  Shift current is a DC current generated from nonlinear light-matter
interaction in a non-centrosymmetric crystal and is considered a promising
candidate for next generation photovoltaic devices. The mechanism for shift
currents in real materials is, however, still not well understood, especially
if electron-hole interactions are taken into account. Here, we employ a
first-principles interacting Green's-function approach on the Keldysh contour
to study photocurrents generated by nonlinear optical processes in real
materials and discover a strong DC shift current at subbandgap excitation
frequencies in monolayer GeS due to strongly bound excitons, as well as giant
enhancement in the shift current coefficients at above bandgap photon
frequencies. Our results suggest that atomically thin two-dimensional materials
may be promising building blocks for next generation shift current devices with
efficiencies beyond the Shockley-Queisser limit.
