Direct and charge transfer mediated photogeneration in polymer-fullerene bulk heterojunction solar cells

We investigated photogeneration yield and recombination dynamics in blends of poly(3-hexyl thiophene) (P3HT) and poly[2-methoxy-5-(30,70-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) with [6,6]- phenyl-C61 butyric acid methyl ester (PC61BM) by means of temperature dependent time delayed collection field (TDCF) measurements. In MDMO-PPV:PC61BM we find a strongly field dependent polaron pair dissociation which can be attributed to geminate recombination in the device. Our findings are in good agreement with field dependent photoluminescence measurements published before, supporting a scenario of polaron pair dissociation via an intermediate charge transfer (CT) state. In contrast, polaron pair dissociation in P3HT:PC61BM shows only a very weak field dependence, indicating an almost field independent polaron pair dissociation or a direct photogeneration. Furthermore, we found Langevin recombination for MDMO-PPV:PC61BM and strongly reduced Langevin recombination for P3HT:PC61BM.Comment: 4 pages, 3 figure

Ätiologie und Behandlung externalisierender Störungen im Kindesalter

Externalisierende Störungen wie Aufmerksamkeitsdefizit/Hyperaktivitätsstörungen (ADHS), Störungen des Sozialverhaltens mit oppositionellem, aufsässigen Verhalten (ODD) und Störungen des Sozialverhaltens (CD) beginnen im frühen Kindesalter. Zur Beschreibung der kindlichen externalisierenden Psychopathologie wird zunehmend eine dimensionale Betrachtung herangezogen. Ätiologisch werden externalisierende Störungen als heterogen betrachtet, und Assoziationen mit Defiziten in verschiedenen psychologischen Funktionen angenommen. Prävalenzraten um 5.4 % verdeutlichen die weite Verbreitung externalisierender Störungen. In klinischer Ausprägung gehen externalisierende Störungen mit erheblichen Beeinträchtigungen für die betroffenen Kinder und ihre Familien einher. Frühzeitige Interventionen sind wesentlich, um den Verlauf positiv zu beeinflussen. Der vorliegende Kumulus befasst sich sowohl mit der Ätiologie externalisierender Störungen im Kindesalter als auch mit der Wirksamkeit frühzeitiger elternzentrierter Interventionen. In Manuskript #1 wurde die Spezifität neuropsychologischer Defizite für ADHS- und ODD-Symptome im Vorschulalter untersucht. Es zeigten sich spezifisch mit ADHS-Symptomen assoziierte Defizite in den Bereichen Verzögerungsaversion und Intelligenz. Diese Zusammenhänge zeigten sich bei Berücksichtigung von Kontrollvariablen und unabhängig von ODD-Symptomen. Exekutive Inhibitionskontrolle und Sensomotorik zeigten keine spezifischen Assoziationen mit ADHS- oder ODD-Symptomen. Für die ODD-Symptome ergab sich kein über die ADHS-Symptome hinausgehender Zusammenhang mit neuropsychologischen Defiziten. Die Ergebnisse werden in den aktuellen Forschungsrahmen der Ätiologie externalisierender Störungen eingeordnet. Spezifische Assoziationen zwischen neuropsychologischen Defiziten einerseits und ADHS- und ODD-Symptomen andererseits scheinen vorzuliegen. Die Identifikation von Subgruppen mit spezifischen Basisdefiziten wird als Ziel zukünftiger Forschung dargestellt. Die Manuskripte #2, #3 und #4 fokussieren die Wirksamkeit elternzentrierter Interventionen zur Behandlung externalisierender Störungen im Kindesalter. Manuskript #2 fasst in einem systematischen Review die Wirksamkeit elternzentrierter Interventionen zur Behandlung psychischer Störungen im Kindesalter zusammen. Hierbei wurden sowohl Belege für die Wirksamkeit elternzentrierter Interventionen zur Behandlung internalisierender als auch externalisierender Störungen gefunden. Die Befundlage zu externalisierenden Störungen erwies sich dabei als deutlich breiter und liefert metaanalytische Ergebnisse zu Auswirkungen auf das kindliche Verhalten und elterliche Charakteristika. Diese umfassendere Befundlage ist allerdings deutlich heterogen. Um eine genauere Aussage über die Höhe der Effekte zu ermöglichen, wurden deshalb in den Manuskripten #3 und #4 die Ergebnisse mittels Meta-Metaanalysen zusammengefasst. Hierbei ergaben sich moderate und stabile Effekte elternzentrierter Interventionen sowohl auf das kindliche Verhalten allgemein als auch spezifisch auf das externalisierende Verhalten. Diese positiven Ergebnisse zeigten sich sowohl im Elternurteil als auch in der Verhaltensbeobachtung, was die Zuverlässigkeit der Ergebnisse untermauert. Hinsichtlich elterlicher Charakteristika zeigten sich in den Analysen moderate und stabile Effekte für das Erziehungsverhalten und die elterliche Wahrnehmung. In den Daten zur Verhaltensbeobachtung bestätigten sich die positiven Effekte auf das Erziehungsverhalten aber nicht. Ein kleiner und stabiler Effekt konnte zudem für die elterliche Beziehungsqualität gefunden werden. Ein ebenfalls kleiner Effekt auf die elterliche psychische Gesundheit zeigte sich im Follow-Up hingegen nicht mehr. In der Diskussion werden die Heterogenität der Ergebnisse und weitere Forschungsansätze erörtert. Insgesamt lässt sich ein positiver Effekt frühzeitiger elternzentrierter Interventionen sowohl auf das kindliche Verhalten als auch elterliche Charakteristika feststellen. Elternzentrierte Interventionen sollten daher als evidenzbasierte Interventionen in der Praxis angewendet werden

