Structure Design And Performance Study Of Two-Dimensional Group Ⅳ-Ⅵ Compounds

With the development of industrialization,the rapid depletion of fossil fuels will give rise to the problems of energy crisis and environmental pollution.Therefore,the environmentally friendly renewable energy and energy storage system with low cost and high efficiency are urgently needed.Solar cells and metal-ion batteries are promising energy conversion and storage devices.The research of the materials that can be utilized as optical absorbers and electrodes is of great interest.Two-dimensional（2D）materials with single atomic layer thickness,large surface area,and short migration distance for photogenerated carriers and metal ions have great potential in the applications of solar cells and metal-ion batteries.Based on first-principles calculations,we theoretically propose three novel 2D group Ⅳ-Ⅵ compounds and calculate their performance as solar cell and electrode materials.The main contents are as follows:Firstly,we design a group Ⅳ oxide monolayer（α-CSiO）by assembly of the organic molecule CH₃OSi H₃.The stabilities are evaluated by calculating its cohesive energy,phonon spectrum,and elastic constants and performing ab initio molecular dynamics simulation.α-CSiO exhibits an unusual in-plane half-auxeticity.Meanwhile,α-CSiO possesses a direct band gap（1.71 e V）,high optical absorption coefficients(～10⁵cm^-1),and high carrier mobility(4.81×10³ cm²V^-1s^-1).Therefore,α-CSiO is expected to have great applications in many fields such as mechanics,electronics,and optics.Secondly,similar toα-CSiO,another carbon-based monolayerα-CGeO is designed by assembly of CH₃OGe H₃.α-CGeO is energetically,dynamically,thermally,and mechanically stable.α-CGeO has a direct band gap of 1.59 e V,which is very close to those of the solar cell materials.Therefore,α-CGeO has good absorption performance in the visible light range.Furthermore,α-CGeO has high electron mobilities and negative Poisson’s ratio.As a result,α-CGeO can be utilized in photovoltaic and mechanical devices.Thirdly,2D group Ⅳ sulfides can also display intriguing properties.SiGeS monolayer based on the skeleton of Si H₃OGe H₃ is more likely to be synthesized than other similar 2D materials.The stabilities of SiGeS are confirmed by evaluating the cohesive energy,phono spectrum,and ab initio molecular dynamics simulation.SiGeS is an indirect band gap（1.95 e V）semiconductor with unusual auxetic phenomenon and remarkable absorption in the region of visible light and ultraviolet.Under strain,SiGeS can be transformed into a direct band gap semiconductor with sizable band gap（～1.5e V）.And the absorption spectrum can cover the whole infrared region under strain.Finally,we evaluated the performance ofα-CSiO andα-CGeO as solar cell materials and anode materials for sodium-and potassium-ion batteries（NIBs and KIBs）due to their moderate band gaps,superior optical properties,and high carrier mobilities.As for solar cells,we propose a series of heterostructure excitonic solar cells based onα-CSiO,α-CGeO,black phosphorene,and transition metal dichalcogenides.The corresponding power conversion efficiencies are estimated to be in the range of 10.5-18.6%.On the other hand,α-CSiO andα-CGeO can serve as anode materials for NIBs and KIBs with high specific capacities,low diffusion barriers,and low open-circuit voltage.The highest specific capacity of K-ion onα-CSiO can be as high as 1433 m A h g^-1,which is higher than those of most other known 2D anode materials for KIBs.