CsSnI3 Perovskite Heterostructure

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Ferroelectric Polarization in CsPbI/CsSnI Perovksite Heterostructure Biao Liu, Mengqiu Long, Meng-Qiu Cai, Xiao-Tao Hao, and Junliang Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04467 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

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The Journal of Physical Chemistry

Ferroelectric Polarization in CsPbI3/CsSnI3 Perovksite Heterostructure Biao Liu 1, Mengqiu Long 1, Meng-Qiu Cai 2, Xiaotao Hao 3, Junliang Yang 1 *

1. Hunan Key Laboratory for Super-microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha 410083, Hunan, China 2. School of Physics and Electronics Science, Hunan University, Changsha 410082, Hunan, China; Synergetic Innovation Center for Quantum Effects and Applications (SICQEA), Hunan Normal University, Changsha 410081, China 3. School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan, China ∗

Corresponding author. E-mail address: [email protected] (J.L.Y)

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ABSTRACT Ferroelectricity is believed to be an important origin for the excellent performance of halide perovskite optoelectronic devices, and building halide perovskite heterostructure is an excellent strategy to enhance performance of interfacial ferroelectric polarization. Herein, all-inorganic perovksite CsPbI3/CsSnI3 heterostructure are constructed for disclosing the interfacial electrical contacts and the ferroelectricity via the first-principles calculations. In CsPbI3/CsSnI3 heterostructure, there are four kinds of electrical contacts, i.e., PbI2-SnI2, CsI-CsI, PbI2-CsI and CsI-SnI2 interfaces. Large cation-anion displacements along z direction are observed for all interface compositions, which indicate a strong ferroelectric field effect in the CsPbI3/CsSnI3 heterostructure. The net polarization displacements of PbI2-SnI2 and CsI-CsI interfaces are smaller than the values of PbI2-CsI and CsI-SnI2 interfaces. The interfacial ferroelectricity drives electron extraction from the perovskite and hinders electron-hole recombination by keeping the electrons and holes separated. The intrinsically interfacial ferroelectric polarization results from the difference of work functions of diverse interfaces and the interface charge transfer. This work suggests that such all-inorganic perovskite heterostructure has significant potential for future optoelectronic applications.

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INTRODUCTION The term perovskite is a ternary family of crystalline materials which have a general chemical formula ABX3. The A and B sites can accommodate inorganic cations of various valence and ionic radius, whereas X site is usually an anion of oxygen or halogen. In the past decades, the most studied perovskite materials are oxide perovskites such as BaTiO3, LaAlO3 and BiFeO3.1-3 They have excellent electrical properties including ferroelectricity, piezoelectricity, pyroelectricity, ferromagnetism and superconductivity.4 Thus, they are promising candidates for nanoscale microelectronic devices, such as sensors, transducers, ultrathin ferroelectric capacitors, and ultrahigh density nonvolatile random access memories.5 In recent years, organic-inorganic hybrid halide perovskites have drawn tremendous attention due to their unprecedented optoelectronic properties, such as high charge carrier mobility, large absorption coefficient, tunable band gap, and long electron-hole diffusion.6-11 The halide perovskites have been widely applied for many optoelectronic

devices,

such

as

solar

cells,

photodetectors

and

light-emitting-diodes.12-14 Recent theoretical and experimental studies shows that ferroelectric polarization can significantly affect the photovoltaic performance of hybrid halide perovskites systems.15-17 For example, Park et al. reported the ferroelectric polarization behavior in CH3NH3PbI3 perovskite in the dark and under illumination by piezoresponse force microscopy measurements.15 The results confirmed the formation of spontaneous polarization in CH3NH3PbI3 in the absence of electric field and the retention of ferroelectric polarization was also observed after removal of the electric field. Daehee Seol et al. demonstrated the screening effect of polarization states on charge redistribution related to the photovoltaic performance of ferroelectric CH3NH3PbI3 thin films using atomic force microscopy.16 The interplay between polarization and injected charges has significant effects on charge transfer, that would greatly influence photovoltaic performance. Normally, the polarizations in ultrathin ferroelectric films are usually reduced 3



