- Email: [email protected]
titania interfaces as a function of the interlayer composition: A theoretical study
Journal Pre-proof Characterization of halide perovskite/titania interfaces as a function of the interlayer composition: A theoretical study Kazhal Shalmashi, Heidar Khosravi, Arash Boochani, Yavar T. Azar
PII: DOI: Reference:
S0022-3697(19)30154-4 https://doi.org/10.1016/j.jpcs.2019.109243 PCS 109243
To appear in:
Journal of Physics and Chemistry of Solids
Received date : 20 February 2019 Revised date : 24 October 2019 Accepted date : 25 October 2019 Please cite this article as: K. Shalmashi, H. Khosravi, A. Boochani et al., Characterization of halide perovskite/titania interfaces as a function of the interlayer composition: A theoretical study, Journal of Physics and Chemistry of Solids (2019), doi: https://doi.org/10.1016/j.jpcs.2019.109243. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.
Journal Pre-proof
of
Characterization of halide perovskite/titania interfaces as a function of the interlayer composition: a theoretical study
pro
Kazhal Shalmashi1 , Heidar Khosravi1,∗, Arash Boochani1 , Yavar T. Azar2
Abstract
In this study, we investigated the electronic structure of halide perovskite/titania interfaces using first-principle methods. Based on the different possible termina-
re-
tions of methylammonium (MA)PbI3 slabs, we investigated two different models comprising MAI/titania and PbI/titania interfaces. In the case of MAI/titania, a new interface model was built due to the possibility of MA cation deprotonation. We also analyzed the key role of the built-in electric field strength in the
urn al P
cell efficiency and the significant effect of deprotonation on the level alignment. Moreover, we investigated band gap narrowing and level alignment as functions of the compositions of the outermost layers. Our findings may have applications in band engineering and they provide a deeper understanding of the atomistic characteristics of perovskite-based photovoltaic and optoelectronic devices. Keywords: Band engineering, Density functional theory, Halide perovskite, Interface
1. Introduction
In recent years, photovoltaic cells based on organic–inorganic metal halide perovskites have been investigated because of their specific properties, such as high efficiency, and their unique optical and electronic properties with respect
Jo
to the conversion of sunlight into electricity [1, 2]. Special properties such as ∗ Corresponding
author: [email protected] of Physics, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran 2 Physics and Accelerators School, NSTRI, Tehran, Iran 1 Department
Preprint submitted to Journal of LATEX Templates
October 24, 2019
Journal Pre-proof
a direct band gap, high optical coefficient, long carrier lifetime, appropriate diffusion length for electrons and holes, and high carrier mobility [3, 4] make
of
this class of materials suitable choices for photovoltaic cells and optoelectronic devices [3, 5, 6]. The power conversion efficiency (PCE) of these materials exceeded 23% in 2019 [7, 8], which is only one decade after the first study
pro
by Miyasaka who achieved about 3.8% in 2009 [5, 9, 10]. This family of hybrid semiconductors has ABX3 stoichiometry [11, 12, 13], where A denotes an organic cation such as methylammonium (MA) or formamidinium (FA), B indicates a metal comprising Pb or Sn, and X is a halide (I, Br, or Cl) [6, 14, 15, 16, 17]. Simple construction and low manufacturing costs are the main motivations for
techniques [18, 19].
re-
research into the optimal compounds and the development of new fabrication
Many efforts have been made to manufacture layered structures to achieve the best PCE in solar cells based on perovskites [20, 21], where alternating toxic components, thermal stability, and engineering the junctions between per-
urn al P
ovskites and transport layers are the most important issues in this area [22, 23]. Among the challenges that need to be addressed to improve the efficiency of cells, the structure of the junctions formed between electron (hole) transport layers and halide perovskites appears to be the most important [24]. The alignment of the valence and conduction band edges in the perovskite relative to the titania bands has a critical role in the direct injection of charge from the active material to the transporting substrate [19]. Many variations of the atomic configurations of these interfaces can be obtained according to the different crystallographic phases of titanium oxide (rutile, anatase, and brookite) and the possible terminations of the perovskite slabs [25, 26]. Geng et al. [27] studied four different interfaces based on PbI (MAI) terminations and the anatase (rutile) phases of titanium oxide. Their calcula-
Jo
tions indicated that among all of the possibilities considered, appropriate cell matching and the formation of bridge bonds leads to more stable configurations in the specific perovskite/rutile interfaces. The built-in electric field present in these heterojunctions is an essential parameter for ensuring the correct level 2
Journal Pre-proof
alignment between perovskite and substrate, where there is a strong correlation between the normal component of this field and the electron transfer rate at the
of
interface [26, 28, 29]. Considering the problems described above, in the present study, we systematically investigated the electronic structure of the most stable MAPbI3 /rutile
pro
interfaces, particularly the interface with a deprotonated MA cation on the titania surface. First, we characterized the geometrical and electronic structures of two PbI- and MAI-terminated slabs, before analyzing the formation of the built-in potential and charge transfer at the interface in detail. Starting from several different configurations, we determined the most stable interfaces in-
re-
volving a rarely considered deprotonated organic cation and utilized them in further investigations of band alignment. Throughout this study, we use the terms X/titania (X = PbI, MAI, MAIdep) to refer to the interfaces formed via the deposition of X-terminated slabs on the titania (rutile) surface and MAIdep denotes the MAI-terminated slab that contains the deprotonated MA.
urn al P
In Section 2, we explain the computational approaches employed in this study. In Section 3, we present the results for the geometrical and electronic structure of the interfaces. In Section 4, we give our conclusions.
2. Computational methods
All of the electronic structures were calculated based on density functional theory [30] and the self-consistent solutions of the Kohn–Sham equations [31]. The cell parameters and atomic positions were optimized using the SIESTA package [32], where the generalized gradient approximation and the Perdew– Burke–Ernzerhof functional [33] were employed to describe the exchange-correlation term. Moreover, the norm-conserving Troullier–Martins pseudopotentials [34]
Jo
for each atomic species were used to consider the interactions of the core-valence electrons. The polarized double-zeta basis set with an energy shift of 50 meV and a mesh cutoff of 200 Ry was used for these calculations. The electronic structures for the optimized geometries were calculated with
3
Journal Pre-proof
the Quantum-ESPRESSO [35, 36] package using a 6 × 6 × 1 Monkhorst–Pack grid for sampling the Brillouin zone. Ultrasoft pseudopotentials were used to
of
describe electron–ion interactions and the plane-wave basis set cutoffs for the wave functions and density were set to 30 and 240 Ry, respectively.
Selecting a standard unique reference potential is a vital step when calculat-
pro
ing the band shifts for periodic slabs. In particular, the planar average potential is calculated as a function of z and the asymptotic value of the potential V (∞) (a point far from the surfaces in a vacuum region) is treated as the reference
(1)
re-
value. The plane-averaged potential is defined as: Z 1 V¯ (z) = V (x, y, z)dxdy, S supercell where S is the surface area of the supercell.
The charge transfer between the substrate and deposited layer is one of the main components of the interlayer dipole and field. Thus, we calculated the averaged charge redistribution after the formation of the interface. Considering
urn al P
the periodic condition in the (x, y) plane, the charge was averaged over this plane and the resulting laterally averaged charge density calculated as: Z 1 ρ¯(z) = ρ(r)dxdy, S
(2)
was employed in a more detailed analysis of the charge displacement. The averaged charges were calculated for the interface as well as for the stand-alone titania and MAPbI3 surfaces at the same relative ionic positions, and their difference was used as a charge transfer descriptor.
3. Results and discussion
The electronic structures calculated for the bulk structures clearly showed
Jo
that the valence band mainly comprised iodine 5p orbitals and the contribution to the conduction band was generally from the 6p orbitals of the lead atoms [9, 37]. In a typical hybrid perovskite, the organic cations have no notable contributions near the band gap [5, 38, 39]. 4
Journal Pre-proof
MAPbI3 crystallizes in the tetragonal phase at room temperature (160-330 ◦
K) [9, 40] with the I4/mcm space group [41, 42]. Two distinct atomic layers
of
in this structure in the [001] direction lead to different possible terminations comprising the so-called MAI-terminated and PbI-terminated slabs, which have different surface properties [25, 43]. These slabs contain excess PbI2 or MAI
pro
layers, and have a non-zero electric dipole at their surfaces, which leads to a significant shift in the conduction and valence bands. The changes in the electronic structures of the slabs were calculated and the results are presented in the following. 3.1. PbI- and MAI-terminated slabs
re-
First, all of the geometrical and electronic parameters were calculated for both the MAI-terminated and PbI-terminated slabs. An apolar model was employed for the orientation of the MA cations to cancel the total dipole moment of the slabs in the z-direction. In this model, two MA cations in each atomic layer
urn al P
were oriented in positive directions and two of them were oriented in negative directions. This model yielded a net zero dipole moment in the z-direction and a unique well-defined value for the asymptotic potential at the two sides of the slab, which could be used as a reference level for the work function calculation [44].
