In situ infrared study of photo-generated electrons and adsorbed species on nitrogen-doped TiO2 in dye-sensitized solar cells

Abstract

Charge transfer between adsorbed dyes and the TiO₂ surface plays a key role in controlling the efficiency of dye-sensitized solar cells (DSSCs). The lack of understanding of charge transfer steps has hindered further development of DSSCs and many solar energy conversion devices/processes. In this study, we used in situ infrared spectroscopy to investigate electron transfer and photo-electric energy conversion processes at the interface, i.e., surface hydroxyls, adsorbed species, as well as the dynamics of photo-generated electrons in TiO₂ and N-TiO₂ in DSSCs. Nitrogen (N-) doping of TiO₂ blocked linear OH, giving more hydrophobic surface characteristics than undoped TiO₂. N-Doping further increased the electron–hole separation caused by solar light on the working electrode and the current density in the DSSC. In situ infrared (IR) studies revealed that N-doping facilitated the electron transfer from the N719 dye (di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2-bipyridyl-4,4-dicarboxylato)ruthenium(II)) to the conduction band in TiO₂, reducing the impedance in the DSSC. Probing N-TiO₂ with adsorbed ethanol showed that shallow traps in N-TiO₂ can be accessed by electrons from adsorbed ethanol. Electron transfer from the N719 dye is significantly faster than that from adsorbed ethanol which involves C–H bond breaking.