Ultrafast carrier dynamics in BiVO₄ thin film photoanode material: interplay between free carriers, trapped carriers and low-frequency lattice vibrations

We explore ultrafast carrier dynamics and interactions of photoexcited carriers with lattice vibrational modes in BiVO₄ photoanode material using time-resolved terahertz spectroscopy and first-principles phonon spectrum calculations. We find that photoexcited holes form bound polaron states by introducing lattice distortion that changes phonon spectrum and suppresses the A_g phonon mode associated with opposite motion of Bi and VO₄ molecular units. At excitation fluence higher than 1 mJ cm² (or 2 × 10¹⁵ cm² per pulse), lattice distortion due to self-localized holes alters the lattice symmetry and vibrational spectrum, resulting in bleaching of THz absorption by A_g phonons. Concurrently, we observe a short lived population of free carriers which exhibit Drude conductivity with mobility on the order of 200 cm² V¹ s¹, orders of magnitude higher than typical carrier mobility in BiVO₄. The anomalously high carrier mobilities are explained in the framework of a Mott transition. This demonstration of enhanced transport suggests how engineering BiVO₄ photoanodes to take advantage of free carrier transport under high excitation conditions may in the future significantly enhance performance of photoelectrochemical devices.