Jiwen Xu

Elucidation of carrier injection and recombination characteristics with impedance and capacitance in organic light-emitting diodes and the frequency effects

Impedance (Z), phase (φ) and capacitance (C) versus bias voltage (V) characteristics are studied to clarify carrier injection and recombination characteristics in organic light-emitting diodes (OLEDs). The Z–V transition starts at a characteristic voltage (Vc), which is strongly frequency dependent, i.e. Vc shifts to a high voltage with increasing measuring frequency. The electron–hole recombination starts at a voltage above Vc revealed by the φ–V and C–V transitions, which correspond to a phase approaching 0, a sharp rise in current density and a decrease in capacitance. Hole injection starts at a low Vc and corresponds to charge carrier accumulation and a slight rise in capacitance. Cole–Cole impedance plots illustrate that the interfacial resistance corresponds to the impedance at ultrahigh frequencies and shows bias independence, while the impedance at low frequencies represents the sum of interfacial resistance and organic stacks, and exhibits considerable bias dependence.

Sintering behavior and refining grains of high density tin doped indium oxide targets with low tin oxide content

High density indium tin oxide (ITO) ceramic targets with low SnO2 content were prepared successfully by sintering co-precipitationally synthesized powders. The sintering behavior, properties and refining grains of the ITO targets were studied in normal pressure oxygen ambience. Higher sintering temperature promoted sintering densification, resulted in abnormal grain growth and decreased bending strength. By a two-step sintering method, the uniform and fine-grained microstructures were obtained. The sintered density of the ITO targets was further improved. Furthermore, the grain size was reduced, and the bending strength was also enhanced.

Microstructures and microwave dielectric properties of (Ba 1− x Sr x ) 4 (Sm 0.4 Nd 0.6 ) 28/3 Ti 18 O 54 solid solutions

Abstract(Ba1−xSrx)4(Sm0.4Nd0.6)28/3Ti18O54 (x = 0.02, 0.04, 0.06, 0.08, 0.1) solid solutions were prepared by the conventional solid-state reaction process. It was found that (Ba1−xSrx)4(Sm0.4Nd0.6)28/3Ti18O54 ceramics are fully composed of BaSm2Ti4O12 and BaNd2Ti5O14 phases for all the compositions. The increasing x value (0.02 ≤ x ≤ 0.1) in ((Ba1−xSrx)4(Sm0.4Nd0.6)28/3Ti18O54 ceramics can not only obtain high Q × f value but also effectively enhance the permittivity (εr). The (Ba1−xSrx)4(Sm0.4Nd0.6)28/3Ti18O54 ceramic with x = 0.08, sintered at 1440 °C for 4 h, shows excellent microwave dielectric properties of permittivity (εr) ≈ 93.19, quality factor (Q × f) ≈ 9770.14 GHz (at 3.415 GHz), and almost near-zero temperature coefficient of resonant frequency (τf) ≈ +4.56 ppm/°C.

Tunable hole injection of solution-processed polymeric carbon nitride towards efficient organic light-emitting diode

Polymeric carbon nitride (CNxHy) has been facilely synthesized from dicyandiamide and functions as a solution-processed hole injection layer in organic light-emitting diodes (OLEDs). The measurements using X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and impedance spectroscopy elucidate that CNxHy exhibits superior film morphology and extra electric properties such as tailored work function and tunable hole injection. The luminous efficiency of CNxHy-based OLED is found to improve by 76.6% in comparison to the counterpart using favorite solution-processed poly(ethylene dioxythiophene):poly(styrene sulfonate) as the hole injection layer. Our results also pave a way for broadening carbon nitride applications in organic electronics using the solution process.

Structure and properties of (1−x)[(K0.5Na0.5)NbO3–LiSbO3]– xBiFe0.8Co0.2O3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1 − x)[0.95(K0.5Na0.5)NbO3–0.05LiSbO3]–xBiFe0.8Co0.2O3(KNN–LS–xBFC) were prepared by a conventional sintering technique. The effect of BFC content on the structure, piezoelectric and electrical properties of KNN–LS ceramics was investigated. The results reveal that the BFC is effective in promoting the sinterability and the electrical properties of the ceramics sintering at low temperature of 1030°C. The ceramics show a single perovskite structure, in which the tetragonal phase decreases while the orthorhombic phase increases with the increase of x. The more the BFC content is, the smaller and homogeneous grains were formed. With the increase of x, the d33 and the kp increase to a maximum value and then slightly decrease, but the Qm increases continuously. As BFC content increases, the Curie temperature Tc and remnant polarization Pr decrease, but the diffusivity of phase transition in KNN–LS ceramics will intensify and the coercive field Ec fluctuate between 1.16 and 1.51 kV mm −1. The samples with x = 0.004 exhibit optimum electrical properties at room temperature (d33 =268 pC N−1, kp = 52%, εr = 1366, tan δ = 2.11%, Tc = 325°C, Pr = 20.4 μC cm−2, Ec = 1.16 kV mm−1).

