Ultrahigh Bulk Photovoltaic Effect Responsivity in Thin Films: Unexpected Behavior in a Classic Ferroelectric Material

The bulk photovoltaic effect (BPE) has drawn considerable attention due to its ability to generate photovoltages above the bandgap and reports of highly enhanced photovoltaic current when using nanoscale absorbers or nanoscale electrodes, which, however, do not lend themselves to practical, scalable implementation. Herein, it is shown that a strikingly high BPE photoresponse can be achieved in an ordinary thin-film configuration merely by tuning fundamental ferroelectric properties. Nonmonotonic dependence of the responsivity (R_SC) on the ferroelectric polarization is observed and at the optimal value of the film polarization, a more than three orders of magnitude increase in the R_SC from the bulk BaTiO₃ value is obtained, reaching R_SC close to 10⁻² A W⁻¹, the highest value reported to date for the archetypical ferroelectric BaTiO₃ films. Results challenge the applicability of standard first-principles-based descriptions of BPE to thin films and the inherent weakness of BPE in ferroelectric thin films.