Pressure-Induced Structural Evolution and Optical Properties of Metal

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Pressure-Induced Structural Evolution and Optical Properties of Metal-Halide Perovskite CsPbCl3 Long Zhang, Lingrui Wang, Kai Wang,* and Bo Zou State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China

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S Supporting Information *

ABSTRACT: Metal-halide perovskites have emerged as the most promising semiconductor materials for advanced photovoltaic and optoelectronic applications. Herein, we comprehensively investigate the optical response and structural evolution of metal-halide perovskite CsPbCl3 (ABX3) upon compression. Band gap realized a pronounced narrowing under mild pressure followed by a sharp increase, which could be ascribed to Pb−Cl bond contraction and inorganic framework distortion, respectively. The transformation of the crystal structure is confirmed and analyzed through in situ high-pressure X-ray diffraction and Raman experiments, consistent with the evolution of optical properties. Combining with the first-principles calculations, we understand the electronic band structure changes and phase transition mechanism, which are ascribed to severe PbCl6 octahedral titling and twisting. Our results demonstrate that the high-pressure technique can be used as a practical tool to modify the optical properties of metal-halide perovskites and maps an innovative strategy for better photovoltaic and optoelectronic device design.

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INTRODUCTION Metal-halide perovskites have attracted much attention because of their excellent promises for high-performance perovskite solar cells (PSCs) and light-emitting diodes (LEDs). As a state-of-the-art photovoltaic semiconductor material, the power conversion efficiency (PCE) of the PSCs has been rapidly enhanced from 3.8% in 2009 to 22.1% in 2017.1,2 Their solution processability with low-temperature treatment, simple thin-film preparation technology, low cost of raw materials, strong light absorption, long diffusion lengths, and other characteristics make them have high potential in applying to various devices and extraordinary commercial competitiveness in the field of semiconductor materials. Compared to organic−inorganic hybrid perovskites, allinorganic cesium lead halide perovskites (CsPbX3, X = Cl, Br and I) exhibit a long-term stability along with remarkable photovoltaic and optoelectronic properties.3 CsPbCl3 as a wide band gap (about 3 eV) semiconductor material with transparency in the visible spectrum, high photoluminescence (PL) quantum yields, narrow emission line widths, high sensitivity to UV radiation, and durable photostability makes it attractive for air-stable LEDs, energy-down-shift (EDS), photodetectors, field effect transistors, and radiation detector applications.4−6 Mn2+-doped CsPbCl3 nanocrystals with strong orange luminescence of Mn2+ not only effectively enhanced PCE by converting UV light to visible light but also eliminated a significant loss mechanism (UV-induced degradation) to improve the stability of PSCs.7 Until now, diversified traditional chemical methods have been used to tune the structural and optical properties of © 2018 American Chemical Society

perovskite materials to improve various physical characteristics; for instance, effectively narrowing the band gap by modifying the perovskite structure with substituting Pb with Sn.8 The absorption and emission spectra of the inorganic cesium lead halide perovskites are readily tunable over the entire visible spectral region by modulating the halide composition.9 However, shortening the carrier lifetime, material instability, large band gap etc issues are existent and undesirable. In recent years, high-pressure techniques as effective and clean means have been widely used to tune the crystal structure and electronic wave functions of metal-halide perovskites. The electronic configuration can be finely modified by the compression of B−X bonds or/and bending of B−X−B bond angles,10,11 thereby improving the photoelectric and photovoltaic properties of perovskite materials. Simultaneous band gap narrowing and carrier lifetime prolongation of MAPbI3 and FAPbI3 under mild pressure already reported in previous literature studies.12,13 In our recent reports, the optical properties of CsPbBr3 exhibit a significant pressure response, suggesting that pressure can flexibly tune electronic configuration in their metal-halide perovskite analogues.14 Numerous novel emergent properties also are observed upon compression, including metallization, piezochromism, enhanced structural stability, and photoresponsiveness.15−22 Received: June 5, 2018 Revised: June 11, 2018 Published: June 15, 2018 15220

DOI: 10.1021/acs.jpcc.8b05397 J. Phys. Chem. C 2018, 122, 15220−15225

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The Journal of Physical Chemistry C

maximum stress, and maximum displacement were set as 0.01 eV/Å, 0.02 GPa, and 5.0 × 10−4 Å, respectively.

