Tuning The Electronic Properties Of All-inorganic Perovskite-based Heterostructures By First-principles Investigations

Photovoltaic cells based on perovskite materials have become one of the most promising solar cells due to their high light absorption coefficient,excellent transport properties of photo-generated carriers,simple preparation process and low preparation cost.Since the first reported photoelectric conversion efficiency of 3.8%for the light absorption layer of solar cells in 2009,the related performance research of perovskite materials has been developed rapidly.Until now,solar cells based on metal halide perovskite materials have achieved a photoelectric conversion efficiency of more than 25%.With the continuous development of material performance requirements for solar cell devices based on perovskite materials and the increasing development of experimental preparation technology,in addition to a wide variety of isolated perovskite materials,heterostructure engineering based on perovskite materials has attracted extensive attention of researchers.As one of the most effective methods to modulate and improve the photoelectric properties of semiconductor materials,constructing heterostructures with unique properties based on the interface coupling between different materials can not only integrate the inherent photoelectric properties of different components under a reasonable lattice mismatch,but also realize different types of energy level alignment in electronic band structure by means of interface interaction.The heterostructure with different band alignment including type-Ⅰ,type-Ⅱ and type-Ⅲ can be generated to promote the efficient transport of photo-generated carriers,making heterostructures with different band alignment be applied in different fields such as light-emitting devices,photovoltaic devices and tunneling field effect devices.Therefore,heterostructure engineering has been regarded as an important way to improve the efficiency of perovskite-based devices in recent years.However,a shaped heterostructure usually exhibits an intrinsic type of electronic energy band alignment,resulting in its relatively single application function.In order to realize the dynamic modulation of electronic properties,especially the band alignment in the heterostructure,to provide more options for the designing of devices with multi-functional applications,the introduction of external or internal modulation strategies is critical.In this thesis,based on density functional theory,the geometric and photoelectric properties of heterostructures consist of different all-inorganic perovskites are systematically investigated by first-principle calculations,the specific content including structural stability,electronic band structure,carrier effective masses,charge density,light absorption coefficient and so on.The effects of different modulation strategies including element substitution,external strains,external electric fields and ferroelectric polarization modulation on the electronic properties of different all-inorganic perovskite-based heterostructure are studied.The findings are listed as follows:1.In the heterostructure system consist of layered two-dimensional material Pd₂Se₃ and three-dimensional all-inorganic perovskite CsSnBr₃ ,the effects of Ni atom replacing Pd atom on the photoelectric properties of Pd₂Se₃ /CsSnBr₃ heterostructure are investigated.The calculated results show that the band alignment of Pd₂Se₃ /CsSnBr₃ heterostructure can switch from type-Ⅰ to type-Ⅱ when the Pd atom of Pd₂Se₃ monolayer is partially replaced by Ni atom.At the same time,the change of band structure caused by the introduction of Ni atom can reduce the average hole effective masses of Pd₂Se₃ /CsSnBr₃ heterostructure greatly.These two results will be beneficial to the efficient separation of photo-generated carriers in the heterostructure,so the related devices can be better applied in the photovoltaic field.In addition,compared with isolated Pd₂Se₃ monolayer and isolated CsSnBr₃ ,the Pd₂Se₃ /CsSnBr₃ heterostructure exhibits higher light absorption coefficient in light absorption spectrum,representing a stronger light absorption performance.As an external impurity atom,it is noted that the introduction of Ni atom does not weaken the excellent optical absorption performance of Pd₂Se₃ /CsSnBr₃ heterostructure,the