Positive Poling Research Articles

Oxygen Vacancy: How Will Poling History Affect Its Role in Photoelectrocatalysis.

Oxygen vacancy (VO) has been recognized to possess an effect to promote the charge separation and transfer (CST) in various n-type semiconductor based photoelectrodes. But how external stimulus will change this VO effect has not been investigated. In this work, external polarization is applied to investigate the effect of VO on the CST process of a typical ferroelectric BiFeO3 photoelectrode. It is found that negative poling treatment can significantly boost VO effect, while positive poling treatment will deteriorate the CST capability in BiFeO3 photoelectrodes. This poling history determined VO effect is rooted in the VO induced defect dipoles, wherein their alignment produces a depolarization electric field to modulate the CST driving force. This finding highlights the significance of poling history in functionalizing the VO in a photoelectrode.

Ferroelectric passivation layer derived high performance AlGaN/GaN heterojunction field-effect transistor

The density of interface states is strongly related to the performance of AlGaN/GaN high-electron-mobility transistors (HEMTs) and is normally attributed to the degradation of the carrier mobility and gate leakage current. The density of interface states is optimized in conventional AlGaN/GaN HEMTs through the use of different passivation layers. However, different passivation layers may create more complex interface structures. In our previous work, ferroelectric polarization was used to regulate the carrier concentration in AlGaN/GaN HEMTs. Herein, we propose a ferroelectric passivation-layer-induced pure field effect modulation within the AlGaN/GaN heterojunction field-effect transistors. After positive poling, the interface trap density (Dit) decreases by 71% and current collapse is reduced. The output current (IDS) increases from 408 to 462 mA/mm and transconductance (gm) increases from 88 to 149 mS/mm. Simultaneously, the carrier mobility in the channel is also greatly improved after positive poling. When negative poling is applied, the gate leakage decreases and the breakdown voltage of the device increases by 55%. Our work provides a simple and effective way to study the density of interface states in GaN device design and optimization.

Reversible charge injection-controlled resistance switching in BiFeO3 ferrodiodes

The ferroelectric diode effect is a promising candidate for resistive memory applications, but the precise role of defects in the current switching mechanism remains unclear. Here, we investigated ferroelectric SrRuO3/BiFeO3/SrRuO3 capacitors and observed strong diode current. The capacitors exhibited preferred polarization orientation toward the bottom electrode in the presence of an imprint field, as evidenced by poor polarization retention of upward polarizations at a bias voltage of 1 V. Interfacial defect-mediated charge injection and trapping enabled by negative voltage poling reduced the built-in field and improved the retention property at the expense of reduced diode current. This phenomenon can be reversed by long-time positive voltage poling, allowing the deeply trapped charges to be expelled out of the trap for the rejuvenation of the diode current. Our study provides experimental evidence that interfacial defects modify the diode current in a manner opposite to that of the switched polarization.

Modulating effect of poling on performance of solar cell based on novel narrow bandgap molecular ferroelectric semiconductor hexane-1,6-diammoniumpentaiodobismuth

In ferroelectric-semiconductor solar cells, the depolarization field formed by the ferroelectric material after poling can constitute a synergistic effect with the built-in electric field of the P–N junction, which can enhance the carrier separation and transport, thus improving the photovoltaic performance, attracting extensive research interest. Hexane-1,6-diammonium pentaiodobismuth (HDA-BiI5) is a narrow bandgap molecular ferroelectric, and solar cells based on it with mesopores and conventional structure have been prepared, but no poling modulation of photovoltaic performance has been studied. In this paper, the effects of poling on the performance of HDA-BiI5 based solar cells with conventional structure are characterized for further studies of HDA-BiI5 based solar cells. The performance can be modulated by poling, improved after positive poling and significantly degraded after negative poling, due to another driving force for charge carrier separation, depolarization filed.

