Structural, Electronic, and Optical Characterizations of the Interface

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Structural, Electronic, and Optical Characterizations of the Interface between CH3NH3PbI3 and BaSnO3 Perovskite: A First-Principles Study Yao Guo, Yuanbin Xue, Cuihuan Geng, Chengbo Li, Xianchang Li, and YongSheng Niu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01088 • Publication Date (Web): 29 May 2019 Downloaded from http://pubs.acs.org on May 30, 2019

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The Journal of Physical Chemistry

Structural, Electronic, and Optical Characterizations of the Interface between CH3NH3PbI3 and BaSnO3 Perovskite: A First-Principles Study Yao Guo1, *, Yuanbin Xue1, Cuihuan Geng1, Chengbo Li2, Xianchang Li2, and Yongsheng Niu2 1

Department of chemical and environmental engineering, Anyang Institute of Technology, Anyang 455000, China

2

Department of mathematics and physics, Anyang Institute of Technology, Anyang 455000, China

*) Corresponding Author: Tel: +86-372-2592066, Fax: +86-372-2592066, e-mail: [email protected]

Abstract The interfaces play a significant role in enhancing the photoelectric properties and stability of the perovskite solar cells (PSCs). In this work, we performed first-principles density-functional-theory (DFT) calculations to examine the interaction between CH3NH3PbI3 and BaSnO3 perovskite. The interfacial structure-property relationships of pristine CH3NH3PbI3/BaSnO3 have been thoroughly explored by theoretical computations. The results of interfacial binding strength and electronic structure indicate that the combination of SnO2/PbI2 has the strongest interfacial interaction and charge transfer capability. The enhanced interfacial interaction between CH3NH3PbI3 and

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BaSnO3 is beneficial for electron-hole separation. Moreover, the lanthanum-doped CH3NH3PbI3/BaSnO3 interfaces are also investigated to explore the impact of lanthanum doping on the properties of CH3NH3PbI3/BaSnO3. The geometric, electronic and optical characteristics of CH3NH3PbI3/BaSnO3 interface with lanthanum dopant at different positions have been evaluated. We found that lanthanum dopant was energetically stable at the sub-surface layer. The doping depth has a negligible influence on the interfacial charger transfer. Lanthanum doping has been demonstrated to be workable for enhancing the n-type conductivity in BaSnO3 while the excellent transport property is preserved. La incorporations effectively reduce the conduction band level of BaSnO3, which will make the interfacial band alignment more suitable for the electron transfer. Finally, the absorption coefficients and inter-band transitions are well estimated to understand the light trapping capacity. This work provides an atomic insight into the interfacial interaction between CH3NH3PbI3 and BaSnO3, which can help us to obtain to new strategies for optimizing the PSCs interfaces.

Keywords: organic-inorganic perovskites, interface, DFT, BaSnO3

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1. Introduction Hybrid halide perovskites have drawn considerable attention in both academic and technical focus areas owing to their high performance and low cost [1-3]. The typical device architecture of perovskite solar cells (PSCs) is the perovskite light absorber inserted between the electron transport layer (ETL) and hole transport layer (HTL) [4]. Despite the dynamically growing photoconversion efficiency, the device stability upon exposure to moisture, heat, and light is still far from satisfactory since the PSCs must have the ability to withstand severe environmental variations before commercialization [5-7]. The stability of perovskite/ETL interface is crucial to the performance of PSCs. To date, the most widely used ETL material is TiO2 for its unique chemical and electronic transport properties [8-11]. However, the device stability and efficiency will be degraded in response to light irradiation [12] when TiO2 ETL is employed in PSCs. The comparatively low electron mobility cannot satisfy further optimization of device performance [13]. To this end, numerous n-type metal oxides have been explored to find potential candidate materials to replace TiO2 and construct more efficient devices [2-4, 14-20]. Barium stannate (BaSnO3), which is an n-type transparent semiconducting perovskite oxide, has a cubic structure and belongs to the Pm3m space group. The band gap energy

