In Situ Raman Spectroscopic Studies of Thermal Stability of All

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In Situ Raman Spectroscopic Studies of Thermal Stability of All-inorganic Cesium Lead Halide (CsPbX3, X=Cl, Br, I) Perovskite Nanocrystals Mengling Liao, Beibei Shan, and Ming Li J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00344 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 1, 2019

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The Journal of Physical Chemistry Letters

In Situ Raman Spectroscopic Studies of Thermal Stability of All-inorganic Cesium Lead Halide (CsPbX3, X=Cl, Br, I) Perovskite Nanocrystals Mengling Liao, Beibei Shan and Ming Li*

School of Materials Science and Engineering, State Key Laboratory for Power Metallurgy, Central South University, Changsha, Hunan 410083, China

*To whom the correspondence should be addressed. E-mail: [email protected] and [email protected]

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ABSTRACT: Thermal degradation becomes the main obstacle for industrial applications of all-inorganic cesium lead halide (CsPbX3, X=Cl, Br, I) perovskite optoelectronic devices. A complete understanding of thermal degradation of CsPbX3 perovskites is required but greatly challenging for achieving optoelectronic devices with long-term stability particularly under extreme settings. Herein, we present an in situ spectroscopic study of thermal stability of CsPbX3 nanocrystals between the cryogenic temperature and high temperature. The low-frequency Raman signatures of CsPbX3 nanocrystals dramatically evolve but differentiate from the halogen atoms at elevated temperatures, acting as potent indicators of their crystalline structures and phase transitions. The merging of doublet Raman bands of CsPbX3 nanocrystals indicates their high-temperature phase transitions. CsPbX3 (X=Br, I) nanocrystals undergo a state of highdegree of disorder with featureless Raman spectra before thermally decomposed. Such understanding is of particular importance for future design and optimization of high-performance CsPbX3 perovskite devices with long-term stability under extreme settings.

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The growing interest in semiconducting lead halide perovskite nanocrystals (NCs) mainly stems from their outstanding photovoltaic and optoelectronic properties for promising applications in photovoltaic cells, light-emitting diodes (LEDs), lasers and photodetectors.1-7 Recently, all-inorganic cesium lead halide (CsPbX3, X=Cl, Br, I) perovskite NCs have been the subject of intense research, mainly owing to their salient features of ultrahigh photoluminescence (PL) quantum yields (~100%), narrow emission bandwidths, size/composition-tunable emission wavelength, broadband absorption, large exciton diffusion length, good electron mobility as well as low-cost fabrication routes.8-10 CsPbX3 NCs are typically achieved by a hot injection method or room-temperature precipitation method commonly used in the literature.3,11-15 Despite the impressive improvement in intrinsic long-term stability in comparison with their organic-inorganic hybrid perovskite counterparts,16,17 these CsPbX3 NCs are extremely sensitive to the external environment (i.e., moisture, solvents, light, temperature, pressure, surface ligands) and undergo dramatical changes in structures and optoelectronic properties accordingly, thereby degrading the device performance.18-25 The structural instability has been one of the most important obstacles for practical applications of CsPbX3 perovskites.26,27 CsPbX3 NCs typically exist in orthorhombic, tetragonal or cubic phases, among which the cubic phase is highly symmetric and thermodynamically more stable than both tetragonal and orthorhombic ones at high temperatures.28 These phases could transform into each other by controlling the temperature. Bulky CsPbCl3 typically displays a thermodynamically favorable orthorhombic phase at ambient temperature, and its phase transition into the tetragonal phase takes place at 42-47 oC and into the cubic phase above 47 oC.29 CsPbBr3 possesses the orthorhombic structure at ambient temperature, which is transformed to the tetragonal phase at 100 oC and then to the cubic phase above 130 oC.28 As for CsPbI3, the black (cubic) perovskite phase that shows the desirable photovoltaic and optoelectronic properties is only stable above 300 oC.30,31 Upon cooling, the black perovskite phase undergoes a phase transition to thermodynamically favorable yellow orthorhombic non-perovskite phase with poor photovoltaic and optoelectronic properties. In practical applications, CsPbX3 perovskite optoelectronic devices (i.e., LEDs, solar cells) may encounter 3 ACS Paragon Plus Environment


