251 research outputs found
Probing the energy levels of perovskite solar cells via Kelvin probe and UV ambient pressure photoemission spectroscopy
This work was supported by the Engineering and Physical Sciences Research Council (grant codes EP/M506631/1, EP/ K015540/01, EP/K022237/1 and EP/M025330/1). IDWS and JTSI acknowledge Royal Society Wolfson research merit awards.The field of organo-lead halide perovskite solar cells has been rapidly growing since their discovery in 2009. State of the art devices are now achieving efficiencies comparable to much older technologies like silicon, while utilising simple manufacturing processes and starting materials. A key parameter to consider when optimising solar cell devices or when designing new materials is the position and effects of the energy levels in the materials. We present here a comprehensive study of the energy levels present in a common structure of perovskite solar cell using an advanced macroscopic Kelvin probe and UV air photoemission setup. By constructing a detailed map of the energy levels in the system we are able to predict the importance of each layer to the open circuit voltage of the solar cell, which we then back up through measurements of the surface photovoltage of the cell under white illumination. Our results demonstrate the effectiveness of air photoemission and Kelvin probe contact potential difference measurements as a method of identifying the factors contributing to the open circuit voltage in a solar cell, as well as being an excellent way of probing the physics of new materials.Publisher PDFPeer reviewe
Impact of Gastrostomy Placement on Nutritional Status, Physical Health, and Parental Well-Being of Females with Rett Syndrome: A Longitudinal Study of an Australian Population
© 2018 Elsevier Inc.
Objectives: To evaluate how age-related trends in nutritional status, physical health, and parental well-being in females with Rett syndrome may be related to gastrostomy placement and to examine the impact of the procedure on mortality.
Study design: We included 323 females from the Australian Rett Syndrome Study and analyzed their demographic, genetic, and child and parental health data collected from over 6 waves of follow-up questionnaire between 2000 and 2011. We used mixed-effects models to estimate the association between repeated measures of outcomes and age, gastrostomy placement and their interaction and Cox proportional hazards regression models to estimate relative risks of mortality for individuals with gastrostomy.
Results: Nearly one-third (30.3%) of the cases underwent gastrostomy placement. Nutritional status based on weight, height, and body mass index (BMI) improved over time, and BMI was greater in individuals with gastrostomy placement than in those without (adjusted ÎČ = 0.87, 95% CI 0.02-1.73). There was no association between gastrostomy placement and individual's physical health outcomes or parental physical and mental health, nor did the age trend of these outcomes vary by gastrostomy insertion status. Nevertheless, among those at risk of suboptimal weight, the all-cause mortality rate was greater in those who had gastrostomy placement compared with those who had not (hazard ratio 4.07, 95% CI 1.96-8.45).
Conclusion: Gastrostomy placement was associated with improvement in BMI in females with Rett syndrome, but its long-term impact on individuals and their families is unclear
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The impact of oxygen on the electronic structure of mixed-cation halide perovskites
Alloyed triple A-cation perovskites containing a mixture of Cs, methylammonium (MA) and formamidinium (FA) cations are attracting intense attention because of their high photovoltaic performance and relative stability. However, there is limited fundamental understanding of their vacancy defect behaviour and influence of molecular oxygen on their electronic and stability properties. In this combined computational-experimental study, we investigate the (FA,MA,Cs)Pb(I,Br)3 model system with its simulated atomistic structure presented for the first time and supported by X-ray diffraction data. We examine how iodide vacancies and O2 molecules influence the local geometry and electronic structure. Our calculations, supported by Kelvin Probe contact potential difference and photoluminescence measurements, show that introduction of O2 leads to a p-doped triple-cation perovskite, and passivates iodide vacancies resulting in enhanced luminescence efficiency. These results have important implications for the performance and stability of mixed-cation perovskites in optoelectronic devices.This work was supported by the EPSRC Programme Grant âEnergy Materials: Computational Solutionsâ (EP/K016288/1) and the HPC Materials Chemistry Consortium for Archer computational time (EP/L000202/1). Z.A.-G. acknowledges funding from a Winton Studentship, and ICON Studentship from the Lloydâs Register Foundation. S.D.S acknowledges the Royal Society and Tata Group (UF150033). K.G. acknowledges the Polish Ministry of Science and Higher Education within the Mobilnosc Plus program (Grant No. 1603/MOB/V/2017/0). M.A. acknowledges funding from the European Unionâs Horizon 2020 research and innovation programme under the Marie SkĆodowska-Curie grant agreement No 841386
Automatic kelvin probe compatible with ultrahigh vacuum
