Enhanced photovoltaic properties in bilayer BiFeO3/Bi-Mn-O thin films

Approaching Charge Separation Efficiency to Unity without Charge Recombination

Improving the efficiency of charge separation (CS) and charge transport (CT) is essential for almost all optoelectronic applications, yet its maximization remains a big challenge. Here we propose a conceptual strategy to achieve CS efficiency close to unity and simultaneously avoid charge recombination (CR) during CT in a ferroelectric polar-discontinuity (PD) superlattice structure, as demonstrated in (BaTiO_{3})_{m}/(BiFeO_{3})_{n}, which is fundamentally different from the existing mechanisms. The competition of interfacial dipole and ferroelectric PD induces opposite band bending in BiFeO_{3} and BaTiO_{3} sublattices. Consequently, the photoexcited electrons (e) and holes (h) in individual sublattices move forward to the opposite interfaces forming electrically isolated e and h channels, leading to a CS efficiency close to unity. Importantly, the spatial isolation of conduction channels in (BaTiO_{3})_{m}/(BiFeO_{3})_{n} enable suppression of CR during CT, thus realizing a unique band diagram for spatially orthogonal CS and CT. Remarkably, (BaTiO_{3})_{m}/(BiFeO_{3})_{n} can maintain a high photocurrent and large band gap simultaneously. Our results provide a fascinating illumination for designing artificial heterostructures toward ideal CS and CT in optoelectronic applications.

Enhanced UV-Vis Photodegradation of Nanocomposite Reduced Graphene Oxide/Ferrite Nanofiber Films Prepared by Laser-Assisted Evaporation

Nanocomposite films of rGO/MFeO3 (M = Bi, La) nanofibers were grown by matrix-assisted pulsed laser evaporation of frozen target dispersions containing GO platelets and MFeO3 nanofibers. Electron microscopy investigations confirmed the successful fabrication of MFeO3 nanofibers by electrospinning Part of nanofibers were broken into shorter units, and spherical nanoparticles were formed during laser processing. Numerical simulations were performed in order to estimate the maximum temperature values reached by the nanofibers during laser irradiation. X-ray diffraction analyses revealed the formation of perovskite MFeO3 phase, whereas secondary phases of BiFeO3 could not be completely avoided, due to the high volatility of bismuth. XPS measurements disclosed the presence of metallic bismuth and Fe2+ for BiFeO3, whereas La2(CO3)3 and Fe2+ were observed in case of LaFeO3 nanofibers. High photocatalytic efficiencies for the degradation of methyl orange were achieved for nanocomposite films, both under UV and visible light irradiation conditions. Degradation values of up to 70% after 400 min irradiation were obtained for rGO/LaFeO3 nanocomposite thin layers, with weights below 10 µg, rGO platelets acting as reservoirs for photoelectrons generated at the surface of MFeO3.

Maximizing Short Circuit Current Density and Open Circuit Voltage in Oxygen Vacancy-Controlled Bi1-xCaxFe1-yTiyO3-δ Thin-Film Solar Cells

Designing solid-state perovskite oxide solar cells with large short circuit current (J_SC) and open circuit voltage (V_OC) has been a challenging problem. Epitaxial BiFeO₃ (BFO) films are known to exhibit large V_OC (>50 V). However, they exhibit low J_SC (≪μA/cm²) under 1 Sun illumination. In this work, taking polycrystalline BiFeO₃ thin films, we demonstrate that oxygen vacancies (V_O) present within the lattice and at grain boundary (GB) can explicitly be controlled to achieve high J_SC and V_OC simultaneously. While aliovalent substitution (Ca²⁺ at Bi³⁺ site) is used to control the lattice V_O, Ca and Ti cosubstitution is used to bring out only GB-V_O. Fluorine-doped tin oxide (FTO)/Bi_1-xCa_xFe_1-yTi_yO_3-δ/Au devices are tested for photovoltaic characteristics. Introducing V_O increases the photocurrent by four orders (J_SC ∼ 3 mA/cm²). On the contrary, V_OC is found to be <0.5 V, as against 0.5-3 V observed for the pristine BiFeO₃. Ca and Ti cosubstitution facilitate the formation of smaller crystallites, which in turn increase the GB area and thereby the GB-V_O. This creates defect bands occupying the bulk band gap, as inferred from the diffused reflection spectra and band structure calculations, leading to a three-order increase in J_SC. The cosubstitution, following a charge compensation mechanism, decreases the lattice V_O concentration significantly to retain the ferroelectric nature with enhanced polarization. It helps to achieve V_OC (3-8 V) much larger than that of BiFeO₃ (0.5-3 V). It is noteworthy that as Ca substitution maintains moderate crystallite size, the lattice V_O concentration dominates GB-V_O concentration. Notwithstanding, both lattice and GB-V_O contribute to the increase in J_SC; the former weakens ferroelectricity, and as a consequence, undesirably, V_OC is lowered well below 0.5 V. Using optimum J_SC and V_OC, we demonstrate that the efficiency ∼0.22% can be achieved in solid-state BFO solar cells under AM 1.5 one Sun illumination.

