Work Function Of Ag Research Articles

2D-modelling of Metal-Si Emitter Interface Assuming Schottky or Ohmic Contact

In this study we investigate, using a two dimensional simulation tool, the characteristics of the metal-Si emitter interface of screen printed and firing through contacts. In the model we assumed that the metal contact to n+ or p+ emitters is either Ohmic or Schottky and the dominant current conduction mechanism flow is directly through Ag-crystallite. The simulation results were then compared with the experimental data for the contact resistance (ρc) and fill factor (FF) of p-type cells with an n+ phosphorous diffused emitter and n-type cells with a p+ boron diffused emitter. The emitters of p-type and n-type cells were metalized using an Ag paste or, in case of p+ doped emitters, by an AgAl paste. From the modeling of Si-emitters contacted by a screen printed Ag paste, a Schottky contact, assuming literature value for Ag work function, agrees with the experimental IV data for n+ emitters, but does not agree for the p+ emitters. Assuming only a Schottky contact at metal-p+ emitter interface, the model fails to estimate simultaneously VOC and FF of the cells contacted by an Ag paste. Thus, the current transport mechanism at Ag- p+ emitter interface may not be dominated by direct metal-Si contact through Ag-crystallites imprints but, possibly, by tunneling through a thin interface glass layer that resulted (in this case) in high contact resistance as observed experimentally. Therefore modeling Ag-p+ emitter interface using an Ohmic contact with a high contact resistance agrees better with the experimental IV data.

Cathodic multilayer transparent electrodes for ITO-free inverted organic solar cells

We demonstrate cathodic multilayer transparent electrodes based on a ZnS/Ag/TiOx (ZAT) structure for ITO-free inverted organic solar cells. A quality solution-based TiOx layer is adopted as an inner dielectric layer to modify the effective work function of Ag, ensuring the ZAT electrode works as a cathode. The effect of the TiOx layer is seen on the open-circuit voltage of a solar cell incorporating this layer, increasing to 900mV from 600mV in the case of a cell with a bare Ag layer for a bulk-heterojunction of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C70-butyric acid methyl ester (PCBM70). The results of a joint theoretical and experimental study indicate that the photocurrent of a ZAT-based solar cell can be significantly enhanced by carefully balancing the optical-spacer and cavity-resonance effects, both of which are modulated by the thickness of the WO3 layer used as a hole-collection layer at the top anode side. ZAT-based inverted solar cells with an optimized structure exhibit a power conversion efficiency as high as 5.1%, which is comparable to that of the ITO-based equivalent.

Effects of Oxide Roughness at Metal Oxide Interface: MgO on Ag(001)

Defects in thin oxide films on metal substrates affect metal work function and determine the chemical and physical properties of an oxide. However, accurately predicting properties of these heterogeneous systems is still challenging. Here we use a new approach to treat a mixed metal/metal oxide system within density functional theory, which is based on the application of the auxiliary density matrix method (J. Chem. Theory Comput.2010, 6, 2348) to calculate the exchange interaction at a sharp interface between the two materials, as implemented in the CP2K code. This method is used to calculate the shift of the Ag work function in the MgO/Ag(001) system as a function of the MgO film morphology as well as charge state, position, and density of oxygen vacancies. An accurate band alignment between metal and oxide allows us to predict the relative stabilities of different charge states of oxygen vacancies in MgO as a function of their position with respect to the interface with Ag. Our results confirm that F+ ...

UV 처리에 의한 T-OLED용 산화전극에 적합한 Ag 박막연구: Nano-Mechanics 특성 분석을 중심으로

The work function of Ag (silver) is too low (~4.3 eV) to be used as an electrode of T-OLED (Top Emission Organic Light Emitting Diode). To solve this weakness, researches used plasma-, UV-, or thermal treatment on Ag films in order to increase the work function (~5.0 eV). So, most of studies have focused only on the work function of various treated Ag films, but studies focusing on nanomechanical properties were very important to investigate the efficiency and life time of T-OLED etc. In this paper, we focused on the mechanical properties of the Ag and film. The Ag was deposited on a glass substrate with the thickness of 150 nm by using rf-magnetron sputter with the power was fixed at 100 W and working pressure was 3 mTorr. The deposited Ag film was UV treated by UV lamp for several minutes (0~9 min). We measured the sheet resistance and mechanical property of the deposited film. From the experimental result, there were some differences of the sheet resistance and surface hardness of Ag thin film between short time (0~3 min) and long time UV treatment. These result presumed that the induced stress was taken place by the surface oxidation after UV treatment.

