Pressure-Induced Structural Evolution and Band Gap Shifts of

Letter pubs.acs.org/JPCL

Pressure-Induced Structural Evolution and Band Gap Shifts of Organometal Halide Perovskite-Based Methylammonium Lead Chloride Lingrui Wang,† Kai Wang,† Guanjun Xiao,† Qiaoshi Zeng,‡ and Bo Zou*,† †

State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China Center for High Pressure Science & Technology Advanced Research, Shanghai 201203, China

‡

S Supporting Information *

ABSTRACT: Organometal halide perovskites are promising materials for optoelectronic devices. Further development of these devices requires a deep understanding of their fundamental structure−property relationships. The effect of pressure on the structural evolution and band gap shifts of methylammonium lead chloride (MAPbCl3) was investigated systematically. Synchrotron X-ray diffraction and Raman experiments provided structural information on the shrinkage, tilting distortion, and amorphization of the primitive cubic unit cell. In situ high pressure optical absorption and photoluminescence spectra manifested that the band gap of MAPbCl3 could be fine-tuned to the ultraviolet region by pressure. The optical changes are correlated with pressureinduced structural evolution of MAPbCl3, as evidenced by band gap shifts. Comparisons between Pb-hybrid perovskites and inorganic octahedra provided insights on the effects of halogens on pressure-induced transition sequences of these compounds. Our results improve the understanding of the structural and optical properties of organometal halide perovskites.

O

The application of high pressure is a straightforward and robust way to explore the structural and electronic properties of materials.14−16 In recent years, research on the effect of pressure on OMHPs has considerably progressed.17−25 Zhao et al. illustrated that the maximum resistance of a MAPbBr3 crystal reaches 5 orders of magnitude higher than the starting value under high pressure.26 Gao et al. reported that the photocurrent of the MAPbI3 crystal at 1 GPa is larger than that under ambient conditions.27 Therefore, the pressure effect on the structural evolution and optoelectronic properties of OMHPbased perovskites is an area of great interest and challenge. Herein, we performed a systematic study to explore the behavior of MAPbCl3 at high pressure levels by combining in situ X-ray diffraction (XRD), Raman, optical absorption, and photoluminescence (PL) measurements. The XRD patterns revealed that MAPbCl3 underwent two structural transitions and subsequent amorphization during compression. The Raman spectra of MAPbCl3 under applied pressure were recorded to study the interplay between the organic MA cation and the corner-sharing inorganic octahedra. Both high-pressure absorption and PL experiments of MAPbCl3 exhibited a red shift followed by a blue shift in wavelength. The pressuredependent optical spectra and band gaps derived from the absorption edge were correlated with structural evolutions. The

rganometal halide perovskites (OMHPs) have gained increasing attention because of their extraordinary optoelectrical properties, low cost, and simple solution-based fabrication procedures.1−4 The power conversion efficiency of OMHP-based solar cells has significantly increased 6-fold from 3.8 to 22.1%.5,6 Many scholars have focused on studying halide perovskite compounds with the general formula AMX3, where A is a large organic molecular cation and M is a divalent metal cation octahedrally coordinated by anion X. This class of materials includes the crystals of MAPbX3 (MA = CH3NH3+, X = Cl, Br, I); MAPbI3 and MAPbBr3 have been studied extensively because of their strong optical absorption and emission across the visible spectra.7,8 Maculan et al. reported that the trap-state density, charge carrier concentration, mobility, and diffusion length of MAPbCl3 are comparable with those of the best quality crystals of MAPbI3 and MAPbBr3.9 Furthermore, MAPbCl3 possesses a wide band gap of 3.11 eV and is visibly transparent but sensitive to the ultraviolet (UV) region. MAPbCl3-based UV photodetectors exhibit high ON−OFF current ratio, fast photoresponse, and long-term photostability.10 MAPbCl3 can also be used in transparent optoelectronics, fire and missile plume detection, and optical communications.11−13 However, MAPbCl3 has been rarely investigated both theoretically and experimentally. The optimal band gap and suitable structure of the materials are key factors for successful application of OMHPs in optoelectronic devices. Hence, researchers should explore effective techniques that further investigate the structural and optical properties of highly efficient photovoltaic perovskites. © 2016 American Chemical Society

Received: October 18, 2016 Accepted: December 5, 2016 Published: December 5, 2016 5273

