Pressure-Induced Structural and Optical Properties of Organometal

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Letter

Pressure-Induced Structural and Optical Properties of Organometal Halide Perovskite Based Formamidinium Lead Bromide Lingrui Wang, Kai Wang, and Bo Zou J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00999 • Publication Date (Web): 20 Jun 2016 Downloaded from http://pubs.acs.org on June 21, 2016

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The Journal of Physical Chemistry Letters

Pressure-Induced Structural and Optical Properties of Organometal Halide Perovskite Based Formamidinium Lead Bromide Lingrui Wang, Kai Wang, and Bo Zou*

State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China

Corresponding Author

*To whom correspondence should be addressed. Bo Zou, E-mail: [email protected].

Contributions

Lingrui Wang, and Bo Zou designed and performed experiments, and analyzed data. Kai Wang assisted in performing experiments. Kai Wang and Bo Zou provided intellectual input. Lingrui Wang, Kai Wang and Bo Zou wrote the manuscript.

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ABSTRACT Organometal halide perovskites (OMHPs) are attracting an ever-growing scientific interest as photovoltaic materials with moderate cost and compelling properties. In this letter, pressure-induced optical and structural changes of OMHP-based formamidinium lead bromide (FAPbBr3) were systematically investigated. We studied the pressure dependence of optical absorption and photoluminescence, both of which showed piezochromism. Synchrotron X-ray diffraction indicated that FAPbBr3 underwent two phase transitions and subsequent amorphization, leading directly to the bandgap evolution with redshift followed by blueshift during compression. Raman experiments illustrated the high pressure behavior of organic cation and the surrounding inorganic octahedra. Additionally, the effect of cation size and the different intermolecular interactions between organic cation and inorganic octahedra result in the fact that FAPbBr3 is less compressible than the reported methylammonium lead bromide (MAPbBr3). High pressure studies of the structural evolution and optical properties of OMHPs provide important clues in optimizing photovoltaic performance and help to design novel OMHPs with higher stimuli-resistant ability. TOC GRAPHICS

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Organometal halide perovskites (OMHPs), as high efficiency light sensitizers in solar cells, have become hotspots in the photovoltaic research field. In a markedly short timeframe, these extraordinary materials have exhibited unprecedented development, now exceeding 22% power conversion efficiency (PCE)1 and offering potential to further boost to 25% in photovoltaic devices.2 Their attributes, such as suitable optical bandgap,3 trap-state density,4 high absorption coefficient5 and long diffusion lengths,6 render OMHPs as one kind of the most competitive materials for applications in lasers, photovoltaics, light-emitting diodes (LEDs) and visible-blind UV-photodetector.7-11 OMHP-based perovskite-type crystal structure presents a basic formula of ABX3, with A = [Cs, CH3NH3 (MA), NH2CH=NH2 (FA)], B = [Ge, Sn, Pb] and X = [Cl, Br, I]. The structure comprises a set of fully corner-sharing [BX6]4- octahedra and organic cations coordinated by 12X anions. By changing the compositions of A, B and X, one can tune their optoelectronic properties to get underlying material properties in perovskite-structured photovoltaics. For instance, several reports have documented to chemically tuning the bandgap of devices based on MA(OA)PbX3 and MASnIxBr3-x with satisfactory PCEs, respectively.12,13 Although the studies of OMHPs have made great stride, the stability of the materials among various environmental factors (moisture, light, temperature, and pressure) is still need to be drilled down into. Wozny and co-workers examined the effect of relative humidity on the structural properties of FAPbI3 thin films.14 Foley and Xu et al. explored the temperature-dependent structural changes of MAPbI3 and FAPbBr3, respectively.15,16 High pressure is another significant factor that can influence structural properties in extreme cases.17-19 However, the effects of strain or pressure on OMHPs are just 3



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beginning to be explored. With the use of diamond anvil cells (DAC), the pressure-induced striking piezochromism and electronic conductivity of 2D Cu-Cl perovskites have been reported recently.20 Similar changes in 3D hybrid perovskites have also been observed. The phase stability and visible light response of MAPbBr3 under high pressure have been studied intensively.21-24 The pressure response of the optical and electronic properties of MAPbI3 has also been investigated to delve into the structure-property relationship of perovskites.24-27 Formamidinium lead bromide (NH2CH=NH2PbBr3, FAPbBr3) possesses a bandgap of 2.23 eV under ambient conditions, which makes it more suitable for tandem application than MAPbBr3.28 Hanusch et al. reported a striking advantage of the FAPbBr3 perovskites, that is, the material could be processed in a planar heterojunction conﬁguration without a mesoporous scaﬀold for charge extraction.29 Zhumekenov et al. showed a long carrier diffusion length of FAPbBr3 crystals to be 19.0 µm, being one of the longest reported values in perovskite materials.6 Hence, exploring the structural and optical properties of FAPbBr3 crystal is essential for utmost utilization of this material in practical applications. In this letter, we investigated the structural stability and optical properties of FAPbBr3 crystal under pressure by means of high pressure spectra and elaborated the structure-property correlation of FAPbBr3 crystal. High pressure optical micrographs, optical absorption and photoluminescence (PL) spectra indicated that FAPbBr3 crystal showed piezochromism. The structural change Pm m → Im