Textured Bi4Ti3O12 Ceramics: One‐Step Spark Plasma Sintering and Their Single‐Crystal‐Like Polar Anisotropy

Texturing is the most effective way to engineer the performance of bismuth layer–structured ferroelectric (BLSF) ceramics by utilizing the strong anisotropy of grains, but the existing processing approaches are still complex. Herein, a one‐step spark plasma sintering (SPS) approach is demonstrated to prepare textured Bi4Ti3O12 (BiT) ceramics with high performance. The degree of orientation can be controlled from 57% to 95% by only adjusting the sintering temperature from 840 to 1000 °C. The largest spontaneous polarization (Ps) of ≈32.2 μC cm−2 is obtained in 95% <001> textured ceramics along the direction perpendicular to the SPS pressure, accompanied with the maximum electro‐strain of ≈0.14% and piezoelectric coefficient of 14.3 pC N−1. Moreover, under the same conditions of poling and illumination, the textured ceramics exhibit unique anisotropic photoresponse in which the photocurrent polarity is dependent on the testing orientations. In particular, a short‐circuit photocurrent density of 8.76 nA cm−2 is achieved along the direction of SPS pressure after positive poling, while the value goes to −79.4 nA cm−2 along direction perpendicular to the SPS pressure. These results are expected to provide a paradigm to achieve highly textured BLSF ceramics for electric and photoelectric applications.

Effect of ferroelectric poling on the photoelectrochemical activity of hematite-BaTiO3 nanowire arrays

Ferroelectric α-Fe2O3/BaTiO3 photoanodes (hematite/BT) were fabricated on FTO and FTO/TiO2 substrates using a hydrothermal process and spin coating along with thermal treatments. The prepared hematite nanowires had length under 1 μm and the BT film was about 18 nm thick. SEM, TEM and XPS investigations prove the formation of α-Fe2O3/BaTiO3 heterojunction structure. The ferroelectric poling of hematite/BT heterojunction was conducted both in propylene carbonate and in air. The photoelectrochemical performance of hematite/BT photoanodes is strongly influenced by the direction of ferroelectric polarization. The positive poling of the hematite/BT prepared on FTO/TiO2 substrate produces a 40.4% photocurrent density enhancement, in comparison with not poled version of the sample. Electrochemical impedance spectroscopy measurements provided usefull information regarding the effect of ferroelectric polarization on the charge transfer kinetics at the photoanode/electrolyte interface.

Electric polarization tune enhanced photoelectrochemical performance of visible light active ferroelectric Bi0.5Na0.5TiO3 nanostructure photoanode

Significantly enhanced photoelectrochemical (PEC) performance of the visible light active photoanodes of ferroelectric Bi0.5Na0.5TiO3 nanostructure has been demonstrated by tuning the electrical polarization of the film. The photocurrent density of unpoled Bi0.5Na0.5TiO3 photoanode increases from 0.48 mA cm−2 at 1 V with respect to Ag/AgCl to 1.21 mA cm−2 after negative poling, whereas it decreases to 0.18 mA cm−2 after positive poling. The incident photon-to-current conversion efficiency (IPCE) of the nanostructure photoanodes has been observed to vary from 5% to 28% just by changing the direction of electric polarization. The change in the PEC performance and enhancement in the IPCE value after switching the polarization state of Bi0.5Na0.5TiO3 photoanodes have been attributed to the modification in the band bending at the interface of semiconductor electrode/electrolyte, leading to facilitate the increment in photogenerated charge transfer efficiency. The modulation in the band bending and variation in the charge transfer method after switching the polarization state are confirmed by Mott-Schottky (M-S) and electrochemical impedance spectroscopy (EIS) measurements. Our work provides a new visible light active ferroelectric nanostructure material for efficient visible light active photoelectrochemical performance.

Linear electro-optic effect in ferroelectric HfO2-based epitaxial thin films

Electro-optic (EO) modulators for silicon photonics using CMOS-compatible materials and processes are in great demand. In this study, epitaxial (100)-undoped HfO2 and Y-doped HfO2 thin films were fabricated on Sn-doped In2O3/yttria-stabilized zirconia(100) substrates at room temperature via magnetron sputtering. EO measurement of the Y-HfO2 film using modulation ellipsometry showed that the phase was changed by 180° after application of positive and negative poling biases, and the modulation amplitude increased linearly with increasing AC electric field, indicating a linear EO effect based on ferroelectricity. The observed results indicate that ferroelectric HfO2-based films are viable candidates for CMOS-compatible EO devices.