DOI: 10.1021/acs.jpclett.6b02420 J. Phys. Chem. Lett. 2016, 7, 5273−5279

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The Journal of Physical Chemistry Letters pressure-induced structural transition sequences of MAPbX3 strongly depended on the halogen X. These observations elucidate the fundamental relationship between structural variation and optoelectronic properties of OMHPs. This study provides an improved understanding of the band gap modulation and structural optical properties of MAPbCl3 under high pressure, and we hope that these findings will be help in improving the efficiency of perovskite-based optoelectronic devices. Figure 1 shows representative XRD data of MAPbCl3 during compression up to 20.0 GPa and decompression (Supporting

Figure 2. Pressure dependence of the unit cell volume of MAPbCl3. The deformation in the unit cell volume is fitted with the third-order Birch−Murnaghan equation of state.

using the third-order Birch−Murnaghan equation of state.17 The change in volume with pressure levels of 0.8 and 2.0 GPa is associated with pressure-induced structural modification. However, no noticeable change was observed before and after acquiring the XRD pattern at 0.8 GPa. Therefore, we suggested that the first phase transition at 0.8 GPa is a cubic to cubic isostructural phase transition, and the second phase transition at 2.0 GPa is associated with a pressure-induced cubic to orthorhombic phase transition. The crystal structure of the new high-pressure phases must be determined to understand the pressure-induced phase transitions. Figure 3 presents the Rietveld refinement profiles Figure 1. Angle-dispersive XRD patterns of MAPbCl3 at various pressure levels. The arrows mark new diffraction peaks appearing at 2.0 GPa.

Information, Figure S1). The first XRD pattern under ambient conditions confirmed that the initial structure showed good 28 phase purity in the cubic phase with space group Pm3m ̅ . As the pressure increased, all diffraction peaks shifted toward higher angles because pressure induced a decrease in the unit cell volume. However, a new set of diffraction patterns appeared suddenly at 2.0 GPa and finally stabilized at 2.4 GPa, indicating the presence of a pressure-induced phase transition. When the applied pressure exceeded 5.6 GPa, a broad background caused by the appearance of diffuse scattering implied the onset of amorphization.17−26 Moreover, the intensity of all of the original peaks decreased abruptly and disappeared with increasing pressure up to 20.0 GPa; this finding indicates that amorphization gradually occurred. The pressure-induced amorphization of MAPbCl3 retained its tilting distortion of the inorganic perovskite skeleton but with highly distorted organic molecules. Upon decompression, the amorphous state sample returned to the initial MAPbCl3 crystalline form. The XRD patterns of MAPbCl3 were subjected to Rietveld refinement; the indexed and refined structural parameters are given in Table S1 (Supporting Information). The structure at low pressure can be indexed into a cubic phase as the original phase, and the structure at 2.4 GPa is an orthorhombic phase. Figure 2 shows the variation in the unit cell volume as a function of pressure. The pressure−volume data were fitted

Figure 3. Crystal structure of MAPbCl3 phases I (cubic, Pm3̅m), II (cubic, Pm3m ̅ ), and III (orthorhombic, Pnma) at 0, 0.8, and 2.4 GPa. 5274

DOI: 10.1021/acs.jpclett.6b02420 J. Phys. Chem. Lett. 2016, 7, 5273−5279

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The Journal of Physical Chemistry Letters

and at around 3000 cm−1 (C−H and N−H stretching).32 At ambient pressure, MAPbCl3 (phase I) exhibited a cubic phase with space group Pm3̅m. In this structure, the Pb atom and the MA cation (with C3v molecular symmetry) belong to the Oh site group symmetry, and the three chlorine atoms lie on the D4h sites. Therefore, all lattice modes in phase I showed broad multiple bands and were inactive in Raman scatting, which were not easily resolved into components.31 As the pressure increased to 0.8 GPa, the spectra in lattice modes remained unchanged and characterized by almost the same degree of disorder. However, a new peak appeared at 998 cm−1, which is associated with rocking of the MA cation, and was accompanied by the disappearance of the original peak at 1007 cm−1. This finding demonstrates that the rotational modes of the MA cations were affected by the surrounding octahedra.31 The crystal compression fully relied on shortening of the Pb−Cl bonds and the volume of voids gradually confined around the MA cations. Hence, the transition from cubic to cubic isostructural relies mainly on the confined rotation of the MA cation.14 When the pressure increased to 1.9 GPa, the sudden appearance of lattice modes denoted with a star in Figure 5

