Halide perovskites: Current issues and new strategies to push material and device stability

Abstract This short review aims at summarizing the current challenges related to poor Perovskite Solar Cells (PSCs) stability which nowadays puts severe constrains on near future device commercialization. As a game changer in the field of photovoltaics (PVs), PSCs are highly efficient and cheap to fabricate. However, they suffer from poor long-term stability upon exposure to heat, moisture, oxygen and light, and combinations thereof. Poor device stability originates from intrinsic instability issues of the perovskite active layer itself, as well as extrinsic factors due to partial degradation of the layers composing the device stack. Here we briefly review the chemical and physical processes responsible for intrinsic material instability, and we highlight possible solutions to overcome it; we then consider the whole device, discussing properties and interactions of the stacked layers. Finally, particular emphasis is placed on the need of shared standards for stability tests, which should include detailed report on experimental conditions over a statistically significant number of samples, allowing for a direct comparison of results across different groups and fostering a rapid advance of our understanding of degradation mechanisms and of the solutions to overcome them

Band gap narrowing in ferroelectric KNbO3-Bi(Yb,Me)O3 (Me=Fe or Mn) ceramics

The direct optical band gap in ferroelectric KNbO3-Bi(Yb,Me)O3 (Me=Fe or Mn) ceramics fabricated by the solid state reaction method varies from 3.2 eV for KNbO3 down to 2.2 eV for 0.95KNbO3-0.05BiYbO3, as revealed by optical spectroscopic ellipsometry. This narrowing of band gap is accompanied by an apparent increase of the room-temperature relative permittivity from 320 for KNbO3 to 900 for 0.95KNbO3-0.05BiYbO3. All compositions studied exhibit dielectric anomalies associated with structural phase transitions and their ferroelectric nature is corroborated by the presence of a sharp mixed mode (at ~190 cm-1) and by a Fano-type resonant dip in their Raman spectra

Bone apparent and material densities examined by cone beam computed tomography and the Archimedes technique: comparison of the two methods and their results

An understanding of bone apparent and material densities and how they vary within bone at the organ level is of great interest in the understanding of degenerative bone conditions and for biomedical engineering applications. The densities of bone tissue have been shown to appreciably influence the mechanical competency of bone tissue. In order to assess the density of bone in the body, it is important to ensure that the parameters being measured in vivo are truly representative of the real values that have been measured in vitro. To assess the densities of bone across the entire spectrum of available porosities, 112 samples from an elephant femur were assessed using the Archimedes method (water displacement) and by micro-computed tomography (μ-CT). Comparisons were drawn between the two methods to determine if the densities calculated by μ-CT were representative of physically measured densities. The results showed that the apparent densities measured over the entire spectrum were very similar but varied in the intermediate regions of bone tissue, probably due to an increased presence of osteoid, increased remodeling, or experimental error as these type of bone is known for the presence of regions of closed cell geometry in the cancellous architecture. It could be argued that the measurements taken by μ-CT are more reliable of bone density values for the mineralized regions of bone as the threshold is defined with respect to the absorption of X-rays by the mineral. In contrast, the Archimedes method thresholds everything with a density value above that of the surrounding medium, 1 (g cm−3) for water, and hence it is more sensitive to the presence of osteoid, soft collagenous matrix, and epithelial cell layers. Further research is required to optimize the parameters of scanning methods for the structural properties of different bone tissue porosities, which hopefully in turn will be able to provide a basis for the development of predictive remodeling models

Structure–property relationships in (1 − x)BaTiO3–xBiGdO3 ceramics

(1 − x)BaTiO3–xBiGdO3 ceramics were prepared by the solid state reaction method. X-ray diffraction and Raman spectroscopy indicate a maximum co-solubility of Bi/Gd in BaTiO3 at x = 0.10 with a change of symmetry from tetragonal to pseudo-cubic at x = 0.08. Backscattered electron images, however, reveal the presence of a secondary phase in x ≥ 0.06. The dielectric behaviour evolves continuously with x from a classical ferroelectric to a typical relaxor and this transition is accompanied by a shift in the permittivity maxima towards lower temperatures. The presence of two dielectric anomalies for x ≥ 0.06 is associated with residual core–shell structures, as revealed by transmission electron microscopy. The dielectric anomaly associated with the core regions remains at ∼120 °C, whereas the other anomaly decreases continuously towards lower temperature with x. This study shows that chemical equilibrium is much more difficult to achieve than in other (1 − x)BaTiO3–xBi[Me]O3 systems, where Me is Yb or Sc

Continuously controllable optical band gap in orthorhombic ferroelectric KNbO3-BiFeO3 ceramics

