Wei Huang

Epitaxial Bi₂FeCrO₆nanoisland arrays with room-temperature multiferroic properties were fabricated by patterned SiO₂template-assisted pulsed laser deposition.

The photoelectric properties of multiferroic double-perovskite Bi2FeCrO6 (BFCO), such as above-bandgap photovoltages, switchable photocurrents and bulk photovoltaic effect, 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 4−fold enhancement of photocurrent density, up to 0.4 mA cm−2 (at +1.23 V vs. RHE), a best performance of stability over four hours 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 optimisation of PEC performance by polarization switching of BFCO, as well as the successful achievement of p−TCOs/n−FE heterojunction photoanodes able to sustain multiple hour-stable water oxidation.

Perovskite multiferroic oxides are promising materials for the realization of sensitive and switchable photodiodes, due to their favorable band gap (< 3.0 eV), high absorption coefficient and tunable internal ferroelectric 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. 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 which is so far the highest reported for any ferroelectric thin-film photodiode. Our work highlights the huge potential for using multiferroic oxides to fabricate highly-sensitive and switchable photodiodes.

The p–i–n heterojunction devices based on intrinsic absorber multiferroic Bi ₂ FeCrO ₆ sandwiched between p-type NiO and n-type Nb-doped SrTiO ₃ were demonstrated.

We 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.

We report a systematic study of the electrical and optical properties of epitaxial perovskite p‐ type In‐doped SrTiO ₃ thin films (SrIn _x Ti _{1− x} O ₃ , 0 ≤ x ≤ 0.15) grown on single‐crystal (100)‐oriented LaAlO ₃ substrates using a hybrid method which combines pulsed laser deposition and molecular beam epitaxy in a range of deposition conditions. X‐ray diffraction analysis confirms the epitaxial growth of high crystal quality films. Four‐point probe and Hall Effect measurements demonstrate that the films are p‐ type semiconductors with a low resistivity of ~10 ⁻² Ω·cm and a high carrier concentration of ~10 ¹⁹ cm ⁻³ . The optical transmittance spectra reveal that the films are highly transparent (˃70%) in the visible region.

Ferroelectric 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 −0.97 V 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.