Thermodynamic Origin of the Photostability of the Two-Dimensional Perovskite PEA2Pb(I1-x Br x )4.

The two-dimensional (2D) mixed halide perovskite PEA2Pb(I1-x Br x )4 exhibits high phase stability under illumination as compared to the three-dimensional (3D) counterpart MAPb(I1-x Br x )3. We explain this difference using a thermodynamic theory that considers the sum of a compositional and a photocarrier free energy. Ab initio calculations show that the improved compositional phase stability of the 2D perovskite is caused by a preferred I-Br distribution, leading to a much lower critical temperature for halide segregation in the dark than for the 3D perovskite. Moreover, a smaller increase of the band gap with Br concentration x and a markedly shorter photocarrier lifetime in the 2D perovskite reduce the driving force for phase segregation under illumination, enhancing the photostability.

Effect of Light-Induced Halide Segregation on the Performance of Mixed-Halide Perovskite Solar Cells.

Light-induced halide segregation hampers obtaining stable wide-band-gap solar cells based on mixed iodide-bromide perovskites. So far, the effect of prolonged illumination on the performance of mixed-halide perovskite solar cells has not been studied in detail. It is often assumed that halide segregation leads to a loss of open-circuit voltage. By simultaneously recording changes in photoluminescence and solar cell performance under prolonged illumination, we demonstrate that cells instead deteriorate by a loss of short-circuit current density and that the open-circuit voltage is less affected. The concurrent red shift, increased lifetime, and higher quantum yield of photoluminescence point to the formation of relatively emissive iodide-rich domains under illumination. Kinetic Monte Carlo simulations provide an atomistic insight into their formation via exchange of bromide and iodide, mediated by halide vacancies. Localization of photogenerated charge carriers in low-energy iodide-rich domains and subsequent recombination cause reduced photocurrent and red-shifted photoluminescence. The loss in photovoltaic performance is diminished by partially replacing organic cations by cesium ions. Ultrasensitive photocurrent spectroscopy shows that cesium ions result in a lower density of sub-band-gap defects and suppress defect growth under illumination. These defects are expected to play a role in the development and recovery of light-induced compositional changes.

Unified theory for light-induced halide segregation in mixed halide perovskites.

Mixed halide perovskites that are thermodynamically stable in the dark demix under illumination. This is problematic for their application in solar cells. We present a unified thermodynamic theory for this light-induced halide segregation that is based on a free energy lowering of photocarriers funnelling to a nucleated phase with different halide composition and lower band gap than the parent phase. We apply the theory to a sequence of mixed iodine-bromine perovskites. The spinodals separating metastable and unstable regions in the composition-temperature phase diagrams only slightly change under illumination, while light-induced binodals separating stable and metastable regions appear signalling the nucleation of a low-band gap iodine-rich phase. We find that the threshold photocarrier density for halide segregation is governed by the band gap difference of the parent and iodine-rich phase. Partial replacement of organic cations by cesium reduces this difference and therefore has a stabilizing effect.

A deep-learning approach to realizing functionality in nanoelectronic devices.

Many nanoscale devices require precise optimization to function. Tuning them to the desired operation regime becomes increasingly difficult and time-consuming when the number of terminals and couplings grows. Imperfections and device-to-device variations hinder optimization that uses physics-based models. Deep neural networks (DNNs) can model various complex physical phenomena but, so far, are mainly used as predictive tools. Here, we propose a generic deep-learning approach to efficiently optimize complex, multi-terminal nanoelectronic devices for desired functionality. We demonstrate our approach for realizing functionality in a disordered network of dopant atoms in silicon. We model the input-output characteristics of the device with a DNN, and subsequently optimize control parameters in the DNN model through gradient descent to realize various classification tasks. When the corresponding control settings are applied to the physical device, the resulting functionality is as predicted by the DNN model. We expect our approach to contribute to fast, in situ optimization of complex (quantum) nanoelectronic devices.

High energy acceptor states strongly enhance exciton transfer between metal organic phosphorescent dyes.

