Fe3O4 interfaces

Accepted Manuscript Prediction on electronic structure of CH3NH3PbI3/Fe3O4 interfaces Xueyao Hou, Xiaocha Wang, Wenbo Mi, Zunfeng Du PII:

S0038-1098(17)30353-8

DOI:

10.1016/j.ssc.2017.10.019

Reference:

SSC 13311

To appear in:

Solid State Communications

Received Date: 10 July 2017 Revised Date:

12 October 2017

Accepted Date: 25 October 2017

Please cite this article as: X. Hou, X. Wang, W. Mi, Z. Du, Prediction on electronic structure of CH3NH3PbI3/Fe3O4 interfaces, Solid State Communications (2017), doi: 10.1016/j.ssc.2017.10.019. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Prediction on Electronic Structure of CH3NH3PbI3/Fe3O4 Interfaces

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Xueyao Hou,a Xiaocha Wangb, Wenbo Mi,a,* and Zunfeng Duc,*

Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparation Technology, School

School of Electrical and Electronic Engineering, Tianjin University of Technology, Tianjin 300384,

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b

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of Science, Tianjin University, Tianjin 300354, China

China

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State Key Laboratory of Hydraulic Engineering Simulation and Safety, School of Civil Engineering,

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Tianjin University, Tianjin 300354, China

*

Author to whom all correspondence should be addressed. E-mail: [email protected] (Wenbo Mi) and [email protected] (Zunfeng Du)

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ABSTRACT

The interfacial electronic structures of CH3NH3PbI3(MAPbI3)/Fe3O4 heterostructures are predicted

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by density functional theory. Four models (MAI/FeBO, PbI2/FeBO, MAI/FeA and PbI2/FeA) are included. Especially, a half-metal to semiconductor transition of Fe3O4 appears in PbI2/FeA model.

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A series of electric field is added to PbI2/FeA model, and a direct-indirect bandgap transition of Fe3O4 appears at a 500-kV/cm field. The electric field can control the bandgap of Fe3O4 in PbI2/FeA

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model by modulating the hybridization. The prediction of spin-related bandgap characteristic in MAPbI3/Fe3O4 is meaningful for further study.

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Keywords: CH3NH3PbI3; Fe3O4; Electronic structure; Bandgap; Electric field effects

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1. Introduction

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Perovskite solar cells (PSCs) have attracted much attention as an ideal candidate of dye-sensitized solar cells (DSSCs) [1,2]. Compared with DSSCs, PSCs has a high absorption coefficient even in a thinner film [3], which meets the commercial requirements of solar cells.

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CH3NH3PbI3 (MAPbI3) is one of the most attractive integrants of PSCs family [4]. MAPbI3 has a

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direct bandgap, a large absorption coefficient and a long diffusion length (reach to 100 nm) for both electrons and holes [3-5], as well as a high stability in dry air [6]. In MAPbI3, the electronic coupling appears between the inorganic sheets and organic components. The ionic and covalent interaction between the Pb2+ and I- creates the inorganic octahedral, while the MA+ head groups

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provide a charge balance to the structure [6]. MAPbI3 exhibits a complex structure with a successive phase transition between orthorhombic, tetragonal (T) and cubic (Oh symmetry) at 165

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and 327 K [7]. Recently, the power conversion efficiency (PCE) of PSCs, a characteristic value of organolead halide perovskites for photovoltaic applications, has increased from 3.8% to 20% [8-11].

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The high PCE of PSCs can be achieved by depositing MAPbI3 on various electrons (holes) transport materials [12-17], which implies that the diversity of electron-hole separation can be obtained by controlling the selective contacts. Although the MAPbI3/oxide heterostructures have received substantial attention, few strategies including the electric field (E-field), chemical doping [17] and functionalization [18] have been proposed and investigated. The intrinsic mechanism remains unclear at this stage, such as the structure, electronic and optical properties [15]. Many studies have focused on the planar configuration of MAPbI3/TiO2 heterostructures and

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ACCEPTED MANUSCRIPT described the uncommon MAPbI3/TiO2 interfaces (such as the MAI-terminated model) [6,13,14]. Magnetite Fe3O4 (FO) has an inverse spinel structure with two types of Fe ions (tetrahedral sites FeA and octahedral sites FeB). With a spin polarization of 100% at Fermi level (EF), FO has potential

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applications in magnetoelectric and spintronic devices [19]. At 125 K, FO undergoes a first-order transition, which is named as Verwey transition [20]. FO can be a hole-extraction layer in PSCs, for its low cost and antioxidant capability [21]. To the best of our knowledge, although a significant

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enhancement of efficiency and stability has been observed in FO-based PSCs [21], the theoretical

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study on MAPbI3/FO heterostructure is lacking. Thus, the prediction of spin-related bandgap characteristic in MAPbI3/Fe3O4 can provide opportunities for developing spintronics. The interfacial electronic structures of MAPbI3/FO heterostructures are predicted by density functional theory. Four models (MAI/FeBO, PbI2/FeBO, MAI/FeA and PbI2/FeA) are included.

