Free Photons for Efficient White OLEDs

Trapped light particles: Dresden physicists use nanostructures to free photons for highly efficient white OLEDs. (Source: S. Reineke et al., Nat. Commun.: CC BY 4.0)

Thanks to intensive research in the past three decades, organic light-emitting diodes (OLEDs) have been steadily conquering the electronics market – from OLED mobile phone displays to roll-out television screens, the list of appli­cations is long. Current OLED research focuses in particular on improving the performance of white OLEDs for lighting elements such as ceiling or car interior lighting. These components are subject to much stricter require­ments in terms of stability, angular emission and power efficiency. Since light-emitting diodes only produce monochrome light, manu­facturers use various additive colour-mixing processes to produce white light.

Since the first development of white OLEDs in the 1990s, numerous efforts have been made to achieve a balanced white spectrum and high luminous efficacy at a practical luminance level. However, the external quantum effi­ciency (EQE) for white OLEDs without addi­tional outcoupling techniques can only reach 20 to 40 percent today. About 20 percent of the generated photons remain trapped in the glass layer of the device. The reason for this is the total internal reflection of the particles at the interface between glass and air. Further photons are wave­guided in the organic layers, while others get ultimately lost at the interface to the top metal electrode.

Numerous approaches have been inves­tigated to extract the trapped photons from OLEDs. An inter­national research team led by Simone Lenk and Sebastian Reineke from the TU Dresden has now presented a new method for freeing the light particles. The physicists introduce a facile, scalable and especially litho­graphy-free method for the generation of controllable nano­structures with directional randomness and dimensional order, significantly boosting the efficiency of white OLEDs. The nano­structures are produced by reactive ion etching. This has the advantage that the topography of the nano­structures can be speci­fically controlled by adjusting the process parameters.

In order to understand the results obtained, the scientists have developed an optical model that can be used to explain the increased effi­ciency of OLEDs. By integrating these nano­structures into white OLEDs, an external quantum efficiency of up to 76.3% can be achieved. For Simone Lenk, the new method opens up numerous new avenues: “We had been looking for a way to specifically manipulate nano­structures for a long time already. With reactive ion etching, we have found a cost-effective process that can be used for large surfaces and is also suitable for industrial use. The advantage lies in the fact that the periodicity and height of the nano­structures can be completely adjusted via the process parameters and that thus an optimal outcoupling structure for white OLEDs could be found. These quasi-periodic nano­structures are not only suitable as outcoupling structures for OLEDs, but also have the potential for further appli­cations in optics, biology and mechanics”. (Source: TU Dresden)

Reference: Y. Li et al.: Tailor-made nanostructures bridging chaos and order for highly efficient white organic light-emitting diodes, Nat. Commun. 10, 2972 (2019); DOI: 10.1038/s41467-019-11032-z

Link: Organic Semiconductors (S. Reineke), Dresden Integrated Center for Applied Physics and Photonic Materials, Technische Universität Dresden, Dresden, Germany

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