This article describes a new type of in situ ultrahighâvacuum compatible kelvin probe based on a voiceâcoil driving mechanism. This design exhibits several advantages over conventional mechanical feedâthrough and (in situ) piezoelectric devices in regard to the possibility of multiple probe geometry, flexibility of probe geometry, amplitude of oscillation, and pure parallel vibration. Automatic setup and constant spacing features are achieved using a digitalâtoâanalog converter (DAC) steered offset potential. The combination of very low driver noise pickâup and dataâacquisition system (DAS) signal processing techniques results in a work function (wfâ ) resolution, under optimal conditions, of <0.1 meV. Due to its high surface sensitivity and compatibility with standard sample cleaning and analysis techniques this design has numerous applications in surface studies, e.g., adsorption kinetics, sample topography and homogeneity, sputter profiles, etc. For semiconductor specimens the high wf resolution makes it eminently suitable for surface photovoltage (SPV) spectroscopy
Generalised Framework for Controlling and Understanding Ion Dynamics with Passivated Lead Halide Perovskites
Metal halide perovskite solar cells have gained widespread attention due to
their high efficiency and high defect tolerance. The absorbing perovskite layer
is as a mixed electron-ion conductor that supports high rates of ion and charge
transport at room temperature, but the migration of mobile defects can lead to
degradation pathways. We combine experimental observations and drift-diffusion
modelling to demonstrate a new framework to interpret surface photovoltage
(SPV) measurements in perovskite systems and mixed electronic ionic conductors
more generally. We conclude that the SPV in mixed electronic ionic conductors
can be understood in terms of the change in electric potential at the surface
associated with changes in the net charge within the semiconductor system. We
show that by modifying the interfaces of perovskite bilayers, we may control
defect migration behaviour throughout the perovskite bulk. Our new framework
for SPV has broad implications for developing strategies to improve the
stability of perovskite devices by controlling defect accumulation at
interfaces. More generally, in mixed electronic conductors our framework
provides new insights into the behaviour of mobile defects and their
interaction with photoinduced charges, which are foundational to physical
mechanisms in memristivity, logic, impedance, sensors and energy storage
Atomistic origins of high-performance in hybrid halide perovskite solar cells
The performance of organometallic perovskite solar cells has rapidly
surpassed that of both conventional dye-sensitised and organic photovoltaics.
High power conversion efficiency can be realised in both mesoporous and
thin-film device architectures. We address the origin of this success in the
context of the materials chemistry and physics of the bulk perovskite as
described by electronic structure calculations. In addition to the basic
optoelectronic properties essential for an efficient photovoltaic device
(spectrally suitable band gap, high optical absorption, low carrier effective
masses), the materials are structurally and compositionally flexible. As we
show, hybrid perovskites exhibit spontaneous electric polarisation; we also
suggest ways in which this can be tuned through judicious choice of the organic
cation. The presence of ferroelectric domains will result in internal junctions
that may aid separation of photoexcited electron and hole pairs, and reduction
of recombination through segregation of charge carriers. The combination of
high dielectric constant and low effective mass promotes both Wannier-Mott
exciton separation and effective ionisation of donor and acceptor defects. The
photoferroic effect could be exploited in nanostructured films to generate a
higher open circuit voltage and may contribute to the current-voltage
hysteresis observed in perovskite solar cells.Comment: 6 pages, 5 figure
Non-intrusive flow diagnostics for unsteady inlet flow distortion measurements in novel aircraft architectures
Inlet flow distortion is expected to play a major role in future aircraft architectures where complex air induction systems are required to couple the engine with the airframe. The highly unsteady distortions generated by such intake systems can be detrimental to engine performance and were previously linked with loss of engine stability and potentially catastrophic consequences. During aircraft design, inlet flow distortion is typically evaluated at the aerodynamic interface plane, which is defined as a cross-flow plane located at a specific upstream distance from the engine fan. Industrial testing currently puts more emphasis on steady state distortions despite the fact that, historically, unsteady distortions were acknowledged as equally important. This was partially due to the limitations of intrusive measurement methods to deliver unsteady data of high spatial resolution in combination with their high cost and complexity. However, as the development of aircraft with fuselage-integrated engine concepts progresses, the combination of different types of flow distortions is expected to have a strong impact on the engineâs stability margin. Therefore, the need for novel measurement methods able to meet the anticipated demand for more comprehensive flow information is now more critical than ever. In reviewing the capabilities of various non-intrusive methods for inlet distortion measurements, Filtered Rayleigh Scattering (FRS) is found to have the highest potential for synchronously characterising multiple types of inlet flow distortions, since the method has the proven ability to simultaneously measure velocity, static pressure and temperature fields in challenging experimental environments. The attributes of the FRS method are further analysed aiming to deliver a roadmap for its application on ground-based and in-flight measurement environments.European Union funding: 88652
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