Exciton management in organic light-emitting diodes (OLEDs) is vital for improving efficiency, reducing device aging, and creating new device architectures. In particular in white OLEDs, exothermic Förster-type exciton transfer, e.g. from blue to red emitters, plays a crucial role. It is known that a small exothermicity partially overcomes the spectral Stokes shift, enhancing the fraction of resonant donor-acceptor pair states and thus the Förster transfer rate. We demonstrate here a second enhancement mechanism, setting in when the exothermicity exceeds the Stokes shift: transfer to multiple higher-lying electronically excited states of the acceptor molecules. Using a recently developed computational method we evaluate the Förster transfer rate for 84 different donor-acceptor pairs of phosphorescent emitters. As a result of the enhancement the Förster radius tends to increase with increasing exothermicity, from around 1 nm to almost 4 nm. The enhancement becomes particularly strong when the excited states have a large spin-singlet character.

Ballistic Phonons in Ultrathin Nanowires.

According to Fourier's law, a temperature difference across a material results in a linear temperature profile and a thermal conductance that decreases inversely proportional to the system length. These are the hallmarks of diffusive heat flow. Here, we report heat flow in ultrathin (25 nm) GaP nanowires in the absence of a temperature gradient within the wire and find that the heat conductance is independent of wire length. These observations deviate from Fourier's law and are direct proof of ballistic heat flow, persisting for wire lengths up to at least 15 µm at room temperature. When doubling the wire diameter, a remarkably sudden transition to diffusive heat flow is observed. The ballistic heat flow in the ultrathin wires can be modeled within Landauer's formalism by ballistic phonons with an extraordinarily long mean free path.

Classification with a disordered dopant-atom network in silicon.

Classification is an important task at which both biological and artificial neural networks excel1,2. In machine learning, nonlinear projection into a high-dimensional feature space can make data linearly separable3,4, simplifying the classification of complex features. Such nonlinear projections are computationally expensive in conventional computers. A promising approach is to exploit physical materials systems that perform this nonlinear projection intrinsically, because of their high computational density5, inherent parallelism and energy efficiency6,7. However, existing approaches either rely on the systems' time dynamics, which requires sequential data processing and therefore hinders parallel computation5,6,8, or employ large materials systems that are difficult to scale up7. Here we use a parallel, nanoscale approach inspired by filters in the brain1 and artificial neural networks2 to perform nonlinear classification and feature extraction. We exploit the nonlinearity of hopping conduction9-11 through an electrically tunable network of boron dopant atoms in silicon, reconfiguring the network through artificial evolution to realize different computational functions. We first solve the canonical two-input binary classification problem, realizing all Boolean logic gates12 up to room temperature, demonstrating nonlinear classification with the nanomaterial system. We then evolve our dopant network to realize feature filters2 that can perform four-input binary classification on the Modified National Institute of Standards and Technology handwritten digit database. Implementation of our material-based filters substantially improves the classification accuracy over that of a linear classifier directly applied to the original data13. Our results establish a paradigm of silicon-based electronics for small-footprint and energy-efficient computation14.

Protease-activated receptor 2 deficiency mediates cardiac fibrosis and diastolic dysfunction.

