Parameters Influencing the Efficiency of Electron Injection in Dye-Sensitized Solar Cells

In this paper we focus upon the electron injection dynamics in complete nanocrystalline titanium dioxide dye-sensitized solar cells (DSSCs) employing the ruthenium bipyridyl sensitizer dye N719. Electron injection dynamics and quantum yields are studied by time-resolved single photon counting, and the results are correlated with device performance. In typical DSSC devices, electron injection kinetics were found to proceed from the N719 triplet state with a half-time of 200 +/- 60 ps and quantum yield of 84 +/- 5%. We find that these injection dynamics are independent of presence of iodide/triiodide redox couple and of the pH of the peptization step used in the synthesis of the TiO2 nanoparticles. They are furthermore found to be only weakly dependent upon the application of electrical bias to the device. In contrast, we find these dynamics to be strongly dependent upon the concentration of tert-butylpyridine (tBP) and lithium cations in the electrolyte. This dependence is correlated with shifts of the TiO2 conduction band energetics as a function of tBP and Li+ concentration, from which we conclude that a 100 meV shift in band edge results in an approximately 2-fold retardation of injection dynamics. We find that the electron injection quantum yield determined from these transient emission data as a function of tBP and Li+ concentration shows a linear correlation with device short circuit density J(SC). We thus conclude that the relative energetics of the dye excited state versus the titanium dioxide acceptor state is a key determinant of the dynamics of electron injection in DSSC, and that variations in these energetics, and therefore in the kinetics and efficiency of electron injection, impact directly upon device photovoltaic efficiency. Finally, we discuss these results in terms of singlet versus triplet electron injection pathways and the concept of minimization of kinetic redundancy.