Research Advances of Ferroelectric Semiconductors of 2D Hybrid Perovskites toward Photoelectronic Applications

INTRODUNTION

Ferroelectric materials are a typical class of polar crystalline compounds with two or more possible directions of spontaneous polarization (P_s), which can be reversed or changed under the action of the external electric field.^[1] The presence of ferroelectric P_s enables abundant macroscopic physical properties, such as dielectric response, piezoelectricity, pyroelectricity, second-order optical nonlinearity, photorefractive effect, etc.^[2-6] All these attributes have been widely employed for practical device applications, including capacitors, random access memories, infrared detectors, piezoelectric sensors, nonlinear optical devices, and so forth. From the perspective of crystallography, the crystal structures of ferroelectric materials should belong to one of ten polar point groups: C₁ (1), C_s (m), C₂ (2), C_2v (mm2), C₃ (3), C_3v (3m), C₄ (4), C_4v (4mm), C₆ (6) and C_6v (6mm). Especially, the ferroelectric order of crystalline materials usually exists only in a certain temperature range. That is, the emergence of ferroelectric P_s closely relates to the paraelectric-ferroelectric structural phase transition with a critical temperature (the Curie temperature, T_c).^{[7, 8]} When the temperature reaches T_c, the compounds undergo very subtle structure changes from a low-symmetric ferroelectric phase (FEP, T < T_c) to a high-symmetric paraelectric phase (PEP, T > T_c), accompanying with remarkable anomalies of macroscopic physical properties.^[9-11] This symmetry breaking is an indispensable indicator for almost all the ferroelectric materials, except for a few cases.^[9-11] Consequently, a deep understanding of the relationship between symmetry breaking and ferroelectricity is quite essential for exploring new ferroelectric families and thereby extending their potential device application.^[12]

Rochelle salt (NaKC₄H₄O₆·4H₂O, sodium potassium tartrate tetrahydrate) is the first molecule-based compound that shows intriguing dielectric properties, defining the concept of ferroelectricity analogous to the ferromagnetism.^[13] Although its ferroelectric properties and structural phase transition were verified experimentally, ^[14] the structural complexity and instability still caused difficulties in the early studies on the origin of its ferroelectric orders. Subsequently, the family of ABX₃-type perovskite oxides gradually became the mainstream of ferroelectric portfolios, such as BaTiO₃, Pb(Zr, Ti)O₃ and LiNbO₃, which have greatly promoted the practical device applications of ferroelectric materials. In term of chemical compositions and crystal structures, the ferroelectric compounds have covered a wide range of inorganic oxides, organic compounds, organic-inorganic hybrids, polymers, and liquid crystals. More recently, with the increasing concerns on environmental and sustainable development, molecular ferroelectrics have attracted renewed interest due to their intriguing characteristics of facile preparation, low cost, environmental friendliness, strong compatibility, etc. In 2013, Xiong et al.^[15] reported a molecule-based binary salt of diisopropylammonium bromide, showing the high T_c and large P_s comparable with that of inorganic oxide counterparts, which greatly accelerates the progress of molecular ferroelectrics toward potential applications. Since then, substantial new types of molecular ferroelectrics with fascinating properties have been synthesized and reported, such as metal-free 3D perovskite ferroelectrics^{[16, 17]} and organic-inorganic hybrid perovskite ferroelectrics.^[18-21]

According to the microscopic mechanisms of symmetry breaking, the ferroelectric phase transitions could be roughly divided into the displacive type and order-disorder type (Scheme 1). Among them, the most of inorganic oxides, such as BaTiO₃, LiNbO₃, PbTiO₃, belong to the displacive type, of which the off-centered displacements of the ions give rise to the spontaneous electric polarization.^[22-24] The typical representative of order-disorder type ferroelectrics is NaNO₂, of which the dynamic reorientation of electric dipoles about the mirror plane accounts for PEP-FEP phase transition.^[25] In reality, these two types of structural phase transitions are not mutually exclusive.^[26] As an important branch of molecule-based ferroelectrics, organic-inorganic hybrid perovskites (OIHPs), in which the order-disorder characteristics of organic cations often afford a driving force to the trigger phase transition, ^[27-29] have recently attracted renewed attentions on account of their infinite structural flexibility and processability. In particular, the 2D family of OIHPs, combining ferroelectric orders with their unique semiconducting merits, would behave as a potential candidate for fabricating the newly-conceptual photoelectronic devices.^{[30, 31]} Up to date, the most well-developed subclass of 2D OIHPs is the Ruddlesden-Popper (RP) structure, which adopts a general chemical formula of (Aʹ)₂(A)_n-1M_nX_3n+1, ^[32] where Aʹ denotes organic spacer, A is perovskitizer, M is divalent metal and X is anion. The n value presents the number of inorganic perovskite layers (Scheme 1c). From a structural design viewpoint, such multilayered architectures can be established by the chemical alloying of mixed-organic cations (A and Aʹ), for which the strong structural compatibility allows diverse physical properties.^{[33, 34]} In the 2D family of OIHPs, the organic sheets and inorganic sheets are arranged alternately to constitute the quantum-confined structure, of which the perovskite components make a crucial role for their semiconducting properties, such as optical bandgap and electrical conductivity. Meanwhile, the organic spacing cations allow molecular freedom of dynamic motions to trigger phase transitions and ferroelectric spontaneous polarization. These structural characteristics of 2D OIHPs thus endow the combination of semiconducting electric conductivity and ferroelectricity. Most interestingly, the subtle chemical modification not only results in the targeted compounds but also optimizes their physical properties. The main design strategies include tailoring the A/Aʹ-site organic cations, modifying inorganic perovskite frameworks, and regulating the number of inorganic layers (i.e., n value); all these methods have been utilized to realize the rational design of new multifunctional ferroelectric materials.^[35-37] In this review, we have summarized the recent advances of 2D OIHP-type ferroelectrics materials, termed ferroelectrics semiconductors. Primarily, the structural changes of symmetry breaking for generating ferroelectric orders are analyzed. The combination and/or coupling of ferroelectric P_s with other semiconducting activities results in new optoelectronic behaviors. Furthermore, the prospects of these 2D ferroelectric semiconductors are also discussed, as well as their future development trend for photoelectronic device applications.

