First-principles Study Of Photogalvanic Effect Photocurrent In Two-dimensional Binary Compounds

Developing and utilizing solar energy is an important way to solve environmental pollution and meet the ever growing global energy demands.To this end,the research and design of solar photovoltaic cells have attracted extensive attention in recent years.The photovoltaic cell based on semiconductor heterojunction is the earliest developed and the most mature photovoltaic conversion technology.Nonetheless,its photoelectric mechanism restrains its conversion efficiency hence cannot exceed the Shockley-Queisser limit.However,recently the photocurrent generated due to asymmetric lattice potential in homogenous non-centrosymmetric materials by second order susceptibilities called bulk photovoltaic effect（BPVE）shift current provides a mechanism in which Shockley-Queisser limitation is feasibly overcome.Nevertheless,limited by the inherent second order process,the photocurrents induced by this mechanism in closed circuits remains comparatively much smaller than p-n junction currents limiting its practical applications to only scientific research.To realize measurable BPVE photocurrent,also known as linear photogalvanic effect（LPGE）,search for possible materials that may host large shift photocurrents is an ongoing process.In addition,various shift photocurrents enhancement schemes are under development stage.Motivated by the recent documented promising findings,herein the first principles calculations are employed to elucidate large shift photocurrents in emerging two-dimensional ferroelectrics（α-AsP,α-NP）.The study also explores applied external excitations in2D group-IV monochalcogenides（strain inα-Ge S,α-Sn S）,and group-V binary compound（electric fields inα-Sb P）,as-well as exotic atom arrangements in two-dimensional ferroelectric materials（vertically arranged van der Waals homostructures of h-BN,β-Ge S）to simulate and documents shift photocurrents with enhanced magnitudes averaging over 200%.For 2D group-V_A binary monolayer,giant and anisotropic linear photogalvanic effect induced photocurrent responses exhibiting several peaks locating in a wide range of photon energies are observed.The strong shift photocurrent response is a fascinating attribute of the in-plane susceptibilities of both monolayers.With a giant amplitude reaching-154μАV^-2 in magnitude in the ultraviolet range,single-layer2Dα-AsP is close to monolayer 2Dα-Ge S in performance but larger than monolayerα-Sn S.Likewise,with the large peak that reaches～87μАV^-2 in magnitude within the visible range,intrinsic monolayer 2Dα-NP performs much better than most well-known 3D materials that host shift currents.In addition,with the response of the 2D monolayers spanning over wide photon energy ranges,the two binary compounds are therefore possible candidate materials for strong BPVE over a wide range of frequencies.Another striking and important feature of the simulated innate photocurrents of the two compounds is the large anisotropic in-plane photocurrent which is attributed mainly to different valley pumping of the Brillouin zone with linear light.Importantly,the shift photocurrent can be largest in the UV visible range,indicating the potential of these materials for optoelectronic applications.With this outstanding attribute,the two binary compounds are promising candidates for nonlinear optical devices and improved BPVE performance of the two binary compounds is achievable.In two 2D monolayer compounds;α-Ge S andα-Sn S,an increase in the applied equibiaxial compressive strain reduces the bandgap,driving the intrinsic zero-biased photocurrent peaks to redshift while simultaneously promoting an increase in its magnitude.The marked enhancement of the shift photocurrent is a consequence of the rapidly evolution of the band characters to the highly delocalized p_z orbital states.Shift photocurrent carrying excited states are more likely to be constructed from a superposition of highly delocalized orbitals rather than heavily localized orbitals.In addition,the enhanced shift photocurrent arises from the reduction in the bond lengths i.e.,stronger bonds as the result of increasing compressive strain which increase the magnitudes of the momentum matrix elements.On the other hand,an increase in the applied equibiaxial tensile strain results to an enhancement as well as a blue-shift of the shift photocurrent spectra of the two monolayer compounds.Direction reversals are also observed in the photocurrent response.On application of compressive strains-4%to-12%,a SPC direction reversal in observed on the entire spectra but more pronounced in photon energies 1.7-2.2 e V.Such prominent changes in the shift photocurrent allow for an opportunity to engineer the direction of the photocurrent using the strain as a controllable knob.Based on our first-principles calculations,it is established that the vertically AB stacked 2D bilayer h-BN andβ-Ge S homostructures exhibit large innate shift photocurrents.With the peak magnitudes reaching～49.3μAV^-2 and～130μAV^-2,respectively,the two slide-bilayer homostructures out-performs most conventional bulk ferroelectrics.When the number of layers is increased from 2l to 5l,the peak magnitude of the out-of-plane shift photocurrent component reaches 142μAV^-2 at photon energy 7.40 e V from the initial～13μAV^-2 in h-BN homostructures.This constitutes a～10-fold increment.Besides,the prominent in-plane component reaches a～3-fold enhancement.The shift photocurrent magnitude of this homostructure