36 research outputs found
Enhanced photovoltaic properties in bilayer BiFeO3/Bi-Mn-O thin films
No full textWe report an external solar power conversion efficiency of ∼1.43% in BiFeO3(BFO)/BiMnO3(BMO) bilayer thin films. Both films are epitaxially grown on (111) oriented niobium doped SrTiO3 (NSTO) single crystal substrates by pulsed laser deposition. By illuminating the BFO/BMO films under 1 Sun (AM 1.5 G), we found a remarkably high fill factor of ∼0.72, much higher than values reported for devices based on BFO or BMO alone. In addition, we demonstrate that the photocurrent density and photovoltage are tunable by changing the polarization direction in the BFO/BMO bilayer, as confirmed by the macroscopic polarization-voltage (P-V) hysteresis loop. This effect is described in terms of a more favorable energy band alignment of the electrode/bilayer/NSTO heterostructure junction, which controls photocarrier separation
Photovoltaic properties of Bi2FeCrO6 films epitaxially grown on (100)-oriented silicon substrates
No full textWe demonstrate the promising potential of using perovskite Bi2FeCrO6 (BFCO) for niche applications in photovoltaics (PV) (e.g. self-powered sensors that simultaneously exploit PV conversion and multiferroic properties) or as a complement to mature PV technologies like silicon. BFCO thin films were epitaxially grown on silicon substrates using an MgO buffer layer. Piezoresponse force microscopy measurements revealed that the tensile strained BFCO phase exhibits a polarization predominantly oriented through the in-plane direction. The semiconducting bandgap of the ordered BFCO phase combined with ferroelectric properties, opens the possibility of a ferroelectric PV efficiency above 2% in a thin film device and the use of ferroelectric materials simultaneously as solar absorber layers and carrier separators in PV devices. A large short circuit photocurrent density of 13.8 mA cm-2 and a photovoltage output of 0.5 V are typically obtained at FF of 38% for BFCO devices fabricated on silicon. We believe that the reduced photovoltage is due to the low diffusion length of photogenerated charge carriers in the BFCO material where the ferroelectric domains are predominately oriented in-plane and thus do not contribute efficiently to the photocharge separation process
Improved photovoltaic performance from inorganic perovskite oxide thin films with mixed crystal phases
No full textInorganic ferroelectric perovskites are attracting attention for the realization of highly stable photovoltaic cells with large open-circuit voltages. However, the power conversion efficiencies of devices have been limited so far. Here, we report a power conversion efficiency of ~4.20% under 1 sun illumination from Bi-Mn-O composite thin films with mixed BiMnO3 and BiMn2O5 crystal phases. We show that the photocurrent density and photovoltage mainly develop across grain boundaries and interfaces rather than within the grains. We also experimentally demonstrate that the open-circuit voltage and short-circuit photocurrent measured in the films are tunable by varying the electrical resistance of the device, which in turn is controlled by externally applying voltage pulses. The exploitation of multifunctional properties of composite oxides provides an alternative route towards achieving highly stable, high-efficiency photovoltaic solar energy conversion
Photoelectrochemical properties of BiMnO 3 thin films and nanostructures
No full textWe report and compare the properties of BiMnO3 (BMO) nanostructures and thin films as photoanodes in photoelectrochemical solar cells. BMO films are grown on Niobium doped SrTiO3 crystalline substrates using pulse laser deposition. Nanoscale patterns of BMO are obtained by depositing through nanostencils, namely shadow masks with nanometer-scale circular apertures. We demonstrate that BMO nanostructures exhibit superior photoelectrochemical properties, compared to BMO thin films when used as photoelectrodes in cells for hydrogen production. A photocurrent density of ∼0.9 mA cm−2 at 0.8 V vs Ag/AgCl (1.38 V vs RHE) under 1 Sun is recorded for BMO nanostructures. On the other hand, BMO films exhibit a photocurrent density of ∼40 μA cm−2 at 0.4 V vs Ag/AgCl (0.98 V vs RHE) under 2 Sun which is four times higher than that recorded under 1 Sun illumination (∼10 μA cm−2 at 0.4 V vs Ag/AgCl). Mott-Schottky analysis evidences n-type characteristics for both BMO thin films and nanostructures. According to band alignment with respect to the redox potential of water, we conclude that both types of photoelectrodes are suitable for oxygen evolution reaction
