Semi-transparent electrode

For spectroelectrochemical and photoelectrochemical studies, optically semi-transparent electrodes have been fabricated by vapour deposition techniques on glass or quartz substrates (Chapter 12). Tin and indium oxides, platinum, and gold have been used. 

In these techniques a beam of photons is directed to the electrode such that it is transmitted or reflected. The majority of the techniques are reflective, since transmission is limited to transparent or semi-transparent electrode materials. 

For webs, the substrate electrode is usually a vapor-deposited, semitransparent metal layer (Ritchie and Fenn, 1987 Chen, 1993). Al, Ni, and Cr are commonly used. The use of semi-transparent electrodes permits the use of rear exposures for erase purposes. In the case of drums, the metal cylinder serves as the electrode. Usually, a thin, less than 1 pm, blocking layer is interposed between the electrode and the photoreceptor to prevent charge injection. This layer must not be so thick that a residual potential builds up during cycling. Many insulating polymers have been used acrylic polymers, epoxy resins, polyamides, polyesters, polyphosphazenes, polysiloxanes, polyurethanes, vinyl polymers, etc. 

Semi-Transparent Electrode Blocking Layer Charge Transport Layer Charge Generation Layer -Blocking Layer... 

Research in our laboratory and by Osa and Fujihira showed that it is possible to covalently attach monolayers of chromo-phores to metal-oxide semiconductor surfaces — with no compromise in quantum efficiency to energy conversion compared with dyes adsorbed from solution (9-11). The quantum efficiency for these systems (ratio of photo-generated current to photons adsorbed in the dye layer, ne/np) is quite low, in the range of 10 5 to 10 4 and argues against device applications of these simple modified electrodes without further improvements, such as linear, multielectrode stacks of dye-modified, semi-transparent electrodes (10). 

In the case of pure electrical measurements for substrates and contact materials Au, Ag, or Pt are preferred due to the p-type behaviour of as-prepared oligothiophenes. If simple band models are assumed for otnT and the contacts, materials like the noble metals with a workfunction of 5.3 eV (Au) or 5.6 eV (Pt) should lead to ohmic contacts whereas materials with low workfunction such as Al (4.28 eV) or Mg (3.66 eV) should form Schottky barriers. (For n-type behaviour, i.e. after n-doping or annealing in air, compare Section 4.2.2, the opposite is true.) Both types of contacts are necessary for electro-optical measurements. Here also one electrode has to be optically transparent. The most common material for the latter purpose is indium-oxide doped tin-oxide (ITO). This material is highly transparent and highly conductive but has the problem that the substrate always exhibits several spikes standing out of the surface. The other type of semi-transparent electrodes are ultra-thin metal films evaporated onto the organic film. 

A schematic view of the cold cathode fabrication process is shown in Fig. 10.18. The cold cathode is fabricated by low pressure chemical vapor deposition (LPCVD) of 1.5 pm of non-doped polysilicon on a silicon wafer or a metallized glass substrate. The topmost micrometer of polysilicon is then anodized (10 mA cnT2, 30 s) in ethanoic HF under illumination. This results in a porous layer with inclusions of larger silicon crystallites, due to faster pore formation along grain boundaries. After anodization the porous layer is oxidized (700 °C, 60 min) and a semi-transparent (10 nm) gold film is deposited as a top electrode. 

The dye sensitised semi-conductor electrode is a transparent conducting sheet of glass coated (5 pm) with nanocrystalline TiOj (diameter 20 nm) doped with a ruthenium bipyridyl complex. The dye absorbs light, becomes excited and injects electrons into the TiOj electrode. The electrons travel into the transparent WO3 hhn and then, to balance the charge, lithium ions from the electrolyte solution insert into the WO3 and in so doing create the coloured species as described above. If the light source is removed then the cell is bleached back to its original colour. However, if the... 

