Inorganic caesium lead iodide perovskite solar cells

The vast majority of perovskite solar cell research has focused on organic–inorganic lead trihalide perovskites. Herein, we present working inorganic CsPbI3 perovskite solar cells for the first time. CsPbI3 normally resides in a yellow non-perovskite phase at room temperature, but by careful processing control and development of a low-temperature phase transition route we have stabilised the material in the black perovskite phase at room temperature. As such, we have fabricated solar cell devices in a variety of architectures, with current–voltage curve measured efficiency up to 2.9% for a planar heterojunction architecture, and stabilised power conversion efficiency of 1.7%. The well-functioning planar junction devices demonstrate long-range electron and hole transport in this material. Importantly, this work identifies that the organic cation is not essential, but simply a convenience for forming lead triiodide perovskites with good photovoltaic properties. We additionally observe significant rate-dependent current–voltage hysteresis in CsPbI3 devices, despite the absence of the organic polar molecule previously thought to be a candidate for inducing hysteresis via ferroelectric polarisation. Due to its space group, CsPbI3 cannot be a ferroelectric material, and thus we can conclude that ferroelectricity is not required to explain current–voltage hysteresis in perovskite solar cells. Our report of working inorganic perovskite solar cells paves the way for further developments likely to lead to much more thermally stable perovskite solar cells and other optoelectronic devices

Powder synthesis and crystal growth of Y2V2O7 under high pressure and its physical properties

Y2V2O7 with pyrochlore structure has been synthesized under a high pressure of 5 GPa at 1200 °C. The crystal structure has been determined by Rietveld analysis of the X-ray diffraction data. The structure is cubic with a=10.0143(8) Å and can be described by the space group F d -3 m (no. 227). Scanning electron microscopy data suggest the efficiency of the grain growth method for single crystal growth of Y2V2O7 under high pressure and high temperature conditions. Magnetic susceptibility data conforming to the Curie-Weiss law in the range 150-300 K with an effective moment peff of 1.76μB/V4+ ion and θ=+68 K indicates the predominance of ferromagnetic interactions. © 2007 Elsevier B.V. All rights reserved

Crystal growth of A2V2O7 (A = Y, Er, and Dy) pyrochlores using high pressure

Single crystals of pyrochlore-type vanadates A2V 2O7 where A = Er, Y, and Dy have been successfully grown under high pressure and high temperature, using a multianvil technique. The temperature gradient method was applied for the crystal growth. The crystal structure of all three compounds has been determined from the Rietveld analysis of X-ray powder diffraction (XRD) data and confirmed a cubic pyrochlore structure in space group Fd3̄m (No. 227). The unit cell parameters were a = 9.9924(1), 10.01438(8), and 10.0372(3) Å for Er2V 2O7, Y2V2O7, and Dy 2V2O7, respectively. Their compositions were investigated by energy dispersive X-ray analysis. A single crystal of Dy 2V2O7 was characterized by a single-crystal XRD method. © 2008 American Chemical Society

Crystal growth of A2V2O7 (A = Y, Er, and Dy) pyrochlores using high pressure

Single crystals of pyrochlore-type vanadates A2V 2O7 where A = Er, Y, and Dy have been successfully grown under high pressure and high temperature, using a multianvil technique. The temperature gradient method was applied for the crystal growth. The crystal structure of all three compounds has been determined from the Rietveld analysis of X-ray powder diffraction (XRD) data and confirmed a cubic pyrochlore structure in space group Fd3̄m (No. 227). The unit cell parameters were a = 9.9924(1), 10.01438(8), and 10.0372(3) Å for Er2V 2O7, Y2V2O7, and Dy 2V2O7, respectively. Their compositions were investigated by energy dispersive X-ray analysis. A single crystal of Dy 2V2O7 was characterized by a single-crystal XRD method. © 2008 American Chemical Society

Powder synthesis and crystal growth of Y2V2O7 under high pressure and its physical properties

Y2V2O7 with pyrochlore structure has been synthesized under a high pressure of 5 GPa at 1200 °C. The crystal structure has been determined by Rietveld analysis of the X-ray diffraction data. The structure is cubic with a=10.0143(8) Å and can be described by the space group F d -3 m (no. 227). Scanning electron microscopy data suggest the efficiency of the grain growth method for single crystal growth of Y2V2O7 under high pressure and high temperature conditions. Magnetic susceptibility data conforming to the Curie-Weiss law in the range 150-300 K with an effective moment peff of 1.76μB/V4+ ion and θ=+68 K indicates the predominance of ferromagnetic interactions. © 2007 Elsevier B.V. All rights reserved

Route to stable lead-free double perovskites with the electronic structure of CH3NH3PbI3: a case for mixed-cation [Cs/CH3NH3/CH(NH2)2]2InBiBr6

