Letter pubs.acs.org/JPCL
Pressure-Induced Structural and Optical Properties of Inorganic Halide Perovskite CsPbBr3 Long Zhang, Qingxin Zeng, and Kai Wang* State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China
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S Supporting Information *
ABSTRACT: Perovskite photovoltaic materials are gaining sustained attention because of their excellent photovoltaic properties and extensive practical applicability. In this Letter, we discuss the changes in the structure and optical properties of CsPbBr3 under high pressure. As the pressure increased, the band gap initially began to red shift before 1.0 GPa followed by a continuous blue shift until the crystal was completely amorphized. An isostructural phase transition at 1.2 GPa was determined by high-pressure synchrotron X-ray and Raman spectroscopy. The result could be attributed to bond length shrinkage and PbBr6 octahedral distortion under high pressure. The amorphization of the crystal was due to the severe distortion and tilt of the PbBr6 octahedron, leading to broken longrange order. Changes in optical properties are closely related to the evolution of the crystal structure. Our discussion shows that high-pressure study can be used as an effective means to tune the structure and properties of all-inorganic halide perovskites.
P
applications in light-emitting diodes,10,11 high-energy detectors,12 lasers,13,14 and other fields.15 In recent decades, the evolution behavior of some allinorganic halide perovskites under high pressure has been reported.16−19 Compared to chemical and other physical modification technologies,20−24 high-pressure techniques as a clean, effective approach can change the crystal structure and electronic configuration precisely to adjust the physical and chemical properties of materials. The effect of pressure on halide perovskites had obtained many gratifying achievements, including optimization of optical and electrical properties,25−28 enhanced structural stability,29 piezochromism,28,30,31 and metallization.32 MAPbI3 and FAPbI3 perovskite can be simultaneously tuned in band gap narrowing and carrierlifetime prolongation under moderate pressure.25,26,33−35 Pressure-treated CH3NH3SnI3 has excellent properties of improved structural stability, increased electrical conductivity, and enhanced photoresponsiveness.29 The pressure effect leads to excellent optimization for the band gap of CsGeBr3.17 Therefore, for CsPbBr3 as a state-of-the-art inorganic halide perovskite material, the stability of the structure upon compression and photoelectric properties of the modification have great scientific value and significance, which will be beneficial to promote the broad application of such materials. Herein, we systematically studied the evolution of the CsPbBr3 crystal structure and the optical properties under high pressure by diamond anvil cells (DACs). CsPbBr3 experienced an isostructural phase transition and subsequent amorphization under compression using in situ high-pressure powder X-ray
erovskite solar cells have attracted enormous research interest as a promising photovoltaic technology owing to their outstanding photoelectric properties and excellent power conversion efficiency. Since the first report on solid-state perovskite solar cells in 2012,1 with a power conversion efficiency of 9.7%, rapid growth to a remarkably high value of 22.1% was obtained in just 4 years.2 A perovskite-type crystal structure of the 3D hybrid perovskites has been fabricated with a formula of AMX3,3 where A is a monovalent inorganic or organic cation (Cs+, Rb+, MA+, or FA+), B is a divalent metal cation (Ge2+, Sn2+, or Pb2+), and X is a halogen anion (Cl−, Br−, or I−). Element substitution is performed in a large number of permutations to tune physical, chemical, optical, and electronic properties, thereby promoting the development of multifunctional materials. Although methylammonium or formamidinium lead trihalide organic−inorganic hybrid perovskites exhibit superior photovoltaic properties and high energy conversion efficiency (especially MAPbI3),4−6 many serious issues should be resolved. The sensitivity to moisture and temperature impedes the long-term stability of perovskite solar cells.7 The substitution of organic cations by Cs+ is a direct, simple, and effective approach to improve the stability of perovskite materials. Zhu et al. confirmed that CsPbBr3, MAPbBr3, and FAPbBr3 perovskite crystals exhibit similar properties for band edge carriers.8 Compared with MAPbBr3based perovskite solar cells, CsPbBr3-based perovskite solar cells had long-term stability and generated high open-circuit voltages, as reported by Cahen.9 Currently, durable stability of perovskite materials is a tricky issue and urgently needs to be resolved; thus the prominent stability of CsPbBr3 makes it a superior candidate for long-term operation of the device. In addition to photovoltaic properties, CsPbBr3 has extensive © 2017 American Chemical Society
