Small Molecules As Acceptors in Organic Solar Cells, Direct
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DISSERTATION SUBMITTED TO THE COMBINED FACULTIES FOR THE NATURAL SCIENCES AND MATHEMATICS OF THE RUPERTO-CAROLA UNIVERSITY OF HEIDELBERG,GERMANY FOR THE DEGREE OF DOCTOR OF NATURAL SCIENCES Put forward by M.Sc. Vincent Lami Born in Komárno (Slovak Republic) Oral examination: 16.10.2019 VISUALISING THE VERTICAL ENERGETIC LANDSCAPE IN ORGANIC PHOTOVOLTAICS Referees: Prof. Dr. Yana Vaynzof Prof. Dr. Annemarie Pucci Visualisierung der vertikalen Energielandschaft in organischer Photovoltaik Das Ziel dieser Doktorarbeit ist das Erlangen eines besseren Verständnisses der En- ergieniveaus in organischen, mehrlagigen Bauelementen. Der erste Teil dieser Ar- beit beschäftigt sich mit einer neuen Klasse von nicht-fullerenen Akzeptoren in or- ganischen Solarzellen, den sogenannten N-Heteroazenen. Eine gezielte Variation der Seitenketten erlaubt eine Verdreifachung der Effizienz, teilweise durch eine verbesserte Oberflächenbeschaffenheit der Schichten. Messungen mit transienter Absorptionsspektroskopie zeigen eine ungewöhnliche, vom elektrischen Feld ab- hängige Ladungsseparation im Gegensatz zu gewöhnlichen Fulleren-Akzeptoren. Im zweiten Teil dieser Arbeit wird die Entwicklung einer neuartigen Methode dargestellt, welche es ermöglicht die Anpassung der Energieniveaus zu unter- suchen. Hierfür wird ultraviolette Photoemissionsspektroskopie mit einem es- senziell schadenfreiem Ätzen mittels Argon Cluster-Atomen kombiniert. Wir zeigen die Effektivität unserer Methode für mehrere hochmoderne, leistungsfähige Photovoltaik-Systeme und demonstrieren, dass deren geschätzte photovoltaische Bandlücken hervorragend sowohl mit der Energie der Charge-Transfer-Komplexen übereinstimmen, als auch mit den zugehörigen Leerlaufspannungen korrelieren. Zu- dem stellen wir die Vielseitigkeit unserer Methode dar, indem wir die Auswirkung von Degradation auf die Entwicklung der Energetik, vertikale Schichtenbildung, Injektionsbarrieren in verborgenen Schichten, Seitenkettenvariationen, Molekül- dotierung und die energetische Anpassung in ternär gemischten Systemen un- tersuchen. Zusammengefasst, diese Arbeit demonstriert das Potenzial und die vielfältige Verwendbarkeit unserer neuartigen Methode, die für ein besseres Ver- ständnis der vertikalen Zusammensetzung und energetischen Anpassung in dünnen, organischen Filmen genutzt werden kann. Dieses Verständnis hat eine entscheidende Rolle für die zukünftige Entwicklung optoelektronischer, organischer Bauelemente. Visualising the Vertical Energetic Landscape in Organic Photovoltaics The aim of this thesis is to achieve a better understanding of the energetic alignment in organic multi-layered devices. The first part of this thesis is dedicated to the in- vestigation of a new class of non-fullerene acceptor materials, N-Heteroacenes, in organic solar cells. Intentionally varying the side chain structure enables a threefold increase in device performance, partly due to an improved active layer morphology. In addition, by employing transient absorption spectroscopy, an uncommon electric field-dependent charge separation is found, starkly different than for the case of con- ventional fullerene acceptors. In the second part of this thesis, a novel method for the investigation of energy level alignment in organic layers is developed, based on com- bining ultra-violet photoemission spectroscopy and essentially damage-free argon gas cluster etching. The efficacy of the technique is shown on several state-of-the-art high-performance photovoltaic systems, with estimated photovoltaic gaps being in excellent agreement with charge transfer state energies, and in direct correlation with corresponding open-circuit voltages. Furthermore, the versatility of the technique is exemplified by its application to study the evolution of the energetic alignment upon environmental degradation, vertical stratification, injection barriers at buried inter- faces, side-chain variation, molecular doping and the energetic alignment in ternary blend systems. This work demonstrates the potential and wide applicability of our novel technique for understanding the vertical composition and energetic alignment in organic thin films. This understanding is crucial towards the future development of optoelectronic organic devices. List of Publications Publications Included in this Thesis [1] V.Lami, D. Leibold, P.Fassl, Y. J. Hofstetter, D. Becker-Koch, P.Biegger, F. Paulus, P. E. Hopkinson, M. Adams, U. H. F. Bunz, S. Huettner, I. Howard, A. A. Bakulin and Y. Vaynzof, N-Heteroacenes as a New Class of Non-Fullerene Electron Acceptors for Organic Bulk-Heterojunction Photovoltaic Devices. Sol. RRL 1, 1700053 (2017). [2] V. Lami, A. Weu, J. Zhang, Y. Chen, Z. Fei, M. Heeney, R. H. Friend and