Current measurement by real-time counting of single electrons

The fact that electrical current is carried by individual charges has been known for over 100 years, yet this discreteness has not been directly observed so far. Almost all current measurements involve measuring the voltage drop across a resistor, using Ohm's law, in which the discrete nature of charge does not come into play. However, by sending a direct current through a microelectronic circuit with a chain of islands connected by small tunnel junctions, the individual electrons can be observed one by one. The quantum mechanical tunnelling of single charges in this one-dimensional array is time correlated, and consequently the detected signal has the average frequency f=I/e, where I is the current and e is the electron charge. Here we report a direct observation of these time-correlated single-electron tunnelling oscillations, and show electron counting in the range 5 fA-1 pA. This represents a fundamentally new way to measure extremely small currents, without offset or drift. Moreover, our current measurement, which is based on electron counting, is self-calibrated, as the measured frequency is related to the current only by a natural constant.Comment: 9 pages, 4 figures; v2: minor revisions, 2 refs added, words added to title, typos correcte

On the frequentist coverage of Bayesian credible intervals for lower bounded means

For estimating a lower bounded location or mean parameter for a symmetric and logconcave density, we investigate the frequentist performance of the $100(1-\alpha)$ Bayesian HPD credible set associated with priors which are truncations of flat priors onto the restricted parameter space. Various new properties are obtained. Namely, we identify precisely where the minimum coverage is obtained and we show that this minimum coverage is bounded between $1-\frac{3\alpha}{2}$ and $1-\frac{3\alpha}{2}+\frac{\alpha^2}{1+\alpha}$; with the lower bound $1-\frac{3\alpha}{2}$ improving (for $\alpha \leq 1/3$) on the previously established ([9]; [8]) lower bound $\frac{1-\alpha}{1+\alpha}$. Several illustrative examples are given.Comment: Published in at http://dx.doi.org/10.1214/08-EJS292 the Electronic Journal of Statistics (http://www.i-journals.org/ejs/) by the Institute of Mathematical Statistics (http://www.imstat.org

Cognitive performance in healthy older adults relates to spontaneous switching between states of functional connectivity during rest

Growing evidence has shown that brain activity at rest slowly wanders through a repertoire of different states, where whole-brain functional connectivity (FC) temporarily settles into distinct FC patterns. Nevertheless, the functional role of resting-state activity remains unclear. Here, we investigate how the switching behavior of resting-state FC relates with cognitive performance in healthy older adults. We analyse resting-state fMRI data from 98 healthy adults previously categorized as being among the best or among the worst performers in a cohort study of >1000 subjects aged 50+ who underwent neuropsychological assessment. We use a novel approach focusing on the dominant FC pattern captured by the leading eigenvector of dynamic FC matrices. Recurrent FC patterns - or states - are detected and characterized in terms of lifetime, probability of occurrence and switching profiles. We find that poorer cognitive performance is associated with weaker FC temporal similarity together with altered switching between FC states. These results provide new evidence linking the switching dynamics of FC during rest with cognitive performance in later life, reinforcing the functional role of resting-state activity for effective cognitive processing.This project was financed by the Fundação Calouste Gulbenkian (Portugal) (Contract grant number: P-139977; project “Better mental health during ageing based on temporal prediction of individual brain ageing trajectories (TEMPO)”), co-financed by Portuguese North Regional Operational Program (ON.2) under the National Strategic Reference Framework (QREN), through the European Regional Development Fund (FEDER) as well as the Projecto Estratégico co-funded by FCT (PEst-C/SAU/LA0026-/2013) and the European Regional Development Fund COMPETE (FCOMP-01-0124-FEDER-037298) and under the scope of the project NORTE-01-0145-FEDER-000013, supported by the Northern Portugal Regional Operational Programme (NORTE 2020) under the Portugal 2020 Partnership Agreement through the European Regional Development Fundinfo:eu-repo/semantics/publishedVersio

Crystalline silicon solar cell with front and rear polysilicon passivated contacts as bottom cell for hybrid tandems