Surface plasmon polariton enhanced electroluminescence and electron emission from electroformed Al-Al2O3-Ag diodes

Electroforming of Al-Al2O3-Ag diodes results in voltage-controlled negative resistance (VCNR) in the current-voltage (I-V) curves. Electroluminescence (EL) and electron emission into vacuum (EM) develop simultaneously. The temperature dependence between 200 and 300 K of VCNR, EL, and EM of Al-Al2O3-Ag diodes with anodic Al2O3 thicknesses between 12 and 41 nm has been studied. I-V curves and VCNR are slightly temperature dependent. The voltage for the onset of EL, VEL, is between ∼1.3 and ∼1.9 V for the range of Al2O3 thicknesses, with small temperature dependence. The density of defects in anodic Al2O3 is >1.5 × 1020 cm−3. Defect conduction bands that form from excited states of F- or F+-centers, oxygen vacancies in Al2O3, determine the value of the barrier height at the Al-Al2O3 interface, ϕA, and they control EM. EM is anomalous. The threshold voltage for EM, VEM, is ∼1.9 to ∼2.5 V for the range of Al2O3 thicknesses, which is less than the work function of Ag, 4.6 eV. EM at 300 K is ∼10−9 A. As temperature is lowered, EM drops to ∼10−12 A at TD ≅ 290 K and recovers to ∼10−9 A at TR ≅ 260 K. The particular values of TD and TR depend on sample preparation and Al2O3 thickness. The source of anomalous EM is electrons that tunnel through the high field region at the Al-Al2O3 interface into defect conduction bands. They gain energy and momentum by combining with surface plasmon polaritons (SPPs) that are generated at the Al2O3-Ag interface by EL photons. EL from Al-Al2O3-Ag diodes with 12 nm or 15 nm of Al2O3 is much larger than EL from diodes with thicker Al2O3 layers. The conducting channel of electroformed diodes with the thinnest Al2O3 acts as a microcavity in which the electromagnetic field due to SPPs stimulates EL from defect centers by the Purcell effect.

Hydrogen Detection with Noble Metal-TiO2 Schottky Diodes

Fast detection of hydrogen at a wide concentration range is desired for many applications. We report detecting hydrogen using Au-TiO2-Ti and Ag-TiO2-Ti diodes. While hydrogen-Au and hydrogen-Ag interactions are very different, at a constant biasing voltage, the measured current in both diodes is highly sensitive to the partial pressure of hydrogen contamination in the surrounding atmosphere. Work function variations were investigated by connecting the I-V specifications to the energy barrier height established at the Au-TiO2 and Ag-TiO2 junctions. Electronic features of the devices were described based on the assumption of two different hydrogen-noble metal interactions: Hydrogen reduces Ag work function by reducing the adsorbed oxygen species from the silver surface, while Au work function is reduced by the same mechanism as well as the direct adsorption of hydrogen species to the gold surface. Both of these mechanisms result in hydrogen detection by Schottky barrier height reduction and current increase.

Adsorption of Ag and Au atoms on wurtzite ZnO (0001) surface

We perform first-principles calculations to investigate various surface structures in the absorption of Ag and Au atoms on wurtzite ZnO (0001) surface. The results show that both Ag and Au atoms prefer to be absorbed on the H 3 sites (the center of Zn–O ring) of the surface, and the most favorable monolayer (ML) coverage is 1. The calculated electron structure shows that the Ag- and Au-adsorbed ZnO (0001) surfaces exhibit metallic characteristics even the ML coverage of the adatoms is very low. Finally, the work functions of Ag- and Au-adsorbed ZnO (0001) surfaces are calculated and discussed for the first time in the present work.

Electronic structures and related properties of Ag–Au bulks and surfaces

First-principles calculation reveals that the Ag–Au bulks are energetically favorable with negative heats of formation within the entire composition range, and the Ag–Au interaction has a negligible effect on electronic structures of Ag–Au bulks. Moreover, it is found out that surface segregation of Ag atoms could reduce the work function of Ag–Au surfaces to an extent of about 0.13–0.60 eV, and composition as well as surface state would be fundamental factors in determining surface segregation of the Ag–Au system, which could clarify the controversy regarding surface segregation of Ag–Au in the literature.

