Solution-processed WOx hole injection layer for efficient fluorescent blue organic light-emitting diode

Solution-processed tungsten oxide (s-WO_x) interfacial layer for efficient hole injection in fluorescent blue organic light-emitting diode (OLED) is demonstrated. The OLED using 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) as emitter shows luminous efficiency of 3.3 cd/A, power efficiency of 2.5 lm/W and external quantum efficiency of 4.6% with Commission Internationale d'Eclairage (CIE) color coordinates of (0.154, 0.102). Using MADN doped 1-4-di-[4-(N,N-diphenyl)amino]styryl-benzene as emitter, luminous efficiency of 10.8 cd/A, power efficiency of 6.4 lm/W and external quantum efficiency of 7.2% with CIE color coordinates of (0.167, 0.283) are achieved. Atomic force microscopy and X-ray photoelectron spectroscopy show that s-WO_x features superior film morphology and non-stoichiometry with slight oxygen deficiency. Current-voltage characteristics and impedance spectroscopy analysis indicate that s-WO_x behaves slightly enhanced hole injection and accordingly contributes to improved device performance in comparison with conventional vacuum thermal evaporation WO_x. Our results pave an alternative way for broadening WO_x application with solution process and advancing fluorescent blue OLEDs.     

Photochemical Hole Burning: A Spectroscopic Study of Relaxation Processes in Polymers and Glasses

Photochemical hole burning is a special type of saturation spectroscopy in the optical domain having many analogies with NMR methods. The holes, which are burnt with laser irradiation, appear as small indentations in the absorption spectra of dye molecules which are doped into a polymer or glass in minute concentrations. Based on their narrow line width, photochemical holes can be regarded as highly sensitive spectroscopic probes. They can be used to detect small perturbations of the system by external parameters, giving rise to line-shifts and broadenings. Besides the many well documented, spectroscopic applications of hole burning, it may offer interesting future developments for the spectroscopy of biomolecules and for high-density data storage.     

Surface finish of open holes on fatigue life

In this paper, the effect of surface finish of open holes on the fatigue life has been studied. Four defects of the surface finish are simulated. They are scratch, void, inclusion and roundness. Firstly, the effect of the four defects on the stress distributions around the holes has been studied by the finite element method (FEM). The fatigue lives are determined based on the stress distributions by the method of nominal stress approach. The results show that the fatigue lives are dependent on the quantity of the surface finished. There are the critical defect values of scratch, void and inclusion, smaller than which there is no effect of the surface finish on the fatigue life. For these three defects, the fatigue lives decrease with the increasing of the values of the defects. It is the same to the defect of roundness, e.g. the bigger roundness tolerance is, the shorter the life is. Further, an approximate quadratic curve has been found for the relationship between the roundness tolerances and their logarithmic fatigue lives.     

Enhanced chemiluminescence from reactions between CdTe/CdS/ZnS quantum dots and periodate

A novel chemiluminescence (CL) performance of CdTe/CdS/ZnS quantum dots (QDs) with periodate (KIO₄) was studied. Effects of concentration and pH on the CL system were investigated. Electron spin resonance (ESR) and the effects of radical scavenger analysis were employed for identification of intermediate species. The CL spectra for this system showed only one maximum emission peak centered around 620 nm, which was similar with photoluminescence (PL) spectra of CdTe/CdS/ZnS QDs. The CL of CdTe/CdS/ZnS QDs was induced by direct chemical oxidation and the possible mechanism could be explained by radiative recombination of injected holes and electrons. This investigation not only provided new sight into the optical characteristics of CdTe/CdS/ZnS QDs, but also broadened their potential optical utilizations.     

The dependence of interfacial properties on the layer number in 1T′/2H-MoS2 van der Waals heterostructures

Metallic 1T′-MoS₂ monolayers are predicted to be efficient hole injection contacts for nanoelectronic devices composed of 2H-MoS₂ monolayers. The layer number can affect the physical properties of two-dimensional materials. In this paper, the dependence of the interfacial properties of 1T′/2H-MoS₂ van der Waals (vdW) heterojunctions on the layer number was studied using density functional theory. The calculation results show that 1T′-MoS₂ forms p-type contacts with 2H-MoS₂, and the p-type Schottky barrier height (SBH) is between -75 and 30 meV and depends on the number of 1T′-/2H-MoS₂ layers. Thus, the efficiency of 1T′-MoS₂ as a hole injection contact for 2H-MoS₂ can be tuned by the layer number. This work provides a method for tuning the contact-resistance in nanodevices composed of 2H-MoS₂.     

