We report on nanoscale strain gradients in ferroelectric HoMnO3 epitaxial thin films, resulting in a giant flexoelectric effect. Using grazing-incidence in-plane x-ray diffraction, we measured strain gradients in the films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. The combination of transmission electron microscopy, electrical measurements, and electrostatic calculations showed that flexoelectricity provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.

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Giant Flexoelectric Effect in Ferroelectric Epitaxial Thin Films

The application of ferroelectric materials (i.e. solids that exhibit spontaneous electric polarisation) in solar cells has a long and controversial history. This includes the first observations of the anomalous photovoltaic effect (APE) and the bulk photovoltaic effect (BPE). The recent successful application of inorganic and hybrid perovskite structured materials (e.g. BiFeO3, CsSnI3, CH3NH3PbI3) in solar cells emphasises that polar semiconductors can be used in conventional photovoltaic architectures. We review developments in this field, with a particular emphasis on the materials known to display the APE/BPE (e.g. ZnS, CdTe, SbSI), and the theoretical explanation. Critical analysis is complemented with first-principles calculation of the underlying electronic structure. In addition to discussing the implications of a ferroelectric absorber layer, and the solid state theory of polarisation (Berry phase analysis), design principles and opportunities for high-efficiency ferroelectric photovoltaics are presented.

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Ferroelectric Materials for Solar Energy Conversion: Photoferroics Revisited

Although there has been significant progress in the fabrication and performance optimization of one-dimensional nanostructure-based photodetectors, it is still a challenge to develop an effective and low-cost device with high performance characteristics, such as a high photocurrent/ dark-current ratio, photocurrent stability, and fast time response. Herein an efficient and low-cost method to achieve high-performance 'visible-blind' microscale ZnS nanobelt-based ultraviolet (UV)-light sensors without using a lithography technique, by increasing the nanobelt surface areas exposed to light, is reported. The devices exhibit about 750 times enhancement of a photocurrent compared with individual nanobelt-based sensors and an ultrafast time response. The photocurrent stability and time response to UV-light do not change significantly when a channel distance is altered from 2 to 100 μm or the sensor environment changes from air to vacuum and different measurement temperatures (60 and 150°C). The photoelectrical behaviors can be recovered well after returning the measurement conditions to air and room temperature again. The low cost and high performance of the resultant ZnS nanobelt photodetectors guarantee their highest potential for visible-blind UV-light sensors working in the UV-A band.

An Efficient Way to Assemble ZnS Nanobelts as Ultraviolet-Light Sensors with Enhanced Photocurrent and Stability

Organic–inorganic hybrid perovskites have been intensively researched in the past decade for their optoelectronic properties. The emergence of Ruddlesden–Popper perovskites, which have mixed dimensionality, has heralded new opportunities for tailor-made semiconductors that combine enhanced stability with useful properties between those of 2D and 3D systems. Inspired by advances in 2D materials research, there is growing interest in molecularly thin versions of these hybrid perovskites, owing to their ease of incorporation into electronic devices. There is, thus, a need to understand thickness-dependent electrical, excitonic and phononic properties that go beyond quantum-confinement effects. Recent studies have shown that, apart from tuning the dimensionality of the system, fine-tuning its thickness also helps to optimize performance in different applications, ranging from third-harmonic generation to photodetectors and spintronics. Owing to their layered structure, the properties of 2D perovskites can be controlled by tuning their thickness. This Review surveys how fine-tuning the thickness of 2D perovskites from the sub-micrometre to the molecularly thin regime helps to optimize their electrical and optical properties for use in different applications.

From bulk to molecularly thin hybrid perovskites

Multiaxial molecular ferroelectrics, in which multiple-directional switching of spontaneous polarization creates diverse properties, have shown many intriguing advantages, making them indispensable complements to conventional inorganic oxides. Despite recent blooming advances, multiaxial molecular ferroelectric with bulk photovoltaic effects still remains a huge blank. Herein, we report a biaxial lead halide ferroelectric, EA4Pb3Br10 (1, EA = ethylammonium), which adopts the unique trilayered perovskite motif with a high Curie temperature of ∼384 K. Particularly, for 1, the distinct symmetry breaking with 4/ mmmF mm2 species leads to the emergence of four equivalent polarization directions in the ferroelectric phase. Based on its biaxial nature, the bulk photovoltaic effect of 1 can be facilely tuned between such multiple directions through electric poling. As far as we know, this is the first report on biaxial hybrid perovskite ferroelectric showing directionally tunable photovoltaic activity. This work provides an avenue to control the bulk physical properties of multiaxial molecular ferroelectrics, and highlights their potential for further applications in the field of smart devices.

An Unprecedented Biaxial Trilayered Hybrid Perovskite Ferroelectric with Directionally Tunable Photovoltaic Effects.