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(Eg) of BaSnO3 is around 3 eV and the electron mobility is comparatively higher than TiO2 [21-23]. The electronic and optical properties of BaSnO3 could be significantly modified through substitutional doping [24-26]. Recently, Shin et al. reported CH3NH3PbI3-based PSCs with the compact and uniform lanthanum-doped (5 mol%) BaSnO3 ETL [27]. The BaSnO3-based PSCs exhibited high efficiency and photo-stability after 1000 hours, whereas the TiO2-based reference PSCs degraded entirely in 500 hours. The developments of the BaSnO3 ETL bring PSCs commercialization one step closer. Nevertheless, the interfacial interactions between CH3NH3PbI3 and BaSnO3 are not well understood. Especially, the role of lanthanum in the interface is not clear in spite of the success of BaSnO3-based PSCs. Up to now the electronic properties of perovskite/TiO2 interface have been widely investigated by DFT calculations [28-34], and experiments are gradually giving a clear picture of CH3NH3PbI3/BaSnO3. An atomic insight into the interfacial interaction between CH3NH3PbI3 and BaSnO3 can contribute to new strategies for optimizing the interface. For this reason, in this study we performed first-principles investigations to characterize the structural, electronic, and optical properties of CH3NH3PbI3/BaSnO3. In addition, the effect of lanthanum on the hetero interface was unveiled by theoretical electronic structure calculations. Our study would constitute the basis for further enhancement and exploitation of BaSnO3-based PSCs.

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2. Computational details and structural modeling First-principles DFT investigations were carried out by means of the Vienna ab initio simulation package (VASP) [35, 36] within the MedeA [37] computational environment. The ion-electron interactions were treated with the projector-augmented wave (PAW) [38] formalism and the Perdew–Burke–Ernzerhof-generalized gradient approximation (PBEGGA) [39] were included for the exchange-correlation functional. The cutoff energy at moderate energy resolution (500 eV) is sufficient for the system. The convergence criteria applied were 10−6 eV for the total energy and 0.01 eV/Å for the atomic forces. The reciprocal space integration over the three-dimensional Brillouin zone utilized the Monkhorst–Pack [40] k-point grid with densities of 0.2 Å−1. The Gaussian smearing of electron density with 0.1 eV width is employed for bulk, surface and interface, respectively. To improve the accuracy of the electronic properties, Hubbard U correction for the on-site Coulomb (for Sn and La) [41,42] is employed to provide not only a better structural representation but also improve the calculated band gap. A dipole correction along the c-axis was applied to avoid interactions between periodic images of the slab. The lattice structures of tetragonal CH3NH3PbI3 and cubic BaSnO3 perovskite bulk have been investigated to construct the surface model. The lattice constant (see Table 1) for

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CH3NH3PbI3 and BaSnO3 are reliable compared with experimental results [43-45]. The CH3NH3PbI3 and BaSnO3 surfaces were modeled by the symmetric five-layer and sevenlayer slabs, respectively. A vacuum layer of 20 Å is added to remove spurious interactions between image slabs. For CH3NH3PbI3 (001)/BaSnO3 (001), we consider two configurations of minimal stress. The small one consisted of √2×√2 CH3NH3PbI3 and 2×2 BaSnO3 slabs and the average lattice mismatch was less than 5.0%. The enlarged one was rotated 45-degree and composed of 2×2 CH3NH3PbI3 and 3×3 BaSnO3 slabs. The average lattice mismatch was as small as ~1%. Moreover, to compare and explore the effect of CH3NH3 orientation, 2√2×√2 CH3NH3PbI3 and 4×2 BaSnO3 were also employed to build the CH3NH3PbI3 (011)/BaSnO3 (001) interface. The various CH3NH3PbI3/BaSnO3 interface structures under investigation are depicted in Figure 1. During the interfacial relaxations, the two BaSnO3 layers at the bottom of the interface were frozen at the perfect lattice site and treated as bulk structures. The remaining layers were taken as CH3NH3PbI3/BaSnO3 hetero-interface and geometry relaxations were performed. The effect of spin-orbit coupling (SOC) [46] was not considered because the previous study on CH3NH3PbI3 [47] indicated that excluding the SOC for Pb-6p orbitals can also estimate the bandgap correctly using PBE-GGA.