Impact of nongeminate recombination on the performance of pristine and annealed P3HT:PCBM solar cells

Transient photovoltage (TPV) and voltage dependent charge extraction (CE) measurements were applied to poly(3-hexylthiophene)(P3HT):[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) bulk heterojunction solar cells to analyze the limitations of solar cell performance in pristine and annealed devices. From the determined charge carrier decay rate under open circuit conditions and the voltage dependent charge carrier densities n(V) the nongeminate loss current jloss of the device is accessible. We found that jloss alone is sufficient to describe the j-V characteristics across the whole operational range, for annealed and, not yet shown before, also for the lower performing pristine solar cells. Even in a temperature range from 300 K to 200 K nongeminate recombination is found to be the dominant and, therefore, performance limiting loss process. Consequently, charge photogeneration is voltage independent in the voltage range studied.Comment: 3 pages, 3 figures; Rapid Research Letter 201

Multiscale Modeling and Simulation of Organic Solar Cells

In this article, we continue our mathematical study of organic solar cells (OSCs) and propose a two-scale (micro- and macro-scale) model of heterojunction OSCs with interface geometries characterized by an arbitrarily complex morphology. The microscale model consists of a system of partial and ordinary differential equations in an heterogeneous domain, that provides a full description of excitation/transport phenomena occurring in the bulk regions and dissociation/recombination processes occurring in a thin material slab across the interface. The macroscale model is obtained by a micro-to-macro scale transition that consists of averaging the mass balance equations in the normal direction across the interface thickness, giving rise to nonlinear transmission conditions that are parametrized by the interfacial width. These conditions account in a lumped manner for the volumetric dissociation/recombination phenomena occurring in the thin slab and depend locally on the electric field magnitude and orientation. Using the macroscale model in two spatial dimensions, device structures with complex interface morphologies, for which existing data are available, are numerically investigated showing that, if the electric field orientation relative to the interface is taken into due account, the device performance is determined not only by the total interface length but also by its shape

Bilayer Interdiffused Heterojunction Organic Photodiodes Fabricated by Double Transfer Stamping