Ferroelectrics are considered next generation photovoltaic (PV) materials. In this work, a switchable and large PV effect is demonstrated in a Pb-free ferroelectric 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-BCT) thin film fabricated by a pulsed laser deposition technique. The material shows a remarkable PV output of 0.81 V due to its morphotropic phase boundary composition. The observed PV effect is analyzed on the basis of the interfacial Schottky barrier and bulk depolarization field. The poling dependent PV studies revealed that although the Schottky and depolarization field contribute to the PV effect, the latter dominates the PV response beyond the coercive field. Additionally, the importance of this compound in the field of a self-biased photodetector is elucidated in terms of calculated photodetector parameters such as responsivity and detectivity. The explored results will bring significant advancement in the field of ferroelectric PV, UV solid state detector applications and also give an additional dimension to the multifunctional ability of the BZT-BCT system.

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Polarization controlled photovoltaic and self-powered photodetector characteristics in Pb-free ferroelectric thin film

Recently the solar energy, an inevitable part of green energy source, has become a mandatory topics in frontier research areas. In this respect, non-centrosymmetric ferroelectric perovskites with open circuit voltage (VOC) higher than the bandgap, gain tremendous importance as next generation photovoltaic materials. Here a non-toxic co-doped Ba1−x(Bi0.5Li0.5)
 x
 TiO3 ferroelectric system is designed where the dopants influence the band topology in order to enhance the photovoltaic effect. In particular, at the optimal doping concentration (x
 opt
  ~ 0.125) the sample reveals a remarkably high photogenerated field EOC = 320 V/cm (VOC = 16 V), highest ever reported in any bulk polycrystalline non-centrosymmetric systems. The band structure, examined through DFT calculations, suggests that the shift current mechanism is key to explain the large enhancement in photovoltaic effect in this family.

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Giant photovoltaic response in band engineered ferroelectric perovskite

The magnetoelectric (ME) effect in composites, a strain-mediated coupling phenomenon between piezoelectric and magnetostrictive phases, has a wide range of technological applications. Here, ME coupling phenomena are explored in $\mathrm{Pb}$-free piezoelectric, $0.5\mathrm{Ba}({\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}){\mathrm{O}}_{3}\text{\ensuremath{-}}0.5({\mathrm{Ba}}_{0.7}{\mathrm{Ca}}_{0.3}){\mathrm{Ti}\mathrm{O}}_{3}$ and piezomagnetic ${\mathrm{Ni}\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ bilayer laminate composites. The direct and converse ME coupling strengths are found to be enhanced at the electromechanical resonance modes, rather than at the off-resonance frequencies. Here, it is proposed to further enhance the ME coupling strength at electromechanical resonance modes by the in-phase superimposition of the radial and second bending modes via varying the bilayer thickness, which, in turn, varies the volume fraction of the bilayer. The proposed enhanced ME coupling is experimentally demonstrated at a theoretically envisaged bilayer thickness of about 1.8 mm. This results in a large direct ME coupling coefficient of $22.5\phantom{\rule{0.1em}{0ex}}\mathrm{V}\phantom{\rule{0.1em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.1em}{0ex}}\mathrm{O}{\mathrm{e}}^{\ensuremath{-}1}$, which is around 100% more than the values observed at individual resonance modes. The results are further validated by calculations from theoretical models. The method adopted in this work gives a roadmap to the significant enhancement of the ME effect in laminate composites.

Engineering Resonance Modes for Enhanced Magnetoelectric Coupling in Bilayer Laminate Composites for Energy Harvesting Applications

The photovoltaic effect seen in ferroelectric oxides is gaining interest for next-generation solar cells, as these materials do not suffer from the same issues regarding carrier recombination and band gap as do the commonly employed semiconductors. The authors demonstrate a large photovoltaic response in a Pb-free mixed titanate-zirconate system by electrically tuning the material's symmetry-breaking structural transformation, and correlate the effect to the preferred structural symmetry via band-structure calculations. These results give welcome insight for engineering ferroelectric oxides for photovoltaic applications.

Large Bulk Photovoltaic Response by Symmetry-Breaking Structural Transformation in Ferroelectric [ Ba ( Zr 0.2 Ti 0.8 ) O 3 ] 0.5 [( Ba 0.7 Ca 0.3 ) Ti O 3 ] 0.5

Atal Bihari Swain

Papers

Polarization controlled photovoltaic and self-powered photodetector characteristics in Pb-free ferroelectric thin film

Giant photovoltaic response in band engineered ferroelectric perovskite

Engineering Resonance Modes for Enhanced Magnetoelectric Coupling in Bilayer Laminate Composites for Energy Harvesting Applications

Large Bulk Photovoltaic Response by Symmetry-Breaking Structural Transformation in Ferroelectric [ Ba ( Zr 0.2 Ti 0.8 ) O 3 ] 0.5 [( Ba 0.7 Ca 0.3 ) Ti O 3 ] 0.5

The role of precursors on piezoelectric and ferroelectric characteristics of 0.5BCT-0.5BZT ceramic