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3. Results and discussion 3.1 CH3NH3PbI3 and BaSnO3 surfaces First, the structural nature of CH3NH3PbI3 and BaSnO3 surfaces were investigated in this study. Along the c-axis, CH3NH3PbI3 is composed of CH3NH3I (MAI) and PbI2 layers and BaSnO3 is consisted of BaO and SnO2 layers. Compared to the polar lattice planes, the (001) plane was non-polar and more stable [48]. In addition, previous studies indicate that the surface states on the CH3NH3PbI3 (001) plane acted as effective hole transfer mediums [49]. Considering the higher stability of the tetragonal phase CH 3NH3PbI3 in previous experiments [4, 20, 27], we selected the tetragonal CH3NH3PbI3 (001) and cubic BaSnO3 (001) slabs in this study. The surface energies [50] are the important issues for understanding the relative stability of various surface terminations. This value could be obtained by the summation of cleavage and relaxation energies, which can be calculated by the following equation: Esurface＝[Ecleavage + Erelax]/2A

(1)

where Esurface represents the surface energy, Ecleavage corresponds to the cleavage energy, Erelax represents the relaxation energy, and A is the surface area. Surface relaxations were performed for all possible terminations and these results are listed in Table 2. For CH3NH3PbI3, the PbI2 termination was found to be energetically more favorable than the

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MAI termination. Moreover, the surface energy difference between BaO and SnO2 termination is quite small and the coexistence of both types of surface terminations can be expected. These results are similar to other theoretical studies on CH3NH3PbI3 and BaSnO3 surfaces in previous reports [49-52].

3.2 CH3NH3PbI3/BaSnO3 interface The CH3NH3PbI3/BaSnO3 interface is built by connecting the cubic BaSnO3 (001) slab with the tetragonal perovskite (001) slab based on various terminations, as mentioned above. Therefore, there are four probable CH3NH3PbI3/BaSnO3 interfaces morphologies for each configuration. The average of CH3NH3PbI3 and BaSnO3 cell parameters are employed to build the interface model. Binding energy [53] of the interface is usually employed as a relevant parameter to estimate the interfacial stability of the CH3NH3PbI3/BaSnO3. The interfacial binding energies can be predicted by the following equation: Ebinding＝EBSO + Eperovskite – Etotal

(2)

Where Ebinding is the binding energy, Etotal, EBSO, and Eperovskite are corresponding energies of the interface, anatase and perovskite surfaces, respectively. For comparison, the corresponded CH3NH3PbI3 (001)/TiO2 (001) interfacial configuration is included for

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comparison. The calculated results are presented in Table 3. The binding strength of CH3NH3PbI3/BaSnO3 is higher than that of CH3NH3PbI3/TiO2 [29]. The relatively large Ebinding indicates that CH3NH3PbI3/BaSnO3 is a somewhat stronger interface. It can be expected that BaSnO3 have better carrier injection properties than other ETL materials. The SnO2/PbI2 interface has the highest binding energy independent of all three interfacial configurations. The terminations of both CH3NH3PbI3 and BaSnO3 can affect interfacial stability. The Bader charge analysis [54] was employed to evaluate the charge transfer between perovskite and ETL and finally to report the theoretical capability of electronhole separation. Negative values mean electron transfer from perovskite to BaSnO3 and positive values mean electron transfer from BaSnO3 to perovskite. The negative maximum value for SnO2/PbI2 interface indicated strong change transfer. Therefore, the SnO2/PbI2 interface with the highest binding energy and strongest charge transfer capability will be selected for subsequent investigations. To analyze the difference in electrostatic effects, the planar and macroscopic averaged electrostatic potentials of CH3NH3PbI3/BaSnO3 were derived from the DFT+U results. The electrostatic potential across the interfaces are displayed in Figure 2. Taking a closer look at the interfacial region, we can found that the 2×2 SnO2/PbI2 has the lowest potential at the interface when compared to the other three interfaces. This will make it easier for

