Field-Induced Antiferroelectric-Ferroelectric Transformation in Organometallic Perovskite Displaying Giant Negative Electrocaloric Effect.

Antiferroelectric materials with an electrocaloric effect (ECE) have been developed as promising candidates for solid-state refrigeration. Despite the great advances in positive ECE, reports on negative ECE remain quite scarce because of its elusive physical mechanism. Here, a giant negative ECE (maximum ΔS ∼ -33.3 J kg-1 K-1 with ΔT ∼ -11.7 K) is demonstrated near room temperature in organometallic perovskite, iBA2EA2Pb3I10 (1, where iBA = isobutylammonium and EA = ethylammonium), which is comparable to the greatest ECE effects reported so far. Moreover, the ECE efficiency ΔS/ΔE (∼1.85 J cm kg-1 K-1 kV-1) and ΔT/ΔE (∼0.65 K cm kV-1) are almost 2 orders of magnitude higher than those of classical inorganic ceramic ferroelectrics and organic polymers, such as BaTiO3, SrBi2Ta2O9, Hf1/2Zr1/2O2, and P(VDF-TrFE). As far as we know, this is the first report on negative ECE in organometallic hybrid perovskite ferroelectric. Our experimental measurement combined with the first-principles calculations reveals that electric field-induced antipolar to polar structural transformation results in a large change in dipolar ordering (from 6.5 to 45 µC/cm2 under the ΔE of 18 kV/cm) that is closely related to the entropy change, which plays a key role in generating such giant negative ECE. This discovery of field-induced negative ECE is unprecedented in organometallic perovskite, which sheds light on the exploration of next-generation refrigeration devices with high cooling efficiency.

Renewing Halogen Substitution Strategy for the Rational Design of High-Curie Temperature Metal-Free Molecular Antiferroelectrics.

Metal-free molecular antiferroelectric (AFE) holds a promise for energy storage on account of its unique physical attributes. However, it is challenging to explore high-curie temperature (Tc) molecular AFEs, due to the lack of design strategies regarding the rise of phase transition energy barriers. By renewing the halogen substitution strategy, we have obtained a series of high-Tc molecular AFEs of the halogen-substituted phenethylammonium bromides (x-PEAB, x=H/F/Cl/Br), resembling the binary stator-rotator system. Strikingly, the p-site halogen substitution of PEA+ cationic rotators raises their phase transition energy barrier and greatly enhances Tc up to ~473 K for Br-PEAB, on par with the record-high Tc values for molecular AFEs. As a typical case, the member 4-fluorophenethylammonium bromide (F-PEAB) shows notable AFE properties, including high Tc (~374 K) and large electric polarization (~3.2 µC/cm2). Further, F-PEAB also exhibits a high energy storage efficiency (η) of 83.6 % even around Tc, catching up with other AFE oxides. This renewing halogen substitution strategy in the molecular AFE system provides an effective way to design high-Tc AFEs for energy storage devices.

Visible-Photoactive Perovskite Ferroelectric-Driven Self-Powered Gas Detection.

Chemiresistive sensing has been regarded as the key monitoring technique, while classic oxide gas detection devices always need an external power supply. In contrast, the bulk photovoltage of photoferroelectric materials could provide a controllable power source, holding a bright future in self-powered gas sensing. Herein, we present a new photoferroelectric ([n-pentylaminium]2[ethylammonium]2Pb3I10, 1), which possesses large spontaneous polarization (∼4.8 µC/cm2) and prominent visible-photoactive behaviors. Emphatically, driven by the bulk photovoltaic effect, 1 enables excellent self-powered sensing responses for NO2 at room temperature, including extremely fast response/recovery speeds (0.15/0.16 min) and high sensitivity (0.03 ppm-1). Such figures of merit are superior to those of typical inorganic systems (e.g., ZnO) using an external power supply. Theoretical calculations and in situ diffuse reflectance infrared Fourier transform spectroscopy measurements confirm the great selectivity of 1 for NO2. As far as we know, this is the first realization of ferroelectricity-driven self-powered gas detection. Our work sheds light on the self-powered sensing systems and provides a promising way to broaden the functionalities of photoferroelectrics.

