High‐Efficiency and Stable Tandem Organic Light‐Emitting Diodes Based on In Situ Coordination‐Activated N‐Doping
Xiao Li, Ziyang Liu, Minqiang Mai, Qingyu Meng, Taijū Tsuboi, Xiangyu Liu, Dongdong Zhang, Lian Duan
Abstract
Abstract Tandem organic light‐emitting diodes (OLEDs), where more than one electroluminescence (EL) units are connected electrically in series by charge generation layers (CGLs), have aroused wide attention for high‐luminance tandem displays. However, the n‐doped layer in commercialized CGL still relies on the active low‐work function alkali metals and suffers from the metal‐migration caused operation stability issues. Here, an advanced CGL structure using in situ coordination‐activated n‐doping by air‐stable silver metal in electron‐transporting ligand is proposed. On the basis of a new developed meta‐linked diphenanthroline‐type ligand, a quasi‐planar tetradentate silver (Ag)–chelate is in situ synthesized during the evaporation process, not only decreasing the ionization energy of Ag for a high n‐doping efficiency to generate small charge generation and electron injection barriers of only 0.48 eV and 0.17 eV in CGL, but also preventing the metal‐migration to enhance device operation stability. Besides a high maximum external quantum efficiency (EQE max ) of 64.5%, the proof‐of‐the‐concept tandem OLEDs with Ag‐doped CGL demonstrate a LT95 (lifetime to 95% of the initial luminance) reaches 155.1 h at 5000 cd m −2 , as 1.72 times longer than the Li‐doped reference.