Scheme 1

Scheme 1.  Schematic illustration of the microscopic mechanism for generating ferroelectric P_s during (a) displacive type and (b) order-disorder type phase transitions. (c) Design strategy for assembling ferroelectric semiconductors in the family of 2D organic-inorganic hybrid perovskites.

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PEROVSKITE-TYPE MONOLAYERED FERROELECTRIC SEMICONDUCTORS (n = 1)

Perovskite-type structures are generally defined by a framework of corner-sharing MX₆ octahedra. This concept refers to the mineral CaTiO₃ that comprises the 3D anionic network with Ca²⁺ cations locating inside the space formed by corner-sharing TiO₈ octahedra. As the low-dimensional subclass, the 2D layered hybrid perovskites can be formed by the conceptual slicing of a 3D prototype lattice along with a certain index (Scheme 1c). For instance, the majority of 2D RP-type structures are the < 100 > -oriented perovskites with the cutting planes perpendicular to crystallographic < 100 > direction. In terms of the thickness of inorganic sheets, the monolayered perovskites (n = 1) belong to the simplest 2D structure of (Aʹ)₂(A)_n-1M_nX_3n+1 family. There is only one type of organic cation (Aʹ) serving as the spacing moiety, which is alternatively arranged with the perovskite sheets. Owing to the remarkable structural flexibility, the monolayered hybrid perovskites allow the incorporation of versatile organic cations, of which the dynamic motions and reorientations give rise to their ferroelectric orders.

(n-BA)₂PbCl₄ (Compound 1). Basic crystal structure of 1 consists of the infinite layers of corner-sharing PbCl₆ octahedra separated by the bilayers of organic n-butylammonium (n-BA) cations. At room temperature, all the organic cations are orderly arranged along the c-axis, which also indicates the direction of P_s at FEP (Figure 1a).^[38] With the temperature increasing, the n-BA⁺ cations become disordered and create the crystallographic m mirror plane at high-temperature PEP. Symmetry breaking with an Aizu notation mmmFmm2 was solidly verified, showing the transformation from centrosymmetric space group Cmca (E, C₂, 2C₂', i, σ_h, 2σ_ν) to the polar one Cmc2₁ (E, C₂, 2σ_ν) at FEP. This process belongs to one of the 88 phase transitions for ferroelectric materials. Intuitively, the evidence for ferroelectricity, i.e., electric polarization versus electric field (P-E) hysteresis loop, was measured along the crystallographic c^FEP-axis (Figure 1b). The saturated P_s of 2.1 μC·cm^-2 and coercive force field (E_c) of 14.3 kV·cm^-1 were obtained at 300 K, falling in the range of other analogue ferroelectrics of OIHPs.

Figure 1

Figure 1.  (a) Project of crystal structures of 1 at FEP (216 K) and PEP (330 K). (b) P-E hysteresis loops measured along the c^FEP-axis direction. Inset: P-E hysteresis loops collected along the a^FEP- and c^FEP-axis directions. (c) CIE chromaticity coordinates of 1 in the color space chromaticity diagram. Inserted: the white light emission picture excited at 336 nm.

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Interestingly, the structural distortion of PbCl₆ octahedra of this OIHP not only contributes to excellent ferroelectricity, but also contributes to the broadband spectral emission.^[39] When subjected to 336 nm excitation, 1 exhibits a wide spectral range of white emission covering the whole visible region with a maximum at 543 nm. As shown in Figure 1c, the emission locates near the yellow side of pure white light with the Commission Internationale de L'Eclairage (CIE) chromaticity coordinate of (0.37, 0.40) and a correlated color temperature of 4423 K corresponding to the "warm" white light, which is advantageous for solid-state lighting and display applications. In addition, the broadband emission of 1 also affords excellent color rendition with high color rendering index (CRI) up to ≈ 86. This value is even higher than the typical fluorescent light source (CRI ≈ 65). The photoluminescence (PL) quantum efficiency (1%) and lifetime (τ₁ ≈ 2.59 ns, τ₂ ≈ 9.68 ns) were also measured for this white-light luminescent ferroelectric. As far as we are aware, the combination of ferroelectricity and broadband white-light emission in the single-phase material might open up a potential pathway toward the integrated optoelectronic devices.

