Photovoltaic Photoelectrochemical Solar Cells

A dye-sensitized solar cell also operates in the photovoltaic mode. It is, however, based on a slightly different operation principle because the absorber is a dye instead of a semiconductor and the charge separation has been attributed to differences in the chemical potential on both sides of the dye [100]. It is therefore considered an excitonic solar cell [101]. 

Photo(electro)catalysis encompasses a wide range of reactions [11,102-105]. First, light-induced water dissociation is considered because of its relevance in solar fuel 

Therefore, two fundamental research strategies can be envisaged (i) the development of robust devices made from abundant (nontoxic) materials that exhibit reduced efficiencies and (ii) the development of devices that use the photonic excess energy as, for instance, in tandem or other third-generation photoactive structures. Here, the theoretical efficiency can increase to values above 40% for a two-junction device, depending on the respective energy gaps. In the former approach, concepts and principles from photosynthesis have already been adapted. Examples include the preparation of macromolecules that contain centers with 

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Two surfaces are conditioned the aforementioned InP(lll) A-face with In atoms protruding a quarter of a monolayer and the In-rich InP(lOO) (2x4) reconstructed surface of a thin homoepitaxial layer, prepared by metal organic vapor-phase epitaxy (MOVPE) on a crystalline InP wafer substrate [241], Whereas the former surface has been successfully conditioned for the development of an efficient and stable photovoltaic photoelectrochemical solar cell (see Section 2.5.2), the latter surface has been employed in the development of an efficient photoelec-trocatalytic structure for light-induced hydrogen generation (see Section 2.6.2). 

For instance, in a photovoltaic or a photoelectrochemical solar cell, a (small) drop in the electrochemical potential over the active phase is necessary for collecting the electrons [6, 9]. However, it leads to a decrease of the work that an electron can perform in the external circuit. 

Conventional solar cells are built from inorganic silicon-like materials. Efficiency of such solar cells is high, but they originate from expensive materials and special techniques are required for their processing. Recently hybrid and photoelectrochemical solar cells [54] have been cost effective alternatives for conventional silicon solar cells. The correspondence between the photon absorption and charge separation events is the point of differentiation between the photovoltaic effect in a semiconductor junction, and that in a photon-induced generation of a chemical potential in natural systems, i.e. photosynthesis. In the latter, and this is very simple but highly relevant in the context of artificial photosynthetic systems, the point in space at which the... 

H. Gerischer, Photoelectrochemical solar cells, in Proceedings of the 2nd Electrochemical Photovoltaic Solar Energy Conference, Ed. by R. van Overstraeten and W. Palz, Reidel, Boston, 1979, pp. 408-431. 

Indium phosphide has been a successful material in the preparation of solid-state, photovoltaic, and photoelectrocatalytic electrochemical solar cells [237-240]. Photovoltaic soUd-state solar cells reach single-junction efHciencies above 24% [237]. When used as a photocathode in photoelectrochemical solar energy conversion, the material has shown excellent stability [239], related to the unique surface chemistry of the polar InP(lll) A-face that exposes In atoms only [240]. The photoelectrochemical conditioning of single-crystalline p-type InP with the aim of preparing efficient and stable photoelectrochemical solar cells for photovoltaic and photoelectrocatalytic operation is described in the following and the induced surface transformations are analyzed employing a variety of surface-sensitive methods. 

In this section, photoelectrochemical solar cells are presented that convert sunlight into electricity. Also, a sohd-state photovoltaic cell with a polycrystalline CIS absorber has been included that is prepared by a photoelectrochemical conditioning procedure and by KCN etching. 

In dye-sensitized inorganic heterojunction solar cells, a monolayer of dye is sandwiched between two wide band gap semiconductors one of them exhibits a p-type and the other an n-type conduction mechanism. Inorganic p-type semiconductors were successfully applied in an attempt to replace the liquid electrolyte in photoelectrochemical solar cells, and in fact the first solid-state dye-sensitized photovoltaic device described in the literature was based on a wide band gap p-type semiconductor material [31]. 

Only a few materials were studied concerning their applicability to dye-sensitized hole injection processes. Among those are different copper(I) compounds (e.g. Cu(I)SCN, Cu(I)I, Cu2(I)0 [33-35]) and nickel(II) oxide [36]. Photovoltaic performances of such devices are orders of magnitudes poorer than those of classical dye-sensitized photoelectrochemical solar cells based on n-type materials. Substantial advantages could arise if an efficient photo-hole injection process would be available. The formation of solid-state tandem solar cells would become feasible, and a quantum step in device efficiency of dye-sensitized solar cells could be at reach. However, because of the poor performance of all known photocathodes, a combination of available photoanodes and photocathodes to a tandem device always results in a device that is photovoltaically less efficient than the photoanode on its own. The concept for electrolyte-based tandem cells exists. However, it contains strong potential to improve the photovoltaic performance in both electrolytic and in solid-state, dye-sensitized solar cells. 

