Juanxiu Xiao

Ferroelectricity of CH3NH3PbI3 Perovskite.

Ferroelectricity has been believed to be an important but controversial origin of the excellent photovoltaic performance of organometal trihalide perovskites (OTPs). Here we investigate the ferroelectricity of a prototype OTP, CH3NH3PbI3 (MAPbI3), both theoretically and experimentally. Our first-principles calculations based on 3-D periodic boundary conditions reveal that a ferroelectric structure with polarization of ∼8 μC/cm(2) is the globally stable one among all possible tetragonal structures; however, experimentally no room-temperature ferroelectricity is observed by using polarization-electric field hysteresis measurements and piezoresponse force microscopy. The discrepancy between our theoretical and experimental results is attributed to the dynamic orientational disorder of MA(+) groups and the semiconducting nature of MAPbI3 at room temperature. Therefore, we conclude that MAPbI3 is not ferroelectric at room temperature; however, it is possible to induce and experimentally observe apparent ferroelectric behavior through our proposed ways. Our results clarify the controversy of the ferroelectricity in MAPbI3 and also provide valuable guidance for future studies on this active topic.

Enhancing the planar heterojunction perovskite solar cell performance through tuning the precursor ratio

Perovskite solar cells (PSCs) have attracted great attention due to their high power conversion efficiencies (PCEs) and low fabrication cost. The composition of the precursor solution determines the compositions of perovskite films. Excess precursor(s) may be used in the solution for the fabrication of perovskite films. However, it is still unclear how an excess precursor like PbI2 affects the structure and properties of the perovskite layer and the photovoltaic performance of PSCs. In this work, we investigated the effect of excess PbI2 that has a large bandgap on the electronic structure and properties of perovskite films and the photophysics and photovoltaic performance of PSCs. The presence of slightly excess PbI2 can affect the crystal structure and thus shift the Fermi level of perovskites. It can increase the open-circuit voltage (Voc) and thus the PCE of PSCs. However, the presence of a large amount of excess PbI2 is detrimental to the photovoltaic performance of PSCs. It can shorten the carrier lifetime, increase the resistance of the perovskite films, and decrease the fill factor (FF) and PCE of PSCs.

Stable ferroelectric perovskite structure with giant axial ratio and polarization in epitaxial BiFe0.6Ga0.4O3 thin films.

Ferroelectric perovskites with strongly elongated unit cells (c/a > 1.2) are of particular interest for realizing giant polarization induced by significant ionic off-center displacements. Here we show that epitaxial BiFe0.6Ga0.4O3 (BFGO) thin films exhibit a stable super-tetragonal-like structure with twinning domains regardless of film thickness and substrate induced strain, evidenced with high resolution X-ray diffractometry (HR-XRD), transmission electron microscopy (TEM) and piezoresponse force microscopy (PFM). The origin of the structural stability of BFGO is investigated by the first-principles calculation. The ferroelectric properties of BFGO are studied by PFM, first-principles calculation and macroscopic polarization-electric field (P-E) hysteresis measurement. A giant ferroelectric polarization of ∼150 μC/cm(2) is revealed by the first-principles calculations and confirmed by experiments. Our studies provide an alternative pathway of employing Ga-substitution other than the extensively studied strain engineering to stabilize the supertetragonal structure in BiFeO3-based epitaxial thin films.

Elucidating the charge carrier transport and extraction in planar heterojunction perovskite solar cells by Kelvin probe force microscopy

Perovskite solar cells have attracted much attention due to their high power conversion efficiency and low fabrication cost. The efficiency of the devices depends on the charge transport and charge extraction at the interface between the perovskite and electron or hole transport material. In this study, we use Kelvin probe force microscopy (KPFM) to investigate the charge carrier generation, transport, and extraction mechanism of several different structures with/without the hole/electron interlayers. It was found that the perovskite films exhibited unbalanced charge-carrier transport and extraction in the p-i-n type structure. Meanwhile, the leakage current under bias conditions can be suppressed by using a [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) capping layer. The time-resolved photoluminescence (TR-PL) results reveal that the electrons have a longer lifetime and longer diffusion length than the holes in the perovskite layer. These results suggest that the electron extraction is more efficient than the hole extraction in the planar heterojunction devices of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/perovskite/PCBM. These findings are beneficial for further optimizing the perovskite solar cells.

