing the latest-generation phosphorescent and thermally activated delayed fluorescence emitters and integrating capping layers and other light extraction modalities. The universality of this approach offers vast opportunities for unlocking the full potential of polariton-based OLEDs.
Research
Mischok, A., Hillebrandt, S., Kwon, S., et al. Highly efficient polaritonic light-emitting diodes with angle-independent narrowband emission. Nat. Photon. (2023). https://doi.org/10.1038/s41566-023-01164-6