The planar averaged electrostatic potentials calculated for both slabs are shown in Fig. 1(a). According to the averaged potentials, there was a difference of ∼ 1.5 eV between the asymptotic potentials for the two layers, which indicates that the surface geometry had an obvious effect on the work functions of the slabs. The shift in the vacuum level was due to the opposite direction of the surface dipoles for the PbI- and MAI-terminated slabs. In the next step, we treated the vacuum level as the energy reference and
Jo
calculated the total density of states (DOS), as shown in Fig. 1(b). The significant effects of termination on the conduction band edge and valence band edge were deduced by inspecting the DOS plots. Furthermore, the Kohn–Sham band gap decreased from 1.9 eV to 1.35 eV from the MAI-terminated to the 5
Journal Pre-proof
PbI-terminated slab, which reflects the sensitivity of the optical property to the surface termination of these slabs. These results are in good agreement with
urn al P
re-
pro
of
those reported previously [24, 45].
Fig. 1. Averaged electrostatic potential energy (a) and total density of states (b) for MAIterminated (blue) and PbI-terminated (orange) slabs.
Based on the results given above, a notable change in the electronic structure of these thin layers can be expected after the formation of junctions with other materials. Thus, we investigated some important aspects of the MAPbI3 /titania (rutile) interface, as described in the following. 3.2. Geometrical parameters of interfaces
Two different terminations comprising MAI- and PbI-terminated slabs were examined for building the interface and the structures obtained were employed in the subsequent analysis. Beginning with several relative displacements of
Jo
the perovskite and titania layers (in the ab plane), different configurations were built and optimized to ensure that the most stable was achieved for each case. After optimizing the atomic positions of these distinct structures, the three most stable geometries were obtained, as shown in Fig. 2.
6
pro
of
Journal Pre-proof
Fig. 2. Interface geometries obtained for PbI/titania (a), MAI/titania (b), and deprotonated
re-
MAI/titania (c).
As shown in Fig. 2, starting from different PbI-terminated structures led to a unique geometry, whereas the partial deprotonation of MA cations at the titania surface yielded two distinct final structures in the case of some MAI-
urn al P
terminated interfaces. This partial deprotonation of MA cations at the titania surface is associated with the formation of the volatile methylamine molecule and it can drastically affect the stability of halide perovskites. The light-induced degradation of the MAPbI3 structure via the formation of CH3 NH2 is one of the main issues that determine the stability of these systems [46]. It should be noted that the first two geometries for PbI- and MAI-terminated interfaces have been reported previously [27], but to the best of our knowledge, the deprotonation of MA on the titania surface and its effect on band alignment were not considered.
According to the scheme described above and given the presence of relatively strong Pb-O and I-Ti bonds at the interface, the PbI/titania interface exhibited better matching between the two outermost layers of titania and the perovskite
Jo
surfaces. In this case, the iodine atoms on the perovskite surface were connected to the TiO2 layer, where they matched closely with the four-fold coordinated titanium atoms on the rutile surface.
7
Journal Pre-proof
By contrast, due to the lack of Pb atoms in the outermost layer of the MAI-terminated surface, only two weak I-Ti bonds were formed per unit cell.
of
Thus, irrespective of any more complicated analysis, the PbI-terminated slab formed more stable interfaces with the rutile (001) substrate. However, under realistic conditions, surface defects and the synthesis parameters do not exclude
pro
the possibility of the other geometries at the interface [24]. Next, we calculated the electronic structures for all three of these distinct structures. 3.3. Electronic structures of the interfaces
We calculated the total DOS and their projections over the atomic orbitals, and they are plotted in Fig. 3 where the energy reference was taken at the
re-
vacuum level (Fermi energy) for comparisons of the total DOS (projected DOS) plots. Analogous to the perovskite slabs, MAI/titania interface had the largest gap (∼1.90 eV) and the PbI/titania interface had the narrowest one (∼1.5 eV). Decomposing the total DOS into the contributions from the PbI2 , MA, and
urn al P
TiO2 atomic orbitals yielded a better understanding of the effects of the composition of the interface on the electronic structures of these systems. In all cases, the atomic orbitals of titania and lead made the dominant contributions to the conduction band edge, and changing the interface components determined the distance between the conduction band edge (titania projected DOS) and the projected DOS over the Pb atoms. The distance between the lower edges of the Pb-projected and titania-projected DOS plots indicated whether the band alignment was appropriate or inappropriate for the efficient charge transfer from perovskite to titania. Similar to the bulk perovskites, the MA cation had no direct effect on the density near the band gap [47, 48] but its deprotonation could significantly change both the band gap and overall shape of the DOS for the MAI/titania interface. These effects can be attributed to the proton-induced
Jo
downshift of the conduction band edge for TiO2 compared with the MAI/titania system. The downshift reduced the distance between the conduction band edge for TiO2 and the valence band edge for perovskite, thereby narrowing the band gap for the interface. A similar band realignment due to the adsorption of pro8
re-
pro
of
Journal Pre-proof
Fig. 3. Comparison of the total DOS determined for the three geometries considered (a). Projection of DOS over atomic orbitals for the PbI framework, organic cation, and substrate
urn al P
for PbI titania (b) MAI/titania (c), and deprotonated MAI/titania (d).
tons on the titania surface was reported for dye-sensitized solar cells [49], which we investigated further in the fat band analyses of these systems. The average electrostatic potential along the z-direction is depicted in Fig. 4. The slope of V¯ (z) in the vacuum region indicated the formation of an effective dipole moment normal to the interface plane. This dipole could be assigned to either the charge transfer between two layers via bond formation or geometrical displacements breaking the symmetry of the starting slabs. Regardless of its origin, the opposite sign of the potential slope in the vacuum region for the MAI/titania and PbI/titania interfaces clearly indicated the opposite shift of the conduction band for perovskite with respect to that for titania in these interfaces. Thus, the difference between the asymptotic averaged potential at the
Jo
right and the left sides of the slab, i.e., ∆V¯ = V (∞) − V (−∞), yielded a quantitative description of the built-in potential at the interface. For PbI/titania, MAI/titania, and MAIdep/titania the values were calculated as +0.29, –0.50, and –0.28 eV, respectively. The signs and magnitudes of these values explain
9
Fig. 4.
pro
of
Journal Pre-proof
Averaged electrostatic potential energy for PbI/titania (dot dashed green),
re-
MAI/titania (dashed cyan), and MAIdep/titania (solid violet).
the band edge alignment in PbI/titania after the formation of the interface compared with the band alignment in the clean slabs in their isolated form.
urn al P
According to these results, the interfacial region experienced an electric field of about ∼ 0.001 a.u. and this caused significant Rashba splitting in the conduction and valence bands of the halide perovskites [6, 24, 50, 51]. Rashba band splitting can change the typical direct gap for perovskites into an indirect gap and slow down the recombination rate for electron–hole pairs at the band edges. This increase in the electron–hole lifetime will lead to the accumulation of carriers, thereby enhancing the rate of charge transfer to the substrate [52]. In order to obtain a better understanding of the impact of interface formation on the charge redistribution, we calculated the difference in the charge density between the interface and its components, i.e., ∆ρ, as well as its isosurface along with planar average, as shown in Fig. 5. The plot indicates that charge accumulation occurred in the interface region together with significant charge
Jo
polarization in the outermost layers of the perovskite. This charge redistribution plays an important role in the formation of the built-in potential and band realignment as a consequence. The detailed band structures were calculated for these three model systems,
10
re-
pro
of
Journal Pre-proof
Fig. 5. Average charge displacement along the normal direction (a) and three-dimensional charge difference isosurface (b). The grey interval denotes the interface region. The yellow
urn al P
and cyan regions represent charge depletion and accumulation, respectively.
as shown in Fig. 6. For all of the interfaces, the band structure is shown as a fat band presentation, where the contributions of iodine and lead to the band structure of the interface are highlighted. When considered with the projected DOS, it is clear from the fat bands that the interface atomic composition had a significant effect on the shape and configuration of the band structure near the gap region.
