Influence of doping on charge carrier collection in normal and inverted geometry polymer: fullerene solar cells

While organic semiconductors used in polymer:fullerene photovoltaics are generally not intentionally doped, significant levels of unintentional doping have previously been reported in the literature. Here, we explain the differences in photocurrent collection between standard (transparent anode) and inverted (transparent cathode) low band-gap polymer:fullerene solar cells in terms of unintentional p-type doping. Using capacitance/voltage measurements, we find that the devices exhibit doping levels of order 1016 cm−3, resulting in space-charge regions ~100 nm thick at short circuit. As a result, low field regions form in devices thicker than 100 nm. Because more of the light is absorbed in the low field region in standard than in inverted architectures, the losses due to inefficient charge collection are greater in standard architectures. Using optical modelling, we show that the observed trends in photocurrent with device architecture and thickness can be explained if only charge carriers photogenerated in the depletion region contribute to the photocurrent

A Simple and Robust Approach to Reducing Contact Resistance in Organic Transistors

Efficient injection of charge carriers from the contacts into the semiconductor layer is crucial for achieving high-performance organic devices. The potential drop necessary to accomplish this process yields a resistance associated with the contacts, namely the contact resistance. A large contact resistance can limit the operation of devices and even lead to inaccuracies in the extraction of the device parameters. Here, we demonstrate a simple and efficient strategy for reducing the contact resistance in organic thin-film transistors by more than an order of magnitude by creating high work function domains at the surface of the injecting electrodes to promote channels of enhanced injection. We find that the method is effective for both organic small molecule and polymer semiconductors, where we achieved a contact resistance as low as 200 Ωcm and device charge carrier mobilities as high as 20 cm2V−1s−1, independent of the applied gate voltage

Spektral Ellipsometrie Studien zur Untersuchung der Morphologie von Polymer/Fullerene Solarzellen

