Types of Photoelectrochemical Devices

Under standard conditions water can reversibly electrolyze at a potential of 1.23 V, a value derived from the following relationship  

There are three general types of photoelectrochemical devices using semiconductor electrodes for the conversion of water into hydrogen [17-54]. 

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To facilitate a self-contained description, we will start with well-estahlished aspects related to the semiconductor energy hand model and the electrostatics at semiconductor-electrolyte interfaces in the dark . We shall then examine the processes of light absorption, electron-hole generation, and charge separation at these interfaces. The steady state and dynamic aspects of charge transfer are then briefly considered. Nanocrystalline semiconductor films and size quantization are then discussed as are issues related to electron transfer across chemically modified semiconductor-electrolyte interfaces. Finally, we shall introduce the various types of photoelectrochemical devices ranging from regenerative and photoelectrolysis cells to dye-sensitized solar cells. 

As Fig. 27 illustrates, there are basically three types of photoelectrochemical devices for solar energy conversion. The first type is regenerative in nature and the species that are photooxidized at the -type semiconductor electrode are simply re-reduced at the counterelectrode (Fig. 27a). Instead of an elec-trocatalytic electrode [291, 292] where the counterelectrode reaction occurs in the dark (this is the situation schematized... 

Fig. 27 Types of photoelectrochemical devices for solar energy conversion, (a),...

There is a special type of photoelectrochemical devices based on donor/acceptor dyads and triads self-assembled on the metal (gold) or metal oxide (TTO and FTO) electrodes. A multistep photoinduced electron transfer occurring in such devices resembles in some way a natural photosynthetic process. The power conversion efficiencies of such devices are negligibly low because the monolayer of the photoactive material does not absorb much light. However, internal quantum efficiencies (IQEs) of such devices approach 80-90% in many cases. 

The optimization of photoelectrochemical devices for solar energy conversion depends on the choice of semiconductor, electrolyte, and cell design. The performance of the cell is strongly dependent upon the design, surface area, and placement of the counterelectrode and current collectors. This type of solar cell may be economical under concentrated illumination or in regions where electrical power has high value. 

Photoelectrochemical Cell - A type of photovoltaic device in which the electricity induced in the cell is used immediately within the cell to produce a chemical, such as hydrogen, which can then be withdrawn for use. 

N-type semiconductors can be used as photoanodes in electrochemical cells Q., 2, 3), but photoanodic decomposition of the photoelectrode often competes with the desired anodic process (1 4 5). When photoanodic decomposition of the electrode does compete, the utility of the photoelectrochemical device is limited by the photoelectrode decomposition. In a number of instances redox additives, A, have proven to be photooxidized at n-type semiconductors with essentially 100% current efficiency (1, 2, 3, 6>, ], 8, 9). Research in this laboratory has shown that immobilization of A onto the photoanode surface may be an approach to stabilization of the photoanode when the desired chemistry is photooxidation of a solution species B, where oxidation of B is not able to directly compete with the anodic decomposition of the "naked" (non-derivatized) photoanode (10, 11, 12). Photoanodes derivatized with a redox reagent A can effect oxidation of solution species B according to the sequence represented by equations (1) - (3) (10-15). 

We saw above that the study of the competition between Fe3+ and H + reduction on illuminated p-GaP led to an increased understanding of the nature of surface electrochemical processes on that material. For many n-type materials, however, the most serious competing reaction with the oxidation of some redox couple in solution is the oxidative corrosion of the semiconductor itself. This has considerable practical consequencies a photoelectrochemical device for the conversion of solar energy must be one in which the desired electrochemical route is overwhelmingly probable compared with semiconductor dissolution. So essential is this requirement, and so difficult has it proved to find satisfactory solutions for n-type semiconductors, that a substantial fraction of the recent literature on semiconductor electrochemistry has been devoted to both practical and theoretical considerations of the problem. 

Further details of these device types as well as nonenefgy-related applications of photoelectrochemical cells (such as in environmental remediation) may be found in the chapters that follow in this volume. 

When replacing the Kquid electrolyte in dye-sensitized solar cells with a sohd p-type conductor, a range of requirements have to be fulfilled to preserve the high external quantum efficiencies common to the original photoelectrochemical devices. 

Only a few materials were studied concerning their applicability to dye-sensitized hole injection processes. Among those are different copper(I) compounds (e.g. Cu(I)SCN, Cu(I)I, Cu2(I)0 [33-35]) and nickel(II) oxide [36]. Photovoltaic performances of such devices are orders of magnitudes poorer than those of classical dye-sensitized photoelectrochemical solar cells based on n-type materials. Substantial advantages could arise if an efficient photo-hole injection process would be available. The formation of solid-state tandem solar cells would become feasible, and a quantum step in device efficiency of dye-sensitized solar cells could be at reach. However, because of the poor performance of all known photocathodes, a combination of available photoanodes and photocathodes to a tandem device always results in a device that is photovoltaically less efficient than the photoanode on its own. The concept for electrolyte-based tandem cells exists. However, it contains strong potential to improve the photovoltaic performance in both electrolytic and in solid-state, dye-sensitized solar cells. 

Fig. 2.25 Examples of possible PEC configurations under illumination. Top row Standard singlesemiconductor devices based on a photoanode (a) or photocathode (b) with a metal counter electrode. Middle row Monolithic devices based on a photoanode (c) or photocathode (d) biased with an integrated p-n junction. Bottom row p-n junction photoelectrochemical device (e), and an n-n heterojunction PEC device based on a photoanode deposited on top of a second n-type semiconductor that boosts the energy of the electrons (f)...

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