Liquid-junction device semiconductors

Photovoltaic devices based on compound semiconductors like CuInSe2, CdTe, and quantum dots as well as liquid junction devices will require improved strategies for stabilization against environmental corrosion and dissolution. 

These techniques are covered in standard text books (20), review articles (21) and books (22) dedicated to the individual techniques. The subject of applications of any of these techniques in the study of properties of semiconductors is too broad to be covered here. We will just provide ashort outline focused on specific applicationsfor study of liquid-junction devices. 

A summary of other modulation techniques such as waveiength moduiation, temperature moduiation and stress moduiation can befound in Ref. 77, These modulation techniques have not yet found any significant applications in the study of semiconductor-liquid junction devices. 

In any case, it is perceived from the above discussion that the problem of longterm chemical stability of polycrystalline semiconductor liquid junction solar cells is far from being solved. Still, as already pointed out in the early research, any practical photovoltaic and PEC device would have to be based on polycrystalline photoelectrodes. Novel approaches mostly involving specially designed PEC systems with alternative solid or gel electrolytes and, most importantly, hybrid/sensitized electrodes with properties dictated by nanophase structuring - to be discussed at the end of this chapter - promise new advances in the field. 

Singh P, Singh R, Gale R, Rajeshwar K, DuBow J (1980) Surface charge and specific ion adsorption effects in photoelectrochemical devices. J Appl Phys 51 6286-6291 Bard AJ, Bocarsly AB, Pan ERF, Walton EG, Wrighton MS (1980) The concept of Fermi level pinning at semiconductor/liquid junctions. Consequences for energy conversion efficiency and selection of useful solution redox couples in solar devices. J Am Chem Soc 102 3671-3677... 

As it has been described in various other review articles before, the conversion efficiencies of photovoltaic cells depend on the band gap of the semiconductor used in these systems The maximum efficiency is expected for a bandgap around Eg = 1.3eV. Theoretically, efficiencies up to 30% seem to be possible . Experimental values of 20% as obtained with single crystal solid state devices have been reported " . Since the basic properties are identical for solid/solid junctions and for solid/liquid junctions the same conditions for high efficiencies are valid. Before discussing special problems of electrochemical solar cells the limiting factors in solid photovoltaic cells will be described first. 

If neither of these goals can be realized, layered semiconductors may not become useful electrode material in either semiconductor liquid junction or Schottky junction devices. Fortunately, evidence is already being obtained that the negative effects due to steps can be at least temporarily and partially alleviated (35, 36). Future development of chemical methods to inhibit deflection of minority carriers to the edges of steps and to reduce the high recombination rates at steps may open the way for the use of polycrystalline layered chalcogenide semiconductors in solar cell devices. 

Note that this is a very simplified case. A liquid junction, dual-layer insulator, trapped charges in the insulator, surface states at the insulator/semiconductor interface, channel doping profile, and multiple connecting metals have been omitted, for the sake of simplicity. They would be present in all real devices and situations, but would not affect the thought analysis in any significant way. 

It is evident from Eq. (94) that the maximum photovoltage depends critically on the exchange current Jo- In the case of pn-junctions, jo is determined by the injection and recombination (minority carrier device). Whereas in Schottky-type of cells jo can be derived from the thermionic emission model (majority carrier device). The analysis of solid state systems has shown that jo is always smaller for minority carrier devices [20,21]. Using semiconductor-liquid junctions, both types of cells can be realized. If in both processes, oxidation and reduction, minority carrier devices are involved, then jo is given by Eq. (37a), similarly as... 

Eq. (11.1) is also valid for pure solid state devices, such as semiconductor-metal contacts (Schottky junctions) and p-n junctions, as described in Chapter 2. The physics of the individual systems occurs only in y o- The main difference appears in the cathodic forward current which is essentially determined by /o. In this respect it must be asked whether the forward current is carried only by minority carriers (minority carrier device) or by majority carriers (majority carrier device). Using semiconductor-liquid junctions, both kinds of devices are possible. A minority carrier device is simply made by using a redox couple which has a standard potential close to the valence band of an n-type semiconductor so that holes can be transferred from the redox system into the valence band in the dark under cathodic polarization. In this case, the dark current is determined by hole injection and recombination (minority carrier device) and /o is given by Eq. (7.65), i.e. 

The liquid-junction photovoltaic cell has the advantages that the junction between electrolytic solution and semiconductor is formed easily and that polycrystalline semiconductors can be used. The principal disadvantage is that the semiconductor electrode tends to corrode under illumination. The electrochemical nature of the cell allows both production of electricity and generation of chemical products which can be separated, stored, and recombined to recover the stored energy. Liquid-junction cells also have the advantages that are attributed to other photovoltaic devices. Photovoltaic power plants can provide local generation of power on a small scale. The efficiency and cost of solar cells is independent of scale, and overall efficiency is improved by locating the power plant next to the load.72... 

The conversion of solar energy into electricity has been accomplished primarily by semiconductor photovoltaic devices. However, liquid junction photovoltaic cells have been developed.Dye-sensitized Ti02 nanoparticles and films are particularly important because of their potential application in liquid junction photovoltaic cells (Figure 22). In these the dye is bound to the surface of a semiconductor nanoparticle. The high surface area of the nanoparticle makes possible the adsorption of sufficient dye for efficient light collection. Dye sensitization can involve direct photoinjection of an electron from the dye to the nanoparticle... 

Other variants of the three types of device operation may be envisioned for semiconductor-liquid junctions. Thus, in the photoelectrolytic mode, the cell reaction clearly is driven (by light) in the contra-thermodynamic direction, that is, AG > 0. However, there are many instances, involving, for example, the photooxidation of organic compounds in which light merely serves to accelerate the reaction rate. Thus these cells operate in the photocatalytic mode. In fact, aqueous suspensions comprising irradiated semiconductor particles may be considered to be an assemblage of short-circuited microelectrochemical cells operating in the photocatalytic mode. 

An alternative to the photochemical reduction is the photoelectrochemical reduction. p-Type semiconductor/liquid junctions are extensively studied as PV devices. The p-type semiconducting electrodes can act as photocathodes for photoassisted... 

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