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136265/1/adom201600784-sup-0001-S1.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136265/2/adom201600784_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136265/3/adom201600784.pd

P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance

Each year we are bombarded with B.Sc. and Ph.D. applications from students that want to improve the world. They have learned that their future depends on changing the type of fuel we use and that solar energy is our future. The hope and energy of these young people will transform future energy technologies, but it will not happen quickly. Organic photovoltaic devices are easy to sketch, but the materials, processing steps, and ways of measuring the properties of the materials are very complicated. It is not trivial to make a systematic measurement that will change the way other research groups think or practice. In approaching this chapter, we thought about what a new researcher would need to know about organic photovoltaic devices and materials in order to have a good start in the subject. Then, we simplified that to focus on what a new researcher would need to know about poly-3-hexylthiophene:phenyl-C61-butyric acid methyl ester blends (P3HT: PCBM) to make research progress with these materials. This chapter is by no means authoritative or a compendium of all things on P3HT:PCBM. We have selected to explain how the sample fabrication techniques lead to control of morphology and structural features and how these morphological features have specific optical and electronic consequences for organic photovoltaic device applications

Photocurrent in Organic Solar Cells

A quite new approach to low-cost mass production of flexible solar cells are organic photovoltaics. Even though the device efficiencies increased rapidly during the last years, further imporvements are essential for a successful market launch. One important factor influencing the device efficiency is the photocurrent of a solar cell, which is defined as the difference between the current under illumination and in the dark. In case of organic bulk heterojunction (BHJ) solar cells it is — in contrast to inorganic devices — dependent on the applied bias voltage. The voltage dependence results in a reduced fill factor and thus an even more pronounced influence of the photocurrent on the device efficiency. It is therefore crucial to understand the underlying processes determining the photocurrent in order to be able to further improve the solar cell performance. In a first step the photocurrent of P3HT:PC61BM devices was investigated by a pulsed measurement technique in order to prevent disturbing influences due to device heating under continous illumination. The resulting photocurrent was hyperbolic tangent like and featured a point symmetry, whose origin and meaning were discussed. In addition, the photocurrent was described by a combined model of Braun–Onsager and Sokel–Hughes theory for field dependent polaron pair dissociation and charge extraction, respectively. After this macroscopic view on the photocurrent, the focus of this work moves to the more basic processes determining the photocurrent: charge photogeneration and recombination. In a comparative study the field-dependence of these was investigated by time-delayed collection field (TDCF) measurements for two well-known reference systems, namely P3HT:PC61BM and MDMO-PPV:PC61BM. It was possible to identify two different dominating scenarios for the generation of free charge carriers. The first one — via a thermalized charge transfer state (CTS) — is clearly influenced by geminate recombination and therefore less efficient. In the second scenario, the free charge carriers are either generated directly or via an excited, “hot” CTS. In addition, clear differences in the nongeminate recombination dynamics of both material systems were found. Similar studies were also be presented with two modern low bandgap polymers which only differ by the bridging atom in the cyclopentadithiophene (PCPDTBT:PC71BM vs. Si-PCPDTBT:PC71BM). Such small changes in the chemical structure were already sufficient to affect the charge photogeneration as well as the morphology of the blend. These findings were set into relation to current–voltage characteristics in order to discuss the origin of the clear differences in the solar cell performance of both materials. Another crucial parameter limiting the solar cell efficiency is the builtin potential of a device. Within the range of semiconducting pn-junctions, Mott–Schottky analysis is an established method to determine the built-in potential. As it was originally derived for abrupt pn-junctions, its validity for organic BHJ solar cells — a bipolar, effective medium — was discussed. Experimental findings as well as the contradictions to Mott–Schottky theory indicated, that a direct transfer of this method to organic photovoltaics is not appropriate. Finally, the results obtained in the framework of the MOPS-project (Massengedruckte Organische Papier-Solarzellen) will be presented, in which the first completely roll-to-roll printed paper solar cells were realized