\u3cp\u3eIn this paper we analyze and model perovskite/c-Si tandem cells with front and rear polySi passivated contacts on the bottom cell. A high-efficiency tandem approach will benefit from the high V\u3csub\u3eoc\u3c/sub\u3e potential of a c-Si bottom cell with front and rear polySi passivated contacts while the combination with a high band gap, semi-transparent, perovskite top cell will largely diminish the UV-Vis parasitic absorption in a polySi front side layer on the c-Si cell. On the other hand since the J\u3csub\u3esc\u3c/sub\u3e is strongly reduced in a tandem bottom cell, free carrier absorption, to which both front and rear polySi layers contribute, will become a relatively more important loss mechanism. We investigate the trade-off between the optical gains and resistive losses from reducing the polySi thickness for cell configurations without transparent conductive oxide (TCO) and also consider the potential of the combination with TCOs. From our optical simulations we conclude that optical losses in the polySi layers of 100 nm and below are limited when applied on the bottom cell. Taking into account resistive losses in the polySi layers of varying thickness the optimal thickness is found to be 50 nm. In combination with the high V\u3csub\u3eoc\u3c/sub\u3e values resulting from the application of polySi passivating contacts this offers a promising route to establish a bottom cell with high efficiency. The combination of very thin polySi with highly transparent TCOs is likely to further improve bottom cell performance.\u3c/p\u3

Highly near-infrared-transparent perovskite solar cells and their application in high-efficiency 4-terminal perovskite/c-Si tandems

\u3cp\u3eIn this contribution the development of highefficiency planar semi-transparent perovskite solar cells (STPSCs) for tandem applications is presented. The ST-PSC absorber layer, and electron and hole transport layers were processed using spin coating. The near-infrared (NIR) transmission of the ST-PSC is optimized by improving ITO material quality and tuning the thickness of component layers in cells for optimal light management, leading to a high NIR transmittance of about 93%. In combination with a SunPower IBC cell of 23.8% single-junction efficiency, a 4-terminal (4T) perovskite/c-Si tandem cell efficiency of 26.1% is achieved. In combination with a metal-wrap-through n-PERT c-Si cell laminate of 18.6% efficiency, a 4T perovskite/c-Si tandem cell efficiency of 24.1% is demonstrated, showing that a very significant efficiency gain can be obtained on lower performance c-Si cells.\u3c/p\u3

Material properties of LPCVD processed n-type polysilicon passivating contacts and its application in PERPoly industrial bifacial solar cells

\u3cp\u3eWe present a detailed material study of n\u3csup\u3e+\u3c/sup\u3e-type polysilicon (polySi) and its application as a carrier selective rear contact in a bifacial n-type solar cell comprising fire-through screen-printed metallization and 6 Cz wafers. The cells were manufactured with low-cost industrial process steps yielding V\u3csub\u3eoc\u3c/sub\u3es from 676 to 683 mV and J\u3csub\u3esc\u3c/sub\u3es above 39.4 mA/cm\u3csup\u3e2\u3c/sup\u3e indicating an efficiency potential of 22%. The aim of this study is to understand which material properties determine the performance of POCl\u3csub\u3e3\u3c/sub\u3e-diffused (n-type) polySi-based passivating contacts and to find routes to improve its use for industrial PERPoly (Passivated Emitter Rear PolySi) cells from the point of view of throughput, performance, and bifacial application. This paper reports on correlations between the parameters used for low pressure chemical vapour deposition (LPCVD), annealing, and doping on optical, structural, and electronic properties of the polySi-based passivating contact and the subsequent influence on the solar cell parameters.\u3c/p\u3

Combination of Advanced Optical Modelling with Electrical Simulation for Performance Evaluation of Practical 4-terminal Perovskite/c-Si Tandem Modules

The perovskite solar cell is considered a promising candidate as the top cell for high-efficiency tandem devices with crystalline silicon (c-Si) bottom cells, contributing to the cost reduction of photovoltaic energy. In this contribution, a simulation method, involving optical and electrical modelling, is established to calculate the performance of 4-terminal (4T) perovskite/c-Si tandem devices on a mini-module level. Optical and electrical characterization of perovskite and c-Si solar cells are carried out to verify the simulation parameters. With our method, the influence of transparent conductive oxide (TCO) layer thickness of perovskite top cells on the performance of tandem mini-modules is investigated in case of both tin-doped indium oxide (ITO) and hydrogen-doped indium oxide (IO:H). The investigation shows that optimization of TCO layer thickness and replacement of conventional ITO with highly transparent IO:H can lead to an absolute efficiency increase of about 1%. Finally, a practical assessment of the efficiency potential for the 4T perovskite/c-Si tandem mini-module is carried out, indicating that with a relatively simple 4T tandem module structure the efficiency of a single-junction c-Si mini-module (19.3%) can be improved by absolute 4.5%.Photovoltaic Materials and Device