The optical band gap of orthorhombic ferroelectric KNbO3 is shown to be continuously controllable via Bi and Fe co-substitution according to a K1-xBixNb1-xFexO3 doping mechanism. Room temperature X-ray diffraction data combined with Raman spectroscopy analysis show the polar orthorhombic crystal structure to persist up to x=0.25, while the bandgap narrows monotonically by 1 eV (~33%). In-situ Raman spectroscopy corroborates the polar nature of all compositions in the temperature range -100 to 200 C. The ability to control the band gap while maintaining the spontaneous polarisation makes the K1-xBixNb1-xFexO3 system interesting for photoinduced processes in a wide temperature range

Band gap evolution and piezoelectric-to-electrostrictive crossover in (1-x)KNbO3-x(Ba0.5Bi0.5)(Nb0.5Zn0.5)O3 ceramics

The band gap of (1-x)KNbO3-x(Ba0.5Bi0.5)(Nb0.5Zn0.5)O3 (0≤x≤0.25) ceramics narrows slightly from 3.22 eV for x=0 to 2.89 eV for x=0.25, in broad agreement with first-principle calculations [Phys. Rev. B 89, 235105 (2014)]. In addition, an unreported piezoelectric-to-electrostrictive crossover is observed in this compositional range, which is accompanied by a continuous decrease of the maximum electric field-induced strain due to the presence of a non-ferroelectric phase. An electrostriction coefficient of 0.023 m4/C2 is measured for x=0.05, whilst no electromechanical response is observed for non-ferroelectric x=0.25, even under an applied electric field of 80 kV/cm

Remarkable impact of low BiYbO3 doping levels on the local structure and phase transitions of BaTiO3

In-situ Raman spectroscopy shows the simultaneous incorporation of small amounts of Bi3+ and Yb3+ into the lattice of BaTiO3 to break the average symmetry inferred from X-Ray powder diffraction analysis and permittivity measurements. In particular, Bi3+ with a stereochemically active lone-pair of electrons induces severe lattice strain and the coexistence of different local crystal symmetries over a wide temperature range, effectively controlling the physical properties, such as the temperature dependence of the permittivity and the Curie temperature. These results show that compositional gradients based in small variations of these two dopants could successfully explain the enhanced thermal stability of the permittivity in core-shell type ceramics, whereas the lower capacitance of the shell can also cap the maximum permittivity at the Curie temperature

Multiferroic and magnetoelectric properties of Pb0.99[Zr0.45Ti0.47(Ni1/3Sb2/3)0.08]O3–CoFe2O4 multilayer composites fabricated by tape casting

A 2-2 type multiferroic composite device encompassing three CoFe2O4 (CFO) layers confined between four Pb0.99[Zr0.45Ti0.47(Ni1/3Sb2/3)0.08]O3 (PZT) layers was fabricated by tape casting. X-ray diffraction data showed good chemical compatibility between the two phases, whereas Scanning Electron Microscopy imaging also revealed an intimate contact between CFO and PZT layers. Under an applied electric field of 65 kV/cm, this multilayer device shows a saturated polarisation of 7.5 C/cm2 and a strain of 0.12%, whereas under a magnetic field of 10 kOe it exhibits a typical ferromagnetic response and a magnetic moment of 33 emu/g. These devices can be electrically poled, after which they exhibit magnetoelectric coupling

Nanoscale Mapping of Bromide Segregation on the Cross Sections of 2 Complex Hybrid Perovskite Photovoltaic Films Using Secondary 3 Electron Hyperspectral Imaging in a Scanning Electron Microscope

Mixed halide (I/Br) complex organic/inorganic hybrid perovskite materials have attracted much attention recently because of their excellent photovoltaic properties. Although it has been proposed that their stability is linked to the chemical inhomogeneity of I/Br, no direct proof has been offered to date. Here, we report a new method, secondary electron hyperspectral imaging (SEHI), which allows direct imaging of the local variation in Br concentration in mixed halide (I/Br) organic/inorganic hybrid perovskites on a nanometric scale. We confirm the presence of a nonuniform Br distribution with variation in concentration within the grain interiors and boundaries and demonstrate how SEHI in conjunction with low-voltage scanning electron microscopy can enhance the understanding of the fundamental physics and materials science of organic/inorganic hybrid photovoltaics, illustrating its potential for research and development in “real-world” applications

Yttrium Iron Garnet/Barium Titanate Multiferroic Composites

Dense multiferroic 0-3 type composites encompassing BaTiO3 and Y3Fe5O12 were fabricated by the solid state reaction method. X-ray diffraction data combined with Scanning Electron Microscopy imaging show virtual immiscibility between the two phases, with the Y3Fe5O12 ferrimagnetic phase well dispersed in the tetragonal BaTiO3 ferroelectric matrix. Raman spectroscopy analyses corroborate the polar nature of the BaTiO3 matrix in composites with a Y3Fe5O12 content as great as 40 wt%. Ferrimagnetism is detected in all composites and no additional magnetic phases are distinguished. Although these dense ceramics can be electrically poled, they exhibit a very weak magnetoelectric response, which slightly increases with Y3Fe5O12 content