Based on the fat band representations, it is clear that in all cases, the valence bands comprised iodine p orbitals, whereas the bottom bands in the conduction region were made of different combinations of titania and lead atoms. In each case, the distance between the bottom of the lead-projected bands and the conduction band edge can be interpreted as a measure of the appropriate
Jo
band alignment in these systems. The energy difference between the bottom of the lead-projected bands and the titania-projected conduction bands determines the driving force for the injection of electrons from perovskite into the titania
11
Journal Pre-proof
substrate. According to this discussion and the calculated fat bands (Fig. 6), the PbI/titania interface is a more suitable heterojunction for efficient charge
of
transfer from perovskite to the transport layer. Moreover, our comparison of the electronic structures in the MAI/titania interface before and after MA deprotonation indicated that the band alignment was better in the deprotonated state.
pro
The adsorption of protons on the TiO2 surface stabilized the conduction band of the substrate but without significantly affecting the position of the perovskite band edges, thereby resulting in a band alignment that was more suitable for charge transfer. Schematic comparisons of the band alignment in the different interfaces are shown in Fig. 6. It should be noted that this increase in the
re-
current density of the protonated MA was accompanied by the negative effect of the voltage drop and it did not necessarily lead to an improved PCE in this case.
Based on our findings, we can conclude that the energy level alignment in the halide perovskite/titania interfaces is sensitive to the interlayer composition.
urn al P
This deeper understanding of the relationship between the composition and band alignment may allow us to tune a desirable open circuit voltage (Voc ) to achieve higher PCEs.
4. Conclusion
In this study, we investigated the electronic structure of the MAPbI3 /titania interface as a function of the interlayer composition using the density functional theory framework. Our comparison of the structures produced with different terminations on perovskite indicated the higher stability and better level alignment of the PbI/titania interface. We calculated the built-in potential profiles for different interfaces and visualized them along the normal direction with re-
Jo
spect to the interface. Despite the better level alignment of MAI-terminated slabs before the formation of junctions, our analysis of the potential profiles indicated that PbI/titania had a more suitable band alignment and a larger driving force for electron transfer after the formation of the interface. Finally,
12
urn al P
re-
pro
of
Journal Pre-proof
Fig. 6. Band structures for PbI/titania (a), MAI/titania (b), and MAI/titania with a deprotonated MA group (c). The fat band representations are presented to show the projected
Jo
weights on the iodine atoms (red) and lead atoms (violet).
13
Journal Pre-proof
the proton-induced downshift of the conduction band for TiO2 explained the
of
significant effect of MA deprotonation on the level alignment at the interface.
References
[1] S. Yang, W. Fu, Z. Zhang, H. Chen, C.-Z. Li, Recent advances in perovskite
pro
solar cells: efficiency, stability and lead-free perovskite, Journal of Materials Chemistry A 5 (23) (2017) 11462–11482.
[2] S. Stranks, Sd stranks, ge eperon, g. grancini, c. menelaou, mjp alcocer, t. leijtens, lm herz, a. petrozza, and hj snaith, science 342, 341 (2013).,
re-
Science 342 (2013) 341.
[3] L. M. Herz, Charge-carrier dynamics in organic-inorganic metal halide perovskites, Annual review of physical chemistry 67 (2016) 65–89. [4] T. Wang, B. Daiber, J. M. Frost, S. A. Mann, E. C. Garnett, A. Walsh,
urn al P
B. Ehrler, Indirect to direct bandgap transition in methylammonium lead halide perovskite, Energy & Environmental Science 10 (2) (2017) 509–515. [5] P. Umari, E. Mosconi, F. De Angelis, Relativistic gw calculations on ch 3 nh 3 pbi 3 and ch 3 nh 3 sni 3 perovskites for solar cell applications, Scientific reports 4 (2014) 4467.
[6] J. S. Manser, J. A. Christians, P. V. Kamat, Intriguing optoelectronic properties of metal halide perovskites, Chemical reviews 116 (21) (2016) 12956–13008.
[7] D. Yang, R. Yang, K. Wang, C. Wu, X. Zhu, J. Feng, X. Ren, G. Fang, S. Priya, S. F. Liu, High efficiency planar-type perovskite solar cells with
Jo
negligible hysteresis using edta-complexed sno 2, Nature communications 9 (1) (2018) 3239.
[8] W. Ke, M. G. Kanatzidis, Prospects for low-toxicity lead-free perovskite solar cells, Nature communications 10 (1) (2019) 965.
14
Journal Pre-proof
[9] T. C. Sum, N. Mathews, Advancements in perovskite solar cells: photophysics behind the photovoltaics, Energy & Environmental Science 7 (8)
of
(2014) 2518–2534. [10] B. Lee, J. He, R. P. Chang, M. G. Kanatzidis, All-solid-state dye-sensitized
pro
solar cells with high efficiency, Nature 485 (7399) (2012) 486.
[11] G. Kieslich, S. Sun, A. K. Cheetham, Solid-state principles applied to organic–inorganic perovskites: new tricks for an old dog, Chemical Science 5 (12) (2014) 4712–4715.
[12] M. I. Dar, N. Arora, P. Gao, S. Ahmad, M. Gr¨ atzel, M. K. Nazeeruddin,
re-
Investigation regarding the role of chloride in organic–inorganic halide perovskites obtained from chloride containing precursors, Nano letters 14 (12) (2014) 6991–6996.
[13] H. S. Jung, N.-G. Park, Perovskite solar cells: from materials to devices,
urn al P
small 11 (1) (2015) 10–25.
[14] T.-B. Song, Q. Chen, H. Zhou, C. Jiang, H.-H. Wang, Y. M. Yang, Y. Liu, J. You, Y. Yang, Perovskite solar cells: film formation and properties, Journal of Materials Chemistry A 3 (17) (2015) 9032–9050. [15] W. S. Yang, J. H. Noh, N. J. Jeon, Y. C. Kim, S. Ryu, J. Seo, S. I. Seok, High-performance photovoltaic perovskite layers fabricated through intramolecular exchange, Science 348 (6240) (2015) 1234–1237. [16] Z.-K. Tan, R. S. Moghaddam, M. L. Lai, P. Docampo, R. Higler, F. Deschler, M. Price, A. Sadhanala, L. M. Pazos, D. Credgington, et al., Bright light-emitting diodes based on organometal halide perovskite, Nature nan-
Jo
otechnology 9 (9) (2014) 687–692.
[17] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, Journal of the American Chemical Society 131 (17) (2009) 6050–6051.
15
Journal Pre-proof
[18] J. Huang, Y. Yuan, Y. Shao, Y. Yan, Understanding the physical properties of hybrid perovskites for photovoltaic applications, Nature Reviews
of
Materials 2 (7) (2017) 17042. [19] W. Yin, L. Pan, T. Yang, Y. Liang, Recent advances in interface engineering
pro
for planar heterojunction perovskite solar cells, Molecules 21 (7) (2016) 837. [20] C.-Y. Chang, C.-P. Wang, R. Raja, L. Wang, C.-S. Tsao, W.-F. Su, Highefficiency bulk heterojunction perovskite solar cell fabricated by one-step solution process using single solvent: synthesis and characterization of material and film formation mechanism, Journal of Materials Chemistry A
re-
6 (9) (2018) 4179–4188.
[21] O. Malinkiewicz, A. Yella, Y. H. Lee, G. M. Espallargas, M. Graetzel, M. K. Nazeeruddin, H. J. Bolink, Perovskite solar cells employing organic charge-transport layers, Nature Photonics 8 (2) (2014) 128.
urn al P
[22] E. M. Hutter, M. C. G´elvez-Rueda, A. Osherov, V. Bulovi´c, F. C. Grozema, S. D. Stranks, T. J. Savenije, Direct–indirect character of the bandgap in methylammonium lead iodide perovskite, Nature materials 16 (1) (2017) 115.
[23] T. Baikie, Y. Fang, J. M. Kadro, M. Schreyer, F. Wei, S. G. Mhaisalkar, M. Graetzel, T. J. White, Synthesis and crystal chemistry of the hybrid perovskite (ch 3 nh 3) pbi 3 for solid-state sensitised solar cell applications, Journal of Materials Chemistry A 1 (18) (2013) 5628–5641. [24] C. Quarti, F. De Angelis, D. Beljonne, Influence of surface termination on the energy level alignment at the ch3nh3pbi3 perovskite/c60 interface, Chemistry of Materials 29 (3) (2017) 958–968.
Jo
[25] W. Geng, C.-J. Tong, Z.-K. Tang, C. Yam, Y.-N. Zhang, W.-M. Lau, L.-M. Liu, Effect of surface composition on electronic properties of methylammonium lead iodide perovskite, Journal of Materiomics 1 (3) (2015) 213–220.
16
Journal Pre-proof
[26] M.-F. Lo, Z.-Q. Guan, T.-W. Ng, C.-Y. Chan, C.-S. Lee, Electronic structures and photoconversion mechanism in perovskite/fullerene heterojunc-
of
tions, Advanced Functional Materials 25 (8) (2015) 1213–1218. [27] W. Geng, C.-J. Tong, J. Liu, W. Zhu, W.-M. Lau, L.-M. Liu, Structures and electronic properties of different ch 3 nh 3 pbi 3/tio 2 interface: A
pro
first-principles study, Scientific reports 6 (2016) 20131.
