TY - GEN A1 - Stolterfoht, Martin A1 - Grischek, Max A1 - Caprioglio, Pietro A1 - Wolff, Christian Michael A1 - Gutierrez-Partida, Emilio A1 - Peña-Camargo, Francisco A1 - Rothhardt, Daniel A1 - Zhang, Shanshan A1 - Raoufi, Meysam A1 - Wolansky, Jakob A1 - Abdi-Jalebi, Mojtaba A1 - Stranks, Samuel D. A1 - Albrecht, Steve A1 - Kirchartz, Thomas A1 - Neher, Dieter T1 - How to quantify the efficiency potential of neat perovskite films BT - Perovskite semiconductors with an implied efficiency exceeding 28% T2 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - Perovskite photovoltaic (PV) cells have demonstrated power conversion efficiencies (PCE) that are close to those of monocrystalline silicon cells; however, in contrast to silicon PV, perovskites are not limited by Auger recombination under 1-sun illumination. Nevertheless, compared to GaAs and monocrystalline silicon PV, perovskite cells have significantly lower fill factors due to a combination of resistive and non-radiative recombination losses. This necessitates a deeper understanding of the underlying loss mechanisms and in particular the ideality factor of the cell. By measuring the intensity dependence of the external open-circuit voltage and the internal quasi-Fermi level splitting (QFLS), the transport resistance-free efficiency of the complete cell as well as the efficiency potential of any neat perovskite film with or without attached transport layers are quantified. Moreover, intensity-dependent QFLS measurements on different perovskite compositions allows for disentangling of the impact of the interfaces and the perovskite surface on the non-radiative fill factor and open-circuit voltage loss. It is found that potassium-passivated triple cation perovskite films stand out by their exceptionally high implied PCEs > 28%, which could be achieved with ideal transport layers. Finally, strategies are presented to reduce both the ideality factor and transport losses to push the efficiency to the thermodynamic limit. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1434 KW - non-radiative interface recombination KW - perovskite solar cells KW - photoluminescence Y1 - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-516622 SN - 1866-8372 IS - 17 ER - TY - GEN A1 - Caprioglio, Pietro A1 - Stolterfoht, Martin A1 - Wolff, Christian Michael A1 - Unold, Thomas A1 - Rech, Bernd A1 - Albrecht, Steve A1 - Neher, Dieter T1 - On the relation between the open‐circuit voltage and quasi‐Fermi level splitting in efficient perovskite solar cells T2 - Postprints der Universität Potsdam Mathematisch-Naturwissenschaftliche Reihe N2 - Today's perovskite solar cells (PSCs) are limited mainly by their open‐circuit voltage (VOC) due to nonradiative recombination. Therefore, a comprehensive understanding of the relevant recombination pathways is needed. Here, intensity‐dependent measurements of the quasi‐Fermi level splitting (QFLS) and of the VOC on the very same devices, including pin‐type PSCs with efficiencies above 20%, are performed. It is found that the QFLS in the perovskite lies significantly below its radiative limit for all intensities but also that the VOC is generally lower than the QFLS, violating one main assumption of the Shockley‐Queisser theory. This has far‐reaching implications for the applicability of some well‐established techniques, which use the VOC as a measure of the carrier densities in the absorber. By performing drift‐diffusion simulations, the intensity dependence of the QFLS, the QFLS‐VOC offset and the ideality factor are consistently explained by trap‐assisted recombination and energetic misalignment at the interfaces. Additionally, it is found that the saturation of the VOC at high intensities is caused by insufficient contact selectivity while heating effects are of minor importance. It is concluded that the analysis of the VOC does not provide reliable conclusions of the recombination pathways and that the knowledge of the QFLS‐VOC relation is of great importance. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 774 KW - electro‐optical materials KW - perovskite solar cells KW - photovoltaic devices KW - thin films Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-437595 SN - 1866-8372 IS - 774 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 -