→ Pnma and amorphization of FAPbBr3 crystal under

compression were responsible for piezochromism. The XRD data showed that FAPbBr3 required higher pressures for phase transitions compared with the reported MAPbBr3, which indicated 4


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that FAPbBr3 is less compressible under external pressure. Raman spectra of FAPbBr3 under applied pressure were obtained to study the high pressure behavior of organic FA cation and the surrounding inorganic [PbBr6]4- octahedra. Our findings offer an attractive route for exploring highly efficient photovoltaic materials and help to design novel OMHPs with higher stimuli-resistant ability.

Figure 1. (a) In situ high pressure optical micrographs in a diamond anvil cell, showing the piezochromism of FAPbBr3 crystal. (b) Corresponding PL photographs under UV irradiation (λex = 355 nm). Considering the remarkable performance of OMHPs in photovoltaics, we investigated the high pressure optical properties of FAPbBr3. Figure 1a shows the representative in situ high pressure optical micrographs of FAPbBr3 crystal in a DAC chamber. Under ambient conditions, the material was transparent with orange color. With further increase of pressure, we observed

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piezochromism manifested in the color change of FAPbBr3 crystal. The color of the sample gradually became red up to about 2.2 GPa, then transforming into yellow and finally becoming colorless above 4.1 GPa. The corresponding PL photographs under UV irradiation are shown in Figure 1b. Pressure-dependent color change could be easily visualized by naked eye as well, the following sequence: green → orange → yellow, then the PL disappeared at approximately 4.1 GPa. The sample was completely reverted to its original color upon decompression. Moreover, the color of FAPbBr3 crystal can be tuned by high pressure to produce an array of translucent colors, and this feature can be used to create colorful solar design.30

Figure 2. Changes in the optical absorptions and PL spectra of FAPbBr3 under pressure. Red arrows indicate the evolution of the absorption and PL spectra as a function of pressure, blue arrows show the shift of the second absorption band. The above optical micrographs showing the piezochromic transitions of FAPbBr3 crystal

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indicated that optical properties change under high pressure. To further inquire into quantitative informations of the phenomena of pressure-dependent color variation, we performed the high pressure optical absorption and PL experiments. As shown in Figure 2, the ambient sample exhibited a steep absorbance at the band edge (532 nm) and the emission was monitored at the maximum PL emission at 543 nm, consistent with that in the previous literature.29 In addition, the Stokes shift was 380 cm-1. Both the absorption edge and PL peak shift were detected upon compression: a gradual redshift occurred blow the 2.2 GPa initially, and followed by persistent blueshift. The sudden changes of the absorption and PL at about 2.2 GPa suggested that the FAPbBr3 crystal underwent a structural change. With further increase of pressure, the steep absorbance weakened and a small bump was observed around 3.0 GPa. The original absorption edge and PL peak disappeared at about 4.1 GPa, whereas the second absorption edge observed at higher energies disappeared completely at 8.0 GPa, leaving only a broad absorption tail at about 400 nm. The weakened absorption and disappearance of PL could be attributed to pressure-induced amorphization.23 The high pressure behavior of absorption and PL spectra of FAPbBr3 crystal presented in Figure 2 matched well with the micrographs color changes shown in Figure 1.

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Figure 3. (a) Tauc plot of FAPbBr3 at 1 atm, 0.6 and 2.2 GPa showing the absorption onset. (b) Pressure dependence of bandgap energy.