Polarization-Dependent Large Photorefractive Effect In A Wide Bandgap 2D Metal Halide Ferroelectric.

Photorefractive effect of ferroelectrics refers to the light-induced change of refractive index, which is an optical controlling avenue in holographic storage and image processing. For most ferroelectrics, however, the small photorefractive effect (10-5 -10-4 ) hinders their practical application and it is urgent to exploit new photorefractive system. Here, for the first time, strong photorefractive effects are achieved in a 2D metal-halide ferroelectric, [CH3 (CH2 )3 NH3 ]2 (CH3 NH3 )Pb2 Cl7 (1), showing large spontaneous polarization (≈4.1 µC cm-2 ) and wide optical bandgap (≈3.20 eV). Notably, under light irradiation, 1 enables a large variation of refractive indices up to ≈ 1× 10-3 , being one order higher than the existing materials and comparable to the state-of-the-art inorganic ferroelectrics. This intriguing photorefractive behavior involves with the sharp variation of polarization caused by photo-pyroelectricity. As the first report of 2D metal-halide photorefractive ferroelectric, this work sheds light on optical controlling of physical properties in electric-ordered materials.

Acquiring Bulk Anomalous Photovoltaic Effect in Single Crystals of a Lead-Free Double Perovskite with Aromatic and Alkali Mixed-Cations.

The bulk anomalous photovoltaic (BAPV) effect of acentric materials refers to a distinct concept from traditional semiconductor-based devices, of which the above-bandgap photovoltage hints at a promise for solar-energy conversion. However, it is still a challenge to exploit new BAPV-active systems due to the lacking of knowledge on the structural origin of this concept. BAPV effects in single crystals of a 2D lead-free double perovskite, (BBA)2 CsAgBiBr7 (1, BBA = 4-bromobenzylammonium), tailored by mixing aromatic and alkali cations in the confined architecture to form electric polarization are acquired here. Strikingly, BAPV effects manifested by above-bandgap photovoltage (VOC ) show unique attributes of directional anisotropy and positive dependence on electrode spacing. The driving source stems from orientations of the polar aromatic spacer and Cs+ ion drift, being different from the known built-in asymmetry photovoltaic heterojunctions. As the first demonstration of the BAPV effect in the double perovskites, the results will enrich the family of environmentally green BAPV-active candidates and further facilitate their new optoelectronic application.

Building Block-Inspired Hybrid Perovskite Derivatives for Ferroelectric Channel Layers with Gate-Tunable Memory Behavior.

Ferroelectric photovoltaics driven by spontaneous polarization (Ps ) holds a promise for creating the next-generation optoelectronics, spintronics and non-volatile memories. However, photoactive ferroelectrics are quite scarce in single homogeneous phase, owing to the severe Ps fatigue caused by leakage current of photoexcited carriers. Here, through combining inorganic and organic components as building blocks, we constructed a series of ferroelectric semiconductors of 2D hybrid perovskites, (HA)2 (MA)n-1 Pbn Br3n+1 (n=1-5; HA=hexylamine and MA=methylamine). It is intriguing that their Curie temperatures are greatly enhanced by reducing the thickness of inorganic frameworks from MAPbBr3 (n=∞, Tc =239 K) to n=2 (Tc =310 K, ΔT=71 K). Especially, on account of the coupling of room-temperature ferroelectricity (Ps ≈1.5 µC/cm2 ) and photoconductivity, n=3 crystal wafer was integrated as channel field effect transistor that shows excellent a large short-circuit photocurrent ≈19.74 µA/cm2 . Such giant photocurrents can be modulated through manipulating gate voltage in a wide range (±60 V), exhibiting gate-tunable memory behaviors of three current states ("-1/0/1" states). We believe that this work sheds light on further exploration of ferroelectric materials toward new non-volatile memory devices.

Soft Multiaxial Molecular Ferroelectric Thin Films with Self-Powered Broadband Photodetection.

