TY - JOUR A1 - Hosseini, Seyed Mehrdad A1 - Roland, Steffen A1 - Kurpiers, Jona A1 - Chen, Zhiming A1 - Zhang, Kai A1 - Huang, Fei A1 - Armin, Ardalan A1 - Neher, Dieter A1 - Shoaee, Safa T1 - Impact of Bimolecular Recombination on the Fill Factor of Fullerene and Nonfullerene-Based Solar Cells BT - A Comparative Study of Charge Generation and Extraction JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - Power conversion efficiencies of donor/acceptor organic solar cells utilizing nonfullerene acceptors have now increased beyond the record of their fullerene-based counterparts. There remain many fundamental questions regarding nanomorphology, interfacial states, charge generation and extraction, and losses in these systems. Herein, we present a comparative study of bulk heterojunction solar cells composed of a recently introduced naphthothiadiazole-based polymer (NT812) as the electron donor and two different acceptor molecules, namely, [6,6]-phenyl-C71-butyric acid methyl ester (PCBM)[70] and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC). A comparison between the photovoltaic performance of these two types of solar cells reveals that the open-circuit voltage (Voc) of the NT812:ITIC-based solar cell is larger, but the fill factor (FF) is lower than that of the NT812:PCBM[70] device. We find the key reason behind this reduced FF in the ITIC-based device to be faster nongeminate recombination relative to the NT812:PCBM[70] system. Y1 - 2019 U6 - https://doi.org/10.1021/acs.jpcc.8b11669 SN - 1932-7447 VL - 123 IS - 11 SP - 6823 EP - 6830 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Würfel, Uli A1 - Perdigon-Toro, Lorena A1 - Kurpiers, Jona A1 - Wolff, Christian Michael A1 - Caprioglio, Pietro A1 - Rech, Jeromy James A1 - Zhu, Jingshuai A1 - Zhan, Xiaowei A1 - You, Wei A1 - Shoaee, Safa A1 - Neher, Dieter A1 - Stolterfoht, Martin T1 - Recombination between Photogenerated and Electrode-Induced Charges Dominates the Fill Factor Losses in Optimized Organic Solar Cells JF - The journal of physical chemistry letters N2 - Charge extraction in organic solar cells (OSCs) is commonly believed to be limited by bimolecular recombination of photogenerated charges. However, the fill factor of OSCs is usually almost entirely governed by recombination processes that scale with the first order of the light intensity. This linear loss was often interpreted to be a consequence of geminate or trap-assisted recombination. Numerical simulations show that this linear dependence is a direct consequence of the large amount of excess dark charge near the contact. The first-order losses increase with decreasing mobility of minority carriers, and we discuss the impact of several material and device parameters on this loss mechanism. This work highlights that OSCs are especially vulnerable to injected charges as a result of their poor charge transport properties. This implies that dark charges need to be better accounted for when interpreting electro-optical measurements and charge collection based on simple figures of merit. Y1 - 2019 U6 - https://doi.org/10.1021/acs.jpclett.9b01175 SN - 1948-7185 VL - 10 IS - 12 SP - 3473 EP - 3480 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Shoaee, Safa A1 - Armin, Ardalan A1 - Stolterfoht, Martin A1 - Hosseini, Seyed Mehrdad A1 - Kurpiers, Jona A1 - Neher, Dieter T1 - Decoding Charge Recombination through Charge Generation in Organic Solar Cells JF - Solar RRL N2 - The in-depth understanding of charge carrier photogeneration and recombination mechanisms in organic solar cells is still an ongoing effort. In donor:acceptor (bulk) heterojunction organic solar cells, charge photogeneration and recombination are inter-related via the kinetics of charge transfer states-being singlet or triplet states. Although high-charge-photogeneration quantum yields are achieved in many donor:acceptor systems, only very few systems show significantly reduced bimolecular recombination relative to the rate of free carrier encounters, in low-mobility systems. This is a serious limitation for the industrialization of organic solar cells, in particular when aiming at thick active layers. Herein, a meta-analysis of the device performance of numerous bulk heterojunction organic solar cells is presented for which field-dependent photogeneration, charge carrier mobility, and fill factor are determined. Herein, a "spin-related factor" that is dependent on the ratio of back electron transfer of the triplet charge transfer (CT) states to the decay rate of the singlet CT states is introduced. It is shown that this factor links the recombination reduction factor to charge-generation efficiency. As a consequence, it is only in the systems with very efficient charge generation and very fast CT dissociation that free carrier recombination is strongly suppressed, regardless of the spin-related factor. KW - charge generation KW - charge transfers KW - non-Langevin recombination KW - spin-related factors Y1 - 2019 U6 - https://doi.org/10.1002/solr.201900184 