Distribution control enables efficient reduced-dimensional perovskite LEDs.

Light-emitting diodes (LEDs) based on perovskite quantum dots have shown external quantum efficiencies (EQEs) of over 23% and narrowband emission, but suffer from limited operating stability1. Reduced-dimensional perovskites (RDPs) consisting of quantum wells (QWs) separated by organic intercalating cations show high exciton binding energies and have the potential to increase the stability and the photoluminescence quantum yield2,3. However, until now, RDP-based LEDs have exhibited lower EQEs and inferior colour purities4-6. We posit that the presence of variably confined QWs may contribute to non-radiative recombination losses and broadened emission. Here we report bright RDPs with a more monodispersed QW thickness distribution, achieved through the use of a bifunctional molecular additive that simultaneously controls the RDP polydispersity while passivating the perovskite QW surfaces. We synthesize a fluorinated triphenylphosphine oxide additive that hydrogen bonds with the organic cations, controlling their diffusion during RDP film deposition and suppressing the formation of low-thickness QWs. The phosphine oxide moiety passivates the perovskite grain boundaries via coordination bonding with unsaturated sites, which suppresses defect formation. This results in compact, smooth and uniform RDP thin films with narrowband emission and high photoluminescence quantum yield. This enables LEDs with an EQE of 25.6% with an average of 22.1 ±1.2% over 40 devices, and an operating half-life of two hours at an initial luminance of 7,200 candela per metre squared, indicating tenfold-enhanced operating stability relative to the best-known perovskite LEDs with an EQE exceeding 20%1,4-6.

Micron Thick Colloidal Quantum Dot Solids.

Shortwave infrared colloidal quantum dots (SWIR-CQDs) are semiconductors capable of harvesting across the AM1.5G solar spectrum. Today's SWIR-CQD solar cells rely on spin-coating; however, these films exhibit cracking once thickness exceeds ∼500 nm. We posited that a blade-coating strategy could enable thick QD films. We developed a ligand exchange with an additional resolvation step that enabled the dispersion of SWIR-CQDs. We then engineered a quaternary ink that combined high-viscosity solvents with short QD stabilizing ligands. This ink, blade-coated over a mild heating bed, formed micron-thick SWIR-CQD films. These SWIR-CQD solar cells achieved short-circuit current densities (Jsc) that reach 39 mA cm-2, corresponding to the harvest of 60% of total photons incident under AM1.5G illumination. External quantum efficiency measurements reveal both the first exciton peak and the closest Fabry-Perot resonance peak reaching approximately 80%-this is the highest unbiased EQE reported beyond 1400 nm in a solution-processed semiconductor.

Chloride Insertion-Immobilization Enables Bright, Narrowband, and Stable Blue-Emitting Perovskite Diodes.

Metal halide perovskites show promise for light-emitting diodes (LEDs) owing to their facile manufacture and excellent optoelectronic performance, including high color purity and spectral stability, especially in the green region. However, for blue perovskite LEDs, the emission spectrum line width is broadened to over 25 nm by the coexistence of multiple reduced-dimensional perovskite domains, and the spectral stability is poor, with an undesirable shift (over 7 nm) toward longer wavelengths under operating conditions, degradation that occurs due to phase separation when mixed halides are employed. Here we demonstrate chloride insertion-immobilization, a strategy that enables blue perovskite LEDs, the first to exhibit narrowband (line width of 18 nm) and spectrally stable (no wavelength shift) performance. We prepare bromide-based perovskites and then employ organic chlorides for dynamic treatment, inserting and in situ immobilizing chlorides to blue-shift and stabilize the emission. We achieve sky-blue LEDs with a record luminance over 5100 cd/m2 at 489 nm, and an operating half-life of 51 min at 1500 cd/m2. By device structure optimization, we further realize an improved EQE of 5.2% at 479 nm and an operating half-life of 90 min at 100 cd/m2.

Anchored Ligands Facilitate Efficient B-Site Doping in Metal Halide Perovskites.

