Electrochemical Impedance Spectroscopy of All-Perovskite Tandem Solar Cells

This work explores electrochemical impedance spectroscopy to study recombination and ionic processes in all-perovskite tandem solar cells. We exploit selective excitation of each subcell to enhance or suppress the impedance signal from each subcell, allowing study of individual tandem subcells. We use this selective excitation methodology to show that the recombination resistance and ionic time constants of the wide gap subcell are increased with passivation. Furthermore, we investigate subcell-dependent degradation during maximum power point tracking and find an increase in recombination resistance and a decrease in capacitance for both subcells. Complementary optical and external quantum efficiency measurements indicate that the main driver for performance loss is the reduced capacity of the recombination layer to facilitate recombination due to the formation of a charge extraction barrier. This methodology highlights electrochemical impedance spectroscopy as a powerful tool to provide critical feedback to unlock the full potential of perovskite tandem solar cells.

Substitution of lead with tin suppresses ionic transport in halide perovskite optoelectronics

Despite the rapid rise in the performance of a variety of perovskite optoelectronic devices with vertical charge transport, the effects of ion migration remain a common and longstanding Achilles' heel limiting the long-term operational stability of lead halide perovskite devices. However, there is still limited understanding of the impact of tin (Sn) substitution on the ion dynamics of lead (Pb) halide perovskites. Here, we employ scan-rate-dependent current-voltage measurements on Pb and mixed Pb-Sn perovskite solar cells to show that short circuit current losses at lower scan rates, which can be traced to the presence of mobile ions, are present in both kinds of perovskites. To understand the kinetics of ion migration, we carry out scan-rate-dependent hysteresis analyses and temperature-dependent impedance spectroscopy measurements, which demonstrate suppressed ion migration in Pb-Sn devices compared to their Pb-only analogues. By linking these experimental observations to first-principles calculations on mixed Pb-Sn perovskites, we reveal the key role played by Sn vacancies in increasing the iodide ion migration barrier due to local structural distortions. These results highlight the beneficial effect of Sn substitution in mitigating undesirable ion migration in halide perovskites, with potential implications for future device development.

End-of-life practices in 11 German intensive care units : Results from the ETHICUS-2 study

BACKGROUND

End-of-life care is common in German intensive care units (ICUs) but little is known about daily practice.

OBJECTIVES

To study the practice of end-of-life care.

METHODS

Prospectively planned, secondary analysis comprising the German subset of the worldwide Ethicus‑2 Study (2015-2016) including consecutive ICU patients with limitation of life-sustaining therapy or who died.

RESULTS

Among 1092 (13.7%) of 7966 patients from 11 multidisciplinary ICUs, 967 (88.6%) had treatment limitations, 92 (8.4%) died with failed CPR, and 33 (3%) with brain death. Among patients with treatment limitations, 22.3% (216/967) patients were discharged alive from the ICU. More patients had treatments withdrawn than withheld (556 [57.5%] vs. 411 [42.5%], p < 0.001). Patients with treatment limitations were older (median 73 years [interquartile range (IQR) 61-80] vs. 68 years [IQR 54-77]) and more had mental decision-making capacity (12.9 vs. 0.8%), advance directives (28.6 vs. 11.2%), and information about treatment wishes (82.7 vs 33.3%, all p < 0.001). Physicians reported discussing treatment limitations with patients with mental decision-making capacity and families (91.3 and 82.6%, respectively). Patient wishes were unknown in 41.3% of patients. The major reason for decision-making was unresponsiveness to maximal therapy (34.6%).

CONCLUSIONS

Treatment limitations are common, based on information about patients' wishes and discussion between stakeholders, patients and families. However, our findings suggest that treatment preferences of nearly half the patients remain unknown which affects guidance for treatment decisions.

