Accelerated Turn-On and High Performance in Light-Emitting Electrochemical Cells Using Highly Charged Iridium Complexes

Light-emitting electrochemical cells based on ionic transition metal complexes are attractive as efficient electroluminescent devices due to their simple, single-layer solution-processed architecture. One challenge for their operation is improving the response times of iridium iTMC LECs, which are highly efficient but slow in their pristine form due to the low ionic conductivity of the singly cationic iridium complexes. To address this challenge, we synthesized a series of triply cationic iridium complexes that produced solid-state films with higher conductivity and wider-bandgap emission than conventional [Ir]⁺ complexes. These [Ir]³⁺ complexes featured alkylated (ethyl (EPP) or propyl (PPP)) 2.3'-bipyridine ligands serving as the cyclometalating units, ĈN, and an ancillary 2,2'-bipyridine ligand, N̂N, which is either unmodified (bpy) or substituted with dimethoxy (meoxy) moieties. These complexes exhibited sky-blue photoluminescence and electroluminescence. LECs from simple pristine films of these [Ir]³⁺ complexes yielded 100-1000-fold faster electroluminescence than a pristine [Ir]⁺ control, but also suffered from correlating 100-1000-fold lower luminance half-lives and lower luminance maxima. Blending the singly and triply cationic complexes enabled enhanced performance by leveraging the benefits of each. In particular, an exemplary 10% EPP bpy [Ir]³⁺ device turns on in 4 s while retaining the luminance and stability characteristics of the [Ir]⁺ control complex. This illustrates a strategy for enhancing the DC response of iTMC LECs without foreign dopants, and further refinement of these ionically conductive [Ir]⁺ emitters could yield even greater gains.