Room temperature ferroelectricity and blue photoluminescence in zero dimensional organic lead iodine perovskites

Abstract

Organic–inorganic hybrid perovskite materials have attracted great attention due to their great application potential in photovoltaics and optoelectronics. Among them, some 2D and 1D lead-iodide-based perovskites were found to exhibit ferroelectricity at room temperature. Yet, no 0D lead-iodide-based perovskites were reported to be room temperature ferroelectric. Here, we report a new lead-iodide-based perovskite material, (DMA)₄PbI₆, which is ferroelectric at room temperature. The spontaneous polarization of (DMA)₄PbI₆ is about 0.3 μC cm⁻². (DMA)₄PbI₆ undergoes a ferroelectric–ferroelectric phase transition around 252 K in the heating process. The ferroelectric–ferroelectric phase transition is a first order phase transition with a hysteresis of 18 K between the heating process and the cooling process. The absorption spectra show that (DMA)₄PbI₆ is a direct band material with a band gap of 2.80 eV. A broadband blue photoluminescence centered at 2.7 eV was observed, which may be attributed to the self-trapped excitons.

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1. Introduction

Hybrid organic–inorganic perovskites (HOIPs) have attracted great attention due to their potential application in photovoltaics, optoelectronics, and pizoelectrics.^1–6 MAPbI₃ is one of the most famous HOIP materials as a good photovoltaic material.^7,8 The power conversion efficiency of MAPbI₃ (CH₃NH₃PbI₃) based solar cells has boosted from 3.8% to 24.2% in just a few years. The ferroelectricity of MAPbI₃ was proved by different methods by different groups.^9–13 Polarization in MAPbI₃ films is believed to be helpful for the separation of photo generated electron–hole pairs in solar cells. The bulk ferroelectric photovoltaic effect may contribute largely to the power conversion efficiency in MAPbI₃ solar cells. MAPbI₃ is a three-dimensional perovskite, which is similar to BaTiO₃. The PbI₆ octahedron shares one I site with nearby six PbI₆ octahedra, forming a three-dimensional grid structure. The three dimensional perovskite structure makes MAPbI₃ have a small band gap of 1.5–1.6 eV. Meanwhile, MAPbI₃ behaves as a semiconductor with good conductivity, which makes it hard to study the ferroelectricity in MAPbI₃. Therefore, the ferroelectricity of MAPbI₃ is still controversial.¹⁴ In addition to MAPbI₃, very recently two 3D perovskites that crystallize in polar structures comprising methylhydrazinium cations have been discovered,^15,16 which may be potential ferroelectric materials with good photoluminescence properties.

Although it is hard to study the ferroelectricity in 3D organic–inorganic halide lead perovskites, some 2D and 1D organic–inorganic halide lead perovskites are found to exhibit room temperature ferroelectricity.^17–19 The band gaps of the 2D/1D ferroelectrics are usually smaller than those of other molecular ferroelectrics, making them good candidates for optoelectronic device materials. Of all the reported hybrid halide lead perovskite ferroelectrics, no one was reported to have blue light emission. In this study, we synthesized a new hybrid halide lead perovskite ((DMA)₄PbI₆(CH₃)₄PbI₆) with a general formula of A₄BX₆. (DMA)₄PbI₆ is a 0 dimensional perovskite, where the individual lead halide octahedra (PbI₆) are completely isolated from each other and surrounded by the wide-band-gap organic ligands (CH₃NH₂CH₃⁺). This isolation allows the bulk crystals to exhibit the intrinsic properties of the individual lead halide units/clusters by inhibiting interactions between the lead halide octahedra.^20–22 0D lead iodide hybrid perovskites have been significantly underexplored, although some early literature studies have demonstrated lead iodide hybrid perovskites with a 0D structure. For example, MA₄PbI₆·2H₂O is reported to have a 0D structure with isolated PbI₆ octahedra.²³ However, this crystal phase is unstable and slowly decomposes into PbI₂.²⁴ The most studied 0D metal halide hybrid perovskites are Sn⁴⁺, Sn²⁺, Bi³⁺, and Te⁴⁺ based octahedra. The properties of 0D organic lead iodine hybrid perovskites have not been reported. Pb²⁺ based 0D iodine hybrid perovskites require four positive monovalent organic molecules to form a stable crystal structure. In the DMA₄PbI₆ structure, the PbI₆⁴⁻ octahedra are completely decoupled by wide band DMA⁺ molecules in all dimensions. The optical properties of the DMA₄PbI₆ crystals may closely resemble those of individual PbI₆⁴⁻ clusters. The unique photophysical properties of PbI₆ based 0D perovskites are of interest for a variety of potential applications, such as optically pumped and electrically driven LEDs, lasers, scintillators, etc. in addition to the intrinsic optical properties, we found that DMA₄PbX₆ is also a room temperature 0D ferroelectric material. Ferroelectricity in 0D perovskites may be helpful to regulate the optical and electrical properties.

