Fabrication of highly efficient pure blue-emitting electroluminescent devices using ZnSe/ZnSexS1-x/ZnS QDs

Abstract

Since electroluminescent (EL) quantum dots (QDs) are considered a key component of the next-generation display, and large-scale production of environment-friendly QDs is required for their wide use in commercial displays. Therefore, several studies on non-cadmium QDs, such as indium phosphide (InP) QDs in the III-V category, graphene QDs, and copper indium sulfide (CuInS2) or silver indium sulfide (AgInS2) QDs in the I-III-VI2 category, have been conducted owing to their non-toxicity and good optical properties. Subsequently, significant results have been reported for green and red colors. However, for synthesis of blue QDs, pure blue emission in the range of 440-460 nm has been achieved with few materials. Among them, zinc selenide (ZnSe) is a promising candidate for synthesizing blue QDs. However, owing to the wide band gap (2.7 eV) of ZnSe, highly effective QDs were attained in the violet region (420-440 nm). Here, for the first time, we have synthesized ZnSe/ZnSexS1-x/ZnS QDs emitting at a wavelength of 444 nm with high photoluminescence quantum yield (PLQY) of 77.2%. Also, full width at half maximum (FWHM) of 23.3 nm ensured its excellent color purity. Use of a gradient intermediate shell of ZnSeS in the original ZnSe/ZnS QDs was the key factor behind this achievement. The intermediate gradient shell of ZnSeS around the core delocalizes the electrons, weakening the quantum confinement effect (QCE), hence rendering the emission color of the QDs tunable from violet to blue by manipulating the ratio of selenium (Se) and sulfur (S) in the composites. A blue emission peak centered at 452 nm was observed for the quantum dot light-emitting diodes (QD-LEDs) fabricated using the above-mentioned QDs, and an external quantum efficiency (EQE) of 5.32%, current efficiency of 1.51 cd/A, and power efficiency of 0.74 lm/W were reported. Furthermore, our fabricated device exhibited a maximum brightness of 3,754 cd/m2 and a half operational time (LT50) with 100 cd/m2 initial luminance of 1.27 h, which are the highest values of these parameters to be reported till date for a blue QD-LED fabricated using ZnSe core based QDs in pure blue region.