Electrolyte Junction Research Articles

Comparison between metal and electrolyte/(III–V) semiconductor interfaces

Abstract Barrier formation at III–V semiconductor (SC)/redox electrolyte junctions is studied. It is found that the Fermi-level pinning due to the redox species in solution is much weaker than that evidenced for solid Schottky barriers. It is also shown that corrosion by water is able to pin the Fermi level of the SC; this is the main difficulty to compare SC/liquid and SC/metal interfaces.

Impedance of metal-solid electrolyte interfaces

The impedance of metal-solid electrolyte junctions display a characteristic frequency dependence in both the real and imaginary terms which can be represented by Z(ω) = B(jω)−n over several decades in ω, where 0<n<1. Impedance measurements on cells using β- and β″-alumina at temperatures from -134 to 400°C combined with scanning electron and optical microscopy of bare and metallized electrolyte surfaces show that the value of the frequency exponent n depends on the surface microstructure and sample temperature. Two recent models based on (1) the fractal geometry of a rough interface and (2) a porous interface provide a qualitative explanation of the experimental results.

Light‐Generated Electrodics—Rotating Dual Electrodes

The generation of current for solution redox reactions below the mass‐transport limiting current at semiconductor electrodes has been shown to be spatially controllable by light‐beam dimension and placement. Thus, a localized electron or hole transfer surface can be created at a semiconductor‐redox electrolyte junction by means of a focused laser beam. Such a spot was scanned over a semiconductor disk in a series of experiments within a rotating ring‐disk electrode geometry to test the transit and collection properties of the local reaction products. By comparison to related theory for the given geometries, the predicted disk and ring currents as a function of electrode size and location were quantitatively confirmed. These microelectrodes ought to have the enhanced radial transport and reduced resistive losses associated with their micrometallic counterparts, but with flexible manipulation of the geometric parameters. Other results include mapping the generation of selected products over the surface of the semiconductor material and establishing the possibility of detecting kinetic processes by monitoring the collection and transit parameters as a function of radial origin.

Laser Flash Induced Charge Carrier Transfer in Polycrystalline n‐TiO2 Semiconductors

AbstractPhotocurrent and photovoltage transients have been measured at n‐TiO2 electrolyte junctions by means of ns‐laser pulse excitation. Two different kinds of build‐up, independent of excitation intensity and external resistance have been observed. It is concluded that the charge carrier transit through the depletion layer is the rate determining step if traps or grain boundaries of the microcrystals acting as such, play a significant role. The result is a longer rise time of either photocurrent or ‐voltage, which exceeds the laser pulse width by orders of magnitude.

Flatband potential determination and surface modifications at semiconductor-liquid junctions

Abstract For n-GaAs/aqueous electrolyte junctions, the surface oxide formation can be revealed by capacitance measurements. It is shown that photoholes are trapped in surface bonds before the transfer to solution. Both phenomenon give rise to a band edge shift with illumination which is minimized by ruthenium chemisorption.

Band‐Edge Shift and Surface Charges at Illuminated n ‐ GaAs / Aqueous Electrolyte Junctions: Surface‐State Analysis and Simulation of Their Occupation Rate

Mott‐Schottky plots for electrolyte junctions under various illuminations show that the semiconductor bandedges are shifted with respect to their position in the dark, the magnitude of the shift increasing with increasing light intensity. Correlatively, the surface‐state capacitance presents a sharp peak; its height increases with illumination. A model for the electron occupation rate of a surface‐state distribution is described in order to interpret these experimental facts. It is shown that such a kinetic model explains only impedance results; chemical modifications at the surface have to be invoked to account for the flatband‐potential shift at the illuminated electrode. The surface‐state capture cross sections are pulled from impedance analysis through the relaxation time .

Solid-state perspectives of the photoelectrochemistry of semiconductor–electrolyte junctions

More than one mechanism is required to account for the photoinduced movement of charge across semiconductor–electrolyte junctions. Distinguishing the various mechanisms, which is here accomplished using a synthesis of theoretical ideas from both solid-state physics and electrochemistry, may have important consequences in research related to potential applications of liquid junction devices in solar energy conversion.

Photoelectrochemical characterization of the p-Cu 2O-non aqueous electrolyte junction

Abstract The current-voltage characteristics of the p -Cu 2 O electrolyte junctions were analyzed. The electrode decomposition process was defined in the dark and under illumination in the liquid ammoniate of sodium iodide (LASI). It was shown that the photodecomposition process is slower in LASI. The photocurrents at the p -Cu 2 O/LASI junctions were described using the Gartner model. Some typical solid parameters such as energy gap and minority carrier diffusion length were determined.

Catalyzed reactions at illuminated semiconductor interfaces

Many desirable minority carrier chemical redox processes are too slow to compete with e−−h+ recombination at illuminated semiconductor/liquid electrolyte junction interfaces. Reductions of H2O to H2 or CO2 to compounds having C–H bonds are too slow to compete with e−−h+ recombination at illuminated p-type semiconductors, for example. Approaches to improve the rate of the desired processes involving surface modification techniques are described. Photoanodes are plagued by the additional problem of oxidative decomposition under illumination with ≥Eg illumination. The photo-oxidation of Cl−, Br−, and H2O is considered to illustrate the concepts involved. Proof of concept experiments establish that catalysis can be effective in dramatically improving direct solar fuel production; efficiencies of &amp;gt;10% have been demonstrated.