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a temperature between the cryogenic temperature (500 oC) in the field during the fabrication and operation of devices, e.g. space exploration, deep drilling.27 Field temperature changes may cause device performance degradation through temperature-induced structural changes or thermal decomposition, which seriously hampers their usage in such robust environments.32-38 Studies showed the dramatical performance degradation of perovskite solar cells upon heating at or above the operation temperature (>150 oC).32, 33, 39 Also, changes in PL emission of CsPbX3 perovskite NCs have been observed at elevated temperatures, indicating temperature-induced performance degradation of LEDs.35-38 To date, substantial efforts have been made to address the thermal instability of CsPbX3 perovskites, and understand their degradation mechanisms.40-44 Remarkable correlations were observed between spectral features of Raman and structural changes of CsPbX3 at temperatures from the ambient temperature to ωTO, where ωLO and ωTO are the frequency of the LO and TO phonon modes, respectively. To implement in situ Raman measurements at temperatures between -190 oC and 500 oC, the as-prepared CsPbX3 NCs were drop-coated onto a cleaned silicon wafer. The temperature profile adopted for the in situ Raman measurements was shown in Figure S2. The starting temperature was set at 25 oC followed by the cooling process, facilitating us to observe the structural evolution at the low temperature region (25 oC to -190 oC)

of the as-prepared CsPbX3 NCs with thermodynamically favorable perovskite structures. All CsPbX3

NCs exhibit sharp low-frequency ( CsPbBr3> CsPbI3. Furthermore, the temperature dependence of Raman shifts of CsPbX3 NCs exhibits a regular redshift with the increased temperature, except the new produced 129 cm-1 and 134 cm-1 Raman bands from thermal decomposition of CsPbBr3 and CsPbI3 NCs (Figure 3). Raman shifts follow a non-linear decrease and an increase in linewidth to follow with the increased temperature. In CsPbCl3 NCs, the doublet TO2 Raman band (72, 89) cm-1 at -190 oC merges into ~72 cm-1 at -70 oC and subsequently redshifts to 58 cm-1; the TO3 Raman band (110,121) cm-1 at -190 oC merges into ~111 cm-1 at 50 oC and then redshifts to 107 cm-1. Similar results can be observed for CsPbBr3 and CsPbI3 NCs with different Raman shifts and merging temperatures (Figure 3B, C). These changes are attributed to the strain from the volume expansion and the increasing disorder of crystal lattices. With the continuing increase in the temperature beyond the decomposition temperature, both 129 cm-1 and 134 cm-1 Raman bands become much sharper, which may be due to the increasing crystallization of CsBr and CsI3.19,52-54 10 ACS Paragon Plus Environment

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Figure 3. Temperature dependent evolution of Raman shifts as a function of temperature of (A) CsPbCl3, (B) CsPbBr3 and (C) CsPbI3 NCs. Note: a and b in (A-C) indicate the temperatures of disappearance of TO1 modes and merging point of all doublet TO modes into their singlet TO modes, respectively; regions marked with pink and light green colors indicate the temperature regions of featureless Raman spectra and thermal decomposition of CsPbX3 NCs, respectively. Decomposition of CsPbCl3 NCs was not observed at temperatures investigated in this work, which may be its relatively high thermal stability.

To further understand the structural evolution of CsPbX3 NCs at elevated temperatures, in situ XRD measurements were performed between -190 oC and 500 oC as well, and the corresponding temperature profile was presented in Figure S7. It should be noted that the distinct XRD intensities were observed at each temperature investigated for all three CsPbX3 NCs, which may originate from different crystal structures and crystallinity. At 25 oC, the as-prepared CsPbCl3, CsPbBr3 and CsPbI3 NCs are indexed as tetragonal, orthorhombic and cubic phases, respectively (Figures 1C and 4). Typically, the tetragonal 11 ACS Paragon Plus Environment


