Method for analyzing the structural change process of lead-based perovskite compounds using raman spectroscopy and brillouin spectrum analysis

Abstract

The present invention relates to a method for analyzing the structural change process of a lead-based perovskite compound through Raman spectroscopy and Brillouin spectrum analysis. The present invention provides a method for analyzing the structural change process of a lead-based halide perovskite, comprising the steps of: preparing a lead-based halide perovskite; analyzing the lead-based halide perovskite by Raman spectroscopy using a standard Raman spectrometer; and analyzing the Brillouin spectrum pattern of the lead-based halide perovskite using a Brillouin spectrometer.

Description

Technology Field

[0001] The present invention relates to a method for analyzing the structural change process of lead-based perovskite compounds through Raman spectroscopy and Brillouin spectrum analysis. Background Technology

[0002] Lead-based halide perovskites (LHPs) using the chemical formula ABX3 consist of CH3NH3 (methylammonium, MA) or CH(NH2)2 (formamidinium, FA) cations at the A site, Pb at the B site, and Cl, Br, or I at the X site. Due to the high compositional flexibility of these perovskites, we can have various similarities and great degrees of freedom in tuning their physical properties. The most widely studied organic-inorganic composition of LHP is MAPbX3 (X = Cl, Br, I). Here, the MA cation is concentrated inside the lead halide cage, whereas inorganic cations such as Cs can also occupy the same position.

[0003] LHPs have demonstrated significant functional importance in photovoltaics and optoelectronics. Furthermore, they have attracted widespread interest due to attractive properties such as long carrier diffusion lengths, low trap density, large absorption coefficients, low-cost manufacturing pathways, and solution processability. Due to these advantages, the use of LHPs in various applications, such as solar cells, photodetectors, and light-emitting diodes, has increased rapidly. For example, recent research on perovskite solar cells has demonstrated remarkable optoelectronic performance, with photoconversion efficiencies increasing dramatically by up to 25% within just a few years. Moreover, LHPs are attractive technical materials that can be used as probes in the fields of photocatalysis and bioimaging. In particular, methylammonium chloride (MAPbCl3) is known for its wide bandgap and ultraviolet photodetector effects.

[0004] Structural changes and phase transitions in hybrid perovskites explain changes in macroscopic properties related to device performance. For example, permittivity is altered due to distortion of the inorganic octahedral structure and symmetry breaking caused by the substitution of organic cations. For these reasons, research on structural phase transitions is very important.

[0005] MAPbCl3 exhibits a high bandgap energy of 2.88 eV, making it transparent and ideal for visible light spectroscopy. As temperature decreases, MAPbCl3 undergoes two major phase transitions: from cubic to tetragonal, and from tetragonal to orthorhombic. Many previous reports have investigated the phase transition temperatures and optical phonon modes of MAPbCl3 using Raman spectroscopy. This indicates that some discrepancies still remain between the reported phase transition temperatures. Furthermore, previous Raman studies lack a complete analysis of optical phonon modes across a wide frequency range as a function of temperature. Therefore, we aimed to investigate the structural phase transitions of this material in greater detail through a complete Raman spectroscopic investigation. Previously, Maleej et al. conducted one of the first Raman spectroscopic studies on MAPbCl3. In that study, the authors reported changes in the Raman spectrum only within a limited frequency range and at a few temperatures. Recently, Nguyen et al. discussed the temperature-dependent changes in the Raman spectrum of MAPbCl3, but there was a lack of discussion regarding low-frequency modes.

[0006] The goal of our research is 10–3500 cm 1The goal was to extend the Raman investigation to all optical phonons of MAPbCl3 across the entire frequency range and a wide temperature range from -190°C to 20°C. To our knowledge, this is the first study to include a detailed discussion of the temperature-dependent Raman modes of MAPbCl3 across the widest frequency range. Since optical phonons investigated by Raman spectroscopy are sensitive to structural changes, detailed mode analysis can clarify the phase transition behavior of this interesting material. Furthermore, in addition to all beneficial properties, the widespread commercial use of LHPs is hindered by rapid decomposition and low chemical and mechanical stability. These perovskites are unstable under harsh environmental conditions, such as high temperature and humidity, leading to device performance degradation. Therefore, it is crucial to monitor the structural change processes of MAPbCl3 using various methods. In this invention, both high-temperature Raman scattering and Brillouin scattering measurements were performed for the first time to monitor the thermal stability, specifically the structural change processes, of MAPbCl3 single crystals. Prior art literature

[0007] Zhao, Y.; Zhu, K. Organic-Inorganic Hybrid Lead Halide Perovskites for Optoelectronic and Electronic Applications. Chem. Soc. Rev. 2016, 45, 655-689.

[0008] Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; et al. Low Trap-State Density and Long Carrier Diffusion in Organolead Trihalide Perovskite Single Crystals. Science 2015, 347, 519-522.

[0009] De Wolf, S.; Holovsky, J.; Moon, S.J.; L per, P.; Niesen, B.; Ledinsky, M.; Haug, F.J.; Yum, J.H.; Ballif, C. Organometallic Halide Perovskites: Sharp Optical Absorption Edge and Its Relation to Photovoltaic Performance. J. Phys. Chem. Lett. 2014, 5, 1035-1039.

[0010] Burschka, J.; Pellet, N.; Moon, S.J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M.K.; Gr tzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316-319.

[0011] Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051.

[0012] Dou, L.; Yang, Y.M.; You, J.; Hong, Z.; Chang, W.H.; Li, G.; Yang, Y. Solution-Processed Hybrid Perovskite Photodetectors with High Detectivity. Nat. Commun. 2014, 5, 5404.

[0013] . Tan, Z.K.; Moghaddam, RS; Lai, M.L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L.M.; Credgington, D.; et al. Bright Light-Emitting Diodes Based on Organometal Halide Perovskite. Nat. Nanotechnology. 2014, 9, 687-692.

[0014] NREL Efficiency Chart. Available online: http: / / www.nrel.gov / ncpv / images / efficiency_chart.jpg (accessed on 27 July 2022).