DMA halide hybrid perovskites have been reported previously, such as DMAPbI₃,^25,26 DMAPbBr₃,²⁷ DMAPbCl₃,²⁷ DMA₇PbBr₁₅,²⁸ and DMA₇PbCl₁₅.²⁸ 0D DMA₄PbI₆ has not been reported before. The optical energy gap energy is 2.80 eV, which is larger than 2.59 eV of DMAPbI₃.²⁵ A broadband photoluminescence centered at 2.7 eV was observed, which may be attributed to the self-trapped excitons.

2. Experimental section

2.1. Crystal growth

1 mmol PbI₂ and 4 mmol MAI or FAI or NH₄I were mixed with 20 ml of DMF. The solution was stirred until the solution became clear. Then 10 ml of hydroiodic acid (57%) and 2 ml of 50% phosphorous acid solution were added dropwise into the solution. Phosphorous acid can protect hydroiodic acid from oxidation. The solution was stirred for about 2 hours. 0D DMA₄PbI₆ single crystals were obtained by slow evaporation of the solution at 60 °C. About two weeks later, crystals appeared in the solution. About a month later, large single crystals were collected.

2.2. Characterization

The elemental analysis was performed on a Heraeus CHN-0-Rapid elemental analyzer. The absorption spectrum was recorded by using a PV Measurements Model QEX10 through using an Integral Sphere Method from 300 nm to 1100 nm. Single crystal X-ray diffraction was carried out by using a Bruker D8 QUEST. Powder X-ray diffraction (PXRD) was conducted on a Rigaku D/MAX 2000 PC X-ray diffractometer. DSC measurements of single crystals were recorded by using a NETZSCH DSC 200F3 in the temperature range of 100–300 K. All electrical tests were carried out on single crystal flakes as shown in Fig. S1 (ESI†). The silver paste is coated on the largest crystal surface as an electrode. The complex permittivity was measured using a Tonghui TH2828A LCR meter. The photoluminescence spectra were detected by time resolved fluorescence spectroscopy (FLSP20, Edinburgh, lhstrcamenss). The absorption spectra were recorded by using a Quantum efficiency tester. Raman spectra were collected using a Horiba Jobin Yvon HR800 spectrometer device by means of a 488 nm laser line. P–E hysteresis loops were recorded on a Precision Premier II (Radiant Technologies, Inc.). Pyroelectric current was measured using a Keithley 6517B. Silver glue is applied on the largest surface of the crystal as an electrode. The temperature is controlled by blowing liquid nitrogen to cool down and by using heating plates to heat. During the pyroelectric current measurement, the heating and cooling rate is 10 K min⁻¹. The IV tests are conducted using a Keithley 2400 at room temperature.

