STUDY OF ELECTRON LOSSES IN NANOSTRUCTURED ANATASE USING TRANSIENT ABSORPTION SPECTROSCOPY

摘要

In order to elucidate electron migration in dye-sensitised nanoporous anatase TiO_2, time-of-flight short-circuit photocurrents and transient absorption spectra between 500 and 2000 nm have been recorded. It is found that electrons in TiO_2 dominate the transient absorption between 900 and 1100 nm, whereas at wavelengths longer than 1100 nm absorption by electrons in the SnO_2:F substrate prevails. To facilitate a qualitative analysis, the absorption cross-sections of electrons in TiO_2 and SnO_2:F have been measured. Combining transient absorption and photocurrent response data, the time-resolved recombination loss can be determined. When the excitation density is below 33.5 μJ/cm~2, on average less than one electron per nanoparticle is injected. Under this condition the IPCE equals unity. When higher excitation densities are applied, more than one electron per nanoparticle is injected, losses become significant, and the IPCE reduces to 40%. The time evolution of the recombination loss reveals that recombination primarily takes place with a few microseconds. One mechanism involves the recombination of electrons with dye cations, occurring in the first 20 ns after laser flash excitation.