Numerical modelling of ion-migration caused hysteresis in perovskite solar cells

We presented one dimensional defect model to simulate hysteresis in perovskite solar cells. It can be numerically simulated in perovskite devices with p+-i-n+ configuration by considering ions migration under influence of different voltage biasing. The availability of mobile ion which contribute towards ion migration, ion accumulation at interfaces and near interface regions are observed to be generating I–V hysteresis in simulations. To simulate ion migration under positive poling we have taken a defect model and reverse the defect polarity for simulating ion migration under negative poling. The hysteresis is observed when the device has either of the condition (a) defects at the interfacial traps at the junction, or (b) defects in p+ or n+ layer in vicinity of the interface region. Both the conditions have yielded I–V hysteresis when we switched polarity of defect to simulate positive and negative poling. This study brings out the prominent role of mobile ions and carrier dynamics in controlling device working. Band diagram and capacitance variation visibly showed the impacts of charge reversal and capacitance simulations of perovskite device. 1D modelling shows that the ionic migration is one of the responsible mechanisms behind the I–V hysteresis in the perovskite solar cell devices

Depolarization electric field and poling voltage‐modulated Pb,La(Zr,Ti)O3‐based self‐powered ultraviolet photodetectors

AbstractTraditional self‐powered ultraviolet photodetectors, which are usually designed based on p‐n junction interfacial effects, exhibit low responsivity and specific detectivity because the photogenerated electrons and holes cannot be separated effectively. Unlike wide band‐gap semiconductor materials, ferroelectrics have large remnant polarization and thus high depolarization electric field throughout the whole bulk region, which can cause effective separation of photogenerated electrons and holes. Based on this, in this study, we prepare Pb0.93La0.07(Zr1‐xTix)0.9825O3 (PLZT) ferroelectric thin films with large remnant polarization and self‐powered ultraviolet photodetectors with Au/PLZT/FTO structure. The results indicate that the photoelectric response performances of the detectors improve as the remnant polarization of the PLZT thin film and positive poling voltage increase. By adjusting the Ti content, due to large remnant polarization of 47.4 μC/cm2 in the PLZT thin films with 80 mol% Ti, the corresponding photodetector exhibits the best self‐powered ultraviolet photoelectric response with the high photo/dark current ratio of 2600, responsivity of 2.05 mA/W, specific detectivity of 5.45 × 1010 Jones, and fast response speed (rise time of 18 ms). These values are superior to those of recently reported self‐powered ultraviolet photodetectors.

Observation of relaxor-ferroelectric behavior in gallium ferrite thin films

We report a detailed investigation on magnetic and electrical properties of the multiferroic GaFeO3 (GFO) thin films grown on SrRuO3 (SRO) buffered SrTiO3 (1 1 1) substrates. The magnetic transition temperature (Tc) of GFO thin films is observed ∼240 K, which is higher by 20 K than in GFO ceramic. The clear and reversible out-of-plane phase-contrast seen in the Piezoresponse Force Microscopy (PFM) phase and amplitude images measured within positive and negative poling confirms the polarization reorientation and hence piezoelectric nature of the compound. The temperature and frequency dependence of permittivity studies demonstrates the relaxor behavior of these thin films. The observed near room temperature (RT) magnetic phase transition with RT piezoelectric nature of these thin films elucidates the possible potential candidates for a multiferroic world with spintronics device applications.

Improved Photovoltaic Effect of p-i-n Structured BiFeO₃ Film Deposited by Radio-Frequency Magnetron Sputtering.

Pure phase polycrystalline BiFeO₃ film was deposited onto FTO substrate by RF magnetron sputtering method. SEM result shows that BiFeO₃ film has the obvious porosity and large clusters which lead to the poor ferroelectric and photovoltaic properties in FTO/BiFeO₃/Ag device. However, these properties are improved in p-i-n structured FTO/TiO₂/BiFeO₃/HTM/Ag device by incorporating the electron and hole transport materials. The hysteresis loop measurement demonstrates the excellent ferroelectric property with large remnant polarization (2Pr = 180 μC/cm²) and low leakage current. The J-V curve shows the short-circuit current density is dozens of times larger than that of FTO/BiFeO₃/Ag device. Moreover, the photovoltaic output depends on the poling field where the positive poling improves the short-circuit current density to -85 μA/cm₂ and the negative poling reduces both the photocurrent and photovoltage. It is believed that the ferroelectric polarization plays a dominant role in the photovoltaic effect.