[28] J. Yin, D. Cortecchia, A. Krishna, S. Chen, N. Mathews, A. C. Grimsdale, C. Soci, Interfacial charge transfer anisotropy in polycrystalline lead iodide perovskite films, The journal of physical chemistry letters 6 (8) (2015)
re-
1396–1402.
[29] H. Zhou, Q. Chen, G. Li, S. Luo, T.-b. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu, Y. Yang, Interface engineering of highly efficient perovskite solar cells, Science 345 (6196) (2014) 542–546.
urn al P
[30] P. Hohenberg, W. Kohn, Inhomogeneous electron gas, Physical Review 136 (1964) B864–B871.
[31] W. Kohn, L. J. Sham, Self-consistent equations including exchange and correlation effects, Physical Review 140 (4A) (1965) A1133. [32] J. M. Soler, E. Artacho, J. D. Gale, A. Garcia, J. Junquera, P. Ordejon, D. Sanchez-Portal, The siesta method for ab initio order-n materials simulation, Journal of Physics: Condensed Matter 14 (11) (2002) 2745. [33] J. P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Physical Review Letters 77 (1996) 3865–3868. [34] N. Troullier, J. L. Martins, Efficient pseudopotentials for plane-wave cal-
Jo
culations, Physical review B 43 (3) (1991) 1993. [35] P. Giannozzi, S. Baroni, N. Bonini, M. Calandra, R. Car, C. Cavazzoni, D. Ceresoli, G. L. Chiarotti, M. Cococcioni, I. Dabo, A. Dal
17
Journal Pre-proof
Corso, S. de Gironcoli, S. Fabris, G. Fratesi, R. Gebauer, U. Gerstmann, C. Gougoussis, A. Kokalj, M. Lazzeri, L. Martin-Samos, N. Marzari,
of
F. Mauri, R. Mazzarello, S. Paolini, A. Pasquarello, L. Paulatto, C. Sbraccia, S. Scandolo, G. Sclauzero, A. P. Seitsonen, A. Smogunov, P. Umari, R. M. Wentzcovitch, Quantum espresso: a modular and open-source soft-
pro
ware project for quantum simulations of materials, Journal of Physics: Condensed Matter 21 (39) (2009) 395502 (19pp). URL http://www.quantum-espresso.org
[36] P. Giannozzi, O. Andreussi, T. Brumme, O. Bunau, M. B. Nardelli, M. Calandra, R. Car, C. Cavazzoni, D. Ceresoli, M. Cococcioni, N. Colonna,
re-
I. Carnimeo, A. D. Corso, S. de Gironcoli, P. Delugas, R. A. D. Jr, A. Ferretti, A. Floris, G. Fratesi, G. Fugallo, R. Gebauer, U. Gerstmann, F. Giustino, T. Gorni, J. Jia, M. Kawamura, H.-Y. Ko, A. Kokalj, E. K¨ u¸cu ¨kbenli, M. Lazzeri, M. Marsili, N. Marzari, F. Mauri, N. L. Nguyen,
urn al P
H.-V. Nguyen, A. O. de-la Roza, L. Paulatto, S. Ponc´e, D. Rocca, R. Sabatini, B. Santra, M. Schlipf, A. P. Seitsonen, A. Smogunov, I. Timrov, T. Thonhauser, P. Umari, N. Vast, X. Wu, S. Baroni, Advanced capabilities for materials modelling with quantum espresso, Journal of Physics: Condensed Matter 29 (46) (2017) 465901. URL http://stacks.iop.org/0953-8984/29/i=46/a=465901 [37] T. Umebayashi, K. Asai, T. Kondo, A. Nakao, Electronic structures of lead iodide based low-dimensional crystals, Physical Review B 67 (15) (2003) 155405.
[38] P. P. Boix, K. Nonomura, N. Mathews, S. G. Mhaisalkar, Current progress and future perspectives for organic/inorganic perovskite solar cells, Mate-
Jo
rials today 17 (1) (2014) 16–23.
[39] Y. Zhao, K. Zhu, Organic–inorganic hybrid lead halide perovskites for optoelectronic and electronic applications, Chemical Society Reviews 45 (3) (2016) 655–689. 18
Journal Pre-proof
[40] Y. Chang, C. Park, K. Matsuishi, First-principles study of the structural and the electronic properties of the lead-halide-based inorganic-organic per-
of
ovskites (ch˜ 3nh˜ 3) pbx˜ 3 and cspbx˜ 3 (x= cl, br, i), Journal-Korean Physical Society 44 (2004) 889–893.
[41] M. T. Weller, O. J. Weber, P. F. Henry, A. M. Di Pumpo, T. C. Hansen,
pro
Complete structure and cation orientation in the perovskite photovoltaic methylammonium lead iodide between 100 and 352 k, Chemical Communications 51 (20) (2015) 4180–4183.
[42] Y. Kawamura, H. Mashiyama, K. Hasebe, Structural study on cubictetragonal transition of ch 3 nh 3 pbi 3, Journal of the Physical Society
re-
of Japan 71 (7) (2002) 1694–1697.
[43] E. Mosconi, E. Ronca, F. De Angelis, First-principles investigation of the tio2/organohalide perovskites interface: The role of interfacial chlorine,
urn al P
The journal of physical chemistry letters 5 (15) (2014) 2619–2625. [44] L. Bengtsson, Dipole correction for surface supercell calculations, Physical Review B 59 (1999) 12301–12304.
[45] D. O. Scanlon, C. W. Dunnill, J. Buckeridge, S. A. Shevlin, A. J. Logsdail, S. M. Woodley, C. R. A. Catlow, M. J. Powell, R. G. Palgrave, I. P. Parkin, et al., Band alignment of rutile and anatase tio 2, Nature materials 12 (9) (2013) 798.
[46] G. Abdelmageed, L. Jewell, K. Hellier, L. Seymour, B. Luo, F. Bridges, J. Z. Zhang, S. Carter, Mechanisms for light induced degradation in mapbi3 perovskite thin films and solar cells, Applied Physics Letters 109 (23) (2016) 233905.
Jo
[47] Q. Chen, N. De Marco, Y. M. Yang, T.-B. Song, C.-C. Chen, H. Zhao, Z. Hong, H. Zhou, Y. Yang, Under the spotlight: The organic–inorganic hybrid halide perovskite for optoelectronic applications, Nano Today 10 (3) (2015) 355–396. 19
Journal Pre-proof
[48] F. Wei, Z. Deng, S. Sun, F. Xie, G. Kieslich, D. M. Evans, M. A. Carpenter, P. D. Bristowe, A. K. Cheetham, The synthesis, structure and electronic
of
properties of a lead-free hybrid inorganic–organic double perovskite (ma) 2 kbicl 6 (ma= methylammonium), Materials Horizons 3 (4) (2016) 328–332. [49] F. De Angelis, S. Fantacci, A. Selloni, M. Gr¨ atzel, M. K. Nazeeruddin,
pro
Influence of the sensitizer adsorption mode on the open-circuit potential of dye-sensitized solar cells, Nano letters 7 (10) (2007) 3189–3195. [50] C. Quarti, E. Mosconi, F. De Angelis, Interplay of orientational order and electronic structure in methylammonium lead iodide: implications for solar
re-
cell operation, Chemistry of Materials 26 (22) (2014) 6557–6569. [51] F. Brivio, K. T. Butler, A. Walsh, M. Van Schilfgaarde, Relativistic quasiparticle self-consistent electronic structure of hybrid halide perovskite photovoltaic absorbers, Physical Review B 89 (15) (2014) 155204.
urn al P
[52] F. Zheng, L. Z. Tan, S. Liu, A. M. Rappe, Rashba spin–orbit coupling enhanced carrier lifetime in ch3nh3pbi3, Nano letters 15 (12) (2015) 7794–
Jo
7800.
20
Journal Pre-proof
Highlights
Jo
urn al P
re-
pro
of
. Interlayer composition and band alignment linked in perovskite/titania interfaces. .PbI-terminated perovskite/titania interface more stable than MAI/titania. .PbI/titania interface level alignment better than that for MAI/titania. .Proton-induced downshift of conduction band for TiO2 affects band alignment.