SERS-activating effect of chlorides on borate-stabilized silver nanoparticles: formation of new reduced adsorption sites and induced nanoparticle fusion

Changes in morphology, surface reactivity and surface-enhancement of Raman scattering induced by modification of borate-stabilized Ag nanoparticles by adsorbed chlorides have been explored using TEM, EDX analysis and SERS spectra of probing adsorbate 2,2'-bipyridine (bpy) excited at 514.5 nm and evaluated by factor analysis. At fractional coverages of the parent Ag nanoparticles by adsorbed chlorides <0.6, the Ag colloid/Cl(-)/bpy systems were found to be constituted by fractal aggregates of Ag nanoparticles fairly uniform in size (10 +/- 2 nm) and SERS spectra of Ag(+)-bpy surface species were detected. The latter result was interpreted in terms of the presence of oxidized Ag(+) and/or Ag(n)(+) adsorption sites, which have been encountered also in systems with the chemically untreated Ag nanoparticles. At chloride coverages >0.6, a fusion of fractal aggregates into the compact aggregates of touching and/or interpenetrating Ag nanoparticles has been observed and found to be accompanied by the formation of another surface species, Ag-bpy, as well as by the increase of the overall SERS enhancement of bpy by factor of 40. The same Ag-bpy surface species has been detected under the strongly reducing conditions of reduction of silver nitrate by sodium borohydride in the presence of bpy. The formation of Ag-bpy is thus interpreted in terms of the stabilization of reduced Ag(0) adsorption sites by adsorbed bpy. The formation of reduced adsorption sites on Ag nanoparticle surfaces at chloride coverages >0.6 is discussed in terms of local changes in the work function of Ag. Finally, the SERS spectral detection of Ag-bpy species is proposed as a tool for probing the presence of reduced Ag(0) adsorption sites in systems with chemically modified Ag nanoparticles.

Improving the Performance of Transparent PLEDs with LiF∕Ag∕ITO Cathode

This work demonstrates the favorable performance of a transparent polymeric light-emitting diode (PLED) using an effective LiF/Ag/indium tin oxide (ITO) cathode. An Ag layer can prevent the underlying emitting layer from damage by bombardment during ITO sputtering. Devices that have an Ag layer have a lower driving voltage and lower leakage current than those without. X-ray photoelectron spectroscopy data show that Ag increases the concentration of Li in the polymer, enhancing its electron injection and transport capacities. The lower leakage current results from the fact that the work function of Ag is lower than that of ITO. The insertion of the Ag layer also enhances the optical characteristics by improving carrier balance and reducing sputtering damage.

Mo O x modified Ag anode for top-emitting organic light-emitting devices

Efficient top-emitting organic light-emitting devices (TOLEDs) using a thin MoOx layer modified Ag as the effective hole-injection anode are demonstrated. With tris-(8-hydroxy quinoline)aluminum as emitting layer and trilayer LiF∕Al∕Ag as semitransparent cathode, the Ag∕MoOx based TOLED shows a tune-on voltage of 2.67V and a maximum current efficiency of 7.27cd∕A, which are much better than those (3.92V, 6.12cd∕A) obtained from Ag∕Ag2O based TOLED and those (5.25V, 3.5cd∕A) obtained from the corresponding bottom-emitting organic light-emitting devices. Contact potential difference measurement shows that the work function of Ag∕MoOx is higher than those of Ag∕Ag2O and ozone-treated indium tin oxide, leading to a stronger hole injection. The good performance of Ag∕MoOx based TOLED is attributed to the efficient hole injection from the Ag∕MoOx anode as well as a microcavity effect.

UPS study of tri(β-naphthyl) phosphine overlayer on Ag(110)

Ultraviolet photoelectron spectroscopic(UPS) measurements of the tri (β-naphthyl) phosphine (TNP) film deposited on the Ag(110) surface were made. The TNP derived valence features are located at 38，63，93and 110 eV below Fermi level, and the top of the highest occupied molecular orbital(HOMO) of TNP is located at 25 eV in binding energy. The work function of Ag(110) is about 43 eV, and upon deposition of TNP, the work function decreases to the saturated value of 38 eV. Based on the UPS measurements, the energy level alignment at the interface of TNP/Ag(110) is given, and the results indicate a weak interaction between TNP and the Ag substrate.

