Role of domain walls in the abnormal photovoltaic effect in BiFeO3

Strain evolution from the ferroelectric to the relaxor state in (0.67 - x)BiFeO3-0.33BaTiO3-xBi(Mg0.5Zr0.5)O3 lead-free ceramics

The BiFeO3-BaTiO3 solid solution exhibits enhanced electric properties due to its modified phase structure with relaxor characteristics and reduced leakage current. Despite these advancements, the underlying mechanism behind the phase transition from a ferroelectric to a relaxor state in BF-BT ceramics remains largely unexplored. Here, the evolution of strain in (0.67 - x)BiFeO3-0.33BaTiO3-xBi(Mg0.5Zr0.5)O3 ceramics is investigated, with a focus on the strain transition from a ferroelectric to a relaxor phase. A strengthening of relaxor behavior is observed in the modified rhombohedral (R) and pseudocubic (PC) phase structure, resulting in optimal strain (Suni = 0.25%, Spos = 0.24%) at x = 0.04. The enhanced strain is attributed to the promotion of domain switching and the presence of strong random fields, with polar nanoregions integrating into a long-range ordered matrix. Furthermore, a gradual increase in strain with rising temperature is noted, driven by increased polarization and the expansion of ferroelectric domains. This study underscores the critical role of structural modifications in augmenting the electric response of BF-BT ceramics, thereby advancing the development of lead-free piezoelectric materials.

In-Sublattice Carrier Transition Enabled Polarimetric Photodetectors with Reconfigurable Polarity Transition

Miniaturized polarimetric photodetectors based on anisotropic two-dimensional materials attract potential applications in ultra-compact polarimeters. However, these photodetectors are hindered by the small polarization ratio values and complicated artificial structures. Here, a novel polarization photodetector based on in-sublattice carrier transition in the CdSb2Se3Br2/WSe2 heterostructure, with a giant and reconfigurable PR value, is demonstrated. The unique periodic sublattice structure of CdSb2Se3Br2 features an in-sublattice carrier transition preferred along Sb2Se3 chains. Leveraging on the in-sublattice carrier transition in the CdSb2Se3Br2/WSe2 heterostructure, gate voltage has an anisotropic modulation effect on the band alignment of heterostructure along sublattice. Consequently, the heterostructure exhibits a polarization-tunable photo-induced threshold voltage shift, which provides reconfigurable PR values from positive (unipolar regime) to negative (bipolar regime), covering all possible numbers (1→+∞/-∞→-1). Using this anisotropic photovoltaic effect, gate-tunable polarimetric imaging is successfully implemented. This work provides a new platform for developing next-generation highly polarimetric optoelectronics.

Bulk photovoltaic effect in ferroelectric nematic liquid crystals

The bulk photovoltaic (BPV) effect in ferroelectric liquid crystals is of increasing scientific interest owing to its great potential for light-energy conversion. The ferroelectric nematic phase exhibits a huge spontaneous polarization that can be aligned to a preferred direction. In this Letter, we investigate the tensorial properties of the BPV effect in the planarly aligned ferroelectric nematic phase of the liquid crystalline material RM734. A steady-state short-circuit photocurrent of ~160 pA and an open-circuit photovoltage of ~50 mV were observed in a cell with a thickness of 5.5 µm under the illumination of ultraviolet light without any bias voltage. Based on the photocurrent measurements in different electrode configurations, the non-zero elements of the BPV tensor were obtained. The BPV effect is attributed to the combination of the spontaneous polarization and the asymmetric distribution of photoinduced charge carriers. This study not only provides an understanding of the bulk PV mechanism in soft ferroelectrics but also promises a wide range of unprecedented, to the best of our knowledge, benefits for light harvesting to engineer marketable photovoltaic devices.

The Ferroelectric Effects of Rhombohedral and Tetragonal BiFeO3 in Photoelectrochemical Water Splitting

The phase of BiFeO3 (BFO) as well as its domain configuration can be tuned by strain engineering. Phase change may greatly influence the properties of the polarization field and hence charge separation. However, the photoelectrochemical properties of different BFO phases have rarely been addressed. Here, the photoelectrochemical study of tetragonal (T-) and rhombohedral (R-) phase BFO films was conducted under visible light illumination. The photocurrent density of R-BFO is 5 times that of T-BFO. A ferroelectric domain study shows that T-BFO features single domain structure in contrast to the polydomain structure of R-BFO. Higher charge separation efficiency is achieved in R-BFO, dominated by the domain walls as conducting pathways for efficient charge separation and transfer. This work provides a fundamental understanding of the photoelectrochemical properties of T- and R-BFO, offering valuable insights for the development of BFO-based materials for solar energy conversion.

Depolarization Field-Induced Photovoltaic Effect in Graphene/α-In2Se3/Graphene Heterostructures

The ferroelectric photovoltaic effect (FPVE) enables alternate pathways for energy conversion that are not allowed in centrosymmetric materials. Understanding the dominant mechanism of the FPVE at the ultrathin limit is important for defining the ultimate efficiency. In contrast to the wide band gap conventional thin-film ferroelectrics, 2D α-In2Se3 has an ideal band gap of 1.3 eV and enables the fabrication of ultrathin and stable heterostructures, providing the perfect platform to explore FPVE in the nanoscale limit. Here, we study the ferroelectric layer thickness-dependent FPVE in vertical few-layer graphene/α-In2Se3/graphene heterostructures. We find that the short-circuit photocurrent is antiparallel to the ferroelectric polarization and increases exponentially with decreasing thickness. We show that the observed behavior is predicted by the depolarization field model, originating from the unscreened bound charges due to the finite density of states in semimetal few-layer graphene. As a result, the heterostructures show enhancement of the power conversion efficiency, reaching 2.56 × 10-3% under 100 W/cm2 in 18 nm thick α-In2Se3, approximately 275 times more than the 50 nm thick α-In2Se3. These results demonstrate the importance of the depolarization field at the nanoscale and define design principles for the potential of harnessing FPVE at reduced dimension.

