Experimentally Calibrated Kinetic Monte Carlo Model Reproduces Organic Solar Cell Current-Voltage Curve

Kinetic Monte Carlo (KMC) simulations are a powerful tool to study the dynamics of charge carriers in organic photovoltaics. However, the key characteristic of any photovoltaic device, its current-voltage ($J$-$V$) curve under solar illumination, has proven challenging to simulate using KMC. The main challenges arise from the presence of injecting contacts and the importance of charge recombination when the internal electric field is low, i.e., close to open-circuit conditions. In this work, an experimentally calibrated KMC model is presented that can fully predict the $J$-$V$ curve of a disordered organic solar cell. It is shown that it is crucial to make experimentally justified assumptions on the injection barriers, the blend morphology, and the kinetics of the charge transfer state involved in geminate and nongeminate recombination. All of these properties are independently calibrated using charge extraction, electron microscopy, and transient absorption measurements, respectively. Clear evidence is provided that the conclusions drawn from microscopic and transient KMC modeling are indeed relevant for real operating organic solar cell devices.Comment: final version; license update

How to Reduce Charge Recombination in Organic Solar Cells: There Are Still Lessons to Learn from P3HT:PCBM

Suppressing charge recombination is key for organic solar cells to become commercial reality. However, there is still no conclusive picture of how recombination losses are influenced by the complex nanoscale morphology. Here, new insight is provided by revisiting the P3HT:PCBM blend, which is still one of the best performers regarding reduced recombination. By changing small details in the annealing procedure, two model morphologies were prepared that vary in phase separation, molecular order and phase purity, as revealed by electron tomography and optical spectroscopy. Both systems behave very similarly with respect to charge generation and transport, but differ significantly in bimolecular recombination. Only the system containing P3HT aggregates of high crystalline quality and purity is found to achieve exceptionally low recombination rates. The high-quality aggregates support charge delocalization, which assists the re-dissociation of interfacial charge-transfer states formed upon the encounter of free carriers. For devices with the optimized morphology, an exceptional long hole diffusion length is found, which allows them to work as Shockley-type solar cells even in thick junctions of 300 nm. In contrast, the encounter rate and the size of the phase-separated domains appears to be less important.Comment: final version, journal reference and DOI adde

Visual assessment of biliary excretion of Gd-EOB-DTPA in patients with suspected diffuse liver disease – A biopsy-verified prospective study

AbstractObjectivesTo qualitatively evaluate late dynamic contrast phases, 10, 20 and 30min, after administration of Gd-EOB-DTPA with regard to biliary excretion in patients presenting with elevated liver enzymes without clinical signs of cirrhosis or hepatic decompensation and to compare the visual assessment of contrast agent excretion with histo-pathological fibrosis stage, contrast uptake parameters and blood tests.Methods29 patients were prospectively examined using 1.5T MRI. The visually assessed presence or absence of contrast agent for each of five anatomical regions in randomly reviewed time-series was summarized on a four grade scale for each patient. The scores, including a total visual score, were related to the histo-pathological findings, the quantitative contrast agent uptake parameters, expressed as KHep or LSC_N, and blood tests.ResultsNo relationship between the fibrosis grade or contrast uptake parameters could be established. A negative correlation between the visual assessment and alkaline phosphatase (ALP) was found. Comparing a sub-group of cholestatic patients with fibrosis score and Gd-EOB-DTPA dynamic parameters did not add any additional significant correlation.ConclusionsNo correlation between visually assessed biliary excretion of Gd-EOB-DTPA and histo-pathological or contrast uptake parameters was found. A negative correlation between the visual assessment and alkaline phosphatase (ALP) was found

Undersökning av dopning i organiska dioder och solceller

Organiska solceller har gjort stora framsteg med avseende på effektivitet och stabilitet under det senaste årtiondet. Oavsiktlig dopning är ett ofta förekommande fenomen i organiska halvledare. Orenheter och defekter i halvledarmaterialet kan ge upphov till dopning. Dessutom kan organiska halvledare dopas över tid då de kommer i kontakt med syre eller fukt. I takt med att andra egenskaper optimeras och effektiviteten stiger blir dopningen en allt viktigare förlustprocess att ta i beaktande i organiska solceller. Målet med detta arbete har varit att bättre förstå dopningens inverkan på organiska dioder och solceller. Tanken var att med hjälp av två olika dopningsmolekyler åstadkomma avsiktlig dopning och på så sätt kontrollerat undersöka och skapa en bättre förståelse av hur dopningen påverkar egenskaperna hos organiska halvledare i dioder och solceller. För att undersöka dopningen har en mätmetod som baserar sig på laddningsextraktion med hjälp av en linjärt ökande spänning, kallad CELIV, använts. Teorin för mätmetoden har utvecklats för att analysera godtyckliga dopningsprofiler och för att ta i beaktande dopningsprofilen vid bestämning av mobiliteten. Metoden har, genom de experiment som utförts, bekräftats fungera väl för undersökning av dopning i organiska tunnfilmsdioder. De två dopningsmolekyler som använts har testats framgångsrikt för dopning av polymererna P3HT och PBTTT. Under arbetets gång kunde en viktig orsak till oavsiktlig dopning identifieras. Som selektivt håltransportlager vid anoden är molybdentrioxid ett av de mest använda materialen. I detta arbete visas att molekyler från ett tunt lager av molybdentrioxid diffunderar in i halvledarlagret och orsakar dopning. Dopningen i P3HT:PCBM till följd av ett molybdentrioxidlager är så hög att även tunna solceller, kring 100 nm, kommer att påverkas negativt på grund av ökad rekombination. Dopningen till följd av diffusion av molybdentrioxid är ett resultat som visar på ett behov att hitta nya alternativa material för håltransport. Ett alternativ kunde vara att använda kraftigt dopade organiska halvledare. För detta ändamål kan den avsiktliga dopning som här testats vara relevant. De experimentella resultat som ingår i denna avhandling bekräftar att CELIV-metoden lämpar sig väl för att mäta dopningskoncentration och dopningsprofiler i organiska dioder samtidigt som man kan erhålla information om laddningstransporten i halvledarlagret

