Conductivity Of Zinc Oxide Research Articles

Improvement of the performance of dye sensitized solar cell using ZnO nanorods array deposited with Ag nanoparticles as photoanode

Well-aligned zinc oxide (ZnO) nanorods (NRs) arrays deposited with Ag nanoparticles (NPs) are prepared by a liquid phase epitaxial growth process followed by a reduction of Ag on the surface of the ZnO NRs. Transmission electron microscopy images show that most Ag NPs are deposited on the upper part of the ZnO NRs, and the overall optical absorption in the range of visible light can be enhanced due to the surface plasmon resonance of the Ag NPs. ZnO NRs with and without Ag NPs are used to assemble dye sensitized solar cells. Devices fabricated from the Ag NPs/ZnO NRs composite arrays exhibit a higher open voltage, short circuit current and fill factor than that fabricated from the bare ZnO NRs array, thus, the overall efficiency of the as-fabricated cell is increased from less than 0.5 % to 0.8 %. The main reason for the enhancement of the device performance may be ascribed to that the electron transfer back from ZnO to the dye and electrolyte is blocked by the Schottky barrier at the Ag/ZnO interface, resulting in a great increase of the electron density at the ZnO conduction band.

An efficient inverted organic solar cell with improved ZnO and gold contact layers

This paper presents a high efficiency (∼3.8%) inverted organic photovoltaic devices based on a P3HT:PCBM bulk heterojunction (BHJ) blend with improved electron- and hole-selective contact layers. Zinc oxide (ZnO) nanoparticle films with different thicknesses are deposited on the transparent electrodes as a nano-porous electron-selective contact layer. A thin gold film is used between the BHJ photoactive layer and the poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), which improves the wettability and significantly enhances the stability of the device (>50days of air exposure). Photovoltaic device parameters such as power conversion efficiency (PCE) and external quantum efficiency (EQE) are systematically examined for inverted devices with different thicknesses of ZnO and gold layers in comparison to the non-inverted and reference inverted devices with no contact layers. The optimized organic devices with ZnO and Au contact layers show exceptional short circuit currents (in excess of 13mA/cm2), in comparison to the reference devices, which is related to increased quantum efficiency of the device observed in measured EQE experiments. These results are important for development of high efficiency and stable all-printed organic solar cells and point out the role of contact layers, in particular, ZnO conductivity and morphology in the device performance.

Structural and electrical properties of V-doped ZnO prepared by the solid state reaction

This paper reports the synthesis, crystal structure and electrical conductivity properties of vanadium (V)-doped zinc oxide (ZnO) powders (i.e. Zn1−2X V X O binary system, x = 0, 0.0025, 0.005, 0.0075 and in the range 0.01 ≤ x ≤ 0.15). I-phase samples, which were indexed as single phase with a hexagonal (wurtzite) structure in the V-doped ZnO binary system, were determined by X-ray diffraction (XRD). The limit solubility of V in the ZnO lattice at this temperature is 3 mol % at 950 °C. The impurity phase at 950 °C was determined as ZnV2O6 when compared with standart XRD data. The research focused on single I-phase ZnO samples which were synthesized at 950 °C because of the limit of the solubility range is widest at this temperature. It was observed that the lattice parameters a and c decreased with V doping concentration. The electrical conductivity of the pure ZnO and single I-phase samples were studied using the four-point probe dc method at temperatures between 100 and 950 °C in an air atmosphere. The electrical conductivity values of pure ZnO and 3 mol % V-doped ZnO samples at 100 °C were 2.75 × 10−6 and 7.94 × 10−5 Ω−1 cm−1, and at 950 °C they were 3.4 and 54.95 Ω−1 cm−1, respectively. In other words, the electrical conductivity increased with V doping concentration.