Realization of High-Performance Self-Powered Polarized Photodetection with Large Temperature Window in a 2D Polar Perovskite

Polarization photodetection taking advantage of the anisotropy of 2D materials shines brilliantly in optoelectronic fields owing to differentiating optical information. However, the previously reported polarization detections are mostly dependent on external power sources, which is not conducive to device integration and energy conservation. Herein, a 2D polar perovskite (CBA)2CsPb2Br7 (CCPB, CBA = 4-chlorobenzyllamine) has been successfully synthesized, which shows anticipated bulk photovoltaic effect (BPVE) with an open-circuited photovoltage up to ≈0.2 V. Devices based on CCPB monomorph fulfill a fascinating self-powered polarized photodetection with a large polarization ratio of 2.7 at room temperature. Moreover, CCPB features a high phase-transition temperature (≈475 K) which prompts such self-powered polarized photodetection in a large temperature window of device operation, since BPVE generated by spontaneous polarization can only exist in the polar structure prior to the phase transition. Further computational investigation reveals the introduction of CBA+ with a large dipole moment contributes to quite large polarization (17.5 µC cm-2) and further super high phase transition temperature of CCPB. This study will promote the application of 2D perovskite materials for self-powered polarized photodetection in high-temperature conditions.

Flexoelectric Engineering of Bulk Photovoltaic Photodetector

The bulk photovoltaic effect (BPVE) offers an interesting approach to generate a steady photocurrent in a single-phase material under homogeneous illumination, and it has been extensively investigated in ferroelectrics exhibiting spontaneous polarization that breaks inversion symmetry. Flexoelectricity breaks inversion symmetry via a strain gradient in the otherwise nonpolar materials, enabling manipulation of ferroelectric order without an electric field. Combining these two effects, we demonstrate active mechanical control of BPVE in suspended 2-dimensional CuInP2S6 (CIPS) that is ferroelectric yet sensitive to electric field, which enables practical photodetection with an order of magnitude enhancement in performance. The suspended CIPS exhibits a 20-fold increase in photocurrent, which can be continuously modulated by either mechanical force or light polarization. The flexoelectrically engineered photodetection device, activated by air pressure and without any optimization, possesses a responsivity of 2.45 × 10-2 A/W and a detectivity of 1.73 × 1011 jones, which are superior to those of ferroelectric-based photodetection and comparable to those of the commercial Si photodiode.

Colossal Ferroelectric Photovoltaic Effect in Inequivalent Double-Perovskite Bi2FeMnO6 Thin Films

Double perovskite films have been extensively studied for ferroelectric order, ferromagnetic order, and photovoltaic effects. The customized ion combinations and ordered ionic arrangements provide unique opportunities for bandgap engineering. Here, a synergistic strategy to induce chemical strain and charge compensation through inequivalent element substitution is proposed. A-site substitution of the barium ion is used to modify the chemical valence and defect density of the two B-site elements in Bi2FeMnO6 double perovskite epitaxial thin films. We dramatically increased the ferroelectric photovoltaic effect to ∼135.67 μA/cm2 from 30.62 μA/cm2, which is the highest in ferroelectric thin films with a thickness of less than 100 nm under white-light LED irradiation. More importantly, the ferroelectric polarization can effectively improve the photovoltaic efficiency of more than 5 times. High-resolution HAADF-STEM, synchrotron-based X-ray diffraction and absorption spectroscopy, and DFT calculations collectively demonstrate that inequivalent ion plays a dual role of chemical strain (+1.92 and -1.04 GPa) and charge balance, thereby introducing lattice distortion effects. The reduction of the oxygen vacancy density and the competing Jahn-Teller distortion of the oxygen octahedron are the main phenomena of the change in electron-orbital hybridization, which also leads to enhanced ferroelectric polarization values and optical absorption. The inequivalent strategy can be extended to other double perovskite systems and applied to other functional materials, such as photocatalysis for efficient defect control.

Light-induced giant enhancement of nonreciprocal transport at KTaO3-based interfaces

Nonlinear transport is a unique functionality of noncentrosymmetric systems, which reflects profound physics, such as spin-orbit interaction, superconductivity and band geometry. However, it remains highly challenging to enhance the nonreciprocal transport for promising rectification devices. Here, we observe a light-induced giant enhancement of nonreciprocal transport at the superconducting and epitaxial CaZrO3/KTaO3 (111) interfaces. The nonreciprocal transport coefficient undergoes a giant increase with three orders of magnitude up to 105 A-1 T-1. Furthermore, a strong Rashba spin-orbit coupling effective field of 14.7 T is achieved with abundant high-mobility photocarriers under ultraviolet illumination, which accounts for the giant enhancement of nonreciprocal transport coefficient. Our first-principles calculations further disclose the stronger Rashba spin-orbit coupling strength and the longer relaxation time in the photocarrier excitation process, bridging the light-property quantitative relationship. Our work provides an alternative pathway to boost nonreciprocal transport in noncentrosymmetric systems and facilitates the promising applications in opto-rectification devices and spin-orbitronic devices.

Intercorrelated Ferroelectricity and Bulk Photovoltaic Effect in Two-Dimensional Sn2P2S6 Semiconductor for Polarization-Sensitive Photodetection

A two-dimensional (2D) ferroelectric semiconductor, which is coupled with photosensitivity and room-temperature ferroelectricity, provides the possibility of coordinated conductance modulation by both electric field and light illumination and is promising for triggering the revolution of optoelectronics for monolithic multifunctional integration. Here, we report that semiconducting Sn2P2S6 crystals can be achieved in a 2D morphology using a chemical vapor transport approach with the assistant of space confinement and experimentally demonstrate the robust ferroelectricity in atomic-thin Sn2P2S6 nanosheet at room temperature. The intercorrelated programming of ferroelectric order along out-of-plane (OOP) and in-plane (IP) directions enables a tunable bulk photovoltaic (BPV) effect through multidirectional electrical control. By combining the capability of anisotropic in-plane optical absorption, a highly integrated Sn2P2S6 optoelectronic device vertically sandwiched with graphene electrodes yields the polarization-dependent open-circuit photovoltage with a dichroic ratio of 2.0 under 405 nm light illumination. The reintroduction of ferroelectric Sn2P2S6 to the 2D asymmetric semiconductor family provides possibilities to hardware implement of the self-powered polarization-sensitive photodetection and spotlights the promising applications for next-generation photovoltaic devices.

The Effect of Multi-Fields Synergy from Electric/Light/Thermal/Force Technologies on Photovoltaic Performance of Ba0.06 Bi0.47 Na0.47 TiO3 Ferroelectric Ceramics via the Mg/Co Substitution at A/B Sites

Currently, it is widely reported that the photovoltaic effect in ferroelectric materials can be promoted by the application of a piezoelectric force, an external electric field, and intense light illumination. Here, a semiconducting ferroelectric composition is introduced, (1-x) Ba0.06 Bi0.47 Na0.47 TiO3 -xMgCoO3 (abbreviated as xMgCo, where x = 0.02-0.08), synthesized through Mg/Co ions codoping. This process effectively narrows the optical bandgaps to a spectrum of 1.38-3.06 eV. Notably, the system exhibits a substantial increase in short-circuit photocurrent density (Jsc ), by the synergy of the electric, light, and thermal fields. The Jsc can still be further enhanced by the extra introduction of a force field. Additionally, the Jsc also shows an obvious increase after the high field pre-poling. The generation of a considerable number of oxygen vacancies due to the Co2+ /Co3+ mixed valence state (in a 1:3 ratio) contributes to the reduced optimal bandgap. The integration of Mg2+ ion at the A-site restrains the loss and sustains robust ferroelectricity (Pr  = 24.1 µC cm-2 ), high polarizability under an electric field, and a significant piezoelectric coefficient (d33  = 102 pC N-1 ). This study provides a novel perspective on the physical phenomena arising from the synergy of multiple fields in ferroelectric photovoltaic materials.