(DFCHA)₂PbI₄ (Compound 2). Compared with lead-chloride counterparts, the lead-iodide hybrid perovskites have shown remarkable advantages of optical absorption and electrical transport properties. To explore ferroelectric semiconductors of lead-iodide family, the method of fluorine modification was used to design organic spacing cations, of which the dynamic motions allow for ferroelectric orders, as verified in compound (DFCHA)₂PbI₄ (where DFCHA is 4, 4-difluorocyclohexylammonium).^[40] Thermal analyses reveal that 2 undergoes a structural phase transition at 377 K (T_c). At the PEP (above T_c), it features a 2D RP-type perovskite structure with the non-polar centrosymmetric space group of Pbca. The 2D inorganic perovskite sheets consist of infinite corner-sharing PbI₆ octahedra, and two layers of organic DFCHA cations occupy the space between adjacent sheets (up, Figure 2a). It is noteworthy that in the unit cell of PEP, two layers of DFCHA cations are linked to lead-iodide sheets with different orientations, thus cancelling out the net polarization. With temperature decreasing below T_c, 2 crystallizes in the polar space group of Cmc2₁ at FEP. Compared to the structure at PEP, all the organic DFCHA cations exhibit the consistent arrangement in the unit cell, and their reorientations break the inversion symmetry. Consequently, the macroscopic polarization could be generated by such an arrangement (down, Figure 2a). This structural change with an Aizu notation of mmmFmm2 is one of the 88 species of ferroelectric phase transitions.^{[41, 42]} The electric switching of P_s verified by lateral piezoresponse force microscopy (PFM) confirms its ferroelectric properties. As shown in Figure 2b, the distinct 180˚ switching of the PFM phase signals with the hysteresis behavior as well as the butterfly-shaped amplitude signals indicates the switchable electric polarization of 2.

Figure 2

Figure 2.  (a) Packing structures of 2 collected at PEP (398 K) and FEP (298 K), projected along the a- and b-axis, respectively. Hydrogen atoms are omitted for clarity. (b) Local lateral PFM switching spectroscopy. (c) Schematic diagram of FEPV measurement with the electrodes aligned parallel and perpendicular to the P_s direction. Short-circuit photocurrent in the different experimental setup (indicated by different colors), respectively.

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Structurally, the inorganic lead-iodide frameworks make an important role to the optical and semiconducting properties of 2. The optical bandgap of ~2.38 eV was obtained from the Tauc equation, of which the valence band maximum and conduction band minimum involve with the electronic states of Pb and I atoms that determine its optical bandgap. Furthermore, ferroelectric photovoltaic (FEPV) effects were achieved in the single crystals of 2 (Figure 2c). Crystal-based device with two electrodes parallel to the polarization direction exhibits a photovoltaic short-circuit photocurrent (I_sc) with the amplitude of ~1 pA under white light (red curve, Figure 2c). Subsequently, large voltage was applied to reverse polarization direction, and the results clearly show that I_sc should stem from the ferroelectric polarization. That is, the FEPV effects in 2 are the main mechanism, thus eliminating the possibility of potential asymmetry caused by depolarization electric field effect, optical rectification effect and electrode asymmetry. Such findings would highlight the potentials of 2 for perovskite solar cells and other optoelectronic device applications.

(BPA)₂PbBr₄ (Compound 3). The modification of organic spacing cations is an effective pathway to trigger ferroelectric orders of the 2D family of hybrid perovskites. Under the guidance of hydrogen/halogen substitution, we have successfully synthesized a new ferroelectric of (BPA)₂PbBr₄ (BPA = 3-bromopropylammonium) that adopts the 2D RP-type motif, ^[43] which consists of the inorganic monolayered perovskite frameworks and interlaced bilayers of organic BPA⁺ spacer (Figure 3a). At the roomtemperature phase, it is interesting that the organic moieties are fully ordered with the C–N bonds orienting along the c-axis. With the temperature increasing above T_c, the inorganic PbBr₆ octahedra transform into the symmetric configuration and BPA⁺ cations become highly-disordered, locating with respect to the crystallographic m mirror plane. This order-disorder phase transition accompanies with the symmetry breaking from a centrosymmetric space group Cmca (the point group mmm, at PEP) to the polar one Cmc2₁ (the point group mm2, at FEP), being reminiscent of possible ferroelectricity. Furthermore, the measurements of temperature-dependent P-E hysteresis loops verify the ferroelectric properties of 3. As shown in Figure 3b, the linear trace reveals its paraelectric behavior above T_c, whereas the typical P-E hysteresis loops with good rectangularity provide direct evidence for its switchable polarization. At 298 K, the P_s and coercive field (E_c) are about calculated as 4.8 μC·cm^-2 and 18.2 kV·cm^-1 respectively, which are on the par with other OHIP ferroelectrics.