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Especially, a half-metal to semiconductor transition of FO appears in PbI2/FeA model. A series of E-field is added to PbI2/FeA model, and a direct-indirect bandgap transition of FO appears at a

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500-kV/cm field. The E-field can control the bandgap of FO in PbI2/FeA model by modulating the hybridization. Our results improve the understanding on the interfacial electronic structure of

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MAPbI3/FO heterostructures and have potential application on spin-photodiode.

2. Calculation Details And Models

The first-principles calculations on all the models are implemented by Vienna Ab-initio Simulation Package code [22,23]. The projector augmented wave method and exchange correlation potential are adopted within Perdew-Burke-Ernzerhof functional (PBE) [24]. PBE functional is

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ACCEPTED MANUSCRIPT adequate to describe bandgap of MAPbI3 due to a cancellation of two effects [25]. On the one hand, Pb atom is a heavy element that incorporates the spin-orbit coupling (SOC), which typically enlarges the bandgap. On the other hand, PBE functional underestimates the bandgap due to its

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self-interaction error. The ignorance of SOC will cancel out the bandgap underestimation effect of DFT using GGA calculation, which can result in a good calculated bandgap. The next section will discuss this effect in more detail. In all the self-consistent calculations on electronic structure, the

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van der Waals correction DFT-D2 method is considered [26]. A plane-wave basis set with a kinetic energy cutoff of 520 eV is used. The valence states are H 1s1, C 2s2 2p2, N 2s2 2p3, O 2s2 2p4, Fe 3d7

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4s1, I 5s2 5p5 and Pb 5d10 6s2 6p2. The initial MFeA and MFeB are settled at 4.5 and -5.0 µ B, respectively.27 For FO with charge ordering, the t2g electron hops from one site to another. The change of the Coulomb energy in this process due to the intrasite electron-electron interaction U and intersite

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interaction V. Thus, the electron interactions between the localized Fe 3d orbitals are taken into account by the on-site Coulomb repulsion term [28] of U=4.5 eV and J=0.89 eV [29].

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The experimental lattice constant of cubic FO with a space group Fd3m is 8.396 Å, which is in well agreement with the reported value of 8.391 Å [30]. In our work, only T-MAPbI3 is considered

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due to its high stability under the ambient condition [4]. According to previous results, the lattice constants of T-MAPbI3 at a space group of I4/mcm are a=b=8.695 Å and c=12.830 Å [7]. The MA+ cations of T-MAPbI3 don’t have fixed positions as those of the orthorhombic phase, but several disordered molecule configurations. On the one hand, the several configurations have similar enthalpies within T unit cell, which is proven by Fan and coworkers [31]. On the other hand, the MA+ cations with deep energy level will have no influence on the states nears the EF. Thus, the configurations will have little effect on the interfacial structure, and heterostructures are stable at

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ACCEPTED MANUSCRIPT room temperature. In previous study, the MA+ cations are disordered between two nonequivalent positions in each cage [7]. The associated octahedral tilting pattern in the most stable configuration is a0a0c- in Glazer notation (without orientational dynamics of MA+ cations). The most energetically

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stable configuration of a0a0c- is selected in our simulation, which is agreed with the previous reports [7,32].

The lattice mismatch is 3.48% by considering the in-plane lattice constant of T-MAPbI3 (1×1)

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and FO (1×1) in (001) plane, respectively. The (PbI2)0, (MAI)0, FeA and Fe2O4 (FeBO) terminations

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of T-MAPbI3 and FO are considered in the calculation. The models include the PbI2/FeA, MAI/FeA, PbI2/FeBO and MAI/FeBO, as shown in Figs. 1(a)-(d). Brillouin-zones (BZ) of pristine MAPbI3 and FO are sampled by 7×7×6 and 5×5×5 Γ-centered grids. All the MAPbI3/FO heterostructures have a 5×5×1 Γ-centered grid. The convergence criteria of 10-5 eV and 0.02 eV/Å are reached. A 15-Å

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vacuum layer is adopted for all the models to avoid the adjacent interlayer interaction. The atoms and z-directional lattice constant of T-MAPbI3 are fully relaxed, but the xy-plane lattice constants of

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pristine T-MAPbI3 are fixed at that of FO. In the MAPbI3/FO heterostructures, the ions at the bottom three layers of FO are fixed as its bulk position, while other atoms are fully relaxed. The E-field has

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been added by dipole layer method (place dipoles in vacuum region of model). In Fig. 1(e), the arrow from the substrate to MAPbI3/FO interface indicates the positive direction of E-field. In the calculations, the E-field is set at -500, -100, 0, 100 and 500 kV/cm.