Polarization Alignment in Polycrystalline BiFeO3 Photoelectrodes for Tunable Band Bending

Polarization in a semiconductor can modulate the band bending via the depolarization electric field (EdP), subsequently tuning the charge separation and transfer (CST) process in photoelectrodes. However, the random orientation of dipole moments in many polycrystalline semiconductor photoelectrodes leads to negligible polarization effect. How to effectively align the dipole moments in polycrystalline photoelectrodes into the same direction to maximize the polarization is still to be developed. Herein, we report that the dipole moments in a ferroelectric BiFeO3 photoelectrode can be controlled under external poling, resulting in a tunable CST efficiency. A negative bias of -40 voltage (V) poling to the photoelectrode leads to an over 110% increase of the CST efficiency, while poling at +40 V, the CST efficiency is reduced to only 41% of the original value. Furthermore, a nearly linear relationship between the external poling voltage and surface potential is discovered. The findings here provide an effective method in tuning the band bending and charge transfer of the emerging ferroelectricity driven solar energy conversion.

Stereochemically Active Lone Pair Induced Polar Tri-layered Perovskite for Record-Performance Polarized Photodetection

Bulk photovoltaic effect, a promising optoelectronic phenomenon for generating polarized dependent steady-state photocurrent, has been widely applied in various photodetectors. However, incorporating stereochemically active lone pair to construct bulk photovoltage in organic-inorganic hybrid perovskite (OIHP) is still elusive and challenging. Herein, bulk photovoltage (1.2 V) has been successfully achieved by introducing the stereo-chemically active lone pair perovskitizer to construct a polar tri-layered hybrid perovskite, namely, (IBA)2 MHy2 Pb3 Br10 (1, IBA=iso-butylamine, MHy=methylhydrazine). Strikingly, owning to the promising bulk photovoltage, 1-based detectors exhibit an ultra-highly sensitive polarized photodetection (polarization ratio of up to 24.6) under self-powered mode. This ratio surpasses all the reported two-dimension OIHP single-crystal photodetectors. In addition, detectors exhibit outstanding responsivity (≈200 mA W-1 ) and detectivity (≈2.4×1013 Jones). More excitingly, further investigation confirms that lone pair electrons in MHy+ result in the separation of positive and negative charges to produce directional dipoles, which further directional alignment to generate bulk photovoltage, thereby resulting in polarization-dependent photocurrent. Our findings provide a new demonstration for polar multilayer materials' construction and may open opportunities for a host of high-sensitive polarized photodetection.

Sub-Nanosecond Reconfiguration of Ferroelectric Domains in Bismuth Ferrite

Domain switching is crucial for achieving desired functions in ferroic materials that are used in various applications. Fast control of domains at sub-nanosecond timescales remains a challenge despite its potential for high-speed operation in random-access memories, photonic, and nanoelectronic devices. Here, ultrafast laser excitation is shown to transiently melt and reconfigure ferroelectric stripe domains in multiferroic bismuth ferrite on a timescale faster than 100 picoseconds. This dynamic behavior is visualized by picosecond- and nanometer-resolved X-ray diffraction and time-resolved X-ray diffuse scattering. The disordering of stripe domains is attributed to the screening of depolarization fields by photogenerated carriers resulting in the formation of charged domain walls, as supported by phase-field simulations. Furthermore, the recovery of disordered domains exhibits subdiffusive growth on nanosecond timescales, with a non-equilibrium domain velocity reaching up to 10 m s-1 . These findings present a new approach to image and manipulate ferroelectric domains on sub-nanosecond timescales, which can be further extended into other complex photoferroic systems to modulate their electronic, optical, and magnetic properties beyond gigahertz frequencies. This approach could pave the way for high-speed ferroelectric data storage and computing, and, more broadly, defines new approaches for visualizing the non-equilibrium dynamics of heterogeneous and disordered materials.

Centimeter-Size Single Crystals of Halide Perovskite Photoferroelectric Solid Solution with Ultrahigh Pyroelectricity Boosted Photodetection

Photoferroelectrics, especially emerging halide perovskite ferroelectrics, have motivated tremendous interests owing to their fascinating bulk photovoltaic effect (BPVE) and cross-coupled functionalities. However, solid solutions of halide perovskite photoferroelectrics with controllable structure and enhanced performance are scarcely explored. Herein, through mixing cage cation, a homogeneous halide perovskite photoferroelectric PA2 FAx MA1-x Pb2 Br7 solid solution (PA, FA and MA are CH3 CH2 CH2 NH3 + , NH2 CHNH2 + and CH3 NH3 + , 0≤x≤1) has been developed, which demonstrates tunable Curie temperature in a wide range of 263-323 K and excellent optoelectrical features. As the component adjusted to x=0.7, the bulk crystal demonstrates ultrahigh pyroelectric coefficient up to 1.48 μC cm-2  K-1 around room temperature. Strikingly, benefiting from the light-induced pyroelectricity and remarkable BPVE, a self-powered and sensitive photodetector based solid solution crystals with boosted responsivity and detectivity over than 1300 % has been achieved. This pioneering work sheds light on the exploration of photoferroelectric solid solutions towards high-performance photoelectronic devices.

Giant Polarization Sensitivity via the Anomalous Photovoltaic Effect in a Two-Dimensional Perovskite Ferroelectric

Polarization sensitivity, which shows great potential in photoelectric detection, is expected to be significantly improved by the ferroelectric anomalous photovoltaic (APV) effect. However, it is challenging to explore new APV-active ferroelectrics due to severe polarization fatigue induced by the leakage current of photoexcited carriers. For the first time, we report a strong APV effect in a 2D hybrid perovskite ferroelectric assembled by alloying mixed organic cations, (HA)₂(EA)₂Pb₃Br₁₀ (1, where HA⁺ is n-hexylammonium and EA⁺ is ethylammonium), which has a large spontaneous polarization ∼3.8 μC/cm² and high a Curie temperature ∼378 K. Its ferroelectricity allows a strong APV effect with an above-bandgap photovoltage up to 7.4 V, which exceeds its bandgap (∼2.7 eV). Most strikingly, based on the dependence on polarized-light angle, this strong APV effect renders the highest level of polarization sensitivity with a giant current ratio of ∼25, far beyond other 2D single-phase materials. This study sheds light on the exploration of APV-active ferroelectrics and inspires their future high-performance optoelectronic device applications.