AIMS: Heart failure with preserved ejection fraction (HFpEF) and pathological cardiac aging share a complex pathophysiology, including extracellular matrix remodelling (EMR). Protease-activated receptor 2 (PAR2) deficiency is associated with EMR. The roles of PAR1 and PAR2 have not been studied in HFpEF, age-dependent cardiac fibrosis, or diastolic dysfunction (DD). METHODS AND RESULTS: Evaluation of endomyocardial biopsies from patients with HFpEF (n = 14) revealed that a reduced cardiac PAR2 expression was associated with aggravated DD and increased myocardial fibrosis (r = -0.7336, P = 0.0028). In line, 1-year-old PAR2-knockout (PAR2ko) mice suffered from DD with preserved systolic function, associated with an increased age-dependent α-smooth muscle actin expression, collagen deposition (1.7-fold increase, P = 0.0003), lysyl oxidase activity, collagen cross-linking (2.2-fold increase, P = 0.0008), endothelial activation, and inflammation. In the absence of PAR2, the receptor-regulating protein caveolin-1 was down-regulated, contributing to an augmented profibrotic PAR1 and transforming growth factor beta (TGF-ß)-dependent signalling. This enhanced TGF-ß/PAR1 signalling caused N-proteinase (ADAMTS3) and C-proteinase (BMP1)-related increased collagen I production from cardiac fibroblasts (CFs). PAR2 overexpression in PAR2ko CFs reversed these effects. The treatment with the PAR1 antagonist, vorapaxar, reduced cardiac fibrosis by 44% (P = 0.03) and reduced inflammation in a metabolic disease model (apolipoprotein E-ko mice). Patients with HFpEF with upstream PAR inhibition via FXa inhibitors (n = 40) also exhibited reduced circulating markers of fibrosis and DD compared with patients treated with vitamin K antagonists (n = 20). CONCLUSIONS: Protease-activated receptor 2 is an important regulator of profibrotic PAR1 and TGF-ß signalling in the heart. Modulation of the FXa/FIIa-PAR1/PAR2/TGF-ß-axis might be a promising therapeutic approach to reduce HFpEF.

Equilibrated Charge Carrier Populations Govern Steady-State Nongeminate Recombination in Disordered Organic Solar Cells.

We employed bias-assisted charge extraction techniques to investigate the transient and steady-state recombination of photogenerated charge carriers in complete devices of a disordered polymer-fullerene blend. Charge recombination is shown to be dispersive, with a significant slowdown of the recombination rate over time, consistent with the results from kinetic Monte Carlo simulations. Surprisingly, our experiments reveal little to no contributions from early time recombination of nonequilibrated charge carriers to the steady-state recombination properties. We conclude that energetic relaxation of photogenerated carriers outpaces any significant nongeminate recombination under application-relevant illumination conditions. With equilibrated charges dominating the steady-state recombination, quasi-equilibrium concepts appear suited for describing the open-circuit voltage of organic solar cells despite pronounced energetic disorder.

Stabilizing Lead-Free All-Inorganic Tin Halide Perovskites by Ion Exchange.

Because of its thermal stability, lead-free composition, and nearly ideal optical and electronic properties, the orthorhombic CsSnI3 perovskite is considered promising as a light absorber for lead-free all-inorganic perovskite solar cells. However, the susceptibility of this three-dimensional perovskite toward oxidation in air has limited the development of solar cells based on this material. Here, we report the findings of a computational study which identifies promising Rb y Cs1-y Sn(Br x I1-x )3 perovskites for solar cell applications, prepared by substituting cations (Rb for Cs) and anions (Br for I) in CsSnI3. We show the evolution of the material electronic structure as well as its thermal and structural stabilities upon gradual substitution. Importantly, we demonstrate how the unwanted yellow phase can be suppressed by substituting Br for I in CsSn(Br x I1-x )3 with x ≥ 1/3. We predict that substitution of Rb for Cs results in a highly homogeneous solid solution and therefore an improved film quality and applicability in solar cell devices.

Interstitial Occupancy by Extrinsic Alkali Cations in Perovskites and Its Impact on Ion Migration.

Recent success in achieving highly stable Rb-containing organolead halide perovskites has indicated the possibility of incorporating small monovalent cations, which cannot fit in the lead-halide cage with an appropriate tolerance factor, into the perovskite lattice while maintaining a pure stable "black" phase. In this study, through a combined experimental and theoretical investigation by density functional theory (DFT) calculations on the incorporation of extrinsic alkali cations (Rb+ , K+ , Na+ , and Li+ ) in perovskite materials, the size-dependent interstitial occupancy of these cations in the perovskite lattice is unambiguously revealed. Interestingly, DFT calculations predict the increased ion migration barriers in the lattice after the interstitial occupancy. To verify this prediction, ion migration behavior is characterized through hysteresis analysis of solar cells, electrical poling, temperature-dependent conductivity, and time-dependent photoluminescence measurements. The results collectively point to the suppression of ion migration after lattice interstitial occupancy by extrinsic alkali cations. The findings of this study provide new material design principles to manipulate the structural and ionic properties of multication perovskite materials.

