Surface Instability of Sn-based Hybrid Perovskite Thin Film

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Surface Instability of Sn-based Hybrid Perovskite Thin Film, CHNHSnI: The Origin of Its Material Instability 3

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YoungMi Lee, Jinwoo Park, Byung Deok Yu, Suklyun Hong, Min-Cherl Jung, and Masakazu Nakamura J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00494 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018

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The Journal of Physical Chemistry Letters

Surface Instability of Sn-based Hybrid Perovskite Thin Film, CH3NH3SnI3: The Origin of Its Material Instability Young Mi Leea,ᵻ, Jinwoo Parkb,ᵻ,+, Byung Deok Yuc, Suklyun Hongb,*, Min-Cherl Jungd,* and Masakazu Nakamurad a

Beamline department, Pohang Accelerator Laboratory, POSTECH, Pohang, 790-784, Republic

of Korea b

Graphene Research Institute and Department of Physics, Sejong University, Seoul, 05006,

Republic of Korea c

Department of Physics, University of Seoul, Seoul, 02504, Republic of Korea

d

Graduate School of Materials Science, Nara Institute of Science and Technology, Nara, 630-

0192, Japan AUTHOR INFORMATION Corresponding Author * [email protected] and [email protected]

ACS Paragon Plus Environment

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ABSTRACT To understand the instability of Sn-based perovskite, CH3NH3SnI3, photoelectron spectroscopy with synchrotron radiation and theoretical calculations, such as density functional theory and abinitio molecular dynamics, were performed. Findings from this experimental and theoretical study highlight the crucial changes of surface-chemical states, the broken chemical bondings in Sn-I, and the depletion of a CH3-NH3+ cation on the surface region. The material instability origin of Sn-based perovskite can be explained by the chemical state instability in the surface.

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Since the introduction in 2009 of hybrid organic-inorganic halide perovskite (OHP) materials in dye-sensitized solar cells, this material research field has grown dramatically in solar-cell applications.1-9 Furthermore, many researchers are exploring new application fields such as resistive memory, light-emitting diodes, and laser utilizing a nano-scale OHP single crystal.5,10,11 This expansion appears decidedly broader and faster than the typical development of materials because it has several good physical properties for application, such as wide absorption photon range, low exciton binding energy, and high carrier mobility.12-14 Recently, the power conversion efficiency (PCE) of OHP-based solar-cells has already reached 22.7 %.15 This is a very competitive value in comparison with CdTe, CIGS, and Si-based solar-cells.15 One of the critical advantages of application fields is the use of cost-effective solution-based fabrication methods, such as spin-casting and printing.16 Subsequently, further development of OHP-based applications can be expected. However, many researchers are still wondering about the exact chemical structure, precise role of each interface, material stability, and the possibility of lead-free organic-inorganic compounds17,18 because OHP is a kind of organic-inorganic hybrid material and researchers are still trying to understand its physical and chemical property originating from its organic and inorganic parts. The primary research issue for application is the development of stable Pb-free OHP, while keeping its physical properties. Notably, a lead-free OHP is critical for avoiding toxicity and environmental impacts from future technologies.19-21 One promising candidate in this direction is Sn (instead of Pb). Its potential has been confirmed with a PCE of 5~9 % in a solar-cell application.22-24 At the same time, however, many researchers have reported the instability of Sn-based hybrid perovskite due to the presence of the 2+ oxidation state of tin (Sn2+) in OHP.25-27 This is most likely a contamination issue after


