Optically transparent electrode electrically conducting

ECDs operate in the diffuse reflectance mode and the basic requirements for their functionality are (i) a primary electrochromic electrode (e.g. a polymer electrode) deposited on a substrate which is both optically transparent and electrically conducting (generally indium-tin-oxide(ITO)-... 

Transparent conducting oxides are widely used as electrodes in thin film optoelectronic devices as solar cells and light emitting diodes because of their transparency for visible light and their high electrical conductivity. Highest optical transparency and electrical conductivity are thus key aspects for such applications. Most work on TCO electrodes is, therefore, dedicated to find deposition parameters, which improve these material parameters. In addition, contact properties are essential for the application of TCOs as electrodes. 

The most extensively studied a-Si H solar cells, due to their highest conversion efficiencies, are those fabricated in the form of p-i-n devices. Typically, the p-a-Si H film is less than 10 nm, the undoped (-a-Si H is between 200 and 700 nm and w-a-Si H layer is approx. 30-50 nm. The layers are deposited on each other in successive plasma reactor chambers connected by vacuum locks. A metaUic electrode is used as a substrate. An opposite optically transparent and electrically conducting electrode (e.g. ITO film) is deposited from the top. Today, the commercial large-area solar cell modules based on a-Si H are fabricated with a stabilized conversion efficiency (the ratio of the maximum power output to the solar energy input) in the 4-6 % range (Green, 2007). 

Figure 9.2 Schematic diagram showing how an electrical contact is fixed with silver paint to the conductive side of an optically transparent electrode. The outer layer of epoxy resin is necessary to impart strength, to insulate the silver paint from the analyte solution and to stop analyte solution seeping between the paint and the conductive layer.

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. 

Film electrodes are generally fabricated from conducting or semiconducting materials, which may be deposited as a result of either a physical or a chemical process (or some combination) onto a suitable substrate, which is typically an insulator. Key factors governing the desired thickness of the film are the electrical resistivity (p) or conductivity (k = 1/p) of the film material, which is a practical consideration in almost all cases, and the optical transparency or reflectance of the material if optical transmission or reflection is also desired. The optimum film thickness for an application involving both electrical and optical considerations will require a trade-off, since a decrease in resistivity (usually desirable) normally is also accompanied by a decrease in light transmission (undesirable for an optically transparent electrode). 

No other material shows as much versatility as an electrode as does electrically conducting, CVD diamond. The material can be used in electroanalysis to provide low detection limits for analytes with superb precision and stability for high current density electrolysis (1-10 A/cm ) in aggressive solution environments without any morphological or micro-structural degradation and as an optically transparent electrode (OTE) for spectroelectrochemical measurements in the ultraviolet-visible (UV-Vis) and infrared (IR) regions of the electromagnetic spectrum. 

The use of electrically conductive diamond as an optically transparent electrode is a new field of research [50,52,117,118]. Diamond possesses attractive qualities as both an electrode and an optically transparent material, making it an obvious choice for use as an OTE in spectroelectro-chemical measurements. Diamond OTEs exhibit several technologically useful properties (1) the possibility of transmission measurements from the near-UV to the far-IR (0.225-100 pm) (2) low background current (3) wide working potential window (4) good responsiveness for many... 

A schematic presentation of one of the most convenient modifications of these cells is given in Fig. 8.2. It represents a glass cuvette 1 (internal dimensions 5x3 cm and 7 x 1.7 cm), made of transparent optical glass, sintered glass filters 2 for creating a definite capillary pressure and electrodes 3 for measuring foam electrical conductivity. 

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. 

Transparent polymer solar cells (i.e., polymer solar cells with transparent electrodes) can be easily fabricated based on inverted architecture and have important application in tandem architectures as well. We can form transparent solar cells by replacing the Al top electrode with 12 nm Au in the inverted structure. The J-V curves for this transparent polymer solar cell, with light incident from ITO and Au side, are shown in Figure 11.17. The difference between the two J-V curves is due to the partial loss by the reflection and absorption at the semitransparent Au electrode. To provide sufficient electrical conductance, Au layer thickness has to be sufficient and the optical loss at Au electrode becomes significant. However, the inverted solar cell structure has the V2O5 layer which is not only transparent but also provides effective protection to the polymer layer. A transparent conductive oxides electrode, such as ITO, can therefore be deposited without compromising device performance. 

There is now sufficient evidence to validate, in principle, the long-held expectation that metallic SWNTs may ultimately be used in transparent electrodes, or at least as alternatives to the ITO technology. In practice, many technical issues from materials (separated metallic SWNTs) to fabrication have yet to be fully addressed. Beyond transparent electrodes, metallic SWNTs may find other applications in which extremely high electrical conductivity and excellent optical properties are both required, or even some in which optical transparency is not necessary. Again for DSSCs, as an example, great benefits for using metallic SWNTs to replace the presently used platinum metal in the cathode may be expected on the basis of available experimental results. " ... 

There are two types of OTEs [384], A metal microgrid with small (10-30-[xm) holes, which allows >50% of the radiation to be transmitted, or an interdigitized array metal electrode [385] may be used for identification of products or intermediates in redox systems [386, 387], The other type is comprised of a metal film deposited on a transparent support. The thickness of this film is a compromise between its electrical conductivity and optical transparency. This type of OTE can be used for surface analysis, particularly when the metal of the working electrode can provide the surface enhancement (Section 3.9.4). 

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