  Tin iodide phosphide (SnIP), an inorganic double-helix material, is a
quasi-1D van der Waals semiconductor that shows promise in photocatalysis and
flexible electronics. However, our understanding of the fundamental
photophysics and charge transport dynamics of this new material is limited.
Here, we use time-resolved terahertz (THz) spectroscopy to probe the transient
photoconductivity of SnIP nanowire films and, with insight into the highly
anisotropic electronic structure from quantum chemical calculations, measure an
electron mobility as high as 280 $cm^2V^{-1}s^{-1}$. Additionally, the THz
vibrational spectrum reveals a photoexcitation-induced charge redistribution
that reduces the amplitude of a twisting mode of the outer SnI helix on
picosecond timescales. Finally, we show that the carrier lifetime and mobility
are limited by a trap density greater than $10^{18}\,cm^{-3}$. Our results
provide insight into the optical excitation and relaxation pathways of SnIP and
demonstrate a remarkably high carrier mobility for such a soft and flexible
material.