Superior ferroelectric photovoltaic properties in Fe -modified (Pb,La) (Zr,Ti)O3 thin film by improving the remnant polarization and reducing the band gap

In order to develop ferroelectric photovoltaic devices with high power conversion efficiency, ferroelectric materials must have simultaneously large remnant polarization and narrow band gap so as to efficiently separate photo-generated carriers and absorb more sunlight. Based on this idea, in this report, we introduce Fe3+ into Pb0·93La0·07(Zr0·6Ti0.4)0.9825O3 ferroelectric thin film to increase the remnant polarization and decrease the band gap of the thin film. In doing so, we prepare Fe3+ doping Pb0·93La0·07(Zr0·6Ti0.4)0.9825O3 thin-film based photovoltaic devices. The experimental results indicate that with increasing the Fe3+ amount, the remnant polarization of the film first improves to the maximum value of 50 μC/cm2 at the 4.8 mol% Fe3+ content and then reduces gradually, while the band gap continuously decreases. In addition, at a negative poling voltage, the device exhibits larger short-circuit current and open-circuit voltage in comparison with those obtained at the positive poling voltage, which is attributed to the depolarization electric field originating from the remnant polarization of ferroelectric thin films in the same direction as the built-in electric field caused by the Schottky barrier. In this report, the most superior photovoltaic performances with the open-circuit voltage of as large as −0.55 V and short-circuit current of as high as 0.4 μA/cm2 are obtained in the device with 4.8 mol% Fe3+ amount and at −5 V poling voltage. This is on account of the improved sunlight absorbing properties and photo-generated carriers separation ability of the device. This work provides a novel idea for designing and preparing ferroelectric photovoltaic devices with high power conversion efficiency.

Impact of Ferroelectric Polarization on Different Semiconductors for Photoelectrochemical Application

Introducing a built-in electric field into photoelectrochemical (PEC) system is considered as an effective strategy to tune interfacial energy band bending and manipulate charge transport direction. Here, we design the hybrid ferroelectric-semiconductor structure photoanodes for water oxidation, and the BaTiO3 is used for ferroelectric while monoclinic WO3, tetragonal TiO2, and triclinic CuWO4 for the semiconductors respectively. After the positive poling, 3 wt% BTO-WO3 and 3 wt% BTO-CuWO4 hybrid photoanodes perform significantly enhanced photocurrent of 68.5% and 62.9% compare with the pristine semiconductors. The efficiencies of charge separation and oxidation kinetics of these hybrid photoanodes increase by 14-19% and 1.38-1.42 fold, and the electron transport time decreases by 16%. Further, Kelvin probe force microscopy measurements indicate that the surface potential of hybrid structure is lower than unmodified samples, suggesting the space charge region near the semiconductor surface has been change...

Effects of the film thickness and poling electric field on photovoltaic performances of (Pb,La)(Zr,Ti)O3 ferroelectric thin film-based devices

The ferroelectric photovoltaic (FPV) effect obtained in inorganic perovskite ferroelectric materials has received much attention because of its large potential in preparing FPV devices with superior stability, high open-circuit voltage (Voc) and large short-circuit current density (Jsc). In order to obtain suitable thickness for the ferroelectric thin film as light absorption layer, in which, the sunlight can be fully absorbed and the photo-generated electrons and holes are recombined as few as possible, we prepare Pb0.93La0.07(Zr0.6Ti0.4)0.9825O3 (PLZT) ferroelectric thin films with different layer numbers by the sol-gel method and based on these thin films, obtain FPV devices with FTO/PLZT/Au structure. By measuring photovoltaic properties, it is found that the device with 4 layer-PLZT thin film (~300 nm thickness) exhibits the largest Voc and Jsc and the photovoltaic effect obviously depends on the value and direction of the poling electric field. When the device is applied a negative poling electric field, both the Voc and Jsc are significantly higher than those of the device applied the positive poling electric field, due to the depolarization field resulting from the remnant polarization in the same direction with the built-in electric field induced by the Schottky barrier, and the higher the negative poling electric field, the larger the Voc and Jsc. At a -333 kV/cm poling electric field, the FPV device exhibits the most superior photovoltaic properties with a Voc of as high as 0.73 V and Jsc of as large as 2.11 μA/cm2. This work opens a new way for developing ferroelectric photovoltaic devices with good properties.

Ferroelectricity of the Orthorhombic and Tetragonal MAPbBr3 Single Crystal.

