Ferroelectricity of the Orthorhombic and Tetragonal MAPbBr3 Single

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Ferroelectricity of the Orthorhombic and Tetragonal MAPbBr3 Single Crystal Zhang-Ran Gao, Xiao-Fan Sun, Yu-Ying Wu, Yi-Zhang Wu, Hong-Ling Cai,* and X. S. Wu* Collaborative Innovation Center of Advanced Microstructures, Laboratory of Solid State Microstructures & School of Physics, Nanjing University, Nanjing 210093, People’s Republic of China

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S Supporting Information *

ABSTRACT: Hybrid organic−inorganic halide perovskites (HOIPs) MAPbBr3 and their ramifications have emerged because of the photovoltaic, optical, and other fascinating performances of HOIPs in recent years. However, many intrinsic properties, such as crystal structure and ferroelectricity, are still controversial. In this work, the ferroelectricity of the orthorhombic and tetragonal MAPbBr3 single crystal was confirmed through the dielectric behavior versus bias electric field ε(E), the temperature-dependent pyroelectric current with positive/negative poling, and the positive-up-negative-down (PUND) measurements. The electric field dependence of dielectric constant curves shows a butterfly type shape in the orthorhombic and tetragonal phase. The pyroelectric current shows two maxima at 155 and 245 K, corresponding to ferroelectric−ferroelectric and ferroelectric−paraelectric phase transitions, respectively. In particular, the direction of the pyroelectric current can be reversed by a positive or negative poling electric field, which is the assertive evidence of ferroelectricity. The PUND measurements act as the most convincing proof of the ferroelectricity of the MAPbBr3 single crystal. This work reports new evidence of the ferroelectric properties of the MAPbBr3 single crystal, which provides the intrinsic property when considering their high power conversion efficiencies.

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(phase II) at 235 K, another tetragonal phase (phase III) at 155 K, and an orthorhombic phase (phase IV) at 150 K. Although MAPbI3 has tetragonal symmetry at room temperature, its space group and ferroelectricity have still been under massive debate in the past several years. Many researchers suggested that MAPbI3 is in nonpolar space group I4/mcm, which discredits the possibility of ferroelectricity at room temperature.22−24 Meanwhile, other studies have shown that MAPbI3 is in the I4cm polar space group at room temperature and has ferroelectricity. Its domain walls have also been reported.25−31 However, some other researchers reported that MAPbBr3 belongs to nonpolar cubic space group Pm3̅m at room temperature.32−34 The space group and spontaneous polarization behavior of the tetragonal or orthorhombic MAPbBr3 single crystal at low temperatures are still being investigated.35−38 The space group of orthorhombic phase IV was reported to be polar Pna21 by Poglitsch and Weber in 198737 and confirmed by Gesi in 1997.35 The recent synchrotron X-ray powder diffraction investigations suggest that the orthorhombic phase is in nonpolar space group Pnma.33,39 Tetragonal phase II was determined to be in space group I4/mcm at first by Poglitsch and Weber.37 However, it has to be noted that polar space group I4cm and nonpolar space group I4/mcm have exactly the same octahedron-tilt system, and the only difference is that I4cm is also polar along

