  Photovoltaic efficiency in solar cells is hindered by many unwanted effects.
Radiative channels (emission of photons) sometimes mediated by non-radiative
ones (emission of phonons) are principally responsible for the decrease in
exciton population before charge separation can take place. Another unwanted
effect is electron-hole recombination at surfaces where in-gap edge states
serve as the non-radiative channels. Topological insulators (TIs), in
particular the most common ones characterized by a Z2 invariant, are rarely
explored from an optoelectronics standpoint, possibly because of their
typically small gaps. Otherwise, they are not much different from small-gap
semiconductors and excitons are also expected to be their simplest excitations.
Here we show that one can take advantage of the non-radiative decay channel due
to the surface states to generate a topologically protected photovoltaic
current. Focusing on two-dimensional TIs, and specifically for illustration
purposes on a Bi(111) monolayer, we show the potential of TI nanoribbons to
generate edge charge accumulation and edge currents under illumination.