[0015] Zhu, Y.; Liu, Y.; Miller, K.A.; Zhu, H.; Egap, E. Lead Halide Perovskite Nanocrystals as Photocatalysts for PET-RAFT Polymerization under Visible and Near-Infrared Irradiation. ACS Macro Lett. 2020, 9, 725-730.

[0016] Zhang, H.; Wang, X.; Liao, Q.; Xu, Z.; Li, H.; Zheng, L.; Fu, H. Embedding Perovskite Nanocrystals into a Polymer Matrix for Tunable Luminescence Probes in Cell Imaging. Adv. Funct. Mater. 2017, 27, 1604382. The problem to be solved

[0017] The present invention aims to provide a method for analyzing the structural change process of lead-based perovskite compounds through Raman spectroscopy and Brillouin spectrum analysis.

[0018] The purposes of the present disclosure are not limited to those mentioned above, and other purposes and advantages of the present disclosure not mentioned may be understood from the following description and will be more clearly understood from the embodiments of the present disclosure. Furthermore, it will be readily apparent that the purposes and advantages of the present disclosure can be realized by the means and combinations thereof set forth in the claims. means of solving the problem

[0019] The present invention provides a method for analyzing the structural change process of a lead-based halide perovskite, comprising the steps of: preparing a lead-based halide perovskite; analyzing the lead-based halide perovskite by Raman spectroscopy using a standard Raman spectrometer; and analyzing the Brillouin spectrum pattern of the lead-based halide perovskite using a Brillouin spectrometer.

[0020] The step of analyzing by the above Raman spectroscopy may include the step of exciting a lead-based halide perovskite using a diode-pumped solid-state laser with a wavelength of 532 nm.

[0021] The above method may further include the step of analyzing the XRD pattern of lead-based halide perovskite using an XRD (X-ray diffraction) spectrometer.

[0022] The present invention also provides a system for analyzing the structural change process of a lead-based halide perovskite, which is performed by a computing device, comprising the steps of: analyzing the lead-based halide perovskite by Raman optical spectroscopy; and analyzing the Brillouin spectrum pattern of the lead-based halide perovskite using a Brillouin spectrometer.

[0023] The step of analyzing by the above Raman spectroscopy may include the step of exciting a lead-based halide perovskite using a diode-pumped solid-state laser with a wavelength of 532 nm.

[0024] The step of analyzing the Brillouin spectrum pattern may include the step of exciting a lead-based halide perovskite using a laser with a wavelength of 532 nm.

[0025] The above system may additionally include a step of analyzing the XRD pattern of lead-based halide perovskites using an XRD (X-ray diffraction) spectrometer.

[0026] The present invention also provides an apparatus for performing a method for analyzing a structural change process of a lead-based halide perovskite, comprising: a processor; and a memory storing at least one instruction executed through the processor, wherein the at least one instruction is executed to irradiate a lead-based halide perovskite with a laser beam and to obtain a Raman scattering spectrum and a Brillouin spectrum using the laser beam scattered from the lead-based halide perovskite.

[0027] The lead-based halide perovskite above is a material having the structural formula APbX3, where A is CH3NH3 (methylammonium, MA) or CH(NH2)2 (formamidinium, FA), and C can be Cl, Br, or I.

[0028] The above lead-based halide perovskite may be a material having the structural formula CH3NH3PbCl3. Effects of the invention

[0029] According to the present invention, a method for analyzing the structure of lead-based perovskite compounds through Raman spectroscopy and Brillouin spectrum analysis is provided. Brief explanation of the drawing

[0030] Figure 1 is a flowchart showing the manufacturing process of a lead-based perovskite compound. FIG. 2 shows a photograph of the synthesized lead-based perovskite compound (a), the basic unit cell structure (b), and the cubic structure. Pm3m This shows the X-ray diffraction pattern (c) of the MAPbCl3 powder. Fig. 3 is cubic 3 The absorption and emission spectra of a single crystal of MAPbCl3 are shown. The inset shows the band gap calculated using the Tauc plot method. Figure 4 shows the temperature-dependent Raman spectrum of a single crystal of MAPbCl3 from room temperature to -196°C, and 10–400 cm⁻¹ 1 (a), 400-1800 cm 1 (b) and 2700-3400 cm 1 (c) Indicates the frequency range. Figure 5 shows the Raman spectra of a single crystal of MAPbCl3 in orthorhombic (a), tetragonal (b), and cubic (c) phases and the best-fit curves. Gray circles and black solid lines represent the raw and total fitted spectra, respectively. The colored solid lines represent the fitting curves of individual Raman modes. Fig. 6 is 10-300 cm 1This shows the temperature dependence of the Raman shift over the frequency range (ad). Phase boundaries are indicated by vertical dashed lines along with the phase transition temperatures. Fig. 7 is 400–1700 cm 1 The temperature dependence of the Raman shift over the frequency range (a–c) is shown. Phase boundaries are indicated by vertical dashed lines along with the phase transition temperatures. Fig. 8 is 2700–3400 cm 1 The temperature dependence of the Raman shift over the frequency range (a–c) is shown. Phase boundaries are indicated by vertical dashed lines along with the phase transition temperatures. Figure 9 compares the Raman spectrum of MAPbCl3 decomposed at room temperature with the Raman spectra of its byproducts (PbCl2 and MACl). The black solid line indicates that the crystal has deteriorated due to exposure to high temperatures, and the green solid line indicates the crystal exposed to ambient conditions for a long time. For comparison, the Raman spectra of PbCl2 and MACl are also shown. Figure 10 compares the Brillouin spectra of the original (a) and decomposed MAPbCl3 (b) cooled to high temperature and RT. Circles and solid lines represent the best fit results to the raw data, respectively. Figure 11 shows the temperature dependence of the frequency (blue) and FWHM (red) of the LA (a) and TA modes (b) measured by Brillouin scattering. Fig. 12 is 10-300cm -1 This shows the temperature dependence of the full width at half maximum (FWHM) over the frequency range (a–d). Phase boundaries are indicated by vertical dashed lines along with the phase transition temperatures. Fig. 13 is 400-1700 cm -1 This shows the temperature dependence of the full width at half maximum (FWHM) in the frequency range (ac). Phase boundaries are indicated by vertical dashed lines along with the phase transition temperatures. Fig. 14 is 2700-3400 cm -1This shows the temperature dependence of the full width at half maximum (FWHM) in the frequency range (ac). Phase boundaries are indicated by vertical dashed lines along with the phase transition temperatures. Fig. 15 is 10–3500 cm -1 This shows the Raman spectrum of MAPbCl3 recorded in a wide frequency range from room temperature (RT) to 200°. Figure 16 shows the thermogravimetric analysis curve for the synthesized MAPbCl3 crystals. Figure 17 shows the Raman spectrum of MAPbCl3 from room temperature (RT) to 200°C, from 10 to 800 cm⁻¹. -1 It is shown within a limited frequency range. The inserted figure shows a single crystal before (Figure below) and after (Figure above) degradation. Figure 18 shows a comparison of the Brillouin spectra of the original MAPbCl3 single crystal (a) and the crystal (b) stored for one month under ambient conditions. Specific details for implementing the invention