erovskite materials have been extensively researched in the past several decades due to their remarkable properties, such as giant magnetoresistance (GMR), ferroelectricity, and superconductivity.1−6 Hybrid organic−inorganic halide perovskites (HOIPs), such as MAPbX3 (MA = CH3NH3+; X = I−, Br−, or Cl−), and their ramifications have emerged because of the photovoltaic, optical, and other fascinating performances of HOIPs in recent years.7−17 Devices based on these materials such as sensors, solar cells, and light-emitting diodes (LEDs) have been performed. For instance, the power conversion efficiency (PCE) of the HIOP MAPbI3 solar cells has increased from 3.8% to >22.1% since 2009, approaching the PCE of a single-crystalline silicon solar cell.18 However, MAPbI3 is unstable in the atmosphere and will decompose under room conditions in 48 h. Meanwhile, MAPbBr3 is a promising alternative to MAPbI3 with a higher stability toward air and moisture. The ultrasensitive detectors based on MAPbBr3 were reported because the photoluminescence (PL) spectrum of the MAPbBr3 crystal can be modulated by different environments.19−21 Although the excellent performance and low manufacturing cost of MAPbX3 are well-known, its intrinsic properties such as crystal structure and ferroelectricity remain controversial. MAPbI3 and MAPbBr3 share similar reversible structural phase transitions. MAPbI3, which has a tetragonal structure at room temperature, experiences a structural transition to a cubic phase above 330 K and an orthorhombic phase below 165 K. MAPbBr3 experiences more structural phase transitions than MAPbI3 does.20 MAPbBr3 is a cubic phase (phase I) at room temperature. It undergoes transitions to a tetragonal phase © 2019 American Chemical Society

Received: March 18, 2019 Accepted: May 1, 2019 Published: May 1, 2019 2522

DOI: 10.1021/acs.jpclett.9b00776 J. Phys. Chem. Lett. 2019, 10, 2522−2527

Letter

The Journal of Physical Chemistry Letters the c-direction. The interpretation of data from diffractionbased techniques depends on the user’s choice of the model to which the diffraction pattern should fit. Small deviations between I4/mcm and I4cm can rarely be clearly distinguished. That is the reason for that the recent article that refused to determine the exact space group of tetragonal phase II.20 Despite the massive amounts of diligent research that have been performed, opinions regarding the aforementioned fundamental properties have been contradictory in the past several decades, and further work is demanded to reconcile these conflicts. Generally, ferroelectric properties exist only below the Curie temperature (TC), at which the crystal lattice must adopt one of 10 polar point groups: 1(C1), m(C1h), 2(C2), mm2(C2v), 3(C3), 3m(C3v), 4(C4), 4mm(C4v), 6(C6), and 6mm(C6v). When the temperature exceeds TC, ferroelectrics will undergo the reversible structural phase transition from the ferroelectric phase to the paraelectric phase, accompanied by heat flow, dielectric and pyroelectric anomaly, ferroelectric domain wall motion, and ferroelectric hysteresis loops. To determine the presence or absence of ferroelectricity, many functionally relevant properties should be performed simultaneously, such as piezoelectricity, pyroelectricity, and P−E hysteresis. Here, high-quality single crystals of MAPbBr3 were grown from the solution. We systematically investigate the intrinsic properties of high-quality MAPbBr3 single crystals by means of differential scanning calorimetry (DSC), dielectric anomalies, temperature-dependent pyroelectricity, bias electric field-dependent dielectric constant, and positive-up-negative-down (PUND) measurements, which disclose the ferroelectricity of the MAPbBr3 single crystal. The high quality of bulk single crystals with minimum structural defects empowered us to investigate the intrinsic properties of these materials. The best way to explore the dielectric and ferroelectric properties of HOIPs would be to exploit the single crystals. The MAPbBr3 single crystal was prepared by the antisolvent method. First, 10.0 mmol of PbBr2 (AR, 99.0%, Aladdin) and 10.0 mmol of MABr (99.99%, pOLED) were dissolved in dimethylformamide (DMF) at room temperature in a small crystallizing basin forming a saturated solution. To confine antisolvent vapor diffusion, the basin was covered with a perforated tinfoil. The covered crystallization basin was then placed inside a deeper, larger, crystallizing basin, which contained methylbenzene (antisolvent). The outer crystallizing basin was sealed with tinfoil to create a balanced antisolvent atmosphere. MAPbBr3 crystals were grown from an equimolar solution by slow vapor diffusion of methylbenzene into the dimethylformamide. DSC was performed in the temperature range of 120−273 K with a heating or cooling rate of 10 K/min under a nitrogen atmosphere on a NETZSCH DSC 200F3 instrument. The single crystals were cut into a thin plate along the ⟨001⟩ direction for dielectric, pyroelectric, and PUND measurement. Silver conductive paste deposited on the plate surfaces was used as the electrodes. The temperature-dependent complex dielectric permittivity at frequencies of 5 Hz to 1 MHz and the bias electric field-dependent dielectric permittivity were measured with a Tonghui TH2828A LCR meter. The pyroelectric current was measured with a constant heating rate on an electrometer (Keithley 6517B). The PUND current was measured by a homemade instrument that contained a waveform generator (Agilent 33500B), a high-voltage waveform amplifier (Trek model 609E-6), and an electrometer