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dramatically due to the depolarization field caused by incomplete interfacial charge screening,

hampering

the

utility

ferroelectrics.18,19

of

Building

perovskite

heterostructure is an effective strategy to enhance ferroelectric polarization at the interfaces.20-22 The method of heterostructure has been extensively studied theoretically and experimentally, which can not only integrate the intrinsic electronic properties of the isolated components, but also create particularly advantageous electronic properties.23-25 Ma et al. design and fabricate a ferroelectric/multiferroic PbTiO3/BiFeO3 system and the PbTiO3 layer near the head-to-tail polarized interface suggest an over ∼70% enhancement of polarization compared with that of bulk PbTiO3 and the polarization in the BiFeO3 layer is also remarkably enhanced.18 Although the ferroelectric polarization of oxide perovskites and their heterostructures are widely studied, the ferroelectricity studies of inorganic halide perovskites and their heterostructures are in their infancy. Interface engineering of all-inorganic halide heterostructure is fascinating. The charge generation, collection, and transport layers are stacked together, producing several interfaces and interfacial regions.26-29 The carrier must transfer across the interfaces to be collected. Thus, for a high efficiency device, aside from having appropriate materials, device structures, and a good film quality, interfaces are the key. Herein, all-inorganic halide CsPbI3/CsSnI3 heterostructure are constructed for disclosing the interfacial electrical contacts and the ferroelectricity via the first-principles calculations, in which there are four kinds of typical electrical contacts, including PbI2-SnI2, CsI-CsI, PbI2-CsI and CsI-SnI2 interfaces. Obvious ferroelectric polarization behaviors are found in CsPbI3/CsSnI3 heterostructure and the ferroelectric polarization is closely linked with the contact characteristics including physical contact and chemical contact. The intrinsical ferroelectric polarization, resulted from the difference of work functions of diverse interfaces and the interface charge transfer, exhibits significant potential for future electronic and optoelectronic applications.

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COMPUTATIONAL METHOD The present calculations have been performed with the vienna ab initio simulation package

(VASP)

code,

based

on

density

functional

theory

(DFT).30,31

Projector-augmented wave method is used to describe the interaction between ion cores and valence electrons. Atomic structures are optimized using the exchange-correlation functional of Perdew, Burke, and Ernzerhof (PBE) combined with the DFT-D3 method of Grimme for describing the Van der Wells (vdW) interaction.32 A plane-wave cutoff of 450 eV is employed in the calculation. Monkhorst-Pack 5×5×1 k-point grid is used to sample the Brillouin zone. All atoms are allowed to be fully relaxed till the atomic Hellmann-Feynman forces are less than 0.01 eV/Å. The convergence criterion of energy in the self-consistency process is adjusted about 1×10-4 eV.

RESULTS AND DISCUSSION The calculated lattice parameters of the cubic CsPbI3 and CsSnI3 slab are a=b=6.269Å and a=b=6.388 Å, respectively, which agrees well with experimental or theoretical results. The CsSnI3 is selected as substrate to match with CsPbI3 slabs. The unit cell of heterostructure system is composed by 1×1 unit cell of CsPbI3 slab and 1×1 unit cell of CsSnI3 along the x and y directions, which can nicely match each other. The lattice mismatch along the x and y directions is ~1.86% for the composed configuration. The CsPbI3 and CsSnI3 nanoplates have 4 layers octahedron, which show similar performance with the bulk.33 The CsPbI3 and CsSnI3 nanoplates have two kinds of interfaces including Pb/Sn-I interface and Cs-I interface, respectively. Therefore, four types of heterostructures are constructed, including PbI2-SnI2, CsI-CsI, PbI2-CsI and CsI-SnI2 interface, respectively, in CsPbI3/CsSnI3 heterostructure. A vacuum region of 15Å in the z direction is used to avoid the interactions between neighboring slabs. The equilibrium geometry of the four kinds of heterostructures are depicted in Figure 1. The relaxed vertical interlayer distances of d1-d4 for the PbI2-SnI2, CsI-CsI, PbI2-CsI and CsI-SnI2 interface equilibrium configuration are 4.13 5



Å, 4.02 Å, 3.16 Å and 3.08 Å, respectively, in the CsPbI3/CsSnI3 heterostructure. The average bond length in the CsPbI3 and CsSnI3 is about 3.15 Å. Therefore, the results show that the PbI2-SnI2 and CsI-CsI interface heterostructure are physical contact (vdW contact) and the PbI2-CsI and CsI-SnI2 interface heterostructure are chemical contact.

Figure 1 Side view of (a) PbI2-SnI2, (b) CsI-CsI, (c) PbI2-CsI and (d) CsI-SnI2 interface heterostructure, respectively. Top and bottom half represent CsPbI3 and CsSnI3 slab, respectively.

