Photovoltaic cells, semiconductor electrodes

The photovoltaic effect is initiated by light absorption in the electrode material. This is practically important only with semiconductor electrodes, where the photogenerated, excited electrons or holes may, under certain conditions, react with electrolyte redox systems. The photoredox reaction at the illuminated semiconductor thus drives the complementary (dark) reaction at the counterelectrode, which again may (but need not) regenerate the reactant consumed at the photoelectrode. The regenerative mode of operation is, according to the IUPAC recommendation, denoted as photovoltaic cell and the second one as photoelectrolytic cell . Alternative classification and terms will be discussed below. 

Photoelectrochemical semiconductor cells are used to convert photon energy into chemical substances or into electricity, the former is a photodectrolytic cell and the latter is a photovoltaic cell. A photoelectrochemical semiconductor cell consists of either a pair of metal and semiconductor electrodes or a pair of two semiconductor electrodes. 

Fig. 8.9 Schematic diagram of PV-electrolysis systems proposed for solar water splitting (a) Electricity generated from photovoltaic cell driving water electrolysis (b) PV assisted cell with immersed semiconductor p/n junction as one electrode.

Photovoltaic cells. As the name suggests, these involve direct conversion of light into electric current. The reaction at the auxiliary electrode is the inverse of that at the semiconductor electrode. In principle, there is no change in electrolyte composition with time. An example would be... 

Dye sensitization of electrodes is an old area of science with a rich history. The field has experienced renewed interest owing to the development of high surface area colloidal semiconductor electrodes. These materials yield impressive solar conversion efficiencies when employed in regenerative solar cells that have already found niche applications and have the real possibility of replacing traditional solid-state photovoltaics. Thus for the first time in history a solar cell designed to operate on a molecular level is useful from a practical point of view. It is also likely that other applications in the growing areas of molecular photonic materials will arise. 

Figure 5. Schematic representation of the principle of the nanocrystalline injection photovoltaic cell showing the electron energy level in the different phases. The cell voltage AK obtained under illumination corresponds to the difference in the Fermi level of the semiconductor and the electrochemical potential of the redox couple (M+/M) used to mediate charge transfer between the electrodes.

Photovoltaic cells based on the sensitization of mesoporous titanium dioxide by Ru(II) complex dyes in conjunction with the I.3 /U redox couple as a mediator have proved very efficient at exploiting this principle. In such systems, the ionic mediator travels back and forth by diffusion from the working electrode to the counterelectrode, to shuttle to the sensitizer the electrons that have gone through the electrical circuit [18, 21, 84]. Recently, solid-state devices have been described where the liquid electrolyte present in the pores of the nanocrystalline oxide film is replaced by a large-bandgap p-type semiconductor acting as a hole-transport medium [85 88]. 

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