(Keithley 6517B). The thickness of the MAPbBr3 single crystal was ∼1 mm. XRD of the powder sample and single crystal was performed at room temperature (Figure 1) to determine the structure and

Figure 1. (a) XRD pattern of the single crystal showing (010), (020), (030), (040), and (050) diffraction peaks. The inset shows the topography of the single crystal. (b) XRD pattern with Rietveld refinement for polycrystalline MAPbBr3. The observed XRD intensities are marked by the black crosses. The Rietveld refinement result is presented as a red solid line. The Bragg reflections ae marked by pink vertical lines, and the differences between the observed and calculated intensities are denoted with green dots.

purity of compounds. The XRD pattern of the powder sample (Figure 1b) can be indexed according to the MAPbBr3 structure in space group Pm3̅m, which proves the purity of the samples. The XRD patterns for the cubic samples at room temperature can be well refined using the standard Rietveld program GSAS with an Rwp factor within 13%. MAPbBr3 single crystals are fairly stable without any decomposition or deliquescence under room conditions for several months. The XRD pattern of the single crystal has only (010), (020), (030), (040), and (050) diffraction peaks, which is proof of the very good crystallinity and the measured plane is along the baxis. DSC measurements play a very important role in interpreting phase transition behaviors. Compared with stable reference materials, ferroelectric crystals will absorb or release latent heat during ferroelectric−paraelectric and/or ferroelectric−ferroelectric phase transitions. The temperaturedependent DSC curve (Figure 2a) shows three endothermic peaks at T3 = 144 K, T2 = 151 K, and T1 = 239 K during the heating run and three exothermic peaks at around T1′ = 233 K, T2′ = 158 K, and T3′ = 154 K during the cooling run. These results reveal three reversible phase transition couples. The value of enthalpy change ΔH can be integral from the peak area of DSC anomalies, and the entropy changes of phase transitions can be calculated as ΔS = ΔH/T. The entropy changes (ΔS) around T1 is ∼1.13 J mol−1 K−1, and the value around T2 and T3 is ∼28.47 J mol−1 K−1. Both entropy changes (ΔS) are estimated via DSC. Three apparent thermal hystereses indicate first-order phase transitions. The appearance of large dielectric anomalies in the vicinity of the phase transition is an essential characteristic of spontaneous polarization. The temperature-dependent permittivities (ε′) (real part of the complex dielectric constant) along 2523

DOI: 10.1021/acs.jpclett.9b00776 J. Phys. Chem. Lett. 2019, 10, 2522−2527

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The Journal of Physical Chemistry Letters

Rakita et al. claimed that the polarization value can be simply obtained by integrating ε over the bias electric field, based on the following equation:31 ΔP = ε0

∫ ε ∂E

Therefore, the ε(E) hysteresis is further direct evidence of ferroelectricity. The real part ε′ of MAPbBr3 still dominates its complex permittivity ε, like PZT40 and Rochelle salt [potassium sodium tartrate, KNa(OC(O)C(OH))2·4H2O]. The real part permittivities ε′ as a function of the electric field of the tetragonal and orthorhombic MAPbBr3 single crystal along the ⟨001⟩ direction are shown in panels a and b of Figure 3, respectively. The dielectric constant has a small value as

Figure 2. (a) Temperature dependence of the differential scanning calorimetry of the MAPbBr3 single crystal along the ⟨001⟩ direction during the heating and cooling runs. (b) Temperature dependence of the real part of the dielectric constant of MAPbBr3 during heating runs. (c) Temperature dependence of the real part of the dielectric constant of MAPbBr3 during cooling runs.