Multiferroic Bi2FeCrO6 based p-i-n heterojunction photovoltaic devices
No full textPhotovoltaic devices made of ferroelectric films are being widely studied, due to their efficient charge separation driven by the internal polarization as well as above-bandgap generated photovoltages. These features may enable power conversion efficiencies (PCE) exceeding the Shockley-Queisser limit, which characterizes conventional semiconductor-based solar cells. However, improving the PCE of such devices is still a challenge, mainly due to the weak charge transport and collection induced by the recombination of photocarriers. Here, we fabricated p-i-n heterojunction devices based on double-perovskite multiferroic Bi2FeCrO6 thin films. The latter act as intrinsic absorbers, sandwiched between hole- and electron-transporting layers, a p-type NiO and an n-type Nb-doped SrTiO3 semiconductor, respectively. Under 1 sun illumination, the optimized p-i-n device yields an open-circuit voltage of 0.53 V and a short-circuit current density of 8.0 mA cm-2, leading to a PCE of ca. 2.0%, a four-fold enhancement compared to that of the i-n device architecture
Epitaxial Bi2FeCrO6 Multiferroic Thin Film as a New Visible Light Absorbing Photocathode Material
No full textFerroelectric materials have been studied increasingly for solar energy conversion technologies due to the efficient charge separation driven by the polarization induced internal electric field. However, their insufficient conversion efficiency is still a major challenge. Here, a photocathode material of epitaxial double perovskite Bi2FeCrO6 multiferroic thin film is reported with a suitable conduction band position and small bandgap (1.9-2.1 eV), for visible-light-driven reduction of water to hydrogen. Photoelectrochemical measurements show that the highest photocurrent density up to -1.02 mA cm-2 at a potential of -0.97V versus reversible hydrogen electrode is obtained in p-type Bi2FeCrO6 thin film photocathode grown on SrTiO3 substrate under AM 1.5G simulated sunlight. In addition, a twofold enhancement of photocurrent density is obtained after negatively poling the Bi2FeCrO6 thin film, as a result of modulation of the band structure by suitable control of the internal electric field gradient originating from the ferroelectric polarization in the Bi2FeCrO6 films. The findings validate the use of multiferroic Bi2FeCrO6 thin films as photocathode materials, and also prove that the manipulation of internal fields through polarization in ferroelectric materials is a promising strategy for the design of improved photoelectrodes and smart devices for solar energy conversion. A new photocathode material Bi2FeCrO6 with a small bandgap (1.9-2.1 eV) and a suitable conduction band position to photoreduce water to H2 is reported. The highest photocurrent density (ca. -1.0 mA cm-2) is obtained at ca. -1.0 V versus a reversible hydrogen electrode. The photocurrent could be further tuned through modulating the ferroelectric polarization
Highly Sensitive Switchable Heterojunction Photodiode Based on Epitaxial Bi2FeCrO6 Multiferroic Thin Films
No full textPerovskite 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
EFFECT OF MULTI-WALLED CARBON NANOTUBES ON THE STABILITY OF DYE SENSITIZED SOLAR CELLS
No full textWe report the improvement of the operational stability of dye-sensitized solar cells (DSSCs) by incorporating
multi-wall carbon nanotubes (MWCNTs) in conventional nanostructured semiconducting TiO2
photoanodes. DSSCs were prepared by adding various concentrations of MWCNTs (up to 1.0% wt.) to TiO2
anatase nanoparticles. Optimization of MWCNT concentration leads to photoconversion efficiency as
high as 4.1% as opposed to 3.7% for pure TiO2 photoanodes. The performance of the solar cells was
measured for 10 consecutive days of continuous ambient light exposure. MWCNT addition results in the
decrease of efficiency from 4.1% to 3.7%, while a decrease from 3.7% to 2.4% was recorded in pure TiO2
photoanodes. These results are encouraging toward the commercial exploitation of DSSCs
Epitaxial magnetite nanorods with enhanced room temperature magnetic anisotropy
No full textNanostructured 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 Fe3O4 nanorods on a SrTiO3 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 Fe3O4 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
Photovoltaic properties of Bi2FeCrO6 epitaxial thin films.
No full textWe 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
