TY - JOUR A1 - Kraffert, Felix A1 - Steyrleuthner, Robert A1 - Albrecht, Steve A1 - Neher, Dieter A1 - Scharber, Markus C. A1 - Bittl, Robert A1 - Behrends, Jan T1 - Charge Separation in PCPDTBT : PCBM Blends from an EPR Perspective JF - The journal of physical chemistry Y1 - 2014 U6 - https://doi.org/10.1021/jp509650v SN - 1932-7447 VL - 118 IS - 49 SP - 28482 EP - 28493 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Pradhan, Basudev A1 - Albrecht, Steve A1 - Stiller, Burkhard A1 - Neher, Dieter T1 - Inverted organic solar cells comprising low-temperature-processed ZnO films JF - Applied physics : A, Materials science & processing N2 - Inverted organic solar cells are fabricated using low-temperature-annealed ZnO film as an electron transport layer. Uniform ZnO films were prepared by spin coating a diethylzinc (DEZ) precursor solution in air, followed by annealing at 100 A degrees C. Organic solar cells prepared on these ZnO films with a 1:1 P3HT:PCBM blend as the active layer show a high power conversion efficiency of 4.03 %, which is more than 10 % higher than the PCE of solar cells comprising ZnO prepared via a high-temperature sol-gel route. Y1 - 2014 U6 - https://doi.org/10.1007/s00339-014-8373-8 SN - 0947-8396 SN - 1432-0630 VL - 115 IS - 2 SP - 365 EP - 369 PB - Springer CY - New York ER - TY - JOUR A1 - Fischer, Florian S. U. A1 - Trefz, Daniel A1 - Back, Justus A1 - Kayunkid, Navaphun A1 - Tornow, Benjamin A1 - Albrecht, Steve A1 - Yager, Kevin G. A1 - Singh, Gurpreet A1 - Karim, Alamgir A1 - Neher, Dieter A1 - Brinkmann, Martin A1 - Ludwigs, Sabine T1 - Highly Crystalline Films of PCPDTBT with Branched Side Chains by Solvent Vapor Crystallization: Influence on Opto-Electronic Properties JF - Advanced materials N2 - PCPDTBT, a marginally crystallizable polymer, is crystallized into a new crystal structure using solvent-vapor annealing. Highly ordered areas with three different polymer-chain orientations are identified using TEM/ED, GIWAXS, and polarized Raman spectroscopy. The optical and structural properties differ significantly from films prepared by standard device preparation protocols. Bilayer solar cells, however, show similar performance. Y1 - 2015 U6 - https://doi.org/10.1002/adma.201403475 SN - 0935-9648 SN - 1521-4095 VL - 27 IS - 7 SP - 1223 EP - 1228 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Roland, Steffen A1 - Neubert, Sebastian A1 - Albrecht, Steve A1 - Stannowski, Bernd A1 - Seger, Mark A1 - Facchetti, Antonio A1 - Schlatmann, Rutger A1 - Rech, Bernd A1 - Neher, Dieter T1 - Hybrid Organic/Inorganic Thin-Film Multijunction Solar Cells Exceeding 11% Power Conversion Efficiency JF - Advanced materials N2 - Hybrid multijunction solar cells comprising hydrogenated amorphous silicon and an organic bulk heterojunction are presented, reaching 11.7% power conversion efficiency. The benefits of merging inorganic and organic subcells are pointed out, the optimization of the cells, including optical modeling predictions and tuning of the recombination contact are described, and an outlook of this technique is given. Y1 - 2015 U6 - https://doi.org/10.1002/adma.201404698 SN - 0935-9648 SN - 1521-4095 VL - 27 IS - 7 SP - 1262 EP - 1267 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Kegelmann, Lukas A1 - Wolff, Christian Michael A1 - Awino, Celline A1 - Lang, Felix A1 - Unger, Eva L. A1 - Korte, Lars A1 - Dittrich, Thomas A1 - Neher, Dieter A1 - Rech, Bernd A1 - Albrecht, Steve T1 - It Takes Two to Tango-Double-Layer Selective Contacts in Perovskite Solar Cells for Improved Device Performance and Reduced Hysteresis JF - ACS applied materials & interfaces N2 - Solar cells made from inorganic organic perovskites have gradually approached market requirements as their efficiency and stability have improved tremendously in recent years. Planar low-temperature processed perovskite solar cells are advantageous for possible large-scale production but are more prone to exhibiting photocurrent hysteresis, especially in the regular n-i-p structure. Here, a systematic characterization of different electron selective contacts with a variety of chemical and electrical properties in planar n-i-p devices processed below 180 degrees C is presented. The inorganic metal oxides TiO2 and SnO2, the organic fullerene derivatives C-60, PCBM, and ICMA, as well as double-layers with a metal oxide/PCBM structure are used as electron transport materials (ETMs). Perovskite layers deposited atop, the different ETMs with the herein applied fabrication method show a similar morphology according to scanning electron