Achieving high power conversion efficiencies (PCEs) in ferroelectric photovoltaics (PVs) is a longstanding challenge. Although recently ferroelectric thick films, composite films, and bulk crystals have all been demonstrated to exhibit PCEs >1%, these systems still suffer from severe recombination because of the fundamentally low conductivities of ferroelectrics. Further improvement of PCEs may therefore rely on thickness reduction if the reduced recombination could overcompensate for the loss in light absorption. Here, a PCE of up to 2.49% (under 365-nm ultraviolet illumination) was demonstrated in a 12-nm Pb(Zr 0.2 Ti 0.8)O 3 (PZT) ultrathin film. The strategy to realize such a high PCE consists of reducing the film thickness to be comparable with the depletion width, which can simultaneously suppress recombination and lower the series resistance. The basis of our strategy lies in the fact that the PV effect originates from the interfacial Schottky barriers, which is revealed by measuring and modeling the thickness-dependent PV characteristics. In addition, the Schottky barrier parameters (particularly the depletion width) are evaluated by investigating the thickness-dependent ferroelectric, dielectric and conduction properties. Our study therefore provides an effective strategy to obtain high-efficiency ferroelectric PVs and demonstrates the great potential of ferroelectrics for use in ultrathin-film PV devices.

Oxygen stoichiometry plays a crucial role in determining the crystalline structure and physical properties of transition metal oxides (TMOs). Tuning the oxygen content via an electrochemical redox reaction can effectively manipulate the functionalities of TMOs, which is harnessed in many cutting-edge energy and information technologies such as fuel cells, [1] rechargeable batteries, [2] supercapacitors, [3] and memory devices. [4] The redox reaction in certain TMOs can be enabled by a so-called topotactic phase transformation, manifesting itself as the insertion/release of a large amount of oxygen ions without breaking the lattice framework. For example, for an ABO 3 perovskite (PV) phase, upon forming ordered oxygen vacancy channels in its lattice, it transforms into an ABO 2.5 brownmillerite (BM) phase. Along with the structural change, many intriguing physical phenomena emerge owing to the couplings between lattice, charge, and Resistive switching (RS) memory has stayed at the forefront of next-generation nonvolatile memory technologies. Recently, a novel class of transition metal oxides (TMOs), which exhibit reversible topotactic phase transformation between insulating brownmillerite (BM) phase and conducting perovskite (PV) phase, has emerged as promising candidate materials for RS memories. Nevertheless, the microscopic mechanism of RS in these TMOs is still unclear. Furthermore, RS devices with simultaneously high density and superior memory performance are yet to be reported. Here, using SrFeO x as a model system, it is directly observed that PV SrFeO 3 nanofilaments are formed and extend almost through the BM SrFeO 2.5 matrix in the ON state and are ruptured in the OFF state, unambiguously revealing a filamentary RS mechanism. The nanofilaments are ≈10 nm in diameter, enabling to downscale Au/ SrFeO x /SrRuO 3 RS devices to the 100 nm range for the first time. These nanodevices exhibit good performance including ON/OFF ratio as high as ≈10 4 , retention time over 10 5 s, and endurance up to 10 7 cycles. This study significantly advances the understanding of the RS mechanism in TMOs exhibiting topotactic phase transformation, and it also demonstrates the potential of these materials for use in high-density RS memories.

The polarization imprint along with the photovoltaic (PV) effect has been studied in Pt/ Pb(Zr 0.3 Ti 0.7)O 3 /SrRuO 3 ferroelectric capacitors. It is shown that the positive DC poling induces the imprint with a downward direction whereas the negative DC poling suppresses the imprint (i.e., rejuvenation). In the polarization up state, the imprinted capacitor exhibits degraded PV properties compared with the rejuvenated one. This may be because the imprint reduces the number of upward domains, thus lowering the driving force for the PV effect. In the polarization down state, however, the rejuvenated capacitor enters the imprinted state spontaneously. This rejuvenation-to-imprint transition can be further aggravated by applying positive voltages and ultraviolet illumination. It is proposed that the domain pinning/depinning, which are associated with the oxygen vacancies and trapped electrons modulated by polarization, voltage, and illumination, may be responsible for the polarization imprint and rejuvenation. Our study therefore sheds light on the correlation between the polarization imprint and the PV effect in the ferroelectrics and also provides some viable suggestions to address the imprint-induced degradation of PV performance.

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Nanoscale phase mixtures in transition-metal oxides (TMOs) often render these materials susceptible to external stimuli (electric field, mechanical stress, etc.), which can lead to rich functional properties and device applications. Here, direct observation and multifield manipulation of a nanoscale mixture of brownmillerite SrFeO2.5 (BM-SFO) and perovskite SrFeO3 (PV-SFO) phases in SrFeOx (SFO) epitaxial thin films are reported. The mixed-phase SFO film in its pristine state exhibits a nanoscaffold structure consisting of PV-SFO nanodomains embedded in the BM-SFO matrix. This nanoscaffold structure produces gridlike patterns in the current and electrochemical strain maps, owing to the strikingly different electrical and electrochemical properties of BM-SFO and PV-SFO. Moreover, electric field control of reversible topotactic phase transformation between BM-SFO and PV-SFO is demonstrated by electric-field-induced reversible changes in surface height, conductance, and electrochemical strain response. In addition, it is also shown that the BM-SFO → PV-SFO phase transformation can be enabled by applying mechanical stress. This study therefore not only identifies a strong nanometric structure–property correlation in the mixed-phase SFO but also offers a new paradigm for the multifield control of topotactic phase transformation.