Organometal halide perovskites exhibit large bulk crystal domain sizes, rare traps, excellent mobilities and carriers that are free at room temperature-properties that support their excellent performance in charge-separating devices. In devices that rely on the forward injection of electrons and holes, such as light-emitting diodes (LEDs), excellent mobilities contribute to the efficient capture of non-equilibrium charge carriers by rare non-radiative centres. Moreover, the lack of bound excitons weakens the competition of desired radiative (over undesired non-radiative) recombination. Here we report a perovskite mixed material comprising a series of differently quantum-size-tuned grains that funnels photoexcitations to the lowest-bandgap light-emitter in the mixture. The materials function as charge carrier concentrators, ensuring that radiative recombination successfully outcompetes trapping and hence non-radiative recombination. We use the new material to build devices that exhibit an external quantum efficiency (EQE) of 8.8% and a radiance of 80 W sr-1 m-2. These represent the brightest and most efficient solution-processed near-infrared LEDs to date.

Perovskite energy funnels for efficient light-emitting diodes

We demonstrate that, via controlled anion exchange reactions using a range of different halide precursors, we can finely tune the chemical composition and the optical properties of presynthesized colloidal cesium lead halide perovskite nanocrystals (NCs), from green emitting CsPbBr3 to bright emitters in any other region of the visible spectrum, and back, by displacement of Cl– or I– ions and reinsertion of Br– ions. This approach gives access to perovskite semiconductor NCs with both structural and optical qualities comparable to those of directly synthesized NCs. We also show that anion exchange is a dynamic process that takes place in solution between NCs. Therefore, by mixing solutions containing perovskite NCs emitting in different spectral ranges (due to different halide compositions) their mutual fast exchange dynamics leads to homogenization in their composition, resulting in NCs emitting in a narrow spectral region that is intermediate between those of the parent nanoparticles.

Tuning the Optical Properties of Cesium Lead Halide Perovskite Nanocrystals by Anion Exchange Reactions

Perovskite quantum wells yield highly efficient LEDs spanning the visible and near-infrared.

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Perovskite light-emitting diodes based on solution-processed self-organized multiple quantum wells

Lead halide perovskite materials have attracted significant attention in the context of photovoltaics and other optoelectronic applications, and recently, research efforts have been directed to nanostructured lead halide perovskites. Collodial nanocrystals (NCs) of cesium lead halides (CsPbX3, X = Cl, Br, I) exhibit bright photoluminescence, with emission tunable over the entire visible spectral region. However, previous studies on CsPbX3 NCs did not address key aspects of their chemistry and photophysics such as surface chemistry and quantitative light absorption. Here, we elaborate on the synthesis of CsPbBr3 NCs and their surface chemistry. In addition, the intrinsic absorption coefficient was determined experimentally by combining elemental analysis with accurate optical absorption measurements. 1H solution nuclear magnetic resonance spectroscopy was used to characterize sample purity, elucidate the surface chemistry, and evaluate the influence of purification methods on the surface composition. We fi...

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Highly Dynamic Ligand Binding and Light Absorption Coefficient of Cesium Lead Bromide Perovskite Nanocrystals

Organic-inorganic hybrid perovskites have cemented their position as an exceptional class of optoelectronic materials thanks to record photovoltaic efficiencies of 22.1%, as well as promising demonstrations of light-emitting diodes, lasers, and light-emitting transistors. Perovskite materials with photoluminescence quantum yields close to 100% and perovskite light-emitting diodes with external quantum efficiencies of 8% and current efficiencies of 43 cd A(-1) have been achieved. Although perovskite light-emitting devices are yet to become industrially relevant, in merely two years these devices have achieved the brightness and efficiencies that organic light-emitting diodes accomplished in two decades. Further advances will rely decisively on the multitude of compositional, structural variants that enable the formation of lower-dimensionality layered and three-dimensional perovskites, nanostructures, charge-transport materials, and device processing with architectural innovations. Here, the rapid advancements in perovskite light-emitting devices and lasers are reviewed. The key challenges in materials development, device fabrication, operational stability are addressed, and an outlook is presented that will address market viability of perovskite light-emitting devices.