Figure 3

Figure 3.  (a) Project of crystal structures of 3 at FEP (298 K) and PEP (390 K). (b) Variable-temperature P-E hysteresis loops measured along the c^FEP-axis. (c) Plot of the resolved I_sc as a function of the polarized-light polarization angles at FEP.

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The strong light-matter interactions in ferroelectric materials lead to new concept of photoelectronic properties. Here, notable FEPV effects were clearly observed in the bulk crystals of 3 under UV illumination with an intensity of 45 mW·cm^-2, showing the open-circuit photovoltage (V_oc) of 0.85 V and I_sc of ~0.5 nA, respectively. Such bulk FEPV responses are mainly derived from the electric P_s, which can create a built-in electric field to separate the photogenerated carriers without applying external bias.^[44-46] Given the quite low dark current (I_dark, 1.2×10^-14 A), a large ratio of current (I_sc/I_dark) up to ~10⁴ was obtained under the irradiation of 377 nm light. In combination of P_s-induced FEPV effects and natural 2D anisotropy, the new prospects of self-driven polarized-light detection for 3 can be expected. Figure 3c displays the resolved I_sc as a function of the polarization angles of polarized-light, giving the maximum polarization ratio (I_max/I_min) of ~6.8. This figure exceeds some other UV polarized-light sensitive materials, such as ZnO, GaN, GeS₂, etc. Besides, the photoresponsivity (R_λ) and detectivity (D*) of the device were estimated as 10^-4 A/W and 10⁷ Jones (0 V bias, 1.5 mW·cm⁻²). These results indicate that the self-powered photodetectors based on single crystals of 3 have remarkable potentials toward the polarization-sensitive photodetection in the UV region.

PEROVSKITE-TYPE BILAYERED FERROELECTRIC SEMICONDUCTORS (A')₂AM₂X₇ (n = 2)

The huge structural flexibility of 2D hybrid perovskites allows for accommodation of diverse ammonium spacing cations, of which a large freedom degree of molecular motions are the key elements to realize ferroelectric properties.^[47] Meanwhile, the modification of inorganic networks and regulation of layer numbers (i.e., n value) are feasible to improve their semiconducting properties. Particularly, the 2D multilayered RP-type hybrid perovskites (Aʹ)₂(A)_n-1M_nX_3n+1 combine the different properties of organic and inorganic components in a single-molecule complex, acting as an ideal platform to expand new physical concepts. For ferroelectric semiconductors, however, a subtle balance should be taken between ferroelectricity and conductivity, because the severe P_s deterioration might be caused by the leakage current of photoexcited carriers. In this section, we will mainly highlight the typical examples of ferroelectric semiconductors of n > 1 multilayered hybrid perovskites.

The photo-induced properties that coexist and/or combine with ferroelectric P_s include two-photon nonlinear optical absorption, polarized-light sensitivity, X-ray detection, self-driven photodetection, etc.

(n-BA)₂(FA)Pb₂Br₇ (Compound 4). The < 100 > -oriented bilayered perovskites (n = 2) are the main members that have been crystallographically characterized, and compound 4 is selected as one of the representative cases of this series. It crystallizes in the polar space group Cmc2₁ and adopts a typical bilayered perovskite motif. As shown in Figure 4a, its ferroelectric structure consists of bilayers [Pb₂Br₇]_∞ stacked along the a-axis, ^[48] and the organic n-BA⁺ cations are distributed between the adjacent [Pb₂Br₇]_∞ sheets. The small-size FA⁺ cations (FA = formamidine) are confined inside the central cavity formed by the PbBr₆ corner-sharing octahedra. The dynamic orderings of or ganic n-BA⁺/FA⁺ cations result in ferroelectric phase transition at 322 K. The alternative stacking of organic and inorganic components resemble the typical 2D quantum-well structure, ^[49] which would exert significant influences on the two-photon absorption characteristics. With temperature increasing beyond its T_c (~322 K), the paraelectric structure transforms to the centrosymmetric space group Cmcm, along with the organic cations becoming disordered. During this phase transition, the crystallographic symmetry elements are halved from 8 (E, C₂, 2C₂', i, σ_h, and 2σ_v) at PEPto4(E, C₂, and 2σ_v) at FEP, revealing thesymmetry breaking with an Aizu notation of mmmFmm2. The measured P-E hysteresis loop demonstrates that 4 has the P_s value of ~3.8 μC·cm^-2 at 313 K, coinciding with the calculated P_s value by the point charge model.

Figure 4

Figure 4.  (a) Packing structures of 4 viewed along the b-axis direction at FEP (293 K) and PEP (330 K). (b) The P-E hysteresis loops measured at different temperatures. (c) Open aperture Z-scan experimental responses.

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The 2D perovskite frameworks of 4 constructed by alternative arrangement of organic and inorganic components resemble 2D quantum wells, of which inorganic sheets are "wells" and organic bilayers act as "barriers". This intrinsic quantum-well structure creates interesting quantum and dielectric confinement effects. Photoexcited charge carriers might be strongly confined in inorganic wells to enhance light-matter interactions, thus leading to abundant photoelectronic properties. According to the Z-scan result, the nonlinear optical absorption provides a large two-photon absorption coefficient of ~5.76×10³ cm·GW^-1 for 4. This figure is almost two orders of magnitude larger than that of the inorganic oxide ferroelectrics. Such results highlight the further exploration of new ferroelectric materials of hybrid perovskites toward nonlinear optical applications.