Pulse-Driven Nonvolatile Perovskite Memory with Photovoltaic Read-Out Characteristics

This paper presents a unique GdFe_0.8Ni_0.2O₃ perovskite thin film for use in pulse-controlled nonvolatile memory devices (combined with a SrTiO₃ (STO) substrate) without the need for an electrical-stressing read-out process. The use of pulse voltage imposes permanent downward/upward polarization states on GFNO, which enables greater energy density and higher energy efficiency than the unpoled state for memory. The two polarization states produce carrier migrations in opposing directions across the GFNO/STO interface, which alter the depletion region of the device, as reflected in photovoltaic short-circuit current density (J_sc) values. Modulating the duration (varying the number of sequential pulses but fixing the pulse width and delay time) and direction of continuous pulse voltage is an effective method for controlling J_sc, thereby allowing the fabrication of nondestructive, light-tunable, nonvolatile memory devices. In experiments, J_sc in the downward polarized state was approximately 6 times greater than that in the upward polarized state. It is promising that more memory states can be enabled by the proposed heterostructure by selecting appropriate pulse trains. Real-time interfacial changes (relative to the nonvolatile characteristics of the device) were obtained by applying synchrotron X-ray techniques simultaneously with pulse characterization. This made it possible to separately probe the electronic and chemical states of the GFNO (a p-type-like semiconductor) and STO (an n-type-like semiconductor) while varying the pulse direction, thereby making it possible to identify the mechanisms underlying the observed phenomena.

Perovskites with d-block metals for solar energy applications

Pb2+ halide organic-inorganic perovskites are excellent semiconductors for use in solar energy applications, but at the expense of robustness and environmental compatibility. Tin (Sn), which sits just above lead in the periodic table, forms pure (or mixed with lead) perovskites when at the 2+ or 4+ oxidation state. It can act as a promising alternative; however, there are still some serious concerns regarding its suitability. This presents a major challenge; viable metal cations have to be identified. A good number of elements, originating from a large range of d-block metal ions, with adequate oxidation states, moderate toxicity, and relative abundance, seem ideal for this purpose. In this review, we present the most characteristic perovskites (conventional perovskites, layered, or double perovskites) that can be formed with the help of these metals. We focus on d-block metal ions with stable oxidation states, such as Ag+ or Ti4+, which have exhibited satisfactory photovoltaic properties until now. Further, we highlight the results involving compounds other than halide perovskites, such as oxides, chalcogenides, and nitrides (as well as oxyhalides, oxysulfides, and oxynitrides); a few of them are ferroelectric (based on Ti4+, Zr4+, Fe3+, and Cr3+) and can yield a photovoltage that exceeds the bandgap of the material. Finally, we present the critical challenges that currently limit the efficiency of these systems and propose prospects for future directions.