3. Results and discussion

As 0D DMA₄PbI₆ is not stable in PbI₂ solution, it is hard to eliminate 0D DMA₄PbI₆ single crystals in the traditional methods. Furthermore, when DMA was added into the solution, it is easier to grow 1D DMAPbI₃ rather than 0D DMA₄PbI₆. Though 0D DMA₄PbI₆ crystals can appear in the solution, 0D DMA₄PbI₆ tends to decompose to form 1D DMAPbI₃ crystals. In the typical method, DMAI and PbI₂ were used as reactants. Here, DMAI comes from the reaction of DMF, HI and H₂O. MAI or FAI or NH₄I was added to the solution to form clusters with PbI₂ in the solution. The growth of 3D MAPbI₃ in DMF solution is hard. So we do not need to worry about growing MAPbI₃ impurities. In this way, single crystals of DMA₄PbI₆ were obtained by slow evaporation at 60 °C. Due to the consumption of MAI or FAI or NH₄I by PbI₂, DMA⁺ is greatly excessive in comparison to the free PbI₆⁴⁻, which contributes to the single crystal growth of DMA₄PbI₆. Fig. S1 (ESI†) shows the photograph of single crystals we acquired. The single crystals present parallelogram-shaped flakes. The side of the parallelogram is about two millimeters long. The thickness of the crystal is 0.5–1 mm.

To ascertain the structure and purity of compounds (DMA)₄PbI₆, we quantitatively measured the mass fractions of carbon, and nitrogen in the compound by means of CHN elemental analysis. The results are C 8.28%, H 2.84%, and N 4.84% for the compound, which are in accordance with the theoretical values (C 8.32%, H 2.78%, and N 4.85%). The measurement error of the mass fractions is about 0.3%. The Raman spectrum was recorded on the surface of single crystal of (DMA)₄PbI₆ (Fig. S3, ESI†). The Raman spectrum corresponds to the vibration spectrum of dimethylamine ions, proving that the organic molecules in the crystal are dimethylamine ions.

Single crystal X-ray diffraction (SCXRD) was performed to obtain the crystal structure of the 0D organic metal hybrid perovskites. The structure of the obtained crystal was solved with Shelxtl 97 package. It is hard to choose the room temperature space group based on the single crystal X-ray diffraction data. Two space groups were suggested, Cmca and Aba2. The CFOM of the two space group is close. According to the room temperature ferroelectric properties, the crystal structure of (DMA)₄PbI₆ was solved in polar space group (Aba2). Fig. 1 shows the crystal structure of (DMA)₄PbI₆. In (DMA)₄PbI₆, the individual PbI₆⁴⁻ octahedra are surrounded by eight DMA⁺ cations. The iodine ions in (DMA)₄PbI₆ are no longer shared between PbI₆⁴⁻ octahedra. At room temperature, N atoms in the DMA cations and 2/3 iodine atoms are disordered. All N atoms in DMA have a 50% probability of appearing in each of the two positions. The disorder of DMA molecules may be caused by the rotation of half of the DMA molecules along the C–C axis. The disorder of 2/3 iodine atoms may be caused by the rotation of half of the PbI₆⁴⁻ along the rest of the I–Pb–I axis. Disorder makes the crystals crystallize in a relatively high symmetry space group Aba2. Compared with Cs₄PbI₆, (DMA)₄PbI₆ could be considered as true 0D organometal halide perovskites due to the complete isolation of the photoactive PbI₆⁴⁻ octahedra by the wide band DMA⁺ cations.^4,22,29 The powder X-ray diffraction pattern of the ball milled crystal powder is in well accordance with the simulated patterns from SCXRD, indicating the purity of the synthesized 0D (DMA)₄PbI₆ crystals.

Fig. 1 Crystal structure of 0D (DMA)₄PbI₆ shown with polyhedron.

Most ferroelectrics exhibit phase transitions. Differential scanning calorimetry (DSC) is sensitive to phase transition. Fig. 2a shows the DSC curve of the powder of 0D (DMA)₄PbI₆. An endothermic peak at 252.5 K and an exothermic peak at 234.2 K can be found from the DSC when scanning at a temperature rate of 10 K min⁻¹, indicating the reversible phase transition happening at around 252 K. This phase transition displays a large hysteresis of 18.3 K, suggesting a first order phase transition. The enthalpy (ΔH) is 3.07 × 10⁴ J mol⁻¹ in the cooling process and 3.32 × 10⁴ J mol⁻¹ in the heating process. The entropy change is 131.1 J mol⁻¹ K⁻¹ (cooling) and 131.5 J mol⁻¹ K⁻¹ (heating), as determined from the area under the heat flow/temperature curve and the peak temperature T_max. The large entropy change indicates that this phase transition belongs to order–disorder phase transition. Considering the disorder of DMA⁺ and iodine atoms in DMA₄PbI₆ at room temperature, the order–disorder phase transition may originate from the rotation of DMA⁺ molecules and PbI₆⁴⁻.

