Photovoltaic-electrochemical systems

A galvanic cell is a system that can perform electrical work when its energy is consumed at the expense of chemical or concentration changes that occur inside the system. There are also systems analogous to galvanic cells based on conversion of other types of energy than chemical or osmotic into electrical work. Photovoltaic electrochemical cells will be discussed in... 

Photo-electrochemical systems use semi-conducting materials (like photovoltaics) to split water using only sunlight. 

Electrochemical photovoltaic cell systems. Photovoltage (llph j, photocurrent (jph j, fill factor FF, efficiency (here rj) Reprinted from R. Memming, Semiconductor Electrochemistry, Wiley-VCH Verlag GmbH, Weinheim (2001). Copyright 2001 with permission from Wiley-VCH Verlag GmbH. 

Background current — (i) Generally, in electrochemical systems, any current other than the wanted one (e.g., see - dark current in photovoltaics). 

E. Peeters, D. Lapadatu, W. Sansen, R. Puers, PHET - an electrodeless photovoltaic electrochemical etchstop technique, ]. Microelectromechanical Systems 1993, 3, 113-123. 

Electrochemical photovoltaic cell systems. Photovoltage photocurrent, fill factor FF, efficiency (here rj )... 

Unlike photovoltaic cells, most photo-electrochemical systems do not operate without an external source. In a more general approach, the potential of the working electrode can be varied with respect to the equilibrium potential Veq (or to the potential of a RE) by means of an external voltage source (e.g. a potentiostat) connected between WE and CE [21, 24]. The current density (indicated here by j) is measured both in the dark and under illumination as a function of the applied potential V. This is described in more detail in Sect. 2.1.2.2. 

In photochemical processes, photon absorption creates a molecular excited state or stimulates an interband electronic transition in a semiconductor that induces a molecular change. Comprehensive reviews, including those by Gratzel [6], Kalyanasundaram [7], and Har-riman [8], have discussed various aspects of photochemical energy conversion. A photoactivated molecular excited state can drive either (1) photodissociation (2) photoisomerisation or (3) photoredox reactions. Processes based on semiconductors may involve photovoltaic or photo-electrochemical systems. 

Starting from the pioneering works of chemical modification of TCO electrodes [242,265,290,291], alotof application possibilities have been developed. These electrodes are widely applied in electrochemistry and electrocatalysis. Electrochemical studies on chemically modified oxide electrodes have been discussed in details in Refs. 283-285. Recent developments in optoelectronic and photovoltaic device systems require control of the chemical and... 

Table 11.1 Electrochemical photovoltaic cell systems. Photovoltage (t/pj,), photocurrent (7ph), fill factor (FF), efficiency q).

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. 

The choice of the R group leads to materials exhibiting properties which can find applications in diverse fields long alkyl chains help solubilization in organic solvents, whereas triethylene glycols have been used as spacers to attach water-solubilizing moieties, or even electron-donor systems such as ferrocene, which may find applications in photovoltaics, or as bio- and electrochemical sensors [33]. 

Multijunction cell technology developed by the photovoltaic industry is being used to develop photo-electrochemical light harvesting systems that generate sufficient... 

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. 

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