Light-emitting electrochemical cells (LECs) show high technical potential for display and lighting utilizations owing to the superior properties of solution processability, low operation voltage, and employing inert cathodes. For maximizing the device efficiency, three approaches including development of efficient emissive materials, optimizing the carrier balance, and maximizing the light extraction have been reported. However, most reported works focused on only one of the three optimization approaches. In this work, a combinational approach is demonstrated to optimize the device efficiency of LECs. A sophisticatedly designed yellow complex exhibiting a superior steric hindrance and a good carrier balance is proposed as the emissive material of light-emitting electrochemical cells and thus the external quantum efficiency (EQE) is up to 13.6%. With an improved carrier balance and reduced self-quenching by employing the host–guest strategy, the device EQE can be enhanced to 16.9%. Finally, a diffusive layer embedded between the glass substrate and the indium-tin-oxide layer is utilized to scatter the light trapped in the layered device structure, and consequently, a high EQE of 23.7% can be obtained. Such an EQE is impressive and consequently proves that the proposed combinational approach including adopting efficient emissive materials, optimizing the carrier balance, and maximizing the light extraction is effective in realizing highly efficient LECs.

Solid-state near-infrared (NIR) light-emitting devices have recently received considerable attention as NIR light sources that can penetrate deep into human tissue and are suitable forb ioimaging and labeling. In addition, solidstate NIR light-emitting electrochemical cells (LECs) have shown several promising advantages over NIR organicl ightemittingd evices (OLEDs). However,a mong the reported NIR LECs based on ionic transition-metal complexes (iTMCs), there is currently no iridium-based LEC that displays NIR electroluminescence (EL) peaksn ear to or above 800 nm. In this report we demonstrate as imple methodf or adjusting the energy gap between the highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO)o fi ridium-based iTMCs to generate NIR emission.We describe as eries of novel ionic iridium complexes with very small energy gaps, namely NIR1-NIR6,i nw hich 2,3-diphenylbenzo[g]quinoxaline moieties mainly take charge of the HOMO energy levels and 2,2'-biquinoline, 2-(quinolin-2yl)quinazoline, and 2,2'-bibenzo[d]thiazole moieties mainly control the LUMO energy levels. All the complexes exhibited NIR phosphorescence, with emission maximau pt o8 50 nm, and have been applied as componentsi nL ECs, showing a maximume xternal quantum efficiency (EQE) of 0.05 %i nt he EL devices. By using ah ost-guest emissive system,w ith the iridiumc omplex RED as the host and the complex NIR3 or NIR6 as guest, the highest EQE of the LECs can be further enhanced to above 0.1 %.Supporting information and the ORCID identification number(s) for the author(s) of this articlecan be found under: https://doi.

Solid‐state light‐emitting electrochemical cells (LECs) have several advantages, such as low‐voltage operation, compatibility with inert metal electrodes, large‐area flexible substrates, and simple solution‐processable device architectures. However, most of the studies on saturated red LECs show low or moderate device efficiencies (external quantum efficiency (EQE) <3.3 %). In this work, we demonstrate a series of five red‐emitting cationic iridium complexes (RED1‐‐RED5) with 2,2′‐biquinoline ligands and test their electroluminescence (EL) characteristics in LECs. The Commission Internationale de l′Eclairage (CIE) 1931 coordinates for the LECs based on these complexes are all beyond the National Television System Committee (NTSC) red standard point (0.67, 0.33). The maximal EQE of the neat‐film RED1‐based LECs reaches 7.4 %. The reddest complex, RED3, is doped in the blue‐emitting host complex, BG, to fabricate host–guest LECs. The maximal EQE of the host–guest LECs (1 wt % complex RED3) reaches 9.4 %, which is among the highest reported for the saturated red LECs.

Near-infrared (NIR) light-emitting devices with organic semiconductors have great potential for bio-imaging, telecommunication, night-vision displays, and chemical sensing. Light-emitting electrochemical cells (LECs) have demonstrated the advantages of a simple and...