Machine learning methods offer great promise for fast and accurate detection and prognostication of coronavirus disease 2019 (COVID-19) from standard-of-care chest radiographs (CXR) and chest computed tomography (CT) images. Many articles have been published in 2020 describing new machine learning-based models for both of these tasks, but it is unclear which are of potential clinical utility. In this systematic review, we consider all published papers and preprints, for the period from 1 January 2020 to 3 October 2020, which describe new machine learning models for the diagnosis or prognosis of COVID-19 from CXR or CT images. All manuscripts uploaded to bioRxiv, medRxiv and arXiv along with all entries in EMBASE and MEDLINE in this timeframe are considered. Our search identified 2,212 studies, of which 415 were included after initial screening and, after quality screening, 62 studies were included in this systematic review. Our review finds that none of the models identified are of potential clinical use due to methodological flaws and/or underlying biases. This is a major weakness, given the urgency with which validated COVID-19 models are needed. To address this, we give many recommendations which, if followed, will solve these issues and lead to higher-quality model development and well-documented manuscripts.

Quantum-confined CsPbBr3 nanoplatelets (NPLs) are extremely promising for use in low-cost blue light-emitting diodes, but their tendency to coalesce in both solution and film form, particularly under operating device conditions with injected charge-carriers, is hindering their adoption. We show that employing a short hexyl-phosphonate ligand (C6H15O3P) in a heat-up colloidal approach for pure, blue-emitting quantum-confined CsPbBr3 NPLs significantly suppresses these coalescence phenomena compared to particles capped with the typical oleyammonium ligands. The phosphonate-passivated NPL thin films exhibit photoluminescence quantum yields of ∼40% at 450 nm with exceptional ambient and thermal stability. The color purity is preserved even under continuous photoexcitation of carriers equivalent to LED current densities of ∼3.5 A/cm2. 13C, 133Cs, and 31P solid-state MAS NMR reveal the presence of phosphonate on the surface. Density functional theory calculations suggest that the enhanced stability is due to the stronger binding affinity of the phosphonate ligand compared to the ammonium ligand.

Extensive studies have focused on improving the operational stability of perovskite solar cells but few surveyed the fundamental degradation mechanisms. One aspect overlooked in earlier works is the effect of the atmosphere on the device performance during operation.Here, we investigate the degradation mechanisms of perovskite solar cells operated under vacuum and a nitrogen atmosphere using synchrotron radiation-based operando grazingincidence X-ray scattering methods. Unlike what was seen in previous reports, we find that light-induced phase segregation, lattice shrinkage, and morphology deformation occur under vacuum. Under nitrogen, only lattice shrinkage appears during the operation of solar cells resulting in a better device stability. The different behavior in nitrogen is attributed a larger energy barrier for lattice distortion and phase segregation. Finally, we find that the migration of excessive PbI2 to the interface between the perovskite and the hole transport layer degrade the performance of devices either under vacuum or nitrogen.3 Solution-processed hybrid halide perovskite materials have attracted strong interest for next-generation thin-film photovoltaic applications due to their high power conversion efficiency (PCE) and low fabrication costs compared to silicon photovoltaics 1 . With solvent engineering, compositional tuning, and surface passivation 2-4 , the highest PCE of perovskite solar cells (PSCs) has reached 25.5 % 5 . Moreover, possibility of fabricating PSCs on flexible substrates opens up promising manufacturing routes, and novel application fields are explored, such as lightweight photovoltaic devices for space applications. Previous studies showed that PSCs were successfully operated in space with low vacuum conditions such as on a highaltitude balloon and a suborbital rocket [6][7][8] . Although these pioneers confirmed the possibility of operating PSCs in space, the operational stability of PSCs is unknown under such conditions.In terrestrial studies, vacuum conditions play a major role in the performance loss of PSCs during operation. Thus, although there is such a rapid increase in the PCE, very significant challenges remain. More research is required to increase the stability of the materials and the longevity of the devices, as long-term operational stability remains the main challenge for realworld applications of hybrid halide perovskite materials. Therefore, investigating the performance degradation mechanism of PSCs under different atmospheric conditions is one key approach to further improving the long-term operational stability of PSCs 9 .Exposure to above-bandgap illumination can cause a loss of phase and structure stability for perovskite materials. For instance, phase segregation introduced by lattice distortion, halide migration, and crystalline reorganization can cause an open-circuit voltage penalty arising from halide segregation 10 . In addition, several studies have indicated that a lattice distortion under illumination originates from light excitation or therm...

received his B.Eng. degree from Imperial College London in 2017 and then received his M.Phil. degree from the University of Cambridge in 2018. He is pursuing his Ph.D. degree at the University of Cambridge with research interests in perovskite light-emitting and imaging algorithm applications. Miguel Anaya is a research fellow at Darwin College and a Marie Curie Fellow in the Cavendish Laboratory at the University of Cambridge. He completed his Ph.D. at the Spanish National Research Council in 2018, with recognition from the Spanish Royal Society of Physics as the Best Thesis in Experimental Physics. He leads a subgroup at the StranksLab focused on the modeling, fabrication, and characterization of perovskite-based light-emitting devices and sensors.

