Edge current generation in 2d topological insulators through exciton dissociation

Exciton dissociation in semiconductor junctions constitute one of the main mechanisms for photovoltaic current generation. The quest for more efficient solar panels has caused lots of efforts to be put into research of materials with enhanced exciton dissociation rates, and alternative photocurrent mechanisms. On the other hand, topological insulators are a promising platform for spintronics, due to the presence of helical edge states which are robust to disorder and have a spin-locked propagation direction. As insulating materials, they are subject to exciton formation, so we conjecture that if an exciton is formed in bulk, it could dissociate on the edges leading to some effective current. In this work, by considering a ribbon of Bi(111) which is known to be a 2d time-reversal topological insulator, we compute the exciton spectrum within tight-binding theory using the Tamm-Dancoff approximation, and show the effect of topology in both the spectrum and the bulk excitonic wavefunction, as well as its consequences over the exciton transition rate to edge states. By performing energy band engineering, modifying the edge bands appropiately through different substractes at each edge, we conclude that it is possible to obtain a non-zero net current from exciton dissociation formed at the bulk.