Control of charge separation and recombination is critical
for
dye-sensitized solar cells and photoelectrochemical cells, and for
p-type cells, the latter process limits their photovoltaic performance.
We speculated that the lateral electron hopping between dyes on a
p-type semiconductor surface can effectively separate electrons and
holes in space and retard recombination. Thus, device designs where
lateral electron hopping is promoted can lead to enhanced cell performance.
Herein, we present an indirect proof by involving a second dye to
monitor the effect of electron hopping after hole injection into the
semiconductor. In mesoporous NiO films sensitized with peryleneimide
(PMI) or naphthalene diimide (NDI) dyes, dye excitation led to ultrafast
hole injection into NiO from either excited PMI* (τ < 200
fs) or NDI* (τ = 1.2 ps). In cosensitized films, surface electron
transfer from PMI–to NDI was rapid (τ = 24
ps). Interestingly, the subsequent charge recombination (ps−μs)
with NiO holes was much slower when NDI–was generated
by electron transfer from PMI–than when NDI was
excited directly. We therefore indicate that the charge recombination
is slowed down after the charge hopping from the original PMI sites
to the NDI sites. The experimental results supported our hypothesis
and revealed important information on the charge carrier kinetics
for the dye-sensitized NiO photoelectrode system.