Work Function Of Cu Research Articles

Solution‐Processed Copper‐Doped Chromium Oxide with Tunable Oxygen Vacancy for Crystalline Silicon Solar Cells Hole‐Selective Contacts

Compared with vacuum evaporation, metal oxides deposited by solution process exhibit advantages, for example, dopants can be easily incorporated in a wide range and the contents are easy to be regulated by changing the composition of precursors. Herein, solution‐processed p‐type copper‐doped chromium oxide (Cu:CrOx) films with tunable oxygen vacancy are demonstrated via a postannealing process. The synthetic and postannealing temperature have a significant impact on the chromium cation oxidation state in the Cu:CrOx films, leading to the variation of work function for the oxide films. By adjusting the work function of Cu:CrOx films, a low contact resistivity of 95 mΩ cm2 can be realized for the Ag/Cu:CrOx/p‐Si contact. Finally, the optimized Cu:CrOx films are used as the hole transporting layer in c‐Si solar cells, showing an increased power conversion efficiency from 15.2% (without Cu:CrOx) to 16.9% (with Cu:CrOx). The results show an effective stage to use p‐type metal oxides as a hole‐selective layer in c‐Si solar cells.

Abnormal behavior of threshold voltage shift in bias-stressed a-Si:H thin film transistor under extremely high intensity illumination.

We report on the unusual behavior of threshold voltage turnaround in a hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) when biased under extremely high intensity illumination. The threshold voltage shift changes from negative to positive gate bias direction after ∼30 min of bias stress even when the negative gate bias stress is applied under high intensity illumination (>400 000 Cd/cm(2)), which has not been observed in low intensity (∼6000 Cd/cm(2)). This behavior is more pronounced in a low work function gate metal structure (Al: 4.1-4.3 eV), compared to the high work function of Cu (4.5-5.1 eV). Also this is mainly observed in shorter wavelength of high photon energy illumination. However, this behavior is effectively prohibited by embedding the high energy band gap (∼8.6 eV) of SiOx in the gate insulator layer. These imply that this behavior could be originated from the injection of electrons from gate electrode, transported and trapped in the electron trap sites of the SiNx/a-Si:H interface, which causes the shift of threshold voltage toward positive gate bias direction. The results reported here can be applicable to the large-sized outdoor displays which are usually exposed to the extremely high intensity illumination.

Effects of secondary β and γ phases on the work function properties of Cu–Be alloys

The work functions of Cu–2.7Be and Cu–2.0Be (wt%) alloys were measured using ultraviolet photoelectron spectroscopy, and their dependence on secondary phase characteristics (composition and distribution) was investigated. The work function properties were significantly altered when the Be content was increased from 2.0 to 2.7 wt%, which was attributed to the introduction of secondary phases (β and γ) in the alloys with higher Be content. Furthermore, the distribution of secondary phases may also have influenced the gradients of the work function curves measured by UPS.

High-performance inverted polymer solar cells based on thin copper film

We report the fabrication of cost-effective indium-free polymer solar cells (PSCs) with an inverted structure that incorporates an ultrathin copper (Cu) film as a bottom cathode via thermal evaporation. The average optical transmittance of the 15-nm Cu coated glass substrate in the visible region of the spectrum was found to be around 80% with a highest value of 84.5%. The Cu electrode was modified by an interfacial layer of an alcohol-/water-soluble conjugated polymer, poly[(9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) interlayer to ensure a very smooth surface. Upon the use of the PFN interfacial layer, the work function of Cu was decreased from 4.68 to 4.31 eV, which can form an Ohmic contact with photoactive layer and facilitate electrode transport and extraction. As a result, a power conversion efficiency of 3.6% was achieved when poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] and a [6,6]-phenyl C71-butyric acid methyl ester blend were utilized as the photoactive layers, demonstrating that the thermally evaporated Cu thin-film electrode can be a promising candidate to replace indium tin oxide for highly efficient PSCs.

