Long‐wavelength light‐emitting electrochemical cells (LECs) are potential deep‐red and near infrared light sources with solution‐processable simple device architecture, low‐voltage operation, and compatibility with inert metal electrodes. Many scientific efforts have been made to material design and device engineering of the long‐wavelength LECs over the past two decades. The materials designed the for long‐wavelength LECs cover ionic transition metal complexes, small molecules, conjugated polymers, and perovskites. On the other hand, device engineering techniques, including spectral modification by adjusting microcavity effect, light outcoupling enhancement, energy down‐conversion from color conversion layers, and adjusting intermolecular interactions, are also helpful in improving the device performance of long‐wavelength LECs. In this review, recent advances in the long‐wavelength LECs are reviewed from the viewpoints of materials and device engineering. Finally, discussions on conclusion and outlook indicate possible directions for future developments of the long‐wavelength LECs. This review would like to pave the way for the researchers to design materials and device engineering techniques for the long‐wavelength LECs in the applications of displays, bio‐imaging, telecommunication, and night‐vision displays.

Recently, the fields of organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs) have improved tremendously with regard to tunable emission, efficiency, brightness, and thermal stability. Imidazole derivatives are excellent deep blue-green light-emitting layers in the OLED or LEC devices. This Review summarizes the major breakthroughs of various electroluminescence (EL) layers with imidazole-containing organic or organometallic derivatives, the molecular design principles, and their light-emitting performances as effective EL materials. The highly tunable chemical structures and flexible molecular design strategies of imidazole-based compounds are advantages that provide great opportunities for researchers. They can provide a good basis for the design and development of new EL materials with narrower emission and higher efficiency. Moreover, imidazole compounds have demonstrated breakthrough performances in thermally activated delayed fluorescence (TADF) properties where triplet excitons are utilized to inhibit anti-intersystem quenching, showing great promise in breaking the theoretical external quantum efficiencies (EQE) limits in traditional fluorescent devices.

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