SN - 2367-198X VL - 3 IS - 11 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Kniepert, Juliane A1 - Paulke, Andreas A1 - Perdigon-Toro, Lorena A1 - Kurpiers, Jona A1 - Zhang, Huotian A1 - Gao, Feng A1 - Yuan, Jun A1 - Zou, Yingping A1 - Le Corre, Vincent M. A1 - Koster, Lambert Jan Anton A1 - Neher, Dieter T1 - Reliability of charge carrier recombination data determined with charge extraction methods JF - Journal of applied physics N2 - Charge extraction methods are popular for measuring the charge carrier density in thin film organic solar cells and to draw conclusions about the order and coefficient of nongeminate charge recombination. However, results from such studies may be falsified by inhomogeneous steady state carrier profiles or surface recombination. Here, we present a detailed drift-diffusion study of two charge extraction methods, bias-assisted charge extraction (BACE) and time-delayed collection field (TDCF). Simulations are performed over a wide range of the relevant parameters. Our simulations reveal that both charge extraction methods provide reliable information about the recombination order and coefficient if the measurements are performed under appropriate conditions. However, results from BACE measurements may be easily affected by surface recombination, in particular for small active layer thicknesses and low illumination densities. TDCF, on the other hand, is more robust against surface recombination due to its transient nature but also because it allows for a homogeneous high carrier density to be inserted into the active layer. Therefore, TDCF is capable to provide meaningful information on the order and coefficient of recombination even if the model conditions are not exactly fulfilled. We demonstrate this for an only 100 nm thick layer of a highly efficient nonfullerene acceptor (NFA) blend, comprising the donor polymer PM6 and the NFA Y6. TDCF measurements were performed as a function of delay time for different laser fluences and bias conditions. The full set of data could be consistently fitted by a strict second order recombination process, with a bias- and fluence-independent bimolecular recombination coefficient k(2) = 1.7 x 10(-17)m(3) s(-1). BACE measurements performed on the very same layer yielded the identical result, despite the very different excitation conditions. This proves that recombination in this blend is mostly through processes in the bulk and that surface recombination is of minor importance despite the small active layer thickness. Published under license by AIP Publishing. Y1 - 2019 U6 - https://doi.org/10.1063/1.5129037 SN - 0021-8979 SN - 1089-7550 VL - 126 IS - 20 PB - American Institute of Physics CY - Melville ER - TY - GEN A1 - Shoaee, Safa A1 - Armin, Ardalan A1 - Stolterfoht, Martin A1 - Hosseini, Seyed Mehrdad A1 - Kurpiers, Jona A1 - Neher, Dieter T1 - Decoding charge recombination through charge generation in organic solar cells T2 - Postprints der Universität Potsdam Mathematisch-Naturwissenschaftliche Reihe N2 - The in‐depth understanding of charge carrier photogeneration and recombination mechanisms in organic solar cells is still an ongoing effort. In donor:acceptor (bulk) heterojunction organic solar cells, charge photogeneration and recombination are inter‐related via the kinetics of charge transfer states—being singlet or triplet states. Although high‐charge‐photogeneration quantum yields are achieved in many donor:acceptor systems, only very few systems show significantly reduced bimolecular recombination relative to the rate of free carrier encounters, in low‐mobility systems. This is a serious limitation for the industrialization of organic solar cells, in particular when aiming at thick active layers. Herein, a meta‐analysis of the device performance of numerous bulk heterojunction organic solar cells is presented for which field‐dependent photogeneration, charge carrier mobility, and fill factor are determined. Herein, a “spin‐related factor” that is dependent on the ratio of back electron transfer of the triplet charge transfer (CT) states to the decay rate of the singlet CT states is introduced. It is shown that this factor links the recombination reduction factor to charge‐generation efficiency. As a consequence, it is only in the systems with very efficient charge generation and very fast CT dissociation that free carrier recombination is strongly suppressed, regardless of the spin‐related factor. