Layer-Dependent Photoabsorption and Photovoltaic Effects in Two-Dimensional ${\mathrm{Bi}}_2{\mathrm{O}}_{2}X$ (X = $\mathrm{S}$, $\mathrm{Se}$, and $\mathrm{Te}$)

Significant photoconductive effects are reported in emergent two-dimensional (2D) ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$. In this work, we investigate the layer-dependent photoresponse properties and photovoltaic effects of 2D ${\mathrm{Bi}}_2{\mathrm{O}}_{2}X$ (X = $\mathrm{S}$, $\mathrm{Se}$, and $\mathrm{Te}$) by first-principles calculations and quantum-transport simulation. The absorbance per layer increases with the decreasing layer number for high-frequency light, so the absorbance density of 2D ${\mathrm{Bi}}_2{\mathrm{O}}_{2}X$ can be elevated by decreasing the layer number. An outstanding open-circuit voltage (1.08 V) among 2D materials is found for the monolayer (ML) ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ p-n junction. The computed responsivities of ML black phosphorous, ${\mathrm{Mo}\mathrm{S}}_2$, and ${\mathrm{W}\mathrm{Se}}_2$ p-n junctions through our methods are in good agreement with experiments. The ML ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ and ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Te}$ p-n junctions show responsivities of 16.8 and 13.6 mA/W, respectively, under AM1.5 sunlight; these values are higher than those of their extensively studied ML ${\mathrm{Mo}\mathrm{S}}_2$ (8.6) and ${\mathrm{W}\mathrm{Se}}_2$ (8.8) counterparts. The ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ film and ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{S}$ p-n junctions also show higher responsivities than those of commercial $\mathrm{Si}$ and $\mathrm{Ga}\mathrm{As}$. Therefore, the 2D ${\mathrm{Bi}}_2{\mathrm{O}}_{2}X$ p-n junctions have prospective applications in photovoltaic devices.

Figure 1

Geometry and band structures of ${\mathrm{Bi}}_2{\mathrm{O}}_{2}X$. (a) Atomic structure of a single unit cell of bulk ${\mathrm{Bi}}_2{\mathrm{O}}_{2}X$ crystal. (b) Side view of ML ${\mathrm{Bi}}_2{\mathrm{O}}_{2}X$ passivated by hydrogen atoms. Purple, green, red, and white balls represent $\mathrm{Bi}$; $\mathrm{S},\mathrm{Se}$, or $\mathrm{Te}$; $\mathrm{O}$; and $\mathrm{H}$ atoms, respectively. (c)–(e) Projected band structures of ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{S}$, ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$, and ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Te}$, respectively, calculated with HSE06 functional. Green, purple, and red dots represent projection weights of $\mathrm{S},\mathrm{Se}$, or $\mathrm{Te}$; $\mathrm{Bi}$; and $\mathrm{O}$ orbitals, respectively.

Figure 2

Optical properties and scaling effect of ${\mathrm{Bi}}_2{\mathrm{O}}_{2}X$. (a)–(c) Absorption spectra of incidence light for ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{S}$, ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$, and ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Te}$, respectively, calculated by TD DFT method. Red (blue) lines represent absorption coefficient in bulk crystals for photon polarization perpendicular (parallel) to $\mathrm{Bi}\mathrm{O}$ layers, and black lines represent absorbance of light (incident from z direction and polarized to x direction) by ML ${\mathrm{Bi}}_2{\mathrm{O}}_{2}X$ (in x-y plane). (d) Average absorbance of each layer for 2D ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ with different layers compared with bulk ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$. (e) Comparison between light absorbance of ML ${\mathrm{Bi}}_2{\mathrm{O}}_{2}X$ calculated by different methods. Red lines, blue lines, and black lines are calculated by TD DFT, GW BSE, and single-particle approximation, respectively. (f) Illustration of photovoltaic device with 2D ${\mathrm{Bi}}_2{\mathrm{O}}_{2}X$ contacted by two electrodes.

Figure 3

Electronic structure of bulk ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Te}$ modulated by strain. Partial density of states (PDOS) of fully relaxed crystal, crystal with in-plane anisotropic strain of 3%, vertical expansion of 2%, and vertical compression of 2%. Total density of states and their projections to s, p_x, p_y, and p_z orbitals are plotted as gray, red, green, purple, and blue lines, respectively.

Figure 4

(a) Device configuration and corresponding LDOS of ML ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ p-n junction. Color scale from blue to red represents LDOS from low to high. (b) Photoinduced carrier distribution of corresponding slightly doped p-n junction simulated by continuous model. Blue and red solid lines denote CBM and VBM of p-n junction, while blue and red dash lines denote photoinduced electron and hole density, respectively. Fermi level is set as zero.

Figure 5

Photovoltaic properties in ML ${\mathrm{Bi}}_2{\mathrm{O}}_{2}X$ p-n junctions under different photon energies. (a) Photocurrent density, (b) responsivity, and (c) EQE at V_bias= 0 V for photon energies from 0 to 5 eV. (d) Responsivity spectra of ML TMDs (${\mathrm{Mo}\mathrm{S}}_2$ and ${\mathrm{W}\mathrm{Se}}_2$) and group V 2D materials (black phosphorous and antimonene) p-n junctions. Experimental responsivities of ML BP and ${\mathrm{Mo}\mathrm{S}}_2$ p-n junctions are denoted by black and red stars, respectively. (e) Comparison of responsivity (red bars) and EQE (blue bars) of calculated ML materials under standard AM1.5 sunlight radiation. Green bar represents experimental data. (f) I-V curve of heavily doped ML ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ p-n junction under AM1.5 illumination. U_OC, I_SC, and W_max represent open-circuit voltage, short-circuit current, and maximal output power of the device, respectively.

Figure 6

Photoresponsivity comparison between multilayer ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{S}$ and ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ p-n junctions (a) at thickness of 100 nm for different incident photon energies. (b) Thickness dependence of ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ and ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{S}$ thin film under AM1.5 sunlight radiation. Dashed lines are saturated values for the bulk limit.