Ultrafast Spectroscopy and Numerical Analysis of Inhomogeneous Electron Injection in Dye-Sensitized Solar Cells: Effect of Electrolyte

Ultrafast electron injection from a photo-excited dye molecule to semiconductor electrode is one of the most important reactions in the dye-sensitized solar cell (DSSC). The reaction occurs in the wide time region from femtoseconds to nanoseconds with a inhomogeneous manner.^[1,2] Since the origin of such inhomogeneity has not been understood well, there still is many problems to improve the photo conversion efficiency of DSSC.^[2]Here, we propose a quantitative reaction model of the electron injection dynamics to discuss the origin of the inhomogeneity. Our experimental results of transient absorption spectroscopy (TAS) were successfully analyzed by the model.

Dye sensitized TiO₂ films with Ru-complex (N719) were prepared. The fs-TAS and ns-TAS measurements were carried out for the films in acetonitrile-based electrolytes. The results were numerically analyzed as a sum of non-adiabatic electron transfer reactions, which supported our experimental finding that the inhomogeneous energy distribution of the energy gap between the donor and acceptor states (Δ) was present.^[2]

Fig.1 shows the results of population time profile of injected electron in DSSC. The profiles are measured by fs-TAS at <1 ns and sub-nsTAS >1 ns. The simulation result based on our model showed a good agreement with the experimental result. Furthermore, the effects of electrolyte with different potential controlling ion (Li⁺) concentrations and the electron concentration in semiconductor electrode were investigated experimentally and the obtained electron injection kinetics were well explained by the numerical simulation, indicating that the model is reasonable.

In some cases, however, the model does not seem to be easily applicable. For example, when we measured electron injection kinetics from excited N719 to TiO₂ in the presence of ionic liquid (IL) as surrounding solvent, less efficiency was observed for some ILs with higher viscosity (> 30 cP). This suggests that not only the driving force but also restriction of molecular reorientation of the sensitizer may affect the reaction.

Some other examples of electron injection dynamics relating to the effect of electrolytes and solvents will be shown and the mechanism will be discussed ^{[3, 4]}.  

Acknowledgement: A part of this work is supported by NEDO Japan.

Fig.1Population time profile of injected electron in DSSC. Solid line shows a simulation result.

References

[1] Y. Tachibana et al., J. Phys. Chem. B, 104, 1198 (2000)

[2] K. Sunahara, A. Furube et al., J. Phys. Chem. C, 115, 22084 (2011)

[3] A. Furube, Z. Wang, K. Sunahara, K. Hara, R. Katoh, and M. Tachiya, J. Am. Chem. Soc., 132, 6614-6615 (2010)

[4] S. Mahanta, A. Furube, H. Matsuzaki, T. N. Murakami, and H. Matsumoto, J. Phys. Chem. C, 116, 20213–20219 (2012)