Potential Distribution and Photovoltage Origin in Nanostructured TiO₂ Sensitization Solar Cells:  An Interference Reflection Study

Abstract

A well-defined modulation reflection spectrum due to a multiple interference process is originated in the TiO 2 dye sensitizated solar cell (DSSC). Experimental evidence is shown that the interference process leading to the reflection spectrum takes place at the n-conducting F:SnO 2 (FTO) layer of the FTO/TiO 2 back contact. Moreover, the interference reflectance spectrum is influenced by the applied potential and illumination bias and disappears when FTO is metallized with 10 Å of platinum. These results show that FTO/TiO 2 cannot be considered as an ohmic but as a rectifying contact where the FTO behaves as a highly doped n-type semiconductor which absorbs an important part of the equilibrium contact potential in the dark. On the basis of our experimental results a new insight on the role of the dark equilibrium contact potential at the FTO/ TiO 2 interface in the processes of electric charge separation and photovoltage generation is given. Evidence is shown that the theoretically maximum attainable photovoltage in a DSSC is in one direction limited by the equilibrium redox potential in the dark, and in the other direction by the (light intensity dependent) bottom of the TiO 2 conduction band.

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

“…1 The interpretation of the interference reflection spectra in thin oxide films has been corroborated by various authors, [17][18][19] as a perturbation of the complex optical dielectric constant of the film. In this section we discuss the derivation of theoretical spectra resulting from such perturbation in the FTO thin film.…”

“…However, this is far from being true, since due to its low dielectric contact ( r ) 9), even when highly doped (N d ) 10 21 cm -3 ), FTO has a non-negligible electric field region (depletion layer) at its interface with the electrolyte, which can be modulated under external bias. [1][2][3] The external potential applied to a DSSC is placed at the FTO/TiO 2 interface (back contact), generating there an externally modulable electrical potential barrier that is able to hinder the injection in the dark of electrons from the FTO substrate to the TiO 2 and to promote the transfer of photoinjected electrons from the TiO 2 to the FTO. 4,5 The energy structure of this potential barrier is very important in determining the behavior of the DSSC both in the dark and under illumination.…”

“…16 Besides electrolyte electrorreflectance, the interference reflection technique is an alternative and powerful experimental method very sensitive to the magnitude of the electric field at the FTO depletion layer. 1 In this paper we develop a new method for an in situ precise estimation of the potential distribution at the FTO/electrolyte interface of the DSSC back contact from experimental interference reflection data, under working conditions of the solar cell. It will be shown that both U 0 and U 1 (and hence C H ) constitute independent parameters that can be determined with a high accuracy.…”

“…It is shown that the interference reflection technique [Turrión, M.;Macht, B.;Tributsch, H.; Salvador, P. J. Phys. Chem.…”

Fabrication and reliability of dye solar cells: A resonance Raman scattering study

“…The overall fabrication process of DSC materials can be established using wet-processing, large area techniques such as blade coating or screen printing [1,2], ink-jet printing, roll-to-roll processing. The overall conversion efficiencies of the best devices made of mesopourous TiO 2 films have now reached 12% [3][4][5][6][7][8][9][10][11][12][13][14][15][16]. It is indeed of fundamental importance the analysis and the optimization of DSC considering the complex interplay and interoperability of the various material constituents [17].…”