doped Pd_1.5Ni_0.5Se₃ /CsSnBr₃ heterostructure and PdNiSe₃ /CsSnBr₃ heterostructure almost exhibit the same optical absorption curve as the original heterostructure.2.In the heterostructure system consist of two-dimensional Ruddlesden-Popper（RP）perovskite Cs₂PbI₂Cl₂ and three-dimensional all-inorganic perovskite Tetra-CsPbBr₃ （Tetra represents the tetragonal phase）,the effects of spontaneous polarization of Tetra-CsPbBr₃ on the electronic band structure of Cs₂PbI₂Cl₂ /CsPbBr₃ heterostructure are investigated.The specific methods include two parts.Firstly,the positive increase in the polarization displacement between anions and cations is simulated by artificially moving the atomic coordinates of cations(Cs⁺and Pb²⁺)in Tetra-CsPbBr₃ along the positive polarization direction,which indicates the enhanced spontaneous polarization of material.The calculated results show that the band gap of Cs₂PbI₂Cl₂ /CsPbBr₃ heterostructure decreases gradually with the increase of spontaneous polarization intensity in Tetra-CsPbBr₃ ,and decreases to zero finally.At the same time,the band alignment of heterostructure switches from initial type-Ⅱ to type-Ⅲ without bandgap.In order to verify the above conclusions,the external strains modulation are applied to find the conditions for enhancing the polarization intensity of Tetra-CsPbBr₃.The results show that the positive enhancement of polarization displacement in Tetra-CsPbBr₃ can be observed as the≥5%tensile strains are applied.When the RP perovskite Cs₂PbI₂Cl₂ is combined with Tetra-CsPbBr₃ under the corresponding tensile strains,the bandgaps of new-constructed heterostructures indeed exhibit a deceasing tendency and eventually close down with the increasing polarization,and the band alignment transition between type-Ⅱ and type-Ⅲ is also observed,which are in agreement with the statement about artificial movement.3.In the heterostructure consist of two-dimensional ferroelectricα-In₂Se₃ monolayer and two-dimensional Ruddlesden-Popper（RP）perovskite Cs₂SnI₂Cl₂ monolayer,the effects of ferroelectric polarization switching（out-of-plane ferroelectric polarization）inα-In₂Se₃ on the electronic band structure ofα-In₂Se₃ /Cs₂SnI₂Cl₂ heterostructure are investigated.The calculated results show that in the heterostructure withα-In₂Se₃ on the upper layer and Cs₂SnI₂Cl₂ on the lower layer,the band alignment of FE1 phase（FE represents the abbreviation of ferroelectric）α-In₂Se₃ /Cs₂SnI₂Cl₂ heterostructure belongs to type-Ⅱ when the direction of out-of-plane ferroelectric polarization inα-In₂Se₃ is downward.However,the band alignment of FE2 phaseα-In₂Se₃ /Cs₂SnI₂Cl₂ heterostructure switches from type-Ⅱ to type-Ⅲ without bandgap when the direction of out-of-plane ferroelectric polarization inα-In₂Se₃ is reversed to upward.In addition,in the FE1 phase and FE2 phaseα-In₂Se₃ /Cs₂SnI₂Cl₂ heterostructures,we find that the external electric field can effectively modulate the electronic band structures of heterostructures,which induces the rich transition behaviour of band alignments.4.In the heterostructure system constructed via Platinum-based transition metal dichalcogenides（TMDs）PtSe₂ and two-dimensional Ruddlesden-Popper（RP）perovskite Cs₂PbI₄ ,we investigate the effect of thickness modulation in PtSe₂ layer on the electronic band structures of PtSe₂/Cs₂PbI₄ heterostructure.The calculated results show that the bandgap of PtSe₂/Cs₂PbI₄ heterostructure decreases obviously,and the band alignment type of heterostructure can be transferred from type Ⅱ to type-Ⅰ as the thickness of PtSe₂ in the heterostructure increases from monolayer（1L,L represents layer）to bilayer（2L）.While the band structure of PtSe₂/Cs₂PbI₄ heterostructure based on trilayer（3L）PtSe₂ exhibits the metallic characteristic with a p-type Schottky barrier of 0.65 e V.In addition,we study the effects of strain engineering on the electronic band structures of PtSe₂/Cs₂PbI₄ heterostructures with different thicknesses in PtSe₂and analyze the variation of electronic properties with external strains.It is found that the external strains can effectively tune the electronic properties such as the bandgap,hole effective mass of 1L-PtSe₂/Cs₂PbI₄ heterostructure and the Schottky barrier height of 3L-PtSe₂/Cs₂PbI₄ heterostructure.However,the electronic properties of 2L-PtSe₂/Cs₂PbI₄ heterostructure are less affected by external strains.