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由两种或多种不同的二维材料组合而产生的层状范德瓦耳斯异质结构具有不同寻常的物理特性, 可用于设计高效光电器件. 本文使用基于密度泛函理论的第一性原理方法统地研究了由二维砷化硼(BAs)和蓝磷砷(I-AsP)单层形成的异质结的几何结构和光电性能. 研究表明, 4种垂直堆叠的BAs/I-AsP异质结构在基态下具有稳定的结构, 且带隙在0.63—0.86 eV之间. 相较于其组份的单层结构, 该异质结构的光学吸收系数得到提升, 并且具备I型能带排列结构. 另外, 通过施加双轴应变和电场可显著地改变异质结构的带隙和能带类型. 在双轴施加–10%—8%的拉伸或压缩应变下, 带隙也随之增大, 在拉伸大于8%时, 带隙开始减小. 电场在–0.5—0.5 V/Å范围内线性地影响带隙, 随着电场增大, 带隙逐渐减小. 双轴应变和电场都可使材料能带排列在I型和II型之间转变. 同时, BAs/I-AsP异质结具有约13%的理论光电转换效率. 可见, 该二维异质结在光伏和光电领域具有广阔的应用前景.In recent years, two-dimensional (2D) materials have attracted considerable attention due to their outstanding optical and electronic properties, and they have shown great potential applications in next-generation solar cells and other optoelectronic devices. In this work, density functional theory (DFT) is used to systematically study the electronic and optoelectronic properties of the heterojunction formed by 2D BAs and I-AsP monolayers, as well as the response of this heterojunction under biaxial strain and electric field. The calculation results show that in the ground state, the four vertically stacked BAs/I-AsP heterostructures all have stable geometric structures, and their band gaps range from 0.63 to 0.86 eV. Compared with their constituent monolayers, these heterostructures have the increased optical absorption coefficients (the absorption coefficient in the x-direction reaches 106 cm–1), and they can effectively separate the photogenerated electron-hole pairs. Of the four structures, the A1 structure exhibits the smallest interlayer spacing, the smallest binding energy, and the highest stability. It has a type-I band alignment and a structure of a direct-band-gap semiconductor with band gaps of 0.86 eV (PBE) and 1.26 eV (HSE06), which can be used in the field of light-emitting diodes. The band gap and band type of the heterostructure can be effectively changed by applying biaxial strain and electric field. Under the application of biaxial tensile or compressive strain in a range of -10% to 8%, the band gap increases accordingly. When the tensile strain is greater than 8%, the band gap starts to decrease. When the biaxial strain ε ≤ –3% and ε > 8%, the heterojunction transitions from a type-I band alignment to a type-II band alignment. Under tensile strain, the absorption spectrum undergoes a red shift, while compressive strain leads to a blue shift of the absorption spectrum. Similarly, the externally applied electric field linearly affects the band gap of the BAs/I-AsP heterojunction in a range from -0.5 to 0.5 V/Å, and the band gap decreases as the electric field increases. When a positive electric field with E≥0.2 V/Å is applied, the band alignment of the heterojunction can also transition from type-I to type-II. The BAs/I-AsP heterojunction has strong absorption properties in the ultraviolet and visible light ranges. Based on the Scharber model, the theoretical power conversion efficiency (PCE) η of the BAs/I-AsP heterojunction is found to be greater than 13%, which is higher than those of 2D heterojunction materials such as Cs3Sb2I9/InSe (η = 3.3%), SiPGaS/As (η = 7.3%) and SnSe/SnS (η = 9.1%). This further expands the application scope of the BAs/I-AsP heterojunction, making it expected to play an important role in the field of photodetectors and solar cells.
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Keywords:
- 2D heterojunction /
- DFT /
- power conversion efficiency /
- modulate
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图 1 单层(a) I-AsP和(b) BAs的俯视图和侧视图
Fig. 1. Top and side views of monolayer (a) I-AsP and (b) BAs.
图 2 BAs/I-AsP异质结4种堆叠构型的俯视图与侧视图
Fig. 2. Top and side views of the four stacking configurations of the BAs/I-AsP heterojunction.
图 3 在T = 300 K下的AIMD模拟中, BAs/I-AsP异质结构(A1构型)的能量与温度变化情况
Fig. 3. Variations of energy and temperature of the BAs/I-AsP heterostructure (A1 structure) in the AIMD simulation at T = 300 K.
图 4 采用PBE和HSE06方法得到的BAs/I-AsP异质结能带结构和投影态密度
Fig. 4. Band structures and projected density of states of the BAs/I-AsP heterojunction obtained by using the PBE and HSE06 methods.
