Topologically protected photovoltaics in Bi nanoribbons

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.