As is well known, the variations of absorption band are directly related to the altered bandgap. We thus obtained the bandgap evolution through the steep absorption spectra at different pressure (Figure 3a). The estimated bandgaps of FAPbBr3 are determined from the extrapolation of the linear region to the energy axis intercept by plotting (αhν)², known as a Tauc plot.31 The Tauc plot of the absorbance spectrum under ambient conditions possesses a band gap of 2.27 eV, thus making the material more suitable for wide bandgap photovoltaics. The evolution of pressure-dependent bandgap is shown in Figure 3b. The rate of change in bandgap upon pressure presented two distinct turning point at 0.6 and 2.2 GPa. While the color of sample gradually changed from translucent orange to red without abrupt change at 0.5 GPa, and the red sample at 2.2 GPa gradually became shallow with increasing pressure (Figure 1). The bandgap is mainly in relation to [PbX6]4- octahedra (Pb-Br bond), as it affects the wave function overlap (band dispersion).24, 32 We suppose that the two distinct turning points in Figure 3b are induced by structural changes at 0.6 and 2.2 GPa. 8


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Figure 4. (a) Representative ADXRD patterns of FAPBr3 crystal at various pressure levels. Arrows mark new diffraction peaks appearing at 0.53 GPa and the asterisk highlights the splitting of the peak at 2.2 GPa. (b) Refined crystal structures of FAPbBr3 at ambient pressure, 0.53 and 2.2 GPa. All H atoms are placed in geometrically idealized positions for purposes of clarity. Green arrows illustrate the rotation direction of [PbBr6]4- octahedra. In order to figure out the correlation between the optical properties and structure of FAPbBr3 crystal, we performed high pressure synchrotron XRD experiments. Figure 4a shows the collected XRD data at different pressure levels during compression to 30.1 GPa and decompression. Figure 4b presents the refined crystal structures of FAPbBr3 at ambient pressure, 0.53 and 2.2 GPa. The refinement details can be seen in Supporting Information. The simulated XRD patterns by Rietveld refinements agreed well with the experimental XRD data at ambient pressure, 0.53 and 2.2 GPa (Supporting Information Figure S1 and Table S1). The XRD pattern

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at ambient pressure confirmed that the initial structure showed good phase purity in the cubic phase with space group Pm m.29 A cubic structure of FAPbBr3 crystal was used as a starting model and the refinement gave cell parameter a = 5.989（4）Å. With increasing pressure, all diffraction peaks shifted to higher angles, indicating a smaller unit cell (Supporting Information Table S2).33 We noted that two phase transformations occurred at 0.53 and 2.2 GPa respectively during compression. The emergence of two new small peaks at 9.5° and 11.3° suggested that the FAPbBr3 crystal underwent a phase transition from phase Pm m to Im

at 0.53 GPa. With

respect to the subtle pressure-induced structure transformation, 0.53 GPa pattern presented a 2 × 2 × 2 supercell cubic unit cell (Im , a ≈ 2a0). Analysis results of the XRD diffraction pattern demonstrate that the first transition is mainly caused by the shrinkage of the [PbBr6]4- octahedra. The [PbBr6]4- octahedral (Pb-Br bond) contraction explains the absorption edge and PL redshift and the optical bandgap reduction. In fact, the same Pm m to Im transition under pressure was observed in ReO3-type doubling of the primitive cubic unit cell, a perovskite framework without occupied cations under pressure.34 This demonstrates the soft nature of organic cations under pressure. As pressure further increased to 2.2 GPa, another phase transformation occurred from phase Im

to Pnma as evidenced by the obvious splitting of the peaks located at around 12.2°.

An orthorhombic distortion with space group Pnma was used to fit the further split diffraction peaks. Our structural analysis shows that compression in the secondary phase transition also occurs through [PbBr6]4- octahedral tilting distortion. For corner-bonded of [PbBr6]4- octahedra in the plane normal to the rotation, a tilt of one octahedron about one axis causes the tilting in the opposite sense of neighboring octahedra.22 The abrupt blueshift in the absorption and PL, as well 10


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as bandgap widening is induced by the tilting distortion of the [PbBr6]4- octahedra in the secondary phase transition. With further increase of pressure, the peak at 13.7° evolved into the doublet band at 3.6 GPa due to pressure dependence of the peak shift at different rates. A broad background (at about 12.6°) appeared above 4 GPa, and the intensity of all original peaks suddenly decreased, indicating that the sample became amorphous.23 Several relative strong diffraction peaks from the residual crystalline state almost disappeared at 30.1 GPa, which indicated that the amorphization was a gradual process with increasing pressure. The sample in the amorphous state returned to the initial cubic after the complete release of pressure (Supporting Information Figure S2). In short, the FAPbBr3 crystal showed the following pressure-induced phase transition sequence: Pm m → Im