Metal halide perovskites exhibit outstanding optoelectronic properties: superior charge carrier mobilities, low densities of deep trap states, high photoluminescence quantum yield, and wide color tunability. The introduction of dopant ions provides pathways to manipulate the electronic and chemical features of perovskites. In metal halide perovskites ABX3, where A is a monovalent cation (e.g., methylammonium (MA+), Cs+), B is the divalent metal ion(s) (e.g., Pb2+, Sn2+), and X is the halide group (e.g., Cl-, Br-, or I-), the isovalent exchange of A- and X-site ions has been widely accomplished; in contrast, strategies to exchange B-site cations are underexamined. The activation energies for vacancy-mediated diffusion of B-site cations are much higher than those for A- and X-sites, leading to slow doping processes and low doping ratios. Herein we demonstrate a new method that exchanges B-site cations in perovskites. We design a series of metal carboxylate solutions that anchor on the perovskite surface, allowing fast and efficient doping of B-sites with both homovalent and heterovalent cations (e.g., Sn2+, Zn2+, Bi3+) at room temperature. The doping process in the reduced-dimensional perovskites is complete within 1 min, whereas a similar reaction only leads to the surface attachment of dopant ions in three-dimensional structures. We offer a model based on ammonium extraction and surface ion-pair substitution.

Picosecond Charge Transfer and Long Carrier Diffusion Lengths in Colloidal Quantum Dot Solids.

Quantum dots (QDs) are promising candidates for solution-processed thin-film optoelectronic devices. Both the diffusion length and the mobility of photoexcited charge carriers in QD solids are critical determinants of solar cell performance; yet various techniques offer diverse values of these key parameters even in notionally similar films. Here we report diffusion lengths and interdot charge transfer rates using a 3D donor/acceptor technique that directly monitors the rate at which photoexcitations reach small-bandgap dot inclusions having a known spacing within a larger-bandgap QD matrix. Instead of relying on photoluminescence (which can be weak in strongly coupled QD solids), we use ultrafast transient absorption spectroscopy, a method where sensitivity is undiminished by exciton dissociation. We measure record diffusion lengths of ∼300 nm in metal halide exchanged PbS QD solids that have led to power conversion efficiencies of 12%, and determine 8 ps interdot hopping of carriers following photoexcitation, among the fastest rates reported for PbS QD solids. We also find that QD solids composed of smaller QDs ( d = ∼3.2 nm) exhibit 5 times faster interdot charge transfer rates and 10 times lower trap state densities compared to larger ( d = ∼5.5 nm) QDs.

Origins of Stokes Shift in PbS Nanocrystals.

Stokes shift, an energy difference between the excitonic absorption and emission, is a property of colloidal quantum dots (CQDs) typically ascribed to splitting between dark and bright excitons. In some materials, e.g., PbS, CuInS2, and CdHgTe, a Stokes shift of up to 200 meV is observed, substantially larger than the estimates of dark-bright state splitting or vibronic relaxations. The shift origin remains highly debated because contradictory signatures of both surface and bulk character were reported for the Stokes-shifted electronic state. Here, we show that the energy transfer among CQDs in a polydispersed ensemble in solution suffices to explain the excess Stokes shift. This energy transfer is primarily due to CQD aggregation and can be substantially eliminated by extreme dilution, higher-viscosity solvent, or better-dispersed colloids. Our findings highlight that ensemble polydispersity remains the primary source of the Stokes shift in CQDs in solution, propagating into the Stokes shift in films and the open-circuit voltage deficit in CQD solar cells. Improved synthetic control can bring notable advancements in CQD photovoltaics, and the Stokes shift continues to provide a sensitive and significant metric to monitor ensemble size distribution.

Tailoring the Energy Landscape in Quasi-2D Halide Perovskites Enables Efficient Green-Light Emission.

Organo-metal halide perovskites are a promising platform for optoelectronic applications in view of their excellent charge-transport and bandgap tunability. However, their low photoluminescence quantum efficiencies, especially in low-excitation regimes, limit their efficiency for light emission. Consequently, perovskite light-emitting devices are operated under high injection, a regime under which the materials have so far been unstable. Here we show that, by concentrating photoexcited states into a small subpopulation of radiative domains, one can achieve a high quantum yield, even at low excitation intensities. We tailor the composition of quasi-2D perovskites to direct the energy transfer into the lowest-bandgap minority phase and to do so faster than it is lost to nonradiative centers. The new material exhibits 60% photoluminescence quantum yield at excitation intensities as low as 1.8 mW/cm2, yielding a ratio of quantum yield to excitation intensity of 0.3 cm2/mW; this represents a decrease of 2 orders of magnitude in the excitation power required to reach high efficiency compared with the best prior reports. Using this strategy, we report light-emitting diodes with external quantum efficiencies of 7.4% and a high luminescence of 8400 cd/m2.

Pure Cubic-Phase Hybrid Iodobismuthates AgBi2 I7 for Thin-Film Photovoltaics.