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 773 KW - charge generation KW - charge transfers KW - non-Langevin recombination KW - spin-related factors Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-437512 SN - 1866-8372 IS - 773 ER - TY - THES A1 - Kurpiers, Jona T1 - Probing the pathways of free charge generation and recombination in organic solar cells T1 - Pfade der Generierung und Rekombination freier Ladungsträger in organischen Solarzellen BT - the role of excess energy and dispersive effects BT - die Rolle von Überschussenergie und dispersiven Effekten N2 - Organic semiconductors are a promising class of materials. Their special properties are the particularly good absorption, low weight and easy processing into thin films. Therefore, intense research has been devoted to the realization of thin film organic solar cells (OPVs). Because of the low dielectric constant of organic semiconductors, primary excitations (excitons) are strongly bound and a type II heterojunction needs to be introduced to split these excitations into free charges. Therefore, most organic solar cells consist of at least an electron donor and electron acceptor material. For such donor acceptor systems mainly three states are relevant; the photoexcited exciton on the donor or acceptor material, the charge transfer state at the donor-acceptor interface and the charge separated state of a free electron and hole. The interplay between these states significantly determines the efficiency of organic solar cells. Due to the high absorption and the low charge carrier mobilities, the active layers are usually thin but also, exciton dissociation and free charge formation proceeds rapidely, which makes the study of carrier dynamics highly challenging. Therefore, the focus of this work was first to install new experimental setups for the investigation of the charge carrier dynamics in complete devices with superior sensitivity and time resolution and, second, to apply these methods to prototypical photovoltaic materials to address specific questions in the field of organic and hybrid photovoltaics. Regarding the first goal, a new setup combining transient absorption spectroscopy (TAS) and time delayed collection field (TDCF) was designed and installed in Potsdam. An important part of this work concerned the improvement of the electronic components with respect to time resolution and sensitivity. To this end, a highly sensitive amplifier for driving and detecting the device response in TDCF was developed. This system was then applied to selected organic and hybrid model systems with a particular focus on the understanding of the loss mechanisms that limit the fill factor and short circuit current of organic solar cells. The first model system was a hybrid photovoltaic material comprising inorganic quantum dots decorated with organic ligands. Measurements with TDCF revealed fast free carrier recombination, in part assisted by traps, while bias-assisted charge extraction measurements showed high mobility. The measured parameters then served as input for a successful description of the device performance with an analytical model. With a further improvement of the instrumentation, a second topic was the detailed analysis of non-geminate recombination in a disordered polymer:fullerene blend where an important question was the effect of disorder on the carrier dynamics. The measurements revealed that early time highly mobile charges undergo fast non-geminate recombination at the contacts, causing an apparent field dependence of free charge generation in TDCF experiments if not conducted properly. On the other hand, recombination the later time scale was determined by dispersive recombination in the bulk of the active layer, showing the characteristics of carrier dynamics in an exponential density of state distribution. Importantly, the comparison with steady state recombination data suggested a very weak impact of non-thermalized carriers on the recombination properties of the solar cells under application relevant illumination conditions. Finally, temperature and field dependent studies of free charge generation were performed on three donor-acceptor combinations, with two donor polymers of the same material family blended with two different fullerene acceptor molecules. These particular material combinations were chosen to analyze the influence of the energetic and morphology of the blend on the efficiency of charge generation. To this end, activation energies for photocurrent generation were accurately determined for a wide range of excitation energies. The results prove that the formation of free charge is via thermalized charge transfer states and does not involve hot exciton splitting. Surprisingly, activation energies were of the order of thermal energy at room temperature. This led to the important conclusion that organic solar cells perform well not because of predominate high energy pathways but because the thermalized CT states are weakly bound. In addition, a model is introduced to interconnect the dissociation efficiency of the charge transfer state with its recombination observable with photoluminescence, which rules out a previously proposed two-pool model for free charge formation and recombination. Finally, based on the results, proposals for the further development of organic solar cells are formulated. N2 - Organische Halbleiter sind eine vielversprechende Materialklasse. Ihre besonderen Eigenschaften sind die gute Absorption, das geringe Gewicht und die einfache Verarbeitung zu dünnen Filmen. Daher