Semiconductor, mesoporous

To detail DSSC technologies, Fig. 18.1 illustrates the modus operandi of DSSCs. Initially, light is absorbed by a dye, which is anchored to the surface of either n- or p-type semiconductor mesoporous electrodes. Importantly, the possibility of integrating both types of electrodes into single DSSCs has evoked the potential of developing tandem DSSCs, which feature better overall device performances compared to just n-or p-type based DSSCs [19-26]. Briefly, n-type DSSCs, such as TiOz or ZnO mesoporous films, are deposited on top of indium-tin oxide (ITO) or fluorine-doped tin oxide (FTO) substrates and constitute the photoanodes. Here, charge separation takes place at the dye/electrode interface by means of electron injection from the photoexcited dye into the conduction band (cb) of the semiconductor [27,28]. A different mechanism governs p-type DSSCs, which are mainly based on NiO electrodes on ITO and/or FTO substrates... 

DAD 14b] Dadashi-Silab S., Tasdelen M.A., Kiskan B. et al., Photochemically nediated atom transfer radical polymerization using polymeric semiconductor mesoporous graphitic carbon nitride , Macromolecular Chemistry and Physics, vol. 215, pp. 675-681,2014. 

Of the various semiconductors tested to date, Ti02 is the most promising photocatalyst because of its appropriate electronic band structure, photostability, chemical inertness and commercial availability. But currently, a variety of nanostmctured Ti02 with different morphologies including nanorods, nanowires, nanostmctured films or coatings, nanotubes, and mesoporous/nanoporous structures have attracted much attention. 

One difficulty with many synthetic preparations of semiconductor NCs that complicates any interpretation of NMR results is the inevitable distribution of sizes (and exact shapes or surface morphologies). Therefore attempts to make semiconductors as a sort of molecular cluster having a well-defined stoichiometry are of interest to learn potentially about size-dependent NMR parameters and other properties. One approach is to confine the semiconductor inside a template, for instance the cuboctahedral cages of the sodalite framework or other zeolite structures, which have been characterized by multinuclear NMR methods [345-347], including the mesoporous channel material MCM-41 [341, 348]. 

Figure 9.7. Illustration of the usage of mesoporous films of transparent conducting oxides for novel types of solar cells. The dark gray areas correspond to ITO, the brighter ones to an oxide deposited onto the TCO matrix. The sphere symbolizes a dye. For instance, such films can be used as porous electrodes to include dyes and to deposit semiconductors such as ZnO. 

An electric field in the semiconductor may also produce passivation, as depicted in Fig. 6.1c. In semiconductors the concentration of free charge carriers is smaller by orders of magnitude than in metals. This permits the existence of extended space charges. The concept of pore formation due to an SCR as a passivating layer is supported by the fact that n-type, as well as p-type, silicon electrodes are under depletion in the pore formation regime [Ro3]. In addition a correlation between SCR width and pore density in the macroporous and the mesoporous regime is observed, as shown in Fig. 6.10 [Thl, Th2, Zh3, Le8]. 

Microcrystallites of direct semiconductors usually show a simple exponential decay of the PL intensity P with time, with time constants r in the ps and ns range at RT. A similar simple exponential decay (r = 20ms at 2 K) is observed for PL from mesoporous silicon of high porosity, which shows a weak confinement effect... 

An interesting question is whether such well-ordered pore arrays can also be produced in other semiconductors than Si by the same electrochemical etching process. Conversion of the macropore formation process active for n-type silicon electrodes on other semiconductors is unlikely, because their minority carrier diffusion length is usually not large enough to enable holes to diffuse from the illuminated backside to the front. The macropore formation process active in p-type silicon or the mesopore formation mechanisms, however, involve no minority carrier diffusion and it therefore seems likely that these mechanisms also apply to other semiconductor electrodes. 

The employment of a mesoporous semiconductor electrode dramatically increases the contact area between the semiconductor and the absorbed photoactive components and electrolyte, and eventually improves the conversion efficiency of the PECs. How-... 

Abstract This review highlights how molecular Zintl compounds can be used to create new materials with a variety of novel opto-electronic and gas absorption properties. The generality of the synthetic approach described in this chapter on coupling various group-IV Zintl clusters provides an important tool for the design of new kinds of periodically ordered mesoporous semiconductors with tunable chemical and physical properties. We illustrate the potential of Zintl compounds to produce highly porous non-oxidic semiconductors, and we also cover the recent advances in the development of mesoporous elemental-based, metal-chalcogenide, and binary intermetallic alloy materials. The principles behind this approach and some perspectives for application of the derived materials are discussed. 

Keywords Chalcogenide Germanium compound Mesoporous semiconductor Self-assembly Zintl compound... 

Recently, we and others demonstrated that appropriate germanide Zintl clusters in non-aqueous liquid-crystalline phases of cationic surfactants can assemble well-ordered mesostructured and mesoporous germanium-based semiconductors. These include mesostructured cubic gyroidal and hexagonal mesoporous Ge as well as ordered mesoporous binary intermetallic alloys and Ge-rich chalcogenide semiconductors. 

A new family of hexagonal mesoporous all-germanium semiconductors was prepared by the surfactant-assisted cross-linking polymerization reaction of Zintl [Geg]" anions (Scheme 1) with Ge(lV) bridges in formamide/ethylenediamine solution (1) [43]. 

Fig. 8 Kubelka-Munk optical absorption spectra of as-prepared mesostructured (black) and mesoporous NU-Ge-1 (red) semiconductors and NU-Ge-1 incorporating into the pores TCNE (blue) and TTF (green line) organic molecules. The recovered optical adsorption spectra of NU-Ge-1 by encapsulation of TCNE-TTF complexes are also given (dashed lines). Inset optical absorption spectrum of NU-Ge-1 encapsulating anthracene...

The ability of using mixed [Geg.nSin]" clusters as starting building blocks, which are soluble in ethylenediamine, allowed us to prepare mesoporous Ge/Si alloy semiconductors. These structures were synthesized as described above by the oxidative self-polymerization of mixed [Geg.nSin]" clusters with the assistance of self-assembled cationic surfactants (3). 

Fig. 13 Optical absorption spectra of mesoporous NU-GeSi-1 (Wtic/t), NU-GeSi-2 (red), NU-GeSi-3 (green) and NU-GeSi-4 (blue line) semiconductors...

Strongly depends on the chemical composition and systematically increases as the Si fraction in the walls increases. Analogous blue-shift in energy band gap of a nanoporous Ge/Si alloy semiconductor relative to pure mesoporous germanium has been observed by Sun et al. [44],... 

So far, the synthesis of mesoporous metal chalcogenides remains an open challenge but progress is being made. A few examples have been reported, including 11-VI group-type semiconductors such as CdS [68, 69], ZnS [70], and CdTe [17]. 

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