Perovskite light-emitting diodes (LEDs) have attracted broad attention due to their rapidly increasing external quantum efficiencies (EQEs) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] . However, most high EQEs of perovskite LEDs are reported at low current densities (< 1 mA cm -2 ) and low brightness. Decrease in efficiency and rapid degradation at high brightness inhibit their practical applications. Here,

toward the optimization of perovskite thin film growth from simple precursors have improved the efficiency and stability of devices to a high quality standard and low cost, placing them on the verge of commercialization. [1][2][3][4][5][6] Nevertheless, a better understanding of what influences their crystalline structure is needed in order to achieve phase purity, manage defects, and ultimately achieve optimal device performances.The dramatic gain in solar cell device efficiency since 2012 is only one of the features making perovskites stand out among other photovoltaic materials. With a Young's modulus around 20 GPa, [7][8][9][10] perovskites are mechanically softer than most other PV materials such as silicon (>160 GPa), [11,12] GaAs (≈85 GPa), [13] CIGS (≈80 GPa), [14,15] and CdTe (≈40 GPa), [16,17] and their structure has been reported to be prone to light-induced, electric-fieldinduced, and temperature-dependent rearrangements. [18][19][20][21][22][23] The workhorse system studied to date, methylammonium lead iodide (MAPbI 3 ), is in a tetragonal phase (TP) at room temperature, but undergoes a transition to a cubic phase at high temperature (≈330 K) and an orthorhombic phase (OP) at low temperature (≈150 K). Recently, we and others [24][25][26] have reported that the structural rearrangement from TP to OP causes a distinct hysteretic change in optical and transport properties as well as device behavior between heating and cooling cycles. This hysteresis could be reduced by scraping the film from the substrate and instead measuring randomly oriented powder samples. [24] These results provide hints that the thermal stability [27] and phase transition can be influenced by the local environment of the film due to interactions between the material and substrate as well as within the bulk film itself. Unless understood and mitigated, such hysteretic changes at low temperature may limit the use of perovskite solar cells in some specific applications, for example, aerospace applications, which require operation at extremely low temperatures [28] (<200 K).State-of-the-art perovskite films are polycrystalline, which leads to microscale inhomogeneities in a number of properties such as morphology and defect distributions [29][30][31][32] and, in turn, to local variations in the electronic environment for charge carriers. Generally, increasing grain sizes in MAPbI 3 films has resulted in improvements in critical performance parameters, such as an increase in carrier mobility and charge collection efficiency, [33,34] along with smaller bandgaps, longer lifetimes, Grain size in polycrystalline halide perovskite films is known to have an impact on the optoelectronic properties of the films, but its influence on their soft structural properties and phase transitions is unclear. Here, temperature-dependent X-ray diffraction, absorption, and macro-and microphotoluminescence measurements are used to investigate the tetragonal to orthorhombic phase transition in thin methylammonium lead iodide films with grain sizes ranging fr...

The unprecedented advancement in power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) has rendered them a promising game‐changer in photovoltaics. However, unsatisfactory environmental stability and high manufacturing cost of window electrodes are bottlenecks impeding their commercialization. Here, a strategy is introduced to address these bottlenecks by replacing the costly indium tin oxide (ITO) window electrodes via a simple transfer technique with single‐walled carbon nanotubes (SWCNTs) films, which are made of earth‐abundant elements with superior chemical and environmental stability. The resultant devices exhibit PCEs of ≈19% on rigid substrates, which is the highest value reported to date for ITO‐free PSCs. The facile approach for SWCNTs also enables application in flexible PSCs (f‐PSCs), delivering a PCE of ≈18% with superior mechanical robustness over their ITO‐based counterparts due to the excellent mechanical properties of SWCNTs. The SWCNT‐based PSCs also deliver satisfactory performances on large‐area (1 cm2 active area in this work). Furthermore, these SWCNT‐based PSCs can retain over 80% of original PCEs after exposure to air over 700 h while ITO‐based devices only sustain ≈60% of initial PCEs. This work paves a promising way to accelerate the commercialization of ITO‐free PSCs with reduced material cost and prolonged lifetimes.

We report here fast A‐site cation cross‐exchange between APbX3 perovskite nanocrystals (NCs) made of different A‐cations (Cs (cesium), FA (formamidinium), and MA (methylammonium)) at room temperature. Surprisingly, the A‐cation cross‐exchange proceeds as fast as the halide (X=Cl, Br, or I) exchange with the help of free A‐oleate complexes present in the freshly prepared colloidal perovskite NC solutions. This enabled the preparation of double (MACs, MAFA, CsFA)‐ and triple (MACsFA)‐cation perovskite NCs with an optical band gap that is finely tunable by their A‐site composition. The optical spectroscopy together with structural analysis using XRD and atomically resolved high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) and integrated differential phase contrast (iDPC) STEM indicates the homogeneous distribution of different cations in the mixed perovskite NC lattice. Unlike halide ions, the A‐cations do not phase‐segregate under light illumination.