In a typical spectroelectrochemical measurement, an optically transparent electrode (OTE) is used and the UV/vis absorption spectrum (or absorbance) of the substance participating in the reaction is measured. Various types of OTE exist, for example (i) a plate (glass, quartz or plastic) coated either with an optically transparent vapor-deposited metal (Pt or Au) film or with an optically transparent conductive tin oxide film (Fig. 5.26), and (ii) a fine micromesh (40-800 wires/cm) of electrically conductive material (Pt or Au). The electrochemical cell may be either a thin-layer cell with a solution-layer thickness of less than 0.2 mm (Fig. 9.2(a)) or a cell with a solution layer of conventional thickness ( 1 cm, Fig. 9.2(b)). The advantage of the thin-layer cell is that the electrolysis is complete within a short time ( 30 s). On the other hand, the cell with conventional solution thickness has the advantage that mass transport in the solution near the electrode surface can be treated mathematically by the theory of semi-infinite linear diffusion. 

Another type of Chi interfacial layer employed on a metal electrode was a film consisting of ordered molecules. Villar (79) studied short circuit cathodic photocurrents at multilayers of Chi a and b built up on semi-transparent platinum electrodes in an electrolyte consisting of 96% glycerol and 4% KCl-saturated aqueous solution. Photocurrent quantum efficiencies of multilayers and of amorphous films prepared by solvent evaporation were compared. The highest efficiency (about 10 electrons/ absorbed photon, calculated from the paper) was obtained with Chi a multilayers, and the amorphous films of Chi a proved to be less efficient than Chi b multilayers. 

Fig. 5.6. Schematic drawing of a bulk heterojunction device. Charge generation occurs throughout the bulk, but the quality of the two transport networks (p-and n-type channels) is essential for the functionality of the blend as an intrinsic, ambipolar semiconductor. Light emission occurs at the semi-transparent ITO electrode. Electron transport on the fullerenes is marked by full arrows and hole transport along the polymer by dotted arrows...

Fig. 9.13. Construction of two optically transparent thin-layer cells, (a) With minigrid electrode (from Ref. 22 with permission) (b) With semi-transparent tin dioxide electrode, and usable in a flow system.

As a result of the work summarized in this chapter, unprecedented spectro-electrochemical sensitivity is now available in the visible wavelength region using broadband, single-mode waveguide platforms overcoated with semi-transparent conductive oxide electrode layers [3-5]. For certain molecular systems, it is now possible to monitor ET events for surface coverages of just a few percent of a monolayer moles/cm ). This work has spawned the development of broad-... 

Figure 5 A schematic of the interdigital time-of-flight configuration. Here, (a) is a semi-transparent plate electrode, (b) the sample, and (c) an interdigital electrode. In the conventional time-of-flight apparatus, the interdigital electrode is replaced by a second plate electrode.

This approach was then generalized for the surface modification of oxide films. For instance, chemical modification of nanoporous Sn02 thin film coated onto a transparent semi-conducting electrode with... 

The pyroelectric detector contains a mono-crystal of deuterated triglycine sulfate (DTGS) or lithium tantalate (LiTaOj), sandwiched between two electrodes, one of which is semi-transparent to radiation and receives the impact of the optical beam. It generates electric charges with small temperature changes. The crystal is polarized proportionally to the radiation received and it acts as a capacitor. 

Harrison et al came to another conclusion [231,232]. They find radical cations, rt-diiners, and di-cations in their MIS device. In this device ITO was etched to form two 5 mm electrodes on the glass substrate. Onto this structure 100 nm 6T was evaporated under various conditions (see below). As insulator evaporated SiO was used. Finally, two semi-transparent gold strips were evaporated as gate electrodes. They form a right-angle with the ITO electrodes. In contrast to the above-mentioned experiment, four MIS diodes are formed in this way. With this device optical spectroscopy of the... 

Figure 3.6 Semi-transparent body of the PEDOT-PSS-based plastic counter electrode.

Fabrication of counter electrodes by simple coahng processes using PEDOT-based printable pastes as described here is an important key for DSSC manufacture with respect to minimum costs and rapid production. Design of semi-transparent counter electrodes also contributes to improved hght uhlization of the flexible ceU and modules. 

The ORDE consists of a quartz rod, polished at both ends, one of which is coated with a quasi-metallic antimony-doped tin oxide film to form a (semi-) transparent disc electrode. Light is shone down the rod and through the disc electrode as it is rotated in solution. The rotation of the ORDE imposes RDE hydrodynamics at the transparent... 

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