Use of a Liquid Electrolyte Junction for the Measurement of Diffusion Length in Silicon Ribbon

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Characterization of the Interface Energetics for N‐Type Cadmium Selenide/Nonaqueous Electrolyte Junctions

Single crystal, n‐type photoanodes have been studied in solutions containing low concentrations of fast, outer‐sphere, one electron redox reagents. A number of redox couples were studied spanning a wide range of redox potentials, . We find that reversible electrochemical response is seen at both dark and illuminated (632.8 nm light) for couples with more negative than −1.2V vs. SCE, e.g., . For couples with positive of −1.2 V vs. SCE we find that is blocking to the oxidation of the reduced form of the redox couple in the dark, but illumination results in its oxidation. The photoanodic current peak in a cyclic voltammogram occurs more negative than at a Pt electrode, the difference between these values is the photovoltage, , taken to approximate the barrier height, . For between −1.2 and −0.1V vs. SCE, increases as increases in a nearly ideal manner. Thus, increases nearly linearly as moves positive of the flatband potential, , of −1.2V vs. SCE. For more positive than −0.1V vs. SCE is constant, independent of . The effect of a number of different etches on the interface energetics of was investigated, since it was previously determined that an oxidizing or reducing etch would yield quite different results for . For , however, the different etches do not give significantly different results with respect to vs. , despite large variation in surface composition deduced from Auger and XPS spectra. The highest obtained is ∼0.8V using and more positive redox couples. In general, with respect to vs. , n‐type more closely mimics the behavior of than, despite the fact that the bandgap of is closer to that of than to .

Photoelectrochemical deposition of metals onto p-silicon using an internal cell

Spatially defined photoelectrochemical deposition of low work function metals onto p-Si has been observed, operating through the local cell M+n+ne−→p-SihνM° ←dark Some M° must be incorporated initially onto the Si surface via a low resistance contact. With M=Zn or Cd, a large band bending is induced at the semiconductor/plating electrolyte junction, resulting in highly efficient imaging. For the more noble metals, Cu and Ni, the bending is very small or nonexistent, and this kind of imaging is not observed.

Current–voltage characteristics of semiconductor–electrolyte junction solar cells

An improved model for the semiconductor–electrolyte solar cell is discussed. Charge transfer kinetics, surface recombination, recombination in the quasi neutral region and in the depletion region, as well as the series and shunt resistance of the cell are included in our model. It is shown that the surface transfer velocity of minority carriers across the semiconductor–electrolyte interface affects primarily the open circuit voltage, the fill factor and power conversion efficiency, and only to a lesser degree the short circuit current.As is the case with nonelectrolytic solid state solar cells, the series resistance of the electrochemical cell reduces the fill factor and the conversion efficiency, while the shunt resistance reduces the open circuit voltage of the cell in addition to reducing the fill factor and the power conversion efficiency.

Photoinduced layer phenomenon caused by iodine formation in MoSe 2: electrolyte (iodide) junctions

Abstract When iodide is photooxidized at a n -type MoSe 2 electrode at a high rate, a dim film is observed to spread over the metallic bright electrode and the reflected light intensity throughout the visible and near ir spectral region drops by 70–80%. The phenomenon can — in a reversible way — be controlled by small potential variations (0 − +0.5 V) and beyond +1.0 V produces electrochemical oscillations, which are paralleled by an additional reflection change which can be seen to propagate across the electrode surface. The phenomenon could be reproduced in absence of light at semiconducting p -type MoSe 2 , at metallic NbSe 2 and at Pt-electrodes which excluded the possibility that the pronounced optical changes are produced by a photoelectrochemical or electrochemical modification of layer-type electrode material in presence of iodide. The reflection anomalies can be explained by physical properties of an iodine layer, which is formed in the iodide-depleted diffusion region at the electrode surface. Evidence is given that the spreading of the dim film is actually a phase transition from dispersed to crystalline iodine with an asymmetrical size distribution within the I − depleted diffusion layer. The resulting gradient of the refraction index works like an anti-reflecting coating. The described phenomenon will have to be considered in attempts to optimize solar cells based on the I − /I 2 redox couple and it might be of some practical interest for antireflecting layer-technology and electro-optical applications.

Photocurrent Spectroscopy of Interface States at a Semiconductor‐Electrolyte Junction

We have investigated the photocurrent induced by subbandgap light irradiation at the n‐Si‐aqueous electrolyte junction. The observed effect is shown to arise from optical processes involving interface states, and mainly from electron transitions from the filled interface states to the conduction band. The dependence of the photocurrent upon anodic voltage is accounted for quantitatively by considering the Schottky barrier lowering effect, already measured from the dark I‐V characteristics. The dependence of the photocurrent upon photon energy enables us to extract a density of states curve for the occupied interface states. It suggests that these states are bound states on the semiconductor side, due to the coulomb potential of adsorbed ions. Complementary observations on oxidized and bandgap irradiated junctions are discussed and are found to fit well in the above picture.