Herein, we study the pressure effect of optical properties of CsPbCl3 in diamond anvil cells (DACs) by absorption and PL spectra. The band gap of materials obtained effectively modified under modest pressure. The evolution of the crystal structure and electronic configuration of CsPbCl3 was tracked and analyzed by in situ X-ray diffraction (XRD), Raman spectra. and first-principles calculations under the highest pressure up to 20.7 GPa. The experimental and theoretical results not only clearly illustrate the modification mechanism of optical properties but also open up a promising strategy for the improvement of metal-halide perovskite materials.

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RESULTS AND DISCUSSION Taking into account the fascinating photoelectric and photovoltaic properties of CsPbCl3 in photoelectricity applications, we implemented in situ UV−vis absorption and PL experiments to explore the optical evolution behavior upon lattice compression. Representative absorption spectra of CsPbCl3 powder at various pressure levels and the absorption spectrum across the entire UV region under ambient conditions, accompanied by a steep absorption edge located at 417 nm, which is associated with the 6s−6p transition for Pb2+ ions, are shown in Figure 1a.23 We observed that the absorption edge

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METHODS Sample Preparation and High-Pressure Generation. The CsPbCl3 sample was purchased from Xi’an Polymer Light Technology Corp and used without further purification. A symmetrical DAC with a pair of 400 μm culets was employed to generate high pressure. A T301 steel gasket was preindented to 45 μm in thickness followed by a center hole with a 150 μm diameter was drilled as a sample chamber. Powder sample and a small ruby ball were placed into the sample chamber and in situ pressure was gauged by the R1 ruby fluorescence method. Silicon oil was used as the pressure-transmitting medium in high-pressure optical absorption, PL, and XRD experiments. In high-pressure Raman measurement, we used argon as the pressure-transmitting medium. Optical Absorption, PL and Raman Measurements. In situ high-pressure absorption spectra were conducted by using a deuterium−halogen light source. In the PL measurement experiment, a 355 nm line of a UV DPSS laser with the output power of 10 mW was used as the excitation light source, and the fiber spectrometer is an Ocean Optics QE65000 spectrometer. The optical micrographs were obtained by using a camera (Canon Eos 5D mark II) equipped on a microscope. In high-pressure Raman measurement, a 785 nm single-mode DPSS laser with the output power of 10 mW was used as the excitation light source and a spectrometer equipped with a charge-coupled device (iHR 550, Syncerity, Horiba Jobin Yvon) was used to collect data. XRD Measurements. In situ high-pressure angle-dispersive XRD experiments with a wavelength of 0.6199 Å beam were carried out at beamline BL15U1, Shanghai Synchrotron Radiation Facility (SSRF), China. CeO2 was used as the standard sample to do the calibration. The 2D images of angle dispersive X-ray diffraction patterns were obtained by using an imaging plate detector, which were then integrated into onedimensional profile with the FIT2D program. Structure refinements were performed by using the Materials Studio program. Computational Methodology. Partial density of state (PDOS) and band structure were conducted with the pseudopotential plane-wave methods based on density functional theory implemented in the CASTEP package at the generalized gradient approximations (GGA)-density functional theory (DFT) level. The exchange−correlation (XC) functional is described by PBE. The starting structure originates from Rietveld refinement based on the XRD data we collected, and the ultrasoft pseudopotentials were used to model the ion−electron interactions of constituent elements. The planewave cutoff energy and Monkhorst−Pack k-points mesh were 1000 eV and 3 × 3 × 2, respectively. The self-consistent field tolerance is 5.0 × 10−7 eV/atom. The maximum force,

Figure 1. (a) Absorption spectra of CsPbCl3 as a function of pressure. Blue and red arrows suggest the moving trend of the initial and second absorption edge under pressure. (b) Band gap evolution of CsPbCl3 as a function of pressure. The illustration shows the selected direct band gap Tauc plots for CsPbCl3 at 1 atm. (c) PL spectra of CsPbCl3 as a function of pressure. (d) PL peak position as a function of pressure.

