| Due to the unique physical and chemical properties,two-dimensional materials have received extensive attentions and have great potential in energy storage and conversion,catalysis and photoelectric devices.Compared with bulk materials,two-dimensional materials have many advantages.Firstly,the band gap of two-dimensional materials can be adjustable with the number of layers and the the carrier mobilities get increasing.As the specific surface area of two-dimensional materials is much larger,therefore,there are more catalytic active sites on the surface.Secondly,with the reduction of dimension,the weakening of the dielectric screening effect in two-dimensional materials results in the enhancement of coulomb interaction between electrons,leading to the significant increase of exciton binding energy,which makes the absorption efficiency enhanced.Finally,the surfaces of the two-dimensional material are naturally passivated without any dangling bonds,which open up many possibilities for the construction of various vertical heterotructures.Among the two-dimensional materials,graphene has strong stability and high carrier mobility,but the property of semi-metallic limits its application in field effect transistors（FETs）.Transition metal disulfide compounds（TMDs）,such as Mo S2,have good stability and suitable direct band gaps in their monolayers,but the low carrier mobilities(200-500 cm2V-1s-1)seriously limit their applications in photovoltaic devices.The two-dimensional black phosphorus thin film has been prepared in the experiment in 2014 with the direct band gaps（0.3-2.0 e V）adjustable with the number of layers.The black phosphporene has high carrier mobility and is expected to be the ideal photoelectric material.However,the black phosphorene is unstable in the air and easily degraded by oxidation,thus damaging the physical and chemical properties,which restricts their applications in photoelectric field.Therefore,it is necessary to search for the two-dimensional photoelectric materials with appropriate direct band gap,high carrier mobility and strong stability.In recent years,the construction of two-dimensional van der Waals（vd W） heterostructure by stacking two or more two-dimensional（2D）materials together have been regarded as one of the most effective means to improve the performance of optoelectronic devices.These vd W heterostructures with more novel physics and unique properties have been widely used in field effect transistors（FETs）,photodetectors,light-emitting diodes and solar cells.More importantly,the vertical vd W heterojunction provides a larger active region that can absorb more photons and produce more excitons,which will increase the light absorption and photocurrent.In addition,the two-dimensional van der Waals heterostructures have very low density hanging bonds and charge traps in the interface,which can easily alleviate the problem of the interface composite center that can affect the photovoltaic efficiency.However,the efficiency of the two-dimensional monolayer based solar cells is much lower,compared with the efficiency of other photovoltaic materials based solar cells.Therefore,improving the photoelectric conversion efficiency（PCE）is still a challenge for the two-dimensional monolayer materials based solar cells.On the other hand,the PCEs of existing 2D van der Waals heterostructure solar cells are not so high,so both the 2D heterostructure solar cells themselves and their photoelectric conversion efficiency need to be expanded and improved.In view of the above problems,the two-dimensional group-VI chalcogenide compounds have been reported with good oxidation resistance and their monolayers are expected to peel off from bulk materials,so we choose them as one of the research object in two-dimensional photoelectric materials.Most recently,two-dimensional transition metal silicon nitrogen monolayer has been successfully prepared in the laboratory by chemical vapor deposition（CVD）method,which has strong stability,excellent mechanical properties and indirect band gap.While in the Mo Si2P4 monolayer,it possesses the direct band gap suitable for ideal photoelectric material,so we choose this two-dimensional transition metal silicon phosphorus compound single-layer material as photoelectric material,and construct the 2D vd W heterostructures to study the electronic and optical properties in photovoltaic applications.The specific research contents of this paper are as follows:1.Recently,theoretical studies