Tailoring Interlayer Charge Transfer Dynamics in 2D Perovskites with Electroactive Spacer Molecules

The family of hybrid organic-inorganic lead-halide perovskites are the subject of intense interest for optoelectronic applications, from light-emitting diodes to photovoltaics to X-ray detectors. Due to the inert nature of most organic molecules, the inorganic sublattice generally dominates the electronic structure and therefore the optoelectronic properties of perovskites. Here, we use optically and electronically active carbazole-based Cz-Ci molecules, where Ci indicates an alkylammonium chain and i indicates the number of CH2 units in the chain, varying from 3 to 5, as cations in the two-dimensional (2D) perovskite structure. By investigating the photophysics and charge transport characteristics of (Cz-Ci)2PbI4, we demonstrate a tunable electronic coupling between the inorganic lead-halide and organic layers. The strongest interlayer electronic coupling was found for (Cz-C3)2PbI4, where photothermal deflection spectroscopy results remarkably reveal an organic-inorganic charge transfer state. Ultrafast transient absorption spectroscopy measurements demonstrate ultrafast hole transfer from the photoexcited lead-halide layer to the Cz-Ci molecules, the efficiency of which increases by varying the chain length from i = 5 to i = 3. The charge transfer results in long-lived carriers (10-100 ns) and quenched emission, in stark contrast to the fast (sub-ns) and efficient radiative decay of bound excitons in the more conventional 2D perovskite (PEA)2PbI4, in which phenylethylammonium (PEA) acts as an inert spacer. Electrical charge transport measurements further support enhanced interlayer coupling, showing increased out-of-plane carrier mobility from i = 5 to i = 3. This study paves the way for the rational design of 2D perovskites with combined inorganic-organic electronic properties through the wide range of functionalities available in the world of organics.

Charge transport in mixed metal halide perovskite semiconductors

Investigation of the inherent field-driven charge transport behaviour of three-dimensional lead halide perovskites has largely remained challenging, owing to undesirable ionic migration effects near room temperature and dipolar disorder instabilities prevalent specifically in methylammonium-and-lead-based high-performing three-dimensional perovskite compositions. Here, we address both these challenges and demonstrate that field-effect transistors based on methylammonium-free, mixed metal (Pb/Sn) perovskite compositions do not suffer from ion migration effects as notably as their pure-Pb counterparts and reliably exhibit hysteresis-free p-type transport with a mobility reaching 5.4 cm² V^-1 s^-1. The reduced ion migration is visualized through photoluminescence microscopy under bias and is manifested as an activated temperature dependence of the field-effect mobility with a low activation energy (~48 meV) consistent with the presence of the shallow defects present in these materials. An understanding of the long-range electronic charge transport in these inherently doped mixed metal halide perovskites will contribute immensely towards high-performance optoelectronic devices.

Tunable Multiband Halide Perovskite Tandem Photodetectors with Switchable Response

Photodetectors with multiple spectral response bands have shown promise to improve imaging and communications through the switchable detection of different photon energies. However, demonstrations to date have been limited to only two bands and lack capability for fast switching in situ. Here, we exploit the band gap tunability and capability of all-perovskite tandem solar cells to demonstrate a new device concept realizing four spectral bands of response from a single multijunction device, with fast, optically controlled switching between the bands. The response to monochromatic light is highly selective and narrowband without the need for additional filters and switches to broader response bands on applying bias light. Sensitive photodetection above 6 × 10¹¹ Jones is demonstrated in all modes, with rapid switching response times of <250 ns. We demonstrate proof of principle on how the manipulation of the modular multiband detector response through light conditions enables diverse applications in optical communications with secure encryption.

Enhanced visible light absorption in layered Cs3Bi2Br9 through mixed-valence Sn(ii)/Sn(iv) doping