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

Plasmonic enhanced Cu2O-Au-BFO photocathodes for solar hydrogen production

A novel Cu2O-Au-BFO heterostructure photocathode was constructed which significantly improved the efficiency of photo-generated carrier transfer for solar hydrogen production. A BiFeO3 (BFO) ferroelectric film was synthesized on top of a Cu2O layer by a sputtering process. The BFO layer acted to protect the Cu2O layer from photochemical corrosion, increasing photoelectrochemical (PEC) stability. The p–n heterojunction between Cu2O and BFO layers enhanced the PEC properties by suppressing charge recombination and improved interfacial charge transfer efficiency. When Cu2O and BFO are interfaced by Au Nanoparticles (NPs) the PEC performance was further enhanced, due to hot-electron transfer at the plasmonic resonance. After positive poling, the depolarization field across the whole volume of BFO film drove electrons into the electrolyte solution, inducing a significant anodic shift, Vop of 1.01 V vs. RHE, together with a significantly enhanced photocurrent density of −91 μA/cm2 at 0 V vs. RHE under 100 mW/cm2 illumination. The mechanism was investigated through experimental and theoretivcal calculations.

Retention characteristics of a nonvolatile latch utilizing a ferroelectric transistor

This paper presents retention characteristics and design recommendations for a nonvolatile latch circuit based on a ferroelectric field-effect transistor. In the design, the use of a ferroelectric transistor in a resistive load inverter allows tuning of the circuit for each ferroelectric transistor size. An estimate of the retention time, due to the unique polarization properties of the ferroelectric layer at the gate, is provided for each transistor size. Data collected for three transistor sizes display the possibility of retaining a stored value beyond a 10-year period, with various size transistors indicating 13–38-year retention periods. Switching the ferroelectric layer polarization between two distinct states by applying either negative or positive poling voltages to the gate of the ferroelectric transistor is explored. Experimental results are presented for each transistor, and detailed retention predictions are shown by measured and extrapolated data. Design parameter recommendations are made for optimization of the retention time of the circuit depending on the transistor size.

Polarization imprint effects on the photovoltaic effect in Pb(Zr,Ti)O3 thin films

The polarization imprint along with the photovoltaic (PV) effect has been studied in Pt/Pb(Zr0.3Ti0.7)O3/SrRuO3 ferroelectric capacitors. It is shown that the positive DC poling induces the imprint with a downward direction whereas the negative DC poling suppresses the imprint (i.e., rejuvenation). In the polarization up state, the imprinted capacitor exhibits degraded PV properties compared with the rejuvenated one. This may be because the imprint reduces the number of upward domains, thus lowering the driving force for the PV effect. In the polarization down state, however, the rejuvenated capacitor enters the imprinted state spontaneously. This rejuvenation-to-imprint transition can be further aggravated by applying positive voltages and ultraviolet illumination. It is proposed that the domain pinning/depinning, which are associated with the oxygen vacancies and trapped electrons modulated by polarization, voltage, and illumination, may be responsible for the polarization imprint and rejuvenation. Our study therefore sheds light on the correlation between the polarization imprint and the PV effect in the ferroelectrics and also provides some viable suggestions to address the imprint-induced degradation of PV performance.

Conjugated Polyelectrolytes Bearing Various Ion Densities: Spontaneous Dipole Generation, Poling-Induced Dipole Alignment, and Interfacial Energy Barrier Control for Optoelectronic Device Applications.

Conjugated polyelectrolytes (CPEs) with π-delocalized main backbones and ionic pendant groups are intensively studied as interfacial layers for efficient polymer-based optoelectronic devices (POEDs) because they facilitate facile control of charge injection/extraction barriers. Here, a simple and effective method of performing precise interfacial energy level adjustment is presented by employing CPEs with different thicknesses and various ion densities under electric poling to realize efficient charge injection/extraction of POEDs. The effects of the CPE ion densities and electric (positive or negative) poling on the energy level tuning process are investigated by measuring the open-circuit voltages and current densities of devices with the structure indium tin oxide/zinc oxide/CPE/organic active layer/molybdenum oxide/gold while changing the CPE film thickness. The performances of inverted polymer light-emitting diodes and inverted polymer solar cells are remarkably improved by precisely controlling the interfacial energy level matching using optimum CPE conditions.