Semiconductor networks

The injected electron flows through the semiconductor network to arrive at the back contact and then through the external load to tlie counterelectrode. At the counterelectrode, reduction of the triiodide, in turn, regenerates iodide [Eq. 

The details of the operating principles of the dye-sensitized solar cell are given in Fig. 2. The photo excitation of the metal-to-ligand charge transfer (MLCT) of the adsorbed sensitizer (Eq. 1) leads to injection of electrons into the conduction band of the oxide (Eq. 2). The oxidized dye is subsequently reduced by electron donation from an electrolyte containing the iodide/triiodide redox system (Eq. 3). The injected electron flows through the semiconductor network to arrive at the back contact and then through the external load to the counter... 

A fascinating aspect of the sensitized colloidal semiconductor films is that injected electrons created throughout the semiconductor network are collected in the external circuit with high efficiency. This implies that carrier transport through the 10 pm thick film occurs with no measurable recombination loss. The mechanisms of carrier transport have been studied in some detail. Carrier transport in a semiconductor film can be described by the continuity equation [155] ... 

Step 6 Electrons can be taken up directly by protonated bases (or amino acids) attached immediately to the surface. Additionally a second mechanism is possible, namely, an electron hopping over a distance to bases (or amino acids) inside the dehydrated parts of adsorbed molecules. Because water is squeezed out, especially for the semiconductor network of the 7r-state, the activation energy for electron transfer into the conduction band should be lowered and thus electrons can move inside the double Y helix (or in the globulus ). 

First, the p-type material needs to have electronic levels into which holes can be injected from the oxidized or excited state of the dye. The redox levels of the dye and the p-type material therefore have to be adapted carefully. An intimate contact between the sensitized metal oxide and the p-type material is vital to assure fast injection and regeneration processes (Fig. 1). This implies either the growth or deposition of one semiconductor inside a preformed, sensitized porous film of its counterpart or the in situ formation of the sensitized composite. Direct formation of the sensitized junction would be appreciable however, charge collection within the two independent semiconductor networks, in which at least one semiconductor is formed from nanometer-sized inorganic semiconductor particles, demands intimate contact between the particles. Reduced... 

Nanostructured electrodes of wide band gap semiconductors can be coated with QDs by a number of methods direct adsorption of QDs on nanostructured semiconductor networks can be achieved by CBD, electrodeposition or spin coating. Alternatively, they can be anchored using linkers, such as mercaptopropionic acid (MPA), which have an acid function which binds to metal oxides, such as Ti02 and ZnO, and a thiol (or amine) group, which binds to CdSe or CdS. When linkers such as MPA are used, a uniform coverage throughout the internal and external... 

To improve the charge transport in the bulk semiconductor network, alternative materials and morphologies have been employed. Kongka-nand et al. have shown that the maximum IPCE for CdSe of the same size (3 nm) assembled on Ti02 nanoparticles and nanotubes differed by... 

Since 1970 the subject of amoiphous semiconductors, in particular silicon, has progressed from obscurity to product commercialisation such as flat-panel hquid crystal displays, linear sensor arrays for facsimile machines, inexpensive solar panels, electrophotography, etc. Many other appHcations are at the developmental stage such as nuclear particle detectors, medical imaging, spatial light modulators for optical computing, and switches in neural networks (1,2). 

Even though silicon is metallic in appearance, it is not generally classified as a metal. The electrical conductivity of silicon is so much less than that of ordinary metals it is called a semiconductor. Silicon is an example of a network solid (see Figure 20-1)—it has the same atomic arrangement that occurs in diamond. Each silicon atom is surrounded by, and covalently bonded to, four other silicon atoms. Thus, the silicon crystal can be regarded as one giant molecule. 

All metals conduct electricity on account of the mobility of the electrons that bind the atoms together. Ionic, molecular, and network solids are typically electrical insulators or semiconductors (see Sections 3.f3 and 3.14), but there are notable exceptions, such as high-temperature superconductors, which are ionic or ceramic solids (see Box 5.2), and there is currently considerable interest in the electrical conductivity ol some organic polymers (see Box 19.1). 

Conventional electrodeposition from solutions at ambient conditions results typically in the formation of low-grade product with respect to crystallinity, that is, layers with small particle size, largely because it is a low-temperature technique thereby minimizing grain growth. In most cases, the fabricated films are polycrystalline with a grain size typically between 10 and 1,000 nm. The extensive grain boundary networks in such polycrystalline materials may be detrimental to applications for instance, in semiconductor materials they increase resistivity... 

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