Molecular Engineering of Ionic Transition Metal Complexes and Counterions for Efficient Flexible Green Light-Emitting Electrochemical Cells

Light-emitting electrochemical cells (LECs) based on ionic transition metal complexes (iTMCs) represent a cost-effective solid-state lighting technology compatible with large-area and industrial-scale manufacturing. To improve the current LEC performance and compete with rivaling light-emitting diode (LED) devices, it is pivotal to design efficient iTMCs/counterion couples that combine high photolumines-cence efficiency with optimized ionic and electron carrier transport. Despite the continuous proposal of novel iTMCs, the investigated counterions are typically limited to the traditional ones, including PF6- and BF4-. In this work, we introduce both rigid and flexible LEC architectures based on a novel single active layer of [Ir-(ppy)(2)(phtz)]-[Et3NTH](+) + X (ppy = 2-phenylpyridine, phtz = S-phenyl-1H-tetrazole and X = lithium bis(trifluoromethane)sulfoneimide (LiTFSI), tetrabutylammonium perchlorate (TBAP), or sodium perchlorate (NaClO4)) sandwiched between a FTO-coated glass or ITO-coated polyethylene terephthalate (PET) anode and Ga:In cathode. Our new Ir-cyclometaled complex with a tetrazole ligand, without salt additives or polymers, shows a bright green electroluminescence emission at 508 nm. The LECs based on the synthesized iTMC and TBAP additive show a current efficiency as high as 1.44 cd/A, a luminance of 503.82 cd/m(2), and an external quantum efficiency of 1.73% at 3.7 V. By using a dual salt additive made of TBAP:LiTFSI (1:1), the LECs further improve the performance of the single salt-based devices, exhibiting a current efficiency of 1.72 cd/A, a luminance of 603.14 cd/m(2), and an external quantum efficiency of 2.06% at 3.6 V. Such improvement of the LEC performance is attributed to the combination of the TBAP anion-iTMC cation size matching and the peculiar electrical properties of the LiTFSI-based solid electrolytes (i.e., high TFSI- mobility), leading to a compact space charge region near the electrodes and low turn-on voltage, respectively.