Optimizing Ag films towards efficient flexible quantum-dot light-emitting diodes

Abstract

Metal film electrodes are frequently adopted in flexible quantum-dot light-emitting diodes (FQLEDs) to form microcavity structures, which accelerate the recombination of excitons and thereby enhancing the efficiency of devices, especially under high driving current density. However, metal films prepared by thermal evaporation grow according to the Volmer-Weber nucleation mechanism, thereby resulting in the formation of numerous metal islands on the target substrate. The serious leakage current and light scattering (hence weakened microcavity effect) caused by these islands lead to inefficient FQLEDs. Here we fabricated a highly uniform and smooth Ag-based flexible electrode by utilizing an ultrathin polyethyleneimine as the nucleation-inducing seed layer. A smooth surface with a root-mean-square of approximately 3.7 nm and an ultralow sheet resistance of 3.65 Ω sq−1 have been obtained for the Ag film with a thickness of only 16 nm. Thus, a high-efficiency FQLED is achieved with the maximum external quantum efficiency up to 25.6 %. Furthermore, this device exhibits ultralow efficiency roll-off, characterized by the external quantum efficiency of over 20 % across an extremely wide luminance range, from 300 to 160,000 cd m−2, meeting all display and lighting requirements.