exhibited a gradual red shift upon compression, followed by the initial absorption edge collapse while the second absorption edge appeared at 1.8 GPa, implying the occurrence of crystal structure transformation based on previous perovskite analogue studies upon compression, altering their electronic landscapes.24−26 With further compression, the second absorption edge sustained a continuous blue shift, until the final complete disappearance, and only left an absorption tail at 4.4 GPa. Upon the release of pressure, the absorption spectra restored to the original state under ambient conditions. We estimated the band gap of CsPbCl3 by extrapolating the linear portion of the (αdhν)2 versus the hν curve in direct band gap Tauc plots (Figure 1b), where α is the absorption coefficient, d is the sample thickness, and hν is photon energy. At ambient pressure, the band gap of CsPbCl3 was elevated to be 2.97 eV, consistent with previous reports of 15221

DOI: 10.1021/acs.jpcc.8b05397 J. Phys. Chem. C 2018, 122, 15220−15225

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The Journal of Physical Chemistry C the literature.27 Band gap exhibits a notable narrowing to 2.92 eV at 1.7 GPa and then experienced a dramatic increase to 3.13 eV at 3.1 GPa. Figure 1c shows the stacked PL spectra of CsPbCl3 as a function of pressure. The ambient CsPbCl3 materials exhibited a bright blue PL with an emission centered at 421 nm, suggesting the relatively small Stokes shift (Supporting Information Figure S1). With increasing pressure, the PL peak shows a red shift in the 0−1.5 GPa region and then a gradual blue shift (Figure 1d), and the second PL peak gradually appeared at 1.9 GPa, owing to the fact that the initial PL peak has been weak at this pressure point. The initial PL peak first completely disappeared at 2.3 GPa, whereas the second PL peak finally disappeared up to 2.7 GPa (PL micrograph, Supporting Information Figure S2). The anomalous evolution behavior of PL coincides with absorption, indicating that significant changes in the electronic configuration associated with the crystal structure. To gain an insight into the structure−property relationships and structural stability of CsPbCl3 upon compression, in situ high-pressure XRD experiments were carried out for accurately tracking pressure effect on the structure changes. According to our collected powder XRD data under ambient conditions, the Rietveld refinement result shows an orthogonal phase (space group: Pbnm, lattice parameters: a = 7.86 Å; b = 7.92 Å; c = 11.24 Å), consistent with the previously reported results (Supporting Information Figure S3, Table S1).28 Figure 2a shows the collected XRD patterns of CsPbCl3 at different pressure and decompression. With the increase of pressure, all

Bragg diffraction peaks are continuously shifted to larger 2θ, while no new peaks emerged and the original peaks disappeared before 1.7 GPa. With further compression, the overlap of diffraction peaks located around 9° is reduced at 2.1 GPa, indicating a phase transition. As the applied pressure exceeds 5 GPa, the broad Bragg diffraction peaks significantly decrease in intensity, accompanied by broadening and disappearance of some initial diffraction peaks, implying the onset of deteriorated crystal crystallinity due to significant disorder of the Pb−Cl network structure under high pressure.21,29 However, despite the presence of largely broad diffuse backgrounds, three distinct Bragg reflections still remain up to 20.6 GPa, suggesting that the inorganic framework still maintains some degree of long-range order under the highest pressure applied in this experiment.30 The deteriorated crystallinity of CsPbCl3 restores to the original crystal structure with minimal hysteresis upon decompression. This memory effect illustrates that in addition to the organic cations present in the organic−inorganic hybrid perovskites, simple Cs+ cations also can play the role of templates.31,32 The variations of cell parameter and volume are shown in Figure 2b,c; a distinct discontinuous reduction of the c parameter and a perceptible drop of the cell volume were observed at 2.1 GPa. The Rietveld refinement exhibits the space group of the high-pressure phase at 2.1 GPa (phase II, space group: Pbnm, lattice parameters: a = 7.63 Å; b = 7.81 Å; c = 11.02 Å) is consistent with the initial phase (Supporting Information Figure S3). Therefore, this is an isostructural phase transition caused by lattice distortion and PbCl6 octahedral titling. CsPbCl3 displays an obvious anisotropic compressibility upon compression.21 The continuity of a axis contraction is slightly affected by the structural transformation. On the contrary, in phase II, b and c axes exhibit a greater compressibility than in phase I. The distorted structure and different pressure effects of isolated Cs+ cations on the in-phase and out-of-phase titling of PbCl6 octahedra can be responsible for this behavior. Fitting their pressure−volume relations to the second-order Birch−Murnaghan equation of state for phase I obtained a bulk modulus K0 of 45.55 GPa.33 The bulk modulus K0 of phase II (K0 = 40.78 GPa) is slightly less than phase I, the pressure-induced lattice softening is mainly attributable to severe octahedral titling in phase II, while volume reduction in phase I is mainly derived from Pb−Cl bond contraction, similar to reported 3D MAPbBr3 halide perovskite.33 The bulk modulus K0 is much less than the oxygen-containing inorganic perovskite such as CaTiO3 with 177 GPa,34 thus revealing the easily compressed characteristic of metal-halide perovskite. The change of the Pb−Cl network structure upon compression redefines electronic configuration, thereby affecting the photoelectric properties of the material.12 The investigations of the vibrational properties of CsPbCl3 are necessary to explore the evolution of the metal-halide sublattice under high pressure. Selected Raman spectra of CsPbCl3 upon compression are shown in Figure 3. Only two distinct Raman vibrational modes in the low-frequency range of 70−280 cm−1 under ambient conditions owing to heavy ions present in the material were observed. According to previous Raman studies of metal-halide perovskite materials, the vibrational peaks v1 and v2 are associated with the vibrational mode of PbCl6 octahedron and motion of Cs+ cations, respectively.35,36 With the increasing pressure, the vibrational peaks v1 and v2 initiate a continuous shift to higher