Lead-free halides with perovskite-related structures, such as the vacancy-ordered perovskite Cs₃Bi₂Br₉, are of interest for photovoltaic and optoelectronic applications. We find that addition of SnBr₂ to the solution-phase synthesis of Cs₃Bi₂Br₉ leads to substitution of up to 7% of the Bi(iii) ions by equal quantities of Sn(ii) and Sn(iv). The nature of the substitutional defects was studied by X-ray diffraction, ¹³³Cs and ¹¹⁹Sn solid state NMR, X-ray photoelectron spectroscopy and density functional theory calculations. The resulting mixed-valence compounds show intense visible and near infrared absorption due to intervalence charge transfer, as well as electronic transitions to and from localised Sn-based states within the band gap. Sn(ii) and Sn(iv) defects preferentially occupy neighbouring B-cation sites, forming a double-substitution complex. Unusually for a Sn(ii) compound, the material shows minimal changes in optical and structural properties after 12 months storage in air. Our calculations suggest the stabilisation of Sn(ii) within the double substitution complex contributes to this unusual stability. These results expand upon research on inorganic mixed-valent halides to a new, layered structure, and offer insights into the tuning, doping mechanisms, and structure-property relationships of lead-free vacancy-ordered perovskite structures.

Optoelectronic Properties of Low-Bandgap Halide Perovskites for Solar Cell Applications

Riding on the coat tails of rapid developments in single-junction halide perovskite solar cells, all-perovskite multijunction solar cells have recently garnered significant attention, with the highest power-conversion efficiency already reaching 25.6%. Much of this progress has been fueled by the rapid rise in the photovoltaic performance of low-bandgap halide perovskite absorbers, materials, which, to date, have only been achievable by the partial or complete substitution of lead with tin. However, much room still exists to develop a more critical understanding of key material properties in these low-bandgap perovskites. Herein, the key optoelectronic properties of absorption, carrier generation, recombination, and transport in these tin-containing perovskites are discussed, showing that intrinsic doping distinctively impacts many of these properties, thereby rendering this class of halide perovskites unique within the family. Current understanding of the mechanisms that degrade optoelectronic performance in these materials and the corresponding devices are also summarized. These collective results highlight an important interplay between doping, defects, and degradation that will need to be controlled. Finally, the current gaps in understanding of these low-bandgap perovskites are outlined, thereby providing guidelines for further research, which will unlock their full potential for realizing a plethora of high-performance optoelectronic devices.

Critical Assessment of the Use of Excess Lead Iodide in Lead Halide Perovskite Solar Cells

It is common practice in the lead halide perovskite solar cell field to add a small molar excess of lead iodide (PbI₂) to the precursor solution to increase the device performance. However, recent reports have shown that an excess of PbI₂ can accelerate performance loss. In addition, PbI₂ is photoactive (band gap ∼2.3 eV), which may lead to parasitic absorption losses in a solar cell. Here we show that devices using small quantities of excess PbI₂ exhibit better device performance as compared with stoichiometric devices, both initially and for the duration of a stability test under operating conditions, primarily by enhancing the charge extraction. However, the photolysis of PbI₂ negates the beneficial effect on charge extraction by leaving voids in the perovskite film and introduces trap states that are detrimental for device performance. We propose that although excess PbI₂ provides a good template for enhanced performance, the community must continue to seek other additives or synthesis routes that fulfill the same beneficial role as excess PbI₂, but without the photolysis that negates these beneficial effects under long-term device operation.

Unveiling spin-glass transition and antiferromagnetic order by μSR studies in spin-chain Sm2BaNiO5

We report the zero-field and longitudinal field muon spin relaxation studies in a spin-chain compound Sm₂BaNiO₅. Two magnetic transitions, that have not been previously detected by the heat capacity and magnetization measurements, are confirmed at 46 and 9 K. The antiferromagnetic order is suggested at 46 K. Analysis of the muon spin polarization unveils the spin-glass transition at 9 K. Time-field scaling relation of the muon spin polarization verifies the spin-spin autocorrelation function following the cut-off power law, which is approximated by the Ogielski form, as employed numerically for characterizing the spin-glasses.

[Non-beneficial therapy and emotional exhaustion in end-of-life care : Results of a survey among intensive care unit personnel]

BACKGROUND

End-of-life care (EOLC) in the intensive care unit (ICU) is becoming increasingly more common but ethical standards are compromised by growing economic pressure. It was previously found that perception of non-beneficial treatment (NBT) was independently associated with the core burnout dimension of emotional exhaustion. It is unknown whether factors of the work environment also play a role in the context of EOLC.