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 (V_OC ) 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.

In-Plane Anisotropic Rectification and Photovoltaic Effects on ZnO Nonpolar (101̅0) Crystal Plane and the Physical Mechanism

The concept of spontaneous electric field (Es) in polar structures is crucial for understanding the physical and chemical properties of compound semiconductors and improving their performanes. However, this concept has not been widely accepted so far. Here, we show the first observation of rectification and photovoltaic effects in the polar [0001] direction on the nonpolar ZnO (1010) crystal plane. However, no rectification and photovoltaic effects are observed in the nonpolar [1210] direction perpendicular to the [0001]. When a stress was applied in the [0001] direction of the ZnO single crystal, the rectification and photovoltaic effects are abated and disappeared. The disappearance of the two effects results from the pressure-induced disappearance of the polar structure. The results fully demonstrated that the rectification and photovoltaic effects arise from the existence of Es in the polar [0001] direction. The Es motivates the directional transfer of the electrons and photocreated charges along the [0001] direction, and the rectification and photovoltaic effects are thus observed. These results provide direct evidence for the polar structure theory and suggest that the polar structures can be employed to develop new types of photovoltaic and other electronic and photoelectronic devices.

Facile Control of Ferroelastic Domain Patterns in Multiferroic Thin Films by a Scanning Tip Bias

BiFeO₃, known as the "holy grail of all multiferroics", provides an appealing platform for exploration of multifield coupling physics and design of functional devices. Many fantastic properties of BiFeO₃ are regulated by its ferroelastic domain structure. However, a facile programable control on the ferroelastic domain structure in BiFeO₃ remains challenging and our understanding on the existing control strategies is also far from complete. This work reports a facile control of ferroelastic domain patterns in BiFeO₃ thin films under area scanning poling by exploiting the tip bias as the control parameter. Combining scanning probe microscopy experiments and simulations, we found that BiFeO₃ thin films with pristine 71° rhombohedral-phase stripe domains exhibit at least four switching pathways solely by controlling the scanning tip bias. As a result, one can readily write mesoscopic topological defects into the films without the necessity to change the tip motion. The correlation between conductance of the scanned region and the switching pathway is further investigated. Our results extend the current understanding on the domain switching kinetics and the coupled electronic transport properties in BiFeO₃ thin films. The facile voltage control of ferroelastic domains should facilitate the development of configurable electronic and spintronic devices.

Precisely Tailoring a FAPbI3-Derived Ferroelectric for Sensitive Self-Driven Broad-Spectrum Polarized Photodetection

Benefiting from superior semiconducting properties and the angle-dependence of the bulk photovoltaic effect (BPVE) on polarized light, the two-dimensional (2D) hybrid perovskite ferroelectrics are developed for sensitive self-powered polarized photodetection. Most of the currently reported ferroelectric-driven polarized photodetection is restricted to the shortwave optical response, and expanding the response range is urgently needed. Here we report the first instance of a FAPbI₃-derived (2D) perovskite ferroelectric, (BA)₂(FA)Pb₂I₇ (1, BA is n-butylammonium, FA is formamidinium). It exhibited a notably high thermostability and broad-spectrum adsorption extending to around 650 nm. Significantly, 1 demonstrated ferroelectricity-driven self-powered polarized photodetection under 637 nm with an anisotropic photocurrent ratio of ∼1.96, ultrahigh detectivity of 3.34 × 10¹² Jones, and long-term repetition. This research will shed light on the development of new ferroelectrics for potential application in broad-spectrum polarization-based optoelectronics.

Dynamic domain boundaries: chemical dopants carried by moving twin walls

Domain walls and specifically ferroelastic twin boundaries are depositaries and fast diffusion pathways for chemical dopants and intrinsic lattice defects. Ferroelastic domain patterns act as templates for chemical structures where the walls are the device and not the bulk. Several examples of such engineered domain boundaries are given. Moving twin boundaries are shown to carry with them the dopants, although the activation of this mechanism depends sensitively on the applied external force. If the force is too weak, the walls remain pinned while too strong forces break the walls free of the dopants and move them independently. Several experimental methods and approaches are discussed.

Suppressing non-radiative recombination in metal halide perovskite solar cells by synergistic effect of ferroelasticity

The low fraction of non-radiative recombination established the foundation of metal halide perovskite solar cells. However, the origin of low non-radiative recombination in metal halide perovskite materials is still not well-understood. Herein, we find that the non-radiative recombination in twinning-tetragonal phase methylammonium lead halide (MAPbI_xCl_3-x) is apparently suppressed by applying an electric field, which leads to a remarkable increase of the open-circuit voltage from 1.12 V to 1.26 V. Possible effects of ionic migration and light soaking on the open-circuit voltage enhancement are excluded experimentally by control experiments. Microscopic and macroscopic characterizations reveal an excellent correlation between the ferroelastic lattice deformation and the suppression of non-radiative recombination. The calculation result suggests the existence of lattice polarization in self-stabilizable deformed domain walls, indicating the charge separation that facilitated by lattice polarization is accountable for the suppressed non-radiative recombination. This work provides an understanding of the excellent performance of metal halide perovskite solar cells.

Giant bulk piezophotovoltaic effect in 3R-MoS2

Given its innate coupling with wavefunction geometry in solids and its potential to boost the solar energy conversion efficiency, the bulk photovoltaic effect (BPVE) has been of considerable interest in the past decade^1-14. Initially discovered and developed in ferroelectric oxide materials², the BPVE has now been explored in a wide range of emerging materials, such as Weyl semimetals^9,10, van der Waals nanomaterials^11,12,14, oxide superlattices¹⁵, halide perovskites¹⁶, organics¹⁷, bulk Rashba semiconductors¹⁸ and others. However, a feasible experimental approach to optimize the photovoltaic performance is lacking. Here we show that strain-induced polarization can significantly enhance the BPVE in non-centrosymmetric rhombohedral-type MoS₂ multilayer flakes (that is, 3R-MoS₂). This polarization-enhanced BPVE, termed the piezophotovoltaic effect, exhibits distinctive crystallographic orientation dependence, in that the enhancement mainly manifests in the armchair direction of the 3R-MoS₂ lattice while remaining largely intact in the zigzag direction. Moreover, the photocurrent increases by over two orders of magnitude when an in-plane tensile strain of ~0.2% is applied, rivalling that of state-of-the-art materials. This work unravels the potential of strain engineering in boosting the photovoltaic performance, which could potentially promote the exploration of novel photoelectric processes in strained two-dimensional layered materials and their van der Waals heterostructures.