of MAPbCl3 at selected pressure levels. Details of the refinements can be seen in Table S2 (Supporting Information). The first XRD pattern under ambient conditions confirms that the initial structure showed good phase purity in the cubic phase with space group Pm3̅m, and the cell parameter was a = 5.6938(9) Å, consistent with previous results.29 As the pressure increased in the phase I range, the Pb−Cl−Pb bond angle remained at 180° and the bond length decreased. The structure of phase II at 0.8 GPa is indexed into a cubic unit cell as phase I; the structure retained the lattice type and space group symmetry Pm3̅m with a lattice constant of 5.6055(8) Å. The phase transition from phase I to phase II is attributed to the cubic to cubic isostructural phase transition. The Pb−Cl−Pb bond angle remained unchanged, and the Pb−Cl bond length continuously decreased during the transition. At 2.4 GPa, an orthorhombic unit cell with space group Pnma was used to fit the split diffraction peaks. The lattice parameters were defined as follows: a = 7.5498(9) Å, b = 7.8352(4) Å, c = 11.0622(7) Å. The Pb−Cl−Pb bond angle deviated from the original 180°, and the [PbCl6]4− octahedra began to tilt, thereby inducing considerable rotations of the MA cations through hydrogenbonding interaction between the methylammonium (MA) group and the chlorine atoms.30 Hence, the MAPbCl3 showed the following structural evolutions under pressure: Pm3m ̅ → Pm3̅m → Pnma. Upon decompression, the amorphous MAPbCl3 returned to the original state, and the XRD pattern could be indexed as the Pm3̅m unit cell. To effectively probe the local structure and the dynamics of the organic MA cation and the halogens, we performed highpressure Raman experiments. The Raman spectra of MAPbCl3 as a function of pressure are shown in Figure 4. The lattice modes (80−160 cm−1) are associated with Pb−X vibrational modes consisting of ionic/covalent interactions in the inorganic framework.31,32 Characteristic internal vibrational modes of MA appeared at high frequencies, namely, below 1000 cm−1 (C−N stretching), at 1400−1600 cm−1 (C−H and N−H bending),

Figure 5. (a) Changes in the optical absorption and (b) PL spectra of MAPbCl3 under pressure. The black arrows indicate the evolution of the absorption and PL spectra as a function of pressure, and the red stars highlight the emergence of new peaks.

might have been related to important changes because of the breakdown of the symmetry and the tilting distortion of the [PbCl6]4− octahedra. The internal modes of the MA cation were also sensitive; all observed Raman spectra became well resolved and showed increased intensity. The red shift of the C−H and N−H bending modes could be attributed to the strengthening of the intermolecular interaction. The interplay of the organic MA cation and inorganic octahedra could be mediated by hydrogen bonding between the MA groups and the halide atoms.30,33 The rotation of the MA cations and the tilting of [PbCl6]4− octahedra can be affected. Hence, the second transition is mainly related to the tilting distortion of the [PbCl6]4− octahedra. With a further increase in pressure, the MA cation became trapped in the squeezed voids and showed distorted shape because of the tilting distortion of the [PbCl6]4− octahedra through the hydrogen-bonding interaction

Figure 4. Selected Raman spectra of MAPbCl3 at elevated pressure levels within 40−250, 900−1150, 1380−1680, and 2700−3300 cm−1. The red upward arrows show the evolution of the modes, and the stars denote the emergence of new peaks. 5275

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ambient conditions to 3.00 eV at 0.64 GPa, increased from 3.01 eV at 0.83 GPa to 3.13 eV at 1.8 GPa, and abruptly increased to 3.15 eV at 1.9 GPa, followed by a continuous increase. These findings demonstrate that the band gap of MAPbCl3 can be fine-tuned to the UV region by pressure, accompanied by the appearance of two turning points originating from the structural changes in the crystal. The band gap of MAPbCl3 was mainly determined by changes in the valence band maximum (VBM) and conduction band minimum (CBM).40,41 The VBM is characterized as an antibonding hybrid state of the Pb-s and the Cl-p orbitals in the [PbCl6]4− octahedral network, and the CBM is characterized as a nonbonding hybrid state of the Pb-p orbitals. When pressure was applied to the cubic phase I of MAPbCl3 perovskite, the [PbCl6]4− octahedra contracted. Therefore, the Pb−Cl bond length decreased, and the coupling of the Pb-s and the Cl-p orbitals enhanced and pushed up the VBM. The CBM is less sensitive to the contraction of the Pb− Cl bond because of its nonbonding characteristic; as such, the band gap showed red shifts with pressure in phase I. As the pressure increased in phase II the Pb−Cl−Pb bond angle remained unchanged and the Pb−Cl bond length continuously decreased. The abrupt increase in band gap is related to the change in electronic state caused by the isostructural transition.36−38 When the pressure was increased to 1.9 GPa, the Pb−Cl−Pb bond angle deviated from the original 180° and the coupling of the Pb-s and Cl-p orbitals was reduced; these findings could explain the subsequent blue jump of the band gap.23 Hence, pressure-driven optical change can be mechanistically elucidated by considering the correlations between band gap and structure. Remarkably, the transition sequences differed among the MAPbX3 family (X = Cl, Br, I), with only the compositional or structural variable of halogen X. The three compounds of MAPbX3 can exist in different phases, as summarized in Figure 7. To understand the observed results, we determined factors