Fig. 2 (a) DSC of (DMA)₄PbI₆ measured with 10 K min⁻¹. (b) The temperature dependent real part of the dielectric constant in the heating process and in the cooling process.

SCXRD of (DMA)₄PbI₆ was also performed below the phase transition temperature. However, the diffraction point of the low temperature phase is so poor that we cannot obtain the low temperature crystal structure of (DMA)₄PbI₆. Twins and defects induced by the phase transition may be responsible for the poor crystal diffraction data.

The complex dielectric constants (ε = ε′ − ε′′, where ε′ and ε′′ are the real and imaginary parts, respectively) of DMA₄PbI₆ were measured on the single crystal. The heating and cooling temperature rate is about 5 K min⁻¹. The temperature dependent dielectric ε′ (Fig. 2b and Fig. S4, ESI†) shows a step-like dielectric anomaly at around phase transition temperature (T₁). As the phase transition has a hysteresis of 18.3 K in the heating and cooling process, the dielectric is bistable around T₁ with a temperature range of about 15 K. As a switchable dielectric, DMA₄PbI₆ can undergo a transition between high and low dielectric states at T₁. Due to the existence of polar DMA molecules, dipolar reorientation in DMA₄PbI₆ may contribute significantly to the dielectric response.^25,30–32 In DMA₄PbI₆, all the DMA molecules are disordered above T₁. The phase transition of DMA₄PbI₆ is an order–disorder phase transition according to the DSC result. The motion of DMA may contribute a lot to the dielectric transition in DMA₄PbI₆.

The ferroelectricity of DMA₄PbI₆ was confirmed by the P–E (polarization–electric field) hysteresis loop. A typical ferroelectric hysteresis loop was observed at 269 K and 210 K with a Sawyer–Tower circuit (Fig. 3). The ferroelectric hysteresis loop measured at 269 K and 210 K indicates that 0D DMA₄PbI₆ is ferroelectric below T₁ and above T₁. At 269 K, the spontaneous polarization P_s is about 0.29 μC cm⁻², the coercive field E_c is about 1.34 kV cm⁻¹, and the remanent polarization P_r is about 0.19 μC cm⁻². The P_S of 0D DMA₄PbI₆ is comparable to that of Rochelle salt. At 210 K, P_s, P_r and E_c are 0.24 μC cm⁻², 0.11 μC cm⁻², and 0.89 kV cm⁻¹, respectively. The spontaneous polarization of DMA₄PbI₆ at 210 K is smaller than that at 269 K, accompanied by a smaller coercive field and remnant polarization. So the high temperature phase is the high polarization state and the low temperature phase is the low polarization state. The switchable dielectric behavior is caused by the ferroelectric–ferroelectric phase transition.

Fig. 3 The ferroelectric hysteresis loop of (DMA)₄PbI₆ measured on the 001 face.

To further determine the ferroelectricity, we measured the I–V characteristics of the material. As shown in Fig. S10 (ESI†), 0-dimensional materials exhibit the I–V characteristic curve of ferroelectric materials at room temperature. Obvious polarization reversal current peaks can be observed around 35 V. The P–E hysteresis loop and I–V characteristic curve of ferroelectrics are adequate to prove the ferroelectricity of 0D DMA₄PbI₆, which is different from the “banana effect”. The I–V characteristic curve of ferroelectric cannot be observed in the banana effect.

The pyroelectric properties of single crystals of DMA₄PbI₆ were determined to confirm the ferroelectricity (Fig. S8 and S9, ESI†). Across the phase transition temperature range, a broad pyrolelectric current peak was detected implying that 0D DMA₄PbI₆ is an improper ferroelectric compound. Polarization changes acquired from the pyroelectric current measurements are larger than that acquired from P–E hysteresis loop measurements, which may be caused by the high conductive properties induced by high defect concentrations.