Received: June 20, 2017 Accepted: July 25, 2017 Published: July 25, 2017 3752
DOI: 10.1021/acs.jpclett.7b01577 J. Phys. Chem. Lett. 2017, 8, 3752−3758
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The Journal of Physical Chemistry Letters
emission centered at 536.8 nm under ambient conditions. The characteristics of the absorption spectra indicated that the CsPbBr3 crystals possessed a direct band gap, which is consistent with the previous literature report.12 The absorption edge and PL peak showed a gradual red shift of up to 1.0 GPa when the pressure was increased. Then, a sudden blue shift of the absorption edge and PL peak occurred at 1.2 GPa, following a persistent blue shift of up to 2.0 GPa. The abrupt anomalous change at the absorption edge and PL peak (Figure 1c,d) implied that the crystal may undergo a structural phase transition at approximately 1.2 GPa,27,36 changing the electronic state of the crystal. With further increase of pressure, the initial absorption edge disappeard at approximately 2.5 GPa, and the second absorption edge sustained a gradual slow blue shift, leaving an absorption tail at 11.0 GPa. This condition may be ascribed to pressure-induced amorphization of the crystal based on the study of such materials under high pressure.31 Upon pressure release, the absorption spectra of amorphous CsPbBr3 recovered the original state at ambient conditions, which suggested that the crystal structure was reversible. The initial PL intensity was slowly and gradually weakened during the red shift of the PL peak (approximately before 1.0 GPa) and then dramatically decreased until it eventually disappeared at approximately 2.5 GPa (the optical micrograph of the PL, Supporting Information Figure S2). The disappearance of the PL intensity coincides with the appearance of amorphization responses in the diffraction patterns; thus, it is attributed to the enhanced nonradioactive processes in the amorphous lattice.37 Initially, the CsPbBr3 crystals were orange at ambient conditions. The color of the sample was slightly deepened at approximately 1.0 GPa and then transformed into light yellow and eventually became colorless and transparent at 2.8 GPa. We clearly observed that the piezochromism of the CsPbBr3 crystals corresponded to evolution in the optical properties of the crystal (optical micrographs, Supporting Information, Figure S3). The band gap of the material is closely related to its absorption edge; therefore, the unusual behavior of the photoelectric properties can be reflected by the evolution of the band gap.26 We assessed the band gap of CsPbBr3 at different pressure levels by extrapolating the linear portion of the (αdhν)2 versus the hν curve in direct band gap Tauc plots (Figure 1c),25 where α is the absorption coefficient, d is the sample thickness, and hν is the photon energy. The band gap of CsPbBr3 was estimated to be 2.32 eV at ambient conditions. The band gap of CsPbBr3 experienced a significant narrowing process before 1.0 GPa, and the subsequent band gap blue shift above 1.2 GPa may originate from the phase transition of the crystal structure.27,36,38 Experimentally, we only estimated the band gap from ambient conditions to 2.0 GPa because the crystal may begin to undergo amorphization after 2.5 GPa.31 The change in the Pb−Br network structure had an unavoidable impact on the optical properties of the CsPbBr3.39 Thus, we carried out high-pressure Raman experiences to track the evolution of the vibrational modes of the metal halide sublattice upon compression. The Raman spectra of CsPbBr3 as a function of pressure are shown in Figure 2. With increasing pressure, all Raman peaks move continuously along the highfrequency direction, and the profile of the spectra remains unchanged. However, when the pressure reaches 1.4 GPa, the Raman spectra exhibit some significant changes; the vibrational mode ν1 is obviously broadened, while the frequencies of all vibrational modes remain almost constant until the pressure
