One-dimensional (1D) nanomaterials with highly anisotropic optoelectronic properties are key components in energy harvesting, flexible electronics, and biomedical imaging devices. 3D patterning methods that precisely assemble nanowires with locally controlled composition and orientation would enable new optoelectronic device designs. As an exemplar, we have created and 3D-printed nanocomposite inks composed of brightly emitting colloidal cesium lead halide perovskite (CsPbX3, X = Cl, Br, and I) nanowires suspended in a polystyrene-polyisoprene-polystyrene block copolymer matrix. The nanowire alignment is defined by the programmed print path, resulting in optical nanocomposites that exhibit highly polarized absorption and emission properties. Several devices have been produced to highlight the versatility of this method, including optical storage, encryption, sensing, and full-color displays.

Two-dimensional (2D) excitons arise from electron-hole confinement along one spatial dimension. Such excitations are often described in terms of Frenkel or Wannier limits according to the degree of exciton spatial...

Colloidal cesium lead halide perovskite nanocrystals exhibit unique photophysical properties including high quantum yields, tunable emission colors, and narrow photoluminescence spectra that have marked them as promising light emitters for applications in diverse photonic devices. Randomly oriented transition dipole moments have limited the light outcoupling efficiency of all isotropic light sources, including perovskites. In this report we design and synthesize deep blue emitting, quantum confined, perovskite nanoplates and analyze their optical properties by combining angular emission measurements with back focal plane imaging and correlating the results with physical characterization. By reducing the dimensions of the nanocrystals and depositing them face down onto a substrate by spin coating, we orient the average transition dipole moment of films into the plane of the substrate and improve the emission properties for light emitting applications. We then exploit the sensitivity of the perovskite electronic transitions to the dielectric environment at the interface between the crystal and their surroundings to reduce the angle between the average transition dipole moment and the surface to only 14°and maximize potential light emission efficiency. This tunability of the electronic transition that governs light emission in perovskites is unique and, coupled with their excellent photophysical properties, introduces a valuable method to extend the efficiencies and applications of perovskite based photonic devices beyond those based on current materials.

Pseudoelasticity in metals is typically associated with phase transformations (e.g., shape memory alloys) but has recently been observed in sub-10 nm Ag nanocrystals that rapidly recovered their original shape after deformation to large strains. The discovery of pseudoelasticity in nanoscale metals dramatically changes the current understanding of the properties of solids at the smallest length scales, and the motion of atoms at surfaces. Yet, it remains unclear whether pseudoelasticity exists in different metals and nanocrystal sizes. The challenge of observing deformation at atomistic to nanometer length scales has prevented a clear mechanistic understanding of nanoscale pseudoelasticity, although surface diffusion and dislocation-mediated processes have been proposed. We further the understanding of pseudoelasticity in nanoscale metals by using a diamond anvil cell to compress colloidal Au nanocrystals under quasihydrostatic and nonhydrostatic pressure conditions. Nanocrystal structural changes are measured using optical spectroscopy and transmission electron microscopy and modeled using electrodynamic theory. We find that 3.9 nm Au nanocrystals exhibit pseudoelastic shape recovery after deformation to large uniaxial strains of up to 20%, which is equivalent to an ellipsoid with an aspect ratio of 2. Nanocrystal absorbance efficiency does not recover after deformation, which indicates that crystalline defects may be trapped in the nanocrystals after deformation.

A chemical passivation strategy to improve durability in GaAs solar cells is described. Trioctylphosphine sulfide (TOP:S) is identified among a promising new class of surfactants for enhanced performance and longevity of GaAs cells. Light‐beam induced current measurements (LBIC) and other studies show treatment with TOP:S mitigates efficiency losses at unpassivated sidewalls and induced fractures.

Solar cell efficiency is maximized through multijunction architectures that minimize carrier thermalization and increase absorption. Previous proposals suggest that the maximum efficiency for a finite number of subcells is achieved for designs that optimize for light trapping over radiative coupling. We instead show that structures with radiative coupling and back reflectors for light trapping, e.g. spectrum-splitting cells, can achieve higher conversion efficiencies. We model a compatible geometry, the polyhedral specular reflector. We analyze and experimentally verify the effects of spectral window and radiative coupling on voltage and power. Our results indicate that radiative coupling with back reflectors leads to higher efficiencies than previously studied architectures for practical multijunction architectures (i.e., #20 subcells).The photovoltaic community is closer than ever to achieving ultra-high multijunction solar cell efficiencies (>50%).1-8 Subcells from III-V compound semiconductors are approaching ideal Shockley-Queisser behavior and emit signicant radiation of photons with energies equal to or above the optical bandgap because nonradiative recombination has been minimized with advanced growth processes.6,9 The optical environment of a solar cell controls where the radiated photons from a subcell are directed and this greatly affects its efficiency.2,3 Thus the optical design of multijunction architectures is crucial for maximizing performance. To date, (1) light trapping and (2) radiative coupling have been investigated as promising optical design strategies. Light trapping inhibits the radiative emission of a subcell in order to reduce the dark current and increase voltage.For example, this can be achieved by including a back reector on a cell.9 By contrast, radiative coupling directs radiative emission between neighboring subcells for reconversion. 2,8Cells that have a high degree of radiative coupling have higher currents and are more tolerant of spectral mismatch because photons can be redistributed and boost carrier generation in the current-limited subcells.10-15 Thus including both strong light trapping and radiative coupling could yield very high efficiencies. However, only geometries that optimize for either strong light trapping or strong radiative coupling have been considered in the previous literature.2 Until now, a proposed structure that only optimizes for light trapping and completely blocks radiative coupling using frequency selective reectors matched to the band gap emission of each subcell has been assumed to be the most efficient structure for discrete numbers of junctions. This 'selective reector' design has been shown to give the highest efficiencies for time symmetric structures comprised of a realistic number Broader contextEven with the recent advances in photovoltaics research, 50% solar cell efficiencies have not yet been achieved. Previous designs have focused on a tandem stack structure where semiconductor layers are epitaxially grown or wafer bonded on top of e...

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