the renaissance of dye - sensitized solar cells

162 nature photonics | VOL 6 | MARCH 2012 | www.nature.com/naturephotonics Dye-sensitized solar cells (DSCs) are attractive because they are made from cheap materials that do not need to be highly purified and can be printed at low cost1. DSCs are unique compared with almost all other kinds of solar cells in that electron transport, light absorption and hole transport are each handled by different materials in the cell2,3. The sensitizing dye in a DSC is anchored to a wide-bandgap semiconductor such as TiO2, SnO2 or ZnO. When the dye absorbs light, the photoexcited electron rapidly transfers to the conduction band of the semiconductor, which carries the electron to one of the electrodes4. A redox couple, usually comprised of iodide/triiodide (I/I3), then reduces the oxidized dye back to its neutral state and transports the positive charge to the platinized counter-electrode5. In 1991, O’Regan and Grätzel demonstrated that a film of titania (TiO2) nanoparticles deposited on a DSC would act as a mesoporous n-type photoanode and thereby increase the available surface area for dye attachment by a factor of more than a thousand1. This approach dramatically improved light absorption and brought power-conversion efficiencies into a range that allowed the DSC to be viewed as a serious competitor to other solar cell technologies6. A schematic and energy level diagram showing the operation of a typical DSC is shown in Fig. 1. During the 1990s and the early 2000s, researchers found that organometallic complexes based on ruthenium provided the highest power-conversion efficiencies7,8. Iodide/triiodide was found to be the most effective redox couple9–13. The record power-conversion efficiency rapidly climbed to 10% in the late 1990s and then slowly settled to 11.5%14– 17. Figure 2 shows a current–voltage curve under 1 Sun illumination, together with a plot of the external quantum efficiency as a function of photon wavelength. The iodide/triiodide system has been particularly successful in DSCs because of the slow recombination kinetics between electrons in the titania with the oxidized dye and the triiodide in the electrolyte, which leads to long-lived electron lifetimes (between 1 ms and 1 s)18–20. Iodide reduces the oxidized dye to form an intermediate ionic species (such as I2) that then disproportionates to form triiodide and diffuses to the counter-electrode, providing two electrons per molecule, as shown in Fig. 1b4,19. The slow recombination and relatively fast dye regeneration rates of the I/I3 redox couple have resulted in near-unity internal quantum efficiencies for a large number of dyes, providing the high external quantum efficiencies shown in Fig. 2a. The small size of the I/I3 redox the renaissance of dye-sensitized solar cells