No abstract

Over the past few years the performance of colloidal quantum dot-light-emitting diode (QLED) has been progressively improved. However, most of QLED work has been fulfilled in the form of monochromatic device, while full-color-enabling white QLED still remains nearly unexplored. Using red, green, and blue quantum dots (QDs), herein, we fabricate bichromatic and trichromatic QLEDs through sequential solution-processed deposition of poly(9-vinlycarbazole) (PVK) hole transport layer, two or three types of QDs-mixed multilayer, and ZnO nanoparticle electron transport layer. The relative electroluminescent (EL) spectral ratios of constituent QDs in the above multicolored devices are found to inevitably vary with applied bias, leading to the common observation of an increasing contribution of a higher-band gap QD EL over low-band gap one at a higher voltage. The white EL from a trichromatic device is resolved into its primary colors through combining with color filters, producing an exceptional color gamut of 126% relative to National Television Systems Committee (NTSC) color space that a state-of-the-art full-color organic LED counterpart cannot attain. Our trichromatic white QLED also displays the record-high EL performance such as the peak values of 23,352 cd/m(2) in luminance, 21.8 cd/A in current efficiency, and 10.9% in external quantum efficiency.

Since the introduction of inorganic ZnO, typically in the form of nanoparticles (NPs), as an electron transport layer (ETL) material, the device performance of electrically driven colloidal quantum dot-light-emitting diodes (QLEDs), in particular, with either Cd-based II−VI or non-Cd-based III−V (e.g., InP) quantum dot (QD) visible-emitters, has been rapidly improved. In the present work, three Zn 1−x Mg x O (x = 0, 0.05, 0.1) NPs that possess different electronic energy levels are applied as ETLs of solution-processed, multilayered I−III−VI type QLEDs that consist of a Cu−In−S, Cu−In−Ga−S, or Zn−Cu−In−S QD emitting layer (EML) plus a common organic hole transport layer of poly(9-vinlycarbazole). The luminance and efficiency of those QLEDs are found to be strongly dependent on the type of ZnMgO NP ETL, resulting in the substantial improvements by means of alloyed ZnMgO ETL versus pure ZnO one. Ultraviolet photoelectron and absorption spectroscopic measurements on a series of ZnMgO NP films reveal that their conduction band minimum (CBM) levels are systematically closer to the vacuum level with increasing Mg content. Therefore, such beneficial effects of alloyed NPs on QLED performance are primarily ascribed to the reduced electron injection barrier between ETL and QD EML that is enabled by the upshift of their CBM levels.

Considering a strict global environmental regulation, fluorescent quantum dots (QDs) as key visible emitters in the next-generation display field should be compositionally non-Cd. When compared to green and red emitters obtainable from size-controlled InP QDs, development of non-Cd blue QDs remains stagnant. Herein, we explore the synthesis of non-Cd, ZnSe-based QDs with binary and ternary compositions toward blue photoluminescence (PL). First, the size increment of binary ZnSe QDs is attempted by a multiply repeated growth until blue PL is attained. Although this approach offers a relevant blue color, excessively large-sized ZnSe QDs inevitably entail a low PL quantum yield. As an alternative strategy to the above size enlargement, the alloying of high-band gap ZnSe with lower-band gap ZnTe in QD synthesis is carried out. These alloyed ternary ZnSeTe QDs after ZnS shelling exhibit a systematically tunable PL of 422–500 nm as a function of Te/Se ratio. Analogous to the state-of-the-art heterostructure of InP QDs with a double-shelling scheme, an inner shell of ZnSe is newly inserted with different thicknesses prior to an outer shell of ZnS, where the effects of the thickness of ZnSe inner shell on PL properties are examined. Double-shelled ZnSeTe/ZnSe/ZnS QDs with an optimal thickness of the ZnSe inner shell are then employed for all-solution-processed fabrication of a blue QD light-emitting diode (QLED). The present blue QLED as the first ZnSeTe QD-based device yields a peak luminance of 1195 cd/m2, a current efficiency of 2.4 cd/A, and an external quantum efficiency of 4.2%, corresponding to the record values reported from non-Cd blue devices.

We explore both the synthesis of Cd-free blue quantum dots (QDs) with high-quality photoluminescence (PL) characteristics and the fabrication of high-efficiency QD light-emitting diodes (QLEDs). True blue (445 nm)-emissive, multishelled ZnSeTe QDs with a high PL quantum yield of 84% and a sharp bandwidth of 27 nm are prepared. To obtain a better electron transport layer (ETL) material, the surface of ZnMgO nanoparticles (NPs) is modified by additional Mg reaction, leading to the possible formation of a Mg­(OH)2 layer on the surface-modified ZnMgO (m-ZnMgO) NPs. The presence of a Mg­(OH)2 overlayer, the origin of the desirably reduced electron mobility, is supposedly responsible for the improved charge balance of the QD emissive layer (EML). The Mg­(OH)2 layer is further found to alleviate the emission quenching of the QD EML. Via combination of blue ZnSeTe QDs and m-ZnMgO NP ETL, highly bright, efficient blue QLEDs with the record luminance of 2904 cd/m2 and an external quantum efficiency of 9.5% are demonstrated.