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Byline: C. Harnagea, C.V. Cojocaru, R. Nechache, O. Gautreau, F. Rosei, A. Pignolet We summarise here our efforts toward the fabrication and characterisation of ferroelectric and multiferroic films and structures. First, we discuss the challenges related with the fabrication and characterisation of nanostructures of functional complex oxides. In particular, to demonstrate the functionality of our films and especially of our structures, we briefly describe atomic force microscopy techniques tailored for local electrical or magnetic characterisation. Piezoresponse Force Microscopy and Magnetic Force Microscopy enable the characterisation of piezoelectric, ferroelectric and magnetic properties at the nanoscale. We then report the fabrication of various functional oxide films by Pulsed Laser Deposition (PLD), in particular the deposition of the conducting oxide electrode SrRuO3 at room temperature. We also describe the fabrication of ferroelectric BaTiO3 and BiFeO3, both in the form of film and mesoscopic (sub-micron size) islands. The formation of ferroelectric structures of arbitrary shape at controlled location was also achieved by nanostencilling, i.e., using a shadow-mask with nanoscale features. Finally, the successful synthesis of Bi2FeCrO6 films by pulsed laser deposition is then detailed; this is a new multiferroic material predicted by ab-initio calculations. The Bi2FeCrO6 films have the correct cationic stoichiometry throughout their thickness and their crystal structure is found to be very similar to that of BiFeO3. Bi2FeCrO6 films exhibit good piezoelectric and ferroelectric properties at room temperature, a property that was not predicted. Magnetic Force Microscopy reveals the presence of magnetic domains and confirms the macroscopic magnetic measurements showing that the Bi2FeCrO6 films do exhibit a saturation magnetisation about one order of magnitude higher than that of BiFeO3 films having the same thickness.

Multiferroic films are increasingly being studied for applications in solar energy conversion because of their efficient ferroelectric polarization-driven carrier separation and above-bandgap generated photovoltages, which in principle can lead to energy conversion efficiencies beyond the maximum value (∼34%) reported in traditional silicon-based bipolar heterojunction solar cells. However, the efficiency reported so far is still too low (<2%) to be considered for commercialization. Here, we demonstrate a new approach to effectively tune the bandgap of double perovskite multiferroic oxides by engineering the cationic ordering for the case of Bi2FeCrO6. Using this approach, we report a power conversion efficiency of 8.1% under AM 1.5 G irradiation (100 mW cm−2) for Bi2FeCrO6 thin-film solar cells in a multilayer configuration. [ABSTRACT FROM AUTHOR]

TiO2 nanowires (NWs) were grown on a (1 0 0)-oriented silicon substrate by pulsed laser deposition. Our growth strategy involves consecutive deposition of a Au/Ti bilayer followed by annealing to promote TiO2 NW growth. Deposition at various laser energy densities suggests that vapor-liquid-solid growth of TiO2 is sensitive to Ti flux. Raman spectroscopy of NWs grown under optimized PLD-conditions reveals the rutile phase of TiO2 NWs. Optical measurements on the TiO2 NW arrays under light excitation at 355 nm wavelength show a strong and sharp photoluminescence peak at 3.41 eV, which we attribute to a direct recombination of photoinduced charge carriers. [ABSTRACT FROM AUTHOR]