Journal Pre-proof
Conflict of Interest and Authorship Conformation Form Please check the following as appropriate: All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
o
This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.
o
The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript
o
The following authors have affiliations with organizations with direct or indirect financial interest in the subject matter discussed in the manuscript:
Author’s name Kazhal Shalmashi
Arash Boochani
Jo
Yavar T. Azar
Affiliation
Department of Physics, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran Department of Physics, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran Department of Physics, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran Physics and Accelerators School, NSTRI, Tehran, Iran
urn al P
Heidar Khosravi
re-
pro
of
o
PII: DOI: Reference:
S0022-3697(19)30154-4 https://doi.org/10.1016/j.jpcs.2019.109243 PCS 109243
To appear in:
Journal of Physics and Chemistry of Solids
Received date : 20 February 2019 Revised date : 24 October 2019 Accepted date : 25 October 2019 Please cite this article as: K. Shalmashi, H. Khosravi, A. Boochani et al., Characterization of halide perovskite/titania interfaces as a function of the interlayer composition: A theoretical study, Journal of Physics and Chemistry of Solids (2019), doi: https://doi.org/10.1016/j.jpcs.2019.109243. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.
Journal Pre-proof
of
Characterization of halide perovskite/titania interfaces as a function of the interlayer composition: a theoretical study
pro
Kazhal Shalmashi1 , Heidar Khosravi1,∗, Arash Boochani1 , Yavar T. Azar2
Abstract
In this study, we investigated the electronic structure of halide perovskite/titania interfaces using first-principle methods. Based on the different possible termina-
re-
tions of methylammonium (MA)PbI3 slabs, we investigated two different models comprising MAI/titania and PbI/titania interfaces. In the case of MAI/titania, a new interface model was built due to the possibility of MA cation deprotonation. We also analyzed the key role of the built-in electric field strength in the
urn al P
cell efficiency and the significant effect of deprotonation on the level alignment. Moreover, we investigated band gap narrowing and level alignment as functions of the compositions of the outermost layers. Our findings may have applications in band engineering and they provide a deeper understanding of the atomistic characteristics of perovskite-based photovoltaic and optoelectronic devices. Keywords: Band engineering, Density functional theory, Halide perovskite, Interface
1. Introduction
In recent years, photovoltaic cells based on organic–inorganic metal halide perovskites have been investigated because of their specific properties, such as high efficiency, and their unique optical and electronic properties with respect
Jo
to the conversion of sunlight into electricity [1, 2]. Special properties such as ∗ Corresponding
author: [email protected] of Physics, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran 2 Physics and Accelerators School, NSTRI, Tehran, Iran 1 Department
Preprint submitted to Journal of LATEX Templates
October 24, 2019
Journal Pre-proof
a direct band gap, high optical coefficient, long carrier lifetime, appropriate diffusion length for electrons and holes, and high carrier mobility [3, 4] make
of
this class of materials suitable choices for photovoltaic cells and optoelectronic devices [3, 5, 6]. The power conversion efficiency (PCE) of these materials exceeded 23% in 2019 [7, 8], which is only one decade after the first study
pro
by Miyasaka who achieved about 3.8% in 2009 [5, 9, 10]. This family of hybrid semiconductors has ABX3 stoichiometry [11, 12, 13], where A denotes an organic cation such as methylammonium (MA) or formamidinium (FA), B indicates a metal comprising Pb or Sn, and X is a halide (I, Br, or Cl) [6, 14, 15, 16, 17]. Simple construction and low manufacturing costs are the main motivations for
techniques [18, 19].
re-
research into the optimal compounds and the development of new fabrication
Many efforts have been made to manufacture layered structures to achieve the best PCE in solar cells based on perovskites [20, 21], where alternating toxic components, thermal stability, and engineering the junctions between per-
urn al P
ovskites and transport layers are the most important issues in this area [22, 23]. Among the challenges that need to be addressed to improve the efficiency of cells, the structure of the junctions formed between electron (hole) transport layers and halide perovskites appears to be the most important [24]. The alignment of the valence and conduction band edges in the perovskite relative to the titania bands has a critical role in the direct injection of charge from the active material to the transporting substrate [19]. Many variations of the atomic configurations of these interfaces can be obtained according to the different crystallographic phases of titanium oxide (rutile, anatase, and brookite) and the possible terminations of the perovskite slabs [25, 26]. Geng et al. [27] studied four different interfaces based on PbI (MAI) terminations and the anatase (rutile) phases of titanium oxide. Their calcula-
Jo
tions indicated that among all of the possibilities considered, appropriate cell matching and the formation of bridge bonds leads to more stable configurations in the specific perovskite/rutile interfaces. The built-in electric field present in these heterojunctions is an essential parameter for ensuring the correct level 2
Journal Pre-proof
alignment between perovskite and substrate, where there is a strong correlation between the normal component of this field and the electron transfer rate at the
of
interface [26, 28, 29]. Considering the problems described above, in the present study, we systematically investigated the electronic structure of the most stable MAPbI3 /rutile
pro
interfaces, particularly the interface with a deprotonated MA cation on the titania surface. First, we characterized the geometrical and electronic structures of two PbI- and MAI-terminated slabs, before analyzing the formation of the built-in potential and charge transfer at the interface in detail. Starting from several different configurations, we determined the most stable interfaces in-
re-
volving a rarely considered deprotonated organic cation and utilized them in further investigations of band alignment. Throughout this study, we use the terms X/titania (X = PbI, MAI, MAIdep) to refer to the interfaces formed via the deposition of X-terminated slabs on the titania (rutile) surface and MAIdep denotes the MAI-terminated slab that contains the deprotonated MA.
urn al P
In Section 2, we explain the computational approaches employed in this study. In Section 3, we present the results for the geometrical and electronic structure of the interfaces. In Section 4, we give our conclusions.
2. Computational methods
All of the electronic structures were calculated based on density functional theory [30] and the self-consistent solutions of the Kohn–Sham equations [31]. The cell parameters and atomic positions were optimized using the SIESTA package [32], where the generalized gradient approximation and the Perdew– Burke–Ernzerhof functional [33] were employed to describe the exchange-correlation term. Moreover, the norm-conserving Troullier–Martins pseudopotentials [34]
Jo
for each atomic species were used to consider the interactions of the core-valence electrons. The polarized double-zeta basis set with an energy shift of 50 meV and a mesh cutoff of 200 Ry was used for these calculations. The electronic structures for the optimized geometries were calculated with
3
Journal Pre-proof
the Quantum-ESPRESSO [35, 36] package using a 6 × 6 × 1 Monkhorst–Pack grid for sampling the Brillouin zone. Ultrasoft pseudopotentials were used to
of
describe electron–ion interactions and the plane-wave basis set cutoffs for the wave functions and density were set to 30 and 240 Ry, respectively.
Selecting a standard unique reference potential is a vital step when calculat-
pro
ing the band shifts for periodic slabs. In particular, the planar average potential is calculated as a function of z and the asymptotic value of the potential V (∞) (a point far from the surfaces in a vacuum region) is treated as the reference
(1)
re-
value. The plane-averaged potential is defined as: Z 1 V¯ (z) = V (x, y, z)dxdy, S supercell where S is the surface area of the supercell.
The charge transfer between the substrate and deposited layer is one of the main components of the interlayer dipole and field. Thus, we calculated the averaged charge redistribution after the formation of the interface. Considering
urn al P
the periodic condition in the (x, y) plane, the charge was averaged over this plane and the resulting laterally averaged charge density calculated as: Z 1 ρ¯(z) = ρ(r)dxdy, S
(2)
was employed in a more detailed analysis of the charge displacement. The averaged charges were calculated for the interface as well as for the stand-alone titania and MAPbI3 surfaces at the same relative ionic positions, and their difference was used as a charge transfer descriptor.
3. Results and discussion
The electronic structures calculated for the bulk structures clearly showed
Jo
that the valence band mainly comprised iodine 5p orbitals and the contribution to the conduction band was generally from the 6p orbitals of the lead atoms [9, 37]. In a typical hybrid perovskite, the organic cations have no notable contributions near the band gap [5, 38, 39]. 4
Journal Pre-proof
MAPbI3 crystallizes in the tetragonal phase at room temperature (160-330 ◦
K) [9, 40] with the I4/mcm space group [41, 42]. Two distinct atomic layers
of
in this structure in the [001] direction lead to different possible terminations comprising the so-called MAI-terminated and PbI-terminated slabs, which have different surface properties [25, 43]. These slabs contain excess PbI2 or MAI
pro
layers, and have a non-zero electric dipole at their surfaces, which leads to a significant shift in the conduction and valence bands. The changes in the electronic structures of the slabs were calculated and the results are presented in the following. 3.1. PbI- and MAI-terminated slabs
re-
First, all of the geometrical and electronic parameters were calculated for both the MAI-terminated and PbI-terminated slabs. An apolar model was employed for the orientation of the MA cations to cancel the total dipole moment of the slabs in the z-direction. In this model, two MA cations in each atomic layer
urn al P
were oriented in positive directions and two of them were oriented in negative directions. This model yielded a net zero dipole moment in the z-direction and a unique well-defined value for the asymptotic potential at the two sides of the slab, which could be used as a reference level for the work function calculation [44].
