Improving the Device Stability by Controlling the Morphology of Quantum Dot Emissive Layer Via a Coating Process in Blue QLEDs

Blue light-emitting CdSe@ZnS/ZnS quantum dot (QD) nanoparticles (NPs) were synthesized and their photophysical properties in both solution and film phases were investigated. The morphological properties of films prepared by different coating methods i. e. single layer coating from low to high concentrations of QD solutions and layer-by-layer (multilayer) coating within constant low QD solution concentration, were also examined in detail. Varying the concentration (1-10 mg/mL) and the number of layers (from 1-16) did not essentially affect the photophysical properties of QD films, although it resulted in a direct increment in QD film thickness. The concentration and layer-dependent films were used as an emissive layer (EML) in QD light-emitting diodes (QLEDs). Although the "6 mg/ml(-1) Layer" QD EML-based device exhibited relatively high device efficiency compared to the "1 mg/ml(-10) Layers" based one at working voltage region, it had similar to 2-fold higher efficiency roll-off at high voltage region. The performance differences for both devices with the same QD EML thickness were attributed to the morphological variations for the QD layer in terms of surface roughness, void density, aggregates/clusters, and trap sites that were directly related to the charge injection balance and Auger recombination.