Solid-liquid junction

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. 

Interfacial electron transfer across a solid-liquid junction can be driven by photoexcitation of doped semiconductors as single crystals, as polycrystalline masses, as powders, or as colloids. The band structure in semiconductors (281) makes them useful in photoelectrochemical cells. The principles involved in rendering such materials effective redox catalysts have been discussed extensively (282), and will be treated here only briefly. 

After photoinduced electron injection, the strong interfacial electric field at the semiconductor solid-liquid junction draws the injected electron (or hole) into the semiconductor and towards the electrical contact. This process facilitates charge separation and reduces the chances of hole-electron recombination. The most important prerequisites for this process include, as described, good redox matching of the dye species excited state and the conduction band of the semiconductor, as well as strong orbital coupling between the immobilized dye and semiconductor. 

Schumacher, R. (1990) The quartz microbalance-a novel approach to the in situ investigation of interfadal phenomena at the solid liquid junction. Angewandte Chemie (International Edition in English), 29, 329-343. 

Electrodeposited surfactant AlPc (41) film also formed solid liquid-junction type cells with redox systems Fe(CN) , benzoquinone/hydroquinone, I3 /I, ... 

Fig. 5.5 SEM surface view and cross section of an electrodeposited, ca. 1 p.m thick, CdSe/li film subjected to accelerated photocorrosion by the apphcation of -0.1 V vs. Pt bias in polysulfide solution under a focused, high-power (1 W cm ) solar illumination for 30 min. The coherence of the as-deposited film morphology is evident. The authors emphasize that, even in this situation, the liquid junction nature prevents the flow of high leakage currents during the process (as it might be the case with a solid junction). (Reprinted from [99], Copyright 2009, with permission from Elsevier)...

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. 

H.J. Yoon, J.H. Shin, S.D. Ixe, H. Nam, G.S. Cha, T.D. Strong, and R.B. Brown, Solid-state ion sensors with a liquid junction-free polymer membrane-based reference electrode for blood analysis. Sens. Actuators B. 64, 8-14 (2000). 

Water is involved in most of the photodecomposition reactions. Hence, nonaqueous electrolytes such as methanol, ethanol, N,N-d i methyl forma mide, acetonitrile, propylene carbonate, ethylene glycol, tetrahydrofuran, nitromethane, benzonitrile, and molten salts such as A1C13-butyl pyridium chloride are chosen. The efficiency of early cells prepared with nonaqueous solvents such as methanol and acetonitrile were low because of the high resistivity of the electrolyte, limited solubility of the redox species, and poor bulk and surface properties of the semiconductor. Recently, reasonably efficient and fairly stable cells have been prepared with nonaqueous electrolytes with a proper design of the electrolyte redox couple and by careful control of the material and surface properties [7], Results with single-crystal semiconductor electrodes can be obtained from table 2 in Ref. 15. Unfortunately, the efficiencies and stabilities achieved cannot justify the use of singlecrystal materials. Table 2 in Ref. 15 summarizes the results of liquid junction solar cells prepared with polycrystalline and thin-film semiconductors [15]. As can be seen the efficiencies are fair. Thin films provide several advantages over bulk materials. Despite these possibilities, the actual efficiencies of solid-state polycrystalline thin-film PV solar cells exceed those obtained with electrochemical PV cells [22,23]. 

Here, C(s) is a solid reduced form of A, and the double line denotes a liquid junction of negligible potential. Also, A is a thermally equilibrated excited (or thexi) state and, as such, is essentially a different chemical species from A. We suppose, therefore, that it is possible to find an electrode M that is reversible to the reduction of A to C(s), but completely polarized with respect to the reduction of A to C(s). Operation of this cell under steady state conditions should then give the desired reversible work available from the photoproduction of A. ... 

Charge Transport in Single Molecular Junctions at the Solid/Liquid Interface... 

In the following we will focus on three molecular electronics test beds as developed and employed for applications at electrified solid/liquid interfaces (1) STM and STS, (2) assemblies based on horizontal nanogap electrodes, and (3) mechanically-controlled break junction experiments. For a more detailed description of the methods we refer to several excellent reviews published recently [16-22]. We will also address specific aspects of electrolyte gating and of data analysis. 

Tao et al. [32] pioneered a technique based on the formation of single molecular junctions between the tip of an STM and a metal substrate. The method was adapted by other groups, modified and applied to a large number of molecular conductance studies at (electrified) solid/liquid interfaces [33, 113-119]. For details we refer to Sect. 2.3. 

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