No abstract

In the study of hybrid quantum dot light-emitting diodes (QLEDs), even for state-of-the-art achievement, there still exists a long-standing charge balance problem, i.e., sufficient electron injection versus inefficient hole injection due to the large valence band offset of quantum dots (QDs) with respect to the adjacent carrier transport layer. Here the dedicated design and synthesis of high luminescence Zn 1−x Cd x Se/ZnSe/ZnS QDs is reported by precisely controlled shell growth, which have matched energy level with the adjacent hole transport layer in QLEDs. As emitters, such Zn 1−x Cd x Se-based QLEDs exhibit peak external quantum efficiencies (EQE) of up to 30.9%, maximum brightness of over 334 000 cd m −2 , very low efficiency roll-off at high current density (EQE ≈25% @ current density of 150 mA cm −2 ), and operational lifetime extended to ≈1 800 000 h at 100 cd m −2 . These extraordinary performances make this work the best among all solution-processed QLEDs reported in literature so far by achieving simultaneously high luminescence and balanced charge injection. These major advances are attributed to the combination of an intermediate ZnSe layer with an ultrathin ZnS outer layer as the shell materials and surface modification with 2-ethylhexane-1-thiol, which can dramatically improve hole injection efficiency and thus lead to more balanced charge injection.

We discuss how the stain left after evaporation of a suspension evolves with heating of the glass or plastic on which the liquid has been deposited. Upon increasing the substrate temperature, it is found that the stain gradually changes from the usually observed ring to an "eye" shape, that is, a combination of the thick central stain and the thin outer ring. Both the size and the relative volume of the central stain increase with the substrate temperature. The main mechanism for this phenomenon is proposed to be an enhanced Marangoni recirculation flow on hot substrates. These findings can be exploited to continuously tune the morphology of coffee stains, with potential applications in self-assembly and ink-jet printing.

Blue nonblinking (>98% "on" time) ZnCdSe/ ZnS//ZnS quantum dots (QDs) with absolute fluorescence quantum yield (QY) of 92% (λ peak = 472 nm) were synthesized via a low temperature nucleation and high temperature shell growth method. Such bright nonblinking ZnCdSe/ZnS//ZnS core/shell QDs exhibit not only good emission tunability in the blue-cyan range with corresponding wavelength from 450 to 495 nm but also high absolute photoluminescence (PL) QY and superior chemical and photochemical stability. Highly efficient blue quantum dotbased light-emitting diodes (QLEDs) have been demonstrated by using nonblinking ZnCdSe/ZnS//ZnS QDs as emissive layer, and the charge−injection balance within the QD active layer was improved by introducing a nonconductive layer of poly(methyl methacrylate) (PMMA) between the electron transport layer (ETL) and the QD layer, where the PMMA layer takes the role of coordinator to impede excessive electron flux. The best device exhibits outstanding features such as maximum luminance of 14,100 cd/m 2 , current efficiency of 11.8 cd/A, and external quantum efficiency (EQE) of 16.2%. Importantly, the peak efficiency of the QLEDs with PMMA is achieved at ∼1,000 cd/m 2 and high EQE > 12% can be sustained in the range of 100 to 3,000 cd/m 2 .

Due to the unique optical properties, colloidal quantum dots (QDs) are excellent candidates for developing next-generation display and solid-state lighting technologies. However, some factors including photoluminescence blinking and Förster resonance energy transfer (FRET) still affect their practical applications. Herein, a series of ZnCdSe-based core/shell QDs with low optical polydispersity have been successfully synthesized by a “low-temperature injection and high-temperature growth” precisely controlled method. The alloyed ZnCdSe core with a certain ratio of Cd and Zn was presynthesized first. Followed by accurate ZnS shell growth, the as-synthesized core/shell QDs are nonblinking with the nonblinking threshold volume of ∼137 nm3. The PL decay dynamics are all single-exponential for both QDs in solutions and close-packed solid films when ZnS shell thickness varying from 2 to 20 monolayers. FRET can be effectively suppressed after growing 10 monolayers of ZnS shell. All of these superb characteristics including nonblinking, single-exponential PL decay dynamics and suppressed FRET can be beneficial to high-quality QD-based light-emitting devices (QLEDs). By applying the ZnCdSe-based core/shell QDs with 10 monolayers ZnS shell, the highest external quantum efficiency of ∼17% was reached, which could compare favorably with the highest efficiency of green QLEDs with traditional multilayered structures.