is also influenced by the odd/even number stacking with odd layers hosting much larger photocurrent than even layers,i.e 2l<4l<3l<5l.However,the photocurrent direction is not influenced by the number of layers.With such robust and colossal shift photocurrent responses elucidated in the two homostructures,the present study provides further theoretical references for the experimental design strategy of novel optoelectronic devices based on van der Waals homostructures.However,with prominent peaks locating in higher photon energies more band tuning methods should be implemented in the homostructures of the two compounds to ensure a redshift of the bandgap hence shift photocurrent to ensure the largest section of the electromagnetic spectrum（visible light）is used.In single-layer 2Dα-Sb P,a strong shift photocurrent response with the prominent in-plane tensor component,xxy reaching 360μAV^-2at photon energy 1.05 e V which is connected to band edge transitions is observed.Likewise,the out-of-plane component,yzz rises to reach 525μAV^-2at 4.3 e V but is related to transitions involving deep bands.Clearly,despite the documented large in-plane polarization,the out-of-plane shift photocurrents tensor elements are larger than those due to the in-plane polarization.A comparison of the shift photocurrent response of single-layer 2Dα-Sb P with those of some previously reported bulk ferroelectrics and also 2D single-layers,our finding suggests that single-layer 2Dα-Sb P can yield much larger shift photocurrent at an ultimate thickness and is a promising 2D material for the bulk photovoltaic effect applications under both the visible light and ultraviolet range.The shift photocurrent response spectra due to in-plane polarization exhibit largely negative peaks at low frequency while the out-of-plane polarization induced response component have both the positive and negative peaks mainly appearing at higher frequencies.The most prominent response component of the out-of-plane excitation in material is～1.5 times larger than the largest non-zero in-plane response component.The investigations of the applied weak external electric fields predicted a negligible effect on the band structure characteristics of this 2D crystal and certainly the inherent shift photocurrent.However,strong applied electric field modifies the band structure of this compound.Accordingly,marked enhancement of both the in-plane and the out-of-plane response tensor components is observed.The component xxy is enhanced to reach 935μAV^-2 at slightly photon energy,i.e.,at 0.70 e V,whereas yzz rises to 1290μAV^-2at 3.5 e V.No shift photocurrent direction change is observed on account of applied external electric fields.It was recently observed that,unlike conventional drift current,shift photocurrent is neither affected by the presence of lattice defects,impurities,or temperature variations,and is also independent of charge mobilities.Therefore,crystal defects and mobility changes(～10⁴ cm²V^-1s^-1)that may arise from the exposure of single-layer 2Dα-Sb P crystal to an applied external electric field do not have an effect on the predicted shift photocurrent responses.The simulated shift photocurrent responses are observed to rise sharply near the indirect bandgap probably originating from band structure rather than localized defects within the crystal lattice due to applied external electric fields.Single-layer 2Dα-Sb P is thus an excellent potential material for strong shift photocurrent.Furthermore,applied external electric field significantly enhances the inherent shift photocurrent responses（more two-fold）of this binary material,thus a proposal that applied external electric field is a viable approach of enhancing the shift photocurrent responses of 2D ferroelectrics is made.Combining the currents results with the benefits of being environmentally friendly material makes single-layer 2Dα-Sb P an excellent candidate for the next-generation solar cells application.The present work therefore hopes to open a pathway to exploring for efficient photovoltaic devices through BPVE based on single-layer 2Dα-Sb P and other similar materialsThis study using first principles calculations reports large shift photocurrents in three emerging 2D ferroelectric monolayers（α-NP,α-AsP andα-Sb P）as-well as two bilayer homostructures（h-BN andβ-Ge S）.With the documented large shift photocurrent,the present work lays a firm foundation for experimental verification and application of bulk photovoltaic effects shift photocurrent,which holds great promise for future efficient photovoltaic devices.Secondly,in this work,the effect of external stimuli was quantified the enhancement of the shift photocurrents of emerging 2D ferroelectrics,viz strain in group-IV monochalcogenides;α-Ge S andα-Sn S and applied static electric field（in 2D binary compound single-layerα-Sb P）.Therefore,these schemes are proposed as viable approaches to realize large measurable bulk photovoltaic effects shift photocurrent,which is essential for attaining improved photovoltaic efficiencies.This study also documents and proposes vertical slide homostructure layering of 2D van der Waals materials as a rich scheme for not only inducing bulk photovoltaic effects shift photocurrent but also increasing its output magnitudes as well as controlling its flow direction.Prominent directional reversal of shift photocurrent spectra with increasing strains is been observed in this investigation.Thus,applied external strain is suggested as a photocurrent direction control knob.