[0031] In this specification, structural changes include degradation and phase transitions.

[0032] The present invention will be explained in more detail below through examples and experimental examples. However, these examples and test examples are intended to illustrate the present invention, and the scope of the present invention is not limited to these examples and test examples.

[0034] Experimental method

[0035] The precursors lead chloride (PbCl2, 99.999%), hydrochloric acid (HCl, 37%, ACS reagent), diethyl ether (HPLC grade, ≥99.9%), dimethyl sulfoxide (DMSO, anhydrous ≥99.9%), methylamine (CH3NH2, 40% in water), and ethanol (anhydrous 99.5%) were purchased from Merck Korea (Seoul). All of these chemicals were used as received without further purification.

[0036] Steps for preparing lead-based halide perovskites: Single crystal synthesis

[0037] The synthesis of MAPbCl3 single crystals consisted of a two-step process. The first step involves preparing methylammonium (MACl, CH3NH3Cl), and the second step involves the crystallization of MAPbCl3. The equation below illustrates the two-step reaction process.

[0038] CH3NH2 + HCl = CH3NH3Cl

[0039] CH3NH3Cl + PbCl2 = CH3NH3PbCl3

[0040] Figure 1 shows a schematic diagram of the overall synthesis process for a single crystal of MAPbCl3. First, to synthesize MACl, methylamine was placed in a round-bottom flask and placed in an ice bath. Subsequently, HCl (24.6 mL) was added dropwise to methylamine (30.6 mL) according to a molar ratio of CH3NH2:HCl = 1.2:1. The purpose of the ice bath was to maintain the reaction temperature. The mixture was continuously stirred for 2 hours until the solution was completely dissolved. Then, the excess solvent was evaporated in a rotary evaporator at 55°C under vacuum. This yielded a white, shiny crystalline MACl powder. The obtained powder was purified by dissolving it in ethanol (200 mL) at 40°C while stirring continuously for 2 hours. After complete dissolution, diethyl ether (200 mL) was added for precipitation. The precipitated powder was filtered from the solution. This purification step was repeated twice. The obtained MACl powder was dried overnight in a vacuum oven at 60°C.

[0041] In the second step, an equimolar solution of the obtained white MACl powder (1M, 2.78 g) and PbCl2 (1M, 0.58 g) was dissolved in DMSO (10 mL) by stirring at 60°C. After complete dissolution, the solution was filtered through a 0.22 μm syringe filter into a crystallization dish. The dish was covered with aluminum foil, and a few holes were punctured in the foil to facilitate crystallization and slow evaporation. The dish was then stored undisturbed at a constant temperature of 100°C for 1 to 2 days. After 1 to 2 days, approximately 5 x 4 x 2 mm 3 Transparent MAPbCl3 crystals were obtained. Then, the crystals were washed with acetone and dried overnight in a vacuum oven at 60°C.

[0042] Specialized technology

[0043] (1) A step of analyzing lead-based halide perovskites by Raman spectroscopy using a standard Raman spectrometer

[0044] A standard Raman spectrometer (LabRam HR800, Horiba Co., Longjumeau, France) was used to perform Raman measurements. The single crystal was excited using a diode-pumped solid-state laser with a wavelength of 532 nm. The irradiated frequency range was 10–3500 cm⁻¹. -1 It was. The Raman spectrometer is equipped with a low-frequency notch filter, which sets the lowest frequency limit to 10 cm. -1 This can be. All measurements are backscattering geometry c ( a , a + b It was performed using an optical microscope (BX41, Olympus Co., Tokyo, Japan) equipped with a 50x objective lens in )c. Here a , b and crepresents the cubic axis. In this geometry, scattered light is collected simultaneously along the same path as the incident light. 520 cm -1 The Raman spectrometer was calibrated before recording measurements using a silicon standard sample having a single peak. Measurements were performed at temperatures ranging from -196°C to room temperature (RT) using a cryostat (Linkam THMS600, Linkam Scientific Instruments Ltd., Surrey, UK) with a temperature stability of 0.1°C. For each measurement, a waiting period of 1 minute 30 seconds was given after reaching the target temperature to reach thermal equilibrium. The intensity of all measured Raman spectra was corrected for the Bose-Einstein thermal factor.

[0045] (2) A step of analyzing the XRD pattern of lead-based halide perovskite using an XRD (X-ray diffraction) spectrometer.

[0046] Powder XRD patterns were obtained using a high-resolution XRD spectrometer (PANalytical, X'pert PRO MPD, Malvern, UK) at Cu K-radiation (λ = 1.5406 Å) in the 2θ angle range of 10–60° at RT. For measurement, single crystals were ground into crystalline powder. XRD patterns were analyzed using PANalytical software (X'pert highscore v1.1).

[0047] (3) A step of analyzing the photoluminescence spectrum pattern of lead-based halide perovskites using a PL (photoluminescent) spectrometer.

