Study On Charged Defects Of Two-Dimensional Materials With Black Phosphorus Structure

A new type of two-dimensional material,black phosphorus,has received extensive attention from domestic and foreign researchers due to its excellent mechanical,electronic and photoelectric properties since it was successfully prepared in 2014.Black phosphorus is a direct band gap semiconductor whose band gap varies with the number of layers,and whose carrier mobility is quite high.It has wide application prospects in many fields such as novel photoelectric devices,spintronic devices,biomedicine and so on.In addition,two-dimensional MX(M=Sn,Ge;X=S,Se)with similar structure to black phosphorus also have potential development in some prospects.Defects play an important role in many aspects of twodimensional materials.In fact,many promising wide band gap semiconductors cannot be used in electronic applications because they cannot achieve the required free carrier density through doping.Therefore,it is of great significance to study the properties of two-dimensional material defects in order to improve the performance of semiconductor devices and design new highperformance devices.In this work,the density functional theory(DFT)calculations were performed using the Vienna Ab-initio Simulation Package(VASP)to investigate the properties of few-layer black phosphorus(BP)and BP-like materials.This paper focuses on the establishment of the evaluation method of charged defects in two-dimensional semiconductor materials,and the application of stacking-fault few-layer BP in thin-film solar cells.The main contents of this paper and the results obtained are as follows:(1)The basic reasons why conventional calculation methods of charged defects in threedimensional semiconductors fail in two-dimensional systems are explored,and an accurate as well as efficient evaluation method is proposed.In the evaluation mechanism,an analytic expression for the asymptotic behavior of charged defects is derived,IE(Lx,Ly,Lz)=IE0+?1/Lx+ ?2/Ly+(?/Lx·Ly)Lz,which is successfully applied to analyze the performances of single vacancy and substitutional defects modified by IVA and VIA elements in BP.In addition,the ionization energy of group IVA elements adsorbed on the surface of BP are also studied.According to the specific position of the defect level in the band,whether the impurity level is deep or shallow,it can be qualitatively judged,which provides a reliable reference for future research.The results show that the ionization energy of the substitutional black phosphorus modified by IVA/VIA group elements are large,as well as the corresponding donor or acceptor energy levels are deep.This indicates that the impurities in the substitutional defect black phosphorus are not easy to ionize at room temperature,and have no contribution to the carriers of semiconductor materials.However,they can act as the recombination center of electrons or holes,which affects the life of minority carriers.(2)In this paper,based on the evaluation mechanism of charged defects we proposed,the defects in four kinds of BP-like materials,MX(M=Sn,Ge;X=S,Se)were systematically studied.According to the specific position of the defect level in the impurity system,whether the impurity level is deep or shallow,it can be qualitatively judged.Our results show that below acceptor levels belong to the shallow level:In GeS,Tl substituted for Ge;In GeSe,Ga,In,Tl substituted for Ge and N,P,As,Sb,V and Nb substituted for Se;In SnS,Tl substituted for Sn;In SnSe,In substituted for Sn,N and Bi substituted for Se.(3)Finally,we study the stacking fault of few-layer black phosphorene,and simply analyze their geometry and band structures.It is found that the existence of 2L-A?-fault leads to the transition of black phosphorene from direct to indirect semiconductor.Then,these few-layer structures can form lateral junctions of type-II alignment as the material of thin-film solar battery.According to the band gap of the donor and the CBM offset between the donor and the acceptor,the power conversion efficiency(PCE)of the solar cell is also calculated.Some of the calculated results are as follows:the PCE of the combination of 2L-A? and 2L-AB is as high as 15.284%;the PCE of the combination of 2L-AB and 3L-A?A is as high as 11.388%;the PCE of the combination of 2L-A? and 3L-ABA is up to 10.255%.In addition,the thin-film solar cell could all be realized by a single material simply by manipulation of van der Waals(vdW)stacking without any chemical modification.All of these results indicate that few-layer phosphorene will have great potential in the application of thin-film solar cells in the future.