Accurate and efficient band gap predictions of metal halide perovskites using the DFT-1/2 method: GW accuracy with DFT expense.

The outstanding optoelectronics and photovoltaic properties of metal halide perovskites, including high carrier motilities, low carrier recombination rates, and the tunable spectral absorption range are attributed to the unique electronic properties of these materials. While DFT provides reliable structures and stabilities of perovskites, it performs poorly in electronic structure prediction. The relativistic GW approximation has been demonstrated to be able to capture electronic structure accurately, but at an extremely high computational cost. Here we report efficient and accurate band gap calculations of halide metal perovskites by using the approximate quasiparticle DFT-1/2 method. Using AMX3 (A = CH3NH3, CH2NHCH2, Cs; M = Pb, Sn, X = I, Br, Cl) as demonstration, the influence of the crystal structure (cubic, tetragonal or orthorhombic), variation of ions (different A, M and X) and relativistic effects on the electronic structure are systematically studied and compared with experimental results. Our results show that the DFT-1/2 method yields accurate band gaps with the precision of the GW method with no more computational cost than standard DFT. This opens the possibility of accurate electronic structure prediction of sophisticated halide perovskite structures and new materials design for lead-free materials.

Charge transport in nanoscale vertical organic semiconductor pillar devices.

We report charge transport measurements in nanoscale vertical pillar structures incorporating ultrathin layers of the organic semiconductor poly(3-hexylthiophene) (P3HT). P3HT layers with thickness down to 5 nm are gently top-contacted using wedging transfer, yielding highly reproducible, robust nanoscale junctions carrying high current densities (up to 106 A/m2). Current-voltage data modeling demonstrates excellent hole injection. This work opens up the pathway towards nanoscale, ultrashort-channel organic transistors for high-frequency and high-current-density operation.

Inhibition of platelet function with clopidogrel is associated with a reduction of inflammation in patients with peripheral artery disease.

BACKGROUND: The reactivity of platelets is increased in patients with peripheral artery disease (PAD). RANTES and sCD40L are chemokines which are stored in the alpha-granules of platelets. Clopidogrel inhibits and thus reduces platelet reactivity. Whether a treatment with clopidogrel is associated with an inhibition of systemic inflammation in patients with PAD has not been thoroughly explored. This study examined the effect of clopidogrel on platelet reactivation and the release of inflammatory chemokines in patients with PAD. METHODS: 40 patients with PAD were randomized into two groups. In the first group A the patients were treated with 100mg acetylsalicylic acid (ASA) and additional placebo for 4weeks. The patients in group B received 75mg/d clopidogrel in addition to ASA 100mg for 4weeks. After obtaining blood at days 0, 7 and 28 the platelet activation was determined by measuring the surface protein expression of CD63, CD62p and thrombospondin (TSP) after stimulation with TRAP and ADP. The release of the chemokines RANTES and sCD40L from platelets was analyzed by ELISA. RESULTS: Platelet activation markers (CD62p and CD63) and chemokine RANTES were significantly reduced in patients with PAD after 7 and 28days after treatment with clopidogrel. No alterations were found in TSP expression and sCD40L during the treatment. CONCLUSION: The treatment with clopidogrel leads to a reduction of platelet reactivity and release of RANTES from the platelets of patients with PAD.

Charge Transport by Superexchange in Molecular Host-Guest Systems.