[0048] PL spectra were measured at room temperature using a PL spectrometer (LabRam HR800, Horiba Co., Longjumeau, France). The slit width was 200 μm, and a 375 nm diode laser was used as the excitation source. Additionally, transmittance was measured using an optical absorption spectrometer (Duetta, Horiba Instruments, Kyoto, Japan) with a 100 μm slit width and a 1 kW output tungsten-halogen lamp as the light source.

[0049] (4) A step of analyzing the Brillouin spectrum pattern of lead-based halide perovskites using a Brillouin spectrometer.

[0050] Brillouin spectra were recorded using a standard tandem multipass Fabry-Perot interferometer (TFP-2, JRS Co., Zurich, Switzerland) with an excitation source of 532 nm. Backscattering geometry was used for measurements using a modified microscope (BH-2, Olympus, Tokyo, Japan). Temperature was controlled using the same temperature steps as those used in the Raman experiment.

[0051] Results and Discussion

[0052] Phase transition of the structure investigated by Raman spectroscopy

[0053] Figure 2a shows a photograph of a grown MAPbCl3 single crystal, and the unit cell of the cubic MAPbCl3 at RT is shown in Figure 2b, where the MA cation is located inside the octahedral space, i.e., between the octahedra. In contrast, Cl is present at the octahedral corners. Figure 2c shows the powder XRD pattern of the MAPbCl3 single crystal measured at RT. The diffraction peaks matched the indices of the cubic phase well, and no additional peaks were observed, confirming that the composition of the single crystal was correctly synthesized. Sharp and distinct diffraction peaks demonstrated the crystalline nature of the sample. The lattice constant obtained from the XRD pattern is 5.67 Å. The Goldschmidt tolerance factor of MAPbCl3 is close to 1 (T ≈ 0.93), which justifies the cubic structure at RT.

[0054] Figure 3 shows the absorption and PL spectra of a single crystal of MAPbCl3 measured at RT. The PL peak was observed at 406 nm, and the spectrum exhibits an asymmetric line shape as previously reported. Furthermore, the peak position shifts to blue relative to the first exciton peak. This may be due to the presence of shallow traps between the band edges. The absorption spectrum shows an absorption edge at 423 nm, corresponding to an optical band gap of 2.93 eV obtained using the Tauc plot method, as indicated in the inset of Figure 3. This is consistent with previously known studies.

[0055] The phonon mode behavior of MAPbCl3 was optically investigated using temperature-dependent Raman spectroscopy to characterize structural phase transitions and determine precise temperatures. Figures 4a–c show the phonon mode behavior over a wide temperature range from 190°C to RT, from 10 to 3500 cm⁻¹. 1This shows the temperature-dependent Raman spectrum in the frequency range. At RT, MAPbCl3 is cubic. As the temperature decreases, it transitions to tetragonal and orthorhombic phases. In the center-symmetric cubic phase at RT, Raman modes are inactive in principle. However, a wide range of Raman modes were observed due to random intrinsic disorder induced by freely rotating MA units and their displacements. Conversely, in the low-temperature orthorhombic phase, the overall symmetry of MAPbCl3 is broken due to the tilting of the PbCl6 octahedrons, resulting in various distinct Raman modes.

[0056] The crystal structure of MAPbCl3 consists of PbCl6 octahedrons and MA cations located in the octahedral space. Therefore, vibrational modes associated with the PbCl6 octahedrons, MA cation motions, and internal modes of the MA cations can be observed across various frequency ranges of the Raman spectrum. We classified the Raman spectrum and associated modes into three regions as shown in Fig. 4. 400 cm⁻¹ 1 The first region below contains Pb-Cl octahedral vibrational modes and other modes associated with the translational motion of the crystal lattice. The second region (400–1600 cm⁻¹) 1 It includes the restricted torsional mode and fluctuation mode of MA cations along with the bending modes of CH3 and NH3. 2700 cm⁻¹ 1 The third region described above consists of internal vibrational modes of MA cations, such as the stretching vibrational modes of CH3 and NH3. The Raman spectra of the three regions shown in Figures 4a-c exhibit distinct changes with temperature. The low-temperature spectrum of the orthorhombic phase (Figure 4a) displays several distinct and well-resolved peaks that broaden and merge as the temperature increases. The Raman spectra of the mid-frequency and high-frequency regions shown in Figures 4b and 4c display various changes in the peak positions and widths of the Raman modes with temperature. The individual mode behaviors and transition temperatures are revealed through curve-fit analysis of these modes.

[0057] Anomalous changes in Raman shift and FWHM (full width at half maximum) indicate important information related to phase transitions. Fitting analysis was performed using Lorentz functions to obtain the accurate Raman shift and FWHM of each Raman mode in the orthorhombic phase. Due to uniformly distributed dynamic disorder of the lattice resulting from the restricted motion initiation of MA cations, Raman peaks are expected to broaden with increasing temperature. Lorentzian line shapes were used to fit all Raman spectra after Bose-Einstein correction, as expressed by the following relationship:

[0058] IR(ν) = I(ν) / [n(v) + 1]

[0059] The corrected Raman intensity IR(ν) is the Bose-Einstein heat factor It was obtained from the Raman intensity I(ν) measured using [the formula]. In the final formula, h and k are Planck's constant and Boltzmann's constant, respectively, and ν and T are frequency and absolute temperature, respectively. Mode designation was performed on the Raman spectrum observed at -196°C. This is because Raman modes are well resolved at low temperatures due to smaller attenuation factors.