GaN-based Light Emitting Diode with Transparent Nanoparticles-Embedded p-Ohmic Electrodes

AbstractOptoelectronics related to GaN-based semiconductors (i.e., InGaN, GaN, and AlGaN) are new technologies that have the potential to far exceed the energy efficiencies of incandescent and fluorescent lighting sources. Among the GaN-based optoelectronic devices like light emitting diode (LED) and laser diode (LD), the GaN-based LEDs are of interest for the next generation illumination because of the representative characteristics such as small, highly radiant, reliably long, and fast responding, compared with the existing general lighting systems.Achievement of high luminous intensity by flip-chip LED (FCLED) with Ag-metallic reflector or using top emitting LED (TELED) with highly transparent ITO contact is required to improve the external quantum efficiency (EQE) and light output of GaN-based LEDs. However, since the work function of Ag and ITO is lower than 5.0 eV, it is difficult to produce low-resistance p-ohmic electrode with Ag-metallic reflector or ITO only.In this study, in order to develop new ohmic contact materials having low contact resistance and high transmittance, transparent nanoparticles-embedded p-ohmic electrode, Mg-doped indium oxide (MIO) (3 nm)/indium tin oxide (ITO) (400 nm) ohmic contact for high brightness TELEDs for solid-state lighting, was suggested. The MIO/ITO contact become ohmic with specific contact resistances of 2.64 × 10-3 Ωcm2 and give transmittance higher than 94.6 % at a wavelength of 450 nm when annealed at 630 °C for 1 min in air. GaN-based LEDs fabricated with the annealed MIO/ITO p-contact layer give a forward-bias voltage of about 3.38 V at injection current of 20 mA. It is further shown that the output power of the LEDs with the MIO/ITO contact is enhanced by about 1.86 times at 20 mA as compared with that of LEDs with the conventional Ni/Au contact.

Tuning of Metal Work Functions with Self-Assembled Monolayers

AbstractWe demonstrate the tuning of metal work functions by chemically modifying the metal surface through the formation of chemisorbed self-assembled monolayers (SAMs) derived from 1H,1H,2H,2H-perfluorinated alkanethiols and hexadecanethiol. The ordering inherent in the SAMs creates an effective, molecular dipole at the metal/SAM interface, which increased the work function of Ag (φAg∼4.4 eV) to 5.5 eV (Δφ∼1.1 eV) for 1H,1H,2H,2H-perfluorinated alkanethiols. Hexadecanethiol on the other hand shifted φAg to 3.8 eV (Δφ ∼0.6 eV). On Au, the SAM of 1H,1H,2H,2H-perfluorodecanethiol raised φAu (4.9 eV) with 0.6 eV to 5.5 eV, whereas hexadecanethiol decreased φAu by 0.8 eV. These chemically modified electrodes were applied in the fabrication of polymer LEDs and the hole injection into poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene (MEH-PPV) was investigated. An Ohmic contact for hole injection between a silver electrode functionalized with the perfluorinated SAMs, and MEH-PPV with a HOMO of 5.3 eV was established. Conversely, a silver electrode modified with a SAM of hexadecanethiol lowered φAg to 3.8 eV blocked the hole injection into PPV, which enables studying the electron transport in composite devices. The electron-only current was measured in a polymer/polymer blend photovoltaic cell based on MDMO-PPV (as donor) and poly[oxa-1,4-phenylene-(1-cyano-1,2-vinylene)-(2-methoxy-5-(3′,7′-dimethyloctyloxy)-phenylene)-1,2-(2-cyanovinylene)-1,4-phenylene] (PCNEPV, acceptor). This method demonstrates a simple and attractive approach to modify and improve metal/organic contacts in organic electronic devices like LEDs, photovoltaic cells, and FETs.

The point of zero charge of adsorbed monolayers: Pt(111) covered by Ag

The pzc of an Ag monolayer on Pt(111) has been determined according to the Gouy Chapman theory by recording the capacitance–potential curves. A value of −0.48 V versus SCE was found which is 0.24 V higher than that of Ag(111). This difference is close to that expected from the difference in work functions of Ag(111) and a monolayer of Ag on Pt(111) (0.26 eV). The pzc of bulk Ag deposits is identical to that of Ag(111), confirming that Ag deposition is epitaxial.