Hall effect and conductivity in zinc oxide (ZnO) doped by thermal diffusion of indium and copper

AbstractMeasurements of the conductivity and the Hall effect of single crystals were performed on zinc oxide (ZnO) samples after a thermal diffusion of indium (In) and copper (Cu) with different time periods. Single crystals of ZnO were growth by hydrothermal method and annealed previously to diffusion process. Diffusion was carried out in ZnO at 1000 ΩC in N2 atmosphere for 3, 6, and 15 h. Hall effect measurements were carried out using the Van der Pauw configuration with a current of 1 mA and a magnetic field of 0.37 T at room temperature. Results showed an increase in donor concentration from 4.6x1014 cm–3 to 1.15×1017 cm–3 when In is diffused and up to 9.9×1015 cm–3 when Cu is introduced in ZnO. Mobility decreased from 134.9 (undoped ZnO) to 44.7 cm2/V s, in In‐doped, and 101.9 cm2/V s, in Cu‐doped. Resistivity decreased with time from 1.29x102 Ω cm to 1.26 Ω cm and 6.25 Ω cm, in the same order as described. Based on measurements, introduction of indium is faster than copper and the higher concentration of donors is result of substitution of Zn by an ion with a different valence. This is a relative simple method to control the conductivity and donor concentration in single crystals of ZnO. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Simulation of pure and defective wurtzite-type ZnO

Changes in the structural and electronic properties of zinc oxide (ZnO) due to the O vacancy and F-centre were studied using a semi-empirical quantum-chemical approach based on Hartree–Fock theory. A periodic supercell of 128 atoms was exploited throughout the study. The semi-empirical parameters for the Zn atom are obtained by reproducing the main properties of the ZnO crystal as well as the first three ionization potentials of the Zn atom. The perturbation imposed by the defect leads to atomic relaxation, which is computed and discussed in detail. It is found that electron density redistribution in the vicinity of defects plays an important role in the determination of atomic movements. The introduction of an oxygen vacancy generates a local one-electron energy level placed below the conduction band while the presence of an F-centre produces a local energy level just above the upper valence band of the material. The deep situation of the local energy level corresponding to the F-centre implies that the F-centre cannot serve as a source of unintentional n-type electrical conductivity in ZnO. Changes in the chemical bonding are observed, showing that it becomes slightly more covalent because of oxygen-vacancy-type defects.

Vertically Aligned ZnO Nanorod Arrays Sentisized with Gold Nanoparticles for Schottky Barrier Photovoltaic Cells

Vertically aligned zinc oxide (ZnO) nanorod arrays coated with gold nanoparticles have been used in Schottky barrier solar cells. The nanoparticles enhance the optical absorption in the range of visible light due to the surface plasmon resonance. In charge separations, photoexcited electrons are transferred from gold nanoparticles to the ZnO conduction band while electrons from donor (I−) in the electrolyte compensate the holes left on the gold nanoparticles. The fill factors of the dye-free Schottky barrier cell reach a value of ∼0.50. However, after incorporation of N719 sensitizing dye, the open circuit voltage increases to 0.63 V from 0.5 V being measured for dye-sensitized solar cells based on the bare ZnO nanorods. The Schottky barrier at the ZnO/Au interface blocks the electron transfer back from ZnO to the dye and electrolyte, and thus increases the electron density at the ZnO conduction band. The efficiency of the gold-coated ZnO nanorod dye-sensitized solar cells is thus increased from 0.7% to 1.2%.

Nanoscale ZnO and Al‐Doped ZnO Coatings on ZnS:Ag Phosphors and their Cathodoluminescent Properties

Blue‐emitting ZnS:Ag phosphors were coated with nanoscale zinc oxide (ZnO) and Al‐doped ZnO (AZO) and the cathodoluminescence (CL) performance of the coated phosphors was investigated. The nanoscale coating was obtained by controlling the hydrolysis reaction of ZnCl2 alcoholic solution using diethylamine as a gradual OH− former, and Al doping was performed to increase the electrical conductivity of ZnO. The coatings were composed of either nanosized particles or a continuous nano layer, and the AZO‐coated phosphor had a more uniformly covered surface. The coated phosphors exhibited improved aging behavior under CL excitation and the AZO coating was more effective at suppressing the degradation, possibly due to its uniformity and high conductivity.

Unambiguous identification of nitrogen-hydrogen complexes in ZnO

Zinc oxide (ZnO) is a wide-band-gap semiconductor with a range of potential applications in optoelectronics. The lack of reliable $p$-type doping, however, has prevented it from competing with other semiconductors such as GaN. In this Brief Report, we report the successful incorporation of nitrogen-hydrogen (N-H) complexes in ZnO during chemical vapor transport growth, using ammonia as an ambient. The N-H bond-stretching mode gives rise to an infrared absorption peak at $3150.6\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Substitutions of deuterium for hydrogen and $^{15}\mathrm{N}$ for $^{14}\mathrm{N}$ result in the expected frequency shifts, thereby providing an unambiguous identification of these complexes. The N-H complexes are stable up to $\ensuremath{\sim}700\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. The introduction of neutral N-H complexes could prove useful in achieving reliable $p$-type conductivity in ZnO.