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Perovskite Materials for Light-Emitting Diodes and Lasers.

We prepare colloidal nanoplatelets of methylammonium lead bromide (MAPbBr3) perovskite and compare the optical signatures of excitons in these two-dimensional systems to spherical perovskite nanocrystals and the corresponding bulk phase. We find that excitonic features that had previously been attributed to quantum confinement in MAPbBr3 nanocrystals are in fact a property of the bulk perovskite phase. Furthermore, we find that higher-energy absorption features originate from two-dimensional nanoplatelets, which are present in the nanocrystal reaction product. Upon further purification, we obtain colloidal nanoplatelets with predominantly single unit cell thickness and submicron lateral dimensions, which are stable in solution and exhibit a sharp excitonic absorption feature 0.5 eV blue-shifted from that of the three-dimensional bulk MAPbBr3 phase, representing a new addition to the growing family of colloidal two-dimensional nanostructures.

Colloidal Organohalide Perovskite Nanoplatelets Exhibiting Quantum Confinement.

https://par.nsf.gov/servlets/purl/10142624

Precipitation of multiple light elements to power Earth's early dynamo

Seismic observations suggest that the uppermost region of Earth's liquid outer core is buoyant, with slower velocities than the bulk outer core. One possible mechanism for the formation of a stably stratified layer is immiscibility in molten iron alloy systems, which has yet to be demonstrated at core pressures. We find immiscibility between liquid Fe-Si and Fe-Si-O persisting to at least 140 GPa through a combination of laser-heated diamond-anvil cell experiments and first-principles molecular dynamics simulations. High-pressure immiscibility in the Fe-Si-O system may explain a stratified layer atop the outer core, complicate differentiation and evolution of the deep Earth, and affect the structure and intensity of Earth's magnetic field. Our results support silicon and oxygen as coexisting light elements in the core and suggest that [Formula: see text] does not crystallize out of molten Fe-Si-O at the core-mantle boundary.

https://www.pnas.org/content/pnas/116/21/10238.full.pdf

Evidence for Fe-Si-O liquid immiscibility at deep Earth pressures.

Laser-heated diamond-anvil cell (LHDAC) experiments reveal electronic changes in KBr at pressures between $\ensuremath{\sim}13--81\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$ when heated to high temperatures that cause runaway heating to temperatures in excess of $\ensuremath{\sim}5000\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The drastic changes in absorption behavior of KBr are interpreted as rapid formation of high-pressure F-center defects. The defects are localized to the heated region and thus do not change the long-range crystalline order of KBr. The results have significant consequences for temperature measurements in LHDAC experiments and extend the persistence of F centers in alkali halides to at least 81 GPa.

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Thermally induced coloration of KBr at high pressures

The melting curve of elemental sulfur was measured to pressures of 65 GPa in a laser-heated diamond-anvil cell using ex situ textural analyses combined with spectroradiometry and benchmarked with laser-power-temperature functions. The melting curve reaches temperatures of $\ensuremath{\sim}1800\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ by 65 GPa and is smooth in the range of 23--65 GPa with a Clapeyron slope of $\ensuremath{\sim}14\phantom{\rule{0.28em}{0ex}}\mathrm{K}/\mathrm{GPa}$ at 23 GPa. This is consistent with melting of a single tetragonal sulfur structure in this range, which is confirmed by in situ x-ray diffraction. An updated equation of state for tetragonal sulfur is determined, and the high-pressure, high-temperature stability region of tetragonal sulfur is reassessed.

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S. M. Arveson

Papers

Colloidal Organohalide Perovskite Nanoplatelets Exhibiting Quantum Confinement.

Precipitation of multiple light elements to power Earth's early dynamo

Evidence for Fe-Si-O liquid immiscibility at deep Earth pressures.

Thermally induced coloration of KBr at high pressures

High-pressure melting curve of sulfur up to 65 GPa