3. Results and Discussion

Firstly, the validity of the models is discussed. Our calculated bandgap for pristine MAPbI3 by

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ACCEPTED MANUSCRIPT GGA-PBE is 1.51 eV. The underestimation of GGA-PBE has no substantial effect on the nature of the interface hybridization. Our method and more accurate hybrid functional+SOC method give a rather similar bandgap (1.51 eV of ours; 1.73 eV of PBE0+SOC; 1.63 eV of experiment) [33,34].

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Second, the band alignment between our method and hybrid functional+SOC differs little, which are all semiconductors with direct bandgap character at Γ point [35,36]. And the method is widely used in investigation of the MAPbI3, and has been proved by many reports [7,14,36]. It is reasonable to

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anticipate that the bandgap underestimation only induces negligible difference in the interfacial

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hybridization and our conclusion.

The binding energy (Eb) of different models is calculated by the equation of Eb=Etotal-EMAPbI3-EFO, where Etotal, EMAPbI3 and EFO denote the total energy of MAPbI3/FO heterostructures, isolated MAPbI3 and FO, respectively. We selected the seven-layer T-MAPbI3

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which has a similar Eb to that of nine-layer [13].The calculated Eb of PbI2/FeA, MAI/FeA, PbI2/FeBO and MAI/FeBO models are -7.28, -5.56, -2.58 and -1.48 eV, respectively. Our result is consistent

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with previous reports from the viewpoint of stability, because they all confirm that the PbI2-termination is more stable than the MAI-termination. The MAPbI3/TiO2 results suggest that the

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interfacial Pb atoms play an important role in the structure stability and electronic properties [13]. And the energy level alignment between MAPbI3 and TiO2 is significantly influenced by the interfacial plane of MAPbI3 [14]. It is found that the PbI2-terminated surface is the most stable than that of MAI [16]. The c/a ratio of bulk T-MAPbI3 in our simulation is 1.44, which is agreed with the experimental result of 1.43 [32]. It is reasonable to believe that our simulation has a reliable basement. In order to analyze the charge transfer and interfacial interaction, the charge densities of

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ACCEPTED MANUSCRIPT PbI2/FeA, MAI/FeA, PbI2/FeBO and MAI/FeBO models are depicted in Figs. 1(e)-(h). The yellow and cyan parts denote the highly localized and depleted electrons, respectively. By combining MAPbI3 with FO, the interfacial electronic structure can be enriched. Charge transfer in two

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FeA-terminated models (Figs. 1(e),(f)) are more obvious than that in two FeBO-terminated models (Figs. (g),(h)). The similar phenomenon has been founded in previous study on FO/BaTiO3, which shows a charge-localized characteristic in two FeBO-terminated models [37]. Moreover, the

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interfacial chemical interactions can be ascribed to the joint contributions of the electrostatic

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interaction and covalent bonding in two FeA-terminated models, which are shown in the yellow and cyan colors. In two FeA-terminated models, the charges transfer from IV-Pb to III-FeA atoms, which will induce a high localization around III-FeA. At the interfaces of two PbI2-terminated models, the charges accumulate along the line of III-FeA and IV-I. Besides, the charge accumulation also

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appears at the second layer of interface (II-FeB atoms). It is shown that the PbI/FeA model has the highest charge transfer, which is consistent with the result of Eb.

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Fig. 2 displays the band structure of different MAPbI3/FO models. The FO part is drawn with blue lines, while the red lines represent MAPbI3. The highest occupied molecular orbitals (HOMOs)

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and the lowest unoccupied molecular orbitals (LUMOs) of FO and MAPbI3 in different models are tested. The bandgap is defined as the energy gap between LUMOs and HOMOs. The FO in PbI2/FeA model shows a significant spin-down bandgap rather than other three models, indicating that FO becomes a semiconductor in PbI2/FeA model, but preserves its half-metallicity in other models. Particularly, the spin-up and spin-down bandgaps of FO in PbI2/FeA model are 1.20 and 0.75 eV, respectively. The MAPbI3 becomes a conductor in PbI2/FeBO model, but maintains its semiconducting characteristic in other models. Although the band bending of the MAPbI3/FO

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bandgap favors the electron-hole excitation because it brings the electronic and vibrational quanta closer to resonance [25]. It meant that the PbI2/FeA model is more suitable for PSCs than other three models.