wird intensiv an der Realisierung organischer Dünnschichtsolarzellen (OPVs) geforscht. Aufgrund der niedrigen Dielektrizitätskonstante organischer Halbleiter sind primäre Anregungen (Exzitonen) jedoch stark gebunden, und es muss ein Typ II-Heteroübergang eingeführt werden, um diese Anregungen in freie Ladungen zu trennen. Daher bestehen die meisten organischen Solarzellen aus mindestens einem Elektronendonator und einem Elektronenakzeptormaterial. Für solche Donator-Akzeptorsysteme sind hauptsächlich drei Zustünde relevant; das Exziton auf dem Donator- oder Akzeptormaterial, der Ladungstransferzustand an der Donator-Akzeptor-Grenzfläche und der ladungsgetrennte Zustand eines freien Elektrons und Lochs. Das Zusammenspiel dieser Zustände bestimmt maßgeblich die Effizienz organischer Solarzellen. Aufgrund der hohen Absorption und der geringen Ladungsträgermobilitäten sind die aktiven Schichten dünn, aber auch die Exzitonendissoziation und die Bildung freier Ladung findet auf kurzen Zeitskalen statt, was die Analyse der Ladungsträgerdynamik erschwert. Im Mittelpunkt dieser Arbeit standen daher zunächst die Installation neuer Versuchsaufbauten zur Untersuchung der Ladungsträgerdynamik in optimierten Solarzellen mit überlegener Empfindlichkeit und Zeitauflösung sowie die Anwendung dieser Methoden auf prototypische photovoltaische Materialien. Im Hinblick auf das erste Ziel wurde ein neuer Aufbau, der optische Methoden wie die zeitaufgelöste Absorptionsspektroskopie (TAS) mit optolektronischen Methoden, wie die zeitaufgelöste Ladungsextraktion (TDCF) kombiniert. Ein wichtiger Teil dieser Arbeit war die Verbesserung der elektronischen Komponenten hinsichtlich der Zeitauflösung und Empfindlichkeit. Zu diesem Zweck wurde ein hochempfindlicher Verstärker zur Ansteuerung und Detektion der Probenantwort in TDCF entwickelt. Dieser Versuchsaufbau wurde im Folgenden auf verschiedene Modellsysteme angewendet, mit besonderem Fokus auf dem Verständnis der Verlustmechanismen, die den Füllfaktor und den Kurzschlussstrom organischer Solarzellen limittieren. Das erste Modellsystem ist ein hybrides Material, das aus anorganischen Quantenpunkten mit organischen Liganden besteht. Messungen mit TDCF ergaben eine schnelle Rekombination freier Ladungsträger, die teilweise durch Fallen unterstützt wurde, während Ladungsextraktionsexperimente unter quasi-konstanter Beleuchtung eine hohe Mobilität zeigten. Mit den gemessenen Parametern konnte die Kennlinie gut mit einem analytischen Modell beschrieben werden. Mit einer weiteren Verbesserung des Aufbaus konnte ein zweites Thema, die detaillierte Analyse der nicht-geminalen Rekombination in einer ungeordneten Polymer-Fulleren-Mischung bearbeitet werden. Die Messungen zeigten, dass hochmobilen Ladungen zu frühen Zeiten eine schnelle, nicht-geminale Rekombination an den Kontakten durchlaufen, was eine scheinbare Feldabhängigkeit der Generierung freier Ladung in TDCF-Experimenten verursachen kann. Andererseits wurde die Rekombination auf längeren Zeitskalen durch dispersive Rekombination in der aktiven Schicht bestimmt, wobei die Ursache der Ladungsträgerdynamik in einer exponentiellen Verteilung der Zustandsdichte liegt. Der Vergleich mit Rekombinationsdaten im stationären Zustand ergab einen sehr schwachen Einfluss nicht-thermalisierter Ladungsträger auf die Rekombinationseigenschaften der Solarzellen unter anwendungsrelevanten Bedingungen. Schließlich wurden temperatur- und feldabhängige Studien zur Erzeugung freier Ladung an drei Donator-Akzeptor-Kombinationen durchgeführt, wobei zwei Donatorpolymere der gleichen Materialfamilie mit zwei verschiedenen Fulleren-Akzeptormolekülen gemischt wurden. Diese besonderen Materialkombinationen wurden ausgewählt, um den Einfluss der Energetik und Morphologie der Mischung auf die Effizienz der Ladungserzeugung zu analysieren. Zu diesem Zweck wurden die Aktivierungsenergien für die Photostromerzeugung für einen weiten Bereich von Anregungsenergien genau bestimmt. Die Ergebnisse beweisen, dass die Bildung freier Ladungen über thermalisierte Ladungstransferzustände erfolgt und keine Aufspaltung mit heißen Exzitonen beinhaltet. Überraschenderweise sind die Aktivierungsenergien vergleichbar mit der thermischen Energie bei Raumtemperatur. Dies führte zu der wichtigen Schlussfolgerung, dass organische Solarzellen nicht aufgrund von hochenergetischen Pfaden gut funktionieren, sondern weil die thermalisierten Ladungstransferzustände schwach gebunden sind. Darüber hinaus wird ein Modell eingeführt, um die Dissoziationseffizienz des Ladungstransferzustands mit seiner mit Photolumineszenz beobachtbaren Rekombination zu verbinden, wodurch ein zuvor vorgeschlagenes Zwei-Pool-Modell für die Bildung und Rekombination von freien Ladungen ausgeschlossen wird. Abschließend werden auf Basis der Ergebnisse Vorschläge zur Weiterentwicklung organischer Solarzellen formuliert. KW - organic solar cells KW - time resolved pump probe spectroscopy Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-429099 ER -