The planar averaged electrostatic potentials calculated for both slabs are shown in Fig. 1(a). According to the averaged potentials, there was a difference of ∼ 1.5 eV between the asymptotic potentials for the two layers, which indicates that the surface geometry had an obvious effect on the work functions of the slabs. The shift in the vacuum level was due to the opposite direction of the surface dipoles for the PbI- and MAI-terminated slabs. In the next step, we treated the vacuum level as the energy reference and
Jo
calculated the total density of states (DOS), as shown in Fig. 1(b). The significant effects of termination on the conduction band edge and valence band edge were deduced by inspecting the DOS plots. Furthermore, the Kohn–Sham band gap decreased from 1.9 eV to 1.35 eV from the MAI-terminated to the 5
Journal Pre-proof
PbI-terminated slab, which reflects the sensitivity of the optical property to the surface termination of these slabs. These results are in good agreement with
urn al P
re-
pro
of
those reported previously [24, 45].
Fig. 1. Averaged electrostatic potential energy (a) and total density of states (b) for MAIterminated (blue) and PbI-terminated (orange) slabs.
Based on the results given above, a notable change in the electronic structure of these thin layers can be expected after the formation of junctions with other materials. Thus, we investigated some important aspects of the MAPbI3 /titania (rutile) interface, as described in the following. 3.2. Geometrical parameters of interfaces
Two different terminations comprising MAI- and PbI-terminated slabs were examined for building the interface and the structures obtained were employed in the subsequent analysis. Beginning with several relative displacements of
Jo
the perovskite and titania layers (in the ab plane), different configurations were built and optimized to ensure that the most stable was achieved for each case. After optimizing the atomic positions of these distinct structures, the three most stable geometries were obtained, as shown in Fig. 2.
6
pro
of
Journal Pre-proof
Fig. 2. Interface geometries obtained for PbI/titania (a), MAI/titania (b), and deprotonated
re-
MAI/titania (c).
As shown in Fig. 2, starting from different PbI-terminated structures led to a unique geometry, whereas the partial deprotonation of MA cations at the titania surface yielded two distinct final structures in the case of some MAI-
urn al P
terminated interfaces. This partial deprotonation of MA cations at the titania surface is associated with the formation of the volatile methylamine molecule and it can drastically affect the stability of halide perovskites. The light-induced degradation of the MAPbI3 structure via the formation of CH3 NH2 is one of the main issues that determine the stability of these systems [46]. It should be noted that the first two geometries for PbI- and MAI-terminated interfaces have been reported previously [27], but to the best of our knowledge, the deprotonation of MA on the titania surface and its effect on band alignment were not considered.
According to the scheme described above and given the presence of relatively strong Pb-O and I-Ti bonds at the interface, the PbI/titania interface exhibited better matching between the two outermost layers of titania and the perovskite
Jo
surfaces. In this case, the iodine atoms on the perovskite surface were connected to the TiO2 layer, where they matched closely with the four-fold coordinated titanium atoms on the rutile surface.
7
Journal Pre-proof
By contrast, due to the lack of Pb atoms in the outermost layer of the MAI-terminated surface, only two weak I-Ti bonds were formed per unit cell.
of
Thus, irrespective of any more complicated analysis, the PbI-terminated slab formed more stable interfaces with the rutile (001) substrate. However, under realistic conditions, surface defects and the synthesis parameters do not exclude
pro
the possibility of the other geometries at the interface [24]. Next, we calculated the electronic structures for all three of these distinct structures. 3.3. Electronic structures of the interfaces
We calculated the total DOS and their projections over the atomic orbitals, and they are plotted in Fig. 3 where the energy reference was taken at the
re-
vacuum level (Fermi energy) for comparisons of the total DOS (projected DOS) plots. Analogous to the perovskite slabs, MAI/titania interface had the largest gap (∼1.90 eV) and the PbI/titania interface had the narrowest one (∼1.5 eV). Decomposing the total DOS into the contributions from the PbI2 , MA, and
urn al P
TiO2 atomic orbitals yielded a better understanding of the effects of the composition of the interface on the electronic structures of these systems. In all cases, the atomic orbitals of titania and lead made the dominant contributions to the conduction band edge, and changing the interface components determined the distance between the conduction band edge (titania projected DOS) and the projected DOS over the Pb atoms. The distance between the lower edges of the Pb-projected and titania-projected DOS plots indicated whether the band alignment was appropriate or inappropriate for the efficient charge transfer from perovskite to titania. Similar to the bulk perovskites, the MA cation had no direct effect on the density near the band gap [47, 48] but its deprotonation could significantly change both the band gap and overall shape of the DOS for the MAI/titania interface. These effects can be attributed to the proton-induced
Jo
downshift of the conduction band edge for TiO2 compared with the MAI/titania system. The downshift reduced the distance between the conduction band edge for TiO2 and the valence band edge for perovskite, thereby narrowing the band gap for the interface. A similar band realignment due to the adsorption of pro8
re-
pro
of
Journal Pre-proof
Fig. 3. Comparison of the total DOS determined for the three geometries considered (a). Projection of DOS over atomic orbitals for the PbI framework, organic cation, and substrate
urn al P
for PbI titania (b) MAI/titania (c), and deprotonated MAI/titania (d).
tons on the titania surface was reported for dye-sensitized solar cells [49], which we investigated further in the fat band analyses of these systems. The average electrostatic potential along the z-direction is depicted in Fig. 4. The slope of V¯ (z) in the vacuum region indicated the formation of an effective dipole moment normal to the interface plane. This dipole could be assigned to either the charge transfer between two layers via bond formation or geometrical displacements breaking the symmetry of the starting slabs. Regardless of its origin, the opposite sign of the potential slope in the vacuum region for the MAI/titania and PbI/titania interfaces clearly indicated the opposite shift of the conduction band for perovskite with respect to that for titania in these interfaces. Thus, the difference between the asymptotic averaged potential at the
Jo
right and the left sides of the slab, i.e., ∆V¯ = V (∞) − V (−∞), yielded a quantitative description of the built-in potential at the interface. For PbI/titania, MAI/titania, and MAIdep/titania the values were calculated as +0.29, –0.50, and –0.28 eV, respectively. The signs and magnitudes of these values explain
9
Fig. 4.
pro
of
Journal Pre-proof
Averaged electrostatic potential energy for PbI/titania (dot dashed green),
re-
MAI/titania (dashed cyan), and MAIdep/titania (solid violet).
the band edge alignment in PbI/titania after the formation of the interface compared with the band alignment in the clean slabs in their isolated form.
urn al P
According to these results, the interfacial region experienced an electric field of about ∼ 0.001 a.u. and this caused significant Rashba splitting in the conduction and valence bands of the halide perovskites [6, 24, 50, 51]. Rashba band splitting can change the typical direct gap for perovskites into an indirect gap and slow down the recombination rate for electron–hole pairs at the band edges. This increase in the electron–hole lifetime will lead to the accumulation of carriers, thereby enhancing the rate of charge transfer to the substrate [52]. In order to obtain a better understanding of the impact of interface formation on the charge redistribution, we calculated the difference in the charge density between the interface and its components, i.e., ∆ρ, as well as its isosurface along with planar average, as shown in Fig. 5. The plot indicates that charge accumulation occurred in the interface region together with significant charge
Jo
polarization in the outermost layers of the perovskite. This charge redistribution plays an important role in the formation of the built-in potential and band realignment as a consequence. The detailed band structures were calculated for these three model systems,
10
re-
pro
of
Journal Pre-proof
Fig. 5. Average charge displacement along the normal direction (a) and three-dimensional charge difference isosurface (b). The grey interval denotes the interface region. The yellow
urn al P
and cyan regions represent charge depletion and accumulation, respectively.
as shown in Fig. 6. For all of the interfaces, the band structure is shown as a fat band presentation, where the contributions of iodine and lead to the band structure of the interface are highlighted. When considered with the projected DOS, it is clear from the fat bands that the interface atomic composition had a significant effect on the shape and configuration of the band structure near the gap region.
