First-principles calculation of shift current bulk photovoltaic effect in two-dimensional α-In2Se3

Shift current is the dominant dc-current response in the bulk photovoltaic effect (BPVE), which is the conversion of solar energy into electricity in the materials with broken inversion symmetry. While the guiding principle of BPVE is a lack of inversion symmetry in a material which also results in ferroelectricity, it is therefore, expected that a significantly large shift current is achieved in ferroelectric materials. In this work, we calculate shift current using first principles in two-dimensional $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{I}{\mathrm{n}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ which has both in-plane and out-of-plane polarization at room temperature. To understand the implications of in-plane and out-of-plane polarization on shift current BPVE, mono- and bilayer structures of $3R$ and $2H$ $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{I}{\mathrm{n}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ are considered in our calculations. It suggests that the in-plane polarization doesn't affect the shift current response of this material. In monolayer, a dominant shift current response of magnitude $750\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{A}/{\mathrm{V}}^{2}$ is obtained along the direction of out-of-plane polarization under uniform illumination of $zz$-polarized light at a photon energy of 4.16 eV. The doping engineering is further implemented to tune the shift current response to visible light. Bismuth (Bi) is used to substitute the indium element at the tetrahedral site thereby introducing more energy levels in the conduction band which importantly are contributed by $p$ orbitals of Bi and take part in the transition process. Consequently, a giant shift current of $1200\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{A}/{\mathrm{V}}^{2}$ is obtained at a photon energy of 2.98 eV. The present study would be an instrumental in understanding the shift current BPVE and would pave the path for designing efficient photovoltaic devices based on $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{I}{\mathrm{n}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ and similar material systems.