Figure 2. (a) Representative synchrotron XRD patterns of CsPbCl3 upon compression of up to 20.7 GPa and decompression. (b) Unit cell parameter and (c) volume evolutions of CsPbCl3 as a function of pressure. 15222

DOI: 10.1021/acs.jpcc.8b05397 J. Phys. Chem. C 2018, 122, 15220−15225

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The Journal of Physical Chemistry C

redefined.12 To further understand the electronic structure behavior upon compression, we implemented first-principles calculations on both charge density and the PDOSs. The crystal structure obtained from Rietveld refinements was an initial input for the calculations. As shown in Figure 4a, with the shrinkage of the lattice, both the calculated θc (Pb−Cl−Pb angle along c axis) and θab (Pb−Cl−Pb angle in ab plane) angles exhibit a continuous slow decrease, confirming octahedral rotation and titling upon compression. However, the θab angle rapidly decreases at 2.5 GPa, and the θc angle begins to increase when the θab angle is reduced to a certain threshold. A torsional rotation of PbCl6 octahedron relative to its neighboring octahedra along the c axis accompanied by deviation from perfect local octahedral symmetry gives rise to a severe distortion of the Pb−Cl inorganic skeleton in threedimensional space. The calculation results coincide with our experimental analysis, implying the existence of isostructural phase transition. This computational evolution mechanism enhances our understanding of the phase transition process of the crystal under high pressure. We can observe significant changes of θc and θab along with the PbCl6 octahedron irregular deformation by the computational crystal structure as a function of pressure (Supporting Information Figure S4). We successfully monitored charge redistribution, which was attributed to Pb−Cl bond contraction and PbCl6 octahedral tilting (Figure 4c). Compared to the ambient structure, the high-pressure structure at 1.5 GPa has more electronic charge density overlapping between the Pb and Cl atoms. The calculated band gap is 2.24 eV under ambient conditions, slightly less than the experimental value. The band gap is underestimated by using the standard DFT XC functional (including GGA).37 Compared to the valence band, the conduction band is obviously more dispersive because of it being more delocalized, and the lowest direct gap is found to be located at the Γ-point of the Brillouin zone (Figure 4d). The VBM is characterized as an antibonding hybridized state, consisting of Cl 3p orbitals and a small amount of Pb 6s

Figure 3. Raman spectra of CsPbCl3 as a function of pressure at room temperature.

frequency due to the crystal structure contraction, while the profile of the Raman spectra remains in its original state up to 1.3 GPa. Above 1.6 GPa, the original Raman modes evidently broaden and dramatically decrease in intensity, suggesting that the phase transition occurred originated from the PbCl6 octahedral tilting and twisting upon compression. Under higher pressure, the progressive deteriorated crystallinity gradually aggravates and cannot clearly distinguishes two Raman peaks under higher pressure until the vibration modes completely disappeared at 10.6 GPa. Upon pressure releasing, the Raman spectrum restored to the original state, indicating that the changes of crystals are reversible, which is in accordance with the XRD diffraction patterns. The photoelectric properties of CsPbCl3 are inevitably affected because of the changes of the lattice structure, leading to the boundary conditions of the electron wave function are