Facile modification of Cu source–drain (S/D) electrodes for high-performance, low-voltage n-channel organic thin film transistors (OTFTs) based on C60

Exploring suitable electrode materials with sufficiently low work function, ambient stability and low-cost is of great technological importance to the development of n-channel OTFTs. Here, we show that the work function of Cu can be effectively reduced from 4.65eV to 4.28eV through surface modification via simply spin-coating a thin layer of branched polyethylenimine (PEI). By exploiting a high-capacitance density gate dielectric (200nF/cm2), low-voltage (3V) C60 TFTs with electron mobility (μe) of 3.2cm2/Vs are demonstrated with PEI modified Cu as source–drain (S/D) electrodes. In contrast, the device with Cu S/D electrodes possesses μe of only 1.0cm2/Vs. The improvement in electrical performance of the PEI modified device is attributed to the efficient electron injection at the Cu/C60 interface which resulted from the reduction in work function of Cu. Moreover, upon PEI modification, the bias stability of the device can be obviously enhanced as compared to the unmodified one, and the resultant device exhibits an excellent thermal stability up to 200°C without appreciable degradation in mobility. The facile modification of low-cost Cu as S/D electrodes for high-performance n-channel OTFTs as well as the low-voltage operation will pave the way for large scale manufacturing of organic electronics.

Characterization of interfacial strength of dissimilar metallic joints using a scanning Kelvin probe

It is very important to understand the interfacial behavior for effective evaluation of welding quality. This letter reports a simple nondestructive characterization method of the interfacial strength of dissimilar metallic joints. Using a scanning Kelvin probe, the electron work function of Cu–steel joints was measured and a strong correlation between the electron work function and the interfacial tensile strength was found.

Triazole, Benzotriazole, and Naphthotriazole as Copper Corrosion Inhibitors: I. Molecular Electronic and Adsorption Properties

The gas-phase adsorption of 1,2,3-triazole, benzotriazole, and naphthotriazole-considered as corrosion inhibitors-on copper surfaces was studied and characterized using density functional theory (DFT) calculations. We find that the molecule-surface bond strength increases with increasing molecular size, thus following the sequence: triazole<benzotriazole<naphthotriazole. This trend is explained in terms of molecular electronegativity and chemical hardness, which decrease monotonously as the molecular size increases. While the electronegativity of triazole is almost degenerate with the work function of Cu(111) surface, the electronegativity of larger acenotriazoles is smaller. The difference in electronegativity between the Cu(111) and the acenotriazoles thus increases with increasing the molecular size, which, together with decreasing the molecular hardness, results in larger molecule-to-metal electron charge transfer and stronger molecule-surface bonds.

Effect of Mo addition on microstructure and vacuum arc characteristics of CuCr50 alloy

The work functions of Cu, Cr, Mo and CrMo were calculated utilizing the first principle. The effect of Mo addition on the microstructure of CuCr50 alloy was characterized by a scanning electron microscope, and the arc motion characteristic was recorded and investigated by a digital high-speed video camera. The calculated results show that the selectivity of breakdown is closely related to the work function of close-packed plane. The experimental results show that Mo addition can strengthen Cr phase, the first breakdown transfers from Cr phase to Cu phase, and the cathode spots present the characteristics of division and deviation, thus avoiding concentrated erosion. In addition, the chopping current is reduced and the breakdown strength is enhanced as well.

The effect of oxygen on the properties of transparent conducting Cu–Al–O thin films deposited by rf magnetron sputtering

Cu–Al–O thin films were prepared by rf magnetron sputtering. By adjusting oxygen partial pressure, the effect of oxygen on optical transmission and electrical properties of the films were investigated. The films exhibit good transparency in the visible range. As oxygen partial pressure is above 30%, the transmittance is about 60–70%. The direct and indirect band gaps are about 3.49 and 1.86 eV, respectively. The linear I– V relations exhibit ohmic contacts between Ag and the films, which suggest that the work function of Cu–Al–O thin films is around 4.26 eV. In the temperature range of 250–310 K, the temperature dependence of the conductivity of Cu–Al–O thin films is of semiconducting thermal-activation type. The average conductivity at room temperature successively increases as the oxygen partial pressure augments. The maximum conductivity is 4.6 × 10 −3 S cm −1 for Cu–Al–O thin film deposited at 40% oxygen partial pressure. When above 40%, the decrease of the conductivity may be due to the inhibition of the carrier transport led by the semiconductor trap states originated from the excess oxygen atoms.

