The Bulk Photovoltaic Effect: Origin of Shift Currents in the Many-Body Picture (2402.02002v2)

Abstract: We show that the shift current in bulk photovoltaics originates from a light-induced shift of charge centers to the many-body excited states. This many-body shift vector is expressed, using a sum-rule, in terms of pairs of nearly-degenerate optically-active excitons overlapping in $k$-space. The shift current can thus be enhanced by nearly-degenerate optically-active and tightly-bound excitons, spread out in $k$-space. $Ab$ $initio$ results for the many-body shift current and shift vector are presented for CdS and MoS$_2$, and exciton-phonon coupling is included through the exciton-phonon self-energies.