Photovoltaic device counter electrode

Dye-sensitized solar cells (DSSCs) are photoelectrochemical solar devices, currently subject of intense research in the framework of renewable energies as a low-cost photovoltaic device. DSSCs are based upon the sensitization of mesoporous nanocrystalline metal oxide films to visible light by the adsorption of molecular dyes.5"7 Photoinduced electron injection from the sensitizer dye (D) into the metal oxide conduction band initiates charge separation. Subsequently, the injected electrons are transported through the metal oxide film to a transparent electrode, while a redox-active electrolyte, such as I /I , is employed to reduce the dye cation and transport the resulting positive charge to a counter electrode (Fig. 17.4). 

In evaporation-intercalation devices solar energy conversion would, at least in the more efficient case of a thermal system, not be converted by exciting electrons and rapidly separating them from holes, but by transferring atoms or molecules across a phase boundary by evaporation which is usually a very efficient process. It is, consequently, neither necessary to use materials which are well crystallized like those developed for photovoltaic cells nor is it necessary to prepare sophisticated junctions. A compacted polycrystalline sheet of a two-dimensional material which is on one side placed in contact with an electrolyte, sandwiched between the layer-type electrode and a porous counter electrode, as it is used in fuel cells, would constitute the central energy conversion unit. Some care would have to be taken to choose an electrolyte which is suitable for intercalation reactions and which is not easily evaporated through leaks in the electrodes. Thin layers of polymeric or solid electrolytes would seem to be promising. 

The photoelectrochemical solar cells form the first family of organic photovoltaic devices. Typically, the active layer of such devices consists of nanostructured and dye sensitized electrodes, whereas the other electrode (counter electrode) is separated by an electrolyte or hole conductor. The highest efficiency of around 11% is achieved in dye sensitized solar cells (DSSCs) using TiO nanostructured electrodes. Schematic layout of a typical DSSC device is shown in Figure 2 ... 

For this introduction, only the principle of energy conversion by such an electrode is shown in Fig. 1.14, where an n-type semiconductor is used for illustration. The redox couple of the electrolyte reacts under illumination with the holes generated by light absorption. The electrons are left in the semiconductor and their accumulation in the bulk leads to a negative voltage difference between the semiconductor and the counter electrode. The latter remains more or less in equilibrium with the redox system in solution. The electric energy of the electrons can be used for external consumption as in a photovoltaic solid state device. 

PEDOTrPSS has also been employed as a coating on the counter electrode and as a solid-state hole conductor in dye-sensitized solar cells (DSSCs) forming its own class of organic photovoltaic devices. 

---