ABSTRACT
Developing green perovskite light-emitting diodes (PeLEDs) with a high external quantum efficiency (EQE) and low efficiency roll-off at high brightness remains a critical challenge. Nanostructured emitter-based devices have shown high efficiency but restricted ascending luminance at high current densities, while devices based on large-sized crystals exhibit low efficiency roll-off but face great challenges to high efficiency. Herein, we develop an all-inorganic device architecture combined with utilizing tens-of-nanometers-sized CsPbBr3 (TNS-CsPbBr3) emitters in a carrier-confined heterostructure to realize green PeLEDs that exhibit high EQEs and low efficiency roll-off. A typical type-I heterojunction containing TNS-CsPbBr3 crystals and wide-bandgap Cs4PbBr6 within a grain is formed by carefully controlling the precursor ratio. These heterostructured TNS-CsPbBr3 emitters simultaneously enhance carrier confinement and retain low Auger recombination under a large injected carrier density. Benefiting from a simple device architecture consisting of an emissive layer and an oxide electron-transporting layer, the PeLEDs exhibit a sub-bandgap turn-on voltage of 2.0 V and steeply rising luminance. In consequence, we achieved green PeLEDs demonstrating a peak EQE of 17.0% at the brightness of 36,000 cd m-2, and the EQE remained at 15.7% and 12.6% at the brightness of 100,000 and 200,000 cd m-2, respectively. In addition, our results underscore the role of interface degradation during device operation as a factor in device failure.
ABSTRACT
A positive aging phenomenon, that is, enhancement of the electroluminescence performance at the beginning of electrical aging, is commonly observed for the state-of-the-art perovskite light-emitting diodes (PeLEDs). The origins of positive aging could fundamentally interfere with those of the operational deterioration processes of PeLEDs (namely negative aging), bringing difficulty in analyzing the degradation mechanisms. This work decouples the positive and negative aging processes of PeLEDs by inserting a thin ionic liquid interlayer between the hole-injection layer and the perovskite layer. This interlayer inhibits ions migration by passivating the halogen vacancies of perovskite films and suppresses interfacial exciton quenching, enabling us to decouple the positive and negative aging processes of PeLEDs while increasing the device efficiency. Inserting an ionic liquid interlayer is also demonstrated to be effective for iodide-based PeLEDs and applicable with the use of other ionic liquids. Our work provides an ideal system for fundamental studies on the degradation mechanisms of PeLEDs.
ABSTRACT
Maintaining the superior lubricating properties of graphene under chemical modification requires a deep understanding of the origin of its friction enhancement. In this study, the DFT calculations were performed to investigate the effects of substrate and tip characteristics on the frictional properties of fluorinated graphene (FGr) on Cu(111) and Pt(111) substrates. The calculation results indicate that the fluorination will increase the geometrical corrugation of graphene and a stronger reactivity between graphene and substrate could confine the geometrical corrugation. The indentation calculations of an Ar atom on the FGr on Cu(111) and Pt(111) illustrate that geometrical corrugation contributes dominantly to the sliding potential energy corrugation. With respect to a reactive 10-atom Ir tip sliding on the FGr on Pt(111), the F atom transfers from graphene to the tip and the friction evolves into a fluorinated Ir tip sliding on the FGr. As a result, the work against the normal load to lift the tip over the geometrical corrugation starts to play a crucial role in contributing to the surface friction. Thus, reducing the geometrical corrugation of graphene after fluorination through a stronger reactive substrate provides a feasible avenue to preserve the lubricating properties of graphene.
ABSTRACT
Triplet excitons can be effectively harvested in organic light-emitting diodes employing thermally activated delayed fluorescence (TADF) molecules as the emitter and host. A design strategy for blue and green emitters with small S1-T1 splitting (ΔEST) is to construct a donor-acceptor (D-A) type molecule with moieties combining a high T1 level with a strong electron-donating/withdrawing character. Here, we report a new kind of TADF emitter with an indolo[2,3-b]indole (IDID) donor. In comparison to other reported indolocarbazole and indoloindole donors, IDID has a higher T1 level, which is comparable to that of the classical donor 9,9-dimethyl-9,10-dihydroacridine (DMAC) for blue TADF emitters. The sky-blue and green TADF emitters based on the IDID donor and a phenyltriazine acceptor exhibit high photoluminescence quantum yields (0.78-0.92) and short TADF lifetimes (1.1-1.7 µs) in doped films. Devices employing these IDID-based emitters offer an external quantum efficiency of 19.2%, which is comparable to that obtained for a device employing an analogous compound with a DMAC donor, while the stability of the former is higher than that of the latter owing to the just-right D-A twisting angles (â¼59°) in the IDID-based emitters leading to a balance between ΔEST and the fluorescence rate. The utilization of host materials with a similar polarity to the emitter is found to be an effective strategy to improve device stability.
ABSTRACT
The inconsistent thermal quenching performance of manganese(II)-doped Cs3Cu2I5 microparticles is exploited in a highly sensitive noninvasive optical thermometer. The ratio of the emissions of Cu(II) and Mn(II) ions in the microparticles is highly temperature dependent in the range from 298 to 498 K. The best absolute and relative sensitivities are 0.547 K-1 and 0.525% K-1, respectively. The emission spectrum, under 300-nm photoexcitation, has emission peaks at 448 and 556 nm. This is the result of energy transfer between the Cu(II) and Mn(II) ions whose efficiency can reach up to 57% when the Mn(II) ion concentration is 2 mol%. The emission color of the microparticles changes from cyan to green when increasing the temperature from 298 to 498 K. Graphical abstract Synthesis of novel Mn(II)-doped Cs3Cu2I5 thermochromic halides with admirable luminescent behaviors for high sensitive ratiometric thermometry and safety sign in high temperature environment.
ABSTRACT
A surface enhanced Raman scattering (SERS) method is described for the determination of vanillin, methyl vanillin and ethyl vanillin at trace levels. Flower-like silver nanoparticles on a silicon wafer are used as the SERS substrate, and the analytes can be specifically and non-destructively recognized by their specific Raman bands. The molecules can be recognized rapidly by identifying the characteristic bands. The SERS spectra of vanillin (C8H8O3) were used as mid-contrast, and specific bands of methyl vanillin and ethyl vanillin (C9H10O3) were acquired at 775 cm-1, 1350 cm-1 and 1282 cm-1, 1382 cm-1, respectively. In addition, by using an improved principal component analysis (PCA) algorithm, the organic molecule can be quantitatively determined. Dissolved in water, vanillin, methyl vanillin and ethyl vanillin still can be detected at a concentration of 10-8 M, at which their characteristic Raman peaks are still visible. The method was successfully applied to the determination of vanillin in milk powder products. Graphical abstract Vanillin can be identified at trace levels by laser irradiation of milk and by using flower-like silver nanoparticles as the surface enhanced Raman scattering (SERS) substrate. Vanillin and its methyl and ethyl derivatives can be quantitatively analyzed by the principal component analysis (PCA) algorithm.
