Surface Photovoltage Spectroscopy of Dye-Sensitized Solar Cells with TiO₂, Nb₂O₅, and SrTiO₃Nanocrystalline Photoanodes:  Indication for Electron Injection from Higher Excited Dye States

Abstract

The onset wavelengths of the surface photovoltage (SPV) in dye-sensitized solar cells (DSSCs) with different mesoporous, wide-band gap electron conductor anode materials, viz., TiO 2 (anatase), Nb 2 O 5 (amorphous and crystalline), and SrTiO 3 , using the same Ru bis-bipyridyl dye for all experiments, are different. We find a clear dependence of these onset wavelengths on the conduction band edge energies (E CB ) of these oxides. This is manifested in a blue-shift for cells with Nb 2 O 5 and SrTiO 3 compared to those with TiO 2 . The E CB levels of Nb 2 O 5 and SrTiO 3 are known to be some 200-250 meV closer to the vacuum level than that of our anatase films, while there is no significant difference between the optical absorption spectra of the dye on the various films. We, therefore, suggest that the blue shift is due to electron injection from excited-state dye levels above the LUMO into Nb 2 O 5 and SrTiO 3 . Such injection comes about because, in contrast to what is the case for anatase, the LUMO of the adsorbed dye in the solution is below the E CB of these semiconductors, necessitating the involvement of higher vibrational and/or electronic levels of the dye, with the former being more likely than the latter. While for Nb 2 O 5 hot electron injection has been proposed earlier, on the basis of flash photolysis experiments, this is the first evidence for such ballistic electron-transfer involving SrTiO 3 , a material very similar to anatase but with a significantly smaller electron affinity. Additional features in the SPV spectra of SrTiO 3 and amorphous Nb 2 O 5 (but not in those of crystalline Nb 2 O 5 ) can be understood in terms of hole injection from the dye into the oxide via intraband gap surface states.

(Photo)electrochemical Methods for the Determination of the Band Edge Positions of TiO2‐Based Nanomaterials

“…Moreover, the Kelvin probe measurements of photoelectrodes immersed into electrolyte have been reported, whereby the electrode and the electrolyte were separated from the Kelvin probe tip by a very thin glass plate [182]. This method has been, for example, used for the estimation of the quasi-Fermi level in the TiO 2 nanocrystalline layers of dye-sensitized solar cells [183].…”

Compact Inverse‐Opal Electrode Using Non‐Aggregated TiO2 Nanoparticles for Dye‐Sensitized Solar Cells

“…[3,13,14] In conjunction with these efforts, recent studies on photoelectrodes have expanded the processing strategies by using periodic nanostructures with long-range ordering to better assure an interconnected morphology of the TiO 2 structure. Such periodic structures have shown promising results for wide bandgap semiconductor materials, [15][16][17][18][19][20][21] core-shell structures, [22,23] nanowire (nanotube) structures, [6][7][8][9][10][11][24][25][26] and inverse-opal (IO) structures. [27][28][29][30][31][32][33] In particular, ordered IO structures of two-dimensional or threedimensional (2D or 3D) colloidal crystals have been successfully utilized in the modification of the absorption bands of dye sensitizers due to the photonic bandgap effect in DSSCs.…”

To investigate opulence of graphene in ZnO/graphene nanocomposites based dye sensitized solar cells

“…Though, DSSCs show lower photoconversion efficiency values but have a higher performance to cost ratio in comparison to silicon-based solar cells [7]. DSSC is a 3rd-generation solar cell [5] having capacitive nature that employs high band gap nanostructured semiconductors like TiO 2 , ZnO [8,9], Nb 2 O 5 , SrTiO 3 [10], SnO 2 [11] and their composites as photoelectrode materials. As ZnO is known to be a wide band gap (3.37 eV) n-type semiconductor that is identical to TiO 2 (3.23 eV) hence similar electron injection actions are expected for both these materials [12] Due to high electron mobility (~107 times more) and lower recombination rate, electron lifetime is considerably higher in ZnO as compared to TiO 2 .…”