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phase of CsPbCl3 NCs has two XRD peaks at 2θ=31.76o and 31.99o, which are assigned to the crystal facets (002) and (200), respectively, while its cubic phase has only a single peak at 2θ=31.91o belonging to the crystal facet (200) (Figure 4A). The double XRD peaks at 2θ=31.76o and 31.99o of the tetragonal phase disappear at 50 oC, and the single peak at 2θ=31.91o of the cubic phase show up at 100 oC (Figure S8). Thus, we clearly see the transition of the tetragonal into cubic phases in CsPbCl3 NCs at 50 oC above, and the orthorhombic-cubic phase transition temperature in CsPbBr3 NCs is beyond 100 oC (Figure 4A). These phase transition temperatures are consistent with those reported in the literature.28,30 However, despite that the thermodynamically stable polymorph of CsPbI3 NCs is the cubic phase at high temperatures, the cubic-orthorhombic phase transition was obviously observed at 200 oC above (Figure 4A). The increasing temperature causes Bragg reflections shifting toward lower 2θ angles, which is due to the thermal volume expansion of crystal lattice. XRD results evidenced again that both CsPbBr3 and CsPbI3 NCs were decomposed into their constituents PbX2 and CsX (or CsX3) at ca. 400 oC and 200 oC above, respectively (Figures 5A). However, we did not observe the thermal decomposition of CsPbCl3 NCs even at 500 oC, and the orthorhombic phase of CsPbI3 NCs did not return to the thermodynamically stable cubic phase as the temperature further increases from 200 oC to 500 oC. These results show the halogen atom dependent thermal stability of CsPbX3 NCs with the thermal decomposition temperature: Cl>Br>I, in agreement with in situ Raman measurements. The state of high-degree of disorder shown in high-temperature Raman measurements was not observed in the in situ XRD measurements. Also, we did not observe the intermediate order-disorder transition at -70 oC for CsPbX3 NCs in the in situ XRD measurements. In addition, we found that CsPbCl3 NCs maintained their initial crystalline structures after the cooling-heating process (25 oC → -190 oC → 25 oC → 500 oC); both CsPbBr3 and CsPbI3 NCs maintained their initial crystalline structures during the cooling process (25 oC → -190 oC → 25 oC) while the thermal decomposition induced by the high-temperatures is irreversible (Figure S9). In addition, significant color changes after in situ XRD measurements were observed: white to black for CsPbCl3 NCs, orange to black for CsPbBr3 NCs, and black to yellow for CsPbI3 NCs (Figure 4B). The TGA-DSC 12 ACS Paragon Plus Environment

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analysis performed under inert atmosphere revealed a mass loss of less than 6% over the 30-500 oC temperature range, which may be due to removal of surface organic ligands (ODE, OAm and OA). TGADSC results again confirmed the high-temperature phase transitions and their thermal decomposition of CsPbX3 (X=Br, I) NCs (Figure S10). These findings can be further confirmed by changes of Raman signatures of CsPbX3 NCs after the cooling-heating process, as shown in Figure 5B.

Figure 4. In situ XRD patterns of CsPbX3 NCs at various temperatures from -190 oC to 500 oC. (A) In situ powder XRD patterns of (i) CsPbCl3, (ii) CsPbBr3 and (iii) CsPbI3 NCs. Standard JCPDS card data of cubic, orthorhombic and (or) tetragonal phases of CsPbX3 NCs are shown as well; The red dot (●) and blue diamond (◊) indicate the decomposed products PbX2 and CsX (or CsX3). The in situ XRD 13 ACS Paragon Plus Environment


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measurement was started at 25 oC, and then the measurement was performed at a temperature interval as the temperature decreasing from 25 oC to -190 oC, followed by returning to 25 oC and implementing the measurement at a temperature interval as the temperature increasing from 25 oC to 500 oC. The XRD measurement was performed after the sample was cooled back to 25 oC as well. (B) Color changes of (i) CsPbCl3 NCs, (ii) CsPbBr3 NCs and (iii) CsPbI3 NCs after in situ XRD measurements.