[0060] [Table 1]

[0061]

[0062]

[0063] Table 1 shows the mode assignments for all Raman modes of MAPbCl3. In the orthorhombic phase, 10–3500 cm⁻¹ -1 A total of 36 modes were observed over a wide frequency range. The grating modes were from 10 to 193 cm. -1 While observed up to [date], individual MA cation modes existed at higher frequencies. In the low-frequency range, at 26, 42, 55, 60, 68, and 77 cm⁻¹ 1 A mode was observed at . The lowest frequency mode of MAPbCl3 is 26 cm⁻¹.-1 It was observed at, and the lowest frequency mode of FAPbCl3 is 37 cm⁻¹ -1 This change in mode frequency was observed in FA + This may be because the cation radius of increased and the lattice of MAPbCl3 became relatively smooth. In the present invention, 26 and 68 cm 1 A new mode of was observed. The inventors identified this mode as MH sharing the same PbCl6 octahedron as MAPbCl3. y Assigned to a lattice free mode based on similarity (in terms of intensity and position) to a similar lattice mode of PbCl3 (methylhydrazine lead chloride). 68 cm -1 A mode was newly observed in the Raman spectrum, which was not seen in previous Raman spectroscopic studies. This mode is likely due to the release of degeneracy at 77 cm⁻¹, caused by the low symmetry of the orthorhombic phase. -1 It appears to be due to octahedral distortion similar to the mode. 93–193 cm -1 The modes observed in are associated with the Pb-Cl bending and stretching modes, as suggested in previous studies. In previous studies for the same frequency range, these modes were indicated as the result of MA cation motion coupled with Pb-Cl octahedral motion, as these modes do not exist when MA cations exist alone in vacuum.

[0064] The torsional mode of the MA cation is 484 cm⁻¹ -1 It appeared in. This mode is highly sensitive to the halide composition. For example, in MAPbCl3, 484 cm⁻¹ 1 , in MAPbBr3, 323cm 1 , 249cm in MAPbI3 1It is found at . Furthermore, this mode is also influenced by changes in the A-site cation. Similar behavior was observed for the torsion mode of FA cations in FA-based halide perovskites. These change trends suggest strong interactions between the organic cation and the halide atom. The MA torsion modes are at 923 and 1265 cm⁻¹. -1 It appeared at. The CN extension mode is 976cm -1 It was found in, and the symmetric and asymmetric bending modes of CH3 and NH3 are 1400–1600 cm -1 It was located within the range. 3000~3200cm -1 The Raman peaks observed at high frequencies include modes associated with the symmetric and asymmetric stretching of CH3 and NH3. The assignment of the observed modes is shown in Table 2 based on previous studies. 68, 2920, and 3201 cm⁻¹ 1 The three peaks located at were not found in previous studies and were therefore not assigned to vibration modes.

[0065] In MAPbCl3, since all three phases have different lattice sizes and symmetries, the Raman modes of each phase are expected to be distinguished by different numbers of allowed modes. The cubic space group Pm3m is. In the case of a tetragonal shape P4 / mmm is. Regarding the orthorhombic phase P222 1 This was proposed previously, but recently Pnma It was proposed. Figures 5a–c show the Lorentzian fitting curves of the orthorhombic, tetragonal, and cubic phases of MAPbCl3, respectively, along with 400 cm⁻¹. -1The low-frequency Raman spectrum observed below [value] is shown. The changes in the Raman spectrum are abrupt, confirming the existence of three different phases in MAPbCl3, as confirmed in previous reports. In the low-temperature orthorhombic phase (Fig. 5a), several sharp and decomposed peaks can be clearly identified. Nevertheless, the peaks combine to form a somewhat broad spectrum in the tetragonal phase (Fig. 5b). In the cubic phase (Fig. 5c), many peaks disappear, and a broad spectral feature remains.

[0066] As shown in Figures 6–8 and 12–14, the Raman shift and FWHM were expressed as a function of temperature to identify the phase transition temperatures. All Raman modes show clear anomalies near -114°C and -110°C, respectively, associated with the phase transitions from orthorhombic to tetragonal and from tetragonal to cubic. Since the tetragonal phase is stable only within a narrow temperature range of ~4°C, slight discrepancies in these phase transition temperatures may occur depending on the type of measurement technique. For convenience, the transition temperature from orthorhombic to tetragonal is denoted as T1, and the transition temperature from tetragonal to cubic is denoted as T2.

[0067] First, 400cm² showing substantial changes depending on crystal symmetry -1 Analyze all the following grid modes. The first and lowest frequency mode is 26 cm. -1 It was observed at , which shows a stepwise change at T2. Similar behavior was observed at 42 cm -1 It appears in the mode of (octahedral torsion). This first moves to higher wavenumbers at T2 and then exhibits a redshift to RT. 56 cm 1 (Octahedral twist mode) and 119 cm 1 Some Raman modes, such as the asymmetric bending mode of Cl-Pb-Cl, exhibit a red shift upon heating and eventually disappear at T2. Additionally, as the temperature increases, 77 cm -1 (Octahedral distortion mode), 144 cm-1 (Symmetric stretching mode of Cl-Pb-Cl) and 193 cm -1 The mode of the (symmetric stretching mode of Pb-Cl) disappears at T1. 93cm -1 The mode is 98cm -1 It exists as a shoulder peak of the mode (symmetric bending mode of Cl-Pb-Cl), exhibits a redshift, and disappears at T1. 98 cm -1 The mode exhibits anomalies near T1 and persists until RT with a weak blue shift. 164 cm 1 The mode (the asymmetric stretching mode of Pb-Cl) exhibits a weak red shift up to T2, where slight anomalies occur, followed by a continuous blue shift up to RT. On the tetragonal and cubic phases at T1, 237 cm⁻¹ 1 A new mode appears at . In the square phase, the appearance of this mode is at 235 cm -1 This is consistent with previous reports that observed a similar mode and associated it with the rotational motion of MA cations. Furthermore, previous studies revealed that this mode does not persist in the cubic phase up to 300 K. Likewise, we were able to observe this mode appearing and disappearing down to -50 K. The origin of this mode may be due to the unlocking of MA cations, which allows for many reorientations causing the emergence of a new mode in a spectrum with broad spectral features.