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the electrons to transfer and accumulate at the interface region. These observations explain and support the results (Table 3) that the SnO2/PbI2 interface has the highest binding energy and strongest charge transfer capability. It is generally accepted that the carrier separation takes place at the ETL/perovskite interface. Hence, larger CB band offset enhances the electron injection into the BaSnO3 layer. The increased binding energy would reduce valence-band offset (VBO) and enhance conduction-band offset (CBO) in CH3NH3PbI3/BaSnO3, which is efficient for the carrier separation. To better understand the electronic structures of CH3NH3PbI3/BaSnO3, we have performed DFT+U calculations on the partial density of states (PDOS). As displayed in Figure 3, the states near conduction-band minimum (CBM) mostly come from the Pb-6p orbital, and the states near valence-band maximum (VBM) are derived mostly from I-5p and O-2p orbitals. The organic cations (MA) provide very few contributions to valenceand conduction-band. The shapes of the DOS distribution for the BaO- and SnO2terminations are similar in general. Careful comparison of the DOS distributions showed that the conduction bands for the two terminations are slightly different. The Ba-5d orbital of SnO2-termination shifts to higher energy level compared to the BaO-termination. It is quite different from the current conventional CH3NH3PbI3/TiO2 interface. The same qualitative shift has also been found in other perovskite surface calculations [55, 56].

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Compared with the 2×2 slab, the perovskite peaks in 3×3 slab shifted towards lower energies. Furthermore, the CH3NH3PbI3 (011)/BaSnO3 (001) interface (4×2 slab) shows more energy shift indicating that the interfacial configuration has strong influence on the interface electronic structures. Given that the hybridization was strong in the interface region, the atomic structure in the interface region will be reconstructed. Therefore, when CH3NH3PbI3 and BaSnO3 form the hetero junction, the periodic crystalline structure at the interface region will be destroyed after reconstruction. This will bring additional interface levels in the band gap. It is known that CH3NH3PbI3 has a narrower band gap compared with BaSnO3. Therefore, the energy-level gap between Pb-6p and I-5p (O-2p) orbitals will determine the photoconversion efficiency of CH3NH3PbI3/BaSnO3 interfaces.

3.3 lanthanum doped CH3NH3PbI3/BaSnO3 interface To elucidate the influence of the La doping on CH3NH3PbI3/BaSnO3 interfaces, the La dopant has been substituted for Ba in the relatively stable BaO/PbI2 and SnO2/PbI2 interface models. The substitution of one Ba atom for La corresponds to the doping ratio of 8.33 at.% (2×2 slab), 5.56 at.% (3×3 slab) and 6.25 at.% (4×2 slab), respectively. The values fall within the range of typical La-doping concentrations in experiments. The interfacial structures of La doping configurations at different depths from the surface are

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shown in Figure 4. In this study, we have only considered two doping positions for the La dopant because the size of the interface models is limited. The surface doping configuration was constructed by the replacement of a Ba atom with a La atom in the surface layer. The sub-surface doping configurations were estimated by replacement of La by Ba situated at the intermediate layer of the interface model. The stability of various doping configurations could be discussed based on the Ebinding of La-doped CH3NH3PbI3/BaSnO3 interface after relaxation. The calculated results in Table 4 indicate the dependence of the La dopant on the depth within the interface layers. The interfacial binding energy, as reference standards for evaluation of interfacial stability, were calculated and summarized. We have found that lanthanum impurities have opposite trends in their doping positions for the BaO/PbI2 and SnO2/PbI2 species. The Ebinding of BaO/PbI2 is steadily increased as the La doping position varies from sub-surface to the surface in BaSnO3. This indicates that La dopant was more likely to exist at the BaSnO3 surface compared to the sub-surface. In contrast, for SnO2/PbI2 interface, table 4 shows that the binding energy of surface doping structure decreased significantly compared with that of sub-surface doping one. The results indicate that the interfacial stability of Ladoped CH3NH3PbI3/BaSnO3 explicitly depends on La doping depth. The Bader charge analysis was employed to characterize the effect of lanthanum doping on charge transfer