Bismuth-based hybrid perovskites are candidates for lead-free and air-stable photovoltaics, but poor surface morphologies and a high band-gap energy have previously limited these hybrid perovskites. A new materials processing strategy to produce enhanced bismuth-based thin-film photovoltaic absorbers by incorporation of monovalent silver cations into iodobismuthates is presented. Solution-processed AgBi2 I7 thin films are prepared by spin-coating silver and bismuth precursors dissolved in n-butylamine and annealing under an N2 atmosphere. X-ray diffraction analysis reveals the pure cubic structure (Fd3m) with lattice parameters of a=b=c=12.223 Å. The resultant AgBi2 I7 thin films exhibit dense and pinhole-free surface morphologies with grains ranging in size from 200-800 nm and a low band gap of 1.87 eV suitable for photovoltaic applications. Initial studies produce solar power conversion efficiencies of 1.22 % and excellent stability over at least 10 days under ambient conditions.

Postoperative Complications of True Dropless Cataract Surgery versus Standard Topical Drops.

Purpose Compare postoperative outcomes in cataract surgery between eyes with standard drop regimen versus dropless protocol by residents. Design Retrospective cohort study between April 1, 2018 and March 31, 2020. Methods The study was performed at Lyndon B. Johnson General Hospital in Houston, Harris County, Texas. A total of 547 eyes (234 dropless vs. 313 standard) with phacoemulsification cataract surgery and minimum of 1-month follow-up with best-corrected visual acuity (BCVA) were included. Dropless received 40 mg sub-Tenon's triamcinolone and intracameral moxifloxacin. Patients were followed at postoperative day 1 (POD1), week 1 (POW1), and month 1 (POM1). Postoperative rate of BCVA better than 20/40 (Good vision) and rate of complications were compared between groups. Results Good vision on POM1 in dropless (77.8%) was noninferior to standard (75.1%, p = 0.80). Complication rate in dropless (28.6%) was noninferior to standard (24.0%, p = 0.13). Intraocular pressure (IOP) elevation on POD1 ( p = 0.041) and anterior chamber (AC) cells on POW1 and POM1 ( p < 0.001) were more frequent in dropless. Mean spherical equivalent at POM1 was better in dropless (-0.37 D [±0.81 D]) compared with standard (-0.61D [±0.77 D], p = 0.001). Early posterior capsular opacification (early PCO) was more frequent in dropless ( p = 0.042). Conclusions Postoperative rate of BCVA better than 20/40 and rate of postoperative complications were noninferior, although dropless had higher rates of AC inflammation, IOP elevation, and early PCO.

Non-traumatic open globe injuries: presenting characteristics and visual outcomes.

PURPOSE: To describe clinical characteristics and visual outcomes of non-traumatic open globe injuries. SETTING: A level 1 trauma centre in a large urban medical centre. DESIGN: Retrospective study. METHODS: Charts of non-traumatic open globe patients admitted to MHH-TMC from 1/2010 to 3/2015 were reviewed for demographics, cause, clinical characteristics, visual acuity (VA) and enucleation. RESULTS: Thirty eyes were included: 15 (50%) were males with a mean age of 47 (±28) years. All presented with zone 1 injury. Twenty-five (83%) had a perforated corneal ulcer. Presenting VA was count fingers (n = 3, 10%) to NLP (n = 6, 20%). Twenty-four (80%) involved infection, 5 (17%) congenital, 3 (10%) chemical burn and 2 (7%) neurotrophic. Conjunctival injection (n = 22, 77%), corneal opacification (n = 20, 71%) and relative afferent pupillary defect (n = 9, 44%) were common. After treatment, 23 (88%) were worse than 6/60 (20/200), 9 (35%) were NLP and 8 (27%) required enucleation. CONCLUSIONS: Often non-traumatic open globe injuries are zone 1 and due to perforated infectious ulcers. Compared to previously reported traumatic injuries, these have higher rates of enucleation (27% vs 8%) and poorer final VA (88% vs 68% worse than 6/60 20/200).

Over the Horizon: The Present and Future of Endovascular Neural Recording and Stimulation.

The past decade has witnessed an explosion in applications for neural recording and stimulation in the treatment of clinical disorders. Neuromodulatory approaches are now a mainstay of care for essential tremor and Parkinson's disease, and are expanding rapidly into a wide range of other neurological and psychiatric diseases. In parallel, advancements in endovascular approaches to cerebrovascular diseases have resulted in minimally invasive techniques that deliver devices to neural tissue in the central and peripheral nervous systems, with significantly improved safety and efficacy. In this review, we discuss the history of endovascular neural recording and stimulation, its current progress, and applications for neurological disease.

Color-pure red light-emitting diodes based on two-dimensional lead-free perovskites.

