Dynamics of Interfacial Charge Transfer States and Carriers Separation in Dye-Sensitized Solar Cells: A Time-Resolved Terahertz Spectroscopy Study

Electron injection from a photoexcited molecular sensitizer into a wide-bandgap semiconductor is the primary step toward charge separation in dye-sensitized solar cells (DSSCs). According to the current understanding of DSSCs functioning mechanism, charges are separated directly during this primary electron transfer process, yielding hot conduction band electrons in the semiconductor and positive holes localized on oxidized dye molecules at the surface. Comparing results of ultrafast transient absorption and time-resolved terahertz measurements, we show here that intermediate interfacial charge transfer states (CTSs) are rather formed upon ultrafast injection from photoexcited Ru(II)− bipyridyl dye-sensitizer molecules into mesoporous TiO2 films. Formation and dissociation of these CTSs were found to strongly depend on their ionic environment and excess excitation energy. This finding establishes a new mechanism for charge separation in DSSCs. It also offers a rationale for the effect of electrolyte composition in liquid-based devices and of ion doping in solid-state solar cells under working conditions. ■ INTRODUCTION Dye-sensitized solar cells (DSSCs) present a promising way in the quest for cost-effective photovoltaics, owing in particular to their ease of fabrication, high efficiency under low-density illumination, and customizable aesthetics. DSSCs belong to the family of hybrid donor−acceptor bulk heterojunction photovoltaic converter systems. They are based on a mesoporous TiO2 film, constituting the photoanode, onto which a monolayer of a molecular sensitizer is adsorbed. The pores of the dyed nanocrystalline film are filled with either a liquid redox-active electrolyte or a solid-state organic hole transport material. In the absence of a built-in electric field at the interface able to separate photogenerated electron−hole pairs, light-induced charge separation in bulk heterojunction cells relies on the kinetic competition between several forward electron transfer, charge transport, and recombination processes. Hence, a detailed investigation of the dynamics of interfacial charge transfer is essential for the understanding of the device functioning mechanism. The kinetics of electron injection from photoexcited heteroleptic Ru(II) bipyridyl dyes into titanium dioxide (TiO2) semiconductor nanoparticles have been extensively studied. Most of the reports describe the kinetics of the primary light-induced, interfacial charge transfer process as being multiexponential and different rationales for the observed kinetic heterogeneity have been proposed. Although a majority of studies report ultrafast electron injection taking place within a sub-100 fs time scale, recent measurements carried out by using transient IR spectroscopy in the presence of an electrolyte suggest that charge separation in Ru(II) complex dye-sensitized solar cells might be much slower. Results obtained here and their interpretation reconcile these apparently contradicting observations. While initial femtosecond photoinduced electron injection is confirmed, actual charge separation to yield free mobile carriers is shown to be hindered by electron trapping in charge transfer states, whose dissociation in the presence of an electrolyte or in contact with a hole transport material can extend over hundreds