diffraction (XRD) and Raman spectroscopy. The optical absorption and photoluminescence (PL) spectra suggest that the optical properties of the CsPbBr3 crystal had undergone an apparent change under high pressure. The partial density of states (PDOS) and the band structure further illustrates the evolution of the optical properties under high pressure. Our research confirms that the optoelectronic properties of CsPbBr3 crystals were modified upon compression. Lattice compressibility and amorphization of the crystal structure enhanced the fundamental understanding of material stability under moderate pressure. In addition, this study provides a scientific theoretical guideline for all-inorganic perovskite photovoltaic materials in future practical applications and introduces a new perspective on engineering routes. Considering the remarkable photovoltaic properties of the promising optoelectronic materials for CsPbBr3, we carried out high-pressure optical absorption and PL experiments to trace the pressure-induced optical evolution. The selected steadystate absorption and PL spectra are shown in Figure 1a,b. The CsPbBr3 crystals showed a steep absorption edge located at 535 nm and bright green PL (schematic in Figure 1b) with an
Figure 1. (a) Absorption spectra of CsPbBr3 as a function of pressure. Green, black, and purple arrows suggest the moving trend of the initial absorption edge and second absorption edge under pressure. (b) PL spectra of CsPbBr3 as a function of pressure. (c) Band gap evolutions of CsPbBr3 as a function of pressure. The illustration shows the selected direct band gap Tauc plots for CsPbBr3 at 1 atm. (d) PL peak position at various pressure levels. Schematic of the PL at ambient conditions in a symmetric DAC. 3753
DOI: 10.1021/acs.jpclett.7b01577 J. Phys. Chem. Lett. 2017, 8, 3752−3758
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Figure 2. Raman spectra of CsPbBr3 upon compression at room temperature.
Figure 3. Representative XRD patterns of CsPbBr3 upon compression up to 20.0 GPa.
reaches 2.1 GPa. The CsPbBr3 crystals begin to be amorphized at a pressure of 2.9 GPa until the vibrational mode ν1 completely disappears at 10.8 GPa.39 The evolutions of Raman spectra are consistent with the changes in absorption and PL spectra at approximately 1.2 GPa, suggesting that the CsPbBr3 crystals may undergo a structural phase transition.18,31,39 The vibrational modes ν1 and ν2 are assigned to the vibrational mode of the PbBr6 octahedron and the motion of Cs+ based on the relevant literature reported.40,41 The vibrational mode ν1 was continuously red shifted, arising from pressure-induced Pb−Br bond contraction above 1.3 GPa. When the pressure reached 1.4 GPa, the vibrational mode ν1 undewent sudden rapid reduction in intensity, and the profile of Raman spectra changed dramatically. The evident changes may imply that the change in the Pb−Br network structure occurred because of the aggravated pressure-induced PbBr6 octahedral distortion. The discontinuous movement of the vibrational mode ν2 at 1.4 GPa may be derived from distortion of the cuboctahedral cavity formed by the nearest-neighbor Br atoms. This distortion changed the relative position of Cs+. The vibrational mode ν2 always existed until the highest pressure in this experiment, suggesting that the distortion and reduction of the cuboctahedral cavity did not damage the interaction between the Cs+ and the Pb−Br network structure. The Raman spectra accurately monitored the changes in the crystal structure under high pressure, but the structural evolution must be further defined by high-pressure XRD analysis. The consistent change in optical properties and Raman spectra suggested the occurrence of structural phase transition. Thus, we carried out high-pressure XRD experiments. We identified the initial crystal structure of CsPbBr3 as a pure orthogonal phase with space group Pbnm based on the refined XRD results at ambient conditions (Supporting Information, Figure S5a). This result is in agreement with previous reports.42 Figure 3 shows the representative XRD patterns of CsPbBr3 upon compression. As the pressure increases, all Bragg diffraction peaks shift to higher angles and partially overlap Bragg diffraction peaks gradually separated with respect to the pressure-induced decrease in lattice constant. During the entire pressurization process, no new peaks appeared, or the initial peaks disappeared until the crystal started to be amorphized.