the ⟨001⟩ direction during the heating and cooling process in a temperature range of 120−273 K are depicted in panels b and c of Figure 2, respectively. The permittivity−temperature curves display a reversible sharp peak at T3 = 144 K and T′3 = 162 K during the heating and cooling process corresponding to the phase transition between the tetragonal and orthorhombic forms. Some small dielectric anomalies occur at temperatures T1 = 240 K and T′1 = 243 K and during the heating and cooling process, corresponding to the phase transition between the cubic and tetragonal phase. The dielectric anomalies with various frequencies are consistent with the DSC curves. The insulating ferroelectrics whose leakage currents are negligible usually show a typical polarization−electric field (P− E) hysteresis loops, which is the direct evidence of ferroelectricity. However, the leakage current will impede the interpretation of P−E loop measurement in some finite resistance ferroelectrics. Fortunately, the relative permittivity dependence of bias electric field curve ε(E) can also indicate the absence or presence of ferroelectricity.40,41 The correct description of the ε(E) curve is connection with other characteristics of the nonlinear response, like the ferroelectric polarization hysteresis loop. Several theoretical models have been proposed to fit the interesting dielectric behavior in ferroelectrics.41−43 The most credible theory is a phenomenological one presented by Eiras et al.,41 which provided a helpful path for judging the existence of ferroelectricity when the P−E hysteresis loop is affected by the conduction mechanism. It suggests that the hysteresis curve ε(E) is a necessary and sufficient condition for the P−E hysteresis loop. The value of the bias electric field, which corresponds to the maximum permittivity in the ε(E) curve, represents the coercive field.

Figure 3. Real part of the dielectric response of the MAPbBr3 crystal along its ⟨001⟩ direction measured at 100 kHz as a function of the applied bias electric field at (a) 170 K and (b) 120 K. The red arrows in panel b indicate the polarization direction of ferroelectric domains.

poling with a positive electric field because all of the domains are polarized up. The number of domains with polarization up decreases, while the number with polarization down increases when the electric field decreases. The dielectric constant increases and reaches a maximum value at a negative electric field where the domains with polarization up are equal to those with polarization down. The value of the bias electric field, which corresponds to the maximum permittivity in the ε(E) curve, represents the coercive field. The dielectric constant decreases with a further decrease in the electric field. All of the domains are polarized down when the electric field reaches a negative saturated value. Another maximum value of the dielectric constant will appear when the electric field reaches the positive coercive field. Therefore, the presence hysteresis loop of the ε(E) curve is direct evidence of ferroelectricity. The MAPbBr3 crystals are known to be ferroelectric below the Curie temperature. The coercive field Ec of MAPbBr3 is ∼1.0 kV/cm for both tetragonal and orthorhombic phases. Because ferroelectrics definitely belong to pyroelectric materials, the spontaneous polarization of ferroelectrics can be obtained from the pyroelectric current. When the 2524

DOI: 10.1021/acs.jpclett.9b00776 J. Phys. Chem. Lett. 2019, 10, 2522−2527

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The Journal of Physical Chemistry Letters temperature-dependent spontaneous polarization changes, the excess free charges appear on the polar surfaces of ferroelectrics. This phenomenon gives rise to the detectable pyroelectric current in the circuit between the two opposite polar surfaces. Herein, the single crystal of MAPbBr3 was cut into a thin plate, which was first poled by an electric field of 400 V (or −400 V) during the cooling process. The pyroelectric currents of MAPbBr3 were measured under zero electric field during the heating processes along the ⟨001⟩ direction. The temperature-dependent pyroelectric currents were recorded (Figure 4), where peaks with values of 0.013 nA

Figure 5. Hysteresis loops of electric polarization measured with a positive-up-negative-down (PUND) measurements at (a) 170 K and (b) 120 K.