Bulk Photovoltage Effect in Ferroelectric BaTiO3

Due to the unusual charge separation mechanism and anomalous photovoltaic effects, the bulk photovoltage effect in ferroelectric semiconductors has attracted a great deal of attention in solar energy conversion, especially in attempts to utilize nonthermalized carriers. Among the various mechanisms that have been proposed for interpreting the photovoltaic effect, a shift mechanism was derived from quantum phenomena, which have been modeled and studied for many years. However, the concurrent shift and ballistic mechanism make investigating the ferroelectric bulk photovoltage effect complex and challenging. Here, taking a tetragonal ferroelectric BaTiO₃ single crystal as a prototype, we report an approach for distinguishing the shift and ballistic mechanism-induced surface photovoltage. The results indicate different effects on the charge separation of the ballistic mechanism and shift mechanisms, as evidenced by surface photovoltage measurement. Interestingly, the shift and ballistic mechanisms afford charge separation in two opposite directions but on the same order of magnitude under monochromatic superband illumination. Our results provide facile and efficient methods for clarifying the shift and ballistic mechanisms in ferroelectrics.

Coupling Enhancement of a Flexible BiFeO3 Film-Based Nanogenerator for Simultaneously Scavenging Light and Vibration Energies

Coupled nanogenerators have been a research hotspot due to their ability to harvest a variety of forms of energy such as light, mechanical and thermal energy and achieve a stable direct current output. Ferroelectric films are frequently investigated for photovoltaic applications due to their unique photovoltaic properties and bandgap-independent photovoltage, while the flexoelectric effect is an electromechanical property commonly found in solid dielectrics. Here, we effectively construct a new form of coupled nanogenerator based on a flexible BiFeO₃ ferroelectric film that combines both flexoelectric and photovoltaic effects to successfully harvest both light and vibration energies. This device converts an alternating current into a direct current and achieves a 6.2% charge enhancement and a 19.3% energy enhancement to achieve a multi-dimensional "1 + 1 > 2" coupling enhancement in terms of current, charge and energy. This work proposes a new approach to the coupling of multiple energy harvesting mechanisms in ferroelectric nanogenerators and provides a new strategy to enhance the transduction efficiency of flexible functional devices.

Extended linear detection range of a Bi0.5Na0.5TiO3 thin film-based self-powered UV photodetector via current and voltage dual indicators

Ferroelectric materials are widely recognized for their ability to generate photovoltaic voltages larger than their bandgap, making them ideal candidates for photodetector applications. Here, we report a self-powered UV photodetector based on a Bi_0.5Na_0.5TiO₃ (BNT) thin film prepared by the sol-gel method. Compared with conventional photodetectors based on a single detection indicator, the demonstrated photodetector realizes UV light intensity detection over a wide linear range using a current and voltage dual indicator detection method. When the UV light intensity is lower than 1.8 mW cm^-2, the voltage can be used to detect the light signal. Conversely, the current can be utilized to detect the signal. This method not only broadens the linear detection range of UV light intensity, making it possible to detect weak UV light of 45.2 nW cm^-2, but also allows the detector to maintain relatively high sensitivity within the detectable range. To investigate the distribution of spatial UV light intensity, a self-powered photodetector array system has been utilized to record the output voltage signals as a map.

Magneto-Electric-Optical Coupling in Multiferroic BiFeO3 -Based Films

Manipulating ferroic orders and realizing their coupling in multiferroics at room temperature are promising for designing future multifunctional devices. Single external stimulation has been extensively proved to demonstrate the ability of ferroelastic switching in multiferroic oxides, which is crucial to bridge the ferroelectricity and magnetism. However, it is still challenging to directly realize multi-field-driven magnetoelectric coupling in multiferroic oxides as potential multifunctional electrical devices. Here, novel magneto-electric-optical coupling in multiferroic BiFeO₃ -based thin films at room temperature mediated by deterministic ferroelastic switching using piezoresponse/magnetic force microscopy and aberration-corrected transmission electron microscopy are shown. Reversible photoinduced ferroelastic switching exhibiting magnetoelectric responses is confirmed in BiFeO₃ -based films, which works at flexible strain states. This work directly demonstrates room-temperature magneto-electric-optical coupling in multiferroic films, which provides a framework for designing potential multi-field-driven magnetoelectric devices such as energy conservation memories.

Nonvolatile ferroelectric domain wall memory integrated on silicon

Ferroelectric domain wall memories have been proposed as a promising candidate for nonvolatile memories, given their intriguing advantages including low energy consumption and high-density integration. Perovskite oxides possess superior ferroelectric prosperities but perovskite-based domain wall memory integrated on silicon has rarely been reported due to the technical challenges in the sample preparation. Here, we demonstrate a domain wall memory prototype utilizing freestanding BaTiO₃ membranes transferred onto silicon. While as-grown BaTiO₃ films on (001) SrTiO₃ substrate are purely c-axis polarized, we find they exhibit distinct in-plane multidomain structures after released from the substrate and integrated onto silicon due to the collective effects from depolarizing field and strain relaxation. Based on the strong in-plane ferroelectricity, conductive domain walls with reading currents up to nanoampere are observed and can be both created and erased artificially, highlighting the great potential of the integration of perovskite oxides with silicon for ferroelectric domain wall memories.

Integrating ferroelectric perovskite oxides on Si is highly desired for electronic applications but challenging. Here, the authors show emergent in-plane ferroelectricity and promising nonvolatile memories based on resistive domain wall in BaTiO₃/Si.

