Theoretical Study On The Design And Optoelectronic Properties Of Two-dimensional Ⅲ-VA And Ⅳ-VA Compounds

The development and utilization of new clean energy become an important way to solve the problem of environmental crisis and resource depletion.Among them,solar energy is an ideal candidate to substitute for fossil fuels in new energy.Meanwhile,solar energy can convert a steady stream of energy into directly used electrical energy or chemical energy that can be stored.Currently,photovoltaic（PV）devices and photocatalytic water splitting are the research hotspots in the development of new energy.However,it is difficult to achieving high power conversion efficiency（PCE）by utilizing bulk semiconductors,and many obstacles appear in the pursuit of an efficient catalyst due to the limitations of bulk materials.But two-dimensional（2D）materials have many obvious advantages compared to bulk counterparts,such as tunable band edges,larger specific surface area,etc.Some high-performance 2D materials have excellent electrical conductivity,high light detection rate,high on-off ratio and carrier mobility,etc.Therefore,designed suitable 2D materials can exhibit excellent device performance in PV cells and photocatalytic water splitting.In this paper,density functional theory（DFT）is employed to study the geometry,electrical and optical properties of 2D III-VA and IV-VA compounds,associated with their applications in optoelectronic devices such as PVs and photocatalytic water splitting.The main research results are as follows:（1）Theoretical study on single-layer solar cells ofⅣ-ⅤA compounds in 2D AX₃ configuration.First,through DFT calculations of physicochemical and photovoltaic properties,the exfoliation energies of AX₃（A=Si,Ge;X=P,As,Sb）are all less than 1 Jm^-2,proving the experimental feasibility to be exfoliated from the corresponding bulk.Second,electronic and optical properties have been systemically investigated.To be specific,the bandgap of monolayer AX₃ falls in the range of 1.12-1.26 e V,which is comparable with silicon（Si）.Significantly,the electron mobility of monolayer AX₃can reach as high as～30,000 cm²V^-1s^-1,which is one order of magnitude higher than that of Si.Furthermore,the optical absorbance of monolayer Si As₃,Si P₃ and Ge As₃ exhibit high coefficient in the visible light.Therefore,we believe that our designed AX₃-based PV system with PCE of 20%can offer great potential in the application of high-performance 2D based PVs.（2）Theoretical study of 2D In Sb/Ga As and In Sb/In P tandem photovoltaic devices.Tandem photovoltaics can reduce thermalization losses in high-bandgap acceptors and exploit the absorption spectrum in low-bandgap acceptors.According to first-principles calculations,the stability,electronic and optical properties of single-layer group-III-V materials（XY,X=Ga and In,Y=N,P,As,Sb,and Bi）are systematically introduced.Due to the high bandgap（E_g）of Ga As and In P being a perfect match with the low E_g of In Sb,In Sb/Ga As-and In Sb/In P-based tandem PVs are constructed.In addition,the complementary absorption spectra of these two subcells can facilitate to achieve high tandem PCE.The photoelectric conversion efficiency is as high as 30.0%by our calculation results verification based on the 2D In Sb/Ga As and In Sb/In P tandem photovoltaics.Furthermore,we have analyzed in detail the influencing factors for PCE and the physical mechanism of the optimized match between the top and bottom subcells in the tandem configurations.Our designed 2D-semiconductor-based PVs can be expected to bring a new perspective for future commercialized high-efficiency energy device.（3）Theoretical study on photocatalytic water splitting of 2D Ga P/Ga As van der Waals heterojunctions.First,the electrical and optical properties of 2D XP/XAs（X=Ga,In）van der Waals heterojunctions and their corresponding monolayers are introduced based on DFT calculations.Multi-junctions have higher carrier mobility and better optical properties.In particular,the Ga P/Ga As van der Waals heterojunction with direct bandgap exhibits more noticeable optical absorption than the single-layer counterparts.Second,intrinsic electric field can be obtained in Ga P/Ga As vd W heterostructure and the excited electrons and holes of Ga P/Ga As vd W heterostructure are present in opposite layers,demonstrating that the Ga P/Ga As vd W heterostructure can spontaneously generate the electron-hole separators.Third,the thermodynamic stability of Ga P/Ga As vd W heterostructure has been confirmed,which shows high potential for experimental implementation.Finally,due to the superior characteristics,Ga P/Ga As vd W heterostructure has been constructed for the photocatalytic water-splitting.Importantly,the photoexcited electrons of Ga P/Ga As vd W heterostructure can spontaneously induce hydrogen half reaction without sacrificial reagents.Moreover,the driving force in oxidation half reaction of Ga P/Ga As vd W heterostructure can be greatly boosted under light illumination.Our constructed 2D-vd W-semiconductors-based devices provide a promising strategy to achieve high efficiency water-splitting photocatalyst.