Ultrafast direct and indirect electron-injection processes in a photoexcited dye-sensitized nanocrystalline zinc oxide film: The importance of exciplex intermediates at the surface

The processes involved in ultrafast electron injection from a photoexcited novel coumarin dye (NKX-2311), an efficient photosensitizer for TiO2-based dye-sensitized solar cells, into the conduction band of a nanocrystalline ZnO film were investigated by observing the femtosecond transient absorption in the visible-to-infrared range (600-5000 nm). After photoexcitation of adsorbed NKX-2311 dye, the stimulated emission and absorption of the singlet excited dye decayed with a 500-fs time constant. These were followed by rises in absorptions of the oxidized dye and conduction-band electrons, indicating a direct electron-injection process. In addition, indirect electron-injection processes involving intermediates were identified. The intermediates showed stimulated emission at longer wavelengths than that of the excited dye; very broad absorptions in the near-IR region (900-1300 nm) were observed immediately after excitation, and they decayed with 1- and 10-ps time constants, leading to further rises in the absorption of electrons in the conduction band. On the basis of their dynamics and spectral features, the species decaying with 1- and 10-ps time constants were assigned to neutral and ionic exciplexes, respectively. The multiplicity of electron-injection processes is due to the presence of various adsorption sites for the dye on the ZnO surface.