Ferroelectricity and ferroelectric resistive switching in sputtered Hf0.5Zr0.5O2 thin films

Ferroelectric properties and ferroelectric resistive switching (FE-RS) of sputtered Hf0.5Zr0.5O2 (HZO) thin films were investigated. The HZO films with the orthorhombic phase were obtained without capping or post-deposition annealing. Ferroelectricity was demonstrated by polarization-voltage (P-V) hysteresis loops measured in a positive-up negative-down manner and piezoresponse force microscopy. However, defects such as oxygen vacancies caused the films to become leaky. The observed ferroelectricity and semiconducting characteristics led to the FE-RS effect. The FE-RS effect may be explained by a polarization modulated trap-assisted tunneling model. Our study not only provides a facile route to develop ferroelectric HfO2-based thin films but also explores their potential applications in FE-RS memories.

Ferroelectricity emerging in strained (111)-textured ZrO2 thin films

(Anti-)ferroelectricity in complementary metal-oxide-semiconductor (CMOS)-compatible binary oxides have attracted considerable research interest recently. Here, we show that by using substrate-induced strain, the orthorhombic phase and the desired ferroelectricity could be achieved in ZrO2 thin films. Our theoretical analyses suggest that the strain imposed on the ZrO2 (111) film by the TiN/MgO (001) substrate would energetically favor the tetragonal (t) and orthorhombic (o) phases over the monoclinic (m) phase of ZrO2, and the compressive strain along certain ⟨11-2⟩ directions may further stabilize the o-phase. Experimentally ZrO2 thin films are sputtered onto the MgO (001) substrates buffered by epitaxial TiN layers. ZrO2 thin films exhibit t- and o-phases, which are highly (111)-textured and strained, as evidenced by X-ray diffraction and transmission electron microscopy. Both polarization-electric field (P-E) loops and corresponding current responses to voltage stimulations measured with appropriate a...

Ferroelectric polarization relaxation in Au/Cu2O/ZnO/BiFeO3/Pt heterostructure

The stability of polarization in ferroelectric BiFeO3 thin film stacked with a p-n junction of Cu2O/ZnO was studied in the Au/Cu2O/ZnO/BiFeO3/Pt heterostructure. It was observed that the downward ferroelectric polarization of BiFeO3 gradually relaxes once the external electric field is removed, which is driven by the depolarization effect induced by the reduction of compensating charges due to the charge redistribution within Cu2O/ZnO. This work contributes to an improved understanding on the polarization behavior in multilayer thin film structures comprising ferroelectrics and p-n junctions for guiding relevant device design and performance analysis.

Direct observation of room-temperature out-of-plane ferroelectricity and tunneling electroresistance at the two-dimensional limit

Out-of-plane ferroelectricity with a high transition temperature in nanometer-scale films is required to miniaturize electronic devices. Direct visualization of stable ferroelectric polarization and its switching behavior in atomically thick films is critical for achieving this goal. Here, ferroelectric order at room temperature in the two-dimensional limit is demonstrated in tetragonal BiFeO3 ultrathin films. Using aberration-corrected scanning transmission electron microscopy, we directly observed robust out-of-plane spontaneous polarization in one-unit-cell-thick BiFeO3 films. High-resolution piezoresponse force microscopy measurements show that the polarization is stable and switchable, whereas a tunneling electroresistance effect of up to 370% is achieved in BiFeO3 films. Based on first-principles calculations and Kelvin probe force microscopy measurements, we explain the mechanism of polarization stabilization by the ionic displacements in oxide electrode and the surface charges. Our results indicate that critical thickness for ferroelectricity in the BiFeO3 film is virtually absent, making it a promising candidate for high-density nonvolatile memories.High temperature perpendicular ferroelectricity in nano thin films is crucial for miniaturization of electronic devices. Here the authors show the presence of stable and switchable out-of-plane ferroelectricity in tetragonal BiFeO3 thin films at the two-dimensional limit and 370% tunneling electroresistance in ferroelectric tunnel junctions.