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properties. As shown in Table 5, all three types of SnO2/PbI2 configurations show similar results that the slight lanthanum doping in BaSnO3 hardly affected charge transfer. Such an effect was found to be insignificant for SnO2/PbI2 interface. Thus, it can be concluded that doping depth has a negligible influence on interfacial charger transfer. To evaluate the influence of lanthanum incorporation on the electronic structures of the CH3NH3PbI3/BaSnO3 interface, the planer averaged electrostatic potential is plotted along the z-axis for various SnO2/PbI2 configurations. It should be noted that the lanthanum existence in the BaSnO3 slab results in fluctuations of the average potential, due to the varying charge densities possessed by the different types of atoms. Compared with the pristine CH3NH3PbI3/BaSnO3 interfaces (Figure 2), the average potential becomes inclined towards the interface region in the surface doping configurations. The discrepancy in average potential can be attributed to the effects of the partial substitution of La for Ba in the surface region. For better understanding of dopant influence on the electronic properties, theoretical analysis of the atom-resolved local density of states (LDOS) for lanthanum dopant and the first nearest-neighbor Ba, Sn and O atoms are performed (seen Figure 6). It is already known that the BaSnO3 has a highly covalent bonding character due to its energetic Sn-O bond. The energy difference between O-2p and Sn-5s orbitals determine the degree of the band gap. In the case of the La-doped

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BaSnO3, the states of the La-4f orbitals could be observed around CBM. The Fermi level shifted toward CBM and the compound showed n-type electrical conducting properties. Thus, the electrical conductivity of BaSnO3 has been greatly enhanced by La doping. Lanthanum substitution is found to introduce donor centers which will reduce electrontransfer resistance and increase hole-extraction at the interface. This will make it easier for the electrons to migrate from the donor level to the conduction band. Therefore, lanthanum substitutions were found to be efficient in enhancing the electrical conductivity of BaSnO3 by decreasing the charge transport resistance while preserving the excellent transport properties. To clarify the atomic redistributions and reveal the nature of the chemical interactions in the interface, the interfacial configurations accompanied with the electron localization function (ELF) images [57] for the pristine and La-doped BaSnO3 were presented and discussed (seen Figure 7). It can be seen that the interaction between the perovskite and BaSnO3 is determined by I and Pb atoms of perovskite bonded to Ba and O atoms in BaSnO3 in the interface region. Moreover, significant geometric distortions can be observed at the CH3NH3PbI3/BaSnO3 interface. As shown in Figure 7, the interfacial plumbum cations apparently diffused into the perovskite surface region. The new O-PbO bonds formed at the interface while the Ba-O bonds were maintained. The partial

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substitution of La for Ba induces a clear lattice contraction compared with undoped material, as observed in La-doped BaSnO3 layers. The analysis of ELF (between 0 and 1) is widely used to illustrate and visualize chemical bonds. The high ELF values (red color) reveal the covalent bonds in small local Pauli repulsion situations, while the low ELF values (blue color) indicate the position with low electron density regions. An ELF value of 0.5 (green color) mean the position has the same electron density as the uniform electron gas. As depicted in Figure 7, the red color of the I and O anions show strong electron localizations, while the green color of the Pb and Ba (or La) cations indicate a uniform electron gas density. In addition, the blue color region between the interfacial anions and cations reveals the strong interactions. Hence, the interaction between BaSnO3 and perovskite is chemical bonding. Furthermore, careful comparison between BaO/PbI2 and SnO2/PbI2 in Figure 7 found that the SnO2/PbI2 implied electron–gas-like characteristics around the interface region. It corresponded to the strong charge transfer at the interface which is consistent with the results of the charge transfer in Table 3. Optical properties, especially strong optical absorption spectra, are very crucial for high performance PSCs [58, 59]. To evaluate the influence of the lanthanum dopant on the optical property, we also analyzed the optical spectra of the pristine and La-doped CH3NH3PbI3/BaSnO3 interfaces in Figure 8. One can see that the absorption spectra of

