Semiconductors liquid-junction

At present, the microwave electrochemical technique is still in its infancy and only exploits a portion of the experimental research possibilities that are provided by microwave technology. Much experience still has to be gained with the improvement of experimental cells for microwave studies and in the adjustment of the parameters that determine the sensitivity and reliability of microwave measurements. Many research possibilities are still unexplored, especially in the field of transient PMC measurements at semiconductor electrodes and in the application of phase-sensitive microwave conductivity measurements, which may be successfully combined with electrochemical impedance measurements for a more detailed exploration of surface states and representative electrical circuits of semiconductor liquid junctions. 

Miller B, Heller A (1976) Semiconductor liquid junction solar cells based on anodic sulphide films. Nature 262 680-681... 

As a matter of fact, semiconductor/liquid junctions provide the most efficient wet chemical method presently known for converting sunlight into electrical or... 

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. 

Chang KC, HeUer A, Schwartz B, Menezes S, Miller B (1977) Stable semiconductor liquid junction ceU with 9 percent solar-to-electrical conversion efficiency. Science 196 1097-1099... 

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... 

Heller A, Chang KC, Miller B (1977) Spectral response and efficiency relations in semiconductor liquid junction solar cells. J Electrochem Soc 124 697-700 Elhs AB, Kaiser SW, Wrighton MS (1976) Optical to electrical energy conversion. Characterization of cadmium sulfide and cadmium selenide based photoelectrochemical cells. J Am Chem Soc 98 6855-6866... 

Similar photovoltaic cells as those described above can be made with semiconductor/ liquid Junctions. The basic function of such a cell is illustrated in terms of an energy scheme in Fig. 2. The system consists of an n-type semiconductor and an inert metal... 

Nakato, Y., Ueda, K., Yano, H., and Tsubomura, H., Effect of microscopic discontinuity of metal over layers on the photo voltages in metal-coated semiconductor-liquid junction photoelectrochemical cells for efficient solar energy conversion, /. Phys. Chem., 92, 2316, 1988. 

Parkinson, B.A., Heller, A., and Miller, B., Effects of cations on the performance of the photoanode in the n-gallium arsenide/potassium selenide (K2Se)-potassium diselenide (K2Se2)-potassium hydroxide/carbon semiconductor liquid junction solar cell, /. Electrochem. Soc., 126,954,1979. 

Bhattacharya, R. N. Pramanik, P. 1982. Semiconductor liquid junction solar cell based on chemically deposited Bi2S3 thin film and some semiconducting properties of bismuth chalcogenides. J. Electrochem. Soc. 129 332-335. 

Similar photovoltaic cells can be made of semiconductor/liquid junctions. For example, the system could consist of an n-type semiconductor and an inert metal counterelectrode, in contact with an electrolyte solution containing a suitable reversible redox couple. At equilibrium, the electrochemical potential of the redox system in solution is aligned with the Fermi level of the semiconductor. Upon light excitation, the generated holes move toward the Si surface and are consumed for the oxidation of the red species. The charge transfer at the Si/electrolyte interface should account for the width of occupied states in the semiconductor and the range of the energy states in the redox system as represented in Fig. 1. 

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. 

Ions that are not chemisorbed do not affect the performance of semiconductor liquid junction solar cells.32 Weakly chemisorbed ions produce inadequate splitting of surface states between the edges of the conduction and valence band and increase rather than decrease the density of the surface states in the band gap and thus the recombination velocity. Bi3+ is an example of such an ion. As seen in Figure 5, it decreases the efficiency of the n—GaAs 0.8M K2Se-0.1M K2Se2-lM KOH c cell.30 Since the chemisorption of Bi3+ is weak, the deterioration in performance is temporary. The ion is desorbed in 10 min. and the cell recovers. 

Schwerzel, R. E. Brooman, E. W. Craig, R. A. V. E. Wood in "Semiconductor Liquid-Junction Sola Cells," edited by Heller, A. (Electrochemical Society, Princeton, 1977) p. 293. 

Inoue, T., Watanabe, T., Fujishima, A., and Honda, K., Competitive Oxidation at Semiconductor Photoanodes, in Semiconductor Liquid--Junction Solar Cells, Heller, A., Ed., The Electrochemical Society, Princeton, N3, 1977, 210. 

Heller, A., ed. "Semiconductor Liquid Junction Solar Cells" The Electrochemical Society, Inc. Princeton, N.J., 1977. 

FIGURE 7. Photoexcitation at an n-type semiconductor-liquid junction. 

Fig. 1. Interfacial energetics at semiconductor-liquid junctions. D is an electron donor and A is...

As for semiconductor/metal contacts, a change in the Fermi level of the liquid phase should result in a different amount of charge transferred across the semicondnctor/liqnid junction. For semiconductor/liquid junctions, the important energetic trends for a series of different liqnid contacts can thns be determined by measuring the solntion redox potential relative to a standard reference electrode system. Within this model, solutions with more positive redox potentials shonld indnce greater charge transfer in contact with n-type semicondnctors. 

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... 

The application of semiconductor-liquid junctions is of special interest for the direct production of a chemical fuel. Especially the production of hydrogen by photoelectrolysis of H2O has been studied by many research groups (compare with [114,194]. It has been demonstrated by many authors that H2-formation is rather easy at semiconductor electrodes. The crucial point is the simultaneous oxidation of H2O. So far, photoelectrolysis was only achieved with SrTiOs, a semiconductor of a large bandgap (Eg = 3.1 eV) [206]. Very recently, photocleavage of HjO was also found with some niobates under open... 

However, just covering the surface by a complete metal layer would lead to a metal-semiconductor Schottky junction which usually exhibits high loss currents. The disadvantage can be avoided by depositing smaU metal clusters. In this type of surface modification, the favourable quality of the semiconductor-liquid junction is kept, as shown for redox reactions by Nakato et al. (see Ref. 

Bard AJ, Wrighton MS (1977) In Heller A (ed) Semiconductor-liquid junctions. Electrochemical Society, Princeton NY, Proc. vol 77-3, p 195... 

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