The Effect of Electron and Hole Transfer Layers on the Electro-Optical Properties of Solution-Processed QD-LED

Abstract

Colloidal quantum-dots-based light emitting diodes (QD-LEDs) have been considered attractive display devices because of the remarkable electrical/optical characteristics of colloidal quantum dots. QD-LEDs have attracted particular interest not only because of their wide-range color tunability, high brightness and good color purity by the narrow emission bandwidth, but also because of their printability with a large size under atmospheric pressure.Nevertheless, achieving optimal control of charge transport/light emission and forming the necessary multilayer device structures is still a major challenge. The hole transport layer (HTL) and electron transport layer (ETL) thickness should be optimized to confine the electron and holes to the recombination zone in an emitting material layer (EML), i.e., the QD layer. For all-solution-processed QD-LEDs, inorganic and organic layers as the ETL and HTL were employed and optimized to achieve improved electroluminescence performances. Red QDs in the core/shell/shell type multi-structure, ZnO nanoparticles, and TiO2 were synthesized using the procedures reported elsewhere. The QD-LEDs were fabricated on patterned ITO glass. As a hole transfer layer, a PEDOT:PSS solution was spin coated. The organic hole transport layer, PVK, was coated on the PEDOT:PSS layer at a different rates to obtain a range of thicknesses. The CdSe/CdS/ZnS QDs (20 mg/mL) were spin coated. On the QD layers, various concentrations of ZnO NPs or TiO2 layers were spin-coated. Finally, 100 nm aluminum, as a cathode layer, was deposited on the ZnO NPs layer by vacuum evaporation As electron charge transport layers and hole charge transport layer, zinc oxide nanoparticles and polyvinyl carvazole (PVK) were applied to all solution-processed quantum dot light emitting diodes. The quantum dot as the emitting layer, and the zinc oxide nanoparticles and titanium oxide, as the carrier transporting layers were synthesized using a solution mediated process. The optimized device using 45 nm zinc oxide nanoparticles as the electron transport layer and 18 nm PVK as the hole transport layer showed a lower turn-on voltage and better luminescence, resulting in a luminance of 13,730 cd/m2, current efficiency of 3.27 cd/A, and EQE of 2.67%.