Iodide/triiodide redox system

Based on extensive screening of hundreds of ruthenium complexes, it was discovered that the sensitizer s excited state oxidation potential should be negative of at least —0.9 V vs. SCE, in order to inject electrons efficiently into the Ti02 conduction band. The ground state oxidation potential should be about 0.5 V vs. SCE, in order to be regenerated rapidly via electron donation from the electrolyte (iodide/triiodide redox system) or a hole conductor. A significant decrease in electron injection efficiencies will occur if the excited and ground state redox potentials are lower than these values. 

The details of die operating principles of the dye-sensitized solar cell are given in Figure. 3. The photoexcitation of the metal-to-ligand charge transfer of the adsorbed sensitizer [Eq. (1)] leads to injection of electrons into the conduction band of the oxide [Eq. (2)]. The oxidized dye is subsequently reduced by electron donation from an electrolyte containing the iodide/triiodide redox system [Eq. 

Papageorgiou et al. reported the first example of an RTIL-based dye-sensitized solar cell system (Gratzel ceU). They found that l-hexyl-3-methylimidazolium iodide, denoted as HMI-I, melts at room temperature [8]. However, the viscosity of HMI-I is very high, at over 1000 mPas at room temperature. Therefore the /sc of DSSC when HMI-I is used (0.75 mA cm at the irradiation of 120,000 Lux [= 1 sun]) is much lower than that when an organic solvent is used (over 15 mA cm at 1 sun). This indicates that the slow diffusion of the iodide/triiodide redox... 

In this chapter, PEC systems using RTILs were introduced. The discussion focused on the relationship between the viscosity of the RTILs and the short-circuit photocurrent. It was shown that a Grotthus-like mechanism observed in a high concentration of iodide/triiodide redox in RTILs compensates for the shortcomings of the relatively high viscosity of RTILs. This is a critical fact for the apphcation of RTILs in actual electrochemical power devices. 

Figure 11. Principle of operation of the dye-sensitized nanocrystalUne solar cell. Photoexcitation of the sensitizer (S) is followed hy electron injection into the conduction band of an oxide semiconductor film. The dye molecule is regenerated by the redox system, which itself is regenerated at the counter-electrode by electrons passed through the load. Potentials are referred to the normal hydrogen electrode (NHE). The energy levels drawn match the redox potentials of the standard N3 sensitizer ground state and the iodide/triiodide couple. (Redrawn from Gratzel [187] with permission from publisher, Elsevier. License Number 2627070632803).

Figure 14-2. Schematic drawing showing the use of dye derivatized semiconductor nanocrystals as light harvesting units. The sensitizer is cis-Ru(SCN)2L2 L = 2,2 -bipyridyl-4,4 -dicurboxylnte) The redox system use to regenerate the dye and transport the positive charges to the counter electrode is the iodide/triiodide couple dissolved in an organic electrolyte or in a room temperature ionic liquid.

The cathode materials employed for the early lithium-based systems were 3.0 V class oxides or sulfides thus, the redox potential for the additive should be located in the neighborhood of 3.2—3.5 V. Accordingly, the first generation redox additive proposed by Abraham et al. was based on the iodine/ iodide couple, which could be oxidatively activated at the cathode surface at 3.20 V and then reduced at the lithium surface. " " " 2° For most of the ether-based solvents such as THF or DME that were used at the time, the oxidation potential of iodide or triiodide occurred below that of their major decompositions, while the high diffusion coefficients of both iodine and iodide in these electrolyte systems ( 3 x 10 cm s ) offered rapid kinetics to shuttle the overcharge current. Similarly, bromides were also proposed.Flowever, this class of halide-based additives were deemed impractical due to the volatility and reactivity of their oxidized forms (halogen). 

The photoproducts can either recombine, or alternatively I2" can react with NiO (i.e. inject a hole) since the redox potential is more positive than that of the top of the valence band. The one-electron oxidation of iodide is thought to be the process that occurs in the regeneration of the ground state dye in n-type DSSCs and is thought to be the reason behind the apparent 0.6 V overpotential required to drive the system. The equivalent oxidation of triiodide lies symmetrically to negative potentials of the formal redox potential of Is"/ " (0.32 V vs NHE in MeCN) according to Equation 3.48 ... 

Alternative p-Type Semiconductors Because the valence band potential of NiO lies only 100 mV more positive than the optimum redox couple for n-type devices, a substantial increase in voltage for the p-type/tandem systems should be achieved if the p-type semiconductor has a valence band with a much lower energy than the triiodide/iodide redox couple. The other requirements for the material include optical transparency (Eg > 3 eV), mechanical and electrochemical stability, good electronic properties (high charge-carrier mobility) and a convenient means of anchoring the dye (e.g. metal oxides and carboxylic acids). Whilst other p-type semiconductors exist, few combine all the properties required and as yet there have been no p-type semiconductors reported that perform better than NiO in a p-type For example, several... 

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