We for the first time employed in situ Raman spectroscopy in combination with in situ XRD measurements to understand the thermal stability of CsPbX3 NCs over a broad temperature range from 190 oC to 500 oC. CsPbX3 NCs undergo successive phase transitions between -190 oC and 500 oC, along with changes of Raman signatures that have important implications for structures and phase transitions. At cryogenic temperatures, CsPbX3 NCs exhibit sharp low-frequency Raman bands, which are assigned to the TO and LO phonon modes. Temperature induced spectral changes include disappearance, broadening and mergence of Raman bands, implying the order-disorder state, phase transition and thermal decomposition of CsPbX3 NCs. These CsPbX3 NCs possess significantly distinct thermal decomposition temperatures, tightly related to the halogen atom type. We did not observe the thermal decomposition of CsPbCl3 NCs even at the highest temperature adopted in this work, while both CsPbBr3 and CsPbI3 NCs begin to decompose at 350 oC and 300 oC above, respectively. Furthermore, a state of high-degree of disorder with featureless Raman spectra was observed before thermal decomposition of CsPbX3 NCs. We found a remarkable agreement between the merging temperature of all doublet TO modes and the temperature of the high-temperature phase transition (tetragonal/orthorhombic-cubic transition) in these CsPbX3 NCs. Thus, we suggest the strong correlation of the merging of doublet TO Raman bands with the high-temperature phase transition in CsPbX3 NCs. In situ Raman spectroscopy combined with in situ XRD measurement presents new insights into the structural evolution of CsPbX3 NCs, which benefits better understanding of the degradation of CsPbX3 perovskite optoelectronic devices and forms foundations of novel strategies for their performance improvement. 14 ACS Paragon Plus Environment

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Figure 5. XRD patterns of degraded CsPbX3 (X=Br, I) NCs and changes of ambient-temperature Raman spectra after in situ Raman measurements. (A) In situ XRD patterns of degraded (i) CsPbBr3 NCs and (ii) CsPbI3 NCs at 450 oC. Standard JCPDS card data of cubic CsBr, orthorhombic PbBr2, hexagonal PbI2 and orthorhombic CsI3 are shown as well. It is worth noting that the 2θ angle difference between the measured XRD patterns at 450 oC and the standard JCPDS card data of CsPbX3 NCs is due to the thermal expansion induced shift of Bragg reflection angles. (B) Raman spectra of (i) CsPbCl3 NCs, (ii) CsPbBr3 NCs and (iii) CsPbI3 NCs measured at (1) 25 oC before the Raman measurement, (2) 25 oC returning from cryogenic temperatures and (3) 25 oC after cooling back from 500 oC.

In summary, this work presents a systematic study on the structural stability of all-inorganic CsPbX3 NCs (X=Cl, Br, I) over a broad temperature range from -190 oC to 500 oC, using in situ Raman spectroscopy combined with in situ XRD measurements. Results demonstrate that Raman signatures of CsPbX3 NCs have important implications for their structural changes and thermal decomposition. The TO Raman bands situated at low-frequency regions (CsPbBr3>CsPbI3. The present work clearly unveils the structural evolution of all-inorganic CsPbX3 NCs at elevated temperatures, and extends our understanding of degradation mechanism of CsPbX3 NCs. Although all-inorganic CsPbX3 NCs may have similar crystal structures to organic-inorganic hybrid lead halide (e.g., CH3NH3PbX3, X=Cl, Br, I) NCs, hybrid lead halide NCs exhibit different Raman signatures and distinct thermal stability. The in-depth understanding of thermal stability of hybrid lead halide NCs over the broad temperature range from -190 oC to 500 oC needs to be carefully examined. We believe that this study will greatly benefit the performance improvement of optoelectronic devices in practical fields over a broad operating temperature range particularly under extreme settings such as space exploration and deep drilling.

ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.

Distribution of edge lengths of CsPbCl3, CsPbBr3 and CsPbI3 NCs; Temperature profile for in situ Raman measurements used in this work; Evolution of Raman spectra of CsPbCl3, CsPbBr3 and CsPbI3 16 ACS Paragon Plus Environment

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NCs at elevated temperatures from -190 oC to 500 oC; Raman spectra at 450 oC and after cooling back to 25 oC from 500 oC; Temperature profiles for in situ XRD measurements; Zoom-in in situ XRD patterns of CsPbCl3 nanocrystals at 50 oC and 100 oC; In situ powder XRD patterns at 25 oC; TGA-DSC analysis

ACKNOWLEDGEMENTS. The authors would like to acknowledge financial support by the National Thousand Young Talents Program of China, National Natural Science Foundation of China (No. 51871246), Innovation-Driven Project of Central South University (No. 2018CX002) and Hunan Provincial Science & Technology Program (No. 2017XK2027). We thank Professor J.W. Huang of the School of Materials Science and Engineering, Central South University for help in in situ XRD measurements.

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