Die Untersuchung organischer Solarzellen stellt ein vielversprechendes interdisziplinäres Forschungsgebiet dar, versprechen sie doch eine günstige Alternative zu anorganischen Solarzellen. Ihr geringes Gewicht und hohe Flexibilität ermöglichen es der organischen Photovoltaik (OPV) in neue Bereiche, wie der Integration in architektonischen Design-Elementen und intelligenter Kleidung, Verwendung zu finden. Dennoch fehlt es noch an kommerzielle Anwendungen, da die OPV fundamentale Probleme überwinden muss. Zu nennen sei hier die geringe Stabilität der organischen Materialien gegenüber Wasser und Sauerstoff bei gleichzeitiger Beleuchtung, als auch morphologische Degradation. Die photoaktive Schicht ist typischerweise eine selbstorganisierte Mischung aus zwei oder mehreren organischen Verbindungen und erfordert daher sehr anspruchsvolle Strukturmanipulation, da die Mischungsmorphologie in äußerstem Maße die Solarzellleistung bestimmt. Es ist von inhärenter Bedeutung die Morphologie aktiv zu steuern, um organische Solarzellen mit höchstmöglicher Effizienz zu produzieren. Das Verständnis zwischen morphologischer Struktur und Solarzelleneigenschaften ist hierzu unentbehrlich. Diese Arbeit hatte zum Ziel, die morphologische Struktur von Polymer/Fulleren-Mischschichten durch ein optisches und daher zerstörungsfreies Verfahren zu untersuchen: winkelvariierende spektroskopische Ellipsometrie (Variable Angle Spectroscopic Ellipsometry (VASE)). Obwohl die dargestellten neu entwickelten Messroutinen und optischen Modelle für beliebige Systeme Anwendung finden können, wurde als zu untersuchendes Materialsystem auf Mischungen des weitverbreiteten Polymers Poly(3-hexylthiophen-2,5-diyl) (P3HT) und dem Fullerenderivat Phenyl-C$61$-Buttersäure-methylester (PCBM) zurückgegriffen. Dies ist dem Gedanken geschuldet die gewonnen Ergebnisse der optischen Modellierung, entsprechend einer indirekten Messung, mit möglichst vielen bereits etablierten Messverfahren zur Morphologieuntersuchung zu vergleichen. Zudem zeigen Polythiophen/Fulleren-Mischungen viele physikalisch höchst interessante Eigenschaften, wie die Kristallisation einer oder beider Komponenten, und die Entmischung der photoaktiven Schicht. %Aufgrund dieser Effekte und der zu erreichenden hohen Solarzelleffizienzen, stellt P3HT/PCBM das Standardmaterialsystem der OPV Forschergemeinschaft dar. In dieser Arbeit wurde die Kristallisation und räumliche Ordnung der Polymerkomponente in P3HT/PCBM-Mischschichten für verschiedene Fulleren-Konzentrationen innerhalb der photoaktiven Schicht untersucht. Es konnte gezeigt werden, dass die Fulleren-Phase stark die Anordnung der Polymer-Komponente beeinflusst. In diesem Zusammenhang wurde ein neues optisches Modell entwickelt, welches die quantitative Unterscheidung zwischen höher und weniger geordneten Polymer-Domänen ermöglicht. Mit diesem Modell ist es erstmals gelungen zu zeigen, dass spinodale Entmischung und das resultierende Fulleren-Konzentrationsprofil über der Schichtdicke die Keimbildung und das Wachstum von geordneten Polymer-Domänen, sowie den Volumenanteil von geordneten Polymerdomänen innerhalb des Filmprofils maßgeblich kontrollieren. Ein Highlight dieser Arbeit ist die Tiefenprofilierung der Fullerene- Konzentration und der Verteilung geordeneter Polymer-Domänen in vollständigen Solarzellstrukturen, im Gegensatz zu der allgemein üblichen Praxis die Morphologie in separat präparierten Schichten auf Silizium oder Glas-Substraten zu untersuchen. Dies ist speziell zur Untersuchung der Grenzflächeninteraktion des Polymer/Fulleren-Gemisches mit der Elektrode während eventueller Temperschritte höchst interessant. Durch die gezeigte zerstörungsfreie Tiefenprofilierung vollständiger Solarzellen wird erstmals die Möglichkeit eröffnet in situ Studien über die Langzeitstabilität der Morphologie und der Elektrodengrenzflächen durchzuführen, da die Möglichkeit besteht dasselbe Bauelement sowohl elektrisch als auch optisch zu Untersuchen. Hierdurch können kleinste morphologische Änderungen direkt mit der Solarzellenleistung korreliert werden.Organic photovoltaics (OPV) are a blooming new research field, as they promise a cheap alternative to inorganic solar cells. Their light weight and flexibility enables them to be used in emerging sectors such as smart clothing and architectural design elements. Nevertheless, commercial applications are missing as OPV still experiences a few fundamental problems. To mention here are the poor stability against water and oxygen when exposed to light, as well as morphological degradation over time. The photoactive layer is typically a self-organized blend of two or more organic compounds which requires highly challenging device engineering as the blend morphology strongly affects the device performance. In order to produce organic solar cells with maximum efficiency, it is of crucial importance to monitor and modify the morphology. Therefore, understanding the correlation between morphological structure and device properties is indispensable. The aim of this work was to investigate the morphological structure of polymer/ fullerene blend films by an optical and non-destructive method, the Variable Angle Spectroscopic Ellipsometry (VASE). Although the measurement routines and optical models developed in my study are adaptable to other OPV systems, the system under investigation consisted of blends of the widely used polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) and the fullerene derivative phenyl-C61-butyric acid methyl ester (PCBM). Polythiophene/fullerene blends show many physically interesting properties, such as the crystallization of one or both of the components and the blend demixing upon thermal annealing. Because of the excellent thermal and chemical stability as well as good light-harvesting and charge-transporting properties, P3HT/PCBM is the standard material system of the OPV research community. Thus all of the indirect results that I have obtained by optical modeling, could be compared with a variety of other more direct measurement techniques existing in the literature. In the first part of the study, the crystallization and spatial order of the polymer component within P3HT/PCBM blends was investigated for various fullerene concentrations within the photoactive blend. It could be shown that the fullerene phase strongly affects the ordering of the polymer component. In this context, a new optical model was developed which enables quantitative distinction between higher ordered and lower ordered polymer domains. This model strongly suggests that spinodal decomposition and the resulting fullerene distribution over the film thickness drive the polymer nucleation and determine the volume fraction of ordered polymer domains over the film thickness. In the final part of the study, the depth profiling of the fullerene concentration and the distribution of highly ordered polymer domains within complete solar cell devices, was investigated. This opens the door for the in situ investigation of solar cells, which is of high interest in order to enable investigations of the interface interactions between the polymer/fullerene blends and the electrode during additional annealing steps, and its influence on the morphology. The shown ability to measure complete solar cells in a non-destructive manner, moreover opens the possibility to correlate the long-term stability studies of the blend morphology and the electrode interfaces during operation to the electric device performance