[0068] The FWHM of these modes shown in Fig. 12 also exhibits clear anomalies at the two transition temperatures. The FWHM is very low in the orthorhombic phase, whereas it becomes much larger in the high-temperature cubic phase. This indicates that the ordered arrangement of MA cations in the orthorhombic phase is responsible for weak anharmonicity and low attenuation of optical phonons. Recent Raman studies have revealed that MA cations are aligned along specific crystallographic directions in the orthorhombic phase. In contrast, the CN axis of MA cations is equivalent in the cubic phase <110> It is disordered along the orientation. In the cubic phase, the free rotational dynamics of MA cations actually induce strong dissonant lattice interactions, as can be seen in the FWHM data, leading to greater phonon decay. Some modes exhibit strong expansion upon heating until they disappear at a specific transition temperature. For example, 77 cm⁻¹ -1 The mode (octahedral distortion mode) first widens significantly and then disappears at T1. In previous studies, the FWHM of the general cage mode of MAPbCl3 was ~40 cm⁻¹. -1 It was found that it can reach, but in the Raman spectrum, the average width is ~20 cm -1 It was observed as such, which is the already known MAPbBr3 (~15cm -1 ) and MAPbI3(~10cm -1 It is similar to the values ​​of other halide perovskites such as ). All low-frequency cubic lattice modes exhibit very broad spectral features attributed to disorder effects caused by rotatable MA cations and heterogeneous local environments.

[0069] 400~1600cm -1 The intermediate frequency region of the range includes internal modes (e.g., torsional, fluctuating, and bending vibration modes) primarily associated with MA cations. The torsional mode (τ) of the MA cation is 484 cm⁻¹. -1 It is located at 483, 484, and 488 cm. In some other studies, this mode is at 483, 484, and 488 cm. -1It was observed in [location]. Since the migration of MA cations is associated with inorganic cages via NH-X hydrogen bonding, altering the halide atom has a significant effect on this mode (e.g., 249 cm⁻¹). -1 488 cm at (MAPbI3) -1 (MAPbCl3)). This mode undergoes significant stepwise hardening at T1, experiences slight changes at T2 with increasing temperature, and then persists until RT. Mode expansion upon heating (see Fig. 13, where the FWHM of all modes is shown in the intermediate frequency range) is MA in a highly symmetric cube. + Cations and Halogen Cl - This may be due to the weakening of hydrogen bonds between them. The FWHM of the torsional mode is the highest among other high-intensity modes at low temperatures, indicating that this mode is highly unharmonic and sensitive to the local environment. Similar behavior has been observed in other bromine and iodine-based MA halide perovskites.

[0070] It is interesting that the torsional mode and first fluctuation mode of the MA cation broaden significantly across the phase transition. This can be attributed to the fact that as the crystal transitions from a low-symmetry orthorhombic phase to an antisymmetric cubic phase, the orientational degrees of freedom of the MA cation increase, thereby altering the surrounding environment for the MA cation within the lattice. At high temperatures, the dynamic disorder of the MA cation allows for a heterogeneous environment where different mode frequencies appear together; this forms broad peaks instead of multiple peaks due to overlap, high damping, and weak intensity. Another possibility is that the size of the octahedral space decreases in the cubic phase, increasing steric hindrance effects and consequently increasing dynamic coupling, which leads to the broadening of the peaks.

[0071] The symmetric and asymmetric bending modes (δs / as) of CH3 are 1421 cm⁻¹, respectively. -1 and 1457cm -1It is located at. Both modes exhibit a blue shift near T1 upon hardening, i.e., heating. The symmetric and asymmetric bending modes (δs / as) of NH3 are 1473 cm⁻¹, respectively. -1 and 1596cm -1 It is located at 1473 cm⁻¹. The temperature dependence of the Raman shift of all these modes is consistent with previous studies. -1 The mode frequency of increases sharply at T1, whereas at 1596 cm -1 The mode frequencies exhibit redshift behavior. However, in this study, both major modes are 1467 cm⁻¹ -1 and 1602cm -1 It exhibits new shoulder peaks and disappears at T-1 and T2, respectively, as the temperature increases. This appears to reflect the low symmetry environment of the orthorhombic system, leading to increased degeneracy. Interestingly, theoretical analysis suggests that this mode splitting is more common in Cl-based halide perovskites. The intensity of various modes changes with the transition temperature. For example, the intensity of the ν(CN) and δs(NH3) modes decreases significantly as the crystal transitions from the orthorhombic to the cubic phase. This implies that, from the perspective of cation kinetics, the orthorhombic phase is more ordered than the cubic phase, and therefore well-defined, strong Raman modes are observed.

[0072] Figures 8a-c are 2700 cm -1 This shows the temperature dependence of the Raman shifts of the above high-frequency modes. Modes in this frequency range are associated with CH3 and NH3 symmetric / asymmetric stretching (νs / as) vibrations. 2819 cm⁻¹ -1 2822cm with shoulder peaks -1 The mode is associated with the asymmetric vibration of the CH coupling. The shoulder peak disappears at T1, whereas at 2822 cm -1 In this mode, Raman movement suddenly increases. 2900cm -1The mode does not exhibit any noticeable abnormalities, except that it softens slightly near T1 when heated. 2920cm -1 The modes continue to increase up to T1 and then disappear. Inconsistencies in mode assignment exist for some modes within this range. For example, 2800–2960cm -1 The modes of the range are known to originate from MA vibrations of phase-separated MACl present on the nanoscale crystal surface. In our results, the overall trend of the Raman shift with temperature is nearly identical to that reported in other studies. The FWHM of these modes shown in Fig. 14 also exhibits clear anomalies at the transition temperature. The clear and noticeable anomalies exhibited by these modes near the phase transition temperature suggest that these modes must be related to bulk properties, namely the internal organic-inorganic structure and the consequent structural phase transition.

[0073] The CH3 asymmetric vibrations in our results are 2967 / 2971 and 3032 cm⁻¹. -1 It exists at 2967cm 1 and 2971cm 1 The mode splits and merges at T1 as temperature increases and persists until RT. However, our results showed that this is directly related to the structural phase transition and can therefore be assigned to the CH3 asymmetric vibration of the bulk MAPbCl3 single crystal, consistent with the mode assignments in previous studies. Furthermore, the CH3 symmetric vibration is 3032 cm⁻¹ -1 It was found in and is 3040cm -1 The shoulder peak disappears at T1, whereas the main peak exhibits a strong increase in Raman shift at T1 and a blue shift to RT. This is evidence that these high-frequency modes are associated with bulk vibration characteristics rather than surface characteristics that have a different configuration from the bulk.

