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 (T
0
= 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.
The role of electrical potential, charge transport, and recombination in determining the photopotential and
photocurrent conversion efficiency (IPCE) of dye-sensitized nanocrystalline solar cells was studied. Electrostatic
arguments and electrical impedance spectroscopy (EIS) are used to obtain information on the electrical and
electrochemical potential distribution in the cell. It is shown that on the macroscopic level, no significant
electrical potential drop exists within the porous TiO2 when it contacts the electrolyte and that the electrical
potential drop at the transparent conducting oxide substrate (TCO)/TiO2 interface occurs over a narrow region,
one or two layers of TiO2. Analyses of EIS and other data indicate that both the photopotential of the cell and
the decrease of the electrical potential drop across the TCO/TiO2 interface are caused by the buildup of
photoinjected electrons in the TiO2 film. The time constants for the recombination and collection of the
photoinjected electrons are measured by EIS and intensity-modulated photocurrent spectroscopy (IMPS). As
the applied bias is varied from short-circuit to open-circuit conditions at 1 sun light intensity, recombination
becomes faster, the collection of electrons becomes slower, and the IPCE decreases. The decrease of IPCE
correlates directly with the decline of the charge-collection efficiency ηcc, which is obtained from the time
constants for the recombination and collection of the photoinjected electrons. Significantly, at open circuit,
η
cc is only 45% of its short-circuit value, indicating that the dye-sensitized nanocrystalline TiO2 solar cell
behaves as a nonideal photodiode.
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