between the organic MA cation and the inorganic octahedra. At 5.8 GPa, most Raman vibrational peaks broadened, indicating that the sample exhibited an onset of amorphization. The pressure-induced amorphization of MAPbCl3 retained its tilting distortion of the [PbCl6]4− octahedra inorganic perovskite skeleton but with highly distorted MA organic molecules. The amorphous MAPbCl3 reverted to its original state during decompression, indicating that the flexible organic MA cations act as templates for the frameworks, leading to structural memory effects.17−26 The compression changed the structures of MAPbCl3, and the optical properties were inevitably affected. To investigate the optical properties of MAPbCl3, we characterized the absorption and PL spectra (Figure 5). At ambient pressure, MAPbCl3 was transparent and colorless. We observed a sharp absorption edge located at 400 nm (Figure 5a) and a PL peak centered close to 405 nm (Figure 5b), which demonstrated that MAPbCl3 is suitable for UV detectors.34,35 The absorption edge and PL peak first experienced a gradual red shift within 0−0.8 GPa, followed by an abrupt blue shift above 0.8 GPa. The optical change corresponded to isostructural transition, which is related to changes in the electronic state.36−38 At around 1.9 GPa, the original absorption edge and PL peak disappeared, and the new absorption edge and PL peak continued to blue shift; these changes could be attributed to cubic to orthorhombic transition. The PL peak in Figure 5b became weaker and undetectable at 5.7 GPa because of pressureinduced amorphization.22,26 The new absorption edge left a broad absorption tail at approximately 340 nm under a pressure of 8.0 GPa (Figure 5a). Upon the release of pressure, the amorphous MAPbCl3 could completely revert to its original state. A thorough analysis of the band gap is necessary to understand the characteristics of MAPbCl3. Figure 6 shows the pressure dependence of the optical band gap. The discrete band gaps of MAPbCl3 at different pressure levels were estimated using Tauc’s plot by plotting (αhν)2 versus hν and by extrapolating the linear portion of the absorption edge to determine the intercept with the energy axis (inset in Figure 6).39 The pressure-driven band gap decreased from 3.06 eV at

Figure 7. Summary of the crystal systems and space groups adopted by MAPbCl3, MAPbBr3,26, and MAPbI323,42 as a function of pressure.

that might influence the structural variables of lead halide perovskites. Upon proceeding down group VIIA (Cl → I), halogens with large atom size show a small Goldschmidt tolerance factor and strong distortion from cubic symmetry.43 Therefore, MAPbI3 is tetragonal and MAPbCl3 and MAPbBr3 are cubic under ambient conditions.44 With increasing pressure, the three crystal symmetries decreased from cubic/tetragonal to orthorhombic. Moreover, the first phase transition points included 0.8 GPa (MAPbCl3), 0.4 GPa (MAPbBr3), and 0.3 GPa (MAPbI3); that is, the smaller the halogen X radius, the higher the transition pressure. This occurrence can be due to the large radius of the halogen X cations and electronegativity to match that of Pb, in the order Cl−, Br−, and I−.45 Hence, the

Figure 6. Derived band gap of MAPbCl3 as a function of pressure. The error bars reflect the variance in extrapolating the band edges. The inset shows the Tauc plot of MAPbCl3 under ambient conditions. 5276