PEEM study of work function changes in Cu, Au and Pd metal surfaces with surface acoustic wave propagation

The effects of surface acoustic wave (SAW) on the work function of Cu, Au and Pd metal surfaces with different surface structures were studied by photoelectron emission microscopy (PEEM). SAW propagation produced bright PEEM images for Cu, Au and Pd metal surfaces consisting of high-index planes and step sites, whereas it yielded dark images for the metals exposing low-index planes, indicating that the SAW enhanced photoemission from rough metal surfaces containing coordinatively-unsaturated metal atoms and lowered that from densely packed smooth metal surfaces. Changes in the PEEM images with SAW-on and SAW-off were reversible and were associated with decreases and increases in the work function of the metal surfaces, respectively. The SAW caused periodic and vertical lattice displacement, and it was demonstrated that large lattice displacement was responsible for work function changes from coincidence between the patterns of photoemission and lattice displacement. A mechanism for work function changes is proposed on the basis of effects on the spatial structures and electronic properties of metal surfaces.

Interface Fermi level pinning in a Cu/p-CuGaS 2 Schottky diode

A Schottky contact to p-type CuGaS 2 that showed the highest rectification ratio of approximately 500 ever reported was realized using a Cu electrode on a HF/HNO 3-treated surface, as well as an excellent Au ohmic contact on a HF-etched surface. The effective Schottky barrier height of 0.9 eV was obtained from the current–voltage and capacitance–voltage characteristics. The value was smaller by 1.1 eV than that calculated from the values of the work function of Cu and electron affinity of CuGaS 2. The results indicated a surface pinning of the Fermi level to certain acceptor-type gap states below the midgap.

Fermi-level pinning at the metal/p-type CuGaS2 interfaces

A Schottky contact to p-type CuGaS2 that showed an extremely high rectification ratio of approximately 500 was realized using a Cu electrode on a HF/HNO3-treated surface, as well as an excellent Au ohmic contact on a HF-etched surface. The effective Schottky barrier height of 0.9 eV was obtained from the current–voltage and capacitance–voltage characteristics. The value was smaller by 1.1 eV than that calculated from the values of the work function of Cu and electron affinity of CuGaS2. The results indicated a surface pinning of the Fermi level to certain acceptor-type gap states below the midgap.

Local variation of the work function of Cu(1 1 1) surface deduced from the low energy photoemission spectra

We have investigated the photoemission spectra very close to the vacuum level of the Cu(1 1 1) surface with high resolution, using a tunable laser as the incident ultraviolet light source. The incident photon energy was tuned very close to the work function of the sample surface. A small emission peak was found ∼100 meV below the vacuum level cutoff that corresponds to the work function. The origin of the small peak is attributed to the emission from the areas whose local work function is depressed below the work function of the majority area. Variation of the spectrum caused by surface roughening and oxygen adsorption suggests that the areas with lower work function are the step edges.

Surface modification of copper due to co-adsorbed oxygen and cadmium probed with optical second harmonic generation

Oxygen adsorbates are unavoidable on Cu in an aqueous environment. In this work, such a polycrystalline Cu surface is decorated with electrodeposited Cd. Optical second harmonic generation is used to probe the time resolved kinetics of co-adsorbed oxygen and Cd on this electrode. The optical data contain contributions from the linear dielectric functions, and second order susceptibility elements of the electrochemical interface. A phenomenological theory is presented to resolve the contributions of these optical parameters in the optical signal of the Cu surface. Electronic effects of the co-deposited oxygen and Cd on Cu are analyzed by testing the coverage dependent features of the interfacial optical parameters. Numerical calculations are performed for this analysis. The results are characterized in terms of a Cd-induced lowering of the surface work function of Cu. The results also demonstrate how second harmonic generation can be used to analyze the relatively complicated electrochemistry of Cu.

