University of Groningen Synthesis of small molecules and π-conjugated polymers and their applications in organic solar cells Wang, Gongbao DOI: 10.33612/diss.99200057 IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2019 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Wang, G. (2019). Synthesis of small molecules and π-conjugated polymers and their applications in organic solar cells. University of Groningen. https://doi.org/10.33612/diss.99200057 Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). 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Download date: 10-10-2021 Synthesis of small molecules and π-conjugated polymers and their applications in organic solar cells Gongbao Wang Synthesis of small molecules and π-conjugated polymers and their applications in organic solar cells Gongbao Wang University of Groningen, The Netherlands ISBN (Printed): 978-94-034-2044-8 ISBN (E-book): 978-94-034-2043-1 This project was carried out in two research groups Chemical Biology which is part of Stratingh Institute for Chemistry and Chemistry of (Bio) Molecular Materials and Devices which is part of Stratingh Institute for Chemistry and Zernike Institute for Advanced Materials, University of Groningen, The Netherlands. This work was funded by Zernike Dieptestrategie, Bonus Incentive Scheme. Printed by: GVO drukkers & vormgevers B.V Front & Back: The cover art is an artistic description of stages of human evolution and designed by Difei Zhou. Original image is a photo taken from the wall of a bicycle shed in Groningen. Copyright © 2019 by G. Wang An electronic version of this dissertation is available at https://www.rug.nl/research/portal. Synthesis of small molecules and - conjugated polymers and their applications in organic solar cells PhD thesis to obtain the degree of PhD at the University of Groningen on the authority of the Rector Magnificus prof. C. Wijmenga and in accordance with the decision by the College of Deans. This thesis will be defended in public on Friday 11 October 2019 at 9.00 hours by Gongbao Wang born on 25 August 1989 in Jianxi, China Supervisor Prof. A.J. Minnaard Co-supervisor Dr. R.C. Chiechi Assessment Committee Prof. J.C. Hummelen Prof. R.M. Hildner Prof. R.J.M. Klein Gebbink Contents 1 Introduction 1 1.1 Organic photovoltaic devices . 2 1.2 휋-conjugated polymers . 3 1.3 The synthesis of 휋-conjugated polymers . 6 1.3.1 Introduction . 6 1.3.2 Kumada cross-coupling . 6 1.3.3 Negishi cross-coupling. 9 1.3.4 Stille cross-coupling . 11 1.3.5 Suzuki cross-coupling . 13 1.3.6 Direct (hetero)arylation polymerization (DHAP) . 15 1.4 Determination of the molecular weight of conjugated polymers via GPC. 19 1.5 Aim and outline of this thesis . 20 References. 21 2 Synthesis of Ene-yne-enes by Nickel-Catalyzed Double SN2’ sub- stitution of 1,6-Dichlorohexa-2,4-diyne 35 2.1 Introduction . 36 2.2 Results and Discussion . 38 2.3 Proposed mechanism . 44 2.4 Conclusion . 46 2.5 Experimental section . 47 2.5.1 General experimental details . 47 2.5.2 General procedure for quantitative NMR experiments . 47 2.5.3 Synthesis of the starting material . 48 2.5.4 General procedure for the double SN2’ reaction of 1,6- dichlorohexa-2,4-diyne . 49 2.5.5 X-ray structure determination of dicobalt complex 10 . 52 References. 55 viii Contents ix 3 Insights into the mechanism of bis(pinacolato)diboron-mediated polymerization for conjugated co-polymers 61 3.1 Introduction . 62 3.2 Results and discussion . 63 3.2.1 Model reaction for polymerization mechanism . 63 3.2.2 Analysis of the whole polymerization process . 64 3.2.3 Mechanism study and explanation . 65 3.3 Conclusions . 67 3.4 Experimental Section . 68 3.4.1 General Experimental details. 68 3.4.2 Synthesis of the Monomer . 68 3.4.3 Model reaction for polymerization mechanism . 70 References. 71 4 Establishment of a green and atom-economical polymerization methodology for 휋-conjugated polymers 74 4.1 Introduction . 75 4.2 Results and Discussion . 77 4.2.1 Exploration of polymerization conditions. 77 4.2.2 t BuLi-mediated polymerization. 78 4.3 Conclusion . 81 4.4 Experimental Section . 82 4.4.1 General experimental details . 82 4.4.2 Synthesis of the starting materials . 83 4.4.3 General Procedure for the t BuLi-mediated polymeriza- tion . 86 References. 86 5 Factors affecting stability of polymer:fullerene solar cells 91 5.1 Introduction . 93 5.2 Results and Discussion . 