microscopy. Further, surface photovoltage spectroscopy measurements indicate comparable perovskite absorber qualities on all ETMs, except TiO2, which shows a more prominent influence of defect states. Transient photoluminescence studies together with current voltage scans over a broad range of scan speeds reveal faster charge extraction, less pronounced hysteresis effects, and higher efficiencies for devices with fullerene compared to those with metal oxide ETMs. Beyond this, only double-layer ETM structures substantially diminish hysteresis effects for all performed scan speeds and strongly enhance the power conversion efficiency up to a champion stabilized value of 18.0%. The results indicate reduced recombination losses for a double-layer TiO2/PCBM contact design: First, a reduction of shunt paths through the fullerene to the ITO layer. Second, an improved hole blocking by the wide band gap metal oxide. Third, decreased transport losses due to an energetically more favorable contact, as implied by photoelectron spectroscopy measurements. The herein demonstrated improvements of multilayer selective contacts may serve as a general design guideline for perovskite solar cells. KW - perovskite solar cell KW - electron contact KW - double-layer KW - regular planar architecture KW - hysteresis KW - fullerene KW - metal oxide Y1 - 2017 U6 - https://doi.org/10.1021/acsami.7b00900 SN - 1944-8244 VL - 9 SP - 17246 EP - 17256 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Albrecht, Steve A1 - Schaefer, Sebastian A1 - Lange, Ilja A1 - Yilmaz, Seyfullah A1 - Dumsch, Ines A1 - Allard, Sybille A1 - Scherf, Ullrich A1 - Hertwig, Andreas A1 - Neher, Dieter T1 - Light management in PCPDTBT:PC70BM solar cells: A comparison of standard and inverted device structures JF - Organic electronics : physics, materials and applications N2 - We compare standard and inverted bulk heterojunction solar cells composed of PCPDTBT:PC70BM blends. Inverted devices comprising 100 nm thick active layers exhibited short circuit currents of 15 mA/cm(2), 10% larger than in corresponding standard devices. Modeling of the optical field distribution in the different device stacks proved that this enhancement originates from an increased absorption of incident light in the active layer. Internal quantum efficiencies (IQEs) were obtained from the direct comparison of experimentally derived and modeled currents for different layer thicknesses, yielding IQEs of similar to 70% for a layer thickness of 100 nm. Simulations predict a significant increase of the light harvesting efficiency upon increasing the layer thickness to 270 nm. However, a continuous deterioration of the photovoltaic properties with layer thickness was measured for both device architectures, attributed to incomplete charge extraction. On the other hand, our optical modeling suggests that inverted devices based on PCPDTBT should be able to deliver high power conversion efficiencies (PCEs) of more than 7% provided that recombination losses can be reduced. KW - Organic solar cells KW - Inverted solar cells KW - PCPDTBT KW - Low band-gap KW - Optical modeling Y1 - 2012 U6 - https://doi.org/10.1016/j.orgel.2011.12.019 SN - 1566-1199 VL - 13 IS - 4 SP - 615 EP - 622 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Albrecht, Steve A1 - Schindler, Wolfram A1 - Kurpiers, Jona A1 - Kniepert, Juliane A1 - Blakesley, James C. A1 - Dumsch, Ines A1 - Allard, Sybille A1 - Fostiropoulos, Konstantinos A1 - Scherf, Ullrich A1 - Neher, Dieter T1 - On the field dependence of free charge carrier generation and recombination in blends of PCPDTBT/PC70BM influence of solvent additives JF - The journal of physical chemistry letters N2 - We have applied time-delayed collection field (TDCF) and charge extraction by linearly increasing voltage (CELIV) to investigate the photogeneration, transport, and recombination of charge carriers in blends composed of PCPDTBT/PC70BM processed with and without the solvent additive diiodooctane. The results suggest that the solvent additive has severe impacts on the elementary processes involved in the photon to collected electron conversion in these blends. First, a pronounced field dependence of the free carrier generation is found for both blends, where the field dependence is stronger without the additive. Second, the fate of charge carriers in both blends can be described with a rather high bimolecular recombination coefficients, which increase with decreasing internal field. Third, the mobility is three to four times higher with the additive. Both blends show a negative field