During the past year, halide double perovskites attracted attention as potential lead-free alternatives to Pb-based halide perovskites. However, none of the compounds discovered so far can match the optoelectronic properties of MAPbI3 (MA = CH3NH3). Here we argue that, from the electronic structure viewpoint, the only option to make Pb-free double perovskites retaining the remarkable properties of MAPbI3 is to combine In and Bi as B(+) and B(3+) cations, respectively. While inorganic double perovskites such as Cs2InBiX6 were found to be unstable due to In(+) oxidizing into In(3+), we show that the +1 oxidation state of In becomes progressively more stable as the A-site cation changes from K to Cs. Hence, we propose the use of MA and FA [FA = CH(NH2)2] to stabilize A2InBiBr6 double perovskites. We show that the optoelectronic properties of A2InBiBr6 are remarkably similar to those of MAPbI3, and explore the mixed-cation (Cs/MA/FA)2InBiBr6 halide double perovskites

On the three-dimensional structure of magnetic skyrmions

Magnetic skyrmions (skyrmions hereafter) are magnetization configurations, whose topological robustness and nano-scale size have led to speculation that they could find use as a next-generation information carrier. Skyrmions have been observed in magnetic multilayer materials that are thin compared to the radius of a skyrmion, and chiral cubic single crystals that can be far larger than any characteristic skyrmion scale. In these single crystals, one would expect that skyrmions could exhibit interesting 3D characteristics. Here, the symmetry of the micromagnetic free energy is investigated. This symmetry permits a complex 3D modulation of a skyrmion string, which we show to be a requirement of a skyrmion coexisting with the conical state. We discuss the implications of this modulation with respect to Thiele’s equation and inter-skyrmion interactions. Further to this internal modulation, we study theoretically and show experimentally that the strings themselves must contort towards the surfaces of their confining crystals

Growth modes and quantum confinement in ultrathin vapour-deposited MAPbI3 films

Vapour deposition of metal halide perovskite by co-evaporation of precursors has the potential to achieve large-area high-efficiency solar cells on an industrial scale, yet little is known about the growth of metal halide perovskites by this method at the current time. Here, we report the fabrication of MAPbI3 films with average thicknesses from 2 – 320 nm by co-evaporation. We analyze the film properties using X-ray diffraction, optical absorption and photoluminescence (PL) to provide insights into the nucleation and growth of MAPbI3 films on quartz substrates. We find that the perovskite initially forms crystallite islands of around 8 nm in height, which may be the cause of the persistent small grain sizes reported for evaporated metal halide perovskites that hinder device efficiency and stability. As more material is added, islands coalesce until full coverage of the substrate is reached at around 10 nm average thickness. We also find that quantum confinement induces substantial shifts to the PL wavelength when the average thickness is below 40 nm, offering dual-source vapour deposition as an alternative method of fabricating nanoscale structures for LEDs and other devices

Growth modes and quantum confinement in ultrathin vapour-deposited MAPbI3 films

Vapour deposition of metal halide perovskite by co-evaporation of precursors has the potential to achieve large-area high-efficiency solar cells on an industrial scale, yet little is known about the growth of metal halide perovskites by this method at the current time. Here, we report the fabrication of MAPbI3 films with average thicknesses from 2 – 320 nm by co-evaporation. We analyze the film properties using X-ray diffraction, optical absorption and photoluminescence (PL) to provide insights into the nucleation and growth of MAPbI3 films on quartz substrates. We find that the perovskite initially forms crystallite islands of around 8 nm in height, which may be the cause of the persistent small grain sizes reported for evaporated metal halide perovskites that hinder device efficiency and stability. As more material is added, islands coalesce until full coverage of the substrate is reached at around 10 nm average thickness. We also find that quantum confinement induces substantial shifts to the PL wavelength when the average thickness is below 40 nm, offering dual-source vapour deposition as an alternative method of fabricating nanoscale structures for LEDs and other devices

Evidence for unidirectional nematic bond ordering in FeSe

The lifting of $d_{xz}$-$d_{yz}$ orbital degeneracy is often considered a hallmark of the nematic phase of Fe-based superconductors, including FeSe, but its origin is not yet understood. Here we report a high resolution Angle-Resolved Photoemission Spectroscopy study of single crystals of FeSe, accounting for the photon-energy dependence and making a detailed analysis of the temperature dependence. We find that the hole pocket undergoes a fourfold-symmetry-breaking distortion in the nematic phase below 90~K, but in contrast the changes to the electron pockets do not require fourfold symmetry-breaking. Instead, there is an additional separation of the existing $d_{xy}$ and $d_{xz/yz}$ bands - which themselves are not split within resolution. These observations lead us to propose a new scenario of "unidirectional nematic bond ordering" to describe the low-temperature electronic structure of FeSe, supported by a good agreement with 10-orbital tight binding model calculations