Based on the fat band representations, it is clear that in all cases, the valence bands comprised iodine p orbitals, whereas the bottom bands in the conduction region were made of different combinations of titania and lead atoms. In each case, the distance between the bottom of the lead-projected bands and the conduction band edge can be interpreted as a measure of the appropriate
Jo
band alignment in these systems. The energy difference between the bottom of the lead-projected bands and the titania-projected conduction bands determines the driving force for the injection of electrons from perovskite into the titania
11
Journal Pre-proof
substrate. According to this discussion and the calculated fat bands (Fig. 6), the PbI/titania interface is a more suitable heterojunction for efficient charge
of
transfer from perovskite to the transport layer. Moreover, our comparison of the electronic structures in the MAI/titania interface before and after MA deprotonation indicated that the band alignment was better in the deprotonated state.
pro
The adsorption of protons on the TiO2 surface stabilized the conduction band of the substrate but without significantly affecting the position of the perovskite band edges, thereby resulting in a band alignment that was more suitable for charge transfer. Schematic comparisons of the band alignment in the different interfaces are shown in Fig. 6. It should be noted that this increase in the
re-
current density of the protonated MA was accompanied by the negative effect of the voltage drop and it did not necessarily lead to an improved PCE in this case.
Based on our findings, we can conclude that the energy level alignment in the halide perovskite/titania interfaces is sensitive to the interlayer composition.
urn al P
This deeper understanding of the relationship between the composition and band alignment may allow us to tune a desirable open circuit voltage (Voc ) to achieve higher PCEs.
4. Conclusion
In this study, we investigated the electronic structure of the MAPbI3 /titania interface as a function of the interlayer composition using the density functional theory framework. Our comparison of the structures produced with different terminations on perovskite indicated the higher stability and better level alignment of the PbI/titania interface. We calculated the built-in potential profiles for different interfaces and visualized them along the normal direction with re-
Jo
spect to the interface. Despite the better level alignment of MAI-terminated slabs before the formation of junctions, our analysis of the potential profiles indicated that PbI/titania had a more suitable band alignment and a larger driving force for electron transfer after the formation of the interface. Finally,
12
urn al P
re-
pro
of
Journal Pre-proof
Fig. 6. Band structures for PbI/titania (a), MAI/titania (b), and MAI/titania with a deprotonated MA group (c). The fat band representations are presented to show the projected
Jo
weights on the iodine atoms (red) and lead atoms (violet).
13
Journal Pre-proof
the proton-induced downshift of the conduction band for TiO2 explained the
of
significant effect of MA deprotonation on the level alignment at the interface.
References
[1] S. Yang, W. Fu, Z. Zhang, H. Chen, C.-Z. Li, Recent advances in perovskite
pro
solar cells: efficiency, stability and lead-free perovskite, Journal of Materials Chemistry A 5 (23) (2017) 11462–11482.
[2] S. Stranks, Sd stranks, ge eperon, g. grancini, c. menelaou, mjp alcocer, t. leijtens, lm herz, a. petrozza, and hj snaith, science 342, 341 (2013).,
re-
Science 342 (2013) 341.
[3] L. M. Herz, Charge-carrier dynamics in organic-inorganic metal halide perovskites, Annual review of physical chemistry 67 (2016) 65–89. [4] T. Wang, B. Daiber, J. M. Frost, S. A. Mann, E. C. Garnett, A. Walsh,
urn al P
B. Ehrler, Indirect to direct bandgap transition in methylammonium lead halide perovskite, Energy & Environmental Science 10 (2) (2017) 509–515. [5] P. Umari, E. Mosconi, F. De Angelis, Relativistic gw calculations on ch 3 nh 3 pbi 3 and ch 3 nh 3 sni 3 perovskites for solar cell applications, Scientific reports 4 (2014) 4467.
[6] J. S. Manser, J. A. Christians, P. V. Kamat, Intriguing optoelectronic properties of metal halide perovskites, Chemical reviews 116 (21) (2016) 12956–13008.
[7] D. Yang, R. Yang, K. Wang, C. Wu, X. Zhu, J. Feng, X. Ren, G. Fang, S. Priya, S. F. Liu, High efficiency planar-type perovskite solar cells with
Jo
negligible hysteresis using edta-complexed sno 2, Nature communications 9 (1) (2018) 3239.
[8] W. Ke, M. G. Kanatzidis, Prospects for low-toxicity lead-free perovskite solar cells, Nature communications 10 (1) (2019) 965.
14
Journal Pre-proof
[9] T. C. Sum, N. Mathews, Advancements in perovskite solar cells: photophysics behind the photovoltaics, Energy & Environmental Science 7 (8)
of
(2014) 2518–2534. [10] B. Lee, J. He, R. P. Chang, M. G. Kanatzidis, All-solid-state dye-sensitized
pro
solar cells with high efficiency, Nature 485 (7399) (2012) 486.
[11] G. Kieslich, S. Sun, A. K. Cheetham, Solid-state principles applied to organic–inorganic perovskites: new tricks for an old dog, Chemical Science 5 (12) (2014) 4712–4715.
[12] M. I. Dar, N. Arora, P. Gao, S. Ahmad, M. Gr¨ atzel, M. K. Nazeeruddin,
re-
Investigation regarding the role of chloride in organic–inorganic halide perovskites obtained from chloride containing precursors, Nano letters 14 (12) (2014) 6991–6996.
[13] H. S. Jung, N.-G. Park, Perovskite solar cells: from materials to devices,
urn al P
small 11 (1) (2015) 10–25.
[14] T.-B. Song, Q. Chen, H. Zhou, C. Jiang, H.-H. Wang, Y. M. Yang, Y. Liu, J. You, Y. Yang, Perovskite solar cells: film formation and properties, Journal of Materials Chemistry A 3 (17) (2015) 9032–9050. [15] W. S. Yang, J. H. Noh, N. J. Jeon, Y. C. Kim, S. Ryu, J. Seo, S. I. Seok, High-performance photovoltaic perovskite layers fabricated through intramolecular exchange, Science 348 (6240) (2015) 1234–1237. [16] Z.-K. Tan, R. S. Moghaddam, M. L. Lai, P. Docampo, R. Higler, F. Deschler, M. Price, A. Sadhanala, L. M. Pazos, D. Credgington, et al., Bright light-emitting diodes based on organometal halide perovskite, Nature nan-
Jo
otechnology 9 (9) (2014) 687–692.
[17] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, Journal of the American Chemical Society 131 (17) (2009) 6050–6051.
15
Journal Pre-proof
[18] J. Huang, Y. Yuan, Y. Shao, Y. Yan, Understanding the physical properties of hybrid perovskites for photovoltaic applications, Nature Reviews
of
Materials 2 (7) (2017) 17042. [19] W. Yin, L. Pan, T. Yang, Y. Liang, Recent advances in interface engineering
pro
for planar heterojunction perovskite solar cells, Molecules 21 (7) (2016) 837. [20] C.-Y. Chang, C.-P. Wang, R. Raja, L. Wang, C.-S. Tsao, W.-F. Su, Highefficiency bulk heterojunction perovskite solar cell fabricated by one-step solution process using single solvent: synthesis and characterization of material and film formation mechanism, Journal of Materials Chemistry A
re-
6 (9) (2018) 4179–4188.
[21] O. Malinkiewicz, A. Yella, Y. H. Lee, G. M. Espallargas, M. Graetzel, M. K. Nazeeruddin, H. J. Bolink, Perovskite solar cells employing organic charge-transport layers, Nature Photonics 8 (2) (2014) 128.
urn al P
[22] E. M. Hutter, M. C. G´elvez-Rueda, A. Osherov, V. Bulovi´c, F. C. Grozema, S. D. Stranks, T. J. Savenije, Direct–indirect character of the bandgap in methylammonium lead iodide perovskite, Nature materials 16 (1) (2017) 115.
[23] T. Baikie, Y. Fang, J. M. Kadro, M. Schreyer, F. Wei, S. G. Mhaisalkar, M. Graetzel, T. J. White, Synthesis and crystal chemistry of the hybrid perovskite (ch 3 nh 3) pbi 3 for solid-state sensitised solar cell applications, Journal of Materials Chemistry A 1 (18) (2013) 5628–5641. [24] C. Quarti, F. De Angelis, D. Beljonne, Influence of surface termination on the energy level alignment at the ch3nh3pbi3 perovskite/c60 interface, Chemistry of Materials 29 (3) (2017) 958–968.
Jo
[25] W. Geng, C.-J. Tong, Z.-K. Tang, C. Yam, Y.-N. Zhang, W.-M. Lau, L.-M. Liu, Effect of surface composition on electronic properties of methylammonium lead iodide perovskite, Journal of Materiomics 1 (3) (2015) 213–220.
16
Journal Pre-proof
[26] M.-F. Lo, Z.-Q. Guan, T.-W. Ng, C.-Y. Chan, C.-S. Lee, Electronic structures and photoconversion mechanism in perovskite/fullerene heterojunc-
of
tions, Advanced Functional Materials 25 (8) (2015) 1213–1218. [27] W. Geng, C.-J. Tong, J. Liu, W. Zhu, W.-M. Lau, L.-M. Liu, Structures and electronic properties of different ch 3 nh 3 pbi 3/tio 2 interface: A
pro
first-principles study, Scientific reports 6 (2016) 20131.