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the surface and sub-surface doping interfaces (2×2 slab) have similar curves except for the slight difference in the absorption peaks around 4.5 eV. The peak around 3 eV could be ascribed to the conduction-to-valence band transition from I-5p (or Pb-6s) states to Pb6p states. In contrast, the relatively large 3×3 slab demonstrated the strongest absorption intensity and the 4×2 slab showed the lowest absorption intensity. The significant difference in absorption intensities can be attributed to the interaction between CH3NH3PbI3 and BaSnO3 at the interface region and the orientation of the organic MA molecules. The degradation of the perovskite layer is consistent with the beforementioned result of interfacial binding strength (Table 3). The optical absorption coefficient of CH3NH3PbI3/BaSnO3 is hardly affected by slight lanthanum dopant. The lanthanum-induced band gap renormalization [60] permits photon transmission to CH3NH3PbI3 in the whole solar spectrum range from UV to visible region, making the lanthanum-incorporated BaSnO3 the ideal ETL material for PSCs. The schematic band alignments of CH3NH3PbI3/BaSnO3 interface were illustrated in Figure 9. As a reference, the vacuum level of the interface system is set to 0 eV. The CBM (Sn-5s states) level of BaSnO3 decrease was induced by the lanthanum doping. The strong coulombic interactions between La3+ dopants and doped-electron will decrease the band gap of BaSnO3, and afterward, the band alignment CH3NH3PbI3/BaSnO3 starts to be suitable for

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the electron transport to electrode.

4. Concluding remarks In summary, the structural, electronic and optical properties of the pristine and La-doped CH3NH3PbI3/BaSnO3 interfaces were investigated by using first-principles calculations. We have found that the SnO2/PbI2 has the highest binding energy and charge transfer, which can be accepted as the most stable interfacial configuration. The enhanced binding energy could reduce the VBO and enhance the CBO between CH3NH3PbI3 and BaSnO3, which is efficient for carrier separation. We have also found that the lanthanum doping exerted

differential

influence

on

the

interfacial

stability

of

La-doped

CH3NH3PbI3/BaSnO3. The interfacial charge transfers were hardly affected by the slight lanthanum doping. Electronic structure calculations reveal that lanthanum doping is effective for enhancing the conductivity of crystalline BaSnO3 while the excellent transport property is preserved. The lanthanum doping lowers the CBM of BaSnO3 and makes the band alignment favorable for electron transport. The band gap and band alignment could be effectively influenced by surface terminations. The results of optical absorption indicate that lanthanum-doped BaSnO3 could be a promising candidate material for electron transporting materials for PSCs. This study provides details of the

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atomic structure and interactions at the CH3NH3PbI3/BaSnO3 interface, which is helpful for further elucidation of the stability mechanisms of PSCs.

Acknowledgment This work was supported by grants from the National Natural Science Foundation of China (Grant No. 11447150).

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[32] Akbari, A.; Hashemi, J.; Mosconi, E.; De Angelis, F.; Hakala, M. First Principles Modelling of Perovskite Solar Cells Based on TiO2 and Al2O3: Stability and Interfacial Electronic Structure. Journal of Materials Chemistry A 2017, 5, 2339. [33] Nemnes, G. A.; Goehry, C.; Mitran, T. L.; Nicolaev, A.; Ion, L.; Antohe, S.; Plugaru, N.; Manolescu, A. Band Alignment and Charge Transfer in Rutile-TiO2/CH3NH3PbI3-xClx Interfaces. Physical Chemistry Chemical Physics 2015, 17, 30417. [34] Mosconi, E.; Grancini, G.; Roldáncarmona, C.; Gratia, P.; Zimmermann, I.; Nazeeruddin, M. K.; De Angelis, F. Enhanced TiO2/MAPbI3 Electronic Coupling by Interface Modification with PbI2. Chemistry of Materials 2016, 28, 3612. [35] Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using A Plane-Wave Basis Set. Computational Materials Science 1996, 6, 15. [36] Kresse, G.; Joubert, D. P. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Physical Review B 1999, 59, 1758. [37] Rozanska, X.; Ungerer, P.; Leblanc, B.; Saxe, P.; Wimmer, E. Automatic and Systematic Atomistic Simulations in the MedeA® Software Environment: Application to EU-REACH. Oil Gas Sci. Technol. – Rev. IFP Energies nouvelles 2015, 70, 405. [38] Blöchl, P. E. Projector Augmented-Wave Method. Physical Review B 1994, 50, 17953. [39] Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Physical Review Letters 1996, 77, 3865. [40] Pack, J. D.; Monkhorst, H. J. "Special Points for Brillouin-Zone Integrations"—A Reply. Physical Review B 1977, 16, 1748. [41] Moreira, E.; Henriques, J. M.; Azevedo, D. L.; Caetano, E. W. S.; Freire, V. N.; Fulco, U. L.; Albuquerque, E. L. Structural and Optoelectronic Properties, and Infrared Spectrum of Cubic BaSnO3 from First Principles Calculations. Journal of Applied Physics 2012, 112, 043703. [42] Xue, Y.; Guo, Y.; Hou, S.; Niu, Y.; The Effect of Oxygen Vacancies on the Properties of Polar and Nonpolar (001) LaAlO3/SrTiO3 Heterostructures. Applied Surface Science 2018, 450, 260.

