DSSC cells viscosity

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... 

Figure 15.6 shows the relationship between Jsc of the Gratzel cell and the viscosity of various liquid media under the same conditions [21]. The Jsc of the DSSC system depends simply on the viscosity of the liquid media. However, as described above, the Jsc of iodide-based RTILs is much higher even where viscosity is higher than 200 mPas at 25°C. As shown in the figure, the amount of iodine (I2) affects the improvement in Jsc- This implies that the diffusion coefficient of the iodide... 

There are two kinds of polymer material that used in quasi-solid/solid state DSSCs. For quasi-solid electrolytes, polyionic liquids have been proposed as solvent and redox couple as solute. They appear in molten salts and present many promising properties, such as, high chemical and thermal stability and high ionic conductivity Their main drawback is related to its high viscosity, which makes the ions diffusion rather slow. As the transport of ions to the counter electrode in an ionic liquid matrix represents a rate-limiting step in DSSC (Bella, 2015), the performance of quasi-solid electrolytes based solar cell is imsatisfled. 

Nowadays, most quasi-solid state DSSCs with ionic liquid electrolyte achieved relatively low cell efficiency as compared to the traditional DSSCs with liquid electrolyte. Despite the former system possessed superior long-term stability than that of the latter, the cell efficiency indeed needs to be further improved. Thereby, the recent challenge in ILs based quasi-solid state DSSCs is on how to lower the viscosity of the ILs and to enhance the diffusion rate of the redox couples. This matter would be a key issue for future study. 

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