Polycrystalline Photoelectrodes

It has been illustrated that polycrystalline materials can be operated in regenerative electrolytic solar cells yielding substantial fractions of the respectable energy conversion efficiency obtained by using single crystals. Pressure-sintered electrodes of CdSe subsequently doped with Cd vapor have presented solar conversion efficiencies approaching 3/4 of those exhibited by single-crystal CdSe electrodes in alkaline polysulfide PEC [84]. 

In 1976, Hodes et al. [85] reported the stabilization of polycrystalline CdSe photoelectrodes (prepared by electrodeposition and subsequent heat treatment) in de-aerated alkaline aqueous or organic (not specified) solution of S , in which some elemental sulfur was dissolved. It was claimed that besides CdSe, other semiconductors such as CdS, CdTe (n- and p-types), ZnSe, or Bi2S3 are also stable as photoelectrodes in a polycrystalline form in the sulfide solution, and that such cells under AMI sunlight are stable over periods of months, a rather exaggerated claim. It was described in addition how a part or all of the converted energy could be stored in a controlled way in the system by the introduction of an electrode of porous silver. 

An interesting idea has been to prepare the photosensitive electrode on site having the liquid play the dual role of a medium for anodic film growth on a metal electrode and a potential-determining redox electrolyte in the electrochemical solar cell. Such integration of the preparation process with PEC realization was demonstrated initially by Miller and Heller [86], who showed that photosensitive sulfide layers could be grown on bismuth and cadmium electrodes in solutions of sodium polysulfide and then used in situ as photoanodes driving the 

Differences in behavior between polycrystalline and single-crystal CdSe electrodes in polysulfide PEC involving the short- and long-term changes in photovoltage and photocurrents have been discussed by Cahen et al. [88], on the basis of XPS studies, which verified the occurrence of S/Se substitution in these electrodes when immersed in polysulfide solution, especially under illumination. The presence of a thin (several nanometers) layer of CdS on top of the CdSe was shown to influence 

Lyden et al. [92] used in situ electrical impedance measurements to investigate the role of disorder in polysulfide PEC with electrodeposited, polycrystalline CdSe photoanodes. Their results were consistent with disorder-dominated percolation conduction and independent of any CdS formed on the anode surface (as verified by measurements in sulfide-free electrolyte). The source of the observed frequency dispersion was located at the polycrystalline electrode/electrolyte interface. 

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In terms of photoelectrode material quality, single crystals comprise a rational choice since their bulk properties can be controlled better and their influence on cell performance may be evaluated in a rather accurate manner, as being microstruc-turally well-defined solids. However, the cost and convenience of single-crystal preparation are not suited to the practical requirement of cheap device components. Polycrystalline photoelectrodes are advantageous in terms of fabrication cost, ability to prepare large areas in one operation, and material economy. 

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. 

An important consideration, from the practical point of view, is that thin film polycrystalline photoelectrodes can be prepared, by various methods, with conversion efficiencies of more than half of those obtained with single crystal electrodes and with better stability characteristics than those obtained with single crystal based PEC s (1,4.,5). ... 

Cell characteristics polycrystalline photoelectrode, Pt counter electrodes, cell configuration as in Fig. 10.6, t/redox of polysulphide electrolyte = -700 mV vs. SCE, l/redox of Se2 /Se = -800 mV vs. SCE, illumination 100 mW cm xenon lamp. Conversion efficiency 4%, i/fiu = 0.45, photovoltage = -400 mV. Charged cell has an open-circuit voltage of 60 mV and initial current through a 100 ohm load across the Pt electrode of 0.5 mA. 

Cell characteristics polycrystalline photoelectrode, Ni counter electrode, basic cell design based on Fig. 10.5, Credox of = 0.500 V vs. SHE, Credox of... 

Studied cell characteristics Polycrystalline photoelectrode, Pt counter electrode, Aredox of Fe +/Fe + = 0.77 V vs SHE, standard hydrogen electrode illumination 40-50 mW cm by a tungsten lamp, initial current during discharge =... 

Studied cell characteristics Polycrystalline photoelectrode, Pt counter electrodes, cell configuration is similar to Fig. 4, ,edox of O2, H+/H2O couple = 1.23 V vs NHE at pH = 1, redox Ag/Ag+= 0.80 V VS NHE, normal hydrogen electrode illumination 500 W Hg lamp, conversion efficiency = 1%, photopotential = 0.28 V vs NHE, open-circuit voltage of the charged cell = 0.28 V, and short-circuit current = 0.3 mA cm . ... 

Studied cell characteristics Polycrystalline photoelectrode, Pt counter electrodes, cell configuration is similar to Fig. 4. Aredox of = -700 mV vs... 

Studied cell characteristics Bipolar series polycrystalline photoelectrode, CoS counter electrodes, cell configuration is as shown in Fig. 4 without the need of switches E or F. itredox of S/S = —0.48 V vs NHE, itredox of SnS/Sn,S = —0.94 V vs NHE. Illumination sunlight, 500 mWhr cm per day, conversion efficiency 6-7%, photovoltage = —600 mV, and storage efficiency >90%. After two weeks of continuous operation the overall solar to electrical efficiency (including conversion and storage losses) is 2-7%. 

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