Bismuth vanadate (BiVO4) is a promising semiconductor material for photoelectrochemical
water splitting showing good visible light absorption and a high photochemical
stability. To improve the performance of BiVO4, it is of
key importance to understand its photophysics upon light absorption.
Here we study the carrier dynamics of BiVO4 prepared by
the spray pyrolysis method using broadband transient absorption spectroscopy
(TAS), in thin films as well as in a photoelectrochemical (PEC) cell
under water-splitting conditions. The use of a dual-laser setup consisting
of electronically synchronized Ti:sapphire amplifiers enable us to
measure the femtosecond to microsecond time scales in a single experiment.
On the basis of this data, we propose a model of carrier dynamics
that includes relaxation and trapping rates for electrons and holes.
Hole trapping occurs in multiple phases, with the majority of the
photogenerated holes being trapped with a time constant of 5 ps and
a small fraction of this hole trapping taking place within the instrument
response of 120 fs. The induced absorption band that represents the
trapped holes is modulated by an oscillation of 63 cm–1, which is assigned to the coupling of holes to a phonon mode. We
find electrons to undergo a relaxation with a time constant of 40
ps, followed by deeper trapping on the 2.5 ns time scale. On time
scales longer than 10 ns, trap-limited recombination that follows
a power law is found, spanning time scales up to microseconds. Finally,
we observe no spectral or kinetic differences by applying a bias voltage
to the PEC cell, indicating that the effect of a voltage and the charge
transfer processes between BiVO4 and the electrolyte occurs
on longer time scales. Our results therefore provide new insights
into the carrier dynamics of BiVO4 and further expand the
application window of TAS as an analytical tool for photoanode materials.
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