The combination of favorable properties of PANI and TiO opens the possibility for various applications of PANI/TiO nanocomposite materials, such as piezoresistivity devices [41], electrochromic devices [99,118], photoelectrochemical devices [43,76], photovoltaic devices/solar cells [44,50,60,61,93,119], optoelectronic devices/UV detectors [115], catalysts [80], photocatalysts [52,63,74,75,78,84,87,97,104,107,121,122,125], photoelectrocatalysts [122,123], sensors [56,61,65,69,85,86,95,120,124], photoelectrochemical [110] and microbial fuel cells [71], supercapacitors [90,92,100,109,111], anode materials for lithium-ion batteries [101,102], materials for corrosion protection [82,113], microwave absorption materials [77,87,89], and electrorheological fluids [105,106]. In comparison with PANI, the covalently bonded PANI/TiO hybrids showed significant enhancement in optical contrast and coloration efficiency [99]. It was observed that the TiO nanodomains covalently bonded to PANI can act as electron acceptors, reducing the oxidation potential and band gap of PANI, thus improving the long-term electrochromic stability [99]. Colloidal... 

The photoelectrochemical solar cells form the first family of organic photovoltaic devices. Typically, the active layer of such devices consists of nanostructured and dye sensitized electrodes, whereas the other electrode (counter electrode) is separated by an electrolyte or hole conductor. The highest efficiency of around 11% is achieved in dye sensitized solar cells (DSSCs) using TiO nanostructured electrodes. Schematic layout of a typical DSSC device is shown in Figure 2 ... 

Semiconductor-Liquid Junction From Fundamentals to Solar Fuel Generating Structures, Fig. 7 Overview of selected output power characteristics of photoelectrochemical solar cells operating in the photovoltaic mode note that here, also 2-electron transfCT redox couples have been used, for which the Marcus-Gerischer theory does not apply the output power characteristics have been normalized and the respective efficiencies are given at each characteristic the conditions (illumination intensity and source) are as follows n-GaAs 95mWcm (sunlight) [15, 17] p-InP ... 

Tennakone K., Kumara G., Wijayantha K., Kottegoda L, Perera V., Aponsu G. Nano-porous solid-state photovoltaic cell sensitized with tannin. Semicond. Sci. Technol. 1998 13 134-138 Tennakone K., Kumara G.R.R.A., Kottegoda I.R.M., Perera V.P.S. An efficient dye-sensitized photoelectrochemical solar cell made from oxides of tin and zinc. Chem. Commun. 1999 15-16... 

Iron sulfide as pyrite (FeS2) has been shown to be a promising photoactive material for photoelectrochemical and photovoltaic solar cells. Whereas a variety of methods have been employed for the preparation of thin films of this material, including CVT, MOCVD, spray-pyrolysis, and sulfidation of either iron oxide or iron, the direct efectrodeposition of FeS2 thin films has proven to be problematic. 

Dye-sensitized solar cells (DSSCs) are photoelectrochemical solar devices, currently subject of intense research in the framework of renewable energies as a low-cost photovoltaic device. DSSCs are based upon the sensitization of mesoporous nanocrystalline metal oxide films to visible light by the adsorption of molecular dyes.5"7 Photoinduced electron injection from the sensitizer dye (D) into the metal oxide conduction band initiates charge separation. Subsequently, the injected electrons are transported through the metal oxide film to a transparent electrode, while a redox-active electrolyte, such as I /I , is employed to reduce the dye cation and transport the resulting positive charge to a counter electrode (Fig. 17.4). 

The formation of amorphous silicon films by electrodeposition from non-aqueous solutions have also been studied [18, 19]. For example, a flat homogeneous silicon film of about 0.25 pm thick can be deposited from 0.2 M SiHCl3-0.03 M Bu4NBr-THF bath on the cathode of Pt, Au, Cu, GC, ITO, etc., although small amounts of impurities (O, C, Cl) are contained. Their use in photovoltaic or photoelectrochem-ical solar cells are promising, although there are still many problems to be solved. 

The promise of photoelectrochemical devices of both the photovoltaic and chemical producing variety has been discussed and reviewed extensively.Cl,, 3,4) The criteria that these cells must meet with respect to stability, band gap and flatband potential have been modeled effectively and in a systematic fashion. However, it is becomirg clear that though such models accurately describe the general features of the device, as in the case of solid state Schottky barrier solar cells, the detailed nature of the interfacial properties can play an overriding role in determining the device properties. Some of these interface properties and processes and their potential deleterious or beneficial effects on electrode performance will be discussed. 

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