When the pressure surpassed 2.4 GPa, some of the original Bragg diffraction peaks disappeared and a broad background originating from the diffuse scattering appeared, suggesting the onset of pressure-induced amorphization.27 All Bragg diffraction peaks disappeared with respect to complete amorphization when the pressure was increased to 20.0 GPa. The variations of cell volume and cell parameters of CsPbBr3 under high pressure are displayed in Figure 4. The appearance
Figure 4. Unit cell volume (a) and cell parameter (b) evolutions of CsPbBr3 under pressure and fit parameters.
of volume collapse and lattice parameter b discontinuity at 1.2 GPa in the process of compression indicates that the structural phase transition occurred. We implemented Rietveld refinement to the crystal structure of the pressure-induced new highpressure phase. Phase II of CsPbBr3 at 1.2 GPa was identified as an orthogonal phase with space group Pbnm (Supporting Information, Figure S5b); thus, the pressure-induced phase transition was ascribed to the orthogonal to orthogonal isostructural phase transition. Similar isostructural phase transitions have been reported in organic−inorganic hybrid perovskites under high pressure.39 We summarize the evolution of some Bragg diffraction peaks as a function of pressure 3754
DOI: 10.1021/acs.jpclett.7b01577 J. Phys. Chem. Lett. 2017, 8, 3752−3758
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The Journal of Physical Chemistry Letters (Supporting Information, Figure S6). The shift rate of partial Bragg diffraction peaks underwent a significant discontinuity at 1.2 GPa. In Phase II, the d value interval of Bragg diffraction peaks with similar d values as Phase I increased rapidly (for instance, the obvious contrast of the (2 1 2) and (1 2 2) Bragg diffraction peak shift rates at Phase I and Phase II). The discontinuity of the shift rate also confirmed that CsPbBr3 underwent a phase transition at 1.2 GPa. As shown in Figure 4a, the bulk modulus K0 of Phase I was estimated to be 18.1 GPa by the fitting cell volume data with the second-order Birch−Murnaghan equation of state P(V ) =
⎡⎛ V ⎞2/3 ⎤⎫ ⎪ ⎪ 3K 0 ⎡⎛ V0 ⎞7/3 ⎛ V0 ⎞5/3⎤⎧ 3 ⎢⎜ ⎟ − ⎜ ⎟ ⎥⎨1 + (K ′ − 4)⎢⎜ 0 ⎟ − 1⎥⎬ ⎪ ⎝ V ⎠ ⎥⎦⎩ ⎢⎣⎝ V ⎠ ⎥⎦⎪ 2 ⎢⎣⎝ V ⎠ 4 ⎭
where P is the pressure, V is the deformed volume, V0 is the initial volume, and K′ is the derivative of the bulk modulus with respect to pressure (we assumed K′ = 4). Compared to organic−inorganic hybrid perovskites such as MAPbI3,43 MAPbBr3,44 MASnI3,45 FASnI3,45 and FAPbI3,25 the slightly higher bulk modulus of CsPbBr3 implies a more sturdy structure at Phase I, which may be beneficial to improve solar cell stability. The K0 value far less than that of oxygencontaining inorganic perovskites such as SrTiO3 and CaTiO346 indicated that CsPbBr3 has a greater degree of modification and compression under mild pressure. The K0 value in Phase II is 37.3 GPa, and a lower compression of Phase II resulted from the sustained compression of the bond length and octahedral serious distortion. The Rietveld refinement bond lengths and bond angles at different pressures are summarized in Table S1. The structure analysis suggests that the volume compression of Phase I was mainly due to Pb−Br bond contraction and octahedral distortion, whereas the volume compression of Phase II was mainly caused by the tilt and rotation of the octahedron. Figure 4b shows the anisotropic compression throughout the entire compression process; the a parameter shows the greatest compressibility compared to b and c. At ambient conditions, a and b parameters are similar, but due to the difference in the compression ratio, the disparity between a and b gradually increases accompanied by increased pressure. Parameters a and c maintain continuous contraction, whereas parameter b abruptly maintains zero compression after 1.2 GPa because of the structural phase transition. This particular phenomenon stems from the special structure of orthogonal phase CsPbBr3. As shown in Figure 5a, the Pb−Br bonding network constitutes a structure similar to the “wine rack” at the ab plane.47 Pb2+ acts as a hinge fulcrum, and the Pb−Br bonds act as arms. The Pb− Pb−Pb angle in the a-axis direction is an obtuse angle at ambient conditions. The degree of obtuse angle gradually increases, and Pb−Br bonds gradually stiffen with increasing pressure. As the obtuse angle increases to the critical value and the Pb−Br bonds are sufficiently stiffened, the tension and the external pressure in the b-axis direction offset each other, resulting in the zero compression phenomenon of the b axis. We proposed the pressure-induced structure evolution mechanism of CsPbBr3 based on the above high-pressure XRD and Raman spectra analysis. The refined Pb−Br netwok structure of CsPbBr3 at ambient conditions and 1.2 GPa is shown in Figure 5b. With the increase of pressure in Phase I, the Pb−Br bond shrinks and all Pb−Br−Pb bond angles decrease simultaneously. All Br− ions initially slightly deviate from the plane and move in the direction of the initial deviation
Figure 5. Pb−Br network structure of CsPbBr3 viewed along the c axis (a) and a axis (b) at ambient conditions and 1.2 GPa, respectively. Gray ball: Pb; blue ball: Br.