[0074] The symmetric / asymmetric stretching modes of NH3 are 3109, 3140, 3156, and 3180 cm -1It is located at 3109 cm, as can be seen in Fig. 8c. -1 At 3180 cm⁻¹, the symmetric stretching mode of NH3 exhibits a blue shift as temperature increases. -1 In the second symmetric stretching mode of NH3, the Raman shift increases sharply at T1 and persists until RT. 3140 cm 1 At 3156 cm⁻¹, the asymmetric stretching mode of NH3 disappears at approximately 170°C. 1 The modes disappear near T1. The significant frequency shift for all CH3 and NH3 modes implies that the hydrogen bond strength changes with temperature, particularly at temperatures near the transition temperature.

[0075] A phase transition from orthorhombic to tetragonal was observed at -114°C, and a phase transition from tetragonal to tetragonal was observed at -110°C; both were previously reported to be first-order phase transitions. The Raman shifts and FWHMs of almost all modes are significantly influenced by the phase transition. The phase transition of MAPbCl3 is characterized by three major anomalies. First, the temperature dependence of low-frequency lattice modes was revealed for the first time. No soft modes were observed, suggesting that the current system is not displaced or that the soft modes are located at the wave vector rather than at the zone center of the first Brillouin zone. Additionally, the low-frequency centered modes observed in MAPbBr3 were not observed, implying that the effect of MA off-centering, which was previously considered the cause of centered modes, plays a different role in the light scattering spectrum of MAPbCl3. Second, the hardening or softening of various modes, such as the CN stretching and MA fluctuation modes of MAPbCl3 at the transition temperature, was nearly identical to that of MAPbBr3. Furthermore, the anomalous phenomena of low-frequency lattice modes were very similar to those of MAPbBr3. This implies that H-halogen interactions, MA freezing / unlocking, and changes at the transition point are very similar between MAPbCl3 and MAPbBr3. Finally, the half-width, which is inversely proportional to the phonon lifetime, changes abruptly across the phase transition. In all modes, the half-width is very small in the orthorhombic phase and increases discontinuously as it passes through the phase transition point. The alignment of MA cations along specific crystallographic directions in the orthorhombic phase transforms into a state of dynamic disorder that unlocks movement within the cavity, resulting in high phonon damping due to high disharmony and a local heterogeneous environment. Nearly identical damping behavior has recently been reported for the MAPbBr3 system. Finally, it is necessary to mention that the phase transition temperature and the temperature range of the tetragonal phase vary significantly depending on the research group and experimental technique.It would be interesting to investigate the effect of impurity levels on the tetragonal stability region in various types of samples, such as single crystals and thin films.

[0076] MAPbCl investigated via Raman and Brillouin scattering 3 The decomposition process of

[0077] Exposure of MAPbCl3 to high temperatures induces structural changes and thermal decomposition. Tracking this process is critical for actual device applications. Raman spectra were measured at high temperatures ranging from RT to 200°C (see Fig. 15). The Raman spectra remained nearly identical from RT to 180°C. However, at 200°C, the spectrum changed abruptly, indicating the thermal decomposition of the material. This temperature is consistent with the thermogravimetric analysis shown in Fig. 16. Another previous study also reported that the temperature at which decomposition begins is 200°C. In the Raman spectrum at 200°C, high-frequency modes disappeared, and low-frequency modes changed abruptly. Furthermore, the appearance of the crystal changed significantly from transparent to a white shade at 200°C (see Fig. 17). Even when the crystal was cooled to RT, the color remained white, indicating that the crystal had undergone irreversible alteration and permanent decomposition. The Raman spectrum of the decomposed sample measured at RT, indicated by the black solid line in Fig. 9, is substantially different from the Raman spectrum of the new sample.

[0078] To verify the effect of time variation on the structural changes of MAPbCl3 and to compare this with the temperature variation described above, Raman spectra were measured after storing the samples under ambient conditions for one month. The Raman spectrum indicated by the green solid line in Fig. 9 differed from the original MAPbCl3 spectrum and was similar to the spectrum of the sample decomposed by high temperature. Therefore, it would be interesting to compare the Raman spectra of possible byproducts with the decomposed / decomposed MAPbCl3. Basically, MAPbCl3 must decompose into two expected byproducts: PbCl2 and MACl. To verify this, the Raman spectra of the two expected byproducts were compared with the Raman spectrum of the decomposed MAPbCl3. Fig. 9 shows a comparison of the Raman spectra of the byproducts with the Raman spectra of the two types of decomposed MAPbCl3. Both spectra of the two decomposed samples showed significant changes in the low-frequency modes and the disappearance of the high-frequency modes compared to the new MAPbCl3 sample. The Raman spectra of the two decomposed MAPbCl3 samples are nearly identical to the Raman spectrum of PbCl2, clearly indicating that a portion of the MAPbCl3 decomposed into PbCl2. The lack of similarity with the MACl spectrum may be due to weak MACl modes hidden beneath strong Pb-Cl modes. Therefore, we can conclude that both temperature-induced and time-induced decomposed MAPbCl3 crystals decompose into PbCl2, which has similar colors consistent with previous reports.