96 5.2.1 Performance of Devices processed without additives: In- itial Power Conversion Efficiency . 96 5.2.2 Stability of devices processed without additives. 97 5.2.3 Performance of Devices Processed with and without Ad- ditives: Initial Power Conversion Efficiency . 106 x Contents 5.2.4 Performance of Devices with and without additives: Sta- bility and Lifetime . 108 5.3 Conclusion . 130 5.4 Experimental . 132 5.4.1 Materials . 132 5.4.2 Solution Processing . 132 5.4.3 Device & Film Fabrication. 132 5.4.4 Characterization . 133 5.4.5 Synthesis of BDT-monomers . 134 References. 136 Summary 144 Samenvatting 146 Acknowledgements 148 1 Introduction 1 2 1. Introduction 1 1.1. Organic photovoltaic devices Organic electronics has been the focus of a growing body of investigation in the fields of physics and chemistry for more than 50 years. Up to only a short time ago, organic electronic and optical phenomena have been the domain of “pure research”, somewhat removed from practical application.[1] The attraction of this field has been the ability to modify chemical structure in ways that could directly impact the properties of the materials when deposited in thin film form. While there was always a hope that organic materials would ultimately have uses in applications occupied by “conventional” semiconductors, for a long time their stability and per- formance fell well short of those of devices based on materials such as silicon or gallium arsenide. That situation changed dramatically in the mid-1980s, with the demonstration of a low voltage and efficient thin film light emitting diode by Ching Tang and Steven van Slyke at Kodak.[2] Although that particular first demonstration was not of sufficiently high performance to replace existing technologies, it never- theless opened the door to the possibility of using organic thin films as a foundation for a new generation of optoelectronic devices. Organic photovoltaic devices, based on 휋-conjugated small molecules or polymers, offer great opportunities for low-cost solar energy conversion, due to several tech- nological advantages of organic materials.[3–7] The 휋-conjugation in these organic molecules and polymers results in typical energy gaps of 1 to 3 eV between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), leading to semiconducting behavior and strong interactions with visible and near-infrared light.The maximum absorption coefficients of typical or- ganic semiconductors are in the order of 105 cm-1, which suggests that the use of 100 nm thick films can absorb most of the incident light. Compared to their inorganic counterparts, these organic electronic materials generally have signifi- cantly lower material costs and can be made into thin films using inexpensive room- temperature processes.[1, 8, 9] Organic materials are intrinsically compatible with flexible substrates and high-throughput, roll-to-roll manufacturing processes; hence they are ideally suited for the fabrication of large-area electronic and optoelectronic devices with low cost.[3],[6],[10, 11] Moreover, the electronic and optical prop- erties of organic semiconductors can be tuned to a large extent with appropriate structure modifications during synthesis, leading to tailored materials properties for specific applications.[6] With low-cost materials and processes, some statistical studies have shown that photovoltaic modules based on organic semiconductors are expected to have a much shorter energy payback time and better environmental 1.2. 휋-conjugated polymers 3 sustainability with less greenhouse gas (CO2) emission than inorganic photovoltaic technologies.[10],[12] In recent years, the performance of organic photovoltaic de- 1 vices, including the efficiency and stability, has steadily improved, resulting from the development of new active materials and material processing/treatment meth- ods, novel device architectures and internal/external optical structures to manage the light. In addition to PV applications, organic semiconductors have been used to produce organic light-emitting devices for lighting and displays,[2],[13, 14] field- effect transistors,[15] photodetectors,[16–18] and memory devices,[19] some of which have already been commercialized.[6] 1.2. 휋-conjugated polymers A conjugated polymer is a carbon-based macromolecule through which the va- lence 휋-electrons are delocalized.