A flexible quasi-solid-state Ni-Zn battery is developed by using tiny ZnO nanoparticles and porous ultrathin NiO nanoflakes conformally deposited on hierar chical carbon-cloth-carbon-fiber (CC-CF) as the anode (CC-CF@ZnO) and cathode (CC-CF@NiO), respectively. The device is able to deliver high performance (absence of Zn dendrite), superior to previous reports on aqueous Ni-Zn batteries and other flexible electrochemical energy-storage devices.

Ferroelectricity has been believed to be an important but controversial origin of the excellent photovoltaic performance of organometal trihalide perovskites (OTPs). Here we investigate the ferroelectricity of a prototype OTP, CH3NH3PbI3 (MAPbI3), both theoretically and experimentally. Our first-principles calculations based on 3-D periodic boundary conditions reveal that a ferroelectric structure with polarization of ∼8 μC/cm(2) is the globally stable one among all possible tetragonal structures; however, experimentally no room-temperature ferroelectricity is observed by using polarization-electric field hysteresis measurements and piezoresponse force microscopy. The discrepancy between our theoretical and experimental results is attributed to the dynamic orientational disorder of MA(+) groups and the semiconducting nature of MAPbI3 at room temperature. Therefore, we conclude that MAPbI3 is not ferroelectric at room temperature; however, it is possible to induce and experimentally observe apparent ferroelectric behavior through our proposed ways. Our results clarify the controversy of the ferroelectricity in MAPbI3 and also provide valuable guidance for future studies on this active topic.

Among the different types of multiferroic compounds, bismuth ferrite (BiFeO 3 ; BFO) stands out because it is perhaps the only one being simultaneously magnetic and strongly ferroelectric at room temperature. Therefore, in the past decade or more, extensive research has been devoted to BFO-based materials in a variety of different forms, including ceramic bulks, thin films and nanostructures. Ceramic bulk BFO and their solid solutions with other oxide perovskite compounds show excellent ferroelectric and piezoelectric properties and are thus promising candidates for lead-free ferroelectric and piezoelectric devices. BFO thin films, on the other hand, exhibit versatile structures and many intriguing properties, particularly the robust ferroelectricity, the inherent magnetoelectric coupling, and the emerging photovoltaic effects. BFO-based nanostructures are of great interest owing to their size effect-induced structural modification and enhancement in various functional behaviors, such as magnetic and photocatalytic properties. Although to date several

REVIEWThis journal is © The Royal Society of Chemistry 2015 J. Mater. Chem. A 1Over the past few years, very interestingly, two subclasses of perovskitesorganic-inorganic halide perovskites and ferroelectric oxide perovskites, have simultaneously become the hotspots in the research field of photovoltaics. Organic-inorganic halide perovskites have launched a new era of low-cost, high-efficiency solar cells, due to their easy solution processability and superior optical and electrical properties for the photovoltaic effect. More recently, a so-called giant switchable photovoltaic effect has been demonstrated in organicinorganic halide perovskites, thus promising a new memristive functionality. On the other hand, the recent renaissance of ferroelectric oxide perovskites for photovoltaics is caused by their fundamentally new photovoltaic mechanisms, which can produce a photovoltage far beyond the bandgap and may even lead to a boost of energy conversion efficiency. In addition, the combination of photovoltaic properties with the ferroic orders may create many novel functionalities for ferroelectric oxide perovskites. Toward the common goals of developing high-efficiency photovoltaics and novel opto-electronic functional devices, these two different subclasses of perovskites shall be brought together into a combined review. In this context, we review both organic-inorganic halide perovskites and ferroelectric oxide perovskites for photovoltaics, focusing on the material nature and the photovoltaic mechanisms. We also discuss their respective unresolved issues, along with useful suggestions for future research.