(n-BA)₂CsPb₂Br₇ (Compound 5). The inorganic perovskite frameworks are chemically modified to design new ferroelectric semiconductors, (n-BA)₂CsPb₂Br₇, ^[50] of which the corner-sharing PbBr₆ octahedra construct the {CsPbBr₃}_n sheets with Cs⁺ cations in the cavities and organic bilayers of n-BA⁺ cations located between adjacent inorganic perovskite sheets (Figure 5a). Compound 5 is a room-temperature ferroelectric that crystallizes in the polar space group Cmc2₁. Above the T_c (412 K), its frameworks adopt the symmetric configuration with Cs⁺ cation occupying the cavity center and organic cations becoming disordered at PEP. The symmetry breaking involves with the disordering of organic cations and displacement of anionic moieties.^[51] The curves of P-E hysteresis loop and current density versus electric field (J-E) confirm that 5 has a large P_s of 4.2 μC·cm^-2 along the polar c-axis direction (Figure 5b).

Figure 5

Figure 5.  (a) Diagram for the crystal structures of 5 at FEP (293 K) and PEP (420 K). (b) The P-E hysteresis loop and the corresponding J-E curve. (c) Polar plot of the normalized angle-resolved photocurrents (λ = 405 nm, 40 nW·cm^-2).

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As expected, the alternative alignment of inorganic and organic layers constructs the 2D Type-I quantum wells. The inorganic sheets of PbBr₆ octahedra distribute along the bc plane, and the unstable ns² lone pairs of Pb²⁺ ions favor mobility of photocarriers that can easily transfer along the c-axis direction. In contrast, the carriers must cross the energy barrier along the a-axis direction, which is not conducive to the carrier transport. Given the built-in electrostatic field induced by P_s, it is conducive to the separation and transport of carriers along the c-axis direction. All these inherent merits of electric and optical anisotropy will benefit the polarized-light detection performances of 5. As shown in Figure 5c, the dichroic photocurrent results of maximum value (I_ph^c) and minimum value (I_ph^a) afford the ratio (I_ph^c/I_ph^a) to be ~1.5, which is comparable to the ratio of other inorganic counterparts. It should be emphasized that even at an incident light power density of ~40 nW·cm^-2, this detector shows highly-sensitive responses to the weak po larized-light. As far as we know, this work provides a bright prospect for the effective polarized-light detection using ferroelectric semiconductors.

(HA)₂CsPb₂Br₇ (Compound 6). Basic structural architecture for 6 adopts a 2D Cs-based perovskite topology, in which the inorganic Cs⁺ cation templates perovskite sheets and the organic HA⁺ (HA = n-hexylammonium) cation behaves as the "spacer" (Fig ure 6a). Compared with the typical perovskite motifs containing organic cations as the perovskitizer, ^[52] electrostatic interactions between PbBr₆ octahedra and Cs⁺ cations afford great possibilities for the occurrence of atomic displacement. Remarkably, the dynamic motions of Cs⁺ cations and tilting of PbBr₆ octahedra, along with the frozen ordering of HA⁺ cations, trigger multiple phase transitions of 6around 303and 349 K. Symmetry breaking occurs with an Aizu notation of 4/mmmFmm2, as deduced by variable-temperature structure changes from paraelectric space group I4/mmm to the polar one Fmm2 and Cmc2₁ at FEP. Ferroelectricity of 6 was further confirmed by the measurement of P-E hysteresis loops along the polar c-axis (Figure 6b). The P-E hysteresis loops keep well-defined rectangular pattern, and the P_s value of 2.5 μC/cm² falls in the range of other analogue ferroelectrics.

Figure 6

Figure 6.  (a) Diagram of packing structures of 6 collected at FEP (200 K) and PEP (370 K). (b) The P-E hysteresis loops measured at different temperatures. (c) One cycle of the ferro-pyro-phototronic photocurrent.

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Considering its temperature-dependent ferroelectric P_s and photosensitive activities caused by inorganic networks, 6 is expected to be a potential candidate that enables the photoinduced pyroelectricity and ferro-pyro-phototronic (FPP) effects.^[53] As shown in Figure 6c, exceptionally large FPP effects are observed in 6, which allow a giant boosting of transient photopyroelectric currents to ~1500% compared with the steady-state photovoltaic signals; this figure-of-merit is superior to the majority of inorganic oxide counterparts. Furthermore, the direction of these FPP effects can be facilely switched and controlled through external electric filed, revealing a crucial role of ferroelectric polarization to this three-channel coupling. The results afford a new avenue to tune physical properties of ferroelectrics and forward their potentials to smart optoelectronic device applications.

PEROVSKITE-TYPE TRILAYERED FERROELECTRIC SEMICONDUCTORS (A')₂A₂M₃X₁₀ (n = 3)

In addition to bilayered perovskite series, the < 100 > -oriented trilayered candidates (A')₂A₂M₃X₁₀ (n = 3) are also investigated as the potential ferroelectric semiconductors. By increasing the thickness of inorganic sheets, the optical and photoelectric properties can be optimized, which also show potentials for sensitive photodetection^[54], polarized-light detection in the solar-blind UV region, multiphoton absorption (MPA), etc.