to further enlarge the displacement. The further displacement of the Br− on the c-axis Pb−Br−Pb bond would exacerbate PbBr6 octahedron tilt (the Br−Pb−Pb angle from 84.280° at ambient conditions to 79.731° at 1.2 GPa),, whereas the Br− displacement at the ab face would cause the octahedral rotation upon compression (Supporting Information, Table S1). The Pb−Br bond was gradually stiffened with the increase in pressure. When the pressure reaches 1.2 GPa, the Pb−Br bond length cannot easily be compressed, and the lattice shrinkage would mainly be attributed to the tilt and rotation of the octahedron, thus leading to the occurrence of structural phase transition. When the pressure exceeds a certain threshold, the long-range order of the lattice begins to be broken due to the severe distortions of the octahedron until complete amorphization at 20.0 GPa. The crystal structure of the orthogonal phase is built up of a highly distorted inorganic skeleton and fillings with monatomic Cs. The pressure-induced amorphization under relatively mild pressure is reasonable. We carried out first-principles calculations on both the PDOS and band structure to fully understand the evolution behavior of the structure and band gap under high pressure. We explored the discontinuity of band gap evolution at 1.5 GPa by theoretical calculation (Figure 6a). The evolutionary trend of calculation was consistent with the experiment, and it ensured the accuracy of the calculation. Figure 6b,c shows a representative PDOS and band structure of CsPbBr3 at 1.5 GPa, indicating that a direct band gap is consistent with the experiment. The valence band maximum (VBM) with antibonding characteristic consisted of a Br 4p orbital and a small amount of Pb 6s orbital, whereas the conduction band minimum (CBM) is mainly composed of a Pb 6p orbital. Cs atoms have almost no contribution to the VBM and the CBM; thus, the evolution of band gap mainly resulted from the change of PbBr6 octahedra. The reduction in the Pb−Br bond would enhance coupling between the Br 4p and Pb 6s orbital and consequently increase band dispersion to increase the VBM.25−27 The nonbonding localized state of the Pb 6p 3755
DOI: 10.1021/acs.jpclett.7b01577 J. Phys. Chem. Lett. 2017, 8, 3752−3758
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EXPERIMENTAL SECTION
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ASSOCIATED CONTENT
Sample Preparation and High-Pressure Generation. The CsPbBr3 sample was purchased from Xi’an Polymer Light Technology Corp and used without further purification. High-pressure experiments were carried out with a symmetric DAC, and the culets size of the diamond anvils was 500 μm. The sample was loaded into the sample chamber of a T301 stainless steel gasket, which was preindented to a thickness of 50 μm and with a 150 μm diameter hole. A small ruby ball was placed in the sample chamber for in situ pressure measurements by the standard ruby fluorescence technique. Silicone oil was used as a pressuretransmitting medium for in situ high-pressure optical absorption, PL, and XRD experiments. For in situ highpressure Raman measurement, we used argon as the pressuretransmitting medium. All experiments were performed at room temperature. Optical Absorption, PL, and Raman Measurements. In situ high-pressure optical absorption spectra measurements were carried out between 250 and 1000 nm using a deuterium halogen light source. The 355 nm line of a UV DPSS laser with an output power of 10 mW was used as the excitation light source, and an optical fiber spectrometer (Ocean Optics, QE65000) was used for PL measurement. A camera (Canon Eos 5D mark II) equipped on a microscope was used to get the micrographs of the sample. In situ high-pressure Raman were conducted by a spectrometer equipped with a liquid nitrogen CCD (iHR 550, Symphony II, Horiba Jobin Yvon). A 785 nm single-mode DPSS laser with an output power of 10 mW was utilized to excite the sample. The resolution of the system was about 1 cm−1. XRD Measurements. In situ high-pressure XRD experiments with a 0.6199 Å wavelength beam were conducted at the 4W2 High Pressure Station at the Beijing Synchrotron Radiation Facility. CeO2 was used as the standard sample to do the calibration. The FIT2D program was used to integrate and analyze the collected 2D images, and Materials Studio