Molecular ferroelectric films (MFFs) offer a good platform for miniaturized electronic devices, which are inseparable from their multiaxial nature. Despite great studies, soft MFFs with broadband photo-electroactivity still remain a huge blank as the photoexcited leakage current will severely deteriorate ferroelectricity, hindering their optoelectronic applications. Here, we constructed the multiaxial MFF of HA2EA2Pb3I10 (1, where EA = ethylammonium and HA = n-hexylammonium) in 2D multilayered perovskites. Eight equivalent polarization directions were observed in 1, as verified by its symmetry breaking (i.e., 4/mmmFm species), which is the maximum among 2D multilayered perovskites and even more than that of classic ceramic BaTiO3. Specially, spin-coated flexible MFFs of 1 are approximately orientated parallel to layered perovskite frameworks, exhibiting in-plane spontaneous polarization (Ps = 1.8 µC/cm2) and broadband absorption (∼1.83 eV). In addition, self-powered broadband detection (∼0.55 µA/cm2 at 637 nm illumination) was achieved on the soft films, revealing their potential for flexible and wearable electronic devices. Our result sheds light on the design of flexible photoelectronic devices and provides an effective way to expand the applications of 2D molecular ferroelectric materials.

Tailoring Interlayered Spacers of Two-Dimensional Cesium-Based Perovskite Ferroelectrics toward Exceptional Ferro-Pyro-Phototronic Effects.

Ferro-pyro-phototronic (FPP) effect is a triple coupling of ferroelectricity, light-induced pyroelectricity, and photo-excitation, which holds a bright promise for next-generation modern optoelectronic devices. However, except for few oxides (e.g., BaTiO3 ), new FPP-active candidates remain extremely scarce due to the knowledge lacking on the underlying role of three coupling components. By tailoring the interlayered spacers, the authors present a series of 2D cesium-based perovskite ferroelectrics, (A')2 CsPb2 Br7 (where A'-site cation is organic spacer), showing remarkable FPP-active properties. As expected, the dynamic ordering and reorientation of spacers along with atomic displacement of Cs+ in the perovskite cavity lead to their ferroelectric polarizations. Particularly, exceptional FPP properties are created through this cooperation; the most FPP-active candidate (n-hexylammonium)2 CsPb2 Br7 endows a giant contrast up to 1500% for photopyroelectric current to photovoltaic signal. This figure-of-merit is far beyond most inorganic oxide counterparts, such as ≈110% for BaTiO3 . Further, the electric switching and controlling of FPP directions confirm a crucial role of ferroelectric polarization to this coupling effect. To the authors' best knowledge, this is the first study on an FPP-active candidate of 2D hybrid perovskites, which affords a new avenue to design ferroelectrics with targeted physical properties and forward their potentials to smart optoelectronic device application.

Anisotropy in a 2D Perovskite Ferroelectric Drives Self-Powered Polarization-Sensitive Photoresponse for Ultraviolet Solar-Blind Polarized-Light Detection.

Polarized-light detection in the deep ultraviolet solar-blind region is indispensable for optoelectronic applications. 2D hybrid perovskite ferroelectrics with inherent anisotropy and bulk photovoltaic effect (BPVE) have become a robust candidate in this portfolio. We here report a wide band-gap 2D perovskite ferroelectric, (isobutylammonium)2 (methylamium)Pb2 Cl7 (1), in which the BPVE behaves as self-driving source for solar-blind ultraviolet polarized-light detection. It exhibits a large dichroism ratio (≈15.7) and superior BPVE-directed photocurrent (≈3.6 µA cm-2 , under 266 nm). These attributes facilitate excellent self-powered polarized-light detection of high on/off contrast (≈103 ) and polarization ratio (≈2.5), beyond those of inorganic oxides (e.g., ZnO, 1.47; GaN, 1.38). This study highlights potential of 2D perovskite ferroelectrics for new optoelectronic application.

A Metal-Free Molecular Antiferroelectric Material Showing High Phase Transition Temperatures and Large Electrocaloric Effects.