We present experimental results on Bi2FeCrO6 (BFCO) epitaxial films deposited by laser ablation directly on SrTiO3 substrates. It has been theoretically predicted by Baettig and Spaldin [Appl. Phys. Lett. 86 012505 (2005)], using first-principles density functional theory, that BFCO is ferrimagnetic (with a magnetic moment of 2 µB per formula unit) and ferroelectric (with a polarization of ∼80 µC/cm2 at 0 K). The crystal structure has been investigated using X-ray diffraction, which shows that the films are epitaxial with a high crystallinity and have a degree of orientation depending on deposition conditions determined by the substrate crystal structure. Chemical analysis, carried out by X-ray microanalysis and X-ray photoelectron spectroscopy (XPS), indicates the correct cationic stoichiometry in the BFCO layer, namely (Bi:Fe:Cr = 2:1:1). XPS depth-profiling revealed that the oxidation state of Fe and Cr ions in the film remains 3+ throughout the film thickness and that both Fe and Cr ions are homogeneously distributed throughout the depth. Cross-section high-resolution transmission electron microscopy images plus selected area electron diffraction confirm the crystalline quality of the epitaxial BFCO films with no identifiable foreign phase or inclusion. The multiferroic character of BFCO is demonstrated by ferroelectric and magnetic measurements showing that the films exhibit ferroelectric and magnetic hysteresis at room temperature. In addition, local piezoelectric measurements carried out using piezoresponse force microscopy (PFM) show the presence of ferroelectric domains and their switching at the sub-micron scale. [ABSTRACT FROM AUTHOR]

We report a large photovoltaic (PV) effect in multiferroic Bi2FeCrO6 (BFCO) films under monochromatic illumination at 635 nm with an intensity of 1.5 mW cm-2. These multiferroic films exhibit a large photocurrent at zero bias voltage and an open-circuit voltage of about 0.6 V. A high PV power conversion efficiency of about 6% for red light is achieved and attributed to a high degree of B-site cationic ordering between Fe and Cr sublattices, the tuning of which is likely to play a key role in further improvements of the PV properties in BFCO. [ABSTRACT FROM AUTHOR]

Due to their low cost and high efficiency, hybrid perovskite solar cells (PSCs) have shown the most outstanding competitiveness among third-generation photovoltaic (PV) devices. However, several challenges remain unresolved, among which the limited stability is arguably the main. Chlorine (Cl) has been widely employed to yield PV performances, but the Cl-doping mechanism and its role in mixed-halide PSCs are not entirely understood. Here, we investigate the effect of Cl-doping using different precursors such as formamidinium chloride (FACl), cesium chloride (CsCl), and lead chloride (PbCl 2 ), which lead to the incorporation of Cl at different sites of the perovskite crystal. We demonstrate that the stability and efficiency of air-processed PSCs are strongly affected by Cl bonding into the cationic chloride precursor. Furthermore, adding potassium thiocyanate (KSCN) leads to the maximum efficiency of 18.1%, improving the operational stability with only 18% PCE loss after 520 h, stored under ambient conditions. Incorporating CsCl and KSCN presents an effective approach to further boost the performance and thermal stability of PSCs by tailoring the composition of the perovskite's composition. Finally, we used the slot-die method to demonstrate that our strategy is scalable for large-area devices that have shown similar performance. Our results show that fully air-processed and stable PSCs with high efficiency for large production and commercialization are achievable.

Ferroelectric materials may be used as effective photoelectrocatalysts for water splitting due to enhanced charge carrier separation driven by their spontaneous polarization induced internal electric field. Compared to other ferroelectric materials, BiFeO 3 exhibits a high catalytic efficiency due to its comparatively smaller bandgap, which enables light absorption from a large part of the solar spectrum and its higher bulk ferroelectric polarization. Here, we compare the photoelectrochemical properties of three different BiFeO 3 morphologies, namely, nanofibers, nanowebs, and thin films synthesized via electrospinning, directly on fluorine-doped tin oxide (FTO) coated glass substrates. A significant photocathodic current in the range from -86.2 to -56.5 μA cm -2 at -0.4 V bias (vs Ag/AgCl) has been recorded for all three morphologies in 0.1M Na 2 SO 4 aqueous solution (pH = 6.8). Among these morphologies, BiFeO 3 nanofibers exhibit higher efficiency because of their larger surface area and improved charge separation resulting from rapid diffusion of photoinduced charge carriers along the axis of the nanofiber. In the case of BiFeO 3 nanofibers, we obtained the highest photocurrent density of -86.2 µA/cm 2 at -0.4 V bias (vs Ag/AgCl electrode) and an onset potential of 0.22 V. We also observed that the onset potential of the photocathodic current can be increased by applying a positive polarization voltage, which leads to favorable bending of band edges at the electrode/electrolyte interface resulting in increased charge carrier separation.