OBJECTIVE

Is the working environment associated with perception of NBT or clinician burnout?

MATERIAL AND METHODS

Physicians and nursing personnel from 11 German ICUs who took part in an international, longitudinal prospective observational study on EOLC in 2015-2016 were surveyed using validated instruments. Risk factors were obtained by multivariate multilevel analysis.

RESULTS

The participation rate was 49.8% of personnel working in the ICU at the time of the survey. Overall, 325 nursing personnel, 91 residents and 26 consulting physicians participated. Nurses perceived NBT more frequently than physicians. Predictors for the perception of NBT were profession, collaboration in the EOLC context, excessively high workload (each p ≤ 0.001) and the numbers of weekend working days per month (p = 0.012). Protective factors against burnout included intensive care specialization (p = 0.001) and emotional support within the team (p ≤ 0.001), while emotional exhaustion through contact with relatives at the end of life and a high workload were both increased (each p ≤ 0.001).

DISCUSSION

Using the example of EOLC, deficits in the work environment and stress factors were uncovered. Factors of the work environment are associated with perceived NBT. To reduce NBT and burnout, the quality of the work environment should be improved and intensive care specialization and emotional support within the team enhanced. Interprofessional decision-making among the ICU team and interprofessional collaboration should be improved by regular joint rounds and interprofessional case discussions. Mitigating stressful factors such as communication with relatives and high workload require allocation of respective resources.

Magnetoelectric Coupling, Ferroelectricity, and Magnetic Memory Effect in Double Perovskite La3Ni2NbO9

We observe ferroelectricity in an almost unexplored double perovskite La3Ni2NbO9. Ferroelectricity appears below ∼60 K, which is found to be correlated with the significant magnetostriction. A reasonably large value of spontaneous electric polarization is recorded to be ∼260 μC/m(2) at 10 K for E = 5 kV/cm, which decreases signifi- cantly upon application of a magnetic field (H), suggesting considerable magnetoelectric coupling. The dielectric permittivity is also influenced by H below the ferroelectric transition. The magnetodielectric response scales linearly to the squared magnetization, as described by the Ginzburg-Landau theory. Meticulous studies of static and dynamic features of dc magnetization and frequency dependent ac susceptibility results suggest spin-glass state below 29 K. Intrinsic magnetic memory effect is observed from zero-field cooled magnetization and isothermal remanent magnetization studies, also pointing spin-glass state below 29 K. Appearance of ferroelectricity together with a significant magnetoelectric coupling in absence of conventional long-range magnetic order is promising for searching new magnetoelectric materials.

Magnetoelectric coupling and exchange bias effects in multiferroic NdCrO3

We report ferroelectricity around  ∼88 K that appears well below T N (∼25 K), unlike other members of RCrO3 series. A synchrotron diffraction study suggests that the occurrence of ferroelectricity in NdCrO3 is coupled to the structural transformation from centrosymmetric Pnma to a non-centrosymmetric Pna21 space group. A strong magnetoelectric coupling is observed in the electric polarization [P(T)]. This coupling is significantly influenced by the magnetic field cooling effect, suggesting an exchange bias effect in P(T). This exchange bias effect is also revealed by the systematic shift of the magnetic hysteresis loops below T(N). The rare occurrence of an exchange bias effect in both the magnetic and electric polarizations associated with a strong magnetoelectric coupling is of fundamental interest, as well as being attractive for technological applications close to liquid nitrogen temperature.

Magnetic memory in nanocrystalline α-Fe2O3 embedded in reduced graphene oxide

Single phase iron oxide (α-Fe₂O₃) of nearly regular ellipsoidal shape, embedded in reduced graphene oxide (RGO) has been prepared by the chemical route. Memory effect is observed in magnetization study.