Antiferroelectric (AFE) materials, featuring an antiparallel alignment of electric dipoles in the adjacent sublattices, are keeping a great promise toward solid-state refrigeration applications on account of their electrocaloric (EC) effects. Although extensive studies have been performed on inorganic oxide counterparts (e.g., PbZrO3 and AgNbO3), metal-free molecular AFE alternatives with the above-room-temperature EC activities are quite scarce but urgently demanded in terms of environmental issues. Herein, we present a new metal-free molecular AFE, cyclohexylmethylammonium bromide (CMB), which exhibits the unusual antiferroelectric-ferroelectric-paraelectric phase transitions around 364 and 368 K upon heating. The phase transition temperatures are much higher than the majority of known molecular AFE materials. The practical utilization level of electric polarization (∼6 µC/cm2) is clearly evidenced by the typical double polarization-electric field hysteresis loops. Strikingly, large positive and negative EC responses with the temperature changes (ΔT) of 4.2 and -3 K are achieved under an electric field of 20 kV/cm. The origin of its antiferroelectricity and EC properties is elucidated by the antipolar reorientation of cations along with displacement of bromine anions, being distinct from the known mechanism of inorganic oxides. Such intriguing AFE behaviors, including large polarization and EC effects, reveal great potentials of CMB for the solid-state refrigeration. This study sheds light on further exploration of new AFE candidates toward environmentally friendly solid-state cooling devices.

Spacer Cation Alloying of a Homoconformational Carboxylate trans Isomer to Boost in-Plane Ferroelectricity in a 2D Hybrid Perovskite.

Two-dimensional (2D) hybrid perovskites of Ruddlesden-Popper (RP) lattices are recently booming as a vigorous class of ferroelectrics, whereas their intrinsic van der Waals gaps exert weak interactions that destabilize the layered motifs. Thus, it is an urgent challenge to reduce interlayered energy gaps to allow an exploration of stable RP ferroelectrics. Here, we propose hydrogen bonds to reduce van der Waals gaps of 2D RP-type perovskites while the ferroelectricity is retained. For the first time, a homoconformational trans isomer has been alloyed as the spacing cation of the 2D ferroelectric (t-ACH)2(EA)2Pb3Br10 (1, where t-ACH is 4-aminomethyl-1-cyclohexanecarboxylate and EA is ethylammonium). Strikingly, the strong O-H···O hydrogen bonds link adjacent spacing sheets to build a quasi-RP motif with a lower energy gap. In terms of ferroelectricity, the mixed-cation alloying has a crucial role in spontaneous polarization (Ps), as verified by structure analyses, quadratic optical nonlinearity, and electric hysteresis loops. The ordering of t-ACH+ and EA+ cations induces mmmFm symmetry breaking, along with an unusual in-plane Ps value of ∼2.9 µC/cm2 in the ac plane. In combination with the anisotropic nature of its 2D motif, this ferroelectricity creates strong linearly polarized light sensitivity with a large dichroism ratio of ∼3.2, far beyond those of most of the inorganic 2D systems. As far as we know, a 2D RP-type ferroelectric with a trans isomer cationic spacer is unprecedented, and the concept of reducing energy gaps via H-bonding interactions will strengthen the layered perovskite structure and shed light on the rational design of stable ferroelectrics toward photoelectric applications.

Two-Dimensional Guanidine-Based Hybrid Perovskites with Strong Dichroism for Multiwavelength Polarization-Sensitive Detection.