Long-Term Stabilization of Organic Solar Cells Using Hindered Phenols as Additives

We report on the improvement of long-term stability of organic solar cells (OPV) using hindered phenol based antioxidants as stabilizing additives. A set of seven commercially available hindered phenols are investigated for use in bulk-heterojunction OPV. Polymer:fullerene films based on poly­(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are characterized with respect to the initial power conversion efficiency and the long-term stability improvement under illumination in ambient conditions. FTIR spectroscopy is used to trace chemical degradation over time. OPV performance is recorded under ISOS-3 conditions, and an improved long-term performance of OPV devices, manifested in increased accumulated power generation (APG), is found for octadecyl 3-(3,5-di-<i>tert</i>-butyl-4-hydroxyphenyl)­propionate. Using this additive, APG is increased by a factor of 3 compared to the reference. Observed differences in the stabilization of tested additives are discussed in terms of energetic trap states formation within the HOMO/LUMO gap of the photoactive material, morphological changes, and chemical structure

A 19-year-old patient with atypical chronic myeloid leukemia

The International Paroxysmal Nocturnal Hemoglobinuria (PNH) Registry (NCT01374360) was initiated to optimize patient management by collecting data regarding disease burden, progression, and clinical outcomes. Herein, we report updated baseline demographics, clinical characteristics, disease burden data, and observed trends regarding clone size in the largest cohort of Registry patients. Patients with available data as of July 2017 were stratified by glycosylphosphatidylinositol (GPI)-deficient granulocyte clone size (&amp;lt; 10%, ≥ 10%-&amp;lt; 50%, and ≥ 50%). All patients were untreated with eculizumab at baseline, defined as date of eculizumab initiation or date of Registry enrollment (if never treated with eculizumab). Outcomes assessed in the current analysis included proportions of patients with high disease activity (HDA), history of major adverse vascular events (MAVEs; including thrombotic events [TEs]), bone marrow failure (BMF), red blood cell (RBC) transfusions, and PNH-related symptoms. A total of 4439 patients were included, of whom 2701 (60.8%) had available GPI-deficient granulocyte clone size data. Among these, median clone size was 31.8% (1002 had &amp;lt; 10%; 526 had ≥ 10%-&amp;lt; 50%; 1173 had ≥ 50%). There were high proportions of patients with HDA (51.6%), history of MAVEs (18.8%), BMF (62.6%), RBC transfusion (61.3%), and impaired renal function (42.8%). All measures except RBC transfusion history significantly correlated with GPI-deficient granulocyte clone size. A large proportion of patients with GPI-deficient granulocyte clone size &amp;lt; 10% had hemolysis (9.7%), MAVEs (10.2%), HDA (9.1%), and/or PNH-related symptoms. Although larger GPI-deficient granulocyte clone sizes were associated with higher disease burden, a substantial proportion of patients with smaller clone sizes had history of MAVEs/TEs

Morphology changes upon scaling a high-efficiency, solution-processed solar cell

Solution processing via roll-to-roll (R2R) coating promises a low cost, low thermal budget, sustainable revolution for the production of solar cells. Poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3′′′-di(2-octyldodecyl)-2,2′;5′,2′′;5′′,2′′′-quaterthiophen-5,5-diyl)], PffBT4T-2OD, has recently been shown to achieve high power conversion efficiency (&gt;10%) paired with multiple acceptors when thick films are spun-coat from hot solutions. We present detailed morphology studies of PffBT4T-2OD based bulk heterojunction films deposited by the volume manufacturing compatible techniques of blade-coating and slot-die coating. Significant aspects of the film morphology, the average crystal domain orientation and the distribution of the characteristic phase separation length scales, are remarkably different when deposited by the scalable techniques vs. spun-coat. Yet, we find that optimized blade-coated devices achieve PCE &gt; 9.5%, nearly the same as spun-coat. These results challenge some widely accepted propositions regarding what is an optimal BHJ morphology and suggest the hypothesis that diversity in the morphology that supports high performance may be a characteristic of manufacturable systems, those that maintain performance when coated thicker than ≈200 nm. In situ measurements reveal the key differences in the solidification routes for spin- and blade-coating leading to the distinct film structures.</p