Efficient molecular ferroelectric photovoltaic device with high photocurrent via lewis acid-base adduct approach

Traditional inorganic oxide ferroelectric materials usually have band gaps above 3 eV, leading to more than 80% of the solar spectrum unavailable, greatly limiting the current density of their devices just atμA cm^-2level. Therefore, exploring ferroelectric materials with lower band gaps is considered as an effective method to improve the performance of ferroelectric photovoltaic devices. Inorganic ferroelectric materials are often doped with transition metal elements to reduce the band gap, which is a complex doping and high temperature fabrication process. Recently, molecular ferroelectric materials can change the symmetry and specific interactions of crystals at the molecular level by chemically modifying or tailoring cations with high symmetry, enabling rational design and banding of ferroelectricity in the framework of perovskite simultaneously. Therefore, the molecular ferroelectric materials have a great performance for both excellent ferroelectricity and narrow band gap without doping. Here, we report a ferroelectric photovoltaic device employing an organic-inorganic hybrid molecular ferroelectric material with a band gap of 2.3 eV to obtain high current density. While the poor film quality of molecular ferroelectrics still limits it. The Lewis acid-base adduct is found to greatly improve the film quality with lower defect density and higher carrier mobility. Under standard AM 1.5 G illumination, the photocurrents of ∼1.51 mA cm^-2is achieved along with a device efficiency of 0.45%. This work demonstrates new possibilities for the application of molecular ferroelectric films with narrow band gaps in photovoltaic devices, and lays a foundation for Lewis acid-base chemistry to improve the quality of molecular ferroelectric thin films to obtain high current densities and device performance.

Incorporating an Aromatic Cationic Spacer to Assemble 2D Polar Perovskite Crystals toward Self-Powered Detection of Quite Weak Polarized Light

Two-dimensional (2D) hybrid perovskites with intrinsic attributes of structural and optical anisotropy are holding a bright promise for polarization-sensitive photodetection. However, studies on self-powered detection to quite weak polarized light remain scarce in this 2D family. By incorporating an aromatic spacer into the 3D cubic prototype, we have successfully assembled a new 2D hybrid perovskite with a polar motif, (FPEA)₂(MA)Pb₂I₇ (FMPI, where FPEA is 4-fluorophenethylammonium and MA is methylammonium). Its unique 2D quantum-well structure allows optical absorption dichroism with a large ratio of ∼3.15, and the natural polarity results in a notable bulk photovoltaic effect. Further, centimeter-size crystals (10 × 10 × 3 mm³) of FMPI were facilely obtained by the temperature cooling method, and its crystal-based detectors enable excellent self-powered detection of quite weak polarized light, showing a notable polarization-sensitive ratio (∼1.5), extremely low detection limit (∼100 nW/cm²), and antifatigued stability. The alloyed aromatic cationic spacers facilitate the polarity and enhanced phase stability. This study paves a way for further exploration of new 2D perovskite candidates toward optoelectronic device applications.

Ferroelasticity in Organic-Inorganic Hybrid Perovskites

Molecular ferroelastics have received particular attention for potential applications in mechanical switches, shape memory, energy conversion, information processing, and solar cells, by taking advantages of their low-cost, light-weight, easy preparation, and mechanical flexibility. The unique structures of organic-inorganic hybrid perovskites have been considered to be a design platform for symmetry-breaking-associated order-disorder in lattice, thereby possessing great potential for ferroelastic phase transition. Herein, we review the research progress of organic-inorganic hybrid perovskite ferroelastics in recent years, focusing on the crystal structures, dimensions, phase transitions and ferroelastic properties. In view of the few reports on molecular-based hybrid ferroelastics, we look forward to the structural design strategies of molecular ferroelastic materials, as well as the opportunities and challenges faced by molecular-based hybrid ferroelastic materials in the future. This review will have positive guiding significance for the synthesis and future exploration of organic-inorganic hybrid molecular ferroelastics.

Flexoelectricity-Enhanced Photovoltaic Effect in Self-Polarized Flexible PZT Nanowire Array Devices

In this investigation, we report the flexoelectricity-enhanced photovoltaic (FPV) effect in a flexible Pb(Zr_0.52Ti_0.48)O₃ nanowire (PZT NW) array/PDMS (polydimethylsiloxane) nanocomposite. The simulation result of density functional theory (DFT) indicated that the FPV effect in PZT NWs can be greatly affected by the interactions of the strain gradients with the internal field generated by self-polarization. We found that when the nanocomposite film was curved down, the photovoltaic current of the aligned PZT-NW/PDMS composite increased by 84.6-fold and 27.6-fold compared with the PZT-nanoparticles/PDMS and randomly aligned PZT-NW/PDMS nanocomposites at the same curvature, respectively. This is mainly ascribed to the increased flexoelectricity in the aligned PZT-NW/PDMS nanocomposite. This study will contribute to a full understanding of the influence of nanoparticle shape on the flexophotovoltaic effect of nanocomposites. It will have potential use in nanocomposites for the study of the FPV effect and associated applications.

Effect of Nd and Mn Co-Doping on Dielectric, Ferroelectric and Photovoltaic Properties of BiFeO3

Bi1−xNdxFe0.99Mn0.01O3 (BNFMO, x = 0.00~0.20) films were epitaxially grown on Nb:SrTiO3 (001) substrates using pulsed laser deposition. It was found that the Nd-doping concentration has a great impact on the surface morphology, crystal structure, and electrical properties. BNFMO thin film with low Nd-doping concentration (≤16%) crystallizes into a rhombohedral structure, while the high Nd-doping (>16%) will lead to the formation of an orthogonal structure. Furthermore, to eliminate the resistive switching (RS) effect, a positive-up–negative-down (PUND) measurement was applied on two devices in series. The remnant polarization experiences an increase with the Nd-doping concentration increasing to 16%, then drops down with the further increased concentration of Nd. Finally, the ferroelectric photovoltaic effect is also regulated by the ferroelectric polarization, and the maximum photocurrent of 1758 μA/cm2 was obtained in Bi0.84Nd0.16Fe0.99Mn0.01O3 thin film. BNFMO films show great potential for ferroelectric and photovoltaic applications.

Realization of In-Plane Polarized Light Detection Based on Bulk Photovoltaic Effect in A Polar Van Der Waals Crystal

2D van der Waals materials are widely explored for in-plane polarized light detection owing to their distinctive in-plane anisotropic feature. However, most of these polarized light-sensitive devices root in their low symmetry of in-plane structure and work depending on external power sources, which greatly impedes the simplification of integrated devices and sustainable development. Bulk photovoltaic effect (BPVE), which separates photoexcited carriers via built-in electric field without an external power source and shows an angle-dependence on light polarization, is promising for self-powered polarized light detection to break through the restriction of in-plane anisotropy. Herein, a 2D lead-free van der Waals perovskite (Cl-PMA)₂ CsAgBiBr₇ (1, Cl-PMA = 4-Chlorobenzylamine) is successfully designed through the dimension reduction strategy. 1 exhibits BPVE with an open-circuited photovoltage up to ≈0.5 V. Driven by the BPVE, self-powered in-plane polarized light detection with a large polarization ratio of 1.3 is obtained for 1. As far as it is known, the first in-plane polarized light detection in hybrid perovskites based on BPVE is realized here. This work highlights the strategy of designing lead-free hybrid perovskite with BPVE and opens an avenue for exploiting in-plane highly sensitive polarized light detection in 2D van der Waals materials.