have shown that ternary Li Al Te2 monolayer can be easily exfoliated from its bulk.However,the wide gap of this material can not meet the the needs of exciton solar cells.Therefore,we propose the 2D ternary ABY2（A=Na,K,Rb;B=In;Y=S,Se）monolayer family.The calculations of phonon spectra and molecular dynamics（MD）simulations show that the 2D ABY2（A=Na,K,Rb;B=In;Y=S,Se）monolayers have good thermodynamic and kinetic stability.The direct band gaps of 2D Na In Se2,KIn S2,KIn Se2 and Rb In S2 monolayers are exactly in the range of candidate materials for exciton solar cells（0.9-1.5 e V）in the calculations of band structures by HSE06 functional.In addition,the 2D ABY2 monolayers have high electron mobilities and strong optical absorption properties.What’s more,the Na In Se2/Ge S2,KIn S2/Ge S2,KIn Se2/Ge S2 and Rb In S2/Sn Se2 systems have type-II band alignments and the highest PCE value reaches to 22.77%.These results indicate that Na In Se2,KIn S2,KIn Se2 and Rb In S2 monolayers can be used as ideal two-dimensional optoelectronic materials and have promising prospects in the field of solar cells.2.Most recently,septuple-atomic-layer 2D Mo Si2N4 has been successfully fabricated experimentally via chemical vapor deposition（CVD）,which exhibits indirect-band-gap semiconducting behavior（～1.94 e V）,high strength and excellent ambient stability.Based on this,we have systematically studied the 2D Mo Si2Z4（Z=N,P,As）family.The calculated phonon spectrum and molecular dynamics simulation have proved the kinetic stability and thermodynamic stability of Mo Si2Z4（Z=N,P,As）monolayer at room temperature,respectively.The exfoliated energy show that it is possible to exfoliate the Mo Si2Z4（Z=P,As）monolayers experimentally from bulk materials in future.High-accuracy G0W0 method has been performed to calculate their electronic band structures and the results show that the Mo Si2Z4 monolayers are direct band gap semiconductors.The Mo Si2P4 monolayer has an appropriate direct band gap of 1.24 e V,which is in the favorable range of semiconductor solar absorption（0.9-1.5 e V）,and can be a good candidate for the material of solar cells.Furtherly,by combining the excellent characteristics of the 2D hexagonal Boron Phosphide（BP）and Mo Si2P4 monolayers based on first-principles calculations,we have designed a stable 2D vd W BP/Mo Si2P4 heterostructure.It is revealed that the BP/Mo Si2P4 heterostructure has an appropriate direct bandgap and type-II band alignment,which promotes the effective separation of photo-induced free carriers.Both the hole mobility and optical absorption intension have been improved dramatically.The PCE of BP/Mo Si2P4heterostructure can reach up to 22.2%,which is the largest one compared with other existing 2D heterostructures above-mentioned.Such a high PCE makes BP/Mo Si2P4 heterostructure a promising candidate for 2D high-efficiency solar cells further.3.Based on first-principles calculations,we combine the excellent properties of Graphene and Mo Si2As4 monolayers and design a stable 2D Graphene/Mo Si2As4 vd W heterostructure.Our results show that the Graphene/Mo Si2As4 heterostructure forms an n-type Schottky contact with a low barrier of 0.12 e V.Compared with graphene monolayer,the optical absorption intensity in the visible region is get stronger.Interestingly,the small effective masses and high carrier mobility(beyond 105 cm2 V-1 s-1)demonstrate that the Graphene/Mo Si2As4heterostructure can be suitable for field-effect transistors.The n-type Schottky contact can be transformed into a p-type one under the electric field of 0.2 V/（?）.Moreover,when the BN is inserted into the Graphene/BN/Mo Si2As4 heterostructure,the opening band gap of graphene is larger than 26 me V,which signifies that this heterostructure may be detected experimentally under room temperatures.The type-II band alignment can be realized in the Graphene/BN/Mo Si2As4heterotrilayers under the electric field of-0.20 V/（?）.These findings demonstrate that the Graphene/Mo Si2As4 heterostructure can be considered as promising candidate for high-efficiency Schottky nanodevices.Our theoretical study provides useful guide for the preparation of high quality 2D function material.We have proposed the high-performance 2D photoelectric semiconductor candidate materials which can been synthesized in experiments in this work.The 2D photoelectric materials will have potential applications in the solar cells and FETs. |
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