Solar liquid junction

Following the same procedure, the kinetic constants have been determined for very different electrochemical conditions. When n-WSe2 electrodes are compared in contact with different redox systems it is, for example, found9 that no PMC peak is measured in the presence of 0.1 M KI, but a clear peak occurs in presence of 0.1 M K4[Fe(CN)6], which is known to be a less efficient electron donor for this electrode in liquid junction solar cells. When K4[Fe(CN)6] is replaced by K3[Fe(CN)6], its oxidized form, a large shoulder is found, indicating that minority carriers cannot react efficiently at the semiconductor/electrolyte junction (Fig. 31). 

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

Tomkiewicz M, Ling I, Parsons WS (1982) Morphology, properties, and performance of electrodeposited n-CdSe in liquid junction solar cells. J Electrochem Soc 129 2016-2022... 

Kressin AM, Doan VV, Klein JD, Sailor MJ (1991) Synthesis of stoichiometric cadmium selenide films via sequential monolayer electrodeposition. Chem Mater 3 1015-1020 Miller B, Heller A, Robbins M, Menezes S, Chang KC, Thomson JJ (1977) Solar conversion efficiency of pressure sintered cadmium selenide liquid junction cells. J. Electrochem Soc 124 1019-1021. 

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. 

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... 

McCann JF, Kainthla RC, Skyllas-Kazacos M (1983) Chemical deposition of Cdi xHgxS thin film electrodes for liquid-junction solar cells. Sol Energy Mater Sol CeUs 9 247-251... 

Robbins M, Bachmann KJ, Lambrecht VG, Thiel EA, Thomson J Jr., Vadimsky RG, Menezcs S, Heller A, Miller B (1978) CulnS2 liquid junction solar cells. J Electrochem Soc 125 831-832... 

Benniston AC, Haniman A (2008) Artificial photosynthesis. Materials Today 11 26-34 Inoue T, Fujishima A, Konishi S, Honda K (1979) Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions of semiconductor powders. Nature 277 637-638 Halmann M (1978) Photoelectrochemical reduction of aqueous carbon dioxide on p-type gallium phosphide in liquid junction solar cells. Nature 275 115-116 Heminger JC, Carr R, Somorjai GA (1987) The photoassisted reaction of gaseous water and carbon dioxide adsorbed on the SrH03 (111) crystal face to form methane. Chem Phys Lett 57 100-104... 

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. 

Despite the potential impact of novel photosynthetic routes based on these developments, the most ambitious application remains in the conversion of solar energy into electricity. Dvorak et al. showed that photocurrent as well as photopotential response can be developed across liquid-liquid junctions during photoinduced ET reactions [157,158]. The first analysis of the output power of a porphyrin-sensitized water-DCE interface has been recently reported [87]. Characteristic photocurrent-photovoltage curves for this junction connected in series to an external load are displayed in Fig. 22. It should be mentioned that negligible photoresponses are observed when only the platinum counterelectrodes are illuminated. Considering irradiation AM 1, solar energy conversions from 0.01 to 0.1% have been estimated, with fill factors around 0.4. The low conversion... 

M. Halmann. Photoelectrochemical reduction of aqueous carbon dioxide on p-type gallium phosphide in liquid junction solar cells. Nature. 1978, 275(5676) 115-116. 

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]. 

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. 

As described in Section 3 of Chapter 4, the stabilization of n-Si electrode by coating with poly(pyrrole) has attracted much attention. The stabilization of a small bandgap n-semiconductor electrode against oxidation is of great value not only to convert visible light into chemical energy, but also to construct liquid-junction solar cells operated under visible irradiation. The poly(pyrrole) film is usually electropolymerized on the semiconductor electrode dipped in the aqueous solution of pyrrole. The remarkable stabilizing effect of poly(pyrrole) film on polycrystalline n-Si is shown in Fig. 22 67). The photocurrent obtained under irradiation in the aqueous solution of... 

In summary, at nanostructured tin-oxide semiconductor-aqueous solution interfaces, back ET to molecular dyes is well described by conventional Marcus-type electron-transfer theory. The mechanistic details of the reaction, however, are remarkably sensitive to the nature of the semiconductor-dye binding interactions. The mechanistic differences point, potentially, to differing design strategies for kinetic optimization of the corresponding liquid-junction solar cells. 

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