→ Pnma, and subsequent

amorphization above 4.0 GPa. A similar pressure-induced amorphization process has been observed in ASnI3 (A = MA, FA) and MAPbX3 (X= I, Br).22,24,35 This process should be associated with the distortion of inorganic perovskite skeleton and highly distorted organic molecules. What is worth mentioning was that MAPbBr3 and FAPbBr3 crystal exhibited similar high pressure XRD behavior and the same following pressure-induced phase transition sequences: Pm m → Im

→ Pnma.23 However, the onset of the Pm m → Im

phase transition at 0.4 GPa

for MAPbBr3 was lower than the one for FAPbBr3 at 0.53 GPa. With further compression, the secondary phase transition (Im

→ Pnma) occurred at 1.8 GPa for MAPbBr3 was also lower

than that of FAPbBr3 crystal. The pressure points of two phase transitions of FAPbBr3 are both higher than MAPbBr3, which demonstrates that the former is less compressible under high 11



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pressure. The stabilization of the perovskite systems is another important aspect that helps to optimize photovoltaic performance.36,37 The elevated transformation point of FAPbBr3 compared with MAPbBr3 under high pressure demonstrate that high pressure as a significant technique to study stability of perovskite systems for further enhancement of their application in photovoltaic devices.

Figure 5. Selected Raman spectra of FAPbBr3 at elevated pressure. The spectra were in the region of (a) 60-1280 cm-1. (b) 1500-1850 cm-1 and 2800-3500 cm-1. Dashed lines are used to indicate the evolution of modes.

Considering the smaller X-ray scattering cross-sections of FA compared with those of Pb and Br atoms, we also performed high pressure Raman experiments of FAPbBr3 crystal to investigate the high pressure behavior of intermolecular interaction between organic FA cation and the surrounding inorganic [PbBr6]4- octahedra. Notably, as presented in Figure 5a, low-frequency vibrational modes (60-380 cm-1) with broad features gradually became resolved 12


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and finally amorphization.38 The band near 521 cm-1 was assigned to the bending H2N−C−NH2 modes, and the band near 1120 cm-1 was assigned to symmetric stretching C−N modes.39,40 In Figure 5b, the bending of C−H and N−H observed in the Raman spectra between 1500 cm-1 and 1750 cm-1 exhibited a substantial redshift. Moreover, C−N antisymmetric stretching vibration appeared at 1720 cm-1, whereas the C−H and N−H stretching modes were observed at 3000-3100 cm-1 and 3200-3500 cm-1, respectively.41-44 Considering that the reduction in the volume and intermolecular distance under high pressure, the redshift of C−H and N−H bending modes could be attributed to the strengthening of the intermolecular interaction between the organic cation and inorganic octahedra.33,45 The trend of N−H stretching modes first staying still then exhibiting a redshift also confirmed this view, whereas the C−H stretching mode exhibited a slight blueshift upon increasing pressure because of the relatively weak interaction with inorganic octahedra (Supporting Information Figure S3). For NH2−C−NH2 bending mode, C−N stretching and C−N bending pattern, no distinct shift in this range was observed below 2.2 GPa. That is to say, the organic FA skeleton had small distorted but merely compressed during the two phase transitions. Broadening of vibrational peaks background, especially after amorphization, can be attributed to the highly distorted soft FA cations under high pressure. From another aspect, with the increase of pressure, the tilting distortion of the [PbBr6]4- octahedra also shrank the octahedral void containing the FA ion through intermolecular interaction, leading to highly distorted FA molecules. To better understand the high pressure behavior of OMHPs, we make a comparison between FAPbBr3 and MAPbBr3 crystal due to their similar phase transition paths and crystallographic 13



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components (Supporting Information Figure S4). The similar high pressure XRD behavior and the similar pressure-induced phase transition paths demonstrate that the pressure-induced phase transition is mainly related to [PbBr6]4- octahedra skeleton, and the soft organic cations simply act as templates for the [PbBr6]4- octahedral frameworks under pressure. However, the pressure points of two phase transitions of FAPbBr3 are both higher than that of MAPbBr3, which demonstrate that the former is less compressible under high pressure. As with the perovskites ABX3, their crystallographic structure formability can be empirically estimated by the Goldschmidt tolerance factor t and an octahedral factor µ. and