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A.;

Weber,

Methylammoniumtrihalogenoplumbates

D. (II)

Dynamic Observed

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Disorder

in

Millimeter-Wave

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[54] Henkelman, G.; Arnaldsson, A.; Jónsson, H. A Fast and Robust Algorithm for Bader Decomposition of Charge Density. Computational Materials Science 2006, 36, 354. [55] Wang, Y. X.; Arai, M.; Sasaki, T.; Wang, C. L. First-Principles Study of the (001) Surface of Cubic CaTiO3. Physical Review B 2006, 73, 035411. [56] Luo, B.; Wang, X.; Tian, E.; Li, G.; Li, L. Structural and Electronic Properties of Cubic KNbO3 (001) Surfaces: A First-Principles Study. Applied Surface Science 2015, 351, 558. [57] Angel, V.; Maurizio, M. Towards A Generalized Vision of Oxides: Disclosing the Role

of

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and Anions

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Determining Unit-Cell Dimensions.

Acta

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Figure captions Figure 1 Schematic illustrations of the pristine CH3NH3PbI3/BaSnO3: (a) √2×√2 CH3NH3PbI3 (001)/ 2×2 BaSnO3 (001) (b) 2×2 CH3NH3PbI3 (001)/ 3×3 BaSnO3 (001) (c) 2√2×√2 CH3NH3PbI3 (011)/ 4×2 BaSnO3 (001). Figure 2 Planar and macroscopic averaged electrostatic potential across the optimized pristine CH3NH3PbI3/BaSnO3: (a) 2×2 BaO/PbI2 (b) 2×2 SnO2/PbI2 (c) 3×3 SnO2/PbI2 (d) 4×2 SnO2/PbI2. Figure 3 Electronic band structures and PDOS of the optimized pristine CH3NH3PbI3/BaSnO3: (a) 2×2 BaO/PbI2 (b) 2×2 SnO2/PbI2 (c) 3×3 SnO2/PbI2 (d) 4×2 SnO2/PbI2. Figure

4

Schematic

illustrations

of

lanthanum

dopant

positioning

at

the

CH3NH3PbI3/BaSnO3: (a) √2×√2 CH3NH3PbI3 (001)/ 2×2 BaSnO3 (001) (b) 2×2 CH3NH3PbI3 (001)/ 3×3 BaSnO3 (001) (c) 2√2×√2 CH3NH3PbI3 (011)/ 4×2 BaSnO3 (001). Figure 5 Planar and macroscopic averaged electrostatic potential across the optimized Ladoped CH3NH3PbI3/BaSnO3: (a) 2×2 SnO2/PbI2 sub-surface doping (b) 2×2 SnO2/PbI2 surface doping (c) 3×3 SnO2/PbI2 surface doping (d) 4×2 SnO2/PbI2 surface doping.

