Strategies for improving performance, lifetime, and stability in light-emitting diodes using liquid medium

The early form of light generation on Earth started with living liquid systems millions of years ago. Recently, artificial light-emissive devices using liquids have started to attract significant research interest for lighting and displays owing to their high-efficiency, flexibility, unique emission recovery, and tuning ability. Light-emitting diodes using liquids have emerged as an interesting and promising scientific and technological route for future lighting and display applications. The two main categories of light-emitting devices include LEDs with active and color conversion layers using liquids. In this review, we first present and discuss a wide variety of fluorophores used in these devices, including colloidal quantum dots, perovskites, fluorescent proteins, and organic compounds. Then, we review the working principle, structure, and fabrication of the organic LED and electrochemiluminescent devices with a liquid active layer, and highlight unconventional designs incorporating, for example, microfluidics, flexibility, and 3D emission tunability. Afterward, we discuss LEDs using a liquid color conversion layer with a special focus on their efficiency progress. Finally, we conclude the review by providing perspectives for the remaining challenges and open opportunities in the field of liquid light-emitting devices.