The influence of hydrogen at pressures up to 2000 Pa on the work function of Cu(111)

(111) surfaces of copper, grown epitaxially on a clean (001) face of a ruthenium crystal, were exposed to hydrogen or deuterium at pressures p up to 2000 Pa, at temperatures from 290 to 440 K. The concomitant changes of the work function phi were recorded photoelectrically. While coadsorbed impurities influenced the absolute value of phi , the slope d phi /dp was found to be roughly constant and independent of the temperature in the pressure range 400<or=p<or=2000 Pa. The slope of the coverage d theta /dp, calculated from low-temperature adsorption data in the literature, is also constant in the above temperature and pressure range. Thus the variation of phi follows the chemisorption of hydrogen atoms due to the tail of the Boltzmann distribution. The effects of impurity adsorption and desorption are discussed.

Identification of metals in scanning tunneling microscopy via image states.

An oscillatory reversal of the contrast between Cu and Mo is observed with scanning tunneling microscopy (STM), using sample bias voltages of $+$5 V and higher. It is attributed to tunneling via a series of discrete states that are induced by a combination of the image potential and the applied field. They are offset in energy due to the different work functions of Cu and Mo. This effect provides a generally applicable mechanism for elemental contrast in STM.

Determination of the Positron Work Functions of Cu(100) and Polycrystalline Fe, Mo, Ni, Pt, Ti and V

Determination of the Positron Work Functions of Cu(100) and Polycrystalline Fe, Mo, Ni, Pt, Ti and V

Calculation of the electronic work function of Cu and Ag from an extended jellium model.

We propose an extended jellium model which includes the most important features of the noble metals. The density of states is modeled by the superposition of a d band and an s-like conduction band. The redistribution of some of the d electrons into the conduction band as an effect of the s-d hybridization is taken into account, and the conduction band includes 1.5 electrons per atom. We show that the occupation number ${Z}_{d}$ of the d band is mainly determined by the ratio ${W}_{d}$/(${\ensuremath{\varepsilon}}_{F}$-${\ensuremath{\varepsilon}}_{d}$), where ${W}_{d}$, ${\ensuremath{\varepsilon}}_{d}$, and ${\ensuremath{\varepsilon}}_{F}$ are, respectively, the width and the center of the d band and the energy of the Fermi level. The shift of the d-band center between the bulk metal and the surface plane is related to the narrowing of the d band of the surface plane, and we deduce an estimate of the d-band contribution to the work function. In the present model the position of the Fermi level and of the d-band center are reproduced, and the result for the work function of Cu and Ag is satisfactory.

Properties of the Schottky Barriers on compensated PbTe〈Ga〉

C-V, I-V and photoelectric characteristics were investigated for some metals with different work functions, deposited onto compensated PbTe〈Ga〉 substrates with low carrier concentrations N, P ⩽ 10 16cm −3. The surfaces of the PbTe〈Ga〉 substrates were prepared by anode-mechanical polishing, that results in enriching of the surface by Te. It was shown that the heights of the potential barriers Ø B of the Schottky contacts prepared by oxidization in air anode-mechanically polished PbTe surfaces do not depend on the metal work function of Cu, Ag, Au and In. The highest values of resistance-area product were obtained for the Cu/ p-PbTe〈Ga〉 contacts (R 0A≅ 1 × 10 4Ωcm 2) and were slightly lower than the R 0A value limited by the thermoemission current.

The Effect of Space Charge on DC Conduction and Light Emission in Dielectric Liquids under Non-Uniform Field

Conduction current characteristics in transformer oil and n-hexane have been studied under negative pointplane electrodes. Experiments were performed with needle electrodes of various materials (W, Cu, Mo, and Ni) with various tip radii (3, 6, 10, 25, and 50 μm) at gap lengths of 1 to 10 mm. From the experimental results, the conduction current was found to be caused by Schottky emission affected by space charge rather than by Fowler-Nordheim emission. Electric field distribution including space charge effect was calculated by the Charge Simulation Method using a digital computer. From the curve-fitting method the following electron mobilities and the effective work functions of W were estimated: 6.0xl0-6 m2/V.s and 1.05 eV for transformer oil, and 10x10-6 m2/V.s and 1.14 eV for n-hexane. The effective work functions of Cu, Mo, and Ni in transformer oil were also estimated at 1.07, 1.06, and 1.08 eV, respectively. Light emission was observed at the point tip in dielectric liquids. The light emission in transformer oil was divided into two distinct regions while in n-hexane there was only one. From the analysis of space-charge-affected field distribution, these light emissions were found to be caused by electronic excitation and photoluminescence.