dependence of mobility, which we suggest to cause bias-dependent recombination coefficients. Y1 - 2012 U6 - https://doi.org/10.1021/jz3000849 SN - 1948-7185 VL - 3 IS - 5 SP - 640 EP - 645 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Albrecht, Steve A1 - Tumbleston, John R. A1 - Janietz, Silvia A1 - Dumsch, Ines A1 - Allard, Sybille A1 - Scherf, Ullrich A1 - Ade, Harald W. A1 - Neher, Dieter T1 - Quantifying charge extraction in organic solar cells: The case of fluorinated PCPDTBT JF - The journal of physical chemistry letters N2 - We introduce a new and simple method to quantify the effective extraction mobility in organic solar cells at low electric fields and charge carrier densities comparable to operation conditions under one sun illumination. By comparing steady-state carrier densities at constant illumination intensity and under open-circuit conditions, the gradient of the quasi-Fermi potential driving the current is estimated as a function of external bias and charge density. These properties are then related to the respective steady-state current to determine the effective extraction mobility. The new technique is applied to different derivatives of the well-known low-band-gap polymer PCPDTBT blended with PC70BM. We show that the slower average extraction due to lower mobility accounts for the moderate fill factor when solar cells are fabricated with mono- or difluorinated PCPDTBT. This lower extraction competes with improved generation and reduced nongeminate recombination, rendering the monofluorinated derivative the most efficient donor polymer. Y1 - 2014 U6 - https://doi.org/10.1021/jz500457b SN - 1948-7185 VL - 5 IS - 7 SP - 1131 EP - 1138 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Kurpiers, Jona A1 - Ferron, Thomas A1 - Roland, Steffen A1 - Jakoby, Marius A1 - Thiede, Tobias A1 - Jaiser, Frank A1 - Albrecht, Steve A1 - Janietz, Silvia A1 - Collins, Brian A. A1 - Howard, Ian A. A1 - Neher, Dieter T1 - Probing the pathways of free charge generation in organic bulk heterojunction solar cells JF - Nature Communications N2 - The fact that organic solar cells perform efficiently despite the low dielectric constant of most photoactive blends initiated a long-standing debate regarding the dominant pathways of free charge formation. Here, we address this issue through the accurate measurement of the activation energy for free charge photogeneration over a wide range of photon energy, using the method of time-delayed collection field. For our prototypical low bandgap polymer:fullerene blends, we find that neither the temperature nor the field dependence of free charge generation depend on the excitation energy, ruling out an appreciable contribution to free charge generation though hot carrier pathways. On the other hand, activation energies are on the order of the room temperature thermal energy for all studied blends. We conclude that charge generation in such devices proceeds through thermalized charge transfer states, and that thermal energy is sufficient to separate most of these states into free charges. Y1 - 2018 U6 - https://doi.org/10.1038/s41467-018-04386-3 SN - 2041-1723 VL - 9 PB - Nature Publ. Group CY - London ER - TY - THES A1 - Albrecht, Steve T1 - Generation, recombination and extraction of charges in polymer T1 - Generierung, Rekombination und Extraktion von Ladungen in Polymer BT - fullerene bulk heterojunction solar cells BT - Fulleren Mischsolarzellen N2 - A dramatic efficiency improvement of bulk heterojunction solar cells based on electron-donating conjugated polymers in combination with soluble fullerene derivatives has been achieved over the past years. Certified and reported power conversion efficiencies now reach over 9% for single junctions and exceed the 10% benchmark for tandem solar cells. This trend brightens the vision of organic photovoltaics becoming competitive with inorganic solar cells including the realization of low-cost and large-area organic photovoltaics. For the best performing organic materials systems, the yield of charge generation can be very efficient. However, a detailed understanding of the free charge carrier generation mechanisms at the donor acceptor interface and the energy loss associated with it needs to be established. Moreover, organic solar cells are limited by the competition between charge extraction and free charge recombination, accounting for further efficiency losses. A conclusive picture and the development of precise methodologies for investigating the fundamental processes in organic solar cells are crucial for future material