Mental Workload in Aircraft and Simulator During Basic Civil Aviation Training

This study investigated mental workload in basic civil aviation training. Heart rate, eye movement, and subjective ratings from 11 students were collected during simulator and aircraft sessions. Results show high correspondence in psychophysiological reactions between the sessions. For some flight segments, heart rate was consistently lower in the simulator, suggesting higher mental workload in the aircraft. Differences in heart rate during rejected takeoff and engine failure indicate that the increase of workload starts in advance of an "unexpected" event in the simulator where it seems to be of preparatory nature, whereas in the aircraft it is more connected to management of the situation

A comparison of two recorders for obtaining in-flight heart rate data

Measurement of mental workload has been widely used for evaluation of aircraft design, mission analysis and assessment of pilot performance during flight operations. Heart rate is the psychophysiological measure that has been most frequently used for this purpose. The risk of interference with flight safety and pilot performance, as well as the generally constrained access to flights, make it difficult for researchers to collect in-flight heart rate data. Thus, this study was carried out to investigate whether small, non-intrusive sports recorders can be used for in-flight data collection for research purposes. Data was collected from real and simulated flights with student pilots using the Polar Team System sports recorder and the Vitaport II, a clinical and research recording device. Comparison of the data shows that in-flight heart rate data from the smaller and less intrusive sports recorder have a correlation of.981 with that from the clinical recorder, thus indicating that the sports recorder is reliable and cost-effective for obtaining heart rate data for many research situations

Recording of Psychophysiological Data During Aerobatic Training

Measuring pilot mental workload can be important for understanding cognitive demands during flight involving unusual movements and attitudes. Data on heart rate, eye movements, EEG, and subjective ratings from 7 flight instructors were collected for a flight including a repeated aerobatics sequence. Heart rate data and subjective ratings showed that aerobatic sequences produced the highest levels of mental workload and that heart rate can identify low-G flight segments with high mental workload. Blink rate and eye movement data did not support previous research regarding their relation to mental workload. EEG data were difficult to analyze due to muscle artifacts

Is "football for all" safe for all? : Cross-Sectional Study of Disparities as Determinants of 1-Year Injury Prevalence in Youth Football Programs

Football (soccer) is endorsed as a health-promoting physical activity worldwide. When football programs are introduced as part of general health promotion programs, equal access and limitation of pre-participation disparities with regard to injury risk are important. The aim of this study was to explore if disparity with regard to parents' educational level, player body mass index (BMI), and self-reported health are determinants of football injury in community-based football programs, separately or in interaction with age or gender. Methodology/Principal Findings Four community football clubs with 1230 youth players agreed to participate in the cross-sectional study during the 2006 season. The study constructs (parents' educational level, player BMI, and self-reported health) were operationalized into questionnaire items. The 1-year prevalence of football injury was defined as the primary outcome measure. Data were collected via a postal survey and analyzed using a series of hierarchical statistical computations investigating associations with the primary outcome measure and interactions between the study variables. The survey was returned by 827 (67.2%) youth players. The 1-year injury prevalence increased with age. For youths with parents with higher formal education, boys reported more injuries and girls reported fewer injuries than expected; for youths with lower educated parents there was a tendency towards the opposite pattern. Youths reporting injuries had higher standardized BMI compared with youths not reporting injuries. Children not reporting full health were slightly overrepresented among those reporting injuries and underrepresented for those reporting no injury. Conclusion Pre-participation disparities in terms of parents' educational level, through interaction with gender, BMI, and self-reported general health are associated with increased injury risk in community-based youth football. When introduced as a general health promotion, football associations should adjust community-based youth programs to accommodate children and adolescents with increased pre-participation injury risk

Impact of a Doping-Induced Space-Charge Region on the Collection of Photogenerated Charge Carriers in Thin-Film Solar Cells Based on Low-Mobility Semiconductors

Unintentional doping of the active layer is a source for lowered device performance in organic solar cells. The effect of doping is to induce a space-charge region within the active layer, generally resulting in increased recombination losses. In this work, the impact of a doping-induced space-charge region on the current-voltage characteristics of low-mobility solar cell devices has been clarified by means of analytical derivations and numerical device simulations. It is found that, in case of a doped active layer, the collection efficiency of photo-generated charge carriers is independent of the light intensity and exhibits a distinct voltage dependence, resulting in an apparent electric-field dependence of the photocurrent. Furthermore, an analytical expression describing the behavior of the photocurrent is derived. The validity of the analytical model is verified by numerical drift-diffusion simulations and demonstrated experimentally on solution-processed organic solar cells. Based on the theoretical results, conditions of how to overcome charge collection losses caused by doping are discussed. Furthermore, the presented analytical framework provides tools to distinguish between different mechanisms leading to voltage dependent photocurrents.Comment: Main text (29 pages, 7 figures) and supplemental material (7 pages, 4 figures