Room temperature ferroelectricity of hybrid organic–inorganic perovskites with mixed iodine and bromine

Ferroelectricity has been reported in organic–inorganic perovskites, and it can enhance the power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) because ferroelectricity can facilitate charge carrier separation and charge transport through the perovskite layer. However, the existence of ferroelectricity in perovskites has been in hot debate, particularly at room temperature. Here, we report the ferroelectric polarization switching of MAPb(I1−xBrx)3 which showed a high dependence on its composition. The ferroelectric behavior of MAPbI3-50% PbBr2 is confirmed with domain switching by Piezoelectric Force Microscopy (PFM) imaging and bias-off “butterfly-like” amplitude loops and piezoresponse hysteresis loops at room temperature. The possible factors, such as film electrical properties attributed to the enhanced room-temperature ferroelectricity in MAPb(I1−xBrx)3 films are clarified by using conductive atomic force microscopy (C-AFM). In addition, the charge separation and charge transport in the perovskite MAPb(I1−xBrx)3 films are further investigated by Kelvin probe force microscopy (KPFM). Finally, the possible influences of polarization orientations, trapping effects and ion migrations within the MAPb(I1−xBrx)3 films on the J–V characteristics of PSC devices are discussed in detail. It discovers that the polarization switching under the positive tip biased condition in the PSCs with 50% PbBr2, which could hinder the photovoltaic performance. These findings help better understand the electronic structure of hybrid organic–inorganic perovskites and provide guidance for the improvement of the PSC performance and other electronic applications of perovskites.

Tuning of current-induced effective magnetic field through Rashba effect engineering in hybrid multiferroic structures

Current-induced effective magnetic fields offer a new pathway through spin orbit interaction (SOI) to switch magnetization and have recently attracted great interest. In the conventional heavy metal/ferromagnetic metal/oxide (HM/FM/Oxide) structure, significant efforts have been made to study the role of the HM in determining effective magnetic fields. However, very little attention has been paid to the oxide layer and its interface with FM, where the Rashba effect may affect the effective field. In this report, we present a pathway to tune the effective magnetic field by engineering the Rashba effect in a hybrid multiferroic multilayer structure. A ferroelectric oxide of BaTiO3, whose polarizations either up or down are controlled by interface engineering, was introduced into the conventional SOI multilayer with the structure of BaTiO3/CoFeB/Pt. The current-induced effective magnetic fields increase by more than 200% when the ferroelectric polarization of BaTiO3 changes from up to down. The changes in the effective magnetic field are mainly attributed to the different Rashba effective fields induced by the opposite ferroelectric polarizations. Our study offers a new path towards controlling the current-induced effective magnetic field and may pave the way for integrating other functional oxides into the spintronic devices.Magnetic materials: oriented toward lower power switchingCombining magnetic materials with oxides offers a route to low-power memories, according to research from scientists in Singapore. Locally switching the magnetization orientation of a permanent magnet is a way of storing binary data. The energy consumption of these magnetic memories is much lower if a local current, rather than an external magnetic field, induces the switch. Current-induced magnetization switching has previously been demonstrated in devices made by layering a heavy metal, a ferromagnetic material, and an oxide material. While many conventional oxide materials have been tried, Jingsheng Chen and Hyunsoo Yang and their colleagues from the National University of Singapore instead used BaTiO3, a ferroelectric oxide which has permanent internal electric fields. They showed that they could increase the current-induced magnetic fields by more than 200% by altering the ferroelectric polarization of the BaTiO3.Over 200% difference of the current-induced effective magnetic field was achieved through engineering the Rashba effect in a hybrid-multiferroic multilayer structure (BaTiO3/CoFeB/Pt), where the polarization of BaTiO3 with either up or down was controlled through interface engineering. Our works offer a new direction towards controlling the current-induced effective magnetic field and may pave the way to integrate other functional oxides into the spintronic devices.