Large-Scale Epitaxial Growth of Ultralong Stripe BiFeO3 Films and Anisotropic Optical Properties

The controlled synthesis of large-scale ferroelectric domains with high uniformity is crucial for practical applications in next-generation nanoelectronics on the basis of their intriguing properties. Here, ultralong and highly uniform stripe domains in (110)-oriented BiFeO₃ thin films are large-area synthesized through a pulsed laser deposition technique. Utilizing scanning transmission electron microscopy and piezoresponse force microscopy, we verified that the ferroelectric domains have one-dimensional 109° domains and the length of a domain is up to centimeter scale. More importantly, the ferroelectric displacement is directly determined on atomic-scale precision, further confirming the domain structure. We find that the unique one-dimensional ferroelectric domain significantly enhances the optical anisotropy. Furthermore, we demonstrate that the purely parallel domain patterns can be used to control photovoltaic current. These ultralong ferroelectric domains can be patterned into various functional devices, which may inspire research efforts to explore their properties and various applications.

Strain and orientation engineering in ABO3perovskite oxide thin films

Perovskite oxides with chemical formula ABO₃are widely studied for their properties including ferroelectricity, magnetism, strongly correlated physics, optical effects, and superconductivity. A thriving research direction using such materials is through their integration as epitaxial thin films, allowing many novel and exotic effects to be discovered. The integration of the thin film on a single crystal substrate, however, can produce unique and powerful effects, and can even induce phases in the thin film that are not stable in bulk. The substrate imposed mechanical boundary conditions such as strain, crystallographic orientation, octahedral rotation patterns, and symmetry can also affect the functional properties of perovskite films. Here, the author reviews the current state of the art in epitaxial strain and orientation engineering in perovskite oxide thin films. The paper begins by introducing the effect of uniform conventional biaxial strain, and then moves to describe how the substrate crystallographic orientation can induce symmetry changes in the film materials. Various material case studies, including ferroelectrics, magnetically ordered materials, and nonlinear optical oxides are covered. The connectivity of the oxygen octahedra between film and substrate depending on the strain level as well as the crystallographic orientation is then discussed. The review concludes with open questions and suggestions worthy of the community's focus in the future.

Fast Photostriction in Ferroelectrics

Light-induced nonthermal strain, known as the photostrictive effect, offers a potential way to excite mechanical strain and acoustic wave remotely. The anisotropic photostrictive effect induced by the combination of bulk photovoltaic effect (BPVE) and converse piezoelectric effect in ferroelectric materials is known as too small and slow for the applications requiring a high strain rate, such as ultrasound generation and high-speed signal transmission. Here, a strategy to achieve high rate dynamic photostrictive strain by utilizing local fast responses under modulating continuous light excitation in the resonance condition is reported. A strain rate of 8.06 × 10^-3  s^-1 is demonstrated under continuous light excitation, which is at least one order of magnitude higher than previous studies on bulk samples as seen in the literature. The significant photostrictive response exists even in depoled ferroelectric material without overall polarization. The theoretical analyses show that fast ferroelectric photostriction can be obtained through the combinational interaction mechanism of local BPVE and local converse piezoelectric effect existing only in the microscopic scale, thus circumventing the slow and low efficient BPVE charging up process across the macroscopic electrical terminals. The achieved fast photostriction and new understandings will open new opportunities to realize future wireless signal transmission and light-acoustic devices.

Enhanced bulk photovoltaic effect in two-dimensional ferroelectric CuInP2S6

The photocurrent generation in photovoltaics relies essentially on the interface of p-n junction or Schottky barrier with the photoelectric efficiency constrained by the Shockley-Queisser limit. The recent progress has shown a promising route to surpass this limit via the bulk photovoltaic effect for crystals without inversion symmetry. Here we report the bulk photovoltaic effect in two-dimensional ferroelectric CuInP2S6 with enhanced photocurrent density by two orders of magnitude higher than conventional bulk ferroelectric perovskite oxides. The bulk photovoltaic effect is inherently associated to the room-temperature polar ordering in two-dimensional CuInP2S6. We also demonstrate a crossover from two-dimensional to three-dimensional bulk photovoltaic effect with the observation of a dramatic decrease in photocurrent density when the thickness of the two-dimensional material exceeds the free path length at around 40 nm. This work spotlights the potential application of ultrathin two-dimensional ferroelectric materials for the third-generation photovoltaic cells.

In-plane ferroelectricity in few-layered GeS and its van der Waals ferroelectric diodes

Two-dimensional ferroelectric semiconductors (2DFeSs) have been attracting extensive research attention on account of their unique properties and versatile applications in random-access memory, digital signal processors, and neuromorphic computing. Germanium sulfide (GeS) is predicted as a typical 2DFeS with a large spontaneous polarization of 484 pC m^-1. Furthermore, the moderate band gap equivalent to 1.63 eV of GeS provides it with significant potential to create a strong bulk photovoltage in the visible light range. However, the fabrication of chemically stable few-to-monolayer GeS has not been reported so far, owing to the strong interlayer force and high chemical reactivity of the surface. Herein we demonstrate a new method for fabricating high quality, air-stable, ultrathin GeS nanoflakes. The electrical characterization confirms the formation of few-layered GeS with a remarkable in-plane ferroelectric hysteresis, which is forbidden by the inversion symmetry in bulk GeS crystals. After applying a coercive field of about 18.1 kV cm^-1, a switchable shift current can also be observed in the polarized GeS nanoflakes under light irradiation. To further enhance the photoresponsivity, few-layered InSe was transferred onto the GeS nanoflakes to form van der Waals ferroelectric diodes. The interfacial perturbation breaking the inversion symmetry results in the enhancement of robust dipoles in the GeS side along the interface, which can be tuned by the in-plane electric field. Overall, this work opens the door for exploring the low-dimensional ferroelectric memory and energy conversion applications based on 2D GeS nanoflakes and provides a deeper understanding of the photovoltaic mechanism with in-plane 2D ferroelectric diodes.

Spontaneous flexoelectricity and band engineering in MS2 (M = Mo, W) nanotubes

Spontaneous flexoelectricity in transition metal dichalcogenide (TMD) nanotubes is critical to the design of new energy devices. However, the electronic properties adjusted by the flexoelectric effect in TMD nanotubes remain vague. In this work, we investigate the effect of flexoelectricity on band engineering in single- and double-wall MS₂ (M = Mo, W) nanotubes with different diameters based on first-principles calculations and an atomic-bond-relaxation method. We find that the energy bandgap reduces and the polarization and flexoelectric voltage increase with decreasing diameter of single-wall MS₂ nanotubes. The polarization charges promoted by the flexoelectric effect can lead to a straddling-to-staggered bandgap transition in the double-wall MS₂ nanotubes. The critical diameters for bandgap transition are about 3.1 and 3.6 nm for double-wall MoS₂ and WS₂ nanotubes, respectively, which is independent of chirality. Our results provide guidance for the design of new energy devices based on spontaneous flexoelectricity.