The influence of niobium doping on the efficiency of n-TiO 2 electrode in water photoelectrolysis

Abstract Quantum efficiencies, flatband potentials and bandgap energies of polycrystalline n-TiO2 electrodes have been studied as a function of the niobium dopant concentration. The experimental data have been analyzed in terms of the physical parameters of the semiconductor by using the Schottky barrier model of the semiconductor electrolyte junction. Density of donor centers generated by substitution of Ti4+ by Nb5+ seems to be tha main parameter controlling quantum efficiences. Optical absorption coefficient and hole diffusion length are not sensibly influenced by the dopant concentration up to levels of about 2%.

A thin-film polycrystalline photoelectrochemical cell with 8% solar conversion efficiency

Photoelectrochemical solar cells (PECs) may become an economic method for conversion of light to electricity, due largely to the simplicity of formation, and quality, of the poly-crystalline semiconductor–electrolyte junction. Within the past few years, conversion efficiencies of thin-layer poly crystalline-based cells have increased from ∼1% to a recently reported 7.3% for the n-GaAs/Sex2− system1. PECs based on the CdSe–polysulphide system have been studied extensively recently. CdTe, with a band-gap, Eg, ∼1.45 eV is better matched to the solar spectrum than CdSe (Eg∼1·75 eV), but is unstable in polysulphide solutions as a photoanode at the current densities to be expected under normal solar irradiation (>10 mA cm−2). A poly sulphide-based PEC is described here which uses poly-crystalline layers of CdSe0.65Te0.35, electrodes of which are prepared by painting a slurry of the semiconductor onto a T1 substrate, and sintering. Subsequent surface treatments of these layers lead to a solar-to-electrical conversion efficiency of up to 8%, with stability comparable to the thin-layer polycrystalline CdSe/polysulphide system2.

Schottky barrier height, photovoltage and photocurrent in liquid-junction solar cells

Abstract The height of the Schottky barrier at a CdS electrode in contact with various redox electrolytes has been measured at electronic equilibrium and the open-circuit photovoltage, V oc , has been found being linearly related to this quantity at constant illumination intensity. The photocurrent-voltage curves of such redox electrolyte junctions with CdS, n-GaAs and n-MoSe2 electrodes have been studied as a function of the light intensity which was varied over three orders of magnitude. The photocurrent of such electrochemical solar cells is at the open-circuit voltage compensated by the forward current through the junction. Its magnitude in comparison to the saturation current gives information on whether the recombination or the forward current controls V oc . The application of such studies for the evaluation of electrochemical solar cells is discussed.

Mercury(II) sulphide: a photo-stable semiconductor

THERE is much interest in photo-electrochemical cells which use a semiconductor–electrolyte junction to transduce light into electrical and chemical energy1. The semiconductors investigated to date fall into two main groups, the first of which contains photostable material but which shows response in the UV rather than the visible region of the spectrum, for example, n-titanium dioxide2, n-strontium titanate3, n-potassium tantalate4, n-tin oxide5 and n-barium titanate6. The second group comprises compounds which respond to the visible region of the spectrum but are photochemically unstable such as n-gallium phosphide7, n-indium phosphide8, n-cadmium sulphide9, and n-cadmium selenide10. Attempts have been made to extend the spectral response of the materials belonging to the first group, by doping11 and chemically affixing dyes to the surface of the semiconductor12. Some success has been attained in stabilising some of the members of the second group; for example, cadmium sulphide and selenide are stable in poly-chalcogenide solutions13. If cells containing a semiconductor–electrolyte junction are to be of any practical use in the harnessing of solar energy then the following criteria for the semiconductor should be met: (1) they should absorb visible radiation; and (2) be photostable and capable of use in powder form. Clearly, if the conduction and valence bands of the semiconductor have energies that facilitate proton reduction and oxidation of hydroxyl ions respectively this would be an added bonus as the cells could be used to either photo-assist or photo-electrolyse the water14. We report here on mercury(II) sulphide (cinnabar) band gap = 2.1 eV15 which fulfils most of the above criteria16.

Semiconductor liquid junction solar cells based on anodic sulphide films

ALTHOUGH good performance has been achieved with established solid state junction solar cells, their cost remains well above that required for large scale terrestrial applications. New concepts1,2 for photovoltaic energy conversion have been introduced which involve the junction between a semiconductor and an electrolyte. These schemes may lead to new possibilities of considerably reduced materials and fabrication costs, providing that many technical problems could be solved. We report here metal–semiconductor–liquid electrolyte junctions in which an n-type sulphide semiconductor is anodically formed in situ on its metal and, under photoexcitation, drives a sulphide–polysulphide redox couple when connected to a suitable cathode. The original contribution in this area was made by Gerischer2 who demonstrated that the cell (single crystal n-CdS/Fe(CN)64−, Fe(CN)63−/SnO2) has an initial conversion efficiency > 5% (ref. 3) for sunlight to electric power, in spite of the 2.4-eV band gap of CdS, which implies that wavelengths > 550 nm (the greater part of the incident solar photons) are ineffective.