Two-dimensional (2D) organic-inorganic hybrid perovskites, benefiting from their natural anisotropy of quantum-well motifs and optical properties, have shown remarkable polarization-dependent responses superior to the 3D counterparts. Here, for the first time, multiwavelength polarization-sensitive detectors were fabricated by using single crystals of a guanidine-based 2D hybrid perovskite, (BA)2 (GA)Pb2 I7 (where BA+ is n-butylammonium and GA+ is guanidium). Its unique 2D quantum-well structure results in strong crystallographic-dependence of optical absorption. Strikingly, our crystal-based photodetector exhibits a prominent photocurrent dichroic ratio (Imax /Imin ) of ∼2.2 at 520 nm, higher than the typical 2D inorganic materials (GeSe, ∼1.09, PdSe2 , ∼1.8). In addition, notable dichroic ratios of 1.29 and 1.23 at 405 nm and 637 nm are also created for the multiwavelength polarized-light detection. The prominent detecting performances, including low dark current (1.6×10-11  A), considerable on/off ratio (∼2×103 ), high photodetectivity (∼3.3×1011 Jones) and responsivity (∼12.01 mA W-1 ), make (BA)2 (GA)Pb2 I7 a promising candidate for polarized-light detection. This work sheds light on the rational engineering of new 2D hybrid perovskites for the high-performance optoelectronic device applications.

Soft Perovskite-Type Antiferroelectric with Giant Electrocaloric Strength near Room Temperature.

Antiferroelectric materials, characterized by an antiparallel array of adjacent dipoles, are holding a bright future for solid-state refrigeration based on their electrocaloric (EC) effects. Despite great advances of inorganic oxides and some organic soft polymers, their EC effects are achieved under quite high electric fields that result in too low EC strengths for practical application. Currently, it is a challenge to exploit soft antiferroelectric with strong EC strengths. Here, by the mixed-cation alloying, we present a new perovskite-type soft antiferroelectric, (isopentylammonium)2CsPb2Br7 (1), which incorporates both an organic spacing cation and an inorganic perovskitizer Cs+ moiety. Remarkably, the synergic cooperativity between the reorientation of the organic spacer and atomic displacement of Cs+ cation triggers its multiple ferroelectric-antiferroelectric-paraelectric phase transitions at 321 and 350 K. Their natural polarization vs electric field hysteresis loops are characterized to confirm ferroelectric and antiferroelectric orders of 1, respectively. It is emphasized that, under a low electric field of 13 kV/cm, the antipolar dipole realignment in 1 endows a giant near-room-temperature EC strength (ΔTEC/ΔE) of 15.4 K m MV-1 at antiferroelectric phase. This merit is on par with the record-high value of BaTiO3 (∼16 K m/MV) but far beyond the state-of-the-art soft polymers. The underlying EC mechanism for 1 is ascribed to the extremely low critical field to switch dipoles, involving the reorientation of the organic spacer and the shift of the Cs+ cation. Besides, notable EC entropy change (∼4.1 J K-1 kg-1) and temperature change (∼2 K) reveal potentials of 1 for solid-state refrigeration. As far as we know, this discovery of near-room-temperature EC strengths is unprecedented in the hybrid perovskite family, which sheds light on the exploration of new soft antiferroelectrics toward high-efficiency refrigeration devices.

Highly-Anisotropic Dion-Jacobson Hybrid Perovskite by Tailoring Diamine into CsPbBr3 for Polarization-Sensitive Photodetection.

2D materials with inherent attributes of structural anisotropy have been well applied in the field of polarization-sensitive photodetection. However, to explore new 2D members with strong polarized-light responses still remains a challenge. Herein, by alloying diamine molecule into the 3D prototype of CsPbBr3 , a new Dion-Jacobson (DJ) type 2D perovskite of (HDA)CsPb2 Br7 (1, where HDA2+ is 1,6-hexamethylenediammonium), containing both inorganic Cs metal and organic cations is designed. The natural anisotropy characteristics of 1 are solidly elucidated by analyzing crystal structure, electric conductivity, and optical properties. Strikingly, distinct polarization-sensitive responses are observed in 1, owing to its strong anisotropy of optical absorption (the ratio of αc /αb ≈ 2.2). Consequently, crystal-based detectors of 1 exhibit fascinating photo-activities to polarized-light, including high detectivity (1.5 × 109 Jones), large dichroism ratio (Iph c /Iph b ≈ 1.6) and fast responding rate (200 µs). All these polarization-sensitive performances along with intriguing phase stability make 1 a potential candidate for polarized-light detection. This work paves a pathway toward new functionalities of DJ-type 2D hybrid perovskites for their future optoelectronic device applications.

