Semiconductor cells

Photoelectrochemical semiconductor cells are used to convert photon energy into chemical substances or into electricity, the former is a photodectrolytic cell and the latter is a photovoltaic cell. A photoelectrochemical semiconductor cell consists of either a pair of metal and semiconductor electrodes or a pair of two semiconductor electrodes. 

LB chlorophyll monolayers (25 mN/m 1.4 nm2/molecule) on Pt electrodes showed low photoactivity, possibly caused by a quenching of excited states by the metal electrode or by total reversibility of electron exchange. Addition of electron acceptors, e.g., quinones, had no effect. The optically transparent tin oxide semiconductor electrode proves to be a much better subphase for the generation of photocurrents. Chlorophyll-coated Sn02 combined with a platinum electrode gave approximately 100 nA/cm. Similar results were obtained with photovoltaic systems of the form mercury droplet/buffer solution/chlorophyll a monolayer/electron acceptor monolayer/aluminum (Fig. 6.9.3). The quantum yield of such monolayer arrangements never exceeded 10" in any of these systems and is thus far away from competitive inorganic semiconductor cells (Norris and Meisel, 1989). 

Figure 6. Scheme of a sensitized semiconductor cell for the conversion of light energy into electricity... 

Fig. 4. Schematic representation of (A) n-type semiconductor photoelectrochemical cell. (B) p-type semiconductor cell.

This device is a handheld CWA detector using Environics patented open loop ion mobility spectrometry" that can operate IMS without the need for desiccant. Unlike conventional IMS detectors, in that M90 the sample does not need to penetrate a membrane before reaching the IMS cell, which makes faster responses possible. It also includes a semiconductor cell for other detection purposes. Chemical identification is based on signal pattern recognition. 

Fig. 3.1 Class p cells (a) A n p junction (b) a metal -insulator - p-type semiconductor cell. The polarity sho m indicates an externally applied forward potential.

Figure 3.23 Semiconductor flat-band potentials and the open circuit photovoltage (Toe) of dye-sensitised semiconductor cells measured under 520 nm monochromatic light. Reprinted from Sayama et al., (1998) . Copyright (1998) American Chemical Society...

There are difficulties in making such cells practical. High-band-gap semiconductors do not respond to visible light, while low-band-gap ones are prone to photocorrosion [182, 185]. In addition, both photochemical and entropy or thermodynamic factors limit the ideal efficiency with which sunlight can be converted to electrical energy [186]. 

Fig. V-14. Energy level diagram and energy scales for an n-type semiconductor pho-toelectrochemical cell Eg, band gap E, electron affinity work function Vb, band bending Vh, Helmholtz layer potential drop 0ei. electrolyte work function U/b, flat-band potential. (See Section V-9 for discussion of some of these quantities. (From Ref. 181.)...

The empirical pseiidopotential method can be illustrated by considering a specific semiconductor such as silicon. The crystal structure of Si is diamond. The structure is shown in figure Al.3.4. The lattice vectors and basis for a primitive cell have been defined in the section on crystal structures (ATS.4.1). In Cartesian coordinates, one can write G for the diamond structure as... 

The combination of electrochemistry and photochemistry is a fonn of dual-activation process. Evidence for a photochemical effect in addition to an electrochemical one is nonnally seen m the fonn of photocurrent, which is extra current that flows in the presence of light [, 89 and 90]. In photoelectrochemistry, light is absorbed into the electrode (typically a semiconductor) and this can induce changes in the electrode s conduction properties, thus altering its electrochemical activity. Alternatively, the light is absorbed in solution by electroactive molecules or their reduced/oxidized products inducing photochemical reactions or modifications of the electrode reaction. In the latter case electrochemical cells (RDE or chaimel-flow cells) are constmcted to allow irradiation of the electrode area with UV/VIS light to excite species involved in electrochemical processes and thus promote fiirther reactions. 

Selenium exhibits both photovoltaic action, where light is converted directly into electricity, and photoconductive action, where the electrical resistance decreases with increased illumination. These properties make selenium useful in the production of photocells and exposure meters for photographic use, as well as solar cells. Selenium is also able to convert a.c. electricity to d.c., and is extensively used in rectifiers. Below its melting point selenium is a p-type semiconductor and is finding many uses in electronic and solid-state applications. 

Mid- and near-infrared Nernst filament globar NaCl or KBr Grating interferometer Golay cell thermocouple bolometer pyroelectric photoconductive semiconductor... 

Defectors are similar in fype fo fhose for fhe far-inffared, namely fhermocouples, bolomefers, Golay cells or phofoconducfive semiconductors. 

It is used as a fluorinating reagent in semiconductor doping, to synthesi2e some hexafluoroarsenate compounds, and in the manufacture of graphite intercalated compounds (10) (see Semiconductors). AsF has been used to achieve >8% total area simulated air-mass 1 power conversion efficiencies in Si p-n junction solar cells (11) (see Solarenergy). It is commercially produced, but usage is estimated to be less than 100 kg/yr. 

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