Figure 6 LDOS of the optimized La-doped CH3NH3PbI3/BaSnO3: (a) 2×2 SnO2/PbI2 sub25



surface doping (b) 2×2 SnO2/PbI2 surface doping (c) 3×3 SnO2/PbI2 surface doping (d) 4×2 SnO2/PbI2 surface doping. Figure 7 ELF of the optimized CH3NH3PbI3/BaSnO3: (a) 2×2 SnO2/PbI2 sub-surface doping (b) 2×2 SnO2/PbI2 surface doping (c) 3×3 SnO2/PbI2 surface doping (d) 4×2 SnO2/PbI2 surface doping. Figure 8 Absorption coefficients of the optimized La-doped CH3NH3PbI3/BaSnO3. Figure 9 Schematic for the band alignment diagram of La-doped CH3NH3PbI3/BaSnO3.

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Table captions Table 1 Lattice constants (in Å) of cubic perovskite and BaSnO3. Table 2 Surface energy (in eV/A2) of CH3NH3PbI3 and BaSnO3 (001). Table 3 Interfacial binding energy per unit cell (in eV) of the optimized pristine CH3NH3PbI3/BaSnO3. Table 4 Interfacial binding energy per unit cell (in eV) of the optimized La-doped CH3NH3PbI3/BaSnO3. Table 5 Interfacial charge transfer (in e) of the optimized CH3NH3PbI3/BaSnO3.

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Fig. 1

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Page 29 of 42

10

Plane-averaged electrostatic potential (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

BaSnO3

Perovskite

Vacuum

(a) 2 ×2 BaO/PbI2

-10 10 0

(b)

10

20

30

2×2 SnO2/PbI2

-10 10

(c)

10

20

30

0

3×3 SnO2/PbI2

-10 10 0

(d)

10

20

30

4 ×2

SnO2/PbI2

-10 0

10

20

30

Distance (Å)

Fig. 2

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4

(a) BaO-PbI2 2×2

Ba Sn O

MA Pb I

(b) SnO2-PbI2 2×2

Ba Sn O

MA Pb I

Ba Sn O

MA Pb I

Ba Sn O

MA Pb I

2

Density of States (1/eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4 2 4

(c) SnO2-PbI2 3×3

2 4 (d) SnO2-PbI2 4×2 2

0

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-5

0 Energy (eV)

5

Fig. 3

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Fig. 4

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BaSnO3 Plane-averaged electrostatic potential (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

10

Perovskite

Vacuum

(a) 2× 2

0

sub-surface

-10 10 0

(b)

10

20

30

surface

(c)

10

20

30

0

0

surface

(d)

10

20

30

4× 2

-10 0

40

3× 3

-10 10

40

2×2

-10 10

Page 32 of 42

surface 10

20

Distance (Å)

30

Fig. 5

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10

(a) 2× 2 sub-surface

La Ba Sn O

MA Pb I

La Ba Sn O

MA Pb I

5 10

Density of States (1/eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b) 2×2 surface

5 10

La Ba Sn O

(c) 3×3 surface

5 10

La Ba Sn O

(d) 4×2 surface

5 0

-10

-5

0

MA Pb I

MA Pb I

5

Energy (eV)

Fig. 6

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Fig. 7

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1.5 Optical Adsorption

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2 2 sub-surface 2 2 surface 33 surface 42 surface

1.0

0.5

2

4

6

8

10

Energy (eV) Fig. 8

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Fig. 9

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lattice

CH3NH3PbI3

BaSnO3

a

8.80

4.19

c

13.05

4.19

Table 1

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surface

Esurface

MAI-terminated (001)

0.013

PbI2-terminated (001)

0.008

BaO-terminated (001)

0.029

SnO2-terminated (001)

0.031

Table 2

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BaO/MAI

BaO/PbI2

SnO2/MAI

SnO2/PbI2

TiO2/PbI2

2×2

-0.36

-0.24

-1.66

1.30

-5.86

3×3

1.15

1.55

1.77

2.16

-

4×2

-2.23

-0.65

-2.08

-0.11

-

Table 3

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surface doping

sub-surface doping

2×2

0.51

0.84

3×3

2.30

-

4×2

-1.17

-

Table 4

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2×2

3×3

4×2

pristine

-0.86

-0.11

-1.03

La-doped

-0.80

-0.12

-0.97

Table 5

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Table of Contents Graphic

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Structural, Electronic, and Optical Characterizations of the Interface

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