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fabrication. When Sn-based OHP is formed in a glovebox or vacuum chamber, however, its degradation is immediately apparent without any contamination elements after a few days. (See Figure S1 in the Supporting Information.) This implies that there is a possibility of material instability. From theoretical study, it is difficult to realize this material instability because of seeing only the bulk structure in the single crystal. If there is a real material instability by itself, it requires the understanding of the instability factor of Sn-based OHP to improve the application performance. This paper endeavours to isolate the causes of the instability of Sn-based OHP with combined studies of surface- and bulk-sensitive photoelectron spectroscopy (PES) with synchrotron radiation and theoretical calculations (based on density functional theory and ab-initio molecular dynamics), from the viewpoint of surface science. Also, we found and confirmed that the surface-chemical instability is closely associated with a critical reason for Sn-based OHP instability. The CH3NH3SnI3 thin film was formed by the sequential evaporation method. A 100 nm layer of SnI2 was deposited on a highly-doped Si(100) substrate followed by the deposition of a 400 nm CH3NH3I layer at room temperature (RT).28 (For details, see Figure S2 in the Supporting Information.) Subsequently, the sample was loaded into the main chamber. PES was obtained at the 10D HRXPS beamline of PLSII. Photon energy was varied from 60 eV to 740 eV to obtain high-quality PES spectra. Photoelectron signals were recorded with a PHOIBOS 150 electron energy analyzer equipped with a two-dimensional charge-coupled device (2D CCD) detector (Specs GmbH), collecting photoelectrons from the surface normal. The base pressure of the main chamber was maintained below 9.8×10-11 Torr. Core-levels (Sn 4d and I 4d) and valence spectra (for N 2s, N-C hybridization, C 2s, and Sn-I hybridization on the surface) were obtained with


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various temperatures, from room temperature (RT) to 120 oC for 2 min. O 1s core-level was also measured to monitor any contamination during the sample transfer. O 1s core-level intensity was below the detection limits of the instruments. Binding energies were calibrated with reference to the Au 4f7/2 level (84.0 eV).29 All calculations were performed using density functional theory (DFT) as implemented in the code of the CASTEP.30,31 We used the On-the-fly pseudopotential generation in CASTEP (OTFG pseudopotential) and the exchange-correlation functional of the spin-polarized PerdewBurke-Ernzerhof expression revised for solids (PBEsol)32 in the generalized gradient approximation (GGA). Electronic wave functions were expanded by plane waves with an energy cut-off of 517 eV. The tetragonal phase of CH3NH3PbI3 and the cubic phase of CH3NH3SnI3 for bulk and surface structures at room temperature were used. Geometry optimization was carried out until the Hellmann-Feynman force acting on the atoms was smaller than 0.03 eV/Å without any symmetry constraint.33,34 In Fig. 3, the binding energy (EBB) per CH3-NH3+ cation in bulk CH3NH3SnI3 (CH3NH3PbI3) is calculated using the following equation

where ETotal is the total energy of bulk CH3NH3SnI3(CH3NH3PbI3), EFrame is the total energy of bulk SnI3(PbI3), N is the total number of CH3NH3+ cation in the unit cell, and Emol is the energy of a CH3-NH3+ single cation. The binding energy (EBS) per CH3NH3+ cation for the CH3NH3SnI3(CH3NH3PbI3) surface is calculated using the following equation


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where ETotal is the total energy for the CH3NH3SnI3(CH3NH3PbI3) surface, and ENm is the total energy for the CH3NH3SnI3(CH3NH3PbI3) surface with Nm CH3-NH3+ cations depleted in the top surface layer. In the DFT-based molecular dynamics simulations, we performed calculations with a Nose thermostat at a target temperature of 350 K under constant temperature and fixed volume (NVT).

Figure 1. (a, b) Sn and (c, d) I 4d core-level spectra with the photon energy of 100 eV (surfacesensitive) and 740 eV (bulk-sensitive) with various temperature treatment. In the bulk-sensitive spectra, there are no changes of the 4d7/5 binding energies in both of them (b and d). However,


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the chemical shifts toward high binding energy are observed clearly in the surface sensitive spectra (a and c).

To compare chemical states between surface and bulk, we used two photon energies with 100 eV (surface-sensitive) and 740 eV (bulk-sensitive) to examine the Sn and I 4d core-level spectra with various temperatures. (Fig. 1) In the case of Sn 4d core-level spectra (Fig. 1a), we found the chemical shifts only in the surface-sensitive measurement. The chemical shifts referenced by the as-received sample are 0.02, 0.06, 0.09, and 0.19 eV for 50, 70, 100, and 120 oC, respectively. As the temperature increased, the Sn chemical states shifted toward the higher binding energy. In contrast, from the bulk-sensitive measurement of Sn 4d core-levels (Fig. 1b), we could not observe any chemical shifts at other temperatures except for the sample with the 120 oC treatment. A similar behavior was observed for the I 4d core-level spectra in the surface-sensitive measurement. The chemical shifts from the as-received sample in the I 4d core-level spectra are 0.06, 0.08, 0.10, and 0.22 eV for 50, 70, 100, and 120 oC, respectively. (Fig. 1c) Furthermore, in the bulk-sensitive measurements of I 4d core-level spectra, we could not observe any chemical shift for the other samples except the 120 oC sample. From these observations, a dramatic change in the surface-chemical state in Sn-based OHP thin film with low-temperature treatment was obtained. This suggests an increased cation environment, as confirmed by the chemical shifts of the Sn 4d and the I 4d core levels toward high binding energy. This is in contrast to the chemical states in bulk keeping their chemical states. Next, the valence spectrum is used to see the change in organic parts on the surface. From the valence spectra with various heat treatments, we can observe N 2s, N-C hybridization, C 2s, and