(ALA)₂(EA)₂Pb₃Br₁₀ (Compound 7). Bulk FEPV effect represents a typical coupling of ferroelectric P_s with photovoltaic behavior. Recently, the concept of polarized-light detection has been achieved through bulk FEPV effects of a 2D trilayered perovskite ferroelectric, (ALA)₂(EA)₂Pb₃Br₁₀ (where ALA is allyammonium and EA is ethylammonium).^[55] Structural analyses reveal that 7 has a trilayered perovskite motif with EA⁺ cation acting as "perovskitizer" inside the cavity of corner-sharing PbBr₆ octahedra, while ALA⁺ behaves as "spacer" in the interlayer space of inorganic sheets. At the high-temperature PEP, compound 7 crystallizes in the nonpolar space group I4/mmm with 16 symmetry elements (E, 2C₄, C₂, 2C₂', 2C₂'', 2S₄, i, σ_h, 2σ_v, 2σ_d). With the temperature cooling down, it undergoes two phase transitions and belongs to the polar space group Cmc2₁ (E, C₂, 2σ_v) at room-temperature FEP (Figure 7a). The reorientation of organic moieties and distortion of PbBr₆ octahedra lead to the separation of positive and negative charge centers, giving rise to the dipole moment (–7.3569×10^-29 cm) and electric polarization along the c-axis. Hence, the symmetry breaking occurs with an Aizu natation of 4/mmmFmm2, of which the four equivalent directions indicate biaxial ferroelectricity for 7. The ferroelectricity was proved by temperature-dependent P-E hysteresis loops with good rectangularity (Figure 7b).

Figure 7

Figure 7.  (a) Crystal structure diagram of 7 projected along the b-axis at FEP (298 K). (b) The P-E hysteresis loops measured along the c^FEP-axis at different temperatures. (c) Polar coordinate of FEPV current I_sc as a function of polarized-light angle under different light intensities at 298 K.

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It is notable that excellent P_s-induced bulk FEPV effects are observed in 7, including near-bandgap photovoltage of ~2.5 V and large current on/off ratio of ~10⁴ under 405 nm irradiation. Figure 7c displays the angle-resolved FEPV I_sc curves under different light intensities and the polarization ratio ω_sc (ω_sc = I_max/I_min) is as high as ~15. As far as we know, this innovative work on the P_s-induced bulk FEPV effect opens up a new pathway for sensitively detecting polarized-light detection, and further expands the application of OIHP-type ferroelectric materials in smart optoelectronic devices.

(EA)₂(MA)₂Pb₃Cl₁₀ (Compound 8). Through the 3D-to-2D reduction of cubic prototype, we have recently designed the 2D ferroelectric of (EA)₂(MA)₂Pb₃Cl₁₀ (8), ^[56] which adopts the trilayered perovskite structure with organic MA⁺ cation acting as "perovskite" and EA⁺ moiety locating in the space between adjacent inorganic sheets (Figure 8a). The ferroelectric structure of 8 crystallizes in the polar space group Cmc2₁. Both organic MA⁺ and EA⁺ cations are ordered with the NH₃ groups tightly anchored to PbCl₆ octahedra by N–H···Cl hydrogen bonds (Figure 8a). The positively charged cations tilt along the c-axis, and the distorted PbCl₆ octahedra deviate towards the opposite direction, thus leading to the generation of dipole moment and electric polarization. With temperature increasing up to 400 K (PEP), its structure transforms into a non-polar space group I4/mmm. Both types of organic cations become highly disordered at PEP, corresponding to the phase transition at 390 K (T_c). Ferroelectric properties of 8 are well confirmed by the P-E hysteresis loops (Figure 8b), which afford the P_s value of ~1.8 μC·cm^-2 at room temperature.

Figure 8

Figure 8.  (a) Variable-temperature crystal structures of 8 at FEP (382 K) and PEP (400 K). (b) The P-E hysteresis loops measured at different temperatures. (c) Polar coordinate diagram of the angle-resolved photocurrent I_sc.

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With respect to semiconducting properties, compound 8 exhibits a wide bandgap (E_g ~3.35 eV) and anisotropic absorbance caused by its inherent 2D motif. Such attributes are favorable to the polarization-sensitive UV photodetection.^{[57, 58]} As expected, the crystal-based devices show excellent polarization-sensitive activities under 266 nm, including large dichroism (~1.38) and high on/off current ratios (~2.3×10³). Figure 8c shows the polar plot of angle-resolved photocurrents under an external bias voltage (10 V). By controlling polarized-light angle, the photocurrents exhibit significant periodic changes. The maximum value (I_max) emerges at 0° and 180° polarization, corresponding to the light polarized along c-axis; when the light is polarized parallel to a-axis (90° and 270°), the minimum photocurrent (I_min) is observed. As the study on the polarization-sensitive UV photodetection, this work provides an effective method for exploring new 2D hybrid perovskite candidate materials for future optoelectronic applications.