Y. Vaynzof, Visualising the Vertical Energetic Landscape in Organic Photovoltaics. Joule (2019). doi:10.1016/j.joule.2019.06.018 [3] J. Zhang, M. H. Futscher, V. Lami, F. U. Kosasih, C. Cho, Q. Gu, A. Sadhanala, A. J. Pearson, B. Kan, G. Divitini, X. Wan, D. Credgington, N. C. Greenham, Y. Chen, C. Ducati, B. Ehrler, Y. Vaynzof, R. H. Friend and A. A. Bakulin, Sequentially Deposited versus Conventional Nonfullerene Organic Solar Cells: Interfacial Trap States, Charge-Transfer States, and Vertical Stratification. in review, (2019). [4] A. Wittmann, G. Schweicher, K. Broch, J. Novak, V. Lami, Y. Vaynzof, E. McNellis, J. Sinova, K. Takimiya, S. Watanabe, J. Takeya and H. Sirringhaus, Tuning Spin Current Injection at Ferromagnet/Non-Magnet Interfaces by Molecular Design. in review, (2019). [5] H. Shekhar, V. Lami, O. Solomeshch, A. Fenigstein, L. Tomer, L. Becky, Y. Vayn- zof and N. Tessler, Doping induced performance enhancement in inverted small molecule organic photodiodes operating below 1V reverse bias - Towards compat- ibility with CMOS for imaging applications. Org. Electron. 67, 1–9 (2019). [6] V. Lami, Y. J. Hofstetter, J. Butscher and Y. Vaynzof, Origin of open-circuit voltage tuning in ternary organic solar cells. in preparation, (2019). Additional Publications [1] E. H. Menke, D. Leibold, V. Lami, Y. J. Hofstetter, M. Mastalerz and Y. Vaynzof, Triptycene-trisaroyleneimidazoles as non-fullerene acceptors - Influence of side- chains on solubility , device morphology and performance. Org. Electron. 47, 211–219 (2017). [2] P. Fassl, V. Lami, A. Bausch, Z. Wang, M. T. Klug, H. J. Snaith and Y. Vaynzof, Frac- tional deviations in precursor stoichiometry dictate the properties, performance and stability of perovskite photovoltaic devices. Energy Environ. Sci. 11, 3380–3391 (2018). [3] A. Weu, T. R. Hopper, V. Lami, J. A. Kreß, A. A. Bakulin and Y. Vaynzof Field- Assisted Exciton Dissociation in Highly Efficient PffBT4T-2OD:Fullerene Organic Solar Cells. Chem. Mater. 30, 2660–2667 (2018). [4] D. Leibold, V. Lami, Y. J. Hofstetter, D. Becker, A. Weu, P. Biegger, F. Paulus, U. H. F. Bunz, P. E. Hopkinson and A. A. Bakulin, Triptycenyl - phenazino - thiadiazole as acceptor in organic bulk- heterojunction solar cells. Org. Electron. 57, 285–291 (2018). [5] P. Fassl, S. Ternes, V. Lami, Y. Zakharko, D. Heimfarth, P. E. Hopkinson, F. Paulus, A. D. Taylor, J. Zaumseil and Y. Vaynzof, Effect of Crystal Grain Orientation on the Rate of Ionic Transport in Perovskite Polycrystalline Thin Films. ACS Appl. Mater. Interfaces 11, 2490–2499 (2019). [6] A. Weu, J. A. Kress, F. Paulus, D. Becker-Koch, V. Lami, A. A. Bakulin and Y. Vayn- zof, Oxygen-Induced Doping as a Degradation Mechanism in Highly Efficient Or- ganic Solar Cells. ACS Appl. Energy Mater. 2, 1943–1950 (2019). [7] D. D. Tune, H. Shirae, V. Lami, R. J. Headrick, M. Pasquali, Y. Vaynzof, S. Noda, E. K. Hobbie and B. S. Flavel, Stability of Chemically Doped Nanotube–Silicon Heterojunction Solar Cells: Role of Oxides at the Carbon–Silicon Interface. ACS Appl. Energy Mater. acsaem.9b01050 (2019). doi:10.1021/acsaem.9b01050 [8] L. M. Falk, K. P. Goetz, V. Lami, Q. An, P. Fassl, J. Herkel, F. Thome, A. D. Taylor, F. Paulus and Y. Vaynzof, Effect of Precursor Stoichiometry on the Performance and Stability of MAPbBr 3 Photovoltaic Devices. Energy Technol. ente.201900737 (2019). doi:10.1002/ente.201900737 [9] D. Becker-Koch, M. Albaladejo Siguan, V. Lami, F. Paulus, H. Xiang, Z. Chen, Y. Vaynzof, The double-edged sword of oxygen exposure of PbS quantum dot solar cells, in review (2019) [10] M. A. Siguan, D. Becker-Koch, A. D. Taylor, Q. Sun, V. Lami, P. G. Oppenheimer, F. Paulus, Y. Vaynzof, Efficient and Stable Hybrid Triple-Cation Perovskite/PbS Quantum Dot Solar Cells, submitted (2019) List of Conference Presentations [1] V. Lami, P. E. Hopkinson, and Y. Vaynzof, Development of a new method to probe the electronic structure of organic photovoltaic materials in a bulk heterojunction, Poster - DPG Spring Meeting 2017, Dresden, Germany [2] V.Lami and Y. Vaynzof, Direct Measurement of the Energetic Landscape of Solution- Processed Organic Photovoltaic Devices, Poster - MRS Fall Meeting 2018, Boston, USA [3] V. Lami, A. Weu, J. Zhang, Y. Chen, Z. Fei, M. Heeney, R. Friend, and Y. Vayn- zof, Visualising the Vertical Energetic Landscape in Organic Photovoltaics, Oral presentation - DPG Spring Meeting 2019, Regensburg, Germany [4] V. Lami, A. Weu, J. Zhang, Y. Chen, Z. Fei, M. Heeney, R. Friend, and Y. Vaynzof, Visualising the Energetic Landscape of Organic Photovoltaic Devices and the Ef- fects of Degradation, Oral presentation - EMRS Spring Meeting 2019, Nice, France Contents Abbrevations xii 1 Introduction1 2 Theoretical Background7 2.1 Physics of Solar Cells..............................7 2.1.1 Characteristic Properties and the Power Conversion