Colloidal perovskite nanocrystals (PNCs) combine the outstanding optoelectronic properties of bulk perovskites with strong quantum confinement effects at the nanoscale. Their facile and low-cost synthesis, together with superior photoluminescence quantum yields and exceptional optical versatility, make PNCs promising candidates for next-generation optoelectronics. However, this field is still in its early infancy and not yet ready for commercialization due to several open challenges to be addressed, such as toxicity and stability. Here, the key synthesis strategies and the tunable optical properties of PNCs are discussed. The photophysical underpinnings of PNCs, in correlation with recent developments of PNC-based optoelectronic devices, are especially highlighted. The final goal is to outline a theoretical scaffold for the design of high-performance devices that can at the same time address the commercialization challenges of PNC-based technology., (© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Epitaxial multiferroic Bi 2 FeCrO 6 nanoisland arrays with a lateral size of ∼100 nm have been successfully fabricated by patterned SiO 2 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., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The photoelectric properties of multiferroic double-perovskite Bi 2 FeCrO 6 (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.

Hybrid organic-inorganic perovskites have shown exceptional semiconducting properties and microstructural versatility for inexpensive, solution-processable photovoltaic and optoelectronic devices. In this work, an all-solution-based technique in ambient environment for highly sensitive and high-speed flexible photodetectors using high crystal quality perovskite nanowires grown on Kapton substrate is presented. At 10 V, the optimized photodetector exhibits a responsivity as high as 0.62 A W -1 , a maximum specific detectivity of 7.3 × 10 12 cm Hz 1/2 W -1 , and a rise time of 227.2 µs. It also shows remarkable photocurrent stability even beyond 5000 bending cycles. Moreover, a deposition of poly(methyl methacrylate) (PMMA) as a protective layer on the perovskite yields significantly better stability under ambient air operation: the PMMA-protected devices are stable for over 30 days. This work demonstrates a cost-effective fabrication technique for high-performance flexible photodetectors and opens opportunities for research advancements in broadband and large-scale flexible perovskite-based optoelectronic devices., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The morphology of hybrid organic-inorganic perovskite films is known to strongly affect the performance of perovskite-based solar cells. CH 3 NH 3 PbI 3-x Cl x (MAPbI 3-x Cl x ) films have been previously fabricated with 100% surface coverage in glove boxes. In ambient air, fabrication generally relies on solvent engineering to obtain compact films. In contrast, this work explores the potential of altering the perovskites microstructure for solar cell engineering. This work starts with CH 3 NH 3 PbI 3- x Cl x , films with grain morphology carefully controlled by varying the deposition speed during the spin-coating process to fabricate efficient and partially transparent solar cells. Devices produced with a CH 3 NH 3 PbI 3- x Cl x film and a compact thick top gold electrode reach a maximum efficiency of 10.2% but display a large photocurrent hysteresis. As it is demonstrated, the introduction of different concentrations of bromide in the precursor solution addresses the hysteresis issues and turns the film morphology into a partially transparent interconnected network of 1D microstructures. This approach leads to semitransparent solar cells with negligible hysteresis and efficiencies up to 7.2%, while allowing average transmission of 17% across the visible spectrum. This work demonstrates that the optimization of the perovskites composition can mitigate the hysteresis effects commonly attributed to the charge trapping within the perovskite film., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