Metal-dependent charge transfer and chemical interaction at interfaces between 3,4,9,10-perylenetetracarboxylic bisimidazole and gold, silver and magnesium

Ultraviolet photoelectron spectroscopy (UPS) is used to investigate interfaces between the organic semiconductor 3,4,9,10-perylenetetracarboxylic bisimidazole (PTCBI) and Mg, Ag and Au. The metals span a range of work function and reactivity that leads to the formation of three different types of interfaces. The PTCBI-on-Au interface is abrupt and unreacted, and the relative position of energy levels across the interface precludes charge exchange and occupation of gap states. The lower work function of Ag leads to a metal-to-organic charge transfer and formation of polaron-like states at the PTCBI-on-Ag interface. Finally, the PTCBI-on-Mg interface shows clear evidence of a strong chemical interaction, which alters the electronic structure of the organic molecules at the interface and results in the formation of a different type of gap states. Dipole barriers consistent with the energetic and chemical characteristics of each interface are seen in all three cases. Finally, the three interfaces exhibit nearly identical Fermi level positions with respect to the organic highest occupied and lowest unoccupied molecular orbitals.

Work function of polycrystalline Ag, Au and Al

Work functions of polycrystalline Ag, Au and Al films were measured by the photoelectric method, which were deposited on amorphous quartz. These films were preferentially oriented, and an oriented plane, i.e. (111), and a degree of orientation were determined precisely by X-ray diffraction. The work functions of well-defined polycrystalline Ag increased from 4.3 5 to 4.6 4 eV after annealing at 500°C, but those of Au and Al remain almost unchanged after annealing at 350°C, i.e. 5.40 and 4.30 eV, respectively. The low work function of Ag before annealing was explained by the appearance of the local density of state near the Fermi edge of Ag, which is due to formation of the stacking fault in f.c.c. Ag during deposition.

The electronic structure of Ag(110)c(4 × 4)C 60 and Au(110)(6 × 5)C 60

C 60 chemisorbs and forms ordered overlayers on both Ag(110) and Au(110)2 × 1: Ag(110)c(4 × 4)C 60 and Au(110)(6 × 5)C 60. We use direct (UPS) and inverse photoemission spectroscopy (IPS) to examine the occupied and unoccupied electronic structure of Ag(110)c(4 × 4)C 60 and IPS to examine the unoccupied electronic structure of Au(110)(6 × 5)C 60. Comparing the data with multilayer spectra we observe clear and unequivocable signatures of final and initial state effects in the spectra. The final state effects take the form of screening of the ionic final state. Charge transfer from Ag(110) into first layer C 60 molecules is directly observed by UPS and the interaction of first layer C 60 molecules with the substrate is further evidenced by a broadening of C 60 features, ascribed to a splitting of molecular orbitals. The work functions of Ag(110)c(4 × 4)C 60 and Au(110)(6 × 5)C 60 are found to be equal to that of bulk C 60. The combination of the work function measurements and the spectroscopy give new insights into the bonding of C 60 on these surfaces.

Where are we in the study of SERS? Role of chemisorption and charge transfer

Abstract The importance of the local mechanism for the surface enhanced Raman scattering (SERS) is derived by a critical re-examination of experimental results mainly for the pyridine-Ag system. Chemisorption involving charge transfer between adsorbed molecule and metal is then predicted as a primary trigger for SERS. The energy diagram of adsorbed pyridine on Ag, constructed with the use of UV photoemission, electron energy loss spectra and the change in Ag work function, enables us to explain the SERS as adsorbate induced resonant Raman scattering via the charge transfer excitations.

Where are we in the study of SERS? Role of chemisorption and charge transfer

The importance of the local mechanism for the surface enhanced Raman scattering (SERS) is derived by a critical re-examination of experimental results mainly for the pyridine-Ag system. Chemisorption involving charge transfer between adsorbed molecule and metal is then predicted as a primary trigger for SERS. The energy diagram of adsorbed pyridine on Ag, constructed with the use of UV photoemission, electron energy loss spectra and the change in Ag work function, enables us to explain the SERS as adsorbate induced resonant Raman scattering via the charge transfer excitations.