It remains a central challenge to the information display community to develop red light-emitting diodes (LEDs) that meet demanding color coordinate requirements for wide color gamut displays. Here, we report high-efficiency, lead-free (PEA)2SnI4 perovskite LEDs (PeLEDs) with color coordinates (0.708, 0.292) that fulfill the Rec. 2100 specification for red emitters. Using valeric acid (VA)-which we show to be strongly coordinated to Sn2+-we slow the crystallization rate of the perovskite, improving the film morphology. The incorporation of VA also protects tin from undesired oxidation during the film-forming process. The improved films and the reduced Sn4+ content enable PeLEDs with an external quantum efficiency of 5% and an operating half-life exceeding 15 hours at an initial brightness of 20 cd/m2 This work illustrates the potential of Cd- and Pb-free PeLEDs for display technology.

Edge stabilization in reduced-dimensional perovskites.

Reduced-dimensional perovskites are attractive light-emitting materials due to their efficient luminescence, color purity, tunable bandgap, and structural diversity. A major limitation in perovskite light-emitting diodes is their limited operational stability. Here we demonstrate that rapid photodegradation arises from edge-initiated photooxidation, wherein oxidative attack is powered by photogenerated and electrically-injected carriers that diffuse to the nanoplatelet edges and produce superoxide. We report an edge-stabilization strategy wherein phosphine oxides passivate unsaturated lead sites during perovskite crystallization. With this approach, we synthesize reduced-dimensional perovskites that exhibit 97 ± 3% photoluminescence quantum yields and stabilities that exceed 300 h upon continuous illumination in an air ambient. We achieve green-emitting devices with a peak external quantum efficiency (EQE) of 14% at 1000 cd m-2; their maximum luminance is 4.5 × 104 cd m-2 (corresponding to an EQE of 5%); and, at 4000 cd m-2, they achieve an operational half-lifetime of 3.5 h.

Machine Learning Accelerates Discovery of Optimal Colloidal Quantum Dot Synthesis.

Colloidal quantum dots (CQDs) allow broad tuning of the bandgap across the visible and near-infrared spectral regions. Recent advances in applying CQDs in light sensing, photovoltaics, and light emission have heightened interest in achieving further synthetic improvements. In particular, improving monodispersity remains a key priority in order to improve solar cells' open-circuit voltage, decrease lasing thresholds, and improve photodetectors' noise-equivalent power. Here we utilize machine-learning-in-the-loop to learn from available experimental data, propose experimental parameters to try, and, ultimately, point to regions of synthetic parameter space that will enable record-monodispersity PbS quantum dots. The resultant studies reveal that adding a growth-slowing precursor (oleylamine) allows nucleation to prevail over growth, a strategy that enables record-large-bandgap (611 nm exciton) PbS nanoparticles with a well-defined excitonic absorption peak (half-width at half-maximum (hwhm) of 145 meV). At longer wavelengths, we also achieve improved monodispersity, with an hwhm of 55 meV at 950 nm and 24 meV at 1500 nm, compared to the best published to date values of 75 and 26 meV, respectively.

Stable Colloidal Quantum Dot Inks Enable Inkjet-Printed High-Sensitivity Infrared Photodetectors.

Colloidal quantum dots (CQDs) have recently gained attention as materials for manufacturing optoelectronic devices in view of their tunable light absorption and emission properties and compatibility with low-temperature thin-film manufacture. The realization of CQD inkjet-printed infrared photodetectors has thus far been hindered by incompatibility between the chemical processes that produce state-of-the-art CQD solution-exchanged inks and the requirements of ink formulations for inkjet materials processing. To achieve inkjet-printed CQD solids with a high degree of reproducibility, as well as with the needed morphological and optoelectronic characteristics, we sought to overcome the mismatch among these processing conditions. In this study, we design CQD inks by simultaneous evaluation of requirements regarding ink colloidal stability, jetting conditions, and film morphology for different dots and solvents. The new inks remain colloidally stable, achieved through a design that suppresses the reductant properties of amines on the dots' surface. After drop ejection from the nozzle, the quantum dot material is immobilized on the substrate surface due to the rapid evaporation of the low boiling point amine-based compound. Concurrently, the high boiling point solvent allows for the formation of a thin film of high uniformity, as is required for the fabrication of high-performance IR photodetectors. We fabricate inkjet-printed photodetectors exhibiting the highest specific detectivities reported to date (above 1012 Jones across the IR) in an inkjet-printed quantum dot film. As a patternable CMOS-compatible process, the work offers routes to integrated sensing devices and systems.