Charge transport in disordered organic semiconductors is generally described as a result of incoherent hopping between localized states. In this work, we focus on multicomponent emissive host-guest layers as used in organic light-emitting diodes (OLEDs), and show using multiscale ab initio based modeling that charge transport can be significantly enhanced by the coherent process of molecular superexchange. Superexchange increases the rate of emitter-to-emitter hopping, in particular if the emitter molecules act as relatively deep trap states, and allows for percolation path formation in charge transport at low guest concentrations.

The effect of clopidogrel on platelet activity in patients with and without type-2 diabetes mellitus: a comparative study.

BACKGROUND: Although antiplatelet therapy involving clopidogrel is a standard treatment for preventing cardiovascular events after coronary stent implantation, patients can display differential responses. Here, we assessed the effectiveness of clopidogrel on platelet function inhibition in subjects with and without type-2 diabetes and stable coronary artery disease. In addition, we investigated the correlation between platelet function and routine clinical parameters. METHODS: A total of 64 patients with stable coronary heart disease were enrolled in the study. Among these, 32 had known type-2 diabetes, whereas the remaining 32 subjects were non-diabetics (control group). A loading dose of 300 mg clopidogrel was given to clopidogrel-naïve patients (13 patients in the diabetes group and 14 control patients). All patients were given a daily maintenance dose of 75 mg clopidogrel. In addition, all patients received 100 mg ASA per day. Agonist-induced platelet aggregation measurements were performed on hirudin-anticoagulated blood using an impedance aggregometer (Multiple Platelet Function Analyzer, Dynabyte, Munich, Germany). Blood samples were drawn from the antecubital vein 24 h after coronary angiography with percutaneous coronary intervention. The platelets were then stimulated with ADP alone or ADP and prostaglandin-E (ADP and ADP-PGE tests, respectively) in order to evaluate clopidogrel-mediated inhibition of platelet function. The effectiveness of ASA was measured by stimulation with arachidonic acid (ASPI test). In addition, maximal platelet aggregation was assessed via stimulation with thrombin receptor-activating peptide (TRAP test). RESULTS: Patients with diabetes exhibited significantly less inhibition of platelet function than patients without diabetes (ADP-PGE test p = 0.003; ASPI test p = 0.022). Administering a clopidogrel loading dose of 300 mg did not result in a lower level of ADP-PGE-induced platelet reactivity in comparison to the use of a 75 mg maintenance dose. Moreover, we observed that ADP-PGE-induced platelet inhibition was positively correlated with fasting blood glucose and HbA1c (p < 0.01). CONCLUSIONS: Patients with type-2 diabetes exhibited increased platelet reactivity compared to patients without diabetes despite combined treatment with clopidogrel and ASA. Using a loading dose of clopidogrel rather than small daily doses was not sufficient for adequately overcoming increased platelet reactivity in patients with type-2 diabetes, highlighting the need for more effective anti-platelet drugs for such patients.

Nicotinamide phosphoribosyltransferase/pre-B-cell colony enhancing factor/visfatin plasma levels and clinical outcome in patients with dilated cardiomyopathy.

BACKGROUND: Nicotinamide phosphoribosyltransferase (Nampt) is an enzyme involved in nicotinamide adenine dinucleotide biosynthesis. Nampt functions as gatekeeper of energy status and survival in cardiac myocytes in animal models of ischemia-reperfusion and might regulate inflammatory processes. Therefore, we performed for the 1st time a clinical study to determine the effects of Nampt on cardiac function in patients with nonischemic dilated (DCM) and inflammatory (DCMi) cardiomyopathy. METHODS AND RESULTS: A total of 113 patients were enrolled in the study and classified into control (n = 25), DCM (n = 38), and DCMi (n = 50) groups. Cardiac functional and inflammatory parameters as well as plasma Nampt and cardiac mRNA and protein Nampt expression were determined at baseline and follow-up after 6 months. Patients with DCM (1.04 ± 0.8 ng/mL; P < .001) and DCMi (1.07 ± 0.7 ng/mL; P < .001) showed significantly increased Nampt plasma concentrations at baseline compared with the control group (0.57 ± 0.5 ng/mL). Patients with higher Nampt concentrations in both heart failure groups showed significant better improvement of cardiac functional parameters (correlation between Nampt plasma levels and the change of left ventricular ejection fraction after 6 months: DCM: r = 0.698, P < .001; DCMi: r = 0.503, P < .001). Moreover, cardiac inflammation did not influence Nampt expression, and Nampt concentrations did not modulate cardiac inflammation in DCMi. A multivariate linear regression model revealed high plasma Nampt expression to contribute to better improvement of cardiac function in patients of both heart failure groups. Moreover, heart failure patients with high plasma Nampt levels showed suppressed cardiac TNF-α and IL-6 mRNA expression after 6 months' follow-up as well as lower B-type natriuretic peptide levels compared with heart failure patients with low Nampt plasma concentrations. CONCLUSIONS: High Nampt expression in patients with nonischemic DCM and DCMi is associated with a favorable outcome and improvement in functional status.

