Liquid-junction device

Deviating from the primary method for pH, measurements for deriving SSs are carried out in cells, separating the solutions by a diffusion-limiting or liquid junction device. Liquid junction potentials forming as a result cannot be determined directly and vary with the composition of the solution forming the junction and the geometry of the junction device. The uncertainty due to the liquid junction potential can be estimated from independent measurements or from theoretical assumptions. 

The primary and the secondary buffers are separated by a liquid junction device, preferably a glass disk of fine porosity. Under these conditions, the contribution of the liquid junction potential to the cell voltage is very small. The increase in uncertainty is also very small. 

Since electrochemical doping occurs in semiconducting polymer films dipped in solution, the liquid-junction device composed of semiconducting polymer films can be doped photochemically (Eqs. (21), (22)). 

Various metal porphyrin complexes (MtP) have also been used to fabricate liquid-junction devices A photocurrent was generated by a liquid-junction Zn-... 

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. 

Among the direct optical spectroscopic techniques, photoluminescence is probably the most widely used in both in situ and ex situ configurations. Since, as with many other techniques, the temperature window in which one can measure in the in situ configuration is limited, one can usually find in the literature comparative studies between in situ and ex situ conditions. A short review article relevant to applications in liquid junction devices was written by Ellis (28). The technique also can... 

Other Modulation Techniques. Electric field modulated photoluminescence in a liquid junction device was reported in Ref. 75. The results agreed with the dead layer model previously discussed. Quantitative comparison of differentiation of Eq. 1 with respect to the eiectrode potentiai and impedance measurements yieided an agreement between the dead Layer and the space-charge iayer. 

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. 

A tunneling junction device was used to determine the water structure at the mercury electrode in an aqueous solution of 0.25MHg2 (N03)2 + 0.3M HNO3. It was found that the structure of water domains is the same as that of hexagonal ice. Hydrogen bonding is a dominant, structuredetermining factor in liquid water near the mercury electrode surface. ... 

In the sheathless interface, the electrical contact is obtained by coating with either a metal [85, 88-90] or a conductive polymer [91] the separation capillary outlet, which is shaped as sharp tip. Also employed are sheathless interfaces in which the electrical contact is established using a metal electrode or a conductive wire inserted into the outlet of the separation capillary [92], A small gap between the separation capillary and the needle of the ionization device filled by a liquid is the approach made to establish the electrical contact in the liquid junction interface [86,87], This arrangement is also realized by making porous through chemical etching the tip [93] or a small section of the wall [94] of the separation capillary at its outlet. 

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... 

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