Mixed Lead Halide Passivation of Quantum Dots.

Infrared-absorbing colloidal quantum dots (IR CQDs) are materials of interest in tandem solar cells to augment perovskite and cSi photovoltaics (PV). Today's best IR CQD solar cells rely on the use of passivation strategies based on lead iodide; however, these fail to passivate the entire surface of IR CQDs. Lead chloride passivated CQDs show improved passivation, but worse charge transport. Lead bromide passivated CQDs have higher charge mobilities, but worse passivation. Here a mixed lead-halide (MPbX) ligand exchange is introduced that enables thorough surface passivation without compromising transport. MPbX-PbS CQDs exhibit properties that exceed the best features of single lead-halide PbS CQDs: they show improved passivation (43 ± 5 meV vs 44 ± 4 meV in Stokes shift) together with higher charge transport (4 × 10-2 ± 3 × 10-3 cm2 V-1 s-1 vs 3 × 10-2 ± 3 × 10-3 cm2 V-1 s-1 in mobility). This translates into PV devices having a record IR open-circuit voltage (IR Voc ) of 0.46 ± 0.01 V while simultaneously having an external quantum efficiency of 81 ± 1%. They provide a 1.7× improvement in the power conversion efficiency of IR photons (>1.1 µm) relative to the single lead-halide controls reported herein.

Ligand cleavage enables formation of 1,2-ethanedithiol capped colloidal quantum dot solids.

Colloidal quantum dots have garnered significant interest in optoelectronics, particularly in quantum dot solar cells (QDSCs). Here we report QDSCs fabricated using a ligand that is modified, following film formation, such that it becomes an efficient hole transport layer. The ligand, O-((9H-fluoren-9-yl)methyl) S-(2-mercaptoethyl) carbonothioate (FMT), contains the surface ligand 1,2-ethanedithiol (EDT) protected at one end using fluorenylmethyloxycarbonyl (Fmoc). The strategy enables deprotection following colloidal deposition, producing films containing quantum dots whose surfaces are more thoroughly covered with the remaining EDT molecules. To compare fabrication methods, we deposited CQDs onto the active layer: in one case, the traditional EDT-PbS/EDT-PbS is used, while in the other EDT-PbS/FMT-PbS is used. The devices based on the new EDT/FMT match the PCE values of EDT/EDT controls, and maintain a higher PCE over an 18 day storage interval, a finding we attribute to an increased thiol coverage using the FMT protocol.

In Situ Back-Contact Passivation Improves Photovoltage and Fill Factor in Perovskite Solar Cells.

Organic-inorganic hybrid perovskite solar cells (PSCs) have seen a rapid rise in power conversion efficiencies in recent years; however, they still suffer from interfacial recombination and charge extraction losses at interfaces between the perovskite absorber and the charge-transport layers. Here, in situ back-contact passivation (BCP) that reduces interfacial and extraction losses between the perovskite absorber and the hole transport layer (HTL) is reported. A thin layer of nondoped semiconducting polymer at the perovskite/HTL interface is introduced and it is shown that the use of the semiconductor polymer permits-in contrast with previously studied insulator-based passivants-the use of a relatively thick passivating layer. It is shown that a flat-band alignment between the perovskite and polymer passivation layers achieves a high photovoltage and fill factor: the resultant BCP enables a photovoltage of 1.15 V and a fill factor of 83% in 1.53 eV bandgap PSCs, leading to an efficiency of 21.6% in planar solar cells.

Perovskite seeding growth of formamidinium-lead-iodide-based perovskites for efficient and stable solar cells.

Formamidinium-lead-iodide (FAPbI3)-based perovskites with bandgap below 1.55 eV are of interest for photovoltaics in view of their close-to-ideal bandgap. Record-performance FAPbI3-based solar cells have relied on fabrication via the sequential-deposition method; however, these devices exhibit unstable output under illumination due to the difficulty of incorporating cesium cations (stabilizer) in sequentially deposited films. Here we devise a perovskite seeding method that efficiently incorporates cesium and beneficially modulates perovskite crystallization. First, perovskite seed crystals are embedded in the PbI2 film. The perovskite seeds serve as cesium sources and act as nuclei to facilitate crystallization during the formation of perovskite. Perovskite films with perovskite seeding growth exhibit a lowered trap density, and the resulting planar solar cells achieve stabilized efficiency of 21.5% with a high open-circuit voltage of 1.13 V and a fill factor that exceeds 80%. The Cs-containing FAPbI3-based devices show a striking improvement in operational stability and retain 60% of their initial efficiency after 140 h operation under one sun illumination.