Two decades ago a research team at EPFL in Lausanne led by Dr M. Gratzel developed a new type of solar cell known as a Dye Sensitized Solar Cell (DSSC), which is based on a completely different principle than conventional solar cells. DSSCs integrate not merely a semiconductor, but rather a series of elements: a titanium dioxide (TiO2) semiconductor, a monomolecular dye film deposited on the semiconductor, and an electrolyte solution. Figure 3.57 shows the structure of a DSSC.
In a DSSC, when a sufficiently energized photon strikes a dye molecule, the photon is absorbed and the molecule enters an excited state, i. e. the energy level of one of the molecule’s valence electrons increases. This excited molecule then injects an electron into the titanium dioxide’s conduction band and is thus ionized.
In the interest of preventing this electron from recombining before long, an electrolytic ‘redox mediator’ (referred to here for reasons of simplicity as M~, but in practice referred to as І3~Л~ or the like) almost instantaneously (in about 10 ns) injects an electron into the ionized dye molecule, thus shifting the molecule to a neutral state and obviating recombination.
The electrons e~ generated by photon absorption are shunted, via the semiconductor, to the bottom electrode and from there, releasing some energy, to the opposite (upper) electrode via the outer electric circuit. This in turn closes off the circuit by virtue of the fact that the electrolytic redox mediator oxidized to I by the dye is diffused to the opposite electrode, where it is reduced to I_ on incorporation of an electron (see [3.8] for a more complete and detailed function description with numerous illustrations).
According to [3.2], a small-scale (0.22 cm2) DSSC achieves zpv — 11.2% efficiency at STC. According to [3.8], the efficiency of such cells in the presence of weak irradiance, G — 100 W/m2, is considerably higher than with G — 1000 W/m2. Moreover, the somewhat simpler production process for DSSCs would presumably allow for inexpensive mass production.
Although there have been reports about DSSCs time and time again over the past two decades in the media and at conferences on photovoltaics, it needs to be borne in mind that DSSCs are still a lab development, and that at the date this book went to press, these modules had yet to be produced commercially and no datasheets were available for them. This technology may well be promising, but its long-term stability and practical usefulness remain to be proven. To the best of my knowledge, DSSCs have yet to be used for long-term energy production. Moreover, experience has shown that it can take a very long time before such new types of solar cells can be successfully produced in a factory. Only time will tell whether the great expectations concerning DSSCs will come to fruition.