We report a comparison of charge transport and recombination dynamics
in dye-sensitized solar cells (DSSCs)
employing nanocrystalline TiO
2 and SnO2 films and address the impact of these dynamics upon photovoltaic
device efficiency. Transient photovoltage studies of electron transport
in the metal oxide film are correlated
with transient absorption studies of electron recombination with both
oxidized sensitizer dyes and the redox
couple. For all three processes, the dynamics are observed to be 2
-3
orders of magnitude faster for the SnO2
electrode. The origins of these faster dynamics are addressed by studies
correlating the electron recombination
dynamics to dye cations with chronoamperometric studies of film electron
density. These studies indicate
that the faster recombination dynamics for the SnO
2 electrodes result both from a 100-fold higher electron
diffusion constant at matched electron densities, consistent with a
lower trap density for this metal oxide
relative to TiO
2, and from a 300 mV positive shift of
the SnO2 conduction band/trap states density of
states
relative to TiO
2. The faster recombination to the redox
couple results in an increased dark current for DSSCs
employing SnO
2 films, limiting the device open-circuit
voltage. The faster recombination dynamics to the
dye cation result in a significant reduction in the efficiency of regeneration
of the dye ground state by the
redox couple, as confirmed by transient absorption studies of this reaction,
and in a loss of device shortcircuit
current and fill factor. The importance of this loss pathway was confirmed
by nonideal diode equation
analyses of device current
-voltage data. The addition of MgO blocking
layers is shown to be effective at
reducing recombination losses to the redox electrolyte but is found
to be unable to retard recombination
dynamics to the dye cation sufficiently to allow efficient dye regeneration
without resulting in concomitant
losses of electron injection efficiency. We conclude that such a large
acceleration of electron dynamics within
the metal oxide films of DSSCs may in general be detrimental to device
efficiency due to the limited rate of
dye regeneration by the redox couple and discuss the implications of
this conclusion for strategies to optimize
device performance.