Origin of Enhancement in Open-Circuit Voltage by Adding ZnO to Nanocrystalline SnO2 in Dye-Sensitized Solar Cells

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Abstract
SnO2 + ZnO working electrodes for dye-sensitized solar cells were made by mixing a nanocrystalline SnO2 colloidal dispersion with ZnO or Zn(CH3COO)2. Addition of ZnO or Zn(CH3COO)2 enhanced the open-circuit voltage (Voc) of the cells with respect to cells containing only SnO2. Dependence of the electron lifetime in the electrodes on short-circuit photocurrent density (Jsc) gave evidence against the assumption that the suppression of back electron transfer to the electrolyte is the origin for the Voc enhancement by addition of Zn. Voc dependence on temperatures indicated a decrease in the combined capacitance of the mixed electrode. The slope of the Voc dependence versus the logarithm of Jsc indicated that the contribution of unpinning of the band to the enhancement of Voc could be neglected. From the cyclic voltammograms of the electrodes, the combined capacitance of the mixed electrode was 1 order of magnitude smaller than that of SnO2. The decrease in the combined capacitance in the mixed electrode could be explained by the decrease in the chemical capacitance of SnO2, thus the shift of the conduction band position toward the vacuum level. X-ray photoelectron spectra of Sn 3d5/2 peaks showed a shift toward lower binding energy with an increasing amount of added Zn. This was attributed to an increase in the surface potential toward the negative direction, which might have resulted from a dipole moment formed by Zn on the surface of SnO2.