Recent advances in the design of dopant-free hole transporting materials for highly efficient perovskite solar cells

This review gives an overview of the latest advances in dopant-free hole transporting materials (HTMs) for perovskite solar cells and discusses the new molecular design strategies towards efficient and stable dopant-free HTMs.     

The competition number of a graph and the dimension of its hole space

The competition graph of a digraph D is a (simple undirected) graph which has the same vertex set as D and has an edge between x and y if and only if there exists a vertex v in D such that (x,v) and (y,v) are arcs of D. For any graph G, G together with sufficiently many isolated vertices is the competition graph of some acyclic digraph. The competition number k(G) of G is the smallest number of such isolated vertices. In general, it is hard to compute the competition number k(G) for a graph G and it has been one of the important research problems in the study of competition graphs to characterize a graph by its competition number. Recently, the relationship between the competition number and the number of holes of a graph has been studied. A hole of a graph is a cycle of length at least 4 as an induced subgraph. In this paper, we conjecture that the dimension of the hole space of a graph is not smaller than the competition number of the graph. We verify this conjecture for various kinds of graphs and show that our conjectured inequality is indeed an equality for connected triangle-free graphs.     

Relationship between the ionic and electronic partial conductivities of co-doped LSGM ceramics from oxygen partial pressure dependence of the total conductivity

La_0.8Sr_0.2Ga_0.85-x Mg_0.15Co _x O_3±δ-materials (further cobalt-doped LSGM), where x varied from 0 to 0.20, were synthesized by means of the conventional powder route. The total conductivity of the La_0.8Sr_0.2Ga_0.85-x Mg_0.15Co _x O_3±δ samples was measured as a function of temperature (400–900 °C) and oxygen partial pressure by means of the impedance technique. The values of the oxygen ionic and the hole conductivities were determined from non-linear regression of the oxygen partial pressure dependence of the total conductivity. It was shown that the substitution of gallium by cobalt in the LSGM results in increasing either the oxygen ionic or the hole conductivity, although the increase of the hole conductivity due to the doping by cobalt is more significant than the increase of the oxygen ionic conductivity. The hole conductivity of the selected compositions was studied by oxygen permeation- and Hebb–Wagner-polarization measurements.     

Analysis of residual stress profiles in plastic materials using the hole drilling method – Influence factors and practical aspects

The current work introduces a procedure to locally measure residual stresses in polymers. For this purpose, the hole drilling method was adapted to the prevailing boundary conditions. Particular attention is given to the analysis of the measured strain signal which is time dependent and strongly influenced by local temperature fluctuations. The different sources of heat prevailing during the measurement and their impact are outlined. By thorough analysis of the viscoelastic and thermal deformation occurring during and after drilling the material, a procedure is proposed to measure the correct strain relaxation from which residual stresses can be calculated. Using quenched polycarbonate samples with known residual stress profiles, the potential of the proposed approach was demonstrated and is discussed.     

Synthesis and properties of new luminescent hole transporting materials of triarylamine with dehydroabietic acid methyl ester moieties

A series of triarylamines based on dehydroabietic acid methyl ester moieties (6a–h) were synthesized for possible application as hole transporting materials for organic electroluminescent devices. The target compounds were characterized by elemental analysis, FT-IR, NMR, and mass spectrometry. Their optical, electrochemical, and thermal properties were investigated using UV–vis, PL spectroscopy, cyclic voltammetry (CV), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC), respectively. CV measurements show that the compounds present suitable HOMO values (in a range of −4.63 to −5.11 eV) for hole injection, which is confirmed by theoretical calculations. All compounds were thermally stable. Organic light-emitting diode devices having 6f, 6g, and 6h as a hole transporting layer showed better performance of maximum brightness, turn-on voltage, and maximum luminous efficiency than a comparable device NPB. These compounds could be excellent candidates for applications in OLED devices.