[0079] Brillouin spectroscopy can be used to investigate low-frequency acoustic phonons sensitive to structural and chemical changes. Previous Brillouin scattering studies have revealed that acoustic phonon behavior is directly associated with low-temperature structural phase transitions. High-temperature Brillouin scattering experiments were performed to compare the acoustic behavior of pristine MAPbCl3 with that of decomposed MAPbCl3. Figure 10a shows the temperature dependence of the Brillouin spectrum of pure MAPbCl3 from room temperature to 200°C. The Brillouin spectrum exhibits significant changes as the sample passes through a specific temperature of approximately 200°C, which is similar to the results of Raman experiments showing significant changes in the Raman spectrum at this temperature. Figure 10b shows the Brillouin spectrum of decomposed MAPbCl3 cooled from 200°C to room temperature. The original MAPbCl3 exhibits two Brillouin doublets corresponding to longitudinal acoustic (LA) and transverse acoustic (TA) modes appearing at ~25 and ~8 GHz, respectively. This is the typical Brillouin spectrum of MAPbCl3 at RT. However, the decomposed MAPbCl3 shown in Fig. 10b differs significantly from the typical spectrum of the fresh sample and exhibits very broad spectral features. Such a broadened spectrum is generally observed in ceramics or powders where the excitation light undergoes multiple reflections and refractions, resulting in a wide allowable momentum transfer range. Figs. 11a and 11b show the temperature dependence of the frequencies of the LA and TA modes and the FWHM, respectively. Both modes show a sharp change at ~200°C, indicating that thermal decomposition begins at this temperature. The present invention demonstrates that Brillouin scattering is a useful tool for monitoring the decomposition process of halide perovskite materials. Figure 18 compares the Brillouin spectra of a new MAPbCl3 single crystal maintained for one month under ambient conditions. In contrast to the new sample, the resonance peaks became asymmetric and broadened in the decomposed sample.In addition, two TA modes appeared at frequencies different from the original crystal. This suggests that long-term exposure to ambient conditions and the resulting degradation process can be investigated through Brillouin scattering.

[0080] Accurate monitoring of structural changes, including degradation processes and phase transitions, is a key parameter to consider in actual device applications. This study, composed of Raman scattering and Brillouin scattering, will help develop a deeper understanding of the structural changes in MAPbCl3 and will also provide valuable data for optimizing device performance by selecting the correct operating temperature.

[0081] conclusion

[0082] Structural changes in halide perovskite MAPbCl3 single crystals during phase transitions and temporal or temperature-induced degradation processes were investigated using a combination of Brillouin and Raman scattering techniques. Raman scattering highlighted new low-frequency lattice modes with temperature dependence, exhibiting distinct and discontinuous anomalies at the transition temperature, similar to the case of MAPbBr3. This is consistent with the similarity of CN stretching and wobble modes between the two systems, suggesting that the temperature variation of H-halogen interactions and the order / disorder of MA cations across the transition point are highly similar. Broadening of linewidths associated with phonon lifetimes was observed at the transition point upon heating for almost all Raman modes, indicating that significant lattice mismatches occurred due to the unlocking of ordered MA cations and the resulting dynamic disorder. High-temperature Raman and Brillouin scattering results showed significant changes at 200°C. Comparing the high-temperature Raman spectra of MAPbCl3 with those of PbCl2 reveals that MAPbCl3 decomposes into PbCl2 at high temperatures above 200°C. Long-term exposure of new samples to ambient conditions for one month induces spectral changes in Raman and Brillouin spectra similar to the high-temperature results. These results demonstrate that a combination of Raman and Brillouin scattering techniques can be a useful tool for monitoring the decomposition process of lead-based halide perovskites.

Claims

Claim 1 A method for analyzing the structural change process of a lead-based halide perovskite, comprising: a step of preparing a lead-based halide perovskite; a step of analyzing the lead-based halide perovskite by Raman spectroscopy using a standard Raman spectrometer; and a step of analyzing the Brillouin spectrum pattern of the lead-based halide perovskite using a Brillouin spectrometer. Claim 2 The method according to claim 1, wherein the lead-based halide perovskite is a material having the structural formula APbX3, where A is CH3NH3(methylammonium, MA) or CH(NH2)2(formamidinium, FA) and C is Cl, Br, or I. Claim 3 In paragraph 2, the method wherein the lead-based halide perovskite is a material having the structural formula CH3NH3PbCl3. Claim 4 A method according to claim 1, wherein the step of analyzing by Raman spectroscopy includes the step of exciting a lead-based halide perovskite using a diode-pumped solid-state laser with a wavelength of 532 nm. Claim 5 A method according to claim 1, further comprising the step of analyzing the XRD pattern of a lead-based halide perovskite using an XRD (X-ray diffraction) spectrometer. Claim 6 A system for analyzing the structural change process of a lead-based halide perovskite, performed by a computing device, comprising the steps of: analyzing the lead-based halide perovskite using Raman optical spectroscopy; and analyzing the Brillouin spectrum pattern of the lead-based halide perovskite using a Brillouin spectrometer. Claim 7 In claim 6, the lead-based halide perovskite is a material having the structural formula APbX3, wherein A is CH3NH3 (methylammonium, MA) or CH(NH2)2 (formamidinium, FA) and C is Cl, Br, or I, a system. Claim 8 In claim 7, the system is a material having the structural formula CH3NH3PbCl3, wherein the lead-based halide perovskite is a material. Claim 9 A system according to claim 6, wherein the step of analyzing by Raman spectroscopy includes the step of exciting a lead-based halide perovskite using a diode-pumped solid-state laser with a wavelength of 532 nm. Claim 10 A system according to claim 6, further comprising the step of analyzing the XRD pattern of a lead-based halide perovskite using an XRD (X-ray diffraction) spectrometer. Claim 11 An apparatus for performing a method to analyze the structural change process of a lead-based halide perovskite, comprising: a processor; and a memory storing at least one instruction executed through the processor, wherein the at least one instruction is executed to irradiate a lead-based halide perovskite with a laser beam and to obtain a Raman scattering spectrum and a Brillouin spectrum using the laser beam scattered from the lead-based halide perovskite. Claim 12 In claim 11, the lead-based halide perovskite is a material having the structural formula APbX3, wherein A is CH3NH3 (methylammonium, MA) or CH(NH2)2 (formamidinium, FA) and C is Cl, Br, or I. Claim 13 In claim 12, the device is a material having the structural formula CH3NH3PbCl3, wherein the lead-based halide perovskite is a material.

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