Resistance switching properties of Ag/ZnMn2O4/p-Si fabricated by magnetron sputtering for resistance random access memory

A resistance random access memory (RRAM) with a structure of Ag/ZnMn2O4/p-Si was fabricated by magnetron sputtering method. Reliable and repeated switching of the resistance of ZnMn2O4 films was obtained between two well-defined states of high and low resistance with a narrow dispersion and 3V switching voltages. Resistance ratio of the high resistance state and low resistance state was found in the range of around 103 orders of magnitude and up to about 103 test cycles. The retention time of Ag/ZnMn2O4/ p-Si device is longer than 106 seconds and the resistance ratio between two states remains higher than 103 at room temperature, showing a remarkable reliability performance of the RRAM devices for nonvolatile memory application. The equivalent simulation circuits for HRS (high resistance state) and LRS (low resistance state) were also studied by impedance spectroscopy.

Effects of Sintering Temperature on Structure and Properties of 0.997(KNN-LS-BF)-0.003V2O5 Lead-Free Piezoelectric Ceramics

Abstract0.997(KNN-LS-BF)-0.003V2O5 lead-free piezoelectric ceramics were prepared by a traditional sintering method. The effects of sintering temperature on the structure and properties of the 0.997(KNN-LS-BF)-0.003V2O5 ceramics were studied. The results show that the sintering temperature exerts a distinct influence on the phase structure and properties. With the increase in sintering temperature from 1040°C to 1060°C, the main crystallographic phase changes from the orthorhombic symmetry to the tetragonal phase, and the optimum dielectric and piezoelectric properties of samples can be obtained when sintering at 1060°C. However, the dielectric and piezoelectric properties of the samples deteriorate when the sintering temperature exceeds 1060°C.

Remarkable improvement of ferroelectric properties and leakage current in BiFeO3 thin films by nd modification

Ferroelectric and leakage properties are important for ferroelectric applications. Pure and Nd-doped (x=0.05-0.20) BiFeO3 thin films were fabricated by sol-gel method on FTO substrates. The phase structure, surface morphology, leakage current, ferroelectric properties, and optical properties of BiFeO3- based thin films were investigated. The substitution of Nd3+ ions for the Bi3+ site converts the structure from rhombohedral to coexisting tetragonal and orthorhombic. Nd doping improves the crystallinity of BiFeO3 thin films. The leakage current of Nd-doped BiFeO3 decreases by two to three orders of magnitude compared with that of pure BiFeO3. Among the samples, 15% Nd-doped BiFeO3 exhibits the strongest ferroelectric polarization of 17.96 μC/cm2. Furthermore, the absorption edges of Bi1-xNdxFeO3 thin films show a slight red-shift after Nd doping.

A new insight into structural complexity in ferroelectric ceramics

The structure of the ferroelectrics has been widely studied in order to pursuing the origin of high electromechanical responses. However, some experiments on structure of ferroelectrics have yielded different results. Here, we report that the controversial phase structure is due to the adaptive diffraction of nanodomains which hides the natural crystal structure, and the electric-field-induced phase transition is that the natural crystal structure reappears due to the coalescent nanodomains or ordering nanodomains by applying a high electric field. The temperature dependence of dielectric constant with different measurement frequencies and X-ray diffraction (XRD) patterns of unpoled, poled, and annealing after poled ceramics in Bi0.5Na0.5TiO3–BaTiO3 (BNT–BT) ceramics authenticate the statement. These results provide a new insight into the origin of structural complexity in ferroelectric ceramics, which is related to the key role of nanodomains.

Electrical properties of lead-free thick film NTC thermistors based on BaFe 0.9 Sn 0.1 O 3 ceramic

Lead free thick film NTC (negative temperature coefficient) thermistors based on perovskite-type semiconducting materials of different compositions BaFe<inf>0.9</inf>Sn<inf>0.1</inf>O<inf>3</inf> + BaBiO<inf>3</inf>, and BaFe<inf>0.9</inf>Sn<inf>0.1</inf>O<inf>3</inf> + BaBiO<inf>3</inf> + CuO were fabricated on alumina substrate by screen printing technology. The evolution of microstructure, electrical performance of these resistors and the influence of thickness of thermistor layer on the thermistor constant and initial resistivity were studied. The thick film samples adhere very well to alumina substrate and show homogeneous microstructure. The electrical resistivity at 22 °C (ρ<inf>22</inf>), thermistor constant (B<inf>25/85</inf>), and activation energy (E<inf>a</inf>) of the NTC thermistors decreases with increasing CuO content. At the almost same thickness, the values of ρ<inf>22</inf>, B<inf>25/85</inf>, and E<inf>a</inf> of the thick film NTC thermistors are 3.197–19.25 kΩ·cm, 3318–4322 K, 0.289–0.376 eV, respectively. As thickness of the films increases, the thermistor constant and the initial resistivity of the thick film NTC thermistors decrease.