The ferroelectric polarization of the CsPbI3/CsSnI3 heterostructure is characterized by uniform displacements of the cations (Pb, Sn and Cs) with respect to the I anions. We introduce the parameter δz as the average cation-anion displacements in a given layer along z direction, i.e., δz=z(Pb/Sn, Cs)-z(I). The calculated δz for each atomic layer of the heterostructure for the PbI2-SnI2, CsI-CsI, PbI2-CsI and CsI-SnI2 interface are showed in Figure 2. Due to the different contact properties of the PbI2-SnI2, 6


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CsI-CsI and PbI2-CsI, CsI-SnI2 interface, the polarization is diverse in the CsPbI3/CsSnI3 heterostructure. For the physical contact of the PbI2-SnI2 and CsI-CsI interface heterostructure, the CsPbI3 and CsSnI3 slabs keep their characteristics of polarization, shown in Figure 2. The surface polarization is greater than the internal polarization. The direction of the polarization for the upper and lower surfaces is

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Figure 2 Layer-by-layer polarization profile as measured by cation-anion vertical displacement (δz) of (a) Pb/Sn-I and (b) Cs-I for PbI2-SnI2, CsI-CsI, PbI2-CsI and CsI-SnI2 interface heterostructure, respectively.

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opposite, therefore, the whole polarization approaches zero. For chemical contact of PbI2-CsI and CsI-SnI2 interface heterostructure, the interfacial Pb, Sn and I atoms are bonding and the interlayer interaction is strong. The dipole is formed between the slabs, that induces a depolarizing field in the heterostructure. The depolarizing field causes the partial polarization to change quickly and the total polarization is no longer as zero. The Pb/Sn-I polarization displacement in the PbI and SnI interface slabs decay quickly, but, it is changed little for the CsI interface slab. The reason is that the CsI interface screen the influence of interface dipole. Therefore, the total polarization direction of PbI2-CsI interface heterostructure is upward and it of CsI-SnI2 interface heterostructure is downward. Moreover, many materials that exhibit a spontaneous electrical polarization are not ferroelectric. We calculate total energy of CsPbI3 and CsSnI3 bulk as a function of Pb/Sn displacement relative to I along z direction, showed in Figure S1. The potential energy surface exhibits a double well profile.34, 35 Therefore, the material is a ferroelectric. The calculated layer-by-layer projected density of states (P-DOS) on I-p, Pb-p and Sn-s orbitals of PbI2-SnI2, CsI-CsI, PbI2-CsI and CsI-SnI2 interface heterostructures are presented in Figure 3, respectively. The layer-by-layer translational displacements of orbitals can reflect the total polarization of the heterostructure. It is shown that the I-p, Pb-p and Sn-s orbitals almost keep the same energy values in the PbI2-SnI2 and CsI-CsI heterostructure. The results show that the total polarization approaches zero in the heterostructure. For the PbI2-CsI and CsI-SnI2 interface heterostructure, the I-p, Pb-p and Sn-s orbitals regularly move from the top to the bottom layer. This is in accordance with their total polarization direction. A conductive state does emerge at the top and the bottom interface. The electron conductive state is located at the bottom layer and the hole conductive state is located at the top layer in the PbI2-CsI interface heterostructure. On the contrary, in the CsI-SnI2 interface heterostructure, the electron conductive state is located at the top layer and the hole conductive state is located at the bottom layer. The results suggest the ferroelectric polarization discontinuity drives electron extraction from the perovskite and hinders electron-hole recombination by keeping the electrons and holes separated. 9



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Figure 3 Layer-by-layer projected density of states (P-DOS) of (a) PbI2-SnI2, (b) CsI-CsI, (c) PbI2-CsI and (d) CsI-SnI2 interface heterostructure, respectively. The green, red and blue shaded curves represent the P-DOS of I-p, Pb-p and Sn-s orbitals. Red dashed lines correspond to the Fermi level, and black lines indicate polarization in the CsPbI3/CsSnI3 heterostructure.

To illustrate the detailed nature of charge transferring at the PbI2-SnI2, CsI-CsI, PbI2-CsI

and

CsI-SnI2

interface,

the

plane-averaged

10


charge

density

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difference ∆ρ(z ) along the perpendicular direction to the interface is plotted, as shown in Figure 4, which can quantitatively study the gain or loss of valence electrons across the interfaces.The plane-averaged charge density difference can be written as