design, efficiency optimization, and the implementation of organic solar cells into commercial products. In order to advance the development of organic photovoltaics, my thesis focuses on the comprehensive understanding of charge generation, recombination and extraction in organic bulk heterojunction solar cells summarized in 6 chapters on the cumulative basis of 7 individual publications. The general motivation guiding this work was the realization of an efficient hybrid inorganic/organic tandem solar cell with sub-cells made from amorphous hydrogenated silicon and organic bulk heterojunctions. To realize this project aim, the focus was directed to the low band-gap copolymer PCPDTBT and its derivatives, resulting in the examination of the charge carrier dynamics in PCPDTBT:PC70BM blends in relation to by the blend morphology. The phase separation in this blend can be controlled by the processing additive diiodooctane, enhancing domain purity and size. The quantitative investigation of the free charge formation was realized by utilizing and improving the time delayed collection field technique. Interestingly, a pronounced field dependence of the free carrier generation for all blends is found, with the field dependence being stronger without the additive. Also, the bimolecular recombination coefficient for both blends is rather high and increases with decreasing internal field which we suggest to be caused by a negative field dependence of mobility. The additive speeds up charge extraction which is rationalized by the threefold increase in mobility. By fluorine attachment within the electron deficient subunit of PCPDTBT, a new polymer F-PCPDTBT is designed. This new material is characterized by a stronger tendency to aggregate as compared to non-fluorinated PCPDTBT. Our measurements show that for F-PCPDTBT:PCBM blends the charge carrier generation becomes more efficient and the field-dependence of free charge carrier generation is weakened. The stronger tendency to aggregate induced by the fluorination also leads to increased polymer rich domains, accompanied in a threefold reduction in the non-geminate recombination coefficient at conditions of open circuit. The size of the polymer domains is nicely correlated to the field-dependence of charge generation and the Langevin reduction factor, which highlights the importance of the domain size and domain purity for efficient charge carrier generation. In total, fluorination of PCPDTBT causes the PCE to increase from 3.6 to 6.1% due to enhanced fill factor, short circuit current and open circuit voltage. Further optimization of the blend ratio, active layer thickness, and polymer molecular weight resulted in 6.6% efficiency for F-PCPDTBT:PC70BM solar cells. Interestingly, the double fluorinated version 2F-PCPDTBT exhibited poorer FF despite a further reduction of geminate and non-geminate recombination losses. To further analyze this finding, a new technique is developed that measures the effective extraction mobility under charge carrier densities and electrical fields comparable to solar cell operation conditions. This method involves the bias enhanced charge extraction technique. With the knowledge of the carrier density under different electrical field and illumination conditions, a conclusive picture of the changes in charge carrier dynamics leading to differences in the fill factor upon fluorination of PCPDTBT is attained. The more efficient charge generation and reduced recombination with fluorination is counterbalanced by a decreased extraction mobility. Thus, the highest fill factor of 60% and efficiency of 6.6% is reached for F-PCPDTBT blends, while 2F-PCPDTBT blends have only moderate fill factors of 54% caused by the lower effective extraction mobility, limiting the efficiency to 6.5%. To understand the details of the charge generation mechanism and the related losses, we evaluated the yield and field-dependence of free charge generation using time delayed collection field in combination with sensitive measurements of the external quantum efficiency and absorption coefficients for a variety of blends. Importantly, both the yield and field-dependence of free charge generation is found to be unaffected by excitation energy, including direct charge transfer excitation below the optical band gap. To access the non-detectable absorption at energies of the relaxed charge transfer emission, the absorption was reconstructed from the CT emission, induced via the recombination of thermalized charges in electroluminescence. For a variety of blends, the quantum yield at energies of