Kates forefoot arthroplasty in rheumatoid arthritis. A 5-year followup study

The longterm results of Kates forefoot arthroplasty in 74 feet of 41 patients with rheumatoid arthritis (RA) after a mean followup of 5.2 years was considered to be good by 38 patients and poor by 3 patients. The average walking distance had doubled. The mean hallux valgus angle was reduced from 46 to 27 degrees. Surgical results proved to depend on the quality of the arc of the remaining stumps, and not on the severity or activity of RA. Reoperations were necessary in 16 feet of 10 patients because of too prominent distal metatarsal stumps. Despite the absence of pain, 28 patients were not satisfied with the function of the hallux. This might be improved by performing arthrodesis of the first metatarsophalangeal joint.

Antimicrobial, insect sterilizing and ovicidal activity of some oxo-vanadium(IV) and oxo-vanadium(V) complexes

Twenty-three newly synthesized mixed-ligand complexes of oxo-vanadium(IV) and oxo-vanadium(V) were studied for antimicrobial activity. Eight of these complexes were found to have microbicidal properties. The complexes [NH4][VO(gl)2]H2O (gl-H2 = glycolic acid) and [VO(ACOAP)(acac-H)]H20 (ACOAP-H2=Schiff base of acetylacetone and orthoaminophenol, acac-H=acetyl-acetone) show broad bactericidal spectra, while the complexes [VO(ACSAM)2]OH (ACSAM-H = Schiff base of acetylacetone and sulphanilamide) and [VO(CSSAM-H)2]H2O (CSSAM-H =Schiff base of 3-carboxy salicylaldehyde and sulphanilamide) possess pronounced antidermatophytic properties. The latter is inhibitory to plant pathogenic fungi as well. Plant tumour producing Agrobacterium tumefaciens is effectively inhibited in vitro by the complex [VO(ACTSC-Na)(acac)]H2O (ACTSC-H2 = condensation product of acetylacetone and thiosemicarbazide). Minimum inhibitory concentrations of all the active complexes are within the values of 0.125-2.00 mg/ml. Out of the 7 active complexes tested for 50% inhibition of conidial germination of Helminthosporium oryzae, a rice plant pathogen, only 1 complex, viz. [VO(acac) (ACACAACD)] (ACACAACD(H)NH4 equal Schiff base of acetylacetone and ammonium 2-amino-1-cyclopentene-1-dithiocarboxylate) shows a positive result. The effective concentration is 0.55 mg/ml. Three vanadium complexes were tested for insect sterilant and ovicidal properties on the red cotton bug, Dysdercus koenigi. The complex [VO(HASA-Na) (acac)]H2O (HASA-H2 = Schiff base of orthohydroxyacetophenone and anthr anilic acid) was found to be a suitable male sterilant.

Antimicrobial, insect sterilizing and ovicidal activity of some cobalt(II) and cobalt(III) complexes

Twenty-one mixed-ligand complexes of cobalt(II) and cobalt(III) have been screened for their antimicrobial, insect sterilizing and ovicidal activities. Three of these cobalt(III) complexes exhibit broad antimicrobial spectra, including against human bacterial pathogens, dermatophytes and plant pathogenic fungi, while one exhibits feeble activity against a human pathogenic bacterium. These results have been compared with the activity of the corresponding cobalt(II) complexes, which have been found to be inactive, while the free ligands show reduced activity compared with the cobalt(III) complexes. Change in biological activity induced by a particular complex appears to be dependent on the composition of the first co-ordination sphere. Two of these complexes showed 50% inhibition of the conidial germination of Helminthosporium oryzae and Alternaria triticina. These results indicate their potential for use against human and plant pathogenic microbes. Minimum inhibitory concentrations of the cobalt(III) complexes were determined. Three of these cobalt(III) complexes have been tested for insect sterilizing and ovicidal activities on Dysdercus koenigi F. Positive sterilizing and ovicidal actions of [Co(BSOP)(NH3)2]NO3 (where BSOP-H2 is the Schiff base derived from salicylaldehyde and orthophenylene-diamine) were obtained. Possible mechanism(s) of all these activities are discussed qualitatively.