Enhanced Switchable Ferroelectric Photovoltaic Effects in Hexagonal Ferrite Thin Films via Strain Engineering

Abstract

Ferroelectric photovoltaics (FPVs) are being extensively investigated by virtue of switchable photovoltaic responses and anomalously high photovoltages of ∼10 V. However, FPVs suffer from extremely low photocurrents due to their wide band gaps (E). Here, we present a promising FPV based on hexagonal YbFeO (h-YbFO) thin-film heterostructure by exploiting its narrow E. More importantly, we demonstrate enhanced FPV effects by suitably exploiting the substrate-induced film strain in these h-YbFO-based photovoltaics. A compressive-strained h-YbFO/Pt/MgO heterojunction device shows ∼3 times enhanced photovoltaic efficiency than that of a tensile-strained h-YbFO/Pt/AlO device. We have shown that the enhanced photovoltaic efficiency mainly stems from the enhanced photon absorption over a wide range of the photon energy, coupled with the enhanced polarization under a compressive strain. Density functional theory studies indicate that the compressive strain reduces E substantially and enhances the strength of d-d transitions. This study will set a new standard for determining substrates toward thin-film photovoltaics and optoelectronic devices.

Lattice strain-enhanced exsolution of nanoparticles in thin films

“…Unlike bulk systems, thin-film heterostructures can induce a lattice strain because of the lattice mismatch between a substrate and a film, which affects many physical properties such as ferroelectricity, electron mobility, ionic conductivity, and electrocatalysis 17–20 . In particular, thin-film oxide fuel cells are attracting renewed attention owing to advantages of low temperature operation and portable device applications 21–24 . Here, we demonstrate an unprecedently high degree of exsolution of nanoparticles in lattice misfit strained epitaxial thin films and achieve a particle density as high as ~1100 particles μm −2 , with a size of only ~5 nm, at a temperature as low as 550 °C.…”

Bulk photovoltaic effect in hexagonal LuMnO3 single crystals

“…It has been shown, for instance, that J sc can be switched by reversing the ferroelectric polarization (P) of the film, which indicates that the Schottky barriers at interfaces with electrodes play a major role [20]. However, to the best of our knowledge, no evidence of BPE has ever been reported neither in h-RMnO 3 , nor in the isostructural h-RFeO 3 [21,22]. Here we report on the photoresponse of h-LuMnO 3 (h-LMO) single crystals, aiming at determining its BPE response which may open new avenues beyond promising opportunities in conventional photovoltaics [23].…”

Single-phase multiferroics: new materials, phenomena, and physics

“…For example, h-RFeO 3 structure can be easily obtained in thin films via the substrate matching route. Indeed, elevated temperature ferroelectricity in (Lu, Yb, Tm)FeO 3 epitaxial films has been demonstrated and the measured remnant polarization P r ∼ 5 μC/cm 2 [ 33 , 34 ], as shown in Fig. 3 .…”

“… Room-temperature ferroelectric hysteresis loops of (a) hexagonal YbFeO 3 films grown on different substrates, and (b) hexagonal LuFeO 3 and TmFeO 3 films grown on Al 2 O 3 (0001) substrates [ 33 , 34 ]. Antiferromagnetic transition temperature T N as a function of (c) lattice parameter c / a , and (d) tolerance factor for hexagonal manganites and ferrites [ 37 , 38 ].…”