Software was employed to index and refine the collected experimental XRD profiles Computational Methodology. Calculations of the PDOS and band structure were performed using the plane-wave pseudopotentials method based on density functional theory implemented in the CASTEP package with the exchange− correlation functional of the generalized gradient approximation (GGA).48 Calculation of the initial structure was orthogonal, which was obtained from the Inorganic Crystal Structure Database, and the ultrasoft pseudopotentials were used to model the ion−electron interactions of constituent elements. The plane-wave cutoff energy and Monkhorst−Pack k-points mesh were 800 eV and 2 × 2 × 1, respectively. The selfconsistent field (SCF) tolerance, maximum force, and maximum stress were set as 5.0 × 10−7 eV/atom, 0.01 eV/Å, and 0.02 GPa, respectively.
Figure 6. (a) Computational band gap evolutions of CsPbBr3 as a function of pressure.
orbital of the CBM is sensitive to the bond length or pressure. On the contrary, distortion of octahedra and a partially broken Pb−Br bond will reduce coupling between Br 4p and Pb 6s orbitals, reducing the VBM. Therefore, the changes in band gap mainly stem from the changes in the VBM upon compression. Through crystal and electronic structural analysis, the competition between the two compression effects, which originated from the bond length contraction and the octahedral distortions, was responsible for the abnormal evolution of the PL and band gap. In Phase I, the contraction of the Pb−Br bond and the distortion of the octahedron are coexistent under pressure, but the effect of bond length contraction is stronger than octahedral distortion. Therefore, the band gap is gradually reduced. On the contrary, in Phase II, the compression of the crystal structure mainly comes from the octahedral tilt and distortion, indicating that the octahedral tilt and distortion strongly affect the electronic structure compared to Pb−Br bond contraction in this regime. Thus, the band gap is increased, which is ascribed to the drop of the VBM. The observed changes in optical absorption and PL spectra completely obtained a reasonable understanding in accordance with the pressure-induced change of structure. Our exploration has evidently proved that high-pressure technology is an effective and practical scientific means to tune the optical properties and structure of the all-inorganic halide perovskite. In summary, we investigated the structure evolution of CsPbBr3 under high pressure by in situ high-pressure powder XRD and Raman spectroscopy. We demonstrated an isostructural phase transition at 1.2 GPa, and the CsPbBr3 compared to organic−inorganic hybrid perovskites has a slightly higher stability in structure. The modification and evolution of optical properties under high pressure were explored by utilizing the in situ high-pressure optical absorption and PL spectra. Combined with the first-principles calculations, a comprehensive understanding of the structural changes significantly affects the properties of the material under high pressure, and the evolution of the band gap was attributed to the distortions and contractions of the Pb−Br octahedra. Our results show that high-pressure technology is an effective approach to regulate the properties of the material by modifying the crystal structure, thus providing a promising direction for practical application in the future.
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01577. PL and optical micrograph, refinement data, and DFT results (PDF) 3756
DOI: 10.1021/acs.jpclett.7b01577 J. Phys. Chem. Lett. 2017, 8, 3752−3758
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kai Wang: 0000-0003-4721-6717 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge funding support from the National Natural Science Foundation of China (No. 21673100, 91227202), Chang Jiang Scholars Program (No. T2016051), Changbai Mountain Scholars Program (No. 2013007), and Jilin Provincial Science & Technology Development Program (No. 20150520087JH). ADXRD experiments were performed at the Beijing Synchrotron Radiation Facility (4W2 beamline), which is supported by the Chinese Academy of Sciences (No. KJCX2SW-N20, KJCX2-SW-N03).
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REFERENCES
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