Hydrogen complexes in ZnO grown by chemical vapor transport

The renewed interest in zinc oxide (ZnO) has provided a great number of details that could lead to a commercially viable alternative to gallium nitride (GaN) for use in high-efficiency, wide band-gap devices. The difficulty in obtaining such devices is that as-grown, large single crystals of ZnO are n type. Past research has shown that hydrogen plays an important role in the n-type conductivity of ZnO. However, other impurities cannot be eliminated as possible donors in as-grown ZnO. In an attempt to study the controlled doping of ZnO, we grew polycrystalline samples via the chemical vapor transport (CVT) method, using graphite as the transport agent. After diffusing hydrogen into these crystals, some clearly showed both the OH-related infrared (IR) absorption peak at 3327 cm −1, as reported in our earlier publications, and the 3611 cm −1 absorption peak reported by Lavrov [Physica B 340–342 (2003) 195]. Additionally, a third hydrogen-related absorption peak appears at 3150 cm −1. Suspecting a contaminant in our original source of graphite, we changed to a high-purity source of graphite. The resulting growth yielded no hydrogen-related absorption peaks after hydrogen diffusion into the sample. However, growing samples in ammonia recreated the 3150 cm −1 IR absorption peak.

Sources of n-type conductivity in ZnO

Zinc oxide (ZnO) is a strong candidate for energy-efficient white lighting and numerous optoelectronic applications. Hydrogen impurities play important roles, good and bad, in the pursuit of reliable p-type doping of ZnO. In pervious work, we identified hydrogen donors with the back-bonded or “anti-bonding” orientation, with an angle of 111° to the c-axis. It is possible, however, that these hydrogen donors are complexed with another defect. Impurities besides hydrogen are also donors in as-grown ZnO. Results from secondary ion mass spectroscopy (SIMS) show significant concentrations of Al in samples of bulk single-crystal ZnO obtained from Cermet, Inc., Ga and B in samples from Eagle-Picher, and Si in both.

Zno-based thin films synthesized by atmospheric pressure mist chemical vapor deposition

An atmospheric pressure mist chemical vapor deposition (mist-CVD) system has been developed to prepare zinc oxide (ZnO)-based thin films. This is a promising method for large-area deposition at low temperatures taking into account of its simplicity, inexpensiveness, and safety. Nominally pure ZnO, Al-doped n-type ZnO (ZnO:Al), and N-doped p-type ZnO (ZnO:N) thin films, as well as Zn 1− x Cd x O and Zn 1− y Mg y O alloy films, have been deposited by this mist-CVD system. The films deposited at the temperatures ranging from 400 to 500 °C were of an acceptable crystallinity with (0 0 2) preferential orientation and homogeneous surface. All the films exhibited high transmittance of about 90% in visible regions and dominant UV emission in the photoluminescence spectra. The n-type ZnO:Al films had a low resistivity of 1.08×10 –3 Ω cm at an optimal Al content of 4 at%. The p-type conductivity was obtained in ZnO:N films annealed at higher temperatures with a resistivity of 72.8 Ω cm, Hall mobility of 2.28 cm 2 V −1 s −1, and hole concentration of 3.76×10 16 cm −3, as confirmed by Hall-effect measurements. A hydrogen-assisted nitrogen-doping mechanism was proposed to answer for the realization of p-type conductivity in ZnO. The films of Zn 1− x Cd x O and Zn 1− y Mg y O ternary alloys were also deposited by this technique. The band gap energies, for instance, were 3.05 eV for Zn 1− x Cd x O ( x=0.06), 3.28 eV for ZnO, and 3.56 eV for Zn 1− y Mg y O ( y=0.11), as confirmed by the optical absorption spectra. The band gap engineering could be readily realized in the ZnO system using mist-CVD.

