Tailoring balance of carrier mobilities in solid-state light-emitting electrochemical cells by doping a carrier trapper to enhance device efficiencies

We demonstrate the improving balance of carrier mobilities in neat-film light-emitting electrochemical cells (LECs) utilizing a cationic transition metal complex (CTMC) as the emissive material and a cationic near-infrared laser dye as the carrier trapper. This low-gap carrier trapper is judiciously chosen such that a significant energy offset in the highest occupied molecular orbital (HOMO) levels between the CTMC and the carrier trapper impedes hole transport in the emissive layers while similar lowest unoccupied molecular orbital (LUMO) levels of these two materials result in relatively unaffected electron transport. Since the CTMC neat films would intrinsically exhibit characteristics of preferred transport of holes, the balance of carrier mobilities would be improved by doping such carrier trapper. Electroluminescent measurements show that the peak external quantum efficiency (EQE) and the peak power efficiency of the neat-film LECs doped with the carrier trapper reach 12.75% and 28.70 lm W^-1, respectively. These device efficiencies represent a 1.4 times enhancement as compared to those of the undoped neat-film LECs and approach the upper limit of EQE (∼15%) that one would expect from the photoluminescence quantum yield of the emissive layer (∼0.75) and an optical out-coupling efficiency of ∼20% from a typical layered device structure, consequently indicating superior balance of carrier mobilities in such a doped emissive layer. These results confirm that the balance of carrier mobilities in the CTMC neat films would be improved by doping a proper carrier trapper and this technique offers a general approach for optimizing device efficiencies of CTMC-based neat-film LECs.