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Sn-I hybridization using the photon energy of 60 eV. (Fig. 2) Interestingly, the N-C hybridization peak starts disappearing over 70 oC. At 120 oC, the peak intensity of N-C hybridization showed only its trace. In the case of N 2s, the peak position was shifted over 70 oC. For the C 2s peak, it is difficult to find any significant change in peak position and intensity. However, the Sn-I hybridization peak is broadened over 70 oC. Additionally, it looks similar to the SnI2 valence structure over 100 oC. From these valence spectra, we can see that the depletion of the CH3-NH3+ organic part occurs in the surface even at the low-temperature of 70 oC. This is experimental evidence is crucial for an understanding of the surface instability of Sn-based OHP thin films.

Figure 2. Valence spectra of the CH3NH3SnI3 thin film with various temperature treatment. At 70 oC, the peak position of N 2s is shifted and the intensity of N-C hybridization peak starts


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decreasing. Over 100 oC, the N-C hybridization peak is almost disappeared, and the chemical states of Sn-I hybridization at the valence edge is changed.

From the surface-/bulk-sensitive core-level and valence spectra measurements, we recognized two significant facts: 1) the change of Sn and I chemical states, and 2) the depletion of CH3NH3+ cation only in the surface without any contamination. To understand the mechanism of surface instability in CH3NH3SnI3, we performed theoretical calculations associated with these two facts. The results of the PES experiment with the synchrotron radiation were confirmed with the changes of chemical states in the surface of the CH3NH3SnI3 thin film. To understand these changes, we performed binding energy and molecular dynamics calculations based on density functional theory. Firstly, the binding energies per CH3-NH3+ cation were calculated in surface and bulk for CH3NH3PbI3 and CH3NH3SnI3. (Fig. 3) The black and red dashed lines are shown to represent the binding energies per CH3-NH3+ cation in the bulk of CH3NH3SnI3 and CH3NH3PbI3, respectively. The black line with the solid black circle and the red line with the solid red square represent calculated binding energies per molecule when Nm CH3NH3+ cations are depleted in the top surface layer of CH3NH3SnI3 and CH3NH3PbI3, respectively. Binding energies per CH3NH3+ cation in the CH3NH3+-depleted surface configuration of CH3NH3SnI3 are compared with those of CH3NH3PbI3. The binding energies per CH3-NH3+ cation in both the surfaces of CH3NH3SnI3 and CH3NH3PbI3 are saturated to the corresponding values in bulk with an increase in the number of depleted molecules, respectively. The binding energy per CH3-NH3+ cation in the depleted surface is seen to be much less for CH3NH3SnI3 than for CH3NH3PbI3 in the whole


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range of CH3NH3+ depletion Nm. In addition, the increase in the binding energy per molecule with Nm increase is seen to be weaker for Sn-based OHP than for Pb-based OHP. These results suggest that CH3-NH3+ cations in the surface of CH3NH3SnI3 tend to be more easily depleted even with a small temperature increase compared to the CH3NH3PbI3 surface.

Figure 3. The binding energy per CH3-NH3+ cation in CH3NH3PbI3 (red) and CH3NH3SnI3 (black) surfaces with Nm CH3-NH3+ cations depleted in the top surface layer. The dashed lines indicate the binding energy per molecule in the bulk structure of each material.