, where RA, RB, and RX are the effective ionic radii for the ion at the A, B, and X

sites, respectively. Thus APbBr3 (A=MA, FA) possess the same µ factor (0.61) and the t factors are calculated to be 0.84 and 0.90 for MAPbBr3 and FAPbBr3 respectively. Simultaneous and synergic compositional modification of the larger FA cation and [PbBr6]4- octahedra lead to a higher degree of space filling and therefore making FAPbBr3 less compressible under high pressure.29,46 Moreover, the intermolecular interaction between organic FA cation and inorganic octahedra are different from that of MAPbBr3, such as the hydrogen bond type and the numbers, which can also contribute to the higher pressures for phase transitions compared with MAPbBr3.47,48 Thereby, the effect of cation size as well as the different intermolecular interactions between organic cation and inorganic octahedra results in the fact that FAPbBr3 is less compressible under external pressure compared with MAPbBr3. In summary, we studied the optical and structural stability of OMHP-based FAPbBr3 crystal

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under pressure. Optical micrographs of FAPbBr3 crystal exhibited piezochromism (translucent orange → red → yellow → colorless) and the corresponding PL change (green → orange → yellow and then disappeared). High pressure synchrotron XRD data confirmed two phase transformations (Pm m → Im → Pnma) then amorphized at about 4.0 GPa, leading directly to the especial band-gap with initial redshift followed by blueshift during compression. According to Rietveld refinement, the first transition is mainly caused by the shrinkage of the [PbBr6]4octahedra, which accounts for the absorption edge and PL redshift upon compression and the optical bandgap reduction. The abrupt blueshift in the absorption and PL, as well as bandgap widening is induced by the tilting distortion of the [PbBr6]4- octahedra in the secondary phase transition. The Raman experiments further illustrate that two phase transitions do not significantly affect the internal structure of the octahedra. Moreover, the soft organic cations act as templates for the inorganic frameworks, leading to the structural memory effect in the reversible amorphization. Particularly, FAPbBr3 is less compressible under external pressure compared with MAPbBr3, which can be attributed to the effect of cation size and different intermolecular interactions between organic cation and inorganic octahedra. Therefore, high pressure studies of the structural stability and optical properties of OMHPs provide important clues in optimizing photovoltaic performance and help to design novel OMHPs with higher stimuli-resistant ability. Experimental Section Sample Preparation and High pressure Generation. Formamidinium lead bromide (FAPbBr3) was purchased from Xi’an Polymer Light Technology Corp. and used without further 15



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purification. Consider that the samples are air-sensitive, the grinding and sample loading processes were carefully conducted in a N2-filled glove-box. High pressure experiments were performed using a symmetric DAC. The sample was loaded into a hole (150 µm in diameter) of the T301 steel gasket, which was preindented to a thickness of 40 µm. A small ruby chip was inserted into the sample compartment for in situ pressure calibration, utilizing the R1 ruby fluorescence method. In high pressure optical absorption, PL and XRD experiments, we used silicone oil as PTM. For high pressure Raman, we employed argon as PTM. And the PTM did not have any detectable effect on the behavior of FAPbBr3 under pressure. All of the measurements were performed at room temperature. Optical and PL Measurements. In situ absorption and PL micrographs of the samples were obtained using a camera (Canon Eos 5D mark II) equipped on a microscope (Ecilipse TI-U, Nikon). The camera can record the photographs under the same conditions including exposure time and intensity. Absorption spectra were measured in the exciton absorption band region using a Deuterium-Halogen light source, and the excitation source, a 355 nm line of a UV DPSS laser with the power of 10 mW was used for PL measurements. The fiber spectrometer is 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 the 4W2 High Pressure Station in Beijing Synchrotron Radiation Facility. CeO2 was used as the standard sample to do the calibration. Then, the collected 2D images were integrated and analyzed using the FIT2D program to gain plots of

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intensity versus 2θ. The XRD patterns were indexed and refined by using the reflex module combined in the Materials Studio Software. Raman Measurements. In situ high pressure Raman spectra were recorded using a spectrometer equipped with liquid nitrogen cooled CCD (iHR 550, Symphony II, Horiba Jobin Yvon). A 671 nm single-mode DPSS laser was utilized to excite the sample and the output power was 10 mW. The resolution of the system was about 1 cm−1.

ACKNOWLEDGMENT This work is supported by NSFC (Nos. 91227202), RFDP (No. 20120061130006), Changbai Mountain Scholars Program (No. 2013007). Angle-dispersive XRD measurement was performed at 4W2 beamline, Beijing Synchrotron Radiation Facility (BSRF) that is supported by Chinese Academy of Sciences (Grant Nos. KJCX2-SW-N03 and KJCX2-SW-N20).

ASSOCIATED CONTENT Supporting Information. Additional Rietveld refinements details, the Structural comparison of FAPbBr3 and MAPbBr3 crystal under ambient conditions. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author *To whom correspondence should be addressed. E-mail: [email protected].

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Notes The authors declare no competing financial interests.

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