Multilayered 2D Cesium-Based Hybrid Perovskite with Strong Polarization Sensitivity: Dimensional Reduction of CsPbBr3.

3D perovskite CsPbBr3 has recently taken a blooming position for optoelectronic applications. However, due to the lack of natural anisotropy of optical attributes, it is a great challenge to fulfil polarization-sensitive photodetection. Here, for the first time, we exploited dimensionality reduction of CsPbBr3 to tailor a 2D-multilayered hybrid perovskite, (TRA)2 CsPb2 Br7 (1, in which TRA is (carboxy)cyclohexylmethylammonium), serving as a potential polarized-light detecting candidate. Its unique quantum-confined 2D structure results in intrinsic anisotropy of electrical conductivity, optical absorbance, and polarization-dependent responses. Particularly, it exhibits remarkable dichroism with the photocurrent ratio (Ipc /Ipa ) of ≈2.1, being much higher than that of the isotropic CsPbBr3 crystal and reported CH3 NH3 PbI3 nanowire (≈1.3), which reveals its great potentials for polarization-sensitive photodetection. Further, crystal-based detectors of 1 show fascinating responses to the polarized light, including high detectivity (>1010 Jones), fast responding time (≈300 µs), and sizeable on/off current ratios (>104 ). To our best knowledge, this is the first study on 2D Cs-based hybrid perovskite exhibiting strong polarization-sensitivity. The work highlights an effective pathway to explore new polarization sensitive candidates for hybrid perovskites and promotes their future electronic applications.

Room-Temperature Ferroelectric Material Composed of a Two-Dimensional Metal Halide Double Perovskite for X-ray Detection.

Although two-dimensional (2D) metal-halide double perovskites display versatile physical properties due to their huge structural compatibility, room-temperature ferroelectric behavior has not yet been reported for this fascinating family. Here, we designed a room-temperature ferroelectric material composed of 2D halide double perovskites, (chloropropylammonium)4 AgBiBr8 , using an organic asymmetric dipolar ligand. It exhibits concrete ferroelectricity, including a Curie temperature of 305 K and a notable spontaneous polarization of ≈3.2 µC cm-2 , triggered by dynamic ordering of the organic cation and the tilting motion of heterometallic AgBr6 /BiBr6 octahedra. Besides, the alternating array of inorganic perovskite sheets and organic cations endows large mobility-lifetime product (µτ=1.0×10-3  cm2 V-1 ) for detecting X-ray photons, which is almost tenfold higher than that of CH3 NH3 PbI3 wafers. As far as we know, this is the first study on an X-ray-sensitive ferroelectric material composed of 2D halide double perovskites. Our findings afford a promising platform for exploring new ferroelectric materials toward further device applications.

Dimensional Reduction of Cs2 AgBiBr6 : A 2D Hybrid Double Perovskite with Strong Polarization Sensitivity.

By dimensional reduction of the 3D motif of Cs2 AgBiBr6 , a lead-free 2D hybrid double perovskite, (i-PA)2 CsAgBiBr7 (1, i-PA=isopentylammonium), was successfully designed. It adopts a quantum-confined bilayered structure with alternating organic and inorganic sheets. Strikingly, the unique 2D architecture endows it highly anisotropic nature of physical properties, including electric conductivity and optical absorption (the ratio αb /αc =1.9 at 405 nm). Such anisotropy attributes result in the strong polarization-sensitive responses with large dichroic ratios up to 1.35, being comparable to some 2D inorganic materials. This is the first study on the hybrid double perovskites with strong polarization sensitivity. A crystal device of 1 also exhibits rapid response speed (ca. 200 µs) and excellent stabilities. The family of 2D hybrid double perovskites are promising optoelectronic candidates, and this work paves a new pathway for exploring new green polarization-sensitive materials.

High-Temperature Antiferroelectric of Lead Iodide Hybrid Perovskites.

