Persistent photovoltage in methylammonium lead iodide perovskite solar cells

Open circuit voltage decay measurements are performed on methylammonium lead iodide (CH3NH3PbI3) perovskite solar cells to investigate the charge carrier recombination dynamics. The measurements are compared to the two reference polymer-fullerene bulk heterojunction solar cells based on P3HT:PC60BM and PTB7:PC70BM blends. In the perovskite devices, two very different time domains of the voltage decay are found, with a first drop on a short time scale that is similar to the organic solar cells. However, two major differences are also observed. 65-70% of the maximum photovoltage persists on much longer timescales, and the recombination dynamics are dependent on the illumination intensity.Comment: 5 pages, 3 figure

Theoretical Perspective on Transient Photovoltage and Charge Extraction Techniques

Transient photovoltage (TPV) is a technique frequently used to determine charge carrier lifetimes in thin-film solar cells such as organic, dye sensitized and perovskite solar cells. As this lifetime is often incident light intensity dependent, its relevance to understanding the intrinsic properties of a photoactive material system as a material or device figure of merit has been questioned. To extract complete information on recombination dynamics, the TPV measurements are often performed in conjunction with charge extraction (CE) measurements, employed to determine the photo-generated charge carrier density and thereby the recombination rate constant and its order. In this communication, the underlying theory of TPV and CE is reviewed and expanded. Our theoretical findings are further solidified by numerical simulations and experiments on organic solar cells. We identify regimes of the open-circuit voltage within which accurate lifetimes and carrier densities can be determined with TPV and CE experiments. A wide range of steady-state light intensities is required in performing these experiments in order to identify their “working dynamic range” from which the recombination kinetics in thin-film solar cells can be determined

Trapping light with micro lenses in thin film organic photovoltaic cells.

We demonstrate a novel light trapping configuration based on an array of micro lenses in conjunction with a self aligned array of micro apertures located in a highly reflecting mirror. When locating the light trapping element, that displays strong directional asymmetric transmission, in front of thin film organic photovoltaic cells, an increase in cell absorption is obtained. By recycling reflected photons that otherwise would be lost, thinner films with more beneficial electrical properties can effectively be deployed. The light trapping element enhances the absorption rate of the solar cell and increases the photocurrent by as much as 25%. (C) 2008 Optical Society of Americ

”Barnen talar två språk och har två kulturer”

Cooperation between school and home is one of the cornerstones in school policies in Sweden, as well as internationally. During the last decades, questions of parental influence and the freedom to choose schools have come to the fore. The importance of well-functioning cooperation between school and home, aiming to support school performance and improve the pupils’ results, is another issue which has been stressed in policy, as well as in research. When implementing a major change, such as developing bilingual education, it is important to have the support and cooperation of the parents. In this paper the views of parents of pupils who received bilingual education in Arabic and Swedish are presented. Data was collected in questionnaires. In addition to information concerning the educational and linguistic background of the family, the parents gave their views on bilingual education in general, and on the bilingual education their children received, in particular. The parents appreciated the bilingual education and emphasized the importance of both mother tongue and Swedish for their children. They regarded bilingualism as an asset and wanted their children to become bilingual and bicultural

Inledning

Föreliggande temanummer i EDUCARE – Tvåspråkig undervisning på svenska och arabiska och mångkulturella storstadsskolor – är huvudresultatet av de studier som genomförts inom forskningsprojektet ”Olika modeller för tvåspråkig undervisning på arabiska och svenska – förutsättningar och konsekvenser” som finansierats av UVK, Vetenskapsrådets utbildningsvetenskapliga kommitté

Quantifying metarefraction with confocal lenslet arrays

METATOYs can change the direction of light in ways that appear to, but do not actually, contravene the laws of wave optics. This direction change applies only to part of the transmitted light beam; the remainder gets re-directed differently. For a specific example, namely confocal pairs of rectangular lenslet arrays with no dead area between lenslets, we calculate here the fractions of power of a uniform-intensity light beam incident from a specific (but arbitrary) direction that get re-directed in different ways, and we derive an equation describing this redirection. This will facilitate assessment of the suitability of METATOYs for applications such as solar concentration. Finally, we discuss similarities between the multiple refraction of light at the lenslet arrays and multiple refraction and reflection of cold atoms at a barrier in the presence of the light fields.Comment: 24 pages, 15 figure