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In order to further understand the electronic structure of MAPbI3/FO, the local density of

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states (LDOS) of interfacial atoms are depicted in Fig. 3. The atomic layers are labeled by Roman numbers. Noticed that the DOS of Pb doesn’t include the 5d orbitals, and its contribution is negligible. This phenomenon can be ascribed to the 5d orbital of Pb is full filled. Actually, the band structure near the bandgap does not depend on the inclusion of 5d orbital of Pb, the conclusion is

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proved by Even et. al [35]. In Fig. 3, the LDOS of MA molecules is not shown because the electronic states of MA molecules don’t contribute to HOMOs or LUMOs of the band edge states

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[25]. The gray parts are the LDOS of pristine MAPbI3. In different models, all the LDOS of interfacial atoms deviate from the pristine MAPbI3 (Fig. 3). And the degree of LDOS deviation of

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the third layer is much weaker than that of the first layer, while the deviation is much stronger in MAPbI3 than FO. The HOMOs of MAPbI3 in heterostructures comes from I 5s and Pb 6p orbitals, and the LUMOs can be ascribed to I 5p and Pb 6s orbitals. In pristine MAPbI3, the HOMOs and LUMOs consist of I 5p and Pb 6p, respectively, which is different from all the MAPbI3/FO heterostructures [38]. The LDOS shape of I atoms in two FeBO-terminated models are similar to the pristine, indicating a weak interfacial interaction. The E-field in MAPbI3 may originate from an accumulation of photogenerated or ionic charges

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ACCEPTED MANUSCRIPT and induce a Raman activity or crystal structure variation [33,39]. Besides, the PbI6 octahedra of MAPbI3 may produce an overall electric dipole [40] which can be influenced by an applied E-field. As a good way to modulating the structure properties, it is necessary to applying E-field to

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heterostructures. Considering the PbI/FeA is the most stable model with lots of charge transfer, great deviation of LDOS and dPb-I, it’s reasonable to apply the E-field on PbI/FeA to investigate the change of electronic structure. Specifically, the FO bandgap at different E-fields is plotted in Fig.

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4(a). Both of spin-up and spin-down bandgaps increase with the increase of negative E-field.

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However, as the positive E-field increases, the spin-up bandgap decreases while the spin-down bandgap increases. The spin-up bandgap keeps its direct characteristic at -500~100 kV/cm, but an indirect bandgap appears at 500 kV/cm. The spin-down bandgap is just the opposite, it is an indirect bandgap at -500~100 kV/cm, but a direct bandgap appears at 500 kV/cm (Fig. 4(b)). The E-field

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can be an effective tool to modulate the size and location of the bandgap (Fig. 4). This property is useful in spintronics if EF can be enhanced by 0.35 eV. The bandgap at -100 kV/cm almost equals to

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that of without field, we ascribe it to the atom alignment of T-MAPbI3 at different fields. The θPb-I-Pb of MAPbI3 at -100 kV/cm is almost same to that of without field (Table 1).

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The LDOS of PbI/FeA model shows a highly localized characteristic at different E-fields (Fig. 5). The coupling strength is related to the wave function overlap. The orbital contribution to HOMOs and LUMOs are diverse at different E-fields. It is shown that HOMOs and LUMOs in MAPbI3 mostly come from I 5p and Pb 6p orbitals at ±500 kV/cm, respectively. However, at -100~100 kV/cm, HOMOs are the combination of Pb 6s and I 5p, while LUMOs are jointly contributed by Pb 6p and I 5s. By visually evaluating the HOMOs-LUMOs overlap in PbI/FeA model at different E-fields, it is clear that the hybridization only occurs between FO and MAPbI3 at

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ACCEPTED MANUSCRIPT 500 kV/cm. Specifically, the hybridizations happens to FeB 3d and I 5p orbitals as well as Pb 6p and FeA 3d orbitals, which can be attributed to the strong wave function overlap at ±500 kV/cm. The bandgap of -100 kV/cm equals to that of without E-field, which should be owing to the ferroelectric

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direction transition. Consistent with the LDOS results (Fig. 5), the magnetic moments (Table 1) of III-FeA, IV-Pb1 and IV-I have changed by 0.04, 0.02 and 0.03 µ B at 500 kV/cm, respectively. At different E-fields, the θPb-I-Pb and dPb-I deviate from the value at zero E-field, revealing a change of

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the inorganic octahedral (Table 1). The above results are all consistent with band structure.

