ULTRAFAST DIRECT AND INDIRECT ELECTRON INJECTION PROCESSES IN PHOTO-EXCITED DYE-SENSITIZED NANOCRYSTALLINE ZnO FILMS

摘要

The processes involved in ultrafast electron injection from photoexcited dyes (N3 and NKX-2311), efficient photosensitizers for dye-sensitized solar cells, into the conduction band of a nanocrystalline ZnO film were investigated by femtosecond transient absorption spectroscopy in the visible-to-infrared range (600-5000 nm). We observed transient absorption signals of the dyes (in the excited and oxidized states), the electrons in the conduction band, and charge transfer complexes consisting of an excited dye and a surface state of ZnO. Direct and indirect electron injection processes were clearly observed. In the direct process, decay of the excited state absorption was simply followed by rises in absorptions of the oxidized dye and conduction-band electrons. In the indirect process, exciplexes, which formed within 100 fs and decayed on 1-100 ps time scale, were observed as intermediate states. The multiplicity of electron-injection processes is due to various adsorption sites for the dye on the ZnO surface.