Antiferroelectrics, characterized by the natural polarization-electric field (P-E) double hysteresis loops, has been developed as a promising branch for energy storage. Here, we present the first antiferroelectric in the booming family of lead iodide hybrid perovskites, (BA)2(EA)2Pb3I10 (1, where BA = n-butylammonium and EA = ethylammonium), which exhibits one of the highest Curie temperatures (∼363 K) for the majority of known molecular systems. Strikingly, its high-temperature antiferroelectricity, triggered by an antipolar alignment of adjacent dipoles, is confirmed by the characteristic double P-E hysteresis loops, thus enabling remarkable energy storage efficiencies in the range of 65%-83%. This merit is almost comparable to those of many inorganic counterparts, suggesting the great potential of 1 for energy storage. Another fascinating attribute is that 1 also acts as a room-temperature biaxial ferroelectric with spontaneous polarization of 5.6 µC·cm-2. As far as we know, this study on the high-temperature antiferroelectric, along with room-temperature biaxial ferroelectricity, is unprecedented for the versatile lead iodide hybrid perovskites, which sheds light on the design of new electric-ordered materials and facilitates their application of high-performance devices.

Two-Dimensional Hybrid Perovskite-Type Ferroelectric for Highly Polarization-Sensitive Shortwave Photodetection.

Two-dimensional (2D) materials have been well developed for polarization-sensitive photodetection, while new 2D members used in shortwave region (>2.5 eV) still remain scarce. The family of 2D hybrid perovskite ferroelectrics, in which the coupling of spontaneous polarization ( Ps) and light benefits dissociation of photoinduced carriers, has shown great potential in this portfolio. Here, we report a new 2D hybrid perovskite ferroelectric, [CH3(CH2)3NH3]2(CH3NH3)Pb2Br7 (1), which exhibits a superior Ps of 3.6 µC/cm2 and a relatively wide bandgap (∼2.55 eV). The unique 2D perovskite motif results in an intrinsic anisotropy of optical absorption (the ratio αc/αa ≈ 1.9 at 405 nm), involving its polarization-sensitive activity. As expected, the strongest photoresponses were observed along the c-axis (i.e., parallel to Ps), along with a large dichroism ratio ( Iphc/ Ipha ≈ 2.0) and highly sensitive detectivity up to ∼109 Jones. Further, crystal-device of 1 shows a fast responding rate (∼20 µs) and excellent antifatigued merits. As pioneering work, 1 is the first polarization-sensitive ferroelectric in the new branch of 2D hybrid perovskites. Such intriguing behaviors make 1 a potential candidate for the shortwave polarized-light detection, which also sheds light on new functionalities for future optoelectronic application of hybrid perovskites.

Highly Oriented Thin Films of 2D Ruddlesden-Popper Hybrid Perovskite toward Superfast Response Photodetectors.

2D hybrid perovskites have shown great promise in the photodetection field, due to their intriguing attributes stemming from unique structural architectures. However, the great majority of detectors based on this 2D system possess a relatively low response speed (≈ms), making it extremely urgent to develop new candidates for superfast photodetection. Here, a new organic-inorganic hybrid perovskite, (PA)2 (FA)Pb2 I7 (EFA, where PA is n-pentylaminium and FA is formamidine), which features the 2D Ruddlesden-Popper type perovskite framework that is composed of the corner-sharing PbI6 octahedra is reported. Significantly, photodetectors fabricated on highly oriented thin films, which exhibit a perfect orientation parallel to 2D inorganic perovskite layers, exhibit a superfast response time up to ≈2.54 ns. To the best of the knowledge, this figure-of-merit catches up with that of the top-ranking commercial materials, and sets a new record for 2D hybrid perovskite photodetectors. Moreover, extremely high photodetectivity (≈1.73 × 1014 Jones, under an incident power intensity of ≈46 µW cm-2 ), considerable switching ratios (>103 ), and low dark current (≈10 pA) are also achieved in the detector, indicating its great potential for high-efficiency photodetection. These results shed light on the possibilities to explore new 2D candidates for assembling future high-performance optoelectronic devices.