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Therefore, one can confirm that a large field can control the bandgap of MAPbI3/FO heterostructures by modulating the hybridization.

By surface reconstruction, the bandgap of FO(001)-termination can be opened, showing a transition from half-metal to semiconductor [41]. This half-metal to semiconductor transition of FO

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may involve the structure changes at the termination [41]. Adsorbing hydrogen (H) can also lead to a similar transition from half-metal to semiconductor in H/FO (001) model, where the charge of a

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deeper layer can be affected by H adsorption [41]. These theoretical results indicate that the half-metallicity to semiconductor transition of FO in PbI/FeA model is reasonable. In the model of

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E-field-controlled bandgap of PbI/FeA heterostructure (Fig. 6), electron-hole pairs arise from the MAPbI3, and FO acts as a carrier extraction layer [21]. The electron-hole pairs will be separated by the built-in E-field of the p-n junction, which further give rise to a photo-voltage (photo-current). FO bandgap varies with the E-fields, which can be a detection of the spin-polarized photo-current. The phenomenon can be used to realize an interfacial electron-transfer model, which has potential applications in spin-photodiode.

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4. Conclusion

The interfacial electronic structures of MAPbI3/Fe3O4 heterostructures are predicted by density

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functional theory. Four models (MAI/FeBO, PbI2/FeBO, MAI/FeA and PbI2/FeA) are included. Especially, a half-metal to semiconductor transition of Fe3O4 appears in PbI2/FeA model. A series of E-field is added to PbI2/FeA model, and a direct-indirect bandgap transition of Fe3O4 appears at a

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500-kV/cm field. The E-field can control the bandgap of Fe3O4 in PbI2/FeA model by modulating

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the hybridization. Our results advance the understanding on the interfacial properties of MAPbI3/FO heterostructures and have potential applications on spin-photodiode. The prediction of spin-related bandgap characteristic in MAPbI3/Fe3O4 is meaningful for experimental research.

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Acknowledgements

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This work is supported by National Natural Science Foundation of China (51671142, U1632152), Key Project of Natural Science Foundation of Tianjin (16JCZDJC37300). It is also

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supported by High Performance Computing Center of Tianjin University, China.

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Table notes

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Table 1 Magnetic moments (µ B) of interfacial atoms, angle (o) for Pb-I-Pb, average Pb-I atomic distances (Å) and the standard deviation of interfacial atoms of PbI-FeA model at different electric

III-FeA

-3.560

IV-Pb

-0.023

IV-I

0.033

θPb-I-Pb

IV

129.800

dPb-I(σ)

IV

3.315(0.177)

0

100

500

-3.560

-3.563

-3.561

-3.525

-0.023

-0.022

-0.024

-0.041

0.032

0.033

0.032

0.002

129.680

129.720

129.890

129.930

3.314(0.178)

3.316(0.179)

3.319(0.179)

3.314(0.183)

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Moments (µ B)

-100

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-500

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Figure captions

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Fig. 1 (a)-(d) are PbI2/FeA, MAI/FeA, PbI2/FeBO and MAI/FeBO models; (e)-(f) are corresponding charge density states at interface of corresponding models, the isosurface are 0.01 where

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yellow and cyan parts are electrons that are accumulating and depleting, respectively.

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Fig. 2 Spin-up and spin-down band structures of pristine MAPbI3 are depicted in left panel, (a)-(d) are band structures of PbI2/FeA, MAI/FeA, PbI2/FeBO and MAI/FeBO, blue lines are FO while red lines are MAPbI3.

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Fig. 3 (a)-(d) are LDOS of PbI2/FeA, MAI/FeA, PbI2/FeBO and MAI/FeBO; DOS of atoms in pristine FO and MAPbI3 is depicted by shaded areas; EF is indicated by vertical lines and set to zero;

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the numbers of layers are displayed by the roman numbers.

Fig. 4 (a) Bandgap of MAPbI3/FeA model at different electric fields, (b) the direct bandgap and indirect bandgap transform.

Fig. 5 (a)-(e) are LDOS of interfacial atoms in PbI2/FeA model at -500, -100, 0, 100, 500 kV/cm electric fields.

Fig. 6 Charge transfer model of MAPbI3/FeA and corresponding spintronic device. 17

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