Flatband Potential of F:SnO₂ in a TiO₂ Dye-Sensitized Solar Cell: An Interference Reflection Study

Abstract

A model for the analysis of interference reflection data of nanoporous TiO 2 dye-sensitized solar cells (DSSC) is presented. It is shown that the interference reflection technique [Turrión, M.;Macht, B.;Tributsch, H.; Salvador, P. J. Phys. Chem. B 2001, 105, 9732] allows one to monitor very precisely the evolution of the depletion layer of the fluor-doped SnO 2 (FTO) conducting substrate at the DSSC back contact with the position of the Fermi level. The model shows that this technique features a much larger sensitivity than the capacitancevoltage Mott-Schottky method for determining both the flatband potential (U 0 ) and the Helmholtz layer capacitance (C H ) at the FTO/electrolyte interface, under working conditions of the DSSC. Under illumination the band-bending in the FTO substrate is controlled by two factors: (a) the position of Fermi level (FTO bulk potential), which is determined by the accumulation of photogenerated electrons in the conduction band of the nanostructured TiO 2 film, and (b) the values of U 0 and C H , which determine the potential distribution at the FTO/electrolyte interface. It is suggested that both parameters U 0 and C H determine a constraint on the maximum photovoltage attainable by the DSSC, as the conduction band edge of the FTO at the FTO/TiO 2 interface cannot be higher than that of the TiO 2 , otherwise the transfer of dye photoinjected electrons from the TiO 2 to the FTO would be hindered. According to our analysis, it can be concluded that the theoretical maximum photovoltage cannot be higher than 0.8 V, a value never surpassed experimentally with untreated DSSC.

Physical aspects of ferroelectric semiconductors for photovoltaic solar energy conversion

“…The built-in voltage bi V ensures that the applied forward bias V can drop somewhere. That is, the VL can be modified as an effect of light without creating kinetic barriers, as long as V V bi  is still positive (Rau et al, 2003,Turrión et al, 2003,Kirchartz and Rau, 2011. Thus, the built-in voltage serves an important role even if like in c-Si solar cells, charge collection is almost entirely driven by diffusion.…”

Facile route to straight SnO2 nanowires and their optical properties

“…As an important ntype semiconductor with wide bandgap (E g = 3.6 eV at 300 K), SnO 2 have been widely used for transparent conductors, gas sensor solar cells, lithium-ion batteries, and electronic devices [4][5][6][7][8][9][10]. Recently, one-dimensional nanowire structure has attracted increasing attentions, owing to its enhanced surface to volume ratio and promising applications for gas sensors [11,12] and electronic nanodevices [13] Up to now, various methods including direct-oxidized growth [14][15][16], molten-salt synthesis [17,18], hydrothermal method [19,20], laser-ablation synthesis [21], carbothermal reduction [22,23], and template method [24], etc.…”

Synthesis, photoluminescence and dielectric properties of O-deficient SnO2 nanowires

“…As an important n-type semiconductor with wide band gap (E g = 3.6 eV at 300 K), SnO 2 has been widely used for transparent conductors, gas sensors, solar cells, lithium-ion batteries, and electronic devices [1][2][3][4][5][6]. Recently, SnO 2 nanowires are considered as an ideal material for constructing nanosized devices due to its high surface to volume ratio and special chemical and physical properties originating from nanosize effects.…”