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Figure 4. The results of ab-initio molecular dynamics calculation of (a) CH3NH3PbI3 and (b) CH3NH3SnI3 surfaces. After 4 ps at 350 K, the surface of CH3NH3PbI3 is keeping its atomic structure, whereas, the Sn-I bonding distance in the CH3NH3SnI3 surface is far enough to be


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broken near the surface. This bond breaking causes the depletion of CH3-NH3+ cations in the surface region, which is consistent with the experimental result.

To confirm this finding in more detail, we also performed ab-inito molecular dynamics simulations based on density functional theory. (Fig. 4) The atomic structures of both CH3NH3PbI3 (Fig. 4a) and CH3NH3SnI3 (Fig. 4b) surfaces are shown after 4 ps at 350 K as the result of molecular dynamics simulation. In the CH3NH3SnI3 structure after 4 ps at 350 K, (Fig. 4b) we can observe the significant structural change in comparison to the CH3NH3PbI3 structure. Notably, there is a large gap between Sn and I in the surface region. This is consistent with the chemical shifts of both the Sn and I 4d core-levels toward the high binding energy in the experimental results with the (surface-sensitive) photon energy of 100 eV. (Fig. 1a and c) If Sn and I have broken bonds, it should be an atomistic behavior which reflects a chemical shift toward high binding energy. This result confirms that Sn-based perovskite has surface instability at room temperature. Interestingly, this is in contrast to the behavior in the surface of Pb-based perovskite, which has weak relaxations and is relatively stable with the original chemical states. Furthermore, we can notice the peak shift of N 2s core-level in the heat treatment after 100 oC. At 100 oC, in fact, the N 2s peak was shifted, but the C 2s peak had no change when referenced by the maximum peak position. To understand the peak shift of N 2s, we performed Mulliken charge analysis. (Table 1) The Mulliken charges of C and N of CH3-NH3+ from the top surface layer of CH3NH3SnI3 are calculated. The N atom shows that the Mulliken charges converge from -0.73e in the surface region to -0.71e in the subsurface region, but the Mulliken charges of the C atom do not change. From the calculations, we can find the reason for the N 2s peak shift


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because the binding energy of N 2s in CH3-NH3+ is measured in the subsurface region after CH3NH3+ cations are depleted in the top surface layer. This is also one of the pieces of evidence of the depletion of a CH3-NH3+ cation in the surface.

Table 1. Atomic Mulliken charges of N and C atoms per layer from the surface (S) to the subsurface regions (S-1, S-2, S-3) in the CH3NH3SnI3 surface structure. Layers

Mulliken charges of N (e)

Mulliken charges of C (e)

S

-0.73

-0.59

S-1

-0.72

-0.59

S-2

-0.71

-0.59

S-3

-0.71

-0.59

To understand the instability of Sn-based perovskite, we performed the PES experiment with the synchrotron radiation and theoretical calculations such as density functional theory and ab-initio molecular dynamics. We found the crucial changes of surface-chemical states, the broken chemical bondings in Sn-I, and the depletion of a CH3-NH3+ cation in the surface region. We confirm that the material instability of Sn-based perovskite can be explained by the origin of chemical state instability in the surface. Additionally, our findings of the surface instability are expected to provide a motivation for further experimental and theoretical studies of another OHP,


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formamidinium tin iodide (FASnI3), exhibiting different behavior from MASnI3 in physical and stability properties.35-37 Additionally, we suggest that the detailed study of thin film quality fabricated by the sequential vacuum deposition method to understand an exact material instability. Such instability in surfaces will cause a critical problem in device performance of applications such as solar-cell, light-emitting diode, and field-effect transistor.


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AUTHOR INFORMATION Notes ᵻ

Young Mi Lee and Jinwoo Park contributed equally to this work

+

The present address: Department of Physics, University of Seoul, Seoul, 02504, Republic of

Korea

ACKNOWLEDGMENT This work was supported by funding from JSPS KAKENHI Grant No. 17K05033 (Japan) and Murata Science Foundation (Japan). Also, this work was supported by Basic Science Research Program (NRF-2015R1C1A2A01054543) and Priority Research Center Program (20100020207) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education in the Republic of Korea. The authors thank Associate Professor Leigh McDowell from the Nara Institute of Science and Technology (NAIST) for valuable suggestions in revising the manuscript.

Supporting Information In this document, the degradation pictures of the Sn-based perovskite thin film and the detailed sequential vacuum evaporation method are included.


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