Epitaxial Bi2FeCrO6 Multiferroic Thin-Film Photoanodes with Ultrathin p-Type NiO Layers for Improved Solar Water Oxidation

The photoelectric properties of multiferroic double-perovskite Bi₂FeCrO₆ (BFCO), such as above-band gap photovoltages, switchable photocurrents, and bulk photovoltaic effects, have recently been explored for potential applications in solar technology. Here, we report the fabrication of photoelectrodes based on n-type ferroelectric (FE) semiconductor BFCO heterojunctions coated with p-type transparent conducting oxides (TCOs) by pulsed laser deposition and their application for photoelectrochemical (PEC) water oxidation. The photocatalytic properties of the bare BFCO photoanodes can be improved by controlling the FE polarization state. However, the charge recombination as well as the limited charge transfer kinetics in the photoanode/electrolyte cause major energy loss and thus hinder the PEC performance. We show that this problem may be addressed by the deposition of an ultrathin p-type NiO layer on the photoanode to enhance the charge transport kinetics and reduce charge recombination at surface-trapped states for increased surface band bending. A fourfold enhancement of photocurrent density, up to 0.4 mA cm^-2 (at +1.23 V vs RHE), a best performance of stability over 4 h, and a high incident photon-to-current efficiency (∼3.7%) were achieved under 1 sun illumination in such p-NiO/n-BFCO heterojunction photoanodes. These studies reveal the optimization of PEC performance by polarization switching of BFCO and the successful achievement of p-TCOs/n-FE heterojunction photoanodes that are able to sustain water oxidation that is stable for many hours.

Hysteretic Characteristics of Pulsed Laser Deposited 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3/ZnO Bilayers

In the present work, we study the hysteretic behavior in the electric-field-dependent capacitance and the current characteristics of 0.5Ba(Zr_0.2Ti_0.8)O₃-0.5(Ba_0.7Ca_0.3)TiO₃ (BCZT)/ZnO bilayers deposited on 0.7 wt % Nb-doped (001)-SrTiO₃ (Nb:STO) substrates in a metal-ferroelectric-semiconductor (MFS) configuration. The X-ray diffraction measurements show that the BCZT and ZnO layers are highly oriented along the c-axis and have a single perovskite and wurtzite phases, respectively, whereas high-resolution transmission electron microscopy revealed very sharp Nb:STO/BCZT/ZnO interfaces. The capacitance-electric field ( C- E) characteristics of the bilayers exhibit a memory window of 47 kV/cm and a capacitance decrease of 22%, at a negative bias. The later result is explained by the formation of a depletion region in the ZnO layer. Moreover, an unusual resistive switching (RS) behavior is observed in the BCZT films, where the RS ratio can be 500 times enhanced in the BCZT/ZnO bilayers. The RS enhancement can be understood by the barrier potential profile modulation at the depletion region, in the BCZT/ZnO junction, via ferroelectric polarization switching of the BCZT layer. This work builds a bridge between the hysteretic behavior observed either in the C- E and current-electric field characteristics on a MFS structure.

Highly Sensitive Switchable Heterojunction Photodiode Based on Epitaxial Bi2FeCrO6 Multiferroic Thin Films

Perovskite multiferroic oxides are promising materials for the realization of sensitive and switchable photodiodes because of their favorable band gap (<3.0 eV), high absorption coefficient, and tunable internal ferroelectric (FE) polarization. A high-speed switchable photodiode based on multiferroic Bi₂FeCrO₆ (BFCO)/SrRuO₃ (SRO)-layered heterojunction was fabricated by pulsed laser deposition. The heterojunction photodiode exhibits a large ideality factor ( n = ∼5.0) and a response time as fast as 68 ms, thanks to the effective charge carrier transport and collection at the BFCO/SRO interface. The diode can switch direction when the electric polarization is reversed by an external voltage pulse. The time-resolved photoluminescence decay of the device measured at ∼500 nm demonstrates an ultrafast charge transfer (lifetime = ∼6.4 ns) in BFCO/SRO heteroepitaxial structures. The estimated responsivity value at 500 nm and zero bias is 0.38 mA W^-1, which is so far the highest reported for any FE thin film photodiode. Our work highlights the huge potential for using multiferroic oxides to fabricate highly sensitive and switchable photodiodes.

Controllable Photovoltaic Effect of Microarray Derived from Epitaxial Tetragonal BiFeO3 Films

Recently, the ferroelectric photovoltaic (FePV) effect has attracted great interest due to its potential in developing optoelectronic devices such as solar cell and electric-optical sensors. It is important for actual applications to realize a controllable photovoltaic process in ferroelectric-based materials. In this work, we prepared well-ordered microarrays based on epitaxially tetragonal BiFeO₃ (T-BFO) films by the pulsed laser deposition technique. The polarization-dependent photocurrent image was directly observed by a conductive atomic force microscope under ultraviolet illumination. By choosing a suitable buffer electrode layer and controlling the ferroelectric polarization in the T-BFO layer, we realized the manipulation of the photovoltaic process. Moreover, based on the analysis of the band structure, we revealed the mechanism of manipulating the photovoltaic process and attributed it to the competition between two key factors, i.e., the internal electric field caused by energy band alignments at interfaces and the depolarization field induced by the ferroelectric polarization in T-BFO. This work is very meaningful for deeply understanding the photovoltaic process of BiFeO₃-based devices at the microscale and provides us a feasible avenue for developing data storage or logic switching microdevices based on the FePV effect.

