Ultrafast Direct and Indirect Electron-Injection Processes in a Photoexcited Dye-Sensitized Nanocrystalline Zinc Oxide Film: The Importance of Exciplex Intermediates at the Surface

摘要

The processes involved in ultrafast electron injection from a photoexcited novel coumarin dye (NKX-2311),an efficient photosensitizer for TiO_2-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.