Bulk photovoltaic effect of an alpha-phase indium selenide (α-In2Se3) crystal along the out-of-plane direction

The photovoltaic effect can occur without metal/semiconductor or semiconductor/semiconductor interfaces in materials with spontaneous electric polarization due to a lack of spatial inversion symmetry. This anomalous phenomenon is known as the bulk photovoltaic (BPV) effect owing to the shift current, which facilitates photoelectric conversion. Alpha-phase indium selenide (α-In2Se3) is a ferroelectric semiconductor with spontaneous electric polarization. Here, we report an experimental demonstration of the BPV effect in α-In2Se3 films along the out-of-plane direction by utilizing graphite films as transparent electrodes. A short-circuit current was detected under white light irradiation, and the sign was inverted upon inverting the direction of electric polarization, thus suggesting its origin to be the shift current, distinct from the effect occurring at the interface. The trend of BPV performances owing to the shift current with respect to the irradiated photon energy is consistent with the dependence of its absorption coefficient. The quantum efficiency of photoelectric conversion for α-In2Se3 along the out-of-plane direction was a few orders of magnitude greater than that of conventional and layered ferroelectric materials and was comparable to that of low-dimensional interface structures and strained two-dimensional materials. Our results pave the way for the innovative design of photoelectric conversion devices.