[28] J. Yin, D. Cortecchia, A. Krishna, S. Chen, N. Mathews, A. C. Grimsdale, C. Soci, Interfacial charge transfer anisotropy in polycrystalline lead iodide perovskite films, The journal of physical chemistry letters 6 (8) (2015)
re-
1396–1402.
[29] H. Zhou, Q. Chen, G. Li, S. Luo, T.-b. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu, Y. Yang, Interface engineering of highly efficient perovskite solar cells, Science 345 (6196) (2014) 542–546.
urn al P
[30] P. Hohenberg, W. Kohn, Inhomogeneous electron gas, Physical Review 136 (1964) B864–B871.
[31] W. Kohn, L. J. Sham, Self-consistent equations including exchange and correlation effects, Physical Review 140 (4A) (1965) A1133. [32] J. M. Soler, E. Artacho, J. D. Gale, A. Garcia, J. Junquera, P. Ordejon, D. Sanchez-Portal, The siesta method for ab initio order-n materials simulation, Journal of Physics: Condensed Matter 14 (11) (2002) 2745. [33] J. P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Physical Review Letters 77 (1996) 3865–3868. [34] N. Troullier, J. L. Martins, Efficient pseudopotentials for plane-wave cal-
Jo
culations, Physical review B 43 (3) (1991) 1993. [35] P. Giannozzi, S. Baroni, N. Bonini, M. Calandra, R. Car, C. Cavazzoni, D. Ceresoli, G. L. Chiarotti, M. Cococcioni, I. Dabo, A. Dal
17
Journal Pre-proof
Corso, S. de Gironcoli, S. Fabris, G. Fratesi, R. Gebauer, U. Gerstmann, C. Gougoussis, A. Kokalj, M. Lazzeri, L. Martin-Samos, N. Marzari,
of
F. Mauri, R. Mazzarello, S. Paolini, A. Pasquarello, L. Paulatto, C. Sbraccia, S. Scandolo, G. Sclauzero, A. P. Seitsonen, A. Smogunov, P. Umari, R. M. Wentzcovitch, Quantum espresso: a modular and open-source soft-
pro
ware project for quantum simulations of materials, Journal of Physics: Condensed Matter 21 (39) (2009) 395502 (19pp). URL http://www.quantum-espresso.org
[36] P. Giannozzi, O. Andreussi, T. Brumme, O. Bunau, M. B. Nardelli, M. Calandra, R. Car, C. Cavazzoni, D. Ceresoli, M. Cococcioni, N. Colonna,
re-
I. Carnimeo, A. D. Corso, S. de Gironcoli, P. Delugas, R. A. D. Jr, A. Ferretti, A. Floris, G. Fratesi, G. Fugallo, R. Gebauer, U. Gerstmann, F. Giustino, T. Gorni, J. Jia, M. Kawamura, H.-Y. Ko, A. Kokalj, E. K¨ u¸cu ¨kbenli, M. Lazzeri, M. Marsili, N. Marzari, F. Mauri, N. L. Nguyen,
urn al P
H.-V. Nguyen, A. O. de-la Roza, L. Paulatto, S. Ponc´e, D. Rocca, R. Sabatini, B. Santra, M. Schlipf, A. P. Seitsonen, A. Smogunov, I. Timrov, T. Thonhauser, P. Umari, N. Vast, X. Wu, S. Baroni, Advanced capabilities for materials modelling with quantum espresso, Journal of Physics: Condensed Matter 29 (46) (2017) 465901. URL http://stacks.iop.org/0953-8984/29/i=46/a=465901 [37] T. Umebayashi, K. Asai, T. Kondo, A. Nakao, Electronic structures of lead iodide based low-dimensional crystals, Physical Review B 67 (15) (2003) 155405.
[38] P. P. Boix, K. Nonomura, N. Mathews, S. G. Mhaisalkar, Current progress and future perspectives for organic/inorganic perovskite solar cells, Mate-
Jo
rials today 17 (1) (2014) 16–23.
[39] Y. Zhao, K. Zhu, Organic–inorganic hybrid lead halide perovskites for optoelectronic and electronic applications, Chemical Society Reviews 45 (3) (2016) 655–689. 18
Journal Pre-proof
[40] Y. Chang, C. Park, K. Matsuishi, First-principles study of the structural and the electronic properties of the lead-halide-based inorganic-organic per-
of
ovskites (ch˜ 3nh˜ 3) pbx˜ 3 and cspbx˜ 3 (x= cl, br, i), Journal-Korean Physical Society 44 (2004) 889–893.
[41] M. T. Weller, O. J. Weber, P. F. Henry, A. M. Di Pumpo, T. C. Hansen,
pro
Complete structure and cation orientation in the perovskite photovoltaic methylammonium lead iodide between 100 and 352 k, Chemical Communications 51 (20) (2015) 4180–4183.
[42] Y. Kawamura, H. Mashiyama, K. Hasebe, Structural study on cubictetragonal transition of ch 3 nh 3 pbi 3, Journal of the Physical Society
re-
of Japan 71 (7) (2002) 1694–1697.
[43] E. Mosconi, E. Ronca, F. De Angelis, First-principles investigation of the tio2/organohalide perovskites interface: The role of interfacial chlorine,
urn al P
The journal of physical chemistry letters 5 (15) (2014) 2619–2625. [44] L. Bengtsson, Dipole correction for surface supercell calculations, Physical Review B 59 (1999) 12301–12304.
[45] D. O. Scanlon, C. W. Dunnill, J. Buckeridge, S. A. Shevlin, A. J. Logsdail, S. M. Woodley, C. R. A. Catlow, M. J. Powell, R. G. Palgrave, I. P. Parkin, et al., Band alignment of rutile and anatase tio 2, Nature materials 12 (9) (2013) 798.
[46] G. Abdelmageed, L. Jewell, K. Hellier, L. Seymour, B. Luo, F. Bridges, J. Z. Zhang, S. Carter, Mechanisms for light induced degradation in mapbi3 perovskite thin films and solar cells, Applied Physics Letters 109 (23) (2016) 233905.
Jo
[47] Q. Chen, N. De Marco, Y. M. Yang, T.-B. Song, C.-C. Chen, H. Zhao, Z. Hong, H. Zhou, Y. Yang, Under the spotlight: The organic–inorganic hybrid halide perovskite for optoelectronic applications, Nano Today 10 (3) (2015) 355–396. 19
Journal Pre-proof
[48] F. Wei, Z. Deng, S. Sun, F. Xie, G. Kieslich, D. M. Evans, M. A. Carpenter, P. D. Bristowe, A. K. Cheetham, The synthesis, structure and electronic
of
properties of a lead-free hybrid inorganic–organic double perovskite (ma) 2 kbicl 6 (ma= methylammonium), Materials Horizons 3 (4) (2016) 328–332. [49] F. De Angelis, S. Fantacci, A. Selloni, M. Gr¨ atzel, M. K. Nazeeruddin,
pro
Influence of the sensitizer adsorption mode on the open-circuit potential of dye-sensitized solar cells, Nano letters 7 (10) (2007) 3189–3195. [50] C. Quarti, E. Mosconi, F. De Angelis, Interplay of orientational order and electronic structure in methylammonium lead iodide: implications for solar
re-
cell operation, Chemistry of Materials 26 (22) (2014) 6557–6569. [51] F. Brivio, K. T. Butler, A. Walsh, M. Van Schilfgaarde, Relativistic quasiparticle self-consistent electronic structure of hybrid halide perovskite photovoltaic absorbers, Physical Review B 89 (15) (2014) 155204.
urn al P
[52] F. Zheng, L. Z. Tan, S. Liu, A. M. Rappe, Rashba spin–orbit coupling enhanced carrier lifetime in ch3nh3pbi3, Nano letters 15 (12) (2015) 7794–
Jo
7800.
20
Journal Pre-proof
Highlights
Jo
urn al P
re-
pro
of
. Interlayer composition and band alignment linked in perovskite/titania interfaces. .PbI-terminated perovskite/titania interface more stable than MAI/titania. .PbI/titania interface level alignment better than that for MAI/titania. .Proton-induced downshift of conduction band for TiO2 affects band alignment.
Journal Pre-proof
Conflict of Interest and Authorship Conformation Form Please check the following as appropriate: All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
o
This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.
o
The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript
o
The following authors have affiliations with organizations with direct or indirect financial interest in the subject matter discussed in the manuscript:
Author’s name Kazhal Shalmashi
Arash Boochani
Jo
Yavar T. Azar
Affiliation
Department of Physics, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran Department of Physics, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran Department of Physics, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran Physics and Accelerators School, NSTRI, Tehran, Iran
urn al P
Heidar Khosravi
re-
pro
of
o