(IA)₂(MA)₂Pb₃Br₁₀ (Compound 9). Regarding the strong light-matter interactions, multiphoton absorption (MPA) is the nonlinear optical process that has potential applications in optical data storage, optical communication, and biological imaging.^[59] We presented the high-order MPA activities in the ferroelectric of (IA)₂(MA)₂Pb₃Br₁₀ (9), ^[60] which adopts a 2D trilayered perovskite structure with an alternative array of inorganic perovskite sheets and organic IA⁺ layers. Structural analyses reveal that order-disordering of IA⁺ and MA⁺ cations affords a driving force to trigger their phase transition at T_c = 305 K, and their reorientation and tilting motions result in electric polarization. Above the T_c, compound 9 belongs to the tetragonal system. Both organic IA⁺ and MA⁺ cations feature the disordered molecular configurations with eight equivalent orientations. In contrast, their crystal structures transform to the orthorhombic system below the T_c. Two types of cations become fully ordered with the protonated NH₃⁺ groups connected to PbBr₆ octahedra through N–H···Br hydrogen bonds. The results of P-E hysteresis loop and J-E curve confirm that compound 9 has a large P_s of 5.0 μC·cm^-2 (Figure 9b), which is on the par with those of other ferroelectrics in hybrid perovskite family.

Figure 9

Figure 9.  (a) Crystal structure of 9 viewed along b-axis at FEP (298 K). It adopts an alternative array of inorganic perovskite sheets and organic cation layers. (b) P-E hysteresis loop and J-E curve measured at room temperature. (c) Energy diagram and PL picture of the crystal wafer during 5PA process.

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The unique 2D trilayered perovskite structure of 9 enables strong dielectric and quantum confinement effects on free carriers, which would be potential for exploring high-order MPA properties. Figure 9c shows the energy diagram of the five-photon absorption (5PA) process of 9. After laser irradiation (≈ 2400 nm), the excitons (i.e., electron-hole pairs) are formed by absorbing five mid-infrared photons at the same time, in which the total energy is sufficient to overcome the energy gap between the ground state (S₀) and excited state (S_i). Subsequently, the interband decay to the lower exciton state (S₁) causes the MPA of about 530 nm to excite PL emission. Here, compound 9 exhibits giant broadband MPA effects involving with its 2D quantum-confined motif. In combination of its ferroelectricity, this study provides a potential optically active ferroelectric candidate in the OHIPs family, which could be used in the broadband multiphoton applications.

LEAD-FREE 2D PEROVSKITE-TYPE FERROELECTRICS

Compared with the developed lead-halogen perovskites, the lead-free counterparts have also attracted interests in term of environmental friendliness and sustainability. One of effective design strategies is element substitution of Pb to expand the family of halide double perovskites, namely, replacing two toxic Pb²⁺ in the crystal lattice using a couple of nontoxic heterovalent (i.e. monovalent and trivalent) metal cations. In particular, the formation of 2D double perovskites can accommodate large and flexible organic cations, such as quasi-spherical molecules and chiral organic ions, which are favorable to the emergence of phase transitions and ferroelectric orders. Regarding the advantages of both organic and inorganic components, the lead-free double perovskite ferroelectrics would hold potentials for diverse applications, ^[61] such as piezoelectric sensor, dielectric capacitor, nonlinear optical device and X-ray detection. In this section, we merely focus on the typical ferroelectric candidates of 2D double perovskites, including their structural phase transitions, ferroelectricity and photoelectric properties.

(R3HQ)₄KCe(NO₃)₈ (Compound 10) and (R3HQ)₄RbEu(NO₃)₈ (Compound 11). By using the low-symmetric chiral cations of R/S-3-hydroxylquinuclidinium (R3HQ and S3HQ), a series of polar 2D rare-earth double perovskites of A₄M^IM^III(NO₃)₈ have been recently synthesized by Ye. et al, and we here take the compound of (R3HQ)₄KCe(NO₃)₈ (10) as an example to discuss the structure-property relationship.^[62] Compound 10 is a ferroelectric with the phase transition at T_c = 320 K. At low-temperature FEP, it belongs to a polar chiral space group P2₁ (symmetry elements: E and C₂). The inorganic frameworks formed by corner-sharing K(NO₃)₆ distorted octahedra and Ce(NO₃)₆ octahedra are alternatively arranged with organic R3HQ layers along the c-axis, thus forming 2D double perovskite motif. Half of the chiral organic cations are parallel to each other along the b-axis, while the other half are antiparallel to each other and perpendicular to the b-axis (Figure 10a). With the temperature exceeding beyond T_c, compound 10 undergoes phase transition and its structure changes to the nonpolar space group B222 (symmetry elements: E, C₂, 2C₂'). Comparison of variable-temperature structures reveals that its phase transition is caused by the order-disordering of NO₃⁻ ions and organic cations. Symmetry breaking analyses confirm structural phase transition of 10 belongs to the Aizu nation of 222F2. As revealed by the P-E hysteresis loop and J-E curve, compound 10 has the P_s value of 4.1 μC·cm^-2 (Figure 10b), solidly confirming its ferroelectric properties.

