Photoactive Semiconductor Material

Many deposition methods are available for fabricating semiconductor electrodes. However, each technique has advantages and drawbacks that should be kept in mind when choosing a fabrication method. For instance, monocrystaUine materials with high PEC performance can often be obtained via molecular epitaxy processes. 

Chen et at, Photoelectrochemical Water Splitting, SpringerBriefs in Energy, DOI 10.1007/978-l-4614-8298-7 3, The Author(s) 2013 

However, such techniques require highly specialized substrates and/or large thermal budgets that might not be compatible with the more common, low cost metallic foUs, or glass substrates. Atomic layer deposition processes can provide precise control over material thickness however, such control requires low deposition rates, which would reduce device throughputs when thicker films are desired. Thus, a balance between semiconductor PEC performance and the cost, speed, and scalability of the material fabrication method must be considered. Commonly used techniques to fabricate semiconductor photoelectrodes include  

This list is by no means exclusive. Other available deposition techniques include sol-gel, powder pressing, etc. 

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To make a breakthrough in household appliances and other consumer product markets UV sensors have to become significantly cheaper while spectral selectivity as a major key feature must be guaranteed. Most of today s UV photodiodes are made from crystalline semiconductor materials. The cheaper materials (Si) lack spectral selectivity, and the wide band gap materials are very expensive. What they all have in common their top performance regarding sensitivity and speed. Crystalline photodiodes have risetimes of often below 1 s. However, the described processes to be sensed here are not faster than some milliseconds or even much slower. In order to obtain a reasonably-priced SiC or GaN photodiode, the photoactive area is often reduced to below 1 mm2 and barely fills the sensor housing. So far, the top sensitivity offered by the semiconductor has been sacrificed for a competitive... 

It had not been realized until recently that electrocatalysis in the water decomposition processes at photoactive semiconductor electrodes was as important as the band-structure properties of the semiconductor material itself. However, it is clear that the effective voltage, beyond the 1.23 V limit, or 1.23 F - hv, required to photoelectrolyze water at some net rate will also be determined, as with metals, by the electrocatalytic properties of the semiconductor surfaces. 

Given that a suitable semiconducting electrode material can be found, what advantages can a PEC system offer compared with an electrolyzer driven by a module of photovoltaic (PV) solar cells With a PEC system at an estimated theoretical efficiency of 25% (corresponding to 1.7 eV), the current density will be around 20mA cm As seen from Figure 4.3, the overpotential rj++ri ) falls off rapidly below 30 mA cm Assuming that the overpotential for water electrolysis is the same in both a PEC cell and an electrolyzer, it follows that the required band gap of the chosen photoactive semiconductor in the PEC cell can be kept as low as 1.6-1.8 eV. Compared with an electrolyzer driven by a solar PV module, each PEC cell... 

In this chapter we emphasize the use of homogeneous photosensitizers as light-harvesting components in artificial photosynthetic systems. Other reviews and monographs highlight the application of semiconductor materials in photosynthetic devices [48-50]. Nevertheless, several aspects of ET and charge separation are common to homogeneous photosensitizers and semiconductor photoactive materials, and these interrelations will be mentioned in later sections. 

It appears that future performance enhancements for flexible cells must rely on adjustments to the function of the substrate itself rather than improved semiconductor material. This means selectively roughening a smooth substrate, or choosing the weave pattern and filaments of a textile substrate, such that it scatters light into the photoactive layer and so enhances optical absorptimi in the thin semiconductor layer. Such improvements may be done on such a fine scale that quantum effects come into play, as they do for colouration of insects, and are perhaps possible for fabrics as well. 

The boundary between effects thus defined is, however, not sharp. If, for instance, light is absorbed by a layer of a photoactive adsorbate attached to the semiconductor electrode, it is apparently difficult to identify the light-absorbing medium as a solution or electrode material . Photoexcited solution molecules may sometimes also react at the photoexcited semiconductor electrode this process is labelled photogalvanovoltaic effect. 

Of the photocatalytic systems and structures composed of a single active material, eventually coupled with redox catalysts and/or metals, only a wide band gap oxide semiconductor, like Pt/Ti02, requiring UV irradiation, showed some photoactivity for water photosplitting. Water splitting with visible light requires the irradiation of multiple band gap photoelectrochemical cells (PEC) or Z-scheme systems (like the photosynthesis system of plants etc.). 

Even without deposition of a metal island, such powders often maintain photoactivity. The requirement for effective photoelectrochemical conversion on untreated surfaces is that either the oxidation or reduction half reaction occur readily on the dark material upon application of an appropriate potential, so that one of the photogenerated charge carries can be efficiently scavenged. Thus, for some photoinduced redox reactions, metallization of the semiconductor photocatalyst will be essential, whereas for others platinization will have nearly no effect. 

In considering photoactivity on metal oxide and metal chalcogenide semiconductor surfaces, we must be aware that multiple sites for adsorption are accessible. On titanium dioxide, for example, there exist acidic, basic, and surface defect sites for adsorption. Adsorption isotherms will differ at each site, so that selective activation on a particular material may indeed depend on photocatalyst preparation, since this may in turn Influence the relative fraction of each type of adsorption site. The number of basic sites can be determined by titration but the total number of acidic sites is difficult to establish because of competitive water adsorption. A rough ratio of acidic to basic binding sites on several commercially available titania samples has been shown by combined surface ir and chemical titration methods to be about 2.4, with a combined acid/base site concentration of about 0.5 mmol/g . 

However, many ternary systems incorporate a second photoactive center in addition to the [FeO ] octahedra in the present case, [NbO ] octahedra. The interaction between such multiple centers has not previously been investigated. In the present work, interband transitions are observed which appear characteristic of niobium centers, together with other transitions characteristic of the iron centers. Since these are homogeneous, singlephase materials, this result suggests that caution should be exercised when applying the conventional band model to such oxide semiconductors. 

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