charge transfer emission was identical to excitations with energies well above the optical band-gap. Thus, the generation proceeds via the split-up of the thermalized charge transfer states in working solar cells. Further measurements were conducted on blends with fine-tuned energy levels and similar blend morphologies by using different fullerene derivatives. A direct correlation between the efficiency of free carrier generation and the energy difference of the relaxed charge transfer state relative to the energy of the charge separated state is found. These findings open up new guidelines for future material design as new high efficiency materials require a minimum energetic offset between charge transfer and the charge separated state while keeping the HOMO level (and LUMO level) difference between donor and acceptor as small as possible. N2 - Die Effizienz von organischen Mischsolarzellen ist in den letzten Jahren durch die Entwicklung neuer halbleitender Materialen beträchtlich gestiegen. Die besten organischen Mischsolarzellen zeigen heute sehr effiziente Ladungsgeneration. Dennoch ist die genaue Funktionsweise der fundamentalen Ladungsgenerationsschritte nicht bis ins Detail verstanden. Zur weiteren Steigerung der Wirkungsgrade und für die kommerzielle Nutzung organischer Mischsolarzellen, sind ein übergreifendes Verständnis der Funktionsweise und die Entwicklung neuer Messmethoden unumgänglich. Die vorliegende Arbeit ist auf diese Fragestellung fokussiert: die Arbeit soll helfen, fundierte Kenntnisse der Ladungsträgererzeugung, der Rekombination und der Extraktion freier Ladungsträger in organischen Mischsolarzellen zu erlangen. Zuerst wurde der Fokus auf Mischsolarzellen mit dem Polymer PCPDTBT gelegt. Dieses Polymer durchmischt stark mit dem Fulleren-Derivat PCBM. Durch Verwendung eines Lösungsmitteladditives kann die Phasenentmischung und damit der Wirkungsgrad deutlich gesteigert werden. Die Generations- und Rekombinationsprozesse wurden mittels zeitverzögerter Sammelfeld-Methode untersucht. Dabei wurde zum ersten Mal eine signifikante Feldabhängigkeit der Ladungsträger-erzeugung entdeckt. Interessanterweise korreliert diese Feldabhängigkeit mit der Domänengröße also dem Grad der Entmischung. In größeren und reineren Polymerphasen ist die Feldabhängigkeit kleiner und die Extraktion verbessert, was zum höheren Wirkungsgrad führt. In einem weiteren Schritt wurde untersucht, wie sich die Fluorinierung des Polymers PCPDTBT auf das Bauteilverhalten auswirkt. Durch Fluorinierung des Polymer-Rückgrats von PCPDTBT wurden zum einen die Energieniveaus abgesenkt, ohne dass sich das Absorptionsverhalten geändert hat. Zum anderen wurde die Phasenentmischung beeinflusst. Mit Fluorinierung entstehen größere, reinere und kristallinere Polymerphasen. Dadurch wird die Generation der Ladungsträger effizienter und die Rekombination stärker unterdrückt. Eindeutige Korrelationen zwischen Phasengröße und Generationseffizienz konnten hierbei gefunden werden. Insgesamt steigt die Bauteileffizienz bei Verwendung von fluoriniertem PCPDTBT von 3.6 auf 6.1% bei gleicher Prozessierung. Durch weitere Optimierung konnte die Effizienz auf 6.6% für fluoriniertes PCPDTBT gesteigert werden. Eine di-Fluorinierung von PCPDTBT limitiert die Bauteileffizienz, speziell den Füll Faktor, trotz der Entstehung noch reinerer Polymerphasen. Eine genauere Analyse der Extraktionseffizienz mittels der genauen Bestimmung der Gleichgewichts-Ladungsträgerdichte für verschiedenen Beleuchtungs- und Feldsituationen zeigte, dass die Fluorinierung die Effizienz der Extraktion deutlich absenkt und dadurch bei di-Fluorinierung die Rekombinationsverluste im Bauteil trotz verlangsamter Rekombination ansteigen. Um weitere fundierte Kenntnisse der Ladungsgeneration zu gewinnen, wurde die Ladungsgeneration für verschiedene Gemische mit veränderten Energieniveaus in Abhängigkeit der Anregungsenergie untersucht. Dabei wurde die wichtige Kenntnis erlangt, dass die Photonenenergie, unabhängig von der Lage der Energieniveaus, keinen Einfluss auf die Effizienz der Generation hat und somit die Bildung freier Ladungsträger aus relaxierten Transferzuständen erfolgt. Dadurch ergeben sich neue Leitlinien für zukünftige Materialeigenschaften mit optimierten Wirkungsgraden. KW - organic solar cells KW - bulk heterojunction KW - charge carrier dynamics KW - charge generation KW - non geminate recombination KW - Generierung von Ladungsträgern KW - nicht geminale Rekombination KW - Extraktion KW - Polymer KW - Fulleren Y1 - 2014 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-72285 ER -