The absorption spectrum of DMA₄PbI₆ was recorded by using an integral sphere method. Fig. 4a shows the absorption spectrum of DMA₄PbI₆ powder. The optical coefficient α of DMA₄PbI₆ around the optical absorption band can be fitted well with Kubelka–Munk function ((αhv)² = C(hv − E_g), E_g is the band gap), indicating that DMA₄PbI₆ is a direct band gap compound. The optical band gap E_g of DMA₄PbI₆ is determined to be 2.80 eV by the Kubelka–Munk function. The band gap of DMA₄PbI₆ is smaller than that of Cs₄PbI₆ (3.38 eV).³³ Due to the correlation between Cs and PbI₆, Cs₄PbI₆ does not exhibit intrinsic luminescence of individual PbI₆ octahedra. Intrinsic luminescence of individual PbI₆ octahedra is characteristic of 0-dimensional materials.

Fig. 4 (a) The absorption spectra of (DMA)₄PbI₆. The inset shows the fitting curve of ((αhv)² = C(hv − E_g)). The optical band gap is determined to be 2.80 eV. (b) The PL spectra excited with 200 nm UV light. The inset shows the digital photograph of the single crystals illuminated with 10 W 365 nm UV light.

Photoluminescence spectra of 0D DMA₄PbI₆ were recorded to explore the intrinsic photoluminescence of PbI₆ octahedra. Fig. 4b and Fig. S5 (ESI†) show the photoluminescence spectra of DMA₄PbI₆ excited with 200 nm UV light. Four photoluminescence peaks, peaking at 340 nm, 362 nm, 455 nm and 553 nm, were found in DMA₄PbI₆. Among the four photoluminescence peaks, 455 nm photoluminescence belongs to the blue emission and is the strongest photoluminescence peak of DMA₄PbI₆. The blue emission is a broadband emission with full width at half maximum (FWHM) of about 80 nm. This FWHM is wider than that of 1D DMAPbI₃. The photoluminescence of 0D DMA₄PbI₆ and 1D DMAPbI₃ is similar (Fig. S5, ESI†). The photoluminescence region of the 0D and 1D compounds is almost the same, except that of the broadband blue emission of the 0D compounds. This means that the main photoluminescence of the 1D and 0D compounds comes from the PbI₆ octahedra. Fluorescence at 553 nm may be derived from iodine atoms.^34–36 The UV and blue fluorescence may be derived from Pb ions.³⁷ The broadband blue emission in 0D DMA₄PbI₆ may be ascribed to the self-trapped excitons of PbI₆.³⁸ Without intermolecular interactions or electronic band formation, 0D perovskites are expected to be the most favorable environment for the formation of self-trapped excited states.^39,40 Interactions between the PbI₆ octahedra in 1D DMAPbI₃ may weaken the self-trapped excitation, resulting in a narrower emission peak in 1D DMAPbI₃ than that in 0D DMA₄PbI₆.

The inset of Fig. 4b shows the microscopy photograph of DMA₄PbI₆ crystals under 365 nm UV light. Under 365 nm ultraviolet light, we can observe that the crystal emits blue-violet light (Fig. 4b inset). It can be observed from the photoluminescence excitation spectra of the blue emission (Fig. S5, ESI†) that deep ultraviolet (UVC) light is more effective in exciting the blue emission compared to all region UV light. 365 nm UV light can only excite weak blue emission.

4. Conclusions

The first stable 0D organic lead iodine perovskite single crystal without a crystallizing solvent was grown using MAI or FAI or NH₄I in addition to PbI₂ in the solution. (DMA)₄PbI₆ exhibits a first order phase transition at T_c (about 260 K), accompanied by a switchable dielectric behavior. The single crystal of (DMA)₄PbI₆ is still stable up to 450 K. PE hysteresis loops show that (DMA)₄PbI₆ is ferroelectric both above T_c and below T_c. (DMA)₄PbI₆ is a room temperature ferroelectric material with a spontaneous polarization of 0.3 μC cm⁻². A broad band blue photoluminescence was found in 0D (DMA)₄PbI₆ which may be ascribed to the self-trapped excitation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Natural Science Foundation of Jiangsu Province, China (BK20170482), the National Natural Science Foundation of China (11874200), and the Major Research Plan (2017YFA0303202).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2016638. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0tc04813e

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