Photovoltaic properties of Bi2FeCrO6 films epitaxially grown on (100)-oriented silicon substrates

Unexpected Phonon Behaviour in BiFexCr1-xO3, a Material System Different from Its BiFeO3 and BiCrO3 Parents

The dielectric function and the bandgap of BiFe_0.5Cr_0.5O₃ thin films were determined from spectroscopic ellipsometry and compared with that of the parent compounds BiFeO₃ and BiCrO₃. The bandgap value of BiFe_0.5Cr_0.5O₃ is lower than that of BiFeO₃ and BiCrO₃, due to an optical transition at ~2.27 eV attributed to a charge transfer excitation between the Cr and Fe ions. This optical transition enables new phonon modes which have been investigated using Raman spectroscopy by employing multi-wavelengths excitation. The appearance of a new Raman mode at ~670 cm⁻¹ with a strong intensity dependence on the excitation line and its higher order scattering activation was found for both BiFe_0.5Cr_0.5O₃ thin films and BiFe_xCr_1−xO₃ polycrystalline bulk samples. Furthermore, Raman spectroscopy was also used to investigate temperature induced structural phase transitions in BiFe_0.3Cr_0.7O₃.

Enhanced photovoltaic response of lead-free ferroelectric solar cells based on (K,Bi)(Nb,Yb)O3 films

Herein, we firstly present the (K,Bi)(Nb,Yb)O₃ inorganic ferroelectric photovoltaic (FPV) film, in which a nearly ideal bandgap of ∼1.45 eV in the center of the solar spectrum and the co-existence of oxygen vacancies as well as ferroelectric polarization were confirmed. Furthermore, a novel cell structure is successfully fabricated by combining charge-transporting TiO₂ nanoparticles, the perovskite sensitizer and a light-absorbing oxide hole p-type NiO conductor to realize a 1 V open circuit voltage, which can be increased to 1.56 V by adjusting the test bias near the coercive voltage. Additionally, under simulated standard AM 1.5G illumination, a fill factor of 86% and a power conversion efficiency of 0.85% are achieved via oxygen vacancy electromigration and polarization switching modulation. It is shown that the obtained power conversion efficiency is one to three orders of magnitude higher than those of pure BiFeO₃ and Pb(Zr,Ti)O₃. The enhanced PV effects are well elucidated using the transformation from a Schottky-like barrier to Ohmic contacts caused by polarization switching and oxygen vacancies. Building upon the above studies, deep insights into the bandgap tunability and PV effects in ferroelectric films with high oxygen vacancy concentration are provided and will facilitate a new versatile route for exploring high PV performance based on inorganic ferroelectric films.

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 patterned Bi2FeCrO6 nanoisland arrays with room temperature multiferroic properties

Epitaxial multiferroic Bi₂FeCrO₆ nanoisland arrays with a lateral size of ∼100 nm have been successfully fabricated by patterned SiO₂ template-assisted pulsed laser deposition. The as-grown island structure exhibits promising multiferroic properties (i.e. ferroelectric and magnetic) even at nanometer dimensions at room temperature. This work demonstrates an effective strategy to fabricate high-density nonvolatile ferroelectric/multiferroic memory devices.

Switchable ferroelectric photovoltaic effects in epitaxial h-RFeO3 thin films

Ferroelectric photovoltaics (FPVs) have drawn much attention owing to their high stability, environmental safety, and anomalously high photovoltages, coupled with reversibly switchable photovoltaic responses. However, FPVs suffer from extremely low photocurrents, which is primarily due to their wide band gaps. Here, we present a new class of FPVs by demonstrating switchable ferroelectric photovoltaic effects and narrow band-gap properties using hexagonal ferrite (h-RFeO3) thin films, where R denotes rare-earth ions. FPVs with narrow band gaps suggest their potential applicability as photovoltaic and optoelectronic devices. The h-RFeO3 films further exhibit reasonably large ferroelectric polarizations (4.7-8.5 μC cm-2), which possibly reduces a rapid recombination rate of the photo-generated electron-hole pairs. The power conversion efficiency (PCE) of h-RFeO3 thin-film devices is sensitive to the magnitude of polarization. In the case of the h-TmFeO3 (h-TFO) thin film, the measured PCE is twice as large as that of the BiFeO3 thin film, a prototypic FPV. The effect of electrical fatigue on FPV responses has been further investigated. This work thus demonstrates a new class of FPVs towards high-efficiency solar cell and optoelectronic applications.

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.

Effects of Plume Hydrodynamics and Oxidation on the Composition of a Condensing Laser-Induced Plasma

High-temperature chemistry in laser ablation plumes leads to vapor-phase speciation, which can induce chemical fractionation during condensation. Using emission spectroscopy acquired after ablation of a SrZrO₃ target, we have experimentally observed the formation of multiple molecular species (ZrO and SrO) as a function of time as the laser ablation plume evolves. Although the stable oxides SrO and ZrO₂ are both refractory, we observed emission from the ZrO intermediate at earlier times than SrO. We deduced the time-scale of oxygen entrainment into the laser ablation plume using an ¹⁸O₂ environment by observing the in-growth of Zr¹⁸O in the emission spectra relative to Zr¹⁶O, which was formed by reaction of Zr with ¹⁶O from the target itself. Using temporally resolved plume-imaging, we determined that ZrO formed more readily at early times, volumetrically in the plume, while SrO formed later in time, around the periphery. Using a simple temperature-dependent reaction model, we have illustrated that the formation sequence of these oxides subsequent to ablation is predictable to first order.