Figure 10

Figure 10.  (a) Crystal structures of 10 collected at FEP (293 K) and PEP (343 K). (b) P-E hysteresis loops and J-E curve measured at 313 K. (c) Emission and excitation spectra of (R3HQ)₄RbEu(NO₃)₈ measured at room temperature.

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This family of 2D double perovskites not only shows room-temperature ferroelectric properties, but also enables tunable photoluminescent (PL) activity due to the rare-earth metals. For instance, the member of (R3HQ)₄RbEu(NO₃)₈ (compound 11) exhibits strong PL emission with very high quantum yield. Owing to the homochirality of organic cations, the local symmetry environment of Eu³⁺ ion is disturbed. Thus, electric dipole transition ⁵D₀ → ⁷F₂ is not absolutely forbidden and the environment of Eu³⁺ is not strictly symmetric, which are favorable to its PL emission (Figure 10c). The weak interactions between organic cations and Eu³⁺ ion might be used to tune PL properties.

(CPA)₄AgBiBr₈ (Compound 12). The lead-free double perovskites have also emerged as the potential candidates for detecting high-energy X-ray radiation. Recently, we have also presented a ferroelectric of 2D double perovskite, (CPA)₄AgBiBr₈ (CPA⁺ = chloropropylammonium, 12), which is sensitive to X-ray radiation.^[63] At low-temperature phase (T_c = 305 K), it belongs to the polar space group Pc (the point group m). The corner-sharing AgBr₆ distorted octahedra and BiBr₆ octahedra consist of inorganic frameworks, which alternate with organic CPA bilayers to form 2D monolayered structure of double perovskite. The directional alignment of organic CPA⁺ cations with terminal halogen atoms make a significant contribution to the construction of polar structure (Figure 11a). At the high-temperature phase (above T_c), it crystallizes in the centrosymmetric space group Pbam (the point group mmm), and the organic CPA⁺ cations become highly disordered with two kinds of molecular geometry. One is distributed over two equivalent orientations with respect to the crystallographic m mirror plane, while the other lies on the special symmetry plane. Along with the distortion decreasing of AgBr₆/ BiBr₆ octahedra, the dipole moments cancel each other out and the bulk P_s disappears. This structural change corresponds to the symmetry breaking of 12 with an Aizu notation of mmmFm. The measured P-E hysteresis loop and J-E curve afford the P_s value of ~3.1 μC·cm^-2 for 12 (Figure 11b), which is comparable with other 2D double perovskite ferroelectrics.

Figure 11

Figure 11.  (a) Crystal structures of 12 at FEP (270 K) and PEP (340 K). (b) The P-E hysteresis loop and J-E curve measured at room temperature. (c) The I-V curve obtained under the X-ray irradiation.

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Interestingly, compound 12 is a semiconductor with the band gap of 2.69 eV. Its 2D layered structure allows efficient separation of excitons into free carriers due to the exposed layer edges.^{[64, 65]} These characteristics suggest the potentials of 12 for high-energy X-ray detection. Under X-ray irradiation, the photocurrent generated under the bias of 10 V is proportional to the dose rate. The slope for this linear fitting determines the sensitivity of X-ray detection is 0.8 μCGy_air^-1·cm^-2. Besides, alternating array of organic cation and inorganic perovskite sheets, like quantum-confined motif, endows a large mobility-lifetime product (μτ = 1.0×10^-3 cm²·V^-1) (Figure 11c). All these results reveal the potentials of 12 for X-ray detection and pave a way for exploring 2D double perovskite ferroelectrics with multifunctional properties.

SUMMARY AND PROSPECT

In this minireview, we have presented recent advances of ferroelectric materials in the family of 2D hybrid perovskites. Structural origin for generating ferroelectricity is analyzed from the aspect of symmetry breaking. It is emphasized that the combination of ferroelectric polarizations with other semiconducting activities gives rise to new optoelectronic properties, which might create a class of ferroelectric semiconductors toward new photoelectronic applications. Up to date, the rational design of new candidates of ferroelectric semiconductors mainly depends on continuous experiments and partial accumulation of experience. How to predict and achieve the ferroelectric-type symmetry breaking is still challenging. Although great efforts have devoted to exploiting new ferroelectric semiconductors, the target-guided optimization of their performances is highly urgent for device applications. As exemplified by P_s-induced bulk ferroelectric photovoltaic effects, one of the most fascinating advantages is the achievement of above-bandgap photovoltages, while the relatively low photocurrent density and conversion efficiency restrict their practical solar-energy generation. Further, more attention should be paid to the coupling between other photoinduced properties and ferroelectric polarization. This is important to realize the new concepts of photoactive properties, such as the combination of photovoltaic and piezoelectric effects (i.e., photostriction). As a multidisciplinary subject involving with chemistry, physics, crystallography and material science, the search for new candidate systems, which are in crystal/ceramic form, thin films or domain-engineered materials, is crucial for deep understanding on photoinduced activities in ferroelectric semiconductors or more generally polar materials. Probably, beyond the tip of iceberg in terms of P_s-induced bulk photovoltaic effects, it is necessary to establish a broad family of new ferroelectric semiconductors that we should exert our endeavors in future.

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