∆ρ( z ) = ρtot − ρCsSnI3 − ρCsPbI3 . Here, ρtot is the plane-averaged density of the combined heterostructure system. ρCsSnI 3 and ρCsPbI 3 are the plane-averaged densities of the free standing CsSnI3 and CsPbI3 slabs, respectively, which are calculated by freezing the atomic positions of the respective components in the combined system. z is the direction normal to the interface. The entire system is neutral. When z is sufficiently far from the heterostructure interface, ∆ρ (z ) closes to zero. It is showed that the charge transfer in PbI2-SnI2 and CsI-CsI interface heterostructure is obviously weaker than PbI2-CsI and CsI-SnI2 interface heterostructure. The electrons are transferred from CsSnI3 slab to CsPbI3 slab in the PbI2-CsI interface, showed in Figure 3(c). The spatial electronic density is redistribution in the interface and an interface dipole layer is formed. As a result, an internal electric field and a depolarizing field are formed. The direction of the internal electric field is the direction of electronic transmission. The direction of ferroelectric polarization is in accordance with the direction of the internal electric field and contrary to the depolarizing field. Contrary to PbI2-CsI interface heterostructure, the electrons are transferred from CsPbI3 slab to CsSnI3 slab in the CsI-SnI2 interface. The direction of internal electric field is from CsPbI3 slab to CsSnI3 slab, which is in accordance with the direction of ferroelectric polarization. Those results validate part discussions in Figure 2. In addition, there exists electrons accumulation and exhaustion layers in top and bottom of the heterostructure in Figure 3(c) and 3(c), this illustrate that the ferroelectric polarization is beneficial to electrons and holes separation and transmission in the CsPbI3/CsSnI3 device.

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Figure 4 The planar-averaged differential charge density ∆ρ(z) of ((a) PbI2-SnI2, (b) CsI-CsI, (c) PbI2-CsI and (d) CsI-SnI2 interface heterostructure along the z direction, respectively. The red and blue colors indicate electron accumulation and depletion, respectively. Gray and yellow dashed area represent the CsSnI3 and CsPbI3 slab, respectively. The vertical green dashed lines are the positions of atomic layers.

The work function represents the ability to bind electrons. A practical way of obtaining work functions from DFT calculations is to track the plane-averaged electrostatic potential into the vacuum, where typically the asymptotic value is reached with a few Å from the surface, shown in Figure S2. ∆V is the work function difference between the asymptotic values of the CsPbI3 slab and CsSnI3 slab via the equation: ∆V = WCsPbI 3 − WCsSnI 3 , where WCsPbI 3 and WCsSnI 3 is the work function of CsPbI3 slab and CsSnI3 slab. The calculated ∆V of the PbI2-SnI2, CsI-CsI, PbI2-CsI and CsI-SnI2 interface heterostructure are -0.9 eV, -0.1 eV, 2.6 eV and -3.3 eV, respectively. The ∆V of PbI2-SnI2 and CsI-CsI interface heterostructure is so small that lead to weak electronic transmission ability, which in turn reduces the total 12


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ferroelectric polarization in the heterostructure. The ∆V of PbI2-CsI and CsI-SnI2 interface heterostructure is obviously greater than PbI2-SnI2 and CsI-CsI interface heterostructure. In the PbI2-CsI interface heterostructure, the CsPbI3 slab has a larger work function than CsSnI3 and thus attracts electrons from the CsSnI3 slab, which in turn enhances the ferroelectric polarization toward the top layer. On the contrary, in the CsI-SnI2 interface heterostructure, the CsSnI3 slab enhances the ferroelectric polarization toward the bottom layer. Therefore, the work function difference is one of the main nature of the diversiform ferroelectric polarization.

CONCLUSIONS In summary, the structural and electrical properties of the CsPbI3/CsSnI3 heterostructures are investigated by first-principles calculations for the first time. Various interfaces contacting have a completely different nature in the heterostructures. The PbI2-SnI2 and CsI-CsI interface heterostructure show physical contact and the PbI2-CsI and CsI-SnI2 interface heterostructure show chemical contact. Owing to the vdW contact of PbI2-SnI2 and CsI-CsI interface heterostructure, the CsPbI3 and CsSnI3 slabs keep their free standing properties in the heterostructure and their total ferroelectric polarization approaches zero. Due to the interface charge reconstruction, the PbI2-CsI interface heterostructure exhibit a ferroelectric polarization along z direction, on the contrary, the CsI-SnI2 interface heterostructure exhibit a ferroelectric polarization opposite z direction. The ferroelectric polarization is beneficial to the electrons and holes separation and transmission. The interface charge transfer and difference of work function are the nature of the induced ferroelectric polarization in the CsPbI3/CsSnI3 heterostructures. This work suggests that such all-inorganic perovskite heterostructures have significant potential for future optoelectronic and photovoltaic applications.

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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XX

Acknowledgments This work was supported by the National Natural Science Foundation of China (51673214), the National Key Research and Development Program of China (2017YFA0206600), the China Postdoctoral Science Foundation (2017M622599), and the Key Projects of Hunan Provincial Science and Technology Plan (2017GK2231).

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CsSnI3 Perovskite Heterostructure

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