Figure 4. (a) First-principles calculated Pb−Cl−Pb bond angle of CsPbCl3 as a function of pressure. (b) First-principles calculated band gap for CsPbCl3 at different pressures. (c) Charge density of CsPbCl3 under ambient conditions and 1.5 GPa. (d−f) Representative band structure and projected density of states for CsPbCl3 under ambient conditions, 2.0 and 5.0 GPa, separately. 15223

DOI: 10.1021/acs.jpcc.8b05397 J. Phys. Chem. C 2018, 122, 15220−15225

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The Journal of Physical Chemistry C Notes

orbitals, whereas the CBM shows mostly a nonbonding Pb 6p character. It is remarkable that the electronic levels from Cs+ cations are located deep away from the VBM and CBM; therefore, they just play a compensating role for the charge valence and do not contribute to the band edge states directly, implying that the optoelectronic properties of CsPbCl3 are mainly determined by PbCl6 octahedra. Upon compression, Pb−Cl bond contraction enhances the coupling between Cl 3p and Pb 6s orbitals (Supporting Information Table S2), which increases band dispersion and pushes up the VBM (Figure 4e).12,33 In contrast, the rotation and tilting of the octahedron can decrease orbital coupling and push down the VBM, therefore increasing the band gap (Figure 4f). The CBM with mostly a nonbonding localized state is not sensitive to bond contraction or pressure. Pb−Cl bond shrinkage is accompanied by the twisting and tilting of PbCl6 octahedra upon compression, continuous red shift of the band gap before 1.7 GPa implies that Pb−Cl bond shrinkage brings into play a more significant effect on the electronic structure than octahedral tilting and twisting in this regime. After structural transformation, the dramatic blue shift in the band gap should be attributed to reduce the coupling between Cl 3p and Pb 6s orbitals by the Pb−Cl inorganic framework distortion.19 As shown in Figure 4b, the calculated band gap red-shifts before 2.0 GPa, followed by a sudden blue shift up to 5 GPa. The evolution trend of the calculated band gap is in good agreement with our UV−vis absorption experiments, confirming the accuracy of the calculated results.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Science Foundation of China (NSFC) (nos. 21725304, 11774120, 21673100, and 91227202), Chang Jiang Scholars Program of China (no. T2016051), Changbai Mountain Scholars Program (no. 2013007), and program for innovative research team (in science and technology) in the university of Jilin Province. Angle-dispersive XRD measurement was performed at the BL15U1 beamline, Shanghai Synchrotron Radiation Facility (SSRF).

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CONCLUSIONS In summary, our results demonstrate that the optical properties of metal-halide perovskites can be tuned obviously under mild pressure by using an ingenious physical tool. Moreover, a structural transformation and reversible deteriorated crystallinity in CsPbCl3 were observed by high-pressure XRD and Raman experiments. Combined with theoretical calculations, the anomalous evolution of band gap is associated with the Pb−Cl bond contraction and PbCl6 octahedral tilting and twisting, which can significantly affect the orbital overlap and electric band dispersion. We also speculate on the structure transition mechanism based on computational Pb−Cl−Pb bond angle changes above 2.0 GPa. Our studies not only provide comprehensive understanding of the structure− property relationships of metal-halide perovskites but also shed light on a new guided route toward better materials-bydesign under ambient conditions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b05397. Absorption and PL spectra of CsPbCl3; PL micrograph; Rietveld refinement results; calculated structural evolutions and Pb−Cl inorganic skeleton of CsPbCl3; and first-principles calculations results (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 86-431-85168882. ORCID

Kai Wang: 0000-0003-4721-6717 Bo Zou: 0000-0002-3215-1255 15224

DOI: 10.1021/acs.jpcc.8b05397 J. Phys. Chem. C 2018, 122, 15220−15225

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DOI: 10.1021/acs.jpcc.8b05397 J. Phys. Chem. C 2018, 122, 15220−15225