Platelet activation and thrombus formation relates to the presence of myocardial inflammation in patients with cardiomyopathy.

BACKGROUND: Patients with cardiomyopathy show a significantly increased risk for thromboembolic events due to a hypercoagulable state and platelet dysfunction. The pathophysiologic mechanism underlying the increasing platelet activity in patients with cardiomyopathy remains unclear. We performed a clinical study to elucidate the link between myocardial tissue alterations and platelet activation in patients with cardiomyopathy. METHODS: A total of 30 patients with suspected cardiomyopathy and 10 healthy control patients were included in our study. Hemodynamic parameters were measured by catheterization and echocardiography. Endomyocardial biopsies were taken to determine myocardial inflammation. Flow cytometry was performed to examine the platelet activation by quantification of p-selectin and thrombospondin expression on platelets. RESULTS: The p-selectin (8.46 ± 3.67 AU) and thrombospondin (26.56 ± 23.21 AU) expression was significantly correlated with the amount of CD3+ T cells (p-selectin: r=0.573, p<0.05; thrombospondin: r=0.488, p<0.05) and the endothelial/interstitial activation (p-selectin: r=0.521, p<0.05; thrombospondin: r=0.39, p<0.05). This was found to be independent of hemodynamic parameters, age, and gender. The platelet activation of patients (n=3) with echocardiographically documented ventricular thrombi was significantly increased (p-selectin: 12.57 ± 5.5 AU vs. 8.1 ± 3.2 AU, p<0.05) and this was associated with elevated myocardial inflammation scores. CONCLUSION: Myocardial inflammation is associated with a significant increase in platelet activation and ventricular thrombus formation independently of the hemodynamic conditions.

Old age and chronic disease: is the emergency medical system the appropriate provider for the elderly?

OBJECTIVE: The use of emergency medical services increases with the age of patients. Some care providers hold on to the prejudice that these alarms are unnecessary or of a lower importance. We assessed the relation of age and age-dependent emergency characteristics, taking into consideration the ratings of emergency physicians on whether or not emergency cases were considered truly in need of emergency physician attendance. METHODS: Emergency physicians dispatched by the Berlin Fire Department evaluated for each case the necessity of emergency physician attendance. Case characteristics such as the day of the week and location of the emergency as well as patient characteristics such as age, sex, prior status, and care dependency were recorded. In addition, whether or not the physician accompanied the patient to the hospital was recorded as a parameter for emergency severity. Analysis was performed using multiple logistic regression modeling. RESULTS: During the 6-month prospective study period, 2702 cases were evaluated. Emergency medical services are used more frequently by older individuals, especially octogenarians. Emergency cases in older individuals were significantly more often rated as in need of emergency physician attendance; however, the rate of patients accompanied by the emergency physician to the hospital did not differ between the age groups. The age of patients, the primary diagnosis, the day and location of the emergency, and the presence of pre-existing dementia showed a significant impact on the necessity of physician-attended emergency missions. CONCLUSION: Despite common prejudices, emergency cases in elder patients are rated more often as in need of emergency physician attendance compared with those involving younger patients.