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

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.

[Yesterday, today and tomorrow].

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

https://iopscience.iop.org/article/10.1088/1361-6463/ab83c3/pdf

Polarization driven self-biased and enhanced UV–visible photodetector characteristics of ferroelectric thin film

The photovoltaic (PV) properties of polycrystalline Bi1−xLaxFeO3 (x = 0–0.3) films have been explored. The X-ray diffraction study reveals that there is a gradual phase transition with the increase in La doping. The composition x = 0.25 is found to be the morphotropic phase boundary (MPB), beyond which the system turns into the non-/antipolar orthorhombic phase. The polarization measurements reveal improved ferroelectric properties with the maximum remanent polarization observed for the x = 0.25 film. A systematic study on the direct and indirect bandgaps of the films has shown a decreasing trend with composition. Interestingly, the PV studies exhibit a maximum open-circuit voltage of 1.30 V for the x = 0.25 film which is three times larger than the value observed for pure BiFeO3 (BFO) (0.47 V). The enhanced PV response in La-doped BFO correlates with the polarization and the change in direct/indirect bandgaps associated with structural instability near the MPB composition. The approach used in this work for enhancing the PV performance in ferroelectric BFO through the combined effects of polarization, bandgaps and competing structures provides a better pathway for improving the ferroelectric PV effect.The photovoltaic (PV) properties of polycrystalline Bi1−xLaxFeO3 (x = 0–0.3) films have been explored. The X-ray diffraction study reveals that there is a gradual phase transition with the increase in La doping. The composition x = 0.25 is found to be the morphotropic phase boundary (MPB), beyond which the system turns into the non-/antipolar orthorhombic phase. The polarization measurements reveal improved ferroelectric properties with the maximum remanent polarization observed for the x = 0.25 film. A systematic study on the direct and indirect bandgaps of the films has shown a decreasing trend with composition. Interestingly, the PV studies exhibit a maximum open-circuit voltage of 1.30 V for the x = 0.25 film which is three times larger than the value observed for pure BiFeO3 (BFO) (0.47 V). The enhanced PV response in La-doped BFO correlates with the polarization and the change in direct/indirect bandgaps associated with structural instability near the MPB composition. The approach used in this work ...

Large photovoltaic response in rare-earth doped BiFeO3 polycrystalline thin films near morphotropic phase boundary composition

Polycrystalline BaTi1-xSnxO3 samples (x = 0.06, 0.07, 0.08, 0.09, 0.10, and 0.11) were synthesized by the solid state technique. The samples exhibit the tetragonal phase at 300 K. In addition, the samples x = 0.06, 0.07, 0.08, and 0.09 also show the orthorhombic phase with enhanced phase fractions upon poling. However, the % orthorhombic phase fractions show an increase up to x = 0.07 and a decrease with an increase in x. The dielectric studies indicate that TC (cubic to tetragonal phase transition) shifts toward lower temperature where the samples x = 0.10 and 0.11 show the tetragonal phase at 300 K. The samples exhibit the maximum remnant polarization and piezoelectric coefficient for x = 0.08. But the bandgap for the x = 0.07 sample shows the value of 2.61 eV before poling and 2.95 eV after poling. A giant photovoltaic (PV) response is seen in the samples with the open-circuit voltage (VOC) as large as 16 V (for x = 0.07). VOC shows a decreasing trend with an increase in the Sn content after x = 0.07, and it did not follow the trend in polarization and the bandgap. The observed results are correlated with the structural symmetry of the compound, and they are validated by the band-structure calculations. The experimental and theoretical studies indicate that the sample with the orthorhombic phase is preferable for the enhanced photovoltaic response in comparison to the tetragonal phase. These studies show a new way to achieve a large photovoltaic response so as to design the system for several device applications such as UV detectors and microactuators.

Enhanced bulk photovoltaic response in Sn doped BaTiO3 through composition dependent structural transformation

The bulk photovoltaic (PV) effect exhibited by non-centrosymmetric systems gained research interest due to the observed large open-circuit voltage. Ferroelectric systems exhibiting anomalous photovoltaic effects are mostly crystallized with multi-phase coexistence. Hence, the computational difficulty in building a multi-phase system restricts the detailed photovoltaic studies through phenomenological and shift current theory. In this work, ferroelectric Ba1−x(Bi0.5K0.5)xTiO3 (BBKT) oxide is designed to crystallize in single-phase tetragonal symmetry with improved polarization characteristics, and it is found to exhibit large PV response. Both experimental and theoretical studies on BBKT samples reveal ∼18% reduction in bandgap compared to the parent BaTiO3. Short-circuit current measured as a function of light intensity and light polarization angle reveal linear and sinusoidal response, respectively. The observed features are in accordance with phenomenological theory. Remarkably, the x = 0.125 sample displays ∼8 times higher open-circuit voltage (7.39 V) than the parent compound. The enhanced PV effect is attributed to the large shift current along the z direction as evidenced by the additional charge-center shift of the valence band occupied by the O-2p orbital and the conduction band occupied by the Bi-6p orbital. Notably, the degenerate Bi-pz state at the conduction band minimum in BBKT favors the large shift current response in the z direction.

Subhajit Pal

Papers

Giant photovoltaic response in band engineered ferroelectric perovskite

Polarization driven self-biased and enhanced UV–visible photodetector characteristics of ferroelectric thin film

Large photovoltaic response in rare-earth doped BiFeO3 polycrystalline thin films near morphotropic phase boundary composition

Enhanced bulk photovoltaic response in Sn doped BaTiO3 through composition dependent structural transformation

Bulk photovoltaic effect in BaTiO3-based ferroelectric oxides: An experimental and theoretical study