Perovskite solar cells (PSCs) have been emerging as a breakthrough photovoltaic technology, holding unprecedented promise for low‐cost, high‐efficiency renewable electricity generation. However, potential toxicity associated with the state‐of‐the‐art lead‐containing PSCs has become a major concern. The past research in the development of lead‐free PSCs has met with mixed success. Herein, the promise of coarse‐grained B‐γ‐CsSnI3 perovskite thin films as light absorber for efficient lead‐free PSCs is demonstrated. Thermally‐driven solid‐state coarsening of B‐γ‐CsSnI3 perovskite grains employed here is accompanied by an increase of tin‐vacancy concentration in their crystal structure, as supported by first‐principles calculations. The optimal device architecture for the efficient photovoltaic operation of these B‐γ‐CsSnI3 thin films is identified through exploration of several device architectures. Via modulation of the B‐γ‐CsSnI3 grain coarsening, together with the use of the optimal PSC architecture, planar heterojunction‐depleted B‐γ‐CsSnI3 PSCs with power conversion efficiency up to 3.31% are achieved without the use of any additives. The demonstrated strategies provide guidelines and prospects for developing future high‐performance lead‐free PVs.
In photonic crystals, two materials with different refractive indexes alternate with a specific periodicity on the opticalwavelength scale. Analogous to the band-gap for electrons in semiconductors, the spatially periodic structure in photonic crystals results in a forbidden stop-band for a particular spectral range of photons, the so-called photonic band-gap. Since being first proposed by Yablonovich and John, [1,2] photonic crystals have been desired to give complete photonic band-gap in the optical regime. In other words, within a specific range of frequencies photons should not be allowed to propagate in all three dimensions inside photonic crystals. Because of the optical scale of the periodicity in refractive index, the fabrication of photonic crystals with complete photonic band-gaps in the optical regime remains a challenge. Physical top-down approaches, including lithographic techniques starting from bulk materials, have proven their efficacy in the long-wavelength radar range, but are difficult to extend to the optical spectrum. Chemical bottom-up techniques, for example self-assembly of highly monodisperse and spherical colloidal particles, offer a viable alternative based on the combined ease of fabrication and low cost. Such photonic crystals from highly ordered colloidal particles are also known as artificial opals. However, in the crystallization of spherical colloids, only two most-stable crystal structures with high filling fraction have been obtained (for face-centered cubic, FCC, and for random hexagonal closed packing, RHCP).It has been suggested by previous studies that for an FCC lattice consisting of colloidal spheres there only exists a pseudo photonic band-gap in the photonic band structure, no matter how high the dielectric contrast, because of a symmetry-induced degeneracy at the W-or U-point of the band structure. [3,4] As suggested by computational studies, this degeneracy can be broken using shape-anisotropic [5] or dielectrically anisotropic [6] colloidal particles as building blocks.Early research in this direction mainly focused on the deformation of colloidal building blocks from spheres to ellipsoids after their being self-assembled into colloidal crystals. This ''post-crystallization treatment'' includes high-energy ion irradiation [7] and uniaxial mechanical stretching, [8,9] leading to colloidal crystals with ellipsoidal silica (SiO 2 ), zinc sulfide (ZnS), and polystyrene (PS) optical atoms. However, these deformation strategies applied on prefabricated colloidal crystals may cause the nonuniform deformation in different parts of the photoniccrystal film, or even to the destruction of the whole periodic superlattice. Direct self-assembly of nonspherical colloidal particles has been studied as an alternative strategy. Polystyrene ellipsoids were used as building blocks for direct self-assembly. The lack of long-range order indicated the difficulty of organizing nonspherical colloids into 3D crystalline lattices.[9] In recent years, Liddell et al. demonstrated the fabrication ...
Nanoactuators and nanomachines have long been sought after, but key bottlenecks remain. Forces at submicrometer scales are weak and slow, control is hard to achieve, and power cannot be reliably supplied. Despite the increasing complexity of nanodevices such as DNA origami and molecular machines, rapid mechanical operations are not yet possible. Here, we bind temperatureresponsive polymers to charged Au nanoparticles, storing elastic energy that can be rapidly released under light control for repeatable isotropic nanoactuation. Optically heating above a critical temperature T c = 32°C using plasmonic absorption of an incident laser causes the coatings to expel water and collapse within a microsecond to the nanoscale, millions of times faster than the base polymer. This triggers a controllable number of nanoparticles to tightly bind in clusters. Surprisingly, by cooling below T c their strong van der Waals attraction is overcome as the polymer expands, exerting nanoscale forces of several nN. This large force depends on van der Waals attractions between Au cores being very large in the collapsed polymer state, setting up a tightly compressed polymer spring which can be triggered into the inflated state. Our insights lead toward rational design of diverse colloidal nanomachines.ctuators are needed to turn energy sources into physical movement. These can be for microrobotics, sensing, storage devices, smart windows and walls, or more general functional and active materials. Such artificial muscles have gained rapidly increasing interest (1, 2) leading to micropropellers (3, 4), gas jets from catalytic surfaces (5), and DNA machines (6). However, the actuation methods, delivery of energy, and forces obtained (typically 10 fN/nm 2 ) are limited so far (7): Magnetic fields are inconvenient to apply locally for actuation, as is >200°C heating to actuate polymer fibers; the nanocatalysis of chemical fuels lacks controllability, whereas DNA machines rely on "fuel" DNA strands to competitively bind and operate on very slow (second) timescales. Piezoelectric-type materials used in high-end instrumentation (such as atomic force microscopy or nanopositioning stages) provide short travel but with inorganic materials that are dense, delicate, expensive, hard to fabricate, and demand high voltages (150-300 V), as is also true for electrostrictive rubbers and relaxor ferroelectrics (8, 9). Many biological systems such as Escherichia coli (10), cilia (11), or nematocysts (12) provide sophisticated models for nanomachines (13). Although molecular motors and artificial muscles from hydrogels (14, 15), colloids (16), or liquid crystalline elastomers (17, 18) successfully mimic such behaviors, they are very slow (on the order of seconds) and the forces generated are very small (∼ pN). This is because either the energy density stored in the system is low or the energy release is inefficient.To overcome this we design a colloidal actuating transducer system with high-energy storage (1,000 k B T/cycle) and fast (>MHz) release mechanism....
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