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Solution processing of semiconductors is highly promising for the high-throughput production of cost-effective electronics and optoelectronics. Although hybrid perovskites have potential in various device applications, challenges remain in the development of high-quality materials with simultaneously improved processing reproducibility and scalability. Here, we report a liquid medium annealing (LMA) technology that creates a robust chemical environment and constant heating field to modulate crystal growth over the entire film. Our method produces films with high crystallinity, fewer defects, desired stoichiometry, and overall film homogeneity. The resulting perovskite solar cells (PSCs) yield a stabilized power output of 24.04% (certified 23.7%, 0.08 cm2) and maintain 95% of their initial power conversion efficiency (PCE) after 2000 hours of operation. In addition, the 1-cm2 PSCs exhibit a stabilized power output of 23.15% (certified PCE 22.3%) and keep 90% of their initial PCE after 1120 hours of operation, which illustrates their feasibility for scalable fabrication. LMA is less climate dependent and produces devices in-house with negligible performance variance year round. This method thus opens a new and effective avenue to improving the quality of perovskite films and photovoltaic devices in a scalable and reproducible manner.

Symmetry is highly relevant with various quantities and phenomena in physics. While the translational symmetry breaks at the edges of two-dimensional hexagonal crystalline flakes, it is usually associated with the breaking of central inversion symmetry that is yet to be observed in terms of physical properties. Here, we report an experiment-theory joint study on in-plane compressed single-crystal monolayer WS

Defects play an important role in the degradation processes of hybrid halide perovskite absorbers, impeding their application for solar cells. Among all defects, halide anion and organic cation vacancies are ubiquitous, promoting ion diffusion and leading to thin-film decomposition at surfaces and grain boundaries. Here, we employ fluoride to simultaneously passivate both anion and cation vacancies, by taking advantage of the extremely high electronegativity of fluoride. We obtain a power conversion efficiency of 21.46% (and a certified 21.3%-efficient cell) in a device based on the caesium, methylammonium (MA) and formamidinium (FA) triple-cation perovskite (Cs0.05FA0.54MA0.41)Pb(I0.98Br0.02)3 treated with sodium fluoride. The device retains 90% of its original power conversion efficiency after 1,000 h of operation at the maximum power point. With the help of first-principles density functional theory calculations, we argue that the fluoride ions suppress the formation of halide anion and organic cation vacancies, through a unique strengthening of the chemical bonds with the surrounding lead and organic cations. Defects and defect migration are detrimental for perovskite solar cell efficiency and long-term stability. Li et al. show that fluoride is able to suppress the formation of halide anion and organic cation vacancy defects by restraining the relative ions via ionic and hydrogen bonds.

Piezoelectricity usually accompanies with flexoelectricity in polar materials which is the linear response of polarization to a strain gradient. Therefore, it is difficult to eliminate piezoelectric effect in determination of pure flexoelectric response. In this work, we propose an analytical method to characterize the flexoelectric coefficient quantitatively at nanoscale in piezoelectric materials by screening piezoelectricity. Our results show that the flexoelectricity reduces the nanopillar stiffness while the piezoelectricity enhances it. With precise design of the shape of the nanopillars and measurement of their stiffness difference, the flexoelectric coefficient can be obtained with the piezoelectric contribution eliminated completely. This approach avoids the measurement of electrical properties with dynamic load, which helps to reduce the challenge of flexoelectric characterization at nanoscale. Our work suggests an alternative strategy for quantitative characterization of flexoelectric properties and design of flexoelectric devices at nanoscale.

Uniaxial and biaxial strain approaches are usually implemented to switch the ferroelastic states, which play a key role in the application of the ferroics and shape memory materials. In this work, by using the first-principles calculations, we found not only uniaxial strain, but also shear strain can induce a novel ferroelastic switching, in which the van der Waals (vdW) layered direction rotates with the ferroelastic transition in layered bulk PdSe2. This is an interesting phenomenon that has never been discovered. The novel three-state ferroelastic switching in layered PdSe2 also occurs under shear strain. Our result shows that the shear strain could be used as an effective approach for manipulating the functionalities of layered materials in potential device applications.

Van der Waals (vdW) layered materials have rather weaker interlayer bonding than the intra-layer bonding, therefore the exfoliation along the stacking direction enables the achievement of monolayer or few layers vdW materials with emerging novel physical properties and functionalities. The ferroelectricity in vdW materials recently attracts renewed interest for the potential use in high-density storage devices. As the thickness going thinner, the competition between the surface energy, depolarization field and interfacial chemical bonds may give rise to the modification of ferroelectricity and crystalline structure, which has limited investigations. In this work, combining the piezoresponse force microscope scanning, contact resonance imaging, we report the existence of the intrinsic in-plane polarization in vdW ferroelectrics CuInP2S6 (CIPS) single crystals, whereas below a critical thickness between 90-100 nm, the in-plane polarization disappears. The Young's modulus also shows an abrupt stiffness at the critical thickness. Based on the density functional theory calculations, we ascribe these behaviors to a structural phase transition from monoclinic to trigonal structure, which is further verified by transmission electron microscope technique. Taken together, these findings demonstrate the foundational importance of structural phase transition for enhancing the rich functionality and broad utility of vdW ferroelectrics.