Ambipolar Diffusion of Photocarriers in Electrolyte-Filled, Nanoporous TiO2†

N. Kopidakis and E. A. Schiff*
Department of Physics, Syracuse University, Syracuse, New York 13244
N.-G. Park, J. van de Lagemaat, and A. J. Frank
National Renewable Energy Laboratory, Golden, Colorado 80401
J. Phys. Chem. B, 2000, 104 (16), pp 3930–3936
DOI: 10.1021/jp9936603
Publication Date (Web): March 18, 2000
Copyright © 2000 American Chemical Society
†

 Part of the special issue “Harvey Scher Festschrift”.

,
*

 Corresponding author e-mail:  Schiff@physics.syr.edu; Fax:  (315) 443-9103.

Abstract

We report transient photocurrent measurements on solar cell structures based on dye-sensitized, porous TiO2 films filled with a liquid electrolyte. The measurements are interpreted as ambipolar diffusion; under most measurement conditions, the ambipolar diffusion coefficient is dominated by electrons diffusing in the TiO2 matrix. We report a strong dependence of the ambipolar diffusion coefficient upon the photoexcitation density, as has been proposed previously. The coefficients vary from 10-8 cm2 s-1 at low density to 10-4 cm2 s-1 for densities of 1018 cm-3. At a specified photoexcitation density, ambipolar diffusion coefficients measured using weak laser pulses and optical bias are about 10 times larger than coefficients measured using large-intensity laser pulses. We describe trapping models for these effects based on an exponential distribution (T0 = 650 K) of electron trap levels in TiO2. We infer an electron recombination cross section less than 2 × 10-27 cm2; this value is nearly 10 orders of magnitude smaller than typical values in compact semiconductors and indicates the extraordinarily effective separation of electrons in the TiO2 matrix from electrolyte ions only nanometers distant.

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History

  • Published In Issue April 27, 2000
  • Received October 13, 1999

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