Solution-Processed Double-Junction Quantum-Dot Light-Emitting Diodes with an EQE of Over 40%

Despite the rapid development in quantum-dot light-emitting diodes (QD-LEDs) with a single junction, it remains a big challenge to make tandem QD-LEDs with high performance. Here, we report solution-processed double-junction tandem QD-LEDs with a high external quantum efficiency of 42.2% and a high current efficiency of 183.3 cd A^-1, which are comparable to those of the best vacuum-deposited tandem organic LEDs. Such high-efficiency devices are achieved by interface engineering of fully optimized single light-emitting units, which improves carriers' transport/injection balance and suppresses exciton quenching induced by ZnO, and design of an effective interconnecting layer consisting of poly(4-butylphenyl-diphenylamine) (poly-TPD)-mixed poly(9-vinylcarbazole) (PVK)/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate/polyethylenimine ethoxylated-modified ZnO.