Nanoscale Bandgap Tuning across an Inhomogeneous Ferroelectric Interface

We report nanoscale bandgap engineering via a local strain across the inhomogeneous ferroelectric interface, which is controlled by the visible-light-excited probe voltage. Switchable photovoltaic effects and the spectral response of the photocurrent were explored to illustrate the reversible bandgap variation (∼0.3 eV). This local-strain-engineered bandgap has been further revealed by in situ probe-voltage-assisted valence electron energy-loss spectroscopy (EELS). Phase-field simulations and first-principle calculations were also employed for illustration of the large local strain and the bandgap variation in ferroelectric perovskite oxides. This reversible bandgap tuning in complex oxides demonstrates a framework for the understanding of the optically related behaviors (photovoltaic, photoemission, and photocatalyst effects) affected by order parameters such as charge, orbital, and lattice parameters.

Recent progress in van der Waals heterojunctions

Following the development of many novel two-dimensional (2D) materials, investigations of van der Waals heterojunctions (vdWHs) have attracted significant attention due to their excellent properties such as smooth heterointerface, highly gate-tunable bandgap, and ultrafast carrier transport. Benefits from the atom-scale thickness, physical and chemical properties and ease of manipulation of the heterojunctions formulated by weak vdW forces were demonstrated to indicate their outstanding potential in electronic and optoelectronic applications, including photodetection and energy harvesting, and the possibility of integrating them with the existing semiconductor technology for the next-generation electronic and sensing devices. In this review, we summarized the recent developments of vdWHs and emphasized their applications. Basically, we introduced the physical properties and some newly discovered phenomena in vdWHs. Then, we emphatically presented four classical vdWHs and some novel heterostructures formed by vdW forces. Based on their unique physical properties and structures, we highlighted the applications of vdWHs including in photodiodes, phototransistors, tunneling devices, and memory devices. Finally, we provided a conclusion on the recent advances in vdWHs and outlined our perspectives. We aim for this review to serve as a solid foundation in this field and to pave the way for future research on vdW-based materials and their heterostructures.

Observation of nanotwinning and room temperature ferromagnetism in sub-5 nm BiFeO3 nanoparticles: a combined experimental and theoretical study

Particle size significantly affects the properties and therefore the potential applications of multiferroics. However, is there special particle size effect in BiFeO₃, which has a spiral modulated spin structure? This is still under investigation for sub-5 nm BiFeO₃. In this report, the structural, electronic and magnetic properties are investigated for chemically synthesized BiFeO₃ nanoparticles with an average size of 3 nm. We observed nanotwinning features in the specific size regime of the nanoparticles (2-4 nm). A weak Bi-O-Fe coordination and weak covalent nature has been observed in the nanoparticles through high-resolution electron energy loss spectroscopy and theoretical analysis, confirming that BiFeO₃ nanoparticles a retain rudimentary R3c phase even at sub-5 nm dimensions. The R3c phase of sub-5 nm BiFeO₃ nanoparticles has also been confirmed using Raman spectroscopy and Raman mapping of the vibrational modes. The nanoparticles display cluster spin glass, room temperature ferromagnetism, and a metamictization-davidite phase. The observation of weak magnetic entropy features confirmed the presence of a weak correlation between the magnetic and ferroelectric components. To support our experimental observations, we have simulated a sub-5 nm BiFeO₃ nanocluster. Using density functional theory, the ferromagnetic ground state and the presence of a weak covalent nature in the nanocluster is established considering the first Brillouin zone, thus confirming our experimental results. Finding of new physicochemical features in sub-5 nm BiFeO₃ would be beneficial for the understanding of the fundamental physical and chemical science as well as potential device development.