is ∼1.0 kV/cm in both tetragonal and orthorhombic phases. Meanwhile, the spontaneous polarization values given by PUND measurements are ∼0.4 and 0.6 μC/cm2 for the tetragonal and orthorhombic phases, respectively, which are consistent with the pyroelectric measurement. This is the most direct and powerful evidence of the ferroelectricity of the MAPbBr3 single crystal. In conclusion, the ferroelectricity of semiconductor MAPbBr3 is investigated in this work. Proving its ferroelectricity is quite challenging because of the leaky current of semiconductor ferroelectric materials. The presence of a traditional P−E hysteresis loop is impeded here for the reasons mentioned above. The butterfly-like electric field− dielectric ε(E) hysteresis curve, the switchable pyroelectric current direction by poling voltage, and the PUND measurements act as the direct evidence of the ferroelectric of orthorhombic and tetragonal MAPbBr3 along the ⟨001⟩ direction. The determination of ferroelectricity of orthorhombic and tetragonal MAPbBr3 provides a necessary property for considering the power conversion efficiency of hybrid organic−inorganic halide perovskites.

Figure 4. Temperature-dependent pyroelectric currents (dots, right coordinate) of the single crystal poled by opposite electric voltages (±400 V) and temperature dependence of polarization (lines, left coordinate) given by integration of pyroelectric currents.

(−0.012 nA) and 0.059 nA (−0.056 nA) were observed at 155 and 245 K, respectively, after poling with a 400 V (−400 V) electric field, indicating the release of screen charges. The inverse of the current direction after the inverse of the poling electric field indicates that the polarization can be switched by an external electric field, which reveals that the crystal has ferroelectricity in the tetragonal and orthorhombic phases. Via integration of the pyroelectric current i in the circuit between two polar surfaces in a heating process, the temperature dependence of spontaneous polarization (Ps) can be obtained: i dT Ps = AR where R is the heating or cooling rate and A is the area of the sample. The spontaneous polarization can reach ∼0.35 μC/ cm2 in the tetragonal phase and ∼0.4 μC/cm2 in the orthorhombic phase. Like the appearance of ferroelectricity in orthorhombic and tetragonal MAPbBr3, the space groups of orthorhombic and tetragonal MAPbBr3 are Pna21 and I4cm, respectively. To confirm the existence of ferroelectricity, precise PUND measurements were taken (Figures S3 and S4), and the polarization−electric field (P−E) hysteresis loops are calculated from the current (Figure 5). As a finite-resistance crystal, the leakage current will impede the interpretation of P−E loop measurement in traditional P−E hysteresis loops measurement. PUND measurement can eliminate the disturbance of the leakage current and provide the true ferroelectric response. Figure 5 shows the P−E hysteresis loop of the MAPbBr3 single crystal given by PUND measurements along the ⟨001⟩ direction in the tetragonal and orthorhombic phase. It confirms the coercive field (Ec) of the MAPbBr3 single crystal

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b00776. Schematic diagram of single-crystal growth for MAPbBr3 (Figure S1), crystal face of the MAPbBr3 single crystal (Figure S2), where the leakage current will impede the interpretation of P−E loop measurement in some finiteresistance ferroelectrics like MAPbBr3, and time−current relationship in PUND measurements (Figures S3 and S4) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 2525

DOI: 10.1021/acs.jpclett.9b00776 J. Phys. Chem. Lett. 2019, 10, 2522−2527

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The Journal of Physical Chemistry Letters ORCID

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Hong-Ling Cai: 0000-0002-2855-3016 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundations of China (11574138, 11874200, and 21427801), the Top-Notch Young Talents Program of China, the National Key R&D Program of China (2016YFA0201104), and Dengfeng Project B of Nanjing University.

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DOI: 10.1021/acs.jpclett.9b00776 J. Phys. Chem. Lett. 2019, 10, 2522−2527

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DOI: 10.1021/acs.jpclett.9b00776 J. Phys. Chem. Lett. 2019, 10, 2522−2527