  In optoelectronic devices such as solar cells and photodetectors, a portion
of electron-hole pairs are generated as so called hot carriers with an excess
energy that is typically lost as heat. The long standing aim to harvest this
excess energy to enhance device performance has proven to be very challenging,
largely due to the extremely short-lived nature of hot carriers. Efforts thus
focus on increasing the hot carrier relaxation time, and on tailoring
heterostructures that allow for hot-carrier extraction on short time- and
length-scales. Recently, semiconductor nanowires have emerged as a promising
system to achieve these aims, because they offer unique opportunities for
heterostructure engineering as well as for potentially modified phononic
properties that can lead to increased relaxation times. In this review we
assess the current state of theory and experiments relating to hot-carrier
dynamics in nanowires, with a focus on hot-carrier photovoltaics. To provide a
foundation, we begin with a brief overview of the fundamental processes
involved in hot-carrier relaxation, and how these can be tailored and
characterized in nanowires. We then analyze the advantages offered by nanowires
as a system for hot-carrier devices and review the status of proof-of-principle
experiments related to hot-carrier photovoltaics. To help interpret existing
experiments on photocurrent extraction in nanowires we provide modelling based
on non-equilibrium Green's functions. Finally, we identify open research
questions that need to be answered in order to fully evaluate the potential
nanowires offer towards achieving more efficient, hot-carrier based,
optoelectronic devices.
