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There are two types of solar cells - those in a solid-state form and those containing liquids. The latter is often called a photoelectrochemical solar cell. Figure 4.1 shows a simplified diagram of a photoelectrochemical, or PEC, solar cell. The top of the figure shows the equilibrium-energy-band diagram for a solar cell material (without a P-N junction) immersed in a solution containing a redox (oxidation-reduction) active species. For example, a Si wafer might be immersed in an acetonitrile-based solution containing iodide as a redox active species. Analogous to the case of the P-N junction described in Chapter 1, the exchange of charge carriers can occur between the redox species and the semiconductor (e.g., N-type Si) to create a depletion region. In the light, electrons can be excited to the conduction band, just like in the case of the P-N junction, and can be collected at the back contact. The role of the redox species, sometimes called the redox mediator, is to replenish the electrons as they are lost and to complete the electrical circuit via a counter electrode. This electrode is usually coated with a suitable catalyst (e.g., C, Pt or Pd) that facilitates the regeneration of the redox species. The redox species therefore goes through a cycle of oxidation and reduction. As shown in Figs. 4.1(a) and 4.1(b), a PEC solar cell operates like one half of a P-N junction. In this regard, it resembles a Schottky barrier solar cell. For both of these types of cells, the equations in Chapters 2 and 3 apply. Practical applications of PEC solar cells have been limited by stability and corrosion issues. If the semiconductor does not absorb light very well itself, a âsensitizerâ can be attached to the surface to act as the light absorber. This is the basis of the dye-sensitized solar cell shown at the bottom of Fig. 4.1. This type of photoelectrochemical cell has come the closest to reaching practical efficiencies and commercial applications. The most efficient (and promising) dye-sensitized solar cell utilizes a semiconductor that is nanocrystalline.
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