ECN publication
Charge transport versus recombination in dye-sensitized solar cells employing nanocrystalline TiO2 and SnO2 films
Published by: Publication date:
ECN Solar Energy 1-8-2005
ECN report number: Document type:
ECN-RX--05-039 Article (scientific)
Number of pages:

Published in: Journal of Physical Chemistry B (American Chemical Society), , 2005, Vol.109, p.12525-12533.


We report a comparison of charge transport and recombination dynamics in dye-sensitized solar cells (DSSCs)

employing nanocrystalline TiO2 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 SnO2 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 TiO2, and from a 300 mV positive shift of the SnO2 conduction band/trap states density of states

relative to TiO2. The faster recombination to the redox couple results in an increased dark current for DSSCs

employing SnO2 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.

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