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The Journal of Physical Chemistry Letters halogen atom X could strongly influence the structural changes in halide perovskite under high pressure. For similar pressureinduced phase transition paths of MAPbBr3 and FAPbBr3 (FA = NH2CHNH2), the small organic cations (MA and FA) simply act as templates and minimally affect the pressure-induced structural transition sequence of perovskites consisting of the same octahedra framework. The comparison of pressureinduced phase transition sequences between the Pb-hybrid perovskites and the sequence is mainly dependent on the structure of the inorganic cage and, particularly, on the halogen atoms. In summary, we investigated the pressure-induced band gap shifts and the associated structure−property relationships of chloride-based hybrid perovskites. The structural transition sequence of MAPbCl3 under high pressure is as follows: Pm3̅m → Pm3̅m → Pnma. The first transition is the cubic to cubic isostructural transition, relying mainly on rotation of the MA cation. The pressure-induced band gap exhibits a red shift in phase I and a subsequent blue shift in phase II. The second transition is related to the tilting and distortion of the inorganic [PbCl6]4− octahedra, leading to a widened band gap. Moreover, the reversible pressure-induced amorphization of MAPbCl3 is related to the flexible organic MA cations. The halide atom X mainly affects the transition sequence of the MAPbX 3 perovskites under high pressure. Studies of the band gap modulation and structural optical properties of MAPbCl3 under high pressure would provide an improved understanding of the intrinsic properties of chloride-based perovskites in the development of OMHP-based optoelectronic devices.

fiber spectrometer was an Ocean Optics QE65000 spectrometer. 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. Then, XRD data were recorded using an imaging plate detector and transformed into one-dimensional XRD patterns with the FIT2D program. Materials Studio with the reflex module was also used to create the initial structures and visualize the results. Raman Measurements. In situ high-pressure Raman spectra were recorded using a spectrometer combined with a liquidnitrogen-cooled CCD (iHR 550, Symphony II, Horiba Jobin Yvon). A single-mode DPSS laser (power output, 10 mW) at 671 nm used as the excitation light source. The resolution of the system was about 1 cm−1.

EXPERIMENTAL SECTION Sample Preparation and High-Pressure Generation. The synthesis of crystalline MAPbCl3 has been described in detail in previous work.46 Lead acetate trihydrate Pb(CH3COO)2·3H2O and methylamine hydrochloride (CH3NH2·HCl) were purchased from Aladdin Co. and used without further purification. Briefly, an excess stoichiometric amount of Pb(CH3COO)2·3H2O was added drop-by-drop to a hot aqueous solution (50 °C) of CH3NH2·HCl. The solution was filtered to remove the precipitation, and the colorless CH3NH3PbCl3 crystals were grown from the slow-cooling filtered solution under vacuum. Consider that the samples are air-sensitive and the grinding and sample loading processes were carefully conducted in a N2filled glovebox. High-pressure experiments were performed with a symmetric diamond anvil cell (DAC). A T301 steel gasket was made from a 250 μm thick piece preindented to 40 μm. Then, a center hole with a diameter of 120 μm was drilled as the sample chamber. Small ruby balls were inserted into the sample compartment for in situ pressure calibration according to the R1 ruby fluorescence method.47 In high-pressure optical absorption, PL and XRD experiments were carried out with silicone oil (150 cst, Aldrich) as the pressure-transmitting medium (PTM); high-pressure Raman measurements with argon were performed. The ruby lines were found to be sharp and well-separated to the highest pressure in our studies. All of the measurements were performed at room temperature. Absorption and PL Measurements. In situ absorption and PL experiments of the samples were conducted by a synthetic IIatype diamond with a high transmittance in the UV region and permit photoemission studies. Absorption spectra (250−1000 nm) were measured using a deuterium−halogen light source, and the excitation source, the 355 nm line of a UV DPSS laser with the power of 10 mW, was used for PL measurements. The

ORCID

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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.jpclett.6b02420. XRD patterns for the MAPbCl3 perovskite by decreasing pressure and refinements structure details (PDF)

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AUTHOR INFORMATION

Corresponding Author

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*E-mail: [email protected].

Bo Zou: 0000-0002-3215-1255 Author Contributions

L.W. and B.Z. designed and performed experiments and analyzed data. K.W., G.X., K.Y., and Q.Z. assisted in performing experiments. B.Z. provided intellectual input. L.W., K.W., G.X., and B.Z. wrote the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the NSFC (Nos. 91227202 and 21673100), the Changbai Mountain Scholars Program (No. 2013007), and the Program for Innovative Research Team (in Science and Technology) at the University of Jilin Province. Angle-dispersive XRD measurement was performed at the 15U1 beamline, Shanghai Synchrotron Radiation Facility (SSRF).

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REFERENCES

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