In recent years, hybrid organic-inorganic halide perovskites have been widely studied for the low-cost fabrication of a wide range of optoelectronic devices, including impressive perovskite-based solar cells. Amongst the key factors influencing the performance of these devices, recent efforts have focused on tailoring the granularity and microstructure of the perovskite films. Albeit, a cost-effective technique allowing to carefully control their microstructure in ambient environmental conditions has not been realized. We report on a solvent-antisolvent ambient processed CH 3 NH 3 PbI 3-x Cl x based thin films using a simple and robust solvent engineering technique to achieve large grains (>5 µm) having excellent crystalline quality and surface coverage with very low pinhole density. Using optimized treatment (75% chlorobenzene and 25% ethanol), we achieve highly-compact perovskite films with 99.97% surface coverage to produce solar cells with power conversion efficiencies (PCEs) up-to 14.0%. In these planar solar cells, we find that the density and size of the pinholes are the dominant factors that affect their overall performances. This work provides a promising solvent treatment technique in ambient conditions and paves the way for further optimization of large area thin films and high performance perovskite solar cells.

Organic-inorganic perovskites have been hailed as promising candidates for optoelectronic and photovoltaic devices, but their operation remains limited to the visible spectrum. Here, we combine single-wall carbon nanotubes, PbS quantum dots and a perovskite to synthesize hybrid devices suitable for operation in both the visible and near-infrared. The photodetectors thus fabricated show responsivities as high as 0.5 A W-1 and 0.35 A W-1 at 500 nm and at 1300 nm, respectively, with an applied bias of 1 V. Moreover, the incorporation of nanotubes within the perovskite matrix facilitates the carrier extraction, resulting in response time under 250 μs, a gain-bandwidth product of 0.1 MHz and detectivities of 1.4 × 1011 Jones and 0.9 × 1011 Jones at 500 nm and at 1300 nm, respectively. This unique approach opens new pathways for the development of low-cost, high-speed and broadband perovskite-based optoelectronic devices for large-scale manufacturing.

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 Bi 2 FeCrO 6 (BFCO)/SrRuO 3 (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.

Thermoelectric Bi 2 Te 3 and Sb 2 Te 3 thin films with high power factor were successfully obtained by pulsed laser deposition (PLD). Here, we demonstrate a well-controlled deposition of Bi 2 Te 3 /Sb 2 Te 3 structures on glass substrates, through a shadow mask with micrometer-scale features. We establish an optimal growth temperature of 45 °C to attain compounds with suitable stoichiometric composition, as well as structural and electrical properties, to achieve high thermoelectric power factor. These films are produced without additional postannealing treatment or added gases. Indeed, crystalline films with Seebeck coefficients of 624 and -78 μV K -1 are obtained for Sb 2 Te 3 and Bi 2 Te 3 , respectively. Microgenerators consisting of four pairs of n-type Bi 2 Te 3 and p-type Sb 2 Te 3 legs connected in series generate a maximum voltage of 50 mV and a power density of around 120 μW cm -2 for a temperature difference of 30 K across the hot and cold ends of the device. This low-temperature and simple PLD-deposited device represents an important step toward practical thermoelectric materials as well as efficient and compact microgenerators for low-temperature energy-harvesting applications.

Nanostructured magnetic materials with well-defined magnetic anisotropy are very promising as building blocks in spintronic devices that operate at room temperature. Here we demonstrate the epitaxial growth of highly oriented Fe 3 O 4 nanorods on a SrTiO 3 substrate by hydrothermal synthesis without the use of a seed layer. The epitaxial nanorods showed biaxial magnetic anisotropy with an order of magnitude difference between the anisotropy field values of the easy and hard axes. Using a combination of conventional magnetometry, transverse susceptibility, magnetic force microscopy (MFM) and magneto-optic Kerr effect (MOKE) measurements, we investigate magnetic behavior such as temperature dependent magnetization and anisotropy, along with room temperature magnetic domain formation and its switching. The interplay of epitaxy and enhanced magnetic anisotropy at room temperature, with respect to randomly oriented powder Fe 3 O 4 nanorods, is discussed. The results obtained identify epitaxial nanorods as useful materials for magnetic data storage and spintronic devices that necessitate tunable anisotropic properties with sharp magnetic switching phenomena.