Enhancing the bulk photovoltaic effect by tuning domain walls in epitaxial BiFeO3films

In this letter, the role of domain wall (DW) on bulk photovoltaic effect (BPV) effect in BiFeO₃(BFO) films was studied by x-ray reciprocal space mapping and conductive atomic force microscope. It was found that the domain structure and DW can be tuned by controlling the epitaxial orientation of BFO film. Remarkably, under 1 sun AM 1.5 G illumination, the 109° DW enhances the transport of photogenerated carriers and simultaneously improves the conductivity and power conversion efficiency (PCE). The short-circuit current density and PCE can reach 171.15μA cm^-2and 0.1127%, respectively. Therefore, our study reveals the correlation between the DW and the BPV effect in BFO film and provides a new pathway to improve the PCE of BFO-based photovoltaic device.

Controllable Distribution and Reversible Migration of Charges in BiFeO3-Based Films on Si Substrates

In ferroelectric-based integrated devices, there are usually buffer layers between ferroelectric films and semiconductor substrates. Here, Bi_x%FeO_3-δ (x = 95, 100, and 105) (BFOx) films are prepared directly on n-Si substrates by the sol-gel method, and the variation of the hysteresis loops with Bi content and heat treatment is investigated. With the help of the dielectric measurement and the composition analysis, a PN heterojunction is believed to exist at the BFOx/Si interface. The Bi/Fe ratio determines not only the type and concentration of charged defects in the films but also the height of the interface barrier and its binding effect on mobile charges. Furthermore, the distribution and the migration of charges can be regulated reversibly by heat treatment. This work reveals the interaction between ferroelectric films and semiconductor substrates, providing an important reference for the design and application of ferroelectric/semiconductor heterostructures.

Oxide and Organic–Inorganic Halide Perovskites with Plasmonics for Optoelectronic and Energy Applications: A Contributive Review

The ascension of halide perovskites as outstanding materials for a wide variety of optoelectronic applications has been reported in recent years. They have shown significant potential for the next generation of photovoltaics in particular, with a power conversion efficiency of 25.6% already achieved. On the other hand, oxide perovskites have a longer history and are considered as key elements in many technological applications; they have been examined in depth and applied in various fields, owing to their exceptional variability in terms of compositions and structures, leading to a large set of unique physical and chemical properties. As of today, a sound correlation between these two important material families is still missing, and this contributive review aims to fill this gap. We report a detailed analysis of the main functions and properties of oxide and organic–inorganic halide perovskite, emphasizing existing relationships amongst the specific performance and the structures.

Effect of polarization rotation on the optical and photovoltaic properties of BiFeO3thin films

Visible-light-active ferroelectric materials are gaining increasing attention due to the unique ferroelectric photovoltaic effect. To boost the light harvesting capability, vast research is devoted to band gap engineering by chemical substitutions, regardless of the side effect on ferroelectric polarization. Here, we focus on how polar order affects the optical and photovoltaic properties. Using BiFeO₃as the model system, we induce the polarization rotation by A-site La substitution, which results in continuous reduction of optical anisotropy of the samples, as revealed by the concerted optical characterizations. This further causes the decrease of angular dependence of ferroelectric photovoltaic effect on the light polarization. The results demonstrate the inner connection of the ferroelectric polarization and optical anisotropy via the lattice degree of freedom.

Growth, Properties and Applications of Bi0.5Na0.5TiO3 Ferroelectric Nanomaterials

The emerging demands for miniaturization of electronics has driven the research into various nanomaterials. Lead-free Bi0.5Na0.5TiO3 (BNT) ferroelectric nanomaterials have drawn great interest owing to their superiorities of large remanent polarization, high pyroelectric and piezoelectric coefficients, unique photovoltaic performance and excellent dielectric properties. As attractive multifunctional ferroelectrics, BNT nanomaterials are widely utilized in various fields, such as energy harvest, energy storage, catalysis as well as sensing. The growing desire for precisely controlling the properties of BNT nanomaterials has led to significant advancements in material design and preparation approaches. BNT ferroelectric nanomaterials exhibit significant potential in fabrication of electronic devices and degradation of waste water, which pushes forward the advancement of the Internet of things and sustainable human development. This article presents an overview of research progresses of BNT ferroelectric nanomaterials, including growth, properties and applications. In addition, future prospects are discussed.

Strongly enhanced and tunable photovoltaic effect in ferroelectric-paraelectric superlattices

Ever since the first observation of a photovoltaic effect in ferroelectric BaTiO₃, studies have been devoted to analyze this effect, but only a few attempted to engineer an enhancement. In conjunction, the steep progress in thin-film fabrication has opened up a plethora of previously unexplored avenues to tune and enhance material properties via growth in the form of superlattices. In this work, we present a strategy wherein sandwiching a ferroelectric BaTiO₃ in between paraelectric SrTiO₃ and CaTiO₃ in a superlattice form results in a strong and tunable enhancement in photocurrent. Comparison with BaTiO₃ of similar thickness shows the photocurrent in the superlattice is 10³ times higher, despite a nearly two-thirds reduction in the volume of BaTiO₃ The enhancement can be tuned by the periodicity of the superlattice, and persists under 1.5 AM irradiation. Systematic investigations highlight the critical role of large dielectric permittivity and lowered bandgap.

Low-Temperature Induced Enhancement of Photoelectric Performance in Semiconducting Nanomaterials

The development of light-electricity conversion in nanomaterials has drawn intensive attention to the topic of achieving high efficiency and environmentally adaptive photoelectric technologies. Besides traditional improving methods, we noted that low-temperature cooling possesses advantages in applicability, stability and nondamaging characteristics. Because of the temperature-related physical properties of nanoscale materials, the working mechanism of cooling originates from intrinsic characteristics, such as crystal structure, carrier motion and carrier or trap density. Here, emerging advances in cooling-enhanced photoelectric performance are reviewed, including aspects of materials, performance and mechanisms. Finally, potential applications and existing issues are also summarized. These investigations on low-temperature cooling unveil it as an innovative strategy to further realize improvement to photoelectric conversion without damaging intrinsic components and foresee high-performance applications in extreme conditions.