Unravelling steady-state bulk recombination dynamics in thick efficient vacuum-deposited perovskite solar cells by transient methods

Accurately identifying and understanding the dominant charge carrier recombination mechanism in perovskite solar cells is of crucial importance for further improvements of this already promising photovoltaic technology. Both optical and electrical transient methods have previously been employed to strive for this warranted goal. However, electrical techniques can be strongly influenced by the capacitive response of the device which overlays with the steady‐state relevant bulk carrier recombination. To ascertain the identification of bulk charge carrier dynamics, it is beneficial to evaluate thicker films to minimize the impact of device capacitance. Herein, the electrical transient response in very efficient planar co‐evaporated n‐i‐p solar cells is studied by varying the active layer thickness from 500 nm to 820 nm and compared to a solution‐processed perovskite device with an active layer of 350 nm. In case of the n‐i‐p devices, the capacitance for the 500 nm solar cell leads to longer perceived decay times in the lower voltage regime, while in the higher voltage regime quite similar kinetics independent of active layer thickness are observed, allowing us to identify the transition from capacitance‐affected to the sought‐after bulk charge carrier dynamics. We show that increasing the perovskite thickness by more than 50 % does not affect the recombination dynamics significantly, confirming the high quality of the vacuum‐processed perovskite solar cells. Finally, it is demonstrated for the first time for perovskite solar cells that the recombination order in both thicker devices is ranging between 1.6 to 2, pointing towards trap‐assisted and free‐carrier recombination under operating conditions. We emphasize that the observed low recombination orders are in strong contrast to earlier literature as well as to the thinner solution‐processed device, which suffers from both shorter carrier lifetimes and a larger device capacitance

Reduced Recombination Losses in Evaporated Perovskite Solar Cells by Postfabrication Treatment

The photovoltaic perovskite research community has now developed a large set of tools and techniques to improve the power conversion efficiency (PCE). One such arcane trick is to allow the finished devices to dwell in time, and the PCE often improves. Herein, a mild postannealing procedure is implemented on coevaporated perovskite solar cells confirming a substantial PCE improvement, mainly attributed to an increased open-circuit voltage (V OC). From a V OC of around 1.11 V directly after preparation, the voltage improves to more than 1.18 V by temporal and thermal annealing. To clarify the origin of this annealing effect, an in-depth device experimental and simulation characterization is conducted. A simultaneous reduction of the dark saturation current, the ideality factor (n id), and the leakage current is revealed, signifying a substantial impact of the postannealing procedure on recombination losses. To investigate the carrier dynamics in more detail, a set of transient optoelectrical methods is first evaluated, ascertaining that the bulk carrier lifetime is increased with device annealing. Second, a drift-diffusion simulation is used, confirming that the beneficial effect of the annealing has its origin in effective bulk trap passivation that accordingly leads to a reduction of Shockley-Read-Hall recombination rates

Особенности работы с иностранными студентами с английским языком обучения на кафедре медицинской и биологической физики

ПРЕПОДАВАТЕЛЬСКИЙ СОСТАВ УЧЕБНОГО УЧРЕЖДЕНИЯОБРАЗОВАНИЕФИЗИКА /ОБУЧИНОСТРАННЫЕ СТУДЕНТЫАНГЛИЙСКИЙ ЯЗЫК /ОБУЧМЕДИЦИНСКАЯ ФИЗИКА /ОБУЧБИОЛОГИЧЕСКАЯ ФИЗИКА /ОБУ

Transparent, Flexible, and Passive Thermal Touch Panel

This work presents a touch panel concept, which is enabled by a novel designof thin film thermocouples. The design offers a simple implementation byutilizing a single thin film to function as an array of thermocouples. Theconcept is demonstrated as a flexible, passive, and highly transparent touchpanel. The passive nature of the thermoelectric touch recognition allows theperformance of the presented sensor to be optimal at moderate sheet resistancevalues of the transparent conductive layers. Hence, the concept is highlypotential for low-cost large-area applications and does not rely on costlylow sheet resistance materials such as indium tin oxide. The demonstratorpresented in this work achieves a signal-to-noise ratio of 20 with a rise timeof 90 ms and is able to distinguish individual touches, sweeping with finger,as well as touching by multiple fingers at the same time. In addition, the conceptmay also be used in other thermal distribution mapping applications.Not peer reviewe