Optical and electrical properties of radical beam gettering epitaxy grown n- and p-type ZnO single crystals

A method has been developed for obtaining single crystal layers of ZnO on ZnS and ZnO single crystal substrates by means of heat treating the crystals in an activated oxygen atmosphere. With this method, an inversion of the type of conductivity in zinc oxide has been accomplished. The fundamental luminescence properties of these layers are investigated. In addition to A-excitons, emission bands corresponding to B- and C-excitons have been registered in the exciton regions of the photoluminescence spectra. It was found that the violet band at 400 nm ascribed to an association of the doubly-charged oxygen vacancy and the singly-charged zinc vacancy is the dominant one in the photoluminescence spectra of ZnO layers with p-type conductivity. In ZnO layers with n-type conductivity, the fundamental visible luminescence bands — green and yellow-orange — are generated by oxygen vacancies and alkali metals, respectively. Hole mobility is determined from Hall effect measurements.

Thermal conductivity of two-phase system ZnO—graphite and MgO—graphite

Thermal conductivity of a two-phase (solid—solid) powder system was measured using a semi-absolute static method. Measurements were made on dish-form samples having fixed porosity. Zinc oxide (ZnO) and magnesium oxide (MgO) powders were chosen to be the continuous solid phase. Each was mixed with different concentrations of graphite (the discontinuous phase). The experimental results have been compared with the available theoretical models for estimating either an effective thermal conductivity of such a system or lower and upper bounds of this property.

High-field miceowave incremental conductivity of zinc-oxide

High-field miceowave incremental conductivity of zinc-oxide

Luminescent transitions and conductivity in zinc oxide

Thermally and IR-stimulated luminescence and conductivity of Cu and Li doped ZnO single crystals have been investigated in the temperature range from 5 to 100 K. It is shown that the stimulation of the green emission is related to the generation of free charge carriers, while the yellow emission is produced mainly by donor–acceptor recombinations. Thermisch und IR-stimulierte Lumineszenz und Leitfähigkeit von Cu and Li dotierten ZnO-Einkristallen wurden im Temperaturbereich zwischen 5 und 100 K gemessen. Es wird gezeigt, daß die Stimulation der grünen Emission mit der Erzeugung von freien Ladungsträgern verbunden ist, während die gelbe Emission hauptsächlich von Donator–Akzeptor-Rekombinationen hervorgerufen wird.

Electrical conductivity of zinc oxide in atomic nitrogen

Electrical conductivity of zinc oxide in atomic nitrogen

The effect of variations in the concentration of atmospheric constituents on the electrical conductivity of zinc oxide

The electrical conductivity of zinc oxide powders exhibits diurnal variations of up to two orders of magnitude when exposed to the atmosphere. Fifteen atmospheric variables were investigated as possible causes of these variations. However, only water vapour, ozone and atmospheric ions induced either dark or photoconductivity changes in zinc oxide. The effects of these constituents were determined by recording their atmospheric concentrations concurrently with the conductivity of zinc oxide photocells and calculating the degree of correlation between the variables. A highly significant correlation was found between zinc oxide conductivity and atmospheric ion concentration. A similar correlation was found between photoconductivity and humidity. However, the humidity varied over a narrow range and its effect, therefore, was relatively small. It is suggested that the variations in conductivity arise primarily from the reversible adsorption of negative ions which promotes a field effect on the mobility of the carriers in zinc oxide.

The effect of variations in the concentration of atmospheric constituents on the electrical conductivity of zinc oxide

The effect of variations in the concentration of atmospheric constituents on the electrical conductivity of zinc oxide

Effect of adsorbed oxygen on changes in zinc oxide conductivity with temperature

Polarization measurements on specially prepared porous nickel electrodes were carried out in a half set-up to determine the intrinsic parameters (exchange current density and charge transfer coefficient) using the spherical grain model developed earlier. The values found for exchange current densities were (6 × 10−6 to 2.8 × 10−4 A cm−2) higher than that reported in the literature which shows the superiority of this electrode. Exchange current density followed an exponential relation with temperature. Apparent activation energies were found to decrease with increasing overpotential and the activation energy corresponding to zero overpotential by extrapolation was found to be 31.4 kJ mol−1.

The Electrophotographic Properties of Zinc Oxide

AbstractMeasurements on the light dependence of the conductivity of zinc oxide in various gases have provided new information about the important role of ionised donors as charge carriers, in addition to free electrons. These results enable one to understand the mechanism of the corona charging process and the flow of charge from the surface to the grounded electrode during exposure.