图 5 (a) BAs/I-AsP异质结界面电位分布; (b)异质结的Bader电荷分析结果(负值代表电荷由BAs层转移至I-AsP层); (c) BAs/I-AsP异质结构沿z方向的平面平均电荷密度差Δρ(z), 其中插图展示了电荷密度差的三维等值面(黄色和蓝色区域分别表示电子增加和减少)
Fig. 5. (a) The interfacial potential distribution of the BAs/I-AsP heterojunction; (b) results of the Bader charge analysis of the heterojunction, the negative values indicate that the charge is transferred from the BAs layer to the I-AsP layer; (c) the planar average charge density difference Δρ(z) of the BAs/I-AsP heterostructure along the z direction of the surface. The inset shows the three-dimensional isosurface of the charge density difference (the yellow and blue regions represent the increase and decrease of electrons respectively).
图 6 BAs/I-AsP异质结构在双轴应变下 (a)带隙变化; (b)能带边缘变化; (c) ε = 4%, (d) ε = –4%的能带结构
Fig. 6. (a) Variation of the band gap; (b) variation of the band edges of the BAs/I-AsP heterostructure under biaxial strain; the energy band structures of the BAs/I-AsP heterostructure with (c) ε = 4% and (d) ε = –4%.
图 7 BAs/I-AsP异质结构在外加电场下 (a)带隙变化; (b)能带边缘变化; (c) E = 0.2 V/Å和(d) E = –0.2 V/Å的能带结构
Fig. 7. (a) Variation of the band gap; (b) variation of the band edges; (c) the energy band structure with E = 0.2 V/Å and (d) the energy band structure with E = –0.2 V/Å of the BAs/I-AsP heterostructure under an externally applied electric field.
图 8 (a)单层BAs、单层I-AsP与BAs/I-AsP异质结沿x轴和z轴的光吸收系数; (b) BAs/I-AsP异质结在不同双轴应变下沿x轴和z轴的光吸收系数变化情况; (c) BAs/I-AsP异质结在不同外加电场作用下沿x轴和z轴的光吸收系数变化情况
Fig. 8. (a) The optical absorption coefficients along the x-axis and z-axis of the monolayer BAs, I-AsP and the BAs/I-AsP heterojunction; (b) variations of the optical absorption coefficients along the x-axis and z-axis of the BAs/I-AsP heterojunction under different biaxial strains; (c) variations of the optical absorption coefficients along the x-axis and z-axis of the BAs/I-AsP heterojunction under different applied electric fields.
图 9 BAs/I-AsP异质结的光电转换效率随导带偏移量$ \Delta {E}_{{\mathrm{c}}} $和施主带隙$ {E}_{{\mathrm{g}}}^{{\mathrm{d}}} $变化的等高线图
Fig. 9. Contour plot of the photoelectric conversion efficiency of the BAs/I-AsP heterojunction as a function of the conduction band offset ($ \Delta {E}_{{\mathrm{c}}} $) and the donor band gap ($ {E}_{{\mathrm{g}}}^{{\mathrm{d}}} $).
表 1 单层BAs、单层I-AsP以及4种堆叠构型的结构参数, 包括晶格常数(其中a = b)、层间距(d)、键长(l)、带隙(Eg)和层间结合能(Eb)
Table 1. Structural parameters for monolayer BAs, monolayer I-AsP, and four stacking configurations in heterostructures, include lattice constants (where a = b), interlayer distance (d), bond length (l), band gap (Eg), and interlayer binding energy (Eb).
a/Å d/Å lB-As/Å(lAs-As/lAs-P/lP-P) Eg/eV Eb/(meV·Å–2) PBE HSE06 BAs 3.39 — 1.96 0.76 1.18 — I-AsP 5.87 — 2.49/2.37/2.24 1.63 2.27 — A1 5.87 3.44 1.96
2.49/2.37/2.240.86 1.26 –0.59 A2 5.87 3.83 1.96
2.49/2.37/2.240.63 — –0.43 A3 5.87 3.60 1.96
2.49/2.37/2.240.79 — –0.52 A4 5.87 3.81 1.96
2.49/2.37/2.240.71 — –0.47 
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表 2 施加双轴应变和外加电场下具有II型能带排列的异质结构的光伏特性相关参数
Table 2. Summary of the photovoltaic characteristic parameters of the heterostructures with type-II band alignment under the biaxial strain and the external electric field.
Strain/% $ {E}_{{\mathrm{g}}}^{{\mathrm{d}}} $/eV $ \Delta {E}_{{\mathrm{c}}} $/eV $ {V}_{{\mathrm{o}}{\mathrm{c}}} $/V $ \eta $/% –4 1.34 0.681 0.359 13.14 –6 1.01 0.558 0.152 7.49 10 1.28 0.634 0.334 12.72 Electric field/(V·Å–1) $ {E}_{{\mathrm{g}}}^{{\mathrm{d}}} $/eV $ \Delta {E}_{{\mathrm{c}}} $/eV $ {V}_{{\mathrm{o}}{\mathrm{c}}} $/V $ \eta $/% 0.2 1.64 0.823 0.522 13.13 0.3 1.66 0.904 0.462 11.26 0.4 1.67 0.976 0.390 9.51 0.5 1.67 1.057 0.315 8.09
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